Mycobacterium tuberculosis are the loners of the bacterial world. They prefer to sequester themselves inside cells, usually isolating in lung macrophages where they mooch off the cell’s lipids as a source of cholesterol. But this reliance has made the bacteria’s cholesterol metabolism pathway an attractive drug target for this difficult-to-treat infection.
Researchers at Cornell University discovered that stimulating M. tuberculosis cyclic AMP production with a new small molecule blocks the bacteria’s ability to metabolize cholesterol, disrupting its pathogenesis in mice (1). The new molecule, which they described in a preprint on bioRxiv, may offer a potential new drug for tuberculosis.
“It is a notoriously difficult disease to treat. The standard of care is six to nine months, daily dosing with two to four drugs,” said Brian VanderVen, a microbiologist and senior author of the study. One reason for this is that the bacteria can invade other cells in the lung or organs where it adapts to survive in those different environments, affecting its susceptibility to treatment.
“We're trying to find drugs that inhibit different subpopulations of bacteria,” VanderVen said. “We decided to ask what drugs we can find that inhibit mycobacteria growth inside of its natural host cell, a macrophage.”
In a 2015 screen, VanderVen’s team identified multiple molecules that inhibited M. tuberculosis growth by activating its adenylyl cyclase, Rv1625c, to synthesize cAMP (2). Out of that screen and another performed in collaboration with Calibr, a division of the Scripps Research Institute, they identified a small molecule agonist of Rv1625c with promising pharmacokinetics, which they called V-59.
The researchers found that stimulating Rv1625c with V-59, or genetically inducing cAMP production in M. tuberculosis prevented the bacteria from metabolizing cholesterol. The team then synthesized an orally available, optimized version of V-59 named mCLB073, which was 17 times more effective at killing M. tuberculosis cells in vitro. When they administered mCLB073 to mice infected with M. tuberculosis, the drug reduced the bacterial burden in the lungs by about a 0.4 log-fold change.
“You'll notice that the in vivo reduction of bacterial numbers is not that impressive,” VanderVen said. Most drugs for tuberculosis reduce the bacterial numbers in the lung by one or two log-fold changes within one week.
Luiz Carvalho, a microbiologist at the Francis Crick Institute who was not involved in the study, agreed with that sentiment. “It's less than half log10, which for the TB community is usually where you draw the line,” he said. He added that it’s possible that the bacteria may rely on other sources of fat during an infection. “If you take the salad from your plate, you're not going to starve to death because you have some carbohydrates and you have some protein there.”
Tanya Parish, a tuberculosis drug discovery researcher at Seattle Children’s Hospital who was not involved in the study, thinks that mCLB073 might still make an effective drug even though it only reduced the bacterial burden by a small amount in a mouse model.
“One thing we do know about targeting cholesterol metabolism is that actually, these molecules have the potential to show more effects when they're combined with other treatments,” she said. “For this type of pathway, you would expect that it wouldn't have a very big effect on its own, but when you put it in combination with other drugs, you get a big boost due to their activity.”
VanderVen’s team has now handed off mCLB073 to the Gates Foundation for safety testing and studying its efficacy in combination with other frontline tuberculosis drugs.
In the tuberculosis community, “one of our key goals is to shorten therapy,” Parish said. “I think this sort of molecule combined with others has the potential to shorten therapy, and I think that's a good thing.”
- Wilburn, K.M. et al. Pharmacological and genetic activation of cAMP synthesis disrupts cholesterol utilization in Mycobacterium tuberculosis. Preprint at https://www.biorxiv.org/content/10.1101/2021.08.03.454881v1.full (2021).
- VanderVen, B.C. et al. Novel Inhibitors of Cholesterol Degradation in Mycobacterium tuberculosis Reveal How the Bacterium’s Metabolism Is Constrained by the Intracellular Environment. PLoS Pathog 11, e1004679 (2015).