Bioactivity of small technetium complexes

Early evidence that small technetium compounds may be subject to active transport processes was provided by the historical serependipitous finding that pertechnetate was handled by the sodium-iodide symporter in the thyroid gland. The feasibility of using biochemical ^99mTc probes for various targets such as enzymes and particularly receptors is due to the tolerance of the target molecules towards metal-based mimics. As the high in-vitro affinities to various neuroreceptors in the nanomolar and subnanomolar range indicate, molecular recognition of complex technetium molecules has become possible. One main issue in developing CNS receptor imaging agents remains the very low or totally absent brain uptake.
The general approach to specific small technetium radiopharmaceutical tracers has not changed much in recent years. Despite substantial progress in technetium chemistry, the number of newly launched ^99mTc radiopharmaceuticals is stagnant, at least in a short-term perspective. The development of biochemically specific, small technetium and rhenium complexes remains therefore a challenging, rewarding and often frustrating activity.