Pimecrolimus – an anti-inflammatory drug targeting the skin
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Abstract: Pimecrolimus is the most recent member of calcineurin inhibitors available for the therapy for inflammatory skin diseases. It targets T-cells and mast cells and inhibits the production and release of cytokines and other inflammatory mediators, as well as the expression of signals essential for the activation of inflammatory T-lymphocytes. Pimecrolimus has a cell-selective mode of action. In contrast to corticosteroids, it does not affect, e.g., Langerhans'cells/dendritic cells (LC/DC), as demonstrated in vitro with human monocyte-derived DC and in vivo with epidermal LC in mice, nor human primary fibroblasts. As shown in vitro with human skin and by comparison of clinical pharmacokinetic data from patients with atopic dermatitis, pimecrolimus permeates less through skin than tacrolimus and much less than corticosteroids. It, thus, has a lower potential for transcutaneous resorption after topical administration, resulting in a lower risk of systemic effects. Pimecrolimus has high anti-inflammatory activity in animal models of skin inflammation, including a model reflecting neurogenic inflammation, but a more favourable balance of anti-inflammatory vs. immunosuppressive activity than tacrolimus. Pimecrolimus does not affect sensitization in a murine model of allergic contact dermatitis and has a lower potency in various models of immunosuppression after systemic administration, compared to tacrolimus. In conclusion, the results of preclinical studies show that pimecrolimus has a selective pharmacological profile, suited for effective and safe treatment for inflammatory skin diseases.
Pimecrolimus (ASM981) is the most recent member of a triad of calcineurin inhibitors – cyclosporin A, tacrolimus, and pimecrolimus – that are now available for the therapy for inflammatory skin diseases. Oral cyclosporin A (Sandimmune®, Neoral®) is used as a second-line therapy for severe cases of psoriasis and atopic dermatitis, and tacrolimus (Protopic® ointment) for the topical treatment for atopic dermatitis (1–4). Topical formulations of cyclosporin A proved to be ineffective (5). For oral tacrolimus (Prograf®), only results from small studies on patients with psoriasis have been published so far and the drug is not registered for the oral treatment for skin diseases (6,7). Whereas cyclosporin A and tacrolimus were originally developed for the prevention of graft organ rejection in transplantation patients, and are widely used in this indication, pimecrolimus was specifically developed for the treatment of inflammatory skin diseases (8). Topical pimecrolimus as cream 1% (Elidel®) proved to be highly effective, safe and well tolerated in patients with atopic dermatitis (9–11). Moreover, early clinical trials have shown that orally applied pimecrolimus was very effective for the treatment for psoriasis and atopic dermatitis (12–14). This review highlights the pharmacological profile of pimecrolimus, and also focuses on the differences to tacrolimus as well as to corticosteroids – the latter being the mainstay of topical therapy for atopic dermatitis and other inflammatory skin diseases over the past 50 years.
Pimecrolimus is a cytokine inhibitor that targets T-cells and mast cells
Activation of T-cells plays a crucial role in the pathogenesis of many inflammatory skin diseases, including atopic eczema/dermatitis. Langerhans' cells (LC) residing in the epidermis capture allergens very efficiently via immunoglobulin E (IgE) bound to the high-affinity FcεRI receptors. After processing, antigens are presented by the LC to antigen-specific T-cells, which become activated to produce and release pro-inflammatory cytokines. This triggers a cascade of events ultimately resulting in the typical features of atopic dermatitis: pruritus and inflammation. Pimecrolimus inhibits the production and release of pro-inflammatory cytokines in T-cells in vitro. This involves T-helper type-1 cell (TH1) as well as T-helper type-2 cell (TH2) cytokines, such as interleukin-2 (IL-2), interferon-γ (INF-γ), IL-4, IL-8 and IL-10, as well as tumour necrosis factor-α (TNF-α). Pimecrolimus like tacrolimus and the corticosteroids betamethasone-17-valerate and dexamethasone is effective at nanomolar concentrations (15,16). In addition, pimecrolimus inhibits T-cell proliferation, as shown in murine and human mixed lymphocyte reactions (MLR) as well as after unspecific or antigen-specific activation of a human T-cell clone, isolated from a patient with atopic dermatitis (15). This inhibitory effect might be indirectly mediated via the downregulation of cytokines, such as IL-2 and IL-4, which are known to function as autocrine and paracrine growth factors for T-cells. However, at higher concentrations, pimecrolimus was found to inhibit T-cell proliferation even in the presence of exogenous IL-2, indicating that it might additionally affect the expression of the IL-2 receptor (15). Most importantly, the effect on T-cell proliferation seems to be specific, because pimecrolimus was reported not to affect the proliferation of B-cells, keratinocytes, endothelial cells or fibroblasts (15). In addition to affecting T-cell proliferation, pimecrolimus also was shown to downregulate the expression of co-stimulatory signals on T-cells that are essential for their differentiation into inflammatory effector T-lymphocytes (17).
