Abstract: 138 Background: Immune checkpoint inhibitors (ICIs) are anti-cancer agents now in routine clinical practice, and may cause immune-related adverse events (irAEs). The proportion of inpatient admissions for irAEs, spectrum of toxicities, management and outcomes are not well described. Methods: Patients with solid tumors admitted to the inpatient oncology service at a tertiary academic center over 3 months, were identified. Patient demographics, treatment details, irAE event and management data, were collected for those who received ICIs, in an IRB-approved database. The diagnosis of irAEs was confirmed by the treating physician and oncologist. Associations between clinical details and irAEs were evaluated using Fisher's exact test. Results: We identified 240 inpatient oncology patients: 53 (22.1%) had received ICIs, and 25% (13/53) were admitted for irAEs. The majority of irAEs requiring admission were high grade (CTCAE grade 1-2: 6/13, 46%; grade 3+: 7/13, 54%), and included: colitis (31%), pneumonitis (23%), skin rash (8%), fever (8%), pancreatitis (8%), fatigue (8%), and renal transplant rejection (8%). Treatment for irAEs included: ICI withhold (2/13, 15%), oral/IV corticosteroids (10/13, 76%), and infliximab (1/13, 8%); with 85% of patients requiring subspecialty consultations. Those with irAEs had a shorter median length of stay vs. other inpatients (5 vs. 6 days). Most irAEs resolved/improved (11/13, 85%), while 15% worsened (1/13) or resulted in patient death (1/13). There was a numerically higher risk of any/grade3+ irAEs for those: treated with combination vs monotherapy (33% vs 23%; 100% vs 40%), age 〉 65 vs 〈 65 (33% vs 15%; 56% vs 50%), and former/current vs. never smokers (31% vs 19%; 63% vs 40%), however differences were not statistically significant. Conclusions: Patients with irAEs constitute a notable proportion of inpatient oncology admissions, with a higher incidence than in reported clinical trials. Initial data suggest that patients treated with combination ICIs, aged 〉 65, and former/current smokers may be more likely to be admitted for irAEs. The majority of irAE admissions require subspecialty consultations, signifying a growing need for multidisciplinary irAE management.

Abstract: Introduction: Previous studies have demonstrated that allogeneic haploidentical (haplo) peripheral blood stem cell transplantation (PBSCT) with post-transplant cyclophosphamide (PTCy) yields improved progression free survival (PFS) when compared to haplo bone marrow transplant (BMT) with PTCy, attributable to lower relapse without an increase in non-relapse mortality (NRM) (Bashey, et al. JCO. 2017). However, haplo PBSCT results in higher rates of graft-versus-host disease (GVHD) which may negate these benefits in older patients who are more susceptible to transplant related toxicity. Thus, evaluation of the outcomes of haplo PBSCT with PTCy in older patients is warranted. Methods: We retrospectively evaluated 121 adult patients with hematologic malignancies treated at the Moffitt Cancer Center with allogeneic T-cell replete PBSCT from a haplo donor followed by PTCy-based GVHD prophylaxis. Data were extracted from the Moffitt BMT Research & Analysis Information Network (BRAIN) database. Myeloablative (n=70, 58%) and reduced intensity (n=51, 42%) conditioning regimens were included. Transplant related outcomes were compared between two age groups: & lt;60 years (n=66) versus & gt;60 years (n=55). Associations with transplant related survival outcomes were assessed using univariate and multivariate Cox proportional hazard survival models. Fine and Gray regression models were used to assess associations of transplant related endpoints with competing