Abstract
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Introduction: REGN5054 is a fully human monoclonal antibody that binds specifically to human and cynomolgus CD8 expressed on T cells. REGN5054 was radiolabeled with 89Zr, a positron emitting radionuclide, for non-invasive immuno-PET detection of CD8 in naive cynomolgus monkeys to determine the clearance and organ uptake after administration.
Methods: REGN5054 was conjugated to the bifunctional chelator p-SCN-Bn-deferoxamine (DFO) for 89Zr radiolabeling. A single dose of 1 mCi 89Zr-DFO-REGN5054 was intravenously injected into cynomolgus monkeys at either 0.5 mg or 2.5 mg total REGN5054 (n = 2/group). The animals [7.8kg (range:5.3-9.8)] were scanned at 2 hours and 1, 4, and 6 days post-injection in a Siemens Biograph mCT PET/CT scanner. Plasma samples were collected at each time point for PK analysis.
Results: Radiolabeling with 89Zr resulted in radiochemical purity of 94.4-98.3% and immunoreactivity of 81.7-94.5%. 89Zr-DFO-REGN5054 was well tolerated in all cynomolgus monkeys. 89Zr-DFO-REGN5054 immuno-PET revealed dose-dependent uptake in T cell rich organs; spleen, lymph nodes, and bone marrow. The 2.5 mg dose of 89Zr-DFO-REGN5054 had slower blood clearance and lower uptake in the spleen, bone marrow, and liver compared to 0.5 mg, indicating the specificity of binding. At day 4 post-injection, the average uptake in the spleen, liver, kidney, bone marrow and right inguinal lymph node was 27.8, 9.9, 2.2, 8.2, and 12, respectively, for animals dosed with 0.5 mg 89Zr-DFO-REGN5054 and 15.4, 6.6, 3.2, 4.8 and 19.6 for animals dosed with 2.5 mg (SUVmean for all except SUVmax for lymph nodes). At day 4, the average tissue-to-blood SUV ratios for spleen, liver, bone marrow and lymph node were 30.9, 10.9, 9.1, and 8.2 for the 0.5 mg dose and 5.6, 2.4, 1.7, and 5.3 for the 2.5 mg dose. Plasma analysis showed a circulating half-life of 12.3 h and 21.2 h for the 0.5 and 2.5 mg 89Zr-DFO-REGN5054 doses, respectively, indicating increased target-mediated clearance for the lower dose.
Conclusions: 89Zr-DFO-REGN5054 immuno-PET was able to reveal cynomolgus CD8 expressed on lymphocytes in the spleen, lymph nodes and bone marrow. These findings support clinical translational studies of 89Zr-DFO-REGN5054 to investigate its utility for non-invasive imaging of CD8+ T cells during immunotherapy.