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T cells modified to express a chimeric antigen receptor (CAR) targeting CD19 can induce potent and sustained responses in children with relapsed/refractory acute lymphoblastic leukemia (ALL). The durability of remission is related to the length of time the CAR T cells persist. Efforts to understand differences in persistence have focused on the CAR construct, in particular the costimulatory signaling module of the chimeric receptor. We previously reported a robust intent-to-treat product manufacturing success rate and remission induction rate in children and young adults with recurrent/refractory B-ALL using the SCRI-CAR19v1 product, a second-generation CD19-specific CAR with 4-1BB costimulation coexpressed with the EGFRt cell-surface tag (NCT02028455). Following completion of the phase I study, two changes to CAR T-cell manufacturing were introduced: switching the T-cell activation reagent and omitting midculture EGFRt immunomagnetic selection. We tested the modified manufacturing process and resulting product, designated SCRI-CAR19v2, in a cohort of 21 subjects on the phase II arm of the trial. Here, we describe the unanticipated enhancement in product performance resulting in prolonged persistence and B-cell aplasia and improved leukemia-free survival with SCRI-CAR19v2 as compared with SCRI-CAR19v1.

Supplementary Data from Modified Manufacturing Process Modulates CD19CAR T-cell Engraftment Fitness and Leukemia-Free Survival in Pediatric and Young Adult Subjects

Supplemental Figure 1 and Figure 2 Legends Supplemental Tables 1-5 Legends

Supplemental Figure 1 and Figure 2 Supplemental Tables 1-5

T cells modified to express a chimeric antigen receptor (CAR) targeting CD19 can induce potent and sustained responses in children with relapsed/refractory acute lymphoblastic leukemia (ALL). The durability of remission is related to the length of time the CAR T cells persist. Efforts to understand differences in persistence have focused on the CAR construct, in particular the costimulatory signaling module of the chimeric receptor. We previously reported a robust intent-to-treat product manufacturing success rate and remission induction rate in children and young adults with recurrent/refractory B-ALL using the SCRI-CAR19v1 product, a second-generation CD19-specific CAR with 4-1BB costimulation coexpressed with the EGFRt cell-surface tag (NCT02028455). Following completion of the phase I study, two changes to CAR T-cell manufacturing were introduced: switching the T-cell activation reagent and omitting midculture EGFRt immunomagnetic selection. We tested the modified manufacturing process and resulting product, designated SCRI-CAR19v2, in a cohort of 21 subjects on the phase II arm of the trial. Here, we describe the unanticipated enhancement in product performance resulting in prolonged persistence and B-cell aplasia and improved leukemia-free survival with SCRI-CAR19v2 as compared with SCRI-CAR19v1.

PURPOSE CD19-targeted chimeric antigen receptor T cells (CD19-CAR) and blinatumomab effectively induce remission in relapsed or refractory B-cell acute lymphoblastic leukemia (ALL) but are also associated with CD19 antigen modulation. There are limited data regarding the impact of prior blinatumomab exposure on subsequent CD19-CAR outcomes. PATIENTS AND METHODS We conducted a multicenter, retrospective review of children and young adults with relapsed or refractory ALL who received CD19-CAR between 2012 and 2019. Primary objectives addressed 6-month relapse-free survival (RFS) and event-free survival (EFS), stratified by blinatumomab use. Secondary objectives included comparison of longer-term survival outcomes, complete remission rates, CD19 modulation, and identification of factors associated with EFS. RESULTS Of 420 patients (median age, 12.7 years; interquartile range, 7.1-17.5) treated with commercial tisagenlecleucel or one of three investigational CD19-CAR constructs, 77 (18.3%) received prior blinatumomab. Blinatumomab-exposed patients more frequently harbored KMT2A rearrangements and underwent a prior stem-cell transplant than blinatumomab-naïve patients. Among patients evaluable for CD19-CAR response (n = 412), blinatumomab nonresponders had lower complete remission rates to CD19-CAR (20 of 31, 64.5%) than blinatumomab responders (39 of 42, 92.9%) or blinatumomab-naive patients (317 of 339, 93.5%), P < .0001. Following CD19-CAR, blinatumomab nonresponders had worse 6-month EFS (27.3%; 95% CI, 13.6 to 43.0) compared with blinatumomab responders (66.9%; 95% CI, 50.6 to 78.9; P < .0001) or blinatumomab-naïve patients (72.6%; 95% CI, 67.5 to 77; P < .0001) and worse RFS. High-disease burden independently associated with inferior EFS. CD19-dim or partial expression (preinfusion) was more frequently seen in blinatumomab-exposed patients (13.3% v 6.5%; P = .06) and associated with lower EFS and RFS. CONCLUSION With the largest series to date in pediatric CD19-CAR, and, to our knowledge, the first to study the impact of sequential CD19 targeting, we demonstrate that blinatumomab nonresponse and high-disease burden were independently associated with worse RFS and EFS, identifying important indicators of long-term outcomes following CD19-CAR.

Chimeric antigen receptor T-cell (CART) therapy has transformed the treatment paradigm for pediatric patients with relapsed/refractory B-cell acute lymphoblastic leukemia (B-ALL), with complete remission rates in key pivotal CD19-CART trials ranging from 65% to 90%. Alongside this new therapy, new toxicity profiles and treatment limitations have emerged, necessitating toxicity consensus grading systems, cooperative group trials, and novel management approaches. This review highlights the results of key clinical trials of CART for pediatric hematologic malignancies, discusses the most common toxicities seen to date, and elucidates challenges, opportunities, and areas of active research to optimize this therapy.

Relapse following chimeric antigen receptor (CAR) T-cell therapy directed against CD19 for relapsed/refractory B-acute lymphoblastic leukemia (r/r B-ALL) remains a significant challenge. Three main patterns of relapse predominate: CD19 positive (CD19pos) relapse, CD19 negative (CD19neg) relapse, and lineage switch (LS). Development and validation of risk factors that predict relapse phenotype could help define potential pre- or post-CAR T-cell infusion interventions aimed at decreasing relapse. Our group sought to extensively characterize preinfusion risk factors associated with the development of each relapse pattern via a multicenter, retrospective review of children and young adults with r/r B-ALL treated with a murine-based CD19-CAR construct. Of 420 patients treated with CAR, 166 (39.5%) relapsed, including 83 (50%) CD19pos, 68 (41%) CD19neg, and 12 (7.2%) LS relapses. A greater cumulative number of prior complete remissions was associated with CD19pos relapses, whereas high preinfusion disease burden, prior blinatumomab nonresponse, older age, and 4-1BB CAR construct were associated with CD19neg relapses. The presence of a KMT2A rearrangement was the only preinfusion risk factor associated with LS. The median overall survival following a post-CAR relapse was 11.9 months (95% CI, 9-17) and was particularly dismal in patients experiencing an LS, with no long-term survivors following this pattern of relapse. Given the poor outcomes for those with post-CAR relapse, study of relapse prevention strategies, such as consolidative hematopoietic stem cell transplantation, is critical and warrants further investigation on prospective clinical trials.

As clinical advances with chimeric antigen receptor (CAR) T cells are increasingly described and the potential for extending their therapeutic benefit grows, optimizing the implementation of this therapeutic modality is imperative. The recognition and management of cytokine release syndrome (CRS) marked a milestone in this field; however, beyond the understanding gained in treating CRS, a host of additional toxicities and/or potential late effects of CAR T cell therapy warrant further investigation. A multicentre initiative involving experts in paediatric cell therapy, supportive care and/or study of late effects from cancer and haematopoietic stem cell transplantation was convened to facilitate the comprehensive study of extended CAR T cell-mediated toxicities and establish a framework for new systematic investigations of CAR T cell-related adverse events. Together, this group identified six key focus areas: extended monitoring of neurotoxicity and neurocognitive function, psychosocial considerations, infection and immune reconstitution, other end organ toxicities, evaluation of subsequent neoplasms, and strategies to optimize remission durability. Herein, we present the current understanding, gaps in knowledge and future directions of research addressing these CAR T cell-related outcomes. This systematic framework to study extended toxicities and optimization strategies will facilitate the translation of acquired experience and knowledge for optimal application of CAR T cell therapies. A host of additional toxicities and/or potential late effects of chimeric antigen receptor (CAR) T cell therapy beyond cytokine release syndrome (CRS) warrant further investigation. Herein, experts in paediatric cell therapy, supportive care and/or study of late effects from cancer and haematopoietic stem cell transplantation present six key focus research areas related to CAR T cell-related outcomes beyond CRS.

Acute lymphoblastic leukemia (ALL) is the most common pediatric malignancy. Advancements in technology that enhance our understanding of the biology of the disease, risk-adapted therapy, and enhanced supportive care have contributed to improved survival rates. However, additional clinical management is needed to improve outcomes for patients classified as high risk at presentation (eg, T-ALL, infant ALL) and who experience relapse. The NCCN Clinical Practice Guidelines in Oncology (NCCN Guidelines) for pediatric ALL provide recommendations on the workup, diagnostic evaluation, and treatment of the disease, including guidance on supportive care, hematopoietic stem cell transplantation, and pharmacogenomics. This portion of the NCCN Guidelines focuses on the frontline and relapsed/refractory management of pediatric ALL.

Introduction: Chimeric antigen receptor (CAR) T-cells redirected against CD19 have demonstrated remarkable clinical activity in children and adults with relapsed/refractory (r/r) B-cell malignancies. The risk of lineage switch (LS) following CD19-directed therapies has been well documented but has been primarily limited to case reports. Additionally, the risk of subsequent malignant neoplasms (SMN) following CAR T-cells has not yet been described. Distinguishing LS (B-ALL to myeloid malignancy) from a therapy-related myeloid neoplasm is both clinically and biologically relevant. The former emerges from a highly refractory leukemic clone, likely resistant to salvage therapy, whereas the latter represents a new malignancy that can be associated with long-term survival. Methods: We conducted a multicenter, retrospective review of children and young adults with r/r B-acute lymphoblastic leukemia (B-ALL) who received either commercial tisagenlecleucel or 1 of 3 investigational murine-based CD19-CAR constructs on clinical trials at 7 US centers between 2012-2019. Patients diagnosed with B-ALL before age 25 years were included and patients who had received any prior CAR product were excluded. Results: Of 420 CAR-treated patients, with a median follow-up of 30.1 months, 12 (2.9%) experienced LS and 6 (1.4%) developed a SMN (Table). The median time to diagnosis of LS following CAR T-cell infusion was significantly shorter compared to diagnosis of SMN (65.5 days vs. 883.5 days; p=0.005). Eleven of 12 patients (91.7%) with LS converted to acute myeloid leukemia (AML). One patient converted to mixed phenotype acute leukemia, B/myeloid type. The leukemia of 10 of 12 patients with LS harbored cytogenetics similar to those at initial diagnosis. For the remaining 2 patients with LS, cytogenetics were unavailable, but the leukemias were considered LS by the treating institution. KMT2Ar rearrangement (KMT2Ar) was a predominant cytogenetic abnormality seen in patients with LS. Overall, 38 of 420 patients (9%) had a KMT2Ar. KMT2Ar was present in 9 of 12 (75%) patients with LS compared to 20 of 408 (7.1%) non-LS patients (p Within the KMT2Ar cohort, 31 (81.6%) patients achieved a complete remission post-CAR. Eight of these patients received a consolidative HSCT (representing 4 first and 4 second HSCTs). No KMT2Ar patient experienced a post-HSCT LS and 3 are alive with a median follow-up of 1164 days post-CAR. In contrast, of the 23 KTM2Ar patients who did not receive HSCT post-CAR, 7 developed LS and 14 are alive with a median follow-up of 864 days post-CAR. Relative contraindications to post-CAR HSCT included a prior HSCT (n=11) or early LS (n=5). Of the 7 CAR non-responding patients with KMT2Ar, 2 (28.6%) had rapid emergence of LS by the first restaging timepoint. There are no long-term survivors following LS, regardless of KMT2A status, dying a median of 123 days (range, 36-594 days) after diagnosis of LS. The 6 SMNs were cholangiocarcinoma, synovial sarcoma, malignant melanoma and 3 therapy-related myeloid neoplasms (MDS/AML), distinguished from LS based on loss of original cytogenetics. Notably, 4/6 (67%) patients that developed a SMN had received an allogeneic HSCT prior to development of SMN. Four patients (67%) remain alive and in remission with a median follow-up of 304 days after diagnosis of SMN, including 2 patients with MDS/AML. Conclusions: In the largest series of pediatric patients treated with CAR T-cell therapy, we show that LS occurs in 2.9% of children. The presence of a KMT2Ar was the biggest risk factor, with 23.7% of these patients experiencing LS. We found that LS can occur very early in a patient's post CAR T-cell course, and despite a variety of treatment approaches, the outcomes for these patients are dismal. Given the predisposition to LS, the role for consolidative HSCT in KMT2Ar patients warrants further study. Limited by a short follow-up period, we saw SMNs in only 1.4% of our patients. Causality is unknown and likely unrelated to CAR-T, but this further supports the long-term safety of CAR T-cells in children with B-ALL. Figure 1 Figure 1. Disclosures Borowitz: Amgen, Blueprint Medicines: Honoraria. Lee: Harpoon Therapeutics: Consultancy; Amgen: Membership on an entity's Board of Directors or advisory committees; BMS: Membership on an entity's Board of Directors or advisory committees; Kite Pharma: Other: trial funding; Gilead: Other: trial funding. Grupp: Novartis, Adaptimmune, TCR2, Cellectis, Juno, Vertex, Allogene and Cabaletta: Other: Study steering committees or scientific advisory boards; Novartis, Roche, GSK, Humanigen, CBMG, Eureka, and Janssen/JnJ: Consultancy; Novartis, Kite, Vertex, and Servier: Research Funding; Jazz Pharmaceuticals: Consultancy, Other: Steering committee, Research Funding. Verneris: jazz: Other: advisory board; Novartis: Other: advisory board; Fate Therapeutics: Consultancy. Gore: Mirati: Current equity holder in publicly-traded company; Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees; Amgen: Consultancy, Current equity holder in publicly-traded company, Honoraria, Membership on an entity's Board of Directors or advisory committees; Roche/Genentech: Consultancy, Honoraria; Clovis: Current equity holder in publicly-traded company; Celgene: Current equity holder in publicly-traded company, Membership on an entity's Board of Directors or advisory committees; Sanofi Paris: Current equity holder in publicly-traded company; Anchiano: Current equity holder in publicly-traded company; Blueprint Medicines: Current equity holder in publicly-traded company. Brown: Novartis: Membership on an entity's Board of Directors or advisory committees; Takeda: Membership on an entity's Board of Directors or advisory committees; Amgen: Membership on an entity's Board of Directors or advisory committees; Kura: Membership on an entity's Board of Directors or advisory committees; KIte: Membership on an entity's Board of Directors or advisory committees. Pulsipher: Equillium: Membership on an entity's Board of Directors or advisory committees; Adaptive: Research Funding; Jasper Therapeutics: Honoraria. Rheingold: Pfizer: Research Funding; Optinose: Other: Spouse's current employment. Gardner: BMS: Patents & Royalties; Novartis: Consultancy.