Mast cells are a heterogeneous group of bone-marrow-derived multifunctional cells, which play an important role in the pathophysiology of allergy, parasite infestation, inflammation, angiogenesis and tissue remodelling. According to their neutral protease content, various subtypes of mast cells may be distinguished: the MCT phenotype containing only tryptase and the MCTC phenotype containing both tryptase and chymase. Both phenotypes express FcεRI and thus may participate in IgE-dependent allergic reactions. In addition to enzymes, mast cells also contain several other inflammatory mediators, such as histamine, cytokines and eicosanoids (18). There are several reports that pimecrolimus may alter the function of activated mast cells. Using a murine mast cell line, rat basophil leukaemia cells and primary human skin mast cells, pimecrolimus was found to inhibit the production of TNF-α and to prevent the release of preformed pro-inflammatory mediators, such as histamine, tryptase, and hexosaminidase, by degranulation from mast cells (15,19,20).
Pimecrolimus has a cell-selective mode of action
In normal human skin, two types of dendritic cells (DC) can be differentiated: LC in the epidermis and dermal DC. Upon inflammation, another type of epidermal DC occurs that has been named inflammatory dendritic epidermal cell (IDEC). It is responsible for the generation of a TH1 immune response as well as for an increased T-lymphocyte reactivity to epidermal cells (EC) (21). According to their capacity to express co-stimulatory molecules and to activate various subsets of T-lymphocytes, DC may be divided into a tolerogenic immature and an immunogenic mature phenotype. Recent findings indicate that there is, in addition, a subset of partially matured DC that can function as inducer of regulatory CD4+ T-cells (22). Therefore, the capacity of pimecrolimus to affect DC differentiation and maturation was investigated in comparison with that of corticosteroids (23,24). In vitro dexamethasone, as well as betamethasone-17-valerate, dose-dependently inhibited the IL-4/granulocyte–macrophage colony-stimulating factor (IL-4/GM-CSF)-induced differentiation of DC from human monocytes. Moreover, the expression of important co-stimulatory molecules, such as CD83 and CD86, on DC also was downregulated in the presence of the corticosteroids indicating their capacity to inhibit the maturation of DC in culture. Corticosteroid-treated DC were shown to have consequently a reduced capacity to stimulate T-cell proliferation. In contrast to corticosteroids, pimecrolimus had no effect on differentiation and maturation nor on the capacity of human and murine DC to activate T-cells. Tacrolimus, however, was reported to impair the maturation of human LC in vitro, as indicated by the inhibition of the expression of various maturation markers (CD25, CD80 and CD40) and the inability to stimulate T-cells (25,26). In patients with atopic dermatitis, treatment with tacrolimus caused depletion of IDEC, but no LC apoptosis (27). In vivo, using murine epidermis, the effect of pimecrolimus and corticosteroids on the density of LC was investigated (28). Topical application of a potent corticosteroid, such as clobetasol-propionate (0.05%), twice daily to murine skin resulted in a complete depletion of LC within 5 days. A similar reduction of the number of LC was observed within 5 days when corticosteroids of medium potency, such as betamethasone-valerate (0.1%), and even of low potency, such as hydrocortisone (1.0%), were used. Regardless of the potency of the corticosteroid used, the repopulation of epidermal LC began only 3–4 weeks after discontinuation of the steroid application and was still incomplete after 5 weeks. By contrast, topical treatment with pimecrolimus (1.0%) for 5 days did not affect the density of epidermal LC at all.
