Abstract A089: Exploiting clonal evolution and liquid biopsy to overcome resistance to anti-EGFR treatment in metastatic colorectal cancer: the CHRONOS trial
Authors: Siena Salvatore , Bardelli Alberto , Sartore-Bianchi Andrea , Lonardi Sara , Leone Francesco , Bergamo Francesca , Tonini Giuseppe , De Braud Filippo , Pietrantonio Filippo , Rimassa Lorenza , Santoro Armando , Siravegna Giulia , Mussolin Benedetta , Rua Francesco , Bonazzina Erica Francesca , Amatu Alessio , Racca Patrizia , Ardizzoni Andrea , Zagonel Vittorina , Marsoni Silvia 
Published: August 2018

Abstract
Background: The EGFR-targeted antibodies cetuximab and panitumumab (MAB-EGFRs) are used for the treatment of RAS-axis wild-type metastatic colorectal cancer (mCRC). Unfortunately, acquired resistance to MAB-EGFRs inevitably develops, dramatically curtailing their clinical use (Misale et al., Cancer Discov 2014). We found that mutations in KRAS, NRAS, and BRAF are the main culprit of this acquired resistance, and that high ctDNA levels of these mutated genes can be found in plasma of progressing patients. However, in preclinical models made resistant to cetuximab, acquired RAS-axis mutated clones decay upon cetuximab withdrawal, allowing tumor cells to regain sensitivity to further anti-EGFR treatment. Paralleling this behavior, RAS-axis WT patients who occasionally benefited from a rechallenge treatment with MAB-EGFRs exhibited corresponding pulsatile levels of mutant RAS ctDNA (Siravegna et al., Nat Med 2015). Collectively, these results suggest that the CRC genome adapts dynamically to intermittent MAB-EGFRs schedules, and provide a molecular rational for rechallenge therapies based on EGFR blockade. Methodology: CHRONOS is a liquid biopsy-driven trial to assess the efficacy of rechallenging with panitumumab (PAN) mCRC patients with ct-DNA-proven RAS-mediated acquired resistance. CHRONOS patients’ selection is based on a progressive enrichment strategy. In the SCREENING phase, multiple wild-type patients [MWTP, i.e., patients not harboring any drivers of resistance to MAB-EGFRs at tumor level (Bertotti et al., Nature 2015)] are identified upfront with a multiplex mCRC-specific tissue-genotyping panel (FUNNEL). In the MOLECULAR phase, MWTP responding to a 1st-line treatment inclusive of MAB-EGFR and developing RAS-mutated ctDNA clones at progression (PD), start 2nd-line chemotherapy and are blood monitored for RAS-related clones' decay. MWTP experiencing a &gt;50% drop in ctDNA RAS levels at 2nd-line PD enter the TRIAL phase and are rechallenged with PAN. The main endpoint is objective response (OR), aiming to achieve a 30% OR rate. 129 MWTP responding and progressing to 1st-line MAB-EGFR-based therapy will be blood-monitored from first to second PD. We expect to find detectable (&gt;3 MAF) ctDNA RAS mutated clones in 80% of cases (N 36), decaying &gt;50% during 2nd-line "EGFR-therapy holiday" (60% of cases). These “ctDNA eligible” patients will receive PAN at 6 mg/kg Q2 weeks until PD or unacceptable toxicity. 27 MWTP will be recruited in TRIAL phase with 6 responses necessary to declare the study positive (β 0.15; α 0.05). Liquid biopsy will be used to correlate the decay kinetics of RAS-extended clone(s) during MAB-EGFR treatment holiday with response to PAN rechallenge, and to link the blood presence of RAS-extended clone(s) during PAN treatment with response and its duration. The tumor ctDNA plasma will be analysed by NGS before and after PAN rechallenge and potential associations identified between different mutated ctDNA clones (RAS or others) and response to PAN rechallenge. Conclusion: This is the 1st trial in mCRC exploiting liquid biopsy and the clonal evolution of tumor cell under treatment pressure, to extend the use of anti-EGFR therapy beyond the point of clinically established resistance in mCRC. (NCT03227926.) Funded by AIRC grant #9970.
Citation Format: Salvatore Siena, Alberto Bardelli, Andrea Sartore-Bianchi, Sara Lonardi, Francesco Leone, Francesca Bergamo, Giuseppe Tonini, Filippo De Braud, Filippo Pietrantonio, Lorenza Rimassa, Armando Santoro, Giulia Siravegna, Benedetta Mussolin, Francesco Rua, Erica Francesca Bonazzina, Alessio Amatu, Patrizia Racca, Andrea Ardizzoni, Vittorina Zagonel, Silvia Marsoni. Exploiting clonal evolution and liquid biopsy to overcome resistance to anti-EGFR treatment in metastatic colorectal cancer: the CHRONOS trial [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2017 Oct 26-30; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2018;17(1 Suppl):Abstract nr A089.