Performance-Modifying Influences in Dry Powder Inhalation Systems

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Abstract
Although dry powder inhaler (DPI) systems offer many advantages over conventional pressurized metered dose aerosols for therapeutic use, there are a number of formulation-related aspects of functionality which can significantly reduce DPI performance. A general introduction is provided to the physical requirements for preparation of efficient and effective therapeutic aerosols and dry powder inhaler systems in particular. Of a number of important DPI design factors highlighted in the introduction, the present study concentrates on two specific ex vivo performance-modifying influences of particle characteristics used in dry powder inhalers. Firstly, powder entrainment characteristics were studied using a model system and it was found that particle-entrainment tube (device) interactions were of two main types, depending on whether coarse or fine lactose particles were involved. In the case of coarse particles (in the range 90–180 μm), entrainment was found to depend on mean linear air velocity (in the range 5–15 ms−1) and tube diameter. For fine particles (in the range 63–90 μm), entrainment was found to be dependent on both tube diameter and pressure drop at the site of entrainment. Further, in the case of lactose carrier particles > 90 μm, entrainment was found to be complete in a small velocity spread, unlike the behavior of finer particles (< 90 μm). Secondly, powder blend homogeneity/stability was found to be a function of the number and nature of the contacting surfaces. Optimum homogeneity of lactose/salbutamol blends was achieved with single contact surface blending. Blends were destabilized by contact with dissimilar second contact surfaces.

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