Efficacy of trastuzumab deruxtecan in treating HER2-low breast cancer leptomeningeal metastasis: a case report.

Abstract accepted at SABCS 2023, poster presented at SABCS 2023 We report the efficacy of trastuzumab deruxtecan (T-DXd) in treating human epidermal growth factor receptor 2 (HER2) low, type ID leptomeningeal breast cancer (LMD) (with positive cerebrospinal fluid [CSF] cytology and hydrocephalus as the only abnormal imaging finding) and the diagnostic and monitoring utilization of a novel microfluidic platform called CNSide™. Breast cancer LMD is associated with poor prognosis, and effective treatments are lacking. Our case highlights two crucial aspects related to the treatment and monitoring of breast cancer LMD. First, T-DXd was chosen based on immunocytochemistry (IHC) data from CSF malignant cells and follow-up revealed effectiveness of T-DXd in treating HER2-low LMD. While the efficacy of T-DXd has been established in treating metastatic HER2-low breast cancer, our case represents, to our knowledge, the first demonstration of T-DXd's effectiveness in HER2-low breast cancer LMD. Second, since this is type 1D LMD with absence of unequivocal measurable radiological disease in both the central nervous system (CNS) and extra-CNS, we employed a novel microfluidic CSF assay to monitor disease response. This novel assay outperformed standard CSF cytology in our case. There is an urgent need to develop CSF tumor cell assessment tool that surpasses the capabilities of conventional CSF cytology. Article highlights T-DXd is highly efficacious in treating HER2 low, leptomeningeal breast cancer. CSF CNSide™ assay utilizes a microfluidic platform and an antibody capture method to capture tumor cells. It uses the same principles used for detecting circulating tumor cells in peripheral blood. CNSide™ assay has a sensitivity of 100% compared with standard CSF cytology which has a sensitivity of 50–60%. CNSide™ assay can be used for facilitating receptor status characterization and guiding treatment. Our case demonstrates efficacy of T-DXd in treating HER2 low leptomeningeal breast cancer and also highlights the clinical utility of CNSide™ assay. Our case aligns well with the emerging literature about the efficacy of T-DXd in treating metastatic HER2-low disease. [ABSTRACT FROM AUTHOR]