The macrolide immunosuppressants in dermatology: mechanisms of action

ARTICLE

Many dermatological conditions are benefited by treatment with immunomodulating drugs. Indeed for the past fifty years, topical or systemic glucocorticosteroids have been the mainstay of immunosuppressive therapy. Despite their undoubted efficacy, glucocorticosteroid use has always been limited by potential for local and systemic side effects, notably skin atrophy.

Over the last fifteen years dermatological therapy has been revolutionised by the use of nonsteroid, immunosuppressive drugs that were primarily developed for use in transplantation medicine. The first of these, cyclosporin A, was isolated from a soil fungus, Tolypocladium inflatum gans, and initially developed as an anti-fungal agent before its immunomodulatory properties were realised. Systemic cyclosporin, commonly used to prevent rejection of transplanted organs, is effective in and licensed for the treatment of common inflammatory dermatoses including atopic dermatitis and psoriasis. Although undoubtedly effective the use of systemic cyclosporin has been limited by concerns regarding side-effects including nephrotoxicity and hypertension. Topical cyclosporin penetrates skin very poorly if at all and cannot be used in place of the systemic formulation. Other drugs used in transplantation medicine, in particular the macrolide immunosuppressants, are being developed for use in dermatological conditions [1].

Macrolides are a group of xenobiotics with a complex macrocyclic structure. They are produced naturally, by various strains of Streptomyces, and inhibit the growth of competing yeasts and fungi [2]. Macrolide antibiotics such as erythromycin have been used for over 40 years, but the immunosuppressant activity of some macrolide compounds was only identified in the last 25 years. Macrolides are usually used to prevent organ transplant rejection [3, 4] and are now being evaluated for their use in the treatment of autoimmune diseases and inflammatory dermatoses [5].

The first macrolide to be developed was tacrolimus, initially known as FK506. It is produced naturally by Streptomyces tsukubaensis. Intravenous and oral tacrolimus is used effectively in transplantation medicine; a topical formulation has been developed which, unlike cyclosporin, penetrates skin well [6]. A number of controlled trials have shown topical tacrolimus to be safe and effective in the treatment of atopic dermatitis [6].

Ascomycins are macrolide immunosuppressants that have a chemical structure and mechanism of action similar to tacrolimus [6]. They are produced by a different strain of Streptomyces, Streptomyces hygroscopicus var. ascomycetius. Initial studies have shown that topical pimecrolimus, an ascomycin derivative formerly known as SDZ ASM 981, is safe and effective in the treatment of atopic dermatitis [7]. An early study has indicated that oral pimecrolimus is effective in the treatment of moderate-to-severe plaque psoriasis [8].

Rapamycin, otherwise known as sirolimus, was isolated from a streptomycete obtained in soil from Rapa Nui (Easter Island) in the 1960s. Like cyclosporin, it was initially investigated for its antifungal properties before its immunosuppressive actions were recognised. It is licensed for use in transplantation medicine and is being evaluated for treatment of inflammatory dermatoses [5].

New macrolides are being developed as their potential in the treatment of inflammatory disorders and transplant medicine is well recognised. A derivative of rapamycin, everolimus (SDZ RAP) is undergoing evaluation for use in transplant medicine [9]. Dunaimycins, a class of macrolide synthesised by Streptomyces diastatochromogenes, have also been identified as immunosuppressants [10]. A recent study has identified a new class of macrolide - efomycine - which blocks selectins thereby inhibiting leucocyte trafficking [11]. Efomycine M appears to be effective in improving inflammation in mouse models of psoriasis [11].

Mechanisms of action

Cyclosporin, pimecrolimus, tacrolimus and sirolimus make up a tetrad of prodrugs that are active after forming complexes with intracytoplasmic proteins called immunophilins: cyclosporin binds cyclophilin whilst pimecrolimus, tacrolimus and sirolimus bind the so-called tacrolimus binding protein (FK-BP). The drugs exert their effects on the target cell via these complexes. The immunosuppressant effects of these drugs appear to derive predominantly from inhibition of T-cell function and it is in these cells that their mechanisms of action have been most extensively studied. There are two main routes whereby the immunophilin-drug complex inhibits T-cell activation.

Inhibition of calcineurin: cyclosporin, tacrolimus and pimecrolimus

Figure 1 shows the pathway via which these compounds inhibit activation of T-cells. Following stimulation by an antigen-presenting cell via the T-cell receptor, intracytoplasmic levels of calcium rise and calmodulin may activate the phosphorylase enzyme, calcineurin. Calcineurin works by dephosphorylating specific cytoplasmic proteins known as Nuclear Factors of Activated T-Cells (NF-AT_c). These proteins, once dephosphorylated, are able to translocate into the nucleus where they may combine with their nuclear subunits (NF-AT_n). The resulting nuclear complex binds to the promoter unit of several genes enabling transcription of pro-inflammatory cytokines and induction of their receptors. As a result, a stimulated T-cell produces crucial proinflammatory cytokines such as interleukin (IL)-2, IL-4, interferon-gamma (IFN-gamma) and transforming growth factor-beta (TGF-beta). Receptors such as IL-2R (CD25) are also upregulated during this process of activation. The calcineurin-NFAT system of signal transduction is utilised in other cells in the immune system including mast cells [12] and neutrophils [13].

