Busulfan (BU) dose adjustment following therapeutic drug monitoring contributes to better outcome of hematopoietic stem cell transplantation (HSCT). Further improvement could be achieved through genotype-guided BU dose adjustments. To investigate this aspect, polymorphism within glutathione S transferase genes were assessed. Particularly, promoter haplotypes of the glutathione S transferase A1 (GSTA1) were evaluated in vitro, with reporter gene assays and clinically, in a pediatric multi-center study (N =138) through association with BU pharmacokinetics (PK) and clinical outcomes. Promoter activity significantly differed between the GSTA1

on behalf of the Pediatric Disease Working Parties of the European Blood and Marrow Transplant Group BU is a key compound of conditioning regimens in children undergoing hematopoietic SCT (HSCT). Inter-individual differences in BU pharmacokinetics (PKs) might affect BU efficacy and toxicity. As BU is mainly metabolized by glutathione S-transferase (GST), we investigated the relationship between GSTA1, GSTM1 and GSTP1 genotypes with first-dose BU PKs, and the relationship with HSCT outcomes in 69 children receiving myeloablative conditioning regimen. GSTM1 null genotype correlated with higher BU exposure and lower clearance in patients older than 4 years (Pp0.04). In accordance with the suggested functional role, GSTA1*A2 haplotype was associated with lower drug levels and higher drug clearance (Pp0.03). Gene-dosage effect was also observed (Pp0.007). GSTA1 haplotypes were associated with HSCT outcomes. Patients with two copies of haplotype *A2 had better event free survival (P ¼ 0.03). In contrast, homozygous individuals for haplotypes *B and *B1 had higher occurrence of veno-occlusive disease (P ¼ 0.009). GSTM1 null individuals older than 4 years had more frequently graft versus host disease (P ¼ 0.03). In conclusion, we showed that GST gene variants influence BU PK and outcomes of HSCT in children. A model for the dosage adjustment with the inclusion of genetic and non-genetic factors should be evaluated in a future prospective validation cohort.

This study demonstrated a significant association between the first-dose pharmacokinetics of Bu and NRM, OS, and EFS. Bu therapeutic drug monitoring provides information that potentially influences outcomes of HSCT in pediatric patients.