The in vitro transport of pergolide from surfactant-based elastic vesicles through human skin: a suggested mechanism of action

Abstract

This paper reports the in vitro transport of pergolide from L-595–PEG-8-L elastic vesicle formulations. Several aspects of vesicular delivery were studied in order to elucidate the possible mechanisms of action and to establish the optimal conditions and drug candidates for usage with L-595–PEG-8-L elastic vesicles. All studies were performed using human skin and flow-through Franz diffusion cells. Pergolide was chosen as model drug. The findings show that there was a strong correlation between the drug incorporation to saturated levels and the drug transport, both of which were influenced by the pH of the drug–vesicular system. The optimal pH was found to be 5.0, giving the highest drug incorporation as well as the highest drug transport. Non-occlusive co-treatment with elastic vesicles improved the skin delivery of pergolide compared to the non-occlusive buffer control by more than 2-fold. However, non-occlusive pre-treatment of skin with empty vesicles did not enhance drug transport. Occlusion improved drug transport from both elastic vesicle as well as buffer solutions due to the fact that water is an excellent penetration enhancer for pergolide. However, in contrast to non-occlusive application, the action of the elastic vesicles themselves was diminished, as occlusive treatments with elastic vesicles showed a lower flux compared to occlusive treatment with the buffer control. Hence, the highest pergolide skin permeation in this study was obtained from an occluded saturated buffer solution, giving a steady-state flux of 137.9 ng/h cm⁻². The volume of application did not have any effect on the drug transport. In conclusion, these results showed no evidence that a penetration enhancing effect is the main mechanism of action. The pH of the drug–vesicular system is an important factor to consider when optimising elastic vesicle delivery systems. Occlusion reduces the actions of elastic vesicles, but could increase the pergolide transport since water is a good penetration enhancer for this particular drug. Based on the results obtained, a mechanism of action for the elastic vesicles was proposed.

Introduction

Despite decades of research, the barrier function of the stratum corneum still remains a problem, which makes the development of new transdermal drug delivery systems an interesting challenge. Vesicular systems have been widely studied as vehicles for dermal and transdermal drug delivery. Their benefits in enhancing drug permeation have well been established [1], [2], [3], [4], [5], [6]. However, the effectiveness of a vesicular system is strongly dependent on its physicochemical characteristics, in particular its thermodynamic state. It has been shown that liquid-state vesicles are more effective in enhancing drug transport compared to gel-state vesicles [7]. In the early 90s novel series of liquid-state vesicles have been developed that contain elastic lipid membranes [8]. It is suggested that the high elasticity of these vesicles could better facilitate drug transport across the skin as compared to vesicles with rigid membranes [9], [10], [11], [12], [13]. In 1998, Van den Bergh et al. introduced a series of elastic and rigid liquid-state vesicles, consisting of the bilayer-forming surfactant L-595 (sucrose laurate ester) and the micelle-forming surfactant PEG-8-L (octaoxyethylene laurate ester) [14]. We have recently reported that L-595–PEG-8-L elastic vesicles were more effective in enhancing the skin permeation of pergolide as compared to L-595–PEG-8-L rigid vesicles [15]. This supports the hypothesis that elastic vesicles are superior to rigid vesicles as skin delivery vehicles.

Several questions concerning elastic vesicles are yet not fully understood. The exact mechanisms by which elastic vesicles enhance drug transport remain a subject for discussion. Furthermore, it is not clear under which conditions elastic vesicles provide the best enhancement of drug transport. However, this information is essential in order to design and develop optimal transdermal delivery systems. Therefore, the studies presented in this paper are aimed to address some of the above mentioned issues.

The mechanism of action of L-595–PEG-8-L elastic vesicles was first investigated by Van den Bergh et al. who reported the effect of vesicle treatment on the penetration of a fluorescent label into human skin in vitro [16]. After treatment with L-595–PEG-8-L elastic vesicles, fluorescent material was localised in a fine meshwork of channel-like structures. In contrast, after treatment with Wasag-7 rigid vesicles or with PEG-8-L micelles, there was a homogeneous intercellular penetration of the fluorescence label. A second study was performed by Honeywell-Nguyen et al. who investigated the in vivo interactions between vesicles and human skin by visualising vesicle-treated skin using the tape-stripping technique combined with electron microscopy [17]. One hour after elastic vesicle treatment, vesicular structures were found up to the ninth tape-strip, where they accumulated in channel-like regions. This was not found in skin treated with rigid vesicles or with micelles. The results of these two studies are in good agreement and suggest that (a) there is a fast partitioning of elastic vesicles into the stratum corneum via channel-like regions and (b) that this could facilitate drug transport into and across the skin. However, it is unclear whether elastic vesicles increase drug transport by simply acting as a penetration enhancer for free drug or whether they serve as ‘carriers’ which transport vesicle-bound drug into the stratum corneum. In other words, is pre-treatment of the skin with empty vesicles sufficient, or is it essential to incorporate drugs into the vesicle solution? In order to assess this, the difference between pre-treatment and co-treatment was investigated.

Secondly, we investigated the drug transport from drug-vesicle solutions at different pH values. At different pH values the model drug pergolide is ionised/unionised to different degrees and therefore has different degrees of lipo- or hydrophilicity. This could affect the amount of drugs that can be incorporated into the vesicle solutions, which subsequently can affect the transdermal drug transport.

Finally, to further optimise vesicle delivery, we investigated the effect of occlusion and of the volume of application. Although the transport of most compounds is increased during occlusive application, it has been suggested that elastic vesicles are most efficient under non-occlusive conditions. Non-occlusive conditions are essential to create a transepidermal osmotic gradient, which is believed to be the driving force for the transport of vesicles into the skin [18]. The effect of the volume of application of L-595–PEG-8-L vesicles has not been investigated in the past. However, this could be an important factor as a higher volume of application could increase the partitioning of vesicles into the skin, thereby increasing the enhancement effect.

Elastic vesicles used in this study consisted of L-595, PEG-8-L and the stabiliser sulfosuccinate in the molar ratio of 50:50:5. The 50:50:5 molar ratio was chosen since this composition would give the most elastic, yet stable vesicle formulation. Furthermore, previous studies have reported that after non-occlusive application this elastic vesicle composition was able to rapidly penetrate into human stratum corneum [17] and to greatly enhance the in vitro penetration of pergolide [15] as compared to rigid vesicle and buffer formulations.