Title : In vitro pharmacokinetics of topically applied compounds from various formulations across porcine skin
Introduction and aim: Topical application delivers active compounds directly to the site of affliction on the skin, circumventing first-pass metabolism that occurs with oral delivery. In this study, the in vitro diffusion parameters of various compounds in liquid, gel and cream formulations, topically applied on excised porcine skin, were determined so as to investigate the efficacy of penetration into and accumulation within the skin.
Methods: HPLC method validation (Kinetex RP- C18 (5µm, 150 x 4.6mm) column) was performed for each active compound (caffeine, theophylline, retinol, co-enzyme Q10 and L-carnitine). A PermeGear 7-in-line flow-through system was used for the in vitro diffusion studies across porcine skin: 1ml Caffeine (2.5%), Theophylline (2%), Retinol (0,3%), Coenzyme Q10 (0.5%) or L-carnitine (2%) within either a cream, gel or liquid formulation was loaded in each donor compartment. PBS (pH 7.4) was pumped through the receptor compartments at 1.5ml/h (32oC). Samples were collected every 30min or 2h (4h or 24h) and analyzed via HPLC. Skin accumulation (%) was performed as follows: excess residue was removed from the skin after experiments, skin was homogenized (10min in 2ml methanol), homogenate centrifuged and the supernatant analyzed.
Results: Steady state FluxSS (ng.cm-2min-1) and Papp (cm.min-1): Caffeine: cream≈liquid≈gel; Theophylline: liquid>cream≈gel; L-carnitine: liquid>gel>cream. Co-enzyme Q10 and retinol did not diffuse across skin. Skin accumulation (%): Caffeine: gel>cream≈liquid; Theophylline: liquid>cream>gel; L-carnitine: liquid>gel>cream; Retinol: cream≈liquid>gel; Co-enzyme Q10: liquid>gel>>cream.
Conclusion: Caffeine indicated the highest diffusion rates across the skin and formulation did not seem to effect penetration. Skin accumulation was also highest (>12%). Theophylline (hydrophilic) indicated variable diffusion rates (very low→high), but much lower than caffeine. Skin accumulation was much lower than caffeine (<1%). L- Carnitine (zwitterion) indicated much lower diffusion rates than the xanthines (caffeine and Theophylline), but skin accumulation (x%) was higher than theophylline (1%<x<2.5%). The two lipophilic compounds (retinol & co-enzyme Q10) did not show any diffusion across the skin but skin accumulation did occur (co-enzyme Q10<0.05% and retinol<0.12%).