Effect of probenecid on riboflavin absorption and excretion in man

An oral dose of probenecid given 1 hr. prior to oral administration of riboflavin or riboflavin‐5′‐phosphate (FMN) to normal humans produced a marked decrease in the excretion rate of the vitamin during the early excretion phase but had no significant effect during the later (post‐absorptive) phase. Oral administration of probenecid prior to intramuscular injection of FMN increased the apparent “half‐life” of the initial rapid phase of riboflavin excretion. The urinary recovery of riboflavin following oral or intramuscular administration of the vitamin was usually somewhat decreased by probenecid. These results are consistent with our earlier suggestion that riboflavin is excreted in part by renal tubular secretion. It appears that probenecid also inhibits the specialized transport process responsible for the intestinal absorption of riboflavin in man. The complex kinetics of riboflavin distribution and elimination in man are discussed and additional evidence is presented in support of enterohepatic cycling of the vitamin.