Search

Activating FLT3 internal tandem duplication (FLT3-ITD) mutations, present in ~30% of AML patients, are associated with poor prognosis. Given that efficacy of FLT3 inhibitors used as monotherapy is limited due to development of secondary FLT3 mutations or activation of alternative survival pathways, new combinatorial strategies are needed to improve outcome in FLT3-ITD AML patients. Constitutive activation of mutated FLT3 often leads to elevated expression of MCL-1, an anti-apoptotic member of the BCL-2 family. Upregulation of MCL-1 underlies the resistance of AML to initially successful targeted therapies, including Bcl-2 inhibition with venetoclax, placing MCL-1 as an attractive target for combinatorial therapies. We have recently shown for the first time that a novel MCL-1 inhibitor S63845 elicited synergistic activity with FLT3 inhibitors AC220, sorafenib and with a multi-kinase inhibitor midostaurin at nanomolar doses in pre-clinical in vitro models of FLT3-ITD AML, including cells resistant to venetoclax (Skwarska A, et al. Cancer Res 2019;79 Suppl 13, Abstract#342). Here, we further explored the therapeutic potential and mechanisms-of-action of dual targeting of FLT3-ITD and MCL-1 in vitro and in vivo. S63845 and midostaurin co-treatment significantly increased apoptosis in FLT3-ITD cells with caspase-3 activation and PARP cleavage within 6 hours of treatment. Analysis of drug combinations using Bliss independence model revealed strong synergistic effect of S63845/midostaurin combination in murine isogenic Ba/F3-FLT3-ITD, Ba/F3-FLT3-ITD-D835Y and Ba/F3-FLT3-ITD-F691L dual mutants with minimal effect in Ba/F3-wt FLT3 cells, suggesting that inhibition of FLT3-ITD could mechanistically contribute to the synergy between midostaurin and S63845. Midostaurin monotherapy caused de-phosphorylation of FLT3-ITD and its downstream targets STAT5, AKT and MAPK, known to upregulate MCL-1 expression and stability (Fig 1A). Consequently, midostaurin significantly reduced Mcl-1 T163 phosphorylation and protein levels exclusively in FLT3-ITD cells. Midostaurin also decreased inhibitory phosphorylation of GSK3, suggesting the involvement of ubiquitin/proteasome axis in MCL-1 degradation. Additionally, pre-treatment with pan-caspase inhibitor zVAD-fmk partially reversed MCL-1 downregulation, implying contribution of caspase-mediated degradation to overall decrease of MCL-1 after midostaurin treatment. While midostaurin alone enhanced degradation of MCL-1, it did not fully deplete the pool of MCL-1. Surprisingly, midostaurin also induced the loss of proapoptotic NOXA, an endogenous inhibitor of MCL-1, which can potentially increase a fraction of free MCL-1. These results provide a strong rationale for targeting residual MCL-1 with selective MCL-1 inhibitor S63845 with the goal to facilitate deeper therapeutic responses in FLT3-ITD AML. Mechanistically, midostaurin increased expression of apoptosis activator BIM and blocked BIM phosphorylation required for its inhibitory sequestration by MCL-1. BIM shRNA knockdown cells were less sensitive to midostaurin/S63845 combination. Similarly, CRISPR/Cas9-mediated knock-out of BAK, a BIM-activated executor of apoptosis, resulted in reduced efficacy of tested combination. These results indicate that activation of BIM/BAX axis has a functional role in the response of AML cells to dual FLT3-ITD and MCL-1 inhibition (Fig 1B). To validate the efficacy of combination in vivo, we used xenotransplantation model of MOLM-14 cells expressing luciferase. Mice were treated for 3 weeks with low doses of midostaurin (25 mg/kg/5 days per week) and with MIK665, structurally optimized version of S63845 Mcl-1 inhibitor with improved pharmacokinetics in mice (Halilovic E, et al. Cancer Res 2019;79 Suppl 13, Abstract#4477). Administration of MIK665 at 25 mg/kg or 35 mg/kg IV 2 days per week in combination with oral midostaurin resulted in a marked delay in the progression of MOLM-14 cell line-derived xenograft and significantly