A crucial function of calcineurin inhibitors is their capacity to inhibit the synthesis of immunomodulating pro-inflammatory cytokines. Pimecrolimus was found to significantly downregulate TNF-α- and IL-8 expression in T-cells (29). It did not, however, interfere with TNF-α synthesis in DC, as shown in a genetically transformed murine cell line, in which the expression of a reporter gene was under the control of the human TNF-α promoter (15). This finding was confirmed in studies with human DC (24). Pimecrolimus also was unable to block the release of IL-8 in a number of cell types, such as keratinocytes, fibroblasts and endothelial cells (15). In a further in vitro study, comparing pimecrolimus and the corticosteroid dexamethasone (30), pimecrolimus did not affect the production of cytokines or chemokines, such as GM-CSF, IL-6, IL-8 (CXCL8), MCP-1 (CCL2), MIP-3α (CCL20), IP-10 (CXCL10) and RANTES (CCL5) in primary human keratinocytes or fibroblasts, and of eotaxin (CCL11) in fibroblasts. There was also no effect on the expression of adhesion molecules, such as ICAM-1 and VCAM-1. By contrast, dexamethasone inhibited IL-6, IL-8, eotaxin and GM-SCF production in fibroblasts. These findings further support the cell-selective mode of action of pimecrolimus, suggesting that this agent acts specifically on cells that are crucial for the effector phase of an inflammatory reaction (Table 1).
Table 1. Pimecrolimus vs. corticosteroids: comparison of pharmacological activities
| || || ||Different mechanism of action||15,23|
| || || ||Pimecrolimus: calcineurin inhibitor|| |
| || || ||Corticosteroids: pleiotropic mode of action involving, e.g. NFκB, AP-1|| |
| || || ||In vitro|| |
| Cytokine production||+||+||Inhibition of anti-CD3-antibody-induced cytokine production and proliferation |
in human peripheral mononuclear cells. IC50 pimecrolimus: 0.35-1.0 nM
| Proliferation||+||+||Relative potency: pimecrolimus ∼ betamethasone-17-valerate ∼ dexamethasone |
|Mast cell activation|
| Cytokine production||+||Not investigated||Ca-Inonophore A23187 + PMA-induced TNFα production in human dermal |
mast cells: ∼80% inhibition by pimecrolimus at 100 nM
| Degranulation||+||(+)||Anti-IgE-induced histamine release by human dermal mast cells: 50% |
inhibition by pimecrolimus at 100 nM, ∼40%
inhibition by dexamethasone at 1000 nM
| differentiation||−||+||Inhibition of GM-CSF + IL-4-induced differentiation of human monocyte- derived dendritic cells: IC50pimecrolimus ?1000 nM, IC50 betamethasone-17- |
valerate, dexamethasone ∼1 nM (with corticosteroids: apoptosis at
the early stage of cell differentiation)
| Maturation||−||+||Inhibition of LPS-induced maturation (CD83, CD86 expression) and IL-|| |
| Cytokine production||−||+||12 production: IC50pimecrolimus ?100 nM, IC50 betamethasone-17-valerate, |
dexamethasone ∼10 nM
|T-cell stimulation |
|−||+||Capacity of dendritic cells to stimulate T-cell proliferation in mixed lymphocyte |
cultures: no effect with pimecrolimus up to100 nM, clear dose-
dependent reduction with betamethasone-17-valerate at 1-10 nM
|−||−||Expression of cytokines/chemokines or adhesion molecules in optimally |
stimulated primary human keratinocytes not affected by pimecrolimus
(up to 10 μM) or dexamethasone (up to 1 μM)
| Cytokines||−||+||Expression of cytokines/chemokines or adhesion molecules in optimally |
stimulated human fibroblasts: no effect with pimecrolimus (up to 10 μM),
inhibition (GM-CSF, IL-6, IL-8, eotaxin) by dexamethasone (IC50 0.64-9 nM)
| Penetration into skin||+||+||Human split-thickness skin; pimecrolimus, betamethasone-17-valerate, |
clobetasole-17-propionate, diflucortolon-21-valerate applied in same
concentration and solvent.
