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Preclinical Rationale: Myelofibrosis (MF) is a chronic myeloproliferative neoplasm for which there are limited therapies. TGFβ plays a pivotal role in the pathobiology of MF by not only promoting bone marrow fibrosis (BMF) and collagen deposition, but also by enhancing the dormancy of normal but not MF hematopoietic stem cells (HSCs). TGFβ has also previously been reported to inhibit normal megakaryocyte (MK) production (Bruno et al Blood 1998). TGFβ1 promotes the synthesis of collagen by normal human mesenchymal stromal cells (MSCs) and activates the TGFβ receptor I/SMAD pathway as well as non-canonical TGFβ pathways. We generated MKs from MF subject mononuclear cells (MNCs) and showed that they elaborated significantly greater levels of TGFβ1 than TGFβ2/3 TGFβ1 treatment reduced the numbers of hematopoietic colonies generated by normal but not MF MNCs. Treatment of MSCs with AVID200, a potent TGFβ1/3 protein trap, significantly decreased MSC proliferation, phosphorylation of SMAD2, and collagen expression. Robust expression of pSMAD2 was observed in the absence of exogenous TGFβ in normal donor or MF-MKs, Addition of AVID200 to -MKs decreased pSMAD2 without affecting total SMAD2/3, indicating that AVID200 blocks the effects of autocrine TGFβ produced by MKs and led to increased numbers of MKs. Moreover, treatment of primary MF MNCs with AVID200 led to increased numbers of progenitor cells with wild type JAK2 and a reduction of mutated colonies. AVID200 blocked TGFβ1-induced p57Kip2 expression and SMAD2 activation by MF MNCs allowing the normal progenitor cells to preferentially cycle, proliferate, and form hematopoietic colonies. Clinical Trial Design: Based on these findings, a phase 1 trial of AVID200 is ongoing in INT-2/high risk MF subjects resistant or intolerant to ruxolitinib; baseline platelet count of ≥ 25 x 109/L, and grade 2/3 BMF. Subjects received intravenous AVID200 (Lots A and B) in dose cohorts of 180 mg/m2 (A), 550 mg/m2 (A), 180 mg/m2 (B) on Day 1 of a 21 day cycle. Cohorts of 3 subjects with a target toxicity rate of 30% were enrolled to estimate the maximum tolerated dose (MTD). A modified toxicity probability interval design was used. Response was assessed by IWG/ELN criteria after 6 cycles of AVID200. Subjects attaining at least a CI or SD with a decrease in BMF by ≥1 grade, continued AVID200. Clinical Trial Results: 10 subjects were enrolled (1 withdrew before receiving treatment) and 9 were treated with AVID200 and were evaluable for DLT assessment [Table1]. Median time after ruxolitinib discontinuation was 3.5 months (0.5-12.2). No DLTs were observed. Grade 3/4 AEs (regardless of attribution) were observed in 6 (66.7%) subjects. Grade 3/4 non-hematologic AEs observed were epistaxis (1, 11.1%), extraocular muscle paresis (1, 11.1%), fatigue (1, 11.1%) and rash (1, 11.1%). Grade 3/4 hematologic AEs were anemia (3, 33.3%) and thrombocytopenia (2, 22.2%) [Table 2]. The median number of cycles received was 5.7 (range 0 - 12). 5 subjects received 6+ cycles and were evaluable. CI occurred in 2 subjects [anemia, spleen and TSS (n=1); TSS (n=1)] 1 of which is still being treated, 2 subjects had SD, 1 subject with 21% blasts prior to study treatment had progressive MPN-BP. 4 subjects failed to reach response evaluation after 6 cycles, 2 had PD due to increasing splenomegaly, 1 subject received an allogeneic transplant and 1 is still being treated [Cycle 2]. The median platelet count at baseline was 114 (range: 42-290) and 159 after cycle 6 [Figure 1]. Maximum changes in platelets from baseline was +64% [range -73%, 169%] in all subjects. 7 subjects had an increase in platelets from baseline during treatment. 2 subjects normalized their platelet count from thrombocytopenic levels. The effect of AVID200 on BMF is currently being examined. 2 subjects remain on treatment. Conclusions: AVID200 a TGFβ1/3 protein trap is well tolerated in advanced MF subjects. Clinical responses were observed at the 550 mg dose and the expansion efficacy cohorts at doses 2 and 3 are enrolling 12 additional subjects. Furthermore, AVID200 therapy improved thrombocytopenia in MF subjects which may be due to AVID200 inhibiting the effects of TGFβ1 on normal MKpoiesis. Updated subject safety and efficacy data along with correlative data will be presented. Disclosures Mascarenhas: Celgene, Prelude, Galecto, Promedior, Geron, Constellation, and Incyte: Consultancy; Incyte, Kartos, Roche, Promedior, Merck, Merus, Arog, CTI Biopharma, Janssen, and PharmaEssentia: Other: Research funding (institution). Kuykendall:Blueprint Medicines: Research Funding; BMS: Research Funding; Incyte: Research Funding; Novartis: Research Funding. Komrokji:Jazz: Honoraria, Speakers Bureau; Abbvie: Honoraria; Agios: Speakers Bureau; BMS: Honoraria, Speakers Bureau; Geron: Honoraria; Incyte: Honoraria; Acceleron: Honoraria; Novartis: Honoraria. Gerds:Gilead Sciences: Research Funding; Imago Biosciences: Research Funding; Sierra Oncology: Research Funding; Celgene: Consultancy, Research Funding; Roche/Genentech: Research Funding; CTI Biopharma: Consultancy, Research Funding; Apexx Oncology: Consultancy; AstraZeneca/MedImmune: Consultancy; Pfizer: Research Funding; Incyte Corporation: Consultancy, Research Funding. Migliaccio:Novartis: Research Funding. O'Connor-McCourt:Forbius: Current Employment. Tremblay:Forius: Current Employment. Nadler:Forbius: Consultancy; Nadler Pharma Associates: Current Employment; Symphogen: Consultancy; Iksuda Therapeutics: Consultancy; Tessa Therapeutics: Consultancy. Mesa:Celgene: Research Funding; Genetech: Research Funding; Samus: Research Funding; Promedior: Research Funding; CTI: Research Funding; LaJolla Pharma: Consultancy; Incyte: Research Funding; Sierra Onc: Consultancy; Abbvie: Research Funding; Novartis: Consultancy. Hoffman:Forbius: Consultancy; Abbvie: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees; Dompe: Research Funding; Protagonist: Consultancy.