risks. Kaplan-Meier curves and cumulative incidence function curves were also plotted. Results: The median age at the time of transplant was 42 years (range: 20-59) for the younger group and 66 years (range: 61-75) for the older group. The median follow-up was 17 months (range: 2-53) for the entire cohort. Younger patients were more likely to receive myeloablative conditioning (83% versus 27%, p & lt;0.001). Baseline characteristics were otherwise similar. Neutrophil engraftment ( & gt;500/uL) by day 30 did not differ significantly between the younger and older group (98% versus 93%, p=0.26). However, the median time to neutrophil engraftment was faster in the younger group versus the older group (16 versus 21 days, p & lt;0.001). Platelet engraftment ( & gt;20,000/uL) by day 90 was achieved in 92% in the younger group versus 76% in the older group (p=0.03). The time to platelet engraftment was faster in the younger group: 28 days versus 36 days (p=0.006). At day 100, the cumulative incidence (CuI) of grade II-IV acute GVHD in younger patients was 42% (95% CI 29-61%) and for older patients was 35% (95% CI 22-55%, p=0.82). The CuI for grade III-IV acute GVHD for the younger and the older groups were 8% (95% CI 4-25%) and 15% (95% CI 7-38%, p=0.23), respectively. At 2 years, the CuI of chronic GVHD was 67% (95% CI 55-82%) for younger patients versus 56% (95% CI 38-82%) for older patients (p=0.20). NRM for the younger group and the older group, respectively, was 6% (95% CI 2-16%) versus 19% (95% CI 11-34%) at 100 days and 14% (95% CI 6-30%) versus 22% (95% CI 13-37%) at 2 years (p=0.17). The CuI of relapse at 2 years was not significantly different between the two age groups, with the younger recipients having a CuI of 42% (95% CI 20-60%) and the older group 31% (95% CI 17-56%, p=0.70). The 2-year DFS was similar between the younger and older group, respectively: 51% (95% CI 36-66%) and 53% (95% CI 37-70%, p=0.72). Similarly 2-year overall survival (OS) for the younger group was 59% (95% CI 44-74%), while the older group was 66% (95% CI 52-80%, p=0.92). In multivariate analysis, NRM was superior in the younger group (HR=0.31, 95% 0.12-0.82, p=0.02). Otherwise, age was not associated with engraftment, risk of acute or chronic GVHD, relapse, DFS, or OS. Conclusion: Our results demonstrate that outcomes following allogeneic haplo PBSCT with PTCy in patients & gt;60 years approximate outcomes in patients & lt;60 years. While NRM was inferior in the older patient group, this difference did not result in significant differences in long term OS or DFS. Instead, other variables such as the hematopoietic comorbidity index and the disease risk index were better indicators of survival outcomes. Additionally, these survival outcomes with haplo PBSCT with PTCy appear to be similar to prior published data with haplo BMT with PTCy in older patients (Kasamon, et al. JCO. 2015). Based on this study, haplo PBSCT with PTCy is an appropriate transplant platform for elderly patients. Disclosures Khimani: Bristol Myers Squibb-Moffitt-Alliance: Research Funding. Nishihori:Novartis: Other: Research support to institution; Karyopharm: Other: Research support to institution. Pidala:Syndax: Consultancy, Membership on an entity's Board of Directors or advisory committees; CTI Biopharma: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Amgen: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Novartis: Research Funding; Takeda: Research Funding; Janssen: Research Funding; Johnson and Johnson: Research Funding; Pharmacyclics: Research Funding; Abbvie: Research Funding; BMS: Research Funding. Bejanyan:Kiadis Pharma: Membership on an entity's Board of Directors or advisory committees.

