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  • Review Article
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Direct small-molecule inhibitors of KRAS: from structural insights to mechanism-based design

Nature Reviews Drug Discovery volume 15, pages 771–785 (2016)Cite this article

Subjects

Key Points

  • The small GTPase KRAS is among the most frequently activated proteins in cancer. Despite this, extensive efforts to target KRAS over the past decades have yet to yield a clinical drug.

  • Advances in drug discovery approaches and technology have renewed hope for targeting this challenging oncogene, and several recent reports have demonstrated substantial progress to this end.

  • Analysis of RAS structure and dynamics suggests that targeting the GDP-bound off state of RAS may be a more viable approach than targeting the GTP-bound on state. Recent data indicating continued flux through the GTPase cycle for some oncogenic mutants make the GDP state even more promising. Several reported molecules now target nucleotide exchange and others allosterically target GTP binding.

  • Efforts to inhibit RAS-effector interactions and to impair RAS localization and trafficking through RAS-binding small molecules have continued to struggle. However, promising molecules have been reported to target these functions indirectly, without binding to RAS.

  • Stabilizing non-productive protein–protein complexes remains an attractive approach to inhibiting RAS; however, little progress has been demonstrated in this area.

Abstract

KRAS is the most frequently mutated oncogene in human cancer. In addition to holding this distinction, unsuccessful attempts to target this protein have led to the characterization of RAS as 'undruggable'. However, recent advances in technology and novel approaches to drug discovery have renewed hope that a direct KRAS inhibitor may be on the horizon. In this Review, we provide an in-depth analysis of the structure, dynamics, mutational activation and inactivation, and signalling mechanisms of RAS. From this perspective, we then consider potential mechanisms of action for effective RAS inhibitors. Finally, we examine each of the many recent reports of direct RAS inhibitors and discuss promising avenues for further development.

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Figure 1: RAS secondary structure and the RAS–GTPase cycle.
Figure 2: Glycine 60 and threonine 35 act as springs anchored at the γ-phosphate of GTP.
Figure 3: Crystal form 1 of HRAS-T35S shown with symmetry mate.
Figure 4: Mutations that block oncogenic transformation, CRAF binding, or phosphoinositide 3-kinase binding.
Figure 5: Crystal structures and mechanisms of action for reported RAS inhibitors.

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Authors and Affiliations

  1. Department of Medicine, Brigham and Women's Hospital, Boston, 02115, Massachusetts, USA

    Jonathan M. L. Ostrem

  2. Department of Cellular and Molecular Pharmacology, Howard Hughes Medical Institute, University of California, San Francisco, 94143, California, USA

    Kevan M. Shokat

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Correspondence to Kevan M. Shokat.

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Competing interests

J.M.L.O. and K.M.S. are joint inventors on a University of California Board of Regents-owned patent application covering inhibitors of KRAS, which has been licensed to Araxes Pharma LLC. They also hold stock in and are consultants to Araxes Pharma LLC.

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Glossary

Effectors

Proteins that bind to the GTP-bound state of RAS in order to transmit downstream signals for proliferation, survival and differentiation.

Intrinsic GTPase activity

GTPase activity (or nucleotide exchange) that is not catalysed by any other protein or chemical.

Transformation

The process by which cells acquire the features of cancer. The degree of transformation refers to the extent to which cells have achieved a cancer-like phenotype.

Tumorigenesis

The initial formation of a cancer, involving the transformation of normal cells into cancer cells.

B factors

Measures of scattering in X-ray crystallography indicating the mobility of the atom. Higher B factors indicate more mobility.

Electron density

A measure of the probability of electrons being present in a region. Because electrons are concentrated around atoms and bonds, in X-ray crystallography a model of the protein structure is built by fitting atoms within an electron density map calculated from the X-ray diffraction pattern.

Nuclear magnetic resonance spectroscopy.

(NMR spectroscopy). A technique based on the absorption and re-emission of electromagnetic radiation by certain isotopes in a magnetic field. This technique may be carried out in solution and may be used to study protein structure, dynamics and interactions.

Hydrogen–deuterium exchange

A technique in which solvent-accessible, labile hydrogen atoms (such as amide hydrogen atoms or side-chain hydrogen atoms) are exchanged for deuterium by incubating a protein in deuterated water (D2O). The exchange can then be evaluated using mass spectrometry or nuclear magnetic resonance spectroscopy. This technique can be used to determine whether regions of the protein are solvent accessible, and by proxy, whether they are mobile.

Dynamics

Proteins are no longer thought to exist as a static shape, but rather as sets of shapes that interconvert on a range of timescales. Dynamics refers to the study of these movements within a protein structure, including the transitions themselves as well as the range of conformations a protein adopts.

Electrophilic

An electron-poor chemical group that interacts with an electron-rich nucleophile to form a covalent bond. In this reaction, the nucleophile donates a pair of electrons to form a bond with the electrophile.

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Ostrem, J., Shokat, K. Direct small-molecule inhibitors of KRAS: from structural insights to mechanism-based design. Nat Rev Drug Discov 15, 771–785 (2016). https://doi.org/10.1038/nrd.2016.139

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