Pharmacogenomics of Gemcitabine Metabolism: Functional Analysis of Genetic Variants in Cytidine Deaminase and Deoxycytidine Kinase

Gemcitabine (dFdC, 2′,2′-difluorodeoxycytidine) is metabolized by cytidine deaminase (CDA) and deoxycytidine kinase (DCK), but the contribution of genetic variation in these enzymes to the variability in systemic exposure and response observed in cancer patients is unclear. Wild-type enzymes and variants of CDA (Lys27Gln and Ala70Thr) and DCK (Ile24Val, Ala119Gly, and Pro122Ser) were expressed in and purified from Escherichia coli , and enzyme kinetic parameters were estimated for cytarabine (Ara-C), dFdC, and its metabolite 2′,2′-difluorodeoxyuridine (dFdU) as substrates. All three CDA proteins showed similar K m and V max for Ara-C and dFdC deamination, except for CDA70Thr, which had a 2.5-fold lower K m and 6-fold lower V max for Ara-C deamination. All four DCK proteins yielded comparable metabolic activity for Ara-C and dFdC monophosphorylation, except for DCK24Val, which demonstrated an approximately 2-fold increase ( P K m ( P