Antiseizure potency and neurotoxicity of the enantiomers of fenfluramine and norfenfluramine in rats and correlations with their concentrations in plasma and brain

Abstract Objective Based on the evidence that fenfluramine‐induced anorexia, weight loss, and cardiovascular toxicity are primarily mediated by the d ‐enantiomers of fenfluramine and its metabolite norfenfluramine, we investigated pharmacokinetic/pharmacodynamic correlations for the active enantiomers of fenfluramine and norfenfluramine in the rat maximal electroshock seizure (MES) test. This is part of a series of studies aimed at optimizing selection of an enantiomerically pure compound for development as a safer alternative to racemic fenfluramine. Methods Median effective concentrations (EC 50 ) in the MES model and median neurotoxic concentrations (TC 50 ) of each enantiomer in plasma and brain were estimated after intraperitoneal administration of d ‐fenfluramine, l ‐fenfluramine, and l ‐norfenfluramine as well as racemates, and compared with the respective median effective (ED 50 ) and neurotoxic (TD 50 ) doses. A newly developed enantioselective assay allowed differentiation of concentrations of each enantiomer after administration of racemates. Protective indexes (PIs) were estimated based on dose (TD 50 /ED 50 ) and concentration (TC 50 /EC 50 ) in plasma and brain. Results Based on ED 50 , all enantiomers tested had comparable potency, but l ‐norfenfluramine had a lower PI than the other enantiomers. Based on EC 50 in plasma and brain, l ‐norfenfluramine and d ‐fenfluramine had the highest potency, but l ‐norfenfluramine was consistently more toxic than the other enantiomers. The PIs of d ‐fenfluramine, l ‐fenfluramine, and l ‐norfenfluramine were 2.7, 2.4, and 1.2, respectively, based on plasma concentrations, and 4.1, 2.8, and 1.2, respectively, based on brain concentrations. Potency estimates for each enantiomer after dosing with racemic fenfluramine and norfenfluramine were consistently lower than after dosing with the pure enantiomers, indicating that both enantiomers contributed to antiseizure activity. Significance Based on these results, l ‐fenfluramine has more favorable properties than l ‐norfenfluramine for potential development as an enantiomerically pure medication. Although our findings are consistent with those previously reported for the MES test in mice and the scn1Lab −/− mutant zebrafish, they differ from those reported for the audiogenic seizures mouse model. Further studies, preferably in disease‐specific models, would be desirable.