Perspective On Research Since The Early 1990s

There has been significant research in the area of particle size characterization of pharmaceutical aerosols since the early 1990s. The focus has been not only on methods but on data presentation, analysis, and requirements for regulatory submissions.

A series of review articles were published by the Inhalation Technology Focus Group of the American Association of Pharmaceutical Scientists describing the range of methods that have been employed to measure the particle size of pharmaceutical inhalation aerosols. These methods include: microscopy [138], impingers [139], impactors [140], holography [141], laser light techniques, including right-angle light scattering [142], time-of-flight [143], diffraction [144], and phase Doppler anemometry [145]. New instruments have been developed, including the Next Generation Inertial Impactor, which has been designed for ease of use [56]. With regard to laser optical techniques, one of the most significant advances is the use of Mie theory in the algorithm for laser diffraction rather than Fraunhofer, which had been employed for almost two decades. Nevertheless, the debate surrounding the appropriateness of the methods, diffraction, time-of-flight, phase Doppler analysis, etc. continues. It is as relevant now as in early '90s to remind the reader of the complementary nature of all sizing methods. There is a need for regulatory and pharmacopoeial standards on which to base the quality of the product, but for development purposes the more approaches that are used to evaluate the product the better is the understanding of its true nature.

Data presentation had rarely been the subject of scrutiny in the years prior to the previous edition of this book. However, in the intervening years it has become clear that no single mathematical fit approximates all particle size distributions [146]. Consequently, the suitability of a log-normal distribution must be assessed prior to deriving the standard parameters of median diameter and geometric standard deviation to describe the data. Moreover, if there is no good mathematical fit to the data, alternative analyses that do not invoke a model may be more appropriate. Under these circumstances, simply stating a fine particle mass (fraction) in addition to the median diameter or presenting the data in blocks of size ranges may be more meaningful. Indeed, the latter approach may be less misleading to any reviewer than using descriptors of a distribution that may be a poor approximation to the raw data.

Following the debate surrounding sampling by inertial impaction in the early 1990s, the apparatus required for propellant-driven metered-dose inhalers and dry powder inhalers has been specified by the USP [33] and EP [147]. In addition, the FDA has issued guidelines on the methods to be employed for both pulmonary and nasal delivery products [148].

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