Intracellular pharmacokinetics of gemcitabine, its deaminated metabolite 2',2'-difluorodeoxyuridine and their nucleotides

Abstract

AIMS: Gemcitabine (2',2'-difluoro-2'-deoxycytidine, dFdC) is a prodrug that has to be phosphorylated within the tumour cell to become active. Intracellularly formed gemcitabine diphosphate (dFdCDP) and triphosphate (dFdCTP) are held responsible for the antineoplastic effects. However, a major part of gemcitabine is converted into 2',2'-difluoro-2'-deoxyuridine (dFdU) by deamination. In the cell, dFdU can also be phosphorylated to its monophosphate (dFdUMP), diphosphate (dFdUDP) and triphosphate (dFdUTP). In vitro data suggest that these dFdU nucleotides might also contribute to the antitumor effects. However, little is known about their intracellular pharmacokinetics (PK). Therefore, objective of this study was to gain insight into the intracellular PK of all dFdC and dFdU nucleotides formed during gemcitabine treatment. METHODS: Peripheral blood mononuclear cell (PBMC) samples were collected from 38 patients receiving gemcitabine, at multiple time-points after infusion. Gemcitabine, dFdU and their nucleotides were quantified in PBMCs. In addition, gemcitabine and dFdU plasma concentrations were monitored. The individual PK parameters in plasma and in PBMCs were determined. RESULTS: Both in plasma and in PBMCs, dFdU was present in higher concentrations than gemcitabine (mean intracellular AUC0-24h1650 vs. 95 μM*h). However, the dFdUMP, dFdUDP and dFdUTP concentrations in PBMCs were much lower than the dFdCDP and dFdCTP concentrations. The mean AUC0-24hfor dFdUTP was 312 μM*h vs. 2640 μM*h for dFdCTP. CONCLUSIONS: This study provides the first complete picture of all nucleotides that are intracellularly formed during gemcitabine treatment. Low intracellular dFdU nucleotide concentrations were found, which questions the relevance of these nucleotides for the cytotoxic effects of gemcitabine.