Abstract 68: Imaging Endothelial PD-L1 expression with ultrasound molecular imaging to stratify immune checkpoint inhibitor responders

Abstract Immune checkpoint blockade therapies (ICTs) have revolutionized oncology through substantial and lasting treatment outcomes, but only in 15-20% of patients. Non-invasive diagnostic tools to select patients that are most likely to respond to ICT and guide treatment regimens, and thus minimize toxicities to non-responder patients, are in high demand. Program death-1 ligand (PD-L1) marker evaluation in the tumor vasculature and tumor microenvironment is an essential criterion for identifying a suitable patient for PD-L1 based ICT, and it also serves as a prognostic marker in tumor response evaluation. Ultrasound molecular imaging (USMI) is an up-and-coming non-invasive imaging modality that enables longitudinal characterization of endothelial-specific markers. We are developing USMI methods to image immune markers expressed specifically on endothelial cells (EC) of tumors as a decision-support tool for guiding ICT strategies. In this work, we demonstrate for the first time the potential of imaging EC-specific immunosuppressive markers such as PD-L1 and investigate whether it can act as a marker for predicting ICT response. Target-ready microbubble (MB) was used (Trust Bio-sonics, Taiwan) to generate PD-L1 antibody (Ab) linked MBs (TMBs). DSPE-PEG was functionalized to anti-PD-L1 Ab or isotype-Ab (control) to generate stable TMBs (~5 × 107 MBs/mL) to target PD-L1 expressing ECs in vivo. We studied the treatment effect of anti-PD-L1-Ab and isotype-Ab using a syngeneic CT-26 tumor in mice and demonstrated the targeting potency of TMB or iso-Ab-MB (NMB). US imaging was performed using Vevo 2100 high-frequency US system with M250 transducer to evaluate and quantify the differentially targeted enhancement parameter (dTE). We observed that TMBs bound PD-L1 expressing ECs in vivo, with high and low enhancement for TMB and NMB, respectively, in tumors validated by dTE quantification through characterization of the slower washout kinetics. Furthermore, TMBs exhibited increased tumor retention and prolonged binding with average differential targeted enhancement values (dTE) (+/- SD) of 5.2 ± 7.4 for TMBs compared to 2.4 ± 3.9 for NMBs (p=0.14). Preliminary results of USMI (dTE values of TMBs) indicate that PD-L1-treated mice showed a higher response (4.5 ± 5.2; n=9) to the treatment group than control mice that did not receive Ab (1.4 ± 0.8; n=4). The tumor growth rate for the treated mice group was 5.4 -fold retarded compared to the control group which correlated with USMI dTE values. The USMI signals correlated with IHC performed on tumor tissue ex vivo from the respective mouse used for US imaging. Overall, our pre-clinical study suggests that USMI using TMBs may contribute to non-invasive assessment of IC expression on tumor ECs, and that it may serve as a predictive biomarker of ICT response. This novel TMB agent could be a game changer for CE USMI-based cancer prognosis in ICB therapy. Citation Format: Arutselvan Natarajan, Farbod Tabesh, Mahsa Bataghva, Ramasamy Paulmurugan, Ahmed Nagy El Kaffas. Imaging Endothelial PD-L1 expression with ultrasound molecular imaging to stratify immune checkpoint inhibitor responders [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 68.