Lactose as a carrier in dry powder formulations: The influence of surface characteristics on drug delivery

The aim of the study was to investigate the interdependence of carrier particle size, surface treatment of the carrier, and inclusion of fines on the drug delivery from dry power inhaler formulations. Two size fractions (<63 and 63-90 mum) of alpha -lactose monohydrate were subjected to treatment with 95% (v/v) ethanol to introduce small asperities or cavities onto the otherwise smooth surface without substantially changing the particle shape. After blending with albuterol sulfate [ALB; volume median diameter (VMD), 1.9 mum; geometric standard deviation (GSD), 1.5], the solvent-treated lactose produced a fine particle fraction (FPF; <6.18 mum) and dispersibility of the drug that was significantly (ANOVA p <0.01) lower than that which resulted from formulations containing untreated lactose of a similar size fraction, after aerosolization at 60 L min(-1) via a Rotahaler. The two size fractions of the treated lactose resulted in similar deposition profiles of ALB. The effects of such surface asperities or cavities of lactose were offset by introducing a small amount (5% w/w) of smaller-sized lactose (5-10 mum) to the powder formulations. The fine lactose increased the FPF and dispersibility of ALB to such a level that all lactose batches, regardless of particle size or whether solvent treated, produced a similar fraction of aerosolized ALB. The inclusion of recrystallized needle lactose (5-15 mum) was superior to micronized lactose in improving the aerosolization of ALB. The findings of this study indicate that the presence and characteristics of the finer fraction of lactose carrier particles dominate over the particle size and surface smoothness of the carrier particles in determining dispersion and deaggregation of drugs from dry powder formulations for inhalation. (C) 2001 Wiley-Liss, Inc. and the American Pharmaceutical Association.