The heparin antithrombin system: a natural anticoagulant mechanism. Structure of the antithrombin-binding site of heparin. Johnson EA, Mulloy B. The molecular weight range of commercial heparin preparations.
Carbohydr Res. The separation of active and inactive forms of heparin. Biochem Biophys Res Commun. The structure of heparin oligosaccharide fragments with high affinity anti- factor Xa activity containing the minimal antithrombin III-binding sequence. Biochem J. Hirsh J, Raschke R. Role of ternary complexes, in which heparin binds both antithrombin and proteinase, in the acceleration of the reactions between antithrombin and thrombin or factor Xa.
J Biol Chem. Bick RL. Heparin and low-molecular-weight heparins. In: Bick RL, ed. Comparative pharmacokinetics of low molecular weight heparin PK and unfractionated heparin after intravenous subcutaneous administration.
Thromb Res. The weight-based heparin dosing nomogram compared with a "standard care" nomogram. Ann Intern Med. Continuous intravenous heparin compared with intermittent subcutaneous heparin in the initial treatment of proximal-vein thrombosis. N Engl J Med. Heparin for 5 days as compared with 10 days in the initial treatment of proximal venous thrombosis. Hemorrhagic complications of intravenous heparin use. Am J Cardiol. Optimal therapeutic level of heparin therapy in patients with venous thrombosis.
Arch Intern Med. Newer strategies for the treatment of heparin-induced thrombocytopenia. Dahlman TC. Osteoporotic fractures and the recurrence of thromboembolism during pregnancy and the puerperium in women undergoing thromboprophylaxis with heparin. Am J Obstet Gynecol.
College of American Pathologists Conference XXXI on laboratory monitoring of anticoagulant therapy: laboratory monitoring of unfractionated heparin therapy. Arch Pathol Lab Med. The British Society for Haematology Guidelines on the use and monitoring of heparin second revision. J Clin Pathol.
Guidelines on the use and monitoring of heparin. Br J Haematol. Bussey HI. Problems with monitoring heparin anticoagulation.
The importance of initial heparin treatment on long-term clinical outcomes of antithrombotic therapy. The emerging theme of delayed recurrence. Suboptimal monitoring and dosing of unfractionated heparin in comparative studies with low-molecular-weight heparin.
The variation of the inhibitory effect of heparins in different plasmas. This large variation in the individual heparin response shows that on a standard dose of any heparin many patients must be over- or under-treated.
That there is nevertheless a well-defined beneficial effect on thrombosis -prevention shows that there must be a significant latitude between the risk of re- thrombosis or bleeding and the actual manifestation of these complications.
This is nothing new: mild hemophilia can go unnoticed until middle age and congenital antithrombin deficiency will not show up until in the late teens. In view of the large variability of response, we surmise that personalization of heparin dosage could considerably reduce the risks of heparin treatment, that of re-thrombosis as well as that of bleeding.
The question is whether current practice can be significantly ameliorated by personalized dosage. We surmise it would, but the cost-benefit relation remains an open question. In conclusion: If one wants to control heparin pharmacokinetics, use aIIa activity, if one wants to know about pharmacodynamics, use thrombin generation. Heparin activities are still expressed in aIIa- and aXa-units relative to a standard, often using clotting times aPTT to establish the equivalence.
When Howell discovered heparin around this was the only possible modus operandi but in this century, it is hopelessly outdated. Therefore, the aPTT—as prescribed by the pharmacopeias, does not detect adulterated heparins, with disastrous consequences 3 , There is a simple and unequivocal manner to determine heparin activity in terms of standard independent, SI-based units A SIU-Xa is defined analogously.
We described a very simple end-point assay to determine these decay constants Because the assay uses a solution of purified antithrombin it will not co-estimate HCII-dependent contaminants. Such contaminants can be quantified by a similar test using heparin cofactor II instead of antithrombin. The concentration of high affinity moieties HA5 can be determined by fluorescence titration As a consequence, of any heparin preparation one can determine the activity in standard independent units per mole of high affinity material.
