High-Dose Toremifene for Fulvestrant-Resistant Metastatic Breast Cancer: A Report of Two Cases

INTRODUCTION: Hormone receptor (HR)-positive metastatic breast cancer (MBC) is usually treated with hormone therapy. In postmenopausal females, aromatase inhibitors (AIs) are usually used as first-line therapy, and fulvestrant is used subsequently. The optimal treatment beyond fulvestrant has not been established. We experienced two cases in which high-dose toremifene (hdTOR) was effective after the failure of AIs and fulvestrant. CASE 1: A 73-year-old female with HR-positive left breast cancer (T2N1M0) underwent preoperative chemotherapy and mastectomy with axillary dissection. Computed tomography (CT) revealed liver tumors in S7 (23 mm) and S8 (25 mm) during adjuvant letrozole therapy, so fulvestrant was started. The tumors initially decreased in size (23 and 22 mm), but then progressed (36 and 25 mm). Treatment was changed to hdTOR, and the tumors shrunk to 26 mm (S7) and 24 mm (S8), and she was stable for 6 months while receiving hdTOR. CASE 2: An 81-year-old female with HR-positive left breast cancer (T2N1M0) underwent left mastectomy and axillary dissection. CT revealed liver tumors in S7 (20 mm) and S8 (11 mm) during adjuvant letrozole therapy, so fulvestrant treatment was started. The tumor in S7 shrunk (13 mm), but the tumor in S8 slightly progressed (13 mm), and both tumors progressed (14 and 18 mm) after 6 months. Treatment was changed to hdTOR, and the tumors slightly shrunk (12 and 14 mm) after 6 months. hdTOR has been continued for 9 months. CONCLUSION: hdTOR was effective for MBC after multiple hormone therapies, likely because it acts via a different mechanism of action.