In vitro permeation study of capsaicin and its synthetic derivatives from ointment bases using various skin types

Nonivamide (NVA) and sodium nonivamide acetate (SNA) both are synthetic analogues of capsaicin. In the present study, in vitro penetration experiments through rat skin from ointment bases were performed in order to establish and develop the transdermal drug delivery system of these two capsaicin analogues. This study was also carried out to evaluate the relative skin permeability of capsaicin and its derivatives through different skin types. As regards the in vitro transdermal absorption of ointment bases, o/w emulsion-type bases (hydrophilic and University of California Hospital (UCH) ointment) revealed better percutaneous absorption effects than the others for both NVA and SNA. However, there was a higher accumulative amount for NVA in the gel base but none for SNA. There were no changes observed in the appearance and texture when the aqueous phase of the three ointment bases (hydrophilic, absorption and UCH ointment) was replaced by pH 4.2 buffer. In the steady-state flux of absorption ointment for NVA and UCH, absorption ointment for SNA was significantly higher (P < 0.05) in pH 4.2 buffer-replaced base than in the original. In the comparison of in vitro permeability through various animal skin types, full-thickness human skin showed the poorest permeability for NVA, SNA and capsaicin. The trends of steady-state flux through the various skin types for capsaicin, NVA and SNA were quite different. However, pig skin could be successfully used as a model to study in vitro percutaneous absorption of these three compounds through human skin.