cialis vs levitra vs viagra Patients and methods Postmenopausal women with HER2 negative, hormone receptor positive tumors and persisting circulating tumor cells CTCs; assessed using the FDA approved CellSearch System, Janssen Diagnostics, LLC after chemotherapy were randomized to 2 years of tamoxifen followed by 3 years of exemestane tamoxifen exemestane group, n 54 or 5 years of exemestane exemestane only group, n 54