Reversal of heparin anticoagulation by recombinant platelet factor 4 in humans

Anti-platelet Factor 4 Antibody-Mediated Disorders: An Updated Narrative Review

Anti-platelet factor 4 (PF4) antibody-mediated disorders are a heterogeneous group of diseases characterized by the presence of highly pathogenic immunoglobulins G directed against PF4 and/or PF4/heparin complexes. These antibodies are able to activate platelets, neutrophils, and monocytes, thus resulting in thrombocytopenia and a hypercoagulable state. Five different forms of anti-PF4 antibody-mediated disorders have been identified: (1) classic heparin-induced thrombocytopenia (HIT) mediated by heparin and certain polyanionic drugs; (2) autoimmune HIT characterized by the presence of anti-PFA/polyanion antibodies that can strongly activate platelets even in the absence of heparin; (3) spontaneous HIT characterized by thrombocytopenia and thrombosis without proximate exposure to heparin, with two subtypes: (a) post-total knee arthroplasty and cardiac surgery using cardiopulmonary bypass or extracorporeal membrane oxygenation and (b) postinfections; (4) vaccine-induced immune thrombotic thrombocytopenia (VITT) characterized by thrombocytopenia, arterial and venous thrombosis, or secondary hemorrhage after receiving adenoviral vector vaccines for coronavirus disease 2019; (5) VITT-like disorders triggered by adenoviral infections. Although extremely rare and largely unknown, there has been growing interest in the VITT syndrome in recent years due to its clinical relevance. Timely detection of these antibodies is crucial for the diagnosis and treatment of anti-PF4 antibody-mediated disorders, via anti-PF4 antibody immunoassays using several antibody capture systems (e.g., enzyme-linked immunosorbent assay-based, particle gel, turbidimetry) and functional assays (e.g., serotonin release assay or heparin-induced platelet activation). We aimed to present the latest on laboratory findings, clinical characteristics, and therapeutic approaches for anti-PF4 antibody-mediated disorders.

Platelet factor 4(PF4) and its multiple roles in diseases

Platelet factor 4 (PF4) combines with heparin to form an antigen that could produce IgG antibodies and participate in heparin-induced thrombocytopenia (HIT). PF4 has attracted wide attention due to its role in novel coronavirus vaccine-19 (COVID-9)-induced immune thrombotic thrombocytopenia (VITT) and cognitive impairments. The electrostatic interaction between PF4 and negatively charged molecules is vital in the progression of VITT, which is similar to HIT. Emerging evidence suggests its multiple roles in hematopoietic and angiogenic inhibition, platelet coagulation interference, host inflammatory response promotion, vascular inhibition, and antitumor properties. The emerging pharmacological effects of PF4 may help deepen the exploration of its mechanism, thus accelerating the development of targeted therapies. However, due to its pleiotropic properties, the development of drugs targeting PF4 is at an early stage and faces many challenges. Herein, we discussed the characteristics and biological functions of PF4, summarized PF4-mediated signaling pathways, and discussed its multiple roles in diseases to inform novel approaches for successful clinical translational research.

Chemokines, molecular drivers of thromboinflammation and immunothrombosis

Blood clotting is a finely regulated process that is essential for hemostasis. However, when dysregulated or spontaneous, it promotes thrombotic disorders. The fact that these are triggered, accompanied and amplified by inflammation is reflected in the term thromboinflammation that includes chemokines. The role of chemokines in thrombosis is therefore illuminated from a cellular perspective, where endothelial cells, platelets, red blood cells, and leukocytes may be both the source and target of chemokines. Chemokine-dependent prothrombotic processes may thereby occur independently of chemokine receptors or be mediated by chemokine receptors, although the binding and activation of classical G protein-coupled receptors and their signaling pathways differ from those of atypical chemokine receptors, which do not function via cell activation and recruitment. Regardless of binding to their receptors, chemokines can induce thrombosis by forming platelet-activating immune complexes with heparin or other polyanions that are pathognomonic for HIT and VITT. In addition, chemokines can bind to NETs and alter their structure. They also change the electrical charge of the cell surface of platelets and interact with coagulation factors, thereby modulating the balance of fibrinolysis and coagulation. Moreover, CXCL12 activates CXCR4 on platelets independently of classical migratory chemokine activity and causes aggregation and thrombosis via the PI3Kβ and Btk signaling pathways. In contrast, typical chemokine-chemokine receptor interactions are involved in the processes that contribute to the adhesiveness of the endothelium in the initial phase of venous thrombosis, where neutrophils and monocytes subsequently accumulate in massive numbers. Later, the reorganization and resolution of a thrombus require coordinated cell migration and invasion of the thrombus, and, as such, indeed, chemokines recruit leukocytes to existing thrombi. Therefore, chemokines contribute in many independent ways to thrombosis.