Abstract: Background: Acute myeloid leukemia (AML) is the most common form of acute leukemia in adults, with an incidence that increases with age, and a generally poor prognosis. The prognosis remains especially grim for those who are older, have secondary AML, or relapsed or refractory (R/R) disease, in which 5-year OS is 5-10%. Therefore, novel therapeutic approaches are needed. CD123(IL3Rα) is a cell surface target that is expressed on normal, committed hematopoietic progenitor cells, and a variety of hematological neoplasms, including AML, myelodysplastic syndrome (MDS), and blastic plasmacytoid dendritic cell neoplasm (BPDCN). UCART123 is genetically modified, allogeneic ("off-the-shelf"), anti-CD123 CAR T cell product candidate in which the TCR alpha constant gene is disrupted to reduce the risk of GvHD, and the CD52 gene is disrupted to permit the use of alemtuzumab for selective and prolonged host lymphodepletion. Also, the CAR is co-expressed with a suicide mechanism (RQR8), which can be activated by using rituximab. In vitro data have demonstrated that UCART123 efficiently targets primary AML cells, with minimal effect on normal progenitors. Also, in PDX mouse models of AML, UCART123 cells can eliminate tumor cells in vivo, prevent relapse, and improve survival; in a competitive BM/AML PDX model, UCART123 cells demonstrated preferential targeting of AML blasts (Guzman; Blood 2016). Methods: AMELI-01 is a phase 1, multi-center clinical trial of UCART123 that employs an mTPI design to evaluate the safety, tolerability and preliminary anti-leukemia activity of UCART123 in patients (pts) with R/R AML. Additional objectives include determination of the MTD; characterization of the expansion, trafficking and persistence of UCART123; assessment of cytokine, chemokine and CRP levels after UCART123 infusion; and assessment of immune cell depletion, reconstitution and immune response. Dose escalation will include up to 28 pts. The dose expansion portion follows a Simon 2-stage design and will enroll up to an additional 37 pts. Eligible pts must be ≤ 65 years of age with R/R AML, adequate organ function, a confirmed donor for potential back-up stem cell transplantation, and no ongoing & gt; G1 toxicity from prior treatment. Pts with APL, prior gene or cellular therapy, & gt; 1 allogeneic SCT, or those with a clinically relevant CNS disorder (including CNS leukemia) are not eligible. Pts receive a lymphodepletion (LD) regimen of either fludarabine and cyclophosphamide (FC) or fludarabine, cyclophosphamide plus alemtuzumab (FCA) starting on Day -5, followed by an infusion of UCART123 at one of 5 dose levels on Day 0. Pts are evaluated for the presence of dose-limiting toxicities (DLT) during a 28-day observation period, which extends to 42 days in the setting of marrow aplasia and/or persistent clinically significant cytopenias without residual AML. DL1 has cleared safety without DLT, and enrollment at the next dose levels are proceeding. ClinicalTrials.gov Identifier: NCT03190278 Monica L. Guzman, et al; Allogeneic TCRα/β Deficient CAR T-Cells Targeting CD123 Prolong Overall Survival of AML Patient-Derived Xenografts. Blood 2016; 128 (22): 765. doi: https://doi.org/10.1182/blood.V128.22.765.765 Disclosures Roboz: Orsenix: Consultancy; Otsuka: Consultancy; Takeda: Consultancy; Trovagene: Consultancy; Cellectis: Research Funding; Jasper Therapeutics: Consultancy; Epizyme: Consultancy; Helsinn: Consultancy; MEI Pharma: Consultancy; Novartis: Consultancy; Janssen: Consultancy; Pfizer: Consultancy; Agios: Consultancy; Celgene: Consultancy; Astex: Consultancy; Amphivena: Consultancy; Abbvie: Consultancy; Array BioPharma: Consultancy; Bayer: Consultancy; Celltrion: Consultancy; Eisai: Consultancy; Jazz: Consultancy; Roche/Genentech: Consultancy; Sandoz: Consultancy; Actinium: Consultancy; Argenx: Consultancy; Astellas: Consultancy; Daiichi