Cyclosporin, tacrolimus and pimecrolimus, when bound to their cognate immunophilins, inhibit the action of calcineurin, thus preventing dephosphorylation of nuclear factors and blocking this path to gene transcription [14]. In stimulated T-lymphocytes, these drugs inhibit activation principally by suppressing IL-2 production and IL-2R expression. In stimulated mast cells, pimecrolimus [15], and to a lesser extent, cyclosporin and tacrolimus, decrease histamine release [12, 15]. It has been demonstrated that tacrolimus inhibits Langerhans' cell function and down-regulates high affinity IgE receptor expression [16]. In histamine-stimulated endothelial cells, tacrolimus and cyclosporin have been shown to decrease production of monocyte chemotactic protein-1 and IL-8 [17].

The calcineurin-NFAT mechanism of signal transduction is utilised in cells outwith the immune system, but its importance in immune activation is borne out by the apparent immune specificity of cyclosporin and the macrolides. The main systemic side-effects of cyclosporin and tacrolimus, namely hypertension and nephrotoxicity, appear to be related to drug-induced endothelin release that is independent of calcineurin inhibition [18].

Inhibition of TOR: sirolimus

The complex formed between sirolimus and its immunophilin FK-BP does not exert its immunosuppressive effect by inhibition of calcineurin. The complex in fact binds to a distinct target protein named Target of Rapamycin (TOR) [19, 20]. TOR proteins are evolutionarily highly conserved kinases that control the cell cycle in response to external stimuli via growth factor receptors. TOR proteins play a crucial role in coordinating the equilibrium between protein synthesis and breakdown in response to nutrient availability. They have been studied extensively in yeasts, metazoans and mammalian cells. Essentially, TOR proteins act on downstream proteins responsible for controlling translation of mRNAs encoding proteins that regulate the cell cycle (Fig. 2). These include translation inhibitors (e.g. 4E-BPs), eukaryotic translation initiators (eIF4GI) and ribosomal S6 kinases (S6Ks) [20].

Inactivation of TOR proteins by sirolimus mimics starvation in yeast, Drosophila, and mammalian cells, and arrests normal progression from G1 phase to S phase of the cell cycle. TOR signalling pathways are utilised by T and B-lymphocytes when activated by IL-2 and other stimuli, rendering these cells sensitive to rapamycin, which effectively blocks activation and clonal expansion. A non-cytotoxic dose of sirolimus inhibits primary and metastatic tumour growth by interfering with angiogenesis: it induces a decrease in production of vascular endothelial growth factor (VEGF) and a block to stimulation of vascular endothelial cells by VEGF [21]. This may also be relevant to its mechanism of action in psoriasis, as angiogenic factors are important to the biology of this disease [22].

Topical and systemic properties of macrolides

Glucocorticosteroids are effective immunosuppressants consequent on binding the glucocorticoid receptor; the resultant complex acts as a transcription factor which blocks expression of genes for pro-inflammatory cytokines. Unfortunately, the same complex also blocks collagen gene expression and as a result prolonged topical treatment may inhibit synthesis of dermal collagen leading to skin atrophy. In contrast, macrolides do not inhibit collagen synthesis and clinical trials indicate that they do not cause skin atrophy when applied topically [23, 24].

Despite its similar mechanism of action, pimecrolimus appears to differ from cyclosporin and tacrolimus in its immune specificity. Data from animal studies indicate that although it is highly effective in models of skin inflammation, pimecrolimus has relatively low activity in models of immunosuppression [25]. Although topical tacrolimus and pimecrolimus improve both atopic and allergic contact dermatitis, they are only effective in psoriasis when applied under occlusion or when administered systemically [25]. The relative "skin specificity" of pimecrolimus may prove advantageous as the systemic immunosuppressant side-effects of cyclosporin and tacrolimus are of concern.

As sirolimus has different mechanisms of action and side-effects from cyclosporin and the other macrolides, it appears to work synergistically with them. A combination of systemic low dose cyclosporin and sirolimus has fewer side-effects than either drug but efficacy equivalent to high dose cyclosporin in preventing transplant rejection and treatment of psoriasis [4, 26]. The discovery of the anti-angiogenic and anti-tumour action of sirolimus is an important development. Where patients have already received potentially tumourigenic therapies such as psoralen plus ultra-violet A photochemotherapy (PUVA) and therefore have limited treatment options available, sirolimus may yet provide a role as it has potential antitumour effects. The dunaimycins and everolimus, which also exert their immunosuppressant effects by inhibiting TOR, have yet to be evaluated for their use in dermatology.

Macrolides: new drugs for the millennium?

An understanding of the mechanisms of action of macrolide immunosuppressants and related drugs has helped identify key processes involved in cellular signalling and has given us insight into disease processes. New, potential immunosuppressant drugs have been identified by harnessing this knowledge [27]. Macrolides with similar mechanisms of action have been demonstrated to have different immune specificities, and justifies a search for novel compounds in this class. The future therapy for inflammatory dermatoses appears to be in these immune-specific drugs as replacements for glucocorticosteroids.

Article accepted on 11/6/02

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