Screening the NCI diversity set V for anti-MRSA activity: cefoxitin synergy and LC-MS/MS confirmation of folate/thymidine biosynthesis inhibition.

New antibacterial agents and agent combinations are urgently needed to combat antimicrobial resistance. A multidimensional chemical library screening strategy was used to identify compounds in the National Cancer Institute (NCI) diversity set V library (1,593 compounds) with anti-methicillin-resistant Staphylococcus aureus (MRSA) activity. In this effort, library compounds were screened for anti-MRSA activity in both their original [un-metabolized (UM)] and human liver microsome-metabolized [post-metabolized (PM)] forms and in the absence and presence of sub-minimum inhibitory concentration (MIC) levels of cefoxitin. This strategy allows for the identification of intrinsically active agents, agents with active metabolites, and agents that can act synergistically with cefoxitin. Sixteen UM compounds with MICs ≤ 12.5 µM were identified. No agents with substantially enhanced activity after microsomal metabolism were found. Several agents showed significant apparent synergy with cefoxitin, and checkerboard assays were used to confirm synergy for four of these (celastrol, porfiromycin, 4-quinazolinediamine, and teniposide). A follow-up comparative screen in the absence and presence of 4-µM thymidine was used to identify three agents as likely folate/thymidine biosynthesis inhibitors. A liquid chromatography-mass spectrometry (LC-MS/MS) assay for deoxythymidine triphosphate (dTTP) was used to confirm these three as suppressing dTTP biosynthesis in MRSA. Bactericidal vs bacteriostatic activity was also evaluated. This study further demonstrates the utility of comparative library screening to identify novel bioactive agents with interesting synergies and biological activities. The identification of several folate/thymidine biosynthesis inhibitors from this small screen indicates that this pathway is a viable target for new drug discovery efforts. IMPORTANCE New antibacterial agents are urgently needed to counter increasingly resistant bacteria. One approach to this problem is library screening for new antibacterial agents. Library screening efforts can be improved by increasing the information content of the screening effort. In this study, we screened the National Cancer Institute diversity set V against methicillin-resistant Staphylococcus aureus (MRSA) with several enhancements. One of these is to screen the library before and after microsomal metabolism as means to identify potential active metabolites. A second enhancement is to screen the library in the absence and presence of sub-minimum inhibitory concentration levels of another antibiotic, such as cefoxitin in this study. This identified four agents with synergistic activity with cefoxitin out of 16 agents with good MRSA activity alone. Finally, active agents from this effort were counter-screened in the presence of thymidine, which quickly identified three folate/thymidine biosynthesis inhibitors, and also screened for bactericidal vs bacteriostatic activity.