Immobilized heparin inhibits the increase in leukocyte surface expression of adhesion molecules

Heparin coatings for improving blood compatibility of medical devices

Blood contact with biomaterials triggers activation of multiple reactive mechanisms that can impair the performance of implantable medical devices and potentially cause serious adverse clinical events. This includes thrombosis and thromboembolic complications due to activation of platelets and the coagulation cascade, activation of the complement system, and inflammation. Numerous surface coatings have been developed to improve blood compatibility of biomaterials. For more than thirty years, the anticoagulant drug heparin has been employed as a covalently immobilized surface coating on a variety of medical devices. This review describes the fundamental principles of non-eluting heparin coatings, mechanisms of action, and clinical applications with focus on those technologies which have been commercialized. Because of its extensive publication history, there is emphasis on the CARMEDA^® BioActive Surface (CBAS^® Heparin Surface), a widely used commercialized technology for the covalent bonding of heparin.

Effects of surface-bound and intravenously administered heparin on cell-surface interactions: inflammation and coagulation

Intravenous administration of heparin and heparin-bonded extracorporeal circuits are frequently used to mitigate the deleterious effects of blood contact with synthetic materials. The work described here utilized human blood in a micro-perfusion circuit to experimentally examine the effects of intravenous and surface-bound heparin on cellular activation. Activation markers of coagulation and of the inflammatory response were examined using flow cytometry; specifically, markers of platelet, monocyte, polymorphonuclear leukocyte (PMN), and lymphocyte activation were quantified. The results indicate that surface-bound heparin reduces the inflammatory response whereas systemically administered heparin does not. This finding has important implications for blood-contacting devices, particularly within the context of recently elucidated connections between inflammation pathways and coagulation disorders. Data presented indicate that surface-bound heparin and intravenously administered heparin play distinct, but vital roles in rendering biomaterial surfaces compatible with blood.

Assessment of Heparin Binding to the AN69 ST Hemodialysis Membrane: I. Preclinical Studies

The AN69 ST membrane was designed to render the surface of the native polyacrylonitrile polymer less cationic. This was achieved by layering the membrane with the polycationic biopolymer polyethyleneimine. This new membrane is able to bind heparin to its surface, through electrical interactions, without altering the reactivity of the sulfonate groups of the membrane, regularly distributed in the membrane bulk. The kinetics of unfractionated or low-molecular-weight heparins were studied in vitro and in vivo in sheep. Encouraging results were obtained indicating that heparin-coated hemodialyzers are potent anticoagulants. Priming the AN69 ST membrane-equipped hemodialyzer with heparin, as in regular hemodialysis, could allow drastic reduction of heparin consumption in hemodialysis.

Effect of complement inhibition and heparin coating on artificial surface-induced leukocyte and platelet activation

BACKGROUND

Exposure of blood to artificial surfaces, as in cardiopulmonary bypass, induces an inflammatory response involving complement, leukocyte and platelet activation. To elucidate the specific role of complement in this process, studies were performed on blood circulated in polyvinyl chloride tubing in the absence and presence of complement inhibitors. Parallel experiments were performed with heparin-coated polyvinyl chloride tubing, which is known to prevent complement and cell activation.

METHODS

A novel experimental model was used, based on human whole blood anticoagulated with lepirudin. Complement activation products, myeloperoxidase, lactoferrin, and thrombospondin were quantified in enzyme immunoassays. Leukocyte CD11b expression and leukocyte-platelet conjugates were detected by flow cytometry.

RESULTS

Increased levels of C3 activation products, alternative pathway convertase, and the terminal SC5b-9 complex, combined with unchanged levels of C1rs-C1-inhibitor complexes and marginal changes in C4 activation demonstrated that complement was activated through the alternative pathway. Granulocyte and monocyte CD11b expression and granulocyte-platelet conjugate formation were efficiently attenuated by blocking either factor D, C3, C5, or C5a receptor. In contrast, monocyte-platelet conjugate formation and release of myeloperoxidase, lactoferrin, and thrombospondin were not reduced by complement inhibition. Heparin-coated polyvinyl chloride tubing efficiently reduced all inflammatory markers studied, except for C1rs-C1-inhibitor complexes, which increased, consistent with the enhancing effect of heparin on C1-inhibitor function. This effect did not, however, reduce fluid-phase classic pathway activation induced by heat-aggregated immunoglobulin G.

CONCLUSIONS

Leukocyte and platelet activation in response to artificial materials occur by mechanisms that vary in their dependence on complement. Heparin coating precludes both the complement-dependent and complement-independent reactions.

