AMG 510, now marketed as sotorasib (brand name Lumakras), works by permanently locking a mutant KRAS protein into its inactive shape so it can no longer tell cancer cells to grow and divide. It was the first drug ever approved to directly target a KRAS mutation, ending decades of frustration during which KRAS was widely considered “undruggable.” The story of how researchers cracked this problem, and the limitations they are still working around, reveals both the promise and the complexity of precision oncology.
Why KRAS Was Considered Undruggable for Decades
KRAS is one of the most commonly mutated genes in human cancers. When it works normally, the KRAS protein acts as a molecular switch: it cycles between an active “on” state (bound to a molecule called GTP) and an inactive “off” state (bound to GDP). Mutations at specific spots on the protein can jam the switch in the “on” position, sending a constant growth signal to the cell. The G12C mutation, in which a glycine at position 12 is replaced by a cysteine, is especially common in non-small cell lung cancer and also appears in colorectal and pancreatic cancers.1PubMed Central. Prognostic and therapeutic impact of the KRAS G12C mutation in colorectal cancer
For nearly four decades, drug designers struggled with KRAS because the protein’s surface is smooth and relatively featureless. It lacks the deep binding pockets that most small-molecule drugs need to grab onto. Hundreds of attempts failed. The breakthrough came in 2013, when researchers in Kevan Shokat’s lab at UCSF discovered a previously unknown pocket on the protein, called the switch-II pocket, that opens only in the GDP-bound (inactive) state. Because only the G12C mutant has a cysteine at position 12, a drug could be designed to form a permanent chemical bond specifically with that cysteine, exploiting a feature normal KRAS cells do not have.2PubMed Central. Allele-specific inhibitors inactivate mutant KRAS G12C by a trapping mechanism
How Sotorasib Traps the Protein in Its Off State
Sotorasib is a covalent inhibitor, meaning it forms an irreversible chemical bond with its target rather than just sitting in a pocket temporarily. Specifically, it contains a reactive chemical group (an acrylamide warhead) that reacts with the cysteine at position 12 on the mutant KRAS protein.3Trends in Pharmacological Sciences. Sotorasib: a KRASG12C inhibitor for non-small cell lung cancer Once bound, the drug wedges itself into the switch-II pocket and holds KRAS in the GDP-bound, inactive conformation. The protein can no longer exchange GDP for GTP and therefore cannot switch on.
A critical detail of the mechanism is that sotorasib can only bind KRAS when the protein is already in the “off” state. Both sotorasib and the related drug adagrasib exploit the strong reactivity of the acquired cysteine, but they require the protein to be holding GDP rather than GTP.4PubMed. Beyond KRAS(G12C): Biochemical and Computational Characterization of Sotorasib and Adagrasib Binding Specificity and the Critical Role of H95 and Y96 The drug-bound protein also becomes resistant to the cellular machinery (nucleotide exchange factors) that would normally reload it with GTP. So once sotorasib traps the protein in its inactive form, the protein stays trapped. Early experiments showed that if the mutant KRAS protein completely lost its ability to return to the GDP-bound state, or if the exchange factors were hyperactive, the drug became less effective, confirming that the trapping mechanism depends on the natural cycling of the switch.2PubMed Central. Allele-specific inhibitors inactivate mutant KRAS G12C by a trapping mechanism
This selectivity is the drug’s biggest advantage and its most significant limitation. Because it relies on a cysteine that only exists in the G12C mutant, sotorasib has no effect on other KRAS mutations like G12D or G12V, which are more common in pancreatic and colorectal cancers. That has spurred a push to develop inhibitors targeting those variants as well.
