Psoriasis

Tazarotene: a review of its pharmacological profile and potential for clinical use in psoriasis

Psoriasis appears to be a T-cell-mediated, HLA-associated genetic skin disease that profoundly alters epidermal differentiation in a reversible manner. The topical treatment of mild-to-moderate stable plaque psoriasis is limited by side-effects, cosmetic problems, and often by unsatisfactory efficacy, while systemic therapy is usually not warranted because of safety concerns. Tazarotene is the first member of a novel acetylenic and non-isomerisable class of retinoids to undergo extensive clinical testing. Tazarotene therapy regulates gene transcription via interaction with specific nuclear retinoic acid receptors (RARs), thereby modulating the three key pathogenic factors in psoriasis. Systemic absorption is minimal and, in contrast to some other retinoids, elimination is rapid. The results of Phase II and Phase III controlled clinical studies have shown tazarotene to be an effective treatment for psoriasis. The clinical response is rapid, and in many patients was sustained for several weeks following discontinuation of therapy. Adverse effects are generally limited to mild-to-moderate local effects, as seen with other topical retinoid therapies. Convenient once-daily application of tazarotene gel is effective first-line monotherapy for mild-to-moderate plaque psoriasis, providing rapid and sustained benefits, while minimal systemic absorption and rapid elimination appear to limit the potential for systemic side-effects.

The pathogenesis of psoriasis and the mechanism of action of tazarotene

The 3 major features of psoriasis--abnormal differentiation of keratinocytes, hyperproliferation of keratinocytes, and infiltration of inflammatory components into the skin--can be quantified by measuring levels of certain biochemical markers. Psoriasis is associated with upregulation or downregulation of several of these markers. Tazarotene helps to normalize the levels of the markers, thereby bringing about clinical improvement.

Oral retinoids--efficacy and toxicity in psoriasis

Psoriasis is a disease of unknown aetiology, affecting approximately 1-3% of the population. In most cases involving relatively localized disease, patients are best managed with either topical therapy alone or topical therapy in combination with UV-phototherapy. However, approximately 35% of patients do not respond well to these conventional treatments or have moderate-to-severe disease requiring more aggressive forms of therapy. The second-generation retinoids, etretinate and its metabolite acitretin, are important additions to the armamentarium of agents used to treat these recalcitrant or severe forms of the disease. Generalized pustular psoriasis generally responds well to high-dose (0.7-1 mg/kg/day) oral retinoid monotherapy. In contrast, increasing small doses of the retinoid are recommended initially in erythrodermic psoriasis in order not to provoke the disease. Long-term clinical experience favours a combination treatment of the retinoid with either topical and/or UV irradiation in chronic plaque-like psoriasis. Both oral retinoids have comparable efficacy and tolerability profiles, and the relapse rates for both drugs are similar. The toxicities associated with both short- and long-term treatment with oral retinoids are significant and include mucocutaneous effects, adverse modulation of serum lipid chemistries, elevation of liver enzymes, and after long-term chronic dosing, skeletal and ligamentous calcification, and hyperostosis. Both etretinate and acitretin, like all retinoids, are known teratogens in animal models, and documented evidence exists for teratogenic activity in humans as well. Consequently, women of childbearing age are strongly advised to avoid pregnancy during treatment and up to 5 years following cessation of therapy with both etretinate and the carboxylic acid metabolite acitretin.

Aetiology and pathogenesis of psoriasis

The hereditary transmission of psoriasis is suggested by epidemiological data and familial association, but remains incompletely defined, not appearing to follow simple autosomal dominant or recessive patterns. The confusion may be due to a multifactorial inheritance, or to inheritance of only a 'predisposition' to disease which requires an environmental stimuli for expression. Recent advances in genetic mapping indicate genetic heterogeneity, and suggest that definition of psoriasis at the level of the gene may soon be possible. Two of the three major pathogenic features of psoriasis--abnormal keratinocyte differentiation and hyperproliferation of keratinocytes--are secondary to altered growth and maturation kinetics related to the normal wound healing process. The third major pathogenic feature--infiltration of inflammatory components into the skin--can be explained by keratinocyte release of a wide variety of cytokines, immune and inflammatory modulators. Three theories have been proposed for the relationship between epidermal keratinocyte and immunocyte activation. The first theory proposes direct activation of epidermal keratinocytes by physical, chemical, or ultraviolet injury, increasing the synthesis and release of cytokines, which trigger T-lymphocyte activation in an antigen-independent fashion. The other two theories propose persistent T-lymphocyte stimulation as a result of either antigen/superantigen presentation by antigen-presenting cells, or as a result of autoreactivity. One or more of these mechanisms may be operative in different patients, at different times, or in response to different environmental stimuli. Also, the genetic heterogeneity of psoriasis suggests that different mechanisms could be linked to different genetic loci. Advances in understanding the aetiology and pathogenesis of psoriasis suggest the possibility of innovative, targeted therapies.