Risk of Relapse Post Reduced Intensity Conditioning Allogeneic Stem Cell Transplant in Patients with High-Risk Myeloid Neoplasms Based on Ptcy Vs TAC/MTX Gvhd Prophylaxis

Introduction: Post-transplant cyclophosphamide (PTCy) has emerged as the new standard of care for graft-versus-host disease (GVHD) prophylaxis following reduced-intensity conditioning (RIC) allogeneic stem cell transplant (alloSCT). The BMT CTN 1703 phase III trial demonstrated superior GVHD/relapse or progression-free survival (GRFS) in patients receiving PTCy, without affecting relapse risk. The primary objective of this study was to compare the risk of relapse in high-risk myeloid neoplasms post RIC/Nonmyeloablative (NMA) transplant using PTCy vs Tacrolimus/Methotrexate (TAC/MTX) as GVHD prophylaxis (PPX).

Methods: We retrospectively reviewed patients with high-risk myeloid neoplasms following RIC/NMA alloSCT who received PTCy or TAC/MTX for GVHD PPX between Jan 2018 and June 2023. Criteria for high-risk myeloid neoplasms included acute myeloid leukemia (AML) with ≥1 of the following (complex or monosomal karyotype, TP53, WT1, FLT3 ITD+/NPM1-, active disease, MRD+, secondary AML), Myelodysplastic syndrome (MDS) with ≥1 of the following (complex karyotype, monosomal karyotype, TP53, RAS pathway mutation, marrow blast of 10% or more) or chronic myelomonocytic leukemia (CMML). Kaplan–Meier and log-rank tests were used to estimate overall survival (OS) and disease-free survival (DFS). Non-relapse mortality (NRM) and relapse incidence (RI) were calculated using competing risk analysis. Factors with P<0.10 in univariate analysis were included in multivariate analysis

Results: Out of 552 alloSCTs, 108 (19.5%) patients met the inclusion criteria. Seventy-two (66.6%) received TAC/MTX, and 36 (33.3%) received PTCy. The median follow-up time was 26 months (range 3-68). The median age at transplant was 65 years (range 62-69) with age comparable between the two groups (p=0.6). Haploidentical transplant accounted for 33.3% of PTCy, while the remainder had matched related or matched unrelated transplant (MRD/MUD). Compared to TAC/MTX group, the PTCy group had more frequent NRAS mutation (19.4% vs 8.3%, p=0.094), but fewer patients with hematopoietic cell transplantation-specific comorbidity index (HCT-CI) ≥ 3(41.7% vs 61.1%, p=0.056). AML MRD+ was more prevalent in the PTCy group (22.2% vs 8.3%, p=0.098). The two groups were otherwise well-balanced in terms of disease distribution, disease risk index, TP53 mutation, and complex monosomal karyotype (CK and MK).

The 1-yr OS, DFS, and GRFS for PTCy vs TAC/MTX were 78.7% vs 72.8% (p=0.9), 53.6% vs 67.9% (p=0.2), 38% vs 40%, (p=0.4), respectively. The 1-yr cumulative incidence of relapse (CIR) was higher in PTCy compared to TAC/MTX (37.6% vs 11.1%, p=0.01). Both haploidentical and MRD/MUD in the PTCy group had similar 1-yr RI (34.7 vs 38.3% p=0.7). Limiting the analysis to MRD/MUD, the PTCy group had a higher risk of relapse compared to TAC/MTX (34.7% vs 10.2%, P=0.007).

Sub-analysis showed a higher 1-yr-CIR in the PTCy cohort for secondary AML (40% vs 0.0%, p=0.01), TP53 mutation (44.4% vs 18.1%, p=0.02), CK/MK (71.4% vs 22.2%, P=0.008), and chromosome 17 abnormality (60% vs 10%, P=0.001), compared to TAC/MTX. Despite of the increased CIR in the PTCy group, only patients with chromosome 17 abnormality receiving PTCy had an inferior DFS compared to TAC/MTX (median 5 vs. 14 months, p=0.008).

In multivariate analysis for 1-yr CIR that included TP53, chromatin modifiers mutations, complex karyotype, high/very high DRI, and PTCy, both PTCy (HR 4.56, 95% CI 1.1-18.07, p=0.031) and high/very high DRI (HR 12.745, 95% CI 1.5-103.4, p=0.017) were independently predictive of post-transplant relapse. However, these factors were not significantly associated with 1-year OS.

Conclusion: In patients with high-risk MN following RIC/NMA alloSCT, those receiving PTCy as GVHD PPX had a higher risk of relapse compared to TAC/MTX, irrespective of the donor source. Notably, an inferior DFS was observed only in patients with chromosome 17 abnormality who received PTCy. Despite of the increased relapse risk in the PTCy group, OS, DFS, and GRFS were comparable to TAC/MTX, likely due to higher risk of GVHD and NRM in the latter group.