Clinical Results in Lung Cancer
Sotorasib’s clinical development centered on the CodeBreaK program. In the pivotal CodeBreaK 100 trial, patients with advanced non-small cell lung cancer (NSCLC) who had already received at least one prior therapy were treated with sotorasib at 960 mg daily. About 37% of patients had their tumors shrink meaningfully (an objective response), and roughly 81% had at least some degree of disease control. The median time before the cancer started growing again was about 6.8 months, and median overall survival was 12.5 months.5PubMed Central. Sotorasib for Lung Cancers with KRAS p.G12C Mutation
Longer follow-up confirmed these results were durable in a subset of patients. A two-year analysis of CodeBreaK 100 found an overall response rate of 41%, with the median duration of those responses stretching past a year. About one in four patients maintained disease control for twelve months or more, and a third were still alive at the two-year mark. That long-term benefit was seen across different levels of PD-L1 expression and even in some patients whose tumors carried additional mutations in STK11 or KEAP1, which are generally associated with a worse prognosis.6PubMed Central. Long-Term Outcomes and Molecular Correlates of Sotorasib Efficacy in Patients With Pretreated KRAS G12C-Mutated Non-Small-Cell Lung Cancer: 2-Year Analysis of CodeBreaK 100
Based on these data, the FDA granted sotorasib accelerated approval in May 2021 for adults with KRAS G12C-mutated locally advanced or metastatic NSCLC who had received at least one prior systemic therapy. It was the first targeted therapy ever approved for a KRAS-mutated cancer.7PubMed Central. Sotorasib: First Approval
Why Colorectal Cancer Responds Differently
One of the more puzzling findings in sotorasib’s development was the sharp drop in activity when it was tried in colorectal cancer. In a phase II trial, the objective response rate was under 10%, though disease control was achieved in roughly 82% of patients and median progression-free survival was four months.8AACR Journals. Sotorasib’s Benefits in Colorectal Cancer Modest The same mutation, the same drug, yet far less tumor shrinkage.
The leading explanation involves feedback signaling through the EGFR pathway. In colorectal tumors, when KRAS is blocked, the cell quickly ramps up EGFR signaling as an escape route, reactivating the very growth pathways that sotorasib is trying to shut down. Lung cancer cells do not rely on this feedback loop to the same degree, which is why they are more vulnerable to KRAS inhibition alone. This understanding set the stage for combination approaches that attack both targets simultaneously.
Combining Sotorasib With EGFR Blockade in Colorectal Cancer
The phase III CodeBreaK 300 trial tested the hypothesis that pairing sotorasib with panitumumab, an antibody that blocks EGFR, could overcome the resistance seen in colorectal cancer. Patients with chemotherapy-refractory, KRAS G12C-mutated metastatic colorectal cancer were randomized to the combination or to the physician’s choice of standard salvage therapy. The combination at the higher sotorasib dose (960 mg) roughly doubled progression-free survival compared to standard care, with a median of 5.6 months versus 2.2 months. The objective response rate with the combination was about 26%, while none of the patients on standard treatment had a measurable tumor response.9PubMed. Sotorasib plus Panitumumab in Refractory Colorectal Cancer with Mutated KRAS G12C
Updated data after longer follow-up showed the response rate in the 960 mg combination arm climbing slightly to about 30%. The overall survival trend favored the combination, though the difference did not reach statistical significance given the sample size and follow-up time. Nevertheless, regulators and oncology guidelines have moved toward recognizing this combination as a new standard of care for this specific patient population.10PubMed Central. Overall Survival Analysis of the Phase III CodeBreaK 300 Study of Sotorasib Plus Panitumumab Versus Investigator’s Choice in Chemorefractory KRAS G12C Colorectal Cancer Patient-reported outcomes also supported the combination, with treated patients maintaining or even improving their quality of life relative to standard chemotherapy options.11The Lancet. Patient-reported outcomes from the phase 3 CodeBreaK 300 study of sotorasib plus panitumumab versus investigator’s choice in KRAS G12C-mutated metastatic colorectal cancer
How Tumors Develop Resistance
Most patients who initially respond to sotorasib eventually see their cancer grow back. Understanding the escape routes tumors use is one of the most active areas of research. Resistance mechanisms fall into a few broad categories.
