Background: CET, a monoclonal antibody targeting the epidermal growth factor receptor, improves overall survival (OS) and progression free survival (PFS) in patients (pts) with KRAS wild-type (WT) chemotherapy refractory CRC. BRAF and PIK3CA mutation status, and PTEN expression levels may further predict benefit from CET therapy. Methods: Available colorectal tumour samples were analyzed from a phase III trial of CET plus best supportive care (BSC) vs BSC alone (NEJM 2007; 357(20)). BRAF and PIK3CA mutations (MUT) identified in tumour-derived DNA using a high resolution melting analysis to identify amplicons with mutations were confirmed by sequencing. PTEN expression by immunohistochemistry (IHC) was performed on tissue microarrays constructed from available tumour blocks. For each biomarker, prognostic (treatment independent) effects were assessed in patients on the BSC alone arm. Predictive effects (benefit from CET) on OS and PFS among all patients and those in the KRAS wild-type subset were examined using a Cox model with tests for treatment-biomarker interaction, adjusting for covariates. Results: Of 401 pts assessed for BRAF status (70% of CO17 population), 13(3%) had mutations. Of 407 pts assessed for PIK3CA status (71% of CO17 population), 61(15%) had mutations. Of 205 pts assessed for PTEN (36% of CO17 population), 148(72%) were negative for IHC expression. No biomarker was prognostic for OS or PFS, and none were predictive of benefit from CET, either in the whole study population or the KRAS WT subset. Conclusions: In chemotherapy-refractory CRC, neither PIK3CA mutation status nor PTEN expression were predictive of benefit from CET therapy. BRAF mutations are uncommon in this setting. Larger sample sizes would be required to determine if BRAF status is predictive for CET benefit.

Biomarker	Prognostic analysis in BSC patients adj HR (MUT vs WT), [for PTEN, negative vs present]
	OS	PFS
BRAF	1.47, p=0.41	1.52, p=0.37
PI3KCA	1.11, p=0.65	1.10, p=0.66
PTEN	1.13, p=0.70	0.99, p=0.98
	Predictive analysis in KRAS WT subset adj HR (CET+BSC vs BSC), interaction p value
	OS	PFS
BRAF	1.39, p=0.69	1.18, p=0.84
PI3KCA	0.79, p=0.63	0.73, p=0.50
PTEN	0.75, p=0.61	2.47, p=0.08