Rapid preparation of 123I-labelled methylene blue and toluidine blue: potential new agents for parathyroid scintigraphy.

Methylene blue and toluidine blue are phenothiazinium dyes used to localize parathyroid glands visually during surgery. For the purpose of assessing the potential of radiolabelled analogues for scintigraphic localization of parathyroids, a rapid and convenient method has been developed for covalently labelling the dyes with iodine-123. Treatment of aqueous methylene blue or toluidine blue with 123I-sodium iodide in the presence of potassium iodate and hydrochloric acid at 100 degrees C for 1 h results in incorporation of 80-90% of the radioiodine into the dye. The chemical and radiochemical impurities are readily removed by passage through a disposable sample preparation column. Using the optimized protocol, a sterile, pyrogen-free multi-dose solution of radioiodinated dye (pH 7, specific activity greater than or equal to 37 MBq mg-1, radioactivity concentration congruent to 37 MBq ml-1, radiochemical purity greater than 99%, overall activity yield available for injection 60-70%) can be prepared within 1.5 h.