PHARMACOKINETICS OF ORALLY AND INTRAVENOUSLY ADMINISTERED RIBOFLAVIN IN HEALTHY HUMANS

The pharmacokinetics and utilization (flavocoenzyme synthesis) of oral ly and intravenously administered riboflavin in healthy humans were as sessed. After the determination of circadian rhythms of riboflavin con centrations in blood plasma and urine of four males and five females ( control period), each of these subjects received three different oral riboflavin doses (20, 40, and 60 mg) and one intravenous bolus injecti on of riboflavin (11.6 mg). Vitamins were administered in a randomized , cross-over design with 2 wk between each administration. Blood plasm a and urine specimens were collected repeatedly over a period of 48 h after each administration. Concentrations of flavocoenzymes and ribofl avin were analyzed in blood plasma; riboflavin was assayed in urine. D uring the control period, a small circadian variation was observed: pl asma concentrations and urinary excretion of riboflavin were low durin g the afternoon (P < 0.05). Pharmacokinetics were calculated using a t wo-compartment open model. The maximal amount of riboflavin that can b e absorbed from a single dose was 27 mg per adult. Half-life of absorp tion was 1.1 h. First-order rate constants describing distribution and elimination of riboflavin were significantly higher after intravenous than after oral administration (P < 0.01). Release of flavocoenzymes into plasma was low compared with the increase of riboflavin concentra tions. 7 alpha-Hydroxyriboflavin was identified in plasma. Clearance d ata indicated that urinary excretion of riboflavin contributes to one- half of the overall removal of riboflavin from plasma. No sex differen ces were observed for any of the pharmacokinetic variables (P > 0.05).