Scientists at Uppsala University have discovered a new class of antibiotics with potent activity against multi-drug resistant (MDR) Gram-negative bacteria. The new compounds target a protein, LpxH, which is used by Gram-negative bacteria in a pathway to synthesize lipopolysaccharide (LPS), a structural component in the outer membrane (OM) of the bacterial cell wall. LPS is implicated in playing a role in the OM’s structural integrity and in maintaining a permeability barrier function against molecules including antibiotics. Through in vivo tests the researchers showed that the new class of compound can successfully treat bloodstream infections in mice.

Reporting on their development in ScienceAntibiotic class with potent in vivo activity targeting lipopolysaccharide synthesis in Gram-negative bacteria,” first author Douglas L. Huseby, PhD, Anders Karlén, PhD, and colleagues, concluded, “This work verifies that the target of these compounds, LpxH in Gram-negative bacteria, is a viable target for antibiotics, and that the chemical series that we have investigated is promising for further development.”

Antibiotics are “an essential pillar of modern medicine,” the authors wrote. Over the last century such drugs have dramatically improved the lives of people around the world. Nowadays we tend to take antibiotics for granted and rely heavily on them to treat or prevent bacterial infections, including for example, to reduce the risk of infections during cancer therapy, during invasive surgery and transplants, and in mothers and preterm babies.” The ready availability of effective antibiotics underpins all of modern medicine,” the team further noted. Increasingly, however, the continuing global rise in antibiotic resistance is threatening their effectiveness. “In many parts of the world, multidrug-resistant (MDR) and even pan-drug-resistant bacterial pathogens are becoming increasingly frequent …” the team continued. “The ever-increasing spread of antimicrobial resistance continuously degrades the efficacy of existing antibiotics, thus requiring the development of innovative compounds and strategies simply to maintain the status quo.”

In response to this threat, World Health Organization (WHO) has issued a list of priority pathogens against which there is an urgent need to discover and develop new antibiotics, the scientists stated. Heading this list as “critical priority” are certain Gram-positive bacterial species, including Escherichia coli and Klebsiella pneumoniae, Pseudomonas aeruginosa and Acinetobacter baumannii. “These species are designated as a critical priority for antibiotic development because they are increasingly resistant to the currently most effective classes of broad-spectrum antibiotics (3rd generation cephalosporins and carbapenems) and because they are responsible for a large proportion of community-acquired and nosocomial infections worldwide,” the authors continued. In fact, they pointed out, no new class of Gram-negative-acting antibiotic has entered the market since fluoroquinolones in the 1970s.

Supported by the EU ENABLE project, a multi-national consortium co-led by researchers at Uppsala University and GlaxoSmithKline, the multinational research team has now developed a new class of antibiotics targeting LpxH, an enzyme involved in the pathway used by some Gram-negative bacteria to synthesize the Lipid A portion of LPS. “Roughly 70% of Gram-negative bacteria, including the WHO critical priority Gram-negative pathogens for R&D of new antibiotics, utilize this enzyme,” the scientists noted in their report. “The lipopolysaccharide synthesis pathway is essential in most Gram-negative bacteria and there is no analogous pathway in humans.” Moreover, they commented, there has been considerable interest in developing antibiotics that target the lipid A synthesis pathway since it is present in all of the WHO-designated critical priority pathogens for R&D of new antibiotics.”

Starting with a series of phenotypic screens, the team describe the development of the unique antibiotic class that targets lipopolysaccharide synthesis, moving from an initial hit to the design of compounds with favorable drug-like properties and potent in vivo activity. The investigators first identified a hit compound, JEDI-852, with good affinity to the LpxH enzyme. They then combined structural features of this compound with those of the one other LpxH inhibitor that was then known, AZ1, to create what they described as a new scaffold with a vastly improved set of properties. They subsequently carried out further development to optimize properties such as solubility, metabolic stability and serum protein binding.

Importantly, since the resulting compound class is completely new and the protein LpxH has not yet been exploited as a target for antibiotics, there is no pre-existing resistance to this class of compounds, the researchers pointed out. This is in contrast to the many ‘me-too’ antibiotics of existing classes currently in clinical development.

“The LpxH inhibitors we have described here, EBL-3599 and EBL-3647, have potent in vitro activity against a diverse range of clinical E. coli and K. pneumoniae isolates, irrespective of the resistance genotype,” they reported. Importantly, the compounds also showed potent in vivo activity. “We demonstrate that these compounds targeting LpxH are also active against E. coli and K. pneumoniae in an in vivo peritonitis model,” the scientists wrote. “During the course of this infection model, bacteria spread to the bloodstream of the mice. The ability of these compounds to strongly reduce the number of bacteria recovered from blood in only a single-dose treatment highlights their potential to treat the most life-threatening infections with Gram-negative MDR pathogens.” Acknowledging that there will be considerable additional work required before compounds of this class will be ready for clinical trials, the team concluded, “Further development of this class of antibiotics could make an important contribution to the ongoing struggle against antibiotic resistance.”

The ENABLE project brought together stakeholders from across Europe representing academia and large and small pharmaceutical companies to pool resources and expertise to advance early-stage antibiotic development. This antibiotic class now continues to be developed in the follow-on project, ENABLE-2, an antibiotic drug discovery platform funded by Swedish Research Council, the National Research Programme on Antibiotic Resistance and Sweden’s innovation agency Vinnova to continue the momentum generated by the original ENABLE project.

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