| Permeation through skin||Low||High||Skin concentration of pimecrolimus after 48 h lower (factor 2-3) but in the |
same range as with the corticosteroids
| || || ||Flux of pimecrolimus through skin much lower (factor 70-110) as with corticosteroids|| |
| || || ||In vivo|| |
|Allergic contact dermatitis|| || |
| Mouse, topical treatment||+||+||Acute, oxazolone-induced allergic contact dermatitis, single dose. |
Pimecrolimus and dexamethasone equipotent in reducing the inflammatory response (minimal effective concentration: 0.004%)
| Pig, topical treatment||+||+||Acute, dinitrofluorobenzene-induced allergic contact dermatitis, two doses||43|
| || || ||Anti-inflammatory effect of pimecrolimus cream 0.3% in the same range as |
clobetasol-17-propionate 0.05%, betamethasone-17-valerate 0.1%,
diflucortolon-21-valerate 0.1%, mometasone-17-(2-furoate) 0.1% and
fluticasone propionate 0.05%; superior to clobetasone-17-butyrate 0.05% and
|Murine epidermal LC|
| Density of population||−||+||Effect on epidermal Langerhans' cells (LC) in mice, treated topically |
twice daily for 5 days with pimecrolimus or corticosteroids in their clinically
| Apoptosis||−||+||Density of epidermal LC population: no effect with pimecrolimus 1%, almost |
complete deletion of LC with clobetasol-propionate 0.05%, betamethasone-
17-valerate 0.1%, hydrocortisone 1%
| || || ||Corticosteroids induce LC apoptosis in situ|| |
|Skin atrophy induction|
| Pig, topical |
|−||+||Pimecrolimus or corticosteroids administered under semiocclusion (Finn |
chambers) 6 h for 13 consecutive days to skin of domestic pigs
| || || ||No sign for skin atrophy with pimecrolimus cream 0.3%, but pronounced |
atrophic effects with clobetasole-17-propionate 0.05%(macroscopic
evaluation, almost complete inhibition of normal growth of cutis, reduction in
epidermal thickness by 33%)
Low capacity of pimecrolimus to permeate through the skin
A drug for the topical treatment for inflammatory skin diseases has to be able to pass through the stratum corneum, the outermost layer of the skin, and to reach therapeutically relevant concentrations in the deeper layers of the epidermis and the dermis. Permeation through skin into the circulation, however, should be kept to a minimum, because it could ultimately result in systemic exposure, potentially followed by systemic adverse events. Extensive topical application or use of high-potency topical corticosteroids may not only cause local side effects, such as skin atrophy, but also have systemic adverse effects, such as hypothalamic–pituitary–adrenal axis suppression, Cushing's syndrome, femoral head osteonecrosis and cataracts (31–35).
Using split human cadaver skin (consisting of the epidermis and major part of dermis) in Franz-type diffusion chambers, the potential of pimecrolimus to permeate through skin was evaluated in comparison with that of various corticosteroids and tacrolimus (36). All compounds were dissolved in the same solvent and were applied in the same concentrations to the epicutaneous side of the skin. Subsequently, the flux through the skin, as well as the concentration in the skin, after 48 h was determined. The concentration of pimecrolimus in the skin was found to be in a range similar to that of corticosteroids and tacrolimus. Pimecrolimus permeated, however, through the skin 70–110 times slower than corticosteroids and 9–10 times slower than tacrolimus. Therefore, it may be concluded that, compared to corticosteroids and tacrolimus, pimecrolimus applied topically is associated with a much lower risk of systemic exposure and subsequent systemic side effects. This notion is supported by data from various pharmacokinetic studies on atopic dermatitis patients using pimecrolimus cream 1%, hydrocortisone cream 1% (Table 2) or 0.1% tacrolimus ointment (36–40). In a 2-week study, comparing pimecrolimus cream 1% and tacrolimus ointment 0.1% twice a day in adult patients with moderate to severe atopic dermatitis, 12% of blood samples from patients in the pimecrolimus group (highest drug concentration 1.51 ng/ml), but 36% of blood samples from patients in the tacrolimus group (highest drug concentration 2.39 ng/ml) had detectable drug levels, i.e. above the limit of quantitation (40). Furthermore, no clinically relevant systemic adverse events have been observed in more than 11 000 patients treated with pimecrolimus cream 1% (Elidel®) in clinical studies and around 11 million patient months of exposure after launch to date (Novartis data on file). Although no relevant data are available, it is reasonable to assume that any compound that has penetrated the skin and is not excreted after systemic uptake will be removed by desquamation in the natural process of skin renewal.
