KM Murali, MD, DM
Wollongong Hospital and University of
Wollongong
Address for Correspondence: Dr.
KM Murali, MD, DM, Senior Staff Specialist and Clinical
Associate Professor, Wollongong Hospital and University of
Wollongong. Email: km_murali@hotmail.com
Keywords: anemia, chronic kidney disease
Introduction
The link between chronic kidney disease (CKD) and anemia has
been known for over 180 years ever since it was highlighted by
the legendary Richard Bright [1]. The key etiological factors
responsible for anemia due to renal disease were identified in
the 1950s as relative deficiency of erythropoietin and reduced
red cell survival as well as abnormalities in iron metabolism
[2]. This knowledge was not translated to therapeutics for the
next three decades and red cell transfusions were the
main-stay of treatment of renal anemia till recombinant human
erythropoietin (rHuEPO) was introduced in the 1980s [3].
Recurrent transfusions, in addition to causing HLA
sensitization, frequently resulted in iron overload with
subsequent parenchymal iron deposition and organ dysfunction -
a well-known scenario in the pre-erythropoetin era and use of
iron chelation therapy was a feasible therapeutic option in
long-term dialysis patients [2]. This situation changed
dramatically with the widespread clinical use of
erythropoiesis stimulating agents (ESA), and polar opposite to
the iron overload problems, most patient on erythropoietin
therapy required oral or parenteral iron supplements to
replenish iron stores for optimal erythropoiesis [4]. Over the
last three decades developments in molecular engineering lead
to the modification of the original rHuEPO to longer acting
analogues such as darbepoetin (Aranesp ®) and methoxy
polyethylene glycol-epoetin beta (Mircera ®) and a raft of EPO
biosimilars [5]. In India, where the cost of medications
constrains the universal access to ESA therapy, biosimilar
erythropoetins offer a relative advantage in that they cost
less than half the price of the premium brands [6].
Nevertheless, the progress in clinically prescribed ESA
therapy since the discovery of rHuEPO has resulted from modest
modification of existing knowledge rather than a revolutionary
break from convention. Over the last few years newer molecules
capable of revolutionizing the therapy of renal anemia have
been or are being developed and tested in clinical trials.
Some of these have the potential to become realistic treatment
options in the near future.
Can there be a better therapy for renal anemia than
erythropoietin?
The need for a better erythropoetic agent has always existed.
The available therapies - rHuEPO and its analogues are costly
and require parenteral administration, which can constrain its
use in special patient groups and locations. Despite the
consistent relationship between low hemoglobin and adverse
patient outcomes in observational studies, randomized
controlled trials have demonstrated an increased risk for
death or adverse outcomes with treatment using ESA targeting
higher hemoglobin [7]. While this paradoxical finding may be
related to the factors such as achieved hemoglobin, the impact
of hypertension or the effect of intravenous iron, it has been
suggested that the thrombotic effects of EPO may well be due
to the drug itself, especially in high doses [8]. The
intermittent administration of high dose ESA cannot be
expected to biologically mimic the low level continuous
physiological secretion of EPO [7].
Iron supplementation is an integral part of ESA therapy in the
treatment of renal anemia [4]. End stage renal failure
patients have reduced intestinal iron absorption, increased
iron loss through haemodialysis and require greater iron
turnover to facilitate ESA-driven red cell production [9].
Subjects with even normal iron stores prior to ESA therapy
require concomitant administration of iron to achieve optimal
erythropoetic response [8]. Intravenous iron is superior to
oral iron therapy when used with ESAs in achieving hemoglobin
targets [9]. In recent years there is a trend towards
increased use of intravenous iron in dialysis patients with
unprecedented increase in ferritin concentrations [10].
Intravenous iron therapy has the potential to exacerbate the
already high level of oxidative stress prevalent in dialysis
patients and has been thought to increase the risk of
infections, cardiovascular events and death [9]. In the
absence of high quality evidence, the impact of this
ubiquitous therapy on mortality and other relevant outcomes in
dialysis patients is difficult to quantify [10].
