High-resolution melting analysis (HRMA) for KRAS mutational study in patients with metastatic colorectal cancer (mCRC pts).

515 Background: The use of anti EGFR monoclonal antibodies cetuximab and panitumumab is restricted to KRAS wild-type (WT) mCRC pts. Standard method for KRAS testing is direct sequencing. To overcome its limited sensitivity, novel techniques have been developed. However there is little concordance on the clinical significance of more sensitive tests. HRMA is very sensitive and economic and allows rapid detection of DNA sequence variations based on specific sequence-related melting profile. Here we report the evaluation of therapeutic effects of cetuximab in direct sequencing tested WT pts, according to their HRMA mutational status. Methods: We retrospectively evaluated 50 (35 males, 15 females) mCRC pts with WT KRAS status by direct sequencing. Median age was 61 years (range 29 – 77). Most (40%) pts had liver metastases and 24% had multi-organ disease. They received second- or third-line cetuximab-containing chemotherapy, with irinotecan (n=25) or with combination regimen (22 FOLFIRI and 3 FOLFOX). Twenty-eight pts were treated in second-line and 22 in third-line. KRAS mutation status was reassessed in all pts using HRMA. Results: All 50 pts were evaluated for tumor response, PFS and OS. In 4 (8%) pts, KRAS mutations were only identified by HRMA. The response rate of HRMA KRAS WT pts was 28.3%, with 1 complete (2.2%) and 12 partial (26.1%). No response was observed in HRMA KRAS MT (3/4 with a progression as best response). Stable disease rates were 30.4% (14/46 pts) and 25% (1/4 pts) in HRMA WT and MT cases respectively. Disease control rate (objective responses plus stable disease) was 58.7% in HRMA KRAS WT pts and 25% in HRMA KRAS MT. No significant correlation was found between HRMA-KRAS status and response rate (p=0.287) or disease control rate (p=0.219). However, median PFS (5.1 versus 2.5 months; HR=0.34, p=0.04) and median OS (11.3 versus 3.2 months; HR=0.11, p=0.03) were significantly longer for HRMA KRAS WT pts compared to HRMA KRAS MT. Conclusions: These data show a significant difference in clinical outcome between HRMA KRAS WT and MT pts receiving a cetuximab-containing second- or third-line treatment, suggesting that techniques more sensitive than direct sequencing could better select pts for cetuximab-based treatment.