Antimicrobial resistance (AMR) is a leading global challenge which is developing faster than we can currently treat it, according to Lisa Dawson, PhD, senior group leader, discovery, at contract research organization (CRO) Charles River. She recently spoke at the European Laboratory Research & Innovation Group’s (ELRIG) Conference 2024 in Manchester U.K.

“Recent estimates state that there will be 10 million deaths a year due to AMR by 2050, but because we don’t have comprehensive AMR data from low and middle income countries available, it could be even higher that this by 2050,” added Dave Chapman, PhD, head of biology at immunotherapy company, Centauri Therapeutics.

Presenters at the conference agreed AMR is reaching crisis point and discussed possible new weapons in the ongoing battle against it, including traditional approaches such as developing new types of antibiotics and innovations including utilizing bacteriocins, neutralizing antibodies, immunotherapy, or phage.

Claudia Zampaloni, PhD, senior principal scientist at Hoffmann-La Roche in Basel Switzerland, discussed her team’s development of zosurabalpin, a new antibiotic to treat carbapenem-resistant Acinetobacter baumannii infections.

“Today the World Health Organization (WHO) and the U.S. Centers for Disease Control and Prevention have identified Acinetobacter baumannii as a critical priority for advancing new antibiotics against because it is difficult to treat and is highly prevalent in hospitals and ICUs,” she said. “This bacterial species also kills between 40–60 percent of patients that become infected with it from bacterial pneumonia or sepsis.”

Zampaloni presented the development path of zosurabalpin noting: “We initially acquired a library of 45,000 macrocyclic peptides (MCPs) with possible antibiotic properties. A high throughput phenotypic screening showed 86 hit compounds that inhibited A.baumannii growth but spared related bacterial pathogens. After some lead optimization and medicinal chemistry on the compounds by Chembiotek, our partner CRO, we had some lead MCP molecules with potent in vitro activity, which we used for in vivo rodent studies.”

According to Zampaloni, mice treated with their first-generation lead MCP in a seven-day mouse sepsis trial survived A.baumannii infection, while untreated mice died. Additionally, in a localized model of infection the first generation MCP almost eradicated the bacterial burden from the thigh of neutropenic mice. However, in their rat mini-tox studies they saw toxicity and deaths following intravenous administration of the first generation MCP. Specifically, this compound caused formation of aggregated low-density lipoprotein/high-density lipoprotein vesicles, which was the most likely cause of the toxicity.

electron microscope image
Cryo-electron microscopy structure of the inner membrane Acinetobacter baylyi LptB2FG complex bound to lipopolysaccharide (LPS) and zosurabalpin, PDB ID: 8frn. [Claudia Zampaloni, PhD/Hoffmann-La Roche]
“Due to the toxicity we developed our next generation lead MCP by altering it to a zwitterionic form, with a balance between positively and negatively charged functional groups,” continued Zampaloni. “This maintains its anti-infective activity but prevents it from reacting with cholesterol/LDL in plasma.”


Zampaloni then presented data from other mice studies using the new MCP-selected clinical candidate they named “zosurabalpin.”1 The data included comparative studies with other antibiotics and pharmacokinetic/pharmacodynamic (PK/PD) studies in pneumonia models using 15 antibiotic-resistant strains of A.baumannii isolated from patients and an in vitro minimum inhibitory concentration (MIC) of zosurabalpin ranging from 0.06 to 8 mg/L. Based on these studies, the research team can calculate a dose that they estimate will cure 90 percent of infected humans.

The pharmacological target of zosurabalpin was also identified and its mode of action was elucidated by cryogenic electron microscopy structural studies. Performed in collaboration with Harvard University, the studies that zosurabalpin’s mechanism of action is to block transport of bacterial lipopolysaccharide from the inner membrane to the outer membrane through inhibition of the LptB2FGC complex, causing damage to the cell membrane followed by cell death.2

“Currently, we have preliminary safety and PK data from completed Phase 1 trials with healthy subjects and critically ill patients, which shows zosurabalpin is safe and well tolerated when given intravenously,” explained Zampaloni. “We are now looking to move into randomized clinical trials in severe Acinetobacter infections. Targeting the LptB2FGC complex is a novel mechanism of action, which could even be utilized in other Gram negative bacteria to develop promising new antibiotics.”

Power of bacteriocins

An interesting alternative to antibiotics was presented by Daniel Walker, PhD, professor at the University of Strathclyde and CSO of Glox Therapeutics, a spinout from the universities of Oxford and Glasgow. Walker stated: “The WHO has identified Gram negative bacteria such as E. coli, Pseudomonas aeruginosa, and some Klebsiella and Acinetobacter species as having widespread AMR and as a critical need for developing new antimicrobials. These Gram negative bacteria are difficult to kill because they have a highly impermeable outer membrane, and it is difficult for many antibiotics to get across it. However, many bacteria naturally produce bacteriocins, which are proteins that have evolved to efficiently cross the outer membrane.”

Bacteriocins, which include colicins and pyocins, are peptides that exert their antibacterial effects mechanisms such as membrane disruption, DNA and RNA degradation, and inhibition of cell wall synthesis.

