Stability and comparative metabolism of selected felbamate metabolites and postulated fluorofelbamate metabolites by postmitochondrial suspensions.

Evidence has been presented suggesting that a reactive metabolite, 2-phenylpropenal (ATPAL), may be responsible for the toxicities observed during therapy with the antiepileptic drug felbamate (FBM). Formation of ATPAL from its unstable immediate precursor, 3-carbamoyl-2-phenylpropionaldedhyde (CBMA) requires the loss of the hydrogen atom at position 2 in the propane chain, and it has been postulated that substitution of this atom with fluorine would prevent the formation of ATPAL. On the basis of this hypothesis, 2-fluoro-2-phenyl-1,3-propanediol dicarbamate (F-FBM) was synthesized and is presently undergoing drug development. To test this hypothesis, we compared the metabolism by human liver postmitochondrial suspensions (S9) in vitro of selected FBM and postulated F-FBM metabolites leading to formation of CBMA or 3-carbamoyl-2-fluoro-2-phenyl-propionaldehyde (F-CBMA). All S9 incubations included GSH as a trapping agent for any reactive metabolites formed. Our results indicated that, in phosphate buffer, pH 7.4, at 37 degrees C, the half-life for 4-hydroxy-5-phenyltetrahydro-1,3-oxazin-2-one (CCMF) was 2.8 and 3.6 h in the presence or absence of GSH, respectively; compared to 4-hydroxy-5-fluoro-5-phenyl-tetrahydro-1,3-oxazin-2-one (F-CCMF) which lost only 2.5% or 4.9% over 24 h under the same conditions. When incubated with S9 in the presence of the cofactor, NAD+, 2-phenyl-1,3-propanediol monocarbamate (MCF) was oxidized to CCMF which was further oxidized to 3-carbamoyl-2-phenylpropionic acid (CPPA). 2-Fluoro-2-phenyl-1,3-propanediol monocarbamate (F-MCF) under similar conditions was stable, and no metabolites were observed. When CCMF was incubated with S9 in the presence of NAD+ cofactor, oxidation to CPPA and reduction to MCF were observed. In addition, a new atropic acid GSH adduct (ATPA-GSH) was identified by mass spectrometry. When F-CCMF was incubated under the same conditions as CCMF, both reduced and oxidized metabolites, F-MCF and 3-carbamoyl-2-fluoro-2-phenylpropionic acid (F-CPPA), respectively, were formed but at significantly lower rates, and no GSH conjugates were identified. Our results support the hypothesis that F-FBM and F-CCMF are not metabolized by S9 in vitro to the known reactive FBM metabolite, ATPAL.