It is therefore obvious that the main-stay therapeutic options
for the treatment of renal anemia are fraught with safety
concerns, prompting the search for a more efficient and less
harmful therapy. Better understanding about the erythropoetic
mechanisms has fueled research that has led to the development
of several newer molecules to treat renal anemia, some of
which have the potential to revolutionize its therapy in the
coming years.
Advances in EPO regulation and Iron homoeostasis
It has been known since 1950s that anoxia to one or both
kidneys stimulate the secretion of erythropoietin [2]. Hypoxic
conditions alter gene expression in the cells, inducing a
myriad of proteins essential for cell survival [11]. These
changes are initiated by a heterodimeric nuclear transcription
factor called hypoxia-inducible factor (HIF), that is made up
of an oxygen regulated alpha-subunit and a constitutive
beta-subunit. When oxygen is available, prolyl hydroxylase
domain containing (PHD) enzymes hydroxylates the alpha-subunit
of HIF, facilitating its binding to Von Hippel-Lindau (VHL)
tumor suppressor protein and subsequent degradation [12], by
the E3 ubiquitin ligase complex [11]. PHD enzymes are
inhibited by a decrease in cellular oxygen and iron levels,
interfering with the hydroxylation of HIF alpha-subunits,
thereby inhibiting their VHL binding and degradation. PHD
inhibitors lead to stabilization of HIF, promoting
transcriptional activation of several genes including that for
EPO [12].
Iron homeostasis is critically linked to erythropoiesis. Since
our body has no mechanism to excrete excess iron, its main
regulation is at the level of intestinal absorption. Hepcidin,
secreted by the liver is the master regulator of iron
homeostasis in humans and other mammals [11]. It is also known
as HAMP (Hepcidin anti-microbial peptide) and was originally
identified as a liver expressed anti-microbial peptide (LEAP)
with activity against a number of bacterial and
fungal species [13]. It is partly regulated by oxygen
and iron levels in the cells - the same factors that regulate
red cell production through HIF, which may thus be the main
link between erythropoiesis and iron homeostasis [11].
Hepcidin regulates iron by altering the cell surface
expression of ferroportin, which is the only known iron
exporter protein at all sites [14]. Hepcidin binds to
ferroportin leading to its internalization and degradation
thereby inhibiting iron transport at all sites - in the
intestine and at the tissue level [13].
Once dietary iron is absorbed by the enterocyte it can be
stored in ferritin or mobilized to circulation by ferroportin.
Once in the blood stream, iron is loaded to the circulating
carrier protein transferrin, which binds to transferrin
receptor on target cells and gets internalized. Inside the
cells, the iron is unloaded from the transferrin, which is
then shuttled back to the circulation and made available for
another round of iron transport. The iron released inside the
cell, can either be utilized for synthetic purposes or stored
mainly as ferritin primarily in the liver. When the
physiologic need arises the iron stored in ferritin can be
recycled through the reticulo-endothelial system, again
transported by ferroportin [14]. By downregulating ferroportin
at all locations, hepcidin inhibits iron mobilization at the
sites of iron absorption (duodenal enterocytes), storage
(mainly hepatocytes) and recycling (macrophages of
reticuloendothelial system) [13]. The net result of its action
is reduced iron availability or utilization.
Patients with chronic kidney disease have high levels of
hepcidin thought to be related to diminished renal clearance
and chronic inflammation [15]. Excess hepcidin may be an
important contributor to the anemia and functional iron
deficiency seen in patients with chronic kidney disease.
Iron status and chronic inflammation are the two major stimuli
that increase hepcidin production in the liver [13]. Iron
mediated hepcidin regulation, works mainly through the
signaling involving proteins belonging to the transforming
growth factor beta superfamily - BMP/SMAD (bone morphogenetic
protein/ sma and mothers against decapentaplegic) pathway,
where haemojuvelin (HJV) plays a key role [16]. Increased
expression of inflammatory cytokines is common in patients
with kidney disease and is thought to be an important
contributor to the renal anemia [17]. Inflammation stimulates
the synthesis of hepcidin through the IL-6/JAK/STAT3
(Interleuki-6/Janus kinase/ Signal transducer and activator of
transcription 3) signaling [16].
