Evaluation of Plasma Detectable Concentrations of Two Lidocaine Transdermal Formulations and Their Analgesic Effect in the Horse

Transdermal absorption of four lidocaine (L) patches (5%) was compared with the transdermal absorption of L cream (5%) to evaluate the pharmacokinetics of the two formulations applied on the same anatomic region under dressing in eight horses. The animals were also assessed for antinociceptive effect after removal of the patches and cream, using a psychophysical method visual analog scale, by pricking the patient's skin to test the response to pain. Horses were randomly assigned to four treatment groups: in groups I and II, four L patches were applied for a period of 24 hours with and without alcohol precleaning, respectively; in group III, 5% L cream was applied every 2 hours over a 24-hour period on the same anatomic site. Group IV was the control. No clinical side effects were noted with either formulation. L was detectable in plasma 6 to 24 hours after application of the patches, and the highest plasma concentrations were reached between 12 and 18 hours. The use of alcohol to preclean the skin appeared to reduce the transdermal drug absorption over time. After L cream application, the peak plasma drug concentration occurred at 24 hours. Nociception assessment after L patch or L cream application showed a decreased response when the L cream was used. The results of this study showed that there was minimal absorption from the L patches compared with the L cream in horses. Also, the L cream treatment reduced significantly the intensity of nociception quality as measured by the visual analog scale.