1. health and fitness
2. disease

Review
New treatments for idiopathic
thrombocytopenic purpura:
rethinking old hypotheses
1.
Introduction
Donald M Arnold†, Ishac Nazi & John G Kelton
2.
Normal platelet physiology
3.
Idiopathic thrombocytopenic
purpura
McMaster University, Michael G DeGroote School of Medicine, Medicine and Pathology and
Molecular Medicine, Hamilton, Ontario, Canada
4.
Conclusion
5.
Expert opinion
Background: The efficacy of thrombopoietin (TPO) mimetics in patients
with idiopathic thrombocytopenic purpura (ITP) reaffirms that impaired
platelet production is an important mechanism. New strategies to reduce
platelet destruction, like rituximab, are also effective. Objectives: To describe the
efficacy and safety of rituximab and the TPO mimetics, romiplostim and eltrombopag, and how they relate to ITP pathogenesis. Methods: Narrative review
summarizing full publications and meeting abstracts. Results/conclusions: A
4-week course of rituximab is associated with a platelet count response in
60% of patients with ITP, and durable responses have been observed.
Subtle increases in infection have been reported. Romiplostim and eltrombopag are each associated with a 60 – 85% response while on treatment.
,
Transient bone marrow reticulin with romiplostim and delevated
liver
a
o
l
n
enzymes with eltrombopag are rare side effects. The application
of
these
ow se.
agents in non-splenectomized patients requires further
u
n d study.
a
al
s c son
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Keywords: autoantibodies, idiopathic thrombocytopenia purpura
us r p,emegakaryocytes, platelets,
d
thrombopoietin
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Expert Opin. Investig. Drugs (2009) 18(6):805-819
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1. Introduction
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The mechanism of thrombocytopenia in ITP is due to increased platelet destruction
by autoantibodies that cause accelerated platelet clearance in the reticuloendothelial
system (RES) and by a relative impairment in platelet production. Conventional
treatments for ITP interfere with platelet clearance or autoantibody formation.
Newer treatments include the CD20 monoclonal antibody rituximab, which
induces B-cell lysis, and a new class of agents, the thrombopoietin mimetics,
which increase platelet production. The efficacy, safety and mechanism of action
of these newer treatments for ITP are the focus of this review.
2.
Normal platelet physiology
Platelets are anucleate cells produced from bone marrow megakaryocytes. Their
function is to prevent bleeding at the site of endothelial injury and to maintain
vascular integrity. Platelets contain a large surface area of phospholipid membrane
that connects with an intracellular cannilicular system. Upon activation, the platelet
surface exposes anionic phosphatidylserine, which serves as a surface upon which
coagulation can occur.
Healthy adults produce about 1 x 1011 platelets per day; however, this rate of
platelet production can increase 10-fold in times of increased demand [1,2]. The
number of circulating platelet is tightly regulated by the hormone thrombopoietin
10.1517/13543780902905848 © 2009 Informa UK Ltd ISSN 1354-3784
All rights reserved: reproduction in whole or in part not permitted
805
New treatments for idiopathic thrombocytopenic purpura: rethinking old hypotheses
(TPO), which acts upon megakaryocytes and hematopoietic
stem cells via the TPO receptor, c-Mpl, to increase the
production of platelets and megakaryocyte progenitors. TPO
is constitutively secreted from the liver, and levels of
TPO available for binding are regulated by the megakaryocyte
mass and the number of circulating platelets, which also
express c-Mpl. After TPO binds to platelets it is internalized,
degraded and removed from circulation [3].
2.1
Normal platelet production
Megakaryocytopoiesis is a complex cellular process that starts
from the proliferation and differentiation of hematopoietic
stem cells and ends with the production of platelets [4]. The
process of cellular proliferation and differentiation requires
interleukins, colony-stimulating factors and TPO to stimulate
megakaryocyte endomitosis and polyploidy [5-7]. Megakaryocyte
progenitor cells undergo cytoplasmic maturation through
the formation of platelet secretory granules and the production
of the demarcation membrane system that permeates the
cytoplasmic space. This extensive membrane system forms
multiple filamentous pseudopod-like structures called proplatelets,
from which platelets are released or ‘shed’ [8-10].
Platelets circulate for 7 – 10 days and are then cleared by
phagocytic cells in the RES. As platelets age, they undergo
physiologic changes including an increase in fibrinogen uptake,
loss of membrane integrity and shedding of CD42b and
GP-VI in a process called senescence [11]; however, the signal
to remove senescent platelets is not known.
2.2
Thrombopoietin
The TPO gene is located on chromosome 3q26.3 – 3q27. TPO,
also known as c-Mpl ligand, was cloned by five independent
groups in 1994 [2,7,12-14]. It is the primary physiological
regulator of megakaryocyte growth and platelet production.
TPO, like other growth factors and interleukins, is necessary
for the survival and proliferation of hematopoietic stem
cells [1,15] and, in conjunction with other growth factors
including the Flt3 ligand, c-kit ligand or interleukin-3, stimulates
megakaryocytopoiesis [16-21]. TPO is a glycoprotein consisting
of 353 amino acids with an average molecular weight of
35.5 kDa and 60 – 70 kDa after glycosylation [22]. The
mature molecule consists of two domains: the amino terminal
domain (active site), and the carbohydrate domain. The
amino terminal 155 residues of thrombopoietin have
20% sequence identity and 25% homology with human
erythropoietin, the main regulatory hormone of red blood cell
production (Figure 1). The carbohydrate-rich carboxy-terminus
of the protein is 177 residues in length, highly glycosylated
and important in maintaining protein stability [2,7,13,22].
