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Immunosuppressive Therapy
for Canine Immune-Mediated
Hemolytic Anemia
❯❯ Suliman Al-Ghazlat, BVSc,
DACVIMa
NYC Veterinary Specialists
Forest Hills, New York
At a Glance
Treatment Options
Page 33
Suggested
Immunosuppressive
Protocol
Page 35
Management of Relapse
Page 40
Abstract: The mortality for dogs with severe immune-mediated hemolytic anemia (IMHA) is unacceptably high, and better immunosuppressive regimens are needed to increase survival. Understanding
the basic immunology of the disease and the mechanisms of action of the available immunosuppressive therapies will help clinicians choose an appropriate immunosuppressive protocol. Prospective,
randomized clinical studies must be conducted to evaluate the efficacy and safety of different combined immunosuppressive modalities to treat canine IMHA and improve patients’ outcomes.
T
he pathogenesis of canine immunemediated hemolytic anemia (IMHA)
involves the production of immunoglobulin (Ig) that recognizes either a self
(autoimmune) antigen or a foreign (immunemediated) antigen associated with red blood
cells (RBCs). This results in sensitization or
opsonization of the RBCs and their subsequent destruction by the complement system,
the mononuclear phagocyte system, or both.1–3
The interaction of RBC surface-bound Ig with
protein crystallizable fragment receptors (FcRs)
on different cells in the immune system leads
to a wide range of immune responses, including antibody-dependent cellular cytotoxicity,
phagocytosis, complement activation, mast cell
degranulation, lymphocyte proliferation, antibody secretion, and enhancement of antigen
presentation. These responses also promote
phagocytosis and clearance of opsonized
RBCs by the mononuclear phagocyte system
cells and lead to intravascular hemolysis by
full activation of complement.1,2,4,5 In severe
cases, clinical signs of anemia progress
rapidly, and animals may present in shock.
Most dogs that succumb to IMHA do so
within the first 2 weeks after onset (acute
phase).3,6–11 A retrospective study of 60 dogs
with IMHA showed a mortality of 52%, with
a good long-term outcome in dogs that survived the first 2 weeks of the disease.11
Treatment Options
Current immunosuppressive therapies for
IMHA act by a variety of mechanisms,
but ultimately they all suppress antibody
production by lymphocytes and/or inhibit
the clearance of opsonized RBCs by macrophages or lysis by the complement system.5,12–14 Because the half-life of IgG (the
antibody class involved in most cases of
canine IMHA) in dogs is approximately 1
week, therapies directed only at suppression of antibody production are unlikely
to affect the outcome in the acute phase
of the disease.13,14
Over the past 25 years, great advances
have been made in the field of immunology, especially in the development
of new immunosuppressive agents (e.g.,
cyclosporine, leflunomide, mycophenolate mofetil) that are more potent, more
selective, and less toxic than glucocorticoids.5,14,15 The wide array of new drugs
offers the opportunity to use combinations
that block different pathways of immune
activation while selecting agents with nonoverlapping toxicity profiles. Nonetheless,
IMHA mortality remains high, ranging
from 22% to 80%.2,3,6–11,16–18 Moreover, multiple retrospective studies have shown that
the addition of cyclosporine, azathioprine,
or cyclophosphamide to glucocorticoids
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did not improve mortality.8,11
IMHA and its therapy can be divided into
three phases: induction of remission, maintenance of remission and prevention of relapse,
and management of relapse. The causes of
death in dogs that die during the acute phase
of IMHA are not well documented, but in addition to lack of response to immunosuppressive
agents, they include thromboembolic disease,
sepsis, and other side effects of therapy, as
well as reactions to antithrombotic therapy and
blood transfusions.8,9,18 One way to improve
outcomes in the acute phase of IMHA is to
focus the induction of immunosuppression
on manipulating FcR/Ig interactions, reducing
phagocytosis, and inhibiting complement. The
causes of death during the maintenance phase
of IMHA therapy are also unclear, but they may
be related to disease recurrences or side effects
of immunosuppression. To improve outcomes
and minimize relapses in this phase, maintenance immunosuppressive therapy should be
potent, specific, and associated with few or no
side effects.
Glucocorticoids
QuickNotes
The mortality for
dogs with severe
IMHA is high, with
most deaths occurring within the first
2 weeks of clinical
disease.
