Stereoselective and regioselective hydroxylation of warfarin and selective O-dealkylation of phenoxazone ethers in human placenta.

The oxidative metabolism of warfarin and a series of phenoxazone ethers was studied in two groups of human placentas which exhibited high or low levels of aryl hydrocarbon hydroxylase (AHH). Warfarin metabolism was stereoselective (mean R/S = 2.48) for the R-enantiomer and regioselective for the 6- and 8- positions in the high AHH group whereas warfarin metabolism in the low AHH group displayed no significant overall stereoselectivity (mean R/S = 1.24) and was regioselective for the 7- position. The high AHH group metabolized the methyl, ethyl, propyl and butyl ethers of phenoxazone rapidly, while the low AHH group catalyzed their biotransformation at very low or negligible rates. Neither group detectably metabolized phenoxazone or pentyloxyphenoxazone whereas both groups metabolized benzyloxyphenoxazone at low but similar rates. Rates of warfarin R-6 and R-8 hydroxylation were highly correlated with metabolism of benzo(alpha)pyrene (r = 0.99) and the C1-C4 phenoxazone ethers (r greater than 0.87), but poorly correlated with metabolism of benzyloxyphenoxazone (r less than 0.50). These data support the use of warfarin and the phenoxazone ethers as sensitive biochemical probes for P-450 isozymes in human extrahepatic tissues. They indicate the presence of a multiplicity of xenobiotic metabolizing P-450's in placental tissue which has not been exposed to inducing agents that elevate AHH.