Effect of CD34+ cell dose on neutrophil and platelet engraftment kinetics in haematopoietic stem cell transplantation – A single-centre experience

Objectives: Haematopoietic stem cell transplantation (SCT) is curative for a number of benign and malignant hematological disorders. CD34 expression on haematopoietic progenitor cells is used to assess stem cell content in peripheral blood stem cell and bone marrow grafts. This study evaluated the relationship between numbers of CD34+ cells infused per kg and the timing of neutrophil and platelet engraftment. Materials and Methods: The effect of cell dose was studied in consecutive HSCT patients transplanted between November 2008 and December 2017. Neutrophil engraftment was defined as the first of 2 consecutive days with an absolute neutrophil count >0.5 × 109/L and platelet engraftment as unsupported platelet count >20 × 109/L for 7 days. Results: Of a total of 131 patients, 26 (19.8%) underwent an autologous SCT, while 105 (80.2%) underwent an allogeneic SCT. The median CD34 dose infused in the auto-SCT group was 5.29 × 106 CD34+cells/kg (IQR = 2.95–10.98) and 6.42 × 106 CD34+cells/kg (IQR = 4.20–9.20) in the allo-SCT group (P = 0.773). The median time to neutrophil engraftment in the auto-SCT group was 11 days (range 9.5–12) and in the allo-SCT group was 15 days (range 13–17), P ≤ 0.001. The median time to platelet engraftment in both groups was similar (12 days). When patients were divided into three groups based on CD34 dose (<5, 5–8 and >8), no difference was observed in the time to ANC or platelet engraftment. Similarly, no differences in time to engraftment were noted in each quartile of CD34 dosage in auto- and allo-SCT. Conclusion: Thus, it was concluded that a cell dose of approximately 5 × 106/kg provides reasonably rapid engraftment, with no advantage seen for a higher cell dose of >5.

Impact of HLA Molecular Mismatch on Haploidentical Hematopoietic Stem Cell Transplantation: A Center for International Blood and Marrow Transplant Research Study

ABSTRACT BACKGROUND The number of haploidentical hematopoietic stem cell transplantations (haplo-HSCT) being performed has substantially increased in recent years. Single-center studies have previously used in silico algorithms to quantitively measure HLA disparity and shown an association of the number of HLA molecular mismatches with relapse protection and/or increased risk of acute graft-versus-host disease (GVHD) in haplo-HSCT. However, inconsistent results from small studies have made it difficult to understand the full clinical impact of molecular mismatch in haplo-HSCT. OBJECTIVE In the current study, we investigated the potential of the HLA class I and II mismatched eplet (ME) score measured by HLAMatchmaker, as well as ME load at a specific locus to predict outcomes in a registry-based cohort of haplo-HSCT recipients. STUDY DESIGN We analyzed data from patients (n= 1,287) who underwent their first haplo-HSCT for acute lymphoblastic leukemia, acute myeloid leukemia, or myelodysplastic syndrome between 2013 and 2017, as reported to the Center for International Blood and Marrow Transplant Research database. ME load at each HLA locus and total Class-I and -II were scored using the HLAMatchmaker module incorporated in HLA Fusion software v4.3, which identifies predicted eplets based on the crystalized HLA molecule models and identifies ME by comparing donor and recipient eplets. RESULTS In the cohort studied, ME scores derived from total HLA Class I or Class II loci or individual HLA loci were not associated with overall survival, disease-free survival, non-relapse mortality, relapse, acute or chronic GVHD (P&lt; .01). An unexpected strong association was identified between total class II ME load in the GVH direction and slower neutrophil engraftment (HR 0.82; 95% CI, 0.75 - 0.91; P &lt; .0001) and platelet engraftment (HR 0.80; 95% CI, 0.72 - 0.88; P &lt; .0001). This was likely attributable to ME load at the HLA-DRB1 locus, which was similarly associated with slower neutrophil engraftment (HR 0.82; 95% CI, 0.73 - 0.92; P = .001) and slower platelet engraftment (HR 0.76; 95% CI, 0.70 - 0.84; P &lt; .0001). Additional analyses suggested that this effect is attributable to matched vs. mismatched in the GVH direction and not to ME load, as there was no dose effect identified. CONCLUSION These findings contradict those of prior relatively small studies reporting that ME load, as quantified by HLAMatchmaker, was associated with haplo-HSCT outcomes. As the study failed to demonstrate the predictive value of ME from HLA molecules for major clinical outcomes, other molecular mismatch algorithms in haplo-HSCT settings should be tested. Disclosures Lee: Pfizer: Research Funding; Novartis: Membership on an entity's Board of Directors or advisory committees, Research Funding; National Marrow Donor Program: Membership on an entity's Board of Directors or advisory committees; Incyte: Research Funding; Janssen: Other; Takeda: Research Funding; Syndax: Research Funding; AstraZeneca: Research Funding; Kadmon: Research Funding; Amgen: Research Funding.