TPO is produced predominantly in the liver, but also in
the kidneys, bone marrow, lungs and placenta [23]. TPO
production is constitutive, meaning that a constant level is
secreted by the liver at all times. TPO binds c-Mpl on platelets,
megakaryocytes and hematopoietic stem cells. After TPO
binds to platelets (and megakaryocytes) it is internalized and
806
degraded [24-30]; thus, during states of thrombocytopenia,
higher levels of free TPO are available for binding megakaryocytes causing more platelets to be produced, and during
states of thrombocytosis, lower levels of free TPO are available for binding (Figure 2) [12,31,32]. The role of TPO as the
principal physiologic regulator of platelet production has been
confirmed in studies of TPO and c-Mpl knockout mice,
which have 5 – 15% normal levels of circulating platelets,
megakaryocytes and megakaryocyte progenitor cells [33,34].
2.3
Thrombopoietin receptor (c-Mpl)
The c-mpl gene is the human homologue of the murine
myeloproliferative leukemia virus oncogene, v-mpl [35]. The
c-mpl gene was cloned in 1992 [36] and shown to encode a
type I transmembrane protein that has substantial homology
with receptors for interleukins and colony-stimulating
factors [35]. C-Mpl mRNA and protein are found in platelets, megakaryocytes and a subpopulation of CD34+ cells;
without c-Mpl, progenitor cells will not differentiate to
megakaryocytes in bone marrow cultures [37]. There are
approximately 30 – 60 c-Mpl receptors per platelet and they
bind TPO with high affinity [38]. Mutations in c-Mpl in
humans causes congenital amegakaryocytic thrombocytopenia,
a hereditary bleeding disorder characterized by decreased or
absent bone marrow megakaryocytes and low platelet
counts [39].
The extracellular domain of the c-Mpl receptor contains
two cytokine receptor homology modules (CRM1 and
CRM2) and a transmembrane domain. CRM1 self-regulates
receptor activity by acting as a brake for cell proliferation;
deletion of this domain results in uncontrolled cell growth in
culture [40]. CRM1 is also the site of TPO binding [25,27] which
induces a conformational change and signal transduction [41,42].
The signaling region of c-Mpl does not possess a tyrosine
kinase domain, tyrosine phosphatase, or other enzymatic
function; signaling occurs via homodimerization of the
receptor and association of various downstream signaling
steps including activation of the intracellular tyrosine kinase JAK2
and the phosphorylation of signal transducers and activators of
transcription (STATs), PI3K, mitogen-activated protein kinases
(MAPKs) and the c-Mpl receptor itself (Figure 3) [43].
3.
Idiopathic thrombocytopenic purpura
Thrombocytopenia (platelet count < 100 x 109/l [44]) is
invariably present in patients with ITP; however, most
patients never bleed. For those who do, bleeding complications range in severity from bruises and petechiae, to lifethreatening intracranial hemorrhage. The prevalence of ITP
is approximately 23 per 100,000 population [45] and refractory, adult-onset ITP is associated with a relative risk of death
of 4.2 (95% confidence intervals [CI] 1.7 – 10.0) compared
with the general population [46]. Death can be the result of
bleeding due to severe thrombocytopenia or perhaps more
frequently from the complications of immune-suppressive
Expert Opin. Investig. Drugs (2009) 18(6)
Arnold, Nazi & Kelton
Receptor-binding domain
TPO
C
C
C
C
NH2
COOH
20% Identical
25% Similar
EPO
Receptor-binding domain
C
CC
C
NH2
COOH
Figure 1. Protein sequence of human thrombopoietin (TPO), the principal platelet regulatory hormone, and human
erythropoietin (EPO), the erythrocyte regulatory hormone. The amino terminus of TPO encoding the active site of the protein has
20% identity (black shading) and 25% similarity with EPO.
C: Disulfide-bonded cysteine residues; COOH: Carboxy terminus; NH2: Amino terminus.
Thrombopoietin
Platelets
Megakaryocyte
Steady state
Thrombocytopenia
Thrombocytosis
Figure 2. Constitutive secretion of thrombopoietin (TPO). Levels of TPO are secreted from the liver at a constant level at all times.
The number of circulating platelets regulates the amount of free TPO available for binding to megakaryocytes and hematopoietic stem
cells to increase platelet production. When the platelet count is low (thrombocytopenia), a higher level of free TPO is available for
megakaryocyte binding, leading to increased platelet production. When the platelet count is high (thrombocytosis), a lower level of free
TPO is available for binding.
Expert Opin. Investig. Drugs (2009) 18(6)
807
New treatments for idiopathic thrombocytopenic purpura: rethinking old hypotheses
with Epstein–Barr virus, HIV, hepatitis C and Helicobacter
pylori infection may offer insight. For example, anti-platelet
antibodies from HIV-positive patients have demonstrated
cross-reactivity with HIV GP-160/120 [56], and anti-platelet
antibodies from H. pylori-positive patients can cross-react
with H. pylori CagA protein [57]. Recent data suggest that
the platelet count increase following H. pylori eradication is
mediated by an increase in inhibitory FcγRIIB receptors on
macrophages and a decrease in their phagocytic potential [58].
Thrombopoietin
3.1.2
P
P
SHC
GRB2
JAK2
P
SOS
P
STAT
Ras
3.2
P42/44
MAPK
Raf1
Signal transduction
Increased platelet production
Figure 3. Signal transduction of the thrombopoietin
receptor in megakaryocytes and hematopoietic stem cells.
