The lengthy and complicated multidrug therapy currently available to treat active tuberculosis (TB) infection has contributed to medical non-adherence and the emerging problems of multidrug-resistant (MDR)- and extensively drug-resistant (XDR)-TB, which are particularly deadly in the setting of HIV co-infection. In addition, persons with HIV are at increased risk for relapse even upon successful completion of anti-TB treatment. The prolonged therapy required to eradicate TB infection is believed to reflect the ability of Mycobacterium tuberculosis (Mtb) to persist within host necrotic granulomas in a non-replicating state characterized by antibiotic tolerance to bactericidal drugs, which predominantly target actively dividing tubercle bacilli. Hyperphosphorylated guanosine ((p)ppGpp), which is synthesized by the stringent response enzyme RelA, plays an important role in bacterial growth restriction and antibiotic tolerance. RelA is essential for long-term Mtb survival during various i vitro and in vivo growth-limiting conditions. Recently, we found that relA-deficient Mtb exhibits enhanced susceptibility to isoniazid during nutrient starvation and in the lungs of chronically infected mice. Our collaborators at GlaxoSmithKline's Tres Cantos Open Lab for Diseases of the Developing World (GSK-DDW) have screened a library of over 2 million compounds in RelA-inhibition assays and whole- cell screens. Of 178 hits representing 18 unique scaffolds, we have found 8 of 12 scaffolds tested to have potent, RelA-specific activity against nutrient-starved Mtb. The goal of this proposal is to validate RelA as a target for Mtb persisters in vivo and develop RelA inhibitors for targeting persistent bacilli, with the ultimate goal of identifyinga lead compound, which can shorten the duration of treatment for TB by awakening persisters and rendering them more susceptible to killing by standard anti-TB drugs. Specifically, we will screen hits using various in vitro assays to select candidates, which will be used for preliminary toxicity studies and to validate the target in vivo. In addition to improving medical adherence and reducing the potential for the development of drug resistance, an abbreviated drug regimen to treat active TB would be particularly useful in HIV co-infected patients, since drug-drug interactions and immune reconstitution may complicate the concurrent management of both infections.
Curing TB requires at least 6 months of therapy with multiple drugs, which lack efficacy against dormant forms of bacteria. In this proposal, we plan to test compounds targeting a TB-specific enzyme, RelA, which, when inhibited, causes the bacteria to keep dividing until they exhaust their nutrient supply and die. Our studies are expected to yield a new small molecule inhibitor, which kills TB bacteria and, when used in combination with other anti-TB drugs, can shorten the time it takes to cure TB. Developing more effective treatments for TB is particularly important for persons with HIV, who are at increased risk for relapse after completing standard treatment.
