Bioavailability and dissolution of proprietary and generic formulations of phenytoin

A comparative study of the bioavailability of seven formulations of phenytoin was carried out on 17 patients with epilepsy who were taking phenytoin regularly as part of their drug therapy. Three patients withdrew for personal reasons. No significant differences were found between Epanutin capsules and other generic formulations. However significant differences were noted between the generic products. Phenytoin BP tablets manufactured by Regent Laboratories (now withdrawn) had a relative bioavailability of only 76% compared with tablets manufactured by A H Cox and Company. In vitro dissolution tests requirements were met by all formulations of generic 100 mg tablets, and it was concluded that in vitro dissolution tests are not reliable indicators of biological equivalence. Significantly higher plasma levels were found with Epanutin Infatabs, but this was accounted for by their higher content of phenytoin, which is present in the acid form rather than the sodium salt.