reduced leukemia tumor burden (Fig 1C). The long-term effect of combination on mice survival is currently being tested and will be reported. Altogether, our results indicate that efficacy of FLT3-ITD inhibitors can be enhanced through combination with low doses BH3 mimetics targeting MCL-1 and provide a rationale for the clinical evaluation of such combinations in FLT3 mutant AML patients. Figure 1 Figure 1. Disclosures Skwarska: Halilovich E, Wang Y, Morris E, Konopleva M, Skwarska A.: Patents & Royalties: Combination of a MCL-1 inhibitor and midostaurin, uses and pharmaceutical composition thereof.. Halilovic: Novartis: Current Employment, Current holder of individual stocks in a privately-held company, Current holder of stock options in a privately-held company. Mistry: Novartis: Current Employment, Current holder of individual stocks in a privately-held company. Derréal: Servier: Current Employment. Banquet: Servier: Current Employment. Daver: Genentech: Consultancy, Research Funding; Trillium: Consultancy, Research Funding; Novimmune: Research Funding; Glycomimetics: Research Funding; Trovagene: Consultancy, Research Funding; FATE Therapeutics: Research Funding; Astellas: Consultancy, Research Funding; Abbvie: Consultancy, Research Funding; Amgen: Consultancy, Research Funding; Hanmi: Research Funding; Gilead Sciences, Inc.: Consultancy, Research Funding; Bristol Myers Squibb: Consultancy, Research Funding; Daiichi Sankyo: Consultancy, Research Funding; Pfizer: Consultancy, Research Funding; ImmunoGen: Consultancy, Research Funding; Sevier: Consultancy, Research Funding; Novartis: Consultancy; Jazz Pharmaceuticals: Consultancy, Other: Data Monitoring Committee member; Dava Oncology (Arog): Consultancy; Celgene: Consultancy; Syndax: Consultancy; Shattuck Labs: Consultancy; Agios: Consultancy; Kite Pharmaceuticals: Consultancy; SOBI: Consultancy; STAR Therapeutics: Consultancy; Karyopharm: Research Funding; Newave: Research Funding. Andreeff: Medicxi: Consultancy; ONO Pharmaceuticals: Research Funding; AstraZeneca: Research Funding; Reata, Aptose, Eutropics, SentiBio; Chimerix, Oncolyze: Current holder of individual stocks in a privately-held company; Karyopharm: Research Funding; Daiichi-Sankyo: Consultancy, Research Funding; Glycomimetics: Consultancy; Novartis, Cancer UK; Leukemia & Lymphoma Society (LLS), German Research Council; NCI-RDCRN (Rare Disease Clin Network), CLL Foundation; Novartis: Membership on an entity's Board of Directors or advisory committees; Syndax: Consultancy; Aptose: Consultancy; Oxford Biomedica UK: Research Funding; Amgen: Research Funding; Breast Cancer Research Foundation: Research Funding; Senti-Bio: Consultancy. Wei: Novartis, Astellas, Pfizer, MacroGenics, AbbVie, Genentech, Servier, Celgene, Amgen, AstraZeneca, Janssen: Honoraria; Novartis, Celgene, AbbVie, Servier, AstraZeneca, and Amgen: Research Funding; Walter and Eliza Hall Institute: Ended employment in the past 24 months. Konopleva: Cellectis: Other: grant support; Stemline Therapeutics: Research Funding; Novartis: Other: research funding pending, Patents & Royalties: intellectual property rights; AstraZeneca: Other: grant support, Research Funding; Eli Lilly: Patents & Royalties: intellectual property rights, Research Funding; F. Hoffmann-La Roche: Consultancy, Honoraria, Other: grant support; Ascentage: Other: grant support, Research Funding; Agios: Other: grant support, Research Funding; Sanofi: Other: grant support, Research Funding; Forty Seven: Other: grant support, Research Funding; Ablynx: Other: grant support, Research Funding; KisoJi: Research Funding; Genentech: Consultancy, Honoraria, Other: grant support, Research Funding; Calithera: Other: grant support, Research Funding; Rafael Pharmaceuticals: Other: grant support, Research Funding; Reata Pharmaceuticals: Current holder of stock options in a privately-held company, Patents & Royalties: intellectual property rights; AbbVie: Consultancy, Honoraria, Other: Grant Support, Research Funding.

