Pretreatment And Postresection Epidermal Growth Factor Receptor (Egfr) Pathway Mutations In A Prospective Phase Ii Trial (Excite) Of Preoperative Cetuximab-Containing Chemoradiation (Crt) In Locally Advanced Rectal Cancer (Larc)

458 Background: Little data exists comparing pre- and post-CRT EGFR pathway mutations in LARC. Methods: Patients had MRI-defined LARC threatening or involving the surgical resection margin. CRT used pelvic radiotherapy (RT) 45Gy in 25 daily fractions with concurrent capecitabine (650mg/m2 bid PO 5 days/week), IV cetuximab (400mg/m2 one week prior then weekly at 250mg/m2 weeks 1-5) and IV irinotecan (weekly at 60mg/m2weeks 1-4). Surgery was 8 weeks after CRT. EGFR pathway mutations were not assessed prospectively. After study completion DNA was extracted from pre-treatment biopsies and post-resection specimens by macrodissecting areas of greatest tumour cell density. KRAS codons 12, 13, 61, 146, NRAS codons 12, 13, 61, PIK3CA codons 542, 545, 546, 1047 and the BRAF V600E hotspot were then pyrosequenced. Results: From 04/09-10/11, 80 patients commenced RT. 76 patients had surgery, with pathological complete response (CR) in 14 (18%) and near-complete (microfoci) in 6 (8%). Four patients had no surgery due to clinical CR. Pre-treatment mutation status was available in 78 patients with a KRAS codon mutation in 34 patients, BRAF in 3, NRAS in 3, PIK3CA in 10 and pathway (any mutation) in 45 (58%). Post-resection data in 54 patients showed 35 mutated codons in 32 patients (59%). 18 patients (33%) showed a discrepancy compared to pre-treatment biopsy. In 17 this was at a single codon, from wild-type (wt) to mutant (mut) in 9 (7x KRAS 12, 1x KRAS 13, 1x PIK3CA 542), from mut to mut in 1 (KRAS 12 c.35G>A to c.34G>T) and mut to wt in 7 (1x KRAS 12, 2x KRAS 13, 2x KRAS 146, 1x PIK3CA 542, 1x PIK3CA 545). One patient changed in 3 codons (mut to wt at KRAS 146 and PIK3CA 545 and wt to mut at KRAS 12). In 12 patients (22%) this changed overall EGFR pathway status (6x wt to mut and 6x mut to wt). Conclusions: Intratumour heterogeneity may explain the EGFR pathway codon changes causing sequencing from single areas of macrodissection to underestimate the tumour genomic landscape and presenting a challenge to personalised medicine. Cetuximab may also drive growth of undetectable mutant clones to detectable levels on pyrosequencing. Clinical trial information: 2007-006701-25.