A Different Way to Kill Bacteria: Experimental Peptide Targets MDR Pathogens

As antimicrobial resistance continues to erode the effectiveness of existing antibiotics, researchers are looking beyond traditional drug discovery for new ways to attack difficult-to-treat pathogens. One experimental approach starts with a molecule that the human immune system already produces. In this Infection Control Today^® (ICT^®) Q&A, Mario Danek discusses D8, a preclinical antimicrobial peptide derived from the immune-signaling protein CXCL10, and how researchers are investigating its ability to kill multidrug-resistant (MDR) bacteria through a mechanism distinct from that of conventional antibiotics. Danek explains the science behind the peptide, its activity against priority hospital pathogens, its potential for inhaled treatment of resistant respiratory infections, and the milestones it must clear before reaching human trials.

ICT: This antimicrobial peptide is derived from CXCL10, a protein already produced by the human immune system. What led researchers to CXCL10, and what does its human origin offer that a traditional antibiotic does not?

Mario Danek: CXCL10 was already known to science as an immune-signaling molecule, an interferon-inducible chemokine the body produces at sites of infection to recruit immune cells. What drew researchers to it is that, beyond that signaling role, it was found to directly kill a broad range of pathogenic bacteria. The part of CXCL10 that resembles a traditional antimicrobial peptide is the C-terminus.That part kills bacteria but has stability and toxicity issues as a standalone peptide.The breakthrough was that when we chopped the C-terminus from CXCL10, the truncated molecule could still kill bacteria. This includes a region that closely resembles classical human antimicrobial peptides, so a host-defense peptide was essentially hiding inside a signaling molecule the immune system already uses. Our lead molecule, D8, is a short peptide taken from the active, N-terminal part of that sequence.

The appeal of a human-origin starting point is that this is an ancient, evolved arm of innate immunity. Bacteria have been exposed to host-defense peptides across evolutionary time, yet they haven’t developed the widespread resistance we see to man-made antibiotics, because these molecules exploit structural vulnerabilities that are far harder to mutate away from. So rather than screening synthetic libraries hoping to find a novel scaffold, we started from a sequence evolution had already optimized to kill pathogens and then engineered it for drug-like stability.

ICT: You’ve said the peptide kills drug-resistant bacteria through a mechanism that conventional antibiotics do not use. Can you explain that mechanism and why it could remain effective against resistant organisms?

MD: D8 comes from the N-terminal region of CXCL10, and that region kills through a mechanism no conventional antibiotic class uses: It dysregulates the bacterial FtsE/X complex, the machinery that controls the enzymes bacteria rely on to build and remodel their cell wall as they grow and divide. Interfering with it causes loss of cell wall integrity, and the cell breaks apart. We see this directly: bacteria engineered to lack the FtsX component are markedly less susceptible to the peptide, but we are still exploring multifactorial causes.

The strongest evidence that this is a genuinely different mechanism comes from colistin, a last-line antibiotic. Bacteria that have become resistant to colistin, through several distinct resistance mutations, are killed by our peptide just as well as colistin-susceptible bacteria. There’s no cross-resistance. That tells you the peptide isn’t relying on the same vulnerabilities existing antibiotics exploit, which is exactly why it can stay effective where those drugs have failed. And because it attacks essential structural machinery rather than a single enzyme target, it’s inherently harder for bacteria to mutate their way around. Furthermore, we know that D8 can efficiently kill a clinical isolate of Klebsiella pneumoniae that is not susceptible to any currently available antibiotic, so the argument can be extended beyond colistin.

ICT: A major concern with any new antimicrobial is whether bacteria eventually develop resistance to it. What have your preclinical studies shown so far about the potential for resistance to emerge?

MD: Our clearest data on this is the colistin finding I just described: Across bacteria carrying several different colistin-resistance mechanisms, including the mobile mcr-1 gene and chromosomal regulatory mutations, the peptide retained full killing activity, with no cross-resistance. That’s meaningful because it shows that resistance to today’s last-line drugs doesn’t carry over to ours. Combined with a mechanism that targets essential, structurally conserved machinery, this gives us a real reason to expect resistance to be slow and difficult to develop. But we’re careful here; no one can honestly call any antimicrobial resistance-proof.