According to Walker, the major advantage of using bacteriocins to treat bacterial infections is that they leave the host microbiome intact. This is because they are targeted and only have antimicrobial properties against strains of the same or related species, often tricking their host into importing them inside the cell where they cause the damage.

bacterium structure
The structure of pyocin S5, a pore-forming bacteriocin that can selectively target and kill isolates of Pseudomonas aeruginosa. [Daniel Walker, PhD/University of Strathclyde]
“At Glox, we are developing pyocins that target P. aeruginosa because this (infection) has a high mortality rate, and it is difficult to treat with most antibiotics,” commented Walker.”


He presented in vivo mouse model data of BALB/c mice infected with P. aeruginosa strains. The data showed that when treatment by inhaled delivery with their lead pyocin molecule S5 reduced colony-forming unit (CFU) infection in the lungs by greater than four-logs (10, 000 fold) five hours post-infection. He also presented mouse model data, which demonstrated that when compared to the antibiotic tobramycin dosed at 300 µg/mL, pyocin S5 was significantly more potent, causing an equivalent reduction in P. aeruginosa CFUs in the lung when dosed at 30 ng/mL.

Walker also presented data of an ex vivo model of the human intestinal microbiome extracted from 10 volunteers, which showed that colicins were able to selectively kill pathogenic E. coli but without affecting the wider intestinal microbial community.

“Our Glox bacteriocins provide an approach which targets specific bacterial strains, while leaving the gut microbiome intact,” said Walker. “Our current challenges are that our IV delivery does not produce optimum pharmacokinetics and our species coverage may not be wide enough. However, we are working on those areas, and in 2023 we were awarded £4.3 million ($5.4 million) in seed funding from Boehringer Ingelheim Venture Fund and Scottish Enterprise to develop platform technologies to engineer bacteriocins with improved coverage and optimized activities.”

Leveraging the immune system

Another interesting alternative to antibiotics was presented by David Chapman, PhD, head of biology at Centauri Therapeutics. He talked about harnessing the body’s own immune system. “Between 35–50 percent of antibiotics falter at Phase III or commercialization, which makes developing antibiotics a tough space to be in,” noted Chapman.

“At Centauri, we are trying to do things differently and have proprietary technology to develop Alphamer® molecules in our ABX01 program to target drug resistant Gram-negative bacteria by triggering the body’s own immune system,” he continued.

binding of Alphamer to a bacterium
Pictogram showing binding of Alphamer to a bacterium; the natural pre-existing antibodies bind to the Alphamer. This induces complement to be activated which drives killing of bacteria by both serum killing and opsonophagocytosis. [David Chapman, PhD/Centauri Therapeutics]
Alphamer molecules have three different parts, including a targeting domain that consists of an antimicrobial peptide that binds to surface molecules on bacteria. The targeting domain is attached via a linker to an effector domain (an alpha-galactose epitope) that binds to naturally occurring antibodies in the body. When the target/linker/effector domains and anti-glycan antibodies are all bound, a complement cascade is activated, directing the immune system to destroy the bacteria.


Chapman presented in vivo neutropenic GTKO mouse model data of mice infected with E. coli or E. coli K12. In the first study, mice were immunized with an anti-galactose antibody, then infected with E. coli K12 and treated with the Alphamer CTX-09. Treated mice showed almost a three-log reduction in bacterial CFU 30 hours post treatment compared with control mice, which were immunized with phosphate buffered saline followed by CTX-09, treatment suggesting that the Alphamer’s potency is enhanced by up to six-fold by the prior presence of the anti-galactose antibody.

In a second study, mice treated with anti-galactose antibody and the Alphamer molecules CTX-107 and CTX-031, also showed a two- to three-log reduction in CFU of bacteria 30 hours post-infection compared to control anti-galactose antibody-untreated mice.

“Our mouse studies indicate that Alphamers have a dual mode of action with immunotherapeutic and anti-bacterial activities in one molecule,” said Chapman. “The Combating Antibiotic-Resistant Bacteria Biopharmaceutical Accelerator (CARB-X) provided $1.4m funding in 2019 and a further $1.9m for continued research in 2022. Furthermore, in 2022 Centauri received Series A investment of £24 million ($32 million) from Boehringer Ingelheim Venture Fund, Evotec SE and Novo Holdings REPAIR Impact Fund to continue the research, identify and progress novel Alphamers to first-in-human clinical trials for difficult-to-treat Gram-negative bacterial infections.”

What’s next?

“We are heading to a period where all antibiotics will become ineffective, and this will have a major impact on our ability to safely perform any surgery or use chemotherapy,” noted Daniel Walker. “We will effectively lose all the gains we have made with modern medicine because there are very few new antibiotics being funded and developed.

“We are moving into this perfect storm because we have a weak pipeline of new treatments available and endemic resistance to the existing antibiotics of last resort such as carbapenem, so we urgently need even more investment in antibiotics and new types of anti-infectives.”

Sue Pearson, PhD, is a freelance writer living in the U.K.


  1. Zampaloni, C., Mattei, P., Bleicher, K. et al. A novel antibiotic class targeting the lipopolysaccharide transporter. Nature 625, 566–571 (2024).
  2. Pahil, K.S., Gilman, M.S.A., Baidin, V. et al. A new antibiotic traps lipopolysaccharide in its intermembrane transporter. Nature 625, 572–577 (2024).



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