Introduction Fanconi anemia (FA) is the most frequent inherited DNA-repair disease in human, driving hematopoietic stem cell (HSC) failure in children and a major predisposition to poor-prognosis myelodysplastic syndrome (MDS) and acute leukemia (AML) in children or young adults. MDS/AML secondary to FA have a dismal prognosis in this frail population with a high chemotherapy-related toxicity. How bone marrow (BM) cells progress to myeloid malignancies in a background of cell intrinsic genomic instability and stem cell exhaustion is still poorly understood. Here we aimed to identify the molecular and functional determinants of BM progression to MDS/AML in FA patients. Methods We studied a cohort of 335 FA patients, representing virtually all FA patients seen in France from 2002 to 2020. We performed longitudinal clinical studies (cytopenia, BM morphology and staging, HSCT, survival), somatic genomics (karyotype, myeloid cancer gene panel, aCGH, WES, WGS), expression analysis by RNAseq on clonal cells, and functional studies (gene modulation in HSPCs, transgenic MDM4 mice, CFU and competitive engraftment experiments). Paired clonal BM and skin fibroblasts samples were available for 62 MDS/AML FA patients; WES and WGS files from age-matched non FA MDS/AML were used as controls. Results 98 out of 335 patients (29%) experienced clonal evolution, first seen at a median age of 13y, including 51 (15%) with blastic evolution (>5% BM blasts, median age 16y). Unbalanced chromosomal translocations rather than point mutations underlaid clonal evolution in comparison to age-matched, sporadic (non-FA) AML cases. The most prominent driver lesion was chromosome 1q duplication (1q+), found in 52% of the clonal FA patients, while other recurrent lesions were gain of 3q (3q+/EVI1; 40%), translocations/del/mut involving the RUNX1 gene (35%), monosomy 7/7q- (31%), and signaling gene mutations (18%). Based on longitudinal studies and ranking models, we evidenced that 1q+ occurred early, yielding preleukemic clonal hematopoiesis, whereas 3q+, -7/del7q, RUNX1 and signaling mutations occurred later along with BM transformation. Regarding genomic instability, WGS analysis of FA AML cells revealed a unique mutational signature that shares features with BRCA-related solid cancers [homologous recombination deficient (HRD)-type substitution signature, accumulation of small/intermediate-size deletions and large structural variants (SV)]. SV breakpoint analysis identified microhomology-mediated end joining (MM-EJ, also known as Alt-EJ) as the preferential DNA repair mechanism in the FA context. Specifically, a fragile site in the 1q pericentromeric repeated region underlaid 1q+ translocations. Next, we found that the MDM4 oncogene, a negative modulator of p53 response located in the minimal 1q duplicated region, was overexpressed in 1q+ but not in clonal non-1q FA cells. We hypothesized that 1q+ may attenuate the FA-associated p53 pathway hyperactivation through increased gene dosage of MDM4. Consistently, RNA-seq of patient cells before and after clonal progression showed p53 pathway activation before clonal evolution and subsequent p53 downregulation along with 1q+. When evaluated in vitro by CFU assay, lentiviral overexpression of MDM4 rescued clonogenicity defect of HSCPs from both FA patients and Fanc-/- mice, at the same level as TP53 knockdown. We produced a transgenic mouse bearing a duplicated Mdm4 locus and showed that MdM4 overexpression conferred an advantage to FA-like HSPCs in competitive transplant