Metabolism and Action of 2′, 2′ -Difluorodeoxycytidine: Self-Potentiation of Cytotoxicity

Abstract

The clinical success of arabinosylcytosine evoked an interest in other deoxycytidine analogues. One such congener is 2′,2′-difluorodeoxycytidine (dFdC., Gemcitabine), which has to pass through a repertoire of steps catalyzed by several cellular enzymes to exert cytotoxicity (1). Once transported into the cell, dFdC is phosphorylated to its active nucleotide metabolites. Several kinases are involved in the intracellular synthesis of dFdC nucleotides, but the first step, the phosphorylation of dFdC to its monophosphate by dCyd kinase, is rate-limiting (1,2). dCyd kinase is therefore the most important enzyme in the activation of dFdC. The activity of this enzyme is tightly regulated by endogenous deoxynucleotide pools, in particular deoxycytidine triphosphate (dCTP) (3). dCTP is of further interest and importance because dFdCTP competes with it for incorporation into the growing DNA strand, a mechanism of dFdC cytotoxicity (4). Cellular dCTP pools are maintained by the de novo pathway, the principal step of which is catalyzed by ribonucleotide reductase. Another enzyme influenced by the cellular dCTP pool is dCMP deaminase, which plays a major role in the elimination of phosphorylated dFdC metabolites (5). The aim of the present investigation was to analyze the role of dFdC metabolites on these key enzymes. A positive influence of dFdC on these enzymes would augment its own cytotoxicity, an action termed self-potentiation (6).