A comparative effectiveness analysis of single-agent cytotoxics in triple-negative metastatic breast cancer patients.

157 Background: There has been considerable progress in treatments for MBC. However, the identification of optimal cytotoxic agents in patients with triple negative disease (negative for hormone receptors, ER/PR and HER-2) remains a therapeutic challenge. A comparative effectiveness analysis of four cytotoxic agents was conducted in patients with TN MBC. Methods: We retrospectively identified 225 patients treated with single agent eribulin (E=47), capecitabine (C=69), gemcitabine (G=56) or vinorelbine (V=53) in 19 community oncology clinics across the US. Data collection included baseline patient characteristics, performance status, duration of current therapy, growth factor use and all dose limiting toxicities. Time to treatment failure (TTF) was measured from the first cycle of chemotherapy until disease progression, discontinuation due to toxicity, or death. TTF was then estimated using the Kaplan-Meier method and Cox proportional hazard modeling adjusted for clustering on the practice site. To control for selection bias that is inherent in observational studies, a propensity score weighted TTF analysis was also conducted. Results: Patients were comparable with respect to age, performance status, duration of disease free survival, presence of comorbidities and hemoglobin level prior to the start of chemotherapy. However, the median lines of therapy for use of C, G, V and E were 2nd, 3rd, 3rd, and 4th, respectively. The median duration of treatment was approximately 2 months with C, G, and E compared to 1.6 months with V. Using eribulin as the reference and adjusting for line of therapy and associated prognostic factors, the propensity score weighted Cox regression analysis did not identify statistically significant differences in TTF: C vs. E: HR = 1.15 (0.75 to 1.76), G vs. E: HR = 0.62 (0.34 to 1.13), V vs. E: HR = 1.0 (0.60 to 1.37). Conclusions: In patients with TN-MBC treated in a community oncology setting, eribulin was utilized in later lines than other agents. However, eribulin demonstrated at least comparable drug activity even when used in more heavily pretreated patients. These findings warrant further analyses in a larger population with an evaluation of biologic heterogeneity.