Fighting drug-resistant tuberculosis: CTP-synthetase and pantothenate kinase as new targets for multitargeting compounds.

Tuberculosis (TB), the infectious disease brought about by Mycobacterium tuberculosis, is afflicting human health worldwide. Epidemiological data indicate 3 billion people latently infected globally, and only in 2014, 1.5 million people died due to this infection. Moreover, TB plague does not show signs to stop, particularly in view of the spread of M. tuberculosis drug-resistant strains (MDR, XDR and TDR), together with patients co-infected with HIV. Thus, considering all these aspects, the research for new antitubercular drugs and the identification of novel targets that could allow the killing of the pathogen through more efficient tools, are surely indispensable. Recently, from the screening of the National Institute of Allergy and Infectious Diseases (NIAID) chemical library, two compounds distinguished themselves for their efficacious antitubercular activity. These molecules, the thiophene-carboxamide 7947882 and the carbamothioyl-propanamide 7904688, displayed activity against the pathogen in vitro, ex vivo, and against a latent model. Genetic and biochemical approaches demonstrated that 7947882 and 7904688 are prodrugs activated by the monooxygenase EthA, already known to be the activator of ethionamide. Moreover, from the sequencing analysis of 7947882 and 7904688 M. tuberculosis spontaneous resistant mutants, the CTP-synthetase PyrG and pantothenate kinase PanK emerged as the putative targets of these compounds. The present work led to the demonstration that PyrG and PanK are the cellular targets of these compounds. Moreover, in view of the importance of finding new drugs targeting more than one cellular function, PyrG and PanK were exploited to perform an in silico screening of the Collaborative Drug Discovery (CDD) compound database, and an in vitro screening of the GSK TB-set chemical library of compounds against the two enzymes. From these screenings, a number of compounds affecting both enzymes emerged, thus strengthening the usefulness of PyrG and PanK for new multitargeting drugs research. Finally, all M. tuberculosis PyrG inhibitors were tested against human CTP-synthetase-1, identifying one compound that inhibits almost exclusively the mycobacterial enzyme, and not the human one, paving the way for new M. tuberculosis PyrG selective inhibitors.