CBAS® Heparin Surface Compendium (PDF 2.5M)

CARMEDA BIOACTIVE SURFACE
CARmEDA BIOACTIVE suRFACE
– a coating technology for dramatic improvement of the performance of artificial materials
of medical devices in contact with blood
INTRODuCTION
The striking increase in clinical interventions utilizing medical devices for permanent implantation as well as short- and long-term therapy has highlighted the
problem of adverse tissue responses to artificial materials. Several strategies have
been used in attempts to attenuate or eliminate the rejection of artificial materials, including the development of new materials that are better tolerated by the
host organism and modification of existing materials to minimize the risk of adverse activation of natural defense mechanisms in the body. One such approach
involves modifying an artificial surface by application of a biocompatible coating,
thus masking the incompatible nature of the foreign material.
The Carmeda® BioActive Surface (CBAS®) is a heparin-based surface-modification
technology designed to enhance the hemocompatibility of devices for use in contact with blood. Its effectiveness has been demonstrated both experimentally and
clinically, and it is the most proven of all commercially available technologies of its
type. The robustness of the CBAS coating in blood-flow and mechanical-abrasion
environments makes it particularly suitable for permanently implanted devices
and those intended for long-term use. CBAS has been applied to devices for
diagnostic purposes and for critical extra-corporeal therapy as well as to devices
for permanent implantation. Products for which the CBAS technology has been
licensed include, among others, vascular grafts, heart-lung machines, ventricular
assist devices, coronary stents, and hemodialysis catheters.
CARMEDA BIOACTIVE SURFACE
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The response of blood to contact with a foreign material.
Intrinsic pathway of the coagulation system and the inhibitory role of antithrombin (AT). Sequence of coagulation factors
and reactions leading to the formation of the insoluble fibrin clot.
CARMEDA BIOACTIVE SURFACE
BLOOD IN CONTACT WITH ARTIFICIAL mATERIALs
Exposure of blood to an artificial material is a signal
of injury to the host defense mechanisms. As a consequence, mechanisms such as hemostasis (stoppage of bleeding) and inflammation (response to the
foreign irritant) are triggered.
thrombotic complications by systemic anticoagulation, usually by administration of heparin intravenously, increases the risk of uncontrollable bleeding
and does not prevent other adverse reactions to the
artificial material.
THROmBOTIC COmPLICATIONs
INFLAmmATORy REACTION
The most obvious and acute defense reaction to the
introduction of an artificial material in the bloodstream is coagulation, leading to clot/thrombus formation. This response may result in malfunctioning
or total obstruction of a medical device and release
of emboli (thrombus fragments causing vessel obstruction downstream) with potentially catastrophic
consequences. On the other hand, counteracting
Exposure of blood or living tissue to a foreign material will trigger an inflammatory response. Extensive
activation of the inflammatory mechanism – for
example, during use of a heart-lung machine – may
lead to a systemic (“whole-body”) inflammatory condition that, if severe, can result in multi-organ failure.
The inflammatory response is linked to activation of
the complement system and white blood cells.
© 009 W. L. Gore & Associates, Inc.
© 009 W. L. Gore & Associates, Inc.
Conventional artificial material.
Thromboresistant (CBAS-coated) material.
Artificial blood vessel after implantation in dog.
THE COAGuLATION mECHANIsm
When blood is exposed to a surface other than the natural vascular wall, the hemostatic mechanism is activated to restore
vessel integrity. Platelets, the blood cells involved in coagulation, adhere to and are activated by the foreign surface along
with activation of the intrinsic pathway of the plasma coagulation system (left). Triggering of the coagulation involves activation of a series of blood components, so called coagulation factors, culminating in the conversion of prothrombin to
enzymatically active thrombin. Finally thrombin converts the
soluble plasma protein fibrinogen to insoluble fibrin which together with the activated platelets forms a solid clot.
In order to avoid unintentional clotting, i.e. thrombosis, the
clotting mechanism is under the control of numerous inhibitors in blood and on the vascular wall, the most important of
which is antithrombin (AT). AT not only inhibits thrombin; it
appears to have the capacity to inactivate essentially all ac-
tivated factors of the coagulation system, as shown to the left.
