Radiosynthesis and evaluation of a fluorine-18 radiotracer [18F]FS1P1 for imaging sphingosine-1-phosphate receptor 1

Assessment of sphingosine-1-phosphate receptor 1 (S1PR1) expression could be a unique tool to determine the neuroinflammatory status for central nervous system (CNS) disorders. Our preclinical results indicate that PET imaging with [(11)C]CS1P1 radiotracer can quantitatively measure S1PR1 expression changes in different animal models of inflammatory diseases. Here we developed a multiple step F-18 labeling strategy to synthesize the radiotracer [(18)F]FS1P1, sharing the same structure with [(11)C]CS1P1. We explored a wide range of reaction conditions for the nucleophilic radiofluorination starting with the key ortho-nitrobenzaldehyde precursor 10. The tertiary amine additive TMEDA proved crucial to achieve high radiochemical yield of ortho-[(18)F]fluorobenzaldehyde [(18)F]12 starting with a small amount of precursor. Based on [(18)F]12, a further four-step modification was applied in one-pot to generate the target radiotracer [(18)F]FS1P1 with 30∼50% radiochemical yield, >95% chemical and radiochemical purity, and a high molar activity (37∼166.5 GBq/μmol, decay corrected to end of synthesis, EOS). Subsequently, tissue distribution of [(18)F]FS1P1 showed a high brain uptake (ID%/g) of 0.48 ± 0.06 at 5 min, and bone uptake of 0.27 ± 0.03, 0.11 ± 0.02 at 5, and 120 min respectively, suggesting no in vivo defluorination. MicroPET studies showed [(18)F]FS1P1 has high macaque brain uptake with a standard uptake value (SUV) of ~2.3 at 120 min. Radiometabolite analysis in nonhuman primate plasma samples indicated that [(18)F]FS1P1 has good metabolic stability, and no major radiometabolite confounded PET measurements of S1PR1 in nonhuman primate brain. Overall, [(18)F]FS1P1 is a promising F-18 S1PR1 radiotracer worthy of further clinical investigation for human use.