Neutralization of low molecular weight heparin by polybrene prevents thromboxane release and severe pulmonary hypertension in awake sheep

A Historical Perspective on the Reversal of Anticoagulants

There has been a landmark shift in the last several decades in the management and prevention of thromboembolic events. From the discovery of parenteral and oral agents requiring frequent monitoring as early as 1914, to the development of direct oral anticoagulants (DOACs) that do not require monitoring or dose adjustment in the late 20th century, great advances have been achieved. Despite the advent of these newer agents, bleeding continues to be a key complication, affecting 2 to 4% of DOAC-treated patients per year. Bleeding is associated with substantial morbidity and mortality. Although specific reversal agents for DOACs have lagged the release of these agents, idarucizumab and andexanet alfa are now available as antagonists. However, the efficacy of these reversal agents is uncertain, and complications, including thrombosis, have not been adequately explored. As such, guidelines continue to advise the use of nonspecific prohemostatic agents for patients requiring reversal of the anticoagulant effect of these drugs. As the indications for DOACs and the overall prevalence of their use expand, there is an unmet need for further studies to determine the efficacy of specific compared with nonspecific pro-hemostatic reversal agents. In this review, we will discuss the evidence behind specific and nonspecific reversal agents for both parenteral and oral anticoagulants.

Reversal Activity and Toxicity of Heparin-Binding Copolymer after Subcutaneous Administration of Enoxaparin in Mice

Uncontrolled bleeding after enoxaparin (ENX) is rare but may be life-threatening. The only registered antidote for ENX, protamine sulfate (PS), has 60% efficacy and can cause severe adverse side effects. We developed a diblock copolymer, heparin-binding copolymer (HBC), that reverses intravenously administered heparins. Here, we focused on the HBC inhibitory activity against subcutaneously administered ENX in healthy mice. BALB/c mice were subcutaneously injected with ENX at the dose of 5 mg/kg. After 110 min, vehicle, HBC (6.25 and 12.5 mg/kg), or PS (5 and 10 mg/kg) were administered into the tail vein. The blood was collected after 3, 10, 60, 120, 360, and 600 min after vehicle, HBC, or PS administration. The activities of antifactors Xa and IIa and biochemical parameters were measured. The main organs were collected for histological analysis. HBC at the lower dose reversed the effect of ENX on antifactor Xa activity for 10 min after antidote administration, whereas at the higher dose, HBC reversed the effect on antifactor Xa activity throughout the course of the experiment. Both doses of HBC completely reversed the effect of ENX on antifactor IIa activity. PS did not reverse antifactor Xa activity and partially reversed antifactor IIa activity. HBC modulated biochemical parameters. Histopathological analysis showed changes in the liver, lungs, and spleen of mice treated with HBC and in the lungs and heart of mice treated with PS. HBC administered in an appropriate dose might be an efficient substitute for PS to reverse significantly increased anticoagulant activity that may be connected with major bleeding in patients receiving ENX subcutaneously.

Andexanet Alfa, the Possible Alternative to Protamine for Reversal of Unfractionated Heparin

The recent shortage of protamine prompted an investigation of alternatives for reversal of unfractionated heparin. Heparin is an anticoagulant utilized in the hospital setting. Available options for anticoagulation include direct oral anticoagulants, vitamin K antagonists, thrombin inhibitors, low-molecular-weight heparins, and heparin. Protamine is the approved reversal agent for heparin with few alternatives under investigation. Although andexanet was designed as an antidote for apixaban and rivaroxaban, in vitro studies show that in a dose-dependent technique, andexanet had near full reversal of heparin, reversed anti–factor Xa activity, and neutralized anticoagulant effects of activated partial thromboplastin time and thrombin time induced by heparin.

Self-assembled multivalent (SAMul) ligand systems with enhanced stability in the presence of human serum

Self-assembled cationic micelles are an attractive platform for binding biologically-relevant polyanions such as heparin. This has potential applications in coagulation control, where a synthetic heparin rescue agent could be a useful replacement for protamine, which is in current clinical use. However, micelles can have low stability in human serum and unacceptable toxicity profiles. This paper reports the optimisation of self-assembled multivalent (SAMul) arrays of amphiphilic ligands to bind heparin in competitive conditions. Specifically, modification of the hydrophobic unit kinetically stabilises the self-assembled nanostructures, preventing loss of binding ability in the presence of human serum - cholesterol hydrophobic units significantly outperform systems with a simple aliphatic chain. It is demonstrated that serum albumin disrupts the binding thermodynamics of the latter system. Molecular simulation shows aliphatic lipids can more easily be removed from the self-assembled nanostructures than the cholesterol analogues. This agrees with the experimental observation that the cholesterol-based systems undergo slower disassembly and subsequent degradation via ester hydrolysis. Furthermore, by stabilising the SAMul nanostructures, toxicity towards human cells is decreased and biocompatibility enhanced, with markedly improved survival of human hepatoblastoma cells in an MTT assay.