Table 2. Blood levels in paediatric patients with atopic dermatitis, treated with pimecrolimus cream 1% or hydrocortisone cream 1%
|Lowest blood concentration recorded (nmol/l)||< 0.64||< 0.64||212|
|Highest blood concentration recorded (nmol/l)||2.8||3.2||2669|
Pimecrolimus is highly effective in animal models of skin inflammation
For the preclinical evaluation of compounds developed for the treatment for inflammatory skin diseases, animal models of allergic contact dermatitis have proved to be very useful (41). Allergic contact dermatitis in pigs is an excellent model, because pig's skin closely resembles human skin with regard to thickness, structure and the skin-associated immune system (41–43). Topical application of pimecrolimus (0.1%) proved to be as effective as that of the potent corticosteroids – clobetasol propionate (0.05%), betamethasone-valerate (0.1%), momethasone (0.1%) and fluticasone (0.05%) – in this pig model of acute contact eczema (43). Pimecrolimus cream 1% also was as active as tacrolimus ointment 0.1% and slightly superior to tacrolimus ointment 0.03% (44). However, unlike corticosteroids, pimecrolimus did not cause skin atrophy in pigs when administered topically under occlusion by using Finn chambers for 6 h on 13 consecutive days. In contrast to betamethasone 0.05% and fluticasone 0.05%, pimecrolimus 0.6% had no effect on skin thickness or texture (43). These findings were confirmed in a randomized, double-blind controlled study in healthy human volunteers, comparing pimecrolimus cream 1% with betamethasone-17-valerate 0.1% cream and triamcinolone acetonide 0.1% cream. Upon histological and sonographic evaluation following a daily application of pimecrolimus cream for 4 weeks, no signs of skin atrophy could be detected in contrast to that of the corticosteroids investigated (45).
Systemic (orally or subcutaneously) as well as topical application of pimecrolimus also was observed to be effective in several rodent models of allergic contact dermatitis (43,46,47). Using the mouse model of oxazolone-induced allergic contact dermatitis, pimecrolimus proved to be superior to cyclosporin A and as potent as tacrolimus (41,47). Taken together, these findings indicate that pimecrolimus is able to inhibit the effector phase of contact hypersensitivity (CHS) as effectively as tacrolimus. However, pimecrolimus had, in contrast to tacrolimus, no effect on the sensitization phase of CHS in mice when administered orally (48). This was showed by evaluating the inflammatory response to hapten challenge in mice that were sensitized by transfer of lymphocytes from mice treated during sensitization. Tacrolimus caused a decrease in the weight and cellularity of the lymph nodes in the treated donor mice and, in a dose-dependent manner, prevented the development of an inflammatory response in the recipients. No such effects were observed with pimecrolimus. Phenotyping of the lymphocytes in the local lymph nodes of mice treated during the sensitization phase confirmed that tacrolimus, unlike pimecrolimus, suppressed cell activation. These data demonstrate that pimecrolimus effectively inhibits skin inflammation in the elicitation phase of CHS, which represents the clinical manifestation of allergic contact dermatitis, but has, in contrast to tacrolimus, no effect on the primary immune response taking place in the induction phase of CHS.
The potent anti-inflammatory activity of pimecrolimus was further confirmed in a rat model of experimental skin inflammation. Magnesium deficiency in hairless rats results in a transient erythematous rash and generalized pruritus, which closely mimics the clinical features of atopic dermatitis. Furthermore, these rats develop histaminaemia, leucocytosis, eosinophilia as well as an inflammatory infiltrate in the skin and mast cell degranulation (49).
Topical or oral treatment with pimecrolimus was able to prevent the development of the skin lesions and to inhibit the histo-and immunopathological changes.
Evidence from several studies supports a role of neuropeptides as mediators of immunity and inflammation. Substance P (SP), a neuropeptide that is released after treatment of the skin with capsaicin and that mediates burning as well as itch, was recently found to significantly contribute to the outcome of a contact hypersensitivity reaction in animals (50). One of the most common adverse events observed following the topical application of tacrolimus and pimecrolimus is a transient sensation of burning and irritation, which possibly is because of the induction of neuropeptide release, such as SP. Indeed, it has been demonstrated in mice that the topical application of both tacrolimus and pimecrolimus results in the release of SP from cutaneous nerves, which coincides with the time frames when itch and inflammation occurs (51,52). Pimecrolimus also significantly reduced the exaggerated ear swelling response observed in neutral endopeptidase knockout mice, indicating that it also downregulates the neuroinflammatory component of contact hypersensitivity, mediated by neuropeptides, such as SP (53).