Background Primary CNS lymphoma (PCNSL) is a rare non-Hodgkin lymphoma that is often associated with immunosuppressed states. The Epstein-Barr Virus (EBV) may play a role in tumor pathogenicity in some cases. The objective of this study was to examine the patient characteristics, tumor pathology, and survival outcomes associated with EBV tumor status in patients with PCNSL. Methods This was a retrospective subset analysis from 17 academic medical centers that included 439 patients of ages 60 years and above with PCNSL (David K et al. ASH 2020). The associations between EBV status and clinical or demographical variables were tested by Fisher's exact test, Wilcoxon rank-sum test, or CMH trend test. Kaplan-Meier estimator was used to estimate survival probability. Survival difference between groups was tested by log-rank test for statistical significance. Confidence interval of survival rate was calculated using Greenwood's formula. Results A total of 247 patients with available EBV status were included in this analysis. Median age was 71 (range 60-84) and 44.5% were male. Notably, none of the patients were HIV-positive. Twenty-five patients (10.1%) had EBV positive tumors as detected by EBER (EBV-encoded RNA) in-situ hybridization or LMP1 immunohistochemistry (IHC), 17 of which were solid organ transplant (SOT)-related post-transplant lymphoproliferative disorders (PTLD) and 8 of which were not PTLD. All EBV-positive non-PTLDs were diffuse large B-cell lymphoma. Three (15%) SOT-related PTLDs were EBV-negative. Patient characteristics analyzed included age at diagnosis, sex, ECOG performance status, history of prior or concurrent malignancies, history of solid organ transplant or autoimmune disease, history of allogeneic stem cell transplant, and immunosuppressive treatment. Of these, only a history of solid organ transplant or autoimmune disease (P Tumor characteristics analyzed included expression of C-MYC, BCL2, CD5, cell of origin markers (BCL6, MUM1, CD10), and CD20 through IHC, C-MYC and BCL2 translocation through FISH, histology, and involvement of brain parenchyma, CSF, spinal cord, and eyes. EBV-positive tumors were associated with low C-MYC (p=0.047) and BCL6 (p=0.0006) expression on immunohistochemistry, but not other factors. There were no significant differences in tumor characteristics between those with EBV-positive PTLD and EBV-positive non-PTLD. Among patients with PTLD, 30% (n=6) did not receive primary chemotherapy, and the most common treatment regimens were high-dose methotrexate (HD-MTX) with or without rituximab (n=5) and rituximab alone (n=3). However, there was no significant difference in outcomes among PTLD patients who received chemotherapy or those who did not. Among EBV-positive non-PTLD patients, only 12.5% (n=1) did not receive primary chemotherapy and the most common treatment regimens were methotrexate/rituximab/temozolomide (n=3) and HD-MTX with or without rituximab (n=3). There was no difference in overall or progression free survival between patients with EBV-positive and EBV-negative tumors, or in outcomes among SOT-related PTLD patients regardless of EBV status. However, patients with EBV-positive non-PTLD PCNSL had better overall survival compared to patients with EBV-positive PTLD and EBV-negative tumors (p=0.033, Figure). Conclusions In this large observational study of older patients with PCNSL, the incidence of EBV positive tumors was overall low and was most commonly associated with SOT-related PTLD. Mycophenolate mofetil was the most common immunosuppressive medication. In those without PTLD, there were no patient or tumor factors that were associated with EBV status. Unexpectedly, non-PTLD EBV-positive PCNSL had superior outcomes to EBV-positive PTLD and EBV-negative PCNSL. Future studies of EBV-positive non-PTLDs are warranted to further evaluate the potential impact of EBV latency and the immune response on the tumor microenvironment. Disclosures Reddy: BMS: Consultancy, Research Funding; Celgene: Consultancy; Abbvie: Consultancy; Genentech: Research Funding; KITE Pharma: Consultancy. Bachanova:Karyopharma: Membership on an entity's Board of Directors or advisory committees; Kite: Membership on an entity's Board of Directors or advisory committees; BMS: Research Funding; FATE: Research Funding; Incyte: Research Funding; Gamida Cell: Membership on an entity's Board of Directors or advisory committees, Research Funding. Bond:Seattle Genetics: Honoraria. Goldlust:Tocagen: Membership on an entity's Board of Directors or advisory committees, Other: travel; BMS: Membership on an entity's Board of Directors or advisory committees, Other: travel; WEX: Consultancy, Other: travel; Cortice Bio: Consultancy, Other: travel; Boston Biomedical: Consultancy; COTA: Other; Novocure: Consultancy, Membership on an entity's Board of Directors or advisory committees, Other: travel, Research Funding, Speakers Bureau. Spurgeon:Gilead: Research Funding; Genentech: Research Funding; Cardinal Health: Honoraria; Bristol-Myers Squibb: Research Funding; VelosBio: Consultancy, Research Funding; Acerta: Research Funding; Janssen: Consultancy, Research Funding; AstraZeneca: Research Funding; Pharmacyclics: Consultancy; Beigene: Research Funding; Verastem: Research Funding; Genmab: Research Funding. Epperla:Verastem Oncology: Speakers Bureau; Pharmacyclics: Honoraria. Karmali:AstraZeneca: Speakers Bureau; Karyopharm: Honoraria; Takeda: Research Funding; BeiGene: Speakers Bureau; Gilead/Kite: Honoraria, Other, Research Funding, Speakers Bureau; BMS/Celgene/Juno: Honoraria, Other, Research Funding, Speakers Bureau. Naik:Celgene: Other: advisory board; Sanofi: Other: advisory board. Smith:Janssen: Consultancy; Celgene: Consultancy, Research Funding; Karyopharm: Consultancy, Research Funding; BMS: Consultancy; FortySeven: Research Funding; Pharmacyclics: Research Funding; Acerta: Research Funding; Genentech/Roche: Consultancy, Other: Support of parent study and funding of editorial support, Research Funding; TG Therapeutics: Consultancy, Research Funding. Rubenstein:Kymera: Research Funding. Kahl:BeiGene: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; AstraZeneca Pharmaceuticals LP: Consultancy, Membership on an entity's Board of Directors or advisory committees; Celgene Corporation: Consultancy; AbbVie: Consultancy; Genentech: Consultancy; Janssen: Consultancy, Membership on an entity's Board of Directors or advisory committees; Acerta: Consultancy, Research Funding; ADC Therapeutics: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Roche Laboratories Inc: Consultancy; Pharmacyclics LLC: Consultancy. Evens:Abbvie: Consultancy, Honoraria; Research To Practice: Honoraria, Speakers Bureau; MorphoSys: Consultancy, Honoraria; Novartis: Consultancy, Honoraria; Pharmacyclics: Consultancy, Honoraria; Merck: Consultancy, Honoraria, Research Funding; Epizyme: Consultancy, Honoraria, Research Funding; Mylteni: Consultancy, Honoraria; Seattle Genetics: Consultancy, Honoraria, Research Funding. Martin:Kite: Consultancy; Morphosys: Consultancy; Regeneron: Consultancy; Incyte: Consultancy; Cellectar: Consultancy; Beigene: Consultancy; Bayer: Consultancy; I-MAB: Consultancy; Sandoz: Consultancy; Janssen: Consultancy; Karyopharm: Consultancy, Research Funding; Teneobio: Consultancy; Celgene: Consultancy.

Methylation profiling in myeloid malignancies such as Acute Myeloid Leukemia (AML) and Chronic Myelomonocytic Leukemia (CMML) has demonstrated the ability to define distinct biological and clinical subgroups, including predicting which patients will respond to therapy with a hypomethylating agent (HMA). The Philadelphia-chromosome negative myeloproliferative neoplasms (MPNs) carry an inherent risk of progression to an accelerated-phase disease (AP; 10-19% blasts in the peripheral blood or bone marrow), as well as to blast phase disease (BP; ≥ 20% blasts in the peripheral blood or bone marrow), which is associated with a poor prognosis. It is unknown whether the methylation profiles of MPN-AP/BP cases may further help identify distinct biological, genomic, and clinical subgroups, including identifying patients more likely to respond to HMA. We recently carried out a phase I/II study to test the safety and efficacy of combination therapy with the JAK1/2 inhibitor ruxolitinib (RUX) and the HMA Decitabine (DAC) in patients with MPN-AP/BP (MPD-RC 109 study; NCT02076191). A total of 46 patients were accrued to the phase I and II studies. 37 patients were evaluable for response. Complete response (CR) occurred in 10%, Complete Response with incomplete count recovery (CRi) in 24%, Partial Response in 24%. 42% of patients had no response to therapy. Using samples available from the MPD-RC 109 study, we sought to assess whether the baseline global methylation profile predicts for response to this regimen. Further, we sought to utilize this dataset to determine if IDH2 mutations (amongst the most common mutations in MPN-AP/BP) are associated with a distinct methylation profile, as has been demonstrated in de novo AML. We carried out a pilot study of 11 MPN-AP/BP patients from the MPD-109 phase I/II trial and performed Enhanced Reduced Representation Bisulfite Sequencing (ERRBS) for DNA methylation quantification at ~3M CpG sites across the genome. Baseline DNA methylation profiles were compared between Responder (R) and Non-responder (NR) patients. Notably, unsupervised analysis using correspondence analysis (COA) demonstrated an almost complete separation of the two groups of patients (Fig 1A), while supervised analysis using a beta binomial model identified 134 differentially methylated regions (DMRs) (FDR25%) between the two groups at diagnosis (Fig 1B). Similar to our prior observation in CMML, response-associated DMRs were depleted from promoter regions (p We next carried out a pilot study to characterize the epigenetic abnormalities of IDH2-mutant MPN-AP/BP cases. For this purpose, we compared the genome-wide DNA methylation profiles of 12 IDH2-mutant to 7 IDH1/2 wild type MPN-AP/BP cases using ERRBS. Unsupervised analysis based on the DNA methylation profiles alone showed a strong trend to naturally segregate mutant from wild-type cases, indicating strong underlying epigenetic differences (Fig.1D). A supervised analysis using the beta binomial method identified 1,477 differentially methylated regions (DMRs) between the two groups (average absolute methylation difference ≥25% and FDR Our data demonstrate that the methylation profile of MPN-AP/BP may predict for response to HMA-based therapy. Such data could be used to guide therapeutic decisions and select patient for whom HMA has the highest likelihood of procuring a response. As well, these findings indicate that IDH2-mutant MPN-AP/BP are epigenetically distinct, and given the preferred targeting of regulatory elements, these epigenetic differences may play a functional role in disease biology. Further validation of these observations is required. Updated data, including analysis of further cases, and RNA-sequencing analysis of gene-expression and pathway enrichment of genes differentially methylated between responders and non-responders, and IDH2 mutated and wildtype cases will be presented at the conference. Disclosures Rampal: Constellation: Research Funding; Pharmaessentia: Consultancy; CTI Biopharma: Consultancy; Promedior: Consultancy; Celgene: Consultancy; Incyte: Consultancy, Research Funding; Abbvie: Consultancy; Galecto: Consultancy; Jazz Pharmaceuticals: Consultancy; Blueprint: Consultancy; Stemline: Consultancy, Research Funding. Mascarenhas:Celgene, Prelude, Galecto, Promedior, Geron, Constellation, and Incyte: Consultancy; Incyte, Kartos, Roche, Promedior, Merck, Merus, Arog, CTI Biopharma, Janssen, and PharmaEssentia: Other: Research funding (institution). Levine:Morphosys: Consultancy; Prelude Therapeutics: Research Funding; Novartis: Consultancy; Amgen: Honoraria; Lilly: Consultancy, Honoraria; Janssen: Consultancy; Astellas: Consultancy; Roche: Consultancy, Honoraria, Research Funding; Celgene: Consultancy, Honoraria, Research Funding; Imago: Current equity holder in private company, Membership on an entity's Board of Directors or advisory committees; C4 Therapeutics: Current equity holder in private company, Membership on an entity's Board of Directors or advisory committees; Isoplexis: Current equity holder in private company, Membership on an entity's Board of Directors or advisory committees; Loxo: Current equity holder in private company, Membership on an entity's Board of Directors or advisory committees; Qiagen: Current equity holder in publicly-traded company, Membership on an entity's Board of Directors or advisory committees; Gilead: Honoraria. Hoffman:Novartis: Membership on an entity's Board of Directors or advisory committees; Protagonist: Consultancy; Forbius: Consultancy; Dompe: Research Funding; Abbvie: Membership on an entity's Board of Directors or advisory committees.

