INTRODUCTION: The effect of amphiphilic non-steroidal anti-inflammatory drugs (NSAIDs), like Ibuprofen (Ibu) a white insoluble powder with pKa = 4.31, on liposomal bilayer as biomembrane model, plays a crucial role in understanding their mechanism at the molecular level [1]. To explain their activity and correlate it with the chemico-physical properties of the drug, the thermal behaviour of the hydrated multilamellar vesicles (liposomes) of dimyristoylphosphatidylcholine (DMPC) in the presence of increasing amounts of both Ibu as well of its Na-salt (Na-Ibu) have been studied at pH = 3.0 and at pH = 7.0 by means of Differential Scanning Calorimetry (DSC) technique. MATERIALS AND METHODS: Liposomes were prepared by mixing the appropriate amount of Ibu or Na-Ibu with DMPC in a NaCl 0.9 % w/w solution buffered at the request pH value up to a final lipid concentration of about 20 % w/w. DSC scans were performed on a Mettler-Toledo DSC 821e calorimeter at a heating rate of 2.0°C/min. RESULTS: The results on Ibu/DMPC liposomes at pH = 3.0 show significant Tm decrease and a ?T1/2 increase when Ibu si added, suggesting that the hydrophobic core is strongly affected by the presence of the drug unionized molecules. Moreover, in the samples where Ibu content is 20 % w/w, the transition disappeared. The ?H behaviour is more complex; indeed initially, up a 2.0 % w/w of added substance, it increases and successively, up to 20 % w/w Ibu content, its values decreased lineally. On the contrary, the measurements on the Ibu/DMPC systems at pH = 7.0 showed noticeably lower values both in Tm decrease as well as in ?T1/2 increase, suggesting that the hydrophobic core is not so strongly affected by the presence of Ibu as it was at pH = 3.0. Moreover the transition is present at any Ibu concentration up to 30 % w/w and also ?H increased up to 20 % w/w system, decreasing only in the most concentrated Ibu containing sample (30 % w/w). Similar results were obtained starting from the very soluble Na-Ibu salt. The data suggest that the interaction Ibu-liposomes is strongly pH-dependent, suggesting that at pH lower than pKa the interaction involves mainly the deeper part of the bilayer. On the contrary, when pH is greater than pKa, the setting up of strong electrostatic interactions between the negative ion on Ibu molecule and the polar moiety of the lipid localize the interaction on the external part of the bilayer. Moreover, the obtaining of the same results even starting from Na-Ibu water solution, confirm the capability of the of liposomes to strip the drug molecules out from water and concentrate within the bilayer. The perturbing effect of Ibu on the membrane structure would alter, indirectly, the function of the membrane proteins, whose function is highly dependent on the membrane structure.
Effect of ibuprofen on the thermal behaviour of DMPC liposomes as a function of pH
A Torreggiani
2008
Abstract
INTRODUCTION: The effect of amphiphilic non-steroidal anti-inflammatory drugs (NSAIDs), like Ibuprofen (Ibu) a white insoluble powder with pKa = 4.31, on liposomal bilayer as biomembrane model, plays a crucial role in understanding their mechanism at the molecular level [1]. To explain their activity and correlate it with the chemico-physical properties of the drug, the thermal behaviour of the hydrated multilamellar vesicles (liposomes) of dimyristoylphosphatidylcholine (DMPC) in the presence of increasing amounts of both Ibu as well of its Na-salt (Na-Ibu) have been studied at pH = 3.0 and at pH = 7.0 by means of Differential Scanning Calorimetry (DSC) technique. MATERIALS AND METHODS: Liposomes were prepared by mixing the appropriate amount of Ibu or Na-Ibu with DMPC in a NaCl 0.9 % w/w solution buffered at the request pH value up to a final lipid concentration of about 20 % w/w. DSC scans were performed on a Mettler-Toledo DSC 821e calorimeter at a heating rate of 2.0°C/min. RESULTS: The results on Ibu/DMPC liposomes at pH = 3.0 show significant Tm decrease and a ?T1/2 increase when Ibu si added, suggesting that the hydrophobic core is strongly affected by the presence of the drug unionized molecules. Moreover, in the samples where Ibu content is 20 % w/w, the transition disappeared. The ?H behaviour is more complex; indeed initially, up a 2.0 % w/w of added substance, it increases and successively, up to 20 % w/w Ibu content, its values decreased lineally. On the contrary, the measurements on the Ibu/DMPC systems at pH = 7.0 showed noticeably lower values both in Tm decrease as well as in ?T1/2 increase, suggesting that the hydrophobic core is not so strongly affected by the presence of Ibu as it was at pH = 3.0. Moreover the transition is present at any Ibu concentration up to 30 % w/w and also ?H increased up to 20 % w/w system, decreasing only in the most concentrated Ibu containing sample (30 % w/w). Similar results were obtained starting from the very soluble Na-Ibu salt. The data suggest that the interaction Ibu-liposomes is strongly pH-dependent, suggesting that at pH lower than pKa the interaction involves mainly the deeper part of the bilayer. On the contrary, when pH is greater than pKa, the setting up of strong electrostatic interactions between the negative ion on Ibu molecule and the polar moiety of the lipid localize the interaction on the external part of the bilayer. Moreover, the obtaining of the same results even starting from Na-Ibu water solution, confirm the capability of the of liposomes to strip the drug molecules out from water and concentrate within the bilayer. The perturbing effect of Ibu on the membrane structure would alter, indirectly, the function of the membrane proteins, whose function is highly dependent on the membrane structure.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
