BIOM-69. THE ONCOGENIC FLIP IN PATIENTS WITH LEPTOMENINGEAL METASTATIC DISEASE (LMD): LONGITUDINAL DETECTION IN CEREBROSPINAL FLUID TUMOR CELLS (CSF-TCS) REVEALS IMPLICATIONS FOR DIFFERENTIAL TREATMENT OF THE LMD TUMOR

Abstract INTRODUCTION Patients with LMD have poor prognosis and limited treatment options. Oncogene amplification of primary, metastatic, and CNS metastatic tumors can be heterogeneous. Therefore, patients with LMD may benefit from assessment of clinically relevant biomarkers in CSF, which may guide the choice of a targeted therapy specifically for the LMD tumor. CNSide is a CLIA-validated laboratory-developed test ordered commercially at the discretion of physicians for CSF-TC enumeration, immunocytochemical (ICC) and fluorescence in situ hybridization (FISH) analysis of oncogene amplification. We longitudinally analyze oncogenes in CSF-TCs in patients with LMD of various primary cancers. METHODS CSF was collected from patients with suspected or confirmed LMD; 613 tests were ordered on 218 individual patients with breast (N=105 patients), lung (N=65), gastrointestinal (N=10), and other cancers. Using CNSide, CSF-TCs were isolated and tested via ICC (ER, PD-L1, and PR), and FISH (ALK, cMET, cMyc, EGFR, FGFR1, HER2, NTRK1, NTRK3, PTEN, RET, and ROS1). RESULTS In patients with lung cancer, ALK was detected in 14% (17/118) of samples, CMET in 61% (78/128), HER2 in 73% (16/22), and RET in 4% (4/90). In patients with breast cancer, HER2 was detected in 39% (65/168) of samples, FGFR1 in 32% (19/60), ER in 26% (44/168) and PR in 4% (5/120). 66 patients underwent 2+ CSF draws; 24 underwent 5+. Among these, there were 13 ICC flips (7 acquired mutations) and 58 FISH probe detection flips (26 acquired mutations). 20/66 patients (30.3%) had at least one flip in their ordered biomarkers. CONCLUSION CNSide can be used to detect oncogene amplification on CSF-TCs of patients with LMD, and mutational status of the LMD tumor may differ from the original tumor biopsy. CSF-TC analysis may provide therapeutic insights that vary from the original tumor and could open the door to additional treatment targets for patients struck with LMD.