Development of novel, highly potent inhibitors of V-RAF murine sarcoma viral oncogene homologue B1 (BRAF): increasing cellular potency through optimization of a distal heteroaromatic group

We describe the design, synthesis, and optimization of a series of new inhibitors of V-RAF murine sarcoma viral oncogene homologue B1 (BRaf), a kinase whose mutant form (V600E) is implicated in several types of Cancer, with a particularly high frequency in melanoma. Our previously described inhibitors with a tripartite A-B-C system (where A is a hinge binding pyrido[4,5-b]imidazolone system, B is an aryl spacer group, and C is a heteroaromatic group) were potent against purified (V600E)BRaf in vitro but were less potent in accompanying cellular assays. Substitution of different aromatic heterocycles for the phenyl based C-ring is evaluated herein as a potential means of improving the cellular potencies of these inhibitors. Substituted pyrazoles, particularly 3-tert-butyl-1-aryl-1H-pyrazoles, increase the cellular potencies without detrimental effects on the potency on isolated (V600E)BRaf. Thus, compounds have been synthesized that inhibit, with low nanomolar concentrations, (V600E)BRaf, its downstream signaling in cells [as measured by the reduction of the phosphorylation of extracellular regulated kinase (ERK)], and the proliferation of mutant BRAF-dependent cells. Concomitant benefits are good oral bioavailability and high plasma concentrations in vivo.