Importance of the formulation in the skin delivery of topical diclofenac: not all topical diclofenac formulations are the same

Purpose: The current study aimed to compare 2 topical diclofenac products (diclofenac diethylamine [DEA] 1.16% emulsion and diclofenac sodium [Na] 5% gel). The quantitative evaluation of skin permeability and the qualitative evaluation of their physical characteristics were performed. Methods: The skin permeability of diclofenac DEA 1.16% emulsion and diclofenac Na 5% gel was compared in vitro using Franz diffusion cells following a single, fixed, 10 mg/cm(2) dose of product applied to a 0.64 cm(2) area of the stratum corneum surface of ex vivo human skin samples. The physical characteristics of the 2 formulations were assessed by rheological measurement and microscopy observation. Results: Diclofenac DEA 1.16% emulsion exhibited a statistically significant higher permeation through human skin at 24 hrs than diclofenac Na 5% gel (554 vs 361 ng/cm(2), respectively; ratio of adjusted geometric means, 1.54 [95% CI, 1.14–2.07]). When expressed as a percentage of the applied dose of diclofenac that permeated through human skin, a 7-fold difference was observed between the diclofenac DEA 1.16% emulsion (0.54%) and the diclofenac Na 5% gel (0.077%). Qualitative composition and physical characterization showed differences between the formulations that may explain some of the permeation data observed. Based on rheological assessments, diclofenac Na 5% gel had a higher viscosity (24.82 Pa.s) than diclofenac DEA 1.16% emulsion (10.29 Pa.s). Conclusion: A topical diclofenac product with a higher concentration of the active ingredient does not necessarily lead to greater absorption relative to a product with lower concentration of the active ingredient but different characteristics. These observations highlight the importance of considering parameters beyond drug concentration, such as composition, which may influence the solubility of the drug and permeation of topical nonsteroidal anti-inflammatory drugs.