34
Glucocorticoids remain the mainstay of immunosuppressive therapy for canine IMHA. Their
activity is largely derived from their ability
to repress the transcription of many genes
responsible for encoding proinflammatory
cytokines and adhesion molecules that influence immune cell trafficking and cellular interactions. The molecular basis of glucocorticoid
action lies in the capacity to diffuse through
the cell membrane and bind to cytosolic steroid receptors, which subsequently undergo
nuclear translocation and modulate transcriptional activation.5,12,19,20 The main effect
of glucocorticoids in the treatment of IMHA
is to suppress complement and phagocytosis
of opsonized RBCs by interfering with the
expression and function of macrophage FcRs,
which is an immediate effect.5,20
Prednisone (2 mg/kg bid) is commonly
used to induce remission in IMHA, and a variety of dosages and protocols have been advocated.1–3,5–11,16–18 There have been no studies to
evaluate the effect of different prednisone dosages on short- or long-term outcomes. Factors
that may influence dosing include patient
size, body condition, breed, disease severity, concurrent illnesses, and the presence of
gastrointestinal (GI) signs at presentation. For
induction, the recommended dose is 2 mg/kg
bid for dogs weighing less than 6 kg and 1.1
mg/kg or 30 mg/m2 bid for those weighing
more than 30 kg.2,17 Remission is signaled by a
stable or rising packed cell volume (PCV) with
no clinical signs of decreased oxygen-carrying
capacity over a period of 7 to 14 days. At this
time, the dose may be decreased by 25% to
50% (depending on the severity of side effects)
and then by 25% every 2 weeks thereafter.2
Once a dose of 0.5 mg/kg sid is reached,
the patient can be switched to alternate-day
prednisone therapy. As the side effects of prednisone at this point are typically mild, decreasing the dose by 25% every 4 to 6 weeks rather
than every 2 weeks is generally advisable.
Prednisone administration can be completely
discontinued if there are no signs of relapse
while the patient is receiving 0.25 mg/kg every
other day for 4 to 6 weeks (Table 1). A complete blood count (CBC) should be obtained
before any decrease in the dose of immunosuppressive agents. A less aggressive tapering
protocol may be advised when glucocorticoids are used as the sole immunosuppressive
agent. Although most dogs can be completely
weaned from glucocorticoids, a few patients
may require lifelong, low-dose therapy to
maintain remission.2,11 This prednisone protocol is based solely on my clinical experience,
and there are no published studies establishing its superiority.
Adverse effects of glucocorticoids include
iatrogenic hyperadrenocorticism, GI ulceration and perforation, recurrent infections,
sepsis, and thromboembolic disease.21–23
Glucocorticoids may also predispose patients
to pancreatitis, although this has not been
proven.24 Based on clinical experience, largeand giant-breed dogs are especially sensitive
to adverse effects of glucocorticoids. In contrast, most dogs weighing less than 6 kg generally experience only mild adverse effects.
Dexamethasone is considered by some clinicians to be superior to prednisone in inducing IMHA remission, but there are no studies
to support this opinion, and (anecdotally) GI
ulceration and pancreatitis are more common
with dexamethasone than with prednisone.13,14
In addition, due to its longer duration of effect
in suppressing the hypothalamic–pituitary–
adrenal axis, dexamethasone is not appropri-
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table 1
Suggested Immunosuppressive Protocol
This protocol is for a 25-kg dog with a body condition score of 5/9, using prednisone for induction of remission and azathioprine to help with
maintenance, and is based on clinical experience.
Day
Protocol
Day 1
Prednisone at 30 mg PO bid or, if the dog cannot tolerate oral medication,
dexamethasone at 9 mg IV sid
Day 5 No GI disturbance for at least 48 hr
Maintain prednisone at 30 mg bid and add azathioprine at 50 mg PO sid
Day 7 PCV is stable or rising for the previous 48 hr and there
Discharge the patient from the hospital and continue the same medications
Day 14 PCV is rising but subnormal, with no signs of
myelotoxicity/hepatotoxicity or serious glucocorticoid side
effects
Obtain a CBC and chemistry profile
Maintain prednisone at 30 mg bid
Decrease azathioprine to 50 mg q48h
Day 21 PCV is normal, with no signs of myelotoxicity or
serious glucocorticoid side effects
Obtain a CBC
Decrease prednisone to 20 mg bid
Maintain azathioprine at 50 mg q48h
Day 28 (week 4) PCV is normal, with no signs of
Obtain a CBC
Maintain prednisone at 20 mg bid
Maintain azathioprine at 50 mg q48h
Week 5 No signs of relapse, myelotoxicity, or hepatotoxicity
Obtain a CBC and chemistry profile
Decrease prednisone to 15 mg bid
Maintain azathioprine at 50 mg q48h
Week 7 No signs of relapse or myelotoxicity
Obtain a CBC
Decrease prednisone to 10 mg bid
Maintain azathioprine at 50 mg q48h
Week 9 No signs of relapse or myelotoxicity
Obtain a CBC
Decrease prednisone to 15 mg sid
Maintain azathioprine at 50 mg q48h
Week 11 No signs of relapse or myelotoxicity/hepatotoxicity
Obtain a CBC and chemistry profile.
