Influence of glutathione S-transferase gene polymorphisms on busulfan pharmacokinetics and outcome of hematopoietic stem-cell transplantation in thalassemia pediatric patients

Abstract

Hematopoietic stem-cell transplantation (HSCT) is currently the only curative therapeutic option for the treatment of thalassemia. In spite of the high cure rate, HSCT can lead to life-threatening adverse events in some patients. Busulfan (Bu) is a key component of the conditioning regimen prior to HSCT. Inter-individual differences in Bu pharmacokinetics (PK) are hypothesized to influence Bu efficacy and toxicity. Since Bu is mainly metabolized by glutathione S-transferase (GST), we investigated the relationship of GSTA1 and GSTM1 genotypes with first-dose PK and HSCT outcomes in 44 children with thalassemia intermedia and thalassemia major. All children received a myeloablative conditioning regimen with IV Bu. Association analysis revealed a relationship between GSTA169C>T (or haplotype *A/*B) and first Bu dose PK that was dependent on sex and Pesaro risk classification (PRC). Among female patients and patients with PRC I-II, homozygous individuals for the GSTA1T-69 allele defining haplotype *B, had higher Bu exposure and lower clearance (P⩽0.01). Association with HSCT outcomes showed that patients with the GSTM1 null genotypes had higher occurrence of regimen-related toxicity (P=0.01). These results suggest that GST genotypes could be useful to tailor the first Bu dose accordingly to improve HSCT outcome.

Figure 1

Influence of GSTA1 C-69 T on the first dose of Bu pharmacokinetics. The pharmacokinetic parameters of Bu in relation to CC, CT and TT genotypes in all studied patients. Respective mean±SD values adjusted for Bu dose (mg/kg) are for logCss (steady state concentration): 2.9±0.08, 2.9±0.1 and 3.0±0.3, and for logAUC (area under the curve): 5.4±0.09, 5.4±0.1 and 5.5±0.3. The number of individuals represented for each genotype group as well as P-values for the difference across groups is given on each plot.

Figure 2

Influence of GSTA1C-69 T genotypes on Bu pharmacokinetics according to sex and Pesaro risk classification. (a) The pharmacokinetic parameters of Bu in female patients with CC, CT and TT genotypes. Respective mean±SD values adjusted for Bu dose (mg/kg) are for logCmax (maximum plasma concentration): 3.0±0.1, 3.1±0.2 and 3.3±0.05; for logCss: 2.8±0.07, 2.9±0.1 and 3.2±0.08; for logAUC: 5.4±0.08, 5.5±0.1 and 5.7±0.1; and for CL per kg (clearance): 4.3±0.9, 3.9±1.4 and 2.0±0.5. (b)The pharmacokinetic parameters of Bu in patients assigned to Pesaro class I–II with CC, CT and TT genotypes. Respective mean±SD values adjusted for Bu dose (mg/kg) are for logCmax: 3.0±0.1, 3.1±0.09 and 3.3±0.05; for logCss: 2.9±0.09, 2.9±0.1 and 3.2±0.1; for logAUC: 5.4±0.09, 5.4±0.1 and 5.7±0.1; and for CL per kg: 4.4±0.9, 4.1±1.3 and 2.0±0.6.

Figure 3

Correlation between dose adjustment and GSTA1-69 genotypes. Dose adjustment presented as ratio of adjusted vs unadjusted Bu dose in relation to CC, CT and TT genotypes in all studied patients. Respective mean±SD values are CC=1.21±0.29, CT=1.08±0.05 and TT=0.83±0.15. The number of individuals represented for each genotype group as well as P-values for the difference across groups is given on each plot.

Figure 4

Association of GSTM1 null genotype with (a) regimen-related toxicity (RRT) and (b) aGvHD. The number of patients in each curve with and without GSTM1 null genotype and number of individuals with (a) RRT or (b) aGvHD (given in the parenthesis), as well as the P-value, estimated by log-rank test for the cumulative RRT and aGvHD incidence between the genotype groups, are indicated on each plot.

Figure 5

Correlation of dose adjustment in (a) and Css in (b) with aGvHD I–IV or aGvHD II–IV and graft failure (a). Presence and absence of a given complication is denoted by + and – sign, respectively. The dose adjustment is depicted by the ratio of adjusted vs unadjusted dose. Mean±SD values for dose adjustment are shown as horizontal lines across each group and for aGvHD I–IV are 1.02±0.29 (−) and 1.24±0.03 (+); for aGvHD II–IV are 1.04±0.26 (−) and 1.49±0.50 (+); and for graft failure are 1.13±0.29 (−) and 0.88±0.21 (+). (b) Css is presented as categorical variable with Css <600 ng/mL, from 600 to 900 ng/mL and >900 ng/mL. The number of individuals represented for each genotype group as well as P-values for the difference across groups is given on each plot.