Introduction: Graft-versus host disease (GVHD) is a major limitation to the success of allogeneic hematopoietic cell transplant (HCT). We hypothesized that the GVHD prophylaxis regimen of post-transplant cyclophosphamide (PTCy), tacrolimus (Tac) and mycophenolate mofetil (MMF) would reduce the incidence of GVHD in patients receiving a matched or single antigen mismatched HCT without an increase in risk of malignant relapse. Methods: This is a phase II study conducted at the University of Minnesota using a myeloablative regimen of either: (A) total body irradiation (TBI, total dose 1320 cGy, administered in 165 cGy fractions, twice a day from days -4 to -1) or (B) Busulfan 3.2mg/kg daily (cumulative AUC 19,000 - 21,000 umol/min/L) plus fludarabine 160mg/m2 days -5 to -2, followed by a GVHD prophylaxis regimen of PTCy (50mg/kg days +3 and +4), Tac and MMF (beginning day +5). The primary endpoint is cumulative incidence of chronic GVHD requiring systemic immunosuppression at 1-year post-transplant. We compared results to our previous myeloablative protocol for matched donors utilizing cyclosporine/methotrexate (CSA/MTX) GVHD prophylaxis. Results: From March 2018 - June 2022, we enrolled and treated 125 pediatric and adult patients with a median follow up of 472 days. Grade II-IV acute GVHD occurred in 16% (95% confidence interval (CI): 9-23%); Grade III-IV acute GVHD was 4% (CI: 0-8%). No patients experienced grade IV GVHD, and there were no deaths due to GVHD before day 100. Only 3 developed chronic GVHD requiring immune suppression, (4%, CI: 0-8%). Two-year overall survival (OS) was 80% (CI: 69-87%), and (graft-versus-host disease-free, relapse-free survival) GRFS 57% (CI: 45-67%), both higher than historical CSA/MTX. The incidence of grade II-IV aGVHD, cGVHD, and NRM were all lower with PTCy/Tac/MMF compared to historical CSA/MTX. One-quarter (25%) experienced relapse (CI: 15-36%) similar to historical CSA/MTX. There was no statistically significant difference in survival outcomes between recipients of matched versus 7/8 donors. Conclusion: Myeloablative HCT with PTCy/Tac/MMF results in extremely low incidence of severe acute or chronic GVHD, the primary endpoint of this clinical trial. Relapse risk is not increased compared to our historical CSA/MTX cohort.

Holtan: Vitrac Therapeutics: Research Funding; Incyte: Research Funding; CSL Behring: Other: Clinical trial adjudication. Gupta: Blue Rock Therapeutics: Membership on an entity's Board of Directors or advisory committees; Vertex Pharmaceuticals: Consultancy. Maakaron: Gilead: Research Funding; CRISPR Therapeutics: Research Funding; Precision BioSciences: Research Funding; Scripps: Research Funding; Fate Therapeutics: Research Funding; ADC Therapeutics: Research Funding. Betts: Brian Betts: Patents & Royalties: WO2019165156; CTI Biopharma: Honoraria; Incyte: Honoraria. Bachanova: AstraZeneca: Membership on an entity's Board of Directors or advisory committees; Incyte: Research Funding; FATA Therapeutics: Research Funding; ADC Therapeutics: Membership on an entity's Board of Directors or advisory committees; Citius Pharma: Research Funding; Karyopharma: Consultancy; Gamida Cell: Membership on an entity's Board of Directors or advisory committees, Research Funding. Miller: ONK Therapeutics: Honoraria, Membership on an entity's Board of Directors or advisory committees; Vycellix: Consultancy, Current holder of stock options in a privately-held company; GT Biopharma: Consultancy, Current holder of stock options in a privately-held company, Research Funding; Fate Therapeutics: Consultancy, Current holder of stock options in a privately-held company, Research Funding. Wagner: ASC Therapeutics: Consultancy; Bluebird Bio: Consultancy; Vertex Pharmaceuticals: Consultancy; Magenta Therapeutics: Consultancy; Rocket Pharmaceuticals, Inc.: Consultancy, Current equity holder in publicly-traded company. Vercellotti: Mitobridge-Astellas: Research Funding; Omeros: Research Funding; CSL-Behring: Research Funding. Weisdorf: FATE Therapeutics: Other: Research Support; Incyte: Other: Research support.

NCT03314974, NCT00176930

This research was supported by NIH grant P30 CA77598 utilizing the Biostatistics Core of the Masonic Cancer Center, University of Minnesota and the National Center for Advancing Translational Sciences of the National Institutes of Health Award Number UL1TR002494.

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This trial was a prospective clinical trial reviewed and approved by the Masonic Cancer Center Protocol Review Committee and Human Subjects Institutional Review Board at the University of Minnesota. All patients signed Institutional Review Board-approved informed consent in accordance with the Declaration of Helsinki.

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