Abstract PD01-03: Phosphorylated p-70S6K predicts tamoxifen resistance in postmenopausal breast cancer patients randomized between adjuvant tamoxifen versus no systemic treatment

BACKGROUND: Activation of the PI3K/MAPK pathways results in anti-estrogen resistance in vitro , however a biomarker that can predict clinical resistance has not yet been identified. Common drivers of these pathways are PIK3CA mutations, loss of PTEN and over-expression of HER2 and IGF-1R. We aimed to test the prognostic and predictive value of PI3K/MAPK pathway drivers as well as downstream activated proteins in postmenopausal breast cancer patients. METHODS: We collected primary tumor tissue from 563 ERα positive breast cancer patients who were randomized between tamoxifen (1–3 years) versus no adjuvant systemic therapy. PIK3CA hotspot mutations were assessed by Sequenom Mass Spectrometry. Immunohistochemistry was performed for expression of PTEN, IGF-1R and the downstream markers p-AKT(Thr308), p-AKT(Ser473), p-mTOR, p-p706SK and p-ERK1/2. Cox proportional hazard models for recurrence free interval were used to assess hazard ratios and interaction between these markers and treatment. RESULTS: No significant interaction between any of the tested PI3K/MAPK pathways drivers and tamoxifen was found. [/bold]However, interactions were identified between tamoxifen and the downstream activated proteins (p-AKT(Thr308), p- mTOR, p-p70S6K and p-ERK1/2). After correcting for multiple testing, p-p70S6K remained significantly associated with tamoxifen resistance. Patients whose tumor did not express p-p70S6K did derive significant benefit from tamoxifen (HR 0.24, 95 % CI= 0.12–0.47, p CONCLUSION: We conclude that the downstream marker of PI3K/MAPK activation p-p70S6K predicts tamoxifen resistance in ERα positive postmenopausal breast cancer patients. Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr PD01-03.