experiments, modeling clonal hematopoiesis. Exposure of 1q+ FA cells to Mdm4 inhibitors raised therapeutic potential. Conclusions The somatic genomic landscape of FA MDS/AML reveals a unique FA mutational signature, characterized by structural rearrangements and copy number abnormalities rather than point mutations. Our results define a canonical oncogenic route towards secondary MDS/AML in FA patients, in which the early modulation of the p53 pathway through 1q+/MDM4 oncogene overexpression plays a pivotal role, raising novel monitoring and therapeutic prospects for the FA patients. Disclosures Sebert: BMS: Consultancy; Abbvie: Consultancy. Dalle: Jazz Pharmaceuticals: Honoraria. Socie: Alexion: Research Funding. Peffault De Latour: Pfizer: Membership on an entity's Board of Directors or advisory committees, Other: Travel support, Research Funding; Amgen: Consultancy, Other, Research Funding; Jazz Pharmaceuticals: Honoraria; Alexion, AstraZeneca Rare Disease: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Travel support, Research Funding.

Introduction: The most common severe toxicity associated with chimeric antigen receptor T-cells targeting CD19 (CART19) is cytokine release syndrome (CRS; PMID: 29972754). Our group and others have published seminal observations on the biology of CRS through cytokine profiling, measuring a small number of analytes (PMID: 27076371, 33434058). Multiple biomarkers including interferon gamma (IFNG), IL-6, and IL-10 have been associated with the development of severe CRS in previous studies (PMID: 33434058). To date, the only biomarker predictive of the development of CRS prior to infusion has been disease burden. To obtain a more robust understanding of CRS biology, we performed comprehensive secretome profiling to measure more than 1400 serum analytes on serial serum samples collected from patients treated with the 41BB-containing CTL019 on two clinical trials. Methods: Serum from patients enrolled on two clinical trials of the CART19 product CTL019 (NCT01626495 & NCT02906371) were obtained serially from pre-infusion to one month post infusion. Patients were categorised as having "minimal" (no CRS, Grade 1, or Grade 2) or "severe" (Grade 3 or 4) CRS. The serum secretome was profiled using the Olink Explore 1536 Analysis platform (Olink, Upsala, Sweden). 1484 proteins were measured from serum via proximity extension assay (PEA) high-multiplex immunoassay. Differential expression analysis, correlation analyses and receiver operating characteristic (ROC) calculations were performed using R (version 4.0.4) in RStudio. Significance was based on a fold change of greater than 2 or less than -2 and a false discovery rate of less than 0.05 calculated using a Benjamini-Hochberg correction. Results: 26 patients (10 NCT01626495 & 16 NCT02906371) were included comprising 128 unique datapoints from baseline to 35 days post-infusion. Thirteen patients had minimal and 13 had severe CRS. Differentially expressed proteins between minimal and severe CRS at the peak timepoint are shown in (A; green represents IFNG responsive proteins). Not surprisingly, proteins involved in IL-6 and IFNG signalling were increased, including biomarkers of hemophagocytic lymphohistiocytosis (HLH) such as VSIG4, CXCL9, CXCL10, CD163. The IL-18 signalling axis was dysregulated at peak CRS in severe patients with markedly elevated IL18 and IL18BP, despite prior reports suggesting IL-18 up-regulation is unique to the late CRS seen with CART22 (PMID: 32925169). Soluble markers of