Mini-ATG/TBI conditioning regimen facilitates cure of acquired aplastic anemia in children via unrelated umbilical cord blood transplantation

Abstract Introduction: Umbilical cord blood transplantation (UCBT) represents a alternative therapeutic option for patients with acquired aplastic anemia (AA) lacking a HLA-matched sibling donor. However, high incidences of graft failure and acute graft-versus-host disease (aGVHD) following UCBT remain major challenges. The optimal conditioning regimen for UCBT in AA is yet to be established. Methods: We enrolled patients with newly diagnosed AA or those refractory to prior Immunosuppressive therapy. This single-center prospective UCBT trial (registered at www.chictr.org.cn #ChiCTR2400088868), initiated in March 2024, compared outcomes using a mini-ATG/TBI-based regimen against historical data from our center prior to March 2024. The mini-ATG/TBI regimen comprised: rabbit ATG (rATG, 2.5 mg/kg) on day -7, total body irradiation (TBI, 3 Gy) on day -7, fludarabine (FLU, 40 mg/m²) on days -6 to -2, and cyclophosphamide (CTX, 40 mg/m²) on days -4 and -2. The historical regimen consisted of TBI (4 Gy) on day -7, FLU (40 mg/m²) on days -6 to -2, and CTX (60 mg/m²) on days -3 and -2. Outcomes evaluated included aGVHD rates, graft failure rate, engraftment rate, time to neutrophil engraftment, time to platelet engraftment, and overall survival. Results: 1.A total of 27 patients were enrolled: 18 in the mini-ATG/TBI group and 9 in the conventional group. The cohort included 13 males and 14 females, with a median age of 7 years (range: 2–15 years). No significant age difference existed between groups (P=0.166). 2.aGVHD: Grade III-IV aGVHD occurred in 2/18 patients (11.11%) and Grade I-II in 5/18 in the mini-ATG/TBI group, compared to 4/9 (44.44%) and 3/9 in the conventional group. The incidence of severe (Grade III-IV) aGVHD was significantly lower in the mini-ATG/TBI group (11.11% vs. 44.44%, P=0.05). 3.Engraftment: One patient in the mini-ATG/TBI group experienced graft failure due to early EBV viremia (within 1month post-transplant) complicated by suspected splenic PTLD and secondary hemophagocytic lymphohistiocytosis (HLH). This patient achieved disease-free survival after salvage haploidentical transplantation using an HLH-directed conditioning regimen. Engraftment was successful in the remaining 26 patients. Engraftment rates of the two groups were 94.12% vs. 100% (P=0.471). Median donor chimerism on day +7 was 71.62% (range: 21.84%–92.75%), with no significant difference between groups (72.25% vs. 63%, P=0.522). Complete donor chimerism (≥99%) was achieved by day +14 in all evaluable patients. Median time to neutrophil engraftment was 15 days (range: 12–30) in the mini-ATG/TBI group and 16 days (range: 13–27) in the conventional group (P=0.411). Median time to platelet engraftment was 25 days (range: 16–67) vs. 24 days (range: 18–36) (P=0.643). In the mini-ATG/TBI group, odd-numbered patients received avatrombopag starting on day +7 to promote platelet engraftment, while even-numbered patients received thrombopoietin (TPO). Subgroup analysis showed no significant difference in platelet engraftment time (27 days vs. 24 days; P=0.456). 4.Transplant Related Complications: Rates of sepsis, transplant-associated thrombotic microangiopathy (TA-TMA), EBV infection, and hemorrhagic cystitis did not differ significantly between two groups. However, CMV viremia incidence differed significantly (6.25% vs. 66.67%, P=0.001), attributed to the widespread use of letermovir for CMV prophylaxis after 2024. 5.Survival: The estimated 2-year overall survival (OS) rate was 92.86% (±6.88%) for the mini-ATG/TBI group and 88.89% (±10.48%) for the conventional group, showing no statistically significant difference (χ²=0.001, P=0.971). Conclusions: These data support the mini-ATG/TBI regimen as an effective conditioning approach for UCBT in acquired aplastic anemia, demonstrating particular utility in pediatric patients or low body weight adults. Key words: Aplastic Anemia; Unrelated Umbilical Cord Blood Transplantation; mini ATG