There is an urgent need for the discovery of new antibiotics with a new mode of action as antimicrobial resistance (AMR) had been one of the major threats to the global public health. Our group has targeted bacterial transcription, a validated but underutilized area, for antimicrobial discovery. We discovered the first bacterial rRNA transcription inhibitor, a novel antimicrobial agent.
We focused on NusB and NusE, essential transcription factors for bacterial rRNA synthesis. Using the highly conserved interface between these factors, we constructed a pharmacophore model for in-silico screening. This led to the discovery of lead compound MC4, which demonstrated antimicrobial activity against S. aureus strains.
To optimize MC4, we carried out target-based de novo design and created over 200 analogues to determine the structure-activity relationship.. Some of these showed significantly improved antimicrobial activity against a range of clinical antibiotic-resistant strains, including MRSA and VRSA. Their minimal inhibitory concentration (MIC) was comparable to marketed antibiotics. We examined the mechanism of MC4 derivatives at both molecular and cellular levels. They inhibited NusB-NusE interaction and the transcription of rRNA promoter. They also disrupted the subcellular localisation of the transcription machinery and significantly reduced total RNA and major rRNA.
Given the concern of resistance development in bacteria, we tested the possibility of resistance to MC4 derivatives. After 30 days of exposure to these derivatives at sub-inhibitory concentrations, no resistant isolates were generated compared to rifampicin control drug which generated 1000-fold of resistance after 5 days (unpublished).
The MC4 derivatives showed a very low level of haemolytic property against human blood cells and high permeability using the Caco-2 cell line. They were proven effective using a MRSA LD90 sepsis mouse infection model, and the PK profiles and tissue distribution were also established (unpublished).
In summary, we have developed first-in-class drug candidates with international patent protection. The preclinical data suggests these compounds have high potential for further development as antimicrobial drug candidates.