RAS-targeted therapies: is the undruggable drugged?

Abstract

RAS (KRAS, NRAS and HRAS) is the most frequently mutated gene family in cancers, and, consequently, investigators have sought an effective RAS inhibitor for more than three decades. Even 10 years ago, RAS inhibitors were so elusive that RAS was termed 'undruggable'. Now, with the success of allele-specific covalent inhibitors against the most frequently mutated version of RAS in non-small-cell lung cancer, KRAS^G12C, we have the opportunity to evaluate the best therapeutic strategies to treat RAS-driven cancers. Mutation-specific biochemical properties, as well as the tissue of origin, are likely to affect the effectiveness of such treatments. Currently, direct inhibition of mutant RAS through allele-specific inhibitors provides the best therapeutic approach. Therapies that target RAS-activating pathways or RAS effector pathways could be combined with these direct RAS inhibitors, immune checkpoint inhibitors or T cell-targeting approaches to treat RAS-mutant tumours. Here we review recent advances in therapies that target mutant RAS proteins and discuss the future challenges of these therapies, including combination strategies.

Fig. 1 |. Clinical development of inhibitors for RAS-mutant tumours.

Activation of receptor tyrosine kinases, such as members of the epidermal growth factor receptor (EGFR) family, promotes the exchange of GDP for GTP in RAS, thereby activating RAS. Inhibition of EGFR can reduce this activation. Inhibition of SOS or SHP2 decreases the rate of GDP–GTP exchange and reduces the GTP-bound RAS population. Mutant RAS proteins accumulate in the GTP-bound state. A number of approaches have been developed to directly inhibit RAS, including covalent allele-specific inhibitors that bind to KRAS-G12C. GTP-bound RAS activates downstream signalling by binding to the RAS-binding domain of effector proteins, such as RAF and p110, to activate the MAPK and PI3K signalling cascades, respectively. Both the MAPK and PI3K signalling cascades can be inhibited at each kinase tier. Data compiled from ClinicalTrials.gov and AccessData.FDA.gov. ^aOnly effective against monomeric BRAF (BRAF-V600E/K). ^bApproved for the treatment of BRAF-mutant melanoma. ^cApproved for the treatment of paediatric patients with NF1 mutations.

Fig. 2 |. Frequency and distribution of RAS mutations in human cancers.

Human cancers differ in which has the most frequently mutated RAS isoform, codon and amino acid substitution. a | Distribution of RAS isoform (KRAS, NRAS and HRAS) mutations across tumour types and the frequency of the RAS mutation by isoform in each tumour type. b | Percentages of KRAS mutations that are in codon 12 by tissue type for pancreatic, colorectal and lung adenocarcinoma, and the percentage of NRAS mutations that are in codon 61 for melanoma. The distributions of amino acid substitutions at the mutated codon (12 or 61) for each tissue type are shown in pie charts beside the relevant organ. Data acquired from The Cancer Genome Atlas (pan-Cancer) from cBioPortal and from Project GENIE (GENIE v7.0 public).

Fig. 3 |. Biochemical features of mutant RAS proteins.

Mutations in codons 12, 13 and 61 disrupt the GTP hydrolysis and guanine exchange rates of RAS proteins. a | Ribbon diagram of HRAS. Switch-I is shown in light blue, switch-II is in green, GppNHp (a GTP analogue) is shown in yellow, and oncogenic hotspot residues G12, G13 and Q61 are shown as red spheres. b | Summary of generalized biochemical disruption of hydrolysis and guanine exchange upon mutations in codon 12 or 61. Generally, mutations in codon 12 disrupt the GTPase activity of RAS and thereby decrease the rate of GTP hydrolysis, so the mutant protein accumulates in the GTP-bound state. Mutations in codon 61 accelerate the rate of GDP–GTP exchange and simultaneously decrease the rate of GTP hydrolysis, so codon 61 RAS mutants also accumulate in the GTP-bound state. c | Biochemical properties of specific amino acid substitutions at KRAS codon 12, 13 or 61, rank ordered by intrinsic hydrolysis. Data acquired from results in Hunter et al.. WT, wild type.

Fig. 4 |. Chemical structures of compounds that bind to KRAS-G12C.

Shokat and colleagues developed the first series of small molecules to bind KRAS-G12C, the most potent of which is compound 12 (REF.). Modification of the linker and hydrophobic binding pocket led to the development of a more potent and cellular active compound, ARS-853 (REF.). Further improvements, such as the introduction of a quinazoline-based series and a fluorophenol hydrophobic binding moiety, enhanced the potency and pharmacological properties and led to the development of ARS-1620 (REF.). Using an alternative orientation of His95 in the switch-II pocket allowed the addition of aromatic rings to enhance the protein–protein interactions with KRAS-G12C, leading to the development of AMG 510 (REF.). Structure-based drug design approaches and optimization led to the development of MRTX849 (REF.).

Fig. 5 |. Structures of RAS surfaces targeted by therapeutics.

Proteins are depicted by surface representation, and compounds and nucleotides are shown as stick models. Each panel is coloured to highlight important surfaces, with switch-I in blue, switch-II in pink and relevant interfaces coloured uniquely. a | HRAS binds to GppNHp, a non-hydrolysable nucleotide, which can be used as a reference for orientation (Protein Data Bank (PDB): 5P21). b | Switch-II pocket (purple) of KRAS-G12C bound to AMG 510 (PDB: 6OIM; compound identifier: MOV). c | Switch-II groove (purple) bound to 2C07 (PDB: 5VBZ; compound identifier: 92V). d | DCAI pocket (teal) of KRAS-G12D bound to the DCAI compound (PDB: 4DST; compound identifier: 9LI). e | Son of Sevenless (SOS) binding interface (red) of HRAS (PDB: 1BKD). f | Proposed HRAS dimerization interface (green) bound by NS1 monobody (gold) (PDB: 5E95). g | RAF–RAS-binding domain (RBD) binding interface (yellow) of HRAS (PDB: 4G0N).