Antibiotic overuse and misuse has led to the spread of antibiotic resistance, a serious pubic health issue that means previously treatable bacterial infections have now become potentially deadly. Yet while we already know that some drug combinations can help to fight multidrug-resistant (MDR) infections, the wider potential use of drug combinations has remained largely unexplored, and they are rarely used in the clinic.
In what they claim is the first large-scale screening study of its kind, scientists at the European Molecular Biology Laboratory (EMBL) in Germany teamed up with researchers in Germany, France, Switzerland, the U.K., and the U.S. to profile the effects of almost 3000 drug combinations – including antibiotics and food additives – on laboratory strains of three MDR bacterial species.
The results highlighted a number of synergistic combinations that were also effective against clinical isolates of MDR bacteria in vitro, as well as in an in vivo insect model. In one of the most effective pairings, a common food additive improved the effectiveness of an antibiotic that is now rarely used because of bacterial resistance against it.
Reporting in Nature, the researchers, headed by the EMBL’s Nassos Typas, Ph.D., say that as well as identifying novel drug combinations that could undergo further testing, their results provide new insights into drug–drug interactions and provide a framework for investigating whether the same interactions occur across different species and between individuals.
“For antibacterial drug therapies, our study shows that non-antibiotic drugs hold promise as adjuvants, offers a new path for narrow spectrum therapies and identifies effective synergies against MDR clinical isolates,” they write in their paper entitled “Species-Specific Activity of Antibacterial Drug Combinations.”
To try and derive the general principles behind drug–drug interactions, the researchers probed the effects of the 3000 drug combinations against each of six laboratory strains of three Gram-negative pathogens, Escherichia coli, Salmonella enterica serovar Typhimurium, and Pseudomonas aeruginosa, which all belong to the highest-risk group for antibiotic resistance. Fifty-nine percent of the drugs were antibiotics, 23% were other human-targeted drugs and food additives, “most of which have reported antibacterial and/or adjuvant activity,” the team notes, while 18% were other compounds that have known bacterial targets.
Results from the screen suggested that of all the interactions identified, about 500 drug combinations improved antibiotic outcome. However, there were 50% more antagonistic, than synergistic interactions. There were also clear patterns, the authors report. “Notably, antagonisms and synergies exhibited a clear dichotomy in our data,” the team writes. “Antagonistic interactions occurred almost exclusively between drugs that target different cellular processes, whereas synergies were also abundant for drugs of the same class or that target the same process.”
Antagonism can be explained by interactions occurring at the drug target level, as the two inhibitors might effectively help the cell to buffer different processes that are disrupted, but the researchers also found that in many cases antagonism occurred because of the effects of one drug decreasing the intracellular concentration of the othe compound.
In contrast with the finding that antagonistic interactions tended to impact on different cellular processes, the team found that synergies often occurred between drugs that targeted the same cellular processes, across all of the three bacterial species. They suggest that while synergism can result when two different drugs attack different parts of the same cellular process, it may also result when the combination impacts, this time positively, on intracellular drug concentrations.
Interestingly, about 80% of the drug–drug interactions were highly conserved within species, yet 13% to 32% were strain specific. And 70% of interactions occurred in only one species, with just 5% occurring showing conservation across all three species, despite the fact that the three bacterial species chosen are relatively closely related. This finding suggests that it may be possible to develop drug combinations that act on specific species, the authors suggest. “Such specificities can be beneficial for creating narrow-spectrum therapies with low collateral damage, by using synergies that are specific to pathogens and antagonisms that are specific to abundant commensals.”
The researchers then tested pairs of drugs that had demonstrated seven of the strongest and most conserved synergistic interactions, against six MDR isolates from human patients. The drug combinations comprised antibiotics, human-targeted drugs, or food additives. Encouragingly, all seven combinations acted synergistically in most of the strains tested, and in some cases combinations of food additives and antibiotics were effective against clinical isolates, even when the additive has no antibacterial activity on its own. “This synergy underlines the importance of exploring the role of food additives in combinatorial therapies,” the authors write.
The strongest synergistic pairing against clinical MDR isolates was that of vanillin, a food additive that gives vanilla its characteristic flavor, and spectinomycin, an antibiotic that has historically been used to treat gonorrhea, but which is now only rarely used because of widespread resistance. “Of the combinations tested, this was one of the most effective and promising synergies we identified,” says Ana Rita Brochado, Ph.D., first author on the paper and research scientist at EMBL. Subsequent tests confirmed that vanillin boosted the antibacterial effect of spectinomycin against E. coli MDR isolates, but antagonized many other drugs, including other antibiotics in the same class as spectinomycin.
While the obvious focus is on identifying combinations that boost the effects of antibiotics, in some instances, dampening antibiotic effects can also be beneficial, Dr. Typas suggests. “Antibiotics can lead to collateral damage and side effects because they target healthy bacteria as well. But the effects of these drug combinations are highly selective, and often only affect a few bacterial species. In the future, we could use drug combinations to selectively prevent the harmful effects of antibiotics on healthy bacteria. This would also decrease antibiotic resistance development, as healthy bacteria would not be put under pressure to evolve antibiotic resistance, which can later be transferred to dangerous bacteria.”
The authors acknowledge that further preclinical studies in different species will be required before it is possible to think about translating the findings into the clinic. Even so, they point out, the large scale of the study provided new insights into the general principles behind drug–drug interactions, which could help the more rational selection of drug pairs in the future. The findings could also offer a basis for carrying similar screens in other microbes. “Some of the principles that we have identified probably go beyond anti-infectives and microbes.”