Purpose: An alternative strategy for the treatment of intractable seizures may be to administer anticonvulsants directly into the brain near the site of a seizure focus using controlled-release polymers. We describe the pharmacokinetics of a phenytoin-ethylene-vinyl acetate (EVAc) controlled-release polymer and report the reduction of seizures in a cobalt-induced rat model of epilepsy with the intracerebral delivery of phenytoin using surgically implanted polymers.
Methods: In the pharmacokinetics study, the drug release rate of 50%-loaded phenytoin-EVAc polymers (n=3) was determined in vitro over 15 weeks initially and then several months later (over a 2-week period after 1 year of in vivo release). In the efficacy study, 85 rats underwent implantation of skull-mounted cortical electrodes for electrocorticography (ECoG) and then underwent application of cobalt chloride to the cerebral cortex for the induction of seizures. Rats in the treatment group (n=9) underwent surgical implantation of phenytoin-EVAc polymers and rats in the control group (n=10) underwent implantation of empty EVAc polymers. In the morbidity study, the potential histologic pathology of the intracerebral delivery of increasing doses of phenytoin from the polymer (10, 20, 30, and 50% loading) was assessed.
Results: Phenytoin was released in vitro from EVAc polymers in a controlled fashion with an initial release of 0.20% of the total loaded dose per week and a continued release of 0.70% of the total loaded dose per week after 365 days of implantation in the brain. The intracerebral controlled-release of phenytoin resulted in a statistically significant reduction in seizure activity in the treatment group as evidenced by lower Racine scores. The four groups of rats (n=5 per group) that underwent intracerebral implantation of 10, 20, 30, or 50%-loaded phenytoin-EVAc polymers displayed expected average weight gain and normal behavior over 365 days. One rat in the 50% group, however, died 354 days after polymer implantation for undetermined reasons.
Conclusions: The intracerebral delivery of phenytoin using an EVAc polymer, which will release this drug for a calculated period of 3.5 years, resulted in a significant reduction in seizures in a rat model of cobalt-induced epilepsy by both behavioral and ECoG criteria. In rats, the long-term interstitial delivery of phenytoin in the brain was not associated with any deleterious effects.