Metabolic profiling of clobazam, a 1,5-benzodiazepine, in rats

The metabolism of the 1,5-benzodiazepine clobazam (CLBZ) was investigated in the rat and in vitro by GC/MS using stable isotope techniques. Coadministration of CLBZ and pentadeuteriophenyl CLBZ to rats facilitated the identification of 4'-hydroxy CLBZ 7,4'-hydroxy N-desmethylclobazam (4'-hydroxy DMC) 5, 3',4'-dihydroxy CLBZ 13, 4'-hydroxy-3'-methoxy CLBZ 14, 3'-hydroxy-4'-methoxy CLBZ 15, and 4'-hydroxy-3'-methoxy DMC 16 in bile as both glucuronide and sulfate conjugates. Metabolites 7, 13, and 14 were present in urine as sulfate conjugates. 4'-Hydroxy CLBZ and 4'-hydroxy-3'-methoxy CLBZ were the major conjugated metabolites in bile and urine, respectively. An unusual in vivo disposition of CLBZ to the O-methyl catechols was discovered. In bile, the para O-methyl catechol 15 constituted 2% of the O-methyl catechols as a glucuronide conjugate, in contrast to constituting 30% (of the O-methyl catechols) as a sulfate. This marks an unprecedented observation of a different catechol O-methyl isomer ratio within the same biological fluid for different conjugate pools. The isotope effect associated with the microsomal N-demethylation of trideuteriomethyl CLBZ was determined. The values of kH/kD were calculated at 5.07 +/- 0.37 (N = 3) and 3.88 +/- 0.23 (N = 4) for control and induced microsomes, respectively.