Pharmacokinetic Studies of [¹⁴C] Urea (3); Feto-placental Transfer and Excretion into Milk after Single Oral Administration of [¹⁴C] Urea in Rats

Abstract

Feto-placental transfer and excretion into milk were investigated in pregnant rats under fasted conditions and lactating rats under non-fasted conditions after a single oral administration of [¹⁴C]urea at a dose of 2 mg/kg.
1. The radioactivity levels in the placenta and fetus were about 1/2 of the maternal plasma concentration and those in the maternal tissues, excluding the stomach, kidney and urinary bladder, were similar to, or lower than the maternal plasma concentration. The observation of whole body autoradiogram at 24 hr after administration showed that the radioactivity had decreased rapidly from the fetus.
2. At 30 min after administration, the radioactivity levels in the plasma and milk reached C_max, and the ratio of the milk to the plasma concentration was 0.8. The major component in the milk and plasma was the unchanged drug. After C_max, the plasma concentration decreased monophasically with t_1/2α of 1.6 hr and the milk concentration decreased biphasically with t_1/2α of 1.8 hr and t_1/2β of 8.2 hr.