Discovery of New Antibacterial Agents for the Treatment of Methicillin-Resistant Staphyloccocus aureus

This thesis describes the efforts toward developing new antibacterial agents for treatment of methicillin-resistant Staphylococcus aureus (MRSA), a gram-positive organism that is a leading cause of hospital-acquired infections. This pathogen is resistant to the β-lactam antibiotics which inhibit the penicillin-binding proteins (PBPs), which are involved in the final stages of bacterial cell-wall (peptidoglycan) biosynthesis. MRSA is able to evade action of these antibiotics by expressing an additional PBP with a low affinity for the β-lactams, which is termed PBP2a. This enzyme has a characteristically closed active-site conformation that is regulated allosterically by a site 60 Å away. We set out to develop new inhibitors of PBP2a by performing docking and scoring at the active site and testing top compounds for antibacterial activity. A quinazolinone with good activity against S. aureus was identified from this screening. Medicinal chemistry optimization of this hit compound was performed to optimize the in vitro antibacterial activity, solubility, stability, pharmacokinetics, and ultimately in vivo efficacy. The mechanism of action for the quinazolinones was investigated through use of a macromolecular synthesis assays, PBP enzyme activity assays, and mRNA antisense susceptibility testing. Additionally, the quinazolinones were tested for synergy with several classes of antibiotics including β-lactams. Two lead quinazolinones were identified that demonstrated good pharmacokinetic properties and thus achieved efficacy in mouse models of infections. Additionally, these quinazolinones were demonstrated to inhibit PBP1 and PBP2a of S. aureus. In the case of PBP2a, the quinazolinones bind to both the allosteric site and the active site in order to accomplish inhibition. Their ability of inhibit PBP2a also allowed for the quinazolinones to achieve synergism with several β-lactam antibiotics, most notably piperacillin and imipenem.