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The effect of carrier surface properties on drug particle detachment from carrier crystals during inhalation with a special test inhaler with basic air classifier has been studied for mixtures containing 0.4% budesonide. Carrier crystals were retained in the classifier during inhalation and subsequently examined for the amount of residual drug (carrier residue: CR). Carrier surface roughness and impurity were varied within the range of their appearance in standard grades of lactose (Pharmatose 80, 100, 110, 150 and 200 M) by making special sieve fractions. It was found that roughness and impurity, both per unit calculated surface area (CSA), tend to increase with increasing mean fraction diameter for the carrier. Drug re-distribution experiments with two different carrier sieve fractions with distinct mean diameters showed that the amount of drug per CSA (drug load) in the state of equilibrium is highest for the coarsest fraction. This seems to confirm that surface carrier irregularities are places where drug particles preferentially accumulate. However, a substantial increase in surface roughness and impurity appears to be necessary to cause only a minor increase in CR at an inspiratory flow rate of 30 l/min through a classifier. At 60 l/min, CR is practically independent of the carrier surface properties. From the difference in CR between 30 and 60 l/min, it has been concluded that particularly the highest adhesive forces (for the largest drug particles) in the mixture are increased when coarser carrier fractions (with higher rugosity) are used. Not only increased surface roughness and impurities may be responsible for an increase in the adhesive forces between drug and carrier particles when coarser carrier fractions are used, but also bulk properties may play a role. With increasing mean carrier diameter, inertial and frictional forces during mixing are increased too, resulting in higher press-on forces with which the drug particles are attached to carrier crystals and to each other.