A Phase II Study of Ruxolitinib Pre-, during- and Post-Hematopoietic Celltransplantation for Patients with Primary or Secondary Myelofibrosis

Background: Myelofibrosis (MF) is a lethal hematological malignancy associated with somatic mutations in JAK2, CALR or MPL. Ruxolitinib is the first JAK1/2 inhibitor approved for treatment of MF. Ruxolitinib does not prevent disease progression and thus, allogeneic hematopoietic stem cell transplantation (HSCT) remains the recommended therapy for eligible patients treated with curative intent. Ruxolitinib discontinuation, in preparation for HSCT is challenging as patients experience return of symptoms/splenomegaly. Therefore, ruxolitinib is often continued during and after HSCT in an off-label fashion, yet little is known about the safety of this approach. In addition, ruxolitinib is now utilized to treat steroid refractory acute and chronic graft versus host disease (GVHD) irrespective of underlying disease. Methods: This is a phase II, multi-center, investigator-initiated trial investigating ruxolitinib given pre-, during- and post-HSCT for patients with primary or secondary MF (NCT03427866). The study utilizes ruxolitinib during and after HSCT in MF patients for one year after HSCT. The accrual goal is 48 patients with 1-year GVHD free and relapse free survival (GRFS) as the primary endpoint. Secondary endpoints include overall and progression free survival, engraftment and incidence of acute and chronic GVHD, respectively. Patients are treated with reduced intensity conditioning with fludarabine (30mg/m 2/day x 5 days) and melphalan (100mg/m 2 or 140mg/m 2 x 1). HSCT grafts are with 7/8 or 8/8 HLA-matched peripheral blood stem cells with tacrolimus and methotrexate as standard GVHD prophylaxis. Results: This pre-planned interim analysis includes 26 MF patients who underwent HSCT between September 2018 and January 2021. An interim analysis was included in the trial design to ensure safety of this approach midway through accrual. Median age was 66 (range, 46-75) and 65% were male. 88% had 8/8 matched related grafts, and 92% had intermediate-2 or high DIPSS risk at the time of transplant. 14 (54%) patients were previously treated with ruxolitinib. At HSCT, 58% had JAK2, 12% CALR, 12% MPL, and 35% ASXL1 mutations (Figure A). There were no unexpected toxicities related to ruxolitinib therapy. The most common grade 3/4 hematologic adverse events (AE) were anemia (n=4), thrombocytopenia (n=3). There were few observed grade 3/4 non hematologic AEs and included infection (n=2) and hypertriglyceridemia (n=1). Median time to neutrophil engraftment was 15 days (range 11-38) after HSCT. All but one patient achieved successful neutrophil engraftment. Median day 30 donor all cell chimerism was 100% (range 95-100). Clinical outcomes are summarized in Figure B. With median follow-up among survivors of 12 months (range 3-24), 1-yr GRFS was 65%. OS, PFS, and cumulative incidence of NRM and disease relapse were 77%, 71%, 13% and 17%, respectively (Figure C). There was no grade IV acute GVHD and only one case of grade III acute GVHD. Cumulative incidence of all chronic GVHD and moderate-severe chronic GVHD was 14% and 5%, respectively. There was no severe chronic GVHD and only one patient developed moderate chronic GVHD. As part of the study, next generation sequencing (NGS) was obtained pre- and 100 days post-HSCT. 14 patients have paired samples, including 6 with ASXL1 mutations. All but one patient, who remains in remission at last follow up, no longer had mutations detected by NGS at day 100 (Figure D). Ongoing studies will assess for the presence of low-level mutation not detectable by clinical NGS testing. Discussion: The interim results of our multicenter study demonstrate safety of ruxolitinib administration pre, during and post-HCT with very favorable engraftment