Allosteric inhibition of factor XIa. Sulfated non-saccharide glycosaminoglycan mimetics as promising anticoagulants

Recent development of sulfated non-saccharide glycosaminoglycan mimetics, especially sulfated pentagalloyl glucopyranoside (SPGG), as potent inhibitors of factor XIa (FXIa) (J. Med. Chem. 2013; 56:867-878 and J. Med. Chem. 2014; 57:4805-4818) has led to a strong possibility of developing a new line of factor XIa-based anticoagulants. In fact, SPGG represents the first synthetic, small molecule inhibitor that appears to bind in site remote from the active site. Considering that allosteric inhibition of FXIa is a new mechanism for developing a distinct line of anticoagulants, we have studied SPGG's interaction with FXIa with a goal of evaluating its pre-clinical relevance. Comparative inhibition studies with several glycosaminoglycans revealed the importance of SPGG's non-saccharide backbone. SPGG did not affect the activity of plasma kallikrein, activated protein C and factor XIIIa suggesting that SPGG-based anticoagulation is unlikely to affect other pathways connected with coagulation factors. SPGG's effect on APTT of citrated human plasma was also not dependent on antithrombin or heparin cofactor II. Interestingly, SPGG's anticoagulant potential was diminished by serum albumin as well as factor XI, while it could be reversed by protamine or polybrene, which implies possible avenues for developing antidote strategy. Studies with FXIa mutants indicated that SPGG engages Lys529, Arg530 and Arg532, but not Arg250, Lys252, Lys253 and Lys255. Finally, SPGG competes with unfractionated heparin, but not with polyphosphates and/or glycoprotein Ibα, for binding to FXIa. These studies enhance understanding on the first allosteric inhibitor of FXIa and highlight its value as a promising anticoagulant.

Comparison of effects on markers of blood cell activation of enoxaparin, dalteparin, and unfractionated heparin in patients with unstable angina pectoris or non-ST-segment elevation acute myocardial infarction (the ARMADA study)

The low-molecular-weight heparins (LMWHs) enoxaparin and dalteparin have shown superior and equivalent efficacy, respectively, over unfractionated heparin (UFH) in patients with unstable angina pectoris (UAP) or non-ST-segment elevation myocardial infarction (NSTEMI). This study aimed to identify markers of blood cell activation that are independent predictors of outcomes at 1 month and to compare the effects of enoxaparin, dalteparin, and UFH on any such markers. In this multicenter, prospective, open-label study, 141 patients with UAP or NSTEMI were randomized to treatment for 48 to 120 hours with enoxaparin (n = 46), dalteparin (n = 48), or UFH (n = 47). Blood samples were taken at the time of randomization and after > or =48 hours of treatment but before catheterization. Multivariate analysis identified increased plasma levels of von Willebrand factor (vWF) and decreased platelet levels of glycoprotein Ib/IX complexes as independent predictors of 1-month adverse outcome (a composite of death, myocardial infarction, and recurrent ischemia). vWF release was strongly related to and may have been released by inflammation as measured by C-reactive protein. Both LMWHs reduced the release of vWF in plasma (as well as C-reactive protein) compared with UFH. Enoxaparin had a more favorable effect on glycoprotein Ib/IX complexes than either dalteparin or UFH. The incidence of the composite clinical efficacy end point was: 13% (enoxaparin), 19% (dalteparin), and 28% (UFH). vWF and its receptor glycoprotein Ib/IX play a key role in acute coronary syndromes. vWF is linked to inflammation and, like glycoprotein Ib/IX, is affected more favorably by the LWMHs than by UFH.

Effects of various anticoagulant treatments on von Willebrand factor release in unstable angina

OBJECTIVES

We tested the hypothesis that different anticoagulant treatments may produce different platelet effects and von Willebrand factor (vWf) release in unstable angina.

BACKGROUND

The early increase of vWf has been reported to be a risk factor for adverse outcome in unstable angina. Anticoagulant drugs play a key role in stabilization of unstable angina, but they may not have the same efficacy and the same effects on acute vWf release.

