Polymorphisms of drug-metabolizing enzymes (GST, CYP2B6 and CYP3A) affect the pharmacokinetics of thiotepa and tepa

Abstract

Aims: Thiotepa is widely used in high-dose chemotherapy. Previous studies have shown relations between exposure and severe organ toxicity. Thiotepa is metabolized by cytochrome P450 and glutathione S-transferase enzymes. Polymorphisms of these enzymes may affect elimination of thiotepa and tepa, its main metabolite. The purpose of this study was to evaluate effects of known allelic variants in CYP2B6, CYP3A4, CYP3A5, GSTA1 and GSTP1 genes on pharmacokinetics of thiotepa and tepa.

Methods: White patients (n = 124) received a high-dose regimen consisting of cyclophosphamide, thiotepa and carboplatin as intravenous infusions. Genomic DNA was analysed using polymerase chain reaction and sequencing. Plasma concentrations of thiotepa and tepa were determined using validated GC and LC-MS/MS methods. Relations between allelic variants and elimination pharmacokinetic parameters were evaluated using nonlinear mixed effects modelling (nonmem).

Results: The polymorphisms CYP2B6 C1459T, CYP3A4*1B, CYP3A5*3, GSTA1 (C-69T, G-52A) and GSTP1 C341T had a significant effect on clearance of thiotepa or tepa. Although significant, most effects were generally not large. Clearance of thiotepa and tepa was predominantly affected by GSTP1 C341T polymorphism, which had a frequency of 9.3%. This polymorphism increased non-inducible thiotepa clearance by 52% [95% confidence interval (CI) 41, 64, P < 0.001] and decreased tepa clearance by 32% (95% CI 29, 35, P < 0.001) in heterozygous patients, which resulted in an increase in combined exposure to thiotepa and tepa of 45% in homozygous patients.

Conclusions: This study indicates that the presently evaluated variant alleles explain only a small part of the substantial interindividual variability in thiotepa and tepa pharmacokinetics. Patients homozygous for the GSTP1 C341T allele may have enhanced exposure to thiotepa and tepa.

Figure 1

Metabolic biotransformation of thiotepa and tepa

Figure 2

Population pharmacokinetic model of thiotepa and tepa. TT centr, central thiotepa compartment; TT per, peripheral thiotepa compartment; T centr, central tepa compartment; T per, peripheral tepa compartment; CL_nonind, non-inducible elimination route of thiotepa; CL_ind, inducible elimination route of thiotepa; CL_tepa, clearance of tepa; CP, cyclophosphamide

Figure 3

(a) Effect of CYP2B6_C1459T on exposure to thiotepa and tepa. (b) Effect of CYP3A4*1B on exposure to thiotepa and tepa. (c) Effect of GSTA1 (C-69T, G-52A) on exposure to thiotepa and tepa. (d) Effect of GSTP1 C341T on exposure to thiotepa and tepa. WT_TT, exposure to thiotepa in wild-type patients; Het_TT, exposure to thiotepa in heterozygous patients; Hom_TT, exposure to thiotepa in homozygous patients; WT_T, exposure to tepa in wild-type patients; Het_T, exposure to tepa in heterozygous patients; Hom_T, exposure to tepa in homozygous patients; WT_com, combined exposure to thiotepa and tepa in wild-type patients; Het_com, combined exposure to thiotepa and tepa in heterozygous patients; Hom_com, combined exposure to thiotepa and tepa in homozygous patients. AUC values of wild-type patients were set at 100% (dashed line)