What's fishy about protamine? Clinical use, adverse reactions, and potential alternatives

Protamine, a highly basic protein isolated from salmon sperm, is the only clinically available agent to reverse the anticoagulation of unfractionated heparin. Following intravenous administration, protamine binds to heparin in a nonspecific electrostatic interaction to reverse its anticoagulant effects. In clinical use, protamine is routinely administered to reverse high-dose heparin anticoagulation in cardiovascular procedures, including cardiac surgery with cardiopulmonary bypass. Despite the lack of supportive evidence regarding protamine's effectiveness to reverse low-molecular-weight heparin, it is recommended in guidelines with low-quality evidence. Different dosing strategies have been reported for reversing heparin in cardiac surgical patients based on empiric dosing, pharmacokinetics, or point-of-care measurements of heparin levels. Protamine administration is associated with a spectrum of adverse reactions that range from vasodilation to life-threatening cardiopulmonary dysfunction and shock. The life-threatening responses appear to be hypersensitivity reactions due to immunoglobulin E and/or immunoglobulin G antibodies. However, protamine and heparin-protamine complexes can activate complement inflammatory pathways and inhibit other coagulation factors. Although alternative agents for reversing heparin are not currently available for clinical use, additional research continues evaluating novel therapeutic approaches.

Thrombocytosis and neutrophilia associated with oxygenator failure and protamine reaction after cardiopulmonary bypass: a case report and literature review

Thrombocytosis has been feared as a source of thrombotic complications during the conduct of cardiopulmonary bypass (CPB) for patients undergoing cardiac procedures. We present a patient urgently requiring repair/replacement of three heart valves that had preexisting myelofibrosis with thrombocytosis (platelet count of 800,000 per µl) and neutrophilia (40,000 per µl). Despite achieving an activated clotting time > 500 s with heparin and antithrombin concentrate administration prior to CPB, the pump oxygenator and reservoir demonstrated significant clot just prior to restoration of the patient's circulation. The patient subsequently suffered a severe protamine reaction that was successfully managed. A review of the literature of similar patients and the relevant cellular and biochemical mechanisms in this setting are presented, with potential therapeutic approaches to prevent such complications noted.

CXCL4 Exposure Potentiates TLR-Driven Polarization of Human Monocyte-Derived Dendritic Cells and Increases Stimulation of T Cells

Chemokines have been shown to play immune-modulatory functions unrelated to steering cell migration. CXCL4 is a chemokine abundantly produced by activated platelets and immune cells. Increased levels of circulating CXCL4 are associated with immune-mediated conditions, including systemic sclerosis. Considering the central role of dendritic cells (DCs) in immune activation, in this article we addressed the effect of CXCL4 on the phenotype and function of monocyte-derived DCs (moDCs). To this end, we compared innate and adaptive immune responses of moDCs with those that were differentiated in the presence of CXCL4. Already prior to TLR- or Ag-specific stimulation, CXCL4-moDCs displayed a more matured phenotype. We found that CXCL4 exposure can sensitize moDCs for TLR-ligand responsiveness, as illustrated by a dramatic upregulation of CD83, CD86, and MHC class I in response to TLR3 and TLR7/8-agonists. Also, we observed a markedly increased secretion of IL-12 and TNF-α by CXCL4-moDCs exclusively upon stimulation with polyinosinic-polycytidylic acid, R848, and CL075 ligands. Next, we analyzed the effect of CXCL4 in modulating DC-mediated T cell activation. CXCL4-moDCs strongly potentiated proliferation of autologous CD4+ T cells and CD8+ T cells and production of IFN-γ and IL-4, in an Ag-independent manner. Although the internalization of Ag was comparable to that of moDCs, Ag processing by CXCL4-moDCs was impaired. Yet, these cells were more potent at stimulating Ag-specific CD8+ T cell responses. Together our data support that increased levels of circulating CXCL4 may contribute to immune dysregulation through the modulation of DC differentiation.

Reversal agents in anaesthesia and critical care

Despite the advent of short and ultra-short acting drugs, an in-depth knowledge of the reversal agents used is a necessity for any anaesthesiologist. Reversal agents are defined as any drug used to reverse the effects of anaesthetics, narcotics or potentially toxic agents. The controversy on the routine reversal of neuromuscular blockade still exists. The advent of newer reversal agents like sugammadex have made the use of steroidal neuromuscular blockers like rocuronium feasible in rapid sequence induction situations. We made a review of the older reversal agents and those still under investigation for drugs that are regularly used in our anaesthesia practice.

