Characterization of irreversible kinase inhibitors by directly detecting covalent bond formation: a tool for dissecting kinase drug resistance.

Targeting protein kinases in cancer therapy with irreversible small-molecule inhibitors is moving to the forefront of kinase-inhibitor research and is thought to be an effective means of overcoming mutation-associated drug resistance in epidermal growth factor receptor kinase (EGFR). We generated a detection technique that allows direct measurements of covalent bond formation without relying on kinase activity, thereby allowing the straightforward investigation of the influence of steric clashes on covalent inhibitors in different resistant kinase mutants. The obtained results are discussed together with structural biology and biochemical studies of catalytic activity in both wild-type and gatekeeper mutated kinase variants to draw conclusions about the impact of steric hindrance and increased catalytic activity in drug-resistant kinase variants.

Proteus in the world of proteins: conformational changes in protein kinases.

The 512 protein kinases encoded by the human genome are a prime example of nature's ability to create diversity by introducing variations to a highly conserved theme. The activity of each kinase domain is controlled by layers of regulatory mechanisms involving different combinations of post-translational modifications, intramolecular contacts, and intermolecular interactions. Ultimately, they all achieve their effect by favoring particular conformations that promote or prevent the kinase domain from catalyzing protein phosphorylation. The central role of kinases in various diseases has encouraged extensive investigations of their biological function and three-dimensional structures, yielding a more detailed understanding of the mechanisms that regulate protein kinase activity by conformational changes. In the present review, we discuss these regulatory mechanisms and show how conformational changes can be exploited for the design of specific inhibitors that lock protein kinases in inactive conformations. In addition, we highlight recent developments to monitor ligand-induced structural changes in protein kinases and for screening and identifying inhibitors that stabilize enzymatically incompetent kinase conformations.

Chemogenomic profiling provides insights into the limited activity of irreversible EGFR Inhibitors in tumor cells expressing the T790M EGFR resistance mutation.

Reversible epidermal growth factor receptor (EGFR) inhibitors are the first class of small molecules to improve progression-free survival of patients with EGFR-mutated lung cancers. Second-generation EGFR inhibitors introduced to overcome acquired resistance by the T790M resistance mutation of EGFR have thus far shown limited clinical activity in patients with T790M-mutant tumors. In this study, we systematically analyzed the determinants of the activity and selectivity of the second-generation EGFR inhibitors. A focused library of irreversible as well as structurally corresponding reversible EGFR-inhibitors was synthesized for chemogenomic profiling involving over 79 genetically defined NSCLC and 19 EGFR-dependent cell lines. Overall, our results show that the growth-inhibitory potency of all irreversible inhibitors against the EGFR(T790M) resistance mutation was limited by reduced target inhibition, linked to decreased binding velocity to the mutant kinase. Combined treatment of T790M-mutant tumor cells with BIBW-2992 and the phosphoinositide-3-kinase/mammalian target of rapamycin inhibitor PI-103 led to synergistic induction of apoptosis. Our findings offer a mechanistic explanation for the limited efficacy of irreversible EGFR inhibitors in EGFR(T790M) gatekeeper-mutant tumors, and they prompt combination treatment strategies involving inhibitors that target signaling downstream of the EGFR.

Displacement assay for the detection of stabilizers of inactive kinase conformations.

Targeting protein kinases with small molecules outside the highly conserved ATP pocket to stabilize inactive kinase conformations is becoming a more desirable approach in kinase inhibitor research, since these molecules have advanced pharmacological properties compared to compounds exclusively targeting the ATP pocket. Traditional screening approaches for kinase inhibitors are often based on enzyme activity, but they may miss inhibitors that stabilize inactive kinase conformations by enriching the active state of the kinase. Here we present the development of a kinase binding assay employing a pyrazolourea type III inhibitor and enzyme fragment complementation (EFC) technology that is suitable to screen stabilizers of enzymatically inactive kinases. To validate this assay system, we report the binding characteristics of a series of kinase inhibitors to inactive p38alpha and JNK2. Additionally, we present protein X-ray crystallography studies to examine the binding modes of potent quinoline-based DFG-out binders in p38alpha.

High-throughput screening to identify inhibitors which stabilize inactive kinase conformations in p38alpha.

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.

Hybrid compound design to overcome the gatekeeper T338M mutation in cSrc.

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.

A new screening assay for allosteric inhibitors of cSrc.

Targeting kinases outside the highly conserved ATP pocket is thought to be a promising strategy for overcoming bottlenecks in kinase inhibitor research, such as limited selectivity and drug resistance. Here we report the development and application of a direct binding assay to detect small molecules that stabilize the inactive conformation of the tyrosine kinase cSrc. Protein X-ray crystallography validated the assay results and confirmed an exclusively allosteric binding mode.

Structural insights into how irreversible inhibitors can overcome drug resistance in EGFR.

Resistance to kinase-targeted cancer drugs has recently been linked to a single point mutation in the ATP binding site of the kinase. In EGFR, the crucial Thr790 gatekeeper residue is mutated to a Met and prevents reversible ATP competitive inhibitors from binding. Irreversible 4-(phenylamino)quinazolines have been shown to overcome this drug resistance and are currently in clinical trials. In order to obtain a detailed structural understanding of how irreversible inhibitors overcome drug resistance, we used Src kinase as a model system for drug resistant EGFR-T790M. We report the first crystal structure of a drug resistant kinase in complex with an irreversible inhibitor. This 4-(phenylamino)quinazoline inhibits wild type and drug resistant EGFR in vitro at low nM concentrations. The co-crystal structure of drug resistant cSrc-T338M kinase domain provides the structural basis of this activity.