Efficacy of macrolides used in combination with ethambutol, with or without other drugs, against Mycobacterium avium within human macrophages

The activities of clarithromycin or roxithromicin used in combination with other antimicrobial drugs were tested in human macrophages experimentally infected with 23 strains of Mycobacterium avium. Overall, clarithromycin–ethambutol–rifampicin was the most active combination tested. The reduction in intracellular viable bacilli was found to be more than 1 log 10 for 95% and more than 2 logs 10 for 65% of the strains. The second most active combination was roxithromycin–ethambutol–rifampicin, which was found to be bactericidal for about 80% and highly bactericidal for 20% of the strains. Others combinations were only bacteriostatic or weakly bactericidal for many of the strains. The addition of a third drug did not necessarily promote enhanced bacterial killing inside the macrophage.