Inactivation of heparin by cationically modified chitosan

This study was performed to evaluate the ability of N-(2-hydroxypropyl)-3-tri methylammonium chitosan chloride (HTCC), the cationically modified chitosan, to form biologically inactive complexes with unfractionated heparin and thereby blocking its anticoagulant activity. Experiments were carried out in rats in vivo and in vitro using the activated partial thromboplastin time (APTT) and prothrombin time (PT) tests for evaluation of heparin anticoagulant activity. For the first time we have found that HTCC effectively neutralizes anticoagulant action of heparin in rat blood in vitro as well as in rats in vivo. The effect of HTCC on suppression of heparin activity is dose-dependent and its efficacy can be comparable to that of protamine-the only agent used in clinic for heparin neutralization. HTCC administered i.v. alone had no direct effect on any of the coagulation tests used. The potential adverse effects of HTCC were further explored using rat experimental model of acute toxicity. When administered i.p. at high doses (250 and 500 mg/kg body weight), HTCC induced some significant dose-dependent structural abnormalities in the liver. However, when HTCC was administered at low doses, comparable to those used for neutralization of anticoagulant effect of heparin, no histopathological abnormalities in liver were observed.

Parenteral anticoagulants: Antithrombotic Therapy and Prevention of Thrombosis, 9th ed: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines

This article describes the pharmacology of approved parenteral anticoagulants. These include the indirect anticoagulants, unfractionated heparin (UFH), low-molecular-weight heparins (LMWHs), fondaparinux, and danaparoid, as well as the direct thrombin inhibitors hirudin, bivalirudin, and argatroban. UFH is a heterogeneous mixture of glycosaminoglycans that bind to antithrombin via a unique pentasaccharide sequence and catalyze the inactivation of thrombin, factor Xa, and other clotting enzymes. Heparin also binds to cells and plasma proteins other than antithrombin causing unpredictable pharmacokinetic and pharmacodynamic properties and triggering nonhemorrhagic side effects, such as heparin-induced thrombocytopenia (HIT) and osteoporosis. LMWHs have greater inhibitory activity against factor Xa than thrombin and exhibit less binding to cells and plasma proteins than heparin. Consequently, LMWH preparations have more predictable pharmacokinetic and pharmacodynamic properties, have a longer half-life than heparin, and are associated with a lower risk of nonhemorrhagic side effects. LMWHs can be administered once daily or bid by subcutaneous injection, without coagulation monitoring. Based on their greater convenience, LMWHs have replaced UFH for many clinical indications. Fondaparinux, a synthetic pentasaccharide, catalyzes the inhibition of factor Xa, but not thrombin, in an antithrombin-dependent fashion. Fondaparinux binds only to antithrombin. Therefore, fondaparinux-associated HIT or osteoporosis is unlikely to occur. Fondaparinux exhibits complete bioavailability when administered subcutaneously, has a longer half-life than LMWHs, and is given once daily by subcutaneous injection in fixed doses, without coagulation monitoring. Three additional parenteral direct thrombin inhibitors and danaparoid are approved as alternatives to heparin in patients with HIT.

Neutralization of heparin activity

Heparin is the mainstay in the treatment and prevention of thrombosis in such diverse clinical settings as venous thromboembolism, acute coronary syndrome, cardiopulmonary bypass, and hemodialysis. However, the major complication of heparin - like that of all anticoagulants - is bleeding. Heparin may need to be reversed in the following settings: clinically significant bleeding; prior to an invasive procedure; at the conclusion of a procedure involving extracorporeal circulation (e.g., cardiopulmonary bypass, dialysis). This chapter discusses protamine sulfate, as well as several other agents that are able to neutralize heparin, including their pharmacological properties, indications, dosing, and efficacy.

The role of the CXC chemokines platelet factor-4 (CXCL4/PF-4) and its variant (CXCL4L1/PF-4var) in inflammation, angiogenesis and cancer

Chemokines are chemotactic cytokines which recruit leukocytes to inflammatory sites. They also affect tumor development and metastasis by acting as growth factor, by attracting pro- or anti-tumoral leukocytes or by influencing angiogenesis. Platelet factor-4 (CXCL4/PF-4) was the first chemokine shown to inhibit angiogenesis. CXCL4L1/PF-4var, recently isolated from thrombin-stimulated platelets, differing from authentic CXCL4/PF-4 in three carboxy-terminally located amino acids, was found to be more potent than CXCL4/PF-4 in inhibiting angiogenesis and tumor growth. Both glycosaminoglycans (GAG) and CXCR3 are implicated in the activities of the PF-4 variants. This report reviews the current knowledge on the role of CXCL4/PF-4 and CXCL4L1/PF-4var in physiological and pathological processes. In particular, the role of CXCL4/PF-4 in cancer, heparin-induced thrombocytopenia and atherosclerosis is described.

