Characterization of a S1PR2 specific ¹¹C-labeled radiotracer in streptozotocin-induced diabetic murine model

Background: Diabetes mellitus is a chronic progressive metabolic disorder that affects millions of people worldwide. Emerging evidence suggests the important roles of sphingolipid metabolism in diabetes. In particular, sphingosine-1-phosphate (S1P) and S1P receptor 2 (S1PR2) have important metabolic functions and are involved in several metabolic diseases. In diabetes, S1PR2 can effectively preserve β cells and improve glucose/insulin tolerance in high-fat diet induced and streptozotocin (STZ)-induced diabetic mouse models. We previously developed a group of potent and selective S1PR2 ligands and radioligands. Methods: In this study, we continued our efforts and characterized our leading S1PR2 radioligand, [¹¹C]TZ34125, in a STZ-induced diabetic mouse model. [¹¹C]TZ34125 was radiosynthesized in an automated synthesis module and in vitro saturation binding assay was performed using recombinant human S1PR2 membrane. In vitro saturation autoradiography analysis was also performed to determine the binding affinity of [¹¹C]TZ34125 against mouse tissues. Type-1 diabetic mouse model was developed following a single high dose of STZ in C57BL/6 mice. Ex vivo biodistribution was performed to evaluate the distribution and amount of [¹¹C]TZ34125 in tissues. In vitro autoradiography analysis was performed to compare the uptake of [¹¹C]TZ34125 between diabetic and control animals in mouse spleen and pancreas. Results: Our in vitro saturation binding assay using [¹¹C]TZ34125 confirmed [¹¹C]TZ34125 is a potent radioligand to recombinant human S1PR2 membrane with a K_d value of 0.9 nM. Saturation autoradiographic analysis showed [¹¹C]TZ34125 has a K_d of 67.5, 45.9, and 25.0 nM to mouse kidney, spleen, and liver tissues respectively. Biodistribution study in STZ-induced diabetic mice showed the uptake of [¹¹C]TZ34125 was significantly elevated in the spleen (~2 fold higher) and pancreas (~1.4 fold higher) compared to normal controls. The increased uptake of [¹¹C]TZ34125 was further confirmed using autoradiographic analysis in the spleen and pancreases of STZ-induced diabetic mice, indicating S1PR2 can potentially act as a biomarker of diabetes in pancreases and inflammation in spleen. Future mechanistic analysis and in vivo quantitative assessment using non-invasive PET imaging in large animal model of diabetes is worthwhile. Conclusions: Overall, our data showed an increased uptake of our lead S1PR2-specific radioligand, [¹¹C]TZ34125, in the spleen and pancreases of STZ-induced diabetic mice, and demonstrated [¹¹C]TZ34125 has a great potential for preclinical and clinical usage for assessment of S1PR2 in diabetes and inflammation.