Topical delivery of diclofenac into and across equine skin from a novel liquid diclofenac epolamine formulation

The aim was to investigate diclofenac delivery into and across equine skin in vitro using Franz diffusion cells from a novel diclofenac epolamine (DIC‐EP; 1.3%) formulation and to compare the results to those of Surpass® (1% diclofenac sodium liposomal cream) and a 1% aqueous solution of diclofenac sodium. Skin was harvested from the lower legs of Freiberger geldings immediately after slaughter and sliced to a thickness of ~2 mm. Skin samples were divided into two groups [Group 1: 1 year old (n = 2) and Group 2: 6–8 years old (n = 3)]. Cumulative permeation of diclofenac in Groups 1 and 2 after 24 h using diclofenac sodium solution was 1.91 ± 0.27 and 1.76 ± 0.34 μg/cm2, respectively. The values for Surpass® and DIC‐EP were 3.2 ± 0.8/3.3 ± 0.7 μg/cm2 and 230 ± 59/89.2 ± 32.5 μg/cm2, respectively. Thus, diclofenac permeation from DIC‐EP was significantly greater and appeared to show an age‐dependent effect. Mathematical modelling showed that the DIC‐EP formulation significantly increased diclofenac partitioning into the skin and a linear correlation was observed between steady‐state flux and the partition parameter. Greater skin deposition of diclofenac was also observed with DIC‐EP. These preliminary results suggest that the DIC‐EP formulation may be effective in treating inflammatory conditions in horses.