Abstract
Triterpenes from the outer bark of birch are known for various pharmacological effects including enhanced wound healing (WH). A birch bark dry extract (TE) obtained by accelerated solvent extraction showed the ability to form oleogels when it is suspended in oils. Consistency of the oleogels and the dissolved amount of triterpenes varies largely with the used oil. Here we wanted to know to what extent different oils and formulations (oleogel versus o/w emulsion) influence WH. Looking at the plain oils, medium-chain triglycerides (MCT) enhanced WH (ca. 1.4-fold), while e.g. castor oil (ca.0.3-fold) or light liquid paraffin (LLP; ca. 0.5-fold) significantly decreased WH. Concerning the respective oleogels, TE-MCT showed no improvement although the solubility of the TE was high. In contrast, the oleogel of sunflower oil which alone showed a slight tendency to impair WH, enhanced WH significantly (ca. 1.6-fold). These results can be explained by release experiments where the release rate of betulin, the main component of TE, from MCT oleogels was significantly lower than from sunflower oil oleogels. LLP impaired WH as plain oil and even though it released betulin comparable to sunflower oil it still results in an overall negative effect of the oleogel on WH. As a further formulation option also surfactant free o/w emulsions were prepared using MCT, sunflower oil and LLP as a nonpolar oil phase. Depending on the preparation method (suspension or oleogel method) the distribution of the TE varied markedly and affected also release kinetics. However, the released betulin was clearly below the values measured with the respective oleogels. Consequently, none of the emulsions showed a significantly positive effect on WH. In conclusion, our data show that the oil used as a vehicle influences wound healing not only by affecting the release of the extract, but also by having its own vehicle effect on wound healing. This is also of importance for other applications where drugs have to be applied in non-polar vehicles because these solvents likely influence the outcome of the experiment substantially.
Highlights
Biological activity of active ingredients is strongly dependent on their vehicle. (1) Release of active ingredients may be different from different vehicles, (2) the vehicles themselves may influence the microenvironment, e.g. by acting as penetration enhancers and (3) they may directly influence the target cells/ physiological processes
In cancer treatment triterpenes are known as potent agents [7–11]. These substances are widely distributed in plants but only the outer bark of the white barked birches contains up to 34% (w/w) betulin, a pentacyclic, lupan type triterpene with two polar hydroxyl groups located on opposite sides of the molecule [12]
While Miglyol (MCT) showed significant wound healing improvement compared to untreated control (1.39±0.15-fold improvement), light liquid paraffin (LLP) (0.47±0.09-fold), castor oil (0.28±0.11) and almond oil (0.58±0.09) showed significant impairment of wound healing compared to untreated control
Summary
Biological activity of active ingredients is strongly dependent on their vehicle. (1) Release of active ingredients may be different from different vehicles, (2) the vehicles themselves may influence the microenvironment, e.g. by acting as penetration enhancers and (3) they may directly influence the target cells/ physiological processes. Oils are often used as vehicles, e.g. in wound dressings, irritation studies or in basic science for the application of non-polar organic ligands or inhibitors. Triterpenes are non-polar biologically active secondary plant metabolites. Their various pharmacological properties like anti-inflammatory, anti-viral, and wound healing effects are well investigated and specified in the literature [1–6]. In cancer treatment triterpenes are known as potent agents [7–11]. These substances are widely distributed in plants but only the outer bark of the white barked birches contains up to 34% (w/w) betulin, a pentacyclic, lupan type triterpene with two polar hydroxyl groups located on opposite sides of the molecule [12]. For an oleogel prepared from TE and sunflower oil it was shown that it has no clear effects on actinic keratosis [17,18], but that it is clearly beneficial for wound healing [2,19,20]
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