In response to the antimicrobial resistance (AMR) global health crisis, physicians have been forced to employ antibiotics of last resort such as colistin, which had previously been removed from clinics due to toxicity concerns. The increase in colistin usage has in turn resulted in the increased incidence of colistin-resistant isolates, making novel therapeutic options for infections caused by these pathogens a priority. We previously reported that sorafenib, a eukaryotic kinase inhibitor, is a colistin adjuvant, augmenting colistin activity against colistin-resistant Klebsiella pneumoniae and Acinetobacter baumannii. Herein, we describe the generation of a library of analogues based upon the sorafenib scaffold, lead compounds from which sensitize colistin-resistant K. pneumoniae, A. baumannii, and Pseudomonas aeruginosa to colistin, and exhibit reduced cytotoxicity compared to sorafenib. We also describe the standalone antimicrobial activity of these sorafenib analogues against methicillin-resistant Staphylococcus aureus (MRSA). Lead compound NDM-773 lowers the colistin minimum inhibitory concentration (MIC) against K. pneumoniae B9 from 512 μg mL-1 to 2 μg mL-1 at 1.5 μM, against A. baumannii 4106 from 1024 μg mL-1 to 1 μg mL-1 at 4 μM, and against P. aeruginosa TRPA162 from 8192 μg mL-1 to ≤0.125 μg mL-1 at 7.5 μM. NDM-773 acts as a standalone antibiotic against MRSA with an MIC of 0.781 μM (0.398 μg mL-1) and a frequency of mutation rate of <10-11 against MRSA BAA-1556. NDM-773 exhibits over three-fold reduction in HepG2 toxicity compared to sorafenib, and has therapeutic indices (TIs) up to 35.6 as a colistin adjuvant, and 68.4 as an MRSA antibiotic.

The prodiginine family constitutes a distinctive class of tripyrrolic natural products notable for their red pigmentation and diverse bioactivities. While the biological activities of the linear prodiginine prodigiosin have been extensively studied, cyclic prodiginines remain comparatively underexplored despite structural features that may confer distinct membrane-perturbing properties and enhanced antibacterial activity. Here, a high-producing colony variant of Streptomyces spectabilis (ISP-5512) is described that yields the cyclic prodiginine metacycloprodigiosin at up to 300 mg L-1 isolated yield, representing the highest reported production to date. This high-yield production enabled a comprehensive biological evaluation of metacycloprodigiosin against a diverse panel of colistin-resistant Gram-negative (CRGN) pathogens as a colistin adjuvant, and demonstrates that 5 µM metacycloprodigiosin reduces the colistin minimum inhibitory concentration (MIC) up to 32 768-fold across A. baumannii, K. pneumoniae, P. aeruginosa, and E. coli isolates harboring both chromosomal and plasmid-mediated (mcr-1) resistance. Initial investigation into the adjuvant action of metacycloprodigiosin reveals that an increase in reactive oxygen species (ROS) could underpin evident colistin potentiation activity in CRGN A. baumannii. These findings establish metacycloprodigiosin as a potent colistin adjuvant, highlight the potential of cyclic prodiginines for combating antimicrobial resistance, and provide insight into structure-activity relationships governing membrane-active antibiotic adjuvants.

Infections in the lungs of cystic fibrosis (CF) patients are often polymicrobial in nature, typically comprising Pseudomonas aeruginosa and Staphylococcus aureus. Compounds that act as an antimicrobial agent against one of these pathogens, and as an antibiotic adjuvant against the other, could provide a valuable approach to treating such infections, however a model that mimics the unique environment found with the CF lung is required for the identification and characterization of such molecules. To address this, we employed a S. aureus/P. aeruginosa coculture screening model in synthetic sputum, and identified compounds from our in-house library that simultaneously have potent anti-S. aureus activity, and potentiate colistin against colistin-resistant P. aeruginosa. The two lead compounds, 12F1 and 12G9, control growth of both species when dosed alongside sub-inhibitory concentrations of colistin, highlighting the potential of using a single molecule as both an antibiotic and antibiotic adjuvant to target multiple species in polymicrobial infections, as well as the importance of conducting activity screens in clinically relevant media.

