Intracellular Quantification and Pharmacokinetics of Pyrimidine Analogues used in Oncology

Pyrimidine analogues form an important class of drugs used in oncology. Before pyrimidine analogues can exert their effect, they have to enter the tumour cell. In the cell, they have to undergo several conversion steps to become available as nucleotides for incorporation into the DNA or RNA. This thesis describes the development of two bio-analytical assays, using the latest analytical techniques, which make it possible for the first time to quantify the intracellular pharmacologically active nucleotides of two drugs within this class, azacitidine and 5-fluorouracil. The nucleotides were quantified in white blood cells. These represent the intracellular activation-machinery and can be easily obtained at multiple time-points after drug administration, unlike solid tumour cells. The developed bio-analytical assays were deployed to study the intracellular pharmacokinetics of the nucleotides involved in patients treated with capecitabine and gemcitabine, respectively. When patients were treated with an identical capecitabine or gemcitabine dose, the intracellular concentrations of the active nucleotides differed considerably. During 14 days of treatment with capecitabine, one of the nucleotides responsible for the effect, 5-fluorouridine triphosphate, was found to accumulate in white blood cells. Gemcitabine is rapidly degraded to 2',2'-difluorodeoxyuridine (dFdU). This thesis demonstrated that the dFdU nucleotide concentrations in white blood cells of patients treated with gemcitabine were very low. This suggests that dFdU has a limited role in the cytotoxic effect of gemcitabine. By gaining more insight into the intracellular nucleotide concentrations in patients treated with pyrimidine analogues, we hope to improve treatment with these agents in the future.