Effects of orally administered antibiotics on the bioavailability of amlodipine

Background: Amlodipine is a representative calcium channel blocker that is frequently prescribed for the treatment of hypertension. In this study, the possibility of drug–drug interactions between amlodipine and coadministered antibiotics (ampicillin) was investigated in rats; thus, changes in the metabolic activities of gut microflora and the consequent pharmacokinetic pattern of amlodipine following ampicillin treatment were characterized. Methods and results: In human and rat fecalase incubation samples, amlodipine was metabolized to yield a major pyridine metabolite. The remaining amlodipine decreased and the formation of pyridine metabolite increased with incubation time, indicating the involvement of gut microbiota in the metabolism of amlodipine. Pharmacokinetic analyses showed that systemic exposure of amlodipine was significantly elevated in antibiotic-treated rats compared with controls. Conclusion: These results showed that antibiotic intake might increase the bioavailability of amlodipine by suppressing gut microbial metabolic activities, which could be followed by changes in therapeutic potency. Therefore, coadministration of amlodipine with antibiotics requires caution and clinical monitoring.