Targeting inactive conformations of protein kinases can potentially have several advantages, including improved cell activity due to reduced ATP competition, efficient pathway inhibition by promoting the unphosphorylated state, and slow off-rate kinetics of binding. A high-throughput affinity screen directed at full-length unphosphorylated SYK identified a class of micromolar inhibitors that appeared selective for the inactive form. X-ray crystallography revealed an unusual binding mode, with the SYK kinase domain in both a DFG-out and C-helix-out state that had not been previously reported. Potency optimization using structure-based design combined with library synthesis identified nanomolar binders that were suitable for further profiling. Despite an unusual binding mode and preference for the inactive form, these compounds were not particularly kinome selective and did not exhibit enhanced cell activity as compared to traditional SYK inhibitors. Although significant conformational rearrangement occurs upon binding, fast off-rate kinetics to both phosphorylated and unphosphorylated SYK was observed. This work provides additional evidence that a non-traditional binding mode and unphosphorylated-form preference does not guarantee the advantages typically associated with inactive-state kinase inhibitors.

Atypical inactive-state inhibitors of spleen tyrosine kinase (SYK)

251st National Meeting (2016)

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