Functional divergence between 2 chemokines is conferred by single amino acid change

CXCL4 and CXCL4L1 are 2 closely related CXC chemokines that exhibit potent antiangiogenic activity. Because interactions with glycosaminoglycans play a crucial role in chemokines activity, we determined the binding parameters of CXCL4 and CXCL4L1 for heparin, heparan sulfate, and chondroitin sulfate B. We further demonstrated that the Leu67/His67 substitution is critical for the decrease in glycan binding of CXCL4L1 but also for the increase of its angiostatic activities. Using a set of mutants, we show that glycan affinity and angiostatic properties are not completely related. These data are reinforced using a monoclonal antibody that specifically recognizes structural modifications in CXCL4L1 due to the presence of His67 and that blocks its biologic activity. In vivo, half-life and diffusibility of CXCL4L1 compared with CXCL4 is strongly increased. As opposed to CXCL4L1, CXCL4 is preferentially retained at its site of expression. These findings establish that, despite small differences in the primary structure, CXCL4L1 is highly distinct from CXCL4. These observations are not only of great significance for the antiangiogenic activity of CXCL4L1 and for its potential use in clinical development but also for other biologic processes such as inflammation, thrombosis or tissue repair.

In vivo neutralization of unfractionated heparin and low-molecular-weight heparin by a novel salicylamide derivative

Unfractionated heparin (UFH) and low-molecular-weight heparin (LMWH) are widely used anticoagulants for surgical and interventional use. Currently, the anticoagulant and bleeding effects of heparin are neutralized by protamine sulfate. There are several problems associated with the use of protamine sulfate, including allergic reactions, cardiovascular effects, heparin rebound, and incomplete neutralization of LMWHs. The objective of this investigation is to characterize the effectiveness of a novel salicylamide-derived heparin antagonist, PMX 60056, in neutralizing the antithrombotic and bleeding effects of UFH and LMWHs. Animals were first anticoagulated using an intravenous injection of UFH or LMWH followed by protamine sulfate or PMX 60056. Established animal models of hemorrhage (rat-tail transection) and thrombosis (jugular vein clamping) were then performed. Blood samples were collected for ex vivo analysis using activated partial thromboplastin time (aPTT), anti-Xa, and anti-IIa assays. We demonstrate that PMX 60056 neutralized the antithrombotic, anticoagulant, and bleeding effects of heparins as effectively as protamine sulfate and may be slightly more efficacious against LMWHs. These results suggest that PMX 60056 may provide an improved approach for the neutralization of UFH and LMWHs.

Strategies to prevent intraoperative lung injury during cardiopulmonary bypass

During open heart surgery the influence of a series of factors such as cardiopulmonary bypass (CPB), hypothermia, operation and anaesthesia, as well as medication and transfusion can cause a diffuse trauma in the lungs. This injury leads mostly to a postoperative interstitial pulmonary oedema and abnormal gas exchange. Substantial improvements in all of the above mentioned factors may lead to a better lung function postoperatively. By avoiding CPB, reducing its time, or by minimizing the extracorporeal surface area with the use of miniaturized circuits of CPB, beneficial effects on lung function are reported. In addition, replacement of circuit surface with biocompatible surfaces like heparin-coated, and material-independent sources of blood activation, a better postoperative lung function is observed. Meticulous myocardial protection by using hypothermia and cardioplegia methods during ischemia and reperfusion remain one of the cornerstones of postoperative lung function. The partial restoration of pulmonary artery perfusion during CPB possibly contributes to prevent pulmonary ischemia and lung dysfunction. Using medication such as corticosteroids and aprotinin, which protect the lungs during CPB, and leukocyte depletion filters for operations expected to exceed 90 minutes in CPB-time appear to be protective against the toxic impact of CPB in the lungs. The newer methods of ultrafiltration used to scavenge pro-inflammatory factors seem to be protective for the lung function. In a similar way, reducing the use of cardiotomy suction device, as well as the contact-time between free blood and pericardium, it is expected that the postoperative lung function will be improved.

