The activities of the fluoroquinolone WIN-57273, 14-OH clarithromycin (a human metabolite of clarithromycin), and minocycline against two virulent strains of Mycobacteium avium complex were evaluated in a model of intracellular infection and compared with that of clarithromycin. Human monocyte-derived macrophages were infected at day 6 of culture. Intracellular CFU at 60 min and intracellular and supernatant CFU on days 4 and 7 were counted after inoculation. The concentrations used, which were equal to peak levels in serum, were 3 pLg of WIN-57273 per ml (MICs for the two strains, 1 a.g/ml), 4 mg of 14-OH clarithromycin per ml (MICs, 8 and 2 Fg/ml, respectively, at pH 7.4), 4 ,g of minocycline per ml (MICs, 64 and 32 Fg/ml, respectively), and 4 ag of clarithromycin per ml (MICs, 2 and 0.5 ILg/ml, respectively, at pH 7.4). On day 7, compared with controls, WIN-57273, minocycline (P < 0.02), clarithromycin, or different combinations of clarithromycin and the other drugs (P < 0.001) slowed the intracellular replication of strain MO-1. 14-OH clarithromycin (P < 0.02), clarithromycin (P < 0.02), 14-OH darithromycin plus darithromycin (P < 0.01), clarithromycin plus minocycline, or clarithromycin plus minocycline plus 14-OH darithromycin (P < 0.001) slowed the intracellular replication of strain LV-2. WIN-57273 was less effective than carithromycin against strain MO-1 (P < 0.05). Clarithromycin plus 14-OH clarithromycin plus minocycline (P < 0.02) was more effective than clarithromycin alone against strain LV-2. Thus, clarithromycin plus minocycline, which corresponds in humans to three active molecules, may exhibit a better efficacy than clarithromycin in this model.Mycobacterium avium complex, a primarily intracellular bacterium which multiplies within phagocytic cells, frequently causes infection in patients with AIDS (7,20 (14). Minocycline is a longer-acting tetracycline derivative whose in vitro activity against M. avium complex was reported by Tsukamura in 1980 (18). Recently, in view of the high activity of a clarithromycin and minocycline combination against Mycobacterium leprae (13), it seemed interesting to test the activity of this combination against M. avium complex. In patients with AIDS treated with clarithromycin (2 g/day), the peak level of clarithromycin in serum was accompanied by a very similar concentration of 14-hydroxy-clarithromycin (1, * Corresponding author. 4). This human metabolite has a longer half-life than clarithromycin, and its trough level is higher than that of clarithromycin. Thus, 14-OH clarithromycin could participate in the therapeutic efficacy of clarithromycin in humans. The efficacy of this metabolite, which is not present in mice, cannot be assessed by clarithromycin treatment of infected mice.The aim of the present study was to evaluate, in a cell model, the activities of WIN-57273, minocycline, 14-OH clarithromycin alone, and 14-OH clarithromycin in combination with other drugs against M. avium complex multiplication within human macrophages. These activities were compar...