Several metabolic barriers in the route leading from intake or endogenous secretion of estrogens to their final site of action influence their biologic potency. Among them, four are clearly defined: (1) intraluminal and mucosal intestinal metabolism, (2) overall clearance from blood, (3) transcapillary exchange, and (4) target tissue metabolism.
Of these four steps, the first two have been extensively studied, whereas the third remains largely unevaluated. The importance of target tissue metabolism of estrogens is discussed here in some detail.
INTESTINAL ABSORPTION AND METABOLISM OF ESTROGENS
Orally administered estrogens are subjected to metabolism by the intestinal flora and in the intestinal wall before entering the portal circulation or the lymphatic system. Conversions of estradiol to estrone and estrone glucuronide in a jejunal loop of a patient were shown by Diczfalusy et al.1 Substitutions in the estradiol molecule, such as introduction of an ethinyl group in the l7α position and further etherification of the phenolic hydroxyl group, limit metabolism. They also increase lipophilicity and uptake by the lymphatic vessels,2 thus reducing the "first passage" of the ingested estrogen through the liver. Longcope and Williams3 concluded that orally administered ethinyl estradiol and mestranol are completely absorbed, since similar blood levels of radioactive compounds are obtained by oral administration or intravenous injection of the estrogens. The absorption of estradiol is also almost complete, as evaluated from the isotope ratios of urinary metabolites following the simultaneous administration of carbon 14-labeled estradiol orally and tritiated estradiol intravenously.4 However, due to intestinal metabolism, the ratio of estradiol to estrone in circulation resulting from oral administration of estradiol is lower than the ratio resulting from intravenous injection of the hormone.
- Copyright © 1978 by the American Academy of Pediatrics