Background: CD19 targeting chimeric antigen receptor (CAR) T cells have induced unprecedented remission rates in high-risk precursor B Acute Lymphoblastic Leukemia (ALL); however recurrent disease with CD19 antigen escape variants is not uncommon. Therefore, we developed a novel CD22 targeting CAR, and following preclinical validation, tested it in a first-in-human pediatric and young adult phase 1 clinical trial, PLAT-04 (NCT03244306). Four subjects were treated at 2 dose levels (DL) (1x10 6/kg (DL1) and 3x10 6/kg (DL2)). The CD22 CAR T cell product (SCRI-CAR22v1) was successfully manufactured (n=4) and no dose limiting toxicity (DLTs), cytokine release syndrome (CRS) or neurotoxicity was observed. However, all subjects had minimal CAR T cell expansion, with 3 of 4 subjects demonstrating persistent or progressive disease at day 21 evaluation despite continued CD22 expression on leukemic blasts. Based on the poor in vivo expansion and lack of activity, enrollment was voluntarily halted to interrogate and optimize the CAR construct for enhanced performance. Methods: Human T cells were transduced to express one of two CD22 CAR constructs. We designed SCRI-CAR22v2, a CD22 CAR that utilizes the same scFv as SCRI-CAR22v1 but with a shorter linker between M971 VH and VL and a shorter hinge with differing transmembrane region, and both using CD8 alpha (Figure A). This construct maintained the truncated EGFR extracellular tag (EGFRt) for tracking and potential in vivo suicide mechanism. The two transduced CAR T cell products were compared preclinically by flow cytometry, chromium release assay and in an in vivo murine model to understand differing T cell activity between the CAR constructs. Additionally, SCRI-CAR22v2 is currently under investigation in a dose finding phase 1 clinical trial, PLAT-07 (NCT04571138). Results: Following use of cetuximab-APC and biotinylated anti-human Fab antibody for surface EGFRt and CAR detection, the SCRI-CAR22v1 expresses lower levels of EGFRt but similar CAR levels on the cell surface demonstrated by MFI (Figure B). Biotinylated, soluble CD22 antigen was also used to evaluate CD22 CAR receptor activity and, as measured by MFI, a higher affinity is suggested via SCRI-CAR22v2 as compared to SCRI-CAR22v1 (Figure B). K562 cells expressing low, medium or high CD22 were used to evaluate the impact of surface antigen expression on the CAR activity level. SCRI-CAR22v2 demonstrates improved targeted cell lysis at all 3 antigen quantity levels by chromium release assay (Figure C). In NSG mice inoculated with Raji tumor cells expressing ffluc, SCRI-CAR22v2 demonstrated improved survival compared to SCRI-CAR22v1 (Figure D) and clearance of Raji tumor cells (Figure E). Based on this promising preclinical data, we initiated enrollment onto PLAT-07, a phase 1 dose finding trial (2x10 5cells/kg (DL1), 5x10 5cells/kg (DL2) and 1x10 6cells/kg (DL3)) of SCRI-CAR22v2. To date, 3 subjects have been enrolled and successfully infused at DL1. All had prior CD19-CAR therapy and 2 lacked CD19 leukemic expression at the time of SCRI-CAR22v2 infusion. At the time of cell infusion, one subject had only extramedullary disease, one had MRD of Conclusions: Despite encouraging preclinical data, SCRI-CAR22v1 demonstrated poor expansion and engraftment in a Phase 1 trial. Notably, minor CAR alterations lead to encouraging in-human activity in early clinical findings. Our experience suggests a shorter linker and hinge as well as incorporation of an CD8 alpha transmembrane region improves the clinical activity of CD22 targeted CAR T cells in subjects with recurrent disease following CD19 CAR T cells. Further evaluation is needed to elucidate the critical CAR components and/or assays at the preclinical level that can best predict which CAR should be brought to the clinic for further evaluation. Figure 1 Figure 1. Disclosures Orentas: Lentigen: Patents & Royalties. Jensen: BMS: Patents & Royalties; Umoja Biopharma: Current holder of stock options in a privately-held company, Membership on an entity's Board of Directors or advisory committees, Research Funding; Bluebird Bio: Research Funding. Gardner: Novartis: Consultancy; BMS: Patents & Royalties.

Introduction: CAR T cells in B-ALL have recently focused on the dual targeting of CD19 and CD22 to enhance long term remissions and prevent antigen negative recurrence that is frequently encountered with single antigen targeting. However, a barrier to this approach has been the retention of dual specificity killing and ongoing persistence. PLAT-05 is a multisite phase 1 trial (NCT03330691) that was undertaken to evaluate the safety and feasibility of SCRI-CAR19x22v1, a dual transduced patient-derived product with lentiviral vectors encoding for either a CD19- or CD22-specific CAR, both with 4-1BB co-stimulation. Early results of the first 27 subjects infused demonstrated feasibility and a favorable safety profile with encouraging CR rates. Products were fractionated evenly between CD19 CAR, CD19+CD22 CAR and CD22 CAR. However, engraftment was predominated by the single CD19 CAR population, leading to unsuccessful eradication of CD19-CD22+ leukemia. This finding led to re-engineering the CD22 CAR construct for enhanced CD22 targeting, and re-initiation of dose finding with the new product, SCRI-CAR19v2. Methods: After enrollment, subjects undergo apheresis followed by a combined CD4/CD8 positive immunomagnetic selection and seeded at a prescribed ratio for co-culture in a closed-system G-Rex bioreactor. Following anti-CD3xCD28 bead stimulation, T cells are transduced with two lentiviral vectors that encode for either a CD19- or CD22-specific CAR. After flu/cy lymphodepletion, CAR T cells are infused at one of three dose levels: 0.5, 1 or 3 X 10 6 CAR T cells/kg. Toxicity is graded according to CTCAEv5 except for CRS and ICANS which are graded per ASTCT criteria. Leukemic response and CAR T cell persistence are evaluated by flow cytometry. Results: 14 subjects enrolled onto PLAT-05 for the SCRI-CAR19x22v2 dose escalation and products were successfully manufactured in all subjects with an average of 8.9 days in culture (range 7-12 days). In contrast to v1 products, the CAR composition of v2 products was skewed in favor of CD22 CAR expression, with median expression of each population as follows: 42% CD22 only, 33% CD19 and CD22, 3.2% CD19 only. Twelve subjects were infused (0.5x10 6/kg n=3, 1x10 6/kg n=3, 3x10 6/kg n=6), 11 of whom had prior exposure to CD19 or CD22 targeted therapies with diverse expression of CD19 and CD22 on the leukemic blasts. No dose limiting toxicities occurred in the 11 fully evaluable subjects (1 subject is pending) and the recommended phase 2 dose was determined as 3x10 6 CAR + cells/kg. CRS was present in 45% of subjects, all grade 1. Neurotoxicity occurred in 45% of subjects, all grade 1 except a single self-limited grade 3 ICANS event (due to a single time point CAPD score). 91% of infused subjects obtained a CR, of which 100% were MRD negative. The non-responder had persistent disease that was CD19-CD22-. The in vivo engraftment of CAR T cells peaked most frequently between day +7 and +14 and was predominated by the CD22 CAR T cells, with some minimal contribution of dual and CD19 CAR T cells. Of the 4 subjects who had previously received an FMC63 based CD19 CAR, expansion was due to solely to the CD22 CARs in all 4 subjects, with apparent rejection of the T cells expressing CD19 CAR. Conclusions: We demonstrate enhanced activity of SCRI-CAR19x22v2 compared to v1, now with dual activity against both CD19 and CD22 demonstrated by elimination of ALL with single antigen expression. We maintained encouraging CR rates with a favorable toxicity profile. Interestingly, the product is predominated by CD22 CAR and CD19/CD22 CAR populations, while in vivo engraftment is predominated by single CD22 CAR expressing T cells. Subjects exposed to prior CD19 murine based CAR rejected the CD19 CAR but engrafted the CD22 CAR with demonstratable activity, a potential advantage of a dual transduced product. The impact of lower CD19 CAR engraftment on durable remissions is unknown. While limited expansion of the CD19 CAR population could be protective against exhaustion, the uneven engraftment of the CAR populations may ultimately lead to single antigen targeting. Optimization of transduction may be required for a more balanced product to maintain dual targeting and give further insight into the behavior of dual-expressing CAR T cells. An expansion cohort is currently underway to further characterize engraftment kinetics and in vivo performance to best inform future development of this product. Figure 1 Figure 1. Disclosures Pulsipher: Jasper Therapeutics: Honoraria; Adaptive: Research Funding; Equillium: Membership on an entity's Board of Directors or advisory committees. Li: Novartis Canada: Membership on an entity's Board of Directors or advisory committees. Jensen: Bluebird Bio: Research Funding; Umoja Biopharma: Current holder of stock options in a privately-held company, Membership on an entity's Board of Directors or advisory committees, Research Funding; BMS: Patents & Royalties. Gardner: Novartis: Consultancy; BMS: Patents & Royalties. OffLabel Disclosure: investigational use of SCRI-CAR19x22 will be discussed

Transitioning CD19-directed chimeric antigen receptor (CAR) T cells from early-phase trials in relapsed patients to a viable therapeutic approach with predictable efficacy and low toxicity for broad application among patients with high unmet need is currently complicated by product heterogeneity resulting from transduction of undefined T-cell mixtures, variability of transgene expression, and terminal differentiation of cells at the end of culture. A phase 1 trial of 45 children and young adults with relapsed or refractory B-lineage acute lymphoblastic leukemia was conducted using a CD19 CAR product of defined CD4/CD8 composition, uniform CAR expression, and limited effector differentiation. Products meeting all defined specifications occurred in 93% of enrolled patients. The maximum tolerated dose was 106 CAR T cells per kg, and there were no deaths or instances of cerebral edema attributable to product toxicity. The overall intent-to-treat minimal residual disease–negative (MRD−) remission rate for this phase 1 study was 89%. The MRD− remission rate was 93% in patients who received a CAR T-cell product and 100% in the subset of patients who received fludarabine and cyclophosphamide lymphodepletion. Twenty-three percent of patients developed reversible severe cytokine release syndrome and/or reversible severe neurotoxicity. These data demonstrate that manufacturing a defined-composition CD19 CAR T cell identifies an optimal cell dose with highly potent antitumor activity and a tolerable adverse effect profile in a cohort of patients with an otherwise poor prognosis. This trial was registered at www.clinicaltrials.gov as #NCT02028455.

