Haploidentical hematopoietic stem cell transplantation with bussulfan, fludarabine and cyclophosphamide and total body irradiation 200cgy conditioning with post-transplant cyclophosphamide and peripheral blood stem cells as an alternative regimen to

Abstract Background: Sickle cell disease (SCD) is a hereditary hemoglobinopathy affecting millions worldwide, characterized by chronic hemolytic anemia, vaso-occlusive crises, and multi-organ complications. In Brazil, it is estimated that between 60,000 and 100,000 individuals live with SCD. Despite advances in supportive care and implementation of the National Policy for Comprehensive Care for People with SCD and Other Hemoglobinopathies, life expectancy remains markedly reduced compared to the general population. Allogeneic hematopoietic stem cell transplantation (allo-HSCT) remains the only established curative therapy for SCD. However, the scarcity of HLA-matched related donors, especially in populations with African ancestry, and the considerable toxicity associated with myeloablative conditioning limit widespread applicability. To address these limitations, haploidentical HSCT using post-transplant cyclophosphamide (PTCy) combined with reduced-intensity conditioning (RIC) has been developed as a promising alternative, potentially expanding transplant access while mitigating toxicity. Initial haploidentical transplantation protocols were hampered by high rates of primary graft rejection, reported as high as 43%, significantly compromising efficacy. Subsequent protocols incorporated thiotepa into non-myeloablative regimens based on the Johns Hopkins platform, achieving improved engraftment rates. However, access to thiotepa is limited in many resource-constrained settings. In light of the high graft rejection rates observed in our previously transplanted patients and the restricted availability of thiotepa, we adapted a RIC regimen consisting of busulfan, fludarabine, and cyclophosphamide, previously employed with satisfactory outcomes and low toxicity in other hematological diseases. Methods: Retrospective analysis of four adult patientes with SCD who underwent haploidentical HSCT using this RIC protocol at two Brazilian institutions between February 2024 and May 2025. Conditioning consisted of fludarabine 30 mg/m²/day for five days, cyclophosphamide 14.5 mg/kg/day for two days, busulfan 3.2 mg/kg/day for two days, and total body irradiation at 200 cGy. Peripheral blood stem cells (PBSC) were the graft source. Graft-versus-host disease (GvHD) prophylaxis included PTCy (50 mg/kg on days +3 and +4), mycophenolate mofetil, and sirolimus. Results: Median recipient age was 23 years (range 16–36), and median donor age was 49 years (range 24–54). All patients were self-identified as Black; three were male. Donors included two fathers, one mother, and one sister. One patient underwent a second transplant following graft failure from a previous haploidentical HSCT. Three donor–recipient pairs were sex-mismatched (two male patientes with female donors). Three patients had major ABO incompatibility. Cytomegalovirus serology was positive in three donor–recipient pairs and negative in one. Median infused CD34+ cell dose was 5.51 × 10⁶/kg (range 4.09–8.00). All patients achieved neutrophil engraftment with a median of 18 days (range 15–28). Toxicities were manageable, including grade II mucositis (n=4), mild sinusoidal obstruction syndrome (n=1), suspected engraftment syndrome (n=1) and grade II cytokine release syndrome (n=1). One patient experienced poor graft function associated with parvovirus infection, which improved with supportive care. No acute GvHD of grade II or higher occurred. One patient developed mild chronic hepatic GvHD, controlled with immunosuppression. Full donor chimerism was maintained in all patientes at last follow-up (median 448 days, range 58–532). Importantly, no transplant-related mortality was observed. Conclusions: Our data support the feasibility and safety of haploidentical HSCT using busulfan, fludarabine, and cyclophosphamide RIC combined with PTCy and PBSC in adult SCD patients. This approach yielded sustained full donor chimerism with low incidence of severe GvHD and graft failure, offering a practical alternative in resource-limited settings where thiotepa is not easily available. Considering the ongoing challenges in donor availability and conditioning toxicity, further studies with larger cohorts and longer follow-up are warranted to confirm the durability of engraftment, late effects, and overall survival benefits. Tailoring conditioning regimens to the realities of diverse healthcare environments remains critical to broadening access to curative therapies for hemoglobinopathies globally.