Pre-Treatment and Post-Treatment Epidermal Growth Factor Receptor Pathway Mutations in a Prospective Phase II Trial (NWCOG EXCITE) of Cetuximab-Containing Chemoradiation in Locally Advanced Rectal Cancer

Pre-operative chemoradiotherapy (CRT) with anti-EGFR antibodies may change the status of EGFR pathway mutations. We assessed the mutational status of a number of EGFR pathway genes before and after CRT in the NWCOG EXCITE trial. Patients with MRI-threatened surgical margins were given pelvic radiotherapy (45Gy) with capecitabine, irinotecan and cetuximab followed by surgery after 8 weeks. DNA was retrospectively extracted from the pre-treatment biopsy and resection specimen by macrodissecting areas of greatest residual tumour. The mutational status of KRAS (codons 12/13/61/146), NRAS (12/13/61), PIK3CA (542/545/546/1047) and BRAF (V600E hotspot) were determined by pyrosequencing. The work is presented on behalf of the NWCOG EXCITE trial investigators and was part-funded by a PathSoc fellowship. 80 patients commenced treatment and 76 underwent surgery with pathological complete response in 14 (18%) and near-complete in 6 (8%). Pre-treatment testing (n=78) detected mutations in KRAS (n=34), BRAF (n=3), NRAS (n=3) and PIK3CA (n=10). Any EGFR pathway mutation was detected in 58%. Following CRT, cases with residual tumour able to be tested (n=54) showed mutations in 32 patients (59%). There was a discrepancy compared to pre-treatment biopsy in 18 cases (33%): from wild-type (wt) to mutant (mut) in 9, from mut to dierent mut in 1 and from mut to wt in 7. One patient changed in 3 codons (mut to wt in KRAS 146/PIK3CA 545 and wt to mut in KRAS 12). In 12 patients (22%) this changed their overall EGFR pathway status (6x wt to mut and 6x mut to wt). Intratumour heterogeneity may explain some of the dierences in EGFR pathway mutations reported between biopsies and resections presenting a challenge to personalised medicine. However, cetuximab may also drive the growth of undetectable mutant clones to detectable levels on pyrosequencing. Further assessment using more sensitive sequencing technologies is currently being employed to investigate these differences.