Over the past decade, the glass formation has become a very attractive strategy to increase solubility and thus bioavailability of active pharmaceutical ingredients (APIs). Nevertheless, despite the advantages coming from the amorphous state, the commercial production of amorphous APIs is still limited. The reason lies in the ability of disordered materials to spontaneously self-arrange under the storage. Therefore, lots of the current research is dedicated to identification and understanding of the critical factors influencing the crystallization process and consequently developing the field of highly stable glassy APIs. As reported many times, among the factors, molecular mobility is considered the most important in controlling the nucleation of an amorphous and supercooled phase. A very powerful experimental tool frequently employed for investigating the molecular dynamics of APIs is broadband dielectric spectroscopy (BDS). However, it should be stressed that the data analysis depends strongly on the intermolecular interactions being decisive in a given system. Specifically, the dielectric results of ionic APIs, where electrostatic forces between cation and anion dominate, dc-conductivity contribution coming from translational motions of ions completely covers the structural relaxation. As a consequence, analysis of the dielectric data collected for ionic APIs is much more complex than it appears for van der Waals drugs.<br/>Herein, using dielectric spectroscopy we characterize the relaxation phenomena of several ionic and nonionic APIs. To describe the ion dynamics of the former ones, we chose the complex electrical modulus M*(f) formalism. We show that this representation is much more convenient in the analysis of dielectric data of ionic APIs than the permittivity formalism ε*(f), traditionally used to describe the molecular dynamics of van der Waals and H-bonded systems, and additionally allows predicting the long-term physical stability of amorphous ionic systems.<br/><br/>

Broadband dielectric spectroscopy for studies of amorphous ionic pharmaceuticals

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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