Decrease prednisone to 10 mg sid
Maintain azathioprine at 50 mg q48h
Week 13 No signs of relapse or myelotoxicity
Obtain a CBC
Decrease prednisone to 5 mg q48h
Maintain azathioprine at 50 mg q48h
Week 17 No signs of relapse or myelotoxicity/hepatotoxicity
Obtain a CBC and chemistry profile
Discontinue prednisone
Maintain azathioprine at 50 mg q48h
Week 23 No signs of relapse or myelotoxicity/hepatotoxicity
Obtain a CBC and chemistry profile
Decrease azathioprine dose to 50 mg q 3 days
Week 27 No signs of relapse
Obtain a CBC
Discontinue azathioprine
Week 30 No signs of relapse
Recommend a CBC q 2 mo for the first year and biannually to triennially thereafter
are no signs of GI disturbance
myelotoxicity or serious glucocorticoid side effects
CBC = complete blood count; PCV = packed cell volume.
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ate for alternate-day therapy. Because the main
effect of glucocorticoids in treating IMHA is
to suppress complement and FcR-mediated
clearance of opsonized RBCs, glucocorticoids
are the drugs of choice in the induction phase
of treatment. However, as glucocorticoids can
have severe side effects when used chronically
at high doses, they are not ideal for maintaining remission and preventing relapse.
Human Intravenous Immunoglobulin
QuickNotes
Glucocorticoids
are the mainstay of
immunosuppressive
therapy for canine
IMHA, but they can
cause serious side
effects.
36
Danazol
Danazol is an androgen derivative used in the
early 1990s as an adjunctive treatment for canine
immune-mediated thrombocytopenia.33,34 Similar
to glucocorticoids, danazol (10 to 15 mg/kg sid)
may inhibit binding of the Fc portion of immunoglobulin to FcRs and hence prevent phagocytosis of opsonized RBCs.33,35 However, in one
preliminary report of a small, prospective study
in dogs with IMHA, adding danazol did not
show any significant beneficial effects compared
with prednisone alone.35 Danazol is an anabolic
steroid and can cause dramatic weight gain in a
short time, especially when used in combination
with prednisone. Furthermore, danazol is expensive and may be hepatotoxic.36 Due to these side
effects of chronic use, danazol is not recommended to maintain remission.
Human IV immunoglobulin (hIVIG) is a sterile, purified IgG preparation manufactured
from pooled human plasma that typically
contains more than 95% unmodified IgG,
which has intact Fc-dependent effector functions, and only trace amounts of IgA and IgM.
Several mechanisms of action have been proposed, including antiidiotypic activity, inhibition of autoantibody production, acceleration Splenectomy
of autoantibody breakdown and removal, and Splenectomy may be considered for patients
suppression of complement activation.25–32 Most with IMHA that fail to respond to, or have severe
importantly, hIVIG may reduce Fc-mediated adverse effects from, immunosuppressive therphagocytosis of IgG-coated RBCs.25,26 One apy or that require long-term, high-dose therapy
study showed that hIVIG binds to canine lym- to remain in remission. The spleen is a major
phocytes and monocytes and inhibits RBC site of autoantibody production and of sequesphagocytosis.26
tration and destruction of IgG-sensitized RBCs.
