Placental transfer and metabolism of 17 alpha-ethynylestradiol-17 beta and estradiol-17 beta in the rhesus monkey.

The synthetic estrogen component of many oral contraceptives, 17 alpha-ethynylestradiol-17 beta (EE2) and the naturally occurring estrogen, estradiol-17 beta (E2) were studied in four pregnant rhesus monkeys (71% term: 108-121 days gestational age). Under ketamine anesthesia, catheters were implanted in the maternal femoral artery and fetal interplacental artery. After simultaneous i.v. administration of [3H]EE2-[14C]E2 to the maternal animal, serial blood samples were drawn from both mother and fetus. The estrogens and metabolites were identified and quantified by the comigration of radioactivity with reference standards in several high-performance liquid chromatography systems and subsequent selective enzyme hydrolysis of the conjugates. Only estrone (E1), E1 sulfate, EE2 and EE2-3 sulfate were observed in the fetal circulation, whereas the major radiolabeled compounds in the maternal circulation consisted of the above plus E2, E1 glucuronide and EE2-3 glucuronide. In order to determine whether the placenta could convert E2 to its metabolite E1, the placentas of three term rhesus monkeys were perfused in situ via the umbilical artery with 120 ml (15 ml/min) of Hanks' balanced salt solution (pH 7.4) containing [3H]E2. High-performance liquid chromatographic analysis of umbilical vein samples revealed that 96% of the E2 was metabolized to E1. These studies indicate that the placenta can metabolize the potent naturally occurring estrogen E2 to the less potent E1. In contrast, the synthetic estrogen EE2 does not undergo this placental metabolic conversion and thus enters the fetal circulation as the parent compound.