Bioavailability and elimination kinetics of the combination furosemide-retard/triamterene

In a pharmacokinetic study on 18 healthy male volunteers the bioavailability and elimination kinetics of furosemide-retard and the combination furosemide-retard/triamterene were investigated and compared with the non-retarded form and another retard form of furosemide. The relative bioavailability of the non-retarded form of furosemide was distinctly reduced by the retardation in the substances investigated. It varied between 42-66% in the plasma and between 37-73% in the urine. In the combination with triamterene the serum concentration time curve of furosemide was only slightly modified but the renal excretion of furosemide was more influenced (36% for furosemide-retard and 25% for furosemide-retard/triamterene). In comparison, the values for the renal excretion of triamterene and OH-TA sulfate were distinctly greater than those obtained after administration of triamterene alone. A renewed increase of the plasma concentration and renal excretion of furosemide were observed between 9 and 10.5 h after administration of furosemide-retard/triamterene. This observation suggests that a further absorption of furosemide occurs as a consequence of the delayed release from distal parts of the small intestine or from the large intestine.