Determination of dissociation constants of (99m)Technetium radiopharmaceuticals by capillary electrophoresis

Capillary electrophoresis was applied to investigate pK(a) values of Tc-99m radiotracers used in nuclear medicine. Therefore, the protonation equilibr ia of the carboxyl groups of Tc-99m-mercaptoacetylglycylglycylglycine (Tc-9 9m-MAG(3)) and Tc-99m-ethylenecysteine dimer (Tc-99m-EC) were studied by pH -dependent determination of electrophoretical velocities. Tc-99m-ethylenecy steine dimer diethyl ester (Tc-99m-ECD) was used as a non-protonable standa rd. The capillary electrophoresis system was equipped with a radioactivity detector. Measurements were performed using a pressure-driven capillary zon e electrophoresis which allowed runs even in the low pH range. For the dete rmination of pK(a) values, the electrophoretical velocities of the analytes were referred to the electrophoretical velocities of tetraphenyle arsonium chloride as a positively charged marker. Calculation of pK(a) values was a ccomplished by non-linear curve fitting of both structure-based equilibria equations and sigmoidal decay functions to the experimental data. Tc-99m-MA G(3) was shown to have a carboxyl group pK(a) value of 4.22. The value for the carboxyl groups of Tc-99m-EC is 2.90 (determined by structure-based equ ilibria equations), which represents a common value for both carboxyl group s. By the use of sigmoidal functions, similar values were elucidated. As ex pected, Tc-99m-ECD shows no protonation step. (C) 1999 Elsevier Science B.V . All rights reserved.