The percutaneous absorption of hydroquinone (HQ) through rat and human skin in vitro

Because of the potential for human contact with photographic developer solutions containing hydroquinone (HQ), the rates of percutaneous absorption of HQ through human stratum corneum and full-thickness rat skin have been measured in vitro using 5% aqueous solutions of HQ as the donor solutions. The studies were performed using infinite doses of aqueous solutions containing ¹⁴C-labeled HQ in Franz-type diffusion cells. The measured absorption rate (mean ± S.D.) of HQ through human stratum corneum was 0.52 ± 0.13 μg/cm²/h, while that for full-thickness rat skin was 1.1 ± 0.65 μg/cm²/h. The ratio (rat/human) of the permeability constants (K_p) was 2.4. Using the definitions suggested by Marzulli et al. (1969) Toxicol. Appl. Pharmacol. Suppl. 3, 76–83, HQ would be classified as ‘slow’ with respect to its absorption through human stratum corneum.