INFORM October 2024

14 • inform October 2024, Vol. 35 (9)

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“This phenotypic screening approach overcomes well-doc umented technical hurdles encountered in target-based screens, specifically by selecting for ‘hits’ that have the ability to cross the bacterial envelope and inhibit essential targets, thereby killing the bacteria,” Bradley said. Optimization of an initial hit from the phenotypic screen led to a potent antibacterial compound, zosurabalpin, but the Roche team did not know how it worked. They collaborated with researchers at Harvard to establish the mechanism of action, demonstrating that MCP block the trafficking of lipo polysaccharides (LPS). LPS is part of the gram-negative bacteria’s armor. Chains of sugars anchored by Lipid A, a hydrophobic phos phoglycolipid, line the outer membrane, helping “create a dense electrolyte mesh that prevents entry of hydro phobic molecules including most antibiotics,” as Harvard companion paper author Daniel Kahne described it on his lab’s website. The bacteria’s inner membrane synthesizes LPS, moving it through other interior layers to the outer membrane. “Surprisingly, this new class of antibiotics binds both to the transport complex in the inner membrane, as well as the LPS itself, preventing its transport to the outer membrane,” Bradley explained. “Consequently, the LPS remains trapped in the inner membrane complex. Without the ability to transport LPS, the bacteria die.” Roche’s zosurabalpin is currently in human clinical tri als, with the goal to assess the safety, tolerability, and phar macology of the molecule. “Zosurabalpin has many features that position it to be a medical breakthrough,” Bradley said. “Future human clinical trials will inform whether it has the potential to address a major gap in the fight against antimicro bial resistance.” EXPANDING THE TOOLBOX FOR ANTIBIOTICS Roche is not alone in exploring membrane lipids as a possi ble venue for new antibiotics approaches, but the field is not exactly crowded. Based on both the science and expectations in the market, an antibiotic must be cheap and effective, but used as sparingly as possible to prevent resistance. Some vari eties may target a limited set of ailments, further constraining potential revenue. “Antibiotics are not particularly lucrative, and so pharma ceutical companies have left the industry. There is almost no one left,” said Doug Huseby, a researcher in the department of medical biochemistry and microbiology at Uppsala University in Sweden. Bradley echoed Huseby’s comments, saying, “Companies working in this space have faced severe financial challenges, because there is not a viable market for new antibiotics based on traditional incentives or reimbursement models.” Besides zosurabalpin, Roche has one other novel antibi otic in the works, Genentech research and early development’s LepB inhibitor (RG6319), which also is focused on the treat ment of carbapenem-resistant gram-negative infections. Both Bradley and Huseby noted a need to fix the economics of anti

biotics through incentives or different commercial models that could stimulate investment. Huseby published research in April that also targets LPS in gram-negative bacteria, albeit in a different way and in a different organism, E. coli . His paper, “Antibiotic class with potent in vivo activity targeting lipopolysaccharide synthesis in Gram-negative bacteria,” was published in the Proceedings of the National Academy of Sciences (https://doi.org/10.1073/ pnas.2317274121). The outer leaflet of gram-negative bacteria is “something that is difficult to overcome, but it also represents a weakness,” Huseby said, because human cells do not have it. “If you can find something to stop the synthesis of this outer membrane, it means it is unlikely to specifically interact with any human sort of pathways.” His team’s research, which is also based on phenotypic screens, inhibits one of nine enzymes needed to make Lipid A, the anchor in LPS. “The importance of LPS, and Lipid A in par ticular, for the viability and virulence of gram-negative bacteria makes the inhibition of Lipid A biosynthesis a promising target for the development of antibiotics,” Huseby and coauthors wrote. Researchers continue to tweak existing antimicrobial mechanisms and use new technologies to search for untapped natural sources of antibiotics. Iterating on current strategies and exploring new ones, like targeting membrane lipids, are both required against an adversary that has billions of years of evolution in its favor, Huseby said. “Bacteria are always trying new things. They have huge pop ulations and a lot of variation, and so there is always pressure for them to come up with solutions to the problems that they are facing,” he said. “In that sense, resistance is almost inevitable.” Christina Nunez is a writer and editor based near Washington, D.C. She writes about science, technology, and innovation for a variety of organizations, including National Geographic and the U.S. Department of Energy. A schematic (a) of lipopolysaccharide (LPS) transport between inner and outer membranes of a bacterium cell. Researchers at Roche discovered that macrocyclic peptides (b) block the transport killing Acinetobacter strains. Source: Pahil, K.S., et al. , Nature , 625, 572-577, 2024.