Abstract 3035: 89Zr-labeled anti-PD-L1 Probody therapeutic CX-072 biodistribution in mice bearing human xenograft or murine syngeneic tumors
Authors: Giesen Danique , Broer Linda N. , Hooge Marjolijn N. Lub-de , Popova Irina , Howng Bruce , Vasiljeva Olga , Vries Elisabeth G. de , Pool Martin 
Published: August 2018

Abstract
BACKGROUND Immune checkpoint inhibiting antibodies have antitumor activity across several tumor types, but are not effective in all patients and can elicit side effects. CX-072, a fully human Probody™ therapeutic currently in a phase 1/2 clinical trial, is reactive to the murine and human programmed cell death-ligand 1 (PD-L1) immune checkpoint. Probody therapeutics are engineered antibodies with target-binding region blocking masking peptides, which can be preferentially cleaved by tumor-associated proteases, yielding fully active antibodies. CX-072 may thus preserve anti-tumor efficacy, while limiting side effects. We radiolabeled CX-072 with the positron emission tomography (PET) isotope zirconium-89 (89Zr) to reveal its tumor targeting properties and whole body distribution using non-invasive PET imaging.
METHODS CX-072 and a non-specific Probody therapeutic control (PbCtrl) were radiolabeled with 89Zr. For in vivo studies, PD-L1 expressing MDA-MB-231 human breast cancer cells were subcutaneously (sc) engrafted in Balb/c nude mice. To assess tracer protein dose dependency of the tumor uptake, mice received 10 μg 89Zr-CX-072 or 89Zr-PbCtrl (~5 MBq) supplemented with 0, 40 or 240 µg of unlabeled CX-072 or PbCtrl. To evaluate 89Zr-CX-072 biodistribution in an immune-competent setting, C57BL6 mice were implanted sc with low PD-L1 expressing MC38 syngeneic murine colon adenocarcinoma cells. All mice underwent serial in vivo PET imaging 1, 3 and 6 days post injection (pi), quantified by mean standardized uptake value (SUVmean) and followed by ex vivo biodistribution. Activated Probody species in tissues were detected by Western capillary electrophoresis.
RESULTS PET imaging revealed increasing 89Zr-CX-072 tumor accumulation between 1-6 days pi, with the highest SUVmean of 1.5 (± 0.2) observed for 10 µg at 6 days pi. Ex vivo biodistribution analysis showed 8.7 % injected dose per gram (%ID/g) tumor uptake for 10 µg 89Zr-CX-072 versus 3.8 %ID/g for 10 µg 89Zr-PbCtrl (P&lt;0.01) in MDA-MB-231 xenografted mice. In the syngeneic MC38 model biodistribution analysis showed modest tumor uptake for 10 μg 89Zr-CX-072 and 89Zr-PbCtrl (6.5 vs 5.5 %ID/g, P=0.24; tumor-to-blood ratio of 0.61 vs 0.45, P&lt;0.05). 89Zr-CX-072 uptake in lymphoid tissues (spleen, lymph nodes) was similar to 89Zr-PbCtrl. Activated Probody species were predominantly detected in tumor with lesser amounts present in lymphoid tissues.
CONCLUSION 89Zr-CX-072 accumulates more in PD-L1-expressing tumor tissues than in lymphoid tissues. A sub-study of an ongoing clinical study (PROCLAIM-CX-072) is designed to validate study drug distribution in patients using a good manufacturing practice (GMP) quality 89Zr-CX-072 tracer.
Citation Format: Danique Giesen, Linda N. Broer, Marjolijn N. Lub-de Hooge, Irina Popova, Bruce Howng, Olga Vasiljeva, Elisabeth G. de Vries, Martin Pool. 89Zr-labeled anti-PD-L1 Probody therapeutic CX-072 biodistribution in mice bearing human xenograft or murine syngeneic tumors [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 3035.