The influence of carrier on 99mTc radiopharmaceuticals.

High specific activity 99mTc-labelled radiopharmaceuticals are required in order to avoid saturating receptor sites and to minimise pharmacologic or toxic effects. The specific activity of 99mTc-pertechnetate is maximised by use shortly after elution from a generator which had been eluted at frequent intervals. Effective specific activity can be maximised by a variety of means. Often is it possible to label a very small amount of precursor with a large amount of 99mTc and use the product without further purification; this is limited by the potency of the ligand and the efficiency of labelling, which in turn is affected by the choice of chelator. A variety of purification techniques have been used, ranging from solvent extraction, solid-phase extraction cartridges, and size-exclusion columns to high-pressure liquid chromatography. Excess unchelated thiol-containing ligands (e.g. N2S2, N3S) can be removed by a thiol-trapping resin. Finally, solid-phase synthesis, in which the precursor is immobilised on a solid support (resin or gold) and only released into solution during chelation of 99mTc, is a promising method. High specific activity will become increasingly important with the next generation of 99mTc radiopharmaceuticals.