hIVIG has been used successfully in mul- A thorough search for an underlying infection
tiple studies to treat a small number of dogs is mandatory before proceeding with splenecwith IMHA.31,32 A total infusion of 0.5 to 2 g/ tomy. Preliminary results of a small, prospective,
kg administered in a divided dose on 2 con- unpublished study showed that splenectomy
secutive days over a period of 6 to 12 hours as an adjunctive therapy is superior to medical
has been beneficial in some refractory cases therapy alone in reducing mortality and shortof canine nonregenerative IMHA.31 No signifi- ening the interval to normal PCVs in dogs with
cant side effects were reported in these lim- severe IMHA.37 In fact, splenectomy may be
effective for both remission induction and mainited samples.31,32
Due to its rapid onset of action and poten- tenance therapy for severe IMHA. It may also
tial to block the phagocytosis of sensitized be helpful in dogs with multiple acute relapses
RBCs, hIVIG may be useful in the induction or in those that only partially respond to mediof remission in dogs with severe IMHA. In the cal immunosuppression. However, as the site
few reported cases, the response was often of clearance of IgM-sensitized RBCs appears
rapid but transient, suggesting that hIVIG is to be the hepatic Kupffer cells, splenectomy is
not ideal for maintaining remission.31,32 Indeed, unlikely to be beneficial in dogs with positive
it is potentially immunogenic in dogs, and Coombs’ test results or IgM autoantibodies.38
repeated infusions could induce acute anaphy- No large-scale studies have evaluated the
laxis. There are no studies on the efficacy and short- and long-term safety of splenectomy in
the safety of repeated hIVIG infusions in dogs. dogs. Due to the cost of surgery and risk of
Although hIVIG is expensive ($600 for a 5-g complications, splenectomy is rarely recombottle), it may be practical and economical to mended as a first-line therapy for severe IMHA.
use for induction of remission if it significantly On the other hand, given the encouraging predecreases hospitalization time and transfusion liminary data, further studies to evaluate the
requirements. Repeated maintenance infusions safety and efficacy of splenectomy in the treatment of canine IMHA are warranted.
are currently not recommended.
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Plasmapheresis
Plasmapheresis is a process whereby the components in plasma believed to cause or exacerbate disease are removed. The remaining
blood components are then combined with
replacement plasma or an inert substitute and
returned to the patient.39,40 The clinical effectiveness of plasmapheresis in the treatment of
autoimmune disease may be due to the partial
removal of autoantibodies, immune complexes,
complement components, proinflammatory
agents, and soluble adhesion molecules.39
Plasmapheresis is most useful for rapidly
reducing plasma concentrations of autoantibodies or immune complexes while other
immunosuppressive measures are applied
to prolong the effect.39 One study reported
encouraging results for plasmapheresis in dogs
with systemic immune-mediated diseases.40
The study suggested that any dog with an
acute immune-mediated disease refractory to
conventional immunosuppressive therapy can
be considered for plasmapheresis.40 Although
there are no reported clinical trials of plasmapheresis for IMHA, it may be helpful in
acute, refractory cases or cases complicated
by concurrent systemic infection.41 However,
availability of plasmapheresis is limited in veterinary medicine, and experience with its use
for treating animals with immune-mediated
diseases is minimal.
on reducing levels of circulating antierythrocyte antibodies. As such, it is probably ineffective for remission induction during the
acute phase of IMHA.
One retrospective study showed no beneficial effect of adding cyclophosphamide
to prednisone in 60 dogs with IMHA,11 and
another study showed higher mortality in dogs
so treated compared with dogs that received
prednisone alone.8 Interpretation of these
studies is complicated by their retrospective
nature, and it is likely that cyclophos­phamide
was used only in the more severe cases.
However, a small, randomized, prospective
trial found no beneficial effects from adding
cyclophosphamide to prednisone in the management of acute IMHA and even suggested
that cyclophosphamide may adversely affect
outcome.10 Indeed, several in vitro and in vivo
experimental studies suggest that cyclophosphamide may paradoxically augment immune
responses through a toxic effect on T regulatory cells.43 Cyclophosphamide also has serious adverse effects, including gastroenteritis,
myelosuppression, sterile hemorrhagic cystitis,
and secondary neoplasia.5,44,45 Based on this
evidence, cyclophosphamide is not recommended for induction of remission in dogs
with IMHA, and the risk–benefit profile should
be considered carefully before it is used for
maintenance therapy.
