Metabolite complex formation of orphenadrine with cytochrome P450. Involvement of CYP2C11 and CYP3A isozymes.

Expression and inhibition of cytochrome P450 (CYP) isozymes capable of forming an orphenadrine metabolite complex were studied in microsomes of untreated and inducer-treated male and female rats. High levels of complex-forming isozymes were found in microsomes of untreated male as compared to female rats. Treatment of male rats with several P450 inducers did not considerably increase the extent of in vitro complex formation. In female rats, however, phenobarbital or dexamethasone treatments led to pronounced induction. The isozyme specificity of complex formation was investigated by several approaches including: 1. inhibition by orphenadrine of isozyme-specific P450 activities, such as hydroxylation of testosterone, O-dealkylation of pentoxy-and ethoxyresorufin and complex formation with triacetyloleandomycin (TAO), 2. inhibition of orphenadrine complex formation by metyrapone, TAO, and cimetidine, and 3. correlation of complex levels with immunochemically, enzymatically, or spectroscopically determined amounts of P450 isozymes. Our data suggest that CYP2C11, a CYP3A isozyme and an unidentified P450 species are involved in complex formation with orphenadrine, but exclude the involvement of CYP1A1/2 and CYP2B1/2. The capability of CYP2C11 to form a metabolite complex with orphenadrine is strongly suggested for the following reasons: 1. Efficient inhibition of testosterone 2 alpha- and 16 alpha-hydroxylation by complex formation with orphenadrine in microsomes of untreated male rats, 2. high expression of orphenadrine-complexing isozymes in untreated male compared to female rats, 3. specific inhibition of in vitro complex formation by cimetidine, 4. suppression of complex-forming isozymes by 3-methylcholanthrene and beta-naphthoflavone, and 5. concomitant induction of complex-forming isozymes, immunodetectable CYP2C11, and testosterone 2 alpha-hydroxylase by stanozolol. That at least one, but not all, CYP3A isozymes is involved in complex formation is concluded from inhibition experiments with TAO that show that orphenadrine complexation can be significantly inhibited in microsomes of dexamethasone-treated, but not in microsomes of untreated rats. Furthermore, complex formation with TAO is not inhibited by orphenadrine in microsomes of phenobarbital (PB)-treated rats. In PB-treated female rats, a further unidentified complex-forming isozyme can be detected that is not inhibited by complex formation with TAO.