Download Update on Atopy Pathogenesis

UPDATE ON THE PATHOGENESIS OF CANINE ATOPIC DERMATITIS:
A COMPARATIVE REVIEW
Rosanna Marsella
University of Florida
INTRODUCTION
Atopic dermatitis (AD) is a chronic inflammatory skin disease affecting over 10% of
human children, and it is the most common cause of occupational disability in adults.1,2 AD also
is seen commonly in companion animals, especially in dogs where its prevalence has been
estimated to approximately 10% of the canine population.1,3,4 Numerous similarities exist
between canine AD and its human counterpart, thus suggesting the canine AD might be used as a
model for the human disease.5,6 Dogs have been used in the past as model for human allergic
diseases. The most popular model has been the Basenji-Greyhound with a recurrent,
nonseasonal pruritic dermatitis and asthma.7 These dogs developed lichenified plaques and
inflammatory nodules and papules. They also developed airway hyperreactivity, with changes in
pulmonary mechanics of the same order as those seen in symptomatic human asthma, and
exhibited a variety of immunological abnormalities similar to the human disease.
Significant confusion still exists on the pathogenesis of AD in both humans and dogs.
Various mediators and cells have been found to play a role in AD in both humans and dogs,8,9
but it is currently unknown, which one is the most important to explain the clinical signs of this
disease. Historically a lot of emphasis was given to the role of IgE, mast cells and histamine in
the pathogenesis of AD. It is clear now that AD is a much more complex disease than a simple
type I hypersensitivity and that different subsets of patients may have different abnormalities that
all lead to the same clinical manifestation. Therefore AD does not have just one immunological
defect but a variety of them. It is also becoming more and more clear that different cells and
mediators play roles at different times and stages of the disease and that AD is more of an Type
IV than originally thought. In this review the old ideas about AD (e.g. type I hypersensitivity)
will be presented in conjunction with the new theories on AD (e.g. biphasic responses, cytokines
and chemokines, T cells imbalance). Newer treatments available for canine AD will also be
discussed.
a.
Definition
A task force of the American College of Veterinary Dermatology has recently revised the
definition of atopy and AD in dogs.10 According to the latest definition canine atopy has been
described as a genetically predisposed tendency to develop IgE-mediated allergy to
environmental allergens (syn. atopic state). In the dog, atopic dermatitis is considered the most
commonly diagnosed atopic disease and AD was defined as a genetically predisposed
inflammatory and pruritic allergic skin disease with characteristic clinical features. Canine AD
is associated most commonly with IgE antibodies to environmental allergens.11
b.
Prevalence of AD and risk factors
In humans, the prevalence of AD in developed countries is approximately 15%, with a
steady increase over the past decades.12 The increased incidence of this disease has been
hypothesized to be linked to an increased indoor allergen load, a decreased microbial load and
changing dietary habits.13 Atopic dermatitis develops as a result of a complex interrelationship
of environmental exposure, genetic background, and immunologic factors. In humans, numerous
genes have been identified as candidate for the development of this disease.14,15,16,17
Immunologic factors that appear to play a role in the pathogenesis of AD include the pattern of
local cytokine release, nature of the antigen, differentiation of helper T cells, IgE antibodies,
infectious agents, and superantigens.18,19,20 The first 6 months of life appear to represent a
critical time window for the initiation of immunological changes resulting in the development of
atopic disease in humans.21,22 The selective development of a Th2 cytokine profile in high-risk
children who develop atopy seems to be due to an increased production of Th2 cytokines,
possibly caused by impaired allergen-induced IFN-gamma production in the neonatal period.23
Furthermore, the decreased allergen-induced IL-10 levels observed in atopic children at 12
months of age may result in a lack of down-regulation of the inflammatory process.
In dogs there are no reliable epidemiological data of the true incidence and prevalence of
AD in the general canine population but it is suspected that canine AD could affect up to 10% of
the population. The typical age of onset of canine AD is between six months and three years
(young adults). Various parameters have been evaluated as risk factors for canine AD. A strong
breed predilection and familial history suggest that genetic factors could play an important role
in canine AD.24 No clear association has been demonstrated between atopy and dog leukocyte
antigen typing although the combination of haplotypes DL-A3 and R15 seemed to be more
common in dogs with AD.25 No significant differences existed in the IgE levels between normal
and atopic dogs and a relationship between serum IgE and Dl-A was not observed.24
THE OLD THEORY: IgE, mast cells, histamine, LT
c.
