Current Pharmaceutical Design - Volume 6, Issue 9, 2000

Treatment of psoriasis is rapidly changing, and there have been dramatic additions such as topical vitamin D analogs and a topical retinoid to the therapeutic armamentarium in recent years. Even though drug therapy is often inadequate and serves only to abate the skin disorder, anthralin of the anthracenone antipsoriatics clears the rash totally. Of all antipsoriatic agents, anthralin has proven to be the most remarkably consistent and time-honored drug for treating the disease. However, anthralin therapy is accompanied by inflammation and staining of the nonaffected skin surrounding a psoriatic lesion. A large body of evidence has shown that the biochemical basis for its mechanism of action at the molecular level is related to its redox chemistry leading to the production of oxygen radicals. These can react with all classes of biological macromolecules and are involved in the principal cellular effects of the anthracenones. This article will focus on current strategies to minimize the potential of skin inflammation by anthralin and the underlying concepts of controlling and manipulating oxygen-radical generation which are essential for the rational design of novel anthracenones. Various studies have shown that by altering the structure of anthralin the in vitro profile can be markedly altered from an only moderate 5-lipoxygenase inhibitor to a more potent inhibitor of this enzyme, which may reflect improved activity against the inflammatory component of psoriasis while the antiproliferative activity is retained. The structural change is also accompanied by significantly diminished oxygen-radical generation and proinflammatory action on the skin. In particular, derivatives bearing a phenylacyl substituent in the critical 10-position of the anthracenone molecule have a favorable overall profile for achieving improved topical therapy of psoriasis.

Natural (all trans-retinoic acid, RA) and synthetic retinoids exhibit potent anti-proliferative, normalization of differentiation and anti-inflammatory activities which appear to account for their therapeutic effects in acne, psoriasis, photoaging, precancerous lesions and established cancers. Although RA has shown considerable promise in dermatologic indications, certain side effects have restricted its use as a choice of agent for chronic administration. Systematic synthesis of receptor-selective retinoids has resulted in two topical drugs, Tazorac/Zorac (tazarotene) and Differin (adapalene). Tazorac is indicated for psoriasis and acne and Differin gel for the treatment of acne. These drugs bind to the retinoic acid receptor (RAR) family members. Various RAR subtype-specific and function-selective retinoids have been synthesized. These retinoids, which are in various stages of pre-clinical development for the treatment of cancers, psoriasis and as an antidote to Accutane-mediated mucocutaneous toxicity, will also be discussed in this review. Discovery of another retinoid receptor, retinoid X receptor (RXR), revealed that RXR-specific retinoids already existed in retinoid chemical libraries. Structure activity relationship studies based upon binding and transactivation assays led to the synthesis of RXR-specific ligands with high affinities for RXR subtypes. These compounds were found to be effective in the treatment of hyperglycemia in animal models of type II diabetes. The discovery of novel retinoids along with an increased understanding of the biological functions and mechanisms of action of retinoid receptors are likely to result in improved treatments for existing responsive indications and identification of new retinoid therapeutic targets.

Psoriasis is a heterogenous skin disease, characterized by epidermal hyperproliferation, abnormal keratinization and inflammation. The heterogeneity of the disease results probably from the interaction of multiple gene abnormalities with environmental factors.The new approaches to drug design have become refocused to the emerging understanding of the role of signaling pathways in health and disease. Protein tyro-sine kinases (PTKs) regulate cell proliferation, differentiation and immune processes. Uncontrolled signaling from receptor and intracellular tyrosine kinases can lead to numerous proliferative diseases: cancer, leukemia, restenosis and psoriasis. Identification of PTKs that play a key role in a defined disease can lead to a selective drug.The balance of signals which regulate the homeostasis of normal epidermis is altered in psoriasis. Several lines of evidence suggest a role for the EGF receptor system in this process. Therefore, blockers of the EGFR kinase were suggested as potent antipsoriasis agents. PTK inhibitors from the tyrphostin family were found to block EGF - dependent cell proliferation. AG 1571 (SU 5271) potently inhibits ligand-induced autophosphorylation of EGF-R, downstream signal transduction events, DNA replication and cell cycle progression at micromolar concentrations, as well as proliferation of keratinocytes isolated from psoriatic lesions in excellent correlation with its EGFR kinase inhibitory activity in these cells. AG 1571 (SU 5271) has been in clinical trials by SUGEN Inc. since early 1997. Overexpressoin of the EGFR is the hallmark of most epithelial cancers. Therefore one can view blockers of the EGFR kinase as becoming universal inhibitors. Tyrphostins are the first signal transduction agents to be used in the clinic. This article summarizes recent progress in the development of PTK inhibitors in the treatment of psoriasis.

Purine nucleoside phosphorylase (PNP) is one of the enzymes comprising the purine salvage pathway, and is responsible for the catalysis of the reversible phosphorolytic cleavage of purine ribonucleosides and 2-deoxyribonucleosides. The pivotal role of PNP in T-cell proliferation has been demonstrated in patients with inherited PNP deficiency, where T-cell levels may be 1-3percent of normal. This observation helped establish the critical role of PNP in T-cells and provided a rationale for developing inhibitors of PNP. Inhibitors of PNP may be useful for treating a variety of T-cell related autoimmune diseases including psoriasis, rheumatoid arthritis and Crohns disease and T-cell cancers. In this manuscript, the x-ray crystal structure of the PNP enzyme is described. Results of a structure-based drug design program aimed at designing small-molecule inhibitors of PNP are also described. Of the many classes of compounds synthesized, studied and reviewed, only one, the 3-pyridinylmethyl-9-deazaguanine (BCX-34, 39) analog has been used in clinical trials. Both topical and oral formulations of BCX-34 were studied in psoriatic patients and the results of these clinical trials are described.

Within the past decade it has been shown that psoriasis can be treated topically with analogs of vitamin-D3 . Impaired differentiation and increased proliferation of keratinocytes are key features in psoriatic lesions together with a local activation of T lymphocytes. Evidence has accumulated showing that analogs of vitamin D3 increase differentiation and inhibit proliferation of keratinocytes. Therefore, analogs of vitamin D3 have been investigated in a number of trials showing improvement of psoriasis. It has been shown that vitamin D analogs are better than their vehicle and show the same potency as potent topical steroids. However, vitamin D analogs have been proven efficacious and without side effects also when used on long term basis. Vitamin D analogs can be used both as monotherapy and in combination topical steroids, UVB, PUVA, retinoids and cysclosporine. The vitamin D3 analog calcipotriol has been investigated in most detail and is available as an ointment, a creme and as a scalp solutation. From clinical studies involving thousands of patients, it can be concluded that calcipotriol is efficacious, safe, well tolerated and can be used on a long term basis. Other analogs are available, however, these analogs have not been studied in greater details yet.