A chemically-modified inactive antithrombin as a potent antagonist of fondaparinux and heparin anticoagulant activity

BACKGROUND

Heparin and its analogs, mediating their anticoagulant activity through antithrombin (AT) activation, remain largely used for the preventive and curative treatment of thrombosis. The major adverse reaction of these drugs is the bleeding risk associated with overdose. Unfractionnated heparin (UFH) can be efficiently and rapidly neutralized by protamine sulfate, but this reversal partially neutralizes low-molecular-weight heparin (LMWH) and is inefficient in reversing fondaparinux. To secure administration of AT-mediated anticoagulants and counteract bleeding disorders, we previously designed a recombinant inactive AT as an antidote to heparin derivatives.

OBJECTIVES

To get around the limited production level of recombinant AT, we propose in this study an alternative strategy to produce a chemically modified inactive AT, exhibiting increased heparin affinity, as an antagonist of heparin analogs.

METHODS

Plasma-derived AT was chemically modified with 2,3 butanedione, a diketone known to specifically react with the arginine side chain. The chemical reaction was conducted in the presence of heparin to preserve basic residues within the heparin binding site from modifications.

RESULTS

AT treated by butanedione and selected for its high heparin affinity (AT-BD) was indeed modified on reactive Arg393 and thus exhibited decreased anticoagulant activity and increased heparin affinity. AT-BD was able to neutralize anticoagulant activity of heparin derivatives in vitro and in vivo and was devoid of intrinsic anticoagulant activity, as assessed by activated partial thromboplastin time assay.

CONCLUSIONS

AT-BD appears to be as efficient as protamine to neutralize UFH in vivo but could be more largely used because it also reverses fondaparinux and LMWH.

A magnetofluorescent nanoparticle for ex-vivo cell labeling by covalently linking the drugs protamine and Feraheme

We synthesized a nanoparticle (NP) for ex-vivo cell labeling and MRI tracking by covalently coupling the C-terminus of a rhodamine-labeled protamine (ProRho) to Feraheme (FH) in order to yield the nanoparticle denoted ProRho-FH. Since protamine can adsorb to certain charged surfaces, we confirmed a covalent interaction between ProRho and FH by heparin affinity chromatography. ProRho-FH lacks a net charge (zeta potential approximately 0) due to the combination of negative FH and positive ProRho charges. ProRho-FH was readily internalized by U87 cells and mouse mesenchymal stem cells as determined by FACS and MR relaxometry. Finally, some 4,000 stem cells were implanted in a mouse brain and imaged by MRI. Due to its lack of net surface charge, ProRho-FH relies on the internalizing properties of the surface guanidinium groups present in the arginine-rich protamine to induce NP uptake. ProRho-FH is a unique cell-labeling agent due to its synthesis using two approved drugs, magnetofluorescence, site-specific covalent attachment chemistry, and lack of surface charge.

Supramolecular protamine/Gd-loaded liposomes adducts as relaxometric protease responsive probes

A new approach to enzyme-responsive MRI agents based on the use of liposomes loaded with a high number of paramagnetic metal complexes (Gd-HPDO3A) is presented. It relies on the disruption of low relaxivity aggregates formed by liposomes and a macromolecular substrate that is selectively cleaved by the enzyme of interest. The interaction of anionic liposomes composed of POPC:CHOL:DPGS and the cationic protein protamine yields a poorly soluble supramolecular assembly endowed with a low relaxivity. The action of the serine protease trypsin causes the digestion of protamine and the consequent de-assembly of the supramolecular adduct. The process is accompanied by an overall relaxation enhancement of solvent water protons as consequence of the dissolution of the aggregated liposomes. The observed increase of relaxivity is linearly dependent on the enzyme concentration. An illustrative example of the possible use of the herein presented responsive agent has been reported. It consists of the entrapment of the supramolecular assembly in alginate microcapsules that have often been used as envelopes for in vivo applications of stem cells and pancreatic islets. The change in the observed longitudinal relaxation rate R(1) (leading to an hyperintense signal in the corresponding MR images) may act as a sensor of the protease activity in the biological environment in which the capsules is located.

