From www.bloodjournal.org by guest on June 17, 2017. For personal use only.
20 NOVEMBER 2014
I VOLUME 124, NUMBER 22
l l l THROMBOSIS & HEMOSTASIS
Comment on Travers et al, page 3183
Polyphosphate: a target for
thrombosis
attenuation
----------------------------------------------------------------------------------------------------Roberto Docampo
UNIVERSITY OF GEORGIA
In this issue of Blood, Travers et al report that a number of synthetic
polyphosphate (polyP) inhibitors are able to reduce thrombosis in mice without
increasing bleeding as much as heparin.1
C
ollectively, arterial thrombosis and
venous thromboembolism are leading
causes of death in the developed world, and
a better understanding of their pathogenic
mechanisms will contribute to the development
of safe and effective drugs. A case in point is
understanding the role of anionic compounds
such as polyP in their pathogenesis.
Steps of the coagulation cascade affected by polyphosphate (polyP). PolyP (red) accelerates factor V
activation by factors Xa and thrombin, accelerates factor IX
back-activation by thrombin, abrogates the ability of TFPI to
inhibit factor Xa, and enhances fibrin polymerization. Fibrin
polymerization and contact pathway activation are preferentially stimulated by long-chain (LC) polyP, like that
present in microorganisms. Modified from Smith and
Morrissey.8 Professional illustration by Luk Cox, Somersault18:24.
PolyP is a linear polymer of from 3 to
hundreds of orthophosphates linked by
high-energy phosphoanhydride bonds and
is found in every organism that has been
investigated, from bacteria to humans.2
In bacteria and eukaryotes, this polymer
accumulates in acidic organelles rich in calcium
and other cations known as acidocalcisomes.3
Recent studies have demonstrated that
polyphosphate is abundant in platelet-dense
granules4 and in mast cell granules,5 which
are therefore considered acidocalcisomes.
In general, polyP present in blood cells is
smaller than that present in microbial cells,
with approximately 60 to 100 orthophosphate
monomers, whereas microbial polyP could
be as large as thousands of monomers. PolyP
has a variety of structural, metabolic, and
regulatory roles that have been the subject of
recent reviews.2,6 Several functions of polyP
are relevant to hematology.
The report that polyP is present in human
platelet-dense granules and is secreted upon
platelet activation4 suggested that this polymer
could have a role in blood coagulation, and
this is indeed the case.7 Initial studies revealed
that polyP accelerates blood clotting by
activating the contact pathway, promoting the
activation of factor V, and abrogating the
function of tissue factor pathway inhibitor
(TFPI). PolyP also delays clot lysis by
BLOOD, 20 NOVEMBER 2014 x VOLUME 124, NUMBER 22
enhancing the thrombin-activatable
fibrinolysis inhibitor (TAFI).7 Subsequent
studies8 revealed that polyP enhances
thrombin generation through multiple points
of the coagulation cascade. In addition to
accelerating factor V activation by factors
Xa and thrombin, it accelerates factor XI
back-activation by thrombin, abrogates the
ability of TFPI to inhibit factor Xa, and
enhances fibrin polymerization (see figure).
Contact pathway activation and fibrin
polymerization are preferentially stimulated
by long-chain polyP, similar to that present
in microbes.8
Another important discovery was that
polyP is a potent proinflammatory agent.9
Stimulation of the contact pathway, which is
dispensable for coagulation in vivo, results
in kallikrein-mediated release of bradikinin
from high-molecular-weight kininogen
resulting in proinflammatory reactions.9
The finding that polyP is also present in
mast cell granules and secreted upon
mast cell activation5 can also explain the
proinflammatory and procoagulant activities of
these cells. Long-chain polyP is also able to
suppress complement via the terminal pathway
by destabilizing C5b,6, reducing the lytic
capacity of the membrane attack complex.8
Contact pathway activation also
contributes to thrombosis.8 Previous studies
had demonstrated that cationic polymers, by
inhibiting anionic compounds such as polyP or
nucleic acids, attenuate thrombosis but their
usefulness is limited because of their toxicity.1
In contrast, several dendritic polymer-based
universal heparin reversal agents (UHRAs)
containing multiple cationic groups with
a protective layer of polyethylene glycol
(PEG) have very low toxicity and still
inhibit anionic compounds.1
By measuring the ability of these novel
UHRA compounds to inhibit polyP-thrombin
binding and polyP-initiated plasma clotting,
Travers et al were able to select 4 inhibitors
for further testing.1 The compounds did
3177
From www.bloodjournal.org by guest on June 17, 2017. For personal use only.
