Heparin
| Heparin | |
| Systematic (IUPAC) name | |
| Identifiers | |
| Chemical data | |
| 12000-15000 g/mol | |
| Pharmacokinetic data | |
| nil | |
| hepatic | |
| 1.5 hrs | |
| ? | |
| Therapeutic considerations | |
| C | |
| ? | |
| i.v., s.c. | |
Heparin, a highly sulfated glycosaminoglycan is widely used as an injectable anticoagulant and has the highest negative charge density of any known biological molecule.[1] It can also be used to form an inner anticoagulant surface on various experimental and medical devices such as test tubes and renal dialysis machines. Pharmaceutical grade heparin is commonly derived from mucosal tissues of slaughtered meat animals such as porcine intestine or bovine lung.[2]
[edit] Heparin structure
Native heparin is a polymer with a molecular weight ranging from 3 kDa to 40 kDa although the average molecular weight of most commercial heparin preparations is in the range of 12 kDa to 15 kDa. Heparin is a member of the glycosaminoglycan family of carbohydrates (which includes the closely related molecule heparan sulfate) and consists of a variably sulfated repeating disaccharide unit. The main disaccharide units that occur in heparin are shown below. The most common disaccharide unit is composed of a 2-O-sulfated iduronic acid and 6-O-sulfated, N-sulfated glucosamine, IdoA(2S)-GlcNS(6S). For example this makes up 85% of heparins from beef lung and about 75% of those from porcine intestinal mucosa.[3] Not shown below are the rare disaccharides containing a 3-O-sulfated glucosamine (GlcNS(3S,6S)) or a free amine group (GlcNH3+). Under physiological conditions the ester and amide sulfate groups are deprotonated and attract positively charged counterions to form a heparin salt. It is in this form that heparin is usually administered as an anticoagulant.
1 unit of heparin is the quantity of heparin required to keep 1 mL of cat's blood fluid for 24 hours at 0°C; it is equivalent approximately to 0.002 mg of pure heparin.
|
GlcA-GlcNAc |
GlcA-GlcNS |
IdoA-GlcNS |
IdoA(2S)-GlcNS |
|
IdoA-GlcNS(6S) |
IdoA(2S)-GlcNS(6S) |
Abbreviations
- GlcA = β-L-glucuronic acid
- IdoA = α-L-iduronic acid
- IdoA(2S) = 2-O-sulfo-α-L-iduronic acid
- GlcNAc = 2-deoxy-2-acetamido-α-D-glucopyranosyl
- GlcNS = 2-deoxy-2-sulfamido-α-D-glucopyranosyl
- GlcNS(6S) = 2-deoxy-2-sulfamido-α-D-glucopyranosyl-6-O-sulfate
Three-dimensional structure
The three dimensional structure of heparin is complicated by the fact that iduronic acid may be present in either of two low energy conformations when internally positioned within an oligosaccharide. The conformational equilibrium being influenced by sulfation state of adjacent glucosamine sugars.[4] Nevertheless the solution structure of a heparin dodecasacchride composed solely of six GlcNS(6S)-IdoA(2S) repeat units has been determined using a combination of NMR spectroscopy and molecular modelling techniques.[5] Two models were constructed one in which all IdoA(2S) were in the 2S0 conformation (A and B below) and one in which they are in the 1C4 conformation (C and D below). However there is no evidence to suggest changes between these conformations occurs in a concerted fashion. These models correspond to the protein data bank code 1HPN.
In the image above:
- A = 1HPN (all IdoA(2S) residues in 2S0 conformation) Jmol viewer
- B = van der Waals radius space filling model of A
- C = 1HPN (all IdoA(2S) residues in 1C4 conformation) Jmol viewer
- D = van der Waals radius space filling model of C
In these models heparin adopts a helical conformation, the rotation of which places clusters of sulfate groups at regular intervals of about 17 angstroms (1.7 nm) on either side of the helical axis.
[edit] Medical use
Heparin is a naturally occurring anticoagulant produced by basophils and mast cells.[6] Heparin acts as an anticoagulant, preventing the formation of clots and extension of existing clots within the blood. While heparin does not break down clots that have already formed (tissue plasminogen activator will), it allows the body's natural clot lysis mechanisms to work normally to break down clots that have already formed. Heparin is used for anticoagulation for the following conditions:
- Acute coronary syndrome, e.g., myocardial infarction
- Atrial fibrillation
- Deep-vein thrombosis and pulmonary embolism
- Cardiopulmonary bypass for heart surgery.
Administration
Details of administration are available in clinical practice guidelines by the American College of Chest Physicians[7]:
Heparin is given parenterally, as it is degraded when taken by mouth. It can be injected intravenously or subcutaneously (under the skin). Intramuscular injections (into muscle) are avoided because of the potential for forming hematomas.
Because of its short biologic half-life of approximately one hour, heparin must be given frequently or as a continuous infusion. However the use of low molecular weight heparin (LMWH) has allowed once daily dosing, thus not requiring a continuous infusion of the drug. If long-term anticoagulation is required, heparin is often only used to commence anticoagulation therapy until the oral anticoagulant warfarin takes effect.
Adverse reactions
A serious side-effect of heparin is heparin-induced thrombocytopenia (HIT syndrome). HITS is caused by an immunological reaction that makes platelets aggregate within the blood vessels, thereby using up coagulation factors. Formation of platelet clots can lead to thrombosis, while the loss of coagulation factors and platelets may result in bleeding. HITS can (rarely) occur shortly after heparin is given, but also when a person has been on heparin for a long while. Immunologic tests are available for the diagnosis of HITS. There is also a benign form of thrombocytopenia associated with early heparin use which resolves without stopping heparin.