Homodimerization of the receptor results in downstream signaling
via JAK2, STATs, PI3K, and MAPKs, causing cellular proliferation.
treatments [46-48]. Overall quality of life of patients with ITP
has been shown to be worse than the general population; for
some, it can be worse than patients with chronic diseases
such as hypertension, arthritis or certain cancers [49].
3.1
Pathogenesis of ITP
The role of increased platelet destruction
3.1.1
Antibodies, typically IgG, directed to platelet glycoprotein
IIbIIIa, IbIX, IaIIa, IV and V (alone or in combination), are
detectable in 60 – 70% of patients with ITP [50]. These
antibodies cause accelerated Fcγ-mediated platelet destruction
in the spleen and other reticuloendothelial organs [51].
Pathogenic, autoreactive B cells responsible for autoantibody
production result from the loss of T-cell tolerance [52]. In ITP,
other T-cell defects have also been described including apoptotic
resistance [53], impairment in T-regulatory cell function [54] and
direct T-cell-mediated platelet lysis [55]. The stimulus for B
cells to become ‘autoreactive’ in patients with ITP is unclear;
however, platelet autoantibody formation in patients infected
808
Role of impaired platelet production
Early studies using autologous, radiolabeled platelets have
shown that platelet turnover is not invariably increased
in patients with ITP, as would be expected [59-61].
Furthermore, levels of TPO measured in patients with ITP
are variable and can be the same, lower or higher than a control
population [62]; however, exceptionally high levels, as seen in
patients with aplastic anemia, are uncommon. Perhaps the
most dramatic evidence of relative platelet underproduction in
ITP is provided by the demonstration of the effectiveness
of TPO agonists [119,64].
Treatment of ITP
Treatments for patients with ITP are aimed at decreasing
autoantibody production or interfering with platelet destruction
(Figure 4). Conventional treatments include corticosteroids,
intravenous immune globulin (IVIg), rhesus immune globulin
(RhIg; for rhesus blood group-positive individuals), immunesuppressant medications such as ciclosporin and azathioprine,
cytotoxic agents and splenectomy. The anti-CD20 monoclonal
antibody, rituximab, has recently been shown to ameliorate
thrombocytopenia in some patients with ITP [65] by interfering
with platelet autoantibody production and possibly through
indirect effects on cellular immunity [66]. TPO mimetics
increase platelet production and will raise the platelet count in
many patients with ITP. A comparison of key features including cost of rituximab and TPO mimetics is shown in Table 1.
3.2.1
Standard treatment for ITP
There is a general agreement among published guidelines [67,68]
that corticosteroids should be used as initial treatment.
Prednisone (1 – 2mg/kg/day) in tapering doses for 4 – 6 weeks
or, less commonly, high-dose dexamethasone (40 mg/day
for 4 days per month) for several cycles [69], are often used.
The addition of IVIG or RhIg is reserved for treatment of
severe thrombocytopenia associated with mucosal or more
severe bleeding.
Corticosteroids are usually tapered following an initial
rise in platelet count to normal levels. The majority of adult
ITP patients will relapse after corticosteroids, which prompts
additional treatment; however a small proportion (10 – 20%) will
have a durable remission. The overall goal of treatment is to
achieve platelet counts that are sufficient for hemostasis, not
necessarily normal. In our clinic, patients with a platelet
count of > 20 – 30 x 109/l and without evidence of bleeding
Expert Opin. Investig. Drugs (2009) 18(6)
Arnold, Nazi & Kelton
Liver
1
Bone marrow
TPO
Spleen
4
Kidneys
6
Macrophage
Platelets
T-Lymphocyte 1
3
Megakaryocyte
5
IgG
2
Lymph node
B-Lymphocyte
Figure 4. Pathophysiology of ITP and targets of therapies. 1. T cells, with surface CD 154, lose tolerance to platelet antigens
(GP-IIbIIIa and GP-IbIX), which are then presented to B cells in the lymph node and spleen. Ciclosporin inhibits T cells, and azathioprine
and mycophenolate mofetil inhibit lymphocyte proliferation; cytotoxic agents interfere at this stage; and anti-CD154 monoclonal
antibody disrupts the CD154 – CD40 interaction. 2. CD20-positive B cells are stimulated to differentiate and produce platelet-reactive
antibodies. Rituximab, an anti-CD20 monoclonal antibody, interferes at this stage; and corticosteroids decrease autoantibody synthesis.
3. Platelet autoantibodies bind to platelets through the Fab terminus. 4. IgG-sensitized platelets undergo FcR-mediated phagocytosis
by reticuloendothelial (RE) cells. Splenectomy removes the primary site of platelet clearance; IVIg and anti-D block RE cells and prevent
platelet clearance; corticosteroids decrease phagocytosis; and vinca alkaloids are toxic to macrophages. 5. Platelet-reactive autoantibodies
target and destroy megakaryocytes, potentially resulting in decreased thrombopoiesis. 6. Levels of thrombopoietin (TPO), constitutively
secreted from the liver, but also from kidneys and bone marrow, are low for the degree of thrombocytopenia in ITP, and platelet
production is impaired. The administration of TPO agonists can overcome platelet underproduction.
Reproduced with permission from [76].
are usually not treated unless a hemostatic challenge such as
surgery is anticipated. For those patients who relapse and
have a platelet count of < 20 x 109/l (about 50% of patients),
most physicians would consider splenectomy [70].
