Ciprofloxacin dimers target gyrase in Streptococcus pneumoniae

We have examined the antipneumococcal activities of novel quinolone dimers in which ciprofloxacin was tethered to itself or to pipemidic acid by linkage of C-7 piperazinyl rings. Symmetric 2,6-lutidinyl- and trans-butenyl-linked ciprofloxacin dimers (dimers 1 and 2, respectively) and a pipemidic acid-ciprofloxacin dimer (dimer 3) had activities against Streptococcus pneumoniae strain 7785 that were comparable to that of ciprofloxacin, i.e., MICs of 2, 1, and 4 to 8 microg/ml versus an MIC of 1 to 2 microg/ml, respectively. Surprisingly, unlike ciprofloxacin (which targets topoisomerase IV), several lines of evidence revealed that the dimers act through gyrase in S. pneumoniae. First, ciprofloxacin-resistant parC mutants of strain 7785 remained susceptible to dimers 1 to 3, whereas a gyrA mutation conferred a four- to eightfold increase in the dimer MIC but had little effect on ciprofloxacin activity. Second, dimer 1 selected first-step gyrA (S81Y or S81F) mutants (MICs, 8 to 16 microg/ml) that carried wild-type topoisomerase IV parE-parC genes. Third, dimers 1 and 2 promoted comparable DNA cleavage by S. pneumoniae gyrase and topoisomerase IV, whereas ciprofloxacin-mediated cleavage was 10-fold more efficient with topoisomerase IV than with gyrase. Fourth, the GyrA S81F and ParC S79F enzymes were resistant to dimers, confirming that the resistance phenotype is largely silent in parC mutants. Although a dimer molecule could bind very tightly by bridging quinolone binding sites in the enzyme-DNA complex, the greater potency of ciprofloxacin against gyrase and topoisomerase IV suggests that dimers 1 to 3 bind in a monomeric fashion. The bulky C-7 side chain may explain dimer targeting of gyrase and activity against efflux mutants. Tethered quinolones have potential as mechanistic tools and as novel antimicrobial agents.