The majority of human bacterial pathogens have the ability to form biofilms in vivo on body tissues and implantable medical devices. Biofilm-mediated chronic bacterial infections are difficult to treat due to their recalcitrance to antimicrobials and immune effectors, often requiring invasive surgical intervention to clear the infection. The difficulty in effectively executing these treatment strategies underscores the need for therapeutic agents that specifically target the biofilm state. To this end, we previously identified two small molecules, JG-1 and M4, that in vitro effectively inhibit and disperse biofilms of Salmonella Typhimurium and members of the ESKAPE pathogen group, including Enterobacter cloacae, Pseudomonas aeruginosa, and Acinetobacter baumannii. In addition to its antibiofilm effects, M4 has a bactericidal effect on Staphylococcus aureus and Enterococcus faecium. While these compounds have promising utility as antimicrobial agents, their mechanism of action remains unknown. By employing multiple techniques including RNAseq, thermal proteome profiling, and site directed mutagenesis, we identified multiple proteins essential to biofilm formation and evaluated their role in the presence of JG-1 and M4 in mutant and wildtype backgrounds. We report that the JG-1 and M4 actions are influenced by proteins important to biofilm maintenance, including OmpA, OmpC, and TrxA. Compound-bacteria interactions cause transcriptional changes that result in biofilm dispersal, and modulation of other virulence mechanisms, including invasion and motility. Additionally, we report that M4 interacts with S. aureus CodY, which promotes cell death, while the specific targets in S. Typhimurium and E. cloacae remain elusive. Collectively, this study presents an empirical investigation into JG-1 and M4’s mechanism of action in S. Typhimurium, E. cloacae, and S. aureus, and how the antibiofilm compounds disrupt microbial community dynamics, ultimately driving biofilm dispersal or cell death.

Penicillin binding protein 4 (PBP4) is essential for Staphylococcus aureus cortical bone osteocyte lacuno-canalicular network (OLCN) invasion, which causes osteomyelitis and serves as a bacterial niche for recurring bone infection. Moreover, PBP4 is also a key determinant of S. aureus resistance to fifth-generation cephalosporins (ceftobiprole and ceftaroline). From these perspectives, the development of S. aureus PBP4 inhibitors may represent dual functional therapeutics that prevent osteomyelitis, and reverse PBP4-mediated β-lactam resistance. A high-throughput screen for small molecules that inhibit S. aureus PBP4 function identified compound 1. We recently described a preliminary structure activity relationship (SAR) study on 1, identifying several compounds with increased PBP4 inhibitory activity, some of which also inhibit PBP2a. Herein, we expand our exploration of phenyl ureas as antibiotic adjuvants, investigating their activity with penicillins and additional cephalosporins against PBP2a-mediated methicillin-resistant S. aureus (MRSA). We screened the previously reported pilot library, and prepared an additional series of phenyl ureas based on compound 1. Lead compounds potentiate multiple β-lactam antibiotics, lowering minimum inhibitory concentrations (MICs) below susceptibility breakpoints, with up to 64-fold reductions in MIC.

An increase in antibiotic resistance and the paucity of new treatment options have led to the present-day antibiotic resistance crisis. Acinetobacter baumannii is categorized by the World Health Organization as a critical priority. Previously, we reported halogenated aryl 2-aminoimidazole (2-AI) adjuvants that potentiate macrolide antibiotics against A. baumannii; however, this class of adjuvants exhibits cytotoxicity toward HepG2 cells. In this study we generate a library of 2-AI-benzimidazole adjuvants that potentiate clarithromycin (CLR), and exhibit reduced cytotoxicity. Lead compounds lower the CLR minimum inhibitory concentration (MIC) across a panel of A. baumannii clinical isolates, and have therapeutic indices (TI) up to 6-fold higher than the parent 2-AIs.

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.

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 β-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.

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 mu g/mL at 30 mu M against Klebsiella pneumoniae 2146, and from 32 to 2 mu g/mL at 5 mu M, 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. Small molecule adjuvants have the potential to expand the spectrum of gram-positive selective antibiotics to encompass gram-negative bacteria. We identifiy a series of 2-aminobenzimidazole (2-ABI) macrolide adjuvants with activity against Acinetobacter baumannii and Klebsiella pneumoniae. Preliminary mechanism of action studies suggests that adjuvants bind to lipopolysaccharide in K. pneumoniae, and modulate lipooligosaccharide biosynthesis, assembly, or transport in A. baumannii. image

Antimicrobial resistance (AMR) has led to a marked reduction in the effectiveness of many antibiotics, representing a substantial and escalating concern for global health. Particularly alarming is resistance in Gram-negative bacteria due to the scarcity of therapeutic options for treating infections caused by these pathogens. This challenge is further compounded by the rising incidence of resistance to colistin, an antibiotic traditionally considered a last resort for the treatment of multi-drug resistant (MDR) Gram-negative bacterial infections. In this study, we demonstrate that adjuvants restore colistin sensitivity in vivo. We previously reported that the salicylanilide kinase inhibitor IMD-0354, which was originally developed to inhibit the human kinase IKK beta in the NF kappa B pathway, is a potent colistin adjuvant. Subsequent analog synthesis using an amide isostere approach led to the creation of a series of novel benzimidazole compounds with enhanced colistin adjuvant activity. Herein, we demonstrate that both IMD-0354 and a lead benzimidazole effectively restore colistin susceptibility in mouse models of highly colistin-resistant Klebsiella pneumoniae and Acinetobacter baumannii-induced peritonitis. These novel adjuvants show low toxicity in vivo, significantly reduce bacterial load, and prevent dissemination that could otherwise result in systemic infection.

