Abstract 3030: Immuno PET imaging of PD-L1 expression in syngeneic and human xenograft tumor mouse models using a site-specific 89Zr labeled PD-L1 antibody
Authors: Christensen Camilla , Kristensen Lotte K , Toftevall Hanna , Nordgren Maria , Agnew Brian J. , Nielsen Carsten H. , Kjaer Andreas 
Published: August 2018

Abstract
Objectives: One of the major checkpoints probed for therapy is programmed cell death protein 1 (PD-1) and its ligand PD-L1. Despite remarkable clinical efficacy, not all patients benefit from immune checkpoint therapy with PD-1/PD-L1 antibodies, and a cohort of non-responding patients exists. Tumor PD-L1 expression measured by immunohistochemistry has been applied as a biomarker for selecting patients to increase the response rate of PD-1/PD-L1 blockage, but the methods are invasive and prone to sampling error. The objective of the current work was therefore to develop an image-based biomarker for non-invasive Positron-Emission Tomography (PET) imaging of PD-L1 expression
Methods: Anti-PD-L1 (clone 6E11, Genentech) was site-specifically conjugated to dibenzocyclooctyne-Deferoxamine (DIBO-DFO) using GlyclNATOR® (EndoS2) and SiteClickTM technology. Site-specific conjugated 6E11 was subsequently radiolabeled with 89Zr.
The immunoreactivity and KD value were determined by cell binding assays. In vitro stability was assessed by high-performance liquid chromatography (HPLC) and the degree of labeling (DOL) was determined by LC/MS. A dose escalating study was performed with or without co-injection with unlabeled 6E11 to determine the optimal mass dose for PET imaging. Longitudinal PET/CT imaging was performed at various time-points after tracer injection in HCC827 tumor (lung adenocarcinoma) bearing animals and ex vivo biodistribution was performed after the last imaging time-point. Additionally, PET/CT imaging studies were carried out in different human xenograft and syngeneic tumor models with varying degree of PD-L1 expression. The syngeneic tumors received either fractionated external radiation therapy (XRT) or mouse Interferon gamma (IFNγ) treatment 3 days prior to 89Zr-DFO-6E11 PET/CT imaging in order to evaluate treatment induced up regulation of PD-L1 expression.
Results: 89Zr-DFO-6E11 was successfully labeled with a radiochemical purity &gt;99% and the KD value was determined to 0.23 nM. The HCC827 tumors were identified by 89Zr-DFO-6E11 PET imaging (3.7 ± 0.2 %ID/g, mean ± SD), and co-injection of unlabeled 6E11 increased the relative tumor uptake. Ex vivo biodistribution confirmed the in vivo results (5.4 ± 1.7 %ID/g) at 144 hours post injection. Non-invasive PET /CT imaging with 89Zr-DFO-6E11 was able to detect a treatment induced up regulation of PD-L1 expression following treatment with XRT or IFNγ.
Conclusions: Site-specific labeling of antibodies with 89Zr is feasible for immuno-PET imaging and tumor PD-L1 imaging by 89Zr-DFO-6E11. PET/CT is an attractive approach for non-invasive whole body visualization of PD-L1 expression and could potentially aid in therapy guidance. Site-specific labeled antibodies create the opportunity to utilize imaging biomarker-driven approaches to achieve best clinical outcomes in immunotherapy.
Citation Format: Camilla Christensen, Lotte K Kristensen, Hanna Toftevall, Maria Nordgren, Brian J. Agnew, Carsten H. Nielsen, Andreas Kjaer. Immuno PET imaging of PD-L1 expression in syngeneic and human xenograft tumor mouse models using a site-specific 89Zr labeled PD-L1 antibody [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3030.