Introduction: Both CD19 CAR T-cells (CD19 CAR) and blinatumomab (blina) effectively induce remission in patients with relapsed/refractory B-cell acute lymphoblastic leukemia (r/r ALL). However, blina use prior to CD19 CAR has raised concerns for increased relapse risk and/or CAR non-response due to selective pressure on CD19. The tisagenlecleucel registration trial in pediatric ALL excluded patients who had received prior blina. Thus, there is limited data regarding the impact of blina on long-term outcomes in patients receiving subsequent CD19 CAR. Methods: This retrospective, multicenter study was performed to evaluate the impact of pre-CAR blina on subsequent CD19 CAR outcomes in pediatric and young adult patients with r/r ALL < 25 years at diagnosis. The primary objective was to evaluate relapse (RFS) and event free survival (EFS) at 6 months following CD19 CAR stratified by blina use. Secondary objectives included: evaluation of RFS at 12 months and response to CD19 CAR. All patients had their first CAR T-cell infusion between the years 2012-2019 and had at least 30 days of follow-up (or an event prior to 30 days). Descriptive statistics were used for baseline demographics and comparison between cohorts. Kaplan-Meier estimates were used to evaluate survival. Results: A total of 420 patients from 7 centers received 1 of 3 CD19 CAR T-cell constructs. (Table 1) The median age at diagnosis and at CAR infusion was 7.1 years and 12.4 years, respectively (range, 0.6-30 years). Amongst 412 patients evaluable for response, a total of 375 (91.0%) patients achieved a complete remission (CR); 363 (96.8%) of whom were minimal residual disease (MRD) negative (by flow cytometry). Thirty-seven (9.0%) of evaluable patients were CD19 CAR non-responders. With a median potential follow-up of 2.3 years (IQR, 1.6-3.3 years), 164 (43.7%) patients experienced relapse. Seventy-five (17.8%) patients received blina prior to CD19 CAR. The median time from last blina use to CD19 CAR was 129 days (IQR, 79-304 days). Blina was associated with an increased risk of CAR non-response; 13/71 (18.3%) blina patients versus 24/341 (7.0%) non-blina patients were non-responders (p=0.0052). Ten of 71 (14.1%) were non-responders to both CD19 CAR and blina; 19 of 29 (66%) blina non-responders achieved remission with subsequent CD19 CAR. Baseline disease status did not differ between blina and non-blina patients at CAR T-cell infusion, although a higher fraction of blina patients harbored KMT2Ar cytogenetics (11/75 (14.7%) versus 22/345 (6.4%), p=0.03). Pre-CAR blina was associated with worse EFS and RFS, but not overall survival (OS). The 6-month RFS for blina and non-blina patients was 63.4% (95% CI, 49.6-74.4%) and 81.1% (95% CI, 76.3-85.0%), respectively (Figure 1A). The 6-month EFS for blina and non-blina patients was 49.7% (95% CI, 37.8-60.5%) and 72.1% (95% CI, 67.1-76.6%), respectively (Figure 1B). Analysis excluding KMT2Ar patients to evaluate for the possibility that these patients represented a higher-risk subgroup and could skew the data, revealed similar EFS and RFS. Amongst 408 patients with pre-CAR CD19 analysis, 6/69 (13.0%) of blina patients versus 21/339 (6.2%) of non-blina patients had CD19 dim/partial/negative disease (p=0.07). Serial CD19 evaluation pre/post blina revealed that 6/52 (11.5%) had CD19 evolution to dim expression. Conclusions: This large, multicenter analysis demonstrate an association with blina use and 1) increased risk of CAR non-response; 2) worse RFS and EFS and 3) a trend towards a higher incidence of pre-CAR CD19 dim disease. While blina non-response did not preclude CD19 CAR response, blina non-responders had lower remission rates to CD19 CAR and a cohort of patients were refractory to both, potentially suggesting resistance to immunotherapeutic CD19 targeting. Additionally, we found that blina may impact CD19 expression, which could subsequently affect response and relapse. Mechanisms of resistance to CD19 CAR include antigen escape or an inherent resistance to T-cell mediated killing. Our data suggest that 1) patients relapsing after or refractory to blina who proceed to CD19 CAR may have an inherent resistance and 2) blina may impact CD19 expression. Ongoing analysis includes detailed analysis of low/dim/partial CD19 expression to delineate the potential impact of blina exposure on leukemic blasts and evaluation of the role of HSCT. Disclosures Gore: Amgen, Novartis, Roche: Membership on an entity's Board of Directors or advisory committees. Brown:Novartis: Membership on an entity's Board of Directors or advisory committees; Servier: Membership on an entity's Board of Directors or advisory committees; Jazz: Membership on an entity's Board of Directors or advisory committees; Janssen: Membership on an entity's Board of Directors or advisory committees. Laetsch:Bayer: Consultancy, Research Funding; Cellectis: Consultancy; Pfizer: Research Funding; Novartis: Consultancy, Research Funding. Gardner:Novartis: Honoraria. Rheingold:Pfizer: Research Funding. Pulsipher:Mesoblast: Honoraria; Novartis: Honoraria; Adaptive: Research Funding; Miltenyi: Honoraria, Research Funding; Bellicum: Honoraria; Jasper: Honoraria.

Immunotherapy with the adoptive transfer of T cells redirected with CD19-specific chimeric antigen receptors (CARs) for B-lineage acute lymphoblastic leukemia (ALL) can salvage >80% of patients having relapsed/refractory disease. The therapeutic index of this emerging modality is attenuated by the occurrence of immunologic toxicity syndromes that occur upon CAR T-cell engraftment. Here, we report on the low incidence of severe cytokine release syndrome (CRS) in a subject treated with a CAR T-cell product composed of a defined ratio CD4:CD8 T-cell composition with a 4-1BB:zeta CAR targeting CD19 who also recieved early intervention treatment. We report that early intervention with tocilizumab and/or corticosteroids may reduce the frequency at which subjects transition from mild CRS to severe CRS. Although early intervention doubled the numbers of subjects dosed with tocilizumab and/or corticosteroids, there was no apparent detrimental effect on minimal residual disease-negative complete remission rates or subsequent persistence of functional CAR T cells compared with subjects who did not receive intervention. Moreover, early intervention therapy did not increase the proportion of subjects who experience neurotoxicity or place subjects at risk for infectious sequelae. These data support the contention that early intervention with tocilizumab and/or corticosteroids in subjects with early signs of CRS is without negative impact on the antitumor potency of CD19 CAR T cells. This intervention serves to enhance the therapeutic index in relapsed/refractory patients and provides the rationale to apply CAR T-cell therapy more broadly in ALL therapy. This trial was registered at www.clinicaltrials.gov as #NCT020284.

OBJECTIVE Immunotherapy for hematologic malignancies with CD19-directed CAR T cells is complicated by neurotoxicity in approximately 40% of patients. We have previously reported evidence of glial injury in pediatric patients with CAR T neurotoxicity by elevated CSF levels of GFAP and S100b. We now hypothesize that NFL is also a useful biomarker of neuronal injury related to abnormal blood-brain-barrier and glial function. METHODS We used the Mesoscale Discovery platform to measure CSF and serum NFL levels in a consecutive cohort of 43 pediatric patients with B cell ALL who received CD19-directed CAR T cells. In addition, we will present expansion cohort measurements of NFL and GFAP (N=95). RESULTS CSF NFL levels prior to CAR T cell infusion positively correlated with the risk of subsequently developing severe neurotoxicity (no neurotoxicity, median 275pg/mL, mild 378pg/mL, severe 951pg/mL, P=0.0182 for severe vs none, P=0.0458 for severe vs mild). During neurotoxicity, mean CSF NFL levels increased to 1179pg/mL (mild neurotoxicity, P=0.0338) and 1345 pg/mL (severe neurotoxicity, P=0.0148), respectively. In serum, pretreatment NFL levels were highly abnormal in many patients (median 368pg/mL, range 10–56,321pg/mL; healthy control median 4pg/mL, range 1–7.5pg/mL). However, there was no correlation with neurotoxicity, history of CNS radiation, peripheral neuropathy, stem cell transplant, or number of prior chemotherapies. Day 7 serum NFL levels did not change significantly (median 439pg/mL, range 5–17,439pg/mL, P=0.3254). CONCLUSION We conclude that CSF NFL is promising biomarker of CAR T neurotoxicity risk and severity. The abnormal baseline serum NFL concentrations remain unexplained and require further study.

3035 Background: Loss of CD19 expression is a major cause of limited durable B-ALL remission following CD19 CAR T cells, which might be overcome by utilization of dual CD19xCD22 CAR T cell targeting. Methods: A Phase I trial (NCT03330691) of SCRI-CAR19x22 was developed using dual transduction of lentiviral vectors encoding for either a CD19- or CD22-specific CAR T cell construct, both with 4-1BB co-stimulation. Manufacturing was performed in a closed G-Rex system with IL-7, IL-15 and IL-21. After lymphodepletion, CAR T cells were infused at 1 or 3 X 106 CAR T cells/kg dose levels. Leukemic response and CAR T cell persistence were evaluated by flow cytometry. Results: Products were successfully manufactured in all 28 enrolled subjects with 7.92 average days in culture (range of 7-11 days) and consisted of an average CD8:CD4 ratio of 3.09 (range 0.19 to 8.9). The cellular product CAR composition was 29% CD19, 31% CD22 and 39% CD19 and CD22 targeting. 13 subjects had prior exposure to CD19 or CD22 targeting therapies with diverse expression of CD19 and CD22 on the leukemic blasts. No dose limiting toxicities occurred in the 27 infused subjects. The recommended phase 2 dose is 3 x 106 CAR+ cells/kg. CRS was present in 80% of subjects, with 85% of CRS being grade 2 or less, and a peak grade of 3 (n = 3). Mild neurotoxicity occurred in 38%, with a single grade 3 event. 84.6% obtained a CR, of which 95% were MRD negative. Of the 4 subjects who did not achieved a CR, 2 had a pre-existing CD19 negative population and one had previously received CAR T cells and rejected SCRI-CAR19x22. There have been 4 relapses with varying CD19 and CD22 expression as follows: 1 CD19-CD22-, 1 CD19+CD22+, and 2 CD19-CD22+. The in vivo engraftment of CAR T cells peaked most frequently between day +7 and +14 and was predominated by the CD19 CAR+ T cells. Conclusions: We demonstrate manufacturing feasibility and safety of SCRI-CAR19x22. While initial efficacy is demonstrated, CD22 activity is poor due to limited expansion of the CD22 CAR-containing components and subjects with pre-existing CD19 negative leukemia fared poorly. Development of a revised CD22 CAR that exhibits a reduction tonic signaling is underway, with plans to explore the new construct in the context of a dual-targeting CD19xCD22 CAR T cell product. Clinical trial information: NCT03330691 .

Together, acute lymphoblastic leukemia (ALL) and acute myeloid leukemia (AML) make up approximately one-third of all pediatric cancer diagnoses. Despite remarkable improvement in the treatment outcomes of these diseases over the past several decades, the prognosis for certain high-risk groups of leukemia and for relapsed disease remains poor. However, recent insights into different types of ‘driver’ lesions of leukemogenesis, such as the aberrant activation of signaling pathways and various epigenetic modifications, have led to the discovery of novel agents that specifically target the mechanism of transformation. In parallel, emerging approaches in cancer immunotherapy have led to newer therapies that can exploit and harness cytotoxic immunity directed against malignant cells. This review details the rationale and implementation of recent and specifically targeted therapies in acute pediatric leukemia. Topics covered include the inhibition of critical cell signaling pathways [BCR-ABL, FMS-like tyrosine kinase 3 (FLT3), mammalian target of rapamycin (mTOR), and Janus-associated kinase (JAK)], proteasome inhibition, inhibition of epigenetic regulators of gene expression [DNA methyltransferase (DNMT) inhibitors, histone deacetylase (HDAC) inhibitors, and disruptor of telomeric signaling-1 (DOT1L) inhibitors], monoclonal antibodies and immunoconjugated toxins, bispecific T-cell engaging (BiTE) antibodies, and chimeric antigen receptor-modified (CAR) T cells.

Background The first step in the production of chimeric antigen receptor T cells is the collection of autologous T cells using apheresis technology. The procedure is technically challenging, because patients often have low leukocyte counts and are heavily pretreated with multiple lines of chemotherapy, marrow transplantation, and/or radiotherapy. Here, we report our experience of collecting T lymphocytes for chimeric antigen receptor T-cell manufacturing in pediatric and young adult patients with leukemia, non-Hodgkin lymphoma, or neuroblastoma. Study design and methods Apheresis procedures were performed on a COBE Spectra machine using the mononuclear cell program, with a collection target of 1 × 109 total mononuclear cells per kilogram. Data were collected regarding preapheresis and postapheresis blood counts, apheresis parameters, products, and adverse events. Results Ninety-nine patients (ages 1.3-25.7 years) and 102 apheresis events were available for analysis. Patients underwent apheresis at a variety of absolute lymphocyte cell counts, with a median absolute lymphocyte count of 944 cells/μL (range, 142-6944 cells/μL). Twenty-two patients (21.6%) had absolute lymphocyte counts less than 500 cells/μL. The mononuclear cell target was obtained in 100% of all apheresis harvests, and chimeric antigen receptor T-cell production was possible from the majority of collections (94%). Mononuclear cell collection efficiency was 65.4%, and T-lymphocyte collection efficiency was 83.4%. Ten patients (9.8%) presented with minor adverse events during the 102 apheresis procedures, with one exception of a severe allergy. Conclusions Mononuclear cell apheresis for chimeric antigen receptor T-cell therapy is well tolerated and safe, and it is possible to obtain an adequate quantity of CD3+ lymphocytes for chimeric antigen receptor T-cell manufacturing in heavily pretreated patients who have low lymphocyte counts.

Several CAR T designs with CD19 specificity have been associated with consistent responses in clinical trials with complete remission (CR) rates ranging from 70-90%. Relevant challenges remain to be addressed, such as production time, early loss of CAR T cells, relapse due to loss of the target antigen, and prevention of severe cytokine release syndrome and neurotoxicity. This review describes constructs, clinical trial results, side effects, and future direction of CAR T-cell therapy in B-ALL.

In spite of advances in the treatment of pediatric acute lymphoblastic leukemia (ALL), a significant number of children with ALL are not cured of their disease. We and others have shown that signaling from the bone marrow microenvironment confers therapeutic resistance, and that the interaction between CXCR4 and stromal cell-derived factor-1 (SDF-1 or CXCL12) is a key mediator of this effect. We demonstrate that ALL cells that upregulate surface CXCR4 in response to chemotherapy treatment are protected from chemotherapy-induced apoptosis when co-cultured with bone marrow stroma. Treatment with the CXCR4 inhibitor plerixafor diminishes stromal protection and confers chemosensitivity. Using xenograft models of high-risk pediatric ALL, plerixafor plus chemotherapy induces significantly decreased leukemic burden, compared to chemotherapy alone. Further, treatment with plerixafor and chemotherapy influences surface expression of CXCR4, VLA-4, and CXCR7 in surviving ALL blasts. Finally, prolonged exposure of ALL blasts to plerixafor leads to a persistent increase in surface CXCR4 expression, along with modulation of surface expression of additional adhesion molecules, and enhanced SDF-1α-induced chemotaxis, findings that may have implications for therapeutic resistance. Our results suggest that while CXCR4 inhibition may prove useful in ALL, further study is needed to understand the full effects of targeting the leukemic microenvironment.