The Centers for Disease Control and Prevention (CDC) reports that hospital acquired infections have increased by 65% since 2019. One of the main contributors is the gram-negative bacterium Acinetobacter baumannii. Previously, we reported aryl 2-aminoimidazole (2-AI) adjuvants that potentiate macrolide antibiotics against A. baumannii. Macrolide antibiotics are typically used to treat infections caused by gram-positive bacteria, but are ineffective against most gram-negative bacteria. We describe a new class of dimeric 2-AIs that are highly active macrolide adjuvants, with lead compounds lowering minimum inhibitory concentrations (MICs) to or below the gram-positive breakpoint level against A. baumannii. The parent dimer lowers the clarithromycin (CLR) MIC against A. baumannii 5075 from 32 μg/mL to 1 μg/mL at 7.5 μM (3.4 μg/mL), and a subsequent structure activity relationship (SAR) study identified several compounds with increased activity. The lead compound lowers the CLR MIC to 2 μg/mL at 1.5 μM (0.72 μg/mL), far exceeding the activity of both the parent dimer and the previous lead aryl 2-AI. Furthermore, these dimeric 2-AIs exhibit considerably reduced mammalian cell toxicity compared to aryl-2AI adjuvants, with IC50s of the two lead compounds against HepG2 cells of >200 μg/mL, giving therapeutic indices of >250.

Gram-negative bacteria are intrinsically resistant to many classes of antibiotics, predominantly due to the impermeability of the outer membrane and the presence of efflux pumps. Small molecule adjuvants that circumvent these resistance mechanisms have the potential to expand therapeutic options for treating Gram-negative infections to encompass antibiotic classes that are otherwise limited to treating Gram-positive infections. Adjuvants that effect increased antibiotic permeation, either by physical disruption of the outer membrane or through interference with synthesis, transport, or assembly of membrane components, and adjuvants that limit efflux, are discussed as potential avenues to overcoming intrinsic resistance in Gram-negative bacteria.

In 2019, five million deaths associated with antimicrobial resistance were reported by The Centers for Disease Control and Prevention (CDC). Acinetobacter baumannii, a Gram-negative bacterial pathogen, is among the list of urgent threats. Previously, we reported 2-aminoimidazole (2-AI) adjuvants that potentiate macrolide activity against A. baumannii. In this study, we identify several of these adjuvants that sensitize A. baumannii to aminoglycoside antibiotics. Lead compounds 1 and 7 lower the tobramycin (TOB) minimum inhibitory concentration (MIC) against the TOB-resistant strain AB5075 from 128 μg/mL to 2 μg/mL at 30 μM. In addition, the lead compounds lower the TOB MIC against the TOB-susceptible strain AB19606 from 4 μg/mL to 1 μg/mL and 0.5 μg/mL, respectively, at 30 μM and 15 μM. The evolution of resistance to TOB and 1 in AB5075 revealed mutations in genes related to protein synthesis, the survival of bacteria under environmental stressors, bacteriophages, and proteins containing Ig-like domains.

Infections caused by methicillin-resistant Staphylococcus aureus (MRSA) are difficult to treat due to their resistance to many β-lactam antibiotics, and their highly coordinated excretion of virulence factors. One way in which MRSA accomplishes this is by responding to environmental stimuli using two-component systems (TCS). The ArlRS TCS has been identified as having a key role in regulating virulence in both systemic and local infections caused by S. aureus. We recently disclosed 3,4'-dimethoxyflavone as a selective ArlRS inhibitor. In this study we explore the structure-activity relationship (SAR) of the flavone scaffold for ArlRS inhibition and identify several compounds with increased activity compared to the parent. Additionally, we identify a compound that suppresses oxacillin resistance in MRSA, and begin to probe the mechanism of action behind this activity.

Antibiotic resistance is a major public health issue due to the growing lack of effective anti-infective therapeutics. New alternatives to treat and prevent biofilm-driven microbial infections, which exhibit high tolerance to clinically available antibiotics, are urgently needed.