The most direct escape involves new mutations in KRAS itself. Researchers analyzing tumors that progressed on KRAS G12C inhibitors found a constellation of secondary KRAS mutations, including changes at the drug-binding pocket (such as H95D, H95Q, H95R, Y96C, and R68S) that physically block the drug from latching on. Other secondary mutations shifted the protein to entirely different oncogenic variants like G12D, G12V, G13D, or Q61H, which sotorasib cannot target. Some tumors also amplified the KRAS G12C gene itself, essentially flooding the cell with more mutant protein than the drug could neutralize.12PubMed Central. Acquired Resistance to KRAS(G12C) Inhibition in Cancer The Y96D mutation in particular disrupts critical interactions between the drug and the switch-II pocket and has been shown to confer resistance in both laboratory models and patient tumors.13PubMed Central. Clinical Acquired Resistance to KRASG12C Inhibition through a Novel KRAS Switch-II Pocket Mutation and Polyclonal Alterations Converging on RAS-MAPK Reactivation
Tumors can also sidestep KRAS entirely. Activation of other members of the RAS family, such as NRAS or HRAS, can re-engage the same downstream growth signals. Loss of the tumor suppressor NF1, which normally helps keep RAS signaling in check, has the same effect. And amplification of receptor tyrosine kinases like MET or EGFR can reactivate growth pathways without going through KRAS at all.14PubMed Central. KRAS-targeted therapies in cancer: novel approaches and overcoming resistance In one clinical report, patients who developed MET amplification after sotorasib treatment were able to respond again when a MET inhibitor (tepotinib) was added to sotorasib, suggesting that these bypass mechanisms can sometimes be addressed by rational drug combinations.15PubMed Central. MET-Driven Resistance to Sotorasib in KRAS G12C–Mutant NSCLC and Response to Combined KRAS and MET Inhibition
The diversity of resistance mechanisms is the central challenge. Because different tumor cells within the same patient can evolve different escape routes simultaneously, a single second-line approach is unlikely to work for everyone. This is why researchers are studying not just new drug combinations but also entirely new classes of KRAS inhibitors that bind the protein in its active (GTP-bound) state, which would avoid some of the vulnerabilities inherent to sotorasib’s mechanism.
Safety Profile and the Immunotherapy Timing Problem
Sotorasib’s side effects are generally manageable compared to traditional chemotherapy, but they warrant close monitoring. In a pooled safety analysis of more than 500 patients with advanced NSCLC, the most common treatment-related side effects were diarrhea (affecting about 31%), nausea (about 15%), and liver enzyme elevations (roughly 12% each for ALT and AST). Dose interruptions and dose reductions resolved over 90% of diarrhea and liver enzyme events.16The Oncologist. Pooled safety analysis and management of sotorasib-related adverse events in KRAS G12C-mutated advanced non-small cell lung cancer
The liver issue deserves special attention, because it intersects with another therapy most of these patients have already received: immune checkpoint inhibitors. Preclinical data suggest sotorasib creates a proinflammatory environment in the tumor, which is helpful for tumor killing but may also reignite immune-mediated toxicity in patients recently treated with immunotherapy.17Journal of Thoracic Oncology and Clinical Research. Severe Immune Checkpoint Inhibitor Hepatitis in KRAS G12C-Mutant NSCLC Potentially Triggered by Sotorasib: Case Report The pooled analysis found that patients who stopped immunotherapy less than three months before starting sotorasib had roughly a 33% rate of treatment-related liver toxicity, compared to about 14% in those who had a longer washout period.16The Oncologist. Pooled safety analysis and management of sotorasib-related adverse events in KRAS G12C-mutated advanced non-small cell lung cancer
Real-world experience from clinical centers has confirmed that hepatotoxicity is the leading reason patients discontinue sotorasib. In one French two-center study, about three-quarters of patients experiencing significant side effects had liver-related toxicity as the primary issue, and the majority of those patients ultimately had to stop the drug permanently.18PubMed. Management of sotorasib-related adverse events and hepatotoxicities following anti-PD-(L)1 therapy: Experience with sotorasib in two French anti-cancer centers and practical guidance proposal This has led to practical guidance recommending careful monitoring of liver enzymes, a graduated dose-reduction strategy, and ideally a gap of at least three months between immunotherapy and starting sotorasib when clinically feasible.