Cyclophosphamide
Azathioprine
Cyclophosphamide is a cytotoxic, myelosup- Azathioprine is a cy totoxic antimetabopressive alkylating agent. It cross-links DNA lite that is conver ted to 6 -mercaptopuhelixes to prevent their separation, thus pre- rine in the liver. It is a purine analogue
venting the formation of a DNA template.5,42 that functions as a competitive purine
Cyclophosphamide is toxic to both resting antagonist, thereby inhibiting cellular proand dividing cells, particularly proliferating liferation.5,12,46 Azathioprine is less toxic to
immune cells (lymphocytes).5 It suppresses resting cells than cyclophos­phamide and
both cell-mediated and humoral immunity, therefore has fewer side effects. It primarily
and it may suppress mononuclear phago- suppresses lymphocyte activation and procytic function.5,12 Historically and anecdot- liferation, reducing antibody production. In
ally, cyclophosphamide has been effective in vivo experimental studies have shown that
inducing and maintaining remission in dogs azathioprine may help to establish antigenwith fulminant IMHA. Several authors have specific tolerance and decrease the risk of
recommended using cyclophosphamide in relapse.47 Azathioprine also suppresses maccases in which intravascular hemolysis or rophage function, which reduces inflammaautoagglutination is present, suggesting that it tory cytokine production and phagocytic
is especially effective in IgM-mediated hemol- efficiency.4,5 The onset of action of azathioysis.5 However, cyclophosphamide principally prine is slow in people but appears to be
targets lymphocyte proliferation and, there- faster in dogs, with immunosuppressive
fore, is unlikely to have an immediate effect effects evident in 2 to 4 weeks.
QuickNotes
Due to its rapid
onset of action,
human intravenous
immunoglobulin
may be a good
choice in the initial
management of
severe IMHA.
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QuickNotes
Cyclophosphamide
is not recommended for induction of remission in
dogs with IMHA.
Several studies have suggested a beneficial effect of azathioprine in the treatment of
canine IMHA.6,11,17 However, as these studies
are retrospective and lack data on azathioprine timing and dosing and because multiple concurrent immunosuppressive and
antithrombotic agents were used, it is difficult
to attribute the improved survival directly to
use of azathioprine. A large, retrospective
study showed that in dogs treated for IMHA
with the combination of prednisone, azathioprine, and ultralow-dose aspirin (0.5 mg/kg
sid), the survival rates at discharge, 1 month,
and 1 year were 88%, 82%, and 69%, respectively, which compared favorably with rates in
previously reported studies (57%, 58%, 34%).17
Due to azathioprine’s slow onset of action, it
is unlikely to play any role in improving shortterm survival. The recommended canine loading dose is 2 mg/kg sid for 5 to 7 days, followed
by 2 mg/kg every other day for maintenance.17
I generally continue azathioprine maintenance
until the patient has been weaned off prednisone for 4 weeks. There is no well-established
dose-tapering protocol for azathioprine, but I
taper the dose gradually over 2 to 3 months by
extending the period of time between doses
by 1 day every 4 weeks (Table 1).
The most common adverse effects of azathioprine therapy are GI disturbances and myelosuppression, particularly in large dogs.46,48,49
Acute pancreatitis and cholestatic hepatopathy have been anecdotally reported in dogs
receiving azathioprine.48,50,51 The prognosis
for azathioprine-induced bone marrow toxicity is good if the drug is discontinued before
severe myelofibrosis occurs.49 Therefore, a
CBC should be obtained every week for the
first month of azathioprine therapy and every
4 weeks for the duration of therapy, and the
drug should be discontinued immediately if
the neutrophil count declines significantly.
Due to its slow onset of immunosuppressive
action, azathioprine is not a drug of choice for
induction of remission. Conversely, due to its
selective immunosuppressive effects, few side
effects, availability, and relatively low cost, it is
a good choice for maintenance therapy.
Cyclosporine A
Cyclosporine A (CsA) is a cyclic polypeptide metabolite of the fungus Tolypocladium
inflatum. It has potent immunosuppressive
38
properties through its blockage of transcription of cytokine genes in activated T cells.