Colistin, typically viewed as the antibiotic of last resort to treat infections caused by multidrug-resistant (MDR) Gram-negative bacteria, had fallen out of favor due to toxicity issues. The recent increase in clinical usage of colistin has resulted in colistin-resistant isolates becoming more common. To counter this threat, we have investigated previously reported compounds, HSD07 and HSD17, and developed 13 compounds with more desirable drug-like properties for colistin sensitization against 16 colistin-resistant bacterial strains, three of which harbor the plasmid-borne mobile colistin resistance (mcr-1). Lead compound HSD1624, which has a lower LogDpH7.4 (2.46) compared to HSD07 (>5.58), reduces the minimum inhibitory concentration (MIC) of colistin against Pseudomonas aeruginosa strain TRPA161 to 0.03 μg/mL from 1024 μg/mL (34,000-fold reduction). Checkerboard assays revealed that HSD1624 and analogues are also synergistic with colistin against colistin-resistant strains of Escherichia coli, Acinetobacter baumannii, and Klebsiella pneumoniae. Preliminary mechanism of action studies indicate that HSD1624 exerts its action differently depending on the bacterial species. Time-kill studies suggested that HSD1624 in combination with 0.5 μg/mL colistin was bactericidal to extended-spectrum beta-lactamase (ESBL)-producing E. coli, as well as to E. coli harboring mcr-1, while against P. aeruginosa TRPA161, the combination was bacteriostatic. Mechanistically, HSD1624 increased membrane permeability in K. pneumoniae harboring a plasmid containing the mcr-1 gene but did not increase radical oxygen species (ROS), while a combination of 15 μM HSD1624 and 0.5 μg/mL colistin significantly increased ROS in P. aeruginosa TRPA161. HSD1624 was not toxic to mammalian red blood cells (up to 226 μM).

Multidrug resistant (MDR) bacterial infections have become increasingly common, leading clinicians to rely on last-resort antibiotics such as colistin. However, the utility of colistin is becoming increasingly compromised as a result of increasing polymyxin resistance. Recently we discovered that derivatives of the eukaryotic kinase inhibitor meridianin D abrogate colistin resistance in several Gram-negative species. A subsequent screen of three commercial kinase inhibitor libraries led to the identification of several scaffolds that potentiate colistin activity, including 6-bromoindirubin-3'-oxime, which potently suppresses colistin resistance in Klebsiella pneumoniae. Herein we report the activity of a library of 6-bromoindirubin-3'-oxime analogs and identify four derivatives that show equal or increased colistin potentiation activity compared to the parent compound.

Ovarian cancer is the sixth leading cause of cancer-related death in women, and both occurrence and mortality are increased in women over the age of 60. There are documented age-related changes in the ovarian cancer microenvironment that have been shown to create a permissive metastatic niche, including the formation of advanced glycation end products, or AGEs, that form crosslinks between collagen molecules. Small molecules that disrupt AGEs, known as AGE breakers, have been examined in other diseases, but their efficacy in ovarian cancer has not been evaluated. The goal of this pilot study is to target age-related changes in the tumor microenvironment with the long-term aim of improving response to therapy in older patients. Here, we show that AGE breakers have the potential to change the omental collagen structure and modulate the peritoneal immune landscape, suggesting a potential use for AGE breakers in the treatment of ovarian cancer.

Multidrug resistant (MDR) bacteria are an increasing public health problem. One promising alternative to the development of new antibiotics is the use of antibiotic adjuvants, which would allow the continued use of FDA-approved antibiotics that have been rendered ineffective due to resistance. Herein, we report a series of dipyrrins and pyrrole derivatives designed as analogues of prodigiosin and obatoclax, several of which potentiate the activity of colistin against Klebsiella pneumoniae, with lead compounds also potentiating colistin against Acinetobacter baumannii and Pseudomonas aeruginosa.

Over the past decades, antibiotic resistance has grown to a point where orthogonal approaches to combating infections caused by resistant bacteria are needed. One such approach is the development of non-microbicidal small molecules that potentiate the activity of conventional antibiotics, termed adjuvants. The diterpene natural product 12(S),16ϵ-dihydroxycleroda-3,13-dien-15,16-olide, which we refer to as (−)-LZ-2112, is known to synergize with oxacillin against methicillin-resistant Staphylococcus aureus (MRSA). To explore this activity, (−)-LZ-2112 was synthesized and the structure confirmed through X-ray analysis. Preliminary structure–activity relationship studies following the synthesis of several analogs identified key structural elements responsible for activity and indicate that scaffold simplification is possible. A preliminary mode of action study suggests mecA plays a role in the adjuvant activity of (−)-LZ-2112.