Role of IgE
In humans most patients with AD exhibit elevated total IgE and allergen specific IgE to
both environmental and food antigens to the extent that anti-IgE have been successfully used to
reduce clinical signs in affected individuals.26 A so-called intrinsic type of AD has also been
described in which normal serum IgE levels and negative immediate-type skin reactions towards
environmental allergens are reported.27 The recently characterized human autoantigen Hom s 1
has been proposed to play a part in the pathogenesis of intrinsic AD.28
In canine AD, the role of IgE is not clear, in that disease expression does not correlate as
closely with the presence of allergen-specific IgE, and allergen exposure. Most dogs with canine
AD have allergen specific IgE demonstrable either by intradermal testing (IDT), or by the use of
serological assays such as the enzyme-linked immunosorbent assay (ELISA)29,30 although the
agreement between these two tests is only 10 greater than that expected by chance alone. 31 In
atopic dogs, IgE are present on the surface of epidermal Langerhans' cells and are suspected to
aid in allergen capture and in its subsequent epidermal penetration.32 Experimental sensitization
in dogs has revealed that the capacity to produce high levels of IgE against a variety of allergens
(high IgE responders), an essential characteristic of the atopic state, is a genetic trait inherited in
a dominant manner.33 In high IgE responder dogs spontaneous development of IgE to inhaled
allergens, such as house dust mites, on the other hand, represents an apparent phenotype very
similar to that observed in human atopic families.34 The full potential of the high IgE response
gene appears to be fulfilled only under some conditions such as early and repeated exposition to
allergens. It is therefore quite possible that the true phenotype of human atopy would also be
inherited in a dominant fashion but not constantly expressed. It is important to note, though, that
increased allergen specific IgE do not necessarily cause disease and that additional factors
influencing mediator release could be involved, such as IgE heterogeneity.35, 36
d.
Immunological abnormalities
In canine AD, abnormalities in T-lymphocyte function are also demonstrable, and it is
possible that a number of other immunological pathways besides IgE synthesis could be involved
in the pathogenesis of this disease.37 Leukocytes, including mast cells, of atopic dogs also have a
greater tendency to release histamine than those of normal and artificially sensitized dogs and
that this is independent of the concentration of total serum IgE or antigen-specific IgE,
suggesting that there may be immunoregulatory abnormalities in atopic dogs intrinsic to the
atopic state as is described in man.38,39,40,41 Other features of canine AD analogous to the human
disease include blunted cyclic adenosine monophosphate (cAMP) responsiveness to beta
adrenergic agents and in vivo release of histamine and slow-reacting substance of anaphylaxis in
response to antigen aerosol challenge.42,43,44
e.
Histamine
Elevated concentrations of histamine have been found in vivo in the skin45 and in the
plasma46,47 of humans with AD, especially during exacerbation of the disease.48,49 Plasma
histamine values tend to return to normal during clinical remission. In vitro, an increased
histamine releasability of peripheral leukocytes after stimulation with a variety of different
substances has been reported.50,51 The difference between patients with AD and normal controls
is generally most pronounced after stimulation with anti-IgE. There is, however, a tendency
towards an increased spontaneous histamine release compared to normal individuals. In
addition, the release of histamine seems to occur more rapidly in patients with AD compared to
normals. The exact mechanism and the clinical impact of increased histamine release in human
AD are largely unclear at the present time.
People with AD can be further subdivided into high and low histamine responders and
these responses showed strong correlation with phosphodiesterase (PDE) activities. Pretreatment of high histamine responders with a PDE inhibitor reduced the histamine release to
normal levels. These findings suggested that increased histamine "releasability" in AD is related
to abnormalities in cyclic nucleotide regulation. Correlation between histamine release and IgE
levels is controversial, as some authors found no correlation,52 while others did.53 Decreased
response to intradermal injection of histamine has been reported in people with AD and it is
thought to be due to down regulation of target structures secondary to the excessive levels
present in affected individuals.54
The importance of histamine as a major mediator of canine AD is controversial. Studies
evaluating serum histamine concentrations found that levels are similar55 or lower56 in dogs with
AD than in healthy controls. On the other hand, cutaneous histamine levels were always greater
in dogs with AD than in normal dogs, but they did not correlate with plasma histamine
concentrations. Histamine release from mast cells isolated from the skin of dogs with AD,
ascaris-sensitive and healthy dogs was also not significantly different. 57 However, the total
histamine content found, per isolated skin mast cell, was reported to be higher in dogs with AD
than in the controls.58 This phenomenon together with the higher concentration of skin mast cell
number in AD lesions might account for the observed increase in cutaneous histamine
concentration.
Mast cells derived from dogs with AD are highly reactive to both non-immunological
(ionophore A23187) and an immunological-like (concanavalin A) stimuli. Furthermore,
histamine net release induced by con A stimulation is clearly enhanced in dogs with AD. In
another study lower mast cell density was demonstrated in lesional and non-lesional skin
samples of dogs with AD than in the skin of healthy controls.59 It was hypothesized that mast
cell granule heterogeneity exists in the dog and that degranulation occurs in dogs with AD.
These results support the hypothesis that mast cells play a major role in causing and, possibly,
modulating AD, through enhanced sensitivity or releasability. However, whether these two
phenomena are primary abnormalities of AD, or only secondary changes, remains undetermined.
The role of allergen in cells degranulation and release of histamine has also been an object of
investigation. In one study an in vitro dose-dependent basophil degranulation was reported in
dogs with AD after incubation with relevant allergen.60 Two peaks of activation were found and
heterogeneity of basophil populations, variations in the saturation of IgE receptors, and different
isotypes of anaphylactic antibodies were considered as possible explanations for the results.
