BORTEZoMIB ( also PS-341 or Velcade): A novel the peptide boronates do not inhibit thiol first-in-class proteasome inhibitor for the proteases. The peptide boronic acids are also up to treatment of multiple myeloma and other cancers 100-fold more potent than their peptide aldehyde analogs, Dipeptide boronic acid bortezomib is of eviewed by P.G. Richardson et al Cancer contro/ particular interest from a clinical perspective. This (2003)10:361-369) small, water-soluble compound is a potent and selective proteasome inhibitor, which offers the (A full text PDF of this article is available at additional advantages of low molecular weight and http://www.moffittusfedu/pub! 61 case of synthesis, Bortezomib is the first molecule this class to reach clinical trials in cance Cancer cells seem to be more sensitive to the proapoptotic effects of proteasome inhibition than patients. are normal cells. It has also been shown that proteasome inhibition enhances the sensitivity of cells to traditional anticancer both in vitro and in vivo preclinical studies (Image removed for copyright reasons. See Figure 1b in Richardson, 2003) The 205 core is a cylindrical complex made up of four stacked rings. The two outer rings bind to the s regulatory particles, and the two inner rings each contain three active sites. These active sites account for the three major proteolytic activities of the proteasome, which have been described as Multiple myeloma is a hematologic malignancy peptide hydrolytic(PGPH) typically characterized by the accumulation of clonal plasma cells at multiple sites in the bone marrow Synthetic inhibitors of the proteasome Although the majority of patients respond to initial peptide aldehydes such as Z-Leu-Ler treatment with chemotherapy and radiation, most (MG132), Z-Ile-Glu(Obut)-Ala-Leu-al(PSr) eventually relapse due to the proliferation of Leu-Nle-aI (ALLN), and peptide vinyl sulfones resistant tumor cells; despite the advent of high Natural proteasome inhibitors include lactacystin, dose chemotherapy with stem-cell transplantation epoxyketones and the TMc-95 cyclic peptides. All of MM remains incurable. This cytotoxic resistance these compounds bind to and directly inhibit active reflects both the inherent characteristics of the sites within the 205 core particle. However,mos MM cell and the protective interactions between the primarily interfere with the chymotrypsin-like tumor and the bone marrow microenvironment activity of the core particle (the rate-limiting step in proteolysis)and appear to have little effect on There is strong evidence that the cell death the other proteolytic activities. Many of these induced by proteasome inhibition is apoptotic. For nhibitors also lack specificity or exhibit example,the cell death observed in MM cells unfavorable kinetics for clinical use. For instance exposed to bortezomib in vitro involved caspase-3 peptide aldehyde inhibitors dissociate rapidly from activation and annexin V binding. Further, gastric the proteasome and are inactivated by oxidization cancer cells treated with MG-132 exhibited signs of being removed from the cell by the multidrug apoptosis, such as cytoplasmic and nuclear shrinkage, ransporter system. Furthermore, they are als chromatin condensation and fragmentation, DNA nhibitors of serine and cysteine proteases, including laddering, upregulation of the proapoptotic protein calpains and cathepsins, which can be undesirable for their use in patients. Others, such as the caspase activation. In laboratory studies, MM cell peptide vinyl sulf ones and natural inhibitors bind the lines were significantly more sensitive to the s irreversibly, which can also be detrimental in proapoptotic effects of bortezomib proteasome he long run. inhibition than were bone marrow cells or peripheral blood mononuclear cells from healthy individuals hese problem were overcome however, by Similarly, other proteasome inhibitors induced aldehyde group of ti apoptosis in chronic lymphocytic leukemia cells and bitors with boronic acid oral squamous cell carcinoma cells at doses that had boronates differ from their aldehyde and no effect on normal human lymphocytes or oral they dissociate more slowly from the epithelial cells, respectively. It has also been noted conferring stable inhibition. Further, the weak in preclinical studies that actively dividing cells are interaction between boron and sulphur means thatBORTEZOMIB (also PS-341 or Velcade): A novel, first-in-class proteasome inhibitor for the treatment of multiple myeloma and other cancers (reviewed by P.G. Richardson et al. Cancer con rol (2003) 10: 361-369) t (A full text PDF of this article is available at http://www.moffitt.usf.edu/pubs/ccj/v10n5/pdf/361.pdf) Cancer cells seem to be more sensitive to the proapoptotic effects of proteasome inhibition than are normal cells. It has also been shown that proteasome inhibition enhances the sensitivity of cancer cells to traditional anticancer agents in both in vitro and in vivo preclinical studies. The 20S core is a cylindrical complex made up of four stacked rings. The two outer rings bind to the 19S regulatory particles, and the two inner rings each contain three active sites. These active sites account for the three major proteolytic activities of the proteasome, which have been described as chymotrypsin-like, trypsin-like, and post-glutamyl peptide hydrolytic (PGPH). Synthetic inhibitors of the proteasome include peptide aldehydes such as Z-Leu-Leu-Leu-al (MG132), Z-Ile-Glu(Obut)-Ala-Leu-al (PSI), Ac-Leu￾Leu-Nle-al (ALLN), and peptide vinyl sulfones. Natural proteasome inhibitors include lactacystin, epoxyketones and the TMC-95 cyclic peptides. All of these compounds bind to and directly inhibit active sites within the 20S core particle. However, most primarily interfere with the chymotrypsin-like activity of the core particle (the rate-limiting step in proteolysis) and appear to have little effect on the other proteolytic activities. Many of these inhibitors also lack specificity or exhibit unfavorable kinetics for clinical use. For instance, peptide aldehyde inhibitors dissociate rapidly from the proteasome and are inactivated by oxidization, being removed from the cell by the multidrug transporter system. Furthermore, they are also inhibitors of serine and cysteine proteases, including calpains and cathepsins, which can be undesirable for their use in patients. Others, such as the peptide vinyl sulfones and natural inhibitors bind the 20S irreversibly, which can also be detrimental in the long run. These problems were overcome, however, by replacing the aldehyde group of the synthetic peptide inhibitors with boronic acid. The peptide boronates differ from their aldehyde analogs in that they dissociate more slowly from the proteasome, conferring stable inhibition. Further, the weak interaction between boron and sulphur means that the peptide boronates do not inhibit thiol proteases. The peptide boronic acids are also up to 100-fold more potent than their peptide aldehyde analogs. Dipeptide boronic acid bortezomib is of particular interest from a clinical perspective. This small, water-soluble compound is a potent and selective proteasome inhibitor, which offers the additional advantages of low molecular weight and ease of synthesis. Bortezomib is the first molecule in this class to reach clinical trials in cancer patients. (Image removed for copyright reasons. See Figure 1b in Richardson, 2003.) Multiple myeloma is a hematologic malignancy typically characterized by the accumulation of clonal plasma cells at multiple sites in the bone marrow. Although the majority of patients respond to initial treatment with chemotherapy and radiation, most eventually relapse due to the proliferation of resistant tumor cells; despite the advent of high￾dose chemotherapy with stem-cell transplantation, MM remains incurable. This cytotoxic resistance reflects both the inherent characteristics of the MM cell and the protective interactions between the tumor and the bone marrow microenvironment. There is strong evidence that the cell death induced by proteasome inhibition is apoptotic. For example, the cell death observed in MM cells exposed to bortezomib in vitro involved caspase-3 activation and annexin V binding. Further, gastric cancer cells treated with MG-132 exhibited signs of apoptosis, such as cytoplasmic and nuclear shrinkage, chromatin condensation and fragmentation, DNA laddering, upregulation of the proapoptotic protein Bax, release of mitochondrial cytochrome c, and caspase activation. In laboratory studies, MM cell lines were significantly more sensitive to the proapoptotic effects of bortezomib proteasome inhibition than were bone marrow cells or peripheral blood mononuclear cells from healthy individuals. Similarly, other proteasome inhibitors induced apoptosis in chronic lymphocytic leukemia cells and oral squamous cell carcinoma cells at doses that had no effect on normal human lymphocytes or oral epithelial cells, respectively. It has also been noted in preclinical studies that actively dividing cells are