September 6, 2011

Researchers decode workings of mysterious critical TB drug

For nearly 60 years, Pyrazinamide has been used in conjunction with other medications to treat tuberculosis, but scientists did not fully understand how the drug killed TB bacteria. Pyrazinamide, or PZA, plays a unique role in shortening the duration of current TB therapy to six months and is used frequently to treat multidrug-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 ribosomal protein S1, or RpsA, and inhibits trans-translation, a process that enables the TB bacteria to survive under stressful conditions. Their findings, published in the Aug. 11 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 nongrowing bacteria called persisters, which are not susceptible to common antibiotics,” said Ying Zhang, senior author of the study and a 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 by an amidase enzyme also identified by Zhang’s group, in 1996. Through a series of experiments, Zhang and his colleagues determined that pyrazinoic acid binds to RpsA, a vital protein in the trans-translation process. Trans-translation is essential for cell survival under stress conditions. Partially synthesized proteins that 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 that 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 shorten 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,” Zhang said.

The study was conducted in collaboration with researchers from Fudan University and the National Institute of Allergy and Infectious Diseases. In addition to Ying Zhang, the authors are Wanliang Shi, Xuelian Zhang, Xin Jiang, Haiming Yuan, Jong Seok Lee, Clifton E. Barry III, Honghai Wang and Wenhong Zhang.

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