Researchers Decode Workings of Mysterious, but Critical TB Drug

For nearly 60 years, Pyrazinamide (PZA) has been used in conjunction with other medications to treat tuberculosis (TB), but scientists did not fully understand how the drug killed TB bacteria. PZA plays a unique role in shortening the duration of current TB therapy to six months and is used frequently to treat multi-drug resistant TB. A new study, led by researchers at the Johns Hopkins Bloomberg School of Public Health, suggests that PZA binds to a specific protein named RpsA and inhibits trans-translation, a process that enables the TB bacteria to survive under stressful conditions. Their findings, published in the August 11, 2011 edition of Science Express, could lead to new targets for developing more effective anti-TB drugs.

“PZA is a peculiar and unconventional drug that works very differently from common antibiotics that mainly kill growing bacteria. PZA primarily kills non-growing bacteria called persisters that are not susceptible to common antibiotics,” said Ying Zhang, MD, PhD, senior author of the study and professor in the Bloomberg School’s W. Harry Feinstone Department of Molecular Microbiology and Immunology. “While PZA works very well in the body against TB, it has no effect on the growing bacteria in a test tube, which has made it difficult to understand just how it works.”

PZA is converted to the active form of pyrazinoic acid (POA) by an amidase enzyme (PncA) also identified by Zhang’s group in 1996. Through a series of experiments, Zhang and his colleagues determined that POA binds to ribosomal protein S1 (RpsA), a vital protein in the trans-translation process. Trans-translation is essential for cell survival under stress conditions. Partially synthesized proteins which are produced under stress conditions are toxic to the bacterial cell. It has developed a mechanism called trans-translation to add a short peptide tag to the partially produced toxic proteins so they can be recognized for degradation by proteases to relieve the toxicity. Inhibition of trans-translation by PZA explains why the drug can eradicate persisting organisms, and thereby shortening the therapy.

“There is renewed interest in PZA because it is the only drug that cannot be replaced among the current TB drugs without compromising the efficacy of therapy. The identification of the drug target RpsA not only offers a new resistance mechanism to PZA but also opens up a way for designing a new generation of antibiotics that target persister bacteria for improved treatment of chronic and persistent infections including TB,” said Zhang.

The study was conducted in collaboration with researchers from Fudan University and the National Institute of Allergy and Infectious Diseases (NIAID). The authors of “Pyrazinamide inhibits trans-translation in Mycobacterium tuberculosis” are Wanliang Shi, Xuelian Zhang, Xin Jiang, Haiming Yuan, Jong Seok Lee, Clifton E. Barry, 3rd, Honghai Wang, Wenhong Zhang, and Ying Zhang.

Funding for the research was provided by NIAID, part of the National Institutes of Health, and the National Key Technologies Research and Development Program of China.