Mechanism of complement activation during extracorporeal blood-biomaterial interaction: effects of heparin coated and uncoated surfaces

In vitro studies with miniaturized rotating circuits and heparinized human blood, as well as long-term extracorporeal membrane oxygenation with either heparin coated (HBS) or uncoated surfaces connected to adult sheep, were performed comparing the impact on complement activation in blood and on surfaces. Analysis of surface bound complement proteins revealed significantly reduced binding of activated C3 and C5b-9 to HBS in vitro, compared with uncoated surfaces, which was probably due to more HBS bound complement inhibitors (C1-Inhibitor, factor H) being present. This was reflected by significantly reduced activation of the alternative pathway (C3bBbP) and terminal complex (SC5b-9) by HBS but slightly increased levels of classic pathway complex (C1rs-C1-inhibitor). These results were confirmed during in vivo study by analysis of hemolytic complement function, activation specific C3 derived split products, and surface bound complement proteins. Increased binding of complement regulators to HBS appears to effectively reduce complement activation by biomaterials, leading to improved long-term biocompatibility.

Effects of new polymer-coated extracorporeal circuits on biocompatibility during cardiopulmonary bypass

An inflammatory response due to bioincompatibility of extracorporeal circuits is a major clinical issue during cardiopulmonary bypass (CPB). By using a swine model, we determined whether new polymer-coated circuits, the blood-contacting surfaces of which are coated with poly(2-methoxyethylacrylate) (PMEA), would reduce the inflammatory response during CPB. Plasma bradykinin levels and the percentages of CD35-positive monocytes in PMEA-coated circuits were significantly lower than those in uncoated circuits during CPB. The amount of proteins adsorbed on the PMEA-coated circuits was significantly lower than that on the uncoated circuits (0.30 microg/cm2 versus 3.42 microg/ cm2). Almost no IgG, IgM, or C3c/d was detected in proteins adsorbed on the PMEA-coated circuits although these proteins were clearly detected in proteins adsorbed on the uncoated circuits. We concluded that PMEA coating could reduce complement activation during CPB by suppressing the adsorption of IgG and IgM, which activate C3 via the classical pathway, on the surface of the circuits.

The concept of glomerular self-defense

The balance between local offense factors and defense machinery determines the fate of tissue injury: progression or resolution. In glomerular research, the most interest has been on the offensive side, for example, the roles of leukocytes, platelets, complement, cytokines, eicosanoids, and oxygen radical intermediates. There has been little focus on the defensive side, which is responsible for the attenuation and resolution of disease. The aim of this review is to address possible mechanisms of local defense that may be exerted during glomerular injury. Cytokine inhibitors, proteinase inhibitors, complement regulatory proteins, anti-inflammatory cytokines, anti-inflammatory eicosanoids, antithrombotic molecules, and extracellular matrix proteins can participate in the extracellular and/or cell surface defense. Heat shock proteins, antioxidants, protein phosphatases, and cyclin kinase inhibitors may contribute to the intracellular defense. This article outlines how the glomerulus, when faced with injurious cells or exposed to pathogenic mediators, defends itself via the intrinsic machinery that is brought into play in resident glomerular cells.

Complement and biocompatibility

Blood contact with foreign surfaces induces a certain degree of activation of the defence systems including complement. Recent data using complement specific inhibitors have shown that complement is not only associated with, but in fact contributes to the whole body inflammatory reaction seen as a complication of cardiopulmonary bypass (artificial surfaces) and is responsible for the hyperacute rejection of xenografts (foreign endothelium). Complement activation products, particularly the terminal SC5b-9 complex (TCC), is a sensitive indicator of bioincompatibility. Modifications of the artificial surfaces by coating with heparin improves both coagulation and complement compatibility. The use of specific complement inhibitors in general improves biocompatibility both of artificial materials and foreign endothelium.

Studies of adsorption, activation, and inhibition of factor XII on immobilized heparin

The aim of the present investigation was to clarify whether immobilized heparin does, as previously suggested, prevent triggering of the plasma contact activation system. Purified FXII in the absence or presence of antithrombin and/or C1 esterase inhibitor as well as plasma was exposed for 1 to 600 seconds to a surface modified by end-point immobilization of heparin. With purified reagents, a process including surface adsorption and activation of FXII occurred within 1 second. In the presence of antithrombin, the resulting surface-bound alpha-FXIIa was inhibited within that time. Likewise, the adsorption of native FXII from plasma was a rapid process. However, the inhibition of surface-bound alpha-FXIIa was slightly slower than with purified components. Nevertheless, neither beta-FXIIa nor FXIa were found in the plasma phase. Exposure of a surface prepared from heparin molecules, lacking antithrombin binding properties, to plasma resulted in surface-bound alpha-FXIIa within 1 second. In the liquid phase, beta-FXIIa was detected after 2.5 seconds and, 12 seconds later, FXIIa and FXIa in complex with the C1 esterase inhibitor appeared. Addition of heparin to plasma prior to surface exposure did not prevent activation of surface-bound FXII, nor did it increase the beta-FXIIa inhibition rate and prevent FXI activation in plasma, although beta-FXIIa and FXIa-AT complex formation occurred. It is concluded that surface-immobilized heparin, unlike heparin in solution, effectively inhibits the initial contact activation enzymes by an antithrombin-mediated mechanism, thereby suppressing the triggering of the intrinsic plasma coagulation pathway.