How the Mutation Is Detected
You cannot receive sotorasib without first confirming that your tumor carries the KRAS G12C mutation. The FDA approval specifies that the mutation must be identified by an FDA-approved test.19Clinical Cancer Research. FDA Approval Summary: Sotorasib for KRAS G12C-Mutated Metastatic NSCLC In practice, the mutation is identified through next-generation sequencing panels run on tumor tissue obtained from a biopsy, or increasingly, from a blood draw.
Blood-based testing, often called liquid biopsy, detects fragments of tumor DNA circulating in the bloodstream. This approach is appealing because it avoids an invasive tissue biopsy, and it can be repeated over time to track how the tumor is responding or evolving. In clinical studies, circulating tumor DNA (ctDNA) analysis has proven useful both for confirming the G12C mutation at baseline and for monitoring treatment response. Researchers found that a rising level of the KRAS G12C mutation in the blood anticipated radiologic disease progression in about 70% of evaluable patients, suggesting ctDNA tracking could eventually give an early warning that resistance is developing.20PubMed Central. Circulating tumor DNA dynamic variation predicts sotorasib efficacy in KRASp.G12C-mutated advanced non-small cell lung cancer
Several sequencing platforms are used in clinical trials and commercial testing, including broad panels that cover hundreds of genes as well as more targeted digital PCR assays that look specifically for the G12C variant. The choice often depends on what is available at a given institution and whether a broader genomic picture is needed for treatment planning. When tissue is available, tissue-based sequencing remains the gold standard, but blood-based confirmation is widely accepted for initiating treatment, especially in patients for whom a biopsy would be risky or impractical.21Journal of Thoracic Oncology. Clinical Utility of Circulating Tumor DNA in Patients With Advanced KRASG12C-Mutated NSCLC Treated With Sotorasib
The Expanding Landscape Beyond Sotorasib
Sotorasib was first to market, but it is not alone. Adagrasib (MRTX849, brand name Krazati) received FDA approval for the same indication and works through a similar mechanism, also binding covalently to the switch-II pocket of GDP-bound KRAS G12C. In its pivotal trial, adagrasib showed a comparable response rate and progression-free survival in NSCLC, and it demonstrated activity in brain metastases, which is a meaningful distinction since lung cancers frequently spread to the central nervous system. Both drugs share the fundamental limitation of only working against the G12C variant and only binding the inactive form of the protein.
Research is now moving in several directions. More potent G12C inhibitors are in development, designed to overcome some of the resistance mutations that defeat sotorasib and adagrasib. Inhibitors that target G12D and G12V variants, which are far more common in pancreatic and colorectal cancers, are also advancing through early trials. And a newer class of “active-state” or “ON-state” KRAS inhibitors, which bind the GTP-loaded protein rather than waiting for the GDP-bound form, could potentially address the trapping limitation that defines sotorasib’s mechanism. Pan-RAS inhibitors that block multiple RAS family members at once are yet another emerging strategy aimed at cutting off the bypass routes tumors use to escape single-target drugs.
Because G12C accounts for only a fraction of all KRAS mutations in cancer, broadening the target space is arguably the most consequential next step. While G12C is the predominant KRAS subtype in lung cancer, the G12D and G12V variants are far more common in colorectal and pancreatic cancers, two of the deadliest solid tumors.14PubMed Central. KRAS-targeted therapies in cancer: novel approaches and overcoming resistance If the same precision targeting strategy can be extended to those mutations, the clinical impact would be substantially larger than what sotorasib alone has achieved. For now, sotorasib’s approval remains a proof of concept that one of the most notoriously difficult targets in oncology is, in fact, druggable, and that concept is reshaping how researchers think about the rest of the RAS family.