Chemically flipped TB peptide becomes more potent and less toxic in an unexpected result
The most common approach to treating tuberculosis involves a combination of antibiotics taken over months. That regimen works - when patients complete it - but resistance is growing, and the search for compounds that act through entirely different mechanisms has become a public health priority. A research team at Penn State and the University of Minnesota Medical School has been working in one such direction, and a recent result surprised them.
The team started with host-defense peptides (HDPs) - short chains of amino acids that the human immune system produces naturally and that have long been studied as potential anti-infective agents. The problem with HDPs as drugs is stability: enzymes in the body degrade them quickly, limiting how long they remain active. To address this, the researchers applied two chemical modifications known to resist enzymatic breakdown. "Backbone inversion" reverses the direction of the peptide's structural framework. "Chirality switching" alters the spatial orientation of amino acid side chains from the natural left-handed form to the right-handed mirror image.
The combination - called retro-inversion - produces a molecule that is structurally mirrored in both senses. Enzymes that readily cut the natural peptide cannot efficiently recognize and cleave the inverted version. The researchers expected improved stability. They did not expect what else they found.
An unexpected gain in potency
"When we compared the original molecule to the one that we did modify, not only was the modified one more stable, but now it was also much more active," said Scott Medina, Korb Early Career Associate Professor of Biomedical Engineering at Penn State and corresponding author on the paper, published in Nature Communications. "That's something that we didn't expect to see."
Using microscopy and structural analysis techniques, the team identified the cause. The retro-inverted form adopts a shape that is more energetically efficient at penetrating the lipid membranes surrounding bacterial cells. Host-defense peptides kill bacteria by physically disrupting these membranes - not by targeting specific biochemical pathways inside the cell. That mechanism distinguishes them from conventional antibiotics, which bacteria can evade by mutating target proteins.
Physical membrane disruption is harder for bacteria to evolve around. A bacterium cannot simply mutate a single protein to resist a molecule that tears apart its outer boundary. "There is an interest in molecules that may be difficult for bacteria to evolve resistance towards, providing a longer span of time for these treatments to be clinically useful," Medina said.
Active against TB, less toxic to human cells
The retro-inverted peptide was specifically more potent against mycobacteria, the family of organisms that includes Mycobacterium tuberculosis. It also showed reduced toxicity to human cells compared to the original molecule - an important property since HDPs can sometimes disrupt mammalian cell membranes as well as bacterial ones.
The researchers do not envision this compound replacing the existing TB drug regimen. Tuberculosis treatment already requires combination therapy to prevent resistance and address different stages of bacterial infection. What the modified peptide might do is enhance that regimen's effectiveness when added as an adjunct.
"We don't envision that this is a drug that's going to entirely replace current TB therapies," Medina said. "Rather, we think the biggest value of our molecule is its potential to enhance the activity of current TB drugs when given together, making the current treatments much more effective."
Scope and remaining work
The study was conducted in laboratory cell and molecular assays rather than in animal models or humans. The jump from demonstrating potency against bacteria in culture to demonstrating therapeutic efficacy in a living organism involves many additional steps - pharmacokinetics, toxicity profiling in animals, and eventually clinical trials. None of that work is reported in this paper. What it establishes is the chemical basis for the observed activity gain and the reduced human cell toxicity, providing a foundation for the next phase of investigation.
Tuberculosis remains one of the leading infectious disease killers globally, with an estimated 1.25 million deaths in 2023 according to the World Health Organization. Drug-resistant TB strains are responsible for a growing fraction of cases. The WHO has flagged M. tuberculosis as a priority pathogen for new treatment development, alongside other resistant bacteria including E. coli, K. pneumoniae, Salmonella, and Acinetobacter.