Is there a risk with pimecrolimus to affect the local or systemic immunosurveillance?
Most of the currently used treatment regimens for atopic dermatitis may impair, at least to some degree, an immune response. Corticosteroids, e.g., were found to inhibit DC functions and to suppress T-cell recruitment into the skin by reducing E-selectin expression on endothelial cells as well as chemokine production. Moreover, they inhibit the synthesis of pro-inflammatory cytokines and chemokines by T-cells, DC and mast cells and decrease the survival of DC, monocytes, T-cells and eosinophils. In the skin, corticosteroids reduce the number of LC by inducing apoptosis (54). Systemic drugs, such as cyclosporin A and tacrolimus, are well-known immunosuppressants, which primarily impair T-cell activation and cytokine production. Cyclosporin A is, thus, used for the treatment for severe atopic dermatitis. Ultraviolet (UV) irradiation, which is widely used in order to treat atopic dermatitis, also has a strong impact on the skin immune system. Accordingly, UV irradiation results in the depletion of epidermal LC and in an altered cytokine profile in the skin. Thus, UVB irradiation stimulates the production of pro-inflammatory cytokines within a few hours, whereas later the synthesis of immunosuppressing factors, such as IL-10 and α melanocyte-stimulating hormone, is upregulated. Because pimecrolimus selectively affects effector mechanisms of inflammation and does not impair a primary immune response, it appears to be advantageous when comparing the conventional immunomodulatory treatments for atopic dermatitis. Even in comparison with tacrolimus in several studies by using relevant animal models, pimecrolimus turned out to exhibit a lower potential for affecting systemic immune reactions (Table 3) (43,47).
Table 3. Pimecrolimus vs. tacrolimus: comparison of pharmacological activities
| || || ||Common mode of action: inhibition of calcineurin||47|
| || || ||In vitro|| |
|T-cell activation|| || || || |
| Proliferation||+||+||Anti-CD3- or antigen-presenting dendritic cell-stimulated||66|
| || || ||proliferation and cytokine production (IL-2, IL-4, IL-5, IL-10,|| |
| Cytokine production||+||+||IFN-γ, TNF-α) of T-cell clones of Th-0 or Th-2 phenotype, derived|| |
| || || ||from an allergic donor. IC50(pimecrolimus/tacrolimus)|| |
| || || ||0.34/0.20 nM (proliferation), 0.18-1.20/0.15-0.63 nM|| |
| || || ||(cytokine production)|| |
|Dendritic cells|| || || || |
| Differentiation||−||+||With pimecrolimus up to 300 nM no inhibition of IL-4 + GM-CSF-||24,26|
| || || ||induced differentiation (CD1a, CD40, CD80 expression) of|| |
| || || ||human monocyte-derived dendritic cells, or capacity of dendritic|| |
| || || ||cells maturated in the presence of pimecrolimus to stimulate T-cell|| |
| || || ||proliferation in mixed lymphocyte cultures|| |
| || || ||Tacrolimus at 10 nM reduces the expression of CD40 and CD80 in|| |
| || || ||cultures of freshly isolated human epidermal LC after 18 and 36 h,|| |
| || || ||respectively, and the capacity of epidermal cells to stimulate the|| |
| || || ||proliferation of allogeneic T-cells is inhibited, when cultured in the|| |
| || || ||presence of tacrolimus (∼50% inhibition at 1 nM)|| |
| || || ||(separate studies, no direct comparison)|| |
|Skin penetration|| || || || |
| Penetration into skin||+||+||Human split-thickness skin. When applied in same concentrations||36,67|
| Permeation through||Low||>Pime-||in same solvent: Similar concentration of pimecrolimus and|| |
| skin|| ||crolimus||tacrolimus in the skin after 48 h, but lower flux of pimecrolimus|| |
| || || ||(factor 9) through skin, compared to tacrolimus. Moreover, when|| |
| || || ||applied as pimecrolimus cream 1% and protopic ointment 0.1 and|| |
| || || ||0.03%, flux of pimecrolimus through skin remains lower than that|| |
| || || ||of tacrolimus (factor 6 and 4.3, respectively)|| |
| || || ||In vivo|| |
|Allergic contact dermatitis|
| Mouse, topical, oral||+||+||Pimecrolimus and tacrolimus (oxazolone-induced, mouse, single||47|
| Rat, oral||+||+||topical, or two oral doses; dinitrofluorobenzene-induced, rat, two oral|| |
| Pig, topical||+||+||doses; dinitrofluorobenzene-induced, pig, two topical doses) inhibit|| |
| || || ||the inflammatory response at challenge with similar potency, as|| |
| || || ||judged by comparison of minimal effective doses|| |
|Immunosuppression|| || || || |
