Investigate the mechanism of action of a Novel Anti-Staphylococcal Agent, SC-5005

Methicillin-resistant Staphylococcus aureus (MRSA) causes serious health problems worldwide due to its’ resistance to most antibiotic. This situation highlights the urgent need of developing novel anti-Staphylococcus agent. In our previous study, we identified a compound called SC-5005, a derivative of anti-cancer drug sorafenib (Nexavar). SC-5005 exhibited high antibacterial activity and low cytotoxicity. However, the selective ratio (antiproliferative activity / antibacterial activity) of SC-5005 is 39.4 only, which is lower than the ideal selective ratio value, 100. Thus, an in-house synthesized compound library of SC-5005 derivatives was screened to identify a better compound. Meanwhile, we keep optimizing the drug vehicle. During the screening, SC-5035 was identified to exhibit higher potency, but it is a pity that SC-5035 failed in in vivo assay. For the drug vehicle optimization, we found SC-5005 has higher solubility in 1,2-Propanediol which could replace our original PEG-400 based vehicle without affecting the antibacterial activity of SC-5005 and avoid PEG-400 induced inflammatory response. .Moreover, we also found a chemical called HRCA that can significantly enhance the antibacterial activity of SC-5005. On the other hand, we also try to figure out how SC-5005 act on S. aureus. Previously, our lab members found that SC-5005 might act on bacterial cell membrane. So we utilized fluorescent Live/Dead staining to observe whether bacterial cell membrane was damage after SC-5005 and SC-5035 treatment. Our result showed that our compounds indeed damage bacterial cell membrane, resulting an orange-colored bacteria. We also tested ATP leakage of S. aureus and observed that intracellular ATP level dropped dramatically after SC-5005 and SC-5035 treatment. The transmission electron microscopy image showed the pore on S. aureus cell membrane after SC-5005 treatment, but the calcein leakage assay result pointed out that the pore formation ability of SC-5005 is not through directly acting on the membrane lipids. In summary, first, we can modify the structure of SC-5005 to enhance its antibacterial activity. Second, 1,2-Propanidiol is a better vehicle for SC-5005 without the side effects of PEG400. Third, SC-5005’s antibacterial activity is by forming pores on bacterial cell membrane.