Background: The detection of acquired KRAS and EGFR extracellular domain mutations have been reported and correlated with acquired resistance to anti-EGFR monoclonal antibodies (MAb). The two original reports of KRAS mutations after EGFR therapy provided different interpretations on whether these mutations were pre-existing in a minor clone and expanded under selective pressure, or were not present in the original tumor and developed through acquisition of new mutations. To address such question, using high-sensitivity assay we re-analyzed twelve archival tissues from KRAS wild type patients that showed the presence of acquired KRAS mutation in plasma circulating free DNA(cfDNA) after progressing on EGFR therapy.

Methods: Plasma and tissue samples were collected from metastatic CRC pts as part of the ATTACC program. Eligible pts had a source documentation of pre-treatment KRAS wild type tumor. KRAS and EGFR ectodomain mutations were analyzed by BEAMing technology (sensitivity of 0.02% mutant to wild type alleles) in DNA extracted from patient tumor tissue collected at the time of diagnosis and from plasma cfDNA collected after progression to anti-EGFR mAb treatment. Acquired mutations were defined as mutations not identified by standard-of-care sequencing of the primary tumor.

Results: Plasma from 77 patients was analyzed for EGFR and KRAS mutation. The S492R EGFR mutation was detected in 4 patients, while 24 patients previously classified as KRAS wild type by standard of care sequencing of the primary tumor had an acquired KRAS mutation in the cfDNA after BEAMing analysis. For 12 of these patients we re-analyzed the untreated primary tumor using BEAMing technology to screen for low-frequency (LF) KRAS mutant clones. LF KRAS mutations were detected in 5 samples (42%), while the other 7 (58%) identified only KRAS wild type alleles. In addition, the analysis identified atypical KRAS mutation, such as codon 61 and 146 (present respectively in 33 and 10% of samples with acquired mutations), and more than one concurrent KRAS codon mutations (29% of samples with acquired mutations) were common features of acquired KRAS mutations compared with primary mutations.

Conclusions: Acquired KRAS mutations are common in clinically KRAS wild type CRC after anti-EGFR therapy, and appear to derive from pre-existing clones in a significant proportion of cases. This data confirm the limit of current technique used for detection of clinically relevant mutations and suggests that the use of more sensitive sequencing technologies may be able to identify clinically relevant low-frequency KRAS mutations.

Citation Information: Mol Cancer Ther 2013;12(11 Suppl):B35.

Citation Format: Maria Pia Morelli, Michael Overman, Arvind Dasari, Ali Kazmi, Edoardo Vilar-Sanchez, Cathy Eng, Bryan Kee, Laurel Deaton, Chris Garrett, Frank Diehl, Dipen Maru, Philipp Angenendt, Scott Kopetz. Acquired KRAS mutations after anti-EGFR therapy partially derive from low-frequency clones detectable in the primary tumor. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr B35.