Plasma/blood pharmacokinetics and metabolism after dermal exposure to para-aminophenol or para-phenylenediamine

The pharmacokinetics and metabolism following dermal application of [14C]-para-aminophenol (PAP) or [14C]-para-phenylenediamine (PPD) were investigated. Groups of rats were treated under occlusion for 24 h with 12.5 mg/kg [14C]-PAP, or for 4 h with 50 mg/kg [14C]-PPD on 10% or 20% of their body surface area, respectively. A female minipig was also treated dermally (24 h, occlusion) with 4.7 mg/kg [14C]-PAP on 10% of its body surface area. Blood and plasma samples were analysed for radioactivity and presence of metabolites. In PAP-treated rats, mean plasma levels at 0.5, 1, 2, 4, 8 or 24 h were 0.16, 0.24, 0.38, 0.50, 0.36 or 0.14 μg [14C]-PAP equivalents/ml, respectively. The plasma half-life was 5.95 h, the Cmax was 0.5 μg/ml, the tmax was 4 h, and the AUC0–∞ was 9.27 μg-equivalents h/ml. No free PAP was detected in the plasma, but 3 metabolites (M1, M2 and M3) were found in 2-, 4- or 8-h samples at ranges from 0% to 17.7% (M1), 27.6% to 45.0% (M2) or 46.9% to 70% (M3) of the total plasma radioactivity. M2 was identified as acetylated PAP (paracetamol, acetaminophen, APAP), whereas M1 and M3 were identified as O-glucuronide or O-sulfate conjugates of APAP, respectively. In the pig, very low levels of radioactivity (Cmax of approximately 10 ng/ml) were found in the blood, and identified as APAP. Analysis of plasma of PPD-treated rats at 4 h after topical treatment revealed levels of 1.41 ± 0.34 μg/ml [14C]-PPD-equivalents in males, and 7.40 ± 1.83 μg/ml in females. Radioactivity, reflected a single metabolite, which was identified to be N,N′-diacetylated PPD. Comparison of the plasma APAP levels in rats or the pig following topical PAP with corresponding human plasma levels after a single oral therapeutic dose of APAP suggested a substantial margin of safety. Overall, the results suggest that topically applied PAP or PPD are metabolised in the skin, presumably by N-acetyltransferase-1 resulting in systemic exposure to acetylated metabolites, and not to their parent arylamines.