This retrospective administrative claims study of women diagnosed with advanced or metastatic breast cancer compared health care costs by receipt of HER2-targeted agents and by disease stage and age group among patients using HER2-targeted agents. Methods: Women aged $18 years and diagnosed with stage III or IV breast cancer were selected from the 2008-2012 Truven Health MarketScan ® databases (Truven Health Analytics Inc., Cambridge, MA, USA) databases using ICD-9-CM (International

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... on of Diseases, 9th Revision, Clinical Modification) codes for nondiagnostic medical claims corresponding to breast cancer and local or distant metastases (earliest diagnosis of stage III or stage IV metastasis was designated as the index date). The 12 months prior to the index date were defined as the pre-index period. The post-index period was variable in length, beginning on the index date and continuing through the end of enrolment, inpatient death, or December 31, 2012, whichever occurred first. Receipt of HER2-targeted agents was defined as at least one claim for trastuzumab or lapatinib in the pre-index or post-index period. The study cohorts were women using or not using HER2-targeted agents, women with stage III or IV breast cancer using HER2-targeted agents, and women using HER2-targeted agents and aged 18-44 years, 45-64 years, or 65+ years at index. Health care costs and utilization were calculated on a per patient per month basis for all-cause and breast cancer-related services by place of service. Generalized linear models were used to estimate total all-cause and breast cancer-related costs. Results: A total of 30,660 eligible women met the study selection criteria, 14.4% of whom received HER2-targeted agents. Users of HER2-targeted agents had significantly lower inpatient utilization but higher outpatient utilization than nonusers, except for emergency room visits. Adjusted total costs were higher for users of HER2-targeted agents than nonusers (US$12,919 versus $8,822, respectively). Among users of HER2-targeted agents, utilization was typically higher for stage IV patients than for stage III patients. Adjusted incremental total per patient per month costs were significantly higher for stage IV patients than for stage III patients (incremental cost $4,519; 95% confidence interval 3,855-5,183), and were highest in patients aged 18-44 years, declining with age. Conclusion: Among patients with advanced or metastatic breast cancer, receipt of HER2targeted agents was associated with greater levels of costs and utilization. Higher costs and utilization in younger patients may indicate receipt of more aggressive treatments.