The inclusion of certain polymers within solid dispersion or lipid-based formulations can maintain drug supersaturation after dispersion and/or digestion of the vehicle, leading to improvements in bioavailability and variability in exposure. This review presents an overview of the fundamental principles that underpin drug precipitation mechanisms, describes the mechanisms by which precipitation may be inhibited, discusses the methods that can be used to identify polymeric precipitation inhibitors (PPIs), and summarizes current literature evidence of the most effective PPIs. Preliminary data from our laboratory is also presented, which describes the precipitation inhibition behavior of 53 polymeric materials using supersaturated solutions of danazol as a model, poorly water-soluble drug. These studies identify a group of PPIs with superior precipitation inhibition qualities, the majority of which are cellulose-based. These new results in combination with previous published data indicate that PPIs represent an appealing new technology with the potential to improve drug absorption for poorly water-soluble drugs. The molecular determinants of polymer utility, however, remain relatively poorly understood, although the cellulose derivates appear, in general, to provide the most benefit. More detailed studies are therefore required to define the parameters that most effectively predict and quantify the drug-polymer relationships that control precipitation inhibition.

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The role of the digestion of lipids in facilitating absorption of poorly water-soluble compounds, such as vitamins, is not only an important nutritional issue but is increasingly being recognized as an important determinant in the effectiveness of lipid-based drug formulations. It has been known for some time that lipids often form complex liquid crystalline structures during digestion and that this may impact drug solubilization and absorption. However, until recently we have been unable to detect and characterize those structures in real time and have been limited in establishing the interplay between composition, digestion, and nanostructure. Here, we establish the use of an in vitro lipid digestion model used in conjunction with synchrotron small-angle X-ray scattering by first confirming its validity using known, nondigestible liquid crystalline systems, and then extend the model to study the real time evolution of nanostructure during the digestion of common formulation lipids. The formation of liquid crystalline structures from unstructured liquid formulations is discovered, and the kinetics of formation and dependence on composition is investigated.

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Cyclosporins are natural or synthetic undecapeptides with a wide range of actual and potential pharmaceutical applications. Several members of the cyclosporin compound family have remarkably high passive membrane permeabilities that are not well-described by simple structural metrics. Here we review experimental studies of cyclosporin structure and permeability, including cyclosporin–metal complexes. We also discuss models for the conformation-dependent permeability of cyclosporins and similar compounds. Finally, we identify current knowledge gaps in the literature and provide recommendations regarding future avenues of exploration.