Copper-64 Labelled Fingolimod for PET Imaging of the Sphingolipid-1-Phosphate Axis in Prostate Cancer

1123 Objectives The sphingolipid-1-phosphate axis including sphingosine kinase (SPhK) and sphingosine-1-phosphate receptors (S1PRs) is implicated in human cancer development and progression.1 Fingolimod (FTY720), a synthetic sphingosine analogue, is an FDA-approved immunmodulating drug, which has been documented as anti-tumor agent by targeting SPhK/S1PRs.2 In this work, we report the development of copper-64 labelled FTY720 for PET imaging of the sphingolipid signaling axis in prostate cancer. Methods To be labeled with 64Cu, FTY720 was conjugated with a chelator,p-SCN-Bn-NOTA. After the in vitro stability and lipophilicity evaluation of the radiotracer, preliminary small animal PET/CT imaging with 64Cu-NOTA-FTY720 was performed in two prostate cancer xenograft models, the SPhK overexpressing PC-3 and the low SPhK expressing LNCaP. The imaging specificity was tested by a blockade experiment of co-injecting the radiotracer with FTY720. Results The conjugate NOTA-FTY720, was obtained in 50.1% yield and characterized by LC-MS, and 1H-NMR. NOTA-FTY720 was labeled with 64Cu in 94.7% radiochemical yield. After HPLC purification and reconstitution with 8% ethanol in saline, 64Cu-NOTA-FTY720 was obtained with > 99.5% radiochemical purity and a specific activity of 403-806 mCi/μmol. The radiotracer remained > 99.0% intact within 24 h incubation in rat serum and its log P value was 1.07. As expected, the radiotracer showed higher uptake in PC-3 xenografts than that in LnCaP xenografts on in vivo PET/CT images at 24 h. Both tumors’ uptakes in the presence of FTY720 were significantly reduced, indicating that the imaging is specifically targeting sphingolipid-1-phosphate axis. Conclusions A radiotracer for noninvasive assessment of the sphingolipid-1-phosphate axis was successfully developed by derivatizing an FDA-approved immunomodulating drug. The preliminary evaluation of the radiotracer 64Cu-NOTA-FTY720 demonstrated its potential utility in prostate cancer imaging and therapy. Further imaging studies and biological validation are ongoing.