Of all treatments, splenectomy is associated with the
highest rate of durable platelet count responses with longterm follow-up. In the largest systematic review of the efficacy
and safety of splenectomy for ITP, Kojouri and colleagues
reported that 66% of 2623 adults achieved a normal platelet
count and responses were durable for a median of 7.3 years [71].
Relapses occurred in 15% of patient (range 0 – 51%) after
a median follow-up of 33 months. Laparoscopic splenectomy
is an increasingly popular surgical approach for the management of ITP [72]. Platelet count responses and the frequency
of missed accessory spleens causing recurrent disease are
similar to open splenectomy, but complications are fewer
and length of hospital stay is shorter [73]. Overall mortality
is approximately 1% after laparotomy and 0.2% after laparoscopic splenectomy [71]. The most frequent perioperative
complications are pleuropulmonary (pneumonia, subphrenic
abscess, pleural effusion) occurring in 4% of patients, major
bleeding in 1.5%, and thromboembolism in 1%. The major
long-term risk is overwhelming sepsis, which occurs with a
frequency of 3.2% overall, with an associated mortality rate of
1.4%, considering all indications for splenectomy [74]. Streptococcus pneumoniae, Neisseria meningitidis and Haemophilus
influenzae type B vaccines are recommended at least 2 weeks
prior to splenectomy [75].
Patients with chronic refractory ITP post splenectomy
have the highest risk of morbidity and mortality including
Expert Opin. Investig. Drugs (2009) 18(6)
809
New treatments for idiopathic thrombocytopenic purpura: rethinking old hypotheses
Table 1. Comparison of new agents used to treat ITP: monoclonal anti-CD20 antibody and thrombopoietin (TPO)
mimetics.
Rituximab
Romiplostim; Eltrombopag
Drug class
Anti-CD2o monocsonal
TPO mimetics
Licensed for ITP
No
Yes
Schedule of administration
Single course of therapy (once
weekly for 4 consecutive weeks)
Maintenance treatment daily
(eltrombopag) or weekly (romiplostim)
Platelet count response (> 50 x 109/l)
62.5% [65]
60 – 85%
Median time to response
5.5 weeks [65]
2 – 3 weeks [64]
Duration of response
Up to 2 years [91] (median, 10.5 months)
Maintained as long as the drug is given
Sustained response after
stopping the drug
Yes
No
Tachyphylaxis
Lower efficacy on re-treatment
has been reported
Not observed
Cost
$16,000 for a 4-week course*
$4125 per month for eltrombopag‡;
$5100 per month for romiplostim§
*USD, for the average 1.73 m2 person (650 mg per dose).
‡USD, 50 mg once/day, according to AWP listings in Price Alert, 15 December 2008.
§USD, 3 µg/kg/week, according to AWP listings in Red Book Update, December 2008.
serious hemorrhage, infection and death [46]. Treatment of
refractory patients with conventional therapies is challenging
and often unsatisfactory because of the lack of effect and/or
the frequency of toxicities [76]. To address the need for safer and
more effective treatments, two new classes of drugs have recently
been introduced: the CD20 monoclonal antibody rituximab,
and the TPO mimetics romiplostim and eltrombopag, which
target platelet destruction and platelet production, respectively.
3.2.2
B-cell-targeted therapy for ITP
Rituximab, a chimeric anti-CD20 monoclonal antibody
approved for use in lymphoma and rheumatoid arthritis [77,78]
has been used in patients with ITP and other autoimmune
diseases [79,80]. Rituximab causes the rapid depletion of CD20+
B cells by Fcγ-mediated cytotoxicity [81], antibody-dependent
cell-mediated cytotoxicity [82] and apoptosis [83]. The typical
dose is 375mg/m2 weekly for 4 weeks (borrowed directly
from the lymphoma indication), although lower doses may
be sufficient for ITP [84]. B-cell depletion lasts for up to
6 months [85,86] and then the B-cell pool is repopulated by
immature (CD38++, CD10+, CD24+) B cells, followed by
naïve (CD 27-) B cells [87,88]. Memory B cells (CD27+) may
remain reduced for up to 2 years [88]. CD20 is expressed on
B cells during maturation from the late pre-B-cell phase
until plasma cell maturation, but is not expressed on stem
cells or mature plasma cells [89]; thus leukopenia and hypogammaglobulinemia are uncommon. In a study of 167 patients
with lymphoma treated with rituximab maintenance,
10.8% developed neutropenia compared with 5.4% of patients
on observation alone (p = 0.07) and median IgG levels were
lower in patients on rituximab (6.3 vs 7.3 g/l) [90]; however,
in only three patients did IgG levels fall below 3 g/l.
810
In ITP, B-cell depletion has been associated with a reduction
in platelet autoantibody levels, and a rise in the platelet
count [85]. In addition, by removing the pathogenic B-cell
pool, rituximab may indirectly cause the removal of autoreactive T cells or the normalization of other cellular immune
defects in patients with ITP, as responses have been associated with an increase in the TH1/TH2 ratio, expression of
Fas ligand and Bcl-2 mRNA, and a decrease in the expression of Bax mRNA in T-helper cells [66].
Rituximab was found to be moderately effective in a
review of publications examining its use in ITP [65]. Of
313 patients, half of whom had splenectomy, 62.5%
(95% CI: 52.6 – 72.5%) achieved a platelet count response
(platelets > 50 x 109/l). Median time to response was 5.5 weeks
(range 2 – 18) and median duration of response was 10.5 months
(range 3 – 20). More recently, rituximab has been investigated
in the early stages of ITP as a means of averting splenectomy.
