New heterocyclic modifiers of oxidative drug metabolism--I. 6-Substituted-2-aminobenzothiazoles.

A series of 6-substituted-2-aminobenzothiazoles(2-AB) was synthesized and evaluated as in vitro inhibitors of microsomal mixed-function oxidase activity (as aminopyrine N-demethylase) from phenobarbitone-induced rat liver. Using physiochemical parameters and multiple regression analysis a quantitative structure-activity relationship (QSAR) was derived in which 82% of the data variance was accounted for in terms of the hydrophobic character of the inhibitor and the molar refractivity of the 2-AB 6-substituent. In contrast, literature equations derived from earlier studies with heterocyclic systems possessing non-polar substitutents underestimated by up to an order of magnitude the potency of the present compounds. Kinetic studies revealed that 6-n-propoxy-2-AB, one of the more potent compounds, was a pure competitive inhibitor of aminopyrine N-demethylase activity (Ki = 60 microM from Dixon analysis), suggesting that the binding of substrate and inhibitor is mutually exclusive at the cytochrome P-450 active site. Binding studies indicated that most 2-AB derivatives elicited mixed type I/reverse type I optical difference spectra in phenobarbitone-induced microsomes. The overlap of these components resulted in non-linear double reciprocal plots of the spectral titrations and precluded the determination of binding parameters. In contrast, the more potent inhibitors (the 6-propoxy and 6-butoxy derivatives of 2-AB) were type I ligands with quite high affinity for ferric cytochrome P-450. Although no quantitative relationship was apparent between inhibition and spectral binding affinity a good correlation (r = 0.93) was observed between inhibition potency (I50) and the capacity of ten 2-AB derivatives to prevent substrate(aminopyrine) binding to cytochrome P-450. These findings suggest that 2-AB derivatives may inhibit microsomal oxidation via a direct competitive effect on substrate binding to cytochrome P-450. The present study also demonstrates that substitution of heterocyclic systems with hydrophilic groups does not necessarily produce weak inhibitors of mixed-function oxidase activity, and that extrapolation of existing QSAR equations to new inhibitor series must be interpreted with caution.