Gemcitabine combined with 5FU may enhance the activity of 5-FU in vivo. Gemcitabine is an inhibitor of ribonucleotide reductase, an enzyme needed for synthesis of deoxynucleotides, and 5-FU interferes with dTTP synthesis by inhibition of thymidylate synthase (TS). It is likely that concomitant administration of gemcitabine and 5FU results in increased cytotoxicity by reducing intracellular dTTP thru two different mechanisms, thereby inhibiting DNA replication and repair. Platinum compounds lead to cell death by forming DNA adducts and causing double strand breaks. By inhibiting DNA synthesis and repair, both gemcitabine and 5-FU potentiate the activity of cisplatin. These interactions underlie the clinical synergism that has been observed with platinum/5FU and platinum/gemcitabine combinations.
