Development and preclinical PET imaging of 68Ga-tin colloid for visualization and quantification of the phagocytic function of Kupffer cells

1618 Objectives: Kupffer cells play a key role in liver immunity, and quantification of the phagocytic function of Kupffer cells could be useful for assessing the mechanisms underlying hepatic diseases. We have developed a simple method for producing 68Ga-tin colloid. The aims of this study were to investigate the optimal conditions for producing the 68Ga-tin colloid and to confirm the usefulness of using this 68Ga-tin colloid in positron emission tomography (PET) for assessing the phagocytic function of Kupffer cells in including non-alcoholic steatohepatitis (NASH) model rats. Methods: To produce the 68Ga-tin colloid, 68Ga solution (1.0 mL), eluted from a 68Ge/68Ga generator, was mixed with tin solution (1.0 mM) commercially available as 99mTc-tin colloid. Sodium acetate solution (NaOAc, 1.0 M, 0.2 mL) was added to adjust the pH to 4.2-4.4. All reactions were performed in glass vials at 25 °C. We changed the labeling time and the mixture volume ratio of the 68Ga solution and the tin solution. The colloidal size was measured by filtrating the obtained solution through three layers of membrane filters (with pore sizes 200, 3000, and 5000 nm). The radioactivity of the filtrate and the filters were measured using a gamma counter. The size change of the colloid was also measured for 0-60 min after pH adjustment. Kupffer cell-depletion was performed by intravenously administering clodronate liposomes in rats. A NASH model was constructed by feeding a methionine- and choline-deficient (MCD) diet to rats for 4 weeks. PET imaging was performed for 30 min after intravenous administration of the optimized 68Ga-tin colloid solution (3 MBq) into the rats. Each Kupffer cell-depleted rat and NASH model rat was twice scanned pre- and post-intervention. The time-activity-curves of the livers, spleens, and blood pool was obtained. The liver uptake was compared between pre- and post-invention PET imaging in each Kupffer cell-depleted rats and NASH model rats. Hematoxylin-eosin (HE) staining and silver impregnation staining were performed to compare the pathological findings between the livers of normal rats and NASH model rats. The data were analyzed with paired samples t-test. Results: The colloidal size increased as the labeling time was longer. The optimal labeling time of these solutions was 30 min. After pH control, the colloidal sizes remained nearly unchanged. The optimal volume ratio of the 68Ga solution and the tin solution was 5:1. More than 90% of the obtained colloid was smaller than 5000 nm. PET imaging of the normal rats revealed that the liver uptake of the 68Ga-tin colloid increased with an increase in the colloidal size. In Kupffer cells-depleted rats, the liver uptake drastically decreased (N=4, P<0.05). NASH model rats showed significantly decreased uptake in the peripheral areas of the livers (N=5, P<0.05). HE and sliver impregnation staining of livers showed fat deposits and mild fibrosis in the NASH model rats. Conclusion: The 68Ga-tin colloid is easily produced by mixing the 68Ga solution and the tin solution. The colloidal size can be altered by changing the time from mixing the 68Ga and tin solution to adding the NaOAc solution. The 68Ga-tin colloid PET enables visualization and quantification of the phagocytic function of Kupffer cells.