3506 Background: Despite advances in molecular segmentation of metastatic colorectal cancer (mCRC), beyond RAS status therapeutic actionability remains confined to the limited subgroups of ERBB2 amplified, BRAF mutated and MSI-H patients. Optimization of available treatments is therefore warranted. Rechallenge with anti-EGFR monoclonal antibodies is often empirically used with some benefit as late-line therapy. We previously found that mutant RAS and EGFR ectodomain clones, which emerge in blood during EGFR blockade, decline upon antibody withdrawal leading to regain drug sensitivity. Based on this rationale, we designed CHRONOS, a multicenter phase II trial of anti-EGFR therapy rechallenge guided by monitoring of the mutational status of RAS, BRAF and EGFR in circulating tumor DNA (ctDNA). To our knowledge, this is the first interventional clinical trial of liquid biopsy for driving anti-EGFR rechallenge therapy in mCRC. Methods: Eligible patients were PS ECOG 0-2 RAS/BRAF WT mCRC having first achieved an objective response and then progression in any treatment line with an anti-EGFR antibody containing regimen, displaying RAS, BRAF and EGFR ectodomain WT status in ctDNA at molecular screening after progression to the last anti-EGFR-free regimen. Clonal evolution in ctDNA was analyzed by ddPCR and next generation sequencing. Panitumumab 6 mg/kg was administered IV every two weeks until progression. The primary endpoint was objective response rate (ORR) by RECIST version 1.1 with independent central review. 27 total patients and 6 responses were required to declare the study positive (power = 85%, type I error = 0.05). Results: Between Aug 19, 2019 and Nov 6, 2020 52 patients were screened by liquid biopsy and 36 (69%) were negative in ctDNA for RAS/BRAF/EGFR mutations. Of these, 27 patients were enrolled in 4 centers. Median age was 64 years (range: 42-80). PS ECOG was 0/50%, 1/46%, 2/4%. Previous anti-EGFR was administered in 1st line in 63%, 2nd in 15% and > 2nd in 22%. Median number of previous treatments was 3. The primary endpoint was met, with 8/27 partial responses (PR) observed (2 unconfirmed) (ORR = 30%, 95% CI: 12-47%). Stable disease (SD) was obtained in 11/27 (40%, 95% CI: 24-59%), lasting > 4 months in 8/11. Disease control rate (PR plus SD > 4 months) was therefore obtained in 16/27 (59%, 95% CI: 41-78%). Median progression-free survival was 16 weeks. Median duration of response was 17 weeks (1 ongoing). Maximal grade toxicity was G3, limited to dermatological and occurring in 19% of patients. ctDNA dynamics were studied in all patients. Conclusions: Liquid biopsy-driven rechallenge with anti-EGFR antibodies leads to further objective responses in one third of patients. Genotyping tumor DNA in the blood to direct therapy can be effectively incorporated in the management of advanced CRCs. Clinical trial information: 2016-002597-12.
423P From clinical trial to bedside: Triplet chemotherapy (FOLFOXIRI-B) in metastatic colorectal cancer