The solubility-permeability interplay when using cosolvents for solubilization: revising the way we use solubility-enabling formulations

Predicting the solubility-permeability interplay when using cyclodextrins in solubility-enabling formulations: model validation

Although the extraordinary solubility advantage afforded by cyclodextrins has led to their widespread use as pharmaceutical solubilizers, several reports have emerged that cyclodextrins may also reduce the apparent permeability of the drug. With the purpose to investigate this solubility-permeability interplay, we have recently developed a mathematical mass transport model that quantitatively explains the impact of molecular complexation on the intestinal permeability. This model enabled excellent quantitative prediction of progesterone P(eff) as a function of HPβCD concentrations in several experimental methods. The purpose of the present study was to challenge the predictive capabilities of this mathematical model, assessing whether the model allows the prediction of literature permeability data, as a model validation method. The mass-transport model was applied to carbamazepine and hydrocortisone, and the predicted permeability (P(eff), P(m) and P(aq)) vs. HPβCD concentration were plotted. Excellent agreement was obtained between literature experimental permeability and the predicted P(eff) values for both compounds at all of the HPβCD concentrations tested. The presented validated model that considers the opposing effects of the formulation on the solubility and the permeability, can lead to a more efficient and intelligent use of molecular complexation strategies; the formulator will be able to a priori strike the optimal solubility-permeability balance to maximize and facilitate the overall oral drug absorption.

The solubility-permeability interplay and its implications in formulation design and development for poorly soluble drugs

While each of the two key parameters of oral drug absorption, the solubility and the permeability, has been comprehensively studied separately, the relationship and interplay between the two have been largely ignored. For instance, when formulating a low-solubility drug using various solubilization techniques: what are we doing to the apparent permeability when we increase the solubility? Permeability is equal to the drug's diffusion coefficient through the membrane times the membrane/aqueous partition coefficient divided by the membrane thickness. The direct correlation between the intestinal permeability and the membrane/aqueous partitioning, which in turn is dependent on the drug's apparent solubility in the GI milieu, suggests that the solubility and the permeability are closely associated, exhibiting a certain interplay between them, and the current view of treating the one irrespectively of the other may not be sufficient. In this paper, we describe the research that has been done thus far, and present new data, to shed light on this solubility-permeability interplay. It has been shown that decreased apparent permeability accompanies the solubility increase when using different solubilization methods. Overall, the weight of the evidence indicates that the solubility-permeability interplay cannot be ignored when using solubility-enabling formulations; looking solely at the solubility enhancement that the formulation enables may be misleading with regards to predicting the resulting absorption, and hence, the solubility-permeability interplay must be taken into account to strike the optimal solubility-permeability balance, in order to maximize the overall absorption.