AT acts by forming a complex with the activated coagulation
factor, resulting in total neutralization of enzymatic activity.
COmPLEmENT ACTIVATION
Another host defense against artificial materials and invading
microorganisms is the complement system. Complement is
activated by essentially any material, but is under strict control by regulating factors on the cells of the host organism
(“self”). As foreign (“non-self”) materials lack this regulatory
function, complement has the capacity to discriminate between “self” and “non-self.” Thus, the presence of any artificial
surfaces in the blood circulation will cause some degree of
complement activation. The principal outcome of this activation is an inflammatory reaction, which is beneficial in the
defense against infectious agents but may represent a serious
complication in the use of medical devices in direct contact
with blood.
CARMEDA BIOACTIVE SURFACE
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HEPARIN
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
Schematic illustration of the heparin molecule with the AT-binding pentasaccharide sequence.
Top: detailed structure of sugar unit components.
Middle: schematic polysaccharide sequence.
Bottom: the heparin molecule represented as a solid line.
O
O
HEPARIN
Because of its well-established effect on blood
coagulation, heparin has been the drug of choice
for anticoagulation for more than half a century. It
is being used for both prophylaxis and treatment of
thrombosis and is routinely administered when artificial materials are introduced in the bloodstream.
mECHANIsm OF ACTION OF HEPARIN
Heparin acts as a catalyst by dramatically increasing
the capacity of a natural coagulation inhibitor in
blood.
HEPARIN COATINGs
Coating artificial materials used in the bloodstream
with functionally active heparin is an efficient technique for improvement of the performance in contact
with blood.
Heparin with active sequence
Conformationally
changed antithrombin
Thrombin
Antithrombin
Thrombin-antithrombin
complex
Catalytic effect of heparin on the inhibitory action of AT.
HEPARIN
Heparin is a natural component of living tissue. Interestingly,
however, there is no unequivocal evidence a natural occurrence of heparin in blood, despite its striking effect on coagulation. Instead, a substance that is both structurally and functionally closely related to heparin has been shown to be present
on all surfaces in the vasculature, where it appears to have a
key role in maintaining the nonthrombogenic properties of
the vessel walls. Chemically, heparin is a linear polysaccharide
with a highly complex structure characterized by a heavy substitution with sulfate groups (left).
mECHANIsm OF ACTION
Heparin exerts its anticoagulant activity by binding to AT,
thereby radically increasing the rate by which AT neutralizes activated coagulation factors. The interaction with AT
depends on a sequence of highly specific structure in the
heparin molecule, the AT-binding sequence (“active sequence”), which is present in about one out of three molecules of standard pharmaceutical preparations of heparin.
After formation of the inactive complex between AT and a
coagulation factor, the comples released and the heparin
molecule is free to bind other AT molecules (above). Thus,
heparin is not itself an inhibitor; instead, it acts as a catalyst
by converting AT from a slow into a highly potent anticoagulant, without being consumed in the inhibitory process.
This catalytic action is fundamental to the creation of a stable nonthrombogenic surface, as the immobilized heparin
is not exhausted or consumed in contact with blood, but
maintains its catalytic activity.
HEPARIN COATINGs
The intention of coating medical devices for use in bloodcontacting applications with heparin is to localize the activity
of this powerful anticoagulant to the surfaces of the foreign
materials that are otherwise likely to cause complications in
contact with blood. Binding of heparin to surfaces, however,
has often been associated with loss of its functional properties and little or no improvement in hemocompatibility. Even
minor modifications of the heparin structure resulting from
the coupling chemistry may interfere with the specific interaction with AT. Accordingly, it is essential to ensure that the
coupling procedure does not compromize the ability of heparin to interact with blood components.
