ABSTRACT
BCS Ⅱ drugs are characterized by low solubility and high permeability. Improving their solubility is considered an important approach to improve its oral absorption. Recent strategies to increase the solubility of poorly-soluble drugs may unexpectedly result in greatly depressed permeability, ultimately leading to failure in improving oral absorption. Based on the mathematics of membrane permeability coefficient of a drug, the membrane/aqueous partition coefficient is dependent on the drug's solubility in the gastrointestinal milieu, suggesting a unique interplay between the solubility and permeability of the drug, and treating the one irrespectively of the other may be insufficient. When we focus on the increase of drug solubility and overlook the efficacy of drug permeability, the positive effect of increased solubility to drug oral absorption might be traded off by depressed permeability. To provide rational formulary designs, by optimizing excipients and evaluation, this review summarizes solubility- permeability interplay for different types of solubilizing techniques, such as cyclodextrin, surfactants-based vehicle, cosolvent, amorphous solid dispersions, other infectors such as P-gp transporters and new techniques for simultaneous evaluation of drug solubility and permeability.
ABSTRACT
Cyclodextrin can increase the solubility of poorly soluble drugs, but also decrease the permeability of poorly soluble drugs in inclusion complexes simultaneously, which partially or completely counteracts the contribution of improvement in solubility to the oral absorption of poorly soluble drugs. If a competing agent is added to the system to compete binding sites of cyclodextrins with drugs, drug permeability can be improved by increasing the concentration of free drugs in the inclusion complex system. In this paper, a rapid in vitro screening method for competing agents of cyclodextrin inclusion complex is proposed based on the principle that good drug permeability is in accord with good cell uptake. The equilibrium constants between drugs and hydroxypropyl-beta-cyclodextrin (HPCD) were determined by phase equilibrium solubility method. Cinnarizine (CN) with a high equilibrium constant was selected as a competing agent, coumarin 6 (C6) and 9-octadecyl berberine (BD) with smaller equilibrium constants were selected as model drugs. Both changes of solubility and uptake by Caco-2 and A549 cells of C6 and BD were investigated different concentrations of CN to the HPCD solution of C6 and BD. The results showed that the uptake of C6 and BD increased in a CN concentration-dependent manner, and the solubility of C6 and BD in HPCD solution decreased with the prolongation of equilibrium time. It might be due to increased free drug concentrations that resulted from the competition of CN for drug binding sites with HPCD. In our study, in vitro cell uptake method was firstly used to validate the ability of CN as a competing agent to increase drug permeability (cell uptake). This method can be used for preliminarily screening of competing agents for drug-cyclodextrin inclusion complexes.