In a single-arm study from France, 60 non-splenectomized
patients with ITP for ≥ 6 months who had failed one or more
previous treatment were followed for up to 2 years after the
administration of rituximab 375mg/m2 once per week for
4 weeks [91]. Baseline platelet counts were < 30 x 109/l. A
good response, defined as a platelet count of ≥ 50 x 109/l with
at least a doubling from baseline, was obtained in 40% of
patients (24/60 [95% CI: 28 – 52%]) at 1 year and in
33.3% (20/60 patients) at 2 years. Sixteen patients experienced
transient side effects, but only one discontinued treatment
because of serum sickness. There were eight severe adverse
events judged not to be related to rituximab, including fatal
myocardial infarction (n = 1), atrial fibrillation (n = 3),
malignancy (n = 2), Guillain-Barré syndrome (n = 1) and
renal colic (n = 1).
Expert Opin. Investig. Drugs (2009) 18(6)
Arnold, Nazi & Kelton
In a randomized trial comparing dexamethasone and dexamethasone plus rituximab in patients with previously
untreated ITP (n = 101), a platelet count response (> 50 x 109/l)
at 6 months was achieved by 36% of patients on dexamethasone alone, and 63% of patients on dex-R (p = 0.004)
yet, a considerable number of patients crossed over to
rituximab [92]. Responses were maintained for a median of
18 months in most responding patients.
Although antibody-producing plasma cells are unaffected
by rituximab, and in most reports infection has not been a
concern [93], the rate of infection may be increased in certain
patient populations. In the systematic review of rituximab in
ITP by Arnold and colleagues, of 306 treated patients,
7(2.3%) developed serious infections, four of which were fatal;
however, a causal association could not be confirmed [65].
In a randomized trial of patients with rheumatoid arthritis
(n = 520), 5.2 serious infections per 100 patient-years
occurred in the rituximab group compared with 3.7 in
controls [94]; and in another trial (n = 161), 5% of treated
patients developed serious infection compared with 2.5% of
controls [78] (test of significance not given). Even in lymphoma,
a recent trial of rituximab maintenance (n = 167) administered
every 3 months for up to 2 years uncovered a higher frequency
of serious infections in treated patients compared with those
on observation (9 vs 2.4%; p = 0.009) [90]. Similarly, in
patients with HIV lymphoma, rituximab may be associated
with an increased risk of bacterial and opportunistic infections [95].
Randomized trials do not provide sufficient power to detect
subtle differences in infectious complications; thus, vigilance
and long term follow-up are required.
One rare life-threatening infection that has been potentially
linked to rituximab is progressive multifocal leukoencephalopathy (PML). PML is a rare demyelinating disease caused
by reactivation of latent polyomavirus JC (JC virus) in the
brain. The syndrome is characterized by rapidly progressive
neurological symptoms including weakness or paralysis,
vision loss, impaired speech and cognitive deterioration.
Immunosuppression and underlying lymphoma are associated
with PML; however, recent data suggest that rituximab may
be an independent risk factor [96]. JC virus is present in a
dormant state in > 80% of adults and disseminates only
when normal cellular immune surveillance is compromised [97]; in particular, JC virus-specific CD8+ cytotoxic T
cells are critical for its containment [98]. The depletion of
CD20+ B cells may remove a unique and efficient population of antigen-presenting cells to cause such a disruption in
cellular immunity [99]. Because of this possible association,
the FDA has issued a black box warning on the label stating:
‘JC virus infection resulting in PML and death has been
reported in patients treated with Rituxan’ [100].
3.2.3
Stimulation of platelet production in ITP
Recombinant thrombopoietin
3.2.3.1
Two recombinant forms of TPO were developed primarily
as a treatment for chemotherapy-induced thrombocytopenia;
recombinant human TPO (rhTPO), which was nearly identical
to endogenous TPO and pegylated recombinant human megakaryocyte growth and development factor (PEG-rHuMGDF)
consisting of the receptor-binding amino terminus conjugated to
a polyethylene glycol moiety [101,102].
Most clinical studies with recombinant TPO were designed
for patients with solid tumors or hematological malignancy [32].
Following myeloablative chemotherapy, recombinant TPO had
no effect on the duration of severe thrombocytopenia or platelet
transfusion requirement [103]; however, with non-myeloablative
treatments, patients receiving recombinant TPO had less
severe thrombocytopenia and, in some studies, required fewer
platelet transfusions [104].
Recombinant TPO was also tested in patients with ITP.
One study reported a gradual increase in the median platelet
count from 15 x 109/l to 77 x 109/l immediately after treatment in 85 patients with chronic ITP [105]. Other groups
reported platelet count improvements in three of four
patients with refractory ITP [106] and in all six patients with
HIV-associated immune thrombocytopenia [107] treated with
PEG-rHuMGDF. Thus, recombinant TPO appeared to be
effective at raising the platelet count in patients with ITP,
supporting the hypothesis that the relative defect in platelet
production could be overcome.
Clinical studies of recombinant thrombopoietin were
halted in 1998 after some patients receiving the pegylated
formulation developed antibodies that cross-reacted with
endogenous TPO, resulting in severe and protracted thrombocytopenia. In a large safety study, 13 of 535 healthy subjects given a single monthly dose of PEG-rHuMGDF for
≤ 3 months developed an anti-TPO antibody and thrombocytopenia, as reported in a recent review article [108]. Two
patients developed pancytopenia [101,109]. Among approximately 650 cancer patients treated with PEG-rHuMGDF,
at least four also developed thrombocytopenia, believed
to be due to autoantibody formation [108]. Three of the
affected individuals were reported in detail [101]: a 49-year-old
female volunteer developed severe thrombocytopenia (nadir
platelet count 6 x 109/l) and bleeding refractory to prednisone, IVIG and platelet transfusions that lasted for approximately 1 year; she subsequently made a full recovery. A
31-year-old female volunteer developed severe thrombocytopenia (nadir platelet count 11 x 109/l) refractory to IVIG
and ciclosporin for nearly 2 years, then recovered. A 61-yearold female with lung caner developed severe refractory
thrombocytopenia (nadir platelet count 2 x 109/l) and died
6 months later of metastatic disease. All three patients had
decreased megakaryocytes on bone marrow examination.
