Interactions of serum albumin, valproic acid and carbamazepine with the pharmacokinetics of phenytoin in cancer patients.

Phenytoin dosing is critical in cancer patients as to decreased absorption secondary to chemotherapy-induced gastrointestinal toxicity, increased phenytoin metabolism in the liver secondary to chemotherapy, extreme patient profile that falls outside the predicted pharmacokinetic population, frequent hypoalbuminaemia and polydrug treatment. A retrospective study to assess the variability of free phenytoin and the free fraction of phenytoin, as well as the influence of comedication on these parameters was performed in cancer patients by using a population approach. Two hundred fifty-eight data pairs of total phenytoin and free phenytoin were analysed from 155 cancer patients on stable phenytoin using non-linear mixed-effect modeling (NONMEM). Total and free phenytoin were determined using a fluorescence polarization immunoassay. An extensive model building procedure was subsequently used for covariate testing on the free fraction of phenytoin. Mean total phenytoin concentration was 11.7 mg/l, free phenytoin 1.25 mg/l and phenytoin free fraction 0.107. Free phenytoin was 2 mg/l on 37 occasions (14.3%). Total and free phenytoin were significantly correlated (r(S)=0.827, P