8O - 89Zr-labeled anti-PD-L1 CX-072 PET imaging in human xenograft and syngeneic tumors

Abstract

Background: Immune checkpoint inhibiting antibodies have antitumor activity across several tumor types, but can also elicit immune-mediated adverse events (imAEs). CX-072 is an investigational antibody prodrug (Probody™ therapeutic), reactive to the murine and human programmed death-ligand 1 (PD-L1) immune checkpoint. CX-072 can be activated to a fully avid antibody by tumor-associated proteases that remove the masking peptides blocking the antigen-binding domain, which may limit peripheral PD-L1 binding and therefore imAEs. CX-072 was radiolabeled with zirconium-89 (89Zr) to study its biodistribution and tumor- versus lymphoid tissue-targeting properties using positron emission tomography (PET). Methods: CX-072, non-specific Probody™ therapeutic control (PbCtrl) and CX-072 parental antibody (CX-075) were radiolabeled with 89Zr. Human PD-L1-expressing MDA-MB-231 tumor-bearing BALB/c nude mice received 10 μg 89Zr-CX-072, 89Zr-PbCtrl or 89Zr-CX-075 (∼5 MBq) supplemented with 0, 40 or 240 µg of respective unlabeled antibody. Biodistribution was also evaluated in C57BL6/J mice bearing PD-L1-expressing MC38 syngeneic colon adenoma tumors. Mice underwent PET scans 1, 3 and 6 days post intravenous injection (pi), followed by ex vivo analysis. Results: In MDA-MB-231 tumors, 89Zr-CX-072 and 89Zr-CX-075 showed comparable uptake of 8.7 ± 1.0%ID/g and 8.8 ± 2.9%ID/g respectively for the 10 µg total protein dose, which is 2.3-fold higher compared to 89Zr-PbCtrl. Autoradiography showed 89Zr-CX-072 localization in PD-L1-positive tumor tissue areas on immunohistochemistry. Activated CX-072 was detected by capillary electrophoresis immunoassay mainly in MDA-MB-231 tumor, with limited amounts found in spleen. Flow cytometry confirmed PD-L1 expression in lymphoid tissues of C57BL6/J mice, while 89Zr-CX-072 uptake in these tissues was similar to 89Zr-PbCtrl and significantly lower compared to 89Zr-CX-075. Conclusions:89Zr-CX-072 accumulates in PD-L1-expressing tumors with minor uptake in murine peripheral lymphoid tissues, confirming that the CX-072 structure limits uptake in non-tumor, PD-L1-expressing tissues. We developed clinical grade 89Zr-CX-072 to study its whole-body distribution in patients. Legal entity responsible for the study: Dept. of Medical Oncology, University Medical Center Groningen (UMCG), The Netherlands. Funding: CytomX Therapeutics Inc. Disclosure: I. Popova, B. Howng, O. Vasiljeva: Employee: CytomX Therapeutics Inc. E.G.E. de Vries: Research funding: CytomX was made available to her institution (UMCG); Advisory boards: Pfizer, Daiichi Sankyo; Other funding: Amgen, Bayer, Chugai, G1 Therapeutics, Genentech/Roche, Nordic Nanovector, Regeneron, Synthon, all payments to UMCG. All other authors have declared no conflicts of interest.