Synthesis of Small Molecule Adjuvants to Combat Antibiotic Resistance Mechanisms

According to the Centers for Disease Control and Prevention (CDC), the U.S. lost a lot of the progress made towards combating the antibiotic resistance crisis back in 2020 due to the effects of the COVID-19 pandemic. They state that during the first year of the pandemic, more than 29,400 people died from a bacterial infection, with 40% of those infections being acquired during a hospital stay. Although determination of the full extent of the national burden of deaths from antimicrobial infections may be hindered from the statistical gaps caused by the pandemic, the 2019 CDC estimate of 2.8 million infections/year resulting in approximately 35,000 deaths, remains some of the strongest data obtained to date. These statistics, the loss of progress made, and the dearth of new antibiotics in the drug pipeline highlight the urgent need for novel ways to treat such infections. The work presented in this document describes multiple projects focusing on the development of antibiotic adjuvants to combat this issue. In-depth discussions detail initial hits found for each project, followed by a structure activity relationship (SAR) study to reveal lead adjuvants. The first two projects focus on utilizing adjuvants to sensitize gram-negative pathogens to gram-positive selective antibiotics. Gram-negative bacteria are more resistant to many antibiotics than gram-positive bacteria, since they possess an outer membrane that acts as a first line of defense to many antibiotics by preventing their entry into the cell. We identified a class of dimeric 2-aminoimidazole (2-AI) adjuvants that dramatically enhance the sensitivity of Acinetobacter baumannii to the macrolide antibiotic clarithromycin, with reductions of minimum inhibitory concentrations (MICs) as high as =512-fold. Activity for these adjuvants was seen across a variety of A. baumannii clinical isolates and among other gram-negative pathogens including: Klebsiella pneumoniae (KP2146), Pseudomonas aeruginosa PAO1, and Escherichia coli (EC25922). Furthermore, the dimeric 2-AI analogs exhibited highly reduced mammalian cell toxicity compared to earlier adjuvants, with IC50 values >200 µg/mL against HepG2 cells, which corresponds to therapeutic indices (TIs) >200. Another project identifies preliminary trends for a second-generation set of 2-AI dimer adjuvants for clarithromycin potentiation against A. baumannii. Lastly, this document describes the efforts made toward synthesizing a set of quinuclidine based natural products for the potentiation of oxacillin against Staphylococcus aureus.