Formulation and topical delivery of niosomes and proniosomes containing α-lipoic acid

Abstract

α-Lipoic acid, the unique and highly potent antioxidant, is a derivative of octanoic acid that originates endogenously. This compound not only possesses the ability to act as an antioxidant itself by scavenging deleterious reactive species, but also increases the functionalities of other antioxidants, such as vitamins E and C. It is identified as an amphiphilic compound attaining both lipophilic and hydrophilic properties, but is considered to have a higher affinity for lipid environments. Therefore, this compound does not only act against reactive species in the aqueous bloodstream, but also in the lipid compartments of cells as well, deeming this unique compound the universal antioxidant (Biewenga et al., 1997:315; Maczurek et al., 2008:1465; Packer et al., 1995:228). Additionally, this endogenous compound has two active states in which it can present itself, i.e. α-lipoic acid (oxidised state) and dihydrolipoic acid (reduced state), forming a powerful redox couple. This redox couple can regenerate other antioxidants, increase the internal glutathione and coenzyme Q10 levels and ultimately enhance the natural antioxidant defences of the human body (Maczurek et al., 2008:1465; Packer et al., 1995:228; Rochette et al., 2013:116). These antioxidative properties may be potentially useful in the treatment or improvement of the signs of skin ageing. The targeted reactive species cause collagen and elastin degradation by altering gene expression pathways in skin cells (Baumann, 2007:246). However, Masaki (2010:89) stated the topical delivery of antioxidants such as α- lipoic acid might possibly be advantageous if included in the treatment of skin ageing. Skin ageing is generally defined as the biological phenomenon including two different aspects, namely chronological or intrinsic ageing and photo- or extrinsic ageing. Atypical skin pigmentation, increased laxity, wrinkling and skin sagging are some characteristics of cutaneous aged skin (El-Domyati et al., 2002:398; Jenkins et al., 2002:801; Masaki, 2010:85). Another important ability of α-lipoic acid is the inhibition of certain transcription factors that are considered vital in the natural inflammatory response, plus the attenuation of additional cytotoxic cytokines produced during this inflammatory reaction. These activities performed by α-lipoic acid are attributed to the anti-inflammatory properties of this compound (Lee & Hughes 2002:409; Maczurek et al., 2008:1465); therefore skin inflammatory conditions may be improved by the addition or treatment with antioxidants such as α-lipoic acid. The most commonly known skin condition, induced or influenced by the inflammatory response is psoriasis. Psoriasis affects almost 2 to 3% of the world’s population and the possible topical delivery of an antioxidant, such as α-lipoic acid used in this study, may be useful in the treatment of this skin disease (Gelfand et al., 2005:23). ~iii~ Transdermal delivery is a fast growing researched topic that is fully deserving of the attention it is gaining from scientists. This route of delivery is easily self-administered, is very convenient to use and is of special importance to the cosmetic industry. Advantages of the transdermal route mainly include the painless and prolonged delivery of active ingredients directly to the targeted area, the circumvention of the first-pass metabolism and the non-invasive nature of the administration itself (Brown et al., 2006:177; Liuzzi et al., 2016:295. Shahzad et al., 2015:2). Although the transdermal delivery route possesses a multitude of advantages, it is not applied widely due to the restricting stratum corneum layer of the skin. This outermost epidermal layer consists of keratinocytes, which are terminally differentiated and known as hydrophilic corneocytes. These flattened corneocytes form a physical barrier with the surrounding lipid medium to keep foreign molecules from penetrating through the skin layers (Kolarsick et al., 2011:205). Aforementioned differences in the solubility of the components are attributed to the barrier function of the skin therefore any potential candidate for topical delivery should possess the necessary solubility properties (Van Smeden et al., 2014:295-296). According to Mahale et al. (2012:47), vesicle systems are formulated for the intended purpose of enhancing the bioavailability of compound and controlling the release of the administered active ingredient. Alexander et al. (2012:33) further explained that vesicle systems act as transporters of active ingredients through the skin and also as penetration enhancers. Hence, to deliver α-lipoic acid to the dermal skin layers, niosomal and proniosomal vesicles were formulated during this study to examine their respective characterisation, release and diffusion profiles. Niosomes are the result of non-ionic amphiphiles assembling themselves to form a closed bilayer structure within an aqueous medium (Sharma et al., 2015:395). For this formation process to initiate a source of energy must be administered, such as an increased temperature or