Resistance mechanisms which inactivate penicillins, cephalo- sporins, aminoglycosides, macrolides, and tetracyclines do not interfere with the antibacterial activity of Avelox. There is no cross resistance between Avelox and these agents. Avelox is effective against b-lactam and macrolide resistant bacteria. Studies in animal models of infection have demonstrated high in vivo activity.

It appears that the C8-methoxy moiety contributes to enhanced activity and lower selection of resistant mutants of Gram-positive bacteria compared to the C8-H moiety present on some other quinolones.

Cross resistance among quinolones has been observed. However, some Gram-positive and anaerobic organisms resistant to other quinolones are susceptible to Avelox.

The presence of the bulky bicycloamine substituent at the C-7 position prevents active efflux, a mechanism of fluoroquinolone resistance. Plasmid-mediated resistance has not been observed to date.