ICT: Which drug-resistant organisms have been tested against the peptide, and have you seen differences in activity against gram-positive vs gram-negative or particularly difficult pathogens?

MD: We’ve tested D8 against a broad panel of antibiotic-resistant clinical pathogens spanning both gram-negative and gram-positive organisms. On the gram-negative side, that includes carbapenem- and drug-resistant Acinetobacter baumannii, Klebsiella pneumoniae, Escherichia coli, Enterobacter, Salmonella Typhi, Shigella, Pseudomonas aeruginosa, and drug-resistant Neisseria gonorrhoeae; on the gram-positive side, vancomycin-resistant Enterococcus, Staphylococcus aureus, and the biodefense pathogen Bacillus anthracis. It’s broadly active across that range.

There are differences worth being candid about. The peptide is most potent against the carbapenem-resistant Enterobacterales and Acinetobacter, some of the hardest, highest-priority hospital pathogens, and against vancomycin-resistant Enterococcus. P aeruginosa and some staph strains are less susceptible and represent the kind of gap we’d work to close in optimization. We’d rather be precise about where it is strongest than claim it kills everything equally.

ICT: For infection prevention professionals facing multidrug-resistant organisms today, where could you envision this therapy fitting—treating active infections, prevention, addressing colonization, or working alongside existing antibiotics?

MD: Our platform delivers the peptide by inhalation, straight to the lung, which points to a clear lead use: treating multidrug-resistant respiratory infections. And that lines up with the biology—the pathogens D8 is most potent against, like carbapenem-resistant Klebsiella and Acinetobacter, are major drivers of MDR hospital pneumonia. We’ve also shown activity in a topical wound-infection setting, so localized treatment of resistant wound infections is a second natural fit. In both cases, the logic is the same: deliver a host-defense peptide directly to the site of infection, at high local concentration, without systemic exposure.

For your readership specifically, the near-term promise is as a targeted option for difficult-to-treat MDR infections where the current toolbox is running out of options, and, in parallel, potentially alongside existing antibiotics, since a mechanism with no cross-resistance is a good partner for combination approaches.

ICT: What are the most important scientific and clinical milestones this peptide must achieve next, and what findings would tell you it has a realistic path toward human trials?

MD: Several of the hardest early questions are already answered, which is what gives us confidence in the path. The peptide is engineered for stability, so it isn’t degraded the way natural peptides are. It survives aerosolization through our device, with its concentration and antimicrobial activity fully intact, and, importantly for lung therapy, pulmonary surfactant doesn’t inhibit it. We’ve shown it kills a broad panel of MDR pathogens, is effective in animal models of both wound and lung infections, and doesn’t rupture red blood cells at active concentrations or exert cytotoxic effects against human cells.

From here, the key milestones are demonstrating efficacy when the drug is delivered by inhalation in vivo, finishing lead optimization (we also have proprietary modifications [additional candidates] that enhance more potent activity, and those are being explored in parallel), and then the formal IND-enabling package: safety and toxicology, pharmacokinetics, and manufacturing scale-up. The finding that would tell us there’s a real path to the clinic is reproducible efficacy from the inhaled product in a relevant infection model at well-tolerated doses. Given where the in vitro and delivery data already are, that’s the next proof point we’re focused on.

ICT: If this succeeds, could CXCL10 represent more than a single candidate—could studying molecules the immune system already uses become a broader strategy for discovering new classes of antimicrobials?

MD: Yes, and we’re already demonstrating it. CXCL10 has multiple antimicrobial regions and is one of a family of immune chemokines with antibacterial activity. More broadly, the immune system is a vast, prevalidated library of molecules that evolution has already tuned to kill pathogens, a much better starting point than screening random chemical space. And we’ve shown the sequence is a workable engineering platform, not a fixed molecule: We’ve built a stabilized D-amino-acid version and more potent dimer forms from it. Our delivery platform is largely agnostic to which molecule in this class we carry, so the vision isn’t a single drug; it’s a pipeline of immune-derived antimicrobials delivered directly where they’re needed. D8 is the first expression of that approach, not the whole of it.