Background: The cancer stem cell hypothesis asserts that there is a small population of cells within a tumor that has the ability to self renew and differentiate, and that these cells drive tumor growth and metastasis but are resistant to conventional cytotoxic chemotherapy. Pathways involved in stem cell growth and differentiation are viable targets for new anticancer therapies. One such pathway, Notch, is inhibited by GSIs which prevent translocation of Notch intracellular domain to the nucleus. Inhibition of GS concurrent with chemotherapy might improve disease control by targeting both stem cells and differentiated cells within the tumor. This Phase Ib clinical trial was designed to determine the MTD of the GSI, MK-0752, in combination with docetaxel, and to evaluate an effect on stem cell markers in serial tumor biopsies. Methods: Eligible subjects had metastatic breast cancer or locally advanced breast cancer that did not respond to anthracycline therapy. Patients with disease that progressed on a taxane, or who had received a taxane within 6 months were excluded. MK-0752 was administered orally on days 1-3 of each 21-day cycle of therapy, in escalating doses. Dose levels (mg/day) 1=300; 2=450; 3=600; and 4=800. Docetaxel 80 mg/m2 IV was administered day 8, with pegfilgrastim day 9 each cycle. Treatment was continued until disease progression, unacceptable toxicity, or symptomatic deterioration. The trial was monitored using the Time to Event Continual Reassessment Method, targeting a 20% toxicity rate. Tumor biopsies were performed at baseline, after 1 cycle, and at treatment completion in a subset of patients. Results: 30 patients were enrolled between Mar 2008 and Jan 2010. Dose limiting toxicities of the combination included diarrhea, hand-foot syndrome, and LFT elevation. 20/30 patients experienced Grade 1 or 2 fatigue. The final estimates and confidence intervals for the probability of dose limiting toxicity at each dose level are summarized in the table: Probability = probability of dose-limiting toxicity 20 enrolled patients had measurable disease by RECIST criteria. Of these, 9 had PR, 8 SD, and 3 PD, for an estimated RR of 45% to the combination. 2 patients have been maintained on therapy in excess of 22 cycles. Conclusions: Dose level 3 was identified for further study in a Phase II randomized trial. Efficacy of docetaxel was not inhibited by MK-0752, as a 45% RR in patients with measurable disease was observed. There is intriguing long term disease stabilization in 2 patients. Evidence of an effect of the combination on the stem cell population was apparent on serial biopsies as presented at SABCS Dec 2009 (Abstract # 48); additional biopsy data will be presented. Citation Information: Cancer Res 2010;70(24 Suppl):Abstract nr P6-15-03.
The realization that many cancers, including breast cancer, are driven by cells which display stem cell properties has significant clinical implications. Furthermore, the demonstrated role of these cells in mediating tumor metastasis and treatment resistance suggests the need to develop strategies to specifically target CSC populations. Cancer stem cell self-renewal and survival pathways represent potential therapeutic targets. These self-renewal pathways are regulated by an interacting network of cell intrinsic pathways, as well as extrinsic factors from the tumor microenvironment. These mircroenvironmental factors include cytokines such as IL-6, IL-8 and TGFb. CSCs maintain the plasticity to transition between epithelial-like MET and mesenchymal-like EMT states, a process regulated by the tumor microenvironment through microRNA circuits. We have demonstrated that previously identified cancer stem cell markers are cancer stem cell state specific. CD44+/CD24- CSCs represent mesenchymal-like stem cells capable of tissue invasion which are largely quiescent. In contrast, Aldehyde dehydrogenase expression identifies a more epithelial-like cancer stem cell state associated with self-renewal. Reversible EMT/MET transitions play a crucial role in mediating tumor metastasis. Preclinical breast cancer models predict that the greatest efficacy of CSC targeting therapeutics will occur when they are used in the adjuvant setting, a concept supported by preclinical models and clinical trials. Tumor regression may reflect effects on bulk cell populations explaining the lack of correlation between tumor shrinkage and patient survival. In contrast, recurrence following adjuvant therapy may be mediated by CSCs, which possesses sufficient self-renewal to form clinically significant metastasies. The important role of HER2 signaling in regulating breast cancer stem cell self-renewal may account for the remarkable clinical efficacy of targeting HER2 in the adjuvant setting. Furthermore, the clinical benefit of such therapies in classically defined HER2-negative breast cancers may be due to selective expression of HER2 in CSCs in the absence of HER2 gene amplification. The clinical benefit of adjuvant trastuzumab in women whose breast cancers are currently classified as HER2-negative is currently being assessed in the randomized national clinical trial B47. These studies may demonstrate the need for reevaluating currently used clinical endpoints and clinical trial designs. Promising new technologies including the isolation and molecular characterization of circulating cancer stem cells may provide the opportunity for real time assessment of the efficacy of CSC targeting agents. A number of agents regulating BCSCs have entered early phase clinical trials which will determine whether effective targeting of CSCs improves patient outcome. Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr BS01-2.
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