Final Report Summary - TARGID (Host target identification of anti-virulence drugs)
Tuberculosis (TB), the disease caused by the slow-growing pathogen, Mycobacterium tuberculosis (M.tb) is a major public health problem throughout the world. In 2014, this complex and chronic disease was responsible for 9.6 million new cases and 1.5 million deaths. However, although most of the TB cases are present in the developing world, tuberculosis remains one of the major pandemic and public health threats on the planet due to immigration and increased international traveling.
The current therapy for TB, introduced more than 40 years ago, consists of the combination of 4 different drugs (rifampicin, isoniazid, ethambutol and pyrazinamide), it lasts at least 6 months and is relatively inefficient by modern pharmaceutical standards. The appearance of drug-resistant TB strains due to inconsistent or partial drug administration makes TB treatment extremely difficult and even longer (up to 2 years) often requiring the prolonged isolation of the individual. In addition to the dissemination of multidrug-resistant (MDR), extensively drug-resistant (XDR), totally drug-resistant (TDR) strains of M.tb synergy with the HIV/AIDS pandemic significantly complicates efficient TB treatment.
M.tb poses a formidable challenge to the scientific community by virtue of its adaptive skills and ability to evade the immune system of the host. The core strategy to persist for decades in humans with the potential of being reactivated is difficult to counter with current means. Since one third of the world’s population is latently infected with M.tb successful anti-TB drugs must also be able to eliminate persistent bacilli, which makes TB therapy more challenging. Finding new druggable targets to treat both active and latent TB is an urgent priority.
The TARGID project formed part of a multidisciplinary anti-virulence drug discovery program focusing on the identification of important host cell signalling pathways that are involved in the pathogenesis, entry and intracellular survival of M.tb. Target identification studies led to the discovery of BTP15, a benzothiophene inhibitor of the histidine kinase MprB that indirectly regulates ESX-1, and BBH7, a benzyloxybenzylidene-hydrazine compound. BBH7 affects Mtb metal-ion homeostasis and revealed zinc stress as an activating signal for EsxA secretion.
In addition to the contribution to the anti-virulence program, in the TARGID project several thousand chemical compounds were tested against M.tb with the aim of identifying new compound-target pairs, which are different from the conventional ones. Molecules which were effective against the active tubercle bacilli and also potent either in the latent model or intracellularly were selected for further target identification. The activity of these molecules was also assessed on several TB clinical isolates including an MDR-TB strain. As a result, a chemical family was discovered which targets a protein (MmpL3) involved in the efflux mechanism of the bacteria. One of these screening molecules is probably a prodrug acting on the folate pathway similarly to the current para-aminosalicylic acid.
Very interestingly the project resulted in the discovery of chemical agents whose mechanisms of action are different from the current drugs under development. One of them probably targets the essential tricarboxylic acid (TCA) cycle of the bacteria. Besides energy production, the TCA cycle provides precursors of several amino acids and also cofactors used in other biochemical reactions. Another molecule was discovered which likely acts on an iron-sulphur protein involved in electron transport.
This Marie Curie project clearly contributed to tuberculosis drug research as the discovery and functional deciphering of new, potential drug targets opens attractive possibilities to combat active/latent/resistant TB and to reduce the risk for society.
The current therapy for TB, introduced more than 40 years ago, consists of the combination of 4 different drugs (rifampicin, isoniazid, ethambutol and pyrazinamide), it lasts at least 6 months and is relatively inefficient by modern pharmaceutical standards. The appearance of drug-resistant TB strains due to inconsistent or partial drug administration makes TB treatment extremely difficult and even longer (up to 2 years) often requiring the prolonged isolation of the individual. In addition to the dissemination of multidrug-resistant (MDR), extensively drug-resistant (XDR), totally drug-resistant (TDR) strains of M.tb synergy with the HIV/AIDS pandemic significantly complicates efficient TB treatment.
M.tb poses a formidable challenge to the scientific community by virtue of its adaptive skills and ability to evade the immune system of the host. The core strategy to persist for decades in humans with the potential of being reactivated is difficult to counter with current means. Since one third of the world’s population is latently infected with M.tb successful anti-TB drugs must also be able to eliminate persistent bacilli, which makes TB therapy more challenging. Finding new druggable targets to treat both active and latent TB is an urgent priority.
The TARGID project formed part of a multidisciplinary anti-virulence drug discovery program focusing on the identification of important host cell signalling pathways that are involved in the pathogenesis, entry and intracellular survival of M.tb. Target identification studies led to the discovery of BTP15, a benzothiophene inhibitor of the histidine kinase MprB that indirectly regulates ESX-1, and BBH7, a benzyloxybenzylidene-hydrazine compound. BBH7 affects Mtb metal-ion homeostasis and revealed zinc stress as an activating signal for EsxA secretion.
In addition to the contribution to the anti-virulence program, in the TARGID project several thousand chemical compounds were tested against M.tb with the aim of identifying new compound-target pairs, which are different from the conventional ones. Molecules which were effective against the active tubercle bacilli and also potent either in the latent model or intracellularly were selected for further target identification. The activity of these molecules was also assessed on several TB clinical isolates including an MDR-TB strain. As a result, a chemical family was discovered which targets a protein (MmpL3) involved in the efflux mechanism of the bacteria. One of these screening molecules is probably a prodrug acting on the folate pathway similarly to the current para-aminosalicylic acid.
Very interestingly the project resulted in the discovery of chemical agents whose mechanisms of action are different from the current drugs under development. One of them probably targets the essential tricarboxylic acid (TCA) cycle of the bacteria. Besides energy production, the TCA cycle provides precursors of several amino acids and also cofactors used in other biochemical reactions. Another molecule was discovered which likely acts on an iron-sulphur protein involved in electron transport.
This Marie Curie project clearly contributed to tuberculosis drug research as the discovery and functional deciphering of new, potential drug targets opens attractive possibilities to combat active/latent/resistant TB and to reduce the risk for society.