Drug Interactions between mycophenolate mofetil and nalidixic acid

MONITOR CLOSELY: Antibiotics which affect beta-glucuronidase producing bacteria in the intestine (e.g., aminoglycosides, cephalosporins, fluoroquinolones, and penicillins) may reduce systemic exposure to mycophenolic acid (MPA) products. The exact mechanism is not known; but is thought to be due to interference with enterohepatic recirculation of the active drug, MPA, via alterations in the gastrointestinal flora that are responsible for regenerating MPA from its glucuronide metabolite. One study reviewed 64 kidney transplant patients taking mycophenolate mofetil (MMF) who received either oral ciprofloxacin (500 mg twice daily for 7 days) or amoxicillin plus clavulanic acid (375 mg three times daily for at least 14 days). This study demonstrated approximately 50% reductions in the median trough MPA concentrations from baseline (MMF alone) in 3 days following the start of oral ciprofloxacin or amoxicillin plus clavulanic acid. The reductions in trough MPA concentrations tended to diminish within 14 days of antimicrobial therapy and cease within 3 days of the discontinuation of antibiotics. It is important to note that the trough level may not accurately reflect changes in the overall MPA exposure as the systemic exposure (AUC) was not evaluated in this study. In a study of 11 healthy volunteers who received a single-dose of MMF 1 gram on day 4 of a 5-day course of dual antibiotic therapy with both norfloxacin and metronidazole, the average AUC of MPA was reduced by 33% compared to the administration of MMF alone. However, when MMF was administered with norfloxacin alone or metronidazole alone (as opposed to the combination of MMF with norfloxacin and metronidazole), the reduction in AUC was not statistically significant. In a study of 12 healthy male volunteers, a single dose of MMF 1.5 grams was administered on day 8 of a 10-day course of trimethoprim 160 mg/sulfamethoxazole 800 mg twice daily and no effect on the bioavailability of MPA was observed.

MANAGEMENT: Close clinical and laboratory monitoring for evidence of diminished immunosuppressive effects of mycophenolic acid products is recommended during concomitant therapy and shortly after antibiotic treatment is completed. Advise patients to report any symptoms of transplant rejection such as a decrease in organ function (e.g., reduced urine output for kidney transplant patients, shortness of breath and/or swelling in heart transplant patients, jaundice in liver transplant patients), and/or flu-like symptoms.

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