Abstract
l-ascorbic acid (AA), commonly known as vitamin C, has been widely used in topical formulations for many years as an antioxidant and anti-aging ingredient. However, the physicochemical properties of AA are not optimal for skin uptake and the molecule is also unstable, readily undergoing oxidation on exposure to air. The compound 3-o-ethyl-l-ascorbic acid (EA) has been developed as a stable vitamin C derivative and has been used in topical products. The aims of this work were to conduct a comprehensive characterisation of physicochemical properties of EA as well as to investigate the influence of various neat solvents on EA skin delivery. Nuclear magnetic resonance (NMR), mass spectroscopy, differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were used to characterise the molecule. The pKa of the compound and the partition coefficient logP(o/w) were experimentally determined. A new HPLC method for analysis of the molecule was also developed and validated. A number of solvents for topical preparations were selected based on their wide use as excipients in topical formulations, their potential to act as skin penetration enhancers and their favourable safety profiles. The solubility and stability of EA was examined. Skin permeation of the molecule in full thickness porcine skin in vitro was investigated using Franz-type diffusion cells. The melting point, log P(o/w) value and pKa value of EA were determined to be 114.39 ± 0.5 °C, −1.07 ± 0.03 and 7.72 ± 0.01 respectively. Skin penetration of EA was evident for the following vehicles 1,2 hexanediol (HEX), glycerol (GLY), propylene glycol (PG), 1,2 pentanediol (1-2P), isopropyl alcohol (IPA), propylene glycol monolaurate (PGML) and propylene glycol monocaprylate (PGMC). Skin uptake but no permeation through the skin was observed for Transcutol® (TC) and dipropylene glycol (DiPG), while no penetration was observed for the solvents 1,5 pentanediol (1-5P) and tripropylene glycol (TriPG). The findings of the permeation experiments confirm the potential of simple formulations to deliver EA to the skin. Studies are ongoing to identify complex vehicles for synergistic enhancement of EA skin penetration. To our knowledge this is the first study to conduct a comprehensive characterization of EA and examine its skin uptake and permeation properties in porcine skin.
Highlights
L-ascorbic acid (AA), or vitamin C, is a water-soluble antioxidant that has been used in personal care formulations for many years
Transcutol® (TC), Capryol 90® or propylene glycol monocaprylate (PGMC) Type II, Lauroglycol 90® or propylene glycol monolaurate (PGML) Type II and Labrafac® or medium chain triglycerides of caprylic (C8) and capric (C10) acids were a kind donation from Gattefossé
Mass loss in the thermogravimetric analysis (TGA) was observed after 190° C (Fig. 2), a temperature above the melting observed in the differential scanning calorimetry (DSC), indicating decomposition
Summary
L-ascorbic acid (AA), or vitamin C, is a water-soluble antioxidant that has been used in personal care formulations for many years. The molecule was shown to protect cells against UV-induced oxidative damage by scavenging free radicals and reactive oxygen species (Colven and Pinnell, 1996; Chan, 1993). The actions of topically applied AA on human skin have been investigated in a limited number of in vivo studies. The ability of AA to protect against UV-induced oxidative damage has been examined by Humbert et al (2003) in a 6-month clinical trial. These authors investigated the antiaging effect of topical AA on the sun-exposed skin of the lower neck and forearm of 19 healthy female volunteers. A commercial cream containing 5% AA was applied daily over a period of 6 months and was reported to lead to a statistically significant reduction in clinical signs of photo-ageing when compared with a placebo formulation
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