Permeation of hypericin across Caco-2 monolayers in absence or presence of quercitrin – a mass balance study

Hypericin is a natural polycyclic quinone found in Hypericum perforatum. Although hypericin reportedly has numerous pharmacological activities, only a limited number of studies have been performed on the absorption and transport characteristics of this compound, presumably, because hypericin is a highly lipophilic compound which is poorly soluble in physiological medium. The major aim of this study was to get a detailed understanding of the exposure and fate of hypericin in the Caco-2 cell system under different experimental conditions.

The permeation characteristics of hypericin (5µM) in absence or presence of the model flavonoid quercitrin (20µM) were studied in the absorptive direction, without or with the addition of 10% FBS to the transport buffer apically.

Following application of hypericin to the apical side of the monolayer only negligible amounts of the compound were found in the basolateral compartment when the experiment was performed with transport buffer. The amount of hypericin in the basolateral compartment increased in the presence of quercitrin (from 0 to 4%). The majority of hypericin was found after cell extraction (44% in absence and 64% in presence of quercitrin). When 10% FBS was added to the transport buffer in the apical compartment to improve the solubility of hypericin in aqueous solution, around 68% of hypericin were bound to serum proteins. Under these experimental conditions, the amount of hypericin in the cells/cell membrane was only 13% in absence, and 18% in presence of quercitrin. The low recovery, and significant amounts of hypericin found after cell extraction, and bound to the surface of the culture dish made a correct estimation of permeability constants impossible. Fluorescence microscopy and imaging analysis revealed that hypericin is mainly accumulated in the cell membrane.

The precise mechanism through which hypericin might overcome the hydrophobic barrier of cell membranes remains to be elucidated. However, our experiments demonstrated that, regardless of the experimental conditions, the permeation characteristics of hypericin improved in presence of the model flavonoid quercitrin.