Activity of moxifloxacin against pathogens with decreased susceptibility to ciprofloxacin.

A panel of 279 clinical isolates of Gram-positive cocci and Gram-negative bacilli with varying levels of resistance to ciprofloxacin were analysed for susceptibility to moxifloxacin, ciprofloxacin, ofloxacin and nalidixic acid. Moxifloxacin was eight- to 32-fold more potent than ciprofloxacin and ofloxacin against staphylococci and Streptococcus pneumoniae, and equivalent to eight-fold more potent against enterococci. Although ciprofloxacin was intrinsically more potent than the other quinolones against highly susceptible Gram-negative isolates, the percentages of Gram-negative isolates susceptible to 1 mg/L of moxifloxacin or ciprofloxacin, or 2 mg/L of ofloxacin were 78%, 80% and 76%, indicating in-vitro equivalence of the agents against a collection that included isolates with diminished quinolone susceptibility. Staphylococci were analysed according to their ciprofloxacin susceptibility status. As ciprofloxacin resistance increased to high levels, all quinolone MICs increased, but moxifloxacin and ofloxacin MICs increased less than ciprofloxacin MICs. In mutational studies moxifloxacin inhibited more mutants (69%) at a concentration of 1 mg/L than did ciprofloxacin (63%) at 1 mg/L or ofloxacin at 2 mg/L (31%). The study indicated that moxifloxacin is more potent than ciprofloxacin and ofloxacin against Gram-positive pathogens, may be comparable in activity against less quinolone-susceptible Gram-negative isolates (other than Pseudomonas aeruginosa), and is less affected than ciprofloxacin by mechanisms responsible for increasing quinolone resistance in staphylococci.