Dual effect of Platelet Factor 4 on the activities of Factor Xa

Platelet Factor 4 (PF4) prevents inhibition of blood coagulation proteases by heparin via formation of a putative enzyme-PF4 complex. To investigate the contribution of the latter, the activity of factor Xa (fXa) was determined in chromogenic assays measuring hydrolysis of a peptide substrate S2765 or cleavage of the macromolecular substrate prothrombin in the activating complex, prothrombinase. Upon preincubation with fXa and heparin, PF4 at about 250 nM decreased the k(cat) of S2765 hydrolysis about fivefold and that of prothrombin activation about 25-fold. In the presence of saturating fVa, inhibition of fXa by PF4 was abolished, while in the presence of limiting fVa, PF4 altered the interaction of fXa with fVa. Interestingly, high concentrations of PF4 restored fXa activity toward S2765 and prothrombin, indicating a dual effect of PF4 on fXa activities. These findings suggest that PF4 in the presence of heparin is an allosteric effector of the prothrombinase complex.

Efficiency of Prostacyclin in the Treatment of Protamine-Mediated Right Ventricular Failure and Acute Pulmonary Hypertension

Protamine is used after cardiopulmonary bypass was stopped in order to reverse the anticoagulant effects of heparin administered during open-heart operations. Adverse hemodynamic responses to protamine are common, ranging from minor perturbations to cardiovascular collapse. The aim of the present study was to investigate whether a prostacyclin is effective in the treatment of protamine-mediated acute pulmonary hypertension and right ventricular failure in the perioperative period of isolated coronary artery bypass grafting (CABG) operations. In sixty-eight (1.78%) of 3800 patients who underwent isolated CABG, acute pulmonary hypertension and right ventricular failure developed during or following the protamine infusion. These 68 patients were included in the study and were randomized into two groups. Thirty-eight of the patients received prostaglandin I(2) (PGI(2)), norepinephrine and dopamine (PGI(2) group), whereas 30 patients received nitroglycerin, norepinephrine and dopamine (control group). Hemodynamic data were recorded before and after the above drug combinations. The mean value of left ventricle ejection fraction significantly increased (p < 0.05) and mean values of central venous pressure, pulmonary artery systolic and diastolic pressure, pulmonary capillary wedge pressure and pulmonary vascular resistance significantly decreased (p < 0.05) in the PGI(2) group. The mean value of pulmonary capillary wedge pressure significantly decreased (p < 0.05) and the mean value of central venous pressure significantly increased (p < 0.05) in the control group. In conclusion, prostacyclin (PGI(2)) is effective in the treatment of protamine-mediated acute pulmonary hypertension and right ventricular failure in the perioperative period in isolated CABG operations. This finding may be an important contribution to the treatment of severe protamine complications during open-heart operations.

Heparin and Low-Molecular-Weight Heparin

This article about unfractionated heparin (UFH) and low-molecular-weight heparin (LMWH) is part of the Seventh American College of Chest Physicians Conference on Antithrombotic and Thrombolytic Therapy: Evidence-Based Guidelines. UFH is a heterogeneous mixture of glycosaminoglycans that bind to antithrombin via a pentasaccharide, catalyzing the inactivation of thrombin and other clotting factors. UFH also binds endothelial cells, platelet factor 4, and platelets, leading to rather unpredictable pharmacokinetic and pharmacodynamic properties. Variability in activated partial thromboplastin time (aPTT) reagents necessitates site-specific validation of the aPTT therapeutic range in order to properly monitor UFH therapy. Lack of validation has been an oversight in many clinical trials comparing UFH to LMWH. In patients with apparent heparin resistance, anti-factor Xa monitoring may be superior to measurement of aPTT. LMWHs lack the nonspecific binding affinities of UFH, and, as a result, LMWH preparations have more predictable pharmacokinetic and pharmacodynamic properties. LMWHs have replaced UFH for most clinical indications for the following reasons: (1) these properties allow LMWHs to be administered subcutaneously, once daily without laboratory monitoring; and (2) the evidence from clinical trials that LMWH is as least as effective as and is safer than UFH. Several clinical issues regarding the use of LMWHs remain unanswered. These relate to the need for monitoring with an anti-factor Xa assay in patients with severe obesity or renal insufficiency. The therapeutic range for anti-factor Xa activity depends on the dosing interval. Anti-factor Xa monitoring is prudent when administering weight-based doses of LMWH to patients who weigh > 150 kg. It has been determined that UFH infusion is preferable to LMWH injection in patients with creatinine clearance of < 25 mL/min, until further data on therapeutic dosing of LMWHs in renal failure have been published. However, when administered in low doses prophylactically, LMWH is safe for therapy in patients with renal failure. Protamine may help to reverse bleeding related to LWMH, although anti-factor Xa activity is not fully normalized by protamine. The synthetic pentasaccharide fondaparinux is a promising new antithrombotic agent for the prevention and treatment of venous thromboembolism.

