Reduzzi, Carolina

e15060 Background: In metastatic Breast Cancer (mBC) Circulating Tumor Cells (CTCs) are an established prognostic indicator of patients (pts) with highly aggressive disease, poor clinical outcomes, and poor response rates to therapy. However, CTCs are typically found in < 20% of mBC pts and many pts without CTCs also progress. Cancer associated macrophage-like cells (CAMLs) are inflammatory tumor macrophages also found in blood, in > 90% of mBC, and are indicators of poor clinical outcomes independent of CTCs. We conducted a prospective study to model CTC & CAML subtypes from 212 mBC pt blood samples prior to induction of new systemic therapies to optimize and validate risk models based on 2-year outcomes of Progression-free survival (PFS) & Overall survival (OS). Methods: An observational multi-institutional prospective study was conducted on 212 mBC pts prior to induction of a new line of therapy who were progressing on their current therapy. Anonymized (7.5ml) whole blood samples were taken prior to therapy induction and filtered by CellSieve filtration. Quantities and subtypes of CTCs & CAMLs were categorized by their expression of CK+/CD45+/CD14+ for CAMLs and CK+/CD45-/CD14- for CTCs. PFS (RECIST v1.1) and OS hazard ratios (HRs) were calculated by censored univariate & multivariate analysis at 2 years. Results: Pt median age was 51 years (range 25-92); TNBC (44%), ER/PR+ (30%), HER2+ (15%). Pts were treated with chemotherapy alone (18%), immune checkpoint inhibitor (46%), targeted (40%), hormone (8%) and unknown (5%). 38% (81/212) pts had ≥1 CTCs which correlated with significantly poorer PFS (HR = 1.9; 95%CI 1.3-2.7; p = 0.0016) and OS (HR = 2.0; 95%CI, 1.3-3.0; p = 00025), with the most significant risk at ≥8 CTCs (14% pts), PFS (HR = 3.8; 95%CI, 2.1-6.9; p < 0.0001) and OS (HR = 7.2; 95% CI, 3.4-14.9; p < 0.0001). Independently, 93% (n = 196/212) of pts had ≥1 CAML, with a threshold of >40um size (84% pts) had significantly poorer PFS (HR = 2.0; 95% CI, 1.2-3.1; p = 0.0032), but not OS (HR = 1.5; 95%CI, 0.9-2.7; p = 0.1169). However, a threshold of >125um size CAML (43% pts) also had significantly poorer PFS (HR = 1.8; 95% CI, 1.2-2.7; p = 0.0013) and OS (HR = 1.6; 95% CI, 1.0-2.5; p = 0.0233). By combining models, the best outcomes were seen in pts with 0 CTCs & 0 CAMLs (mPFS = 12.6 & mOS = 21.3), followed by 0 CTCs & ≥40um CAMLs (mPFS = 5.2 & mOS = 17.5) and 0 CTCs & ≥125um CAMLs (mPFS = 4.9 & mOS = 15.2). Poorer outcomes were seen with ≥1 CTCs (mPFS = 3.3 & mOS = 13.5) and the worst outcomes with ≥8 CTCs (mPFS = 2.5 & mOS = 4.8). Conclusions: CTCs were uncommon in mBC pts but correlated with very poor clinical outcomes. In parallel analysis, CAMLs were common with CAML size correlating with increasingly poorer outcomes. By combining CTC & CAML subtypes, mBC pts were more accurately stratified by risk of progression and death. Additional multivariate studies correlating treatment class and tumor response rates are ongoing.

e14535 Background: Anti-programmed cell death ligand PD-L1/PD-1 immune checkpoint inhibitors (ICIs) are approved for metastatic breast cancer (mBC) patients (pts) with a tumor combined positive score (CPS) ≥10% in their tissue. However, many pts with < 10% CPS may also respond to ICIs. Recent studies in lung cancer have shown that circulating tumor cells (CTCs) and tumor-macrophage fusion cells (TMFCs) both express, and may upregulate, PD-L1 which is predictive for ICI responses. We initiated a prospective study to monitor PD-L1 in CTCs & TMFCs in mBC pts prior to receiving systemic ICIs to correlate their PD-L1 expression to progression free survival (PFS) and overall survival (OS) at 24 months. Methods: We conducted a multi-institutional prospective pilot study of n = 102 pts with pathologically confirmed mBC, having failed ≥1 prior lines of therapy, prior to receiving ICIs (n = 52 Pembrolizumab, n = 39 Retifanlimab, n = 5 Atezolizumab, and n = 1 Nivolumab). Anonymized blinded peripheral blood samples (7.5ml) were procured prior to ICI induction (T0) and (~40 days after ICI induction (T1). TMFCs & CTCs were isolated using Cellsieve microfilters with average PD-L1 expressions measured then categorized as negative/low or high. Tumor CPS was categorized as negative/low (< 10%) or high (> 10%). We compared TMFC PD-L1 to tissue CPS by linear regression. TMFC/CTC PD-L1 expression and tissue PD-L1 CPS were compared to pts’ PFS and OS by Cox proportional univariate and multivariate analysis over 24 months. Results: Of n = 102 pts, median age = 59 years (range 32-77); 42% (43/102) were ER+, 33% (33/102) were PR+, 5% (5/102) were HER2+, and 56% (57/102) were triple-negative. 32% (32/102) had an ECOG score of 0, 25% (26/102) had ECOG 1, and 43% of scores were unavailable. N = 42 pts had available tumor CPS data, which did not correlate to TMFC/CTC PD-L1 at T0 or T1. Pts with high CPS did not significantly predict for PFS (HR = 0.78; 95% CI, 0.3-1.9; P = 0.7627) nor OS (HR = 0.54; 95% CI, 0.2-1.7; P = 0.4471). Pts with high TMFC/CTC PD-L1 at T0 also did not significantly predict for PFS (HR = 0.74; 95% CI, 0.5-1.2; P = 0.2799) nor OS (HR = 0.84; 95% CI, 0.4-1.6; P = 0.7220). However, of the n = 82 pts who provided T1 samples, high CTC/TMFC PD-L1 at T1 was significant for better PFS (HR = 2.55; 95% CI, 1.6-4.2; P = 0.0004) and better OS (HR = 2.06; 95%CI, 1.1-3.8; P = 0.0309). Conclusions: In this study of mBC pts treated with PD-L1 ICIs, tissue CPS did not correlate with PD-L1 expression in CTCs or TMFCs. Further, neither CPS nor baseline PD-L1 in CTC/TMFCs predicted for clinical outcomes in ICI therapies. However, PD-L1 in CTC/TMFCs ~40 days post-induction of ICI did predict for better response rates. While this study suggests predictive value of monitoring PD-L1 in blood during ICI therapies, further studies are required to refine and validate these findings.