Animal Studies

Brownish guinea pigs have melanocytes in their skin and the skin pigmentation is enhanced by ultraviolet (UV) light irradiation, similar to the human situation. The preventative effect of EA on skin pigmentation was investigated by applying EA topically, on the back, for 6 weeks and irradiating by UV for first 2 weeks [8]. The appearance of skin to which EA was applied became similar to normal skin. The melanin content of the skin to which EA had been applied was reduced, not only in the basal layers but also in the stratum spinosum, -granulosum, and -corneum, in comparison with the same structures in control sections to which EA had not been applied. Tyrosinase activity was similar. Furthermore, application of EA to the skin after UV-light irradiation had almost the same affect as applying EA concurrently with the initial irradiation.

According to the results of the studies using the brownish guinea pig, EA is a more efficient skin whitener and suppressor of pigmentation than arbutin or kojic acid, other active skin whiteners, at the same dose level (1%) (Fig. 4).

Figure 4 Comparison of effects of some commercially available agents in preventing skin pigmentation induced by UV-light irradiation. Samples were applied for 4 weeks after UV-light irradiation (eight times): (a) before application;(b) after application for 4 weeks; (upper left) ellagic acid, (upper right) vehicle, (lower left) arbutin, (lower right) kojic acid.

Figure 4 Comparison of effects of some commercially available agents in preventing skin pigmentation induced by UV-light irradiation. Samples were applied for 4 weeks after UV-light irradiation (eight times): (a) before application;(b) after application for 4 weeks; (upper left) ellagic acid, (upper right) vehicle, (lower left) arbutin, (lower right) kojic acid.

Figure 5 Effects of ellagic acid on UV-light induced pigmentation. Samples were applied for 4 weeks (a) after UV-light irradiation (eight times): (upper left) hydroquinone;(upper right) vehicle only;(lower left) control—no EA applied; (lower right) ellagic acid. After application was terminated (b), the same area was irradiated again (c).

Figure 5 Effects of ellagic acid on UV-light induced pigmentation. Samples were applied for 4 weeks (a) after UV-light irradiation (eight times): (upper left) hydroquinone;(upper right) vehicle only;(lower left) control—no EA applied; (lower right) ellagic acid. After application was terminated (b), the same area was irradiated again (c).

Furthermore, the efficacy of EA was almost the same as that of hydroquinone (HQ), a well-known depigmentation agent (Fig. 5). When the same animals were subjected to UV irradiation again after completion of the application phase, normal skin pigmentation was observed in the EA-applied area as well as in the control areas, but only slight pigmentation was seen in the HQ-treated skin. The results of these investigations indicated that EA was not injurious to melanocytes but was a good inhibitor of tyrosinase activity. In comparison, HQ may be toxic to melanocytes.

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