Significant Association Between GSTA1 Genotype and Busulfan Pharmacokinetics in Patients Conditioned for Hematopoietic Stem Cell Transplantation

Abstract

Introduction: Busulfan (BU) is frequently used in high-dose conditioning regimens before hematopoietic stem cell transplantation (HSCT). Due to a narrow therapeutic range and large pharmacokinetic variability, BU dosing is usually guided by therapeutic drug monitoring. Glutathione S-transferases (GST) play an important role in the metabolism of BU, and GST gene variants may explain some of the interindividual variability in pharmacokinetics. The objective of the present study was to evaluate the association between GSTA1, GSTM1, and GSTT1 genotypes and BU pharmacokinetics. Methods: From February 2008 to January 2012, 114 adult patients (age 16-65 yrs) who received oral BU pre-HSCT were enrolled in this study. BU was given every 6 hrs from day -7 to -4 starting with fixed doses (˜1 mg/kg) the first day, followed by dose adjustments to approach a therapeutic target steady-state concentration (Css) of 900 ng/mL. Blood samples were drawn at 0, 30, 60, 90, 120, 180, 240 and 300 min after the 1st, 5th and 9th doses (day 1, 2 and 3, respectively) and BU plasma concentrations were measured by liquid chromatograpy with UV detection. GSTA1 haplotype was determined by melt curve genotyping analyzing the c.-69C>T (rs3957357) variant. Gene copy numbers of GSTM1 and GSTT1 were determined by quantitative real-time PCR using albumin as an internal reference gene. The genotyping assays were run on the LightCycler® 480 instrument. Results: Genotype frequencies were 39.5%, 35.1% and 25.4% for GSTA1C/C (*A/*A), C/T (*A/*B) and T/T (*B/*B), respectively. The proportion of patients carrying 0, 1 and 2 GSTM1 and GSTT1 gene copies were 43.9%, 45.6% and 10.5% and 15.8%, 44.7% and 39.5%, respectively. BU exposure (Css) was significantly different between GSTA1 genotype groups (P≤0.01). No significant associations were observed between GSTM1 and GSTT1 copy numbers, age or gender and BU pharmacokinetics. Stratified according to GSTA1 genotype, *B/*B and *A/*B carrieres demonstrated median 14% and 9% higher (P≤0.01) dose adjusted BU Css on day 1, and 22% and 15% higher (P≤0.04) Css on day 2, compared to *A/*A patients. Clearance/bioavailability (Cl/F) was median 13% and 9% lower (P≤0.02) on day 1 and 22% and 15% (P≤0.04) lower on day 2 among *B/*B and *A/*B vs. *A/*A carrieres, respectively. Furthermore, the *B/*B and *A/*B groups demonstrated BU exposure significantly above the therapeutic target Css of 900 ng/mL on day 1 and 2 (P≤0.001). The initial BU dose was median 1.0 (0.8-1.1) mg/kg in all GSTA1 groups. Following two days of dose adjustments, BU doses the third day of therapy were 14% and 8% lower (P≤0.03) among *B/*B and *A/*B patients compared to *A/*A patients. No significant differences were observed in BU Css or Cl/F between GSTA1 groups on day 3. Conclusion: The data demonstrate an association between GSTA1 genotype and BU Css and Cl/F the first two days of BU conditioning therapy. Genotyping GSTA1 prior to HSCT may allow better prediction of oral BU kinetics, reduce the need for dose adjustments and thereby improve clinical outcomes following high-dose BU conditioning.