In another study, histamine release from canine peripheral leukocytes was measured in
dogs with AD, healthy controls, and artificially sensitized dogs after immunological challenge
with Dermatophagoides farinae antigen and anti-IgE.61 The total cell histamine content of
similar leukocyte preparations was not significantly different between the dogs with AD and the
healthy controls. At all dilutions of antigen a higher amount of histamine was released from the
leukocytes of dogs with AD than was seen in the control group. No histamine release in
response to D. farinae was seen in the sensitized dogs although a statistically significant increase
in serum D. farinae-specific IgE could be demonstrated after sensitization. Histamine release in
response to anti-IgE antibodies was significantly greater in dogs with Ad than in the healthy
controls or in the sensitized dogs. No significant difference in total serum IgE between the
groups was found. The authors concluded that peripheral blood leukocytes of dogs with AD
have a greater tendency to release histamine than those of normal and artificially sensitized dogs
and that this is independent of the concentration of total serum IgE or antigen-specific IgE.
Moreover, they suggested that there might be immunoregulatory abnormalities in dogs with AD
intrinsic to the atopic state as is described in man.
The effects of histamine on lymphocyte functions of dogs with AD has also been
investigated.62
More specifically, spontaneous, histamine-induced and Con A-induced
suppression of mitogenesis of autologous responder cells was studied in normal dogs and in dogs
with AD. Histamine-induced suppression was significantly decreased in the dogs with AD, as
was the Con A-induced suppression. Total numbers of histamine type 1 or type 2 receptors were
not different for cells from normal dogs and dogs with AD. Furthermore, spontaneous
suppression was significantly greater for dogs with AD.
Finally, cutaneous responsiveness to histamine before and 6 hours after intradermal
injection of significant allergen was measured in dogs with AD.63 Histamine responsiveness
increased markedly after antigen, and was correlated with the intensity of neutrophil influx at 6
hours, and to a much greater degree with mononuclear cell influx at 6 hours. In conclusion, there
is evidence that histamine may play a role in canine AD but whether the abnormalities reported
in affected animals are primary or secondary, remains to be elucidated. Further studies are
necessary to define the role of histamine in this disease.
f.
Leukotrienes
Leukotrienes (LT) are thought to play a role in the pathogenesis of AD in people.64,65
Enhanced spontaneous and stimulated release of LTB4 and LTC4 has been reported from leukocytes
of patients with AD when compared to normal controls.66 In addition, the activity of the enzyme
LTA4 hydrolase is increased in peripheral leukocytes from patients with AD.67,68 Levels of this
enzyme correlate with the severity of the disease and are reduced after improvement of skin lesions.
Leukotriene B4 is elevated in lesional skin of patients with AD when compared to non-lesional skin
of the same patients or to skin of healthy controls.69 In patients with AD the release of LTB4 after
allergen challenge is significantly higher than the control skin.70
Increased production of LT has been reported in the skin of patients with AD after allergen
specific challenge.71 Of all the suphido-LT, LTC4 is the one released in the greatest quantity being
more than 85% of the total sulphido-LT released after allergen stimulation.72 Maximum
concentration of LTC4 is reached 4-6 hours after challenge and correlates with the severity of late
phase cutaneous reaction (LPR). Measurement of suphido-LT release after specific allergen
stimulation has been used as an adjunctive test in patients with AD due to the good correlation
between LT release and other parameters of allergic sensitization.73
Few studies have been done to measure cutaneous LT concentrations in veterinary
dermatology.74 Histamine and sulphido-LT release was reported in the Basenji Greyhound model
after allergen challenge75,76 and it was concluded that suphido-LT might be important mediators in
allergic airway constriction. In another study, cutaneous LTB4 was measured in various canine
inflammatory skin diseases (pyoderma, seborrhea, AD, and flea allergy) and was found elevated in
all these diseases.77 This lack of specificity of LT for canine AD does not lessen the pathogenetic
implications of these findings since different diseases may trigger similar inflammatory responses.78
Positive correlation between LTB4 concentrations and pruritus was also found in that study.47
Increased levels of LTB4 were detected in erythematous, hypertrophic, hyperpigmented skin
confirming previous in vitro studies suggesting a mitogenic effect of LT on epidermal cells and
melanocytes.79,80
Sulphido-LT production in the skin of dogs with AD and healthy controls after stimulation
(saline, lipopolysaccharide and an allergen of reference) was evaluated in a pilot study.1 No
significant differences between groups were found for any of the stimuli. In addition, no differences
in sulphido-LT concentrations were detected between normal skin, non-lesional skin of dogs with
AD, and within the group of dogs with AD, between lesional and non-lesional skin. These
preliminary results do not seem to support a role for sulphido-LT in dogs with AD. However,
definite conclusions cannot be drawn due to the small number of dogs evaluated (n=13 per group)
and the large individual variations in sulphido-LT concentrations that seem to exist.
The importance of LT in AD is suggested by the reported beneficial effects of essential fatty
acids supplementation in dogs.81,82 One of the proposed mechanisms of action of such therapy is a
competition of the essential fatty acids with arachidonic acid (AA) for cycloxygenase (CO) and
lipoxygenase (LO) enzymes resulting in a modification of LT synthesis and reduced production of
highly pro-inflammatory products (e.g. LTB4).83 In conclusion, controversies still exist on the
importance of LT in canine AD and further studies are necessary to better elucidate their role in this
disease.
1
Marsella, R., Nicklin, C.F. Investigation on sulfido-leukotriene synthesis from peripheral mononuclear
cells and skin of normal and atopic dogs. In the Proceedings of the 15th Annual Meeting of the
American Academy and American College of Veterinary Dermatology, Maui, Hawaii, 1999: 101-102.
THE NEW THEORY: CYTOKINES, T CELLS AND BIPHASIC RESPONSES
G.