Parviz Lalezari: an autobiography

Doctor Parviz Lalezari, currently a clinical professor of Medicine and Pathology at Albert Einstein College of Medicine in New York, describes highlights of his research career since 1958. He became the director of the blood bank at Montefiore Hospital in New York City in 1961, director of the Division of Immunohematology until 1996, and then until 2001, was President and chief executive officer of the Bergen Community Regional Blood Center in New Jersey. Doctor Lalezari was born in Iran in 1931, and after graduation from Medical School, he came to the United States in 1956. His initial research was on leukocyte antibodies. After modifying the available antibody detection techniques, he discovered that like hemolytic disease of the newborn and neonatal immune thrombocytopenia, fetal-maternal neutrophil incompatibility can cause neonatal neutropenia. He identified the targets of these antibodies and showed that they were expressed only on peripheral blood neutrophils. Doctor Lalezari also discovered that a common form of neutropenia in early childhood was caused by development of autoantibodies, which surprisingly were directed against the same neutrophil-specific antigens involved in fetal-maternal incompatibility. In 1959, a heparin-neutralizing drug (Polybrene) was introduced to be used after open-heart surgery. Lalezari discovered that Polybrene, a quaternary ammonium polymer, reacted with sialic acid molecules on the red blood cell (RBC) surface, causing the RBCs to aggregate. Later, realizing that the repelling forces generated by the RBC surface membrane charges were responsible for failure of the small IgG antibody molecules to agglutinate the RBCs, he used Polybrene to neutralize the RBC surface negative charge to allow the IgG antibody molecules to induce hemagglutination. This became The Polybrene test, which is to be used in RBC antibody detection.

Strategies for improving the functionality of an affinity bioreactor

Heparin employed in extracorporeal blood circulation (ECBC) procedures (e.g. open heart operations) often leads to a high incidence of bleeding complications. Protamine employed in heparin neutralization, on the other hand, can cause severe adverse reactions. We previously developed an approach that could prevent both heparin- and protamine-induced toxic side effects concomitantly. This approach consisted of placing a hollow fiber-based bioreactor device containing immobilized protamine (termed a "protamine bioreactor") at the distal end of the ECBC procedure. This protamine bioreactor would remove heparin after heparin served its anticoagulant purpose in the ECBC device, thereby eliminating heparin-induced bleeding risks. In addition, this protamine bioreactor would prevent protamine from entering the patients, thereby aborting any protamine-induced toxic effects. Both in vitro and in vivo studies have successfully demonstrated the feasibility of this approach. Despite promises, early findings also revealed two shortcomings that must be overcome for the protamine bioreactor to be applied clinically. The first drawback was that the cyanate ester linkages, involved in conjugating protamine to the bioreactor device, were unstable and prone to hydrolysis, resulting in the leakage of a significant amount of protamine into circulation during application of the protamine bioreactor. The second deficiency was that the capacity of the protamine bioreactor in heparin removal was rather low, owing to the limited surface area of the hollow fibers for protamine immobilization and subsequently heparin adsorption. In this paper, we present novel strategies to overcome these two limitations. A new conjugation method based on the use of 4-(oxyacetyl)phenoxyacetic acid (OAPA) as the activating reagent was employed to yield stable linkages, via the abundant arginine residues of protamine, onto the hollow fibers. Results showed that while the amount of protamine immobilized on each gram of fibers was relatively comparable between the OAPA and the previous CNBr activation methods (7.45 mg/g versus 7.69 mg/g fibers), there was virtually no detectable leaching of immobilized protamine from the bioreactor by the OAPA method, comparing to 35% leaching of protamine by the previous CNBr method following 72 h of storage of the bioreactor in PBS buffer at 37 degrees C. To improve the capacity and functionality of the protamine bioreactor, two novel approaches were adopted. Long chain and high molecular weight poly-lysine was linked to the hollow fibers, prior to protamine coupling, to create multiple layers of immobilized protamine for subsequent heparin adsorption. In addition, a poly(ethylene glycol) (PEG) chain was inserted between protamine and the hollow fibers to yield a three-dimensional, free dynamic motion for immobilized protamine. Preliminary observations indicated that a four- to five-fold enhancement in heparin adsorption was attained by utilizing each of these new approaches. Aside from their current use, these new strategies can also be employed generically to improve the functionality of any affinity-type bioreactor. Indeed, efforts have been made recently in utilizing these approaches to develop a clinically usable GPIIb/IIIa bioreactor for the treatment of immune thrombocytopenic purpura (ITP)-an autoimmune disease.

Structural Basis for Antagonism by Suramin of Heparin Binding to Vaccinia Complement Protein,

Suramin is a competitive inhibitor of heparin binding to many proteins, including viral envelope proteins, protein tyrosine phosphatases, and fibroblast growth factors (FGFs). It has been clinically evaluated as a potential therapeutic in treatment of cancers caused by unregulated angiogenesis, triggered by FGFs. Although it has shown clinical promise in treatment of several cancers, suramin has many undesirable side effects. There is currently no experimental structure that reveals the molecular interactions responsible for suramin inhibition of heparin binding, which could be of potential use in structure-assisted design of improved analogues of suramin. We report the structure of suramin, in complex with the heparin-binding site of vaccinia virus complement control protein (VCP), which interacts with heparin in a geometrically similar manner to many FGFs. The larger than anticipated flexibility of suramin manifested in this structure, and other details of VCP-suramin interactions, might provide useful structural information for interpreting interactions of suramin with many proteins.

