Highly loaded interactive mixtures for dry powder inhalers: Prediction of the adhesion capacity using surface energy and solubility parameters

In order to correlate drug adhesion properties of a highly loaded interactive mixture for the use in dry powder inhalers with the surface energy and to establish a link to the solubility parameter, surface free energy was detected for micronized substances (salbutamol sulfate, salbutamol base, theophylline and α-lactose monohydrate) using inverse gas chromatography (IGC). Interactive mixtures with coarse crystalline α-lactose monohydrate as a carrier were prepared at loading levels from 7.5 to 20% (w/w) and analyzed with respect to their adhesion capacity (CA) using the air jet sieving method. Solubility parameters were taken from literature or calculated. As a result the CA was independent of the drug load and correlated linearly with volume specific surface energy interaction (SEIV) values of the adherents (R2 = 0.98498). A link between SEIV and the size normalized solubility parameter (δtot/d50) was found. Consequently, plotting δtot/d50 versus CA resulted also in a strong linear relationship (R2 = 0.99140). Overall a powerful tool was established to judge and quantify adhesion properties of highly loaded interactive mixtures even for estimates in early preformulation at a time where just the molecular structure of the active ingredient is known.