SYNTHESIS AND RADIOLABELING OF -METHOXY-N-(1-AZABICYCLO[2.2.2]OCT-3-YL)BENZAMIDE, THE ACTIVE ENANTIOMER OF [I-125] IODOZACOPRIDE, AND REEVALUATION OF ITS 5-HT3 RECEPTOR AFFINITY

We report an improved synthesis of unlabeled (S)-iodozacopride, the ra diolabeling of (S)-[I-125]iodozacopride via deschloro-(S)-zacopride, a nd a re-evaluation of its affinity for the 5-HT3 receptor, Unlabeled ( S)-iodozacopride was prepared in seven steps from 4-aminosalicylic aci d via alkaline hydrolysis of its 4-acetamide derivative, Catalytic hyd rogenation of (S)-iodozacopride gave deschloro-(S)-zacopride, identica l to that obtained from (S)-3-aminoquinuclidine and 4-amino-2-methoxyb enzoic acid via its corresponding 1-imidazole derivative, Radioiodinat ion to produce (S)-[I-125]iodozacopride was accomplished by treatment of deschloro-(S)-zacopride with 5 mCi sodium (125)iodide and chloramin e-T in hydrochloric acid, Purification of the reaction products using an HPLC system capable of detecting chlorinated side-products revealed a mixture of 2.1 mCi (1.3 nmol) (S)-[I-125]iodozacopride and (S)-zaco pride (1.5 nmol), Saturation analysis of the binding of the purified ( S)-[I-125]iodozacopride to whole rat brain homogenates gave an estimat ed K-D of 1.10 +/- 0.07 nM. As anticipated, this is approximately half the K-D reported for binding of racemic [I-125]iodozacopride, and dif fers from the previously reported value by an order of magnitude. Anal ysis of the apparent binding affinity of a 1:1 mixture of (S)-[I-125]i odozacopride and (S)-zacopride suggests that the previous result may h ave been confounded by contamination of the product with unlabeled (S) -zacopride, Competition analysis of the displacement of (S)-[I-125]iod ozacopride binding by unlabeled (S)-iodozacopride and (S)-zacopride ga ve K-i values of 0.95 and 0.21 nM, respectively.