Characteristics of radiolabelled versus unlabelled inhaler formulations.

The total and regional deposition patterns of aerosols released from pressurized metered dose inhalers (MDIs) may be determined by gamma scintigraphy. Owing to difficulties in chemically labelling drug molecules themselves, the formulation may be radiolabelled with a suitable gamma-ray-emitting radionuclide (usually 99mTc). Validation measurements must then be performed to check that the drug formulation has not been altered significantly by the radiolabelling process and that the radiolabel acts as an adequate marker for the drug across the full range of particle sizes. These radiolabelling techniques have proved widely applicable, not only to pressurized MDIs, but also to dry powder formulations.