Efficacy analysis of disitamab vedotin (RC-48) in the treatment of HER2-low metastatic breast cancer: a case report

HER2-low breast cancer, defined as immunohistochemistry (IHC) 1+ or 2+ with negative in situ hybridization (ISH), has emerged as a clinically relevant therapeutic subgroup. We report the case of a 40-year-old woman with heavily pretreated HER2-low metastatic breast cancer who received disitamab vedotin (RC-48) after failure of multiple prior systemic therapies. During the disease course, HER2 status changed dynamically from HER2-low at initial diagnosis to HER2–0 after metastasis, and then to HER2-low again after further progression, highlighting the biological heterogeneity of metastatic breast cancer and the importance of repeat biopsy. Following RC-48 treatment, chest-wall cutaneous lesions regressed markedly, tumor markers showed partial biochemical improvement, and imaging demonstrated substantial shrinkage of visceral metastatic lesions. The time to treatment failure (TTF) was 4.6 months. After treatment discontinuation following a COVID-19-associated septic shock event, the patient declined further anticancer therapy and subsequently died 13.7 months after RC-48 initiation. Therefore, survival outcomes should be interpreted cautiously. As the first China-developed anti-HER2 antibody-drug conjugate (ADC), RC-48 may provide clinically meaningful benefit in selected patients with HER2-low metastatic breast cancer. This case also underscores the importance of reassessing HER2 status during disease evolution and supports the potential value of ADC-based therapy in later-line treatment.