checkpoint inhibition, including soluble PDL1 (CD274) and LAG3 were also highly elevated. Finally, biomarkers of endothelial damage, such as PLAT, TMSB10 and CALCA were significantly elevated in patients with severe CRS. Pathway analysis revealed significant dysregulation in targetable cytokine, chemokine, and signalling pathways (B). A volcano plot of differentially expressed proteins at pre-infusion (C) identified a single protein, MILR1, as a candidate biomarker that was highly differentially expressed in patients who would subsequently develop severe CRS. MILR1 expression decreased over time (D). An ROC of MILR1 as a predictor for development of severe CRS (E) demonstrated pre-infusion elevated MILR1 could accurately predict development of severe CRS (sensitivity 88%, specificity 97%, AUC=0.977). We identified correlates of MILR1 at pre-infusion and found that MILR1 correlated most highly with soluble FLT3 (R=0.86, p Conclusions: With comprehensive secretome profiling we made multiple novel insights into the biology of CRS after CART19 and identified several potentially targetable proteins and pathways that could mitigate severe CRS. Similar secretome profiling in patients who developed neurotoxicity will also be shown. We identified two novel pre-infusion biomarkers that demonstrate significant capacity to predict the development of severe CRS following CART19 infusion. The inverse relationship apparent between FLT3 and FLT3LG that persists over time is an important finding that implies a potential biological role for FLT3/FLT3 ligand in the development of severe CRS. Mechanistic studies exploring the role of MILR1 and FLT3 in the initiation of CRS are ongoing. Figure 1 Figure 1. Disclosures Lambert: Novartis, shionogi, argenx, Rigel, octapharma: Consultancy; Rigel, Novartis, Sysmex, octapharma: Research Funding. Bassiri: Kriya Therapeutics: Consultancy, Current holder of individual stocks in a privately-held company; Guidepoint Global: Consultancy. Levine: Vycellix: Membership on an entity's Board of Directors or advisory committees; Tmunity Therapeutics: Other: Co-Founder and equity holder; Ori Biotech: Membership on an entity's Board of Directors or advisory committees; Immusoft: Membership on an entity's Board of Directors or advisory committees; Immuneel: Membership on an entity's Board of Directors or advisory committees; Avectas: Membership on an entity's Board of Directors or advisory committees; Akron: Membership on an entity's Board of Directors or advisory committees; In8bio: Membership on an entity's Board of Directors or advisory committees. Maude: Wugen: Consultancy; Novartis Pharmaceuticals Corporation: Consultancy, Research Funding. June: Tmunity, DeCART, BluesphereBio, Carisma, Cellares, Celldex, Cabaletta, Poseida, Verismo, Ziopharm: Current equity holder in publicly-traded company; Novartis: Patents & Royalties; AC Immune, DeCART, BluesphereBio, Carisma, Cellares, Celldex, Cabaletta, Poseida, Verismo, Ziopharm: Consultancy. Barrett: Tmunity Therapeutics: Current Employment. Grupp: Novartis, Kite, Vertex, and Servier: Research Funding; Novartis, Roche, GSK, Humanigen, CBMG, Eureka, and Janssen/JnJ: Consultancy; Novartis, Adaptimmune, TCR2, Cellectis, Juno, Vertex, Allogene and Cabaletta: Other: Study steering committees or scientific advisory boards; Jazz Pharmaceuticals: Consultancy, Other: Steering committee, Research Funding. Teachey: Janssen: Consultancy; NeoImmune Tech: Research Funding; Sobi: Consultancy; BEAM Therapeutics: Consultancy, Research Funding.

