Follicular concentrations of azelaic acid after a single topical application

Follicular concentrations of azelaic acid (AzA) were determined in vivo using a rapid, non-invasive method, after a single topical application of 20% (w/w) AzA cream, in order to establish whether the in vitro antimicrobial effects observed in previous studies are relevant in vivo. Preweighed amounts of 20% (w/w) AzA cream were applied over demarcated areas on the forehead and back of nine young adults, and samples were taken over a period of 5 h. AzA was removed from the skin surface by washing with acetone, and follicular casts were collected using cyanacrylate gel. The samples were centrifuged to remove particulate matter, and the supernatants derivatized for analysis by HPLC. Although the results showed wide-ranging variability, the follicular concentration increased as the amount present on the surface declined. The maximum follicular concentrations of AzA attained ranged from 7.5 to 52.5 ng (μg of follicular casts)⁻¹ and 0–5 to 23–4 ng (μg of follicular casts)⁻¹ in samples taken from the back and forehead, respectively. Assuming an average density of follicular material of 0.9 g ml⁻¹, the mean maximum follicular concentration attained on the back was between 36 and 251 mmol/l, and on the forehead was between 2 and 112 mmol/l, and indicates that the concentration of AzA attained in follicular casts after a single topical application is comparable with the concentration required to inhibit the growth of Propionibacterium acnes and Staphylococcus epidermidis, in vitro.