Risk-benefit assessment of therapies for Mycobacterium avium complex infections

Mycobacterium avium complex (MAC) is an important pathogen that can cause chronic lung disease in immunocompetent patients and disseminated disease in patients with the acquired immunodeficiency syndrome (AIDS). Treatment of MAC with antituberculosis drugs was unsatisfactory, but the introduction of the newer macrolides, clarithromycin and azithromycin, and of rifabutin has greatly improved the outcome of treatment regimens for MAC. However, these agents are also associated with many new treatment-related adverse effects and potential drug-drug interactions. Rifamycins [rifampicin (rifampin) more than rifabutin] induce cytochrome P450 enzymes and accelerate the metabolism of clarithromycin and HIV protease inhibitors. Conversely, clarithromycin inhibits these enzymes, resulting in increased rifabutin toxicity. The net results are treatment regimens that may be extremely difficult to tolerate, especially for elderly or debilitated patients. Clarithromycin and azithromycin must be administered in combination with other agents such as ethambutol to prevent the emergence of macrolide resistance. Unfortunately, not all patients respond to the combination of a macrolide, rifabutin and ethambutol, and many have significant adverse effects (mostly gastrointestinal) with this regimen. For some patients the treatment is worse than the disease. The same 3-drug regimen is also effective therapy for disseminated MAC in AIDS patients, in whom the additional problem of a rifamycin/protease inhibitor interaction may be present. Fortunately, as opposed to pulmonary MAC disease in immunocompetent patients, disseminated MAC disease is a diminishing problem because of effective prophylactic regimens for MAC and improved antiretroviral therapy for HIV. Significant progress has been mode in the treatment of MAC disease with the introduction of the newer macrolides. It is to be hoped that even better drugs that are more active against MAC and are associated with less toxicity and drug-drug interactions will be introduced in the future.