rates and no unexpected toxicities of ruxolitinib use. In addition, we demonstrate superior PFS, OS and GRFS compared to historical observations. Incidence of severe acute and chronic GVHD are thus far minimal, indicating excellent GVHD control with prophylactic and continued ruxolitinib use. Figure 1 Figure 1. Disclosures Hobbs: Bayer: Research Funding; Incyte Corporation: Research Funding; Celgene/Bristol Myers Squibb: Consultancy; AbbVie.: Consultancy; Merck: Research Funding; Novartis: Consultancy; Constellation Pharmaceuticals: Consultancy, Research Funding. Byrne: Karyopharm: Research Funding. Defilipp: Incyte Corp.: Research Funding; Regimmune Corp.: Research Funding; Omeros, Corp.: Consultancy; Syndax Pharmaceuticals, Inc: Consultancy. Chen: Gamida: Consultancy; Incyte: Consultancy. OffLabel Disclosure: Will describe the use of ruxolitinib in the ongoing clinical trial.

Post-Transplant Cyclophosphamide, Abatacept, and Short Course of Tacrolimus Combination (CAST) Is Safe and Seems Highly Effective in Preventing Graft-Versus-Host Disease Following Haploidentical Peripheral Blood Stem Cell Transplantation

The introduction of post-transplant cyclophosphamide (PTCy) has circumvented the need for T-cell depletion following haploidentical stem cell transplantation (SCT). By expanding the donor pool for patients from certain ethnic minorities, this has addressed to some degree an important health care disparity issue in SCT. However, a recent registry study showed increased incidence GvHD and inferior outcomes in patients receiving haploidentical SCT with PTCy, tacrolimus and mycophenolate mofetil for GvHD prevention as opposed to matched unrelated donor SCT with PTCy-based GvHD prevention. Seeking to improve the results of GvHD prevention in the setting of haploidentical SCT, we examined a combination of PTCy, abatacept and a short course of tacrolimus (CAST). Abatacept is a recombinant soluble fusion protein composed of the extracellular domain of cytotoxic T-lymphocyte associated antigen-4 (CTLA-4) fused to the Fc region of IgG1. Abatacept blocks CD28-CD80I86 axis and prevents T-cell co-stimulation. In early studies, abatacept has shown promising results when added to methotrexate and tacrolimus in matched and mismatched donor SCT. We initiated a phase Ib-II clinical trial for patients with hematological malignancies undergoing haploidentical SCT. Patients received G-CSF mobilized peripheral blood grafts from related haploidentical donors. GvHD prevention consisted of PTCy 50mg/kg IV on day +3 and +4 with forced hydration, abatacept 10mg/kg IV on day +5, +14 and +28 and tacrolimus. Tacrolimus was started on day +5 at 0.02mg/kg/day by continuous IV and adjusted thereafter to maintain a trough level of 5-12ng/mL. Tacrolimus taper was planned to begin on day +60 and complete by day +90 in the absence of GvHD. All patients received standard supportive care including levofloxacin until neutrophil engraftment, posaconazole until day +75, acyclovir for 1 year and, if CMV positive by serology, letermovir until day +100. Pneumocystis Jiroveci prophylaxis was started after neutrophil engraftment and continued until 6 months post-transplant. G-CSF was administered routinely until neutrophil engraftment. Since September 2020, 19 patients were enrolled. Three patients are too early in their post-transplant course and were excluded from this analysis. Patients' characteristics are summarized in the table. All but 2 patients received cryopreserved products. Median times to ANC and platelet engraftment were 18.5 days (14-30) and 28.5 (16-61). All 16 patients achieved full whole blood donor chimerism by day +30. There was no secondary graft failure. With a median follow-up was 149.5 days (41-308) with 10 patients having &gt;120 days and 8 &gt;180 days of follow-up, 4 patients developed skin acute GvHD (all