An essential protein for normal stem cell renewal also promotes the growth of breast cancer stem cells when it’s overproduced in those cells, report researchers at The University of Texas MD Anderson Cancer Center. In mouse and lab experiments, the team also discovered that two drugs block the cascade of molecular events that they discovered, thwarting the formation of breast tumor-initiating cells.
The details appear in a paper titled “EZH2 Promotes Expansion of Breast Tumor Initiating Cells through Activation of RAF1-β-Catenin Signaling,” published in the February edition of Cancer Cell.
“Overexpression of the EZH2 protein has been linked to breast cancer progression, but the molecular details of that connection were unknown,” says senior author Mien-Chie Hung, Ph.D., professor and chair of MD Anderson’s department of molecular and cellular oncology. “Tumor-initiating cancer cells that arise from the primary cancer stem cells also are thought to drive cancer progression. This research connects EZH2 to the growth of breast tumor-initiating cells.”
The molecular chain of events that improves self-renewal, survival, and growth of breast cancer stem cells can be initiated by oxygen-starved portions of a tumor, Dr. Hung explains. Lack of oxygen in the tumor’s environment activates the HIF1a(alpha) protein, which in turn activates the EZH2 gene by binding to the gene’s promoter region, Dr. Hung’s team found.
This leads to high expression of EZH2 in breast cancer stem cells, which then reduces levels of the tumor suppressor RAD1. With RAD1 unable to repair damage, amplified RAF1 triggers the MEK-ERK-β-Catenin pathway, a well-known cancer promoting molecular pathway.
The researchers correlated this cascade of events with high-grade tumors in a sample of 168 human breast cancer tumors. It also increased the number of tumor-initiating cells in culture.
The team tested five anticancer drugs against a culture of breast cancer cells and in tumor samples of human breast cancer in a mouse model. Nexavar (Bayer HealthCare and Onyx Pharmaceuticals), which is a RAF inhibitor, eliminated more cancer stem cells and blocked tumor formation better than the other four, the researchers found.
Since Nexavar inhibits multiple targets, the researchers also tested an experimental drug called AZD6244, which reportedly specifically inhibits the MEK-ERK kinase cascade launched by RAF1. They found that the drug eliminated EZH2-promoted breast cancer stem cells and blocked the formation of precancerous mammospheres.
“The drugs’ inhibition of the breast tumor-initiating cells reveals a previously unidentified therapeutic effect for RAF1-ERK inhibitors to prevent breast cancer progression,” Dr. Hung notes.
AZD6244 was identified as a MEK inhibitor by Array BioPharma. In December 2003, AstraZeneca licensed the compound along with Array’s entire MEK program for development in oncology. Array reported having earned $21.5 million in up-front and milestone payments through its deal with AstraZeneca. Further development milestones could reach $75 million, and the firm will also earn royalties.
Although AstraZeneca initiated its first Phase II study with AZD6244 in June 2006, by December 2007 it decided not to move the drug into Phase III for that indication. The Phase II trial compared AZD6244 to Temodar in the treatment of stage III/IV melanoma patients.
AstraZeneca also reported that Phase II trials comparing AZD6244 to Alimta in non-small-cell lung cancer did not demonstrate superior efficacy for either drug. Similar results were seen in a mid-stage study of AZD6244 compared to Xeloda in metastatic colorectal cancer.
According to Array, AstraZeneca is currently recruiting patients for Phase II trials in NSCLC that will combine AZD6244 with Taxotere and compare it to Taxotere alone in KRAS mutation-positive NSCLC. The firm is also enrolling patients for a Phase II BRAF mutation positive melanoma study, where the candidate will be evaluated in combination with DTIC.
In June 2009, AstraZeneca entered an alliance with Merck & Co. to determine the efficacy of combining Merck's MK-2206 with AZD6244. NCI is also reportedly conducting clinical trials in collaboration with AstraZeneca.