Background: While immunotherapy withCD19 specific CAR T cells for relapsed/refractory (R/R) B-ALL achieves MRD negative remission in nearly all patients, relapse occurs in approximately half of patients and is frequently associated with early loss of CAR T cell persistence. Low CD19 antigen burden in the bone marrow prior to lymphodepletion and rapid contraction of CAR T cells in the blood after engraftment are predictive of early loss of CAR T cell persistence. We hypothesize that episodic antigen exposure using CD19t T cell antigen presenting cells (T-APCs) can trigger CD19 CAR T cell proliferation and re-activation in vivo, resulting in more durable CAR T cell persistence and diminished risk of CD19+ relapse. Here we report our experience to date using T-APCs following CD19 CAR T cell therapy for children and young adults with R/R B-ALL on clinical trial PLAT-03 (NCT03186118). Methods: Patient-derived T-APC products were manufactured from cryopreserved CD4/CD8 selected T cells. Cells were then activated with anti-CD3/CD28 beads, transduced with a lentiviral vector to express a truncated human CD19 (CD19t), and expanded in culture for 10 days. Subjects Results: Fourteen subjects (8-26 yrs) have been enrolled; 8 with low CD19 antigen burden, 5 with rapid CAR T cell contraction, and 1 subject with early loss of CAR T cell persistence on a predecessor study who received a re-infusion of CD19 CAR T cells on this study followed by T-APCs. T-APCs were successfully manufactured in 14/14 subjects. Two subjects lost CAR T cell persistence prior to T-APC infusion and were ineligible to receive T-APCs. To date, 11 subjects have received at least one dose of T-APCs. One of 11 subjects experienced a grade 3 febrile infusion reaction within hours of the 2nd dose of T-APCs, prohibiting further dosing. There were no other related adverse events (AEs) > grade 2 among the 11 subjects, and no cytokine release syndrome or neurotoxicity has been observed. An increase in detectable CD19 CAR T cells occurred in all subjects following T-APCs, and T-APCs can be transiently detected following infusion (Figure 1). Of the 10 treated subjects with low CD19 antigen burden or rapid T cell contraction, 8/10 had CAR T cell persistence beyond Day 63, as evidenced by B cell aplasia. At last follow-up, 5/10 have ongoing B cell aplasia, with a median follow up of 8.8 months (range, 2-18.5 months). The estimated 1-year leukemia free survival (LFS) is 69.2%. Conclusion: This first-in-human study of CD19t T-APCs demonstrates the ability to successfully manufacture T-APCs from stored apheresis products collected for CAR T cell production. In 11 subjects receiving at least one T-APC dose to date, there has been one T-APC infusion reaction and no other significant associated toxicity. Early evidence of efficacy demonstrated by secondary expansion of CAR T cells suggests the potential of CD19t T-APCs to enhance durable CD19 CAR T cell persistence. Figure 1 Disclosures Gardner: Novartis: Honoraria. Jensen:Bluebird Bio: Research Funding; Juno Therapeutics, a Celgene Company: Research Funding.

Background: The youngest patients referred for CAR T cell therapy are those with relapsed or refractory (R/R) KMT2A-rearranged infant B-ALL. Infants with relapsed ALL following Interfant-99 therapy have a dismal reported 3-yr OS of 20.9%, indicating the need for novel therapies. Smaller patient size, heavily pre-treated disease and high leukemia burden are often characteristics of this subgroup of patients that pose unique challenges to apheresis and manufacture of a T cell product. Additionally, reports of KMT2A-rearranged leukemia undergoing lineage switch following CD19-targeting pressure raises concern for an increased risk of myeloid leukemia relapses after B-lineage targeted CAR T cell therapy in this population. Here we report our experience using CAR T cell immunotherapy for patients with R/R infant ALL enrolled on clinical trials PLAT-02 (NCT02028455) and PLAT-05 (NCT03330691). Methods: PLAT-02 is a phase 1/2 trial of CD19-specific (FMC63scFv:IgG4hinge:CD28tm:4-1BB:ζ) CAR T cells. PLAT-05 is a phase 1 trial of CD19xCD22 dual specific CAR T cells, transduced with two separate lentiviral vectors to direct the co-expression of the CD19-specific CAR above and a CD22-specific CAR (m971scFv:IgG4hinge-CH2(L235D)-CH3-CD28tm:4-1BB:ζ). Eligible subjects on both studies have R/R B-ALL, an absolute lymphocyte count ≥100 cells/µL, and were at least 1 year of age. In addition, subjects on PLAT-02 were ≥ 10kg, and ≥ 8kg on PLAT-05. For cell manufacture, apheresis products were immuno-magnetically selected for CD4 and CD8 cells. Selected T cells were activated with anti-CD3/CD28 beads, transduced, and grown in culture with homeostatic cytokines to numbers suitable for clinical use. Infant ALL subjects received a range of 5x105 to 10x106 CAR+ T cells/kg following lymphodepleting chemotherapy. Disease response assessments were required at Day 21 and Day 63 following CAR T cell infusion. Adverse events were graded according to CTCAEv4, except CRS which was graded according to 2014 Lee criteria. Results: Eighteen subjects with R/R infant ALL have enrolled on PLAT-02 (n=14) or PLAT-05 (n=4), with a median age of 22.5 months at enrollment (range: 14.5 - 40.1 months). Of these, 2 (11.1%) had primary refractory disease, 8 (44.4%) were in 1st relapse, 7 (38.9%) were in 2nd relapse and 1 (5.6%) was in 3rd or greater relapse. Ten subjects (55.6%) had an M2 marrow or greater at enrollment prior to apheresis, and 9/18 had a history of hematopoietic cell transplant (HCT). The mean ALC was 1309 cells/µL (range 253-6944). Successful CAR T cell products were manufactured in 17/18 subjects, including in 9/9 subjects with no prior history of HCT. Of these, 16/17 subjects with available products were infused, with a median follow up of 26.9 months. One subject died of disease complications prior to CAR T cell infusion. Of the 16 treated subjects, 1 is pending disease and toxicity assessments. The maximum grade of CRS was 3 and occurred in two of 15 evaluable subjects (13%) and neurotoxicity was limited to a maximum grade of 2. Fourteen of 15 (93.3%) achieved an MRD negative complete remission (MRD-CR) by Day 21. Of the 14 subjects with an MRD-CR, 6 went on to HCT with 1 subsequent CD19 negative relapse. Of the 8 subjects who did not proceed to HCT, 1 developed lineage switch at one month following CAR T cells, and 1 died of infectious complications with aplasia. A "wait and watch" approach was taken for the remaining 6 subjects, and 2 developed CD19+ relapse. The incidence of lineage switch among the infant ALL group was 1/15 (6.7%). The estimated 1-year LFS was 66.7% and 1-year OS was 71.4%. Conclusion: This is the largest reported cohort to date of R/R infant B-ALL subjects treated with CAR T cell therapy. We report successful manufacture and administration of a CAR T cell product in the significant majority of infant subjects. Toxicity and MRD-CR rates are comparable to that of non-infant ALL subjects. In our experience, subjects with R/R infant ALL are not at increased risk for lineage switch relapse compared with the entire study populations following B-antigen targeting CAR T cell immunotherapy. Numbers in this report are too small to make definitive conclusions about the value of consolidative HCT. However, the LFS of this cohort is remarkably higher when compared with historical controls. Future work is focused on overcoming feasibility issues for the smallest of subjects, to enable a larger number of these cases to access CAR T cell therapy. Disclosures Pulsipher: Amgen: Other: Lecture; Bellicum: Consultancy; Miltenyi: Research Funding; Medac: Honoraria; Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Jazz: Other: Education for employees; Adaptive: Membership on an entity's Board of Directors or advisory committees, Research Funding; CSL Behring: Membership on an entity's Board of Directors or advisory committees. Wayne:AbbVie: Consultancy; Spectrum Pharmaceuticals: Consultancy, Research Funding; Servier: Consultancy; Kite, a Gilead Company: Consultancy, Research Funding. Jensen:Bluebird Bio: Research Funding; Juno Therapeutics, a Celgene Company: Research Funding. Gardner:Novartis: Honoraria.

Background:Pediatric patients with relapsed or refractory CD19+non-Hodgkin lymphoma (NHL) have poor outcomes despite use of chemotherapy and hematopoietic stem cell transplant (HSCT). Clinical trials of CD19 CAR T-cells have demonstrated efficacy in salvaging adult patients with relapsed and refractory NHL. Objectives:The objectives of this analysis is to assess the safety, toxicity, feasibility and efficacy of SCRI-CAR19v1 for pediatric patients with relapsed or refractory NHL. Design/Methods:The ongoing phase 2 trial (NCT02028455) has enrolled and treated 8 pediatric subjects with CD19+NHL. Subjects underwent apheresis, with their CD4 and CD8 T cell subsets prepared immunomagnetically. T cells were stimulated with anti-CD3xCD28 bead stimulation, and then transduced with a SIN lentiviral vector to direct co-expression of the FMC63scFv:IgG4hinge:CD28tm:4-1BB:ζ CAR and the selection/tracking/suicide construct EGFRt. The transduced cells were propagated using recombinant human cytokines to numbers suitable for clinical use. Subjects received lymphodepletion of fludarabine and cyclophosphamide followed by 1x106CD19 CAR T-cells/kg as a 1:1 ratio of CD4 and CD8 cells. Response was assessed at 3 and 9 weeks. Adverse events were graded according to CTCAEv4 except cytokine release syndrome (CRS) was graded according to Lee et al. Results: Treated subjects had relapsed or refractory diffuse large B cell lymphoma (DLBCL) (4/8), Burkitt's lymphoma (2/8), gray zone B cell lymphoma (1/8), primary mediastinal B cell lymphoma (PMBCL) (1/8), and ranged from 4-18 years old. Two subjects received prior hematopoietic stem cell transplant (HSCT); the subject with PMBCL received auto- and allogeneic HSCT and a subject with Burkitt's received autologous HSCT. Five subjects received prior immunotherapy with brentuximab, nivolumab, rituximab, and/or obinutuzumab. One subject had received ibrutinib. No subject had received prior CAR T-cells. CD4 and CD8 products were successfully manufactured and infused for all subjects. All subjects had expansion of CAR T-cells in the peripheral blood, bone marrow and CSF, with ongoing persistence at last check (range 14 days - 9 months). Toxicity information through day 30 revealed the occurrence of mild CRS in 4 subjects (grade 1 n=3, grade 2 n=1), and one case of severe CRS (grade 3). Mild neurotoxicity was observed in 2 subjects (grade 1 n=1, grade 2 n=1) with no occurrence of severe neurotoxicity. Response assessment at 3 weeks (n=6) revealed anti-tumor responses in 5 subjects, including complete response (CR) by week 9 (n=2, both DLBCL). CR was not sustained in either subject despite ongoing CAR T cell persistence. One of these subjects had a PET avid lesion proven by biopsy to be necrotic tissue but subsequently developed CD19+recurrence at that site. The other subject developed a new CD19+site of disease at six months; however, achieved a 2nd CR 3 weeks after receiving a second infusion of the originally manufactured CAR T-cells. One partial response (PR) subject experienced clearance of marrow disease with stable lymphoma but developed CD19 negative progression at 9 weeks. Updated enrollment, toxicity and response assessments will be presented. Conclusion:SCRI-CAR19v1 therapy demonstrates efficacy in pediatric patients with relapsed and refractory NHL and appears to be well tolerated with less severe toxicities than observed for pediatric patients with CD19+leukemia. Persistence of the CAR T-cells is excellent with no early loss of CAR T-cell engraftment reported to date. Although early responses were observed, these were not durable perhaps reflecting biologic/immunologic differences between B cell lymphomas in children in comparison to NHL in adults. Disclosures Pulsipher: Adaptive Biotech: Consultancy, Research Funding; Amgen: Honoraria; CSL Behring: Consultancy; Novartis: Consultancy, Honoraria, Speakers Bureau. Park:Bristol-Myers Squibb: Membership on an entity's Board of Directors or advisory committees. Jensen:Juno Therapeutics, Inc.: Consultancy, Patents & Royalties, Research Funding.