CsA inhibits the phosphatase activity of calcineurin, thereby preventing activation of the
nuclear factor of activated T cells (NFAT).5,12,52
NFAT activation and nuclear translocation are
required for the transcription of many cytokines, particularly interleukin-2 (IL-2), which
is necessary for the proliferation and maturation of T cells.5,52,53 CsA also enhances expression of transforming growth factor-β (TGF-β),
which is a potent inhibitor of IL-2–stimulated
T cell proliferation and generation of antigenspecific cytotoxic lymphocytes.5,52 Studies indicate that CsA blocks the activation of C-Jun N
terminal kinases and p38 signaling pathways
triggered by antigen recognition, making CsA
a highly specific inhibitor of T cell activation.12
In addition, by suppressing the production of
interferon γ, CsA may inhibit the phagocytic
capabilities of macrophages.5,53
The efficacy of CsA has been demonstrated
in cats and dogs undergoing renal transplantation and in dogs with immune-mediated
disorders.54 One retrospective study showed
improved outcome in dogs with IMHA that
received CsA and glucocorticoids compared
with glucocorticoids alone, but the difference
did not reach statistical significance.8 This
finding suggests that CsA may be superior to
glucocorticoids alone, as CsA was likely added
only in the more severe cases. Preliminary
results from a prospective study showed no
beneficial effect from combining CsA with
prednisone on the survival of dogs with IMHA
in the first 28 days after onset.55 However, four
of the 19 dogs in the prednisone-only group
experienced relapses compared with none in
the combination group. This study has not
yet been published, and the number of subjects was small. In addition, a relatively small
dose of CsA was used (4 mg/kg sid), and there
was no mention of monitoring drug levels.
Generally, a dosing regimen of 5 to 10 mg/kg
sid is recommended for dogs, and plasma concentrations should be periodically monitored
to achieve an effective but safe trough level
(400 to 500 ng/mL); an optimal level has not
been established.5
The most common side effects of CsA in
dogs include GI irritation, gingival hyperplasia, antibiotic-responsive dermatitis, and
papillomatosis.5,54,56 Malignancies, including
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lymphoma and squamous cell carcinoma,
have been noted as well.57,58 In addition,
anaphylaxis has been reported in people
and dogs receiving IV CsA.59 CsA-associated
nephrotoxicosis is a well known complication in people but appears to be rare in dogs
and cats whose serum CsA levels are maintained within the therapeutic range (400 to
500 ng/mL).54 CsA is expensive and requires
blood level monitoring, which usually makes
it cost-prohibitive for large dogs. However,
due to its potent and specific suppressive
effects on lymphocyte proliferation and
autoantibody production and tolerable side
effects even when used chronically, it may be
a good choice for maintenance. In addition,
due to its suppression of phagocytosis, it may
be beneficial during the induction phase of
therapy. 5,53
Mycophenolate Mofetil
Mycophenolate mofetil (MMF) is a fermentation product of several Penicillium spp that
is metabolized completely in the plasma and
liver into its active metabolite, mycophenolic
acid (MPA).60–62 MPA is an effective, reversible
inhibitor of inosine monophosphate dehydrogenase (IMPDH), which is a key enzyme
in de novo purine biosynthesis.12,52,60–62 Most
other cell lines can maintain their function
through the salvage pathway of purine biosynthesis alone, but proliferating lymphocytes
depend on both the salvage pathway and the
de novo pathway. Due to the high specificity of MPA for IMPDH, MPA is a very selective lymphocyte inhibitor.60,61,63 Blockade of
IMPDH by MPA was shown to deplete the
guanosine pool in lymphocytes and to inhibit
T- and B-cell proliferation, differentiation of
alloreactive cytotoxic T cells, and antibody
responses.60–62 Other mechanisms that may
contribute to the immunosuppressive effects
of MPA include induction of apoptosis of activated T cells and impairment of the maturation of dendritic cells.64
MMF has been shown to be safe and effective in prolonging the survival of canine
experimental renal allografts and, anecdotally,
has been used successfully in several cases of
canine immune-mediated disease.61,62 The recommended starting MMF dose for dogs is 20
to 40 mg/kg/day PO divided into two or three
doses.62 Experimental studies showed that
dogs are especially sensitive to the adverse GI
effects (e.g., diarrhea, vomiting) of this drug.61
There are no reports on the use of MMF to
manage canine IMHA, but based on its mechanism of immunosuppressive action, it should
be investigated for maintenance of remission.
Leflunomide
Leflunomide is a synthetic isoxazole derivative
that is metabolized in the gut and liver.60 The
active metabolite of leflunomide, A77 1726,
reversibly inhibits dihydro-orotate dehydrogenase, the rate-limiting enzyme in the de
novo synthesis of pyrimidines.12,52,60,65–67 Again,
lymphocytes need both the salvage and the
de novo pathways of pyrimidine synthesis to
meet the high demand for pyrimidines during lymphocyte activation and proliferation.
