Topical application of ectoparasiticides for flea and tick control is a major focus for product development in animal health. The objective of this work was to develop a quantitative structure permeability relationship (QSPeR) model sensitive to formulation effects for predicting absorption and skin deposition of five topically applied drugs administered in six vehicle combinations to porcine and canine skin in vitro. Saturated solutions (20 μL) of (14) C-labeled demiditraz, fipronil, permethrin, imidacloprid, or sisapronil were administered in single or binary (50:50 v/v) combinations of water, ethanol, and transcutol (6 formulations, n = 4-5 replicates per treatment) nonoccluded to 0.64 cm(2) disks of dermatomed pig or dog skin mounted in flow-through diffusion cells. Perfusate flux over 24 h and skin deposition at termination were determined. Permeability (logKp), absorption, and penetration endpoints were modeled using a four-term Abrahams and Martin (hydrogen-bond donor acidity and basicity, dipolarity/polarizability, and excess molar refractivity) linear free energy QSPeR equation with a mixture factor added to compensate for formulation ingredient interactions. Goodness of fit was judged by r(2) , cross-validation coefficient, coefficients (q(2) s), and Williams Plot to visualize the applicability domain. Formulation composition was the primary determinant of permeation. Compounds generally penetrated dog skin better than porcine skin. The vast majority of permeated penetrant was deposited within the dosed skin relative to transdermal flux, an attribute for ectoparasiticides. The best QSPeR logKp model for pig skin permeation (r(2) = 0.86, q(2) s = 0.85) included log octanol/water partition coefficient as the mixture factor, while for dogs (r(2) = 0.91, q(2) s = 0.90), it was log water solubility. These studies clearly showed that the permeation of topical ectoparasiticides could be well predicted using QSPeR models that account for both the physical-chemical properties of the penetrant and formulation components.

This study investigated the effects of vehicles on penetration and retention of lidocaine applied to sheep skin in vitro. Thoracic skin from two sheep was clipped of wool and stored at -20 °C, until used. Skin samples were defrosted and mounted in Franz-type diffusion cells, and then one of the following formulations, each saturated with lidocaine, was added: sodium lauryl sulphate (SLS) 0.5% in water, SLS 1% in water, dimethyl sulphoxide (DMSO) 50% in water (wt/wt), DMSO 100%, isopropyl myristate 100% (IPM), water alone, diethylene glycol monoethyl ether (DGME) 50% in water (wt/wt) and DGME 100%. The penetration of lidocaine in each skin sample was measured over 8 h. Significantly greater lidocaine skin concentrations and flux (J(SS)) were achieved with the nonaqueous vehicles, DMSO 100% (P < 0.00001 and P < 0.01, respectively), followed by DGME 100% and IPM (P < 0.00001 and P < 0.01, respectively). The lag time (t(lag)) for lidocaine penetration in the DMSO 100% vehicle was significantly shorter (P < 0.01) compared with all other vehicles except water. Improved transdermal penetration of lidocaine in the DMSO 100% vehicle was likely due to skin barrier disruption, as determined by differences in pre- and post-treatment transepidermal water loss (TEWL). This study has shown that nonaqueous vehicles enhanced penetration of lidocaine in sheep skin to a greater extent than aqueous vehicles, which has implications for topically applied local anaesthesia in sheep.