How Sotorasib Works: Locking KRAS G12C in Its Off State
KRAS was considered undruggable for decades. Sotorasib is one of the first drugs to hit it, and it does so by exploiting the specific chemistry of one mutation: the glycine-to-cysteine change at codon 12 (G12C). It covalently attaches to that new cysteine and holds KRAS in its inactive form.
Quick Answer
KRAS was considered undruggable for decades. Sotorasib is one of the first drugs to hit it, and it does so by exploiting the specific chemistry of one mutation: the glycine-to-cysteine change at codon 12 (G12C). It covalently attaches to that new cysteine and holds KRAS in its inactive form.
The Switch That Sotorasib Jams
KRAS is a molecular switch: it is active when bound to GTP and inactive when bound to GDP. It cycles rapidly between the two. Sotorasib binds a pocket that is only present in the GDP-bound (inactive) state, and it reacts covalently with the mutant cysteine 12 that sits beside that pocket.
Once bound, the drug prevents KRAS from exchanging GDP for GTP, so the switch cannot flip back on. Downstream RAF-MEK-ERK signalling falls.
Why It Is Specific to G12C
The covalent bond needs a cysteine at position 12. Normal KRAS has a glycine there, and other common mutations (G12D, G12V, G12R) substitute different residues, none of which offers the reactive thiol group sotorasib requires.
So sotorasib is inactive against KRAS G12D or G12V tumours, and testing must identify the exact codon-12 change, not just report KRAS as mutated.
Activity and Limits
In previously treated KRAS G12C non-small-cell lung cancer, a single-arm trial (CodeBreaK 100) reported responses in roughly a third of patients with a median duration of around a year. Activity in KRAS G12C colorectal cancer is lower as a single agent, thought to reflect rapid reactivation of upstream signalling through EGFR.
These figures are context-specific. Response rates, durability and the value of combinations differ by tumour type and line of therapy.
Resistance Mechanisms
Resistance is frequently polyclonal. Reported routes include secondary KRAS mutations (including at other codons), new alterations in NRAS, BRAF, MAP2K1 or RTK genes, KRAS G12C amplification, and epithelial-mesenchymal or histologic changes. Some tumours simply reactivate the MAPK pathway upstream.
This diversity is why combination strategies — for example with an EGFR antibody in colorectal cancer, or with SHP2 or MEK inhibitors — are an active research focus.
Dosing, Toxicity and Monitoring
Sotorasib is an oral tablet taken once daily. Acid-reducing drugs lower its absorption, so proton-pump inhibitors and H2 blockers are generally avoided and antacids are separated in time; strong CYP3A4 inducers also reduce exposure.
The most consistent toxicity is hepatocellular — rises in ALT, AST and bilirubin — which can be more frequent and more severe when immune-checkpoint inhibitor therapy was given shortly before. Liver enzymes are checked before starting and regularly during treatment, with dose interruption or reduction for significant elevations. Diarrhoea, nausea, fatigue and, less often, drug-induced pneumonitis also occur.
What the KRAS Report Must Specify
A KRAS result must state the exact codon-12 substitution. 'KRAS mutation detected', or even 'KRAS codon 12 mutation', is insufficient, because only G12C creates the reactive cysteine sotorasib needs; G12D, G12V and G12R tumours will not respond.
Broad testing remains worthwhile, since co-occurring alterations in KEAP1, STK11, TP53 or the PI3K pathway influence prognosis and the rationale for combinations. At progression, repeat sequencing looks for the polyclonal resistance changes — secondary RAS-pathway mutations, amplifications, new receptor alterations — that determine whether another targeted approach is realistic.
Key Takeaways
- ·Sotorasib covalently binds mutant cysteine 12 and traps KRAS in its inactive GDP-bound state.
- ·It works only for KRAS G12C, not G12D, G12V or other codon-12 changes.
- ·Resistance is usually polyclonal, often through renewed MAPK-pathway signalling.
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Frequently asked questions
Does sotorasib work for any KRAS mutation?
No. It needs the cysteine created by the G12C substitution to form its covalent bond, so it is inactive against G12D, G12V, G12R and other codon-12 changes. Testing must report the exact codon-12 alteration.
Why does sotorasib work less well in colorectal cancer than lung cancer?
Colorectal tumours rapidly reactivate KRAS signalling through EGFR after the drug binds, so single-agent activity is limited; combining sotorasib with an anti-EGFR antibody addresses that feedback.
Is KRAS G12C resistance usually caused by one mutation?
No. Acquired resistance is frequently polyclonal, mixing secondary RAS-pathway mutations, gene amplification, new receptor alterations and histologic change within the same tumour.
References
Continue Reading
Sotorasib vs Adagrasib: Two KRAS G12C Inhibitors Compared
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KRAS G12C in Colorectal Cancer: Biomarker Context
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KRAS Mutations: Constitutive MAPK Signalling and Targeted Therapy
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MAPK Pathway Resistance: A Framework for Reading Resistance Reports
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Gene Mutations That Cause Disease
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Liquid Biopsy vs Tissue Testing: What Each Sample Can Tell You
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