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Tamoxifen is one of the most frequently used drugs in the treatment of estrogen receptor positive breast cancer. Although tamoxifen significantly increases patient survival in the adjuvant setting, a major factor that decreases its efficacy in the recurrent setting is drug resistance. The aim of our study is to identify a protein signature predicting outcome of tamoxifen treatment using high resolution mass spectrometry.

A total of 112 snap frozen breast tumor tissues were gathered from patients that had received tamoxifen as first line treatment upon recurrence. Samples were divided into a discovery (n= 56; Erasmus Medical Center) and validation sets (n=56; multi-center cohort). Based on progression of disease from start of tamoxifen therapy, patients were categorized having either Good (time-to-progression; ttp>6 months) or Poor (ttp<6 months) outcome. Approximately 4,000 tumor cells (∼200ng protein) were …