Sankyo: Consultancy; AstraZeneca: Consultancy. DeAngelo:Blueprint Medicines Corporation: Consultancy, Research Funding; Forty-Seven: Consultancy; Amgen: Consultancy; Abbvie: Research Funding; Glycomimetics: Research Funding; Shire: Consultancy; Takeda: Consultancy; Pfizer: Consultancy; Novartis: Consultancy, Research Funding; Jazz: Consultancy; Autolos: Consultancy; Incyte Corporation: Consultancy; Agios: Consultancy. Sallman:Agios, Bristol Myers Squibb, Celyad Oncology, Incyte, Intellia Therapeutics, Kite Pharma, Novartis, Syndax: Consultancy; Celgene, Jazz Pharma: Research Funding. Guzman:SeqRx: Honoraria; Cellectis: Research Funding. Kantarjian:Delta Fly: Honoraria; Novartis: Honoraria, Research Funding; Adaptive biotechnologies: Honoraria; Ascentage: Research Funding; Amgen: Honoraria, Research Funding; BMS: Research Funding; Daiichi-Sankyo: Honoraria, Research Funding; Pfizer: Honoraria, Research Funding; Abbvie: Honoraria, Research Funding; Jazz: Research Funding; Actinium: Honoraria, Membership on an entity's Board of Directors or advisory committees; Sanofi: Research Funding; BioAscend: Honoraria; Janssen: Honoraria; Oxford Biomedical: Honoraria; Immunogen: Research Funding; Aptitute Health: Honoraria. Konopleva:Genentech: Consultancy, Research Funding; Rafael Pharmaceutical: Research Funding; Ascentage: Research Funding; AstraZeneca: Research Funding; Agios: Research Funding; Amgen: Consultancy; Sanofi: Research Funding; F. Hoffmann La-Roche: Consultancy, Research Funding; Stemline Therapeutics: Consultancy, Research Funding; Forty-Seven: Consultancy, Research Funding; Eli Lilly: Research Funding; Kisoji: Consultancy; Ablynx: Research Funding; AbbVie: Consultancy, Research Funding; Reata Pharmaceutical Inc.;: Patents & Royalties: patents and royalties with patent US 7,795,305 B2 on CDDO-compounds and combination therapies, licensed to Reata Pharmaceutical; Calithera: Research Funding; Cellectis: Research Funding. Bejanyan:Kiadis Pharma: Membership on an entity's Board of Directors or advisory committees. Esteva:Cellectis: Current Employment. Garton:Cellectis: Current Employment. Backhouse:Cellectis: Current Employment. Galetto:Cellectis: Current Employment. Brownstein:Cellectis: Current Employment. Pemmaraju:Blueprint Medicines: Honoraria; Roche Diagnostics: Honoraria; MustangBio: Honoraria; AbbVie: Honoraria, Research Funding; SagerStrong Foundation: Other: Grant Support; Plexxikon: Research Funding; Novartis: Honoraria, Research Funding; LFB Biotechnologies: Honoraria; Stemline Therapeutics: Honoraria, Research Funding; Celgene: Honoraria; Affymetrix: Other: Grant Support, Research Funding; Incyte Corporation: Honoraria; DAVA Oncology: Honoraria; Samus Therapeutics: Research Funding; Cellectis: Research Funding; Daiichi Sankyo: Research Funding; Pacylex Pharmaceuticals: Consultancy.

Abstract: Introduction: The gut microbiome is a potentially modifiable factor in treatment related outcomes in allogeneic hematopoietic cell transplant (HCT). Prior studies have linked pre- or mid-treatment gut microbiome diversity with risk for treatment related morbidity and mortality. However, these studies have been limited by the inclusion of one or only a few institutions and the lack of longitudinal sampling with high quality metadata. These limitations complicate the interpretation of microbiome alterations over the course of HCT. Methods: To overcome these, we devised and implemented a large-scale biospecimen collection protocol in conjunction with BMT CTN 1703, a randomized, multicenter, Phase III trial of tacrolimus/methotrexate vs. post-transplant cyclophosphamide/tacrolimus/mycophenolate mofetil in reduced intensity conditioning (RIC) allogeneic HCT (NCT03959241). Patients enrolled on 1703 were optionally co-enrolled in the companion immune and microbiome profiling study, 1801 (MI-IMMUNE). This involved blood, urine and stool