| Sensitization to||−||+||Tacrolimus suppresses the sensitization of mice to oxazolone dose||48|
| allergens|| || ||dependently with a minimal effective dose of 4 × 30 mg/kg orally,|| |
| || || ||whereas pimecrolimus has no effect up to 4 × 120 mg/kg orally|| |
| || || ||(parameters: lymph node hyperplasia, antigen-specific proliferative|| |
| || || ||response of lymph node cells, inflammation at challenge)|| |
| Graft-vs.-host||Pimecrolimus < Tacrolimus|| ||Pimecrolimus is less potent than tacrolimus in animal models for|| |
| reaction|| || ||immunosuppression (localized graft-vs.-host reaction, rat, four|| |
| Renal transplantation||Pimecrolimus < Tacrolimus|| ||subcutaneous doses; allogeneic kidney transplantation, rat, 14 oral|| |
| Antibody formation||Pimecrolimus < Tacrolimus|| ||doses; antibody formation against sheep red blood cells, rat, four|| |
| || || ||subcutaneous doses) as judged by comparison of minimal effective|| |
| || || ||doses (factors 66, 15 and 48, respectively)||47|
|Tissue distribution|| || || || |
| Concentration in skin||Pimecrolimus > Tacrolimus|| ||Rat, two oral doses of 25 mg/kg. Exposure (AUC0-24 h)||56,68|
| Concentration in lymph nodes||Pimecrolimus < Tacrolimus|| ||pimecrolimus/tacrolimus: skin 10 162/5 729 ng.h/g, axillary lymph|| |
| || || ||node 11 644/507 785 ng.h/g|| |
In the localized graft-vs.-host reaction in rats, a strong local T-cell activation is induced by injecting spleen cells from an MHC-disparate donor into one footpad of a recipient. The T-cells home to the regional lymph node where they react vigorously to alloantigens present on the host's cells, which ultimately results in an enlargement of the lymph node. Using this model, pimecrolimus after subcutaneous administration proved to be 66 times less immunosuppressive than tacrolimus. When the immune response of a recipient rat to the graft from an allogeneic donor was investigated in a rat model of kidney transplantation, pimecrolimus was 15 times less potent than tacrolimus and also turned out to require a dose three times higher than that of cyclosporin A to achieve a similar protecting effect. When assessing the effect on TH-cell-assisted B-cell activation in rats, subcutaneous application of pimecrolimus was found to be 48 times less effective than that of tacrolimus, as calculated from the doses required to achieve 50% inhibition of antibody formation (47). Finally, comparing the effect of oral treatment on the immunization of rats with keyhole limpet hemocyanin, pimecrolimus was found to be less potent than tacrolimus by a factor of >10 (55).
Pimecrolimus, thus, has a much lower potential than tacrolimus to affect systemic immunoreactions, although it binds to the same receptor – macrophilin-12 – and was found to be equipotent in animal models of skin inflammation (Table 3). One possible explanation for this difference might be the different tissue distribution of the two compounds. After oral administration to rats, somewhat higher concentrations of pimecrolimus were observed in the skin, compared to those of tacrolimus, whereas much higher amounts of tacrolimus were observed in the lymph nodes (56). However, at present, one cannot exclude the possibility that other factors are involved. Although macrophilin-12 is the predominant isoform, various forms of macrophilin – the cytosolic receptor of pimecrolimus and tacrolimus – exist, which are differently expressed in various tissues (57). Four isoforms of the nuclear factor of activated T-cells being substrates for calcineurin have been identified so far. They appear to be differently expressed in immunocompetent as well as other cells (58). Finally, the tacrolimus–macrophilin complex, as well as the cyclosporine–cyclophilin complex, has been reported to inhibit T-cell activation also by a calcineurin-independent pathway (59,60). Additional evidence has been provided recently by a pharmacogenomic analysis, showing that the distinct clinical effects of pimecrolimus and tacrolimus in allergic contact dermatitis in rats are associated with a different change of gene expression in the skin (61). Although changes in the expression of many genes induced at the inflammatory reaction to challenge were normalized similarly by both compounds, only pimecrolimus inhibited the expression of several genes related to inflammatory or fibrotic processes (e.g. PF4, galectin3, heme oxygenase, alpha-2 macroglobulin and versican).
