A hand wearing a latex glove holds up a petri dish containing colonies of bacteria for antibiotic testing. In the background, there is a stack of petri dishes and a rack containing test tubes.
A new study has identified a triple-combination therapy that offers hope for controlling antimicrobial-resistant bacteria. Image credit: jarun011, Getty Images.

Novel triple drug combination effective against antibiotic-resistant bacteria

Scientists at the Ineos Oxford Institute (IOI) have found a new potential combination therapy to combat antimicrobial resistance (AMR) by targeting two key bacterial enzymes involved in resistance. The findings have been published in the journal Engineering.

With the increase of deadly drug-resistant infections, it is crucial that we support research and innovation to accelerate the discovery of new drugs that overcome resistance and result in interventions that save the lives of those with the greatest need. Finding new and innovative ways of combatting antibiotic resistance, such as developing new drug combinations using novel agents with existing antibiotics, is valuable in the fight against infectious disease.

Timothy Jinks, Head of Intervention in Wellcome’s Infectious Disease team

Meropenem is a critical antibiotic used to treat serious multidrug-resistant infections like sepsis when other antibiotics such as penicillin have failed. However, this last resort drug is becoming less effective at treating infections due to antimicrobial resistance (AMR).

One effective strategy to restore the activity of the antibiotic is to use a combination therapy to counter bacterial resistance mechanisms. An antibiotic combination treatment includes an antibiotic and an inhibitor. The inhibitor prevents bacterial enzymes such as metallo-β-lactamases (MBLs) and serine-β-lactamases (SBLs) from breaking down the antibiotic before it has its desired effect to treat the infection.

Research to date has largely focused on developing SBL inhibitors and these are now widely used in clinics and hospitals. Scientists at the IOI are developing new MBL inhibitors to be used in combination therapies.

This new study looked at a combination of three drugs: the β-lactam antibiotic meropenem, a newly developed MBL inhibitor called indole-2-carboxylate 58 (InC58), and an SBL inhibitor called avibactam (AVI).

This study builds on our previous work to develop broad spectrum metallo β-lactamase inhibitors. Here we combatted multiple resistance mechanisms simultaneously to great effect, and this is a great example of how chemistry and microbiology teams can collaborate to develop new potential therapies. This combination therapy works very well in the lab and the next challenge will be to show that this works in infection models and ultimately in a hospital setting.

Dr Alistair Farley, IOI Scientific Lead and a co-author for the study

In hospital settings, it is difficult to determine whether a strain of bacteria causing an infection is producing SBLs or MBLs or if it is harbouring both resistance mechanisms. This is the first study to examine the combination of a carbapenem antibiotic with two inhibitors targeting SBLs and MBLs separately.

The team tested the effectiveness of the combination of all three compounds, compared to a combination of meropenem with either InC58 or AVI alone, on 51 strains of meropenem-resistant bacteria.

Researchers compared the minimum inhibitory concentration (MIC) of the different drug combinations. The MIC is the lowest concentration of a drug that is able to prevent the visible growth of a bacterial strain. An antibiotic with a low MIC value is more effective than one with a high MIC. The MIC50 is defined as the MIC value that inhibits the growth of at least 50% of the bacterial colonies.

The study found that the triple-drug combination was more effective at stopping growth of bacteria in the lab than either of the dual-drug combinations. The combination of meropenem with InC58 and AVI at a concentration of 4 mg/L lowered the MIC50 against all the bacterial isolates tested to 0.5 mg/L. This was 64 times lower than the MIC50 of meropenem combined with AVI alone (32 mg/L) and 4 times lower than the MIC50 of meropenem combined with InC58 alone (2 mg/L). This demonstrates a broad spectrum of antibacterial activity against different strains of MBL- and SBL-producing bacteria.

Diagram using circles to illustrate the minimum inhibitory concentrations. The circle to illustrate the combination of meropenem + InC58 + AVI has an area 4 times smaller than meropenem + InC58, and 64 times smaller than meropenem + AVI.Comparison of the minimum inhibitory concentration of the different drug combinations.
Genetic analysis was done on the bacterial mutants which showed resistance to the effects of the new InC58 and meropenem combination. Resistance was correlated with mutations in two genes associated with changes to porins (channels on bacterial outer membrane) and copper permeability in bacteria. This information helps scientists to understand how resistance to new drug combinations including MBL inhibitors like InC58 could develop in the future.

These findings suggest a potential new combination therapy for meropenem-resistant infections, and whilst this works very well in the lab further development is needed to show that this may also be effective in a hospital setting. The findings provide a benchmark for the activity of a single ideal molecule which can evade bacterial resistance mechanisms. Such novel treatments could significantly extend the antibacterial activity of carbapenems, and possibly other β-lactam antibiotics.

The study ‘The Triple Combination of Meropenem, Avibactam, and a Metallo-β-Lactamase Inhibitor Optimizes Antibacterial Coverage Against Different β-Lactamase Producers’ has been published in Engineering.

You can learn more about the work of the Ineos Oxford Institute on their website.

This work was supported by the Ineos Oxford Institute for Antimicrobial Research, the Biotechnology and Biological Sciences Research Council and the Wellcome Trust.