Mass Spectrometry Reveals How Drugs and Tuberculosis Bacteria Interact

Advanced analytical techniques including mass spectrometry have given drug developers a new understanding of how tuberculosis bacteria interact with long established, but poorly understood, drugs such as para-aminosalicylic acid (PAS) and sulfonamide.

During most of the 50 years it has been in use as an antituberculosis agent, it was assumed that PAS acted in Mycobacterium tuberculosis by blocking folate synthesis. This effect was thought to be due to inhibition of the enzyme dihydropteroate synthase (DHPS) caused by PAS mimicking the substrate, p-aminobenzoate (PABA).

Investigators at Weill Cornell Medical College (New York, NY, USA) revisited well-established drugs like PAS and followed the drug's behavior inside bacterial cells with advanced analytical methods such as mass spectrometry.

They reported in the November 1, 2012, online edition of the journal Science that inhibitors of DHPS inhibited growth of M. tuberculosis only weakly due to their intracellular metabolism. PAS, by contrast, served as a replacement substrate for DHPS. Products of PAS metabolism at this and subsequent steps in folate metabolism inhibited those enzymes, competing with their substrates. PAS was thus a pro-drug that blocked growth of M. tuberculosis when its active forms were generated by enzymes in the pathway they poisoned.

Mass spectrometry was also used to study the behavior of sulfonamide (sulfa), an 80-year-old class of antibacterial agents that is ineffective against tuberculosis. It had been thought that the drug could not enter M. tuberculosis cells. However, data obtained from mass spectrometry showed that the drug did enter the cells but was subsequently degraded by cytoplasmic enzymes. If the sulfa molecule could be modified to resist these enzymes, it might be possible to develop it into an effective anttuberculosis drug.

"The development of antibiotics has been stalled for several decades and many infectious microbes have become drug-resistant," said senior author Dr. Kyu Y. Rhee, associate professor of medicine and associate professor of microbiology and immunology at Weill Cornell Medical College. "We must restock the antibiotic pipeline and our study findings provide a powerful new approach for doing just that."

"Current TB treatments are long and complex, lasting a minimum of six months, and often resulting in treatment failures and the paradoxical emergence of multidrug resistance," said Dr. Rhee. "We are still using the antibiotics that were first developed for TB about 50 years ago. The power of mass spectrometry is now evident, and we cannot wait to use it to test all of the current cocktail of drugs used to treat TB to find ways to improve them. Best of all will be the use of this tool to design and test the much-needed next generation of effective anti-TB agents."

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