• Klaus P Hoeflich, Carol O'Brien, Zachary Boyd, Guy Cavet, Steve Guerrero, Kenneth Jung, Tom Januario, Heidi Savage, Elizabeth Punnoose, Tom Truong, Wei Zhou, Leanne Berry, Lesley Murray, Lukas Amler, Marcia Belvin, Lori S Friedman, and Mark R Lackner.
• Translational Oncology, Genentech, Inc., South San Francisco, California 94080, USA.
• Clin Cancer Res. 2009 Jul 15; 15 (14): 4649-64.

PurposeThe pathways underlying basal-like breast cancer are poorly understood, and as yet, there is no approved targeted therapy for this disease. We investigated the role of mitogen-activated protein kinase kinase (MEK) and phosphatidylinositol 3-kinase (PI3K) inhibitors as targeted therapies for basal-like breast cancer.Experimental DesignWe used pharmacogenomic analysis of a large panel of breast cancer cell lines with detailed accompanying molecular information to identify molecular predictors of response to a potent and selective inhibitor of MEK and also to define molecular mechanisms underlying combined MEK and PI3K targeting in basal-like breast cancer. Hypotheses were confirmed by testing in multiple tumor xenograft models.ResultsWe found that basal-like breast cancer models have an activated RAS-like transcriptional program and show greater sensitivity to a selective inhibitor of MEK compared with models representative of other breast cancer subtypes. We also showed that loss of PTEN is a negative predictor of response to MEK inhibition, that treatment with a selective MEK inhibitor caused up-regulation of PI3K pathway signaling, and that dual blockade of both PI3K and MEK/extracellular signal-regulated kinase signaling synergized to potently impair the growth of basal-like breast cancer models in vitro and in vivo.ConclusionsOur studies suggest that single-agent MEK inhibition is a promising therapeutic modality for basal-like breast cancers with intact PTEN, and also provide a basis for rational combination of MEK and PI3K inhibitors in basal-like cancers with both intact and deleted PTEN.

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