Traditional test methods for the aerodynamic particle size distribution (APSD) measurements of orally inhaled drug products (OIDPs) utilise cascade impactors, due to a number of unique features (see our other animation for how these work). One drawback of cascade impactors is that in order to operate correctly they require a steady, fixed volumetric flow rate to be used. Real patients do not inhale with a constant flow rate, so for routine cascade impactor testing a compromise is reached whereby a constant flow rate is selected, being broadly representative of the mid-inhalation flow rate rate achieved during inhalation. The problem is that the performance of some OINDPs, such as dry powder inhalers (DPIs) can be sensitive to flow rate acceleration, the peak inspiratory flow rate (PIFR) and total inhalation volume. The animation "System for Improved In Vitro-In Vivo Correlations (IVIVCs) of Inhaled Drug Products" explains how, through the use of the Mixing Inlet and a breath simulator, a realistic patient breath profile can be generated through the inhaler, whilst a constant flow rate can be maintained through the cascade impactor. The smooth merging of the two flow regimes is achieved using the Mixing Inlet, ensuring that the aerosol cloud generated by the inhaler, under patient realistic conditions, is not modified as it passes into the cascade impactor at a constant flow rate for subsequent size fractionation and analysis. Such a system provides an alternative to the traditional USP and Ph. Eur. based systems, which were primarily conceived for quality control (QC) purposes, allowing the acquisition of more patient relevant data, in-vitro, during research and development (R&D). This has particular value when undertaking bioequivalence testing for generic products. The animation also features the adult version of the Alberta Idealised Throat (AIT), which is an improved representation of the human throat geometry, based on considerable academic research. This replaces the simplified, right-angled induction port used in traditional test methods. We hope you enjoy it! Please note the animation is silent throughout.