The isolated normothermic hemoperfused porcine forelimb as a test system for transdermal absorption studies

For transdermal absorption studies, a porcine skin model that maintains the characteristics of the skin as close to physiological conditions as possible should be established because of the close similarity of porcine and human skin. Perfusion flow, glucose concentration, and oxygen supply were varied and organ resistance and lactate concentration were measured as parameters for organ performance. Transdermal absorption studies were performed using nitroglycerin or estradiol as test solutes. By the use of a standardized perfusion apparatus, limbs were perfused with a normothermic blood/electrolyte mix over a 6-h period. Organ resistance and lactate concentration were lower in limbs perfused with high flow as compared to perfusion with low flow. However, high perfusion flow caused an initial increase of hemolysis in the venous perfusate resulting in a higher concentration of free hemoglobin compared to low perfusion flow conditions. Glucose level in the perfusate was maintained within the porcine physiological range. By applying optimized conditions, the transdermal absorption of nitroglycerin or estradiol released from transdermal therapeutic systems was investigated. A time-dependent accumulation of the substances in the perfusion medium was observed, indicating the continuous uptake via the skin. After 6 h (nitroglycerin) or 5 h (estradiol) of perfusion, test substances accumulated to concentrations comparable to those described in humans. In conclusion the perfused porcine limb may serve as a reliable test system for transdermal absorption studies. The model is suited to the replacement of animal testing and has a higher predictive potential because of the similarities of porcine and human skin.