Younger children with nonmalignant disease have increased incidence of mixed myeloid chimerism after allogeneic hematopoietic cell transplantation with busulfan-based conditioning.

Successful allogeneic hematopoietic cell transplantation (HCT) for genetic nonmalignant diseases (NMD) is dependent upon elimination of host hematopoietic stem cells (HSCs) and replacement with donor HSCs in stable numbers sufficient to correct the underlying disease. Donor myeloid chimerism (DMC) in peripheral blood (PB) is a surrogate marker for bone marrow HSC engraftment due to the rapid turnover of PB myeloid cells. Busulfan is commonly used during conditioning for patients with NMD, though its optimal exposure to avoid inadequate DMC is not fully known. Younger patients with NMD have more mixed chimerism compared to older patients when receiving melphalan-based conditioning, possibly due to increased marrow reserve; we hypothesized that a similar phenomenon would occur with busulfan-based conditioning. We performed a retrospective analysis of 105 consecutive patients with NMDs (median age of 3.9 years) undergoing first allogeneic HCT with myeloablative (cAUC ≥55 mg*hr/L) busulfan-based conditioning from 2007 to 2023 and evaluated risk factors for the development of mixed (<95%) and minimal (≤20%) DMC. Receiver operating curve analysis demonstrated that age <2.9 years was the cut-off associated with mixed DMC. The cohort was therefore divided into two age groups: <3 years (43.8% of patients) and ≥3 years; there was no difference in busulfan exposure between the groups. The 3-year cumulative incidence of developing mixed DMC was 20.3% (95% CI, 12.1% to 28.5%). Age was associated with mixed DMC by 3 years post-HCT: 45.2% (95% CI, 29.7% to 60.7%) in those <3 years versus 1.9% (95% CI, 0.1% to 5.4%) in those ≥3 years (P < .001). Busulfan exposure was also associated with mixed DMC: 24.8% (95% CI, 14.8% to 34.8%) in those with cAUC 55 to 74.9 mg*hr/L versus 5% (95% CI, 0.1% to 8.3%) in those with cAUC ≥75 mg*hr/L (P = .03). In patients ≥3 years of age, there was no impact of busulfan exposure on development of mixed DMC. The 3-year cumulative incidence of developing minimal DMC was 5.5% (95% CI, 0.8% to 10.2%). Only young age was significantly associated with minimal DMC by 3 years post-HCT: 12.8% (95% CI, 2.2% to 23.4%) in those <3 years versus 0% (95% CI, 0% to 6.1%) in those ≥3 years (P = .008). There was no impact of age or busulfan exposure on immunologic graft rejection, acute or chronic graft-versus-host-disease, or transplant-related mortality. There was no difference in sinusoidal obstruction syndrome (SOS) incidence based on busulfan exposure: 13.7% (95% CI, 6.3% to 21.1%) for a cAUC of 55 to 74.9 mg*hr/L compared to 20.8% (95% CI, 4.5% to 37.1%) for a cAUC of ≥75 mg*hr/L (P = .33). However, the incidence of SOS was impacted by patient age: 32.9% (95% CI, 19.2% to 46.6%) in patients <3 years compared to 1.7% (95% CI, 0.1% to 5%) in patients ≥3 years (P < .001). Age <3 years at time of HCT is the major risk factor for mixed and minimal DMC in patients with NMD who receive busulfan-based conditioning. Patients ≥3 years of age may not require busulfan exposures above 55 to 60 mg*hr/L to develop full DMC. Conversely, to avoid development of mixed DMC, patients <3 years may benefit from either HCT delay (when feasible) or higher (≥75 mg*hr/L) busulfan exposure, albeit carrying a high-risk for SOS development.