New research shows that a TB antibiotic can be inactivated by its own target.

Crick and Imperial researchers have found that a key antibiotic widely used to treat drug-resistant tuberculosis does not work as expected – a finding which could be used to develop new drugs. The study was part-funded by the Francis Crick Institute, Imperial College London, and the Wellcome Trust.

The research, published in Nature Chemical Biology, found that contrary to current understanding, an antibiotic used to treat tuberculosis is unable to permanently prevent one of the enzymes it targets from functioning. By uncovering how this enzyme is re-activated, the research could lead to the development of improved versions of the drug which could be used against antibiotic resistant bacteria.

For more than 50 years, the antibiotic D-cycloserine has been used to treat cases of TB that are resistant to first-line drugs. It had been thought that this drug in part worked by irreversibly preventing an enzyme, alanine racemase, from helping to build TB bacteria’s cell walls.

However, to their surprise the researchers found that a while after being exposed to D-cycloserine about 10% of the enzyme started working again. Through detailed studies using complementary techniques including enzyme kinetics, chemical synthesis and structural biology, the researchers discovered that the enzyme inactivates and destroys the drug, in a process called hydrolysis.

Fortunately, the drug can still treat TB because it also inhibits another enzyme involved in building TB cell walls, called D-Ala:D-Ala ligase. “If it weren’t for the drug also blocking a separate target, it would be ineffective against TB,” says Luiz Pedro Carvalho, group leader in the Mycobacterial Metabolism and Antibiotic Research Laboratory at the Crick. “For decades, it’s been thought this drug worked in one way, that its action could not be reversed, so it’s rather incredible that we’ve found it gets inactivated by one of its targets.”

“Proving the mechanism of antibiotic destruction also required the synthesis of a proposed key D-cycloserine reaction product using organic chemistry. This allowed unambiguous confirmation of the chemical identity of the enzymatic products and demonstrated reversibility of inhibition”, said Miha Homsak, a PhD student working between the labs of Prof Ed Tate in the Department of Chemistry at Imperial College and at the Francis Crick Institute. Prof Tate adds: “This is an outstanding example of how multiple disciplines can come together through the Imperial-Crick partnership to solve important challenges in antimicrobial resistance”.

These findings could lead to the development of new antibiotics, for example by making similar molecules that cannot be hydrolysed. Various bacteria have the same enzyme or a version of it, so such drugs could provide a new way to treat these diseases.

“D-Cycloserine destruction by alanine racemase and the limit of irreversible inhibition” is published in Nature Chemical Biology, DOI 10.1038/s41589-020-0498-9

Photo credit: James Archer - Medical Illustrator