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Fluoroquinolones are broad-spectrum antibacterial agents based on the structure of nalidixic acid. For nearly five decades it has been known that fluoroquinolones inhibit bacterial growth by blocking the enzymatic action of type II topoisomerases such as DNA gyrase and topoisomerase IV. Only recently has it been discovered that some fluoroquinolones are capable of a mechanism that results in fragmented DNA and leads to rapid bacterial cell death. This mechanism is not well understood. Presented here are studies towards understanding the structure activity relationship (SAR) of fluoroquinolones, specifically to determine what leads to the novel mechanism termed "rapid lethality." This work is based on the hypothesis that structurally unique fluoroquinolones interact with the DNA-topoisomerase complex in a unique manner that ultimately leads to rapid cell death.
The first approach to understand SAR for killing was to evaluate the effect of a ring fusion between N-1 and C-8 of the fluoroquinolone core. Known lethal fluoroquinolones are substituted by N-1 cyclopropyl and C-8 methoxy, but some clinically important fluoroquinolones contain a 2-methylmopholino moiety between these two positions. Novel fluoroquinolones were synthesized and clinically available agents were obtained to create a panel of drug molecules with one of six C-7 substituents and either the morpholine ring system or N-1 cyclopropyl and C-8 methoxy. Bacteriostatic and bactericidal activities of these compounds were determined. Bactericidal studies were conducted both in the presence and absence of chloramphenicol, a protein synthesis inhibitor used to simulate non-growing bacteria. Lethality in the presence of chloramphenicol is also important when considering co-administration of fluoroquinolones with other antibiotic classes.
In a second study, fluoroquinolones were synthesized with a C-2 thioalkyl substitution. Substitutions at the C-2 position are severely lacking in clinical fluoroquinolones, with only prulifloxacin, a newly developed antibiotic, being substituted by an N-1 to C-2 thiazetidine ring structure. Analogs of ciprofloxacin and moxifloxacin were synthesized such that the N-1, C-2, and C-8 positions were substituted with cyclopropyl, thioethyl/thioisopropyl, and methoxy groups, respectively. The compounds were then evaluated for antibiotic activity against three different bacterial strains to evaluate the contribution of the C-2 thioalkyl substituent to antibacterial activity.
In a third study, quinazoline-2,4-diones, a new antibiotic class structurally and mechanistically similar to fluoroquinolones, were modified at the C-4 position in an effort to understand the binding interaction between these compounds and the target enzyme. Importantly, the quinazoline-2,4-diones typically retain activity against bacterial cells known to be resistant to fluoroquinolones and are less likely to select for resistant mutants. In this study, the C-4 carbonyl was replaced with either a thiocarbonyl or a hydroxylimine and the new compounds, bearing C-7 substituents common to potent antibiotic fluoroquinolones and quinazolines, were evaluated for activity against bacterial cells.
Despite the findings of recently published X-ray crystallography, it was determined that one of the greatest determinants in antibiotic activity of fluoroquinolones is the C-7 substituent. Additionally, there is increasing evidence that the C-2 carbonyl of quinazoline-2,4-diones affords the increase in activity against resistant mutants by creating a unique binding interaction. Collectively, the conclusions reached here add to our understanding of the structure activity relationship of the fluoroquinolone antibiotic class for rapidly killing bacterial cells and overcoming resistant mutants.