grade I). No patient developed grade II-IV acute GvHD. Two patients developed skin chronic GvHD (limited, both moderate). Both cases were diagnosed following COVID-19 vaccination. Fifteen patients completed tacrolimus taper by day +90. Two patients received systemic steroids, one for treatment of cGvHD. The remaining patients required no further immunosuppressive therapy beyond day +90. CMV activation rate was 25%. One patient had EBV reactivation and required preemptive therapy with 2 weekly rituximab doses. There were no cases of adenovirus, HHV-6 virus or BK virus reactivation. Four patients developed renal insufficiency (3 in the setting of acute sepsis and 1 with thrombotic microangiopathy, which resolved after tapering off tacrolimus. One patient with adult T-cell leukemia/lymphoma relapsed and died. All other patients are alive and well. In summary, our preliminary results suggest that CAST with shortened course of tacrolimus is feasible and seems to offer very promising outcomes with low rates of acute GvHD. The study is accruing actively and the results of a larger cohort with longer follow-up will be presented at the meeting. If confirmed, by improving the outcomes of haploidentical SCT, this regimen may further address a health care disparity issue, offering almost every patient in need of allogeneic SCT an alternative donor option with equal outcomes. Figure 1 Figure 1. Disclosures Al-Homsi: Daichii Sanyko: Consultancy; Celyad: Other: Advisory Board. Abdul-Hay: Abbvie: Consultancy; Servier: Other: Advisory Board, Speakers Bureau; Jazz: Other: Advisory Board, Speakers Bureau; Takeda: Speakers Bureau; Amgen: Membership on an entity's Board of Directors or advisory committees. OffLabel Disclosure: Abatacept - off label use as GvHD prevention Cyclophosphamide - off label use as GvHD prevention

Outcomes of Haploidentical Salvage Transplantation Using Post-Transplant Cyclophosphamide for Graft Failure Following Allogeneic Hematopoietic Stem Cell Transplantation

Introduction: Graft failure (GF) is a fatal complication after allogeneic stem cell transplantation (SCT). Although salvage SCT is the only curative therapy for GF, optimal donors and strategies for this procedure have not yet been established. Although in the last decade haploidentical donors have emerged as alternative donors, only limited data are available regarding the outcomes after haploidentical salvage SCT using post-transplant cyclophosphamide (PTCy). Therefore, this nationwide retrospective study aimed to evaluate the transplant outcomes and risk factors for survival after haploidentical salvage SCT using PTCy on behalf of the Transplant Complications Working Group of the Japan Society for Transplantation and Cellular Therapy. Methods: Clinical data were provided by the nationwide database of the Japanese Data Center for Hematopoietic Cell Transplantation. Patients who were diagnosed with GF and underwent a second or higher allogeneic SCT from the haploidentical related donor (≥2 antigen-mismatch), using PTCy as graft-versus-host disease (GVHD) prophylaxis, between 2011 and 2019 were included. Organ failure was defined as either ejection fraction ≤50%, serum creatinine ≥2 mg/dL, bilirubin ≥1.5 × upper limit of normal, or aspartate aminotransferase/alanine aminotransferase ≥2.5 × upper limit of normal. Overall survival (OS) probabilities were estimated using the Kaplan-Meier method and differences among groups were analyzed using the log-rank test. The multivariate analysis for OS was performed using the Cox proportional hazard regression model. Factors from the univariate analysis that demonstrated significance with P values &lt; 0.1 were included in the multivariate analysis. Results: A total of 33 patients were included in the study. The median age was 34 years (range, 2-67), while performance status (PS) was 0-1 in 21 