Purinylpyridinylamino-based DFG-in/αC-helix-out B-Raf inhibitors: Applying mutant versus wild-type B-Raf selectivity indices for compound profiling

One of the challenges for targeting B-RafV600E with small molecule inhibitors had been achieving adequate selectivity over the wild-type protein B-RafWT, as inhibition of the latter has been associated with hyperplasia in normal tissues. Recent studies suggest that B-Raf inhibitors inducing the ‘DFG-in/αC-helix-out’ conformation (Type IIB) likely will exhibit improved selectivity for B-RafV600E. To explore this hypothesis, we transformed Type IIA inhibitor (1) into a series of Type IIB inhibitors (sulfonamides and sulfamides 4–6) and examined the SAR. Three selectivity indices were introduced to facilitate the analyses: the B-RafV600E/B-RafWT biochemical (bS), cellular (cS) selectivity, and the phospho-ERK activation (pA). Our data indicates that α-branched sulfonamides and sulfamides show higher selectivities than the linear derivatives. We rationalized this finding based on analysis of structural information from the literature and provided evidence for a monomeric B-Raf-inhibitor complex previously hypothesized to be responsible for the desired B-RafV600E selectivity.

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