The hairless guinea-pig as a model for treatment of cumulative irritation in humans

Background: The effect of six skin-care formulations (SCFs) on experimentally induced cumulative irritation was studied in hairless guinea-pigs (HLGPs) and in human volunteers (HVs). The formulations were a basic cream, a carbomer cream and four modifications of the carbomer cream, containing either 10% isopropyl palmitate (IPP cream), 10% glycerol (glycerol cream), 19.5% canola oil (canola oil cream) or 0.5% (-)-alpha-bisabolol (bisabolol cream). Methods: In HLGP, irritant dermatitis was induced with 30 min daily exposure for 4 days to 0.5% sodium lauryl sulfate aq. (SLS). In HVs, irritant dermatitis was induced with 10 min daily exposure for 5+4 days (no irritation on weekends) to 3% SLS aq. on the right and 30% nonanoic acid (NON) in n-propanol on the left volar forearm. Clinical scoring was performed daily; evaporimetry (total epidermal water loss (TEWL)), hydration and colorimetry were measured at baseline (day 0) in the middle and at the end of treatment. Treatments were applied twice daily. The basic cream and the IPP cream were excluded from testing in HLGP because they were known from previous studies to be irritant in HLGP, while all formulations were known to be equally and well tolerated locally in humans. Results: All formulations worsened the skin irritation in HLGP: the glycerol cream the least, the canola oil cream the most, while the bisabolol cream and the carbomer cream were indistinguishable. In humans, the glycerol cream was better than 'No Treatment' after cumulative irritation with both SLS and NON. The basic cream was better tolerated in humans than was expected from previous testing in HLGPs. Conclusion: In conclusion, the results from the studies in HLGPs and HVs are in agreement with regard to ranking of the SCFs. Further, the glycerol cream showed a positive treatment effect on both SLS- and NON-irritated skin in HVs.