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Antimicrobial Agents and Chemotherapy, November 2000, p. 3112-3117, Vol. 44, No. 11
0066-4804/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

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

Victoria J. Heaton,¹ Jane E. Ambler,^2, and L. Mark Fisher^1,*

Molecular Genetics Group, Department of Biochemistry, St. George's Hospital Medical School, University of London, London SW17 0RE,¹ and SmithKline Beecham Pharmaceuticals, Harlow, Essex CM19 5AW,² United Kingdom

Received 15 May 2000/Returned for modification 31 July 2000/Accepted 25 August 2000

We investigated the roles of DNA gyrase and topoisomerase IV in determining the susceptibility of Streptococcus pneumoniae to gemifloxacin, a novel fluoroquinolone which is under development as an antipneumococcal drug. Gemifloxacin displayed potent activity against S. pneumoniae 7785 (MIC, 0.06 µg/ml) compared with ciprofloxacin (MIC, 1 to 2 µg/ml). Complementary genetic and biochemical approaches revealed the following. (i) The gemifloxacin MICs for isogenic 7785 mutants bearing either parC or gyrA quinolone resistance mutations were marginally higher than wild type at 0.12 to 0.25 µg/ml, whereas the presence of both mutations increased the MIC to 0.5 to 1 µg/ml. These data suggest that both gyrase and topoisomerase IV contribute significantly as gemifloxacin targets in vivo. (ii) Gemifloxacin selected first-step gyrA mutants of S. pneumoniae 7785 (gemifloxacin MICs, 0.25 µg/ml) encoding Ser-81 to Phe or Tyr, or Glu-85 to Lys mutations. These mutants were cross resistant to sparfloxacin (which targets gyrase) but not to ciprofloxacin (which targets topoisomerase IV). Second-step mutants (gemifloxacin MICs, 1 µg/ml) exhibited an alteration in parC resulting in changes of ParC hot spot Ser-79 to Phe or Tyr. Thus, gyrase appears to be the preferential in vivo target. (iii) Gemifloxacin was at least 10- to 20-fold more effective than ciprofloxacin in stabilizing a cleavable complex (the cytotoxic lesion) with either S. pneumoniae gyrase or topoisomerase IV enzyme in vitro. These data suggest that gemifloxacin is an enhanced affinity fluoroquinolone that acts against gyrase and topoisomerase IV in S. pneumoniae, with gyrase the preferred in vivo target. The marked potency of gemifloxacin against wild type and quinolone-resistant mutants may accrue from greater stabilization of cleavable complexes with the target enzymes.

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^* Corresponding author. Mailing address: Molecular Genetics Group, Department of Biochemistry, St. George's Hospital Medical School, University of London, Cranmer Terrace, London SW17 0RE, United Kingdom. Phone: 44 208 725 5782. Fax: 44 208 725 2992. E-mail: lfisher{at}sghms.ac.uk.

Present address: Bayer plc, Stoke Poges, Slough SL2 4LY, United Kingdom.

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Antimicrobial Agents and Chemotherapy, November 2000, p. 3112-3117, Vol. 44, No. 11
0066-4804/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

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