Covalent-Allosteric Inhibitors to Achieve Akt Isoform-Selectivity.

Isoforms of protein kinase Akt are involved in essential processes including cell proliferation, survival, and metabolism. However, their individual roles in health and disease have not been thoroughly evaluated. Thus, there is an urgent need for perturbation studies, preferably mediated by highly selective bioactive small molecules. Herein, we present a structure-guided approach for the design of structurally diverse and pharmacologically beneficial covalent-allosteric modifiers, which enabled an investigation of the isoform-specific preferences and the important residues within the allosteric site of the different isoforms. The biochemical, cellular, and structural evaluations revealed interactions responsible for the selective binding profiles. The isoform-selective covalent-allosteric Akt inhibitors that emerged from this approach showed a conclusive structure-activity relationship and broke ground in the development of selective probes to delineate the isoform-specific functions of Akt kinases.

An Unusual Intramolecular Halogen Bond Guides Conformational Selection.

PIK-75 is a phosphoinositide-3-kinase (PI3K) α-isoform-selective inhibitor with high potency. Although published structure-activity relationship data show the importance of the NO2 and the Br substituents in PIK-75, none of the published studies could correctly determine the underlying reason for their importance. In this publication, we report the first X-ray crystal structure of PIK-75 in complex with the kinase GSK-3ß. The structure shows an unusual U-shaped conformation of PIK-75 within the active site of GSK-3ß that is likely stabilized by an atypical intramolecular Br⋅⋅⋅NO2 halogen bond. NMR and MD simulations show that this conformation presumably also exists in solution and leads to a binding-competent preorganization of the PIK-75 molecule, thus explaining its high potency. We therefore suggest that the site-specific incorporation of halogen bonds could be generally used to design conformationally restricted bioactive substances with increased potencies.

Covalent Lipid Pocket Ligands Targeting p38α MAPK Mutants.

A chemical genetic approach is presented to covalently target a unique lipid binding pocket in the protein kinase p38α, whose function is not yet known. Based on a series of cocrystal structures, a library of 2-arylquinazolines that were decorated with electrophiles were designed and synthesized to covalently target tailored p38α mutants containing artificially introduced cysteine residues. Matching protein-ligand pairs were identified by MS analysis and further validated by MS/MS studies and protein crystallography. The covalent ligands that emerged from this approach showed excellent selectivity towards a single p38α mutant and will be applicable as suitable probes in future studies of biological systems to dissect the function of the lipid pocket by means of pharmacological perturbations.

Covalent Allosteric Inhibitors of Akt Generated Using a Click Fragment Approach.

Akt is a protein kinase that has been implicated in the progression of cancerous tumours. A number of covalent allosteric Akt inhibitors are known, and based on these scaffolds, a small library of novel potential covalent allosteric imidazopyridine-based inhibitors was designed. The envisaged compounds were synthesised, with click chemistry enabling a modular approach to a number of the target compounds. The binding modes, potencies and antiproliferative activities of these synthesised compounds were explored, thereby furthering the structure activity relationship knowledge of this class of Akt inhibitors. Three novel covalent inhibitors were identified, exhibiting moderate activity against Akt1 and various cancer cell lines, potentially paving the way for future covalent allosteric inhibitors with improved properties.

Cellular model system to dissect the isoform-selectivity of Akt inhibitors.

The protein kinase Akt plays a pivotal role in cellular processes. However, its isoforms' distinct functions have not been resolved to date, mainly due to the lack of suitable biochemical and cellular tools. Against this background, we present the development of an isoform-dependent Ba/F3 model system to translate biochemical results on isoform specificity to the cellular level. Our cellular model system complemented by protein X-ray crystallography and structure-based ligand design results in covalent-allosteric Akt inhibitors with unique selectivity profiles. In a first proof-of-concept, the developed molecules allow studies on isoform-selective effects of Akt inhibition in cancer cells. Thus, this study will pave the way to resolve isoform-selective roles in health and disease and foster the development of next-generation therapeutics with superior on-target properties.

Spotlight on AKT: Current Therapeutic Challenges.

The protein kinase B (Akt) exemplifies an important switch of cell death and survival within the PI3K/Akt signaling pathway, which renders Akt a valuable target in diseases such as cancer. Herein, we give a short overview of clinical applications involving Akt, outline promising and innovative approaches to investigate the role of this kinase in diseases, and highlight the current challenges that require thorough investigation to set the groundwork for successful therapeutic strategies.

Structural and chemical insights into the covalent-allosteric inhibition of the protein kinase Akt.

The Ser/Thr kinase Akt (Protein Kinase B/PKB) is a master switch in cellular signal transduction pathways. Its downstream signaling influences cell proliferation, cell growth, and apoptosis, rendering Akt a prominent drug target. The unique activation mechanism of Akt involves a change of the relative orientation of its N-terminal pleckstrin homology (PH) and the kinase domain and makes this kinase suitable for highly specific allosteric modulation. Here we present a unique set of crystal structures of covalent-allosteric interdomain inhibitors in complex with full-length Akt and report the structure-based design, synthesis, biological and pharmacological evaluation of a focused library of these innovative inhibitors.

Preclinical Efficacy of Covalent-Allosteric AKT Inhibitor Borussertib in Combination with Trametinib in KRAS-Mutant Pancreatic and Colorectal Cancer.

Aberrations within the PI3K/AKT signaling axis are frequently observed in numerous cancer types, highlighting the relevance of these pathways in cancer physiology and pathology. However, therapeutic interventions employing AKT inhibitors often suffer from limitations associated with target selectivity, efficacy, or dose-limiting effects. Here we present the first crystal structure of autoinhibited AKT1 in complex with the covalent-allosteric inhibitor borussertib, providing critical insights into the structural basis of AKT1 inhibition by this unique class of compounds. Comprehensive biological and preclinical evaluation of borussertib in cancer-related model systems demonstrated a strong antiproliferative activity in cancer cell lines harboring genetic alterations within the PTEN, PI3K, and RAS signaling pathways. Furthermore, borussertib displayed antitumor activity in combination with the MEK inhibitor trametinib in patient-derived xenograft models of mutant KRAS pancreatic and colon cancer. SIGNIFICANCE: Borussertib, a first-in-class covalent-allosteric AKT inhibitor, displays antitumor activity in combination with the MEK inhibitor trametinib in patient-derived xenograft models and provides a starting point for further pharmacokinetic/dynamic optimization.