Carrier electrochemistry of pertechnetate: Application to radiopharmaceutical labelling by controlled potential electrolysis at chemically inert electrodes

Abstract

A method is reported for labelling radiopharmaceuticals with 99 m Tc by direct reduction of 99 m TcO 4 − at chemically inert electrodes without contamination of the agent by any reducing agent. This is distinct from previous electrolytic labelling methods, in which reduction of TcO 4 − was mediated by chemically reactive tin or zirconium species produced from the electrode metal. From polarographic and voltammetric measurements using carrier concentrations of the long lived isotope 99Tc, appropriate ranges of electrode potential for electrolytic reduction were determined for platinum, gold, mercury and glassy carbon electrodes in a variety of complexing and non-complexing media. The reduction potentials proved to depend only on the pH and the electrode composition and not on the presence of complexing agents. Thus from polarograms or voltammograms of the substrate to be labelled in buffers of varying pH one can determine whether or not there exists a combination of electrode potential and pH for which pertechnetate is reduced but not the substrate. In alkaline media pertechnetate was reduced at approximately −0.8 V (referred to the saturated calomel electrode) for all four electrodes. In acid media reduction occurred more readily (at less negative potentials) at mercury than at gold, platinum, or glassy carbon. In acid media the reduction potential depended on pH for all four electrodes and reduction occurred concurrently with hydrogen evolution on platinum and gold. Mercury was found to have a much wider potential range for electrolytic reduction of technetium than did platinum, gold or glassy carbon at any pH. From these data reaction conditions were predicted and experimentally confirmed for the electrolytic labelling of tetracycline and EDTA with carrier free 99 m Tc.