Quinolone Mode of Action — New Aspects

Summary

The interactions of quinolones with the complex of DNA gyrase and DNA have been elucidated by the sequencing of additional mutant gyrA and gyrB genes that produce altered quinolone susceptibility. Strong patterns have emerged in Escherichia coli in which amino acids between positions 67 and 106 of the gyrase A subunit (GyrA) and at positions 426 and 447 of the gyrase B subunit (GyrB) have been consistently identified as important for quinolone action. The susceptibility patterns and changes in amino acids 426 and 447 in mutant resistant GyrB proteins suggest direct electrostatic interactions with quinolones at these positions. The small size and the polar nature of the serine at position 83 of the E. coli GyrA protein are particularly important for determining enzyme sensitivity and bacterial susceptibility to quinolones. Norfloxacin and ciprofloxacin bind most stably to a complex of DNA gyrase and DNA rather than to either component alone, and reduction of norfloxacin binding to complexes containing resistant GyrA proteins confirms the biological relevance of this direct measure of quinolone interaction with the gyrase-DNA complex. Although recent crystallographic studies have expanded and refined information about gyrase structure at the atomic level, direct determination of the sites of quinolone binding within the gyrase-DNA complex awaits further studies.

Although quinolones have little activity against E. coli topoisomerases I and III, topoisomerase IV, a recently described enzyme thought to be involved in chromosome segregation into daughter cells, has homology with GyrA and GyrB, particularly in regions important for quinolone action, and is inhibited by some quinolones in vitro. The role of topoisomerase IV in quinolone action on bacteria, however, remains to be determined. Many antibacterial quinolones are selective in their antagonism of bacterial DNA gyrase relative to its eukaryotic homologue topoisomerase II. Recently, new quinolone structures, in particular those with a 7-hydroxyphenyl or an isothiazolo ring bridging positions 2 and 3, have been shown to have enhanced activity against topoisomerase II, approaching that of other antitumour agents known to target this enzyme. Limited structure-activity relationships are emerging.

The events within the bacterial cell that determine or occur after the interaction of quinolones with the gyrase-DNA complex and that are important for bacterial killing remain elusive. In chemostat cultures, increasing growth rates result in an increasing loss of viability after exposure to quinolones. Newly divided cells appear to be particularly susceptible to quinolone killing regardless of growth rate, suggesting that killing may be maximal at certain stages of the cell cycle. Cell lysis after quinolone exposure has been associated with peptidoglycan degradation and changes in peptidoglycan composition, suggesting a role for autolysins. These findings and the concordant effects of some mutants in reducing killing by both β-lactams and quinolones suggest that for both classes of drugs there may be Some overlap in the final pathways leading to cell death.