Novel skin-whitening agent

DETAILED DESCRIPTION OF THE INVENTION The active ingredient in the skin-whitening agent of the present invention is the dithiazole-monomethine-cyanine compound of the formula (I) wherein R is a straight chain or branched chain alkyl having 1 to 12 carbon atoms, such as methyl, ethyl, propyl, isopropyl, n-butyl, sec-butyl, tert-butyl, n-pentyl, n-hexyl, n-heptyl, n-octyl, n-decyl, n-undecyl and n-dodecyl; X is a halogen atom such as chlorine, bromine or iodine, or a residue of an acid such as perchloric acid, p-toluenesulfonic acid, nitric acid, nicotinic acid, or orotic acid.
Among the compounds of the present invention, one particularly preferred is a compound of the formula (I) wherein R is n-heptyl and X is iodine, i.
e.
4,4'-dimethyl-3,3'-di-n-heptyl-2,2'-monomethinethiazolocyanine iodide [i.
e.
di-(3-n-heptyl-4-methyl)thiazolylmethine] (hereinafter referred to as "pionine") in view of good absorbability (well balanced hydrophilic and lipophilic properties) and also easier preparation thereof.
It is known that the above preferred compound, pionine has some pharmacological activities such antibacterial activity and an activity for prevention of peroxidation of lipid in biomembrane, and this compound has been used in cosmetics as an antiseptic or antimicrobial agent at a concentration of 20 ppm.
However, it has never been known that this compound is remarkably effective for preventing the pigmentation of melanin.
It has newly been found by the present inventors that the compounds of formula (I) are effective for preventing some abnormal melanin conditions, e.
g.
an abnormal enhancement of melanosome functions due to ultraviolet irradiation and a remarkable activation of tyrosinase, which induce enhanced color of skin, and hence, are useful for preventing dyschromia such as stains and freckles by administering continuously.
In spite of the requirement of a long period of time for the treatment of stains and freckles, the skin-whitening agent containing as an active ingredient the above compounds of the present invention can be used very safely without undesirable side effects which are usually observed in the conventional melanin-bleaching agents, hormones or tyrosinase inhibitors