Introduction: Treatment of older patients (pts) with PCNSL is challenging due to the prevalence of comorbidities, frailty, and complexities with delivery of chemotherapy (CT). The optimal induction CT and consolidation regimens for older PCNSL pts is unknown. Moreover, there are few large scale prognostication studies available, including analysis of geriatric assessments (GA). We analyzed detailed characteristics, treatment patterns and outcomes with prognostication across 17 academic centers. Methods: We conducted a large, RW retrospective study of newly diagnosed PCNSL pts (1/2008-1/2019) ages ≥ 60 years (yrs). Survival rates were estimated by Kaplan-Meier with differences assessed by log rank test. We detailed Cumulative Index Rating Scale-Geriatric (CIRS-G) scores & other GAs. Univariate associations were derived via Cox model with variables p Results: Among 491 initial cases, n=450 cases were verified for diagnosis & follow-up. Clinical features included: median age 71 yrs (60-88); male 47%; elevated LDH 30%; creatinine clearance 1 site). Cerebral involvement predominated in 75% with deep structure involvement in 20% & cerebellum in 5%. CSF involvement was documented in 13% of pts (unchecked in 26%). For GA at diagnosis, the median CIRS-G score was 6 (range 0-27) and impaired self-care activities of daily living (ADLs) were noted in 36%. Furthermore, geriatric syndrome (ie, dementia, delirium, depression, and/or falls) was present in 45% of pts. Induction therapy included CT in 91% of pts (of whom 82% had rituximab (Rtx)) and radiation therapy (RT) in 8%. The most common chemotherapy regimens were: high-dose methotrexate (HD MTX) or HD MTX with Rtx (MR) in 38%; HD MTX/procarbazine/vincristine (MPV) +/- Rtx 30%; HD MTX/temozolomide/Rtx (MTR) 22%; Rtx alone 2%; and HD MTX/cytarabine/thiotepa/Rtx (MATRIX) in 2%. Median MTX dosing for all pts was 3.5 g/m2 (range 1-8 g/m2), and by 3 most common regimens (all g/m2): MTR 5.1; MR 5.4; MPV 3.1 (P With 42 month median follow-up (1-125), 3-yr PFS & OS for all pts were 38% & 52%, respectively (Fig 1A/1B). On MVA, factors associated with inferior PFS were: advancing age (continuous HR 1.05, P Conclusions: Older pts with PCNSL have suboptimal outcomes, with 2/3 progressing in the first several years. GA is an important prognostic tool, and could be used to stratify pts in future investigations. In addition, use of Rtx, increasing MTX dose, and the MTR regimen were associated with improved outcomes. Disclosures Reddy: Genentech: Research Funding; Abbvie: Consultancy; KITE Pharma: Consultancy; Celgene: Consultancy; BMS: Consultancy, Research Funding. Bachanova:Gamida Cell: Membership on an entity's Board of Directors or advisory committees, Research Funding; Karyopharma: Membership on an entity's Board of Directors or advisory committees; Kite: Membership on an entity's Board of Directors or advisory committees; FATE: Research Funding; BMS: Research Funding; Incyte: Research Funding. Bond:Seattle Genetics: Honoraria. Goldlust:COTA: Other; BMS: Membership on an entity's Board of Directors or advisory committees, Other: travel; Tocagen: Membership on an entity's Board of Directors or advisory committees, Other: travel; Novocure: Consultancy, Membership on an entity's Board of Directors or advisory committees, Other: travel, Research Funding, Speakers Bureau; Boston Biomedical: Consultancy; Cortice Bio: Consultancy, Other: travel; WEX: Consultancy, Other: travel. Spurgeon:Beigene: Research Funding; Janssen: Consultancy, Research Funding; Pharmacyclics: Consultancy; Bristol-Myers Squibb: Research Funding; Genentech: Research Funding; Gilead: Research Funding; Acerta: Research Funding; AstraZeneca: Research Funding; Genmab: Research Funding; VelosBio: Consultancy, Research Funding; Cardinal Health: Honoraria; Verastem: Research Funding. Epperla:Verastem Oncology: Speakers Bureau; Pharmacyclics: Honoraria. Karmali:Takeda: Research Funding; Karyopharm: Honoraria; AstraZeneca: Speakers Bureau; BeiGene: Speakers Bureau; BMS/Celgene/Juno: Honoraria, Other, Research Funding, Speakers Bureau; Gilead/Kite: Honoraria, Other, Research Funding, Speakers Bureau. Naik:Celgene: Other: advisory board; Sanofi: Other: advisory board. Martin:Celgene: Consultancy; Bayer: Consultancy; Janssen: Consultancy; Sandoz: Consultancy; I-MAB: Consultancy; Teneobio: Consultancy; Beigene: Consultancy; Cellectar: Consultancy; Incyte: Consultancy; Kite: Consultancy; Morphosys: Consultancy; Karyopharm: Consultancy, Research Funding; Regeneron: Consultancy. Smith:Genentech/Roche: Consultancy, Other: Support of parent study and funding of editorial support, Research Funding; TG Therapeutics: Consultancy, Research Funding; FortySeven: Research Funding; Karyopharm: Consultancy, Research Funding; Pharmacyclics: Research Funding; BMS: Consultancy; Janssen: Consultancy; Celgene: Consultancy, Research Funding; Acerta: Research Funding. Rubenstein:Kymera: Research Funding. Kahl:ADC Therapeutics: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; AbbVie: Consultancy; Celgene Corporation: Consultancy; AstraZeneca Pharmaceuticals LP: Consultancy, Membership on an entity's Board of Directors or advisory committees; Genentech: Consultancy; Pharmacyclics LLC: Consultancy; Roche Laboratories Inc: Consultancy; BeiGene: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Janssen: Consultancy, Membership on an entity's Board of Directors or advisory committees; Acerta: Consultancy, Research Funding. Evens:Merck: Consultancy, Honoraria, Research Funding; Research To Practice: Honoraria, Speakers Bureau; Seattle Genetics: Consultancy, Honoraria, Research Funding; Pharmacyclics: Consultancy, Honoraria; Abbvie: Consultancy, Honoraria; Epizyme: Consultancy, Honoraria, Research Funding; MorphoSys: Consultancy, Honoraria; Novartis: Consultancy, Honoraria; Mylteni: Consultancy, Honoraria.

The goal of therapy for essential thrombocythemia (ET) and polycythemia vera (PV) patients is to reduce thrombotic events by normalizing blood counts. Hydroxyurea (HU) and interferon-α (IFN-α) are the most frequently used cytoreductive options for ET and PV patients at high-risk for vascular complications. Myeloproliferative Disorders Research Consortium 112 was an investigator-initiated, phase 3 trial comparing HU to pegylated IFN-α (PEG) in treatment naïve, high-risk ET/PV patients. The primary endpoint was complete response (CR) rate at 12 months. A total of 168 patients were treated for a median of 81.0 weeks. CR for HU was 37% and 35% for PEG (p=0.80) at 12 months. At 24/36 months, CR was 20%/17% for HU and 29%/33% for PEG. PEG led to a greater reduction in JAK2V617F at 24 months, but histopathologic responses were more frequent with HU. Thrombotic events and disease progression were infrequent in both arms, while grade 3/4 adverse events were more frequent with PEG (46% vs. 28%). At 12 months of treatment there was no significant difference in CR rates between HU and PEG. This study indicates that PEG and HU are both effective treatments for PV and ET. With longer treatment PEG was more effective in normalizing blood counts and reducing driver mutation burden, while HU produced more histopathologic responses. Despite these differences, both agents did not differ in limiting thrombotic events and disease progression in high-risk ET/PV patients. (Funded by the National Cancer Institute, 5P01CA108671-09; clinicaltrials.gov number (NCT01259856). [Abstract copyright: Copyright © 2022 American Society of Hematology.]

