Sequential monitoring of tumor macrophage fusion cells in the circulation of metastatic breast cancer and their prognostic value.

3054 Background: Tumor Macrophage Fusion Cells (TMFCs) are hybrids fusions of tumor cells and macrophage/immune cells that have recently been documented in advanced solid tumors, such as metastatic Breast Cancer (mBC). Further, TMFCs are found in the blood of patients (pts) as a CD45+/CD14+ binucleated subtype of standard circulating tumor cells (CTCs), which are Cytokeratin+ (CK+) & CD45 negative, but distinct from highly hyperploidy cells known as cancer associated macrophage-like cells. However, no study has explored the clinical meaning of TMFCs in blood. To better elucidate the clinical meaning of TMFCs, we prospectively pooled 137 mBC pt samples prior to induction of new therapy, ie baseline (BL), and after therapy induction (FU). Both TMFCs or standard CTCs were identified and enumerated to analyze their prognostic value for progression-free survival (PFS) and overall survival (OS). Methods: In this prospective study, we collected 7.5 mL blood samples from n=137 mBC pts enrolled in prospective trials from multiple institutions before starting new treatment lines for newly progressive mBC. If possible, an optional FU sample was collected (n=73) after BL (median=4.9 weeks). TMFCs & CTCs were isolated using a CellSieve microfilter and differentiated by staining for CK, CD45, CD14, and DAPI. Cox proportional regression hazard ratios (HRs) with 95% Confidence Intervals (CI) for PFS & OS by univariate & multivariate analyses were based on RECIST v1.1, determined by local institutional pathologist over 24 months (M). Results: TMFCs were detected in 26.3% (n=36/137) at BL and associated with significantly worse PFS (HR=3.3, CI 1.9-5.7, p<0.001) & OS (HR=2.7, CI 1.4-5.1, p=0.005). TMFCs were also detected in 27.4% (n=20/73) at FU (n=73) and prognostic for PFS (HR=5.2, CI 2.3-11.9, p<0.001) & OS (HR=5.1, 1.8-14.2, p=0.004). Further, pts with increases in TMFCs between BL and FU had worse PFS (HR=4.3, CI 1.9-10.1, p=0.001) & OS (HR=4.5, CI 1.6-12.7, p=0.011). In contrast, pts with CTCs were found in 39.4% (n=54/137) at BL, which were prognostic for worse PFS (HR=2.0, CI 1.3-3.2, p=0.005) and not prognostic for OS (HR=1.7, CI 0.9-3.0, p=0.103). At FU, pts with CTCs were found in 38.4% (n=28/73), which were prognostic for worse PFS (HR=3.6, CI 1.8-7.3, p=0.001) & OS (HR=3.2, CI 1.3-8.0, p=0.021). Overall at BL, pts without CTCs or TMFCs had a 5.7 M mPFS & >24M mOS, CTCs alone had a 5.0 M mPFS & 20.3 M mOS, TMFCs had 2.0 M mPFS & 5.8 M mOS. At FU, pts without CTCs or TMFCs had a 5.8 M mPFS & 14.5 M mOS, CTCs alone had a 3.3 M mPFS & 14.6 M mOS, while TMFCs pts had a 1.7 M mPFS & 4.8 M mOS. Conclusions: TMFCs appear to represent a CTC subgroup with additional prognostic value to standard CTCs, that may correlate with significantly faster PFS and OS versus pts with CTCs alone, or without CTCs. Subtyping treatment regimens is ongoing to determine if the TMFC populations are responsive to specific therapy types in these mBC pts.