Purpose: Epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors, gefitinib and erlotinib, lead to significant tumor regressions in 10% to 15% of non-small cell lung cancer (NSCLC) patients with EGFR activating mutations. However, 30% to 40% of NSCLC patients, majority of whom are EGFR wild-type, develop stable disease following EGFR tyrosine kinase inhibitor therapy. EGFR-directed antibodies (cetuximab) are effective treatments for head and neck squamous cell carcinomas, which seldom contain EGFR mutations. The determinant(s) of efficacy of EGFR-targeted therapies in EGFR wild-type cancers is not well defined.

Experimental Design: We examined the relationship of EGFR ligands, EGF, transforming growth factor-α,and amphiregulin and the efficacy of gefitinib and cetuximab in EGFR wild-type NSCLC (n = 10) and head and neck squamous cell carcinoma (n = 4) cell lines. We compared amphiregulin expression using immunohistochemistry in EGFR wild-type NSCLC patients (n = 24) that developed either stable or progressive disease following erlotinib or gefitinib treatment.

Results: Cell lines which produced ≥20 pmol/L amphiregulin, as detected by an ELISA, were significantly more likely to be growth inhibited by both gefitinib and cetuximab than those that produced minimal or no amphiregulin. In these cell lines, both cetuximab and gefitinib led to cell cycle arrest at the G₁-S boundary and was associated with preferential inhibition of extracellular signal-regulated kinase 1/2 but not Akt signaling. Amphiregulin expression was significantly higher in NSCLC patients that developed stable disease compared with those that developed disease progression following gefitinib or erlotinib treatment.

Conclusions: Amphiregulin expression may help select EGFR wild-type patients who are likely to develop stable disease from EGFR-targeted therapies.