Fluoroquinolones: structure and target sites

The quinolones are a potent group of drugs that target the essential bacterial enzymes DNA gyrase and topoisomerase IV. DNA gyrase is the primary target of Gram negative organisms however, it is topoisomerase IV that is the primary target of Gram positive organisms. Within these enzymes is a highly conserved region centered round the active site where resistance mutations occur. These mutations are almost always identical, irrespective of organism. In spite of the homology of this region, amino acid sequence analysis shows that there are defined differences between the Gram groups, particularly in topoisomerase IV, and it is speculated that herein lies the origin of target preference. Since the first quinolone nalidixic acid was developed, the quinolones have undergone structural modifications, in particular the addition of a fluorine at position 6, to produce the fluoroquinolones. This has seen their potency and pharmakokinetic profile greatly increase. In vitro selection of resistance mutations has allowed the observation of how resistance is acquired and some of the modifications in newer fluoroquinolones have resulted in the shift of primary target from topoisomerase IV to gyrase with Gram positives. Curiously, purified topoisomerase IV is still more sensitive even if gyrase is the primary target. Gyrase remains the primary target for Gram negatives.