''Old labels in new clothes'' - Optimisation of pharmaceutical formulations using radionuclide imaging

The use of gamma-emitting radiopharmaceuticals is confined mainly to nuclear medicine applications. The technique of gamma scintigraphy, however, can provide the biological scientist with a powerful tool with which to monitor the in vivo behaviour of radiolabelled substances (Hardy and Wilson, 1981). Radionuclide imaging is proving to be a valuable technique in pharmaceutical research, where it is used for the optimisation of drug formulations (Wilson and colleagues 1981). Several approaches can be adopted for radiolabelling of drug preparations. The obvious approach is to replace a stable atom with a gamma-emitting isotope. Unfortunately this is rarely practicable, since the majority of drug molecules do not comprise elements having suitable radioisotopes. The second possibility is to add to the preparation a tracer amount of a radiopharmaceutical having similar physicochemical properties to the active ingredient. The efficacy of many pharmaceutical products is influenced by the nature of the dosage form, and the physical characteristics of the active ingredients such as particle size, solubility and viscosity. Often these parameters can be investigated using appropriately radiolabelled model systems. As an alternative to radiolabelling the drug preparation it may be possible to observe the effect of the drug by monitoring changes in the distribution of conventionally administered radiopharmaceuticals. Examples illustrate the diversity of the applications of radiopharmaceuticals in drug formulation studies