The Influence of Cholesterol on the Properties and Permeability of Hypericin Derivatives in Lipid Membranes.

The promising photosensitizing properties of hypericin, a natural quinine substituted with hydroxyl and alkyl groups, have led to the proposal that it can be utilized in photodynamic therapy. Neither the detailed mechanism behind the powerful action of hypericin, arising as a result of light excitation, nor the intracellular localization and transportation of the molecule is yet fully understood. The behavior of hypericin derivatives in a pure dipalmitoylphosphatidylcholine (DPPC) lipid membrane has recently been studied theoretically by means of molecular dynamics simulations. Natural membranes however contain many important constituents-cholesterol being one of the most essential-that influence the function and structure of the membrane, and thereby also the behavior of drug molecules therein. In the present study, we investigated hypericin and its brominated derivatives in membranes containing 9 and 25 mol % cholesterol. The results show that the presence of cholesterol in the membrane affects the permeability of the hypericin molecules and does so differently for the various molecules in the two membranes. Hypericin containing one bromine was found to exhibit the lowest free energy profile for the transport process into the lipids, and also the highest permeability coefficients, indicating that this molecule displays the fastest and easiest diffusion in the membranes. All three molecules were found to accumulate most preferably close to the polar headgroup region in both membranes.