Hopes for a new category of agents recently hailed as “a triumph of drug discovery” have been dimmed somewhat by new data showing many types of acquired resistance.
KRAS is one of the most frequently mutated oncogenes in human cancer and has long been thought “undruggable” – but novel drugs acting specifically on the KRAS G12C mutation have shown promise in clinical trials.
Early results with the experimental KRAS inhibitors sotorasib and adagrasib were deemed promising, but new data presented at the American Association for Cancer Research Annual Meeting 2021: Week 1 (Abstract LB002) have splashed cold water on that enthusiasm.
The efficacy of these new drugs looks to be threatened by the development of resistance caused by a wide range of genomic and histologic mechanisms.
Mark M. Awad, MD, PhD, from the Dana-Farber Cancer Institute in Boston, reported data from 30 patients with non–small cell lung cancer (NSCLC) or colorectal cancer (CRC) bearing the KRAS G12C mutation who had disease progression while being treated with adagrasib in clinical trials. Investigators found multiple on-target KRAS alterations and off-target bypass mechanisms of acquired resistance to adagrasib in these patients.
“Diverse mechanisms confer resistance to the KRAS G12C inhibitors, including secondary KRAS mutations, MAP [mitogen-activated protein] kinase pathway alterations, acquired genomic rearrangements, and histologic transformation,” Dr. Awad said in a mini-symposium presentation.
“Several cases displayed multiple resistance mechanisms, and novel combinatorial strategies will be necessary to delay or overcome resistance in KRAS G12C-mutant cancers,” he said.
Inactivating KRAS
The KRAS G12C mutation is a glycine-to-cysteine substitution that results in the oncogene being switched on in its active form.
But the mutation has been considered too tough to target because of the KRAS gene’s strong binding affinity for guanosine triphosphate, an essential building block of RNA synthesis, and by a lack of accessible drug-binding sites.
Sotorasib and adagrasib are small-molecule, specific, and irreversible inhibitors of KRAS that interact with a “pocket” on the gene’s surface that is present only in an inactive conformation of KRAS. The drugs inhibit oncogenic signaling and tumorigenesis by preventing cycling of the oncogene into its active form.
Multiple mutations, histologic transformations
Dr. Awad and colleagues studied biopsy samples and circulating tumor DNA (ctDNA) from 30 patients both at baseline and after administration of adagrasib monotherapy. The patients all had initial responses to the drug but then experienced disease progression.
The investigators used mutagenesis screens to identify mechanisms of resistance to KRAS G12C inhibitors.
The 30 patients included 23 with NSCLC and 7 with CRC. Eighteen had unknown mechanisms of resistance, and putative resistance mechanisms were identified in the other 12 patients. Of this latter group, seven appeared to have single resistance mechanisms, and five had multiple mechanisms of resistance.
One patient with NSCLC who had radiographic evidence of response followed by progression was found to have had a novel KRAS Y96C mutation, and three had novel KRAS mutations in other gene regions, with multiple concurrent alterations in genes implicated in other forms of cancer, such as PTEN, BRAF, and MAP2K1.
The investigators also identified amplifications of the KRAS G12C allele, and MET.
In two patients, NSCLC underwent histologic transformation from adenocarcinoma at baseline to squamous cell carcinoma at the time of acquired resistance to the drug. No genomic resistance mechanisms were detected in either of these patients, Dr. Awad said.
“In several cases, we see multiple mechanisms or co-occurring alterations in each individual patient, with the suggestion that perhaps the multiple mutations or resistance mechanism may be more common in the colorectal population, particularly with acquired gene fusions, than in the lung cancer population, although larger datasets will be needed to confirm this observation,” he said.