|NIAID Researchers Show How Promising TB Drug Works
Scientists from the National Institute of Allergy and Infectious Diseases (NIAID),
part of the National Institutes of Health, have determined how a promising drug
candidate attacks the bacterium that causes tuberculosis (TB). Published online
this week in Proceedings of the National Academy of Sciences, the finding may
help scientists optimize the drug candidate, PA-824, which targets Mycobacterium
tuberculosis (M. tb).
“PA-824, now in early stage clinical trials, holds promise for shortening the
TB treatment regimen, which is currently cumbersome and lengthy,” says NIAID
Director Anthony S. Fauci, M.D. “This new finding will allow a streamlined approach
for making improved versions of the drug.”
“Previously, we were flying blind in trying to optimize PA-824 in a rational
way because we didn’t know which M. tb protein was the target of PA-824’s action,” says
NIAID scientist Clifton Barry, III, Ph.D., who headed the research team.
In preclinical testing, PA-824 showed evidence of being effective against both
actively dividing and slow-growing M. tb, giving rise to optimism that the compound
may be useful in treating both active and latent TB. (For information about the
first clinical trial of PA-824, see June 14, 2005, NIAID press release: http://www3.niaid.nih.gov/news/newsreleases/2005/tb_pa_824.htm.)
PA-824 must be chemically activated in the bacterium before it exerts its anti-tubercular
effect, notes Dr. Barry. Earlier research had sketched out the first few steps
in this process, but Dr. Barry and his colleagues wanted to pinpoint the precise
protein that binds PA-824 and transforms it into a lethal molecule for TB.
The scientists approached the problem indirectly by searching for M. tb mutants
that resisted the killing power of PA-824. The team confirmed previous research
suggesting that resistance usually occurs when M. tb lacks components called
FGD1 and F420, neither of which interacts directly with the drug.
Next, the investigators screened for PA-824-resistant M. tb that retained sensitivity
to a close relative of PA-824. Within this subgroup of PA-824-resistant bacteria,
the team identified those mutant strains with FGD1 and F420. The investigators
reasoned that resistance to PA-824 in mutants possessing FGD1 and F420 must be
due to a mutation in the M. tb protein that directly interacts with PA-824.
But determining exactly which of M. tb’s thousands of proteins was changed in
these mutants proved difficult, says Dr. Barry. Conventional genetic techniques
for comparing normal and mutant strains of M. tb failed, so the team turned to
a specially modified microarray-based technique, called comparative genome sequencing,
developed by NimbleGen Systems, Inc. (Madison, WI). This was the first time the
technique has been used to identify a protein involved in TB drug resistance,
notes Dr. Barry.
Using the NimbleGen technique, which effectively re-sequences the entire genome
of the bacterium, the scientists quickly pinpointed the protein altered in the
PA-824-resistant mutant strains of M. tb. In the past, such a complete genome
comparison might have taken many months of work; this new technology enables
scientists to zero in on the specific genetic difference between mutant and normal
bacterial strains in just days, says Dr. Barry.
The scientists found a total of four PA-824-resistant mutant strains: two lacked
the newly described M. tb protein altogether, while the remaining two mutants
evidently acquired resistance to PA-824 through a mutation that made the protein
unable to bind to the drug, Dr. Barry says.
With the discovery of the specific protein that interacts with PA-824, Dr. Barry
and colleagues, including researchers at the Novartis Institute for Tropical
Diseases in Singapore, have information they can use to produce improved PA-824
relatives and accelerate the pace of new TB drug development.
NIAID is a component of the National Institutes of Health, an agency of
the U.S. Department of Health and Human Services. NIAID supports basic and
applied research to prevent, diagnose and treat infectious diseases such as
HIV/AIDS and other sexually transmitted infections, influenza, tuberculosis,
malaria and illness from potential agents of bioterrorism. NIAID also supports
research on transplantation and immune-related illnesses, including autoimmune
disorders, asthma and allergies.
The National Institutes of Health (NIH) — The Nation's Medical Research
Agency — includes 27 Institutes and Centers and is a component of
the U. S. Department of Health and Human Services. It is the primary Federal
agency for conducting and supporting basic, clinical, and translational medical
research, and it investigates the causes, treatments, and cures for both common
and rare diseases. For more information about NIH and its programs, visit http://www.nih.gov.