Sequential monitoring of PD-L1 on circulating stromal cells in blood predicts PFS in NSCLC patients undergoing immunotherapy after definitive chemoradiation.

8534 Background: Cancer Associated Macrophage-Like cells (CAMLs) are circulating stromal cells in the blood of patients (pts) with solid tumors that are phagocytic macrophages that may represent the inflammatory state of the tumor microenvironment. Previously, we demonstrated CAMLs ≥50µm after chemo-radiation therapy (CRT) in NSCLC is associated with worse progression free survival (PFS) and overall survival (OS). We also showed that PDL1 expression in CAMLs is dynamic & can change with CRT, difficult to assess with repeat biopsies, but possible with liquid biopsy. For this study we evaluated whether CAML properties can predict response to CRT with/without immunotherapy (IMT) agents in unresectable NSCLC. Methods: A single blind multi-year prospective study was undertaken to test the relationship of PDL1 expression and ≥50µm CAML size to PFS/OS in NSCLC, pre and post CRT with (n = 96) and without (n = 72) anti-PDL1/PD1 IMT. This included atezolizumab (prospective single arm NCT02525757) n = 39, durvalumab n = 52 or pembrolizumab n = 5 both after 2018 FDA approval. We recruited 168 pts with pathologically confirmed unresectable NSCLC prior to CRT. Blood samples 15 mL were taken at baseline (BL), CRT completion (T1), and ̃1 month after CRT (T2) (with n = 96 or without n = 72 IMT). Blood was filtered by CellSieve filtration and CAMLs quantified for size ( < 49 µm or ≥50 µm) and PDL1 expression to evaluate PFS and OS hazard ratios (HRs) by censored univariate and multivariate analysis at 24 months. Results: CAMLs were found in 90% of all samples, average 5.8 CAMLs/15mL. At BL, ≥50µm CAMLs did not predict PFS in CRT/IMT pts (HR 1.6, p = 0.220) nor CRT alone (HR 1.3, p = 0.593). However, after completion of CRT (T1) ≥50µm CAMLs predicted PFS in CRT/IMT pts (HR 2.7, p = 0.003) and CRT alone (HR 2.5, p = 0.015). In primary tumor biopsies, PDL1 expression > 1% did not predict CRT/IMT response (PFS HR 1.8, p = 0.262 & OS HR 2.3, p = 0.158). At BL, high CAML PDL1 did not predict PFS in CRT/IMT pts (HR 1.4, p = 0.427) nor CRT alone (HR 1.1, p = 0.982). Further, at CRT completion (T1), high CAML PDL1 only trended for better PFS in CRT/IMT pts (HR 1.7, p = 0.137), but not CRT alone (HR 1.1, p = 0.972). At T2, however, pts with continuously high CAML PDL1 had significantly better PFS with IMT (HR 3.2, p = 0.002) vs CRT alone (HR 1.4, p = 0.616). While ≥50µm CAMLs at BL did not predict 24 month progression, ≥50 µm CAMLs after CRT (with or without 1 cycle of anti-PDL1 IMT) was 84% accurate at predicting progression. Further subtyping and analysis is ongoing to evaluate OS and PDL1 in the CAML populations. Conclusions: Our data suggests that in unresectable NSCLC, ≥50 µm CAMLs after completion of CRT is prognostic regardless of IMT use. PDL1 expression in CAMLs also appears to predict for response to consolidated IMT after CRT. Additional studies are needed to validate these findings.