2‐Mercaptonicotinoyl glycine prevents UV‐induced skin darkening and delayed tanning in healthy subjects: A randomized controlled clinical study

Abstract Background Chronic nonextreme sun exposure induces two mechanisms of skin pigmentation, causing immediate darkening and delayed tanning. A new molecule, 2‐mercaptonicotinoyl glycine (2‐MNG), has been shown in vitro to inhibit both immediate darkening and new melanin synthesis via covalent conjugation of the thiol group of 2‐MNG to melanin precursors. Objective To evaluate 2‐MNG in preventing both mechanisms in vivo. Methods In a randomized, intra‐individual and controlled study, 33 subjects with melanin‐rich skin were exposed to UV daylight on designated areas on the back and treated with a cosmetic formula containing 0.5% or 1% 2‐MNG alone or 0.5% 2‐MNG in association with lipohydroxy acid (LHA, 0.3%) plus Mexoryl‐SX (MSX, 1.5%). The respective vehicles were used as controls and 4‐n‐butyl‐resorcinol (4‐n‐BR, 2.5%) as a positive reference. Results 2‐MNG alone significantly reduced immediate darkening and inhibited new melanin production when compared with vehicle, with higher performance at 1% than at 0.5%. 2‐MNG at 0.5% in association with LHA and MSX showed significantly higher performance than 2‐MNG 0.5% alone. 2‐MNG at 0.5% and 1% showed significantly better performance than 4‐n‐BR. Conclusions 2‐MNG inhibited both UV‐induced skin pigmentation mechanisms in vivo. The association of 2‐MNG with LHA plus MSX showed the highest efficacy on melanin‐rich skin with pigmentation induced by UV exposure.