Purpose: Percutaneous absorption studies are crucial in the development of cosmetic and pharmaceutical products. Human in vivo percutaneous absorption studies are the most relevant and preferred approach to obtain data pertaining to man. However, there are certain safety and ethical concerns associated with in vivo human studies, particularly if the active or a component of the preparation has potential side-effects or irritant properties. An alternative way of investigating skin absorption is an in vitro permeation study using excised skin. Human skin is considered to be the “gold standard”, however there are difficulties accessing human tissue and as a consequence, surrogates for human skin including tissue from various mammals, rodents, and reptiles have been used instead. Porcine skin is regarded as the closest model to human skin because of the similarities of porcine skin physiology, anatomy and histology to human skin. However, the permeability of porcine skin is influenced by storage conditions, sample handling, and preparation technique. Hence, a reliable, robust, more accessible and efficient alternative for evaluating permeation behavior would be of great benefit in formulation development for the personal care and pharmaceutical industries. Previously we have examined a synthetic stratum corneum model, the Parallel Artificial Membrane Permeability Assay (PAMPA) and shown that it is a promising screening tool to predict human skin permeation of a lipophilic compound, ibuprofen. Here, we investigated the in vitro permeation behavior of phenylethyl resorcinol (PR) using human skin, porcine skin, and the Skin PAMPA model. PR is a potent tyrosinase inhibitor and is a novel safe, stable and efficient skin-lightening compound. However, there are no published reports about the skin penetration or absorption of the molecule. The aims of the present work were to (i) examine the skin penetration of PR and (ii) determine the feasibility of using the PAMPA assay to develop optimal formulations of PR.
Methods: A series of single solvent systems containing 1% (w/v) of active were selected for evaluation. The solvents selected were dimethyl isosorbide (DMI), propylene glycol (PG), Transcutol○R(TC), glycerol, octyl salicylate (OSAL), and isopropyl myristate (IPM). These solvents were selected as they have been used as penetration enhancers or solubilizers in topical or transdermal preparations. Franz cell permeation studies were performed under finite dose conditions for 24 h at 32 ± 1 °C. For human skin and porcine skin, a dose of 5 l/cm2 was used with a volume of 1 l per well, corresponding to 3.3 l/cm2 applied in the Skin PAMPA model. The PAMPA permeation study was conducted for 2.5 h at 32 ± 1 °C.
Results: At 24 h, the cumulative amount of PR that permeated through human skin ranged from 3 to 23 g/cm2 (Fig. 1). The permeation from PG and OSAL was significantly higher than from all other vehicles (p<0.05). The cumulative amount of PR that permeated through porcine skin at 24 h ranged from 4 to 26 g/cm2. The permeation from the Skin PAMPA model at 2.5 h ranged from 30 to 43 g/cm2. For the Skin PAMPA model, all formulations showed a plateau in permeation of the active at 2.5 h, and the corresponding percentages of PR that permeated exceeded 85 %. A linear correlation (Fig. 2) was observed between the permeation of PR in the Skin PAMPA model at 2.5 h and human skin at 24 h (r2 =0.79).
Conclusion: As reported previously, the Skin PAMPA model is more permeable than human skin or porcine skin. The 1 l dose appears to represent realistic finite dose conditions for PR in the Skin PAMPA model and a time of 2.5 h appears to be adequate for screening PR formulations. In vitro permeation behavior in human skin and the Skin PAMPA model will be further investigated using a range of PR formulations. In vivo studies using Confocal Raman spectroscopy (CRS) will also be conducted to evaluate any in vitro-in vivo correlation between the in vitro models and human studies.