Clinical Pharmacology Unit (UPC)

Impact of GSTA1, GSTM1 and GSTP1 Gene Polymorphisms on Pharmacokinetics of Busulfan IV in Children Receiving a Hematopoietic Stem Cell Transplant

Busulfan is one of the main drugs used in preparatory conditioning regimens for hematopoietic stem cell transplants. Busulfan has a narrow therapeutic index. In sub-therapeutic concentrations, its activity is insufficient and is associated with an elevated risk of transplant rejection and relapse. In higher doses than commonly used, mortal (mainly hepatic) toxicity risk rises significantly. The use of an intravenous formula allows for more precise dosages by eliminating absorption factors, but many patients still require important dose adjustments. Busulfan dosage by high-performance liquid chromatography (HPLC) is carried out on 7 serial plasma samples (following the first drug administration). Necessary adjustments are then made, usually prior to administering the 5th of 16 required doses. Busulfan is metabolized by enzymes, including glutathione S-transferase (GST). These enzymes can have variable activities resulting from polymorphisms or genetic variants. The proposed hypothesis is that functional polymorphisms of the genes coding for GST contribute to the interindividual variability observed in Busulfan administration. A greater understanding of the impact of these polymorphisms on Busulfan pharmacokinetics could improve transplant success, reduce its risks and provide a better use of the drug.

Busulfan dosage by HPLC is done on 7 serial plasma samples for each patient receiving a hematopoietic stem cell transplant. Pharmacokinetic data is obtained by using the WinNonlin (Pharsight) computer application and conducting a noncompartmental analysis. Dosage of tetrahydrothiophenium, a Busulfan metabolite generated by GST, will be determined by gas chromatography and mass spectrometry, then analyzed by ChemStation. Alpha GST blood levels will be measured by ELISA, using HEPKIT alpha (Biotrin International). To determine the role of certain GST polymorphisms and their impact on the pharmacokinetics of Busulfan, we propose to analyze polymorphisms in genes GSTA1, GSTM1 and GSTP1. We will study five polymorphisms of the GSTA1 gene promoter (polymorphisms: T –631 G, A -513 G, G -115 A, C -69 T et G -52), along with the GSTM1 deletion polymorphism and two polymorphisms in the GSTP1 coding region. Genotyping will be done through the specific oligonucleotide hybridization technique (ASO).

By examining the relationship between tested polymorphisms and transplant prognosis, VOD and rejection risks, it may be possible to establish an algorithm that could predict, based on these polymorphisms, the optimal Busulfan IV dose required for pediatric patients receiving a hematopoietic stem cell transplant.