D-penicillamine, a potent melanogenesis inhibitor lacks any depigmenting effect
on black guinea A The first randomized,, evaluator-blinded, vehicle-controlled,, in vivo study
M. Sharifian, F. Sari-Aslani, B. Hemmatinejad, M. K. Fallahzadeh, B. Kasraee, M. J. Khoshandish,
R. Miri, S. Mohammadi-Samani, F. Jowkar, M. R. Namazi
KEY WORDS
D-penicillamine, depigmentation, tyrosinase, melanin
-Abstract
D-penicillamine is a melanogenesis inhibitor. This in vivo study on ten black guinea pigs using a 5% D-penicillamine ointment showed its lack of any skin-lightening effect. The potential reasons for this ineffectiveness are discussed in the paper, which could be very helpful for researchers exploring new skin-lightening agents.
Eumelanin derives from the precursor dopaqui-none (ortho-quinone of 3,4-dihydroxyphenylalanine), which is formed via the oxidation of L-tyrosine by the melanogenic enzyme tyrosinase. Dopaquinone is a highly reactive ortho-quinone that plays pivotal roles in the chemical control of melanogenesis. Dopaqui-none undergoes intramolecular cyclization to form cyclodopa, which is then rapidly oxidized by a redox reaction with dopaquinone to give dopachrome (and dopa). Dopachrome then gradually and spontaneously rearranges to form 5,6-dihydroxyindole and to a lesser extent 5,6-dihydroxyindole-2-carboxylic acid, the ratio of which is determined by dopachrome tautomer-ase (tyrosinase-related protein-2). Oxidation and the subsequent polymerization of these dihydroxyindoles leads to the production of eumelanin (1).
Penicillamine is the drug of choice for the treatment of Wilson's disease due to its copper-chelating effect. This drug is a cysteine, doubly substituted with
methyl groups (Figure 1). A free sulphydryl group acts as the copper-chelator. Penicillamine also induces the production of metallothionein, an intracellular cys-teine-rich protein that is an endogenous chelator of metals (2). Tyrosinase, one of the essential enzymes in the synthesis of melanin, needs copper for its proper function. D-penicillamine, by chelating copper ions, can inhibit the activity of tyrosinase (3).
Moreover, penicillamine is a one-electron donor and prevents the production of dopachrome by reducing an oxidation product of dopa, probably a free radical. During the reaction the -SH group of penicillamine is oxidized (4). This can further contribute to its melanogenesis-inhibitory effect. Because non-pigmented melanoma cells are much more sensitive to gamma radiation and, given the melanogenesis inhibitory effect of D-penicillamine, this agent has recently been employed in vitro as a radiation sensitizer for killing melanoma cells (5). Penicillamine has also
been shown to inhibit melanoid pigment production by trichophyton species (6).
This study was conducted to evaluate the skin-lightening effect of topical D-penicillamine. Ten black guinea pigs were randomly divided into two groups of five pigs, groups A and B. Five-percent D- Penicillamine ointment with Eucerin as the base was made and applied on the right ears of group A pigs and left ears of group B pigs twice daily for three months. The other ears received the vehicle (Eucerin) as a placebo. The pigs' ears were evaluated for any color change weekly by two observers blinded to the treatment identification. Contrary to expectations and the strong theoretical grounds, no color change was detected by observers at any point in the study. Microscopic examination of drug- and placebo-treated sites using H & E and Masson-Fontana's stains also revealed no noticeable change.
Although D-penicillamine is a melanogenesis inhibitor, our study, which is the first in vivo study on the subject, did not reveal any lightening effect of this agent. The reason for the negative result of our study despite the strong theoretical background could be insufficient absorption of D-penicillamine through the stratum corneum. Nonpolar molecules lighter than 500 Dalton are believed to penetrate well through the stratum corneum, which is the major skin barrier (7), and D-penicillamine is a small molecule of 149.21 Dalton molecular weight. However, because D-peni-cillamine possesses both acidic and basic groups and
Figure 1. Chemical structure of D-penicillamine.
can produce different ionic species depending on the pH, we surmise that in the acidic pH of mammalian skin it is highly polar and is thus unable to penetrate the skin well.
Given the results of our study, we discourage future in vivo and human studies on this agent for skin lightening and suggest, for the first time, that even small molecules, if having an amino group, do not penetrate well into the skin due to polarization by absorbing a proton in the acidic pH of the skin surface.
Acknowledgement
We thank Iran's National Elite Organization for its support.
References
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authors ' Mohammad R. Namazi, Shiraz Skin Research Center, Shiraz University of addresses Medical Sciences, Shiraz, Iran, corresponding author, E-mail: namazi_mr@
yahoo.com
Maryam Sharifian, Shiraz Skin Research Center, same address
Fatemeh Sari-Aslani, Pathology Department, same address Bahram Hemmatinejad, Pharmacy School, same address Mohammad K. Fallahzadeh, Shiraz Health Policy Research Center, same address
Mohammad J. Khoshandish, Dermatology Department, same address Ramin Miri, Pharmacy School, same address SoleymanMohammadi-Samani, Pharmacy School, same address Farideh Jowkar, Dermatology Department, same address Behrooz Kasraee, Department of Dermatology, Geneva University Hospital, Geneva, Switzerland