Differences in Response to Topical Irritants in Haired and Hairless Guinea Pigs

The hairless guinea pig offers the possibility of performing irritant studies without the use of depilatory agents or clipping. Studies have shown a response to allergens and simple irritants comparable to that of the haired guinea pig but with differences depending on substance and concentration used. Histoanatomical studies have demonstrated differences in cutaneous structure in the two strains, differences that might influence the response to complex low‐grade irritants such as composite vehicles. The purpose of this study was to compare the usability of hairless (HLGP) and clipped haired guinea pigs (CGP) in tolerability studies of composite formulations. The tolerability of six selected skin care formulations (SCF), known to cause a differentiated irritative response in the HLGP, was studied in 15 male CGPs and 15 male HLGPs. All animals were treated on a 5 × 5 cm area on each side of the dorsal trunk twice daily for 4 consecutive days with SCF. The tolerance of the different SCF was assessed by clinical assessment, measurement of transepidermal water loss (TEWL), and colorimetry (a*‐parameter). The results obtained using clinical scoring and noninvasive measurements were consistent for the HLGP. Colorimetry was found to be unsuited for the evaluation of cutaneous irritation in the CGP over a period of days as regrowth of fur will obfuscate the underlying erythema. Both species were able to differentiate between SCFs in relation to skin tolerance, and although the response pattern was somewhat different in the two species, the ranking of the SCF was essentially the same. However, HLGP appears to be a more suitable model for tolerability testing of composite formulations due to the avoidance of clipping, being both time‐consuming and having the risk of affecting the clinical outcome.