patients (64%). At salvage transplantation, 12 (36%) were receiving treatment for active infection, and 5 (15%) had organ failure. The median interval from SCT to salvage SCT was 49 days (range, 26-1,468), and 21 patients (68%) underwent salvage SCT within 100 days after previous SCT. Conditioning regimens consisted of fludarabine (Flu)/ cyclophosphamide (Cy)-based in 10 (31%), Flu/melphalan (Mel)-based in 10 (31%), and Flu/busulfan (Bu)-based in 7 (21%). The total dose of PTCy was 75-100 mg/kg in 26 patients (84%) and 40-50 mg/kg in 5 patients (16%). Most patients (84%) received tacrolimus plus mycophenolate mofetil as GVHD prophylaxis in addition to PTCy. Previous SCT was cord blood transplantation in 22 patients (67%) and haploidentical transplantation in 6 patients (19%), of which 4 patients (13%) received PTCy. The median time for neutrophil engraftment was 18 days, and the cumulative incidence of neutrophil engraftment at 30 days was 82%. Specifically, a patient who had donor-specific human-leukocyte antigen-antibody successfully achieved neutrophil engraftment at 22 days after salvage SCT. The median OS was 359 days, while the OS at 1 year was 47% (Figure A). In the univariate analysis, the OS of patients who received 75-100 mg/kg PTCy was significantly better than those who received 40-50 mg/kg PTCy (61% vs. 0% at 1 year, P = 0.022, Figure B). After adjusting for PS and the presence of active infection and organ failure, 75-100 mg/kg PTCy was significantly associated with better OS (hazard ratio, 0.30; P = 0.036). Although the differences were not significant, patients who received Flu/Mel-based conditioning exhibited numerically better OS than those who received Flu/Cy- or Flu/Bu-based conditioning (80% vs. 40% vs. 57%, P = 0.21). Conclusions: Haploidentical salvage SCT using PTCy offers promising survival outcomes and can be a substantial option for GF after allogeneic SCT. An adequate dose of PTCy (i.e., 75-100 mg/kg) needs to be administered to achieve long-term survival. Figure 1 Figure 1. Disclosures Nakamae: PPD-SNBL K.K: Research Funding; Bristol-Myers Squibb Company: Honoraria, Research Funding; Alexion: Research Funding; Sumitomo Dainippon Pharma Co., Ltd.: Honoraria; Simon-Kucher & Partners: Honoraria; Takeda Pharmaceutical Company Limited.: Honoraria; Astellas Pharma Inc.: Honoraria; Otsuka Pharmaceutical Co., Ltd: Honoraria; ONO PHARMACEUTICAL CO., LTD.: Honoraria; CMIC HOLDINGS Co., Ltd: Research Funding; Novartis: Honoraria, Research Funding; Pfizer Japan Inc.: Honoraria. Ichinohe: Celgene: Honoraria; Novartis Pharma K.K.: Honoraria; Repertoire Genesis Inc.: Honoraria, Research Funding; Takara Bio Inc.: Research Funding; Zenyaku Kogyo Co.: Research Funding; Taiho Pharmaceutical Co.: Research Funding; Sumitomo Dainippon Pharma Co.: Honoraria, Research Funding; Otsuka Pharmaceutical Co.: Research Funding; Nippon Shinyaku Co: Research Funding; Ono Pharmaceutical Co.: Honoraria, Research Funding; Kyowa Kirin Co.: Honoraria, Research Funding; FUJIFILM Wako Chemicals.: Honoraria, Research Funding; Daiichi Sankyo: Research Funding; Eisai Co.: Honoraria, Research Funding; CSL Behring: Honoraria, Research Funding; Chugai Pharmaceutical: Research Funding; Bristol-Myers Squibb: Honoraria; Takeda Pharmaceutical Co.: Honoraria; AbbVie Pharma: Research Funding; Astellas Pharma: Honoraria, Research Funding. Atsuta: Mochida Pharmaceutical Co., Ltd.: Speakers Bureau; Astellas Pharma Inc.: Speakers Bureau; AbbVie GK: Speakers Bureau; Kyowa Kirin Co., Ltd: Honoraria; Meiji Seika Pharma Co, Ltd.: Honoraria. Nakasone: Takeda Pharmaceutical: Honoraria; Otsuka Pharmaceutical: Honoraria; Bristol-Myers Squibb: Honoraria; Celgene: Honoraria; Pfizer: Honoraria; Novartis: Honoraria; Janssen Pharmaceutical K.K.: Honoraria; Eisai: Honoraria; Chugai Pharmaceutical: Honoraria; Nippon Shinyaku: Honoraria.

