Simultaneous PET/MRI measurement of primary T cells biodistribution in naïve mice following adoptive cell transfer

277 Objectives: The aim of the study was to non-invasively determine the biodistribution of adoptively transferred T-cells in mice models of cancer. We wanted to investigate whether there is a difference between the trafficking of primary human T-cells in naive NOD scid IL-2R-gammanull (NSG) mice and primary mouse T-cells in naive Balb/c mice. Animals underwent simultaneous PET/MRI imaging up to 5 days following injection of 89Zr-labeled T-cells in an MR Solutions 7 Tesla scanner with a PET camera. Tissues and organs of interest were dissected and their activity were measured using a gamma counter to validate PET quantification. Methods: Cell labeling and injection: For both the primary human and mouse T-cells, cells were labeled with 89Zr-[oxinate]4 (89Zr) in Hanks buffered saline solution (HBSS) for 45 min following a modified protocol from Weist, et al1. NSG (n=3) and Balb/c (n=3) mice were anesthetized with 1.5% isoflurane and cells labeled with 89Zr (12e6 cells, with cell specific activity = 3µCi per million cells) were injected via the tail vein. Percent viability for the human and mouse T-cells were 64±6% and 88±5%, respectively, 3 hours after labeling. Twenty-four hours post injection, viability was 93±3% for human T-cells and 87±6% for mouse T-cells. MRI and PET image acquisition: A two-dimensional anatomical Fast Spin Echo (FSE) T1-weighted was acquired for identifying regions of interest. Imaging parameters are as follows: TE/TR=11ms/3556ms, slice thickness=1mm, 1 average, echo train length=4, field of view =72 mm x 72 mm, matrix size = 512 x 252, flip angle=90°. To track 89Zr-labeled cells, PET images were acquired using the MR Solutions PET insert installed in the MR solutions 7T MRI scanner. Animals were anesthetized with isoflurane (1.5%), and respiration and body temperature were monitored. A list mode static acquisition protocol, calibrated for 89Zr, was obtained for 10 min on post-injection day (PID) 0-2, and 20 min on PID5. Reconstruction was performed using 3D-OSEM resulting in a 0.28 × 0.28 × 0.28 mm voxel dimension. Regions of interest (ROIs) were manually defined using VivoQuant software over anatomical MRI images and overlaid onto PET images. Gamma counting: Organs and tissues of interests were dissected on PID8 for gamma counting on a Perkins Elmer Wizard 2480 gamma counter. Results: Overall, primary human T-cells and primary mouse T-cells showed similar biodistribution after adoptive transfer. 89Zr-labeled cells (%ID/g) were present in the lungs initially after injection and decreases by PID5. Increasing levels of cells were then detected in the liver and spleen. There was slight increase in non-specific binding to bone. Minimal signal from 89Zr-labeled cells were detected in brain and muscle. Conclusions: Primary human and mouse T-cells have similar spatial and temporal biodistribution within one week after intravenous injection. Signal is detected mainly in the lungs, liver, spleen, and some in bone. One week after injection, label can be detected in the blood, spleen and bone. This suggest that viable T-cells persist in blood. 89Zr signal in bone and spleen usually indicate free 89Zr binding to bone or expired cells ending up in the spleen. We now have a basis for quantifying efficacy of adoptively transferred T-cells in vivo using PET (89Zr). Reference: 1. Weist, M.R., Starr, R., Aguilar, B., Chea, J., Miles, J.K., Poku, E., Gerdts, E., Yang, X., Priceman, S.J., Forman, S.J., Colcher, D., Brown, C.E., Shively, J.E., 2018. PET of Adoptively Transferred Chimeric Antigen Receptor T Cells with (89)Zr-Oxine. J Nucl Med 59, 1531-1537.