New protamine quantification method in microtiter plates using o-phthaldialdehyde/N-acetyl-l-cysteine reagent

Protamine is a well-known excipient in pharmaceutics. It represents a peptide consisting of exclusive aliphatic amino acids, hence it cannot be quantified by UV-spectroscopy (lambdamax 280 nm). A new and sensitive quantification method based on the derivatisation of protamine with ortho-phthaldialdehyde (OPA) in the presents of 2-mercaptoethanol (ME) or N-acetyl-L-cysteine (NAC) in basic aqueous solution using 96-well microtiter plates are introduced in this report. The resulting isoindol derivatives reveal a fluorescence excitation (maximum lambdaex 345 nm) and emission (maximum lambdaem 450 nm) spectra. Derivatives of OPA/NAC reagent were found to be useful for protamine quantification in pharmaceutical nanoparticle preparation containing DNA. A sufficient stability of the isoindol derivatives was shown. It was possible to determine protamine free base, protamine sulphate and protamine chloride with limits of detection less than 1.1 microg/ml.

Dextran sulfate included in factor Xa assay reagent overestimates heparin activity in patients after heparin reversal by protamine

A lack of correlation between activated partial thromboplastin time (aPTT), thrombin time (TT) and anti-factor Xa (AXa) activity was observed in patients after cardiac surgery with cardiopulmonary bypass (CBP). Indeed, AXa activity measured by the chromogenic assay, Coamatic Heparin, was higher than expected with regard to results obtained in coagulation assays. To account for this discrepancy, another AXa chromogenic assay was tested. First, AXa activity was measured with two chromogenic assays (Coamatic Heparin and Rotachrom Heparin) in plasma samples of 25 patients undergoing cardiac surgery at two time points after heparin reversal by protamine. AXa activity was significantly higher when measured with Coamatic Heparin than with Rotachrom Heparin in samples collected just after protamine infusion (p<0.01). Next, since Coamatic( Heparin contains dextran sulfate (DXS) to reduce the influence of heparin antagonists such as platelet factor 4 (PF4), whereas Rotachrom Heparin does not, we hypothesized that the dextran sulfate contained in the reagent might explain this discrepancy. We therefore performed in vitro studies consisting in neutralizing unfractionated heparin (UFH) with protamine and measuring AXa activity with the two chromogenic assays. An AXa activity was still measurable with Coamatic Heparin after neutralization, thus strongly suggesting that dextran sulfate dissociates protamine/heparin complexes. We conclude that Coamatic Heparin assays should be avoided when measuring AXa activity in plasma samples immediately after protamine infusion, as inaccurate results may lead to inadequate management of heparin reversal.

Novel concatameric heparin-binding peptides reverse heparin and low-molecular-weight heparin anticoagulant activities in patient plasma in vitro and in rats in vivo

Patients given unfractionated heparin (UFH) or low-molecular-weight heparin (LMWH) for prophylaxis or treatment of thrombosis sometimes suffer serious bleeding. We showed previously that peptides containing 3 or more tandem repeats of heparin-binding consensus sequences have high affinity for LMWH and neutralize LMWH (enoxaparin) in vivo in rats and in vitro in citrate. We have now modified the (ARKKAAKA)n tandem repeat peptides by cyclization or by inclusion of hydrophobic tails or cysteines to promote multimerization. These peptides exhibit high-affinity binding to LMWH (dissociation constant [Kd], ≈ 50 nM), similar potencies in neutralizing anti–Factor Xa activity of UFH and enoxaparin added to normal plasma in vitro, and efficacy equivalent to or greater than protamine. Peptide (ARKKAAKA)3VLVLVLVL was most effective in all plasmas from enoxaparin-treated patients, and was 4- to 20-fold more effective than protamine. Several other peptide structures were effective in some patients' plasmas. All high-affinity peptides reversed inhibition of thrombin-induced clot formation by UFH. These peptides (1 mg/300 g rat) neutralized 1 U/mL anti–Factor Xa activity of enoxaparin in rats within 1 to 2 minutes. Direct blood pressure and heart rate measurements showed little or no hemodynamic effect. These heparin-binding peptides, singly or in combination, are potential candidates for clinical reversal of UFH and LMWH in humans.

