Synthesis and preliminary evaluation of [ ¹⁸F]-labeled 2-oxoquinoline derivatives for PET imaging of cannabinoid CB ₂ receptor

Introduction: The cannabinoid receptor type 2 (CB ₂) is an important target for development of drugs and imaging agents for diseases, such as neuroinflammation, neurodegeneration and cancer. Recently, we reported synthesis and results of in vitro receptor binding of a focused library of fluorinated 2-oxoquinoline derivatives as CB ₂ receptor ligands. Some of the compounds demonstrated to be good CB ₂-specific ligands with Ki values in the nanomolar to subnanomolar concentrations; therefore, we pursued the development of their ¹⁸F-labeled analogues that should be useful for positron emission tomography (PET) imaging of CB ₂ receptor expression. Here, we report the radiosynthesis of two ¹⁸F-labeled 2-oxoquinoline derivatives and the preliminary in vitro and ex vivo evaluation of one compound as a CB ₂-specific radioligand. Methods: 4-[ ¹⁸F]fluorobenzyl amine [ ¹⁸F]-3 was prepared by radiofluorination of 4-cyano-N,N,N-trimethylanilinium triflate salt followed by reduction with LiAlH ₄ and then coupled with acid chlorides 11 and 12 to afford [ ¹⁸F]-13 and [ ¹⁸F]-14. In vitro CB ₂ receptor binding assay was performed using U87 cells transduced with CB ₂ and CB ₁ receptor. Ex vivo autoradiography was performed with [ ¹⁸F]-14 on spleen and on CB ₂- and CB ₁-expressing and wild-type U87 subcutaneous tumors grown in mice. Results: The radiochemical yields of [ ¹⁸F]-13 and [ ¹⁸F]-14 were 10%-15.0% with an average of 12% (n=10); radiochemical purity was >99% with specific activity 1200 mCi/μmol. The dissociation constant Kd for [ ¹⁸F]-14 was 3.4 nM. Ex vivo autoradiography showed accumulation of [ ¹⁸F]-14 in the CB ₂-expressing tumor. Conclusion: Two new [ ¹⁸F]-labeled CB ₂ ligands have been synthesized. Compound [ ¹⁸F]-14 appears to be a potential PET imaging agent for the assessment of CB ₂ receptor expression; however, poor solubility restrain its use in vivo.