Breaking News: TB Edition!

This week, my lab experienced one of the most satisfying parts of the scientific process–the end! A paper that two of our members wrote about their work (with the input of collaborators, of course) was published in the Proceedings of the National Academy of Sciences, the second most highly cited scientific journal in the world. The work described in this publication aimed to better understand how bedaquiline, a medication approved by the FDA in 2012, works to kill Mycobacterium tuberculosis (Mtb), the bacteria that cause TB.

Bedaquiline (BDQ) was essentially the first new TB drug to be approved worldwide since 1974. That’s a gap of almost FORTY years! (Because rifapentine, approved in 1998, is so closely related to other approved TB therapies, it was not really considered a “new” drug) [1].

On a molecular level, exactly how a drug works to accomplish its goal is called its mechanism of action (MOA). Experiments to determine the MOA of a given drug candidate are a crucial part of the pharmaceutical development process, and those are the kinds of experiments this paper describes.

These studies provide information and evidence that regulatory agencies will use to decide if a drug should be approved and under which circumstances it should be used. Even after approval and introduction to patients, continuing MOA investigations can help to explain and prevent side effects and other drug-related toxicities. The development of BDQ represented an especially exciting advance in the TB field because its primary MOA is completely new; there are no other TB drugs that target the particular process that BDQ does.

Bedaquiline is marketed as Sirturo by Johnson & Johnson [2].

BDQ blocks Mtb’s ability to make ATP, the molecule that all cells use to store the energy they need to live. Without the production of this crucial energy carrier, Mtb cells can no longer survive, making BDQ a very effective treatment for tuberculosis. Human cells also make ATP, but this process takes place inside of specialized energy production facilities called mitochondria.

Side note: Mitochondria is a plural word. The singular form is mitochondrion! Thanks, Greek.

You probably learned in biology class that mitochondria are “the powerhouse of the cell,” and their function of producing ATP is exactly what gives them this nickname.

“The powerhouse of the cell” [3].

Evolutionary biologists have strong evidence to believe that modern-day human mitochondria actually evolved from bacteria. Ancient bacteria formed a mutually beneficial relationship with other cells that engulfed them—an event called endosymbiosis. This idea originated in the 1970s with Dr. Lynn Margulis, a scientist at Boston University, whose theories about endosymbiotic evolution were initially met with skepticism but are now widely accepted. Her contributions to the field are so important that Discover magazine has named her one of the 50 most important women in science [4].

Lynn Margulis (1938-2011) [5].

Despite the fact that this evolutionary mashup of cells occurred billions of years ago, the mitochondria present within each of your body’s trillions of cells continue to share many features with bacteria. Such attributes include having a double-layered membrane that keeps all of their components contained, possessing their own reserves of DNA, and reproducing themselves using a method called binary fission [6]. If you’re interested, more information about the evidence of the bacterial origin of mitochondria can be found by clicking on the picture of Dr. Margulis!

While this story about the link between bacteria and mitochondria is pretty neat from the standpoint of evolution, it can present some problems when scientists like me attempt to discover new antibiotics. Because the chemical processes that take place inside Mtb are so similar to the processes that happen in mitochondria, drugs that target Mtb cells can have unwanted side effects on the cells of patients taking these drugs. In this case, BDQ blocks ATP synthesis in Mtb, but it probably affects this process in human mitochondria as well.

These similarities may explain some of the side effects observed in clinical trials of people taking BDQ, including joint pain, nausea, vomiting, and disruptions of the heartbeat rhythm. Yikes. The side effect profile of BDQ has caused it to be a high-risk, high-reward therapy for TB. For people whose TB disease will not respond to normal, safer therapies because of resistance, BDQ presents an opportunity for them to get better. But this approach comes at a cost; namely, intense side effects and an increased risk of death.


I’ve explained all of this background about endosymbiosis, mitochondria, BDQ, and how it works because it provides a motivation for the research my lab undertook. If we better understand what BDQ does to cells, we can design better strategies about how to use it in patients in order to reduce their risk of adverse effects.

All of the experiments in our lab’s paper were conducted on a laboratory strain of Mtb called H37Rv, but many of the findings were duplicated in strains of bacteria that actually came from patients. This gives us confidence that what we found in our research is broadly applicable to TB outside of the lab! Using an analytical technique called metabolomics, a hallmark of our lab’s work, we carried out an unbiased survey of all of the molecular changes that happen inside of Mtb when we treat it with BDQ. In addition to the expected reduction in ATP, we also found some interesting—and surprising—results.

Continuing the analogy from my first post, BDQ is a knight in shining armor that enters the Mtb fortress and takes out the emperor’s henchman in charge of ATP production. This was already known by TB scientists. But the henchmen inside the castle do not operate in isolation—they communicate with each other, and these interactions so complex and numerous that they are still not well understood! Our research unearthed an unexpected connection between the ATP henchman and the crony in charge of making a building block of the castle called glutamine. When BDQ takes out the ATP henchman, the chain of command to the glutamine builders is taken out, and glutamine production suffers as a result. Without these building blocks, the castle is weakened even further.

Once we realized the existence and importance of this connection, our lab did some tests using drugs in combination: BDQ, to take care of the ATP henchman, and another that sought out and destroyed the glutamine builders. We found that this approach was even more effective at bringing down the castle of Mtb than treating with either drug alone.

The work described in our lab’s paper is exciting and novel because the reduction in glutamine synthesis caused by blocking ATP production has never been reported. Moreover, it presents an opportunity for smarter strategies of using BDQ in TB patients. The ability to combine BDQ with an antibiotic that targets glutamine synthesis would allow for lower doses of BDQ, which would reduce the likelihood of dangerous side effects, while being even better at treating TB than BDQ alone!

There is still a lot of work to be done in this area. For starters, no currently FDA-approved antibiotics target the glutamine pathway in Mtb, which is a hurdle that we’d need to overcome in order to bring our treatment strategy to fruition. However, the scientific approach taken in this paper opens the door to carry out similar MOA studies for other antibiotics, which could eventually lead to much safer and more effective treatment regimens for patients fighting infectious diseases.


Sources:

  1. Deoghare, S. Indian J. Pharmacol. 2013 Sep-Oct; 45(5): 536-537.
  2. Image: Stop TB Partnership (tbonline.info)
  3. Image: Mighty Mitochondria via Twitter (@funkylittlebean)
  4. Svitil, Kathy (13 November 2002). “The 50 Most Important Women in Science”. Discover.
  5. Image: Harvard Book Store
  6. https://evolution.berkeley.edu/evolibrary/article/_0_0/endosymbiosis_04

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