Using Small Molecule Adjuvants to Combat Bacterial Resistance Mechanisms

Bacterial infections caused by multidrug resistant (MDR) organisms are one of the greatest global health threats. However, insufficient resources are allocated to the development of novel strategies to combat this growing issue by pharmaceutical companies and governments worldwide. As a result, antibiotic resistance threatens to outpace antimicrobial development potentially reverting society to a post-antibiotic era. As a result, new approaches to identify effective therapeutics are essential. One such approach, is the identification of antibiotic adjuvants that re-sensitize resistant bacteria to an antibiotic, broaden the spectrum of an antibiotic, or decrease the needed antibiotic dose, while possessing limited or no antimicrobial activity itself. This dissertation focuses on the development of these small molecule adjuvants.

Firstly, this work describes the biofilm inhibitory activity of a novel 6-substituted 2-aminoquinazoline library against Mycobacterium smegmatis, which is a model organism for Mycobacterium tuberculosis. Two lead compounds were identified from a structure-activity relationship (SAR) study as low micromolar inhibitors of M. smegmatis biofilms.

Further, these compounds were both found to be nontoxic alone, suggesting this scaffold may be effective to model new adjuvants. This work also discusses a SAR study conducted against M. smegmatis using the natural product meridianin D as the lead compound. Several derivatives improved upon the biofilm inhibitory and dispersal properties of meridianin D. Notably, replacement of the 2-aminopyrimidine moiety attached to the indole with a 2-aminoimidazole improved antibiofilm activity. Also, halogen substitution and position on the indole impacted activity. However, the SAR revealed that meridianin D analogues lack β-lactam potentiation activity.

Next, this work expands on the concept of using natural products, such as meridianin D, as antibiotic adjuvants. To explore this concept, the National Cancer Institute (NCI) Natural Product Library Set IV was screened for adjuvant activity using four classes of antibiotics (β-lactams, aminoglycosides, macrolides, and polymyxins) against three bacterial pathogens (methicillin-resistant Staphylococcus aureus (MRSA), Acinetobacter baumannii, and Klebsiella pneumoniae). Several natural product adjuvants were identified in combination with colistin against colistin-resistant strains of A. baumannii and K. pneumoniae. Clorobiocin, novobiocin, and prodigiosin were determined to be the most effective adjuvants.

Lastly, this work discusses the antibiofilm properties of AR-12 against M. smegmatis and M. tuberculosis which has been found to selectively kill these pathogens only when in the biofilm state. It also briefly discusses the synthesis and results of a preliminary SAR study using AR-12 as the lead compound.