Clinically significant drug interactions with new immunosuppressive agents.

Tacrolimus (FK506), mycophenolate mofetil, sirolimus (rapamycin), gusperimus, and monoclonal antibody preparations are new immunosuppressive agents, some of which are already approved for clinical use, while others are currently undergoing clinical trials. The present article provides an overview of adverse drug interactions between these immunosuppressants and other drugs which may be used concomitantly. Preliminary data suggest that a pharmacodynamic interaction can occur between tacrolimus and nonsteroidal anti-inflammatory drugs, associated with an increased risk of nephrotoxicity. Erythromycin, clarithromycin, clotrimazole, fluconazole, ketoconazole, and danazol have been shown to increase tacrolimus blood concentrations, while rifampicin (rifampicin) was found to decrease tacrolimus blood concentrations. Evidence from experimental studies suggest that several other drugs also known to affect cytochrome P450 activity may have significant effects on the pharmacokinetics of tacrolimus. On the other hand, tacrolimus itself may inhibit the metabolism of coadministered drugs. This interaction may be attributed to the enhanced renal impairment which has been observed in patients treated with tacrolimus and cyclosporin. The bioavailability of mycophenolic acid, the active metabolite of mycophenolate mofetil, has been reported to be reduced by aluminium/magnesium hydroxide-containing antacids and cholestyramine. Mycophenolic acid, sirolimus and gusperimus may impair bone marrow function and this adverse effect may be enhanced by concomitant administration of other myelosuppressive drugs. There is some evidence that coadministered sirolimus and cyclosporin cause an increase in each other's blood concentrations. An increased risk of central nervous system adverse effects has been described following the combined use of indomethacin and the monoclonal antibody muromonab CD3 (OKT3).