Risk Factors and Outcomes of ICU Admission Following Allogeneic Hematopoietic Stem Cell Transplantation

Background: Patients undergoing allogeneic hematopoietic stem cell transplantation (allo-hsct) can develop complications such as life-threatening infections, multi-system organ failure, ICU admission and ventilator support in the immediate post-transplant period. Whereas outcomes of these complications, particularly ICU admission and ventilator support, are known to be poor, little is known about the risk factors leading to them. Methods: We conducted a retrospective study to analyze the impact of pre-transplant risk factors on acute inpatient complications, focusing on ICU admission, ventilator support and multi-system organ failure, following allo-hsct at the University of Alabama at Birmingham (UAB) between 2008 and 2016. Mortality rates and survival outcomes of patients admitted to the ICU were also analyzed. Pre-transplant individual comorbidities were defined as per Sorror's HCT-CI. Results: There were 304 patients included with a median age of 52y (18-72y). There were 51% male and 82% Non-Hispanic white patients. The most common indication for transplant was AML (45%). Donor type was matched-unrelated, haploidentical and matched-related in 53%, 35% and 12% of cases, respectively. Majority of the patients received myeloablative conditioning (74%). The prevalence of health behaviors and comorbidities at the time of transplant is shown in Table 1. There were 39% patients with HCT-CI score of ≥3, 23% with moderate pulmonary compromise, 22% with a psychiatric disorder, 13% with severe pulmonary compromise, 13% with diabetes mellitus (DM), 10% with cardiac abnormalities and 6% with infection at the time of transplant. During the initial hospitalization, 33 (11%) patients required ICU admission, 29 (10%) required ventilator support, 33 (11%) developed multi-system organ failure, 79 (26%) developed bacterial infections and 15 (5%) developed fungal infections. The median time to neutrophil engraftment was 13 days (7-48 days). In multivariable analysis (Table 2), risk factors for ICU admission included pre-transplant infection (HR 6.50, 95% CI 1.82-23.26, p=0.004), pre-transplant DM (HR 4.14, 95% CI 1.56-10.97, p=0.004), time to neutrophil engraftment (HR 1.13, 95% CI 1.05-1.21, p=0.0007), donor type (ref: matched related donor; haplo: HR 0.24 95% CI 0.07-0.82, p=0.02) and HSCT era (ref: 2008-2010; 2010-2013: HR 0.18 95% CI 0.04-0.88, p=0.03; 2014-2016: HR 0.12 95% CI 0.03-0.4, p=0.0006). Risk factors for ventilator support included pre-transplant infection (HR 10.09, 95% CI 2.44-41.64, p=0.001), pre-transplant DM (HR 3.61, 95% CI 1.31-9.91, p=0.01), time to neutrophil engraftment (HR 1.17, 95% CI 1.11-1.23, p&lt;0.0001) and HSCT era (ref: 2008-2010; 2010-2013: HR 0.21 95% CI 0.06-0.81, p=0.02; 2014-2016: HR 0.07 95% CI 0.02-0.31, p=0.0005). Risk factors for multi-system organ failure included pre-transplant DM (HR 4.38, 95% CI 1.64-11.74, p=0.003), time to neutrophil engraftment (HR 1.13, 95% CI 1.08-1.19, p&lt;0.0001) and HSCT era (ref: 2008-2010; 2010-2013: HR 0.21 95% CI 0.05-0.80, p=0.003; 2014-2016: HR 0.16 95% CI 0.05-0.48, p=0.001).Risk factor for bacterial infection included HSCT era (ref: 2008-2010; 2010-2013: HR 0.30 95% CI 0.14-0.65, p=0.002; 2014-2016: HR 0.24 95% CI 0.12-0.49, p&lt;0.0001) and for fungal infection included