Resistance to therapy is one of the most significant challenges in the treatment of acute myeloid leukemia (AML). While great efforts have uncovered genetic mechanisms of resistance to certain AML-directed therapies, to date, treatment resistance in AML has only partially explained by acquired genetic alterations. Here, we performed genome-wide CRISPR/Cas9 screens to identify drug-gene interactions that modulate therapeutic response to treatments commonly used in AML. Interestingly, our findings uncovered several genes that regulate pre-mRNA splicing whose loss strongly synergized with venetoclax, a BH3 mimetic that blocks the antiapoptotic protein BCL-2. To further delineate the role of RNA processing in response to AML treatments, we performed secondary CRISPR screens with a domain-focused gRNA library targeting 490 RNA processing factors in the presence of various AML drugs. Overall, these genetic screens identified a number of RNA splicing factors whose loss-of-function sensitized AML cells to BCL2 inhibition (Fig. A). Among the top gene candidates whose loss promoted venetoclax efficacy was the splicing factor RBM10 (Fig.B). Strikingly, loss of RBM10 exclusively synergized with venetoclax-based treatments across AML therapeutics, including in TP53 mutant lines (Fig.C-D). Moreover, RBM10 loss restored venetoclax sensitivity to AML cell line variants with acquired venetoclax resistance. Interestingly, while many RNA splicing factors are pan-essential, generation of an Rbm10 conditional knockout mouse revealed that Rbm10 is completely dispensable for steady-state normal hematopoiesis (Fig.E). Since RBM10 has not been studied previously in hematopoiesis, we mapped the impact of RBM10 on mRNA expression and splicing using RNA-seq and direct RNA binding partners genome-wide by eCLIP-Seq (Fig. F). RBM10 loss was strongly associated with downregulation of BCL2A1, an anti-apoptotic factor whose expression is correlated with venetoclax resistance in AML (Fig.G-H). This was dependent on RBM10's ability to bind RNA and expression of BCL2A1 cDNA fully rescued the growth-inhibitory effect of RBM10 KO-venetoclax treated AML cells. Overall, the above data support RBM10 as a synthetic lethal vulnerability in venetoclax therapy. Beyond RBM10, our genetic screens also identified several splicing factors belonging to the family of serine and arginine-rich (SR) proteins whose loss synergized with venetoclax treatment (Fig. I). SR proteins are essential for pre-mRNA splicing and are substrates for phosphorylation by conserved family of kinases, such as Cdc2-like kinases (CLKs) and (dual-specificity tyrosine-regulated kinases) DYRKs. We therefore utilized a series of selective pan-CLK/DYRK1A inhibitors, including SM09419 and SM08502, that potently suppress SR protein phosphorylation. Interestingly, BCL2 is one of the top genetic dependencies upon DYRK1A genetic suppression in prior work from the DepMap (Fig. J). Pharmacologic inhibition of CLK/DYRK1A exhibited high in vitro efficacy at nanomolar range across a diverse range of AML subtypes including cell lines with acquired venetoclax resistance (Fig.K). Consistent with this, combined SM09419 and venetoclax displayed synergistic anti-leukemic effects and venetoclax-sensitive AML cell lines (Fig.L). Taken together, these data support the notion of targeting CLK/DYRK1A in the context of BCL2 inhibition. In this study, we systematically defined gene interactions that mediate the response to a wide range of AML drugs. Recent studies have begun to show that dysfunctional RNA processing promotes AML development. However, the role of RNA processing in modulating drug responsiveness in AML is not well understood. Here, we have uncovered that synthetic lethal targeting of splicing factors, such as RBM10, increases sensitivity of AML cells to BCL2 inhibition. Therapeutically, pharmacologic inhibition of SR protein function via inhibiting CLK/DYRK1A-mediated phosphorylation of splicing factors is an effective strategy used in combination with venetoclax or to overcome venetoclax resistance. Overall, our findings underscore the central importance of RNA splicing in drug response and provides a therapeutic rationale for modulating RNA splicing to enhance current AML therapies. Figure 1 Figure 1. Disclosures McMillan: Prizer: Ended employment in the past 24 months. Bossard: Biosplice Therapeutics: Current Employment. Aifantis: AstraZeneca: Research Funding; Foresite (FL2020-010) LLC: Consultancy. Abdel-Wahab: H3B Biomedicine: Consultancy, Research Funding; Foundation Medicine Inc: Consultancy; Merck: Consultancy; Prelude Therapeutics: Consultancy; LOXO Oncology: Consultancy, Research Funding; Lilly: Consultancy; AIChemy: Current holder of stock options in a privately-held company, Membership on an entity's Board of Directors or advisory committees; Envisagenics Inc.: Current holder of stock options in a privately-held company, Membership on an entity's Board of Directors or advisory committees.

Increased expression of fetal globin (HBG) has been shown to reduce clinical severity of beta-globin disorders and increase survival in sickle cell disease (SCD), through improved globin chain balance in beta thalassemia and reduced HbS polymerization. Pharmacologic approaches are considered most feasible for a majority of patients. Both proportions of F-cells and quantity of hemoglobin F (HbF)/cell are important for therapeutic effects. PB-04 was identified in a high-throughput screen of US and EU-approved drugs to activate HBG gene transcription, without cytotoxicity, to induce fetal globin expression (gamma globin mRNA, F-cells, HbF/cell, HbF), and to suppress or displace 4 repressors of the fetal globin gene promoter (BCL11A, LSD-1, KLF-1, and HDAC-3) in hemoglobinopathy patients' erythroid progenitors. PB-04 enhanced HbF in progenitors from hydroxyurea (HU)-treated patients with sickle cell disease. In in vivo studies, PB-04 induced fetal globin expression >20-fold with oral dosing in anemic baboons, and by 3.5-fold in mice transgenic for 2 copies of the human β-globin gene locus. This agent has been approved and in broad use for Parkinson's Disease treatment for 5 decades in Europe and Canada, to enhance the PK of an active Parkinson's agent, solely in a combination formulation. Because of its safety with chronic dosing up to 1300 mg/day, PB-04 is therefore of interest for repurposing in the treatment of thalassemia and SCD. This Ph1b trial (NCT004432623) will evaluate safety, tolerability, PK, and preliminary efficacy (fetal globin expression) with at least 3 escalating dose cohorts in up to 24 beta thalassemia intermedia (BTI) patients and 12 patients with sickle cell disease, > 18 years, both genders, with 12 weeks of treatment and 4 weeks of follow-up. Doses to be explored were selected from safe and active human equivalent doses in preclinical toxicology and efficacy studies in 2 species. Primary inclusion criteria include clinical diagnosis of BTI, including HbE beta thalassemia, with at least one beta globin gene mutation, or diagnosis of HbSS or HbSb thalassemia (after dose selection); total Hb levels 6-10 gm/dL. Exclusion criteria include red blood cell (RBC) transfusion within 2 months prior to administration of study medication, receiving regular transfusions, and hepatic or renal function > 3 x institutional ULN. Primary endpoints include occurrence, severity, and duration of adverse events by CTCAE v5.0 criteria and PK parameters. Secondary endpoints include change from baseline in assays of HbF expression, total Hb, markers of hemolysis . Exploratory endpoints include select polymorphisms associated with basal fetal globin expression and in vitro responses to the study drug. Statistical analysis will assess the rate of adverse events, PK parameters by dose, and changes in F-cells, F-reticulocytes, HbF/cell, HbF (% and total), compared to 2 averaged baseline values using the Wilcoxon signed rank test. Changes will be analyzed by mixed effect models during the 12-week dosing and 4-week follow-up period. Currently 3 dose cohorts with BTI are enrolled. This study, if successful, will provide a rational basis for definitive trials of an established safe oral therapeutic, with no overlapping toxicities with other experimental or approved agents, for combined use with HU and other therapeutics which enhance red cell viability in the globin disorders. The expertise and contributions of Jim Cradock of NCATS TRND to this work before his passing are gratefully acknowledged. Disclosures Kuo: Agios: Consultancy, Membership on an entity's Board of Directors or advisory committees; Bioverativ: Membership on an entity's Board of Directors or advisory committees; Celgene: Consultancy; Apellis: Consultancy; Bluebird Bio: Consultancy; Pfizer: Consultancy, Research Funding; Novartis: Consultancy, Honoraria; Alexion: Consultancy, Honoraria. Sheth: CRISPR: Consultancy; Bluebird bio: Consultancy; Bristol Myers Squibb: Consultancy, Research Funding; Chiesi: Consultancy; Imara: Research Funding; Agios: Consultancy; Dispersol: Research Funding. Al-Samkari: Moderna: Consultancy; Amgen: Research Funding; Rigel: Consultancy; Argenx: Consultancy; Novartis: Consultancy; Dova/Sobi: Consultancy, Research Funding; Agios: Consultancy, Research Funding. Pace: Imara Inc.: Consultancy. Kuypers: Forma Therapeutics, Inc.: Research Funding. Nouraie: Phoenicia BioScience Inc.: Consultancy. Perrine: Agios Pharmaceuticals: Consultancy; Emmaus Medical: Consultancy.

