Radiosynthesis and Biodistribution of (123)I-Labeled Antagonists of the Histamine H3 Receptor as Potential SPECT Ligands

4-lodo-N-(2-morpholinoethyl)benzamide (1) is a new benzamide that is an analogue of the antidepressant moclobemide. The synthesis of (1) is described and the radiolabelling conditions with Na123I and Na125I were optimized using the Cu(I)-added exchange labelling reaction. The reaction was found to perform best in the presence of Cu+ and a stannous reducing agent, in the absence of Cu2+ and potassium iodide, and at [H+ ] == 1.8-7.9 mM with a ligand (1) concentration = 2.6-5.6 mg/mL cold kit. Above a [H+ ] of 7.9 mM, the hydrolysis of (1) gave 4-iodo[125I]benzoic acid in high amounts. The radiochemical conversion was routinely >95% and >98% after anion exchange Sep-Pak treatment. The radiolabelled product is stable at room temperature for at least 4 h.