Introduction: Chimeric antigen receptor (CAR)-modified T cells targeting CD19 can induce potent and sustained responses in children with relapsed/refractory acute lymphoblastic leukemia (rrALL). We previously reported a robust intent-to-treat product-manufacturing success rate of 100% in minimally selected, heavily pretreated patients with ALL in our Pediatric and young adult Leukemia Adoptive Therapy (PLAT)-02 phase 1 study (NCT02028455). Following completion of the Phase 1 study, a minimal change in CAR T cell manufacturing (henceforth referred to as SCRI-CAR19v2) was introduced with product infusion in 21 subjects.We sought to compare efficacy and toxicity of the subjects who received SCRI-CAR19v2 with those subjects enrolled on the Phase 1 portion of the trial (Gardner et al., Blood 2017). Methods: All subjects underwent apheresis, and CD4 and CD8 T cell subsets are selected immunomagnetically. SCRI-CAR19v2 manufacturing changes from the previously published phase 1 platform included no longer selecting cells in culture based of EGFRt expression and changing the manufacturer of the anti-CD3/CD28 bead stimulation. All patients received fludarabine/cyclophosphamide lymphodepletion (LD) followed by bedside thaw of CD4 and CD8 T cell products and infusion of each product at a 1:1 ratio for a total of 1x106 CAR-T cells/kg. Cytokine release syndrome (CRS) was graded according to Lee et al. and treated with our early intervention strategy of tocilizumab and dexamethasone for persistent, mild CRS. Results: Twenty-one subjects with CD19+ rrALL received SCRI-CAR19v2, with a median age of 13 years (range, 8-17 years), 10 subjects (47.6%) had a history of at least 1 prior allogeneic transplantation, with a range of 335 days from most recent transplantation before enrollment. The disease burden included 7 (33.3%) subjects having M3 bone marrow (BM), 5 (24%) having active CNS involvement at time of LD. Thirteen (62%) subjects had a high antigen burden (>15% CD19 antigen expressing cells in BM by flowcytometry prior to LD). CAR T cell products were released on all 21 subjects. 86% (18/21) of subjects had a documented minimal residual disease (MRD)-negative complete remission (CR) within 21 days following CAR-T cell therapy. The duration of functional CAR-T cell persistence, as measured by ongoing B-cell aplasia (BCA) was 61.1% at 1 year, compared to 20.6% for Phase 1 subjects (p=0.03). With a minimum of 1-year follow up (range 12-18 months), the 12-month event-free survival (EFS) and overall survival were 76% (95% CI 59-97) and 86% (95% CI 72-100), respectively; compared to 50.8% (95% CI, 36.9%-69.9%) and 69.5% (95%CI, 55.8%-86.5%) of phase 1(p=0.14 and 0.23, Figure 1). Of the 5 leukemic relapses, 1 lacked CD19 and 2 recurred as AML. Any grade CRS was seen in 71% (15/21) of infused subjects, with a maximum grade of 2. Although the grading scale from phase 1 to 2 was changed from CTCAE to Lee et al, there did not appear to be any differences in the rate of those requiring low dose vasopressors. Any grade neurotoxicity was seen in 67% (14/21) with grade ≥3 in 24% (5/21), which was similar to the phase 1, however, there was a single event of grade 5 cerebral edema, which had not previously been seen. Comparison of SCRI-CAR19v2 phenotype and functional attributes to SCRI-CAR19v1 revealed evidence of increased CD4 differentiation in the SCRI-CAR19v2 products (lower expression of CCR7, CD27, CD127, high expression of LAG-3 and TIM-3) while SCRI-CAR19v1 products showed a lower frequency of LAG-3, PD-1 and the percentage of cells secreting IFNg, suggesting a less activated phenotype. Conclusions:A small manufacturing change was associated with a more effector driven phenotype of the SCRI-CAR19v2 T cell product, which appears to result in longer CAR-T cell persistence (prolonged BCA). Although the overall rates of CRS and neurotoxicity appeared similar between the two groups, the single event of fatal cerebral edema invoked concern that it could be related to the more effector driven phenotype. Much of the discussion surrounding the toxicity profiles and long term persistence of CAR T cell products has focused on the construct itself, including the FcFv and co-stimulatory molecules. However here we show that a change in the manufacturing platform itself can have an impact on the in vivo performance of the CAR T cell product, potentially effecting both the toxicity profile and the ability to enhance long term persistance. Figure. Figure. Disclosures Pulsipher: Amgen: Honoraria; CSL Behring: Consultancy; Novartis: Consultancy, Honoraria, Speakers Bureau; Adaptive Biotech: Consultancy, Research Funding. Park:Bristol-Myers Squibb: Membership on an entity's Board of Directors or advisory committees. Jensen:Juno Therapeutics, Inc.: Consultancy, Patents & Royalties, Research Funding.

Background: CD19 chimeric antigen receptor (CAR) T cell therapy has demonstrated robust responses in refractory/relapsed subjects with CD19+ acute lymphoblastic leukemia (ALL). Our Phase 1 clinical trial demonstrated a minimal residual disease (MRD) negative complete remission (CR) rate of 93% at 21 days following SCRI-CAR19v1 (a CD19 specific CAR T cell product) infusion (PMID: 29171004). Though remission is frequently attained, approximately half of patients recur. Controversy exists regarding the benefit of hematopoietic cell transplant (HCT) following CD19 CAR T cell therapy. Although not mandated by the study, our current institutional recommendation for relapsed/refractory patients without a history of allogeneic HCT is to undergo a HCT once in remission following SCRI-CAR19v1. Additionally, we have recommended HCT to those who have a short duration of persistence of SCRI-CAR19v1 in vivo regardless of prior HCT status. We report here the leukemia free survival (LFS) of subjects who proceeded to HCT following remission after SCRI-CAR19v1 infusion. Methods/Results: We analyzed the first 64 subjects on our Phase 1/2 trial, PLAT-02 (NCT02028455), with follow-up of ≥1 year to evaluate the potential benefit of consolidative HCT. We excluded subjects that did not respond (n=5) or relapsed prior to day 63 (n=9). Thirty-two of the evaluated subjects were treated on the Phase I dose finding portion of the study and 18 were treated on the Phase 2 portion at dose level of 1x106cells/kg. Of the 50 evaluable subjects, 33 had a history of at least one prior HCT, whereas 17 had no history of HCT. Figure A demonstrates the LFS for patients that did and did not undergo HCT following SCRI-CAR19v1. There is a trend towards improved LFS (p-value 0.057; Figure B) in patients who underwent first HCT following SCRI-CAR19v1. Of the 17 patients without a history of HCT, 3 did not pursue HCT following CAR therapy. Of those 3 subjects, only 1 remains in remission at 28 months. The other 2 subjects relapsed at 6 and 7 months with CD19+ and CD19- disease, respectively. Of the 14 patients that underwent first HCT after SCRI-CAR19v1, 2 relapsed following HCT. One had evidence of MRD by flow at the time of HCT and the other subject relapsed with CD19- disease. The role of second HCT following CAR therapy remains unclear. Of the 33 subjects with a history of HCT, 10 underwent a second HCT following SCRI-CAR19v1 infusion, and 5 are alive and remain in remission. The recurrences included 2 lineage switches and 2 CD19+ (1 was MRD positive by deep sequence prior to HCT), and one transplant related mortality (TRM). Of the 23 subjects that did not undergo a second HCT, 8 remain in remission (p-value not significant (NS); Figure C). We previously reported that subjects with a short duration of B cell aplasia (BCA) ≤63 days following SCRI-CAR19v1 have an increased risk of relapse. Here we show that subjects with short BCA who attained a CR and did not relapse prior to day 63 demonstrate a clear benefit of consolidative HCT (p-value 0.007; Figure D). Of the 15 subjects with short BCA (regardless of HCT history), 6 did not pursue HCT and all recurred (5 CD19+, 1 CD19-). Of the short BCA subjects that underwent HCT, 1 subject died of TRM (2ndHCT for this subject) and 2 subjects relapsed following HCT (1 was MRD positive prior to HCT). All events in the consolidative HCT group have occurred by 20 months following SCRI-CAR19v1. However, we continue to see late relapses following CAR T cell therapy in the group who did not proceed with HCT. Three subjects have relapsed beyond 2 years: 1 with CD19+ disease at 27months, 1 with CD19- disease at 41 months, and 1 patient with lineage switch AML at 38 months. Further analysis is needed to understand the continued long-term risk of relapse following CD19 CAR T cell therapy and the potential role and timing for consolidative HCT in patients with a previous HCT. Conclusions: We demonstrate a trend towards improved LFS for subjects without a history of HCT who undergo a consolidative HCT following CD19 CAR T cell therapy on PLAT-02. In addition, HCT appears to benefit subjects who attain a CR but are at increased risk of relapse with short-term BCA. Currently, the benefit of second HCT following CD19 CAR T cell therapy is unclear and may be restricted to those that have short functional persistence of SCRI-CAR19v1. Late relapses after SCRI-CAR19v1 have only occurred in those without consolidative HCT, but longer follow up is needed to confirm these findings. Figure Figure. Disclosures Jensen: Juno Therapeutics, Inc.: Consultancy, Patents & Royalties, Research Funding.

Background: Immunotherapy with CD19 specific chimeric antigen receptor (CAR) T cells for relapsed/refractory acute lymphoblastic leukemia (ALL) demonstrated a minimal residual disease (MRD) negative remission rate of 93% on the phase 1 portion of Pediatric Leukemia Adoptive Therapy (PLAT)-02. However, the 1-year event free survival (EFS) was 50.8%, largely due to recurrence of disease which was frequently associated with early loss of CAR T cell persistence. Subjects on PLAT-02 with Methods: Eligible subjects for PLAT-03 (NCT03186118), a pilot study of CD19t T-APCs, must have previously enrolled on the phase 2 portion of PLAT-02 and have a stored apheresis product available. Cohorts include subjects that have an identified predictive factor to lose CAR T cell persistence early, or have already experienced early loss of CAR T cell persistence before 6 months and can receive a reinfusion of CD19 CAR T cells prior to planned T-APCs. Patient-derived T-APC products are manufactured with selected and cryopreserved CD4 and CD8 cells from the subject's original apheresis product. The T cells are activated and transduced with a lentiviral vector to express the CD19t transgene. Transduced cells are expanded in culture for 10-14 days, cryopreserved and release-tested. Subjects Results: Seven subjects (ages 9-23 years) have enrolled on PLAT-03; 5 enrolled based on low CD19 antigen burden at the time of lymphodepletion, and 2 based on rapid CAR T cell contraction between Days 10 and 14. T-APC products were successfully manufactured in 5/5 subjects, with two products pending manufacture. One of 5 subjects was unable to receive his planned T-APCs, as he lost CAR T cell persistence prior to T-APC infusion and failed to engraft a second infusion of CD19 CAR T cells. Of the 4 subjects that have received at least one dose of T-APCs, there have been no related adverse events (AEs) > grade 2, and no fever or cytokine release syndrome (CRS) has been observed. Grade 1-2 AEs that are at least possibly related to T-APCs have included hypogammaglobulinemia, nausea, vomiting and headache. Two subjects have completed T-APC treatment; one received all 5 available T-APC doses (subject S001), and one experienced loss of CAR T cell persistence after two T-APC doses and therefore was not eligible for further doses. Data is shown for subject S001, a 20 year old man with multiply relapsed B-ALL. He enrolled on PLAT-03 due to a low CD19 antigen burden of 4.79% by flow cytometry (inclusive of both normal and malignant CD19+ cells) just prior to lymphodepletion. He had minimal toxicity (grade 1 CRS) following CD19 CAR T cell infusion, and subsequently received 5 monthly doses of T-APCs. An increase in the quantity of detectable CAR T cells (EGFRt+ cells) was observed following T-APCs as shown below, and the subject has ongoing CAR T cell persistence at 8 months, as evidenced by B cell aplasia. Conclusion: The first-in-human, pilot study PLAT-03 introduces CD19t T-APCs in an attempt to enhance durable CAR T cell persistence and leukemia remission following CD19 CAR T cell immunotherapy. T-APCs have been successfully manufactured from stored apheresis products collected for CAR T cell production. Four subjects have been successfully treated with at least one T-APC infusion to date, without any significant toxicity and with early evidence of efficacy. Accrual of subjects is ongoing to further assess the safety and feasibility of administering CD19t T-APCs, and to examine the impact of T-APCs on CD19 CAR T cell efficacy. Figure. Figure. Disclosures Park: Bristol-Myers Squibb: Membership on an entity's Board of Directors or advisory committees. Jensen:Juno Therapeutics, Inc.: Consultancy, Patents & Royalties, Research Funding.

