The purpose of this article is to review the suitability of the analytical and statistical techniques that have thus far been developed to assess the dissolution behavior of particles in the respirable aerodynamic size range, as generated by orally inhaled products (OIPs) such as metered-dose inhalers and dry powder inhalers. The review encompasses all analytical techniques publicized to date, namely, those using paddle-over-disk USP 2 dissolution apparatus, flow-through cell dissolution apparatus, and diffusion cell apparatus. The available techniques may have research value for both industry and academia, especially when developing modified-release formulations. The choice of a method should be guided by the question(s) that the research strives to answer, as well as by the strengths and weaknesses of the available techniques. There is still insufficient knowledge, however, for translating the dissolution data into statements about quality, performance, safety, or efficacy of OIPs in general. Any attempts to standardize a dissolution method for compendial inclusion or compendial use would therefore be premature. This review reinforces and expands on the 2008 stimulus article of the USP Inhalation Ad Hoc Advisory Panel, which "could not find compelling evidence suggesting that such dissolution testing is kinetically and/or clinically crucial for currently approved inhalation drug products."
Compendial methods determining dry powder inhaler (DPI)-emitted aerosol aerodynamic particle size distribution (APSD) collect a 4-L air sample containing the aerosol bolus, where the flow, which propagates through the cascade impactor (CI) measurement system from the vacuum source, is used to actuate the inhaler. A previous article described outcomes with two CIs (Andersen eight-stage cascade impactor (ACI) and Next-Generation Pharmaceutical Impactor (NGI)) when the air sample volume was ≤4 L with moderate-resistance DPIs. This article extends that work, examining the hypothesis that DPI flow resistance may be a factor in determining outcomes. APSD measurements were made using the same CI systems with inhalers representing low and high flow resistance extremes (Cyclohaler® and HandiHaler® DPIs, respectively). The ratio of sample volume to internal dead space (normalized volume (V*)) was varied from 0.25 to 1.98 (NGI) and from 0.43 to 3.46 (ACI). Inhaler resistance was a contributing factor to the rate of bolus transfer; the higher resistance DPI completing bolus relocation to the NGI pre-separator via the inlet when V* was as small as 0.25, whereas only ca. 50% of the bolus mass was collected at this condition with the Cyclohaler® DPI. Size fractionation of the bolus from either DPI was completed within the ACI at smaller values of V* than within the NGI. Bolus transfer from the Cyclohaler® capsule and from the HandiHaler® to the ACI system were unaffected by the different flow rise time observed in the two different flow controller systems, and the effects the ACI-based on APSD measurements were marginal.
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