EFFECT OF SKIN SURFACE LIPID ON THE SKIN PERMEATION OF LIDOCAINE FROMPRESSURE-SENSITIVE ADHESIVES

Pressure sensitive adhesives (PSA) tapes containing different concentr ations of lidocaine were prepared by a general casting method using st yrene-isoprene-styrene block copolymer, and the in vitro skin permeati on of lidocaine from each tape was evaluated using diffusion cell and excised hairless rat skin. The skin permeation was proportionally incr eased by up to 40% lidocaine in the PSA tape and did not change after this concentration. Although the bending point of the steady-state flu x via skin concentration curve was found at 40%, saturated concentrati on or solubility of lidocaine in the tape was estimated to be about 20 % by differential scanning calorimetry (DSC) measurement. In addition, the steady-state flux of lidocaine through skin from water or silicon e fluid suspension (92 or 120 mu g/cm(2).h, respectively) was very sim ilar to those of 40, 50 and 60% tapes (105, 101 and 112 mu g/cm(2).h, respectively). Decrease in the concentration in tapes during the perme ation experiment explained only part of these phenomena. To analyze th em further, the drug free PSA tape with or without (control) skin surf ace lipid was affixed to 50% lidocaine PSA tape for 48 h, and the amou nt of lidocaine crystal in the layered tapes was measured by DSC. The amount was found to be lower in the lipid-containing tape than in the lipid-free tape, suggesting that skin surface lipid can dissolve lidoc aine crystal or solid in PSA tape to decrease its thermodynamic activi ty. Thus it is important to follow the concentration and thermodynamic activity of lidocaine in PSA tape, skin and the interface between the two layers to exactly assess its skin permeation flux.