Application of vesicles to rat skin in vivo: a confocal laser scanning microscopy study.

A major problem in (trans)dermal drug delivery is the low penetration rate of most substances through the barrier of the skin, the stratum corneum. One of the methods to increase the penetration rate across the skin is encapsulation of a (model) drug in lipid vesicles. In this study fluorescently labelled liposomes were applied on rat skin, in vivo. Bilayer labelled gel-state and liquid-state liposomes (conventional or with flexible bilayers) were non-occlusively applied on the dorsal area in the neck of the rat for 1, 3 or 6 h. Micelles were used as a control formulation. The penetration pathway and penetration depth of the lipophilic fluorescent label into the skin was visualised by confocal laser scanning microscopy (CLSM). During the first 3 h of application almost no differences in penetration depth were observed, when the label was applied in the various formulations. After 6 h application, it was clear that the label applied in micelles and gel-state liposomes did not penetrate as deep into the skin as the label applied in liquid-state vesicles. Among the liquid-state vesicles, the suspension with the most flexible bilayers showed the highest fluorescence intensity in the viable epidermis and dermis, 6 h post-application. Thus the vesicular form and the thermodynamic state of the bilayer and to a smaller extent the flexibility of the bilayer influence the penetration depth of the label into the skin at longer application periods. These results are in good agreement with CLSM results obtained from in vitro experiments with human skin.

Interactions between liposomes and human skin in vitro, a confocal laser scanning microscopy study.

One major problem in (trans)dermal drug delivery is the low penetration rate of drugs through the barrier of the skin. Encapsulation of a drug in lipid vesicles is one strategy to increase the penetration rate of a drug across the skin. In this study, the interactions between fluorescent-labelled liposomes and skin are visualized by confocal laser scanning microscopy (CLSM). Bilayer labelled gel-state and liquid-state liposomes (conventional or with flexible bilayers) were non-occlusively applied on human skin in vitro. The penetration pathway and penetration depth of the lipophilic fluorescent label into the skin were visualized. From the CLSM images, it was clear that the label applied in micelles and gel-state liposomes did not penetrate as deep into the skin as the label applied in liquid-state vesicles. Among the liquid-state vesicles, the suspensions with the flexible bilayers showed the highest fluorescence intensity in the dermis. Thus, the thermodynamic state of the bilayer and, to a smaller extent, the flexibility of the bilayer influence, strongly the penetration depth of the label into the skin. The label applied non-occlusively in flexible liposomes penetrated deeper into the skin than after occlusive application.

Human recombinant apolipoprotein E-enriched liposomes can mimic low-density lipoproteins as carriers for the site-specific delivery of antitumor agents.

Progressive hypocholesterolemia is a feature associated with a number of cancers of different origin, and it is caused by the high expression of low-density lipoprotein (LDL) receptors (LDLrs) on many tumor cell types. Selective delivery of chemotherapeutics using LDL as a carrier has therefore been proposed, but the endogenous nature of LDL hampers its pharmaceutical application. In the current study, we explored the possibility of synthesizing liposomes that mimic LDL from commercially available lipids and proteins. Small unilamellar liposomes were created (28.9 +/- 0.9 nm) and complexed with 5.8 +/- 0.7 molecules of human recombinant apolipoprotein E (apoE). On intravenous injection into rats, the liposomes retained their aqueous core, structural integrity, and the majority of the preassociated apoE. [3H]Cholesteryl oleate-labeled apoE-enriched liposomes showed a relatively long serum half-life (>5 hr), and a low uptake by cells of the reticuloendothelial system was observed (<0.8% of the injected dose at 30 min after injection). Pretreatment of rats with 17alpha-ethinyl estradiol, which induces the expression of the LDLr on the liver and adrenals, led to a 2.5-fold accelerated serum clearance (t1/2 = 123 +/- 10 min) and a selectively increased uptake of liposomes by the liver (2.0-fold) and adrenals (3.8-fold). The liver association of the liposomes was coupled to the lysosomal uptake route, similarly as for LDL. In vitro studies using B16 melanoma cells showed that the liposomes bound exclusively to the LDLr via their apoE moiety (90,000 liposomes/cell), with a 14-fold higher affinity (Kd = 0.77 +/- 0.09 nM) than LDL itself. Because of their favorable properties, we anticipate that these apoE-enriched liposomes are advantageous compared with native LDL in the development of a selective LDLr-targeted antitumor therapy.