Case Report: Unusual Clinical Response to Trastuzumab and Pertuzumab in Metastatic Her-2 low breast cancer

Background: HER2-low breast cancer represents a biologically distinct subset of breast cancer currently underserved by targeted therapies. In this case report, we describe the clinical course of a patient with metastatic HER2-low breast cancer who exhibited an exceptional response to first-line combination therapy with Pertuzumab, Trastuzumab and Paclitaxel. Case: A 32 year old female with a history of stage III HER2 positive breast cancer, previously treated with neoadjuvant HER2 targeted therapy and chemotherapy, presented with progressive disease involving liver, lung, bone and brain metastases. Due to urgent need to start treatment she was treated with Her-2 targeted therapy based on her original biopsy. Subsequent biopsies from the metastatic disease in liver showed that patient had Her-2-low disease. Her liver function tests normalized after 1 cycle of treatment. After three cycles of treatment, the patient demonstrated a marked reduction in tumor burden as assessed by radiologic imaging and reported a significant improvement in her quality of life, with minimal adverse events. Discussion: Tumor heterogeneity, which can lead to variations in HER2 expression across different metastatic sites, presents a significant challenge in predicting treatment responses. This case highlights the importance of integrating clinical judgement with pathological findings when tailoring treatment. This case highlights potential benefit of Pertuzumab and Trastuzumab in Her-2-low tumors. Conclusion: Pertuzumab and Trastuzumab are not established as standard of care treatments for HER2-low breast cancer; this case suggests potential role in metastatic disease. Further research is necessary to elucidate the molecular mechanisms underlying this subgroup and optimize therapeutic strategies.