Cytokines
The local cytokine profile especially appears to be a critical determinant in the AD skin
lesion. A biphasic pattern of cytokine expression has been well demonstrated during the
development of human AD.84 Acute skin lesions show a prevalence of CD4+ Th2 lymphocytes
and eosinophils able to produce and release IL-4 and IL-13, whereas macrophages are recruited
during the chronic phase, associated with the presence of Th1-type cytokines (IL-2, IL-12, IFN). In human AD skin-seeking T cells, secreting IL-4, IL-5 and IL-13, play an important role
in the development of high IgE levels and increased survival and maturation of eosinophils.85
Interestingly, this Th2-like response is primarily found in the acute skin response while in the
chronic skin lesions, there is conversion to a Th1-like response with increased -IFN and IL-12.
Other cytokines that have been reported to play an important role in human AD are IL-6
and TNF-.86 IL-6 has an obligatory role in IL-4 induced human IgE synthesis87 and is released
after allergen challenge in humans with AD both in vivo and in vitro.88 IL-6 production is found
to correlate with the size of cutaneous late phase reactions (LPR) 59 and, may be important in
amplifying the inflammatory response to antigen and in determining the degree of clinical signs
in humans. In addition, expression of TNF- is up-regulated 2 hours after allergen challenge in
patients with AD.89
Recently, a role for IL-18 in AD has been shown. 90 IL-18 is stored as a biologically
inactive precursor form (pro) in various cell types, including macrophages and keratinocytes.
IL-18 was initially designated as -IFN inducing factor and thought to play an important role in
the Th1 cell response, primarily through its ability to induce -IFN production and as a costimulant for IL-12. It is currently known, however, that IL-18 has a more complicated
pleiotropic role than simply the induction of -IFN production. IL-18, in the presence of IL-3,
directly stimulates basophils and mast cells to release pro-inflammatory mediators in an IgEindependent manner. Furthermore, IL-18 can induce the production of IgE by triggering the
synthesis of both IL-13 and IL-4.91 Serum IL-18 levels in patients with AD are increased when
compared to controls and correlate directly with the severity of the clinical disease. The ability
of IL-18 to stimulate either -IFN or IL-4 production, depending on the presence of other
cytokines (IL-12 and IL-2, respectively), is unique. This property is maintained even in the
absence of T cell antigen receptor engagement. For these reasons IL-18 is an intriguing cytokine
and the most recent studies are focusing on its role in AD.
Finally, it is important to note that chemokines are key regulators of the selective
migration of leukocytes expressing specific chemokine receptors on the cell surface. Several
studies have shown an increased expression and release of chemokines, such as RANTES
(CCL5), eotaxins (CCL11, 24, 26), monocyte chemoattractant protein (MCP)-4 (CCL13),
macrophage-derived chemokine (MDC) (CCL22), and thymus and activation regulated
chemokine (TARC) (CCL17), in cutaneous lesions of AD and/or sera in relation to the presence
of inflammatory cells and/or to the activity of the disease. 92 Th1 cells predominantly express
chemokine receptors CCR5 and CXCR3, whereas Th2 cells preferentially express CCR3, CCR4,
and CCR8. As CCR4 is strongly expressed on Th2 cells, these cells are attracted by ligands for
CCR4, such as TARC and MDC. In human AD, it was reported that the number of CD4+ T
cells expressing CCR4 is increased in peripheral blood mononuclear cells,93 and that there are
increased levels of serum TARC.94 These findings suggest that chemotaxis of cells expressing
CCR4 is induced by TARC, resulting in the selective migration of Th2 cells into lesional skin of
human AD.
In recent decades, the prevalence of allergic diseases including bronchial asthma, hay
fever and atopic dermatitis, has risen steadily in high-income countries. The underlying
mechanisms for this phenomenon remain largely unknown. Since the natural mutation rate is
low, altered environmental and lifestyle conditions are thought to play an important role.
Epidemiological and clinical studies have provided indirect evidence that infections may prevent
the development of atopy and atopic disease. This is referred to as the "hygiene hypothesis".
According to the hygiene hypothesis, viral and/or bacterial infections could inhibit the T-helper
(Th)-2 immune response associated with atopic reactions by stimulating a Th-1 response
involved in defense of bacterial infections and delayed-type hypersensitivity reactions. In
particular, the prenatal period and early childhood are considered to be critical for the
establishment and maintenance of a normal Th-1/Th-2 balance. On the other hand, several
studies suggested that infections exacerbate established allergic diseases, e.g. bronchial asthma,
airway hyperresponsiveness and atopic dermatitis. Therefore, viral and/or microbial infections
and/or their products may have bidirectional effects on the development of allergy and asthma.
There are only a few studies evaluating cytokine expression in the skin of dogs with
naturally occurring AD, and numerous similarities with the human counterpart have been
observed. These dogs had chronic lesions thus evaluation of the early phases of AD was not
possible. In one study using a reverse-transcriptase polymerase chain reaction (RT-PCRs), IL-4
and IL-5 cytokine-gene transcripts were detected more commonly in atopic skin than in healthy
controls while IL-2 mRNA was amplified more often from normal control specimens. 95 In
another study using RT-PCRs, an over-expression of IL-4 mRNA and reduced transcription of
Transforming Growth Factor-beta (TGF-) was found in atopic dogs when compared to
controls.96 Significantly higher levels of -IFN, TNF- and IL-2 mRNA were also seen in
lesional compared with non-lesional and healthy skin. There were no significant differences in
IL-10, IL-6, IL-12p35 or IL-12p40 transcription between groups. Authors concluded that cAD is
associated with over-production of IL-4 and that tolerance in healthy individuals may be related
to higher levels of TGF-.