If one, for convenience, wants to use a standard, than any heparin preparation with a chain-length distribution of between 30 and 45 sugar units — kD can be used, because in the range the inhibitory power is for all practical purposes constant per high-affinity molecule Figure 4. The ideal heparin is the lowest molecular weight heparin that has a good inhibitory potency, i. The longer the heparin the shorter the half life time and the lower the bioavailability.
The best heparin therefore presumably has MW-distribution of 10—20 kD. The pure Choay domain, i. Present day synthetic efforts still focus on aXa-activity 41 — 43 and attain a chain length of 12 sugar residues.
It is clear, in the light of the above that a pure Choay-domain would be over fold more effective than molecules with anti-factor Xa activity only. We hope that this paper might lead to rounding up experts in the field in order to start and perform a collaborative study that proves the suitability of ETP for this purpose.
HH wrote the first draft. SB and RA commented on it and the final text is the result of several amelioration loops. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Eikelboom JW, Hirsh J. Monitoring unfractionated heparin with the aPTT: time for a fresh look. Thromb Haemost. Contaminated heparin associated with adverse clinical events and activation of the contact system.
N Engl J Med. Laurencin CT, Nair L. The FDA and safety—beyond the heparin crisis. Nat Biotechnol. Identification of patients at low risk for recurrent venous thromboembolism by measuring thrombin generation.
High rate of unprovoked recurrent venous thrombosis is associated with high thrombin-generating potential in a prospective cohort study. J Thromb Haemost. Thrombin generation profiles in deep venous thrombosis. The thrombogram in rare inherited coagulation disorders: its relation to clinical bleeding.
Evaluation of thrombin generating capacity in plasma from patients with haemophilia A and B. Prediction of bleeding risk in patients taking vitamin K antagonists using thrombin generation testing. The influence of oral contraceptives on the time-integral of thrombin generation thrombin potential. Resistance to APC and SHBG levels during use of a four-phasic oral contraceptive containing dienogest and estradiol valerate: a randomized controlled trial.
Genetic determinants of thrombin generation and their relation to venous thrombosis: results from the GAIT-2 Project. The calibrated automated thrombogram CAT : a universal routine test for hyper- and hypocoagulability. Pathophysiol Haemost Thromb. Calibrated automated thrombin generation measurement in clotting plasma. Proposal for standardized preanalytical and analytical conditions for measuring thrombin generation in hemophilia: communication from the SSC of the ISTH.
Evaluation of a standardized protocol for thrombin generation using the calibrated automated thrombogram: a Nordic study. A review of commercially available thrombin generation assays. Res Pract Thromb Haemost. Hemker HC. A century of heparin: past, present and future. Kinetics of heparin action. Ann NY Acad Sci. It is a basic tenet in biochemistry that every activator will have a cognate inhibitor and this is true for the coagulation system. The natural inhibitors fall into two main groups, endothelial or hepatic, based on their synthetic site.
An alternate classification could be to separate the inhibitors into those that aim to inhibit thrombin production and those that directly inhibit this enzyme.
This latter protease has been investigated as a means of preventing thrombus formation or extension. First, thrombin is responsible for the generation of aPC. The active site of the thrombin cleaves the protein C moiety to release aPC Fig. This clever device allows thrombin to be converted from a procoagulant to an anticoagulant protein. Similar to a genetic absence of protein C, this is not a lethal gene. However, patients with the Leiden mutation are at substantially increased risk of venous thrombosis, 4 and myocardial infarction in certain populations.
Genomic mapping suggests that these proteins developed and evolved together. We also increasingly recognize a functional relationship between these proteins. One example is that cleavage by thrombin, to release the tethered ligand of the thrombin receptor, occurs at a site with structural and amino acid sequence homology with protein C. TFPI is a relatively new addition to the ranks of the inhibitors of coagulation. TFPI is synthesized and released from the endothelium and appears to be the main inhibitor of the TF pathway in vivo.
Thrombin production is inhibited and slowed as follows. Individuals with congenital TFPI deficiency have not been identified. Mice homozygous for deletion of K1 in the TFPI gene die in utero implying that such a deficiency is not compatible with life. Of interest is that the inhibitor site of the K1 domain differs by only 1 amino acid residue from the inhibitor Kunitz domain of aprotinin.
