Transfer characteristics of triamterene and its analogs. Central nervous system, placenta, and kidney

Earlier in vivo studies revealed a low concentration of triamterene in the brain of guinea pigs and baboons, and a transfer of the drug from the fetus to the mother. Additional investigations have been performed to characterize further the transport system(s) for triamterene in the central nervous system (CNS), placenta, and kidney. In guinea pigs a very low brain to free plasma concentration ratio (0.1) was achieved 3.5 min after drug administration and was maintained during 180 min of drug infusion. The cerebrospinal fluid (CSF) concentration was similar to the concentration of the drug in the brain. A higher brain to free plasma concentration ratio was gradually reached in dogs studied with nanogram per ml and microgram per ml concentrations of triamterene in CSF. Administration of triamterene to fetal and maternal sheep revealed a placental extraction (E) from fetal plasma to placenta 20 times greater than that from maternal plasma to placenta. The E from fetal plasma to placenta was unaffected by a triamterene concentration in the maternal circulation 10 times that in the fetus. These findings and studies of renal clearance support an active transfer of triamterene by the CNS, placenta, and kidney; the physiologic substrate for these systems is unknown.