Tuberculosis still remains a major threat for public health, with one-third of the world population latently infected, and more than one million deaths per year according to the WHO. Among the factors that have accounted for the increase in the global tuberculosis burden, most important are the synergy with the HIV/AIDS pandemics and the spread of drug resistance, including the emergence of the so-called extensively drug-resistant Mycobacterium tuberculosis strains (XDR-TB). The efficacy of the currently available antibiotic therapy is therefore limited and the development of new, more potent drugs against tuberculosis of the utmost importance. Our group is member of the international consortium ‘More Medicines for TB’ (MM4TB), funded by the EC under the FP7 programme, a cluster that takes together leading European academic teams and pharmaceutical companies working in the field of TB drug discovery. The consortium is essentially devoted to compound-based drug development, in which hits from either synthetic or natural compound libraries are selected through whole cell screenings for their antibacterial activity, and their respective targets are first identified genetically (i.e. whole genome sequencing of resistant mutants), then validated chemically (i.e. generation of conditional mutants and study of their resistance profile).
As a member of the MM4TB consortium, the Alzari’s team provides structural and biochemical characterization of newly identified targets from M. tuberculosis, to help progression of the drug design pipeline. Crystal structures of the relevant, pharmacologically validated targets, alone or in complex with the original hits, natural substrates or analogues are indeed crucial tools not only for hit-to-lead progression but also for parallel target-based drug discovery, i.e. by virtual screening or fragment-based approaches.
During the last few years, we have been working on several targets, including protein kinases and signalling elements, cell wall mannosyltransferase PimA, and DNA gyrases. More recently the CTP synthase PyrG, coded by an essential gene, was identified as the target of a new compound from the thiophenecarboxamide chemical class. We determined the crystal structure of PyrG both in apo form and in complex with a series of substrates/analogues (UTP, AMP-PCP, or the glutamine analogue 6-diazo-5-oxo-L-norleucine). We are currently working at the cocrystallization with the active metabolite (as the original compound acts as a prodrug) and new molecules selected by a virtual screening approach, in collaboration with G. Riccardi’s group at the University of Pavia (Italy) and Sean Ekins from Collaborative Drug Discovery (Burlinghame, CA, USA).