KRAS Mutations: Constitutive MAPK Signalling and Targeted Therapy

The KRAS GTPase Cycle and Oncogenic Mutations

KRAS functions as a molecular switch, cycling between GDP-bound (inactive) and GTP-bound (active) states. Intrinsic GTPase activity hydrolyses GTP→GDP within seconds, normally terminating the signal. GAP proteins (NF1, RASA1) accelerate this hydrolysis 100,000-fold. Oncogenic KRAS mutations at codons 12, 13, and 61 impair intrinsic and GAP-stimulated GTPase activity by inserting bulky side chains that sterically clash with the catalytic glutamine (Q61) or the GAP arginine finger — locking KRAS in the GTP-bound active state.

KRAS G12C substitutes the small glycine with cysteine, creating a reactive thiol group in the switch II pocket that is exploited by covalent inhibitors. G12D (aspartate, dominant in pancreatic cancer) and G12V (valine, common in lung and pancreatic) each impair GTPase by different steric mechanisms. G12R (arginine) is the most common KRAS mutation in pancreatic cancer after G12D and specifically blocks GAP arginine finger interaction.

Why KRAS Was Undruggable — and How G12C Inhibitors Broke the Barrier

Traditional KRAS inhibitor approaches failed because: (1) KRAS's picomolar affinity for GTP means competitive inhibitors cannot displace GTP at physiological concentrations; (2) KRAS lacks deep substrate-binding pockets exploitable by small molecules; (3) the protein is relatively small and structurally plastic, with its active/inactive conformations differing primarily in flexible switch I/II loops rather than rigid cavities.

The breakthrough came from exploiting an allosteric pocket in the GDP-bound (inactive) KRAS G12C that is not present in other KRAS mutants or wild-type KRAS. Covalent inhibitors (sotorasib/AMG-510, adagrasib/MRTX849) form an irreversible thioether bond with G12C's Cys12, trapping KRAS in the GDP-bound inactive conformation. Because the covalent bond is irreversible, the drug outcompetes GTP despite its higher concentration, bypassing the affinity problem entirely.

Clinical Results and Resistance Mechanisms

Sotorasib achieved 37% objective response rate and 6.8-month median PFS in previously treated KRAS G12C NSCLC (CodeBreaK 100 trial), leading to FDA approval in 2021. Adagrasib achieved 43% ORR and 6.5-month median PFS in the KRYSTAL-1 trial. Both demonstrated meaningful activity in a cancer type with no previously approved targeted therapy for KRAS-mutant disease.

Resistance to G12C inhibitors arises through multiple mechanisms: secondary KRAS mutations (Y96D, H95 mutations disrupting inhibitor binding), acquired KRAS amplification, KRAS G12C copy-number gains, and bypass activation through EGFR, MET, or RAS pathway reactivation. Vertical combination strategies (G12C inhibitor + EGFR inhibitor, G12C inhibitor + SHP2 inhibitor) aim to prevent bypass-track resistance.

KRAS G12D, G12V, and the Non-G12C Targeting Challenge

KRAS G12C accounts for only ~25% of KRAS-mutant NSCLC and ~1% of pancreatic cancer — the most common KRAS mutation in pancreatic cancer is G12D (~36%), and in colorectal cancer G12D and G12V together account for the majority. These mutations insert aspartate or valine at position 12, impairing GTPase activity through different steric mechanisms than G12C but lacking the reactive cysteine thiol that enables covalent inhibitor chemistry. Conventional competitive inhibition is defeated by KRAS's picomolar GTP affinity and abundant cellular GTP.

MRTX1133 is a non-covalent KRAS G12D inhibitor in Phase 1/2 trials, exploiting a G12D-specific binding pocket formed by the aspartate side chain. Pan-RAS(ON) inhibitors — such as RMC-6236 — target the GTP-bound active state of KRAS regardless of the specific codon 12 mutation, using a non-covalent interaction with Switch II pocket residues that are accessible in the GTP-bound but not GDP-bound state. This 'RAS(ON)' strategy is conceptually opposite to G12C covalent inhibitors (RAS(OFF)) and may be applicable across multiple KRAS mutations simultaneously. Early clinical data from RMC-6236 in pancreatic and lung cancers with G12V and G12D mutations are being evaluated.

Combination Strategies: SHP2, EGFR, and MEK Inhibitors

Single-agent G12C inhibitor responses are limited (~35–43% ORR) and resistance is nearly universal within 12 months, motivating rational combination strategies. SHP2 inhibitors (RMC-4630, TNO155, JAB-3068) block the phosphatase that activates RAS GEFs (SOS1/SOS2), reducing the flux of upstream RTK signalling that reloads G12C with GDP — the substrate for covalent inhibitor binding. By reducing GDP-KRAS G12C availability, SHP2 inhibitors theoretically reduce the competing GTP-loading and maintain more G12C in the inhibitor-trapped GDP state, preventing bypass through upstream RTK reactivation.

EGFR-mediated feedback reactivation is the dominant bypass track in G12C NSCLC: upon KRAS inhibition, tumour cells upregulate EGFR ligands and surface EGFR, reactivating upstream RAS signalling. Adagrasib plus cetuximab (anti-EGFR antibody) achieved 46% ORR in previously treated KRAS G12C NSCLC in KRYSTAL-1 — substantially higher than single-agent adagrasib alone — confirming that EGFR bypass is actionable. In colorectal cancer, where EGFR feedback is even more pronounced, KRAS G12C inhibition plus EGFR blockade is essentially required for meaningful response: sotorasib+panitumumab achieved 26% ORR vs ~10% single-agent sotorasib in KRAS G12C mCRC.

Key Takeaways

• ·KRAS oncogenic mutations (G12C, G12D, G12V, G12R, G13D, Q61H) impair intrinsic and GAP-stimulated GTPase activity by sterically hindering catalytic glutamine Q61 or the GAP arginine finger — locking KRAS in the GTP-bound active state.
• ·KRAS G12C inhibitors (sotorasib, adagrasib) exploit a unique Switch II allosteric pocket accessible only in GDP-bound G12C, forming an irreversible thioether bond that circumvents KRAS's picomolar GTP affinity — a conceptual breakthrough after 40 years of failed inhibitor attempts.
• ·KRAS mutations render tumours resistant to anti-EGFR antibodies (cetuximab, panitumumab) in colorectal cancer — mandating RAS/BRAF testing before anti-EGFR therapy — by bypassing EGFR-dependent signalling.
• ·KRAS G12D and G12V (dominant in pancreatic and colorectal cancer) cannot be targeted by G12C covalent chemistry; MRTX1133 (G12D) and pan-RAS(ON) inhibitors (RMC-6236) are in early clinical development for these previously undruggable mutations.
• ·EGFR-mediated feedback bypass is the dominant G12C inhibitor resistance mechanism; adagrasib+cetuximab (46% ORR in NSCLC) and sotorasib+panitumumab confirm that EGFR co-blockade is required for optimal G12C inhibitor activity.