The mRNA expression of a chemokine (TARC) and several cytokines (IL-1, IL-4, IFN and TNF- was also investigated in the skin of both dogs with AD and healthy dogs.97
TARC mRNA was selectively expressed in lesional skin of the dogs with AD, but not in nonlesional skin of the dogs with AD or the normal skin of the healthy dogs. The expression levels
of IL-1, -FN and TNF- in lesional skin were also significantly higher than those in nonlesional skin of dogs with AD. These results suggested that TARC and inflammatory cytokines
(IL-1, -IFN and TNF- could play roles in the pathogenesis of cAD, as they do in human AD.
The transcription of the gene encoding CC chemokine receptor 4 (CCR4), which is
selectively expressed on Th2 cells and plays an important role in the trafficking of Th2 cells into
inflammatory sites, was also investigated in dogs with natural AD. 98 It was found that CCR4
mRNA was preferentially expressed in lesional skin of dogs with AD, together with the mRNA
of its ligand, TARC.
ALTERNATIVE TREATMENTS FOR CANINE ATOPIC DERMATITIS
MISOPROSTOL
Misoprostol (Cytotec, Searle, Skolkie, IL, USA) is a PGE1 analog99 that has potent
antiallergic effects.100 It inhibits late phase cutaneous reaction (LPR) in atopic people after
allergen challenge101 and appears to be an effective treatment for atopic dermatitis in dogs.102
The rationale behind the use of misoprostol for allergic diseases is that PGE elevates cAMP,
which in turn selectively blocks the secretion of cytokines by Th1 cells.103 It also inhibits
lymphocyte proliferation, granulocyte activation, and synthesis of pro-inflammatory cytokines
(Interleukin-1, Tumor Necrosis Factor ) which are thought to play a role in allergic reactions.104
Misoprostol has the potential of being an effective drug also for the treatment of acute and
chronic asthma.105
In people with atopic dermatitis misoprostol selectively inhibited the late- but not the
immediate-phase response after allergen challenge. In the same study misoprostol significantly
inhibited eosinophil chemotaxis, suppressed the production of granulocyte-macrophage colonystimulating factor by lymphocytes, and reduced the number of inflammatory cells in the dermis.
In a study of 20 dogs with atopic dermatitis, significant improvement was detected when
misoprostol was given at 3-6mcgkg-1 three times daily for 30 days. Pruritus decreased by half in
56% of the dogs and skin lesions improved similarly in 61% of cases. Improvement was seen
after at least one week of continuous administration. Adverse effects consisted in mild vomiting
and diarrhea in some patients.
The efficacy of this therapy has been further evaluated in an unpublished, blinded,
placebo-controlled trial2. Twenty dogs with atopic dermatitis were given either 3 weeks of
placebo or misoprostol at 5mcg kg-1 orally three times daily. Skin lesions were graded by
investigators and pruritus was scored by the owners. Both pruritus and clinical score were
significantly improved after misoprostol therapy but not after placebo administration. During
misoprostol treatment the median decrease of both pruritus and skin lesions was 30%.
CYCLOSPORINE
Cyclosporine A (Sandimmune, Sandoz, East Hanover, NJ, USA) is a fungal metabolite of
Tolypocladium inflatum gams that blocks T cell proliferation by suppressing cytokine
production,106 including IL-2, IL-1, IL-6, and TNF-. It also suppresses histamine release,107
and epidermal proliferation.108 Oral cyclosporine A is very effective in severe cases of atopic
dermatitis in people.109 However, concerns over systemic toxicity have limited its use. To
overcome this problem the efficacy of topical application of cyclosporine was also investigated
in people but with poor success.110 The only times in which clinical efficacy was achieved with
topical application of cyclosporine was when systemic levels were obtained. 111 The lack of
efficacy of topical cyclosporine was attributed to the insufficient skin penetration of the drug.112
In dogs cyclosporine suppresses histamine release from mast cells 113 and there are results
of its successful use for treatment of refractory atopic dermatitis. A literature search identified
four clinical trials investigating the efficacy of CsA for treatment of canine AD. One paper
described results from a small open trial of two week duration, while the three others were larger
blinded randomized controlled trials.114 ,115,116 Altogether, these four trials had enrolled 311 dogs
with AD diagnosed by standard methods. Of 311 dogs, 207 (67%) were treated with CsA
proemulsion concentrate (Neoral, Novartis Pharma; Atopica, Novartis Animal Health, Basel,
Switzerland) while the remaining dogs were treated with either placebo, prednisolone or
methylprednisolone.
All four trials reported that CsA administered at 5.0 mg kg-1 day-1 led to a decrease of
clinical signs of AD. The median reduction from baseline of investigator-graded lesional scores
varied between 52 and 67%. Similarly, improvement from baseline of owner-assessed pruritus
scores fluctuated between 36 and 100%. Induction of treatment with the lowest dosage of CsA
(2.5 mg kg-1 day-1) was found to be no more effective than placebo. At 5.0 mg kg-1 day-1, CsA
administration led to significantly greater reduction in lesion and pruritus scores than the intake
of its vehicle.
