Comparison of the response to topical irritants in hairless guinea pigs and human volunteers

Small rodent laboratory animals lack the complex cutaneous structure and function of human skin, resulting in "all or none'' responses to mild irritants so that the animals may show a less discriminative reaction pattern compared to human volunteers (HV). However, histological studies suggest that the skin of the hairless guinea pig (HLGP) is more similar to human skin than to the skin of haired guinea pigs and other rodents. We compared the tolerance pattern of six composite topical formulations with weak irritant potential in 20 human volunteers (HV) and in 15 male HLGPs. The skin care formulations (SCF), with and without either isopropyl palmitate, glycerol, canola oil, or (-)-alpha-bisabolol, were selected because they were known to cause a differentiated irritative response in HLGP. The HLGPs were treated twice a day on a 5 x 5 cm area on each flank with a SCF for four consecutive days. The irritant effects were quantified by clinical assessment, measurement of transepidermal water loss (TEWL), and colorimetry (a*-parameter). In humans the tolerability was evaluated clinically using the chamber scarification test. The ranking of the formulations was similar in the two models. However, HLGPs were statistically more sensitive to the formulations. Negative results in HLGPs are predictive of good tolerability in humans; however, positive results in the HLGPs do not necessarily indicate that a topical formulation cannot be used in humans.