Development of therapeutic approaches to overcome resistance in Gram-negative bacteria
The emergence and dissemination of Gram-positive and Gram-negative bacteria that are resistant to all currently available antimicrobials remain one of the pressing central challenges of this century and remains a major threat to public health and global economic recovery and growth. Since ancient times, bacteria have developed multimodal resistance mechanisms to all known antimicrobials. These include poor membrane permeability, membrane modification, suppression of porin expression, efflux, antibiotic inactivation by antibiotic-modifying enzymes, antibiotic target modification by mutations or methylation, biofilm formation and others that are used in concert to negate the activity of antibiotics.
The World Health Organization declared carbapenem-resistant Pseudomonas aeruginosa, carbapenem-resistant Acinetobacter baumannii and carbapenem-resistant Enterobacteriaceae as the three most critical pathogens that pose the greatest threat to human health. These Gram-negative pathogens are frequently multidrug-resistant-MDR (resistant to ≥ 3 different antibiotic classes) and have become resistant to all or almost all of our antibiotic armamentarium. Despite significant investments into antibiotic discovery in the past, no new antibiotic class with novel mode(s) of action against Gram-negative bacteria has been approved in half a century. In the absence of new antibiotics, approaches that restore or revive existing antibiotics against antibiotic-resistant pathogens are needed. This can be achieved by using helper molecules termed antibiotic adjuvants. Antibiotic adjuvants typically possess weak to no antibacterial activity on their own but are able to either impede antibiotic resistance mechanisms or potentiate antibiotic action. Importantly, antibiotic and adjuvant possess a synergistic relationship. In the presence of a multidrug-resistant pathogen, both, antibiotic (alone) or adjuvant (alone) show poor activity but when combined display potent antibacterial activity and ideally kill the pathogen in a short period of time. An adjuvant may be an efflux pump inhibitor to prevent extrusion of drugs, a membrane permeabilizer to increase the number of molecules that penetrate the membrane or an enzyme inhibitor to prevent degradation of drugs before reaching their targets.
Several approaches to design antibacterial adjuvants are currently investigated in our group.
Aminoglycosides like Tobramycin (TOB) induce multimodal, pleiotropic effects on the bacterial cell and kill bacteria like Pseudomonas aeruginosa by two concentration-dependent mechanisms. At low concentrations (≤ 4 ug/mL), TOB binds to the 30S ribosomal subunit leading to misreading of RNA, disruption of protein synthesis, and ultimately accumulation of truncated and nonfunctional proteins that leads to bacterial death. However, at higher concentrations (≥ 8 ug/mL), TOB kills P. aeruginosa by outer membrane disruption. Moreover, aminoglycosides are poor substrates for most efflux pumps that makes them ideal precursors for drug development. Recently, we have demonstrated that outer membrane disruption of TOB can be uncoupled from its ribosomal effects by appending hydrophobic moieties onto the C-5 position, thereby generating amphiphilic aminoglycosides with weak or no protein translation inhibitory effects but potent outer membrane-disrupting and efflux inhibitory properties. We are currently designing do novo aminoglycoside-based conjugates that rescue antibiotics from resistance.
(cover page of J. Med. Chem. issue 18, 2016)
Selected Publications: Antimicrob. Agents Chemother.2020, 64, e02055-19. J. Med. Chem.2019, 62, 9103-9115. Eur. J. Med. Chem.2019, 174, 16-32. J. Antimicrob. Chemother.2019, 74, 2640-2648. Eur. J. Med. Chem.2019, 175, 187-200. Clin. Microbiol. Rev.2018, 31 (3). J. Med. Chem.2017, 60, 3913-3932. J. Med. Chem.2017, 60, 3684-3702. ACS Infect. Dis.2017, 3, 941-954. J. Med. Chem.2016, 59, 8441-55. Angew. Chem. Int. Ed.2016, 55, 555-559.
Aminoglycosides and host defense peptides (antimicrobial peptides) share a common polybasic pharmacophore. However, in contrast to host defense peptides, aminoglycosides do not possess immunomodulatory properties. Moreover, aminoglycoside antibiotics are devoid of an amide-based peptide backbone that promote drug likeness. We have shown that conversion of tobramycin into amphiphilic tobramycin boost the innate immune response, specifically the recruitment of immune cells required for the resolution of infections. We are currently investigating structure activity relationships to develop amphiphilic aminoglycosides that combine immunomodulation with antibiotic potentiating effects.
Antimicrobial peptides enhance outer membrane permeability of compounds into Gram-negative bacteria via the self-promoted uptake (SPU) mechanism. SPU is a process by which polycationic (or polybasic) molecules, preferably those containing primary amine functional groups, displace the divalent cations (Ca2+ or Mg2+) that stabilize the lipopolysaccharide (LPS) membrane packing of the phosphate groups of lipid A in the outer membrane. This metal ion displacement leads to a localized disruption of LPS thereby enhancing the passage of molecules into the periplasm. The best known peptide-based antibiotic adjuvant is pentabasic polymyxin B nonapeptide (PMBN). PMBN is derived from polymyxin B by enzymatic hydrolysis of the lipid-diaminobutyric acid portion at the peptide N-terminus and possesses reduced toxicity in animals when compared with polymyxin B and E. An analog of PMBN, the tribasic SPR741 is currently in clinical development. We are currently developing PMBN-based peptidomimetics that retain the potent adjuvant properties of PMBN.
(graphic abstract from ACS Infect. Dis. 2020, 6, 1413-1426)
Salicylanilides is an emerging privileged scaffold with potent antibacterial, antiviral and antitumor properties but unsuitable pharmacokinetic properties. We recently have discovered that salicylanilides are potent antibiotic adjuvants which rescue polymyxins from bacterial resistance. Polymyxins are considered drugs of last resort to treat complicated multidrug-resistant bacterial infections with no other treatment option. However, polymyxin resistance is rising at an alarming rate. We are currently designing salicylanilide analogs with improved pharmacokinetic/pharmacodynamic properties and enhanced adjuvant properties to restore colistin susceptibility in colistin-resistant bacteria.
