Abstract B57: Comprehensive molecular characterization of acquired resistance to anti-EGFR monoclonal antibodies (MoAbs) in patients with metastatic colorectal cancer (mCRC)
Authors: Pietrantonio Filippo , Berenato Rosa , Perrone Federica , Gloghini Annunziata , Tamborini Elena , Picciani Benedetta , Busico Adele , Settanni Giulio , Volpi Chiara Costanza , Gualeni Ambra Vittoria , Pellegrinelli Alessio , Milione Massimo , Caporale Marta , Niger Monica , Di Bartolomeo Maria , de Braud Filippo 
Published: May 2016

Background: Acquired resistance to anti-EGFR MoAbs (cetuximab and panitumumab) represents a challenge in the treatment of mCRC, but its molecular mechanisms are not completely understood. Even in presence of RAS-BRAF-PI3KCA ”quadruple wt” and HER-2/MET negative status for protein expression and gene amplification, pts who primarily respond to anti-EGFR MoAbs will eventually develop secondary resistance. Prior retrospective and small series uniquely investigated a single biomarker, showing in some cases the emergence of KRAS mutations,HER-2 or MET amplifications. Our study aimed at comprehensively describing all known molecular alterations possibly associated with acquired resistance.

Methods: Pts with mCRC were prospectively treated with cetuximab- or panitumumab-based therapy until progressive disease. All archival tumors were defined as RAS-BRAF-PI3KCA”quadruple wt” by Sanger sequencing, as well as HER-2/MET negative by both immunohistochemistry (IHC) and in-situ hybridization (ISH). At the time of disease progression, tumor re-biopsy was performed on the most accessible site of metastasis, as institutional procedure for a wide phase 1 screening program. On both archival tissue and re-biopsy, next generation sequencing of 50 genes' hotspot regions included in the Hotspot Cancer Panel v2 (Life Technologies) was performed by using the Ion Torrent Personal Genome Machine platform (Life Technologies). Moreover,HER-2/MET status were repeated on tumor re-biopsy by IHC and ISH.

Results: Seventeen pts were recruited. All had prior objective response to anti-EGFRs. Next-generation sequencing confirmed RAS-BRAF-PI3KCA wt status on archival tumors. The results of our analyses on tumor re-biopsies are shown in the table:

IDNGS: Acquired mutations (% mutant alleles)cellularity% mutant alleles normalized for cellularityHER2 ISHMET ISH#1KRAS Q61H (37%)80%46%--#2-Amplified-#3BRAF V600E (13%)90%14%--#4NRAS Q61R (13%)40%32%--#5--Amplified#6KRAS Q61K (4%)70%6%&lt;10% cells Amplified-#7---#8---#9KRAS G12R (17%)50%34%--#10-Amplified-#11-Amplified-#12---#13---#14---#15--Amplified#16-Amplified-#17---

Acquired RAS or BRAF mutations were found in 4 (23%) and 1 (6%) cases, respectively. Acquired HER-2 or MET amplification were found in 4 (23%) and 2 (12%) cases, respectively. As shown for patient #6, some degree of intra-tumor heterogeneity may exist due to concomitant presence of low represented RAS-mutated and HER-2 amplified sub-clones. In some cases (35%), a detectable acquired mechanism of resistance remains unknown.

Conclusions: Based on our results, currently known molecular alterations associated with acquired resistance were mainly mutually exclusive. In a relevant subset of cases additional molecular profiling is warranted.

Citation Format: Filippo Pietrantonio, Rosa Berenato, Federica Perrone, Annunziata Gloghini, Elena Tamborini, Benedetta Picciani, Adele Busico, Giulio Settanni, Chiara Costanza Volpi, Ambra Vittoria Gualeni, Alessio Pellegrinelli, Massimo Milione, Marta Caporale, Monica Niger, Maria Di Bartolomeo, Filippo de Braud. Comprehensive molecular characterization of acquired resistance to anti-EGFR monoclonal antibodies (MoAbs) in patients with metastatic colorectal cancer (mCRC). [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr B57.