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Prospective analysis of microRNA 31-3p (miR31-3p) as a predictive biomarker of response to anti-epidermal growth factor receptor (anti-EGFR) monoclonal antibodies (mABs) in patients with metastatic colorectal cancer (mCRC).

548 Background: RAS status in tissue and/or liquid biopsies and tumour location (sidedness) are predictive markers of patients’ response to anti-EGFR mABs. MiR-31-3p expression has been correlated with clinical benefit from anti-EGFR mABs with chemotherapy. We aimed to validate the predictive power of miR-31-3p in a prospective cohort of chemo-refractory mCRC patients treated with single agent anti-EGFR mABs in the PROSPECT-C trial (NCT02994888). Methods: MiR-31-3p was tested by in-situ hybridization in 91 pre-treatment (PT) core biopsies from 45 mCRC patients. Sequential tissue biopsies obtained PT, at time of best response and at disease progression were tested to monitor changes in miR-31-3p expression over treatment. In 34 patients miR-31-3p, gender, sidedness, previous lines of treatment and RAS status in PT cell free (cf) DNA were evaluated in multivariate Cox and logistic regression models. Results: Patients with low miR-31-3p expression in PT biopsies showed better overall response rate (ORR: 58.3% vs 22.2%), median progression free survival (mPFS: 4.21 vs 2.27 months) and overall survival (mOS: 4.21 vs 2.27 mo) compared to those with high miR-31-3p expression (Hazard Ratio for PFS high vs low: 1.96; 95% CI: 0.98-3.88; p: 0.06 and HR for OS high vs low: 2.14; 95% CI: 1.04- 4.40; p: 0.04) adjusting for age, gender and sidedness. There was significant association between ORR and miR-31-3p expression (χ2 test p: 0.029). MiR-31-3p expression was an independent predictor of response, adjusting for gender, previous treatment lines and RAS (odds ratio: 0.14; 95% CI: 0.02-1.00, p: 0.048). A trend towards improved mPFS (5.10 vs 2.27 mo) and mOS (15.23 vs 4.67 mo) was noted in miR31-3p low vs high group in cfDNA RAS wild-type patients. No significant change in miR-31-3p expression were observed in archival tissue and sequential tissue biopsies. Conclusions: Our study validates the role of miR-31-3p as potential predictive biomarker of selection for anti-EGFR mABs. Further studies are needed to test if miR-31-3p combined with cfDNA RAS test can identify best responders in specific clinical niches.