Interferon-alpha (IFN), the first approved immunotherapy for cancer, remains an effective therapy for patients with myeloproliferative neoplasms (MPN). The mechanisms of action of IFN on MPN cells are poorly understood, particularly in patients with CALR mutated (MUT) MPNs, who often exhibit clinical but not molecular responses. Previously, by developing Genotyping of Transcriptomes (GoT) that captures mutation status and single-cell RNA-seq (scRNA-seq) in high-throughput, we observed that CALR mutations led to cell identity-dependent effects on CD34 + cells, including a strong megakaryocytic progenitor (MkP) differentiation bias and fitness. We hypothesized that the IFN effects may be cell identity and mutation status dependent; thus we applied GoT to serial bone marrow aspirates (BM) from 5 patients with CALRmutated ET treated with pegylated-IFN-alfa2a who participated in MPD-RC-111/112 clinical trials. To capture the transcriptional impact of IFN, we removed experimental batch effects with Cell Hashing, in which CD34 + cells from serial BM were uniquely labeled and combined for the same GoT experiment (Fig. 1A). Cell clustering based on transcriptomic data alone revealed that the cells on active treatment clustered based on cell identity and IFN effects (Fig. 1B). When off therapy for 3 weeks, the strong transcriptional effects of IFN were largely lost (Fig. 1B). Next, we batch corrected and integrated across time points for each BM sample (Fig. 1C). We observed that IFN caused large shifts in the composition of wildtype (WT) and MUT cell subsets (Fig. 1D). IFN resulted in a dramatic expansion of WT lymphoid progenitors with a corresponding diminution of other progenitors (Fig. 1E). MUT cells at baseline were enriched for MkPs, compared to WT cells; after treatment, we observed an expansion of the immature myeloid (IMP) and neutrophil progenitors, with a less striking expansion of lymphoid progenitors (Fig. 1E). As IFN has been reported to induce cell cycling of murine hematopoietic progenitor cells, we examined whether a differential increase in proliferation by IFN underlies the differentiation shifts in WT and MUT cells. Cell cycle gene expression of ProB cells increased after treatment similarly in MUT and WT cells, while cell cycle expression of IMPs was increased to a greater extent in MUT cells (Fig. 1F), consistent with the differential shifts in populations. Next, we performed differential expression analysis between baseline and treated WT and MUT cells, respectively. We observed enrichment of the IFN pathways post-therapy, whereas TNF-a signaling was downregulated (Fig. 1G). Uniquely in the MUT cells, TGF-b signaling was downregulated, which may underlie improvements in marrow fibrosis following IFN therapy (Fig. 1G). Finally, as the differentiation biases of IFN persisted after discontinuation, we hypothesized that IFN results in chromatin remodeling of the earliest hematopoietic stem progenitor cells (HSPCs), with respect to transcription factor (TF) accessibility. We leveraged single nuclei chromatin accessibility (snATAC-seq) as a powerful measure of TF regulatory activities. We developed GoT-ATAC, an adaptation of the Multiome platform (10x Genomics), to capture snRNA-seq, snATAC-seq and somatic genotyping within the same cells in high-throughput (Fig. 1H). We applied GoT-ATAC to CD34 + cells from the same clinical trial cohorts (Fig. 1I, n = 3 patients: 3 baseline, 2 treated) and identified the expected enrichment of IRFs and STAT2 in treated HSPCs (Fig. 1J). Accessibility of BCL11A, critical for early lymphoid development, was increased in treated MUT and WT HSPCs. We also identified enhanced motif accessibility of PU.1 which can associate with IRF and is essential for myeloid and lymphoid differentiation. Uniquely within the treated MUT cells, we observed enhanced CEBPA motif enrichment, which regulates myeloid differentiation, together with PU.1. In conclusion, GoT revealed that IFN reshapes the differentiation landscape by promoting early lymphoid development and, uniquely in MUT cells, myeloid differentiation, providing a novel mechanism of actions underlying the effects of IFN in MPN patients. Downregulations of TNF-a and TGF-b signaling were other key molecular consequences of IFN. Lastly, GoT-ATAC demonstrated that IFN governs master regulators of hematopoietic differentiation as a function of the underlying mutational status. Figure 1 Figure 1. Disclosures Mimitou: Immunai: Current Employment. Smibert: Immunai: Current Employment. Hoffman: AbbVie Inc.: Other: Data Safety Monitoring Board, Research Funding; Novartis: Other: Data Safety Monitoring Board, Research Funding; Protagonist Therapeutics, Inc.: Consultancy; Kartos Therapeutics, Inc.: Research Funding.

TGFβ plays a pivotal role in the pathobiology of myelofibrosis (MF) by not only promoting bone marrow fibrosis (BMF) but also by enhancing the dormancy of normal but not MF hematopoietic stem cells (HSCs). TGFβ has also previously been reported to inhibit normal megakaryocyte (MK) production (Bruno et al Blood 1998). TGFβ1 promotes the synthesis of collagen by normal human mesenchymal stromal cells (MSCs). Treatment of MSCs with AVID200, a potent TGFβ1/3 protein trap, significantly decreased MSC proliferation, phosphorylation of SMAD2, and collagen expression. Robust expression of pSMAD2 was observed in the absence of exogenous TGFβ in normal donor or MF-MKs, Addition of AVID200 to MKs decreased pSMAD2 without affecting total SMAD2/3 and led to increased numbers of MKs. Treatment of MF MNCs with AVID200 also led to increased numbers of progenitor cells with wild type JAK2 and a reduction of mutated colonies. A phase 1b trial of AVID200 (NCT03895112) was performed and completed in INT-2/high risk MF patients resistant/intolerant to ruxolitinib (rux); baseline platelet count of ≥ 25 x 10 9/L, and grade 2/3 BMF. Subjects received AVID200 intravenously on Day 1 of a 21 day cycle. Response was assessed by IWG/ELN criteria after 6 cycles of AVID200. Subjects attaining at least a CI or SD with a decrease in BMF by ≥1 grade, continued AVID200. We previously presented the results of the dose escalation study (Mascarenhas ASH 2020) demonstrating that AVID200 was well tolerated without dose limiting toxicities at 3 tested dose levels (Lots A and B) in dose cohorts of 180 mg (A), 550 mg (A)/70 mg (B), and 180 mg (B). Here we report updated safety and efficacy results of the phase 1b dose expansion stage at the two highest doses tested (70 mg (B) and 180 mg (B). Twenty-two subjects were enrolled (1 withdrew before receiving treatment) and 9 were treated with AVID200 in the dose escalation phase and 12 in the dose expansion phase [Table1]. Median time after rux discontinuation was 7.4 months (0.5-59.9). The most common mutations observed at baseline in this cohort included JAK2V617F (71%), TET2 (29%) ASXL1 (24%) and CALR (19%). (Fig 1) No DLTs were observed and Grade 3/4 AEs were observed in 16 (76.2%) subjects. Grade 3/4 non-hematologic AEs were observed in 8 (38.1%) subjects and included one subject in each case (epistaxis, mucositis, extraocular muscle paresis, fatigue, rash, duodenal hemorrhage, gastric hemorrhage, urinary tract infection, and syncope). Grade 3/4 hematologic AEs were anemia (6; 28.3%) and thrombocytopenia (2; 14.3%) [Table 2]. No fatal events were observed. The median number of cycles received was 5 (range 2 - 13) and 7 (33%) patients received more than 6 cycles. For dose levels 2-3 at cycle 7, a CI was attained in one subject at dose level 2 [anemia, spleen and TSS], 5 subjects had SD, 3 subjects had PD and two subjects with 10% and 15% blasts at screening developed MPN-BP while on study based on central review. Reasons for discontinuation by local PI included PD (n=8), lack of response (n=5), study completed (n=2), other (n=2), patient decision (n=1). Median % change in palpable spleen length was +10% (range -70% to +150%) and TSS change was -50% (-100% to +185.7%) The median platelet count at baseline was 114 x 10 9/L (range: 28-695) and 215 x 10 9/L (range: 66-263) after cycle 6 in 7 evaluable subjects (Fig 2A). Notably, 17 subjects had an increase in platelets from baseline during treatment and two subjects normalized their platelet counts. Maximum changes in platelets from baseline across all cycles was +63.8% [range -15.7%, +505.5%] (Fig 2B). Paired bone marrow biopsy pathology samples for 12 subjects were available for central review and showed no significant changes in BMF score or MK histo topography at end of treatment compared to baseline. All patients had elevated plasma levels of TGF β1, but not TGFβ2/β3 levels as detected by ELISA, which were dramatically reduced 21 days after the last dose of AVID200. AVID200 a TGFβ1/3 protein trap is well tolerated and clinical responses at cycle 7 of therapy in this advanced MF patient population were limited as judged by IWG/MRT response criteria. However, AVID200 therapy resulted in significant reduction in serum TGFβ levels and improvements in platelet counts indicating that TGF β1 plays a pivotal role in MF leading to thrombocytopenia which can be reversed with AVID200 therapy. We conclude that AVID200 may best be employed in combination therapy approaches in thrombocytopenic MF patients. Figure 1 Figure 1. Disclosures Mascarenhas: Constellation: Consultancy, Membership on an entity's Board of Directors or advisory committees; Promedior: Consultancy, Membership on an entity's Board of Directors or advisory committees; Incyte: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Geron: Consultancy, Research