Abstract S5-07: SWOG S0500 – A randomized phase III trial to test the strategy of changing therapy versus maintaining therapy for metastatic breast cancer patients who have elevated circulating tumor cell (CTC) levels at first follow-up assessment

Introduction: CTC are detectable in approximately 75% of patients with metastatic breast cancer (MBC), and are elevated (≥5CTC/7.5 ml whole blood (WB)) in about half. Elevated CTC at baseline are associated with a worse prognosis, and a decline in CTC levels suggests response to therapy. We initiated a prospective randomized clinical trial (SWOG S0500) to test whether a change in chemotherapy after failure to clear CTC after one cycle of first line chemotherapy would improve outcomes of patients starting first line cytotoxic chemotherapy for MBC. Methods: Patients initiating first-line chemotherapy for MBC were enrolled. All patients had measurable or evaluable disease that included bone metastases. Patients with elevated CTC at baseline and who continued to have elevated CTC after 21 days of therapy were randomly assigned to either continue initial therapy until progression or to change to a second line chemotherapy (physician choice) immediately at cycle 2. Patients with elevated CTC at baseline, which were not elevated at the 21-day follow-up were maintained on their initial therapy. Patients without elevated CTC at baseline were followed in an observation arm. The primary endpoint was overall survival, and progression-free survival was a secondary endpoint. It was expected that approximately 500-650 patients would have to be screened to enroll 120 patients in the randomized trial. Power was 81% with 2-sided α = 0.05 to detect a 70% increase in median overall survival for patients randomized to change therapy. Three interim analyses were planned during the course of the trial. Results: From 10/1/2006 until 3/15/2012, 624 patients were registered of whom 612 were eligible. A baseline CTC was obtained in 593 (97%) eligible patients of whom 317 (53%) had elevated CTC at baseline. Thirty-one patients (10%) with elevated baseline CTC did not complete the follow-up CTC due to death, progression, withdrawal, or assay failure. Of the 286 remaining patients, 123 patients (43%) continued to have elevated CTCs after the first cycle of chemotherapy and were randomly assigned to either maintain original chemotherapy (n = 64) or switched to new chemotherapy (n = 59). Final outcome results will be analyzed in October 2013 and will be reported if released by the SWOG Data Safety Monitoring Board. Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr S5-07.