Quantifying PD1 Saturation by PDL1 in Tumor Tissue Using a Novel RNA Aptamer-Based Assay

Therapeutic agents targeting the PD1–PDL1 interaction, commonly called PD1 blockade, are of great clinical value; however, predicting which patients will benefit has been inconsistent, in part, due to a lack of reliable biomarkers. Quantifying PD1 saturation by PDL1 in tumor tissue has the potential to serve as a biomarker; unfortunately, few diagnostic technologies are available to reliably quantify PD1 saturation in clinical biospecimens. Here, we report on a novel bioassay based on RNA aptamers, called the PD1 LIRECAP assay, that allows for quantification of the saturation of PD1 by PDL1 in formalin-fixed, paraffin-embedded (FFPE) tumor biospecimens. The assay is technically straightforward, high-throughput capable and reproducible. Results showed that quantification of PD1 saturation determined by PD1 LIRECAP assay correlates closely with PD1-mediated signaling and PD1–PDL1 proximity. Analysis of sarcoma FFPE biospecimens confirmed the assay to be consistent and revealed significant differences between patients as well as considerable intratumoral heterogeneity in PD1 saturation by PDL1. We conclude that this novel PD1 LIRECAP platform is technically feasible, reproducible and has the potential to be a superior predictive biomarker assay to predict the outcome of PD1/PDL1-based therapy. Similar assays based on this platform could be used in other systems and settings to quantify the interaction between two molecules.