Liquid biopsy to stratify metastatic breast cancer progression risk using multi-analyte cell subtyping prior to systemic therapy.

Abstract

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.