Supplementary Material for: Durable and Drastic Response to the Trastuzumab, Letrozole, Abemaciclib, and Goserelin Combination as First-line Therapy in HER2-positive and Hormone Receptor-Positive Metastatic Breast Cancer: A Case Report

Introduction: Chemotherapy combined with anti-HER2 targeted therapy is currently a standard treatment for advanced HER2/HR-positive breast cancer (BC), although evidences showed that HR expression compromised effectiveness of the treatment. While cyclin dependent kinase (CDK) 4/6 inhibitors combined with endocrine therapy is a key therapy for the BC with HR expression, data on the effectiveness and safety of cyclin-dependent kinase 4/6 inhibitors combined with Trastuzumab and endocrine therapy as a first-line treatment for HER2-positive and HR-positive metastatic BC are limited. Case presentation: Here we report a case of a 46-year-old premenopausal woman diagnosed with stage 4 HER2/HR-positive invasive ductal carcinoma from both right and left breast with hypermetabolic activities in multiple lymph nodes, adrenal, bone, and skin. Interventions: because of patient refusal to use chemotherapy, she was started on Goserelin + Abemaciclib + Letrozole + Trastuzumab. Outcomes: the patient’s symptoms were relieved with near resolutions at the primary breast mass and nearly all of the metastatic sites and metabolic resolution was observed in bone lesions. The disease was under control for 57 weeks. During the treatment, neutropenia (Grade 3) and thrombocytopenia (Grade 2) occurred during treatment, but improved after symptomatic treatment. Conclusion: We believe that Trastuzumab, hormone suppression, and Palbociclib is a practicable and effective treatment for HER2-positive and HR-positive metastatic BC in premenopausal patients who cannot tolerate first-line chemotherapy.