sampling before conditioning (PCON), then weekly starting on day 0 through day 77 (through day 84 for blood), and on days 98, 180, 270, 365 and 730. For all enrolled participants where the donor consented, residual donor cells were saved from the empty hematopoietic stem cell product bag for later analysis. Additionally, the protocol included one-time blood, urine and stool sample collection from consented matched related donors (MRDs) prior to stem cell collection. Starting with protocol version 4.0 on February 1, 2021, participation in 1801 stool collection was required for the first six out of eighteen sample collection timepoints. Participation in later stool and urine timepoint collections remained optional. Here we review the feasibility of creating a multi-institutional biobank. Additionally, we assess the success of our strategies by calculating sample collection compliance and standard deviation in compliance across centers for each timepoint. Results: On June 18, 2021, BMT CTN 1703/1801 closed to accrual with 431 patients enrolled on 1703; 323 patients from 36 centers were co-enrolled on 1801. 304 (94%) provided study samples, making this the largest prospective microbiome and immune profiling study in HCT patients to date. As of July 6, 2021, 3,683 blood, 2,668 urine, and 2,098 stool samples had been collected. Across the first 6 timepoints for all participating centers, blood, urine and stool sample collection averaged 93%, 82%, and 74% compliance, respectively. Of the 99 (30%) patients enrolled on 1801 with a MRD, 34 (34%) donors consented to sample collection. Sample collection compliance was lower for MRDs than for patients on the study with 76%, 74%, and 62% of expected blood, urine and stool samples collected, respectively, from this group. For stool collection exclusively, a median of 5 samples were collected per patient across the first 6 timepoints (median of 6 across all timepoints) with 93 (31%) of patients completing a full sample set through Day 28. 139 (46%) patients provided at least one sample after day 28; these represented 37% of the total samples collected to date. The PCON sample, which provides an important measure of pre-treatment gut microbiome diversity, had the third highest compliance with 74% of patients providing a sample. Surprisingly, Day 28 had the lowest compliance (66%) and highest standard deviation (37%) possibly because this timepoint often falls around the time of hospital discharge. Between PCON and day 28, the standard deviation between sites in the average collection compliance (24%) and number of samples collected per patient (1.1) was small indicating the successful adoption of stool collection across institutions. Table 1 summarizes sample collection statistics. Conclusion: Overall this study has resulted in a large, novel biobank of blood, urine and stool samples from patients undergoing RIC allogeneic HCT at 36 centers across the US. This will serve as a valuable resource for investigating the role of the gut microbiome in long term health outcomes following HCT. Although the results of 1801 are forthcoming given ongoing sample collection, the size and composition of the biobank to date clearly demonstrate the feasibility of implementing multi-institutional stool collection. This study represents a critical step towards the large-scale adoption of microbiome sampling as a diagnostic tool. Figure 1 Figure 1. Disclosures Chhabra: GSK: Honoraria. Clark: Kadmon: Consultancy. Horowitz: Mesoblast: Research Funding; Shire: Research Funding; Vertex: Research Funding; Stemcyte: Research Funding; Vor Biopharma: Research Funding; Janssen: Research Funding; Miltenyi Biotech: Research Funding; Kiadis: Research Funding; Sobi: Research Funding; Kite/Gilead: Research Funding; Pfizer, Inc: Research Funding; Jazz Pharmaceuticals: Research Funding; Magenta: Consultancy, Research Funding; Medac: Research Funding; Novartis: Research Funding; GlaxoSmithKline: Research Funding; Daiicho Sankyo: Research Funding; Xenikos: Research Funding; Omeros: Research Funding; Orca Biosystems: Research Funding; Pharmacyclics: Research Funding; Regeneron: Research Funding; Tscan: Research Funding; Takeda: Research Funding; CSL Behring: Research Funding; Genentech: Research Funding; Gamida Cell: Research Funding; Chimerix: Research Funding; Bristol-Myers Squibb: Research Funding; bluebird bio: Research Funding; Astellas: Research Funding; Amgen: Research Funding; Allovir: Consultancy; Actinium: Research Funding; Sanofi: Research Funding; Seattle Genetics: Research Funding. Jenq: Microbiome DX: Consultancy; Merck: Consultancy; Prolacta: Consultancy, Membership on an entity's Board of Directors or advisory committees; Kaleido: Consultancy, Current holder of stock options in a privately-held company, Membership on an entity's Board of Directors or advisory committees, Research Funding; Seres: Consultancy, Current holder of stock options in a privately-held company, Membership on an entity's Board of Directors or advisory committees, Patents & Royalties, Research Funding; LisCure: Consultancy, Membership on an entity's Board of Directors or advisory committees; MaaT Pharma: Consultancy, Membership on an entity's Board of Directors or advisory committees; Karius: Consultancy. Levine: Equillium Bio: Membership on an entity's Board of Directors or advisory committees; Incyte: Consultancy, Research Funding; Kamada: Research Funding; Biogen: Research Funding; Omeros: Membership on an entity's Board of Directors or advisory committees; Symbio: Membership on an entity's Board of Directors or advisory committees; Jazz Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees; Talaris Therapeutics: Membership on an entity's Board of Directors or advisory committees; Viracor: Patents & Royalties: GVHD biomarker patent with royalties from Viracor; Mesoblast: Consultancy, Research Funding; X4 Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees. Murthy: CRISPR Therapeutics: Research Funding. Riches: ATARA Biotherapeutics: Other: Payment; BioIntelect: Membership on an entity's Board of Directors or advisory committees; Jazz Pharmaceuticals: Other: Payment. Sung: Merck: Research Funding; Novartis: Research Funding; Enterome: Research Funding; Seres: Research Funding; AVROBIO: Consultancy; Abbott Nutrition: Honoraria; Clasado: Other: Research Product; DSM: Other: Research Product. Al Malki: Neximmune: Consultancy; Jazz Pharmaceuticals, Inc.: Consultancy; Rigel Pharma: Consultancy; Hansa Biopharma: Consultancy; CareDx: Consultancy. Rezvani: Kaleido: Other: One-time scientific advisory board; Nohla Therapeutics: Other: One-time scientific advisory board; Pharmacyclics-Abbvie: Research Funding; US Department of Justice: Consultancy. Bolaños-Meade: Incyte Corp: Consultancy. Holtan: Incyte: Consultancy, Research Funding; Generon: Consultancy. Saber: Govt. COI: Other. Hamadani: Sanofi, Genzyme, AstraZeneca, BeiGene: Speakers Bureau; Janssen, Incyte, ADC Therapeutics, Omeros, Morphosys, Kite: Consultancy; Takeda, Spectrum Pharmaceuticals and Astellas Pharma: Research Funding. Kean: Bluebird Bio: Research Funding; Bristol Myers Squibb: Patents & Royalties: From clinical trial data, Research Funding; Vertex: Consultancy; Novartis: Consultancy; Gilead: Research Funding; Regeneron: Research Funding; EMD Serono: Consultancy. Perales: Cidara: Honoraria; Servier: Honoraria; Incyte: Honoraria, Other; Equilium: Honoraria; Takeda: Honoraria; Novartis: Honoraria, Other; Nektar Therapeutics: Honoraria, Other; NexImmune: Honoraria; MorphoSys: Honoraria; Omeros: Honoraria; Karyopharm: Honoraria; Sellas Life Sciences: Honoraria; Merck: Honoraria; Miltenyi Biotec: Honoraria, Other; Medigene: Honoraria; Bristol-Myers Squibb: Honoraria; Kite/Gilead: Honoraria, Other; Celgene: Honoraria.