Mixed phenotype acute leukemia (MPAL) is a rare group of acute leukemias defined by immunophenotypic expression of more than one hematopoietic cell lineage. The World Health Organization (WHO) diagnostic immunophenotypic criteria for MPAL rely on the intensity of lineage-defining antigen expression, predominantly a qualitative assessment, and are often difficult to apply to a phenotypically heterogeneous leukemia. Cases of MPAL defined by isolated myeloperoxidase (isoMPO) expression on otherwise typical acute lymphoblastic leukemia (ALL) are variably diagnosed as MPAL or ALL based on the incompletely defined criteria for assigning MPO expression. We hypothesized that quantitative criteria for antigen intensity could be developed and applied in a uniform manner across flow cytometry instruments, reagents, and analysis software to enable a consistent approach to diagnosing MPAL and better defining isoMPO. We previously reported on a multicenter cohort identified by respective institutions as MPAL with subsequent central review according to 2008 WHO criteria (Oberley et al 2020). Of these, 100 had suitable dot plots for reevaluation (89: B/Myeloid, 10: T/Myeloid, 1: B/T). Antigen expression was concurrently reviewed by two hematopathologists to reach consensus (BLW, AEK). The cohort was divided a priori into randomly selected training and testing cases (n=30/n=70). Classification criteria were generated in the training cohort for WHO lineage-defining antigen expression (myeloid: cMPO, CD64, CD14; B: CD19, T: cCD3) from 10 cases and then refined through review of an additional 20 cases. Positive antigen expression was classified as maximal intensity approaching that of the mature normal counterpart population (NCP) (cMPO: neutrophils; CD64, CD14 and CD11c: monocytes; CD19: B cells; cCD3: T cells) either by 1) range of expression recapitulating maturation of the NCP, irrespective of the percentage on the leukemic population (Figure 1A); or 2) uniform expression above background on a discrete (sub)population (Figure 1B). To account for technical variation within and among laboratories, maximal antigen intensity on the leukemic population was measured in 0.5 log increments and normalized to the maximal intensity of the NCP. An intensity of ≥50% of the NCP above background was defined as sufficient for MPAL lineage assignment and Using this approach, 41/98 (42%) cases previously classified as MPAL remained classified as MPAL: 31 as B/Myeloid (25 by maturational MPO expression, 6 by maturational CD64 and/or CD14 expression); 9 as T/Myeloid (8 by maturational MPO expression, 1 with maturational CD64, CD14 and CD11c expression); and 1 as B/T. No cases in the cohort showed uniform expression ≥50% of the NCP. The remaining 57 showed isolated low-level MPO expression (maximal intensity Novel semiquantitative immunophenotypic criteria applied to a large cohort of acute leukemias originally diagnosed as MPAL was able to objectively identify a large subset as having dim, uniform MPO expression (isoMPO). Our approach emphasizes antigen expression recapitulating maturational expression similar to their non-leukemic cellular counterparts, normalizes absolute intensities to internal positive and negative control populations to minimize technical variability between observers and assays, and as per the 2017 WHO, does not require a specific percent threshold of positivity. While requiring validation, this is a critical first step toward establishing a reproducible delineation of these complex cases to practically implement the WHO classification to support treatment decisions and ongoing investigation into MPAL genomics and outcomes (available for this cohort by ASH). Figure 1 Figure 1. Disclosures Oberley: Caris LIfe Science: Current Employment. Orgel: Jazz Pharmaceuticals: Consultancy.

