p16 Hypermethylation and KRAS Mutation Are Independent Predictors of Cetuximab Plus FOLFIRI Chemotherapy in Patients with Metastatic Colorectal Cancer

Purpose Hypermethylation of the CpG island of p16(INK4a) occurs in a significant proportion of colorectal cancer (CRC). We aimed to investigate its predictive role in CRC patients treated with 5-fluorouracil, leucovorin, irinotecan (FOLFIRI), and cetuximab. Materials and Methods Pyrosequencing was used to identify KRAS mutation and hypermethylation of 6 CpG island loci (p16, p14, MINT1 MINT2, MINT31, and hMLH1) in DNA extracted from formalin-fixed paraffin-embedded specimens. Logistic regression and Cox regression were performed for analysis of the relation between methylation status of CpG island methylator phenotype (CIMP) markers including p16 and clinical outcome. Results Hypermethylation of the p16 gene was detected in 14 of 49 patients (28.6%) and showed significant association with KRAS mutation (Fisher exact, p=0.01) and CIMP positivity (Fisher exact, p=0.002). Patients with p16-unmethylated tumors had significantly longer time to progression (TTP; median, 9.0 months vs. 3.5 months; log-rank, p=0.001) and overall survival (median, 44.9 months vs. 16.4 months; log-rank, p=0.008) than those with p16-methylated tumors. Patients with both KRAS and p16 aberrancy (n=6) had markedly shortened UP (median, 2.8 months) compared to those with either KRAS or p16 aberrancy (n=11; median, 8.6 months; p=0.021) or those with neither (n=32; median, 9.0 months; p < 0.0001). In multivariate analysis, KRAS mutation and p16 methylation showed independent association with shorter UP (KRAS mutation: hazard ratio [HR], 3.21; p=0.017; p16 methylation: HR, 2.97; p=0.027). Conclusion Hypermethylation of p16 was predictive of clinical outcome in metastatic CRC patients treated with cetuximab and FOLFIRI, irrespective of KRAS mutation.