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 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 microgram/ml) compared with ciprofloxacin (MIC, 1 to 2 microgram/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 microgram/ml, whereas the presence of both mutations increased the MIC to 0.5 to 1 microgram/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 microgram/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 microgram/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.

Activity of gemifloxacin against penicillin- and ciprofloxacin-resistant Streptococcus pneumoniae displaying topoisomerase- and efflux-mediated resistance mechanisms

Nine penicillin-resistant Streptococcus pneumoniae clinical isolates from Northern Ireland, resistant to ciprofloxacin (MICs, 2 to 64 microg/ml) through topoisomerase- and/or reserpine-sensitive efflux mechanisms, were highly susceptible to gemifloxacin (MICs, 0.03 to 0. 12 microg/ml). Two strains (requiring a ciprofloxacin MIC of 64 microg/ml) carried known quinolone resistance mutations in parC, parE, and gyrB, resulting in S79F, D435V, and E474K changes, respectively. Thus, gemifloxacin is active against clinical strains exhibiting altered topoisomerase and efflux phenotypes.

Molecular cloning and characterization of the human topoisomerase IIalpha and IIbeta genes: evidence for isoform evolution through gene duplication

Human DNA topoisomerase II is essential for chromosome segregation and is the target for several clinically important anticancer agents. It is expressed as genetically distinct alpha and beta isoforms encoded by the TOP2alpha and TOP2beta genes that map to chromosomes 17q21-22 and 3p24, respectively. The genes display different patterns of cell cycle- and tissue-specific expression, with the alpha isoform markedly upregulated in proliferating cells. In addition to the fundamental role of TOP2alpha and TOP2beta genes in cell growth and development, altered expression and rearrangement of both genes are implicated in anticancer drug resistance. Here, we report the complete structure of the human topoisomerase IIalpha gene, which consists of 35 exons spanning 27.5 kb. Sequence data for the exon-intron boundaries were determined and examined in the context of topoisomerase IIalpha protein structure comprising three functional domains associated with energy transduction, DNA breakage-reunion activity and nuclear localization. The organization of the 3' half of human TOP2beta, including sequence specifying the C-terminal nuclear localization domain, was also elucidated. Of the 15 introns identified in this 20 kb region of TOP2beta, the first nine and the last intron align in identical positions and display the same phases as introns in TOP2alpha. Though their extreme 3' ends differ, the striking conservation suggests the two genes diverged recently in evolutionary terms consistent with a gene duplication event. Access to TOP2alpha and TOP2beta gene structures should aid studies of mutations and gene rearrangements associated with anticancer drug resistance.

Identification of yeast DNA topoisomerase II mutants resistant to the antitumor drug doxorubicin: implications for the mechanisms of doxorubicin action and cytotoxicity

Doxorubicin is a therapeutically useful anticancer drug that exerts multiple biological effects. Its antitumor and cardiotoxic properties have been ascribed to anthracycline-mediated free radical damage to DNA and membranes. Evidence for this idea comes in part from the selection by doxorubicin from stationary phase yeast cells of mutants (petites) deficient in mitochondrial respiration and therefore defective in free radical generation. However, doxorubicin also binds to DNA topoisomerase II, converting the enzyme into a DNA damaging agent through the trapping of a covalent enzyme-DNA complex termed the 'cleavable complex.' We have used yeast to determine whether stabilization of cleavable complexes plays a role in doxorubicin action and cytotoxicity. A plasmid-borne yeast TOP2 gene was mutagenized with hydroxylamine and used to transform drug-permeable yeast strain JN394t2-4, which carries a temperature-sensitive top2-4 mutation in its chromosomal TOP2 gene. Selection in growth medium at the nonpermissive temperature of 35 degrees in the presence of doxorubicin resulted in the isolation of plasmid-borne top2 mutants specifying functional doxorubicin-resistant DNA topoisomerase II. Single-point changes of Gly748 to Glu or Ala642 to Ser in yeast topoisomerase II, which lie in and adjacent to the CAP-like DNA binding domain, respectively, were identified as responsible for resistance to doxorubicin, implicating these regions in drug action. None of the mutants selected in JN394t2-4, which has a rad52 defect in double-strand DNA break repair, was respiration-deficient. We conclude that topoisomerase II is an intracellular target for doxorubicin and that the genetic background and/or cell proliferation status can determine the relative importance of topoisomerase II- versus free radical-killing.