Evidence that coal tar is a mixed inducer of microsomal drug-metabolizing enzymes.

Topical application of coal tar (U.S.P.) to neonatal rats resulted in the induction of hepatic cytosolic glutathione-S-transferase and microsomal epoxide hydrolase and aminopyrine N-demethylase activities. Analogous to the effect of the polychlorinated biphenyl Aroclor 1254, treatment of neonatal rats with coal tar resulted in a one nm shift to the blue region in the wavelength maximum of the hepatic microsomal hemoprotein cytochrome P-450. These results demonstrate that therapeutic coal tar contains both type I and type II inducers of hepatic drug-metabolizing enzymes.