Comparative in vitro and in vivo activity of the C-8 methoxy quinolone moxifloxacin and the C-8 chlorine quinolone BAY y 3118.

The C-8 methoxy quinolone moxifloxacin is highly bactericidal against wild-type and first-step gyrase- and topoisomerase IV-resistant mutants. This finding led to the hypothesis that the C-8 methoxy group may lower the propensity for resistance development compared with quinolones possessing different substituents at the C-8 position. Therefore, resistance development of the C-8 methoxy quinolone moxifloxacin was compared with that of its structural analogue BAY y 3118 (chlorine moiety at the C-8 position), with Staphylococcus aureus used as the test organism. The spontaneous emergence of resistance was quantified by counting the number of colonies growing on drug-free medium compared with moxifloxacin- or BAY y 3118-containing media. The multistep emergence of quinolone resistance was encountered by growing S. aureus over 8 passages in drug-containing medium. Human serum concentrations were simulated in an in vitro model over 84 h (dosing every 24 h), and total and resistant S. aureus were quantified. Spontaneous mutation frequencies of 6x10-11 for moxifloxacin and 4x10-7 for BAY y 3118 were observed. Multistep resistance to moxifloxacin developed slowly (2-fold rise) but rapidly against BAY y 3118 (>16-fold rise). No resistance against moxifloxacin developed in this model, whereas resistance to BAY y 3118 began to develop after 4 h. Thus, as the C-8 moiety was the only difference, the 8-methoxy group on moxifloxacin appeared to significantly lower the propensity for quinolone resistance development.