Acid-base equilibria of 2,4-diiodo-6-methylphenylearbamoylmethyl iminodiacetic acid and its labeling with technetium-99m

Abstract

The acid-base equilibria of a novel hepatobiliary imaging agent, 2,4-diiodo-6-methylphenylcarbamoylmethyl iminodiacetic acid (DIIODIDA) were studied. The potentiometrically determined acidity constants of the second carboxylic group, amino and amide groups were pK(2) = 2.52 +/- 0.02; pK(3) = 5.86 +/- 0.06 and pK(4) = 10.9 +/- 0.1. The determinations were performed at 25 degrees C and an ionic strength of 0.1 mol/dm(3) (NaCl). The acidity constants (pK(1) = 1.3 +/- 0.4) corresponding to the first carboxylic group was determined indirectly, on the basis of equilibrium constants obtained in a heterogeneous system, at 25 degrees C and an ionic strength 1 mol/dm(3) (HCl, NaCl). DIIODIDA was labeled with teclinctiurn-99m, and the influence of pH on the yield of labeling was investigated. It was found that labeling within the pH range from 5.5 to 6.5 provided a radiopharmaceutical of high radiochemical purity ( GT 98%).