A Route Dependent Bioavailability in Active Metabolite Suggested in Riboflavin-5'-Phosphate Pharmacokinetics

The pharmacokinetics of riboflavin-5'-phosphate (FMN) and the metabolite riboflavin (FR) in the rat was studied with measurement of the plasma levels after the injection into femoral and portal vein of the FMN dose of 1 and 10 mol. Area under the curve (AUC) of FR plasma level vs. time after FMN injection into portal vein was found to be larger than the case of femoral vein. This route dependency of AUC in active metabolite was successfully explained from viewpoint of the compartment theory of linear pharmacokinetics. The AUC of FR was also varied by the dose of FMN. This dose dependency was analyzed to be the decrease in total clearance of FR with increasing FMN dose. The model dependent rate constants in FMN and FR pharmacokinetics did not vary with the dose and route but the distribution volume of FR did. The per cent of FR excreted in bile and urine in 24 hours after injection of FMN did not vary with the dose and route of FMN injection.