These circulating factors include a number of serine protease inhibitors or serpins. This superfamily of proteins plays a major role in the regulation of coagulation, fibrinolysis, and inflammation. Although hydrolysis is attempted by the protease it cannot be completed and a tight complex is formed which is rapidly cleared from the circulation.
In vivo , glycosaminoglycans such as heparan sulphate on the endothelial cell surface are the initiators of the enhanced ATIII inhibitory function. This combination produces the physiological effect of a vascular surface with profound anticoagulant properties. The congenital forms can be divided into those associated with an absence or reduction of ATIII in the plasma and those associated with an amino acid sequence that bestows inappropriate inhibitory activity to the molecule.
Both these defects are associated with an increased risk of thromboembolic disease. Pregnancy represents an interesting example and model of reduced ATIII activity that is relevant to other clinical arenas. Replacement or enhancement of ATIII concentrations has been suggested in a number of these conditions.
Concentrates from human sources have been available for some time and have shown some benefits in patients with sepsis syndrome. HCII is the second plasma thrombin inhibitor. The endothelial glycosaminoglycan, dermatan sulphate has a specific binding site for HCII.
Included in this category are recombinant agents equivalent to some naturally occurring proteins and totally synthetic agents. A most important point to note is that at times these drug therapies will produce prothrombotic or hypercoagulable states.
Warfarin therapy is associated with cutaneous thrombosis. The first group of antithrombin drugs discussed are not direct inhibitors of thrombin but aim to slow thrombin generation and presentation. Reduction in clotting factor activity is produced when patients are given vitamin K antagonists. The first oral anticoagulant used was dicoumarol that was isolated from spoilt clover. The three widely used drugs are warfarin, phenprocouman, and acenocouman.
Warfarin is the best known of this class of agent and is used prophylactically in atrial fibrillation, venous thrombosis, pulmonary embolism, and in patients with prosthetic heart valves. The INR was developed by the World Health Organization in the early s to eliminate problems in oral anticoagulant therapy caused by variability in the sensitivity of different commercial sources and different batches of thromboplastin.
The ISI is a measure of the response to a thromboplastin preparation and is typically between 2 and 2. This adds some confusion when discussing results from studies of the effects of an anticoagulant regimen on outcome.
An INR of 2. An INR of 3. Warfarin can be given intravenously but is usually given orally and is well absorbed. Peak plasma concentrations of warfarin occur about 90 min after ingestion. This takes about 8—24 h following ingestion of warfarin. The peak effect of a single dose occurs at 36—72 h and lasts about 5 days. The S form is about five times more potent as a vitamin K antagonist than the R form and is oxidized in the liver to hydroxywarfarin that is excreted in the bile.
The R form is metabolized to warfarin alcohols, which are excreted by the kidney. Given the above confounding variables it is not surprising that the biological effect of warfarin to prolong the PT can be significantly altered by a multitude of other therapeutic interventions as shown in Table 2.
Vitamin K antagonists prevent this process. In other words, the plasma concentration of these factors is normal but their function is impaired. With normal liver function, a dose of vitamin K will reverse this functional defect in about 4—6 h.
Fresh frozen plasma will transiently reverse the effect of warfarin although this requires volumes of about 10—15 ml kg —1. The risks of thromboembolism if the patient is not anticoagulated must be balanced with the risks of excessive intraoperative bleeding. For example, in a survey of patients having dental surgery, the incidence of significant bleeding 0. Although the likelihood of significant bleeding may be greater with more major procedures such as bowel or orthopaedic surgery, this must be balanced against the magnitude of the thrombotic risk and its effects.
Although it is common practice to stop warfarin up to a week before surgery, and substitute the more readily reversible heparin, there is little current evidence to show this is an absolute necessity. This is especially the case in patients with prosthetic cardiac valves.
This value was not associated with increased bleeding or increased thrombotic episodes over the transient period of drug withdrawal. One further area of concern is the patient taking oral anticoagulants whom becomes pregnant. In particular, the management of women with prosthetic heart valves during pregnancy poses a particular challenge, as there are no available controlled clinical trials to provide guidelines for effective antithrombotic therapy.
