Enhanced skin permeation of a new capsaicin derivative (DA-5018) from a binary vehicle system composed of isopropyl-myristate and ethoxydiglycol

DA-5018, a recently synthesized capsaicin analog, appears to possess potent analgesic activity when administered topically. The objective of this study is to test the Feasibility of the topical administration of this compound. Specifically, our goal was to identify vehicle system that permit a reasonable transdermal permeation of the compound in mice. Among the vehicles examined, isopropyl myristate (IPM) showed the largest in vitro permeability across the intact skin (83.6 +/- 5.42 mul/cm(2)/h). However, due tea due limited solubility of DA-5018 in IPM (0.53 mg/ml), the maximal flux from the IPM medium remained at only 44.3 +/- 2.87 mug/cm(2)/hr. In order to increase the flux, addition of better solvents for DA-5018 was attempted, under the assumption that flux is the result of both solubility and permeability. Ethoxydiglycol (EC) and oleic acid (OA) were selected as examples of good solvents. The addition of EC or OA to IPM at a 1:1 volume ratio resulted in a comparable increase in the solubility of the compound (i.e., to 61.1 and 50.2 mg/ml for EG and OA, respectively). However, the addition of EG at a 1:1 volume ratio, for example, increased the flux 6.3 fold (i.e., 279 mug/cm(2)/hr), while OA, at a 1:1 volume ratio, decreased the flux 5 fold (i.e., 9.2 mug/cm(2)/hr). The mechanism of this discrepancy between EC and OA was investigated by measuring the permeabilty of DA-5018 across the stratum corneum-removed skin of the mouse, under the hypothesis that the viable skin layer may serve as a barrier for the permeation of lipophilic substances such as DA 5018. The permeability of DA-5018, from the medium of EC or OA, across the viable skin differed greatly for EC (0.41 mul/cm(2)/hr) and OA (0.086 mul/cm(2)/hr), suggesting that a higher permeability across the viable skin layer is needed for the second solvents. The maximum flux across the intact skin was achieved for DA-5018 when EG was added to IPM at a 1:1 volume ratio. Thus, the use of a binary system appears to be the best approach for realizing the transdermal delivery of DA-5018 at a reasonable rate.