The emergence of drug resistance remains a fundamental challenge in the development of kinase inhibitors that are effective over long-term treatments. Allosteric inhibitors that bind to sites lying outside the highly conserved ATP pocket are thought to be more selective than ATP-competitive inhibitors and may circumvent some mechanisms of drug resistance. Crystal structures of type I and allosteric type III inhibitors in complex with the tyrosine kinase cSrc allowed us to employ principles of structure-based design to develop these scaffolds into potent type II kinase inhibitors. One of these compounds, 3c (RL46), disrupts FAK-mediated focal adhesions in cancer cells via direct inhibition of cSrc. Details gleaned from crystal structures revealed a key feature of a subset of these compounds, a surprising flexibility in the vicinity of the gatekeeper residue that allows these compounds to overcome a dasatinib-resistant gatekeeper mutation emerging in cSrc.

Small molecule kinase inhibitors are an attractive means to modulate kinase activities in medicinal chemistry and chemical biology research. In the physiological setting of a cell, kinase function is orchestrated by a plethora of regulatory processes involving the structural transition of kinases between inactive and enzymatically competent conformations and vice versa. The development of novel kinase inhibitors is mainly fostered by high-throughput screening initiatives where the small molecule perturbation of the phosphorylation reaction is measured to identify inhibitors. Such setups require enzymatically active kinase preparations and present a risk of solely identifying classical ATP-competitive Type I inhibitors. Here we report the high-throughput screening of a library of approximately 35000 small organic molecules with an assay system that utilizes enzymatically inactive human p38alpha MAP kinase to detect stabilizers of the pharmacologically more desirable DFG-out conformation. We used protein X-ray crystallography to characterize the binding mode of hit compounds and reveal structural features which explain how these ligands stabilize and/or induce the DFG-out conformation. Lastly, we show that although some of the hit compounds were confirmed by protein X-ray crystallography, they were not detected in classic phosphorylation assays, thus validating the unique sensitivity of the assay system used in this study and highlighting the potential of screening with inactive kinase preparations.

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