Calcineurin Inhibitors: A Novel Approach to Canine Atopic Dermatitis
The introduction of calcineurin inhibitors represents a major addition to the armamentarium of drugs available to veterinary clinicians for the management of allergic skin diseases. Both cyclosporine and tacrolimus have been proven to be well tolerated and effective for the treatment of atopic dermatitis in dogs. Although broad spectrum in their mechanism of action, they lack the major adverse effects of glucocorticoids and provide an appealing alternative to traditional therapies. The purposes of this article are to review clinically relevant information regarding these agents and to provide tips for maximizing the benefit obtained from these therapies.
Introduction
Calcineurin inhibitors such as cyclosporine have been used in the treatment of a variety of inflammatory skin diseases, such as atopic dermatitis and psoriasis, for many years.1–5 Unfortunately, the severe adverse effects of systemically administered cyclosporine in humans (e.g., hypertension) have limited the application of this therapy to only severe and refractory cases.6–8 To overcome this problem, the use of topical cyclosporine was investigated. Unfortunately, poor results were obtained with topical cyclosporine for the management of cutaneous diseases, and the lack of efficacy was attributed to the large size of the molecule.910 It was only when topical calcineurin inhibitors with an excellent safety profile and efficacy became available (e.g., tacrolimus) that the use of these types of drugs was expanded to milder and/or pediatric cases.
The reasons behind the use of calcineurin inhibitors for diseases such as atopic dermatitis are numerous. Traditionally, atopic dermatitis has been classified as a type I hypersensitivity, and most of the emphasis in both the diagnostic and therapeutic strategies has been placed on mast cell degranulation and immunoglobulin E (IgE) levels. Over time, however, researchers have realized that both human and canine atopic dermatitis is a very complex, multifaceted disease, and that IgE may only represent an epiphenomena rather than the primary immunological aberration. More specifically, it has become increasingly clear that this disease involves an imbalance in T cell populations and cytokine production, and new treatments are aimed at the correction of this lymphocytic imbalance.1112 By modulating T cell proliferation and activation, calcineurin inhibitors can block the majority of the reactions triggered by the allergic response.
Cyclosporine
Mechanism of Action
Cyclosporin Aa is a fungal metabolite isolated from the Tolypocladium inflatum fungus. Studies aimed to elucidate the mechanism of action of cyclosporine have shown that this molecule binds to cytosolic proteins of the cyclophilin family. This complex has a high affinity for calcineurin, a key enzyme in T cell activation. By blocking the calcineurin activity, cyclosporine prevents the induction of genes encoding for cytokines (e.g., Interleukin [IL] 2 and 4) and their receptors, thus affecting humoral and cellular immune responses.13
By this mechanism, cyclosporine effectively inhibits the activation of many key cells involved in cutaneous allergic inflammation, such as mast cells, eosinophils, lymphocytes, Langerhans’ cells, and keratinocytes. More specifically, cyclosporine inhibits mast cell degranulation, survival, and cytokine production in rodents, humans, and dogs.14–16 Cyclosporine also inhibits eosinophil function and survival and inhibits most functions of T-lymphocytes, especially lymphocyte activation and proliferation.1920 In addition, cyclosporine decreases the number of Langerhans’ cells in the epidermis and inhibits the lymphocyte-activating functions of these antigen-presenting cells.2122 This latter effect, although favorable in the allergic process, has raised concerns regarding the potential risk for increased incidence of infections and the development of neoplasia in humans.2324 Finally, functions of keratinocytes are also affected by cyclosporine, which is manifested as a decrease in cytokine production.25
Efficacy for Canine Atopic Dermatitis
Several studies have been published evaluating the efficacy and safety of cyclosporine in dogs, and these have led to the acceptance of cyclosporine as a treatment with comparable efficacy to the glucocorticoids.26 In a pilot study, cyclosporine was administered orally to dogs at high doses (10 to 20 mg/kg), and a decrease of pruritus was evident within a few days of therapy.27 After remission occurred, the dosage was tapered to the lowest possible every-other-day dose. In another study, cyclosporine was given orally once daily at 5 mg/kg for 6 weeks, and its efficacy was compared to oral prednisolone at 0.5 mg/kg.28 Skin lesions were graded by the investigators using a clinical scoring system, and pruritus was assessed by the owners.28 At the end of therapy, pruritus was decreased by 78%, and skin lesion scores were decreased by 58%.28 No significant differences in efficacy were noted between prednisolone and cyclosporine therapies.
