The physico-chemical basis of radiolabelling metered dose inhalers with 99mTc

Over the past decade, a number of different approaches have been proposed for the radiolabelling of metered dose inhalers (MDIs) in order to permit gamma scintigraphic imaging of emitted aerosols following inhaled administration. More recently, methods have been described for the indirect 99mTc-labelling of drugs incorporated into commercial MDI products. This paper attempts to rationalise such methods from a surface chemical perspective in order to: I) elucidate the mechanism of association of the radiolabel and drug; 2) predict the appropriateness of the techniques to all MDI products and; 3) propose in vitro testing methods to validate the labelling efficiency prior to in vivo evaluation.