Dermal penetration of propylene glycols: Measured absorption across human abdominal skin in vitro and comparison with a QSAR model

The dermal penetration of undiluted monopropylene glycol (MPG) and dipropylene glycol (DPG) has been measured in vitro using human abdominal skin under conditions of infinite dose application, and the results compared with predictions from the SKINPERM QSAR model (ten Berge, 2009). The measured steady-state penetration rates (J_ss) for MPG and DPG were 97.6 and 39.3 μg/cm²/h, respectively, and the permeability coefficients (K_p) were 9.48 × 10⁻⁵ cm/h for MPG and 3.85 × 10⁻⁵ cm/h for DPG. In comparison, the SKINPERM model slightly over-predicted J_ss and K_p for MPG and DPG by between 2.6- and 5.1-fold, respectively. The model predictions of 254 μg/cm²/h and 24.6 × 10⁻⁵ cm/h for MPG, and 202 μg/cm²/h and 19.8 × 10⁻⁵ cm/h for DPG were in fairly good agreement with the measured values. Further, the model predicted a J_ss of 101 μg/cm²/h and a K_p of 9.9 × 10⁻⁵ cm/h for the homologue tripropylene glycol. Assuming that the measured J_ss was the same under conditions of finite dose application (taken to be 10 μL/cm²) and was maintained over a 24-h period (both conservative assumptions), the relative dermal absorption of the applied dose was estimated to be 23% (0.96%/h) for MPG and 9% (0.39%/h) for DPG. However, the extrapolation for MPG may be further overestimated due to possible residence in the stratum corneum under infinite conditions of exposure that would not be applicable to a finite loading dose.