Data from genomic and histologic analyses in the KRYSTAL-1 study show that a diversity of on-target and off-target mechanisms can confer resistance to KRASG12C inhibition and support the need for development of additional KRAS inhibitors with alternative modes of binding and different allele specificities. According to Dr. Andrew Aguirre, Dr. Mark Awad of the Department of Medical Oncology, Dana–Farber Cancer Institute in Boston, MA, US and colleagues development of effective combination therapy regimens will be required to fully combat resistance mechanisms that emerge during treatment with adagrasib or sotorasib. They published their findings on 24 June 2021 in The New England Journal of Medicine.

KRAS glycine-to-cysteine amino acid substitutions at codon 12 (KRASG12C) mutations occur in approximately 13% of lung adenocarcinomas and approximately 3% of colorectal adenocarcinomas and less commonly in cancers of the uterus, pancreas, breast, bladder, cervix, and ovaries.

Little is known about the clinical mechanisms of resistance to KRASG12C inhibition. Despite the clinical benefit observed in patients treated with KRASG12C inhibitors, acquired resistance to single agent therapy occur in most patients. Among patients with KRASG12C-mutant cancers treated with adagrasib in the KRYSTAL-1 study, the investigators performed histologic and genomic analyses that compared pretreatment samples with those obtained after the development of resistance.

They used a deep mutational scanning screen with a library of KRASG12C missense variants to systematically define possible second-site mutations that confer resistance to KRASG12C inhibition.

In total, 38 patients were included in the analysis, 27 with non–small cell lung cancer, 10 with colorectal cancer and 1 with appendiceal cancer. Putative mechanisms of resistance to adagrasib were detected in 17 patients (45% of the cohort), of whom 7 (18% of the cohort) had multiple coincident mechanisms.

Acquired KRAS alterations included G12D/R/V/W, G13D, Q61H, R68S, H95D/Q/R, Y96C, and high-level amplification of the KRASG12C allele.

Acquired bypass mechanisms of resistance included MET amplification; activating mutations in NRAS, BRAF, MAP2K1, and RET; oncogenic fusions involving ALK, RET, BRAF, RAF1, and FGFR3; and loss-of-function mutations in NF1 and PTEN.

In 2 of 9 patients with lung adenocarcinoma for whom paired tissue-biopsy samples were available, histologic transformation to squamous cell carcinoma was observed without identification of any other resistance mechanisms.

By using an in vitro deep mutational scanning screen, the study team systematically defined the landscape of KRAS mutations that confer resistance to KRASG12C inhibitors.

In this study involving patients who initially had a response to adagrasib or who had a long period of stable disease in response to the drug but then had progression, diverse mechanisms of acquired resistance were found in 45%. Unlike resistance to the tyrosine kinase inhibitors, the cancer cells use many mechanisms to overcome KRASG12C inhibition and new therapeutic strategies are required to delay and overcome this drug resistance.

The study was funded by Mirati Therapeutics and grants from multiple non-profit organisations.


Awad MM, Liu S, Rybkin II, et al. Acquired Resistance to KRASG12C Inhibition in Cancer. N Engl J Med 2021;384:2382-2393. DOI: 10.1056/NEJMoa2105281