In another study, the efficacy of tapering doses of cyclosporine was compared to methylprednisolone over a 4-month period.29 The mean induction dose of both drugs (5 mg/kg for cyclosporine and 0.75 mg/kg for methylprednisolone) was tapered over time according to the clinical response. At the end of the study, the mean estimated reduction rates in lesion scores were 52% and 45%, and the mean reduction rates in pruritus scores were 36% and 33% for the cyclosporine and methylprednisolone treatment groups, respectively.29 These percentages were not significantly different between groups. Cyclosporine-treated dogs had a higher frequency of gastrointestinal disorders (mainly vomiting), while methylprednisolone-treated dogs were more susceptible to infections. It is interesting to note that the percentage of improvements that occurred in this study with lower doses of cyclosporine were inferior to results of a previous study, which implied the efficacy of this drug is dose dependent.29
In another study, the efficacy of two doses of cyclosporine (5 mg/kg and 2.5 mg/kg) was specifically evaluated and compared to a placebo.30 After 6 weeks, mean reductions in lesion severity compared to baseline scores were 34%, 41%, and 67% for dogs treated with the placebo, 2.5 mg/kg cyclosporine, and 5 mg/kg cyclosporine, respectively.30 Similarly, mean reductions in pruritus scores were 15%, 31%, and 45%, respectively.30 Reductions in skin lesion and pruritus scores were significantly higher for dogs given cyclosporine at 5 mg/kg than for dogs given the placebo. Reductions in skin lesion and pruritus scores were higher for dogs that had a history of nonseasonal atopic dermatitis and were treated with cyclosporine at the highest dosage.30
In another study, the remission of clinical signs after cessation of either cyclosporine or methylprednisolone therapy was evaluated.31 During the 2 months following treatment cessation, 87% of the dogs treated with methylprednisolone relapsed (after a mean period of 28 days), whereas 62% of the dogs treated with cyclosporine relapsed (after a mean period of 41 days).31 Both skin lesions and pruritus increased markedly in the dogs treated with methylprednisolone when compared to those treated with cyclosporine.
Adverse Effects
In contrast to humans, dogs appear to tolerate cyclosporine and do not have many adverse effects. In all the studies evaluating the efficacy of cyclosporine for atopic dermatitis in dogs, the drug was well tolerated. Side effects were observed in approximately 20% of cases and consisted primarily of gastrointestinal signs such as vomiting and diarrhea.2829 Gingival hyperplasia, hirsutism, papillomatosis [Figures 1, 2], and bacterial infections are other possible side effects.2829 While bone marrow suppression and nephropathy are reported as adverse effects in dogs, these have not been observed in the clinical trials where cyclosporine was used at lower dosages (5 mg/kg).32
In a 1-year oral toxicity study in dogs, normal beagles were administered cyclosporine at up to 45 mg/kg per day.33 Reported side effects at this dosage included emesis, diarrhea, anorexia, weight loss, generalized cutaneous papillomatosis, chronic hyperplastic gingivitis, and periodontitis.33 There was no evidence of hepatotoxic, nephrotoxic, or myelotoxic effects. All changes were reversible after a 12-week recovery period. The maximum safe (i.e., nontoxic effect) level was set at 15 mg/kg.33
In humans, one of the concerns regarding long-term therapy with cyclosporine is the increased risk of development of neoplasia, especially lymphoma.34–36 The same concern also exists in the dog.37 In a recent, still unpublished study, the author retrospectively evaluated the incidence of lymphoma in cyclosporine-treated dogs (regardless of the dose used and disease treated) and compared it to the rate found in the general hospital population.b It was found that no dogs placed on cyclosporine therapy during the 6-year study period were later diagnosed with lymphoma. These results seem to suggest that development of neoplasia may not be as much of a concern for dogs as it is for humans.
Current Recommendations
The currently recommended dose of cyclosporine for dogs with atopic dermatitis is 5 mg/kg orally once daily. The veterinary formulation (i.e., Atopica) is a microemulsified preparation with the identical properties of Neoral, which ensures more consistent bioavailability than Sandimmune.38 After an initial induction phase of ≥4 weeks, the frequency and/or the dose can be tapered in many atopic dogs. One main advantage of cyclosporine is that it does not act on glucocorticoid receptors, so it is a suitable option for dogs that have developed resistance to glucocorticoids.
