Association of somatic kRAS gene mutations and clinical outcome from a phase 2 mCRC trial of panitumumab

B39 Background: Panitumumab, a fully human monoclonal antibody directed against the epidermal growth factor receptor (EGFr), has demonstrated benefit/efficacy for the treatment of mCRC. Identifying markers of responsiveness would increase the benefit to patients who may respond to the therapy while reducing unnecessary exposure to patients who would not respond. Metastatic colorectal carcinoma patient samples from an open label phase 2 clinical trial evaluating the safety and efficacy of panitumumab were used to explore gene mutations that could be correlated with clinical outcome. The objective of this study was to associate responsiveness, or lack there of, with mutations in EGFR, BRAF, KRAS mutations, or polymorphisms in cyclin D1. Methods: Thirty-seven patients were assessed for gene mutations in EGFR, BRAF, KRAS and a gene polymorphism in cyclin D1. Genomic DNA was isolated from dissected FFPE tumor sections (pretreatment). PCR was performed on EGFr (exons 18, 19, 20, 21, and 23), BRAF (exons 11 and 15), KRAS (exons 2 and 3), and PCR products were pooled and subcloned. More than 30 colonies per exon were sequenced and resolved on a Genetic Analyzer. Subsequent PCR and genomic DNA sequencing confirmed the existence of mutations. PCR products spanning exon 4 of cyclinD1 were purified for direct sequencing and analysis on a genetic analyzer. Best objective response using RECIST criteria (tumor assessments every 6 weeks) by local review were linked to genetic mutation data. Results: No mutations were found in the kinase domain of EGFR. Two patients (1with progressive disease [PD], 1with partial response [PR]) harbored a V600E mutation in BRAF. Thirteen patients harbored KRAS mutations in exon 2 (3 patients had stable disease [SD], and 10 patients had PD). Cyclin D1 polymorphisms G/G and A/G genotypes of were found in patients with PR, SD, or PD and the A/A polymorphism was found in patients with SD or PD. Conclusion: Similar to previous publications1,2, we did not find any EGFR kinase domain mutations in the 37 mCRC samples tested. Our data also indicate that although sequencing revealed all three cyclin D1 polymorphisms, no polymorphism could be used to predict responsiveness to panitumumab treatment. Although the sample size is small, our data indicate that no patients who achieved a PR and only 3 of 13 patients who achieved a SD in response to panitumumab treatment harbored KRAS mutations. In contrast, 10 of 20 patients who had PD harbored mutations in exon 2 of the KRAS gene, suggesting that mutations in KRAS may potentially help predict patients less likely to respond to treatment with panitumumab. These findings warrant sequencing a larger set of samples to correlate KRAS mutations with patient responsiveness to the treatment panitumumab. 1Barber TD, Vogelstein B, Kinzler KW, Velculescu VE. Somatic mutations of EGFR in colorectal cancers and gliobastomas. N Engl J Med. 2004 Dec 30;351(27):2883. 2Moroni M, Veronese S, Benvenuti S, Marrapese G, Sartore-Bianchi A, Di Nicolantonio F, Gambacorta M, Siena S, Bardelli A. Gene copy number for epidermal growth factor receptor (EGFR) and clinical response to anti-EGFR treatment in colorectal cancer: a cohort study. Lancet Oncol. 2005 May;6(5):279-86.