Pharmacokinetics of gemcitabine and its primary metabolite in dogs after intravenous bolus dosing and its in vitro pharmacodynamics

Gemcitabine is a chemotherapeutic agent used to treat a variety of cancers in humans and dogs. In this study, the plasma pharmacokinetics of gemcitabine and its inactive metabolite, 2',2'-difluorodeoxyuridine (dFdU), were investigated in dogs after intravenous bolus gemcitabine doses of 3, 10, and 30 mg/kg. Furthermore, the intracellular accumulation of the active metabolite gemcitabine triphosphate, as a surrogate pharmacodynamic endpoint, was also determined in vitro in canine melanoma cells. Gemcitabine was characterized by linear kinetics, while dFdU dose proportionality remains unknown. The average gemcitabine clearance was 0.560 L/h.kg and volume of distribution at steady-state of 1.27 L/kg. The average terminal elimination half-life, depending on dose, ranged from 1.75 to 3.23 h. Plasma concentrations of dFdU peaked at approximately 2 h post-dosing. In vitro intracellular gemcitabine triphosphate accumulation was saturated with increasing extracellular gemcitabine concentrations. These data can be used to rationally design gemcitabine dosage regimes for canine oncology patients and as a basis for future investigations on the in vivo intracellular accumulation of gemcitabine triphosphate in dogs.