Potent antipneumococcal activity of gemifloxacin is associated with dual targeting of gyrase and topoisomerase IV, an in vivo target preference for gyrase, and enhanced stabilization of cleavable complexes in vitro

We investigated the roles of DNA gyrase and topoisomerase TV in determining the susceptibility of Streptococcus pneumoniae to gemifloxacin, a novel fl uoroquiuolone which is under development as an antipneumococcal drug. Gemif loxacin displayed potent activity against S. pneumoniae 7785 (MIC, 0.06 mug /ml) compared with ciprofloxacin (MIC, 1 to 2 mug/ml). Complementary geneti c and biochemical approaches revealed the following. (i) The gemifloxacin M ICs for isogenic 7785 mutants bearing either parC or gyrA quinolone resista nce mutations were marginally higher than wild type at 0.12 to 0.25 mug/ml, whereas the presence of both mutations increased the MIC to 0.5 to 1 mug/m l. These data suggest that both gyrase and topoisomerase IV contribute sign ificantly as gemifloxacin targets in vivo. (ii) Gemifloxacin selected first -step gyrA mutants of S. pneumoniae 7785 (gemifloxacin MICs, 0.25 mug/ml) e ncoding Ser-81 to Phe or Tyr, or Glu-85 to Lys mutations. These mutants wer e cross resistant to sparfloxacin (which targets gyrase) but not to ciprofl oxacin (which targets topoisomerase IV). Second-step mutants (gemifloxacin MICs, 1 mug/ml) exhibited an alteration in parC resulting in changes of Par C hot spot Ser-79 to Phe or Tyr. Thus, gyrase appears to be the preferentia l in vivo target. (iii) Gemifloxacin was at least 10- to 20-fold more effec tive than ciprofloxacin in stabilizing a cleavable complex (the cytotoxic l esion) with either S. pneumoniae gyrase or topoisomerase IV enzyme in vitro . These data suggest that gemifloxacin is an enhanced affinity fluoroquinol one that acts against gyrase and topoisomerase IV in S. pneumoniae, with gy rase the preferred in vivo target. The marked potency of gemifloxacin again st wild type and quinolone-resistant mutants may accrue from greater stabil ization of cleavable complexes with the target enzymes.