Purpose: Animal models in which seizures are elicited by chemical or electrical means are commonly used for identification and preclinical testing of novel antiepileptic drugs (AEDs). Such models have been successful in discovering all the new AEDs. However, despite the high efficacy of AEDs against elicited seizures in rodent models, a significant proportion of epilepsy patients with spontaneous recurrent seizures is resistant to these drugs. It is not known whether drug testing in rodent models with spontaneous recurrent seizures would yield a more predictive result with respect to AED efficacy in the clinic. This led us to test one of the novel AEDs, levetiracetam (LEV), in a rat model of temporal lobe epilepsy (TLE) with spontaneous recurrent seizures.
Methods: Wistar rats were subjected to pilocarpine-induced status epilepticus and recorded for spontaneous recurrent seizures in the months after pilocarpine treatment. A group of rats with frequent spontaneous seizures was used for the drug trial with LEV. The experimental protocol for drug testing in these rats was as follows. For 2 weeks, rats received subcutaneous implantation of osmotic minipumps filled with saline (predrug control period), followed by a 2-week period with implantation of LEV-filled minipumps (drug period), after which pumps were replaced by drug-free pumps for 2 weeks (postdrug control period). The LEV concentration in the pumps during the drug period was adjusted to give daily doses resulting in the maximal plasma concentration range determined previously in patients with TLE during prolonged treatment with LEV. During the 6 weeks of the experiment in epileptic rats, seizures were recorded by video monitoring.
Results: Average seizure frequency during the pre- and postdrug control period in a group of eight epileptic rats was 21 and 25 seizures. This was reduced to an average seizure frequency of 8 seizures during the 2 weeks of treatment with LEV. However, the individual response of rats to LEV varied markedly from complete seizure control to no effect at all, although plasma drug levels were within the therapeutic range in all rats. When seizure frequency was separately calculated for the first and second week of treatment, the significant anticonvulsant effect determined in the first week was partially diminished in the second week, suggesting that tolerance may have developed in some of the rats.
Conclusions: The data demonstrate that interesting results can be obtained by drug testing in epileptic rats, giving a more realistic prediction of clinical efficacy than results from drug testing in animal models with elicited seizures. Thus, although drug trials in rats with spontaneous recurrent seizures are laborious and time-consuming, such trials should be added to the preclinical characterization of novel AEDs.