Preparation of 6-¹²⁵I-labeled amiloride derivatives

Amiloride and certain of its derivatives are effective inhibitors of Na H antiporters and of epithelial Na channels. We describe a simple method for the preparation of a variety of pharmacologically active 6-iodoamiloride derivatives that are labeled with ¹²⁵I at high specific radioactivity. 6-Dechloroamiloride derivatives (bearing a hydrogen atom instead of the chlorine at the 6 position of the amiloride molecule) are reacted with ¹²⁵ICl, prepared by the oxidation of the iodide in Na¹²⁵I preparations. The ¹²⁵I-labeled derivatives are separated from free ¹²⁵I by anion exchange chromatography, or purified by thin layer chromatography. Both 6-dechloroamiloride and 5-(N-alkyl)-6-dechloroamiloride derivatives can be labeled by this method, with yields varying between 10 and 70%, depending on the ICl concentration and the structure of the 5-N-alkyl group. Efficient radiolabeling at high specific radioactivity also depends on the use of freshly prepared batches of ¹²⁵I. Using carrier-free ¹²⁵I, [¹²⁵I]6-iodoamiloride and [¹²⁵I]6-iodo-5-(N-tert-butyl)amiloride were prepared with yields of 27 and 22%, respectively. Potential applications of the ¹²⁵I-labeled amiloride derivatives include ligand binding and affinity labeling experiments.