Adverse drug events were reported in 14 to 81% of study subjects. They were
categorized as mild to moderate in most patients, and consisted most commonly of intermittent
vomiting (14-42%) and diarrhea (16-18%). Skin infections developed in 29% of subjects
enrolled in the largest and longest clinical trial. In this trial, the severity of adverse drug events
led to interruption of treatment in 6% of CsA-treated dogs. Occasional events led to
discontinuation of treatment in other trials, but the causation of CsA was not proven
unequivocally.
PHOSPHODIESTERASE INHIBITORS
Increased phosphodiesterase (PDE) activity has been reported in atopic dogs117 and
118
people.
Increased PDE activity seems to be responsible for the deficient cAMP response in
atopic people119 and dogs120,121 Atopic PDE in man has higher sensitivity to a variety of enzyme
inhibitors,122,123,124 than healthy controls suggesting an increased therapeutic advantage. Various
PDE isoenzymes exist.
In mast cells, macrophages, T-lymphocytes, eosinophils and
keratinocytes PDE-IV appears to be the predominant isoenzyme and is involved in cell activation
and secretion. PDE-IV plays also a role in mediator release from epithelial cells and
neuropeptides from sensory nerves and regulation of airway caliber and bronchial smooth muscle
function.125 Because the selective PDE IV inhibitors markedly inhibit eosinophil infiltration
induced by cytokines,126 it is suggested that they might have anti-inflammatory effects and be
useful in the treatment of allergic diseases.
In humans a topical PDE IV inhibitor significantly reduced inflammation and the
production of IL-4 and PGE2 with atopic dermatitis in a double-blind, placebo controlled study
over a 28-d period.127 In atopic dogs, the efficacy of an oral PDE-4 inhibitor, arofylline,
(Almirall, Prodesfarma, Barcelona, Spain) was recently evaluated.128 The efficacy of arophylline
at 1mgkg-1 twice daily for four weeks was compared to the efficacy of prednisone (0.5mgkg -1
twice daily for the first week, once daily for the second week and then every other day for two
additional weeks). Pruritus and skin lesions were evaluated and graded on a scale from 0 to 3
before and weekly during the study. In all cases there was a progressive clinical improvement
and no significant difference was noted among treatments. However, many dogs had adverse
effects including vomiting, diarrhea and anorexia.
Pentoxifylline (PTX, Trental,® Hoechst-Russel Pharmaceutics, Trenton, NJ, USA) is
another PDE inhibitor that has multiple immunomodulatory properties and has been shown to
suppress TNF-, IL-1 and IL-6, IL-12 production in rodents and humans.129,130 It decreases
leukocyte adhesion and aggregation (by suppression of the expression of adhesion molecules on
endothelial cells) and increases neutrophil motility and chemotaxis. It inhibits B cell activation
(by suppressing IL-6 synthesis) and T cell activation. Suppression of TNF- by PTX has been
also reported in the dog.131
Pentoxifylline (10mgkg-1 twice daily) has been used for canine AD in a double-blinded,
placebo controlled, crossover clinical trial for 4 weeks.132 A washout period of 2 weeks was
allowed in between treatments. Clinical signs were evaluated and scored by the investigator and
owners. During PTX treatment, scores of pruritus and erythema decreased significantly.
Pentoxifylline did not affect intradermal skin test reactivity to house dust mite at 15 minutes
(allergen of reference of the study). The drug was well tolerated and no adverse effects were
reported. In people adverse effects are occasionally reported and include vomiting, diarrhea,
nausea, and rarely angina, agitation, dizziness, headache, and tremors. Adverse effects are
usually dose-related and can be decreased by lowering the dose.
Due to the fact that pentoxifylline is rapidly eliminated after oral administration (no drug
is detectable after 4 hours) in dogs133 it is reasonable to recommend a more frequent
administration of the drug (three times daily instead of twice daily) to increase the efficacy of
this treatment.
LEUKOTRIENE INHIBITORS
Leukotrienes play an important role in inflammatory and allergic reactions,134,135 thus
numerous compounds with the specific property of inhibiting lypoxygenase (LO) enzymes or
antagonizing LT receptors have been developed in recent years.136,137,138,139 Many of the older
compounds had poor absorption thus had limited clinical application. However, the most recent
compounds have good oral absorption and have been successfully used in the management of
atopic disease in people.140,141,142
Dogs have been used in several studies to evaluate LO inhibitors in model of airway
hyperresponsiveness.143,144 A compound called MK-0591 caused a significant, dose-dependent
inhibition of LT synthesis in dogs.145 This drug exhibited anti-asthmatic effects in allergic dogs
with airway hyperresponsiveness induced by ragweed allergen.146 Another compound that would
have the potential of being useful in the treatment of inflammatory and allergic conditions is A69412. It showed excellent bioavailability (>60%) in dogs and had a relatively long half-life (6
hours).147 Nearly complete suppression of LT production was noted through 16 hours after a single
oral dose of 5mg/kg in the dog.
