Role of partition coefficients in determining the percutaneous penetration of salicylic acid and formaldehyde under varying occlusion durations.
BACKGROUND
Occlusion is widely utilized to enhance the percutaneous penetration of applied drugs in clinical practice; however, occlusion does not increase the penetration of all chemicals.


OBJECTIVE
This study determines: (1) whether occlusion enhances the percutaneous penetration of the lipophilic salicylic acid or the hydrophilic formaldehyde when compared to non-occlusion, (2) evaluate whether occlusion duration affects the penetration of compounds and (3) establish to what extent occlusive films in clinical practice interact with topically-applied chemicals and possibly hinder penetration.


METHODS
Separately, single doses of [14C]-formaldehyde and [14C]-salicylic acid were applied onto human skin overlying diffusion cells under non-occlusion as well as various occlusive time periods (1, 4 and 8 h). The percent dose penetrating into each compartment as well the percent dose adhering to the plastic wrap were determined.


RESULTS
The radioactivity recovery as percent of applied dose of [14C]-salicylic acid was significantly higher under occlusion versus non-occlusion in the epidermis, dermis and receptor fluid after 24 h (p < 0.05). For [14C]-formaldehyde, no significant statistical differences were observed between occlusion versus non-occlusion. The plastic wrap often used to enhance the penetration of topically applied drugs does not itself substantially adhere to the tested chemicals.


CONCLUSIONS
Occlusion duration, previously undocumented for in vitro studies, impacted the percutaneous penetration of the lipophilic salicylic acid more so than the hydrophilic formaldehyde. A strong correlation between occlusion-enhanced penetration and partition coefficients was observed, but we do not wish to overgeneralize these results until more compounds of varying physical--chemical properties are studied.
Dermatology Department, University of California , San Francisco, CA , USA.
Farhaan
F
Hafeez
Audris
A
Chiang
Xiaoying
X
Hui
Howard
H
Maibach
D000279:Administration, Cutaneous; D005557:Formaldehyde / Q000008:administration & dosage / Q000737:chemistry / Q000493:pharmacokinetics*; D006801:Humans; D057927:Hydrophobic and Hydrophilic Interactions; D066298:In Vitro Techniques; D010539:Permeability; D020156:Salicylic Acid / Q000008:administration & dosage / Q000737:chemistry / Q000493:pharmacokinetics*; D012869:Skin Absorption; D013997:Time Factors
D005557:Formaldehyde; D020156:Salicylic Acid
Barrier function; in vitro; occlusion; partition coefficient; percutaneous penetration; stratum corneum