Introduction: Advances in chimeric antigen receptor (CAR) T cell therapy have yielded complete remission (CR) rates in relapsed/refractory B-ALL (rrB-ALL) of 70-95%. However, disease recurrence after CD19 or CD22 CAR therapy is greater than 50% at 1 year, and approximately half of recurrences are due to antigen escape. To reduce antigen escape and optimize the durability of remission, we sought to design a CAR T cell product with dual specificity that is capable of simultaneously targeting both CD19 and CD22. Preclinical testing of our bi-specific CAR showed a preference for signaling through CD22 over the CD19 CAR. In contrast, dual transduced T cells signaled through both the CD19 and CD22 CAR with lytic activity and cytokine production similar to single transduced CAR T cells of the same specificity. Therefore, we opted to move forward with dual transduced T cells for clinical use. We are currently testing SCRI-CAR19x22v1 in PLAT-05 (NCT03330691), a phase 1 clinical trial for pediatric and young adult patients with CD19+CD22+ rrB-ALL, with the primary objectives to determine the feasibility of manufacturing products with dual specificity, to assess the safety of the cryopreserved product infusion, and to describe the full toxicity profile. Methods: Subjects undergo apheresis, after which the CD4 and CD8 T cell subsets are immunomagnetically selected and seeded at a prescribed ratio for co-culture in a closed-system G-Rex bioreactor. Following anti-CD3xCD28 bead stimulation, T cells are transduced with two separate SIN lentiviral vectors that direct the expression of a CD19-specific FMC63scFv:IgG4hinge:CD28tm:4-1BB:ζ CAR with an Her2tG tag and expression of a CD22-specific m971scFv:IgG4hinge:CH2(L235D)-CH3:CD28tm:4-1BB:ζ CAR with an EGFRt tag, creating three distinct populations of CAR T cells (anti-CD19, anti-CD22, and anti-CD19x 22). Transduced cells are expanded in serum free media formulation with IL-7, IL-15, and IL-21. Following lymphodepleting chemotherapy, cryopreserved products are thawed and infused at the protocol-prescribed dose level. Cytokine release syndrome (CRS) is graded according to Lee et al. (Blood 2014) and is treated according to our early intervention strategy of tocilizumab and dexamethasone for persistent, mild CRS. Results: Seven subjects (ages 1-26 yr) with rrB-ALL have been enrolled with 4 treated at dose level 1 (1 x 106 CAR T cells/kg) and 3 treated at dose level 2 (3 x 106 CAR T cells/kg). The mean culture time was 7.9 days (range 7-11) and subjects received infusions with a mean CD8:CD4 ratio of 1.7 (range 0.2 - 3.1). CD8 CAR composition, on average, consisted of 21.6 % CD19 CAR, 37.8 % CD22 CAR, and 40.6 % CD22xCD19 CAR T cells. CD4 CAR composition, on average, consisted of 25.8 % CD19 CAR, 30.6 % CD22 CAR, and 43.6 % CD22xCD19 CAR T cells (Figure). Peak engraftment occurred between days 7 and 14 for all patients and was predominantly composed of the CD19 CAR population with median peak values for CD19 CAR, CD22 CAR, and CD19xCD22 CAR T cell populations of 9.1%, 1.2%, and 2.4%, respectively. A CR was achieved in 5/7 (71%) subjects by day 21, 4 of which were minimal residual disease negative. The two subjects without a CR did not exhibit evidence of CAR T cell engraftment; one had previously received CD19 CAR T cells, and the other had progressive disease and pursued alternative therapy at day 10. Therapy was well tolerated with no dose limiting toxicities. CRS occurred in 5 subjects (Grade 1) with 2 of these subjects experiencing mild neurotoxicity (Grade 1). Four subjects received tocilizumab +/- dexamethasone, and two of these received multiple doses of dexamethasone. Conclusions: Preclinical testing showed superior efficacy against both CD19 and CD22 when using two separate CARs and dual transduction, compared to a single bi-specific CAR. Preliminary analysis of PLAT-05 supports feasibility of product manufacturing, and toxicity and response rates that are consistent with the reported CD19 CAR T cell experience. While the infused SCRI-CAR19x22v1 products consist of a near-uniform distribution of the 3 distinct populations, we observed selective in vivo expansion of the CD19 CAR T cell population. Further investigation is required to understand the mechanism of CD19 CAR dominance in vivo. Continued accrual of subjects is ongoing to further assess the impact of dual antigen targeting on the prevention of antigen escape and the potential to provide a more durable remission. Figure. Figure. Disclosures Park: Bristol-Myers Squibb: Membership on an entity's Board of Directors or advisory committees. Jensen:Juno Therapeutics, Inc.: Consultancy, Patents & Royalties, Research Funding.

Purpose: To determine a safe and biologically active dose of quizartinib (AC220), a potent and selective class III receptor tyrosine kinase (RTK) FLT3 inhibitor, in combination with salvage chemotherapy in children with relapsed acute leukemia. Experimental Design: Quizartinib was administered orally to children with relapsed AML or MLL-rearranged ALL following 5 days of high-dose cytarabine and etoposide (AE). A 3+3 dose escalation design was used to identify a safe and biologically active dose. Plasma inhibitory assay (PIA) testing was performed weekly to determine biologic activity. Results: Toxicities were consistent with intensive relapsed leukemia regimens. One of 6 patients experienced a dose-limiting toxicity (DLT) at 40 mg/m2/day (elevated lipase) and 1 of 9 had a DLT (hyperbilirubinemia) at the highest tested dose of 60 mg/m2/day. Of 17 response evaluable patients, 2 had complete response (CR), 1 complete response without platelet recovery (CRp), 1 complete response with incomplete neutrophil and platelet recovery (CRi), 10 stable disease (SD), and 3 progressive disease (PD). Of 7 FLT3-ITD patients, 1 achieved CR, 1 CRp, 1 Cri, and 4 SD. FLT3-ITD patients, but not FLT3 wild-type (WT) patients, had significantly lower blast counts post-quizartinib. FLT3 phosphorylation was completely inhibited in all patients. Conclusions: Quizartinib plus intensive chemotherapy is well tolerated at 60 mg/m2/day with near complete inhibition of FLT3 phosphorylation in all patients. The favorable toxicity profile, pharmacodynamic activity, and encouraging response rates warrant further testing of quizartinib in children with FLT3-ITD AML. Clin Cancer Res; 22(16); 4014–22. ©2016 AACR.

The importance of the cell surface receptor CXCR4 and the chemokine stromal cell-derived factor-1 (SDF-1/CXCL12) is well-established in normal and malignant hematopoiesis. The Protein Epitope Mimetic POL5551 is a novel and potent antagonist of CXCR4. POL5551 efficiently mobilizes hematopoietic stem and progenitor cells, but its effects in acute lymphoblastic leukemia (ALL) have not been reported. Here, we demonstrate that POL5551 is a potent antagonist of CXCR4 in pre-B and T cell ALL cell lines and pediatric ALL primary samples. POL5551 has activity at nanomolar concentrations in decreasing CXCR4 antibody binding, blocking SDF-1α-mediated phosphorylation of ERK1/2, inhibiting SDF-1α-induced chemotaxis, and reversing stromal-mediated protection from chemotherapy. POL5551 is significantly more effective at inhibiting CXCR4 antibody binding than the FDA-approved CXCR4 inhibitor plerixafor in ALL cell lines and primary samples. We also show that treatment with POL5551 in vitro and cytarabine +/− POL5551 in vivo modulates surface expression of adhesion molecules, findings that may guide the optimal clinical use of POL5551. Finally, we demonstrate that POL5551 increases sensitivity to cytarabine in a xenograft model of a high-risk pediatric ALL, infant MLL-rearranged (MLL-R) ALL. Therefore, disruption of the CXCR4/SDF-1 axis with POL5551 may improve outcomes in children with high-risk ALL.

Introduction: Multiply recurrent pre-B-cell ALL, and particularly relapse following allo-HSCT, has dismal outcomes due in large part to ineffectual therapies. The primary objectives of the Phase 1 portion of the PLAT-02 study (NCT02028455) were to determine the feasibility of manufacturing products of defined composition and transgene expression, the safety of the cryopreserved T cell product infusion, and to describe the full toxicity profile, including development of clinically significant GVHD in the post-allo-HSCT cohort. Methods: Subjects on the PLAT-02 study undergo apheresis, with their CD4 and CD8 T cell subsets prepared immunomagnetically. Following anti-CD3xCD28 bead stimulation, T cell lines are transduced with a SIN lentiviral vector that directs the co-expression of the FMC63scFv:IgG4hinge:CD28tm:4-1BB:ζ CAR and the selection/tracking/suicide construct EGFRt. Transduced cells are propagated using recombinant human cytokine cocktails to numbers suitable for clinical use over 10-22 days, during which time they are subjected to EGFRt immunomagnetic positive selection. Shortly following lymphodepleting chemotherapy, cryopreserved CD4/EGFRt+ and CD8/EGFRt+ T cell products are thawed and infused at the bedside such that patients receive a 1:1 ratio of EGFRt+ CD4 and CD8 T cells at the protocol-prescribed dose level. Results: 45 subjects have been enrolled and 43 have been treated from dose level 1 (5 x 105 CAR-T cells/kg) through 4 (10 x 106 CAR-T cells/kg). Therapeutic T cell products were released on all 45 enrolled subjects, with 1 subject requiring a second apheresis. Two subjects died of disease prior to their infusion. All 43 infused subjects received lymphodepletion chemotherapy prior to T cell infusion (cyclophosphamide, n=27; fludarabine/cyclophosphamide; n=14 cyclophosphamide/etoposide n=1; fludarabine n=1,). 91% (39/43) of subjects received infusions at the desired 1:1 CD4:CD8 ratio and their infusions were well tolerated with only 1 related AE >grade 2. 93% (40/43) of subjects had a documented MRD-negative CR within 21 days following CAR-T cell therapy. The 12 month event-free survival (EFS) is 50.8% (95% CI 36.6, 69.9) and 12 month OS is 69.5% (95% CI 55.8, 86.5). All responding subjects exhibited in vivo expansion of CAR-T cells. The % of CAR T cell expansion over time is not impacted by dose level or lymphodepletion but is impacted by disease burden (p=0.004) and total CD19 antigen burden (p=0.001) at the time of lymphodepletion. The median duration of functional CAR-T cell persistence as measured by ongoing B-cell aplasia (BCA) is impacted by the total CD19 antigen burden in the bone marrow at time of lymphodepletion (>15% vs Conclusions: Infusions of defined composition CD4:CD8 CD19 CAR/EGFRt+ T cells/kg produce high rates of MRD-negative CR in pediatric and young adult B-cell ALL patients. Based on intent to treat analyses without a proliferation screen, we have found it is feasible to generate CAR products from each of the enrolled subjects. Despite the high rates of MRD-negative CR, the durability of remission is highly influenced by the functional persistence of CAR-T cells. Strategies to enhance persistence are currently being investigated, including episodic antigen stimulation through subject derived tAPC engineered to express truncated CD19. Disclosures Gardner: Amgen: Honoraria. Li:Juno Therapeutics: Employment, Equity Ownership. Jensen:Juno Therapeutics, Inc: Consultancy, Equity Ownership, Membership on an entity's Board of Directors or advisory committees, Research Funding.

3048Background: CD19 CAR-T cell has proven efficacy for treatment of aggressive ALL. However, limited CAR-T cell persistence and/orCD19 loss are barriers to therapeutic success. Methods: A Phase I ...

Background: WT1 is a zinc finger transcriptional regulator and acts as a tumor suppressor gene in various cell types. WT1 mutations are reported in approximately 10% of both adult and pediatric patients with acute myeloid leukemia (AML), and at a lower frequency in patients with myelodysplastic syndome (MDS). Reported mutations consist of insertions, deletions or point mutations, and are thought to alter WT1 DNA-binding ability and result in a loss of function. WT1 mutations are associated with FLT3/ITD mutations in AML, suggesting possible leukemogenic cooperativity, and yet WT1 mutations have been independently associated with treatment failure and a poor prognosis. Recently, a physical interaction demonstrated between WT1 and TET2 suggests a common functional pathway, and explains the mutual exclusivity of these mutations in AML. Despite these observations, the functional contribution of WT1 mutations in hematologic malignancies is not entirely understood. To our knowledge, we are the first to describe here a hematologic phenotype in a WT1 mutant mouse model and in a novel WT1 mutant x FLT3/ITD crossbred mouse model. Methods: Knock-in WT1 mutant mice are heterozygous for missense mutation R394W in the DNA-binding domain, which has been described in cases of human AML. Mice with a heterozygous 18-bp ITD knocked into the FLT3 gene were crossbred with the WT1 mutant mice, and Kaplan-Meier survival analysis was performed across genotypes. CBCs and BM cytospin morphology from moribund mutant mice were compared to wild type controls. To create a transplant model, 2e6 whole BM cells from each genotype were injected into lethally irradiated congenic mice. Competitive transplants were performed by injecting a 1:1 ratio of CD45.1 wild type (control) cells with CD45.2 WT1 mutant or wild type (test) cells into lethally irradiated C45.1 recipients. Results: We noted an expansion of lineage negative cells and various progenitor cell compartments in WT1 mutant (WT1mut) BM relative to wild type (wt); including the megakaryocyte-erythroid progenitor (MEP) compartment. WT1mut BM cells from two-month old mice showed an increased ability to serially replate in methylcellulose culture compared to wt BM cells, demonstrating aberrantly enhanced self-renewal capacity. WT1mut mice demonstrated a trend towards an inferior late survival compared to wt in survival analysis, and several moribund WT1mut mice were found to have anemia and erythrodysplasia. Most ITD mice developed a fatal myeloproliferative neoplasm (MPN), as previously described. Interestingly, double mutant mice (WT1mut+ITD) had an inferior survival compared to ITD (p Conclusion: BM cells from mice with the leukemogenic WT1 mutation R394W demonstrate enhanced self-renewal of hematopoietic progenitor cells, which could potentially prime cells for leukemic transformation upon acquisition of cooperative events. Importantly, transplanted mice with WT1 mutant BM consistently develop MDS, manifested as anemia and erythrodysplasia and contributing to a trend in decreased survival. Mice with both this WT1 mutation and a FLT3/ITD mutation develop a mixed MDS/MPN phenotype, which is a discrete diagnostic entity, and results in a more aggressive disease and inferior survival to mice with ITD mutations alone. These data provide new and important insights into the aberrant functional effects of WT1 mutations on hematopoiesis, and are the first to characterize the hematopoietic phenotype of a WT1 mutation in vivo. Disclosures No relevant conflicts of interest to declare.

