Disposition of Finasteride. (III). Transfer into Fetus and Milk in Rats after Single Oral Administration.

The transfer of finasteride into the fetus and milk was investigated after single oral administration of 14C-finasteride at a dose of 5 mg/1.85 MBq/kg to pregnant and lactating rats, respectively.1. At 2 hr after single oral administration to 12-day and 18-day pregnant rats, the whole-body autoradiograms showed that the radioactivity in the fetus was lower than that in the maternal blood. In the fetal tissues, the radioactivity in adrenal gland and liver was similar to or higher than that in maternal blood. At 48 hr after dosing, the radioactivity was not detected in the fetus.2. Following oral administration to 18-day pregnant rats, the radioactivity in the fetus was lower than that in the maternal plasma at 2 hr after dosing, and accounted for 0.11% of the administered dose. Although the radioactivity in the fetal liver at 2 hr after dosing was 1.5 times higher than that in the maternal plasma, the radioactivity in the other tissues was almost the same or lower. At 48 hr after the dose, the radioactivity in the fetal liver and kidney decreased to less than 1 % of the radioactivity at 2 hr after dosing, while the radioactivity was not detected in the other fetal tissues.3. The radioactivity in the milk reached its peak (2.86 μg eq./ml) at 2 hr after single oral administration to lactating rats. From 2 to 24 hr, the radioactivity in the milk declined similarly to that in plasma with a terminal half-life of 3.8 hr. The ratios of radioactivity concentrations to those in plasma remained virtually unchanged for up to 8 hr after administration.