Funding; Forbius: Research Funding; Genentech/Roche: Consultancy, Membership on an entity's Board of Directors or advisory committees; Sierra Oncology: Consultancy, Membership on an entity's Board of Directors or advisory committees; Celgene/BMS: Consultancy, Membership on an entity's Board of Directors or advisory committees; PharmaEssentia: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Roche: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Galecto: Consultancy; Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Prelude: Consultancy; Kartos: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; CTI Biopharm: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Geron: Consultancy; Merck: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; AbbVie: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Gilead: Consultancy, Membership on an entity's Board of Directors or advisory committees; Merus: Research Funding. Palmer: PharmaEssentia: Research Funding; Sierra Oncology: Consultancy, Research Funding; Incyte: Research Funding; CTI BioPharma: Consultancy, Research Funding; Protagonist: Consultancy, Research Funding. Kuykendall: Celgene/BMS: Honoraria; Pharmaessentia: Honoraria; Novartis: Honoraria, Speakers Bureau; Protagonist: Consultancy, Research Funding; Incyte: Consultancy; Abbvie: Honoraria; Blueprint: Honoraria. Mesa: Genentech: Research Funding; Promedior: Research Funding; Samus: Research Funding; Gilead: Research Funding; CTI: Research Funding; Abbvie: Research Funding; Sierra Oncology: Consultancy, Research Funding; Celgene: Research Funding; Novartis: Consultancy; Pharma: Consultancy; CTI: Research Funding; Constellation Pharmaceuticals: Consultancy, Research Funding; AOP: Consultancy; La Jolla Pharma: Consultancy; Incyte Corporation: Consultancy, Research Funding. Rampal: Stemline: Consultancy, Research Funding; Memorial Sloan Kettering: Current Employment; BMS/Celgene: Consultancy; Abbvie: Consultancy; CTI: Consultancy; Novartis: Consultancy; Disc Medicine: Consultancy; Blueprint: Consultancy; Pharmaessentia: Consultancy; Incyte: Consultancy, Research Funding; Jazz Pharmaceuticals: Consultancy; Constellation: Research Funding; Kartos: Consultancy; Sierra Oncology: Consultancy. Gerds: PharmaEssentia Corporation: Consultancy; Sierra Oncology: Consultancy; CTI BioPharma: Research Funding; Constellation: Consultancy; Celgene/Bristol Myers Squibb: Consultancy; AbbVie: Consultancy; Novartis: Consultancy. Yacoub: Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; CTI Biopharma: Membership on an entity's Board of Directors or advisory committees; ACCELERON PHARMA: Membership on an entity's Board of Directors or advisory committees; Agios: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Dynavex: Current equity holder in publicly-traded company; Cara: Current equity holder in publicly-traded company; Ardelyx: Current equity holder in publicly-traded company; Seattle Genetics: Honoraria, Speakers Bureau; Incyte: Consultancy, Honoraria, Speakers Bureau; Hylapharm: Current equity holder in publicly-traded company. Talpaz: Imago: Consultancy; Constellation: Membership on an entity's Board of Directors or advisory committees; Bristol Myers Squibb: Membership on an entity's Board of Directors or advisory committees; Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Takeda: Other: Grant/research support ; Celgene: Consultancy. Komrokji: Acceleron: Consultancy; Taiho Oncology: Membership on an entity's Board of Directors or advisory committees; AbbVie: Consultancy; BMSCelgene: Consultancy, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; PharmaEssentia: Membership on an entity's Board of Directors or advisory committees; Novartis: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Geron: Consultancy; Jazz: Consultancy, Speakers Bureau. Kremyanskaya: Astellas: Research Funding; Astex: Research Funding; Chimerix: Research Funding; Bristol Myers Squibb: Research Funding; Constellation: Research Funding; Protagonist Therapeutics: Consultancy, Research Funding; Incyte: Research Funding. Salama: Mayo Clinic: Current Employment, Other: Mayo Clinic had the contractual work for the central pathology review for this study and I was one of the reviewing pathologists; Constellation Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees. Hoffman: Kartos Therapeutics, Inc.: Research Funding; Protagonist Therapeutics, Inc.: Consultancy; Novartis: Other: Data Safety Monitoring Board, Research Funding; AbbVie Inc.: Other: Data Safety Monitoring Board, Research Funding. OffLabel Disclosure: AVID200 is a TGFb trap and is in clinical testing for fibrotic diseases. It does not have an approved indication at this time.

Recently, the protein LZTR1 (leucine zipper-like transcriptional regulator 1) was discovered as an adaptor for a cullin 3 complex responsible for ubiquitin-mediated degradation of RAS proteins. While these data provided a novel mechanism for RAS protein regulation, there is considerable controversy as to which RAS paralogs are physiologic substrates of LZTR1. In parallel, dysregulated LZTR1 expression via aberrant splicing and mutations in both LZTR1 as well as the RAS GTPase and LZTR1 substrate RIT1 were identified in patients with clonal hematopoietic disorders. However, the effects of these alterations on normal and maliganant hematopoiesis have not been evaluated. Here we utilized a series of genetically engineered murine models for germline and conditional deletion of LZTR1, RIT1, and expression of oncogenic RIT1 mutant which revealed a key role for LZTR1 in the regulation of hematopoietic stem cell (HSC) self-renewal and delineated a series of LZTR1-regulated substrates in hematopoietic cells. Consistent with a role for LZTR1 alterations in the Noonan Syndrome, germline homozygous deletion of Lztr1 was associated with lethality between embryonic day 17.5 and birth. Lztr1-/- fetuses had massive dyserythropoiesis and apoptosis of fetal liver hematopoietic cells. Competitive transplantation of E14.5 Lztr1 null fetal liver or bone marrow from 6-week-old Mx1-cre Lztr1 conditional knockout (cKO) mice resulted in striking increased self-renewal in primary and secondary competitive transplantation assays in vivo (Fig.A-B). Interestingly, recipient animals reconstituted with Lztr1-/- cells developed fatal myeloid and lymphoid leukemias characterized by anemia, thrombocytopenia, and increased myeloid and B-lymphoid cells (Fig.C-D). In order to identify the LZTR1 substrates responsible for effects on HSCs, we evaluated levels of all RAS GTPases in Lztr1 null HSCs. This revealed elevated KRas, NRas, MRas, and Rit1 protein in LZTR1 KO cells (Fig.E), with RIT1 being most prominently elevated. Evaluation of a cohort of 4,113 patients with hematologic malignancies identified 41 patients with somatic RIT1 mutations, the majority of which cluster in the switch II region and escape LZTR1-mediated ubiquitination, resulting in RIT1 protein accumulation (Fig.F-H). Given that the impact of RIT1 mutations on hematopoiesis is unknown, we next compared Lztr1 cKO with conditional expression of one of the most common leukemia-associated RIT1 mutants that escapes LZTR1-mediated ubiquitin (Rit1 M90I). Both Lztr1 cKO and Rit1 M90I conditional expression conferred GM-CSF hypersensitivity to HSCs in vitro, cytokine independent growth to human AML cell lines in vitro, and strong competitive self-renewal in vivo (Fig. I-J). Consistent with RIT1 mutations being found primarily in myeloid neoplasm patients, aged Mx1-cre Rit1M90I/WT mice developed fatal MPN, MDS, and mixed MDS/MPN disorders (Fig.K), which were serially transplantable into sublethally irradiated recipients. Despite convergent effects of LZTR1 and RIT1 on clonal HSC advantage, LZTR1 null cell lines did not solely require RIT1 for HSC advantage as revealed by Lztr1/Rit1 double KO mice. We therefore next carried out a series of experiments in RAS-less cells and whole genome CRISPR screens to delineate factors required for LZTR1 mediated hematopoietic transformation. This revealed that KRAS as well as MRAS and its RAF phosphatase partner SHOC2 were selective dependencies for LZTR1-mediated transformation. These data indicate that multiple RAS GTPases as well as RAF activation are required for LZTR1-mediated transformation (Fig.L). While considerable prior research has evaluated oncogenic alleles of RAS which alter RAS-GTP hydrolysis on hematopoiesis, the role of modulating RAS protein abundance on hematopoiesis is unknown. Here we identify RAS proteolysis as a novel regulator of HSC function, define the spectrum of RIT1 mutations in leukemia, and identify LZTR1 and RIT1 mutations as drivers of leukemogenesis. The discovery of RAS proteolysis as a novel driver of leukemogenesis has important therapeutic implications given efforts to therapeutically degrade RAS family members. Finally, the clinical importance of K/NRAS mutations on resistance to therapies in AML motivates future studies on the potential clinical impact of LZTR1 and RIT1 alterations in myeloid neoplasm patients. Figure 1 Figure 1. Disclosures Abdel-Wahab: H3B Biomedicine: Consultancy, Research Funding; Merck: Consultancy; Foundation Medicine Inc: Consultancy; Prelude Therapeutics: Consultancy; LOXO Oncology: Consultancy, Research Funding; Lilly: Consultancy; AIChemy: Current holder of stock options in a privately-held company, Membership on an entity's Board of Directors or advisory committees; Envisagenics Inc.: Current holder of stock options in a privately-held company, Membership on an entity's Board of Directors or advisory committees.