The major repressors of hepcidin, thereby improving iron
availability or iron utilization, include erythropoiesis,
hypoxia and hormones like testosterone. Though EPO
therapy leads to reduction of hepcidin, it has been suggested
that the effect is not direct, because EPO does not repress
hepcidin when co-treated with inhibitors to erythropoiesis
[13]. However hepatocytes have EPO receptors and direct
inhibition of hepcidin synthesis has also been proposed
[11]. Hypoxic conditions lead to inhibition of PHD
enzyme resulting in stabilization of HIF, which can repress
hepcidin indirectly through its effect on erythropoiesis and
directly through hypoxia responsive elements in the hamp gene
[13]. Though hypoxia can inhibit hepcidin expression through
non-HIF pathways, HIF appears to be the key molecule that
links erythropoiesis and iron homeostasis[11].
Future erythropoiesis stimulating agents
Several novel modulators of erythropoiesis targeting newly
recognized pathways are in varying stages of development. Some
of them may be available for routine clinical use in the near
future. These drugs can be broadly categorized as agents
targeting hypoxia inducible transcription pathways, modulators
of hepcidin expression and agents targeting proteins belonging
to TGF-beta superfamily [18].
PHD inhibitors / HIF stabilizers
Small-molecule drugs that inhibit PHD enzymes, resulting in
stabilization of HIF and increased production of endogenous
erythropoietin have been developed [19]. There are three
subclasses of PHD enzymes, PHD 1, 2 and 3. Inhibition of PHD2
is capable of inducing near maximal EPO production in the
kidneys, while inhibition of all three PHDs was needed to
induce extra -renal, predominantly hepatic EPO production
[18]. The stimulation of endogenous erythropoietin production
by the HIF stabilizers may offer the advantage of increasing
the EPO levels above a certain threshold on a pulsatile daily
basis, mimicking natural secretion of erythropoietin which may
be safer than administering pharmacological concentrations of
erythropoeic proteins [5]. This may be particularly appealing
since higher doses erythropoietin have been consistently
associated with poor clinical outcomes [7].
Another potential advantage is their beneficial effect with
respect to iron utilization. Conventional EPO requires replete
iron stores for optimal erythropoiesis, often necessitating
the intravenous administration of iron. HIF stabilizers
by inhibiting hepcidin lead to better iron absorption and
utilization with the potential to reduce the need for
intravenous iron [18]. Another distinct advantage of these
small-molecule HIF stabilizers is that they are orally active,
which would be expected to be associated with better product
stability and lower the cost of production and distribution
[19]. Roxadustat (AstraZeneca) Vadadustat (Akebia),
Daprodustat (GlaxoSmithKline) and Molidustat (Bayer) are
currently undergoing phase 2 or phase 3 trials [20].
It is also possible that PHD inhibitors compared to EPO have a
beneficial effect on the high cardiovascular risk observed in
patients with chronic kidney disease. Such benefits
could be argued to result from the perceived favourable
effects in comparison to EPO, on cardiovascular risk factors
including blood pressure, glucose tolerance and lipid profile
as well as the proposed ability to protect organs from
ischaemic injury [19]. The results of clinical trials that are
currently underway evaluating whether PHD inhibitors/ HIF
stabilizers offer any advantage with respect to cardiovascular
endpoints at comparable haemoglobin levels will define the
role of these agents as an alternative to ESA and iron therapy
in the treatment of CKD patients with anaemia [20].
PHD Inhibitors/HIF stabilizers have the potential to cause
several adverse effects. Some of these are extension of their
physiologic effect, while others are related to the off-target
effects from the inhibition of other 2-oxoglutarate dependent
enzymes [19]. Higher levels of hemoglobin has been associated
with increased risk of thrombotic events in patients treated
with ESA [7] and the effect is unlikely to be different with
PHD inhibitors. Persistent HIF activation in patients with
genetic mutations involving HIF pathway may be linked to the
development of cancers (VHL disease associated with renal cell
cancer and pheochromocytoma) and pulmonary hypertension
(Chuvash polycythemia and HIF2a mutations are associated with
severe pulmonary hypertension) [19]. However the concern that
PHD inhibitors/HIF stabilizers may cause these adverse events
were not substantiated in phase 2 trials studying these drugs
[20].
