Total-Body PET and Highly Stable Chelators Together Enable Meaningful 89Zr-Antibody PET Studies up to 30 Days After Injection

The use of Zr-89-antibody PET imaging to measure antibody biodistribution and tissue pharmacokinetics is well established, but current PET systems lack the sensitivity needed to study Zr-89-labeled antibodies beyond 2-3 isotope half-lives (7-10 d), after which a poor signal-to-noise ratio is problematic. However, studies across many weeks are desirable to better match antibody circulation half-life in human and nonhuman primates. These studies investigated the technical feasibility of using the primate mini-EXPLORER PET scanner, making use of its high sensitivity and 45-cm axial field of view, for total-body imaging of Zr-89-labeled antibodies in rhesus monkeys up to 30 d after injection. Methods: A humanized monoclonal IgG antibody against the herpes simplex viral protein glycoprotein D (gD) was radiolabeled with Zr-89 via 1 of 4 chelator-linker combinations (benzyl isothiocyanate-DFO [DFO-Bz-NCS], where DFO is desferrioxamine B; DFO-squaramide; DFO-Bz-NCS, where DFO* is desferrioxamine*; and DFO-squaramide). The pharmacokinetics associated with these 4 chelator-linker combinations were compared in 12 healthy young male rhesus monkeys (-1-2 y old, 3 1 kg). Each animal was initially injected intravenously with unlabeled antibody in a peripheral vessel in the right arm (10 mg/kg, providing therapeutic-level antibody concentrations), immediately followed by approximately 40 MBq of one of the Zr-89-labeled antibodies injected intravenously in a peripheral vessel in the left arm. All animals were imaged 6 times over a period of 30 d, with an initial 60-min dynamic scan on day 0 (day of injection) followed by static scans of 3045 min on approximately days 3, 7, 14, 21, and 30, with all acquired using a single bed position and images reconstructed using time-offlight list-mode ordered-subsets expectation maximization. Activity concentrations in various organs were extracted from the PET images using manually defined regions of interest. Results: Excellent image quality was obtained, capturing the initial distribution phase in the whole-body scan; later time points showed residual Zr-89 mainly in the liver. Even at 30 d after injection, representing approximately 9 half-lives of Zr-89 and with a total residual activity of only 20-40 kBq in the animal, the image quality was sufficient to readily identify activity in the liver, kidneys, and upper and lower limb joints. Significant differences were noted in late time point liver uptake, bone uptake, and whole-body clearance between chelator-linker types, whereas little variation (10%) was observed within each type. Conclusion: These studies demonstrate the ability to image Zr-89-radiolabeled antibodies up to 30 d after injection while maintaining satisfactory image quality, as provided by the primate mini EXPLORER with high sensitivity and long axial field of view. Quantification demonstrated potentially important differences in the behavior of the 4 chelators. This finding supports further investigation.