Leflunomide specifically suppresses activated
T and B cells while other cells maintain their
basal cell division. Experimental transplantation studies showed that A77 1726 prevents
antibody production.60 Leflunomide also inhibits the production of proinflammatory cytokines (e.g., IL-1, tumor necrosis factor α) and
augments the production of the antiinflammatory cytokine TGF-β, culminating in significant
antiinflammatory effects.43,52,61,65
One study reported promising results of the
use of leflunomide for treatment of immunemediated and inflammatory diseases in dogs
refractory to conventional therapy.65 In this
study, 26 dogs with different immune-mediated diseases were treated with leflunomide,
including six dogs with IMHA. Five of these six
dogs responded well to leflunomide therapy,
and the one dog that died had severe myelofibrosis that was probably related to the primary
disease. The initial dose of leflunomide was
4 mg/kg/day, but the dose was adjusted to
obtain a target trough A77 1726 plasma level
of 20 µg/mL.67 Most of the dogs had mild to
moderate disease or partial response to conventional therapy, affording sufficient time for
leflunomide to exert its immunosuppressive
effects.
Leflunomide is not commonly used in veterinary medicine, so adverse effects are not
well documented. Adverse effects reported
in a small number of dogs were rare and
mild, including decreased appetite, lethargy,
mild anemia, and self-limiting GI signs.65
However, most of the dogs in the study were
QuickNotes
An immunosuppressive regimen is only
one part of a comprehensive therapeutic approach to
improve the outcome of dogs with
IMHA.
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concurrently receiving other immunosuppressive agents that may have contributed to
these effects. Based on this single study of
dogs with IMHA, it seems that leflunomide
may be an effective immunosuppressive drug
with minimal side effects and should be investigated for maintenance of remission of canine
IMHA.
Mizoribine
Mizoribine is an imidazole nucleoside, and its
active metabolite (mizoribine monophosphate)
is a potent inhibitor of IMPDH.52,68 Therefore,
like MMF, it blocks purine biosynthesis in B
and T cells and inhibits their proliferation.
The antiproliferative effect of mizoribine is
linked to a decrease in guanine ribonucleotide pools. Mizoribine has been approved in
Japan for renal transplant recipients and is currently undergoing clinical testing in Europe as
a substitute for azathioprine in human renal
transplant recipients. It appears to be safe and
(unlike azathioprine) to have minimal myelotoxicity or hepatotoxicity. The availability of
this drug is very limited, and little is known
about its safety and efficacy in veterinary medicine, but theoretically, it could be employed
for the maintenance of IMHA remission.
Management of Relapse
The relapse rate for canine IMHA is not well
documented, but retrospective studies suggest
that it is relatively high (13% to 20%).8,9,11,16,17
Clinicians must monitor patients for relapse
(Table 1), make a prompt diagnosis, assess the
severity and the acuteness of the relapse, and
institute an appropriate immunosuppressive
protocol. The choice of regimen depends on
the severity of the relapse. A patient with an
acute relapse (e.g., drop in PCV from normal
to 25% or lower) should be hospitalized for
supportive care and aggressively treated to
reinduce remission. Immunosuppressive doses
of glucocorticoids are the mainstay of therapy
for relapse; adjunctive induction therapies for
severe, refractory cases include hIVIG, splenectomy, and plasmapheresis.
On the other hand, a patient with a mild
relapse while being weaned off immunosuppressive therapy can be managed less aggressively. For example, a dog in which the PCV
drops from 40% to 35% after the prednisone
dose is decreased from 1.5 to 1 mg/kg/day
40
can be managed by increasing the dose to the
previous level. This dog can be managed as
an outpatient if it is asymptomatic for anemia,
but its CBC should be reevaluated in 1 week
to assess the efficacy of therapy. Importantly,
a drop in PCV for a dog with IMHA does not
always signal a relapse. Other causes of a
decreased PCV in dogs receiving immunosuppressive therapy for IMHA include sepsis and
GI hemorrhage. Failure to identify patients
with these complications can be detrimental
and potentially fatal.
