Excipient-API interactions in dry powder inhalers

There remains a paucity of predictive models to evaluate the suitability of excipients or excipient mixtures for dry powder inhalers because a large number of interdependent variables affect both formulation and inhaler performance. The problem is compounded by empirical studies that are performed under different experimental conditions which make data comparison difficult. An easily calculable molecular parameter, the Parachor, relates structural constants to surface tension. When applied in conjunction with results obtained from inverse gas chromatography, the Parachor can be used to calculate adhesive and cohesive surface energies between excipients and active pharmaceutical ingredients. Values calculated from the Parachor are consistent with qualitative hypotheses and agree reasonably well with published quantitative results. Theability to both achieve and predict the free particle fraction from Parachor derived surface energy data represents a new paradigm worthy of further perusal.