Healthcare Cost Patterns in CDK4/6 Inhibitor-Treated HR+/HER2- Metastatic Breast Cancer

Cyclin-dependent kinase 4/6 inhibitors (CDK4/6i) are standard first-line treatment for hormone receptor-positive, human epidermal growth factor receptor 2-negative (HR+/HER2-) metastatic breast cancer, yet the cost architecture across the full treatment trajectory remains poorly characterized. This study aimed to characterize phase-specific healthcare costs using a Japanese nationwide claims database. We conducted a retrospective cohort study using the DeSC-IQVIA integrated Claims data (April 2014–May 2025). Patients with HR+/HER2- metastatic breast cancer who initiated first-line CDK4/6i were identified. Healthcare costs were decomposed into drug, non-drug inpatient, and non-drug outpatient components across eight treatment phases. Death-anchored analysis evaluated cost structure over the final 12 months of life among decedents. Among 5435 patients (median age 72.3 years), active treatment phases showed drug-dominated cost structures: first-line per-patient per-month (PPPM) cost was 482,936 Japanese yen (JPY) (drug 85%–90%), second-line 361,615 JPY (81%), and extended chemotherapy 255,922 JPY (66%). A marked inversion occurred in the palliative phase (PPPM 233,824 JPY; drug 11%, inpatient 70%) and terminal phase (PPPM 412,149 JPY; drug 17%, inpatient 69%). Death-anchored analysis of 1111 decedents identified a transition 2 months before death, when inpatient costs first exceeded drug costs. Individual fixed-effects models estimated the CDK4/6i-associated cost increment at 239,815 JPY/month (p < 0.001), corresponding to a total treatment-associated cost of 2.99 million JPY per patient. The cost architecture of CDK4/6i-treated metastatic breast cancer follows a predictable trajectory from drug-dominated active treatment to hospitalization-dominated end-of-life care, with a structural transition approximately 2 months before death. These findings support phase-specific healthcare resource allocation and economic modeling.