not activate complement, showed low
levels of platelet activation, did not cause
fibrinogen activation as did other polybasic
compounds, and had antithrombotic
activity in 2 mouse models of arterial
thrombosis causing less bleeding than
heparin.1 Surprisingly, the best polyP
inhibitor in vitro (UHRA 8) did not
perform as well as others tested in vivo,
and the authors cannot conclusively rule
out that polyP binding is the only basis
for their ability to inhibit thrombus
formation in vivo.1
Interestingly, the mode of action of these
compounds differs from that of conventional
antithrombotics. Because long-chain polyP
is more effective in triggering the contact
pathway of blood coagulation and is abundant
in microbes, a potential clinical use of these
compounds in sepsis and disseminated
intravascular coagulation is envisaged.1
Overall, this study dramatically changes
our insights regarding the role of polyP in
thrombosis and sets the stage for the pursuit
of new clinically useful compounds.
Conflict-of-interest disclosure: The author
declares no competing financial interests. n
REFERENCES
1. Travers RJ, Shenoi RA, Kalathottukaren MT,
Kizhakkedathu JN, Morrissey JH. Nontoxic
polyphosphate inhibitors reduce thrombosis while sparing
hemostasis. Blood. 2014;124(22):3183-3190.
2. Rao NN, Gómez-Garcı́a MR, Kornberg A. Inorganic
polyphosphate: essential for growth and survival. Annu Rev
Biochem. 2009;78:605-647.
3. Docampo R, de Souza W, Miranda K, Rohloff P,
Moreno SN. Acidocalcisomes - conserved from bacteria to
man. Nat Rev Microbiol. 2005;3(3):251-261.
4. Ruiz FA, Lea CR, Oldfield E, Docampo R. Human
platelet dense granules contain polyphosphate and are
similar to acidocalcisomes of bacteria and unicellular
eukaryotes. J Biol Chem. 2004;279(43):44250-44257.
5. Moreno-Sanchez D, Hernandez-Ruiz L, Ruiz FA,
Docampo R. Polyphosphate is a novel pro-inflammatory
regulator of mast cells and is located in acidocalcisomes.
J Biol Chem. 2012;287(34):28435-28444.
6. Moreno SN, Docampo R. Polyphosphate and its
diverse functions in host cells and pathogens. PLoS
Pathog. 2013;9(5):e1003230.
7. Smith SA, Mutch NJ, Baskar D, Rohloff P, Docampo
R, Morrissey JH. Polyphosphate modulates blood
coagulation and fibrinolysis. Proc Natl Acad Sci USA.
2006;103(4):903-908.
8. Smith SA, Morrissey JH. Polyphosphate: a new player
in the field of hemostasis. Curr Opin Hematol. 2014;21(5):
388-394.
9. Müller F, Mutch NJ, Schenk WA, et al. Platelet
polyphosphates are proinflammatory and procoagulant
mediators in vivo. Cell. 2009;139(6):1143-1156.
© 2014 by The American Society of Hematology
3178
l l l CLINICAL TRIALS & OBSERVATIONS
Comment on Bolaños-Meade et al, page 3221
The end of knight-errantry
in
GVHD studies
----------------------------------------------------------------------------------------------------Marcos de Lima
CASE WESTERN RESERVE UNIVERSITY
In this issue of Blood, Bolaños-Meade et al reported the results of the randomized
phase 3 study Blood and Marrow Transplant Clinical Trials Network (BMT
CTN) 0802.1 It compared the addition of mycophenolate mofetil to steroids vs
steroids/placebo to treat newly diagnosed acute graft-versus-host disease
(GVHD). Unfortunately, it failed to show a significant difference in outcomes.
