Synthesis and initial in vivo evaluation of F-18 radioligands for S1PR1 PET imaging

Introduction: Sphingosine-1-phosphate receptor 1 (S1PR1) is an important biomarker of neuroinflammation. A clinically suitable S1PR1 PET imaging tracer will be powerful for investigating S1PR1 expression due to inflammatory responses in different diseases such as multiple sclerosis (MS), Parkinson’s disease and atherosclerosis. Building on our previous development of PET radiotracers for imaging S1PR1, we developed new S1PR1 specific PET radioligands with improved properties.

Methods: Sixteen novel compounds bearing an oxadiazole core were designed, synthesized and characterized. Competitive binding assays against [³²P]S1P were performed to determine binding affinities of these compounds toward S1PR1 and binding specificity for S1PR1 over S1PR2-5. The corresponding precursors for three potent ligands were synthesized and radiolabeled with fluorine-18. These radiotracers then were evaluated with microPET imaging using a Focus 220 microPET scanner to collect dynamic scans after intravenous injection into male macaques. Biodistribution studies were conducted in Sprague-Dawley rats euthanized at 5, 30, and 60 min post intravenous radiotracer injection.

Results: Sixteen fluorine-containing compounds were synthesized in moderate to good yields. The in vitro binding assay results indicated that three of these compounds (TZ8251, TZ8252, and TZ82112) had binding affinities of less than 10 nM for S1PR1 and were selective for S1PR1 over S1PR2-5. The F-18 labeling for these promising candidates were successfully achieved with good radiochemical yields (22~35%), high chemical and radiochemical purity (>95%), and high molar activity ((>37 GBq/mmol, decay corrected to EOS). MicroPET imaging revealed that [¹⁸F]TZ8251, and [¹⁸F]TZ8252 achieved a maximum radioactivity uptake in non-human primate brain of 1.5, and 2.3 SUV at 120 min, respectively, while the [¹⁸F]TZ82112 showed the highest peak brain uptake of 3.1 SUV at 7 min, followed by a favorable slow radioactivity washout. Biodistribution studies indicated that [¹⁸F]TZ82-112 had a good brain uptake up to 0.68~0.93 %ID/gram from 5 to 60 min. No defluorination was observed, evidenced by no increase of the bone uptake.

Conclusions: Out of sixteen new S1PR1 compounds, three promising candidates were successfully radiolabeled with fluorine-18, named [¹⁸F]TZ8251, [¹⁸F]TZ8252, and [¹⁸F]TZ82112. The initial evaluation by microPET imaging study in non-human primate and biodistribution in SD rat demonstrated that [¹⁸F]TZ82112 had excellent blood-brain barrier penetration, favorable radioactivity kinetics and did not defluorinate in vivo. Further characterization in neuroinflammatory disease models and radiometabolism studies are warranted for clinical applications in humans.