Development and evaluation of oxadiazole core analogues for S1PR1 PET imaging study

1036 Objectives: Sphingosine-1-phosphate receptor 1 (S1PR1) is an important target for PET imaging of neuroinflammation as well as for therapeutic intervention. A clinically suitable S1PR1 PET radiotracer will be very useful for investigating the changes of S1PR1 expression for inflammatory response in different diseases, such as multiple sclerosis (MS), Parkinson’s disease, and atherosclerosis. Building on our previous efforts on development of PET radiotracers for imaging S1P1, herein, we reported our recent progress on optimization of the lead structure and further evaluation in vitro and in animal models of four new radiotracers for imaging inflammatory response in diseases. Methods: Novel S1PR1 ligands containing a trifluoromethyl group and oxadiazole core were synthesized and characterized. In vitro binding affinities were determined by competing against the binding of [32P]S1P using recombinant human S1P1 membrane. The corresponding precursors of radiotracers with high potency and selectivity for S1PR1 were synthesized with good chemical yield. The [18F]trifluoromethyl radiosynthesis was realized by employing a two-step one-pot procedure. The first step involved the [18F]trifluoromethylation incorporation by using [18F]CF3Cu generated in situ from methyl chlorodifluoroacetate, CuI, TMEDA and [18F]fluoride with the aryl iodide precursor (150 oC, 20 min), followed by removal of protecting groups with 4 M hydrochloride acid aqueous solution (150 µL, 110 oC, 5 min), In addition, other 18F-fuorobenzyl alkyl alcohol analogues were synthesized using conventional nucleophilic reaction to introduce the F-18 isotope. Each of the four F-18 labeled radiotracers were purified using reverse phase semi-preparative HPLC, and then formulated using 10% of ethanol in saline. Results: Four oxadiazole core containing new fluorous compounds were synthesized in moderate to good yield. The S1PR1 binding assay results showed these four compounds had binding affinity less than 20 nM and were selective for S1PR1 over S1PR2-5. The F-18 labeled radiotracers were successfully achieved with good yield. The specific activity of the [18F]trifluoromethyl phenyl containing radiotracers were moderate although the specific activity of 18F-fuorobenzyl alkyl alcohol radiotracers are high (>10 mCi/µM) . Conclusions: A series of new oxadiazole S1PR1 specific analogues was designed and synthesized. The in vitro competitive binding affinity assay showed four compounds exhibited high binding potency towards S1PR1 (IC50