pre-transplant pulmonary compromise (ref: no compromise; severe pulmonary compromise HR 5.16, 95% CI 1.05-25.4, p=0.04). For patients admitted to the ICU, the 60-day and 6-month mortality was 58% and 67%, respectively. No deaths were attributed to relapse disease. The median overall survival for patients admitted to the ICU was 1.4 months (Figure 1). Conclusion: Patients with DM and infection at the time of HSCT and delayed neutrophil engraftment during transplant are at an increased risk for ICU admission, ventilator support and multi-system organ failure following allo-hsct. Patients admitted to the ICU are also at a high risk for early mortality leading to poor survival. Optimizing glycemic control and delaying transplant until resolution of infection, if the underlying disease would allow, may help improve both morbidity and mortality in transplant recipients. Figure 1 Figure 1. Disclosures Di Stasi: Syndax Pharmaceutical: Honoraria, Membership on an entity's Board of Directors or advisory committees; University of Alabama at Birmingham: Current Employment.

Post-Transplant Cyclophosphamide Is Associated with Improved Clinical Outcomes in HLA-Mismatched Unrelated Donor Hematopoietic Cell Transplantation

The current standard of care to prevent graft versus host disease (GVHD) after HLA mismatched unrelated donor (URD) allogeneic hematopoietic cell transplantation (HCT) is tacrolimus, methotrexate (MTX) and anti-thymocyte globulin (ATG). Recently, an approach based on administration of post-transplant cyclophosphamide (PT-Cy) demonstrated promise in a prospective trial for HLA mismatched URD HCT. Here, we compared tacrolimus/MTX/ATG (ATG group) treatment, with that of PT-Cy, mycophenolate mofetil, and tacrolimus or sirolimus (PT-Cy group) after HLA mismatched URD HCT, at two academic centers with similar peri-transplant supportive care practices. Subjects that underwent HCT from a URD mismatched at one or more loci among HLA-A, -B, -C, and -DRB1 from 2010-2020 were included. The primary endpoint of our study was one-year GVHD-free, relapse-free survival (GRFS). Of 128 total subjects included (ATG: 46; PT-Cy: 82), the median age was 54.7 (range: 21.0 - 72.0) in the ATG group and 60.3 (21.0 - 75.2) in the PT-Cy group. Notably, 74 subjects (57.8%) belonged to a racial/ethnic minority, while 53 (41.4%) received a bone marrow (BM) graft, and 26 (20.3%) received a highly mismatched (i.e., &lt;6/8 HLA) graft. A higher proportion of PT-Cy subjects received BM (50% vs. 26.1%, P= 0.01) and highly mismatched grafts (30.5% vs. 2.2%, P=0.001). The groups were well matched for key demographic data, including indication for HCT, high-risk disease, and HCT comorbidity index. The rate of primary neutrophil engraftment at day 28 post HCT was similar between the two groups: 91% (83%-99%), in the ATG group, and 90% (84%-97%), in the PT-Cy group. The median day of neutrophil engraftment was +17 in the ATG, and +20 in the PT-Cy groups. Platelet engraftment at day 100 was 84.8% (74.4% - 95.2%) and 86.6% (79.2% - 94%) in the ATG and PT-Cy groups, respectively. Among subjects that survived 3-months post HCT and had donor chimerism testing, full donor chimerism (≥95% donor) was achieved in 24/26 (92.3%) patients in the ATG group, and 61/68 (89.7%) subjects in the PT-Cy group. Overall, the one-year GRFS was 16% (95% confidence interval: 8% - 31%) vs. 