Skin bleaching preparations 409 negro skin as well as in the treatment of various disorders of pigmentation. This formu- lation of hydroquinone was more potent than standard preparations. SCREENING COMPOUNDS FOR THEIR DEPIGMENTING EFFECT Of animal models, the guinea-pig has been found to be most useful for screening topically applied chemical compounds for their depigmenting effect (11, 12, 21). Creams contain- ing concentrations of 1 to 10K of various test chemicals have been applied to the epilated skin of pure black guinea-pigs for five days each week for periods of one month and their depigmenting potency assessed and compared with control areas treated with the cream base only (12). The assessment has been made visually, but also can be measured using a reflectance spectrophotometer. Of compounds tested using this bioassay method, Bleehen et al. (12) found that 4- isopropylcatechol was the most potent. However, when used in concentrations of 3• or more it was irritant to the skin and, like other substituted phenols, it was a sensitizer. So far, no depigmenting compound has been discovered which is reliable, effective and completely safe. All the compounds potent in producing cutaneous depigmentation have certain drawbacks, particularly their irritant effect on the skin. Other animals with black skins such as pigs have been used to screen compounds but the ultimate test for these compounds is their effect on man. The variation in the response to different species of animals to a drug is often considerable and what may be a potent depigmenting compound for guinea-pigs may not be for man. MODES OF ACTION OF DEPIGMENTING COMPOUNDS Exogenous compounds can interfere with the biological processes involved in the pro- duction and transfer of pigment granules. The possible modes of action are :- The compounds may selectively destroy the melanocytes. They may inhibit the formation of melanosomes and alter their structure. Inhibit the biosynthesis of tyrosinase. Inhibit the formation of melanin. They could interfere with the transfer of melanosomes. They could have a chemical effect on melanin or enhance the degradation of melano- somes in keratinocytes. Recent experimental studies have shown that a number of substituted phenols both in vivo (12, 21, 22) and in vitro (23, 24) have a specific melanocytotoxic effect. Hydro- quinone produces similar toxic effects on functional melanocytes affecting not only the formation, melanization and degradation of melanosomes, but also producing disrup- tion of membraneous cytoplasmic organelles (25). Studies on the melanocytotoxic action of the monomethyl ether of hydroquinone and on 4-isopropylcatechol have shown that both compounds have dose dependent lethal effects on cultures of normal guinea-pig melanocytes as shown by Riley (23) and malignant melanocytes by Bleehen (24). It seems likely that these compounds are con- verted by tyrosinase to form highly toxic oxidation products and electron spin resonance studies indicate that these metabolites are probably fi'ee radicals (23). These free radicals initiate a chain reaction of lipid peroxidation resulting in the irreversible damage of the lipoprotein membranes of the melanocyte and producing the death of this cell.

410 S. S. Bleehen CLINICAL EXPERIENCE OF VARIOUS SKIN BLEACHING PREPARATIONS Hydroquinone It is curious that the depigmenting effect of hydroquinone on the skin of man was dis- covered by chance. The manufacturers of a sunscreen containing this compound found that their preparation was mainly being bought for its effect as a skin bleaching cream (26). Though many workers Spencer (27) and Fitzpatrick et al. (28) have found prepara- tions containing 25/o to 5•o hydroquinone to be effective in producing cutaneous de- pigmentation, our own experience has been most disappointing. Higher concentrations of hydroquinone, while they are more potent, frequently irritate the skin and if used for a protracted period, can induce an exogenous ochronosis and produce pigmented colloid milium as shown by Findlay et al. (5). A suggested formula using 5•o hydroquinone, 0'1•o retinoic acid and 0'1•o dexa- methasone (20) does seem to be promising and is effective in producing depigmentation of human skin. Over the past year, a similar formulation of hydroquinone has been used but instead of dexamethasone, 1 •o hydrocortisone has been substituted. The formulation used in Sheffield is as follows :- Hydroquinone Hydrocortisone B.P. Retinoic Acid Butylated hydroxytoluene Methylated spirit 74 o.p. } Polyethylene glycol 300 •o w/v 5'00 1.00 0.10 0.05 q.s..100 ml 47.00 This preparation has been found to be effective in the treatment of various hypermelanotic conditions. A total of sixteen patients have been treated (Table II), thirteen of these being female and three male. Most of the female patients had melasma (chloasma) and this was mainly due to taking the contraceptive pill (Fig. 1). Two of the male patients had melasma, the other had post-inflammatory hyperpigmentation. The preparation was applied once or twice daily to the hypermelanotic areas and of the twelve patients with melasma, ten were significantly improved (Fig. 1), (Fig. 2). Two of these patients did not show much change even after 3 months of therapy. One patient with post-inflammatory hyper- pigmentation following lichen planus failed to respond and thought that she was made worse. Most of the patients have complained of skin irritation, especially during the early weeks of therapy. In some, erythema and scaling of the treated areas of skin were apparent. A skin bleaching effect was noted within 1 month in a third of the patients but the majority took much longer to respond to treatment and in one case, satisfactory depigmentation occurred only after 3 months of therapy. Table 1I. Disorders of pigmentation treated with new formulation of hydroquinone No. of patients Disorder treated No. improved Melasma 12 10 Post-inflammatory hyperpigmentation 2 1 Vitiligo 2 0