Paramagnetic thermosensitive liposomes for MR-thermometry.

MR-thermometry methods have been developed for the guidance and control of thermal therapies such as thermal ablation or regional hyperthermia. However, they are limited to the measurement of temperature changes and, thus, cannot be used to assess absolute temperature values. Paramagnetic thermosensitive liposomes are innovative contrast agents offering the potential to overcome these limitations. They are composed of a gadolinium- or manganese-based compound enclosed by a phospholipid membrane with a distinct gel-to-liquid crystalline phase transition temperature (Tm). At this temperature, the phospholipid membrane changes from a gel-phase to a liquid-crystalline phase which is associated with an increased transmembrane permeability towards solutes and water. Under these conditions, both types of paramagnetic thermosensitive liposomes demonstrate a significant increase in longitudinal (T1) relaxivity, attributed to the release of paramagnetic material from the liposome and/or to the increased water exchange rate between the liposome interior and exterior. Paramagnetic thermosensitive liposomes have already been successfully studied in animal models and have demonstrated a clear correlation between tissue temperature changes and signal intensity changes in MRI. Nevertheless, before entering clinical trials they have to be studied in more detail with regard to dose, pharmacokinetics and toxicity.

Time-resolved infrared ATR measurements of liposome transport kinetics in human keratinocyte cultures and skin reveals a dependence on liposome size and phase state.

A novel in vitro method for studying the permeation kinetics of superficially applied liposomes or vesicles through layers of human skin or keratinocytes on a solid support is presented, employing attenuated total reflection infrared spectroscopy. The method is applied to investigate transport kinetics of unilamellar vesicles of dimyristoylphosphatidylcholine (DMPC) through cultured human keratinocyte layers and through human skin. We find a strong resemblance of the qualitative features of the permeation kinetics of small unilamellar DMPC vesicles for skin and keratinocytes. Detailed studies of the vesicles transport through keratinocyte layers show that DMPC vesicles with an average diameter of 55 nm can readily permeate through the layer at 37 degrees C with a diffusion constant of D = (4.0 +/- 0.8) x 10(-15) m2/second, whereas larger vesicles of twice that diameter do not permeate at all. In contrast, liposomes containing a chemical permeation enhancer permeate through the layer significantly faster [D = (7.0 +/- 0.5) x 10(-15) m2/second] than the small DMPC vesicles despite their five-times-larger diameter. Moreover, the transport of the DMPC vesicles depends drastically on their phase state. No permeation was observed for small DMPC vesicles at a temperature of 10 degrees C when the lipid is in the crystalline phase state. Our results indicate that keratinocyte culture layers can pose a significant permeation barrier for vesicles. The permeation mechanism can be explained by diffusion of the vesicles through small pores and gaps in the layer, presumably driven by transdermal osmotic gradients.

Lipid transfer between small unilamellar vesicles and single bilayers on a solid support: self-assembly of supported bilayers with asymmetric lipid distribution.

The transfer of lipids between small unilamellar vesicles of either dimyristoylphosphatidylcholine (DMPC), dimyristoylphosphatidylglycerol (DMPG), or dioctadecyl diammonium bromide (DODAB) and a single bilayer on a solid support of chain perdeuterated DMPC-d54 has been studied by time-resolved ATR infrared spectroscopy, deuterium NMR, and DSC. The IR method was used for measuring the transfer kinetics and the amount of lipid transferred to the supported bilayer, while NMR was employed for the assessment of molecular order and for the occurrence of lipid asymmetries due to the transfer. We find that the composition of a supported planar dimyristoylphosphatidylcholine (DMPC-d54) bilayer can be modified by incubation with high concentrations of sonicated vesicles consisting of the donor lipid. Three cases were studied. First, the incubation was done with DMPC as donor lipid. The kinetics of this process is double exponential and comparatively slow, with a half-time in the range of several hours. The activation energy was estimated as 50 +/- 2 kJ/mol. In a second set of measurements, cationic DODAB or anionic DMPG was used as donor lipid. The kinetics of this transfer is 1 order of magnitude faster than for DMPC and can be described by a single exponential. For DMPG transfer, we obtained an activation energy of 35 +/- 2 kJ/mol. Independent of the headgroup charge of the donor lipid, 25-35% of the (acceptor) DMPC in the supported bilayer is not accessible for exchange with the donor lipid. The transfer of either DMPG or DODAB causes drastic changes of the phase transition behavior of the supported bilayer without significantly altering the lipid packing density of the lipids.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction of myelin basic protein with single bilayers on a solid support: an NMR, DSC and polarized infrared ATR study.

The interaction of myelin basic protein (MBP) with single bilayers on a solid support (planar and spherical support) is studied by deuterium nuclear magnetic resonance (2H-NMR), differential scanning calorimetry (DSC) and polarized attenuated total reflection infrared spectroscopy (ATR-IR). The single bilayer consisted of either DMPC or of a binary mixture of DMPC with 10-20 mol% of an acidic phospholipid (DMPG, DMPS or DMPA). All methods applied indicate that MBP strongly interacts with the binary lipid systems but not with the pure DMPC bilayers. The interaction is predominantly electrostatic in nature and does not depend on the choice of a particular acidic lipid (for the binary systems). In particular, the results give no indication for a hydrophobic interaction of MBP with the membrane. Our data provide evidence that, in contrast to previous findings, no demixing and/or domain formation in the binary systems is induced due to the MBP coupling. The infrared order parameter was determined for both lipid components of the binary systems and shows a remarkable change for both lipids due to the interaction with MBP while the NMR order parameter remained essentially unchanged. This is discussed in terms of the different timescales characteristic for both methods. The single supported bilayer responds to the MBP coupling as a whole although only 50% of the bilayer surface is accessible to the protein, indicating a strong coupling between the two bilayer leaflets via the hydrophobic chain region. Moreover, the asymmetric coupling of MBP to the single supported bilayer does not result in a significant redistribution of lipids between the two bilayer leaflets. NMR relaxation time measurements in the headgroup and chain region of DMPG and DMPC suggest that the lateral diffusion coefficient of the acidic lipid decreases significantly due to the coupling with MBP while the zwitterionic DMPC is not affected.