Bortezomib

The ubiquitin-proteasome pathway is a homeostatic proteolytic pathway for intracellular protein degradation: proteins marked with a poly-ubiquitin chain are degraded to small peptides and free ubiquitin by the proteasome, which is a large multimeric protease. Aberrant proteasome-dependent proteolysis, as seen in some malignancies, can lead to uncontrolled cell division, leading to tumorigenesis, cancer growth, and spread.

Bortezomib is a reversible inhibitor of the 26S proteasome, which is made up of a 20S core complexed with a 19S regulatory complex. Individual β-subunits allow specific catalytic action of the 20S core. In mammalian cells, bortezomib is a potent inhibitor of the proteasome’s chymotryptic-like activity, which is attributed to the β5-subunit of the 20S core particle. Bortezomib binds to the active site of the threonine hydroxyl group in the β5-subunit. A probing study showed bortezomib also binding to and inhibiting the β1-subunit, which mediates the caspase-like activity of the proteasome, and β1i-subunit, which is an altered subunit that is expressed to form immunoproteasomes in response to cell stress or inflammation. By inhibiting the proteasome-mediated degradation of key proteins that promote cell apoptosis, bortezomib induces a cell cycle arrest during the G2-M phase. It is believed that multiple mechanisms, other than proteasome inhibition, may be involved in the anticancer activity of bortezomib. The anticancer activity of bortezomib was largely associated with suppression of the NF-κB signalling pathway, resulting in the downregulation of anti-apoptotic target genes and expression of anti-apoptic proteins. This may be explained by bortezomib preventing uncontrolled degradation of IκB, which is an inhibitory protein of NF-κB. NOXA, which is a pro-apoptotic factor, induced by bortezomib selectively in cancer cells; thus, it is suggested to be another key mechanism of bortezomib.