Warfarin is teratogenic and should not be given in the first trimester of pregnancy. However, subcutaneous s. A recent literature review 12 suggested that the regimen associated with the lowest risk of valve thrombosis was the use of oral anticoagulants throughout pregnancy. Although this approach was associated with warfarin embryopathy in 6.
Heparin is a naturally occurring negatively charged sulphated polysaccharide with a complex structure. Heparin was originally isolated from liver during investigations to ascertain if the phospholipid component of cephalin would cause clotting.
Since the discovery of heparin in by McLean, 29 numerous physiological actions have been proposed for this agent.
This concept is strengthened when we consider first, that heparin alone has no direct effects on coagulation, and secondly, is found in lower orders of the animals, such as molluscs, which lack a coagulation system.
Standard preparations of heparin are unfractionated UFH , derived from either porcine intestine or bovine lung and prepared as either calcium or sodium salts. The number and sequence of the saccharides is variable, with molecular weights ranging from to 30 Da, with a mean of 15 Da representing 40—50 saccharides in length.
There is no apparent difference between any of the available forms of UFH with respect to their pharmacology or anticoagulant profile. There are significant variations between the different commercial preparations according to the method used in their production as shown in Table 3.
The binding of heparin to ATIII is highly specific, reversible, and does not inactivate the heparin molecule. ATIII has an intrinsic low level of activity, mediated by an arginine centre that binds to activate serine proteases of the coagulation cascade. Binding with heparin dramatically increases this inhibitory effect. This action reduces the half time of inhibition of thrombin in plasma to 10 ms.
First, heparin attaches to a small, high affinity site on ATIII to produce a conformational change at the reactive site Fig. The second effect of heparin is via a larger, but low affinity, site that extends from the pentasaccharide site to the pole of the ATIII molecule near the active centre.
The active centre carries a positive charge, which tends to repel proteases with a positive charge at the active centre such as thrombin. Addition of the negative charge carried on the heparin will neutralize this effect Fig. This may explain the observation that the addition of heparins with a longer chain length thus more available negative charges has a much greater effect to inhibit thrombin than it does to inhibit plasmin or factor Xa.
Increased negativity will also not affect proteases with a neutral active site such as plasmin, or to a lesser extent factor Xa.
The third effect is also specific to the inhibition of thrombin, which has charged exosites away from the active centre. In this circumstance, the role of heparin is to bring together the protease and its inhibitor a process termed approximation rather than just producing the conformational change.
These tend to be inhibited directly by the heparin—ATIII complex and approximation does not need to take place. First, heparin can activate the other major circulating antithrombin, HCII. This activation does not require the pentasaccharide sequence but does require heparins of greater than Da or 24 saccharide units in length. Similar to the mechanism of action of UFH and LMWH, the pharmacokinetics and dynamics have a number of differences and some similarities.
In particular, neither significantly cross the placenta. Moreover, the plasma concentrations of heparin are not uniformly related to the anticoagulant effect produced and there is a wide variability in dose—response effects in patients. The pharmacokinetics of UFH are complex.
Heparins are poorly absorbed from the gastrointestinal tract and can cause haematomas after intramuscular injection. They are therefore usually administered by s. However, similar levels of anticoagulation can be achieved, with onset delayed by 1 or 2 h, by the s. The heterogeneity of heparin molecules produces great variability in the plasma concentration of the agent in relation to the dose administered.
After injection, plasma levels initially decline rapidly a result of redistribution and uptake by endothelial cells. The first three of these also reduce its bioavailability and activity.
Raised concentrations of these proteins may account for the heparin resistance seen in malignancy and inflammatory disorders. The rapid, saturable phase of heparin clearance is thought to be a result of cellular degradation by macrophages, which internalize the heparin, then depolymerize and desulphate it.
Saturation occurs when all the receptors have been utilized and further clearance depends on new receptor synthesis. Significant plasma levels can only be achieved by saturation of these receptors with a loading dose. The slower phase of heparin elimination is a result of renal excretion. Moreover, LMWHs are not subject to the rapid degradation that UFH suffers as they are not inactivated by platelet factor 4 and do not bind to endothelial cells or macrophages.
