Retrospective evaluation of ADCC activity and cetuximab response in KRAS wild-type metastatic colorectal cancer patients (mCRC)

Introduction: Cetuximab is a IgG1 monoclonal antibody against epidermal growth factor receptor (EGFR) used in mCRC and in Head and Neck Cancer treatment. The proposed working mechanism of cetuximab is thought to include antibody-dependent cell-mediated cytotoxicity (ADCC). Cetuximab has been restricted to mCRC patients (pts) with wild-type RAS. Whether ADCC is associated with EGFR expression and/or mutational status of RAS and BRAF in mCRC remains unclear. Methods: We retrospectively identified, from March 2008 to September 2014, 41 mCRC pts who received chemotherapy with cetuximab (9 received cetuximab in first, 26 insecond and 6 inthird line). Peripheral blood samples were collected at start of therapy and during treatment. ADCC was evaluated over time of ex vivoNK-dependent activity measuring LDH release by a non radioactive cytotoxicity assay. Genotyping of FcgRII, FcgRIII and rs61764370 SNPs was done on DNA extracted from total peripheral blood using the appropriate "allelic discrimination assay". RAS(codon 12-13-59-61-146) and BRAF(V600E) genotypes were determined by pyrosequencing in patients' tumoral Formalin Fixed Paraffin Embedded (FFPE) tissues archived at diagnosis. Results: Median ADCC activity at treatment start for all 41 mCRC pts was 68.5% (range 10-99%). Correlation with OS and PFS was evaluated only in the sub-group of 26 ptstreated with cetuximab in second line. All pts resulted KRASwt. For this latter group, median follow-up was 13 months (range 3-37) for OS and 5.5 months (range 2-37) for PFS. Pts performing ADCC activity above the median value showed an improved OS compared to pts with ADCC activity below this value (median 21 vs12 months; p = 0.045; Long-rank Mantel-Cox Test). Correlation in terms of OS and PFS with FcgR and rs61764370 genotypes resulted significant for FcgRII in PFS where allele frequencies were 52% for A and 48% for G. Pts carrying alleles with A presented a longer PFS in comparison with GG genotype (median 8 vs3 months; p = 0.04;Long-rank Mantel-Cox Test). This effect resulted even amplified when PFS was evaluated in FcgRII favourable alleles stratified for FcgRIII. Pts presented with both FcgRII AA/AG and FcgRIII TT performed better than all the other subgroups (median 11 vs 5 months; p= 0.03;Long-rank Mantel-Cox Test). Conclusion: These results indicate a link between ADCC activity, FcgR genotypes and efficacy of cetuximab in KRAS wild-type mCRC pts. Our results should be confirmed by further large prospective studies.