A Relative Permittivity Approach for Fast Drug Solubility Screening of Solvents and Excipients in Lipid-Based Delivery

Drug solubility screening in solvents and lipids is central for the development of lipid-based formulations (LBFs), and any guidance to reduce the experimental workload would be highly desirable. Solubility parameters are interesting as they can be predicted in silico for a drug but they are hardly predictable for complex lipids. This paper uses a new approach to convert an in silico drug solubility parameter to an estimated relative permittivity, ε_r. Diverse solvents and lipid-based excipients were then experimentally tested for ε_r and solubility using fenofibrate as model. The typical excipients and solvents used in LBFs showed an ε_r range of about 2-24, and good solubility of fenofibrate was indeed evidenced in vicinity of its estimated relative permittivity 13.2 ± 2.7. Mixtures of promising excipients were studied subsequently, and the obtained ε_r was predictable based on the known values of the individual components. The novel permittivity approach has demonstrated its usefulness, it has much potential in early development for ranking of suitable excipients, and it gives an initial orientation to design formulations. Future research may clarify further opportunities and limits of the novel approach for LBFs.