Evaluation of ⁸⁹Zr-Labeled Anti-PD-L1 Monoclonal Antibodies Using DFO and Novel HOPO Analogues as Chelating Agents for Immuno-PET

Programmed death ligand 1 (PD-L1) is a type 1 transmembrane immunosuppressive protein that is expressed on a wide range of cell types, including cancer cells. Anti-PD-L1 antibodies have revolutionized cancer therapy and have led to improved outcomes for subsets of cancer patients, including triple-negative breast cancer (TNBC) patients. As a result, PET imaging of PD-L1 protein expression in cancer patients has been explored for noninvasive detection of PD-L1 expressing tumors as well as monitoring response to anti-PD-L1 immune checkpoint therapy. Previous studies have indicated that the in vivo stability and in vivo target detection of antibody-based radio-conjugates can be dramatically affected by the chelator used. These reports demonstrated that the chelator HOPO diminishes 89Zr de-chelation compared to DFO. Herein, we report an improved HOPO synthesis and evaluated a series of novel analogues for thermal stability, serum stability, PD-L1-specific binding using the BT-549 TNBC cell line, PET imaging in vivo, as well as biodistribution of 89Zr-labeled anti-PD-L1 antibodies in BT-549 xenograft murine models. A new chelator, C5HOPO, demonstrated high stability in vitro and afforded effective PD-L1 targeting in vivo via immuno-PET. These results demonstrated that an improved HOPO chelator is an effective chelating agent that can be utilized to image therapeutically relevant targets in vivo.