Eliminating oncogenic RAS: back to the future at the drawing board

Abstract

RAS drug development has made enormous strides in the past ten years, with the first direct KRAS inhibitor being approved in 2021. However, despite the clinical success of covalent KRAS-G12C inhibitors, we are immediately confronted with resistances as commonly found with targeted drugs. Previously believed to be undruggable due to its lack of obvious druggable pockets, a couple of new approaches to hit this much feared oncogene have now been carved out. We here concisely review these approaches to directly target four druggable sites of RAS from various angles. Our analysis focuses on the lessons learnt during the development of allele-specific covalent and non-covalent RAS inhibitors, the potential of macromolecular binders to facilitate the discovery and validation of targetable sites on RAS and finally an outlook on a future that may engage more small molecule binders to become drugs. We foresee that the latter could happen mainly in two ways: First, non-covalent small molecule inhibitors may be derived from the development of covalent binders. Second, reversible small molecule binders could be utilized for novel targeting modalities, such as degraders of RAS. Provided that degraders eliminate RAS by recruiting differentially expressed E3-ligases, this approach could enable unprecedented tissue- or developmental stage-specific destruction of RAS with potential advantages for on-target toxicity. We conclude that novel creative ideas continue to be important to exterminate RAS in cancer and other RAS pathway-driven diseases, such as RASopathies.

Keywords: RAS; cancer; drug development.

Figure 1.. Overview of small molecule inhibitors targeting RAS.

(A) Selected SII-P small molecule inhibitors based on the 4-piperazin-1-yl-pyrimidine scaffold (green highlights). The common acrylamide warhead of KRAS-G12C inhibitors (top row) is highlighted in blue. Adagrasib served as a starting point for additional inhibitors (arrows), including covalent G12R- and G12S-inhibitors, with an α,β-diketoamide warhead or a strained β-lactone electrophile, respectively (purple). Note that the exact stereochemistry of displayed inhibitors has been largely omitted. (B) Crystal structure of GDP-KRAS-G12C in complex with ARS-1620 (PDB ID 5V9U). The RAS structure can be divided into the N-terminal effector lobe (grey), with the switch I and switch II regions labelled in green, and the allosteric lobe (pink). The allosteric binding sites P1–4 are indicated with circles. (C) Current experimental small molecule inhibitors (here those with an affinity <500 µM) target predominantly P1. The RAS affinity and selectivity is indicated for each compound (cpd). References are in brackets after the names [48,53,63–68]. The full list of small molecule inhibitors is contained in Supplementary File S1.

Figure 2.. Overview of macromolecular RAS binders.

Crystal structure of GDP-KRAS (PDB ID 4OBE). Effector and allosteric lobes, as well as allosteric binding sites are indicated as in Figure 1. The names of macromolecular RAS binders are highlighted in the same colour as their binding sites, with more detailed binding site information given in brackets.

Figure 3.. Timeline of notable RAS drug development events since 2007.

Arrowheads mark publications of binders and sites with colours corresponding to those used for binding sites in Figures 1 and 2.