Abstract: Introduction: Higher infused total nucleated cell dose (TNC) in allogeneic bone marrow transplant (BMT) with post-transplant cyclophosphamide (PTCy) resulted in improved overall survival (OS) in a recent report (McCurdy, et al. BBMT. 2018). As many centers prefer peripheral blood stem cell grafts (PBSCT) with PTCy, the optimal cell dose with this platform requires elucidation. Methods: We retrospectively evaluated 144 consecutive adult patients with hematologic malignancies treated at the Moffitt Cancer Center between 2012-2018 with allogeneic T-cell replete PBSCT followed by PTCy-based graft-versus-host disease (GVHD) prophylaxis. Data were extracted from the Moffitt BMT Research & Analysis Information Network (BRAIN) database. Both myeloablative (n=87) and reduced intensity (n=57) conditioning regimens were included. For analyses, CD34+ cell dose was stratified into low ( 〈 5 x106/kg), intermediate (5-10 x 106/kg) and high ( 〉 10x106/kg). TNC was stratified as higher or lower than the median. CD3+ T cell dose was evaluated as a continuous, linear variable. Associations with transplant related survival outcomes were assessed using univariate and multivariate Cox proportional hazard survival models. Fine and Gray regression models were used to assess associations of transplant related endpoints with competing risks. Kaplan Meier curves and cumulative incidence function curves were also plotted. RESULTS: The median follow-up for the entire cohort was 24 months. The median age at PBSCT was 58 (range: 21-76) years. Donors were haploidentical (n=72, 50%), matched unrelated (n=39, 27%), matched related (n=18, 13%), and mismatched unrelated (n=15, 10%). Median CD34+ cell dose was 8.12 x 106/kg (range: 2.27 - 22.4 x 106/kg). Stratified CD34+ dose was low for 16 patients (11.1%), intermediate for 93 patients (64.6%), and high for 35 patients (24.3%). Median CD3+ dose was 2.89 x 108 (range: 2.33 x 105 - 6.54 x 108/kg). Median TNC was 9.66 x 108/kg (range: 3.82 x 108 - 1.04 x 109/kg). Overall, successful neutrophil engraftment by day 30 was achieved in 94% (95% CI 91-98%) and platelet engraftment ( 〉 20,000/mL) by day 100 in 87% (95% CI 81-93%) of all patients. The likelihood of neutrophil recovery was similar in the high and low CD34+ cell groups, while it was higher for the intermediate CD34+ cell group (HR=1.52, 95% CI 1.0-2.2; p=0.03). Platelet recovery was similar in the intermediate and high CD34+ cell groups, but significantly lower in low CD34+ cell group (HR=0.38, 95% CI 0.2-0.8; p=0.01). CuI of grade II-IV acute GVHD at day 100 was 39% (95% CI 32-48%) and of grade III-IV acute GVHD was 10% (95% CI 6-17%). CuI of chronic GVHD at 2 years was 50% (95% CI 41-59%). The CD34+ cell dose had no significant impact on either acute or chronic GVHD. Nonrelapse mortality (NRM) at 1 year was 19% (95% CI 13-27%). By CD34+ cell dose, the rates of NRM for low, intermediate, and high dose were 46% (95% CI 25-85%), 15% (95% CI 10-25%), and 15% (95% CI 7-34%), respectively (p=0.002). Relapse rate was 30% at 2 years with no differences across dose levels. Progression free survival (PFS) probability at 2 years was 47% (95% CI 39-57%) for the entire group, 0% (95% CI 0%) for the CD34+ low group, 53% (95% CI 42-65%) for the intermediate group and 51% (95% CI 35-73%) for the high dose group (p=0.0003). Two-year OS for the entire group was 57% (95% CI 49-67%), and was inferior in the low group (0%, 95% CI 0%) with no difference between the intermediate (61%, 95% CI 50-73%) and high (67%, 95% CI 52-87%) CD34+ cell dose groups (p=0.0002). In multivariable modeling, the low CD34+ cell dose group was associated with worse NRM (HR=4.1, 95% CI 1.2-14.5, p=0.03), PFS (HR=2.9, 95% CI 1.3-6.5, p=0.01), and OS (HR=3.2, 95% CI 1.4-7.6, p=0.01) compared with the high group, while there were no differences between the high and intermediate groups. Increasing CD3+ dose was also associated with improved NRM (HR=0.8, 95% CI 0.69-0.97, p=0.02), PFS (HR=0.7, 95% CI 0.5-0.9, p=0.02), and OS (HR=0.7, 95% CI 0.5-0.9, p=0.004). TNC had no impact on survival outcomes. CONCLUSION: In patients receiving allogeneic PBSCT with PTCy, CD34+ cell doses below 5 x 106 cells/kg yielded inferior OS and PFS, attributable to higher NRM. Doses of 5-10 x 106 cells/kg and 〉 10 x 106 cells/kg resulted in comparable outcomes. Higher CD3+ dose/kg was also associated with improved survival outcomes. Overall, this study supports targeting a CD34+ dose of 〉 5 x 106 cells/kg for allogeneic PBSCT with PTCy. Disclosures Nishihori: Novartis: Research Funding; Karyopharm: Research Funding. Bejanyan:Kiadis Pharma: Other: advisory board.