The
26S proteasome is a large intracellular
adenosine 5'-triphosphate-dependent
protease that identifies and degrades
proteins tagged for destruction by the
ubiquitin system. The orderly degradation of cellular
proteins is critical for normal cell cycling and function, and inhibition of the
proteasome pathway results in cell-cycle arrest and apoptosis. Dysregulation of this enzymatic system may also play a role in
tumor progression, drug resistance, and altered immune surveillance, making the
proteasome an appropriate and novel therapeutic target in
cancer.
Bortezomib (formerly known as
PS-341) is the first
proteasome inhibitor to enter clinical practice. It is a boronic aid
dipeptide that binds directly with and inhibits the enzymatic complex.
Bortezomib has recently shown significant preclinical and clinical activity in several
cancers, confirming the therapeutic value of
proteasome inhibition in human
malignancy. It was approved in 2003 for the treatment of advanced
multiple myeloma (MM), with approximately one third of patients with relapsed and refractory MM showing significant clinical benefit in a large clinical trial. Its mechanism of action is partly mediated through
nuclear factor-kappa B inhibition, resulting in apoptosis, decreased angiogenic
cytokine expression, and inhibition of
tumor cell adhesion to stroma. Additional mechanisms include
c-Jun N-terminal kinase activation and effects on
growth factor expression. Several clinical trials are currently ongoing in MM as well as several other
malignancies. This article discusses
proteasome inhibition as a novel therapeutic target in
cancer and focuses on the development, mechanism of action, and current clinical experience with
bortezomib.