728 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS to reversible interference with melanogenesis (6, 7) although the exact mechanism is unknown. In low dosage, hydroquinone does not appear to exert a cytocidal action on melanocytes, whereas the related compound monobenzone causes progressive depigmentation which can become per- manent (8), a change attributed to the destruction of melanocytes. The methyl ether of hydroquinone causes variable changes in the colour of black guinea pig skin, depending on the supplier of the compound (9), whereas the ethyl ether causes rapid depigmentation and when treatment is continued for 4 days melanocytes lack pigment although they remain DOPA positive, after prolongation of treatment to 3 weeks the DOPA reaction becomes negative. Cessation of treatment at this stage is followed by recovery, but if application is prolonged irreversible depigmentation results. Similar changes are caused by the methyl derivative. The implication of these observations may be that these compounds have cumulative pro- perties and over a period of time become cytocidal. The relationship between para-(p)-hydroxyanisole induced depigmenta- tion, vitiligo and the chemical specificity involved was studied by Riley (10). Activity is confined to three closely related compounds and although their mode of action is uncertain the similarity between para-(p)-hydroxyanisole suggests that substituted phenols may inhibit the synthesis of tyrosinase or its action. At concentrations greater than 1 mm the three structural isomers of hydroquinone have an inhibitory effect on protein synthesis (11), below this level the ortho- and meta-derivatives stimulate protein synthesis. In- hibition of RNA production is caused by hydroxyanisole at the 5 mM level, a dose which also blocks the respiratory chain probably by interfering with the ubiquinone requiring step. A relationship seems to exist between the depigmenting activity of the isomeric forms of hydroxyanisole and the extent to which they act as substrates for tyrosinase and it has been suggested that the cytotoxic action of substituted phenols, including 4-hydroxyanisole is due to their oxidation by tyrosinase to free radicals capable of initiating lipid peroxidation (12). Labelled compounds are incorporated in melano- cytes initially around the melanosomes and subsequently in pigment in adjacent keratinocytes. Unfortunately, attempts to define precisely the site of labelling by electron microscope autoradiography were unsuccessful. However, the close similarity between the distribution of tritiated 4-hy- droxyanisole and the distribution of the DOPA reaction product suggests that it acts as a substrate for tyrosinase. Cell damage may be due to a com- pound produced by oxidation by tyrosinase, a view supported by the pro- tection afforded by copper binding tyrosinase inhibitors such as glutathione

EFFECTS OF SKIN LIGHTENING COMPOUNDS 729 and sodium diethyl dithiocarbamate. Glycylglycine, a copper binding com- pound, is said to be an effective protective agent. Electron spin resonance spectra indicate that 4-hydroxyanisole causes the formation of a new species of free radical in the epidermis and it seems likely that this initiates lipid peroxidation in melanocytes leading to their destruction. One of the earliest lesions in epidermal tumours is protrusion of pseudo- pods through areas of the basement membrane, a process termed micro- invasion (13). These lesions occur in the early stages of tumours induced by local application of the carcinogen dimethylbenzanthracene, whereas non- carcinogenic irritants fail to produce them (14). Since an initial report that 4-hydroxyanisole (methyl ether of hydroquinone) and 3-hydroxyanisole caused micro-invasion, a number of related compounds have been investi- gated. 7cPhenyl-4-hydroxyanisole and butylated hydroxyanisole cause similar changes and the implication is that these compounds may be carcinogenic (13). However, encroachment of keratinocyte pseudopods into the dermis apparently resolves when treatment is discontinued, dis- tinguishing the effect of hydroxyanisole from that produced by the carcino- gen 7-12 dimethylbenz-(a)-anthracene (15). Although the monoethyl ether of hydroquinone is a potent depigmenting agent, little is known of its precise mode of action (16). Electron microscope studies reveal that epidermal melanocytes are destroyed and subsequently removed by macrophages. Topical application of 8-hydroxyquinoline causes depigrnentation in several strains of mice, rather surprisingly females are mainly affected. This com- pound appears to act selectively on pigment cells at a certain stage of their mitotic cycle and consequently a variegated response occurs (17). The mono- ethyl ether of hydroquinone causes pigment to disappear from the epidermis and fur of the guinea pig contrasting with the action of N-mercaptoethyl dimethylamine which acts on only the pigment in the epidermis (18). Pro- longed treatment with the monoethyl ether causes irreversible depigrnenta- tion comparable to that observed in man after use of hydroquinone mono- benzyl ether. The unpredictability of the effect of the latter compound and the frequent progressive depigrnentation after cessation of treatment, has discouraged its use as a skin lightener. Leuco-melanoderma or leucoderma developing from use of this compound is resistant to corrective treatment. Some of the adverse effects attributed to the mono-benzylether derivative may be due to hydroquinone (19). Experimentation with human volunteers has involved some 380 white and negro males (13). Compounds were applied twice daily, 7 days each week for periods up to 4 months. After 2 months treatment with hydroquinone derivatives, at 10•o level, these compounds were