No abstract

The specific cytochrome P450 (P450) isoforms mediating the biotransformations of clobazam (CLB) and those of its major metabolites, N-desmethylclobazam (NCLB) and 4-hydroxyclobazam were identified using cDNA-expressed P450 and P450-specific chemical inhibitors. Among the 13 cDNA-expressed P450 isoforms tested, CLB was mainly demethylated by CYP3A4, CYP2C19, and CYP2B6 and 4-hydroxylated by CYP2C19 and CYP2C18. CYP2C19 and CYP2C18 catalyzed the 4-hydroxylation of NCLB. The kinetics of the major biotransformations were studied: CYP3A4, CYP2C19, and CYP2B6 mediated the formation of NCLB with K m ‫؍‬ 29.0, 31.9, and 289 M, V max ‫؍‬ 6.20, 1.15, and 5.70 nmol/min/nmol P450, and intrinsic clearance (CL int ) ‫؍‬ 214, 36.1, and 19.7 l/min/nmol P450, respectively. NCLB was hydroxylated to 4-hydroxydesmethylclobazam by CYP2C19 with K m ‫؍‬ 5.74 M, V max ‫؍‬ 0.219 nmol/min/ nmol P450, and CL int ‫؍‬ 38.2 l/min/nmol P450 (Hill coefficient ‫؍‬ 1.54). These findings were supported by chemical inhibition studies in human liver microsomes. Indeed, ketoconazole (1 M) inhibited the demethylation of CLB by 70% and omeprazole (10 M) by 19%; omeprazole inhibited the hydroxylation of NCLB by 26%. Twentytwo epileptic patients treated with CLB were genotyped for CYP2C19. The NCLB/CLB plasma metabolic ratio was significantly higher in the subjects carrying one CYP2C19*2 mutated allele than in those carrying the wild-type genotype. CYP3A4 and CYP2C19 are the main P450s involved in clobazam metabolism. Interactions with other drugs metabolized by these P450s can occur; moreover, the CYP2C19 genetic polymorphism could be responsible for interindividual variations of plasma concentrations of N-desmethylclobazam and thus for occurrence of adverse events. Materials and MethodsChemicals and Reagents. Clobazam and N-desmethylclobazam were obtained from Laboratoires Roussel-Uclaf/Sanofi-Synthelabo France (Paris, France). 4Ј-Hydroxyclobazam and 4Ј-hydroxy-N-desmethylclobazam were synthesized and kindly provided by Laboratoires Biocodex (Montrouge, France).

Patients carrying the CYP3A*1B allele may have enhanced docetaxel clearance and may be underexposed, whereas those carrying GSTP1*A/*B and 3435TT genotypes may have excessive hematologic toxicity. Further studies are warranted to determine the usefulness of genotyping before docetaxel treatment.

ABSTRACT:A metabolic interaction between stiripentol (STP), an anticonvulsant agent that inhibits the activity of several cytochromes P450 (P450s), and clobazam (CLB), a 1,5-benzodiazepine, used in association with STP in severe myoclonic epilepsy in infancy was observed in vivo. This interaction was characterized in vitro using cDNA-expressed CYP3A4 and CYP2C19 (main P450 involved in CLB metabolism) to calculate K i and IC 50 Clobazam (CLB) is a 1,5-benzodiazepine and an antiepileptic agent, frequently used as an add-on therapy in patients with refractory epilepsy (Shorvon, 1995). Stiripentol (STP) is an anticonvulsant agent whose clinical efficacy was demonstrated as an add-on treatment to clobazam and valproate in severe myoclonic epilepsy (SMEI) in infancy (Chiron et al., 2000;Thanh et al., 2002). Changes in the plasma concentrations of CLB and its main metabolites were observed when STP was added to the treatment. Indeed, 4Ј-hydroxynorclobazam (OH-NCLB) mean plasma concentrations decreased on average 83%, whereas those of CLB and norclobazam (NCLB) significantly increased on average 173%. It is known that CLB can be first either demethylated to NCLB or hydroxylated to 4Ј-hydroxyclobazam (OH-CLB); then, NCLB and OH-CLB can be transformed to OH-NCLB (Volz et al., 1979). Because STP is known to be an inhibitor of several P450s (Tran et al., 1997), it might be responsible for the inhibition of the metabolism of CLB. Therefore, it seemed relevant to characterize the effect of STP on CLB metabolism. In a previous article, Giraud et al. (2004) identified the main P450 involved in clobazam metabolism. Hydroxylation of CLB into OH-CLB and demethylation of OH-CLB into OH-NCLB were minor pathways. CYP3A4 and CYP2C19 were found to be the major P450s involved in CLB demethylation, whereas the CYP2C19 was the major P450 involved in the NCLB hydroxylation pathway. The present study provides in vivo data on stiripentol interaction with clobazam and in vitro characterization of the inhibitory effects of STP on CLB metabolism pathways mediated by CYP3A4 and CYP2C19 in comparison with specific inhibitors (ketoconazole and omeprazole, respectively) are presented. Materials and MethodsIn Vivo Study. The detailed procedure of the study was described in a previous article (Chiron et al., 2000). Briefly, the epileptic patients participated to a randomized, placebo-controlled, add-on trial designed to test the efficacy of stiripentol in association with clobazam (0.5 mg/kg/day) and valproate (30 mg/kg/day) in SMEI. After a baseline period of 1 month, placebo or stiripentol (50 mg/kg/day) was added to valproate and clobazam during a double blind period of 2 months. Minimum plasma concentrations of CLB and NCLB were measured at steady state during the 3rd week of the baseline period (P1) and the 7th week (P2) of the double blind period. The primary endpoint was the percentage of responders on stiripentol and on placebo, defined as having experienced at least a 50% reduction of clonic (or tonic-clonic) seizure rate during the 2nd mo...