Progression-free survival (PFS) of patients with HER2-negative, estrogen-receptor (ER)-low metastatic breast cancer (MBC) with the addition of lapatinib to letrozole: Biomarker results of EGF30008

1018 Background: Lapatinib (Lap), an oral, dual inhibitor of EGFR/HER-2, is approved with capecitabine for treatment of HER-2+ MBC for recurrence after taxane, anthracycline, and trastuzumab-based therapy. Letrozole (Let) has activity in hormone-receptor positive (HR+) BC. It has been hypothesized that peptide growth factor signaling and hormone signaling pathways interact and dual targeting of these pathways results in therapeutic synergy. We report a blinded analysis of HER-2, ER, and progesterone receptor (PR) expression for patients (pts) with HR+ MBC at first diagnosis or post-adjuvant relapse and response to Lap + Let versus Let. Methods: 1286 pts were randomized 1:1 to 2.5 mg Let daily plus either 1500 mg Lap daily or placebo. The primary endpoint was PFS in HR+, HER-2+ MBC pts; secondary endpoints included PFS in the intent-to-treat population. Blinded, centralized commerical laboratory analysis of archived tumor tissue for HER-2 (IHC and/or FISH) and academic laboratory semi-quantitative analysis...