Synthesis and radiolabeling of (S)-4-amino-5-iodo-2-methoxy-N-(1-azabicyclo[2.2.2]oct-3-yl)benzamide, the active enantiomer of [125I]iodozacopride, and re-evaluation of its 5-HT3 receptor affinity.
We report an improved synthesis of unlabeled (S)-iodozacopride, the radiolabeling of (S)-[125I]iodozacopride via deschloro-(S)-zacopride, and a re-evaluation of its affinity for the 5-HT3 receptor. Unlabeled (S)-iodozacopride was prepared in seven steps from 4-aminosalicylic acid via alkaline hydrolysis of its 4-acetamide derivative. Catalytic hydrogenation of (S)-iodozacopride gave deschloro-(S)-zacopride, identical to that obtained from (S)-3-amino-quinuclidine and 4-amino-2-methoxybenzoic acid via its corresponding 1-imidazole derivative. Radioiodination to produce (S)-[125I]iodozacopride was accomplished by treatment of deschloro-(S)-zacopride with 5 mCi sodium 125iodide and chloramine-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)-[125I]iodozacopride and (S)-zacopride (1.5 nmol). Saturation analysis of the binding of the purified (S)-[125I]iodozacopride to whole rat brain homogenates gave an estimated KD of 1.10 +/- 0.07 nM. As anticipated, this is approximately half the KD reported for binding of racemic [125I]iodozacopride, and differs from the previously reported value by an order of magnitude. Analysis of the apparent binding affinity of a 1:1 mixture of (S)-[125I]iodozacopride and (S)-zacopride suggests that the previous result may have been confounded by contamination of the product with unlabeled (S)-zacopride. Competition analysis of the displacement of (S)-[125I]iodozacopride binding by unlabeled (S)-iodozacopride and (S)-zacopride gave Ki values of 0.95 and 0.21 nM, respectively.
Hewlett
W A
WA
Department of Psychiatry, Vanderbilt University School of Medicine, Nashville, TN 37232, USA.
de Paulis
T
T
Mason
N S
NS
Schmidt
D E
DE
Trivedi
B L
BL
Zhang
Z J
ZJ
Ebert
M H
MH
D010131:Aminosalicylic Acid / Q000737:chemistry; D000818:Animals; D001549:Benzamides / Q000138:chemical synthesis* / Q000378:metabolism*; D001667:Binding, Competitive; D001921:Brain / Q000378:metabolism; D019086:Bridged Bicyclo Compounds, Heterocyclic / Q000138:chemical synthesis* / Q000378:metabolism*; D002851:Chromatography, High Pressure Liquid; D007457:Iodine Radioisotopes; D007553:Isotope Labeling; D008297:Male; D051381:Rats; D017207:Rats, Sprague-Dawley; D011985:Receptors, Serotonin / Q000378:metabolism*; D044406:Receptors, Serotonin, 5-HT3; D012702:Serotonin Antagonists / Q000138:chemical synthesis* / Q000378:metabolism; D013237:Stereoisomerism
D001549:Benzamides; D019086:Bridged Bicyclo Compounds, Heterocyclic; D007457:Iodine Radioisotopes; D011985:Receptors, Serotonin; D044406:Receptors, Serotonin, 5-HT3; D012702:Serotonin Antagonists; D010131:Aminosalicylic Acid; C055971:zacopride