Liquid Biopsy Assessment of Circulating Tumor Cell PD-L1 and IRF-1 Expression in Patients with Advanced Solid Tumors Receiving Immune Checkpoint Inhibitor.

Background: Reliable biomarkers that can be serially monitored to predict treatment response to immune checkpoint inhibitors (ICIs) are still an unmet need. Here, we present a multiplex immunofluorescence (IF) assay that simultaneously detects circulating tumor cells (CTCs) and assesses CTC expression of programmed death ligand-1 (PD-L1) and interferon regulatory factor 1 (IRF-1) as a candidate biomarker related to ICI use.

Objective: To assess the potential of CTC PD-L1 and IRF-1 expression as candidate biomarkers for patients with advanced epithelial solid tumors receiving ICIs.

Patients And Methods: We tested the IF CTC assay in a pilot study of 28 patients with advanced solid tumors who were starting ICI. Blood for CTC evaluation was obtained prior to starting ICI, after a single cycle of therapy, and at the time of radiographic assessment or treatment discontinuation.

Results: At baseline, patients with 0-1 CTCs had longer progression-free survival (PFS) compared to patients with ≥ 2 CTCs (4.3 vs 1.3 months, p = 0.01). The presence of any PD-L1+ CTCs after a single dose of ICI portended shorter PFS compared to patients with no CTCs or PD-L1- CTCs (1.2 vs 4.2 months, p = 0.02); the presence of any PD-L1+ or IRF-1+ CTCs at time of imaging assessment or treatment discontinuation also was associated with shorter PFS (1.9 vs 5.5 months, p < 0.01; 1.6 vs 4.7 months, p = 0.05). CTC PD-L1 and IRF-1 expression did not correlate with tumor tissue PD-L1 or IRF-1 expression. Strong IRF-1 expression in tumor tissue was associated with durable (≥ 1 year) radiographic response (p = 0.02).

Conclusions: Based on these results, CTC PD-L1 and IRF-1 expression is of interest in identifying ICI resistance and warrants further study.