[I. Metabolism by mouse tissue homogenates in vitro (author's transl)].

Mouse liver homogenates are shown to metabolize tritiated fentanyl intensively and in a nearly quantitative manner. The biotransformation activity is located in the microsomal fraction. NADH and NADPH are required as coenzymes for at least two enzymatic reactions leading to the products of oxidative desalkylation, phenylacetic acid and norfentanyl, and to four other metabolites, some of which are considered as products of aromatic hydroxylation, e.g. p-hydroxy(phenethyl)fentanyl. The degree of fentanyl biotransformation and the product distribution are demonstrated to be pH-dependent, the oxidative desalkylation reaction preferring higher pH. There is no information about the pharmacological activities of the unknown metabolites. Kidney and adrenal gland, too, are able to metabolize fentanyl, the oxidative desalkylation being of only minor importance. Other tissues and the serum prove to be inactive. The results of the present paper, in accordance with recently published work, may suggest a greater importance of fentanyl metabolism for the kinetics of effects and side effects than has been recognized before.