KRAS mutation is highly predictive of cetuximab resistance in metastatic colorectal cancer

Abstract

10502 Background: In metastatic colorectal cancer (MCRC), no molecular predictive markers to cetuximab response have been yet established. The aim was to evaluate whether KRAS gene mutations, EGFR immunochemistery (IHC) and EGFR gene copy number correlate with response to cetuximab. Methods: 59 patients with MCRC treated by cetuximab between July 2004 and December 2005 were retrospectively included. Clinical data were collected and tumour response was evaluated according to RECIST criteria. EGFR IHC was performed using the Dako kit. The EGFR gene copy number was determined by FISH (Fluorescence in-Situ Hybridization). Detection of KRAS gene mutations on exon 2 was performed by sequencing of extracted paraffin-embedded DNA and then by 2 methods, SNaPshot and PCR-LCR, specifically developed to detect small fractions of mutated tumor cells. Response to cetuximab was studied according to clinical data, IHC, FISH and KRAS mutation analysis using the Fischer exact test. Predictive factors of response were determined by logistic regression. Skin reactions were collected but not considered for this analysis as regards the lack of accurate grading in a retrospective study. Times to progression (TTP) were calculated using the Kaplan-Meier method and compared with log-rank test. Results: 12 patients (20.3%) responded to cetuximab (2 patients with complete response and 10 patients with partial response), 19 (32.2%) had stable disease and 28 (47.5%) were in disease progression. A KRAS mutation was detected in 22/59 tumours and, in 6 cases, was missed by sequencing analysis but detected using the SNaPshot and PCR-LCR assays. No KRAS mutation was found in responders patients. KRAS mutation was associated with disease progression (p = 0.0005) and TTP was significantly decreased in mutated KRAS patients (3 vs 5.5 months, p = 0.015). There was no correlation between EGFR IHC and cetuximab response. No EGFR gene copy number increase was detected in responders patients. Predictive factors of cetuximab resistance were KRAS mutation (p=0.003; OR:0.10; 95IC:0.22–0.40) and age<60 (p=0.024; OR:0.13; 95IC:0.02–0.77). Conclusions: KRAS mutation is highly predictive of cetuximab resistance in MCRC. Our study also highlights the need of sensitive methods to ensure an efficient mutation detection. No significant financial relationships to disclose.