Few clinical trials have been done to explore the efficacy of LT inhibitors in the treatment
of dogs with AD. In one study WY-5029 did not improve clinical signs in atopic dogs at oral dose
of 25mgkg-1 for 10 days.148 One possible explanation was the short duration of the trial, since a
massive inflammatory response exists in the skin of symptomatic individuals. In another study
evaluating a compound with antileukotriene, antihistaminic, antiseritonergic, and mast cell
stabilizing actions (AHR-13268) for the management of clinical signs in atopic dogs significant
improvement was noted, suggesting a potential for treatment.149 The authors suggested that this
treatment might be more effective as preventative therapy in a situation where the release of
inflammatory mediators has not occurred rather than to induce remission of pruritus.
Zileuton (A-64077, Zyflo®, Abbott Laboratories, IL, USA) is a 5-LO inhibitor that
suppresses LT formation.150 This drug is selective for 5-LO151 and does not interfere with other
eicosanoid metabolizing enzymes. In placebo-controlled clinical trials utilizing allergic rhinitis
human patients challenged with specific antigens, a single oral dose of zileuton (3 hours prior to
challenge) significantly decreased sneezing and congestion.152,153 Zileuton has also been used
for the treatment of atopic dermatitis in people with good results in a open study. 154 Significant
decrease of erythema was noted while only moderate improvement of pruritus was found.
Zileuton is a safe drug in human medicine and no differences were noted in the
occurrence of adverse effects between human patients receiving zileuton and those that were
given placebo.155,156 Side effects included headache, abdominal pain, asthenia and nausea. The
most serious adverse effect of this therapy is elevation of liver enzymes and monitoring of such
enzymes is recommended while receiving this medication. No adjustment of the dose is required
in cases of renal impairment.157
Zileuton also suppresses LT production in vivo in dogs.158 At oral doses ranging from
0.5 to 5mgkg-1 Zileuton suppresses LTB4 synthesis in whole blood of more than 90% while LTE4
production in the urine is only inhibited by 50%. In another study Zileuton was given orally at
2mgkg-1 and found to be rapidly absorbed and bioavailable (peak levels being achieved within 1
hour of oral administration), with a half-life (t1/2) of approximately 7.5 hours.159 In a pilot
clinical trial Zileuton has been used in 8 dogs with atopic dermatitis at 5 mgkg-1 three times daily
for 6 weeks without any adverse effect.3 Significant decrease in the erythema was noted but no
significant effect on pruritus was seen in the Zileuton group (n=5) when compared to the placebo
(n=3). Only 2 dogs seemed to have a marked clinical response to the Zileuton treatment, but this
difference was not statistically significant when compared to the placebo. At this dose, plasma
concentrations considered to be therapeutic in humans should have been achieved in these dogs
and the length of the study should have been sufficient to alter LT synthesis. Unfortunately, the
number of dogs in which this therapy has been used is too small to draw conclusions on the
clinical efficacy of this drug for canine atopic dermatitis.
TOPICAL TACROLIMUS
Tacrolimus (FK-506) is a macrolide lactone produced by the fungus Streptomyces
tsukubaensis and is chemically distinct from cyclosporine A, yet has similar activity. 160 Topical
tacrolimus was approved for use in humans with AD in Japan in November 1999 and was
launched on the US market in 2001. Tacrolimus inhibits the T lymphocyte response to antigens
and the production of the cytokines responsible for T cell proliferation (Interleukin-2)
Tacrolimus achieves this by forming a complex with a cyclophilin-like protein (FKBP12), and
this complex, in turn, inhibits the ability of calcineurin to dephosphorylate the transcription
factor required for the activation of IL-2 and IL-4 gene transcription.161 Tacrolimus also inhibits
other T cell derived cytokines, such as IL-3, IL-4, IFN-, and TNF-
inflammation.162 In addition to lymphocytes, tacrolimus down-regulates cytokine expression in
other cells that have tacrolimus binding proteins and that are important in allergic skin
inflammation. These include mast cells, basophils, eosinophils, keratinocytes and Langerhans’
cells.163,164,165 Topical tacrolimus leads to profound phenotypic and functional alterations of
epidermal antigen-presenting dendritic cells in patients with AD and down-regulates the
expression of the high affinity IgE receptor (FcepsilonRI) in Langerhans cells.166 Tacrolimus
inhibits the expression of IL-2R (CD25), the costimulatory molecules CD80 (B7.1) and CD40,
and both classes of MHC.167
A significant advantage of tacrolimus over cyclosporine is its efficacy after topical
application.168 This difference between tacrolimus and cyclosporine is due to tacrolimus’
smaller molecular weight and its greater ability to permeate the skin.169 Tacrolimus is therefore a
3
Crow, D., Marsella, R., Nicklin, C.F. Double blinded pilot study on the efficacy of Zileuton for canine
atopic dermatitis. In the Proceedings of the 4th World Congress of Veterinary Dermatology.
powerful topical immunomodulator that can effectively decrease inflammation and allergic
reactions. Topical tacrolimus has been used successfully to decrease signs of AD in humans,
both adults and children.170,171,172 Clinical improvement is marked and rapid and appears to be
directly proportional to the concentrations used.173 A mean percent improvement of 83% of
clinical scores was reported with 0.1% ointment and a significant difference from placebo was
evident as early as day 8.174 When a 0.3% ointment was used, improvement of clinical lesions
was observed on day 3 in 81% of the adult patients and 88% of the children.175 By day 8,
clinical improvement was evident in 94% of the adults and 100% of the children.
