Paracetamol, 3-monoalkyl- and 3,5-dialkyl derivatives. Comparison of their microsomal cytochrome P-450 dependent oxidation and toxicity in freshly isolated hepatocytes.

The effects of 3-monoalkyl- and 3,5-dialkyl-substitution on the cytotoxicity of paracetamol (PAR) in rat hepatocytes was studied. PAR is known to be bioactivated by the hepatic microsomal cytochrome P-450 containing a mixed-function oxidase system presumably to N-acetyl-para-benzoquinone imine (NAPQI), a reactive metabolite which upon overdosage of the drug causes depletion of cellular glutathione (GSH) and hepatotoxicity. The four 3-mono- and the four 3,5-di-alkyl-substituted derivatives of PAR investigated in this study (R = CH3, C2H5, C3H7, C4H9) interacted with cytochrome P-450 giving rise to reverse type I spectral changes. Like PAR, all derivatives underwent cytochrome P-450-mediated oxidation to NAPQIs. In contrast to induction by phenobarbital, induction of cytochrome P-450 by 3-methylcholanthrene enhanced the microsomal oxidation of PAR and its derivatives. The NAPQIs formed from PAR and the 3-mono-alkyl derivatives by microsomal oxidation were found to conjugate with GSH and to oxidise GSH to GSSG. The NAPQIs formed from the 3,5-dialkyl-substituted derivatives, however, only oxidized GSH to GSSG. PAR and the 3-monoalkyl derivatives were found to deplete cellular GSH to about the same extent and to be equally toxic in freshly isolated hepatocytes from 3-methylcholanthrene treated rats. In contrast, the 3,5-di-alkyl-substituted derivatives of PAR did not affect the GSH levels and were not toxic in the hepatocytes, even at higher concentrations. It is suggested that the difference between the way of reacting of 3,5-dialkyl-NAPQIs and NAPQIs from PAR and 3-monoalkyl derivatives with thiols of cellular GSH and protein could account for the observed difference between the toxicity of the 3,5-dialkyl- and the 3-monoalkyl-substituted derivatives of PAR.