Transgenic mice studies demonstrate a role for platelet factor 4 in thrombosis: dissociation between anticoagulant and antithrombotic effect of heparin

The platelet-specific chemokine platelet factor 4 (PF4) is released in large amounts at sites of vascular injury. PF4 binds to heparin with high affinity, but its in vivo biologic role has not been defined. We studied the role of PF4 in thrombosis using heterozygote and homozygote PF4 knock-out mice (mPF4(+/-) and mPF4(-/-), respectively) and transgenic mice overexpressing human PF4 (hPF4(+)). None of these lines had an overt bleeding diathesis, but in a FeCl(3) carotid artery thrombosis model, all showed impaired thrombus formation. This defect in thrombus formation in the mPF4(-/-) animals was corrected by infusing hPF4 over a narrow concentration range. The thrombotic defect in the mPF4(+/-) and mPF4(-/-) animals was particularly sensitive to infusions of the negatively charged anticoagulant heparin. However, the same amount of heparin paradoxically normalized thrombus formation in the hPF4(+) animals, although these animals were anticoagulated systemically. Upon infusion of the positively charged protein, protamine sulfate, the reverse was observed with mPF4(+/-) and mPF4(-/-) animals having improved thrombosis, with the hPF4(+) animals having worsened thrombus formation. These studies support an important role for PF4 in thrombosis, and show that neutralization of PF4 is an important component of heparin's anticoagulant effect. The mechanisms underlying these observations of PF4 biology and their clinical implications remain to be determined.

Structural insight into how an anti-idiotypic antibody against D3H44 (anti-tissue factor antibody) restores normal coagulation

6A6 is a murine monoclonal antibody raised against the humanized anti-tissue factor antibody D3H44. 6A6 is able to completely neutralize the anticoagulant activity of D3H44 in tissue factor-dependent functional assays, such as endotoxin-induced whole blood clotting, prothrombin time, as well as factor X and factor IX activation. ELISA-type assays further showed that 6A6 binds to an epitope with critical determinants on the V(L) domain of D3H44. The possibility that the anti-idiotypic 6A6 might carry an "internal image" of the original antigen (tissue factor) was examined using the X-ray structure of the 6A6-Fab/D3H44-Fab complex determined at 2.5A resolution. We find that 6A6 structurally mimics tissue factor only so far as it combines with the antigen recognition surface of D3H44. While 6A6 contacts both V(L) and V(H) domains of D3H44, as does tissue factor, there is more contact with the D3H44 V(L) domain and less with the D3H44 V(H) domain relative to the tissue factor contacts on D3H44. Additionally, there is an almost total lack of correspondence between 6A6 and tissue factor at the level of amino acid side-chain functional groups. Despite the fact that both tissue factor and 6A6 are composed largely of beta-sheets, they present fundamentally different elements of secondary structure to D3H44; tissue factor presents beta-sheets edge-on, while 6A6 uses mostly loops. Finally, the finding that 6A6 competes with tissue factor for D3H44 binding raises the possibility of using 6A6 as an antidote for D3H44 anticoagulant therapy. To this end, we constructed a chimeric murine/human 6A6-Fab, which effectively neutralized D3H44 and fully restored tissue factor function in enzymatic assays.

Platelet factor 4 enhances generation of activated protein C in vitro and in vivo

Platelet factor 4 (PF4), an abundant platelet alpha-granule protein, accelerates in vitro generation of activated protein C (APC) by soluble thrombin/thrombomodulin (TM) complexes up to 25-fold. To test the hypothesis that PF4 similarly stimulates endothelium-associated TM, we assessed the influence of human PF4 on thrombin-dependent APC generation by cultured endothelial monolayers. APC generated in the presence of 1 to 100 microg PF4 was up to 5-fold higher than baseline for human umbilical vein endothelial cells, 10-fold higher for microvascular endothelial cells, and unaltered for blood outgrowth endothelial cells. In an in vivo model, cynomolgus monkeys (n = 6, each serving as its own control) were infused with either PF4 (7.5 mg/kg) or vehicle buffer, then with human thrombin (1.0 microg/kg/min) for 10 minutes. Circulating APC levels (baseline 3 ng/mL) peaked at 10 minutes, when PF4-treated and vehicle-treated animals had APC levels of 67 +/- 5 ng/mL and 39 +/- 2 ng/mL, respectively (P <.001). The activated partial thromboplastin time (APTT; baseline, 28 seconds) increased maximally by 27 +/- 6 seconds in PF4-treated animals and by 9 +/- 1 seconds in control animals at 30 minutes (P <.001). PF4-dependent increases in circulating APC and APTT persisted more than 2-fold greater than that of controls from 10 through 120 minutes (P < or =.04). All APTT prolongations were essentially reversed by monoclonal antibody C3, which blocks APC activity. Thus, physiologically relevant concentrations of PF4 stimulate thrombin-dependent APC generation both in vitro by cultured endothelial cells and in vivo in a primate thrombin infusion model. These findings suggest that PF4 may play a previously unsuspected physiologic role in enhancing APC generation.