CARMEDA BIOACTIVE SURFACE
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CARmEDA BIOACTIVE suRFACE (CBAs)
Carmeda’s patented End-point attachment™ method permits heparin molecules to be attached to a
foreign surface by a stable covalent linkage while
preserving the specific functional activity of this
anticoagulant. The coating is non-leaching, and its
chemical structure allows the immobilized heparin
to preserve its antithrombogenic properties on
artificial surfaces for extended periods of time. As a
natural heparin-like substance is present on all surfaces of the vasculature, the functionally active CBAS
coating can be considered to mimic properties of
the only truly blood-compatible surface: the natural
vessel wall.
CHEmIsTRy OF CBAs
PRImING OF THE mATERIAL suRFACE
Prior to attaching heparin to an artificial material,
the surface must be primed by applying a polymeric base matrix using a layer-by-layer technique. A
cationic amino polymer is adsorbed to the material surface, followed by an anionic polymer and a
polymeric amine. Additional layers of anionic and
cationic polymers are applied to achieve optimal
functional characteristics and coverage of the underlying material.
END-POINT ATTACHmENT OF HEPARIN
The CBAS technology is based on a minor chemical
modification of heparin that results in formation of a
reactive aldehyde group at one end of the linear molecule. These groups are then reacted with primary
amino groups incorporated on the material surface
by the priming procedure, leading to formation
of Schiff ’s bases, which are reduced to yield stable
covalent bonds. (bellow)
Priming of the material surface by stepwise adsorption
of positively and negatively charged polymers.
End-point attached heparin.
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CARMEDA BIOACTIVE SURFACE
CBAs FuNCTION
End-point attachment thus allows preservation
of the functional activity of heparin throughout
the coupling procedure. Because of its linkage by
a single bond at one terminus (“end point”), the
immobilized heparin molecule extends from the
surface into the liquid phase, enabling it to interact
with molecules in the circulating blood with retained
specificity. Hence, the immobilized bound heparin
can express its potent anticoagulant activity locally,
on the biomaterial surface.
Heparin with active sequence
Antithrombin
Experiments with surfaces prepared from a heparin
fraction completely devoid of anticoagulant activity (i.e. lacking the specific AT-binding sequence)
demonstrated that the nonthrombogenic properties
of CBAS are critically dependent on exposure of the
functionally intact AT-binding sequence of the immobilized heparin.
Conformationally
changed antithrombin
Thrombin
Thrombin-antithrombin
complex
Continuous neutralization of activated coagulation factors, such as thrombin, by CBAS in the presence of antithrombin.
CARMEDA BIOACTIVE SURFACE
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OTHER HEPARIN COATINGs
Many techniques for coating the surfaces of artificial materials with heparin have demonstrated unsatisfactory performance in contact with blood because the functional properties of heparin were poorly preserved.
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Poor stability
Essentially inactive
IONIC BONDs
COVALENT CROssLINKING
The first developed and simplest way to immobilize
heparin is by ionic bonding; the negative charge of
the molecule is used to attach it to a positively charged surface. Because of the inherently weak nature
of ionic bonds, multiple interactions are required to
retain heparin on the surface, resulting in poor exposure of the specific AT-binding sequence. However,
the limited stability of the binding may allow a slow
release of heparin, which reduces the thrombo-genicity of the surface for a short period of time only.
Stable covalent bonding can be achieved by using
crosslinking agents or reactive groups in the heparin
molecule. As with ionic bonding, however, preservation of the functional properties of heparin is poor.
Successful in short-term applications
Superior end-point bonding by Carmeda only
HEPARIN COmPLEXEs
END-POINT BONDING
Surface modifications based on heparin complexes
have essentially the same shortcomings as ionic
bonding: poor exposure of the functional AT-binding
sequence and limited coating stability.
End-point immobilization of heparin (“end- point
attachment”), which is used for the CBAS coating, is
superior to most other immobilization techniques
with respect to preserving the functional properties
of the bound heparin molecules and providing a stable, non-leaching coating with long-term bioactivity
and excellent performance in contact with blood.
