Читать книгу Successful Drug Discovery, Volume 5 - Группа авторов - Страница 25

The discovery of carfilzomib started from a regular literature search. Craig Crews (Yale University) was searching for new project ideas and reviewed past issues of the Journal of Antibiotics. He came across a compound called epoxomicin (Figure 1.12) [76] that caught his attention. The compound was isolated from an Actinomycete strain and displayed interesting cytotoxic activity against various cancer cell lines, as well as activity in an in vivo B16 leukemia model. The stereochemistry was not determined, but it had an exposed epoxide as a rather unusual structural feature. The molecule was actually discovered by Japanese researchers at BMS, but as the mechanism of action was unknown and the drug‐like properties of the compound were rather poor, BMS decided to drop the project. Crews did not intend to start a drug discovery project, but was rather interested in applying emerging chemical biology techniques to the molecule and unravel its mode of action. He completed the first total synthesis of the molecule, which also allowed determination of the previously unknown stereochemistry [77]. By employing a biotinylated derivative, he determined that epoxomicin specifically targets the proteasome [78]. The proteasome, a protein complex of about 1700 kDa, is responsible for degradation of misfolded proteins and exists in all eukaryotic cells and archaea, as well as in several prokaryotes. Shutting down the proteasome will significantly disturb normal cellular processes and will quickly kill the cell. However, cells with high replication rates should be more dependent on optimal functionality of the proteasome, thus opening an opportunity for cancer therapy. Several proteasome inhibitors, natural and synthetic, had already been reported in the literature [79], but their structures usually contained very reactive warheads, resulting in insufficient compound selectivity. Crews showed that epoxomicin, in contrast to other compounds with reactive warheads, selectively inhibited the proteasome. He started a collaboration with German Nobel laureate Robert Huber (Max Planck Institute for Biochemistry, in Martinsried, Germany) and solved the crystal structure of epoxomicin bound to the proteasome [80]. They discovered that epoxomicin reacted with the N‐terminal threonine moiety of the 20S proteasome (Figure 1.12), forming a stable morpholino ring through successive epoxide opening by the terminal amino group and attack of the nucleophilic hydroxyl group on the ketone of epoxomicin. This specificity was remarkable, which prompted Crews and his coworkers to derivatize and improve epoxomicin.

Figure 1.12 Epoxomicin binding to the 20S‐ribosome.

Source: Based on Hanada et al. [76].

After several rounds of optimization, first systematically varying the individual positions of the tetrapeptide and then combining the optimized residues in one molecule, they came up with a compound they later termed YU‐101 (Figure 1.13) [81]. The compound did show significantly enhanced activity compared with epoxomicin and PS‐341 (bortezomib), a dipeptidyl boronic acid derivative of epoxomicin developed by a biotech company called ProScript. Bortezomib was later acquired by Millennium Pharmaceuticals and became the FDA approved medication Velcade™, used for treatment for treatment of multiple myeloma and mantle cell lymphoma. YU‐101 was licensed to Proteolix, a startup company founded by Craig Crews and Raymond J. Deshaies (California Institute of Technology). Proteolix was dedicated to the discovery of drugs targeting the proteasome. Scientists at Proteolix continued optimization and finally selected carfilzomib for preclinical and later clinical development. Proteolix was acquired by Onyx in 2009 for a nominal value of US$ 810 million. Carfilzomib (Kyprolis™, Figure 1.13) was approved by the FDA in July 2012 for treatment of advanced multiple myeloma and in 2015/2016 in combination with dexamethasone or lenalidomide and dexamethasone for treatment of refractory melanoma.

Figure 1.13 From epoxomicin to carfilzomib.