Purpose: Accumulating evidence supports the existence of breast cancer stem cells (BCSC), which are characterized by their capacity to self-renew and divide indefinitely and resistance to conventional therapies. The Notch pathway is important for stem cell renewal and is a potential target for BCSC-directed therapy.

Experimental Design: Using human breast tumorgraft studies, we evaluated the impact of gamma secretase inhibitors (GSI) on the BCSC population and the efficacy of combining GSI with docetaxel treatment. The mouse experimental therapy paralleled a concurrent clinical trial in patients with advanced breast cancer, designed to determine the maximum-tolerated dose of the GSI, MK-0752, administered sequentially with docetaxel, and to evaluate BCSC markers in serial tumor biopsies.

Results: Treatment with GSI reduced BCSCs in MC1 and BCM-2147 tumorgrafts by inhibition of the Notch pathway. GSI enhanced the efficacy of docetaxel in preclinical studies. In the clinical trial, 30 patients with advanced breast cancer were treated with escalating doses of MK-0752 plus docetaxel. Clinically, meaningful doses of both drugs were possible with manageable toxicity and preliminary evidence of efficacy. A decrease in CD44⁺/CD24⁻, ALDH⁺, and mammosphere-forming efficiency were observed in tumors of patients undergoing serial biopsies.

Conclusions: These preclinical data show that pharmacologic inhibition of the Notch pathway can reduce BCSCs in breast tumorgraft models. The clinical trial shows feasibility of combination GSI and chemotherapy, and together these results encourage further study of Notch pathway inhibitors in combination with chemotherapy in breast cancer. Clin Cancer Res; 19(6); 1512–24. ©2012 AACR.