Introduction: Income, education, and health insurance coverage have been shown to influence access to appropriate oncology care, impacting detection and treatment. We sought to evaluate the role of area-based measures of socioeconomic status in contributing to outcome disparities on the two most recent Children's Oncology Group (COG) Phase 3 clinical trials for acute myeloid leukemia (AML), AAML0531 and AAML1031. We hypothesized that pediatric AML patients from low income and low education zip codes have inferior five-year overall survival (OS) and event-free survival (EFS) relative to patients from middle income and more educated zip codes. Methods: Patients enrolled on AAML0531 and AAML1031 were included. Patients with Down syndrome (n=5), FLT3/ITD high allelic ratio (n=264), in AAML1031 Arm D (n=332), and patients whose zip code was not able to be mapped to a US Census area were excluded (n=60). Patients were observed from enrollment on study through last available follow up. Zip code level median annual household income was the primary exposure and was categorized as follows: Poverty: $56,516. Secondary exposures of interest included zip code level educational attainment and insurance type (Medicaid Only vs other insurance) at AML diagnosis. Standard descriptive statistics were used to compare patient characteristics by levels of exposure; the Kaplan Meier method was used to estimate OS (defined as time from study entry to death) and EFS (time from study entry until failure to achieve CR during induction, relapse, or death). Cox proportional hazards models were used to estimate hazard ratios (HR) for OS and EFS. Measures of association were adjusted for known risk factors for mortality including cytogenetic/mutation risk group, gemtuzumab (GO) receipt, race, and age. Logistic regression analyses were used to estimate odds ratios (OR) for early mortality (defined as death during induction). Results: Of 2387 patients enrolled on AAML0531 and AAML1031, 1726 met inclusion criteria for the overall analysis. Due to missing covariate data, 1467 patients were included in the final model. Race/ethnicity differed significantly by area-based income, area-based education, and insurance type with a higher proportion of Black and Hispanic patients living in poverty, low income, and low education areas, and having Medicaid only insurance. Lower area-based income was associated with lower OS (43% in poverty vs. 61% in low income vs. 68% in middle/high income; p = 0.004) and EFS (34% in poverty vs. 46% in low income vs. 54% in middle/high income; p = 0.005), shown in Figure 1. Lower area-based educational attainment was also associated with lower OS (58% in Quartile 4 (lower education) vs. 70% in Quartile 1 (higher education); p = 0.005 across quartiles) and EFS (44% in Q4 vs. 54% in Q1; p = 0.03 across quartiles). Patients with Medicaid Only insurance had lower OS (59 ± 5% vs. 66 ± 3%: p = 0.01) but similar EFS (48 ± 5% vs. 50 ± 3%: p = 0.33). In a full multivariable model, differences in survival by area-based educational attainment and insurance type resolved suggesting that observed crude associations were explained by confounding by area-based income combined with established risk factors. Patients from middle/high income areas experienced 25% lower risk of mortality compared to patients from low income areas (OS: crude HR 0.74 95% CI 0.62, 0.89; adjusted HR 0.79 95% CI 0.63, 0.99) with similar differences in EFS (crude HR 0.79 95% CI 0.69, 0.92; adjusted HR 0.77 95% CI 0.65, 0.89). There was no meaningful confounding of the income-survival association detected as evidenced by unchanged magnitudes of association following adjustment for area-based education, insurance, and established risk factors. Area-based low income was associated with both higher risk of early death (crude OR: 2.43 95% CI 1.04, 5.69) and treatment-related mortality on therapy (11.1 ± 10.5% vs. 3.7 ± 2.5%, p = 0.03) compared to area-based middle/high income. Conclusions: Lower area-based income and education were associated with significantly inferior EFS and OS among patients with AML on the last two Phase 3 COG trials. Moreover, zip-code based low SES is an independent risk factor for mortality in pediatric AML. Additional studies to understand mechanisms of observed socioeconomic disparities in treatment outcomes will inform interventions that may mitigate these inequities. Disclosures Fisher: Pfizer: Research Funding; Astellas: Other: Data Safety Monitoring Board Chair for an antifungal study; Merck: Research Funding.

