The Solubility-Permeability Interplay in Using Cyclodextrins as Pharmaceutical Solubilizers: Mechanistic Modeling and Application to Progesterone

A quasi-equilibrium mass transport analysis has been developed to quantitatively explain the solubility-permeability interplay that exists when using cyclodextrins as pharmaceutical solubilizers. The model considers the effects of cyclodextrins on the membrane permeability (Pm) as well as the unstirred water layer (UWL) permeability (Paq), to predict the overall effective permeability (Peff) dependence on cyclodextrin concentration (CCD). The analysis reveals that: (1) UWL permeability markedly increases with increasing CCD since the effective UWL thickness quickly decreases with increasing CCD; (2) membrane permeability decreases with increasing CCD, as a result of the decrease in the free fraction of drug; and (3) since Paq increases and Pm decreases with increasing CCD, the UWL is effectively eliminated and the overall Peff tends toward membrane control, that is, Peff ≈ Pm above a critical CCD. Application of this transport model enabled excellent quantitative prediction of progesterone Peff as a function of HPβCD concentrations in PAMPA assay, Caco-2 transepithelial studies, and in situ rat jejunal-perfusion model. This work demonstrates that when using cyclodextrins as pharmaceutical solubilizers, a trade-off exists between solubility increase and permeability decrease that must not be overlooked; the transport model presented here can aid in striking the appropriate solubility-permeability balance in order to achieve optimal overall absorption.