Abstracts

Rationale: Efficacy and tolerability of a drug are both significantly influenced by its pharmacokinetics (PK). Thus, extensive preclinical PK assessment of antiseizure drugs is essential in identifying safe and effective novel compounds. Although the Epilepsy Therapy Screening Program has routinely tested antiseizure drug (ASD) prototypes for efficacy in various rodent seizure models, there is limited data to compare efficacy to brain and plasma concentrations of the drug. We used efficacy testing alongside sample collection and bioanalysis of plasma and brain concentrations of carbamazepine (CBZ), clobazam (CLB), valproic acid (VPA), and levetiracetam (LEV) as a proof-of-principle study on how PK can be useful to guide efficacy studies using rodent seizure models.

Methods: We performed selected in vivo efficacy assays of CBZ, CLB, VPA, and LEV followed by bioanalyses of brain and plasma levels of drugs in adult, male CF-1 mice and male Sprague-Dawley rats. We selected assays (6Hz 44 mA, 6Hz 80 V, and maximal electroshock seizure (MES) tests) and drug doses (ED₅₀-low dose and TD₅₀-high dose) that provided a reasonable therapeutic dose range in these assays based on previous studies. Plasma and brain samples were collected between 0.25 and 8 hour time-points after efficacy and tolerability assessments. CLB, CLB metabolite -N-desmethylclobzam (N-CLB), CBZ, CBZ metabolite -carbamazepine epoxide (CBZE), VPA, and LEV concentrations in both plasma and brain matrices were quantified using liquid chromatography-tandem mass spectrometry. The mean concentrations of each analyte at each time point were determined and used to calculate PK parameters in both plasma(p) and brain(b) using non-compartmental analysis in Phoenix WinNonLin.

Results: N-CLB concentrations [area under the concentration-time curve (AUC)_p-22,277 ng*hr/mL] were approximately 10 fold greater than CLB [AUC_p-2263 ng*hr/mL] and the observed efficacy of CLB in MES test corresponded with N-CLB levels in mice. However, N-CLB levels [AUC_p-196 ng*hr/mL] in rats were approximately 20 fold lower than CLB [AUC_p-4059 ng*hr/mL]. CBZE plasma exposures were approximately 70% of CBZ in both mice and rats, contributing to the high and prolonged effect of CBZ. VPA showed a relatively shorter half-life (t_1/2) in both mice and rats, which correlated with a sharp decline in its efficacy.

Conclusions: Our data suggest that species-specific variations in PK profiles of ASDs in various animal models of epilepsy may underlie the differences in observed drug efficacy in these models. This study also illustrates the potential role of active metabolites in ASD efficacy and further sets the stage for therapeutic monitoring of other drugs and their metabolites.

Funding: Please list any funding that was received in support of this abstract.: This project has been funded by Federal funds from the National Institute of Neurological Disorders and Stroke, Epilepsy Therapy Screening Program, National Institutes of Health and Department of Health and Human Services, under Contract No. HHSN271201600048C.