METHODS

We studied 154 patients enrolled in several clinical trials testing four different anticoagulant treatments in unstable angina or non-Q-wave myocardial infarction. Patients were treated during at least 48 h by either intravenous unfractionated heparin, one of two different low molecular weight heparins (enoxaparin or dalteparin) or the direct thrombin inhibitor PEG-hirudin. All patients received aspirin but no Ib/IIIa inhibitors.

RESULTS

The release of vWf over the first 48 h (delta vWf) did not relate to the baseline clinical characteristics. At 30 days of follow-up, delta vWf was sevenfold higher in patients with an end point (death, myocardial infarction, revascularization) than in patients free of events (+53 +/-7% vs. +7 +/-14%, p = 0.004). The same trend was present for each component of the composite end point with the highest levels for one-month mortality (+87 +/- 32% vs. +26 +/- 8%, p = 0.09). The vWf values did not increase over 48 h in patients receiving either enoxaparin or PEG-hirudin (+10 +/- 9% and -5 +/- 20%, respectively). A serious rise ofvWf was measured in unfractionated heparin-treated patients (+87 +/- 11%), which differed significantly from the enoxaparin group (p = 0.0006) and PEG-hirudin group (p < 0.0001). In dalteparin-treated patients, delta vWf was elevated (+48 +/- 8%) and did not differ from the unfractionated heparin group (NS).

CONCLUSIONS

We confirm that, in unstable angina patients, a rise of vWf over the first 48 h is associated with an impaired outcome at 30 days. Moreover, the four different anticoagulant treatments tested here do not provide the same protection with regards to vWf release, which may have important prognostic implications and explain different results observed in recent clinical trials.

Perioperative management of calves undergoing implantation of a left ventricular assist device

OBJECTIVE

To describe perioperative management of calves that underwent left lateral thoracotomy, aortic cross-clamping, partial left heart bypass and implantation of a left ventricular assist device.

SAMPLE POPULATION

A total of 43 healthy castrated male calves, weighing 121 +/- 24 kg.

RESULTS

Diazepam (mean +/- SD, 0.26 +/- 0.07 mg/kg), ketamine (5.9 +/- 2.17 mg/kg) and isoflurane were used in the anesthetic management of calves undergoing implantation of a left ventricular assist device in the descending thoracic aorta. Other adjunctive agents administered were fentanyl (11 +/- 5.4 microg/kg), lidocaine (4.9 +/- 3.19 mg/kg), bupivacaine (0.75%) and butorphanol (0.49 +/- 0.13 mg/kg). None of the calves regurgitated at induction or during intubation. A tube was used to drain the rumen and prevent bloat during the procedure. Partial left heart bypass was used to perfuse the caudal half of the body during the period of aortic cross clamp and device implantation. Initial mean systemic blood pressure was 96 +/- 25 mm Hg, and pressures measured in the auricular artery increased during aortic cross-clamping and bypass. Vasoconstrictor therapy was required to treat caudal arterial hypotension during the procedure in 9 calves. Mean systemic arterial pressures returned to baseline values by the end of the anesthetic period. Initial mean pulmonary arterial pressures (PAP) were 22 +/- 3 mm Hg. A significant but transient increase in pulmonary arterial pressure occurred after both heparin and protamine administration.

CONCLUSIONS

The described anesthetic protocol was effective for thoracotomy and implantation of an intra-aortic left ventricular assist device in normal calves. Partial left ventricular bypass was a useful adjunct during the period of aortic cross clamp. The doses of heparin and protamine administered were effective. Responsibility to monitor oxygenation of the cranial half of the animal continues during the bypass period as hypoxemia due to pulmonary dysfunction will not be detected by the perfusionist.

Adverse events after protamine administration in patients undergoing cardiopulmonary bypass: risks and predictors of under-reporting

A retrospective cohort study of patients undergoing cardiopulmonary bypass over 2 years at a single hospital was performed to determine the rate and predictors of attributing events to protamine and reporting them to a well-developed, hospital-based adverse drug reaction (ADR) program. Overall, 123 (12.9%) of 952 procedures were associated with an adverse event. Thirteen percent of these events were attributed to protamine in the medical record and 2.4% were reported to the ADR program. Only 19% of events attributed to protamine were reported. Even after excluding events with other measurable etiologies, the incidence of adverse events (2.6%) was still higher than that of attributed (0.8%) or reported (0.3%) events. Events that manifest as pulmonary hypertension occurred sooner after protamine, or were severe were significantly more likely to be attributed to protamine. These predictors demonstrated similar relationships with the probability of reporting events. Reliance on reporting of adverse events could not only underestimate the risk of these events, but might bias studies by identifying non-representative events.

