Identification of a 2-Aminobenzimidazole Scaffold that Potentiates Gram-Positive Selective Antibiotics Against Gram-Negative Bacteria.

The development of novel therapeutic approaches is crucial in the fight against multi-drug resistant (MDR) bacteria, particularly gram-negative species. Small molecule adjuvants that enhance the activity of otherwise gram-positive selective antibiotics against gram-negative bacteria have the potential to expand current treatment options. We have previously reported adjuvants based upon a 2-aminoimidazole (2-AI) scaffold that potentiate macrolide antibiotics against several gram-negative pathogens. Herein, we report the discovery and structure-activity relationship (SAR) investigation of an additional class of macrolide adjuvants based upon a 2-aminobenzimidazole (2-ABI) scaffold. The lead compound lowers the minimum inhibitory concentration (MIC) of clarithromycin (CLR) from 512 to 2 µg/mL at 30 µM against Klebsiella pneumoniae 2146, and from 32 to 2 µg/mL at 5 µM, against Acinetobacter baumannii 5075. Preliminary investigation into the mechanism of action suggests that the compounds are binding to lipopolysaccharide (LPS) in K. pneumoniae, and modulating lipooligosaccharide (LOS) biosynthesis, assembly, or transport in A. baumannii.

Synthesis, Stereochemical Resolution, and Analogue Synthesis of Variabiline, an Aporphine Alkaloid That Sensitizes Acinetobacter baumannii and Klebsiella pneumoniae to Colistin.

Increasing antimicrobial resistance, coupled with the absence of new antibiotics, has led physicians to rely on colistin, a polymyxin with known nephrotoxicity, as the antibiotic of last resort for the treatment of infections caused by Gram-negative bacteria. One approach to increasing antibiotic efficacy and thereby reducing dosage is the use of small-molecule potentiators that augment antibiotic activity. We recently identified the aporphine alkaloid (±)-variabiline, which lowers the minimum inhibitory concentration of colistin in Acinetobacter baumannii and Klebsiella pneumoniae. Herein, we report the first total synthesis of (±)-variabiline to confirm structure and activity, the resolution, and evaluation of both enantiomers as colistin potentiators, and a structure-activity relationship study that identifies more potent variabiline derivatives. Preliminary mechanistic studies indicate that (±)-variabiline and its derivatives potentiate colistin by targeting the Gram-negative outer membrane.

Development of phenyl-urea-based small molecules that target penicillin-binding protein 4.

Staphylococcus aureus has the ability to invade cortical bone osteocyte lacuno-canalicular networks (OLCNs) and cause osteomyelitis. It was recently established that the cell wall transpeptidase, penicillin-binding protein 4 (PBP4), is crucial for this function, with pbp4 deletion strains unable to invade OLCNs and cause bone pathogenesis in a murine model of S. aureus osteomyelitis. Moreover, PBP4 has recently been found to modulate S. aureus resistance to ß-lactam antibiotics. As such, small molecule inhibitors of S. aureus PBP4 may represent dual functional antimicrobial agents that limit osteomyelitis and/or reverse antibiotic resistance. A high throughput screen recently revealed that the phenyl-urea 1 targets PBP4. Herein, we describe a structure-activity relationship (SAR) study on 1. Leveraging in silico docking and modeling, a set of analogs was synthesized and assessed for PBP4 inhibitory activities. Results revealed a preliminary SAR and identified lead compounds with enhanced binding to PBP4, more potent antibiotic resistance reversal, and diminished PBP4 cell wall transpeptidase activity in comparison to 1.