Human antithrombin III-derived heparin-binding peptide, a novel heparin antagonist

In the blood coagulation cascade, human antithrombin III (hAT III) acts as an inhibitor of serine proteases such as thrombin and factor Xa, and this anticoagulatory glycoprotein requires the binding of heparin for its activation. In this study, we synthesized the polypeptides corresponding to the proposed heparin-binding sites including the (41-49), (286-301) and (123-139) regions of hAT III, and examined their interactions with heparin by means of physicochemical and biochemical methods. All the synthetic peptides had a high affinity toward heparin, evidenced by the fact that they were eluted from a heparin-agarose column at the high salt concentration range of 520-700 mM. In addition, hAT III (123-139) attenuated the effect of heparin on the activation of hAT III, whereas other HBPs did not, suggesting that only hAT III (123-139) could interact with the active site of heparin. On the basis of these results, we prepared novel hAT III (123-139)-related derivatives as potent heparin antagonist candidates, and examined the influence of several modifications on their activity in vitro. The results provided new findings about the structure-activity relationship of hAT III (123-139), and led us to the successful development of a potent antagonist for heparin.

A less toxic heparin antagonist--low molecular weight protamine

A new thirteen amino acid peptide, named low molecular weight protamine (LMWP), was obtained through the enzymatic digestion of native protamine. Both in vitro and in vivo results showed that LMWP fully maintained the heparin neutralization function of protamine but had much lower immunogenicity and antigenicity. Unlike protamine, neither LMWP nor LMWP/heparin complexes caused significant blood platelet aggregation in rats. These results suggest that LMWP can be used as a substitute for protamine for developing a new generation of nontoxic heparin antagonists.

Mechanisms responsible for the failure of protamine to inactivate low-molecular-weight heparin

Protamine is unable to completely reverse the anticoagulant effect of the low-molecular-weight heparins (LMWH), a fact of clinical importance given the rapid increase in use of LMWH in clinical practice. This investigation sought to determine the mechanism by which LMWH were able to resist protamine-mediated inactivation. Affinity fractionation of LMWH by passage through a protamine column, with subsequent determination of molecular mass and sulphate charge density, demonstrated that the protamine-resistant fraction in LMWH is an ultra-low-molecular-weight fraction with low sulphate charge density. This group of molecules was not found in unfractionated heparin, even when species of similar molecular mass were compared. We then determined that different commercially available LMWH varied in their ability to be neutralized by protamine, and that this variability correlated with the total sulphate content of the LMWH. We conclude that reduced sulphate charge, not molecular mass, is the principle reason that protamine is unable to fully inactivate LMWH. Furthermore, different LMWH vary in their ability to be neutralized by protamine, suggesting that product-specific recommendations for neutralization might be developed.

Low molecular weight protamine (LMWP) as nontoxic heparin/low molecular weight heparin antidote (I): preparation and characterization

Low molecular weight protamine (LMWP) appears to be a promising solution for heparin neutralization without the protamine-associated catastrophic toxic effects. The feasibility of this hypothesis was proven previously by using a peptide mixture produced from proteolytic digestion of protamine. To further examine the utility of this compound as an ultimate nontoxic protamine substitute, detailed studies on the purification and characterization of LMWP including the precise amino acid sequence, structure-function relationship, and possible mechanism were conducted. A number of LWMP fragments, composed of highly cationic peptides with molecular weights ranging from 700 to 1900 d, were prepared by digestion of native protamine with the protease thermolysin. These fragments were fractionated using a heparin affinity chromatography, and their relative binding strengths toward heparin were elucidated. Five distinct fractions were eluted at NaCl concentration ranging from 0.4 to 1.0 M and were denoted as TDSP1 to TDSP5, in increasing order of eluting ionic strength. Among these 5 fractions, TDSP4 and TDSP5 contained 3 LMWP peptide fragments, and they were found to retain the complete heparin-neutralizing function of protamine. By using a peptide mass spectrometry (MS) fingerprint mapping technique, the amino acid sequences of the microheterogeneous LMWP fragments in all these 5 elution fractions were readily identified. A typical structural scaffold made by arginine clusters in the middle and nonarginine residues at the N-terminal of the peptide sequence was observed for all these LMWP fragments. By aligning the sequences with the potency in heparin neutralization of these LMWP fragments, it was found that retention of potency similar to that of protamine required the presence of at least 2 arginine clusters in the LMWP fragments; such as the sequence of VSRRRRRRGGRRRR seen in the most potent LMWP fraction-TDSP5. The above finding was further validated by using a synthetic LMWP analogue-CRRRRRRR-and it was found that its heparin-neutralizing ability was increased by changing from a monomeric to a dimeric structure of this analogue peptide. Based on these results, the structural requirement for a compound to function as an effective heparin antidote and the possible mechanism involved in heparin neutralization were established.