Introduction Severe COVID-19 disease is associated with a hyperinflammatory, pro-thrombotic state and a high mortality. A thrombotic phenotype rather than a coagulopathy is suggested and we undertook plasma exchange to determine its effects on organ function and thrombo-inflammatory markers. Methods Plasma exchange was carried out in seven critically ill adults with severe COVID-19 respiratory failure (PaO2:FiO2 ratio 800 IU/L and D dimer>1000 µg/L (or doubling from baseline). Patients received a daily single volume 3 litre plasma exchange for a minimum of five days. No other immunomodulatory medications were initiated during this period. Effects on organ function, thrombo-inflammatory markers and complications were monitored. Seven patients matched for age and baseline biochemistry were a comparator group. Results Coagulation screening revealed no evidence of coagulopathy. However, von Willebrand Factor (VWF) activity, antigen and VWF antigen:ADAMTS13 ratio, Factor VIII and D-dimers were all elevated. Following five days of plasma exchange, plasma levels of all the above, and ferritin levels, were significantly reduced (p Conclusion Plasma exchange was associated with an improvement in oxygenation, decreased incidence of AKI, normalisation of lymphocytes and reduction in circulating thrombo-inflammatory markers including D-Dimer and VWF Ag:ADAMTS13 ratio. Disclosures Thomas: Ablynx: Honoraria, Other: Advisory Board; Bayer: Honoraria, Speakers Bureau; Sanofi: Honoraria, Other: Advisory Board. Scully:Takeda: Consultancy, Speakers Bureau; Alexion: Consultancy, Speakers Bureau; Sanofi: Consultancy, Speakers Bureau; Shire/Takeda: Other: Advisory Board, Research Funding, Speakers Bureau; Novartis: Other: Advisory Board, Speakers Bureau; Takeda: Speakers Bureau; Ablynx/Sanofi: Consultancy, Other: Advisory Board, Speakers Bureau.

Background Thalassemia Syndromes (TS) are commonly classified as transfusion-dependent-thalassemia (TDT) or non-transfusion-dependent thalassemia (NTDT) at diagnosis on the basis of requirement for lifelong regular transfusion therapy for survival. However, data from observational studies and expert opinion suggest that these categories may reflect a wide spectrum rather than a dichotomy, and may actually be interchangeable at many parts of the disease journey. Thus, an evaluation of alternate clusters to classify TS patients remains of merit. Aims The aim of this study was to cluster TS patients on the basis of possible clinical indicators of phenotype severity (IPhS) using suitable algorithms and to determine whether these are able to detect cohorts with different clinical phenotypes. Methods Representatives from thirteen international centers from seven countries agreed on 19 IPhS to be collected for a retrospective study. Data from 7910 TS patients were collected. NbClust R Packagewas performed for exploring the existence of a substructure inside the studied TS population, determining the best number of clusters. Unsupervised Random Forest (RF)clustering and the Partitioning Around Medoids (PAM)algorithms were performed to define the clusters. The most important IPhS in defining clusters were selected according to the Gini index. Kaplan-Meier (K-M) survival curves of the identified clusters, defined by the selected IPhS, were used to represent the risk of death for these clusters. Results NbClust method showed the existence of three possible clusters. The RF-PAM procedure defined three distinct clusters with a classification error rate of 4.3% (Fig 1). Moreover, the most important IPhS were patient age, mean serum ferritin level, age at diagnosis, age at first transfusion, age at first iron chelation, and number of complications. K-M curves showed statistically significant differences in survival among the three clusters (p Conclusions The observation of statistically significant differences in survival between the three newly identified clusters but not the original TDT-NTDT classification confirms that the latter classification is interchangeable, and a new triad classification system is required. These findings warrant further evaluation in prospective studies to determine specific thresholds for IPhs indicators that can aid physicians in assigning classes and tailoring care, in order to improve survival in TS patients. Disclosures Meloni: Chiesi Farmaceutici S.p.A.: Other: speakers' honoraria. Pistoia:Chiesi Farmaceutici S.p.A.: Other: speakers' honoraria. Vichinsky:Novartis: Consultancy, Research Funding; Bluebird Bio: Consultancy, Research Funding; Agios Pharmaceuticals: Consultancy, Research Funding; Pfizer: Consultancy, Research Funding; GBT: Consultancy, Research Funding.

Background: CD3/CD19 cell depletion (Barfiled RC, et al, Cytotherapy, 2004), αβ T-cell/CD19 cell depletion (Locatelli F, et al, Blood, 2017), CD34+ positive selection (Aversa F, et al, NEJM, 1998) are designed to deplete T cells and reduce AGVHD following allogeneic stem cell transplantation (AlloSCT). These approaches achieved low rates of AGVHD, but the grafts had few T and B cells. To improve immune reconstitution we undertook an alternative approach to addback small numbers and percentages of immune cells in the final HSCT product. We previously reported a very low incidence of AGVHD in pediatric recipients receiving CD34 enriched HPC products with peripheral blood mononuclear cells (PBMNC) addback containing a fixed dose of 2 x 105 CD3/kg from MUD donors (Geyer/Cairo et al, BJH, 2012). Recently we demonstrated that despite a 5 log depletion of T cells, PBMNC addback (fixed at 2 x 105 CD3/kg) facilitated rapid hematopoietic engraftment, high levels of donor chimerism and immune reconstitution with a low probability of Grade II-IV AGVHD. Patients had a 1 yr OS of 90% following familial haploidentical (FHI) CD34 Enriched Stem Cell Transplantation in patients with SCD (Cairo, JAMA Pediatr, 2020). Objective: To determine the final immune cell concentration following CD34 enrichment and PBMNC (2 x 105 CD3/kg) addback and determine the effect on engraftment and T and NK cell immune reconstitution. Methods: Patients and/or their guardians signed written informed consents and/or assents (NCT NCT02675959). CD34+ enrichment was performed using a CD34+ reagent system (CliniMACS; Miltenyi Biotec). Mononuclear cells (2 × 105 CD3 cells/kg of recipient body weight) were removed from the leukapheresis collection prior to CD34+ enrichment and were cryopreserved as a source of MNC addback (T cells). The addback products were analyzed for CD3+CD56- T cells, CD3-CD56+ NK cells, CD3+CD56+ NKT cells, Lin-CD123+ HLA-DR+ DC cells and Lin-CD11c+ HLA-DR+ DC cells by multicolor flow cytometry analysis. Th1/Th2 cytokines were measured by multiplex assays. T cell activity was measured by viral T cells IFN-g and plasma cytokines. NK function was measured by NK receptor expression by flow cytometry analysis and in vitro cytotoxicity. Results: We identified in the PBMNC addback, mean+SEM white blood cell (WBC) percentage of: CD3+ CD56- T cells = 56.4±5%; CD3- CD56+ NK cells = 4.6±1%; CD3+ CD56+ NKT cells = 5.1±0.6%; CD19+ B cells = 29.9±3.5%. Lin- WBC consisted of: CD123+ HLA-DR+ DC cells = 18.4±8.2%; CD11c+ HLA-DR+ DC cells = 6.0±3.0%. There were 20.0+9.1e6 T cells, 1.1+0.3e6 NK cells, 1.6+0.7 e6 NKT cells, 8.6+2.5e6 B cells, 1.2+0.6e6 CD123+DC and 0.8+0.5e6 CD11c DC in the final infused products (Fig.1). We found that percentages of IFN-g+ in CD4 cells in response to CMV (pp65), ADV (hexon) and EBV (BZLF1), ranged from 0.2%+0.1% to 0.5%+0.1%, while percentages of IFN-g+ in CD8 cells in response to the antigens ranged from 0.7%+0.3% to 3.7%+1.8% when examined at days 180, 270 and 365. NK (CD3- CD56+) reconstitution was extremely rapid and occurred as early as day 30 (35.5±8.6%, 2710+1624.4 cells/ul total cells; p Conclusion: PBMNC addback to CD34 enriched HPC products, with a final dose of 2 × 105 CD3 cells/kg, led to stem cell products with a diverse mixture of T, NK, NKT, DC1, and DC2 cells. Immune reconstitution following PBMNC addback to CD34 enriched cells resulted in excellent CD4 and CD8 responses to CMV, ADV and EBV, and rapid functional NK cell reconstitution (Supported by FDA R01FD004090 (MSC)). Disclosures Baxter-Lowe: CHLA: Current Employment, Membership on an entity's Board of Directors or advisory committees, Patents & Royalties: Patents related to HLA typing, Research Funding. Johnson:Miltenyi Biotec: Research Funding; Cell Vault: Research Funding. Cairo:Miltenyi: Research Funding; Technology Inc/Miltenyi Biotec: Research Funding; Jazz Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Nektar Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees, Research Funding.

