Inhibitory effect of stiripentol on clobazam metabolism by cDNA-expressed human CYP

Objective To characterize in vitro the interaction observed in vivo between stiripentol (STP), an anticonvulsant agent that inhibits the activity of several cytochromes P450 (CYP), and clobazam (CLB), a 1,5-benzodiazepine used in association with STP in Severe myoclonic epilepsy in infancy. Methods cDNA expressed CYP3A4 and CYP2C19 (main P450 involved in CLB metabolism, Giraud et al.2004) were used to calculate Ki and IC50 of stiripentol in comparison with ketoconazole (CYP3A4 inhibitor) and omeprazole (CYP2C19 inhibitor). Results STP inhibited N-demethylation of CLB to N-desmethylclobazam (NCLB) mediated by CYP3A4 (non-competitively) and CYP2C19 (competitively) with Ki=1.59± 0.07 and 0.516± 0.065μM and IC50=1.58 μM[CI95%=1.20–2.08] and 3.29 μM[CI95%=1.87–5.79] respectively. STP inhibited also more strongly the hydroxylation of NCLB to 4'-hydroxy-N-desmethylclobazam by CYP2C19 (competitive- interaction with Ki=0.139± 0.025 μM and IC50=0.276 μM[CI95%=0.206–0.371]). The inhibitory effect of STP on CLB demethylation by CYP3A4 was much weaker than that of ketoconazole (ketoconazole IC50=0.023 μM[CI95%=0.016–0.033]) while its effect on NCLB hydroxylation by CYP2C19 was much higher than that of omeprazole (omeprazole IC50=2.99 μM[CI95%=2.11–4.24]). Conclusions The major inhibitory effect of STP on CLB and mostly NCLB biotransformations in vitro is consistent with the changes of CLB and NCLB plasma concentrations observed in vivo in children treated by the association CLB/STP. Clinical Pharmacology & Therapeutics (2005) 77, P49–P49; doi: 10.1016/j.clpt.2004.12.079