Abstract PS11-03: Comparison of next-generation sequencing (NGS) results from the cerebrospinal fluid, peripheral blood, and systemic metastatic tumor tissue of patients with metastatic breast cancer (MBC) and leptomeningeal disease (LMD)

Abstract Introduction. Leptomeningeal disease (LMD) is a devastating complication of advanced malignancies and occurs in 5-15% of patients (pts) with metastatic breast cancer (MBC). Molecular characterization of the primary tumor to identify actionable mutations is standard of care, and targeted therapies have significantly improved survival. In contrast, metastatic tumors to the central nervous system (CNS) are not routinely assessed for the presence of actionable mutations, so the frequency and concordance of actionable mutations centrally vs. peripherally is not well characterized. In this single-center prospective study, we collected cerebrospinal fluid (CSF) in pts with known or suspected LMD and performed next generation sequencing (NGS) to evaluate the status of actionable mutations in the CSF and compared these results with NGS from matched peripheral blood and systemic metastatic tumor samples. Methods. We enrolled sixteen (16) pts with MBC and suspected or confirmed LMD at a single academic center from 2021-2023 in this non-therapeutic prospective study and collected CSF, peripheral blood, and archival systemic tumor samples from each pt. We used NGS to evaluate the status of actionable biomarkers in the CSF, blood, and tissue using the following NGS platforms: CNSide (Biocept) for CSF; Guardant 360, Foundation One CDx and CNSide for blood; and Caris and the UCSF CLIA-validated UCSF500 panel for the systemic metastatic tumor samples. We also evaluated CSF tumor cell enumeration and HER2 status in a subset of pts (CNSide). Results. Of the sixteen pts with MBC and suspected or confirmed LMD, 81% (13/16) were ultimately determined to be LMD-positive based on positive CSF cytology and/or convincing MRI imaging, including 6 pts with HR+/HER2- MBC, 6 pts with metastatic triple negative breast cancer (mTNBC), and 1 pt with HR+/HER2+ MBC; 3 pts were determined to be LMD-negative based on similar criteria. Of the 6 pts with HR+/HER2- MBC and LMD, NGS detected actionable mutations in the CSF in 67% (4/6), including two pts with PIK3CA E545E, one pt with both PIK3CA E542K and ERBB2 amplification, and 1 pt with ESR1 Y537S. Matched NGS analysis of the peripheral blood and/or systemic metastatic tissue showed concordance with PIK3CA E542K detected across tissue types in two pts, but discordance across tissue types in the other pts, including ESR1 detected systemically in two pts but not in the CSF. Of the 6 pts with mTNBC and LMD, CSF NGS detected actionable mutations in the CSF in 17% (1/6) of pts, including one pt with PIK3CA E542K; analysis of blood NGS for this pt is pending. 3 pts (50%) had mutations in TP53 in the CSF (1 pt with R273H and two pts with K132R), which were concordant across tissue types in all 3 pts. NGS did not detect any variants in the CSF of the one pt with HR+/HER2+ MBC and LMD. CSF tumor cell enumeration and central HER2 status will also be reported. Conclusion. This study demonstrates that NGS can detect actionable mutations in the CSF of pts with MBC and LMD. We observed concordance and heterogeneity in the status of actionable mutations between the CSF, peripheral blood, and systemic metastatic tumor tissue. Larger studies are needed to assess the clinical utility of these observations, particularly with the development of several novel targeted agents that are CNS-penetrant. Citation Format: Laura Huppert, Lindy Her, Christine Hodgdon, Susie Brain, Carol Simmons, Jo Chien, Melanie Majure, Hope Rugo, Mark Jesus Magbanua, Ron Balassanian, Michelle Melisko. Comparison of next-generation sequencing (NGS) results from the cerebrospinal fluid, peripheral blood, and systemic metastatic tumor tissue of patients with metastatic breast cancer (MBC) and leptomeningeal disease (LMD) abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PS11-03.