Long-term outcomes of patients surviving beyond 2 years post–allogeneic stem cell transplantation.

6508 Background: Advances in hematopoietic stem cell transplantation (HSCT) have been driven by progress in supportive care, conditioning regimens, and graft-versus-host disease (GvHD) prophylaxis. Post-transplant morbidity and mortality are most pronounced in the first two years after HCT. Long-term outcomes have been previously described in a CIBMTR study, which reported a 10-year survival of 85% for those who survived the first two years. Post-transplant cyclophosphamide (PTCy) has revolutionized transplant outcomes. However, its impact on long-term survival remains unclear. Methods: We included patients undergoing HSCT from Jan 2015 to Dec 2023 who were alive and without relapse two years after HSCT. We aimed to compare long-term outcomes in patients receiving PTCy-based GvHD prophylaxis to previous conventional GvHD prophylaxis strategies. Overall survival (OS) was calculated using the Kaplan–Meier method. The incidence of non‐relapse mortality (NRM) and relapse were estimated using Fine & Gray's competing risk analysis. Results: Of the 1,401 patients, we included 571 patients alive and relapse-free two years after HCT (Table 1). The median follow-up post-HSCT was 4.8 years (3-5.9). The 5-year OS was 91% (95% CI: 88 – 94). The causes of death were: disease recurrence (17, 3%), infection (8, 1.4%), GvHD (4, 0.7%), second primary malignancies (3, 0.5%), and cardiac complications (3, 0.5%). The use of PTCy was associated with improved OS, 91.7% (95% CI: 88 – 95) vs. 88.7% (95% CI: 81 – 93), p=0.05. NRM at 5 years was 4.9% (95% CI: 3 – 7) and was significantly lower in patients receiving PTCy (2.9 % vs. 10.4%, p<0.001). Relapse risk at 5 years was 9.2% (7 – 13) and was not significantly different between the groups, 10.6% in recipients of PTCy vs. 5.6%, p=0.06. On MVA accounting for confounding variables, PTCy was independently associated with improved OS [HR: 0.46 (0.2 – 0.8), p=0.01], and NRM [HR: 0.23 (0.1 – 0.5), p<0.001]. Age >60 years at HSCT was associated with increased mortality [HR: 2.8 (1.5 – 5.3), p=0.001]. The development of chronic GvHD was protective against relapse [HR: 0.5 (0.3 – 0.9), p=0.04] (Table 1). Conclusions: The long-term outcomes for patients alive and relapse-free 2 years after HSCT are excellent. Relapse remained the most common cause of death even after 2 years. PTCy-based GvHD prophylaxis was associated with improved NRM and OS without an impact on disease relapse. Baseline characteristics and multivariable analysis. PTCy Others p Age, years, median (IQR) 57 (40 – 64) 52 (37 – 62) 0.04 Diagnosis, n (%)AMLMDSMF 231 (54)57 (13)46 (11) 60 (43)19 (14)5 (4) 0.78 Donors, n (%)HLA-matched siblingHLA-matched unrelatedHLA-mismatched unrelatedHaploidentical 90 (21)220 (51)38 (9)84 (19) 69 (49)60 (43)8 (7)2 (1) <0.01 GVHD prophylaxis, n (%)CnI + MTXATG + CnI + MTXAlemtuzumab + CnIPTCy-ATG-CnIPTCy- CnI – MMF ---383 (89)48 (11) 43 (31)73 (52)24 (17)-- - Cryopreservation, n (%) 100 (23) 52 (36) 0.004 CD34 + x 10 6 /kg, median (IQR) 7.2 (6 – 8.1) 7.3 (5.9 – 7.6) 0.76 MVA OS HR 95% CI p Age >60 years (vs <60 years) 2.84 1.5 – 5.3 0.001 PTCy (vs non-PTCy) 0.45 0.2 – 0.8 0.01 NRM Age more than 60 years (vs <60 years) 2.50 1 – 6.2 0.04 PTCy (vs non-PTCy) 0.23 0.1 – 0.5 <0.01 Relapse Age >60 years (vs <60 years) 1.47 0.7 – 3.0 0.29 PTCy (vs non-PTCy) 1.54 0.4 – 5.4 0.49 <

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