Whichever of these techniques is chosen, it is necessary to ensure that the label behaves in an aerodynamic manner in the same way as the drug particles. The most rigorous way to do this is to fire the device into a multistage classifier (normally an Andersen sampler with 8-10 stages) and verify that the distribution of radiolabel activity in the various stages is the same as the distribution of drug measured by direct chemical assay. If the label distributes similarly to the drug in the classifier, it is a reasonable assumption that it will do so in vivo, although it is not impossible that differences may still exist. For example, while the classifier operates in normal air, the air in the lung has nearly 100% humidity. This can lead to differences in particle growth rates that are not seen in vivo.

Generally, it is not necessary to perform a measure of label binding, as is the case in oral formulations, since it must be assumed that the micronized drug particle will dissolve instantaneously on contact with the pulmonary mucosa, and absorption of the drug and label will be rapid. Thus in most cases anything other than an immediate measure of drug/ label distribution is of little value. The exception is for formulations such as liposomes and microparticles, in which the delivery agent is postulated to remain intact in the lung for a period of time and hence clearance can be measured17.

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