Mechanism of action of the novel small molecule IMP-1700 against Staphylococcus aureus

Abstract

Antibiotic resistance is a major health challenge globally. Despite the need for novel therapeutics with new targets or to improve existing antibiotics, there is a low number in the pipeline. IMP-1700 is a novel small molecule which was recently shown to re-sensitise methicillin-resistant Staphylococcus aureus (MRSA) to the DNA damaging antibiotic ciprofloxacin via targeting the DNA repair complex RexAB. However, IMP1700 exhibited direct antibacterial activity against S. aureus which was suggestive of an alternative target given that RexAB is non-essential. Serial passaging was used to generate S. aureus isolates exhibiting an increased IMP-1700 minimum inhibitory concentration (MIC). Whole-genome sequencing revealed that these isolates had a single mutation in GyrA, indicating that DNA gyrase was a target for the bacteriostatic activity of IMP-1700. Work in this thesis tested whether IMP-1700 potentiated the activity of antibiotics other than ciprofloxacin including co-trimoxazole, daptomycin and cloxacillin. Assessment of IMP-1700 bactericidal synergy against S. aureus was most potent with daptomycin. DNA gyrase was found to mediate the synergy between IMP-1700 and daptomycin. In silico docking predicted that IMP-1700 interacts with a second target in the GyrB subunit via the trifluoromethyl group. The importance of this moiety for synergy with daptomycin was confirmed by an SAR approach. DNA gyrase is crucial in the DNA replication and chromosome segregation stages of staphylococcal cell division. Although daptomycin primarily targets the bacterial membrane, studies have also found that it binds to the septum. IMP-1700 was shown to increase the number of cells in growth phase 3 (complete septum) and enhance daptomycin binding at the septum. Together these data demonstrate that targeting DNA gyrase can disturb the staphylococcal cell cycle and improve the efficacy of daptomycin. Because daptomycin is used against serious staphylococcal infections, these results provide promise where clinical treatment failure occurs in ~23% of cases.