Abstract: Second-derivative absorption spectra and steady-state fluorescence quenching were used to investigate the interaction between three anti-inflammatory drugs (NSAIDs), namely, indomethacin, sulindac and tolmetin, with egg phosphatidylcholine (EPC) liposomes, which served as a membrane mimetic system. The absorbance of drugs decreased with the addition of EPC liposomes and no shift was observed in the spectra, which indicated that drug molecules did not move to the less polar media constituted by hydrocarbon, but bound on the surface of phosphatidylcholine bilayer. Drugs had the ability to displace, in a dose-dependent manner, 1-anilino-8-naphthalene-sulfonate (ANS), a fluorescent anionic probe previously bound to the head group of phosphatidylcholine molecules. The existence of drugs in liposomes could inhibit the binding of ANS obviously. These factors suggested that the anti-inflammatory drugs could reach the surface of the bilayer, probably with the negative carboxyl group anchored near the phospholipid headgroup, and with the rest of the molecule buried partly in the membrane and aligned with the phospholipid aliphatic tails. These results provided evidence that NSAIDs reduced the ability of surface-active phospholipids to form a hydrophobic protective layer. The strongest interaction was observed between sulindac and liposomes.

Key words: Liposome, Non-steroidal anti-inflammatory drugs, Second-derivative absorption spectra, Fluorescence spectroscopy, ANS