XL281

XL281 is a novel small molecule drug designed to specifically inhibit RAF kinases, which lie immediately downstream of RAS and are key components of the RAS/RAF/MEK/ERK kinase signaling pathway. Genetic lesions that activate this pathway are common in human tumors, with activating mutations in K-Ras occurring in 30 percent of tumors and activating mutations in B-RAF occurring in approximately 60 percent of melanomas. The RAS/RAF/MEK/ERK pathway also plays a key role in the transmission of growth-promoting signals downstream of receptor tyrosine kinases. This suggests that deregulation of this pathway plays a pivotal role in the progression of many human tumors, and that inhibition of the pathway may provide clinical benefit in the treatment of cancer. In preclinical studies, XL281 showed potent inhibition of B-RAF, mutationally activated B-RAF and C-RAF, and did not interact with kinases outside of the RAF family. XL281 displays high oral bioavailability and strongly inhibits RAS/RAF/MEK/ERK signaling in human tumor models.