applied mechanical stirring. Niosomal vesicle formation can be done by one of many preparation methods depending on the ingredients’ physicochemical properties. This specific vesicle system is considered the preferred choice for cosmetic products due to the fact it causes less skin irritation compared to more conventional vesicle systems (Mahale et al., 2012:47; Sharma et al., 2015:393). Proniosomes, the dry form of niosomes, has the upper hand on physical stability that makes this type of vesicle system easier to transport and store, and the dosing more accurate (Kumar & Rajeshwarrao, 2011:214). The granular powder is prepared by the slow spray-coating of a water soluble carrier such as sorbitol, which is then hydrated with water prior to administration (Mahale et al., 2012:50-51). A reliable and accurate high performance liquid chromatography (HPLC) method for the analysis of α-lipoic acid detection was developed and validated. The optimisation and characterisation proved that both dispersions formed large unilamellar vesicles (LUV) for the intended entrapment of α-lipoic acid. Results obtained also revealed acceptable pH and zeta- potential values for both dispersions. The experimental values determined for the aqueous solubility and octanol-buffer distribution coefficient (log D) of α-lipoic acid was 0.33 mg/ml and - 1.21, respectively. Neither of the determined values indicated favourable permeation through the skin. The membrane release studies revealed desirable amounts of active ingredient released during the 6 h release studies from both vesicle dispersions. Therefore, both the niosomal and hydrated proniosomal dispersions successfully released the α-lipoic acid with an average flux of 467.49 ± 51.82 μg/cm².h and 332.01 ± 49.04 μg/cm².h, respectively; hence, the average flux value of the niosomal dispersion was slightly higher than that of the hydrated proniosomal dispersion. Results from the skin diffusion studies confirmed that the niosomal dispersions transported the α-lipoic acid to a better degree compared to the hydrated proniosomal dispersions. Both dispersions delivered the active ingredient transdermally and topically, but to different extents in each individually examined skin layer. Statistical significant differences were identified between the concentrations detected in the epidermis-dermis after the two dispersions were applied to the donor phase. This aforementioned skin layer is the intended target site due to the presence of collagen and melanin, and being the metabolic region in the skin. The average concentrations of active ingredient detected in the epidermis-dermis were 5.077 ± 1.47 μg/ml for the niosomal dispersion and 2.854 ± 1.43 μg/ml for the hydrated proniosome dispersion. Keeping the characterisation and diffusion profiles of the separate vesicle systems in mind, the decision was made to execute further clinical efficacy studies on the niosomal dispersion. After the clinical efficacy studies were conducted on human volunteers, the results obtained were analysed and compared to relative controls. Two studies were done, namely an anti- ageing study that took place over 28 days and an erythema study with a duration of seven days. During the anti-ageing study, α-lipoic acid’s effect on skin hydration levels, skin topography (roughness, scaliness, smoothness and wrinkling) and skin elasticity (maximum recovery, elastic recovery, viscous recovery and viscoelastic recovery) were evaluated. The results obtained identified the improvement of several parameters such as hydration level, skin scaliness, maximum recovery, elastic recovery and viscous recovery. Unfortunately, other parameters were negatively influenced by the topical application of a α-lipoic acid formulation, i.e. skin roughness, smoothness, wrinkling and the viscoelasticity of the tested skin areas. ~v~ The erythema study was aimed to examine the anti-inflammatory abilities of α-lipoic acid compared to several control groups. The results obtained did in fact identify an anti- inflammatory effect portrayed by the α-lipoic acid formulation. However, the control group showed similar results to that measured from the topical formulation containing α-lipoic acid. To conclude, both the niosomal and hydrated proniosomal dispersions consisted of vesicles successfully entrapping the α-lipoic acid. The characterisation experiments also indicated the dispersions were stable with acceptable characteristics according to the specific criteria for topically applied substances. The niosomal dispersion delivered the active ingredient more efficiently to the targeted dermal layers and clinical efficacy studies were conducted. Some parameters measured, during the anti-ageing study, improved after treatment with the active test formulation (ATF), whilst other parameters remained the same or even decreased. No statistical significant differences were identified between the ATF and the control group during the anti-inflammatory study. Suboptimal concentrations of α-lipoic acid, compared to the recommended daily dosage, may be an attributing factor to the small therapeutic effects observed during the clinical efficacy studies.