A study on the labeling of o-iodohippuric acid with 123I

Recent emphasis on the replacement of 131I with 123I, in the ionic form and incorporated into organ and function specific compounds, prompted an investigation of the radioiodination of the renal function and imaging agent, o-iodohippuric acid. One of the more rapid and efficient methods is one in which the o-iodohippuric acid, in the molten state, is labeled with 123I. The effects of volume and chemical composition of 123I solutions on the labeling efficiency of this “melt” method were determined. The results showed that a concentration of NaOH greater than 0.6 molecular equivalent (meq) per 200 mg of o-iodohippuric acid caused a rapid decrease in labeling efficiency resulting from a change in the pH of the reaction mixture from acidic to basic. Adjustment of the pH to below approximately 5 before the reaction resulted in quantitative labeling yields. When the 123I was in saline, the yield was independent of the volume of saline. As a result of these studies, a method for fast and efficient labeling of o-iodohippuric acid with 123I was developed and is described.