Despite its potency, topical tacrolimus is safe and minimally absorbed. Tacrolimus
ointment (0.3%) was applied to up to 30% of the total body surface area of adults and children
and the highest blood level of tacrolimus was 1.6 ng/ml, which is substantially below
concentrations (>20ng/ml) that have been associated with an increased risk of toxicity.176 The
bioavailability of topically applied tacrolimus was calculated to be less than 0.5% relative to
intravenous administration. Also, with repetitive applications, no significant accumulation was
found. Adverse effects were minor and self-limited. The most common adverse effects were a
burning sensation and erythema. However, these symptoms occurred equally in both the vehicle
and tacrolimus groups and none of them occurred after the first 3 days of treatment, despite
uninterrupted drug applications. No significant changes in laboratory profile have been
associated with the administration of tacrolimus ointment. 177
The efficacy and safety of 0.3% tacrolimus lotion in dogs with AD was evaluated. 178 In
this double blinded, randomized placebo controlled crossover study, eight dogs with wellcharacterized AD were selected. Dogs were randomly assigned to be treated with either
compounded 0.3% tacrolimus lotion (Prograf, Fujisawa, Deerfield, IL; maximum dosage: 0.3 m
kg-1 day-1) or placebo lotion (vehicle) once daily for four weeks. After a two-week wash out,
treatments were reversed. At the end of the trial, there were no significant differences in ownerassessed pruritus scores within treatment groups (week four compared to baseline) or between
interventions. After four weeks, investigator-graded erythema scores, but not pruritus scores,
were significantly lower in dogs treated with tacrolimus compared to those receiving the placebo
vehicle. After four weeks, and compared to baseline, clinician-assessed pruritus scores were
significantly lower in tacrolimus-treated dogs, but not in those receiving placebo. Thirty-eight
percent of dogs treated with tacrolimus exhibited a greater than 50% improvement from baseline
in erythema scores, while 13 to 50% of subjects achieved the same benchmarks for owner and
investigator assessed pruritus, respectively. At trial’s end, there were no differences in complete
blood counts or chemistry panels between tacrolimus and placebo-treated dogs. Adverse drug
events following tacrolimus lotion application were not reported.
TOPICAL CAPSAICIN
In humans there is increasing evidence that neuropeptides, such as Substance P (SP), are
involved in the pathogenesis of AD.179 ,180 In humans, SP release is triggered by various stimuli,
including histamine. Intradermal injection of SP causes wheal and flare reactions. SP has an
influence on the immunomodulation of AD patient by regulating IFN-gamma production, either
directly or indirectly. Receptors for SP have been identified on human mast cells and their
stimulation triggers degranulation and histamine release and further release of SP. Substance P is
released after allergen challenge in humans with atopic disease and significantly contributes to
the recruitment of eosinophils in allergic rhinitis. Distribution and density of several
neuropeptides were examined in lesional and non-lesional skin of atopic patients and in normal
controls by immunohistochemistry. Substance P immunoreactivity was present in all atopic
patients but not in the controls. The distribution density of the cutaneous nerve fibers was higher
in patients with AD than in normal controls, and the diameter of these fibers was larger, because
of the large number of axons in each nerve fiber. These studies support a role for neuropeptides
in the pathogenesis of human AD, and an imbalance between various neuropeptides (vasoactive
intestinal polypeptide versus SP) might reflect diverse roles in the modulation of AD lesion.
The effect of SP on proliferation and cytokine expression of peripheral blood
mononuclear cells in response to allergens (Dermatophagoides farinae) was also studied in
patients with AD.20 Upon stimulation with relevant allergens, peripheral blood mononuclear
cells from patients with AD proliferated while the ones from healthy controls did not and it was
noted that SP promoted the allergen induced proliferation. It was concluded that SP modifies
immune responses of atopic T cells by promoting proliferation and altering cytokine profiles, and
therefore, modulates the clinical manifestations of AD. Therefore, neuropeptides, neuropeptide
receptors, neuropeptide-degrading enzymes and neurotrophins participate in a complex,
interdependent network of mediators that modulate skin inflammation, wound healing and the
skin immune system.
Capsaicin is an alkaloid derived from the seed and membranes of plants of the nightshade
family (active principle of chili pepper). The exact mechanism of action is unknown. The
effects of capsaicin on SP appear to be principally on type C sensory neurons. These
unmyelinated slow-conducting fibers of the type C group have been implicated in mediating
cutaneous pain and itch sensation. Repeated application of topical capsaicin abolishes pain and
itch. The efficacy of twice daily topical application of capsaicin (0.025%) for the management
of pruritus in dogs with atopic dermatitis (AD) was evaluated in double blinded, placebocontrolled study.181 Twelve dogs with AD were randomly assigned to either 0.025% capsaicin or
vehicle lotion applied twice daily for 6 weeks. After a 4-week wash-out period, treatments were
switched. Significant improvement was reported by owners (p=0.0006), but not by investigators.
Owners noted temporary worsening of pruritus after the first week of capsaicin therapy. Overall
capsaicin was well tolerated. Substance P (SP) concentrations in the skin did not correlate with
the severity of the pruritus and did not significantly change over time and between treatments.
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