Utilizing Flux to Inform Formulation Strategy Decisions in Preclinical Studies

In preclinical settings, flux experiments can serve multiple purposes, such as (1) to select a formulation that provides higher flux, (2) to establish use-times for supersaturating formulations such as spray dried dispersion suspensions, and (3) to explain variation in exposures in in vivo experiments. In the current manuscript, a total of five internal model compounds are shown to portray the scenarios where flux measurements have been used. The overarching aim of this study was to provide a decision-making framework for identifying optimal formulations based on flux. Two different diffusion set ups were explored: 96-well PermeaPad plates and the Pion μFLUX diffusion system. The 96-well PermeaPad plates were initially tested as a drug-sparing method for conducting the flux assay. However, undetectable concentrations in the receiver compartment hindered its application across compounds, needing to pivot to a larger size diffusion set up, using the Pion μFLUX and measuring concentrations by mass spectrometry. This study highlights the advantages and limitations of each diffusion setup. It underscores the importance of amorphous solubility values and MDCK passive permeability in determining whether PermeaPad plates will produce detectable concentrations in the receiver compartment through mass spectrometry. This study displays a good correlation between in vitro flux experiments and in vivo exposures in animal species and shows drawbacks of using dissolution data alone. Lastly, this work emphasizes the importance and potential use of flux assays to screen formulations or to explain formulation differences seen in vivo.