Effusion Reduction after Early Trastuzumab Deruxtecan Therapy for Unresectable HER2-Low Breast Cancer in an Older Patient: A Case Report.

INTRODUCTION: In recent years, HER2-low breast cancer has emerged as a distinct subtype, and the anti-HER2 antibody-drug conjugate trastuzumab deruxtecan (T-DXd) has become a promising treatment option. We report the case of an older patient with advanced HER2-low breast cancer resistant to endocrine therapy, who demonstrated a favorable clinical response to the early introduction of T-DXd.

CASE PRESENTATION: A 70-year-old woman was diagnosed at the age of 64 years with stage IV right breast cancer (invasive lobular carcinoma; T1cN1M1 with pleural effusion positive for malignant cells). The patient received endocrine therapy and a CDK4/6 inhibitor; however, disease progression was observed, including increased pleural and ascitic effusion. Cytotoxic chemotherapy was discontinued owing to adverse events and impaired activities of daily living (ADL). Low HER2 expression was confirmed in ascitic cell block specimens, and T-DXd therapy was initiated. After 4 courses of T-DXd, the pleural and ascitic effusions were almost resolved, and no further paracentesis was required. The patient's ADL improved, and she experienced clinical benefit for 15 months without any severe adverse events.

CONCLUSIONS: T-DXd is potentially a safe and effective treatment option for older patients with HER2-low breast cancer and for those with impaired ADL. Further accumulation of cases and investigations of long-term outcomes are warranted.