How do you defeat an opponent who has acquired an effective new defence mechanism? Either develop a more powerful weapon, or find a way to undermine his clever new defence device. In the war against superbugs, this is the equivalent of either developing new drugs, or make them susceptible again to existing drugs. Well, now scientists have discovered a way to do this for drug-resistant bacteria that have acquired an ingenious defence mechanism: efflux pumps. These pumps enable the bugs to expel antibiotic drugs from their bodies; that is until a team of chemists from Brown University comes along and blocks their pumps, making them vulnerable again to antibiotics.

Dr Jason K. Sello, assistant professor of chemistry at Brown University in Providence, Rhode Island in the US, and colleagues, write about how they synthesized a new compound called BU-005 and used it to block efflux pumps that bacteria use to expel an antibacterial agent called chloramphenicol, in the 15 December issue of Bioorganic and Medicinal Chemistry.

Sitting in the cell walls or membranes of bacteria, efflux pumps are proteins that spot and expel drugs that breach those membranes. In some cases, the pumps have become so advanced they can recognize and expel drugs with totally different structures and mechanisms.

Sello told the press that:

“This turns out to be a real problem in clinical settings, especially when a bacterial pathogen acquires a gene encoding an efflux pump that acts on multiple antibiotics.”

“In the worst case scenario, a bacterium can go from being drug-susceptible to resistant to five or six different drugs by acquiring a single gene,” he explained.

The journey that Sello and colleagues have undertaken is not new: many scientists have been pursuing the goal of disarming drug-resistant bacteria by inactivating their efflux pumps. The problem is there are different classes of bacteria, and different types of efflux pump, and there is also the added problem of when you find a compound that can disarm them, can you make it easily and in a way that is likely to be cost-effective for mass use?

Sello and his team seem to have ticks in all these boxes: they have discovered BU-005, a compound in a class called C-capped dipeptides, that appears to disarm a family of drug-efflux pumps used by Gram-positive bacteria, which include the dangerous MRSA and tuberculosis strains. Until their discovery, scientists thought C-capped dipeptides only worked against drug-efflux pumps used by Gram-negative bacteria.

Nearly all bacteria are either gram-positive or gram-negative. Gram refers to Gram’s staining method, whereby because they have more layers of peptidoglycan in their cell walls than gram-negative bacteria, gram-positive bacteria retain the stain. E.coli and Salmonella are Gram-negative bacteria; Staphylococcus aureus and its superbug variant MRSA are Gram-positive, as is tuberculosis (TB).

A company called MPEX Pharmaceuticals recently discovered that certain C-capped dipeptides could block efflux pumps of the RND family, which are responsible for much of the drug resistance in Gram-negative bacteria.

Sello and his team decided to see if any C-capped dipeptides could block the pumps of the major facilitator superfamily (MFS) of drug efflux pumps, which are used mostly by Gram-positive bacteria.

They developed a new, shorter, two-step process to screen a a collection of structurally diverse C-capped dipeptides for compounds with new or enhanced activities. Their new screening method used a chemical process called the Ugi reaction.

From a collection of nearly 100 C-capped dipeptides they had prepared and tested, they found BU-005 prevented the MFS efflux pump family in the bacterium Streptomyces coelicolor (a relative of the human pathogen Mycobacterium tuberculosis) from expelling chloramphenicol, one of the oldest antibacterial drugs.

“Our findings that C-capped dipeptides inhibit efflux pumps in both Gram-positive and Gram-negative bacteria should reinvigorate interest in these compounds,” said Sello.

,”Moreover, our simplified synthetic route should make the medicinal chemistry on this class of compounds much simpler,” he added.

Brown University funded the study.

Written by Catharine Paddock PhD