Topical
pain relief products differ in the type of
drug, concentration, and formulation. All these factors influence the
drug transit through the skin barrier, and its eventual retention in the skin as a reservoir for subsequent release. In addition, the
drug potency can be different, which is important for the product efficacy. We studied here ex vivo human skin permeation and retention of five over-the-counter
NSAID gels containing 2.32%
diclofenac (
DIC) and 5-10%
etofenamate (ETF). The potency of the permeated/retained
drug amounts were compared using a composite parameter, the Index of Relative Topical Anti-inflammatory Activity (IRTAA), which is calculated as the product of the skin permeation/retention and the
drug relative potency. The IRTAAs of the
DIC gel were 94-667-fold higher and 72-208-fold higher for transdermal delivery and skin retention, respectively, than IRTAAs of the ETF
gels. These superior IRTAAs indicate that
DIC delivered by this topical formulation would achieve a higher bioactivity and would form a potent
drug reservoir relevant for its subsequent long-lasting release.