In order to decrease the cost of therapy, clinicians frequently administer ketoconazole concurrently. Ketoconazole suppresses cytochrome P450 enzymes, which are important in the metabolism of cyclosporine.39 By administering these two drugs concurrently, it is often possible to decrease the dosage of cyclosporine needed to achieve a clinical benefit.40 The most commonly used protocol is the administration of ketoconazole at 5 mg/kg twice daily, which often allows a 50% to 75% reduction in the cyclosporine dose.4142
Another aspect of cyclosporine therapy is the issue of laboratory monitoring of cyclosporine blood levels. In a clinical trial evaluating cyclosporine for the treatment of canine atopic dermatitis, a single blood sample was collected in dogs that had received cyclosporine for 28 days. No significant correlation was found between clinical improvement and cyclosporine blood concentrations.43 Considering the large margin of safety of cyclosporine in dogs, the limited interindividual variability, and the lack of correlation between blood concentrations and clinical response, routine monitoring of blood cyclosporine concentration does not appear necessary in dogs with atopic dermatitis.4445
Tacrolimus
Mechanism of Action
Tacrolimusc is another calcineurin inhibitor currently approved in the United States for use in humans with moderate to severe atopic dermatitis.4647 The mechanism of action of tacrolimus is very similar to that of cyclosporine, as it leads to the suppression of calcineurin and gene transcription.48 Tacrolimus inhibits T cell response to antigens and the production of the cytokines responsible for T cell proliferation (i.e., IL-2). Tacrolimus also inhibits other T cell-derived cytokines, such as IL-3, IL-4, interferon gamma (IFN-γ), granulocyte macrophage colony-stimulating factor, and tumor necrosis factor alpha (TNF-α), which contribute to allergic inflammation.49 In addition to lymphocytes, tacrolimus down-regulates cytokine expression in other cells that have tacrolimus-binding proteins and are important in allergic skin inflammation. These cells include mast cells, basophils, eosinophils, keratinocytes, and Langerhans’ cells.5051 Topical tacrolimus leads to profound phenotypic and functional alterations in epidermal antigen-presenting dendritic cells in animals with atopic dermatitis, down-regulates the expression of high-affinity IgE receptors on Langerhans’ cells, and decreases the number of inflammatory dendritic epidermal cells.52 Finally, topical tacrolimus significantly inhibits T cell-mediated keratinocyte apoptosis.53 Keratinocyte apoptosis in atopic dermatitis seems to be mediated by IFN-γ released from activated T cells and contributes to the severity of clinical signs.54
Efficacy for Canine Atopic Dermatitis
Tacrolimus ointment has been extensively evaluated for atopic dermatitis in humans and has been found to be a safe and efficacious treatment for long-term use in pediatric and adult humans with the disease.5556 Only a few studies exist on the use of tacrolimus in veterinary medicine. An early double-blinded, placebo-controlled pilot study was conducted in dogs using a compounded lotion (0.3%), and tacrolimus was found to decrease clinical signs of atopic dermatitis.57 The improvement in clinical signs, however, was not perceived as significant by the owners.57
After the release of the commercial ointment, a double-blinded, placebo-controlled clinical trial was instituted using the commercial product (0.1%) on affected areas once daily.58 In this study, tacrolimus ointment significantly decreased severity of symptoms at the end of the 4-week trial. The decrease in clinical signs was seen as early as 2 weeks after the start of treatment and became statistically significant after 3 weeks of daily application.58 Dogs with localized disease responded better than dogs with generalized disease. Investigator scores improved by >50% in 58% of the dogs in the tacrolimus treatment group.58 When scores were evaluated according to the extent of the disease, dogs with generalized disease had an average decrease of 24%, while dogs with localized disease had an average decrease of 60%.58 The percent improvement (60%) observed in dogs with localized disease was similar to the improvement in clinical scores reported with cyclosporine treatment and with topical triamcinolone therapy.59 Owner-generated scores improved by >50% in 41% of the dogs during tacrolimus treatment. All the dogs that had >50% improvement according to the owners had similar improvement in investigator scores. Owner-generated scores in the localized disease group decreased an average of 58%, while scores in the generalized disease group decreased an average of 19%.59
Adverse Effects
Long-term studies of tacrolimus in humans have demonstrated that tacrolimus is well tolerated and safe.60 Tacrolimus ointment is not atrophogenic, which is a major advantage over the use of topical glucocorticoids, and it is associated with minimal systemic absorption. No significant changes in laboratory parameters have been reported with long-term topical tacrolimus therapy in people.61 The most common adverse effect in humans is a transient skin-burning sensation and pruritus at the site of application. Both of these sensations spontaneously resolve after a few days of therapy.
