Clinical utility of serial circulating tumor DNA (ctDNA) to identify acquired resistance to anti-EGFR antibodies in metastatic colorectal cancer (mCRC).

Abstract

158 Background: Real-world evidence informing on the value of incorporating serial ctDNA into clinical practice is lacking. We aimed to explore the utility of serial ctDNA for identifying mechanisms of resistance and new targets for patients (pts) receiving anti-EGFR therapy for mCRC. Methods: We retrospectively reviewed 2801 pts with mCRC who had ≥1 alteration detected on a Guardant360 assay (Guardant Health) and subsequently had serial ctDNA assessments with known intervening therapies. We compared detection of new MAPK pathway alterations ( RAS/EGFR/BRAFV600 SNV or indel, MET or ERBB2 amplification) among patients who initially lacked these alterations by whether patients received anti-EGFR antibody alone (panitumumab or cetuximab), in combination with chemotherapy, or if they had never received an anti-EGFR antibody between ctDNA assays. Patients with a subsequent ctDNA assay after therapy initiation were included for analysis of acquired alterations. Results: Of 2801 pts, 1308 (47%) harbored ≥1 MAPK alteration in their first ctDNA timepoint and were excluded from analysis. Of the remaining 1493 without a MAPK alteration prior to therapy, 588 (39.4%) had a MAPK alteration detected in a subsequent ctDNA timepoint. Among 447 of 1493 pts (31.8%) with subsequent exposure to anti-EGFR antibodies, acquisition of a MAPK alteration occurred in 229/447 (51.2%). This was more common (OR 2.01. 95% CI 1.61-2.51, P<0.0001) than in patients without exposure to anti-EGFR antibodies (359/1046 [34.3%]). Acquired MAPK alterations were more common among those receiving single agent antibodies (42/66, 63.6%) than anti-EGFR plus chemotherapy (187/381, 49.1%) (OR 1.82, 95% CI 1.05-3.08, P=0.030). Patients with anti-EGFR antibody exposure were found to have higher blood TMB scores in the first assay after treatment compared to patients treated with chemotherapy alone (11.5 vs. 8.8 mut/MB, respectively; P=0.001), with no significant blood TMB difference between those receiving single agent anti-EGFR versus anti-EGFR plus chemotherapy (median 12.4 vs. 11.5 mut/MB; P=0.52). Patients who acquired MAPK alterations after anti-EGFR therapy had higher blood TMB scores compared to patients who did not acquire MAPK alterations (median 14.3 vs 8.6 mut/MB, respectively; p<0.0001). Conclusions: In this large cohort of pts with mCRC and treatment information, we demonstrate serial ctDNA identified a large proportion of patients acquiring resistance alterations that may impact response to future therapies. These results suggest serial ctDNA analysis may provide additional molecular insights for patients receiving anti-EGFR therapy and inform future anti-EGFR rechallenge strategies.