Pharmacokinetics of lidocaine following the application of 5% lidocaine patches to cats

Lidocaine patches have been used to provide local analgesia in dogs and cats. We conducted this study to assess the systemic and local absorption of lidocaine from topical patches in cats. Eight 2-year-old cats received either intravenous lidocaine at 2 mg/kg or one 700 mg lidocaine patch placed on the lateral thorax for 72 h, in a cross-over randomized repeated measures design. Plasma was collected at specific times and the skin was biopsied at the time of patch removal for the quantitative analysis of lidocaine and its major metabolite, monoethylglycinexylidide (MEGX), by gas chromatography with mass spectrometry. Percent absorption time plots for systemic lidocaine appearance were constructed using the Loo-Riegelman method. Approximately, constant rate absorption was observed from 12-72 h after patch application at a mean +/- SD rate of 109 +/- 49 microg/kg/h, resulting in steady-state lidocaine plasma concentrations of 0.083 +/- 0.032 microg/mL and MEGX concentrations of 0.012 +/- 0.009 microg/mL. Overall bioavailability of transdermal lidocaine was 6.3 +/- 2.7%, and only 56 +/- 29% of the total lidocaine dose delivered by the patch reached systemic circulation. Skin lidocaine concentrations were much higher than plasma concentrations, at 211 +/- 113 microg/g in the thoracic skin beneath the patch and 2.2 +/- 0.6 microg/g in the contralateral thoracic skin without the patch. As both lidocaine and MEGX were recovered from contralateral skin, it is likely that lidocaine accumulated in the skin from low systemic concentrations of circulating lidocaine over the 72-h period of patch application. Plasma lidocaine concentrations remained well below systemically toxic concentrations, and no obvious clinical side effects were observed in any of the cats. The low systemic absorption rate coupled with high local lidocaine concentrations on the skin support the safe use of lidocaine patches in cats.