Danish researchers have used pioneering organic chemistry to greatly reduce the cellular toxicity of a forgotten class of antibiotics to which bacteria develop little resistance.
Well before COVID-19 dominated the headlines, researchers, doctors and healthcare organizations around the world were concerned about antimicrobial resistance.
In the long term, antimicrobial resistance is a much greater threat to global health than COVID-19, because if drugs no longer kill bacteria, procedures as simple as surgery may become too dangerous because infections can no longer be eradicated.
We therefore desperately need new types of antibiotics. Promising new candidates can now be produced based on a very ingenious method developed by Danish researchers, who use pioneering organic chemistry to cut and paste some naturally occurring substances and turn them into potent antibiotics.
The technology and the novel antibiotics have been presented in Nature Chemistry.
“We have known about this class of antibiotics for a long time, but people have not been able to use them because they can harm human cells. However, we have greatly reduced their harmful properties, so they can become attractive candidates to combat the problem of increasing antimicrobial resistance,” explains Thomas B. Poulsen, Associate Professor, Department of Chemistry, Aarhus University.
Antibiotics that people cannot use
The new research focused on polyether ionophores, which are potentially useful antibiotics that have been known for many years. They very effectively eradicate especially gram-positive bacteria such as staphylococci, streptococci and Listeria species and also have antiparasitic properties.
The ionophores work by binding to membrane systems and moving ions from one side of the membranes to the other. Over time, this destroys the vital ion gradients, and then the bacteria die.
“The bacteria also appear to develop little resistance to the way the polyether ionophores attack them,” says Thomas B. Poulsen.
This may sound like good news. Nevertheless, researchers have known for many years about the antibiotic effect of polyether ionophores, but they have never been considered for treating human bacterial infections because they also affect human cell membranes and are potentially toxic to both the cell and the body as a whole.
“Polyether ionophores have been used to combat parasitic infections in animals, but in their current form they will not be approved for treating people. Their cellular toxicity prevents this – the basic therapeutic window is simply not wide enough. Using them for people requires finding a way to make them more selective against bacteria and minimizing the side-effects on human cells. This is exactly what we have done,” explains Thomas B. Poulsen.
Leapfrogging complex chemical synthesis pathways
Thomas B. Poulsen and his colleagues, including researchers from the Department of Biological and Chemical Engineering at Aarhus University, developed a revolutionary technology to alter the properties of polyether ionophores.
Researchers normally have to follow a long series of synthesis steps to change the properties of complex organic molecules and arrive at the final compounds.
This means not simply transforming A into B, with A being commercially available chemicals, but rather transforming A into B, then C, etc. until reaching Z. The chemical synthesis steps can be so numerous and so complex that, although they may appear possible in theory, they can be practically impossible to carry out in the real world.
Further, researchers cannot know in advance whether a specific polyether ionophore will selectively harm bacteria and not human cells, so they need to be able to discover new polyether ionophores more rapidly and efficiently.
Chemical modules reconstructed to form a new antibiotic
Instead of trying to synthesize ionophores with attractive properties from scratch, Thomas B. Poulsen took a short cut and recycled what nature has already developed by deconstructing the polyether ionophores with chemical scissors and then arbitrarily reconstructing them.
Thomas B. Poulsen explains that polyether ionophores are constructed in modules, which he has chemically separated and reassembled like Lego bricks.
This enabled the researchers to construct polyether ionophores with new properties. Some have not been useful, but others have become more selective than usual.
“This was a proof-of-concept study showing that the useful components in complex organic molecules can be gathered and then reconstructed in a novel way to create a whole new class of polyether ionophores. This circumvents the extremely long synthesis sequences we would otherwise have had to develop. In fact, we probably would not have tried at all,” says Thomas B. Poulsen.
New antibiotics do not affect human cells
Playing with these molecular Lego bricks results in new antibiotics that retain the antimicrobial activity of the natural polyether ionophores but are more selective against bacteria than human cells.
The researchers characterized the properties of the compounds through various experiments.
The researchers investigated whether one of the newly developed antibiotics could eliminate bacteria without also inducing toxicity to human cells and found that it could do this 3–10 times better than the known polyether ionophores used for comparison.
Some compounds may not be directly toxic to our cells but can still affect them, and they are still inappropriate to use since antibiotics ideally should only kill bacteria and not affect us.
Consequently, in a second experiment, the researchers used an advanced staining technique, called cell painting, to investigate whether the new antibiotic affected human cells at all even though it did not kill them.
“However, this experiment showed that not only did our structural modifications reduce the undesired cellular toxicity of the original polyether ionophores, the new compounds also did not affect human cells in other ways. This is again different from the original polyether ionophores that can have multiple effects at concentrations at which they are not overtly toxic,” explains Thomas B. Poulsen.
Thomas B. Poulsen elaborates that a few polyether ionophores are used in agriculture and are therefore available on an industrial scale, so this provides many opportunities to design new antibiotics for human use.
Testing in animals – and then people
The next step in Thomas B. Poulsen’s research is to conduct more studies in animals with the newly developed antibiotics or the next generation that is already being developed. Later, the plan is to conduct human clinical trials.
Many major international actors are actively seeking new types of antibiotics that can hopefully alleviate the global antimicrobial resistance crisis. Thomas B. Poulsen’s synthetic polyether ionophores or technology for developing new classes of antibiotics may be able to help to resolve this crisis.
“We are looking at a future in which millions of people might die each year from antibiotic-resistant infections. Everyone is therefore searching for new antibiotics that can be deployed to combat antibiotic-resistant bacteria. The polyether ionophores have the properties that are needed – if we can learn to tame them,” concludes Thomas B. Poulsen.