54% (44% - 66%, p &lt; 0.001) in the ATG and PT-Cy groups, respectively. The adjusted, multivariate hazard ratio (HR) for GRFS was 0.34 (0.21 - 0.55, p &lt; 0.001), in association with the use of PT-Cy. The one-year overall survival (OS) in the ATG group was 45% (32% - 62%) vs. 75% (66% - 85%, p &lt; 0.001) in the PT-Cy group. There was no difference in 2-year relapse incidence (27%, 14-39% vs. 19%, 10% - 28%, p = 0.2), whereas 1-year non-relapse mortality (NRM) was increased with ATG-based prophylaxis: (38%, 23% - 52% vs. 16%, 9% - 25%, p = &lt;0.001). The use of PT-Cy was associated with improved GRFS for patients treated with BM (HR = 0.27, 0.11 - 0.62; p = 0.002) and G-CSF mobilized, peripheral blood derived (PB) grafts (HR = 0.39, 0.21 - 0.71; p = 0.002). BM grafts were not an independent predictor of GRFS (HR: 1.2, 0.8 - 1.9; p = 0.5) in a multivariate Cox regression model. The use of PT-Cy, compared to ATG, was associated with lower rates of grade 3-4 acute GVHD in the entire cohort (ATG: 22%, 18% - 45% vs. PT-Cy: 15%, 8% - 23%; p = 0.03), and in the sub-cohorts of patients transplanted with PB (ATG: 33%, 17% - 49% vs. PT-Cy: 20%, 9% - 33%) and BM grafts (ATG: 25%, 1% - 50% vs. PT-Cy: 10%, 1% - 19%). Chronic GVHD requiring systemic immune suppression was more frequent in the ATG/PB group (26%, 13% - 42%) than in the ATG/BM (8%, 0% - 24%), the PT-Cy/PB (8%, 2% - 19%), and the PT-Cy/BM (10%, 1% - 19%) groups. Despite greater HLA-mismatching, PT-Cy following mismatched URD HCT resulted in superior GRFS, OS and lower NRM and GVHD rates compared to ATG-based GVHD prophylaxis. These results indicate that PT-Cy is the preferred method to prevent GVHD after mismatched URD HCT and confers a benefit in patients treated with either PB or BM-derived allografts. Combinatorial approaches, using the T-cell co-stimulatory blockading agent abatacept, in addition to tacrolimus/methotrexate, may also prevent GVHD in this population, and can be compared to a PT-Cy-based approach in a prospective clinical trial. Figure 1 Figure 1. Disclosures Jimenez: AbbVie: Research Funding; Takeda: Research Funding. Perales: MorphoSys: Honoraria; Bristol-Myers Squibb: Honoraria; Celgene: Honoraria; Karyopharm: Honoraria; Merck: Honoraria; Equilium: Honoraria; Miltenyi Biotec: Honoraria, Other; Medigene: Honoraria; Kite/Gilead: Honoraria, Other; Incyte: Honoraria, Other; Cidara: Honoraria; Takeda: Honoraria; Nektar Therapeutics: Honoraria, Other; Servier: Honoraria; Sellas Life Sciences: Honoraria; Omeros: Honoraria; NexImmune: Honoraria; Novartis: Honoraria, Other. Sauter: Precision Biosciences: Consultancy; Genmab: Consultancy; Celgene: Consultancy, Research Funding; Kite/Gilead: Consultancy; Bristol-Myers Squibb: Research Funding; GSK: Consultancy; Gamida Cell: Consultancy; Novartis: Consultancy; Spectrum Pharmaceuticals: Consultancy; Juno Therapeutics: Consultancy, Research Funding; Sanofi-Genzyme: Consultancy, Research Funding. Beitinjaneh: Kite/Gilead: Other: Ad Board Event Attendee. Ponce: CareDx: Consultancy, Honoraria; Ceramedix: Consultancy, Honoraria; Seres Therapeutics: Consultancy, Research Funding; Kadmon pharmaceuticals: Consultancy, Honoraria; Takeda Pharmaceuticals: Research Funding; Generon Pharmaceuticals: Consultancy. OffLabel Disclosure: Off-label use of Cyclophosphamide for Graft-versus-Host Disease prevention will be discussed in the abstract.