Conclusion
Canine patients with severe IMHA have a
guarded prognosis. Despite the introduction of
several new immunosuppressive agents, mortality remains high.2,3,5–7,12–16 Glucocorticoids
are the mainstay of IMHA therapy for induction of remission, but the addition of more
potent immunosuppressive agents to prednisone therapy may be warranted in severe
cases and in patients with major adverse
glucocorticoid effects. Because azathioprine,
CsA, leflunomide, and MMF are unlikely to
have immediate beneficial effects in dogs
with IMHA but may have immediate adverse
reactions, I generally wait to add one of these
agents to glucocorticoid therapy until the PCV
is stable and the dog exhibits no GI disturbances. At my institution, the most common
immunosuppressive agents used in combination with prednisone for severe IMHA are CsA
and azathioprine. CsA is faster acting and has
fewer side effects than azathioprine, but it can
be cost prohibitive in large dogs. Preliminary
results seem promising for leflunomide and
discouraging for cyclophosphamide, but there
are few published prospective, randomized
trials evaluating the use of various immunosuppressive agents used in combination with
prednisone versus prednisone alone.
Immunosuppressive therapy is only one part
of a comprehensive approach to improving the
outcome of IMHA. Other therapies being studied include antithrombotic agents; one large,
retrospective study showed improved survival
in dogs with IMHA that were treated with
ultralow-dose aspirin to prevent thromboembolic complications.17 The timing, volume, and
type of oxygen-carrying support provided by
blood products or blood substitutes is another
area that requires further investigation.
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Immunosuppressive Therapy for Canine IMHA CE
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3 CE
CREDITS
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regarding the applicability of this program.
CE Test 3
1.Which statement regarding dogs with
IMHA is correct?
a.Most dogs die during the maintenance
phase of disease management.
b.All dogs can be successfully weaned off
immunosuppressive therapy.
c.Dogs that survive the first 2 weeks of
the disease generally have a good longterm prognosis.
d.To avoid relapses, dogs should never
be weaned off immunosuppressive
medications.
2.Which statement regarding glucocorticoid therapy for treating IMHA is correct?
a.Prednisone at 2 mg/kg bid is a good
dose for large- and giant-breed dogs,
whereas small-breed dogs should not
receive more than 1.2 mg/kg bid.
b.It has been proven that dexamethasone
is superior to prednisone for inducing
remission.
c.Dexamethasone is not appropriate for
alternate-day therapy.
d.Prednisone at doses higher than 2
mg/kg bid is associated with better
outcomes.
3.Which statement regarding immunosuppressive agents used to treat canine
IMHA is correct?
a.A zathioprine is a good drug to induce
remission, but it can cause bone marrow suppression.
b.Hepatotoxicity is a common side effect
44
of cyclophosphamide.
c.Secondary malignancies have been
reported in dogs receiving cyclosporine.
d.Cyclosporine is a potent cytotoxic
immunosuppressive drug.
4.Which statement regarding the mechanism of action of immunosuppressive
agents is incorrect?
a.A zathioprine is a competitive purine
antagonist.
b.Mizoribine blocks the purine biosynthetic pathway.
c.Leflunomide blocks the biosynthesis of
pyrimidine.
d.Cyclosporine works by blocking the
biosynthesis of pyrimidine.
5.Which is not a reported side effect of
cyclosporine therapy in dogs?
a. GI disturbances
b. infection
c. development of secondary neoplasia
d. aplastic anemia
6.Which is not a side effect of azathioprine
therapy in dogs?
a. pancreatitis
b. bone marrow toxicity
c. gingival hyperplasia
d. GI disturbances
7.Which is not a side effect of
glucocorticoids?
a. GI ulceration and perforation
Compendium: Continuing Education for Veterinarians® | January 2009 | CompendiumVet.com
b. increased risk of infection
c. bone marrow suppression
d.increased risk of thromboembolic
disease
8.Which patient is the most suitable candidate for hIVIG treatment?
a.a dog with a 3-day history of mild weakness, a PCV of 26%, total bilirubin of 1.5
mg/dL, and 2+ spherocytes
b.a dog experiencing an IMHA relapse
with PCV of 24% and a history of
receiving an hIVIG dose 1 month earlier
c.a dog that has been hospitalized for
IMHA for 3 days and received prednisone 2 mg/kg bid and three blood
transfusions
d.a dog presenting with acute IMHA, a
PCV of 20%, and a history of congestive heart failure
9.Which immunosuppressive therapy
is unlikely to be effective in the first 2
weeks of treating IMHA?
a. glucocorticoids
c. azathioprine
b. splenectomy
d.hIVIG
10. In dogs with IMHA, administration of
which immunosuppressive drug has
been associated with a worse outcome
in retrospective studies?
a.cyclosporine
b.leflunomide
c.cyclophosphamide
d.mycophenolate mofetil