CARMEDA BIOACTIVE SURFACE
PROPERTIEs OF CBAs
REDuCED THROmBOGENICITy
By suppressing the coagulation mechanism on a
coated surface, CBAS reduces or eliminates thrombotic complications related to the exposure of blood
to artificial materials. CBAS-coated surfaces have
been shown to effectively neutralize activated coagulation factors such as thrombin and FXa.
Studies have also revealed that factors involved in
the initiation of clotting are inhibited by CBAS, meaning that the trigger mechanism of the coagulation
system is suppressed by the immobilized heparin.
Because of the minimal challenge to the coagulation
system presented by CBAS-coated medical devices,
the risk of thrombosis-related complications is reduced to a degree that may allow decreased systemic
anticoagulation.
The CBAS coating is highly robust and will withstand
virtually all expected fluid flow conditions and many
mechanical challenges. For example, CBAS has been
shown to remain bound and bioactive for months
or even years in long-term blood flow applications.
It also withstands, i.e. the mechanical challenge of
balloon expansion of coronary stents in stenosed
arteries.
DECREAsED INFLAmmATORy REsPONsE
Several studies have shown near-elimination of
complement activation in blood exposed to CBAScoated surfaces. Moreover, activation of white cells
in blood exposed to CBAS-coated surfaces is low.
Clinical studies have confirmed that CBAS-coated
medical devices cause less complement activation
than uncoated equipment.
PLATELET COmPATIBILITy
ANTImICROBIAL PROPERTIEs
Compared with conventional foreign materials,
CBAS-coated surfaces cause minimal binding and
activation of platelets.
Clinical studies have indicated a reduced infection
rate in patients treated with CBAS-coated central
venous catheters compared with uncoated catheters. Hypothetically, due to the nonthrombogenic
properties of CBAS the catheter surface is less likely
to be fouled by clot-related material, making it less
prone to support microbial growth.
ROBusT COATING
Covalent bonding of heparin results in stable attachment to the artificial surface, with no leaching
into the circulation, and creates the potential for
long-term use of medical devices with CBAS-coated,
blood-exposed surfaces.
mETHODs FOR CHARACTERIZING HEPARIN-COATED suRFACEs
The easiest way to visualize a heparin coating is by
staining with specific dyes. Staining is useful for assessing the outcome of the coating process, including the uniformity of the coat.
In addition to using dyes to evaluate coating process
results, Carmeda employs in-house chemical methods to measure the total amount of heparin bound
to a coated surface.
CARmEDA BIOACTIVITy AssAy
Coating with heparin does not automatically confer
thromboresistance. Studies have shown that preservation of the functional activity of heparin during
the coupling procedure is a prerequisite of thromboresistance. As a tool for evaluation of the immobilized heparin, Carmeda developed the proprietary
BioActivity Assay which measures the highly specific
interaction between heparin and AT.
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CARMEDA BIOACTIVE SURFACE
This assay has been shown to correlate well with
other in vitro and ex vivo hemocompatibility tests.
Thus, the Carmeda BioActivity Assay is an effective
screening method and predictor of in vivo performance.
PERFORmANCE IN CONTACT WITH BLOOD
Other techniques developed in-house include
models for screening of the performance in contact
with whole blood or plasma, reflecting properties
of artificial surfaces related to clotting and inflammation.
Carmeda also makes use of additional in-house
and externally available methods for testing of the
surface chemistry and hemocompatibility of artificial
materials.
In summary:
Heparin coating of artificial surfaces by the CBAS technology entails the following benefits:
• decreased thrombus formation
• reduced risk of device malfunction
• reduced loss of platelets
• attenuated inflammatory response
• may allow reduced need for systemic anticoagulation, with reduced risk of bleeding
and need for transfusion
• possible reduction in rate of infection
Disclaimer
While the information contained in this document is believed to be correct, Carmeda and the directors and employees of Carmeda
disclaim any and all liability for the contents of, or omissions from, this document. No representations or warranties are made as to the
accuracy and completeness of any statements, claims or interpretations of the document.
Carmeda® and CBAS® are registered trademarks of Carmeda AB. © 2009 Carmeda AB
CARMEDA BIOACTIVE SURFACE
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