UFH, and this guarantees a more predictable anticoagulant action. They are almost completely absorbed following s. Their distribution volume is close to the blood volume. Similar to UFH they are partially metabolized by desulphation and depolymerization. Monitoring is routinely performed during therapy with UFH for a number of reasons.
There is a marked variation in the initial anticoagulant response to a fixed dose of UFH. The risk of recurrent thromboembolism is reduced if the effect of heparin is maintained above the lower therapeutic limit. Direct measurement of heparin concentration is not possible, although protamine titration can be used to ascertain blood levels.
The therapeutic range most commonly quoted is an aPTT between 1. However the commercially available kits for measurement of aPTT differ in their sensitivity to heparins. This suggests that the protamine titration method of monitoring may be more robust. These properties allow once daily s. Although haemorrhage is rare with prophylactic doses of either UFH or LMWH given alone, it is a frequent complication of therapeutic heparin administration.
The greater the dose of heparin and therefore the greater its anticoagulant effect, the greater the risk of haemorrhage. When comparable doses are used, the risks are similar using either the continuous i. Heparin will also impair platelet aggregation and inhibits platelet function by direct binding to platelets. A review of this complication estimated the daily frequencies of fatal, major, and all types of haemorrhage in patients receiving therapeutic anticoagulation as 0.
It plays a crucial role in mediating procoagulant and anticoagulant processes to maintain proper blood flow. It is a versatile drug used for the treatment of atrial fibrillation, acute coronary syndrome ACS , arterial thrombosis, deep vein thrombosis, and pulmonary embolism, and in the prevention of thrombosis during cardiopulmonary bypass and extracorporeal membrane oxygenation.
It is also used as adjunctive pharmacotherapy in percutaneous coronary intervention PCI. Overdosing of heparin can lead to dangerous bleeding complications.
This article reviews the physiology, pharmacology, therapeutic applications, and clinical data on heparin in the setting of PCI. In addition, different dosing regimens and the efficacy of heparin monotherapy compared with bivalirudin in the setting of PCI are discussed. Although it does not break down preformed clots like tissue plasminogen activator, it instead potentiates the progression of the body's natural clot lysis mechanisms to prevent the formation of clots.
Heparin plays a vital role in the complex network of serine proteases that convert proenzymes to their active forms. Thrombin, or factor lla, is the final serine protease that cleaves fibrinogen to form fibrin, the foundation of a clot when combined with a platelet plug Figure 1. Vascular injury exposes these serine proteases to tissue factor and collagen, procoagulant stimuli that activate the coagulation cascade.
These include antithrombin AT , heparin cofactor II, and protein C inhibitor, which are members of a class of proteins called serpins short for serine protease inhibitors. Heparin must bind to both the coagulation enzyme and AT to inhibit thrombin. Thus, heparin's activity against thrombin is size dependent and requires at least 18 saccharide units.
Heparin molecules with fewer than 18 saccharides lack the chain length to bridge between AT and thrombin. However, binding to the enzyme is not required for the inhibition of factor Xa; only the pentasaccharide binding site is required.
AT and the other serpins possess a reactive loop that mimics a serine protease substrate sequence. A procoagulant protease that cleaves this loop is then trapped in an inactive complex with the serpin. When heparin binds to AT, a conformational change in the enzyme inhibitor increases the flexibility at its reactive site loop and activates it.
Once the enzyme is inactivated, heparin attached to the AT is released so that it can act again on another free serpin available. Commercial preparation of animal-derived heparin is derived from tissue extract from pig intestines and cow lungs. In addition, heparin can lead to complications including anaphylaxis, bleeding, thrombocytopenia, and osteopenia.
Heparin is administered parenterally because it is not absorbed in the gut due to its high negative charge and size. Intramuscular injections are avoided because of the risk of developing hematoma.
Heparin is primarily excreted by the reticuloendothelial system in a rapid dose-dependent manner, but the extent of reticuloendothelial saturation is difficult to ascertain. As endothelial cell binding of heparin is saturated, a higher burden is placed on the kidneys as they clear the drug from the bloodstream at a slower rate.
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