Preclinical PET imaging with the novel human antibody Zr-DFO-REGN3504 sensitively detects PD-L1 expression in tumors and normal tissues.

Background: Programmed cell death protein 1/programmed death-ligand 1 (PD-1/PD-L1) blocking antibodies including cemiplimab have generated profound clinical activity across diverse cancer types. Tumorous PD-L1 expression, as assessed by immunohistochemistry (IHC), is an accepted predictive marker of response to therapy in some cancers. However, expression is often dynamic and heterogeneous, and therefore not reliably captured by IHC from tumor biopsies or archival samples. Thus, there is significant need for accurate whole-body quantification of PD-L1 levels.

Methods: We radiolabeled the novel human anti-PD-L1 antibody REGN3504 with zirconium-89 (Zr) using the chelator p-SCN-Bn-Deferoxamine to enable non-invasive immuno-positron emission tomography (immuno-PET) of PD-L1 expression. PET imaging assessed the localization of Zr-REGN3504 to multiple human tumor xenografts. Mice genetically humanized for PD-1 and PD-L1 were used to assess the biodistribution of Zr-REGN3504 to normal tissues and the estimated human radiation dosimetry of Zr-REGN3504 was also determined. Pharmacokinetics of REGN3504 was assessed in monkeys.

Results: Clear localization of Zr-REGN3504 to human tumor xenografts was observed via PET imaging and ex vivo biodistribution studies demonstrated high (fourfold to sixfold) tumor:blood ratios. Zr-REGN3504 specifically localized to spleen and lymph nodes in the PD-1/PD-L1 humanized mice. Zr-REGN3504 immuno-PET accurately detected a significant reduction in splenic PD-L1 positive cells following systemic treatment with clodronate liposomes. Radiation dosimetry suggested absorbed doses would be within guidelines for other Zr radiolabeled, clinically used antibodies. Pharmacokinetics of REGN3504 was linear.

Conclusion: This work supports the clinical translation of Zr-REGN3504 immuno-PET for the assessment of PD-L1 expression. Future clinical studies will aim to investigate the utility of Zr-REGN3504 immuno-PET for predicting and monitoring response to anti-PD-1 therapy.