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Studies were conducted to elucidate the processes involved in the biological disposition of diethylstilbestrol (DES). The apparent biological half‐life of [ 14 C] DES decreased with increasing age in immature rats. The half‐life of DES In adult rats (14 min) was reached between the 15th and 25th day after birth.
Comparison of the intestinal absorption of [ 14 C] DES and [ 14 C] DE5 monoglucuronide showed the unconjugated drug to be absorbed much more rapidly. Hydrolysis of the conjugate took place in the lower intestine, and this permitted the drug to undergo enterohepatic cycling.
Placenta/transfer of [ 14 C] DES was studied in pregnant rats at mid and late gestation. The drug and its metabolites were restricted in their movement from mother to fetus, and relatively low fetal levels were found. Only 0.0006% of the maternal dose of DES was found as the unchanged drug in each fetus at 90 min after drug administration. Conjugated metabolites were found in the fetus just prior to birth but not in the 13‐day‐old fetus.
Analysis of plasma and urine after oral administration of both [ 3 H] DES and [ 14 C] DES monoglucuronide to two human subjects showed that the glucuronide was absorbed more slowly than DES. Similar metabolic products were found in the urine after administration of DES or its glucuronide conjugate. This and other Indirect evidence indicated that the conjugate was hydrolyzed to DES in the gastrointestinal tract prior to absorption. The data showed that the biological disposition of DES in humans was not extensively different from that observed in laboratory animals.