Heterogenous and Low Expression of HER2 in Breast Cancer Overcome by DS-8201a in a Heavily Treated Patient: Case Report and Review of the Literature.

Breast cancer is highly heterogenous with temporal and spatial heterogeneity making it necessary for rebiopsy. DS-8201a, a new potential therapy for human epidermal growth factor receptor 2 (HER2) low expression breast cancer, had been proved that it could overcome heterogenous HER2 expression in a preclinical setting. In January 2014, a 23-year-old woman was presented with a lump in the right breast with bone metastasis, diagnosed as infiltrating ductal carcinoma, estrogen receptor (ER)+, progesterone receptor (PR)+, HER2 immunohistochemistry (IHC) 2+, and fluorescence in situ hybridization negative. The patient received a series of therapies including surgery, radiotherapy, endocrine therapy, target therapy, and chemotherapy. The longest progression-free survival was 17 months after surgery. Biopsy of liver metastasis in February 2020 showed triple negative (HER2-, ER-, PR-), which was quite different from the initial diagnosis in 2014, so retesting was performed and the results showed ER-, PR+ by 10%, HER2 IHC score of 1+, indicating heterogeneity of HER2 expression. In May 2020, DS-8201a treatment was initiated and continued for 10 cycles until November 2020. Remarkable relief in symptoms was observed after the first dose. A reduction in the metastatic lesion size (liver and brain) and improved liver function was observed during the therapy. This case indicated the heterogeneity of breast cancer, and impressive efficacy of DS-8201a in a heavily treated patient with HER2-low and HER2 heterogeneity.