Introduction Thalassemia Syndromes (TS) are a group of inherited haemoglobin disorders characterized by different phenotype severity falling among heterozygote state, no transfusion dependent thalassemia (NTDT) and transfusion dependent Thalassemia (TDT) (Graffeo et al, 2018; Taher & Saliba, 2017). Several factors, independently by genotype and globin chain unbalance, modulate the severity of ineffective erythropoiesis (Rivella et al, 2015). Considering the complexity of this pathophysiology, one tool to evaluate patients on an individual basis is needed. The aim of this study was to develop a severity scoring system with a view to initiate timely interventions that would prevent any further complications. Methods An International Health Repository (IHR) protocol was approved on May 25th, 2017 by the Italian Ethical Committee (EudraCT Code Numbers 2017-004457-17 and 143AOR2017). Subsequently, an International Working Group (IWG) on Thalassemia was formed and it met in Palermo, Italy, on September 15th and 16th, 2017. Indicators of phenotype severity thought to be most pertinent in defining disease severity were shared among the IWG based on their expertise as well as on expert opinion reported on literature. These data were collected and stored on IHR platform (www.sanitasicilia.eu/IWG ). In order to define the prognostic score (PS) a retrospective multicenter case-control study was carried ahead. Overall, clinical findings of 7910 patients who attended the IWG centres from 1976 to 2018 were collected (Tab. 1). The study was based on 5657 cases that developed complications and 2253 controls that didn't develop any complications. The term "patient" was used both for cases and for controls. The variables used for the scoring system development were reported in Table 2. The PS was built using a Conditional Logistic Regression Model (CLRM) (DW. Hosmer, Jr., 2013). The full data-set was split into two data-set containing Group A and Group B patients. Group A included transfused TDT and NTDT patients, while Group B included only no transfused NTDT patients. This was necessary because of the variable "age at first transfusion" was absent in no transfused patients. The two PS will refer as Score A and Score B, respectively. The Stata 12 (StataCorp, College Station, TX, USA) was used for all analyses. Results Overall, the analysis was performed on 5657 cases (2812 Females, age at follow up 34.7±12.9) with a mortality of 8.5% and 2253 controls (1136 Females, age at follow up 22.1±12.8) with a mortality of 1.9%. Table 3 shows complications and deaths in the full data set. Moreover, it even suggests, as single patient may have more than one complication and that the cardiac complications followed by the cancer and infection were the most common. Table 4 shows the statistically significant variables for the calculation of the score. The age of the patient and at first diagnosis, the mean Hb, AST, ALT and the Left Ventricular Ejection Fraction (LVEF) were the statistically significant variables at CLRM analysis, for the Group A. The formula for the Score A=1.1x(Age)+0.9x(Age at first diagnosis)+1.3x(Hb mean)+1.006x(AST)+1.02x(ALT)+ 0.9x(EF). The age of the patient, the mean Hb and the LVEF were the statistically significant variables at CLRM analysis for the Group B. The formula for the Score B= 1.05x(Age)+1.001x(Hb mean) +0.9x(EF). Finally, Table 5 shows the application of these formulas to the full data set, defining five well-distinguished prognostic categories (Very low, Low, Intermediate, High, Very high). Conclusions Following appropriate validation, we propose that the severity scoring system described here could be developed into a practical tool for widespread clinical application, not only to evaluate patient status and classify disease subgroups, but also to inform treatment decisions and monitor patient progress in response to therapy. Finally, the use of this scoring system may help to select appropriate candidate for innovative treatment. Disclosures Pepe: Chiesi Farmaceutici S.p.A., ApoPharma Inc., and Bayer: Other: No profit support. Vichinsky:bluebird bio: Consultancy, Research Funding; GBT: Consultancy, Research Funding; Pfizer: Consultancy, Research Funding; Novartis: Consultancy, Research Funding; Agios: Consultancy, Research Funding.

Background: Allogeneic stem cell transplantation (AlloSCT) from an HLA-matched sibling donor is the only known curative therapy in patients with high-risk SCD (Talano/Cairo, EJH, 2015). Unfortunately only about 15% of high risk patients with SCD have an HLA-matched unaffected sibling donor. T cell depletion has been employed to reduce AGVHD e.g., CD3/CD19 cell depletion (Barfiled RC, et al, Cytotherapy, 2004), αβ T-cell/CD19 cell depletion (Locatelli F, et al, Blood, 2017), CD34+ positive selection (Aversa F, et al, NEJM, 1998). MUD transplantation in high-risk SCD recipients has shown unexpectedly high rates of CGVHD (Shenoy et al, Blood, 2016). We reported a very low incidence of acute and chronic GVHD in pediatric recipients receiving CD34 enriched HPC products with PB MNC addback with 2 x 105 CD3/kg from MUD donors (Geyer/Cairo et al, BJH, 2012). Furthermore, rapid NK cell reconstitution after AlloSCT is associated with a significant improvement in 1yr OS (Pical-Izard, BBMT, 2015; Dunbar et al, Hematologica, 2008). Recently, we reported promising results for high-risk SCD patients at 1 year follow-up after FHI CD34 enriched/PBMNC with addback AlloSCT with the probability of 1-year overall survival (OS) n=17; 88.2% (CI95: 60.6-96.9) (Talano/Cairo, ASH, 2017), expanding the donor pool and hopefully improving outcomes for high-risk patients with SCD. Objective: To investigate donor chimerism, immune cell reconstitution and NK cell function in high-risk patients with SCD following AlloSCT using FHI CD34 enrichment/PBMNC (2 x 105 CD3/kg) addback. Methods: Twenty-one eligible SCD patients (2- Results: There was 100% engraftment of neutrophils and platelets. The median day post-HISCT to neutrophil and platelet engraftment was +9 and +19, respectively. Whole blood donor chimerism (mean±SEM) at 1-year, 2-year, and 3-year post-HISCT was 97±1%, 97±1%, 97±1%, respectively (Fig.1). Donor chimerism for CD71+ RBCs (mean±SEM) at 1-year, 2-year, 3-year post-HISCT was 97±2%, 98±1%, 98±1%, respectively (Fig.1). Immune reconstitution of CD3, CD4, CD8, and CD19 was evaluated. The time to recovery of minimally normal levels post-HISCT of CD3 (800 cells/ul), CD4 (400 cells/ul), CD8 (200 cells/ul), and CD19 (200 cells/ul), was approximately 365, 365, 270, and 60 days post-HISCT (Fig.2), respectively. Probability of Grade II-IV AGVHD, CGVHD and 1 year EFS/OS was 6.2%, 6.7% and 90%, respectively. NK reconstitution was rapid and peaked at d+30 (36±9%, 2710cells/ml). NK cytotoxicity against K562 at a E:T=10:1 peaked at d+30 (26±3%) and d+180 (28±3%) vs at pre-t (16±2%) (p Conclusion: Despite a 5 log depletion of T cells, the PBMNC addback (fixed at 2 x 105 CD3/kg) facilitated rapid donor chimerism and immune reconstitution with a low probability of Grade II-IV AGVHD. The rapid NK reconstitution may have in part contributed to the excellent 1yr OS in the FHI study. (Supported by FDA R01FD004090 (MSC)). Disclosures Cairo: Jazz Pharmaceuticals: Other: Advisory Board, Research Funding, Speakers Bureau; Osuka: Research Funding; Miltenyi: Other: MTA.