3.2.3.2
Thrombopoietin mimetics
To overcome the problem of cross-reactive antibodies,
investigators sought to identify molecules that would activate the
c-Mpl receptor, yet were structurally dissimilar to TPO. Two
such agents, romiplostim (Nplate™, previously known as
AMG-531; Amgen) and eltrombopag (Promacta™, previously
Expert Opin. Investig. Drugs (2009) 18(6)
811
New treatments for idiopathic thrombocytopenic purpura: rethinking old hypotheses
known as SB-497115; GlaxoSmithKlein) have now been
approved for use in ITP in the United States and other
countries. Both agents are effective in approximately 70%
of patients with ITP, and the platelet count responses are
maintained as long as the drug is administered. Table 2 is a
comparison of romiplostim and eltrombopag.
3.2.3.2.1
Romiplostim (AMG-531, Nplate)
Romiplostim is a 60 kDa TPO ‘peptibody’ composed of
two identical polyglycine linked peptide sequences, which
are combined with two disulphide-bonded human IgG1
kappa heavy chain constant regions (Fc fragment). The
peptide was selected from a screening library of molecules
with a tertiary structure capable of binding c-Mpl, but
with no amino acid sequence homology with TPO. The
carrier Fc portion prolongs the circulatory half-life of the
molecule by allowing it to bind to the neonatal Fc salvage
receptor and undergo endothelial recirculation [110]. The
protein structure of the molecule requires parenteral
administration. Romiplostim competes with human TPO
for the same binding site on c-Mpl (Figure 5) and induces
signal transduction via rapid tyrosine phosphorylation
of Mpl, JAK2, and STAT5. In vitro studies indicate
that romiplostim is capable of inducing growth of
megakaryocyte colonies [110].
Initial clinical studies with romiplostim in healthy volunteers
demonstrated a dose-dependent increase in the platelet count
starting on day 4 – 9 following subcutaneous administration
and peaking by day 12 – 16. Those who received the highest
dose (10 µg/kg i.v.) achieved a mean peak platelet count
of 1380 x 109/l, nearly fourfold higher than the upper limit
of normal [111]. Neutralizing anti-TPO antibodies were
not observed.
A Phase I study in patients with ITP for ≥ 3 months who
had failed ≥ 1 prior treatment and had a baseline platelet
count of < 30 x 109/l (without corticosteroids) or < 50 x 109/l
(with corticosteroids) demonstrated a dose-dependent platelet
count response. One of 12 patients on low dose romiplostim
(1 µ/kg or lower) achieved a platelet count of > 50 x 109/l
compared with 7 of 12 who received higher doses (3 – 10
µ/kg) [112]. Another dose-finding study enrolled 16 patients
with ITP and demonstrated that 8 of 11 patients receiving
one or two weekly doses of romiplostim (≥ 1 µg/kg) achieved
the target platelet count of 50 – 450 x 109/l with doubling
from baseline [113]. In a Phase II randomized, placebocontrolled trial, the target platelet count was reached in
12 of 16 patients receiving weekly doses of romiplostim at
1 and 3 µg/kg [112].
Two pivotal Phase III trials (published in the same report),
one in splenectomized (n = 63) and the other in
non-splenectomized (n = 62) patients, showed a significant
increase in the proportion of patients achieving a durable
platelet count response (platelets > 50 x 109/l for 6 of the
last 8 weeks of treatment) by 6 months compared with
placebo: 16/42 (38%) versus 0/21 (0%) in the splenectomized
812
group (difference in proportion, 38%; 95% CI, 23.4 – 52.8;
p = 0.0013) and 25/41 (61%) versus 1/21 (5%) in the nonsplenectomized group (56% difference; 95% CI, 38.7 – 73.7,
p < 0.0001) [64]. Thrombosis occurred in one patient on
placebo and in two patients on romiplostim. The one patient
on placebo died of pulmonary embolism. One patient on
romiplostim developed a popliteal artery thrombosis, and
the other had a cerebrovascular accident at week 21; both
patients had previous histories of vascular disease and platelet
counts that were below the normal range. One patient on
romiplostim developed increased bone marrow reticulin that
returned to baseline upon discontinuation of the drug.
An interim report of a follow-up, open-label extension
study comprising 142 patients treated with romiplostim for
a mean of 69 weeks (up to 156 weeks) showed that 87%
(124/142) of patients treated achieved a platelet count
response (> 50 x 109/l and double baseline value) at one or
more points during the study, and that 84% (27/32) were
able to discontinue or reduce the dose of concurrent
ITP medications [114]. Bleeding events decreased from
42% (60/142) during the first 24 weeks of treatment to
20% (13/65) by weeks 48 – 72. Of the 16 patients with
bone marrow samples available, eight had increased reticulin.
