In Vivo PET Imaging of Zr-Labeled Natural Killer Cells and the Modulating Effects of a Therapeutic Antibody.

Natural killer (NK) cells can kill cancer cells via antibody-dependent cell-mediated cytotoxicity (ADCC): a tumor-associated IgG antibody binds to the Fcγ receptor CD16 on NK cells via the antibody Fc region and activates the cytotoxic functions of the NK cell. Here, we used PET imaging to assess NK cell migration to human epidermal growth factor receptor 2 (HER2)-positive HCC1954 breast tumors, examining the influence of HER2-targeted trastuzumab antibody treatment on NK cell tumor accumulation. Human NK cells from healthy donors were expanded ex vivo and labeled with [Zr]Zr-oxine. In vitro experiments compared the phenotypic markers, viability, proliferation, migration, degranulation, and ADCC behaviors of both labeled (Zr-NK) and unlabeled NK cells. Female mice bearing orthotopic human breast HCC1954 tumors were administered Zr-NK cells alongside trastuzumab treatment or a sham treatment and then scanned using PET/CT imaging over 7 d. Flow cytometry and γ-counting were used to analyze the presence of Zr-NK cells in liver and spleen tissues. Zr cell radiolabeling yields measured 42.2% ± 8.0%. At an average specific activity of 16.7 ± 4.7 kBq/10 cells, Zr-NK cells retained phenotypic and functional characteristics including CD56 and CD16 expression, viability, migration, degranulation, and ADCC capabilities. In vivo PET/CT studies indicated predominant accumulation of Zr-NK cells in the liver and spleen. Ex vivo analyses of liver and spleen tissues indicated that the administered human Zr-NK cells retained their radioactivity in vivo and that Zr did not transfer to cells of murine soft tissues, thus validating this Zr PET method for NK cell tracking. Notably, Zr-NK cells migrated to HER2-positive tumors, both with and without trastuzumab treatment. Trastuzumab treatment was associated with an increased Zr-NK cell signal at days 1 and 3 after injection. In vitro, Zr-NK cells maintained key cellular and cytotoxic functions. In vivo, Zr-NK cells trafficked to HER2-postive tumors, with trastuzumab treatment correlating with enhanced Zr-NK infiltration. This study demonstrates the feasibility of using PET to image Zr-NK cell infiltration into solid tumors.