AGENTS AFFECTING PIGMENTATION
Hydroquinone,
monobenzone (Benoquin, the monobenzyl ether of hydroquinone), and mequinol (the
monomethyl ether of hydro-quinone) are used to reduce hyperpigmentation of the
skin. Topical hydroquinone and mequinol usually result in temporary
lighten-ing, whereas monobenzone causes irreversible depigmentation.
The
mechanism of action of these compounds appears to involve inhibition of the
enzyme tyrosinase, thus interfering with the bio-synthesis of melanin. In
addition, monobenzone may be toxic to melanocytes, resulting in permanent loss
of these cells. Some percu-taneous absorption of these compounds takes place,
because monobenzone may cause hypopigmentation at sites distant from the area
of application. Both hydroquinone and monobenzone may cause local irritation.
Allergic contact dermatitis to these compounds can occur. Prescription
combinations of hydroqui-none, fluocinolone acetonide, and retinoic acid (Tri-Luma)
and mequinol and retinoic acid (Solagé) are more effective than their
individual components.
Trioxsalen and
methoxsalen are psoralens used for the repigmenta-tion of depigmented macules
of vitiligo. With the recent develop-ment of high-intensity long-wave
ultraviolet fluorescent lamps, photochemotherapy with oral methoxsalen for
psoriasis and with oral trioxsalen for vitiligo has been under intensive
investigation.
Psoralens
must be photoactivated by long-wavelength ultravio-let light in the range of
320–400 nm (ultraviolet A [UVA]) to produce a beneficial effect. Psoralens
intercalate with DNA and, with subsequent UVA irradiation, cyclobutane adducts
are formed with pyrimidine bases. Both monofunctional and bifunctional adducts
may be formed, the latter causing interstrand cross-links. These DNA
photoproducts may inhibit DNA synthesis. The major long-term risks of psoralen
photochemotherapy are cata-racts and skin cancer.
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