Low molecular weight protamine as nontoxic heparin/low molecular weight heparin antidote (III): preliminary in vivo evaluation of efficacy and toxicity using a canine model

Heparin employed in cardiovascular surgeries often leads to a high incidence of bleeding complications. Protamine employed in heparin reversal, however, can cause severe adverse reactions. In an attempt to address this clinical problem, we developed low molecular weight protamine (LMWP) as a potentially effective and less toxic heparin antagonist. A homogeneous 1880-d peptide fragment, termed LMWP-TDSP5 and containing the amino acid sequence of VSRRRRRRGGRRRR, was derived directly from protamine by enzymatic digestion of protamine with thermolysin. In vitro studies demonstrated that TDSP5 was capable of neutralizing various anticoagulant functions of both heparin and commercial low molecular weight heparin preparations. In addition, TDSP5 exhibited significantly reduced crossreactivity toward mouse sera containing antiprotamine antibodies. TDSP5 showed a decrease in its potential in activating the complement system. All of these findings suggested the possibility of markedly reduced protamine toxicity for TDSP5. In this article, we conducted preliminary in vivo studies to further demonstrate the feasibility and utility of using LMWP as a nontoxic clinical protamine substitute. Dogs were chosen as test animals because they were known to magnify the typical human response to protamine. By using a full spectra of biological and clinical assays for heparin, including the anti-IIa and anti-Xa chromogenic assays and the activated partial, thromboplastin time and TCT clotting assays, TDSP5 showed that it could completely neutralize all these different anticoagulant functions of heparin in dogs. Although administration of protamine in dogs produced a significant reduction in mean arterial blood pressure (-14.9 mm Hg) and elevation in pulmonary artery systolic pressure (+5.0 mm Hg), the use of TDSP5 in dogs did not elicit any statistically significant change in any of the variables measured. Furthermore, the use of LMWP also significantly reduced the protamine-induced transient thrombocytopenic and granulocytopenic responses. The white blood cell counts and platelet counts decreased to 82.1% and 60.0% of baseline, respectively, in dogs given intravenous protamine compared to 97.8% and 88.6% of baseline in dogs receiving TDSP5. These preliminary findings indicated that LMWP could potentially provide an effective and safe means to control both heparin- and protamine-induced complications.

Poly-L-lysine amplification of protamine immobilization and heparin adsorption

We previously reported the development of a cellulose fiber based blood filter device containing immobilized protamine (termed protamine filter) that could be used to control both heparin- and protamine-induced complications during extracorporeal therapy. To achieve enhanced heparin adsorption on the fibers, we examined the possibility of utilizing the poly-L-lysine based amplification method to augment protamine loading on the fiber, as well as to create multiple layers of immobilized protamine for heparin interaction. Results show that such a method yielded about a threefold increase in protamine loading and, consequently, about a fourfold enhancement in heparin adsorption when compared with the control without poly-L-lysine amplification. This technological improvement may facilitate development of a new generation of protamine filters with capacity and efficacy suitable for various clinical applications in extracorporeal heparin removal.

Diagnosis of heparin-induced thrombocytopenia in the vascular surgery patient

BACKGROUND

Recent studies have shown that patients with heparin-induced thrombocytopenia (HIT) form immunoglobulin G (IgG) and/or IgM antibodies directed against a complex of platelet factor 4 (PF4) and heparin. This recognition has resulted in the development of enzyme-linked immunosorbent assays (ELISAs) that use the heparin/PF4 complex as the antigen. This study describes the use of a standardized ELISA to assess antibody formation in five patients suspected of having HIT.

METHODS

Five patients received heparin for treatment of arterial or venous thrombotic disorders. All patients had the ELISA performed to detect IgG or IgM antibodies directed against heparin-PF4, as well as the 14C serotonin release assay, when HIT was clinically suspected.

RESULTS

HIT was diagnosed in four patients and ruled out in a fifth by using the ELISA. All patients had a 40% decrease in platelet count that returned to normal after heparin cessation. Only one of the four patients who tested positive by ELISA for IgG antibodies also tested positive by the 14C serotonin release assay. Treatment was significantly altered by the ELISA results in all five patients.

CONCLUSIONS

It is likely that the ELISA is more sensitive in the diagnosis of HIT than the more traditional aggregation tests, and it may emerge as a new gold standard. Prospective studies in which serial laboratory testing is combined with measurement of clinical outcomes are needed and will eventually provide a greater understanding of the full spectrum of HIT and the clinical settings that precipitate thrombosis in the vascular surgery patient.