In two published studies evaluating tacrolimus as a treatment for canine atopic dermatitis, tacrolimus was detected in the blood of animals receiving the active ingredient, but levels were well below the level of toxicity, and no adverse effects were reported in any of the dogs.5758 No changes in complete blood counts or biochemical parameters were detected between or within treatment groups.5758
Development of secondary skin infections is always a concern when immunomodulatory therapy is used. Interestingly, tacrolimus in humans has not predisposed to secondary skin infections; in fact, it appears to decrease bacterial colonization of the skin.6263 Since tacrolimus does not have antibacterial properties per se, it was hypothesized that the decrease in cutaneous colonization arose from a decrease in inflammation and restoration of an efficient barrier function of the skin. In the few existing studies in dogs, no increased incidence of skin infections has been noted.
Calcineurin Inhibitors and IgE
Interestingly, both tacrolimus and cyclosporine paradoxically increase IgE synthesis in humans and rodents.64–66 It would be interesting to evaluate the effects of long-term treatment with these calcineurin inhibitors on serum allergy testing in dogs. A short-term study (i.e., 21 days) using cyclosporine at 5 mg/kg found no effect on serum allergen-specific IgE levels, but it is possible that prolonged treatment might produce different effects.67 Tacrolimus and cyclosporine do not appear to affect immediate skin test reactivity in dogs.6668 The effect on late-phase reactions has only been evaluated with topical tacrolimus, and some suppression was noted.68 No withdrawal is recommended when skin testing is done for the purpose of evaluating only immediate reactions (i.e., within 10 to 15 minutes), but a 4-week withdrawal may be necessary for evaluation of late-phase reactions (i.e., after 4 to 6 hours). To the best of the author’s knowledge, no information is available on the effects of cyclosporine therapy on late-phase reactivity in the dog.
Financial Considerations
Calcineurin inhibitors are expensive drugs. The monthly cost of topical tacrolimus treatment for a 50-lb dog is approximately $120. The monthly cost of cyclosporine therapy at 5 mg/kg is approximately $210. Based on these calculations, tacrolimus may be an appealing treatment option for dogs with localized disease or in cases in which systemic use of a calcineurin inhibitor might be a concern.
Conclusions and Future Directions
Calcineurin inhibitors represent an important breakthrough in the never-ending quest for alternative treatments to manage chronic cases of atopic dermatitis. Because many dogs develop adverse effects and resistance to glucocorticoids or may not respond to allergy vaccine and other traditional therapies (e.g., antihistamines, essential fatty acids), it is important to constantly evaluate new therapeutic options in these animals. Dogs with localized disease may be controlled with daily tacrolimus applied to the affected areas, while dogs with generalized disease may require systemic treatment with cyclosporine. Although both of these calcineurin inhibitors do not appear to increase the risk for skin infections, it is important to always evaluate the possibility of secondary infections in affected dogs. Atopic dogs are prone to skin infections, which can seriously decrease the efficacy of other therapies.
Since the clinical term “atopic dermatitis” encompasses a broad spectrum of abnormalities that may be different for each animal, future research is aimed at the identification of tests that can determine the specific abnormality present and can predict or tailor treatments to the individual animal. This approach would allow the use of more targeted therapies rather than the generalized application of broad-spectrum immunomodulants.
Sandimmune, Neoral, Atopica; Novartis, East Hanover, NJ 07017
Santoro D, Marsella R. Investigation of possible correlation between atopic dermatitis, the use of cyclosporine A and the development of lymphomas in dogs. Submitted as abstract to the North American Veterinary Dermatology Forum, 2005.
Protopic; Fujisawa, Deerfield, IL 60015
Citation: Journal of the American Animal Hospital Association 41, 2; 10.5326/0410092
Citation: Journal of the American Animal Hospital Association 41, 2; 10.5326/0410092
A 6-year-old, spayed female mixed-breed dog that developed papillomatosis (black lesions) after 4 months of cyclosporine therapy at 5 mg/kg per os. Lesions improved after decreasing the dosage of cyclosporine and administration of antibiotics.
Papillomatosis of the right periocular skin in a dog treated with oral cyclosporine. Photo courtesy of Dr. Mona Boord, Animal Dermatology Clinic, San Diego, CA.
