Regional differences in transdermal penetration of fentanyl through equine skin

The rate and regional differences for the penetration of fentanyl through equine skin was investigated in vitro using a commercial transdermal therapeutic system (TTS) or ‘patch’. Skin collected from the thorax, groin and leg (dorsal metacarpal) regions of five horses was placed in diffusion cells and a fentanyl TTS applied to each skin sample. Drug penetration through each skin sample over 48 h measured using high performance liquid chromatography (HPLC). Cumulative penetration (μg/cm²) was plotted against time (h) and used to regress the steady state flux (μg/cm²/h) of fentanyl through each skin site. Results showed similar fluxes for both the thorax (2.32 ± 0.17 μg/cm²/h and groin (2.21 ± 0.11 (μg/cm²/h) regions, but significantly lower flux (P = < 0.05) for the leg region (1.56 ± 0.120 μg/cm²/h. Interestingly, there was a significantly longer lag time for the penetration of fentanyl through the groin region (7.87 ± 0.51 h) compared to the other two sites (5.66 ± 0.97 h and 5.75 ± 0.43 h for the thorax and leg regions respectively). The results suggest that a fentanyl TTS applied to the leg region may have a small but significantly lower amount of fentanyl available systemically, compared to patches applied to the thorax or groin regions, which may affect the level of analgesia subsequently achieved in the horse.