Suspension versus solution metered dose inhalers: different products, different particles?

More than 50 years of research in the field of inhaler design has generated a plethora of portable pressurised metered dose inhalers (pMDIs). During this period innovative formulation and device design has not only increased the diversity of pMDIs that are available, it has also improved their efficiency in targeting drug to the lung. Through this research a greater understanding of drug-propellant interactions has evolved. In this article the manner in which propellant-drug interactions have been elucidated will be explored and the impact on product performance exemplified. In a context of suspension and solution systems, the differences between currently available products will be highlighted and the potential consequences for product equivalence in terms of pharmacokinetic and pharmacodynamic responses in patients considered.