Evaluation of metered dose inhaler spray velocities using phase Doppler anemometry (PDA)

Droplet velocity is an important parameter which can significantly influence inhalation drug delivery performance. Together with the droplet size, this parameter determines the efficiency of the deposition of MDI products at different sites within the lungs. In this study, phase Doppler anemometry (PDA) was used to investigate the instantaneous droplet velocity emitted from MDIs as well as the corresponding droplet size distribution. The nine commercial MDI products surveyed showed significantly different droplet velocities, indicating that droplet velocity could be used as a discriminating parameter for in vitro testing of MDI products. The droplet velocity for all tested MDI products decreased when the testing distance was increased from 3 cm to 6 cm from the front of mouthpiece, with CFC formulations showing a larger decrease than HFA formulations. The mean droplet diameters of the nine MDIs were also significantly different from one-another. Droplet size measurements made using PDA (a number-based technique) could not be directly compared to results obtained using laser light scattering measurements (a volume-based technique). This work demonstrates that PDA can provide unique information useful for characterizing MDI aerosol plumes and evaluating MDI drug delivery efficiency. PDA could also aid the evaluation of in vitro equivalence in support of formulation or manufacturing changes and in evaluation of abbreviated new drug applications (ANDAs) for MDIs.