I
n writing this commentary, I could not
avoid the temptation to make a literary
analogy. That brings me to Alonso Quixano,
aka Don Quixote, a complex character that,
according to one of the possible interpretations
of Miguel de Cervantes’ masterpiece, reflects
the end of chivalry, a man who represented
the transition of times. In his highly
unorthodox ways, Quixote seemed to get
more and more “rational” toward the end
of the book. Cervantes, as an author, made
the leap from chivalric romance to modern
literature.2 I believe we are witnessing
a similarly important transition in our field
of hematopoietic stem cell transplantation.
Steroids remain the standard of care for
the treatment of GVHD. This statement
has held true since the 1970s, and GVHD
remains a major cause of treatment failure
for recipients of allogeneic transplants. The
paucity of randomized, multicenter studies
in this field reflects both the complexities of
the problem and the lack of a collaborative
instrument to mediate and coordinate such
efforts in the United States. The latter
changed dramatically with the creation of
the BMT CTN. As a matter of fact, the
study reported here stems from a previous
prospective randomized phase 2 study
(BMT CTN 0302) that evaluated GVHD
response rates to pentostatin, mycophenolate
mofetil, denileukin diftitox, or etanercept
added to steroids for the first-line treatment
of acute GVHD. The best outcomes were
observed with mycophenolate, and included
a day 56 GVHD-free survival of 71%.3
Although somewhat frustrating in that
the hypothesis that led to the phase 3
trial turned out to be refuted, the logical
sequence of questions posed here point
to the inability of underpowered studies
(usually performed by single institutions)
to replace larger randomized studies
(ideally multicenter), and also reinforce
the need to support the infrastructure
that makes such studies possible.
The benefit to our patients is clear.
In the big scheme of disease prevalence,
hematologic malignancies (the main
indication for allogeneic transplantation)
are a relatively small fraction of the
universe of conditions that need treatment
improvement. GVHD occurs in a fraction
of those patients, and due to the small
numbers, attracts less attention from the
public and medical community at large
(although it certainly does not feel like an
orphan disease to our patients and to hospital
staff who take care of them). Therefore, in
order to answer critical questions and to
achieve strength in numbers, cooperative
efforts are needed. The role of the National
Heart, Lung, and Blood Institute (NHLBI)
in supporting the BMT CTN enterprise
cannot be overemphasized.
As for the trial itself, the mycophenolate
arm had more advanced disease patients
than the placebo group, likely explaining
the decreased disease-free survival in the
former subgroup (10% inferior, although
this did not reach statistical significance).
BMT CTN 0802 conclusions do not necessarily
apply to “alternative” donor cord blood or
haploidentical transplants, or to pediatric
patients, given the underrepresentation of these
patients in the population studied here. It is
interesting to know that day 56 GVHD-free
survival, the primary endpoint for 0802, was
BLOOD, 20 NOVEMBER 2014 x VOLUME 124, NUMBER 22
From www.bloodjournal.org by guest on June 17, 2017. For personal use only.
2014 124: 3177-3178
doi:10.1182/blood-2014-09-601641
Polyphosphate: a target for thrombosis attenuation
Roberto Docampo
Updated information and services can be found at:
http://www.bloodjournal.org/content/124/22/3177.full.html
Articles on similar topics can be found in the following Blood collections
Free Research Articles (4545 articles)
Information about reproducing this article in parts or in its entirety may be found online at:
http://www.bloodjournal.org/site/misc/rights.xhtml#repub_requests
Information about ordering reprints may be found online at:
http://www.bloodjournal.org/site/misc/rights.xhtml#reprints
Information about subscriptions and ASH membership may be found online at:
http://www.bloodjournal.org/site/subscriptions/index.xhtml
Blood (print ISSN 0006-4971, online ISSN 1528-0020), is published weekly by the American Society
of Hematology, 2021 L St, NW, Suite 900, Washington DC 20036.
Copyright 2011 by The American Society of Hematology; all rights reserved.