Preclinical characteristics of gemcitabine.

Gemcitabine (2',2'-difluorodeoxycytidine, dFdC) is a nucleoside analogue of deoxycytidine in which two fluorine atoms have been inserted into the deoxyribofuranosyl ring. Once inside the cell gemcitabine is rapidly phosphorylated by deoxycytidine kinase, the rate-limiting enzyme for the formation of the active metabolites gemcitabine diphosphate (dFdCDP) and gemcitabine triphosphate (dFdCTP). Gemcitabine diphosphate inhibits ribonucleotide reductase, which is responsible for producing the deoxynucleotides required for DNA synthesis and repair. The subsequent decrease in cellular deoxynucleotides (particularly dCTP) favours gemcitabine triphosphate in its competition with dCTP for incorporation into DNA. Reduction in cellular dCTP is an important self-potentiating mechanism resulting in increased gemcitabine nucleotide incorporation into DNA. Other self-potentiating mechanisms of gemcitabine include increased formation of active gemcitabine di- and triphosphates, and decreased elimination of gemcitabine nucleotides. After gemcitabine nucleotide is incorporated on the end of the elongating DNA strand, one more deoxynucleotide is added, and thereafter the DNA polymerases are unable to proceed. This action, termed "masked chain termination", appears to lock the drug into DNA because proof-reading exonucleases are unable to remove gemcitabine nucleotide from this penultimate position. Incorporation of gemcitabine triphosphate into DNA is strongly correlated with the inhibition of further DNA synthesis. Compared with ara-C, gemcitabine serves as a better transport substrate, is phosphorylated more efficiently, and is eliminated more slowly. These differences, together with self-potentiation, masked chain termination and the inhibition of ribonucleotide reductase, which are not seen with ara-C, may explain why gemcitabine is, and ara-C is not, active in solid tumours. This unique combination of metabolic properties and mechanistic characteristics suggests that gemcitabine is likely to be synergistic with other drugs that damage DNA, and also with other modalities such as radiation.