Many activities of cancer cells such as growth, metastasis, invasion and the like are caused via intracellular signal transduction from RTK: receptor tyrosine kinases (EGFR, HER2 etc.), which is activated by stimulation by growth factors and mutation, and the activation signal thereof is transmitted downstream via RAS protein. As the intracellular signal transduction pathway via Ras, Ras/Raf/MEK/ERK pathway is best known, which is deeply involved in the control of various cell functions such as cell proliferation, cellular motility, transformation, apoptosis (cell death) resistance and the like.
To block the pathway, inhibitors of growth factor receptors, for example, epithelial growth factor receptor (EGFR) inhibitors gefinitib (trade name: Iressa), and erlotinib (trade name: Tarceva), and human epithelial growth factor receptor type 2 (HER2) inhibitory antibody trastuzumab (trade name: Herceptin) are placed on the market in recent years. They have been reported to be effective for the treatment of some cancer types in clinical practices, such as lung cancer, breast cancer and the like. In addition, it has been shown that inhibitory antibody bevacizumab (trade name: Avastin) against vascular endothelial growth factor (VEGF) inhibits activation of VEGFR in the intratumoral neovascular endothelial cells and shows an antitumor activity. These medicaments suppress signal transduction system at the downstream when showing a tumor growth inhibitory activity in cancer to be the target cells and vascular endothelial cells, through inhibition of receptor enzyme activity and inhibition of receptor activation.
On the other hand, the Ras/Raf/MEK/ERK pathway is well known to cause highly frequent mutations in cancer. Ras gene is reported to undergo an activation type mutation at codon 12, 13 or 61 of various carcinomas, for example, about 90% of the total of pancreatic cancer, about 35% of non-small cell lung cancer, about 30% of liver cancer and the like, and there are many reports on the correlation between Ras mutation and developing malignant tumor.
With regard to Raf gene, activation mutation in kinase domain of B-Raf in cancer has been reported. It is known that B-Raf mutation, particularly V600E, occurs in various carcinomas, for example, about 60% of the total of malignant melanoma, about 30% of thyroid cancer, about 15% of colorectal cancer and the like. Particularly, B-Raf (V600E) kinase has about 13-fold MEK phosphorylation activity as compared to wild-type B-Raf kinase, and the activity of B-Raf is deeply involved in the growth of cancer having a mutation in B-Raf.
In these cancers, inhibitions of the upstream growth factor receptor activity and Ras cannot suppress signal transduction system downstream of Raf kinase, which is constantly activated. In this case, since suppression of the downstream signal (Raf/MEK/ERK signal transduction system) cannot be expected, a tumor growth suppressive activity cannot be expected, either. For example, melanoma showing highly frequent B-Raf mutation is highly metastatic and the 5 year survival rate is about 6%, for which no promising therapeutic drug exists at present.
In the Ras/Raf/MEK/ERK pathway, Raf kinase is the most downstream molecule to be activated by mutation. A compound inhibiting Raf activity is considered to be effective as a therapeutic drug for any cancer caused by mutation of growth factor receptor or excessive activation by ligand stimulation, or cancer caused by activation type mutation of Ras.
Raf is a serine/threonine kinase, and is known to include three isoforms of A-Raf, B-Raf and c-Raf. Raf is activated by Ras and phosphorylates the downstream molecule MEK. The activated MEK further phosphorylates ERK to transmit the signal further downstream. Of three isoforms, B-Raf kinase shows an extreme strong activity of phosphorylating MEK in the basal state, which is about 15- to 20-fold that of A-Raf, c-Raf kinase activity. To undergo process of activation, moreover, c-Raf requires phosphorylation of the 338th serine in the activation loop to obtain the maximum activity (same for A-Raf). However, B-Raf is known to be easily activated as compared to A-Raf and c-Raf, since the corresponding sequence is always phosphorylated.
A compound that inhibits B-Raf kinase activity and mutant B-Raf kinase is considered to suppress cell proliferation particularly in cancer with poor prognosis. Accordingly, such compound becomes an effective therapeutic drug for cancer for which a growth factor receptor enzyme activity inhibitor is ineffective.
As Raf inhibitors, sorafenib-related derivatives (patent documents 1-3, non-patent document 1), benzylidene derivative (patent document 4), imidazole derivatives (patent documents 5-8), pyridylfuran derivatives (patent documents 9-12), benzazole derivatives (patent documents 13-15), thiazolopyridine derivatives (patent documents 16 and 17) and the like are known.