Introduction: Patients who harbor the Philadelphia (Ph+) chromosome t(9;22) translocation account for approximately 20-30% of adult ALL and 2-5% of pediatric ALL. Prior to approval and use of imatinib, a small molecule TKI which targets the Ph+ chromosome BCR-ABL1, these patients had poor survival & EFS - with long term survival rates in the 20% range. With the addition of imatinib and later generation TKIs to chemotherapy backbones and bone marrow transplant, EFS & survival rates have substantially improved - surpassing 50% in studies in adults and even higher in children. However, resistance to imatinib and other TKIs has become a significant problem in Ph+ ALL, especially in adults. ABL1 kinase domain mutations are the dominant form of TKI resistance, however other resistance mechanisms include upregulation of parallel pathways such as SRC family kinases, MAPK and BCL6 pathways. BCL6 is an oncogene that suppresses transcription of tumor suppressor genes such as p53 and CDNK1A. Interestingly, BCL6 has been shown to be upregulated and activated through deacetylation following imatinib treatment in Ph+ ALL, likely leading to its role in resistance. Histone deacetylase inhibitors (HDACi) have been shown to act synergistically with TKIs in imatinib sensitive and resistant Ph+ leukemia though multiple mechanisms including attenuation of BCR-ABL1 levels and other downstream proliferation promoting pathways. We have shown that HDACi treatment acetylates (and thus inactivates) BCL6 in Ph+ ALL, and that the combination of HDACis and TKIs leads to synergistic effects in vitro and in vivo (using xenograft models). Methods: In vitro WST-1 cell viability assays were carried out on TOM1 cells (non-ABL1 mutant, imatinib sensitive Ph+ ALL) and NALM1 cells (non-ABL1 mutant, imatinib resistant CML lymphoid blast crisis) with imatinib and entinostat (a HDACi). Synergy was assessed using Calcusyn software. Western blots were performed assessing BCL6 expression and acetylation, and expression of downstream effectors of apoptosis. Two separate in vivo xenograft mouse experiments were performed transplanting TOM1 and NALM1 cells into Nod SCID Gamma (NSG) mice. Cohorts of TOM1 mice were treated with imatinib 50mg/kg BID, entinostat 15mg/kg QD, imatinib plus entinostat combination, or vehicle control. In the NALM1 mice we added a higher dose imatinib cohort (100 mg/kg BID) due to known imatinib resistance. Results: In vitro, there was substantially more synergy of the imatinib/entinostat combination in imatinib-resistant NALM1 cells vs. the imatinib-sensitive TOM1 cells. Average Combination Index (CI) values in TOM1 cells across multiple entinostat and imatinib doses was 1.2 (CI: =1 suggest additive effect, 1 = antagonism), while the CI in NALM1 cells at the same dose combinations was 0.53. We noted BCL6 upregulation and decreased BCL6 acetylation - signs correlating with resistance - in Western blots of NALM1 and TOM1 cells treated with imatinib, while exposure to entinostat caused increased acetylation of BCL6 and increased expression of downstream tumor suppressors. In the imatinib-sensitive TOM1 xenograft trial, the combination displayed a significant reduction in bone marrow leukemic blast involvement versus control following 6 weeks of dosing as measured by flow cytometry (36.9% mean decrease, p=0.001). There was a trend toward decreased bone marrow involvement between the combination treatment and other active treatment arms. There was no difference in peripheral blood blast percentage between arms. In the imatinib-resistant NALM1 xenograft trial, the combination showed a significant decrease in peripheral blood blast percentage in the combination arms versus all other arms after only two weeks of therapy (p=0.0008). Conclusions: Upregulation of activated BCL6 is a known mechanism of resistance in Ph+ ALL that may be abrogated by acetylation of BCL6 with HDACi, as our in-vitro data suggests. Further, we have shown in xenograft models of Ph+ acute lymphoblastic leukemia that combination therapy with HDACi + imatinib, even in imatinib-resistant leukemia, has significant activity. Interestingly, the combination appears more active in resistant disease than in imatinib-sensitive disease. This combination could prove a viable strategy to attenuate imatinib- (and perhaps other TKI-) resistance in Ph+ ALL relapse, particularly in cases not driven by ABL1 kinase domain mutations. Disclosures No relevant conflicts of interest to declare.

Background: The WT1 gene encodes for a zinc finger-containing transcription factor involved in differentiation, cell cycle regulation and apoptosis. WT1 expression is developmentally regulated and tissue-specific, with expression maintained in the kidney and in CD34+ hematopoietic progenitor cells. WT1 mutations are reported in approximately 10-15% of both adult and pediatric patients with acute myeloid leukemia (AML), and have been associated with treatment failure and a poor prognosis. Reported mutations consist of insertions, deletions or point mutations; and occur primarily in exon 7 or exon 9 of the WT1 gene. These mutations are thought to alter WT1 DNA-binding ability or result in a loss of function. Despite these observations, the functional contribution of WT1 mutations in leukemogenesis is still largely undetermined. Results and Methods: We have shown that transduction and expression of wild type WT1 in murine 32D cells enhances granulocytic differentiation upon treatment with G-CSF, and that expression of mutant WT1 inhibits this effect. To investigate this in a human AML cell model, we transduced U937 cells with the same WT1 vectors. Strikingly, shortly after transduction, U937 cells expressing wild type WT1 spontaneously differentiate towards a mature monocytic phenotype, but U937 cells expressing mutant WT1 do not differentiate and maintain an immature phenotype (Fig A). This relative block in U937 differentiation with mutant WT1 expression was overcome with differentiation-inducing treatment with all-trans retinoic acid (ATRA). These results suggest that mutant WT1 alters the ability of myeloid cells to terminally differentiate. We obtained a novel knock-in WT1 mutant (WT1mut) mouse model that is heterozygous for the missense mutation R394W in exon 9, and homologous to exon 9 mutations observed in human AML. We evaluated cohorts of two-month old mice and noted an expansion of lineage negative cells and various progenitor cell compartments; particularly, the megakaryocyte-erythroid progenitor (MEP) compartment; in WT1mut bone marrow (BM) relative to wild type. We also found that lineage negative WT1mut BM cells from two-month old mice show higher in vitro colony-forming capacity and an increased ability to serially replate in methylcellulose culture compared to wild type BM cells. Flow cytometry of WT1mut cells at tertiary replating revealed an immature, largely c-kit+ population, suggesting an aberrantly enhanced self-renewal capability of myeloid progenitors in WT1mut mice. Furthermore, survival analysis of the WT1mut mice demonstrates inferior survival compared to wild type, and several WT1mut mice were found to have anemia and myelodysplasia. To address the possibility of germ line WT1mut syndromes causing renal failure and anemia, and thereby influencing survival, we transplanted BM from each genotype into lethally irradiated congenic mice. Upon engraftment with donor marrow, the expression of WT1mut is confined to the hematopoietic system in this model. The Kaplan-Meier survival curve, based on absolute age of the BM, shows statistically significant decreased survival of WT1mut BM transplant recipients compared to wild type BM recipients (Fig B). Anemia and dysplasia were also seen in these WT1mut BM recipients; findings that are suggestive of dysfunctional hematopoiesis, and may be secondary to the changes in progenitor cell self-renewal and differentiation we have observed. Conclusions: Leukemogenic WT1 mutations confer enhanced self-renewal of hematopoietic progenitor cells and a block in terminal myeloid differentiation in vitro, which could potentially prime cells for leukemic transformation upon acquisition of cooperative events. Mice with WT1 mutant bone marrow develop anemia and evidence of myelodysplasia, which may contribute to their decreased survival. These data provide new and important insights into the aberrant functional effects of WT1 mutations on hematopoiesis, and are the first to characterize the hematopoietic phenotype of a WT1 mutation in vivo. Figure: (A) U937 cells expressing wild type WT1 spontaneously differentiate, demonstrated here by gain of monocytic markers CD11a and CD11b as measured by flow cytometry, whereas cells expressing mutant WT1 vectors 101 and 126 remain undifferentiated. (B) Mice transplanted with WT1mut bone marrow have inferior survival compared to mice transplanted with wild type bone marrow. Figure:. (A) U937 cells expressing wild type WT1 spontaneously differentiate, demonstrated here by gain of monocytic markers CD11a and CD11b as measured by flow cytometry, whereas cells expressing mutant WT1 vectors 101 and 126 remain undifferentiated. (B) Mice transplanted with WT1mut bone marrow have inferior survival compared to mice transplanted with wild type bone marrow. Disclosures No relevant conflicts of interest to declare.

Cytoplasmic nucleophosmin (NPMc + ) mutations and FMS-like tyrosine kinase 3 ( FLT3 ) internal tandem duplication (ITD) mutations are two of the most common known molecular alterations in acute myeloid leukemia (AML); they frequently occur together, suggesting cooperative leukemogenesis. To explore the specific relationship between NPMc+ and FLT3/ITD in vivo, we crossed Flt3/ITD knock-in mice with transgenic NPMc+ mice. Mice with both mutations develop a transplantable leukemia of either myeloid or lymphoid lineage, definitively demonstrating cooperation between Flt3/ITD and NPMc+. In mice with myeloid leukemia, functionally significant loss of heterozygosity of the wild-type Flt3 allele is common, similar to what is observed in human FLT3/ITD+ AML, providing further in vivo evidence of the importance of loss of wild-type FLT3 in leukemic initiation and progression. Additionally, in vitro clonogenic assays reveal that the combination of Flt3/ITD and NPMc+ mutations causes a profound monocytic expansion, in excess of that seen with either mutation alone consistent with the predominance of myelomonocytic phenotype in human FLT3/ITD+/NPMc+ AML. This in vivo model of Flt3/ITD+/NPMc+ leukemia closely recapitulates human disease and will therefore serve as a tool for the investigation of the biology of this common disease entity.

Background FLT3 is expressed in most human acute leukemias. When activated by FL, wild type (wt) FLT3 dimerizes and initiates downstream signals that result in proliferation and inhibition of apoptosis and differentiation. Activating FLT3 mutations (internal tandem duplications (ITDs) or point mutations) are common in AML and rare in ALL. ITD mutations confer a poor outcome in AML. In vitro, mutant FLT3 signaling can be further enhanced by binding of FL. Peripheral blood (PB) plasma FL levels rise in adults with AML, peaking about two weeks after initiation of chemotherapy. We sought to determine plasma levels of FL in pediatric patients after chemotherapy, and the functional effect of various levels of FL on both wt and mutant FLT3 leukemia cells. Methods FL levels were measured using FL ELISA on plasma samples (n=352) isolated from PB of children (n=75) enrolled on 4 multi-center acute leukemia clinical trials. Functional studies were performed on AML and ALL cell lines with wt FLT3 (HL60, RS4;11, SEMK2, and KOPN-8) and mutant FLT3 (MOLM14, MV4-11, and HB-1119). 72 hr etoposide IC50 was determined by WST-1 for each line. Cells were plated (250,000 cell/mL) for 72hr at etoposide IC50 in RPMI 1640 along with increasing concentrations of recombinant human FL (62.5 to 4,000 pg/ml). Cell cycle and apoptosis were analyzed using propidium iodide staining and annexin V/7-AAD binding, respectively. To explore the mechanism of FL effects, Ba/F3-ITD cells were incubated for 72hr in serum-free conditions with either 4,000 pg/mL (“high”), 62.5 pg/mL (“low”), or no FL. After washing, total and phosphorylated FLT3 protein levels were determined by Western blot. Results Pediatric patients receiving chemotherapy for the treatment of acute leukemia demonstrate a pattern of plasma FL rise with low levels at baseline (mean 41 pg/ml) and peak levels at day 11-14 following initiation of therapy (mean: 1,190 pg/mL; max: 5,783 pg/mL)(Fig 1A). Cell lines with FLT3 activating mutations selectively demonstrate resistance to etoposide-induced apoptosis (Fig 1B) and G2/M cell cycle arrest (Fig 1C) at low concentrations of FL (62.5 pg/mL). Dose-dependent reduction of etoposide resistance is seen with increasing concentrations of FL up to 4,000 pg/mL, suggesting that optimal etoposide-induced killing of FLT3-mutant leukemias may occur when FL plasma levels are at their peak. Ba/F3-ITD cells pre-incubated with peak concentrations of FL showed diminished baseline FLT3 phosphorylation, suggesting that the interaction of FL and FLT3/ITD exhibits substrate inhibition kinetics and results in a loss of FLT3/ITD-induced activation with high level FL exposure, thus providing a mechanistic basis for the observed loss of etoposide resistance. Conclusions Plasma FL rises to peak levels 11-14 days after initiation of chemotherapy. Through substrate inhibition of mutant FLT3 enzymatic activity, peak FL levels may reduce the etoposide resistance that characterizes FLT3-mutant leukemia cells exposed to pre-chemotherapy levels of FL. Thus, introduction of etoposide in a “time sequential” manner during periods of peak plasma FL levels may enhance killing of residual chemoresistant FLT3-mutant leukemia cells. Disclosures: No relevant conflicts of interest to declare.

