The target of the screen is protein splicing, which is an essential step in the expression of genes that are interrupted by an intein and occurs in the pathogenic mycobacteria but in no other bacteria associated with humans. In Mycobacterium tuberculosis, the recA, dnaB and sufB genes are interrupted by closely related inteins. Two of these genes (dnaB and sufB) are essential for growth and the third (recA) predisposes to drug-resistance mutations. Protein splicing inhibitors, by disabling two separate vital functions and at the same time suppressing mutation rates, should therefore be highly effective anti-TB drugs that would be unlikely to elicit drug-resistance, suggesting that protein splicing would be an effective antimycobacterial target. This program was initiated in 2004 by screening about 85,000 compounds from commercial libraries at the Institute of Chemistry and Cell Biology (ICCB) at Harvard Medical School and continued recently under an NIH Roadmap R03 grant by screening about 290,000 compounds of the MLPCN library at the Broad Institute. Most of the inhibitors identified in these screens were electrophiles, which covalently modified the catalytic cysteine residue of the inteins, and were not suitable for long-term drug use. Medicinal chemistry efforts are therefore needed to attenuate inhibitor reactivity as well as additional screening efforts focused on different classes of compounds such as natural products.
Protein Splicing Inhibitors