Clonal abnormalities by immunophenotyping (performed
on three patients) or cytogenetic analysis (five patients) were
not found. Follow-up bone marrow examinations done in
two patients showed improvement of reticulin in one and
no change in the other. Twelve thrombotic events were
reported in seven (4.9%) patients; eight events occurred
with platelet counts < 400 x 109/l. During the study, three
patients died: one each of cardiac arrest, post-splenectomy
sepsis and hepatocellular carcinoma. Transient antibodies
against romiplostim, but not against TPO, were detected in
one patient.
3.2.3.2.2
Eltrombopag (SB-497115; Promacta, Revolade®)
Eltrombopag is a nonpeptide, small-molecule TPO agonist
with a molecular weight of 564 kDa that is orally bioavailable.
It is a hydrazone molecule containing an acid group, lipophilic
groups and a metal chelate group [115,116]. It binds c-Mpl at
the transmembrane domain of the heterodimer receptor, a
binding site distinct from TPO (Figure 5) [117], and induces
signaling for megakaryocyte proliferation and differentiation
in a manner similar to TPO [115].
Initial studies in humans demonstrated that eltrombopag
produced a dose-dependent increase in platelet count to 20%
above normal baseline values at daily doses of 30 – 75 mg [118].
A Phase II study enrolled 118 patients with ITP who had a
platelet count < 30 x 109/l and who failed at least one standard
treatment. The proportion of patients who achieved a platelet
count > 50 x 109/l after 6 weeks of oral daily doses
of eltrombopag 30, 50 or 75 mg was 28, 70 and 81%, respectively, compared with 11% on placebo [63]. Bleeding symptoms
also improved. Serious adverse events included one patient
with cataract progression and one death due to cardiorespira-tory
Expert Opin. Investig. Drugs (2009) 18(6)
Arnold, Nazi & Kelton
Table 2. Comparison of romiplostim and eltrombopag, 2 thrombopoietin (TPO) mimetics recently licensed by the
FDA for the treatment of patients with idiopathic thrombocytopenic purpura (ITP).
Romiplostim
Eltrombopag
Binding site on TPO receptor
Membrane-distal cytokine receptor homology
module (CRM1) of TPO receptor
Transmembrane domain of
TPO receptor
Competition for binding
by endogenous TPO
Yes
No
Dosing frequency
Weekly
Daily
Route of administration
Subcutaneous injection
Oral pill
Safety concerns
Transient bone marrow reticulin; thrombosis
Increase in liver enzymes
failure; however, the relationship to the medication was
uncertain.
In a Phase III trial, 110 patients with ITP and a platelet
count < 30 x 109/l were randomized to eltrombopag 50 mg/day
(with escalation to 75 mg if response was not achieved after
3 weeks) or placebo [63]. After 6 weeks, 43 (59%) of patients
on eltrombopag and 6 (16%) on placebo had a platelet
counts ≥ 50 x 109/l (odds ratio [OR] 9.61; 95% CI, 3.31
– 27.86; p < 0.0001). Median platelet count increased to 53
x 109/l by day 15 for patients on eltrombopag, which was
sustained for the 6-week treatment period, whereas median
platelet count for patients on placebo did not increase significantly from baseline. Fewer patients in the eltrombopag
group than in the placebo group had bleeding symptoms, as
measured by the WHO bleeding scale, at day 43 (20 [39%]
vs 18 [60%]; OR 0.27 [95% CI, 0.09 – 0.88]; p = 0.029) or
at any point in time during the course of treatment
(46 [61%] vs 30 [79%]; OR 0.49 [95% CI, 0.26 – 0.89];
p = 0.021). There was no difference in health-related quality
of life. Increases in liver enzyme tests were noted in six
patients on eltrombopag, and new or progression of existing
cataracts were reported in three patients on eltrombopag
(two were progression) and one on placebo (progression).
All patients with cataracts had been previously treated with
corticosteroids.
In another Phase III trial, 197 patients with ITP and a
platelet count < 30 x 109/l were randomized to eltrombopag
(50 – 75 mg per day) or placebo [119]. Patients on eltrombopag were eight times more likely to achieve platelet counts
of 50 – 400 x 109/l during the 6-month treatment period
compared with placebo (OR [95% CI] = 8.2 [4.32 – 15.38];
p < 0.001). Treated patients also had fewer bleeding episodes,
were more likely to reduce their regular ITP medications,
and were less likely to need rescue treatments. A higher incidence
of hepatobiliary laboratory abnormalities were reported in
patients on eltrombopag (13 vs 7%).
In a report of long-term follow-up of 207 patients treated
with eltrombopag (range of treatment duration, 3 – 523 days),
79% of patients achieved a platelet count of ≥ 50 x 109/l or
higher. Median platelet counts remained at ≥ 50 x 109/l
throughout the observation period of the study for nearly all
responding patients. Bleeding symptoms improved. Adverse
events were mostly mild or moderate and included headache
(15%), upper respiratory tract infection (13%), diarrhea (10%),
and nasopharyngitis (9%). Six thromboembolic events were
reported during the study. No clinically relevant changes on
patient bone marrow examinations were detected [120].
3.2.4
New drugs under investigation
Other TPO mimetics
3.2.4.1
ARK-501 (formerly YM-477, AkaRx and MGI Pharma) is
another TPO agonist that has no sequence homology with
endogenous TPO discovered through screening peptide
libraries. AKR-501 binds to the c-Mpl receptor at a site
similar to eltrombopag, at His499 in the transmembrane
domain. It is formulated as a once-daily oral pill. A Phase I
study in healthy volunteers showed a dose-dependent
increase in platelet count in healthy volunteers and demonstrated that the drug was well tolerated [54,121]. Single doses
resulted in a platelet count rise that may have been more
substantial than the other oral TPO agonist eltrombopag.