Various 2-oxoglutarate dependent enzymes, structurally similar
to the PHD enzymes are involved in methylation of RNA, DNA and
histone as well as hydroxylation of ankyrin repeats and matrix
formation [19]. Inhibition of these enzyme could cause a
variety of off-target effects, which are not currently known.
Phase 2 trials have not demonstrated any serious pattern of
side effects and phase 3 studies of these agents are currently
underway[18]. It is quite possible that that these agents
could become the mainstay therapy of renal anaemia within the
next decade if the phase 3 trials turn out to maintain
haemoglobin without causing excess risk of cardiovascular or
other systemic adverse effects in comparison to EPO therapy.
Hepcidin modulators for treatment of renal anaemia
Drug candidates that directly inhibit hepcidin are in much
earlier phases of development in the therapeutics of renal
anaemia [18] compared to PHD inhibitors. Indirect, but
significant suppression of hepcidin is achieved by PHD
inhibitors that stabilize HIF which may offer a dual advantage
of erythropoietin stimulation and improved iron utilization
[19]. Though heparin, vitamin D and erythropoietin use also
lower hepcidin, none of these options will significantly lower
the levels in CKD patients at doses used in standard clinical
practice [18]. Drugs that sequester hepcidin
(antibodies, anticalins), inhibit BMP/SMAD/HJV pathway or
IL6/STAT3 pathway or agents that affect hepcidin transduction
(siRNA/shRNA) or stabilize ferroportin have the potential to
be effective in conditions where hepcidin excess is a cause or
contributor to anaemia [16].
Most of the drugs that repress hepcidin are being developed to
target anaemia of inflammation and are not specific to treat
anaemia of renal disease [18]. Trials to evaluate these
agents in patients with renal disease have a narrow scope and
the trial initiated by Ferrumax Pharmaceuticals, Inc.
evaluating sHJV.Fc (FMX-8), in patients with renal anaemia
interfering with haemojuvelin pathway, was terminated due to
an inability to recruit patients that meet the inclusion
criteria [21]. Nevertheless, hepcidin will remain an important
therapeutic target in the future and hepcidin modulation -
increasing or deceasing hepcidin expression, offers promise in
the treatment of a number of hematological disorders with link
to iron metabolism, like hereditary hemochromatosis,
polycythemia vera, anaemia of inflammation and iron-refractory
iron deficiency anaemia [14].
Other candidates in the pipeline
Sotatercept is a molecule that trap activins, a family of
protein belonging to TGF-beta superfamily and is being
evaluated in the therapy of anaemia due to renal disease
[22]. The drug was initially used in a trial to improve
bone mineral density in post-menopausal women and
serendipitously noted to increase haemoglobin [18]. The
drug caused only mild adverse effects and its use was not
associated with development of anti-drug antibodies
[22]. The agent promotes late stage erythroid precursors
and has a mechanism of action distinctly different from that
of EPO [18]. Clinical trials are underway using Luspatercept
(ACE 536) an agent analogous to Sotatercept, acting as a
ligand trap for members in the TGF-beta superfamily
[22].
Synthetic peptides that mimic EPO, like Peginesatide,
Pegolsihematide and EMP antibody fusion protein as well as EPO
gene therapy have also raised some expectations as potential
novel approaches to treatment of renal anaemia [18]. Of this
Peginesatide was recalled only one year after it was rolled
out for routine clinical use in the USA, due to serious side
effects [22].
Conclusions
Recent advanced in the understanding of pathways in the EPO
regulation and iron homeostasis have led to development of
novel treatment strategies, that offer great promise in the
therapeutics of anaemia of renal disease. PHD
inhibitors/HIF stabilizers offer the best hope of
revolutionizing the management of renal anaemia within the
next decade, but hepcidin inhibitors may capture a niche area
in the treatment of renal anaemia patients with certain
characteristics. In the meantime, erythropoietin and their
longer acting analogues remain the mainstay of therapy more
than three decades after they were first rolled out for
clinical use. The future however looks bright and the light at
the end of the tunnel is fast approaching.
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