SLC28A3 genotype and gemcitabine rate of infusion affect dFdCTP metabolite disposition in patients with solid tumours

Background:Gemcitabine is used for the treatment of several solid tumours and exhibits high inter-individual pharmacokinetic variability. In this study, we explore possible predictive covariates on drug and metabolite disposition.Methods:Forty patients were enrolled. Gemcitabine and dFdU concentrations in the plasma and dFdCTP concentrations in peripheral blood mononuclear cell were measured to 72 h post infusion, and pharmacokinetic parameters were estimated by nonlinear mixed-effects modelling. Patient-specific covariates were tested in model development.Results:The pharmacokinetics of gemcitabine was best described by a two-compartment model with body surface area, age and NT5C2 genotype as significant covariates. The pharmacokinetics of dFdU and dFdCTP were adequately described by three-compartment models. Creatinine clearance and cytidine deaminase genotype were significant covariates for dFdU pharmacokinetics. Rate of infusion of <25 mg m-2 min-1 and the presence of homozygous major allele for SLC28A3 (CC genotype) were each associated with an almost two-fold increase in the formation clearance of dFdCTP.Conclusion:Prolonged dFdCTP systemic exposures (≥72 h) were commonly observed. Infusion rate <25 mg m-2 min-1 and carriers for SLC28A3 variant were each associated with about two-fold higher dFdCTP formation clearance. The impacts of these covariates on treatment-related toxicity in more selected patient populations (that is, first-line treatment, single disease state and so on) are not yet clear.