Totrombopag (SB-559448; GSK and Ligand) and LGD-4665
(Ligand) are other TPO mimetics that have completed
Phase I trials in healthy subjects [122].
3.2.4.2
FcR-blocking monoclonal antibodies
Fc receptors in the spleen and other reticuloendothelial
organs are the site of opsonization of autoantibody-coated
platelets in ITP. Several monoclonal antibodies against the
Fc receptor are currently in clinical trials; MDX-33
(Medarex), a humanized anti-FcγRI monoclonal, and GMA-161
(MacroGenics and Genzyme), a humanized anti-FcγRIII
monoclonal, have shown favorable safety and efficacy in
patients with ITP [123]. Other molecules under investigation
target FcR signaling by other mechanisms, including inhibition
of Syk [124] and soluble FcγRIIb.
4.
Conclusion
The clinical presentation of ITP is heterogeneous; however,
patients with persistent, severe thrombocytopenia have the
greatest risk of bleeding. The mechanism of thrombocytopenia
Expert Opin. Investig. Drugs (2009) 18(6)
813
New treatments for idiopathic thrombocytopenic purpura: rethinking old hypotheses
A.
B.
Thrombopoietin
Romiplostim
Eltrombopag
Figure 5. Binding of romiplostim and eltrombopag to c-Mpl. A. The ligand for the thrombopoietin receptor (c-Mpl) binds at the
membrane distal cytokine receptor homology module (CRM1); B. Romiplostim binds to the thrombopoietin receptor at the same site as
TPO, and eltrombopag binds at the transmembrane domain, distinct from the TPO binding site.
in ITP is multifactorial, involving platelet autoantibodies,
autoreactive B cells and the loss of normal T-cell regulation,
as well as a relative impairment in platelet production for
the degree of thrombocytopenia. Most therapies for ITP
target platelet autoantibody production and interfere with
platelet destruction, including the anti-CD20 monoclonal
antibody, rituximab. Although randomized clinical trials are
lacking and ITP is not a licensed indication, rituximab is
frequently used to treat patients with chronic ITP because
of its potential efficacy and favorable safety profile. The use
of rituximab as a splenectomy-sparing agent is appealing,
but requires further research to establish safety. Subtle alterations
in cellular immunity may predispose patients to opportunistic
infections, including PML; however, this association has not
been confirmed.
The TPO mimetics target platelet production. Two of
these agents, romiplostim and eltrombopag, have now
been licensed for ITP in the United States. Clinical data
demonstrate that approximately 70% of patients treated with
these agents are able to achieve a platelet count response,
including some patients with chronic, severe ITP. The
efficacy of these agents appears to be more pronounced in
non-splenectomized patients.
5.
Expert opinion
In countries where funding is not restrictive, patients with
ITP are frequently treated with rituximab, even though data
from randomized trials are lacking. The rationale for the use
of rituximab in the early stages of ITP is to interrupt the
814
self-sustainability of the disorder by transient B-cell
depletion, and reversion of T-cell defects back to normal.
Results from one randomized trial in this population is
promising [92], with others currently underway. The potential
for long-term risk of infection following rituximab is
concerning, however, especially as patients are typically
young and otherwise healthy. Further research into the
effects of rituximab on cellular and humoral immunity are
needed.
The effectiveness of the TPO mimetics has been confirmed
in Phase III clinical trials and the safety profile seems acceptable
even after longer-term (2-year) use. This class of agents represents
the most significant advance for the care of chronic refractory
patients with ITP since the discovery of IVIG in the early
1980s, and has brokered a shift in our understanding of
pathophysiology. In addition, it is rekindling interest in
platelet regulation by TPO, a unique yet primitive feedback
system. Although efficacy data in patients who have not had
a splenectomy is equally (if not more) compelling, the
acceptability and safety of chronic maintenance therapy with
megakaryocyte growth factors in this population remains
unsettled. Bone marrow reticulin formation with romiplostim
is of particular concern given that, theoretically, malignant
transformation is plausible; however, no evidence of clonality
has ever been documented. The possible increase in thrombotic
complications following TPO mimetics deserves careful
attention, as does the baseline risk of thrombosis in ITP
patients overall, which may be higher than previously
thought. Longer-term safety data will certainly aid in treatment
decisions for patients with early stage ITP.
Expert Opin. Investig. Drugs (2009) 18(6)
Arnold, Nazi & Kelton
TPO mimetics that activate the c-Mpl receptor at distinct
sites can act synergistically to increase platelet production
and may potentially be used in combination for the most
refractory patients. The side-effect profiles of romiplostim
and eltrombopag appear to be non-overlapping. Costeffectiveness analyses for rituximab and for the TPO agents
for both splenectomized and non-splenectomized patients
with ITP, like the one by Oatis and colleagues [125], are
needed. Finally, future trials of new agents in ITP should
focus not only on platelet count outcomes, but on bleeding
and quality of life.
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We thank Aurelio Santos for his help with the design of
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Declaration of interest
Dr Arnold received research funding from Roche for a clinical
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Affiliation
Donald M Arnold†, Ishac Nazi & John G Kelton
†Author for correspondence
McMaster University,
Michael G DeGroote School of Medicine,
Medicine and Pathology and Molecular Medicine,
1200 Main Street W, Hamilton, Ontario,
L8N3Z5, Canada
Tel: +905 521 2100; Fax: +905 521 4971;
E-mail: arnold@mcmaster.ca
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