Radiolabeling of a cyclic RGD (cyclo Arg‐Gly‐Asp‐d‐Tyr‐Lys) peptide using sodium hypochlorite as an oxidizing agent

A simple and rapid nonradioactive iodide labeling/radiolabeling method for peptides, using an inexpensive oxidizing agent such as sodium hypochlorite and a cyclic peptide, cRGDyK (cyclo Arg‐Gly‐Asp‐d‐Tyr‐Lys), was developed in this work. Labeling reaction was optimized by conducting experiments under variable ratios of the reagents, the reaction times, and the pH. The study demonstrated that radiolabeling of the cyclic peptide was fast and pH independent. Monoiodinated and di‐iodinated cRGDyK were formed under all conditions and varied with the ratio of the reagents and the reaction time. Total percent of the iodinated cRGDyK (monoiodinated and di‐iodinated cRGDyK) varied between 44 and 100 depending on the reaction conditions. Excess cyclic peptide over equal molar ratio of sodium iodide and sodium hypochlorite yielded in predominant amounts of monoiodinated cRGDyK, ie, >60% under 2:1:1 ratio and ~88% under 5:1:1 ratio of cRGDyK:sodium iodide:sodium hypochlorite.