OBJECTIVES: Eribulin is a non-taxane microtubule dynamics inhibitor that has demonstrated improved overall survival in a phase III study relative to commonly used therapies in late-line metastatic breast cancer (MBC). This budget impact model (BIM) was developed to help US payers make informed decisions regarding addition of eribulin for the management of MBC. METHODS: From the perspective of US commercial payers, an Excel-based BIM was constructed upon a hypothetical plan of 1 million members.  600 women are estimated to be newly diagnosed with breast cancer; of these 5% with MBC and 24% are initiated on 3rd-line chemotherapy, 47% of these on monotherapy. A Markov decision-analytic model was used to trace movement of patients receiving eribulin or another 3rd-line single-agent chemotherapies through different MBC states (treatment initialization, stable, response, progression, death). The model used a two-year time horizon with one-week Markov cycle lengths. The efficacy, market shares, and medical and drug costs were derived using published and internally available data sources. Model probabilities were based on discontinuation, progression free survival, and overall survival rates from the EMBRACE trial. Market share of eribulin was assumed to be 5% in year-one and 10% in year-two of the model.  Model outcomes included: total costs over 2 years pre- and post-inclusion of eribulin, net costs, cost per member per month (PMPM), cost per patient per month (PPPM) and median overall survival. RESULTS: Of the 1 million plan members, approximately 3 incident cases of MBC requiring 3rd line treatment were identified each year. Budgetary impact PMPM was estimated to be