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The effect of penetration enhancers on the percutaneous absorption of thyrotropin releasing hormone (TRH) across human epidermis was studied using flow-through diffusion cells with infinite dose technique. Steady-state penetration of TRH under control conditions, 0.27±0.01 μ g/cm 2 ×h, was achieved within 4 h and maintained for 40 h. The presence of 50% ethanol increased the permeation of TRH to 0.83±0.02 μ g/cm 2 ×h. The addition of individual terpenes as 3% cineole, carveol and menthone in combination with 47% ethanol increased the penetration of TRH to 0.92±0.03, 1.07±0.02 and 1.05±0.03 μ g/cm 2 ×h, respectively. The major effect of terpene compared to the addition of ethanol alone is not the increase in maximum flux, but a more rapid arrival at steady-state flux. This effect is most pronounced for cineole where maximum flux was registered 6 h after application. When calculated as total amount of penetrated TRH during the 40-h study period, the values measured in the presence of cineole, carveol and menthone with ethanol were 33.9±0.8, 38.0±1.6 and 33.3±4.6 μ g/cm 2 , respectively. With a dermal patch area of 20 cm 2 this corresponds to a delivered dose in the order of 400 μ g TRH per 24 h. This demonstrates the feasibility of achieving transdermal delivery of small peptides as TRH at physiologically relevant amounts by passive systems using penetration enhancers. |
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