Influence of patient characteristics and renal function on factor Xa inhibition pharmacokinetics and pharmacodynamics after enoxaparin administration in non-ST-segment elevation acute coronary syndromes

BACKGROUND

The current standard of care for patients with non-ST-segment elevation acute coronary syndromes (ACS) includes antithrombotic therapy with aspirin and heparin. Although emerging data suggest that low-molecular weight preparations offer distinct advantages over unfractionated heparin, limited information on patient-related factors that may influence dosing, safety, and efficacy is available.

PURPOSE

The purpose of our study was the determination of the impact of patient age, sex, body weight, and renal function on factor Xa inhibition pharmacokinetics and pharmacodynamics after enoxaparin administration in patients with ACS.

METHODS AND RESULTS

Patients enrolled in the TIMI 11A trial received a full complement of antiischemic therapy, aspirin, and enoxaparin (30 mg intravenously, followed by weight-adjusted doses of either 1 mg/kg or 1.25 mg/kg subcutaneously every 12 hours). Before and after the third and last doses, blood samples were obtained from 445 patients for measurement of anti-Xa activity. The mean apparent clearance, distribution volume, and plasma half-life were 0.733 L/h, 5.24 L, and 5 hours, respectively. Among a wide range of clinical and laboratory covariates, creatinine clearance emerged as the most influential factor on apparent clearance, area under the curve, and anti-Xa activity. Patients with marked renal impairment (creatinine clearance <40 mL/min) had higher trough and peak anti-Xa activity compared with those with normal renal function and were more likely to have major hemorrhagic events.

CONCLUSION

The pharmacokinetic and pharmacodynamic profiles after enoxaparin administration are consistent across a broad range of patients with ACS. Dose adjustments or anti-Xa coagulation monitoring or both will be necessary rarely in routine clinical practice, with the exception of patients with severe renal insufficiency.

The systemic vasodilatory action of protamine: is it inhibited or mediated by heparin?

The administration of protamine to neutralize the circulating heparin is common practice in cardiovascular surgery. The use of this drug is sometimes associated with hemodynamic alterations of varying degree and intensity (systemic hypotension, pulmonary hypertension and even cardiogenic shock). An intrinsic action of protamine has been suggested to be the cause of these vascular reactions. This action is blocked when protamine forms a complex with heparin, although in other cases it appears that the heparin-protamine complex is the factor responsible for these hemodynamic alterations. The aim of this experimental study was to characterize the vasodilatory action of protamine on the systemic circulation, determining whether or not it is dose-dependent; to analyze the role of endothelium; and to evaluate whether this vasodilatory effect is modified by the presence of heparin.

MATERIALS AND METHODS

The abdominal aorta was dissected from eight New Zealand rabbits and then sectioned into vascular rings for study in an organ chamber. Mechanical disruption of endothelium was performed on some rings (n = 14). Once submaximal contraction was reached (ClK 80 mM), protamine sulfate with a final concentration in the organ chamber of 80-400 micrograms/ml was added to one of the groups (n = 12). In the second group (n = 12), equal concentrations of protamine were tested in the presence of heparin at a final concentration of 100 U/ml.

RESULTS

The mean vasodilatation reached in the group of rings exposed only to protamine was 95.4 +/- 1.5% with respect to the submaximal contraction induced with ClK. In the second study group, the rings were exposed to protamine at equally increasing concentrations (80-400 micrograms/ml) but with the presence of heparin in the organ chamber. The mean vasodilatation in this group was 90 +/- 1.5. No statistically significant differences in vasodilatation were found between this group and the protamine without heparin group. On the other hand, in the endothelium-denuded rings (n = 14) exposed to isolated protamine and to protamine-heparin, no vasodilatory response was observed.

