RO4987655 is a highly selective and small molecule inhibitor of MEK with IC50 value of 5nM [1].

RO4987655 is developed with a unique 3-oxo-[1,2]oxazinan-2-ylmethyl group at the 5-position.It is highly selective against MEK over other 400 kinases. Meanwhile, RO4987655 is a non- competitive inhibitor of MEK. It shows higher affinity for MEK both in the absence and the presence of ATP. RO4987655 has a strong anti-proliferation efficacy in various tumor cells including COLO205, HT29, QG56, MIA and C32 with IC50 values of 0.86nM, 1.7nM, 9.5nM, 3.3nM and 8.4nM, respectively. In addition, RO4987655 is found to have antitumor activity in a wide range of human cancer xenograft models. In the HT-29 human colon cancer xenograft, RO4987655 shows a strong inhibition of pERK formation as well as tumor regression [1, 2].

RO4987655 is now in phase I trials. It shows clinical activity in both patients with BRAF wild-type melanoma and BRAF V600-mutated melanoma. It is also efficacious in patients with KRAS-mutated non-small cell lung cancer but not KRAS-mutated colorectal cancer [2].

References:
[1] Isshiki Y, Kohchi Y, Iikura H, Matsubara Y, Asoh K, Murata T, Kohchi M, Mizuguchi E, Tsujii S, Hattori K, Miura T, Yoshimura Y, Aida S, Miwa M, Saitoh R, Murao N, Okabe H, Belunis C, Janson C, Lukacs C, Schück V, Shimma N. Design and synthesis of novel allosteric MEK inhibitor CH4987655 as an orally available anticancer agent. Bioorg Med Chem Lett. 2011 Mar 15;21(6):1795-801.
[2] Zimmer L, Barlesi F, Martinez-Garcia M, Dieras V, Schellens JH, Spano JP, Middleton MR, Calvo E, Paz-Ares L, Larkin J, Pacey S, Venturi M, Kraeber-Bodere F, Tessier JJ, Eberhardt WE, Paques M, Guarin E, Meresse Naegelen V, Soria JC. Phase I expansion and pharmacodynamic study of the oral MEK inhibitor RO4987655 (CH4987655) in selected advanced cancer patients with RAS-RAF mutations. Clin Cancer Res. 2014 Jun 19. pii: clincanres.0341.2014.