Intracellular targets of moxifloxacin: a comparison with other fluoroquinolones

The in vitro activity of the novel 8-methoxyquinolone, moxifloxacin, against Streptococcus pneumoniae was evaluated, and the intracellular targets of this agent were studied. Analysis of mutant strains selected with moxifloxacin demonstrated that first-step mutants bore amino acid substitutions at position 81 in the GyrA subunit of DNA gyrase. This suggests that, unlike older fluoroquinolone agents such as ciprofloxacin and levofloxacin, but similar to other C-8 substituted quinolones like sparfloxacin and gatifloxacin, moxifloxacin targets the GyrA subunit of DNA gyrase as an initial lethal event. Such a mechanism results in high activity against increasingly common S. pneumoniae strains bearing substitutions in DNA topoisomerase IV. Moxifloxacin was active with an MIC of Phe/Tyr substitution in ParC. The moxifloxacin MIC for strains with mutations in the structural genes for both DNA gyrase subunit GyrA and DNA topoisomerase IV subunit ParC did not exceed 2 mg/L, a level within clinically achievable serum concentrations for this agent. We also found that moxifloxacin is a poor substrate for active efflux in S. pneumoniae. Therefore, the high activity of moxifloxacin against S. pneumoniae appears to be a result of both enhanced activity against DNA gyrase and topoisomerase IV, and reduced efflux from the bacterial cell.