Deuterium isotope effects on the metabolism and toxicity of phenacetin in hamsters

Both p-[1,1- ²H]ethoxyacetanilide (phenacetin-d ₂) and p-[2,2,2- ²H]ethoxyacetanilide (phenacetin-d ₃) have been prepared, and the effects on the metabolism and toxicity of p-ethoxyacetanilide (phenacetin) in the hamster, brought about by substituting deuterium for hydrogen on the ethyl group, have been examined. The rate of de-ethylation of phenacetin to p-hydroxyacetanilide (acetaminophen) by hamster liver microsomes was decreased by substituting deuterium for hydrogen in the α-methylene position of the p-ethoxy group (kH/k[d ₂]= 1.86), whereas the rate was not significantly decreased by substituting deuterium for hydrogen in the β-methyl group (kH/k[d ₃] = 1.2). After i.p. injections of hepatotoxic doses of phenacetin and its deuterated analogs in the 3-methylcholanthrene-pretreated hamster, measurements of acetaminophen blood levels for 6 hr revealed that significantly more acetaminophen was formed from phenacetin than from either phenacetin-d ₂ (AUC[H]/AUC[d ₂] = 2.71) or phenacetin-d ₃ (AUC[H]/AUC[d ₃] = 1.32). Phenacetin was similarly more hepatotoxic than was phenacetin-d ₂, although no significant difference was observed between the hepatotoxicity caused by phenacetin and phenacetin and phenacetin-d ₃, as measured by the extent of hepatic necrosis, glutathione depletion in the hamster liver, and the covalent binding of radiolabel to hepatic tissue macromolecules. In contrast, methemoglobin formation in the hamster was significantly less over an 8-hr period after the administration of phenacetin than of phenacetin-d ₂ (AUC[H]/AUC[d ₂] = 0.75); no significant difference was found after the administration of phenacetin-d (AUC[H]/AUC[d ₂] = 0.94).