The search for new therapeutic agents has been greatly aided in recent years by a better understanding of the structure of enzymes and other biomolecules associated with diseases. One important class of enzymes that has been the subject of extensive study is protein kinases.
Protein kinases constitute a large family of structurally related enzymes that are responsible for the control of a variety of signal transduction processes within the cell. Protein kinases are thought to have evolved from a common ancestral gene due to the conservation of their structure and catalytic function. Almost all kinases contain a similar 250-300 amino acid catalytic domain. The kinases may be categorized into families by the substrates they phosphorylate (e.g., protein-tyrosine, protein-serine/threonine, lipids, etc.).
The processes involved in tumor growth, progression, and metastasis are mediated by signaling pathways that are activated in cancer cells. The MAPK or Raf-Mek-ERK pathway plays a central role in regulating mammalian cell growth by relaying extracellular signals from ligand-bound cell surface tyrosine kinase receptors such as erbB family, PDGF, FGF, and VEGF receptor tyrosine kinase. Activation of the ERK occurs via a cascade of phosphorylation events that begins with activation of Ras. Activation of Ras leads to the recruitment and activation of Raf, a serine-threonine kinase. Activated Raf then phosphorylates and activates MEK1/2, which then phosphorylates and activates one or both of ERK1 and ERK2. When activated, one or both of ERK1 and ERK2 phosphorylates several downstream targets involved in a multitude of cellular events including cytoskeletal changes and transcriptional activation. The ERK/MAPK pathway is one of the most important for cell proliferation, and human tumor data suggest that the ERK/MAPK pathway is frequently activated in many tumors. Ras genes, which are upstream of one or both of ERK1 and ERK2, are mutated in several cancers including colorectal, melanoma, breast, lung, and pancreatic tumors. High Ras activity is accompanied by elevated ERK activity in many human tumors. In addition, activating mutations of BRAF, a serine-threonine kinase of the Raf family, are associated with increased RAF, MEK, and ERK kinase activity. Tumors types with the most frequent mutations in BRAF include melanomas (60%), thyroid cancers (greater than 40%) and colorectal cancers.
Many diseases are associated with abnormal cellular responses, proliferation and evasion of programmed cell-death, triggered by protein kinase-mediated events as described above. Accordingly, there remains a need to find protein kinase inhibitors useful as therapeutic agents.