Selection of clarithromycin-resistant Mycobacterium avium complex during combined therapy using the beige mouse model.

Sixteen weeks of treatment with clarithromycin (CLARI) alone displayed significant bactericidal activity against Mycobacterium avium complex infection in beige mice. Only two combined regimens, CLARI combined with an initial 4 or 8 weeks of amikacin (AMIKA), displayed activity greater than that displayed by CLARI alone. Four other combined regimens, CLARI combined with ethambutol (EMB), rifabutin (RBT), or both EMB and RBT during the entire 16 weeks of treatment or with AMIKA administered in an initial 2-week course showed bactericidal activity not significantly greater than that of CLARI alone. After 16 weeks of treatment, CLARI-resistant mutants were isolated from the majority of mice that had been treated with CLARI alone, CLARI-RBT, CLARI-EMB, or CLARI-EMB-RBT, as was the case for untreated controls, but the frequencies of occurrence of mutants were significantly greater in the groups treated with these combinations or CLARI alone. On the other hand, no CLARI-resistant mutants were isolated from the mice that had been treated with the combination of CLARI plus an initial 4 or 8 weeks of AMIKA and were isolated from only a tiny proportion of mice that had been treated with CLARI plus an initial 2 weeks of AMIKA. Therefore, only treatment with CLARI combined with an initial 4 or 8 weeks of AMIKA but not combined with RBT or EMB or both, could enhance the activity of the drug treatment and prevent the selection of CLARI-resistant mutants.