Because pimecrolimus, in contrast to corticosteroids and tacrolimus, was shown not to affect DC/LC and to have no effect on the primary immune response, it is expected not to interfere with the development of an antigen-specific pool of memory T-cells, a prerequisite for the establishment of a local immunosurveillance. Accordingly, patients with atopic dermatitis, treated with pimecrolimus cream 1% for up to 1 year, showed a normal response pattern to a range of common bacterial and fungal antigens, indicating that pimecrolimus does not impair the existing immunosurveillance (62). This is further supported by data from clinical studies, showing that in children with atopic dermatitis during long-term treatment with pimecrolimus, the incidence of skin infections (bacterial, fungal and viral) was not increased in comparison with that in the vehicle-treated control group (62,63).
A major concern of any immunomodulating therapy is the potential risk of the development of UV-mediated skin cancer. Therefore, the risk of photocarcinogenicity associated with topical pimecrolimus treatment and exposure to sunlight was investigated in a standard murine model. After 40 weeks of daily UV exposure and concomitant treatment with pimecrolimus cream, no increase in the incidence of precancerous lesions was observed in comparison with that in the vehicle-treated animals (64). Moreover, in a recent study, it has been demonstrated that topical application of tacrolimus and pimecrolimus prevents the formation of thymidine dimers after UV-exposure (65). These findings, together with the low capacity of pimecrolimus to affect a primary immune response, indicate that the photocarcinogenic potential of topically applied pimecrolimus is very low.
In conclusion, these results imply that with pimecrolimus the risk of impairing the local immunosurveillance is much lower than with corticosteroids or tacrolimus. In view of the lower potency of pimecrolimus to affect systemic immunoreactions, compared to that of tacrolimus, and considering that it shows only minimal percutaneous resorption, topical pimecrolimus is likely to have also no potential for systemic immunosuppression.
Pimecrolimus is a novel calcineurin inhibitor with a selective pharmacological profile. Inhibiting the activation of T-cells and the production of inflammatory cytokines, it interferes with a key event in the pathomechanism of atopic dermatitis and other inflammatory skin diseases, such as psoriasis and contact eczema. Pimecrolimus shows high activity in animals with experimental allergic contact dermatitis – a model for a T-cell-mediated inflammatory skin disease. The inhibition of both mast cell activation and release of pro-inflammatory mediators may contribute to its anti-inflammatory effects observed in models of allergic contact dermatitis as well as in the inflammatory condition induced by low-magnesium diet. The cell-selective mode of action implies that pimecrolimus has a lower potential to cause side effects and consequently a higher safety. This is supported by the lack of atrophogenicity seen in animals as well as in humans. Pimecrolimus penetrates into the skin to a similar degree as corticosteroids and tacrolimus, but shows a much lower permeation through skin. This indicates that pimecrolimus after topical application will be delivered to the compartments of skin where its activity is required but that only minimal amounts will enter the circulation. This minimizes the potential for systemic exposure and systemic side effects. After oral administration, pimecrolimus was shown to have higher affinity to the skin than tacrolimus, similar anti-inflammatory activity, but a much lower potential to affect systemic immune reactions. These results, together with the observation that pimecrolimus does not affect DC and does not impair the primary response in allergic contact dermatitis, demonstrate that topical pimecrolimus has a more favourable balance of anti-inflammatory vs. immunosuppressive potential than tacrolimus and corticosteroids.
It is concluded that the results of the preclinical studies show that pimecrolimus has a selective activity profile, suited for effective and safe treatment for inflammatory skin diseases.