Permeation Characteristics of 8-Methoxypsoralen Through Human Skin; Relevance to Clinical Treatment

Abstract

Abstract The permeation characteristics through human skin of 8-methoxypsoralen (8-MOP) and its physical attributes were investigated. The log octanol/water partition coefficient and saturated aqueous solubility of 8-MOP at 32°C were 1.98 and 55.8 μg mL−1 respectively. 8-MOP showed Fickian diffusion, with its flux being linearly related to the concentration of drug in the donor solution. The permeability coefficient of 8-MOP through human skin from different concentrations of aqueous solutions and a 2.6 μg mL−1 bath lotion (as used in clinics) were statistically identical with mean values of 1.76 ± 0.12 × 10−2 and 1.70 ± 0.32 × 10−1 cm h−1 respectively (P ≥ 005). An ethanol/water (1:1 w/v) receptor solution did not improve the clearance of 8-MOP from the dermis when compared with an aqueous vehicle. Complete removal of the stratum corneum by tape stripping from full-thickness membranes produced a threefold increase in the flux of 8-MOP thus suggesting that the main barrier to 8-MOP permeation resides in the stratum corneum although the aqueous epidermal and dermal tissue provide a significant resistance to transdermal drug permeation. The equilibrium uptake of 8-MOP into psoriatic plaques and the 8-MOP aqueous/plaque partition coefficient were found to be more than twofold greater than for normal stratum corneum. The absorption of 8-MOP from the total applied topical dose (396 mg) was assessed as ∼0.25% and only 2.5% of an oral dose, a significant reduction in the possible toxic hazard. The peak concentration of 8-MOP permeating through the skin was observed at about 35 min after limited exposure for 15 min. Our results suggest that following a 15 min bath in the drug solution, there may be a need for an interval of about 20 min before patients are irradiated to ensure the optimization of photosensitizer with UVA irradiation (PUVA) therapy. Alternatively, UV irradiation could be applied at a lower flux over a longer time.