Pharmacogenetics and Pharmacokinetics in high-dose alkylating chemotherapy

Abstract

High-dose chemotherapy in combination with peripheral blood progenitor cell transplantation has been developed as a possible curative treatment modality in several solid tumours. A frequently used high-dose regimen in the Netherlands is the CTC regimen, which is a 4-day course of cyclophosphamide, thiotepa and carboplatin. High-dose chemotherapy is demanding for the patient. Severe and sometimes life-threatening toxicities may occur after high-dose chemotherapy. Therefore, treatment optimization in order to prevent or reduce toxicity is necessary. Pharmacogenetics focuses on the question to what extent variability in genetic background is responsible for the observed interindividual variability in pharmacokinetics and toxicity and might provide a tool for the a priori identification of patients at risk for severe toxicity. The objectives of this thesis were to identify relations between the genotype of the metabolising enzymes involved in cyclophosphamide, thiotepa and carboplatin metabolism, pharmacokinetics and toxicity in order to identify patients at risk for severe toxicity or under-treatment and, when possible, to develop a strategy for safe dosing of the agents included in the high-dose CTC regimen. Thiotepa is metabolised by cytochrome P450 and glutathione S-transferase enzymes. Polymorphisms of these enzymes may affect elimination of thiotepa and tepa, its main metabolite. Clearance of thiotepa and tepa was shown to be predominantly affected by the GSTP1 C341T polymorphism. Patients homozygous for the variant allele had an increased exposure to thiotepa and tepa of 45% compared to patients with the wild-type genotype. The presently evaluated variant alleles, however, only explained a small part of the substantial interindividual variability in thiotepa and tepa pharmacokinetics. Cyclophosphamide is metabolised by cytochrome P450, glutathione S-transferase and aldehyde dehydrogenase enzymes. The presently evaluated variant alleles in the genes encoding the enzymes important in cyclophosphamide metabolism did not explain the interindividual variability in cyclophosphamide and 4-hydroxycyclophosphamide pharmacokinetics. To obtain more insight into the interindividual variability in toxicity observed after treatment with CTC chemotherapy, relations between polymorphisms in drug metabolising enzymes and toxicity were studied. Sixteen selected polymorphisms in nine genes (CYP2B6, CYP2C9, CYP2C19, CYP3A4, CYP3A5, GSTA1, GSTP1, ALDH1A1 and ALDH3A1) of putative relevance in cyclophosphamide, thiotepa and carboplatin metabolism were assessed. Patients heterozygous for the ALDH3A1*2 and ALDH1A1*2 allele had an increased risk of haemorrhagic cystitis and liver-toxicity, respectively, compared to patients with wild-type alleles when treated with a high-dose chemotherapy combination of cyclophosphamide, thiotepa and carboplatin. The ALDH enzymes are important in the intracellular detoxification of cyclophosphamide. Therefore, although no effect of polymorphisms in these enzymes on the plasmapharmacokinetics of cyclophosphamide could be demonstrated, differences in susceptibility to toxicity might occur due to differences in intracellular detoxification of the active cyclophosphamide metabolites. In conclusion, although the pharmacokinetics of the components of the CTC regimen are not affected by the variability in the presently evaluated genes to a large extent, it is apparent that variability in genes encoding drug metabolising enzymes does affect the occurrence of toxicity. Pharmacogenetic approaches have potential for identifying patients who are at a higher risk of experiencing toxic side-effects in high-dose chemotherapy.