Introduction: Prophylactic T cell depletion with antithymocyte globulin (ATG) remains a standard of care for GVHD prophylaxis during allotransplant (ASCT). Although the optimal ATG dosing strategy is still unknown, recent studies have reported that recipient absolute lymphocyte counts (ALC) at the time of ATG administration may predict survivals in ASCT with unrelated donors, suggesting that the dose (especially at the cut off of Materials & Methods: All adults transplanted in our department between 01/2009 and 03/2019 with a Purine analogue/Busulfan/ATG based conditioning regimen and PBSC as source of graft from a matched or 9/10 mismatched donor were eligible. Reduced-intensity conditioning (RIC) regimen consisted of fludarabine 30mg/m²/day (d) from d-6 to d-2, busulfan 3,4 mg/kg/d from d-4 to d-3 and ATG (Thymoglobuline, Sanofi, Lyon, France) 2,5 mg/Kg/d, d-2 and d-1 (FB2A2) or the same but with clofarabine 30mg/m²/d in replacement of fludarabine with 1 or 2 d of ATG (CloB2A2/CloB2A1). Reduced-toxicity myeloablative conditioning regimens (RT-MAC) consisted of the same as FB2A2 but with 3 or 4 d of busulfan instead of 2 (FB3A2/FB4A2). All grafts were administered freshly on the day of the collection while patients (pts) had already received ATG. GVHD prophylaxis was ciclosporine alone for pts with a sibling donor while pts grafted with a matched (MUD) or a 9/10 mismatch (mmUD) unrelated donor received ciclosporine+MMF. We exhaustively looked at pts for whom a blood differential was available at the time of ATG administration in order to study the impact on OS, DFS and GRFS (no grade 3-4 acute GVHD, no moderate/severe chronic GVHD and no relapse) of a profound lymphopenia vs not. Results: Of 395 eligible pts, 116 (median follow-up for alive pts: 49 months) were documented with a differential at time of ATG administration, confirming that this analysis is not a routine practice in our department and probably in many centers. RIC was administered in 80 (69%) of the pts including 39 FB2A2, 12 CLOB2A2 and 29 CLOB2A1. RT-MAC was administered in 36 (31%) pts, including 27 FB3A2 and 9 FB4A2. Seventy-six pts had a myeloid disease while 40 had a lymphoid disease. Donor types were siblings (n=33), MUD (n=70) or 9/10 mmUD (n=13). For the entire cohort, 4y OS, DFS and GRFS were 56.2% (47-66), 40.9% (32-51) and 34.5% (26-45), respectively. No difference in survivals was observed between lymphoid vs myeloid pts, pts transplanted with sibling vs other donors, pts receiving a RIC vs a MAC or a CloB2 vs a FB2 RIC regimen. Median ALC at time of start of conditioning was .915x109/L (range: .010-15.780). No difference in terms of survivals was observed when considering pts under this threshold vs others. ROC curve analysis failed to identify a cut-off allowing to predict better survivals according to ALC at the time of ATG administration (ALC/ATG). Median ALC/ATG was .070x109/L (range: 0-2.300). No difference in terms of survivals was observed when considering pts under this threshold vs others. The same was true when considering .100x109/L as ALC/ATG cut-off. Regarding MAC, the median ALC/ATG was .100x109/L with no difference in survivals between pts under or above this value. The same was true for RIC with ALC/ATG cut-offs < median (.055x109/L) or .100 x109/L as only 4 of them received 1d of ATG vs 23 2d. The dose of CD34+ and CD3+ T cells infused had no impact also on survivals. Conclusion: This study demonstrates that profound lymphopenia at the time of ATG administration as part of a RIC as well as a RT-MAC Purine analogue/Busulfan/ATG based conditioning regimen has no impact on outcomes. Moreover, a reduced dose of ATG in RIC pts with profound lymphopenia at the time of ATG administration did not translate into better survivals. Other unknown factors rather than recipient lymphopenia remain to be discovered to optimize individualized dosing of ATG. Disclosures Peterlin: AbbVie Inc: Consultancy; Astellas: Consultancy; Jazz Pharma: Consultancy; Daiichi-Sankyo: Consultancy. Moreau:Celgene: Consultancy, Honoraria; Janssen: Consultancy, Honoraria; Takeda: Consultancy, Honoraria; AbbVie: Consultancy, Honoraria; Amgen: Consultancy, Honoraria. Le Gouill:Celgene: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Travel support; Roche-Genentech: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Travel support. Chevallier:Daiichi Sankyo: Honoraria; Incyte: Consultancy, Honoraria; Jazz Pharmaceuticals: Honoraria.