CONCLUSION

Our results show that the administration in vitro of protamine induces endothelium-dependent vasodilatation of the systemic circulation. Likewise, this relaxing effect mediated through endothelium is not blocked when protamine forms a complex with heparin in comparable concentrations of both drugs. Based on these preliminary findings, we believe that in high-risk patients the prevention of systemic vasodilatation and cardiovascular collapse produced by protamine should move towards the use of other substances that can neutralize the anticoagulant effect of heparin or towards pre-medication guidelines that prevent these secondary effects in the case of protamine administration.

Heparin neutralization with methylene blue, hexadimethrine, or vancomycin after cardiopulmonary bypass

There are no clinically available alternatives for reversing heparin in protamine-allergic patients. This study examined the ability of methylene blue, hexadimethrine, and vancomycin to reverse circulating heparin so that these compounds can be carefully examined in future placebo-controlled studies in humans. Heparin activity in blood obtained from extracorporeal circuits was reversed by adding protamine (13.5, 27.0, 81.1, 135.1, and 270.3 micrograms/mL), methylene blue (13.5, 27.0, 135.1, 202.7, 270.3, 337.8, 405.4, 473.0, 540.5, and 810.8 micrograms/mL), hexadimethrine (6.8, 13.5, 20.3, 27.0, 81.1, and 135.1 micrograms/mL), or vancomycin (13.5, 27.0, 135.1, 270.3, 540.5, and 810.8 micrograms/mL), and activated clotting times (ACTs) were measured with kaolin (n = 18). Heparinase-ACT was obtained to determine complete reversal. Heparin concentrations were 3.3 +/- 0.3 U/mL with ACT values of 485 +/- 97 s. The ACT at a protamine concentration of 81.1 micrograms/mL and at hexadimethrine concentrations of 81.1 and 135.1 micrograms/mL was not statistically different from heparinase-ACT; however, methylene blue or vancomycin did not reverse the anticoagulation at any concentrations. Hexadimethrine can reverse heparin-induced anticoagulation after cardiopulmonary bypass as well as protamine, although methylene blue or vancomycin did not neutralize heparin in vitro.

Restoration of the normal coagulation process: advances in therapies to antagonize heparin

A number of naturally occurring anticoagulants exist that preserve normal blood fluidity and limit blood clot formation to vascular injury sites, thus acting as regulators of hemostasis. The protein C/protein S pathway is one system that acts to modulate thrombin formation. The activation of protein C by thrombin is accelerated more than 1,000-fold at the endothelial surface by thrombomodulin localized on the endothelial cell. Activated protein C then binds to its co-factor, protein S, and the protein C/protein S complex exerts its antithrombotic function by inactivating the coagulation factors Va and VIIIa. Patients deficient in protein C and protein S may be particularly vulnerable to thrombotic events after cardiac surgery. In addition, several studies suggest that reductions in protein C and protein S concentrations, as well as thrombomodulin, occur during cardiopulmonary bypass (CPB). The possibility of a low anticoagulant potential when heparinization is reversed may be an important factor in the subsequent morbidity associated with thrombotic complications. Aprotinin is a serine protease inhibitor that in vitro binds competitively with the serine protease-activated protein C. However, aprotinin in the clinical setting has not been reported to alter levels of protein C in patients undergoing CPB. Reversal of the heparinization needed for CPB is almost universally performed with protamine. However, protamine has many deleterious effects. Recombinant platelet factor 4 (rPF4) has been proposed as an alternative to protamine. We investigated the effective heparin neutralization dose of rPF4 vs. the standard agent protamine in human blood activated through exposure to the CPB circuit. Activated clotting time (ACT) measurements suggested a 2:1 (w/w) reversal ratio for rPF4 and protamine. The first human open-label phase 1 trial of rPF4 reported no serious side effects and no important hemodynamic effects. Doses of 2.5 and 5.0 mg/kg were uniformly effective in reversing the anticoagulant effect of heparin and reducing the ACT to <200 s by 5 min after administration. Repeated monitoring of the ACT did not detect a rebound effect of heparin.

The clinical experience with antiangiogenic agents

Antiangiogenic agents have been recently recognized to be potentially useful in the treatment of malignant processes. There has been a resulting flood of these agents into the clinical research field, forcing researchers to consider the unique problems which these agents pose. As these are agents with a novel, angiostatic mechanism of action, new clinical trial designs may become necessary to move these agents from the phase 1 level on to clinical use. Currently, several of these agents are being or have been tested in clinical trials. In this article, we review the available clinical data with antiangiogenic agents from other investigators, and our own experience with pentosan polysulfate.