The human inflammatory response

Inflammation is a protective response of vascularized tissue normally elicited toward nonself-determinants or tissue injury. Inflammation functions as part of normal host surveillance mechanisms to destroy or quarantine both harmful agents and damaged tissue. Most forms of inflammation are exaggerated out of proportion to the stimulus, because humoral amplification systems recruit additional components of the immune system initiating the production of proinflammatory mediators, including cytokines synthesized by activated macrophages. These act as secondary messengers to induce synthesis and expression of specific adhesion molecules on endothelial cells and white cells. Polymorphonuclear leukocytes play a central role in the acute inflammatory response. Anaphylaxis, an immediate hypersensitivity reaction to substances administered in the perioperative period, serves as a useful model for some of the problems of acute inflammation because there are important interrelationships with the cardiovascular system, endothelium, and coagulation. Mast cells and basophils produce the acute inflammation associated with anaphylaxis mediated by immunoglobulin E (IgE) antibodies, whereas the immunoglobulin G (IgG)-antigen interaction activates the complement cascade to generate anaphylatoxins, specifically C5a. Activation of white cells causes release of proteolytic enzymes, production of oxygen-derived free radicals, and the synthesis of a variety of lipid mediators. Protamine sulfate is one of the most common causes of life-threatening anaphylactic reactions during cardiac surgery. Differing responses occur, dependent on the presence of either IgE or IgG antibodies with the activation of the mast cell or the complement system, respectively. The many different amplifying pathways that can be recruited during anaphylaxis and the array of mediators involved are important when therapeutic intervention is considered. The challenge for the future will be to identify a pharmacologic agent that will arrest the inflammatory cascade and thus prevent further amplification and resultant host injury.

Protamine-induced pulmonary venoconstriction in heparinized pigs

Reversal of heparin anticoagulation with protamine may be associated with acute pulmonary vasoconstriction. The specific site of pulmonary vasoconstriction has not been determined. This study was designed to determine the site of protamine-induced pulmonary vasoconstriction and the role of nitric oxide (NO) after protamine injection. Pigs were anesthetized and instrumented with catheters for monitoring pulmonary arterial, systemic arterial, and central venous pressures. Pulmonary capillary pressure was estimated using the arterial occlusion concept, while left atrial pressure was estimated from the equilibrium wedge pressure. Hemodynamic measurements were made during baseline, before and after heparin (200 U/kg), at peak pressure response after protamine injection (2 mg/kg), and 10 and 30 min thereafter. In the control group, pulmonary vascular resistance (PVR) values during baseline and after heparin were identical (2.7 +/- 0.4 mm Hg.L-1.min-1). At peak protamine response (1-2 min) PVR increased to 8.0 +/- 1.6, but returned to baseline value after 10 min (2.8 +/- 0.3) and remained stable for 30 min (2.2 +/- 0.3). The increase in PVR after protamine was primarily due to an increase in venous resistance from 1.0 +/- 0.2 to 4.9 +/- 1.4 mm Hg.L-1.min-1, and a much smaller increase in arterial resistance from 1.7 +/- 0.3 to 3.4 +/- 0.6 mm Hg.L-1.min-1. A second group was treated with nitrow-L-arginine (LNA, 20 mg/kg) to inhibit NO release, and then heparin and protamine were administered as in the first group. Heparin had no effect on pressures, but protamine increased PVR by the same magnitude as in Group 1.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential effects of a heparin antagonist (hexadimethrine) or chlorate on amphiregulin, basic fibroblast growth factor, and heparin-binding EGF-like growth factor activity

