Comparison of Nonrelapse Mortality After Haploidentical Hematopoietic Stem Cell Transplantation With Post-transplant Cyclophosphamide Versus Single Umbilical Cord Blood Transplantation in Hematologic Disease

Unrelated cord blood transplantation (UCBT) and haploidentical transplantation using posttransplant cyclophosphamide (PTCy-haplo) are alternatives for patients lacking a human leukocyte antigen-matched donor. CD34⁺ cell counts in cord blood affect transplantation outcomes, particularly nonrelapse mortality (NRM). The primary objective of this study was to compare the transplantation outcomes after UCBT and PTCy-haplo focusing on CD34⁺ cell counts in cord blood. This retrospective study used data from 2014 to 2020 from a Japanese nationwide database. UCBT cases were divided into those with UCBT with higher (UCB-H; ≥.84 × 10⁵/kg) and lower (UCB-L; <.84 × 10⁵/kg) CD34⁺ cell counts, depending on the median CD34⁺ cell count. The study cohort comprised cases of PTCy-haplo (n = 1142), UCB-H (n = 3185), and UCB-L (n = 3172). In the multivariate analysis, neutrophil engraftment was significantly better in the PTCy-haplo than in the UCB-H (hazard ratio [HR], .64; 95% confidence interval [CI], .57 to .70; P < .001) and UCB-L groups (HR, .45; 95% CI, .41 to .50; P < .001). The UCB-H group showed similar NRM (HR, 1.19, 95% CI, 1.00 to 1.43, P = .051) and OS (HR, 1.05, 95% CI, .94 to 1.18, P = .38) compared with PTCy-haplo, whereas UCB-L was significantly associated with poor NRM (HR, 1.35, 95% CI, 1.13 to 1.61, P = .001) and OS (HR, 1.13, 95% CI, 1.01 to 1.26, P = .038). In contrast, the UCB-H (HR, .86; 95% CI, .75 to .98; P = .027) and UCB-L groups (HR, .80; 95% CI, .70 to .92; P = .001) were associated with lower relapse rate. Regarding the graft-versus-host disease (GVHD), the UCB-H and UCB-L groups were identified as significant risk factors for the development of grade II-IV acute GVHD (UCB-H: HR, 1.73; 95% CI, 1.51 to 1.99; P < .001; UCB-L: HR, 1.55; 95% CI, 1.35 to 1.78; P < .001) and grade III-IV acute GVHD (UCB-H: HR, 2.28; 95% CI, 1.78 to 2.91; P < .001; UCB-L: HR, 1.85; 95% CI, 1.44 to 2.37; P < .001), but neither were associated with the incidence of all-grade GVHD (UCB-H: HR, 1.12; 95% CI, .95 to 1.32; P = .16; UCB-L: HR, 1.08; 95% CI, .91 to 1.27; P = .37) or extensive chronic GVHD (UCB-H: HR, .86; 95% CI, .68 to 1.09; P = .21; UCB-L: HR, .88; 95% CI, .69 to 1.12; P = .31). Furthermore, higher NRM in UCB-L was attributed to higher infection-related mortality (HR, 1.50; 95% CI, 1.15 to 1.95; P = .003) but not GVHD-related mortality (HR, 1.15; 95% CI, .82 to 1.62; P = .43), whereas UCB-H was not a significant risk factor for both infection-related mortality (HR, 1.29; 95% CI, .99 to 1.69; P = .06) and GVHD-related mortality (HR, 1.28; 95% CI, .90 to 1.80; P = .16). UCB-H offered similar NRM and OS to PTCy-haplo, whereas UCB-L had worse outcomes. Our results can provide useful information for optimal donor selection.