Dry powder inhalers: Mechanistic evaluation of lactose formulations containing salbutamol sulphate

The purpose of this study was to evaluate the relationships between physicochemical properties and aerosolisation performance of different grades of lactose. In order to get a wide range of physicochemical properties, various grades of lactose namely Flowlac®100 (FLO), Lactopress anhydrous®250 (LAC), Cellactose®80 (CEL), Tablettose®80 (TAB), and Granulac®200 (GRA) were used. The different lactose grades were carefully sieved to separate 63-90 μm particle size fractions and then characterised in terms of size, shape, density, flowability, and solid state. Formulations were prepared by blending each lactose with salbutamol sulphate (SS) at ratio of 67.5:1 (w/w), and then evaluated in terms of SS content uniformity, lactose-SS adhesion properties, and in vitro aerosolisation performance delivered from the Aerolizer®. Sieved lactose grades showed similar particle size distributions (PSDs) and good flow properties but different particle shape, particle surface texture, and particle solid state. Content uniformity assessments indicated that lactose particles with rougher surface produced improved SS homogeneity within DPI formulation powders. Lactose-SS adhesion assessments indicated that lactose particles with more elongated shape and the rougher surface showed smaller adhesion force between lactose and salbutamol sulphate. Lactose powders with higher bulk density and higher tap density produced smaller emission (EM) and higher drug loss (DL) of SS. In vitro aerosolisation for various lactose grades followed the following rank order in terms of deposition performance: GRA > TAB > LAC ≈ CEL > FLO. Linear relationships were established showing that in order to maximize SS delivery to lower airway regions, lactose particles with more elongated shape, more irregular shape, and rougher surface are preferred. Therefore, considerable improvement in DPI performance can be achieved by careful selection of grade of lactose included within DPI formulations.

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