Amphiregulin (AR) and heparin-binding EGF-like growth factor (HB-EGF) are two recently identified members of the EGF family. Both AR and HB-EGF share with EGF the ability to interact with the type-1 EGF receptor; however, AR and HB-EGF differ from EGF in that both of these mitogens bind to heparin while EGF does not. To determine whether interactions with heparin-like molecules on the cell surface influence binding of AR and HB-EGF with EGF receptors and the subsequent mitogenic activity exerted by these growth factors, murine AKR-2B and Balb/MK-2 cells were treated with either an inhibitor of proteoglycan sulfation (chlorate) or a heparin antagonist (hexadimethrine). As expected, neither treatment significantly altered the specific binding of 125I-EGF on AKR-2B cells. Interestingly, treatment with either chlorate or hexadimethrine inhibited the ability of AR to compete with 125I-EGF for cell surface binding and also attenuated AR-mediated DNA synthesis. Thus, as has been suggested for other heparin-binding growth factors such as basic fibroblast growth factor (bFGF), the interaction of AR with an EGF-binding receptor appears to be facilitated by interaction with cell-associated sulfated glycosaminoglycans or proteoglycans. Unexpectedly, however, neither chlorate nor hexadimethrine treatment caused an inhibition of HB-EGF-induced mitogenic activity. Chlorate treatment did not significantly alter the ability of HB-EGF to compete with 125I-EGF for cell surface binding sites, however, heparin and hexadimethrine reduced the ability of HB-EGF to compete for 125I-EGF binding. These results suggest that, in AKR-2B cells, HB-EGF may mediate its mitogenic response at least in part through a receptor which appears to be selective for HB-EGF and permits HB-EGF-mediated mitogenic responses in the presence of hexadimethrine or heparin. Finally, hexadimethrine inhibited the specific binding and mitogenic activity of bFGF, suggesting that this cationic polymer can function as an antagonist of heparin-binding mitogens other than AR.

Effective and less toxic reversal of low-molecular weight heparin anticoagulation by a designer variant of protamine

PURPOSE

This investigation assessed protamine reversal of heparin anticoagulation by formation of a protamine-heparin alpha-helix by use of a new designer-variant protamine [+18BE] that was made from an existing protamine variant [+18B] whose non-alpha-helix-forming amino acid proline (P) was replaced by an alpha-helix-forming glutamic acid (E). The rate of administration of the new [+18BE] variant protamine on efficacy and toxicity in comparison to that of [+21] standard protamine and [+18B] was also studied.

METHODS

Acetyl-EAA(K2A2K2A)4K2-Amide [+18BE] was administered intravenously in a 1:1 dose to low-molecular-weight heparin (LMWH)-anticoagulated (intravenous 150 IU antifactor Xa/kg) dogs over 10 seconds or 3 minutes (n = 7, each group). Reversal efficacy was documented by measuring activated clotting time, thrombin clotting time, antifactor Xa, and antifactor IIa. Toxicity was defined by measuring systemic blood pressure, heart rate, cardiac output, pulmonary artery pressure, and oxygen consumption. Measurements were made at baseline, after administration of LMWH, before its reversal, and for 30 minutes thereafter. Results were compared with those after LMWH reversal with [+21] standard protamine and the [+18B] variant. A total toxicity score (TTS) was calculated for each compound from maximal declines in blood pressure, heart rate, cardiac output, and oxygen consumption.

RESULTS

LMWH anticoagulation reversal was significantly (p < 0.01) less toxic over 10 seconds and 3 minutes with the [+18BE] designer variant (TTS -2.3, -2.2) compared with the [+21] standard protamine (TTS -6.4, -7.2). Percent LMWH reversal at 3 minutes revealed [+18BE] to have antifactor Xa activity as high as 91%, compared with 68% for protamine [+21], when given over 3 minutes (p < 0.05).

CONCLUSIONS

This investigation documents that a new designer variant of protamine [+18BE] has superior efficacy compared with [+21] standard protamine for reversal of LMWH anticoagulation and that this occurs with a highly favorable toxicity profile.

Protamine and protamine-insulins exacerbate the vascular response to injury

Endothelial cells and smooth muscle cells produce heparinlike compounds that are growth inhibitory for vascular smooth muscle cells, and it has been suggested that these compounds play a regulatory role that is perturbed with vascular injury. Indeed, exogenous heparin preparations effectively suppress smooth muscle cell proliferation following injury imposed on vascular endothelium. We now report that protamine, an agent that binds heparin and negates its anticoagulant properties, has potent stimulatory effects on vascular smooth muscle cell proliferation. The administration of protamine, alone or as part of commonly used insulin preparations, stimulated the proliferation of cultured smooth muscle cells, exacerbated vascular smooth muscle cell proliferative lesions in laboratory rats, and interfered with the growth-inhibitory effects of heparin in culture and in vivo. These results confirm the importance of endogenous heparinlike compounds in arterial homeostasis and may require reconsideration of protamine use following vascular reparative procedures and in diabetics.