Abstract 3605 Background: AC220 is a novel class III receptor tyrosine kinase (RTK) inhibitor that is potent and highly selective for mutant and wild type (WT) FLT3 and other class III RTK's including KIT, CSF1R, RET and PDGFR. In childhood acute myeloid leukemia (AML), ∼18% of children have FLT3 internal tandem duplication mutations (FLT3-ITD), and ∼10% high WT FLT3 expression. FLT3-ITD is associated with poor prognosis. In childhood acute lymphoblastic leukemia (ALL), the highest levels of FLT3 mRNA expression occur in cases of infants (80%) and childhood ALL with MLL rearrangements (MLL-r) (5%), both conferring poor prognosis.1,2 Study Design: TACL 2009–004is a first-in-children study using AC220 in combination with cytarabine and etoposide. Children > 1 month and < 21 years of age with relapsed/refractory AML or MLL-rearranged ALL are eligible. A standard 3+3 dose escalation design is utilized. The three doses tested (25, 40 and 60 mg/m2/day) are significantly lower than those tested in adults. Dose escalation past 60 mg/m2 occurs only if adequate biologic activity as determined by a plasma inhibitory assay (PIA) is not achieved. Intravenous (IV) cytarabine (1 gm/m2/dose every 12 hours) and IV etoposide (150 mg/m2/dose daily) are given over 5 days. AC220 is administered once daily as an oral solution on days 7–28. Patients can receive up to 2 courses of therapy. PIA testing is performed at trough time points weekly during exposure to AC220 to determine biologic activity. Results: To date, 13 patients (pts) were enrolled and 12 are evaluable for toxicity and response. One pt died from infectious complications (not drug-related) after a single dose of AC220 and was replaced. Median age at study entry was 10.2 years (range 11 mo – 20 yrs), average number of prior regimens was 2.8 (range 1–5), and 5 pts had prior stem cell transplant. Nine pts had relapsed AML, 2 had relapsed MLL-r ALL, and 1 had secondary AML. Of patients with AML, 4 had FLT3-ITD mutations and one had a D835 mutation. Toxicities were consistent with intensive relapsed leukemia regimens. Across all dose levels, non-hematologic toxicities ≥ grade 3 attributed to AC220 included vomiting (n=1), elevated transaminases (n=1), anorexia (n=2), and infection (n=3). One pt experienced a dose-limiting toxicity (DLT) on dose level 2 (40 mg/m2/day) of recurrent grade 3 elevated lipase. Dose level 2 was expanded to 6 pts without additional DLTs. Of 3 pts treated at 60 mg/m2/day, there have been no DLTs. Near total (>99%) inhibition of FLT3 phosphorylation by PIA is seen in every patient across all dose levels. Of 12 pts evaluable for response to date, 1 patient achieved a complete response (CR), 3 achieved complete response with incomplete neutrophil and platelet recovery (CRi), 5 had stable disease (SD), and 3 had progressive disease (PD). Responses in the 4 FLT3-ITD pts include 1 CR, 2 CRi and 1 SD. The FLT3-ITD patient with SD had reduction in marrow blasts without peripheral blood count recovery. An additional 6 pts will be enrolled at 60 mg/m2/day to complete safety evaluation and confirm biologic activity. Conclusions: AC220 plus intensive chemotherapy is well tolerated at doses up to 60 mg/m2/day with near complete inhibition of FLT3 phosphorylation in all pts tested to date. Response rates to date in pre-treated children with relapsed FLT3-ITD AML are encouraging. Disclosures: Off Label Use: AC220 in relapsed/refractory pediatric acute leukemia. Gammon:Ambit Biosciences: Employment.

Abstract 1410 Background: Activation of CXCR4 by the chemokine SDF-1 (CXCL12) results in the migration of leukemia cells to marrow niches that may contribute to chemoresistance and relapse. We previously showed that in vitro chemotherapy (chemo) treatment modulates CXCR4 expression in leukemia cell lines and primary pediatric AML samples, and that chemo-induced increases in surface CXCR4 (s-CXCR4) results in increased chemotaxis toward an SDF-1 gradient and decreased chemo-induced apoptosis when co-cultured with human marrow stroma feeder layers. We hypothesized that 1) CXCR4 inhibition by plerixafor (P) would sensitize leukemias to chemo through the interruption of leukemia-stromal cell signaling and 2) the degree of chemo-induced s-CXCR4 upregulation would be a predictive biomarker of the efficacy of P as a chemosensitizer. Because B-precursor ALL are known to highly express CXCR4, we tested these hypotheses in vitro using ALL cell lines and in vivo using a xenograft model of a high-risk pediatric leukemia, infant ALL. In Vitro Methods/Results: ALL cell lines were pretreated for 72 hours with araC (A), dauno, vcr, and vehicle control (C). Chemo pretreatment induced upregulation of s-CXCR4 compared to C. Viable cells were then isolated using Ficoll and plated off stroma (O), on stroma (S), or pretreated with P for 30 minutes prior to plating on stroma (P+S). Cells were then treated for an additional 72 hours with full dose ranges of chemo. Apoptosis was measured with Annexin V/7-AAD, and IC50 was calculated. Overall, IC50 values were highest in S, followed by P+S, then O, demonstrating that upregulation of s-CXCR4 leads to stromal protection, and that stromal protection is diminished by treatment with P. Cells with higher levels of s-CXCR4 upregulation had greater differences between S IC50 and O IC50, compared to cells with lower s-CXCR4 upregulation, suggesting that the degree of s-CXCR4 upregulation is predictive of the degree of stromal protection. Cells with higher s-CXCR4 upregulation also had greater differences between S IC50 and P+S IC50, suggesting that P diminishes stromal protection more effectively in leukemias that highly upregulate s-CXCR4 in response to chemo. Xenograft Methods/Results: Infant ALL patient samples were transplanted into sublethally irradiated NOG mice. After 3 weeks, we treated cohorts (n=5) with single doses of P, A, P followed by A 4 hours later (P+A), or C. We dosed A below the maximal tolerated dose (MTD) to facilitate assessment of P+A synergy. Mice were sacrificed 4 weeks post treatment and cells were isolated from bone marrow (BM), spleen, liver, and peripheral blood (PB) and analyzed by FACS. Leukemic blasts were defined as human CD19+ and CD45+. S-CXCR4 MFI was measured in the blast population. Overall, leukemic burden was similar in C, A, and P, consistent with conservative dosing of A and minimal direct anti-leukemic effect of P. A resulted in increased blasts in spleen and liver compared to C, possibly due to higher levels of s-CXCR4, while P resulted in increased blasts in liver, possibly due to mobilization of blasts. The key finding was that P+A resulted in decreased blasts in BM, spleen, liver, and PB, demonstrating a synergistic effect between P and A. Interestingly, P+A led to a higher reduction in blasts in a sample with A-induced s-CXCR4 upregulation, compared to a sample that did not upregulate s-CXCR4 in response to A. In all treatment cohorts, s-CXCR4 expression was highest in PB blasts, followed by liver, and BM/spleen. Conclusions: Chemo-induced s-CXCR4 upregulation confers stromal-mediated chemoprotection in vitro that can be reversed by P. In vivo, P is an effective chemosensitizer. S-CXCR4 expression is increased in blasts located outside the BM, suggesting that blasts migrating from BM into PB upregulate s-CXCR4 as they home to new niches. Extramedullary disease may develop as a result of chemo-induced upregulation of s-CXCR4 or through mobilization of blasts by P alone. Importantly, P+A resulted in decreased leukemic burden in our infant ALL xenografts, suggesting that chemo-induced increases in s-CXCR4 and P-induced blast mobilization can be overcome. Finally, the efficacy of P as a chemosensitizer was predicted by the degree of chemo-induced s-CXCR4 upregulation, identifying a biomarker with the potential to identify optimal patients for CXCR4 inhibition. P in combination with chemo may thus prove useful in the treatment of high-risk pediatric ALL. Disclosures: No relevant conflicts of interest to declare.

Abstract 2437 Introduction/Background: Wilms tumor 1 (WT1) gene encodes for a zinc finger-containing transcription factor thought to play a role in differentiation, cell cycle regulation and apoptosis. WT1 expression is developmentally regulated and tissue-specific, with adult expression maintained in kidney cells and early CD34+ hematopoietic progenitor cells. Inactivating mutations of this tumor suppressor gene are well described in sporadic Wilms tumor and as germline mutations in WAGR and Denys-Drash syndromes. Approximately 10% of adult and pediatric patients with cytogenetically normal acute myeloid leukemia (CN-AML) harbor WT1 mutations. Some studies suggest that patients with WT1 mutations may have a worse overall prognosis, particularly in combination with other poor prognostic indicators, such as FLT3ITD mutations. Interestingly, WT1 and FLT3ITD mutations are commonly found together, suggesting they may cooperate to cause AML. Despite these clinical observations, the functional contribution of WT1 mutations to leukemogenesis, both alone and in cooperation with FLT3ITD, is still largely undetermined. Methods/Results: Ba/F3 cells were stably transfected with the WT1 gene, including 3 different engineered mutated vectors: two missense point mutations (WT1mut101 and WT1mut146) and one nonsense insertional mutation (WT1mut126), which produces a truncated protein. These mutations have been described in pediatric patients with acute leukemia. The expression of each WT1 vector was confirmed by PCR and cDNA sequencing. We compared the proliferative rate of WT1 wildtype (WT1wt) to WT1mutated (WT1mut) transfected cells using trypan blue cell counting, and saw an early increased proliferative rate for high-expressing WT1mut compared to WT1wt and WT1 empty vector (EV). To further investigate these differences, we performed cell cycle analysis with propidium iodide (PI) staining. After synchronizing all cell lines by arresting cells in G0-G1 phase, the cells were seeded at equal concentrations and assayed for cell cycle changes at various time points. Interestingly, all WT1mut cell lines consistently showed earlier entry into S phase and therefore a decreased G1/S ratio at 24 hrs after synchronization, compared to EV and WT1wt (Fig A). This suggests that the “check-point” controlling entry into S phase is altered in cells expressing WT1 mutations, which manifests as a proliferative advantage in these cells. Next, we stably transfected Ba/F3 cells with a FLT3ITD construct, and co-transfected additional cells with both WT1mut and FLT3ITD constructs. All cells were synchronized by arresting in G0-G1 phase, achieved with 24 hrs of serum and cytokine starvation and treatment with 20nM of CEP-701 (ensuring G0-G1 arrest of cells with FLT3 vectors). The cells were then washed and re-suspended in serum and cytokine-containing media, and seeded at equal concentrations. We observed that FLT3ITD cells and WT1mut126 cells had decreased and nearly equivalent G1/S ratios at early time points compared to EV and WT1wt cells, conferring similar early proliferative advantages for cells with these mutations. Interestingly, we found that cells co-transfected with both mutations enhanced this effect: the absolute number of WT1mut126+FLT3ITD cells undergoing transition into S phase was increased and this effect was seen at an earlier time point compared to WT1mut126, FLT3ITD, WT1wt or EV cells (Fig B). Conclusions: The functional and contributory role of WT1 mutations in leukemogenesis has yet to be characterized. Our preliminary in vitro data with a cell line transfected with WT1 vectors suggests that cell cycle regulation, and therefore proliferation, is aberrant in cells expressing mutated WT1. Consistent with previous reports, our data reaffirms a proliferative advantage in cells transfected with FLT3ITD mutations. We showed that these cells transition to S phase at an early time point, conferring an early proliferative advantage, and do so at an equivalent rate and time to WT1mut cells. In addition, we found that cells co-transfected with both WT1mut126+FLT3ITD demonstrated an earlier and more pronounced entry into S phases compared to either individual mutation alone. These observations deserve further investigation, as they may help explain how mutated WT1 contributes to the initiation and progression of leukemia, and how WT1 and FLT3ITD mutations may cooperate in leukemogenesis. Disclosures: No relevant conflicts of interest to declare.

// Colleen E. Annesley 1 , Cara Rabik 2, 3 , Amy S. Duffield 2, 4 , Rachel E. Rau 5 , Daniel Magoon 2 , Li Li 2 , Vicki Huff 6 , Donald Small 2, 3 , David M. Loeb 7 and Patrick Brown 2, 3 1 Department of Pediatrics, University of Washington, Seattle, WA, USA 2 The Sidney Kimmel Comprehensive Cancer Center at Johns Hopkins, Johns Hopkins University School of Medicine, Baltimore, MD, USA 3 Department of Pediatrics, Johns Hopkins University School of Medicine, Baltimore, MD, USA 4 Department of Pathology, Johns Hopkins University School of Medicine, Baltimore, MD, USA 5 Department of Pediatrics, Baylor College of Medicine, Houston, TX, USA 6 Department of Molecular Genetics/Cancer Genetics, University of Texas M.D. Anderson Cancer Center, Houston, TX, USA 7 Current affiliation: Departments of Pediatrics and Developmental and Molecular Biology, Albert Einstein College of Medicine, Bronx, NY, USA Correspondence to: Patrick Brown, email: pbrown2@jhmi.edu Colleen E. Annesley, email: colleen.annesley@seattlechildrens.org Keywords: WT1; Wilms tumor 1; myelodysplastic syndrome; AML; FLT3 Received: August 28, 2018 Accepted: September 05, 2018 Published: October 19, 2018 ABSTRACT Wilms tumor 1 (WT1) is a zinc finger transcriptional regulator, and has been implicated as both a tumor suppressor and oncogene in various malignancies. Mutations in the DNA-binding domain of the WT1 gene are described in 10–15% of normal-karyotype AML (NK-AML) in pediatric and adult patients. Similar WT1 mutations have been reported in adult patients with myelodysplastic syndrome (MDS). WT1 mutations have been independently associated with treatment failure and poor prognosis in NK-AML. Internal tandem duplication (ITD) mutations of FMS-like tyrosine kinase 3 ( FLT3 ) commonly co-occur with WT1 -mutant AML, suggesting a cooperative role in leukemogenesis. The functional role of WT1 mutations in hematologic malignancies appears to be complex and is not yet fully elucidated. Here, we describe the hematologic phenotype of a knock-in mouse model of a Wt1 mutation (R394W), described in cases of human leukemia. We show that Wt1 +/R394W mice develop MDS which becomes 100% penetrant in a transplant model, exhibit an aberrant expansion of myeloid progenitor cells, and demonstrate enhanced self-renewal of hematopoietic progenitor cells in vitro . We crossbred Wt1 +/R394W mice with knock-in Flt3 +/ITD mice, and show that mice with both mutations ( Flt3 +/ITD / Wt1 +/R394W ) develop a transplantable MDS/MPN, with more aggressive features compared to either single mutant mouse model.