Turning Back the Tuberculosis Tide

An ancient scourge, tuberculosis has made a comeback in recent years.

By | May 23, 2005

<p>FACING THE TB CHALLENGES:</p>

Photos courtesy of WHO/STB/Colors Magazine/J. Mollison

Tuberculosis, a publication prepared for the World Health Organization by editors and photographers from Colors Magazine, attempts to put a human face on the TB epidemic with pictures of patients from around the world.

An ancient scourge, tuberculosis has made a comeback in recent years. According to a recent World Health Organization report, tuberculosis (TB) incidence increased by one percent in 2003.1 Though one-third of the world's population carries latent TB, roughly eight million people per year experience progression to the active form of the disease. Prevalence and mortality have gone down, however, since the early 1990s, meaning that while the number of cases has increased, TB control programs are finding and curing increasing numbers.

But the situation has deteriorated rapidly in Africa, the former Soviet Union, and parts of Eastern Europe due to the HIV epidemic, and poor healthcare systems that contribute to serious multidrug resistance problems. According to a recent report from Doctors Without Borders, approximately 4% of all patients with TB worldwide are resistant to at least one current first-line drug.2 In parts of Eastern Europe, multidrug resistant (MDR) TB could be spreading by as fast as 250,000 to 400,000 new cases each year.3 Yet a new TB drug hasn't been developed in nearly 40 years.

A potentially significant factor in the disease's spread: Treatment for active TB can last more than six months. Early discontinuation may contribute to the emergence of MDR strains. A new push in TB research, fueled by an enhanced recognition of the problem from academia to industry, has pointed to a straightforward charge: Shorten the treatment regimen.

But finding novel ways to target Mycobacterium tuberculosis is not so straightforward. Researchers must delve into the biological details of an organism that for years has been neglected because cures were believed to be effective, and because the bulk of TB cases are in the developing world where drug companies have little chance of turning a profit.

"It's a double-layered problem," says Mel Spigelman, director of research and development at the Global Alliance for TB Drug Development, a not-for-profit organization that works to develop drugs through partnership with government, academic, and industry scientists. Beyond finding the compounds that work against the best targets, researchers need to design the combination of therapies that bring about the fastest cures.

WHERE THEY SLEEP

Attacking the latent bacilli may be one path to shorter regimens, according to Clifton Barry, head of the tuberculosis research section at the National Institute of Allergy and Infectious Diseases. During the latent state, the bacterium doesn't replicate and may be more drug-tolerant if not outright drug-resistant, as drugs typically target processes that are important only during replication, such as cell-wall synthesis. Barry says the importance of the latent stage is being increasingly recognized, as he noticed at a recent Keystone meeting that he co-organized. "It's something that's emerging as a consensus in the field. We have to understand the biology of this nonreplicating state of the bacterium to both inform drug discovery for latent disease and to shorten the course of chemotherapy."

Barry's lab, along with the Novartis Institute for Tropical Diseases and the TB Alliance, is helping to design a drug based on the compound PA824 that could have such an effect. Latent bacteria might languish in an anaerobic state, and traditional drugs targeting facets of high growth, such as cell-wall synthesis, might fail to shorten therapy. These quiescent bugs do maintain some metabolic function, however, such as the ability to maintain and energize the membrane and acquire nutrients. Moreover, latent M. tuberculosis may subsist using different carbon sources from those grown in the lab. Barry is also using whole-cell screening to uncover agents that kill nonreplicating bacteria; he then investigates their mechanism of action in hopes of finding drug targets.

Metabolic targets are generally a neglected area of study in bacterial pathogenesis, according to John McKinney, an associate professor at Rockefeller University in New York. "We have little idea of how these organisms acquire and assimilate nutrients in vivo," says McKinney. In 2000, his lab reported finding that in vivo the organism switches from sugar to fat as its carbon source.4 The key: isoci-trate lyase (ICL), an enzyme essential for metabolizing fatty acids and mediating the bug's fat-burning potential. Disrupting a gene encoding ICL (icl1) severely attenuated the pathogen in vivo.

In more recent work, McKinney's group has shown that this pathway is actually even more important than previously thought. Knocking out icl1, as reported in the 2000 paper, impaired persistence but not growth. Another gene, icl2, when knocked out with icl1, had a synergistic effect by stymieing persistence and growth, resulting in rapid elimination of the organism in vivo. McKinney says it's too early to know if targeting the ICL enzymes would effectively inhibit the organism during its latent phase. GlaxoSmithKline and the TB Alliance are now investigating potential drugs based on targeting isocitrate lyase.

Targeting the nutrient supply of M. tuberculosis has become a topic of interest for many TB investigators, according to Eric Rubin, an assistant professor of immunology and infectious diseases at the Harvard School of Public Health. In hopes of infiltrating the organism's stubborn, lipid-rich, hydrophobic cell wall, researchers are looking to crucial transport molecules, the bug's specialized mechanisms for shuttling nutrients and waste. Rubin's group identified several potential targets.5

ENERGY PRODUCTION

Other promising targets involve inhibiting the bacterium's energy production centers. Researchers at Johnson & Johnson Pharmaceutical Research and Development (J&JPRD) recently identified an active new compound, R207910, that appears to interfere with ATP synthesis, though the precise mechanism has not yet been elucidated.6 After identifying R207910 from a large chemical library, they sequenced resistant M. tuberculosis bacteria and pinpointed the ATP synthase gene as the only one consistently mutated. Though the gene's main function is making ATP, it also maintains pH homeostasis in the protoplasm. "Which of the two actions or maybe a combination of both is responsible for the killing, we currently don't know," says Koen Andries, a distinguished research fellow at J&JPRD in Belgium.

His group has no direct evidence that R207910 kills latent bacilli, although mouse studies suggest that it might, perhaps via interference with intracellular pH homeostasis. Andries and colleagues showed that certain combination therapies that included their compound achieved the same bacterial load reduction in one month as is observed in two months with current, standard therapies.

Work from researchers at the University of Pennsylvania, though more preliminary, also suggests ATP synthase as a target.7 "Two different approaches ended up on basically the same enzyme pathway," says senior author Harvey Rubin, a professor of medicine and microbiology at Penn. Rubin and coworkers stopped the bacteria from multiplying by inhibiting an enzyme called type II NADH dehydrogenase, an early player in the ATP pathway. They then found that a drug called phenothiazine, used previously to treat schizophrenia, killed the bacteria in cultures and suppressed its growth in mice that had acute TB infection. Though phenothiazine is a nonstarter as a drug candidate because of its toxicity, these results suggest a possible therapeutic strategy. Rubin notes that if he can reduce ATP concentrations, then enzymes that require ATP will be disrupted as well. "We really believe that if we can knock off this particular enzyme, then we might have a way to shorten therapies, even in the dormant stage," says Rubin.

No target, say researchers, can insure that the organism will not mutate and build up resistance. Still, many are optimistic that with the increased focus on TB, it's only a matter of time before modern biological tools will yield, for the first time in 40 years, fruitful, novel drugs that can have a major public health impact. McKinney notes that current, standard drugs were based only on the study of the organism's growth in a test tube. "We've been going after the wrong targets, so we've got drugs that are suboptimal," says McKinney. "If we go after the right targets, do things in a more rational way, we're bound to come up with other drugs." According to the TB Alliance's Spigelman, his organization and its partners will have six new drugs in clinical trials this year.

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