Ideally, new drugs should target essential pathways in M. tuberculosis that are not currently targeted by first- and second-line drugs. Consequently, there is an urgent need for the development of new antitubercular therapies that are effective against resistant as well as persistent forms of tuberculosis. (1) Resistant strains are not susceptible to the standard drugs, and although MDR-TB is treatable using second-line drugs, such treatments are costly, toxic, and/or not readily available. According to the World Health Organization (WHO), approximately 480,000 people developed MDR-TB in 2014 and the cure rate of those patients was only 50%. (1) Although the mortality rate has dropped by 47% since the 1990s, the emergence of multidrug resistant (MDR-TB) and extremely drug resistant (XDR-TB) strains has complicated our ability to control the disease. In 2014, 9.6 million patients were diagnosed with TB infection and ∼1.5 million people died. Mycobacterium tuberculosis, the causative agent of tuberculosis (TB), has plagued mankind for centuries and is one of the world’s deadliest infectious diseases.
Compounds did not increase cell permeability, dissipate membrane potential, or inhibit an unrelated mycobacterial enzyme, suggesting a specific mode of action related to the LepB secretory mechanism. Inhibition of LepB activity was observed for a number of compounds in a biochemical assay using cell membrane fraction derived from M. tuberculosis. A number of chemical modifications around the hydrazone moiety resulted in improved potency. We conducted a limited structure–activity relationship determination around a representative PHY compound with differential activity (MICs of 3.0 μM against the LepB-UE strain and 18 μM against the wt) several analogues were less potent against the LepB overexpressing strain. We identified the phenylhydrazone (PHY) series as having higher activity against the LepB-UE strain. We screened 72,000 compounds against both the Lep-UE and wild-type (wt) strains. We developed a target-based whole cell assay to screen for potential inhibitors of LepB, the sole signal peptidase in Mycobacterium tuberculosis, using a strain engineered to underexpress LepB (LepB-UE). During protein export, the signal peptidase LepB catalyzes the cleavage of the signal peptide and subsequent release of mature proteins into the extracellular space.
The general secretion (Sec) pathway is a conserved essential pathway in bacteria and is the primary route of protein export across the cytoplasmic membrane.
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