Design, synthesis, and biological evaluation of multiple F-18 S1PR1 radiotracers in rodent and nonhuman primate.

Here we report our design and synthesis of 28 new fluorine-containing compounds as potential F-18 radiotracers for CNS imaging of sphingosine-1-phosphate receptor 1 (S1PR1), and determination of their in vitro binding potency and selectivity toward S1PR1 over other S1PR subtypes. Nine potent and selective compounds, 7c&d, 9a&c, 12b, 15b, and 18a-c with IC(50) values ranging from 0.6-12.3 nM for S1PR1 and weak binding toward S1PR2, 3, 4, and 5, were further (18)F-radiolabeled to produce [(18)F]7c&d, [(18)F]9a&c, [(18)F]12b, [(18)F]15b, and [(18)F]18a-c. Multi-step F-18 radiochemistry procedures were investigated for radiosynthesis of [(18)F]7c&d and [(18)F]9a&c, and the presumed intermediates were synthesized and authenticated by analytic HPLC. We then performed nonhuman primate (NHP) PET brain imaging studies for eight radiotracers: [(18)F]7c&d, [(18)F]9a, [(18)F]12b, [(18)F]15b, and [(18)F]18a-c. Three radiotracers, [(18)F]7c, [(18)F]7d, and [(18)F]15b, had high NHP brain uptake with standardized uptake values (SUVs) at 2 h post-injection of 2.42, 2.84, and 2.00, respectively, and good brain retention. Our ex vivo biodistribution study in rats confirmed [(18)F]7d had a high brain uptake with no in vivo defluorination. Radiometabolic analysis of [(18)F]7c and [(18)F]7d in rat plasma and brain samples found that [(18)F]7c has a more favorable metabolic profile than [(18)F]7d. However, the trend of increased brain uptake precludes [(18)F]7c as a suitable PET radiotracer for imaging S1PR1 in the brain. Further structural optmization is warranted to identify a highly S1PR1-specific radiotracer with rapid brain uptake kinetics.