Rarer side effects include alopecia and osteoporosis with chronic use.
As with many drugs, overdoses of heparin can be fatal. In September 2006, heparin received worldwide publicity when 3 prematurely-born infants died after they were mistakenly given overdoses of heparin at an Indianapolis hospital.[8]
Treatment of overdose
In case of overdose, protamine sulfate can be given to counteract the action of heparin, in the same amount as heparin.
Mechanism of anticoagulant action
Heparin binds to the enzyme inhibitor antithrombin III (AT-III) causing a conformational change which results in its active site being exposed. The activated AT-III then inactivates thrombin and other proteases involved in blood clotting, most notably factor Xa. The rate of inactivation of these proteases by AT-III increases 1000-fold due to the binding of heparin.[9]
AT-III binds to a specific pentasaccharide sulfation sequence contained within the heparin polymer
GlcNAc/NS(6S)-GlcA-GlcNS(3S,6S)-IdoA(2S)-GlcNS(6S)
The conformational change in AT-III on heparin binding mediates its inhibition of factor Xa. For thrombin inhibition however, thrombin must also bind to the heparin polymer at a site proximal to the pentasaccharide. The highly negative charge density of heparin contributes to its very strong electrostatic interaction with thrombin.[1] The formation of a ternary complex between AT-III, thrombin and heparin results in the inactivation of thrombin. For this reason heparin's activity against thrombin is size dependent, the ternary complex requiring at least 18 saccharide units for efficient formation.[10] In contrast anti factor Xa activity only requires the pentasaccharide binding site.
Chemical structure of fondaparinux
This size difference has led to the development of low molecular weight heparins (LMWHs) and more recently to fondaparinux as pharmaceutical anticoagulants. Low molecular weight heparins and fondaparinux target anti factor Xa activity rather than anti thrombin (IIa) activity, with the aim of facilitating a more subtle regulation of coagulation and an improved therapeutic index. The chemical structure of fondaparinux is shown to the left. This is a synthetic pentasaccharide whose chemical structure is almost identical to the structure the AT-III binding pentasaccharide sequence that can be found within polymeric heparin and heparan sulfate.
With LMWH and fondaparinux, there is a reduced risk of osteoporosis and heparin-induced thrombocytopenia (HIT). Monitoring of the APTT is also not required and indeed does not reflect the anticoagulant effect, as APTT is insensitive to alterations in factor Xa.
Danaparoid, a mixture of heparan sulfate, dermatan sulfate and chondroitin sulfate can be used as an anticoagulant in patients who have developed HIT. Because danaparoid does not contain heparin or heparin fragments cross-reactivity of danaparoid with heparin-induced antibodies is reported as less than 10%.[11]
The effects of heparin are measured in the lab by the partial thromboplastin time (aPTT), (the time it takes the blood plasma to clot).
Heparin's exact physiological role is still unclear, because blood anti-coagulation is mostly achieved by endothelial cell-derived heparan sulfate proteoglycans.[12] Heparin is usually stored within the secretory granules of mast cells and only released into the vasculature at sites of tissue injury. It has been proposed that rather than anticoagulation the main purpose of heparin is in a defensive mechanism at sites of tissue injury against invading bacteria and other foreign materials.[13]
History
Heparin is one of the oldest drugs currently still in widespread clinical use. Its discovery in 1916 predates the establishment of the United States Food and Drug Administration, although it did not enter clinical trials until 1935.[14] It was originally isolated from canine liver cells, hence its name (hepar or "ηπαρ" is Greek for "liver"). Heparin's discovery can be attributed to the research activities of two men, Jay McLean and William Henry Howell.
In 1916 McLean, a second-year medical student at Johns Hopkins University, was working under the guidance of Howell investigating pro-coagulant preparations, when he isolated a fat soluble phosphatide anti-coagulant. It was Howell who coined the term heparin for this type of fat soluble anticoagulant in 1918. In the early 1920s, Howell isolated a water soluble polysaccharide anticoagulant which was also termed heparin, although it was distinct from the phosphatide preparations previously isolated. It is probable that the work of McLean changed the focus of the Howell group to look for anticoagulants, which eventually led to the polysaccharide discovery.
Between 1933 and 1936, Connaught Medical Research Laboratories, then a part of the University of Toronto, perfected a technique for producing safe non-toxic heparin that could be administered to patients in a salt solution. The first human trials of heparin began in May 1935 and by 1937 it was clear that Connaught's heparin was a safe, easily available and effective blood anticoagulant. Prior to 1933, heparin was available, but in small amounts, was extremely expensive, toxic and consequently of no medical value.[15]
For a full discussion of the events surrounding heparin's discovery see Marcum J. (2000).[16]
Novel drug development opportunities for heparin
As detailed in the table below, there is a great deal of potential for the development of heparin like structures as drugs to treat a wide range of diseases in addition to their current use as anticoagulants.[17][18]
| Disease states sensitive to heparin | Heparins effect in experimental models | Clinical status | |
| Reduces cell activation and accumulation in airways, neutralizes mediators and cytotoxic cell products, and improves lung function in animal models | Controlled clinical trials | ||
| Effective in animal models | - | ||
| Effects as for adult respiratory distress syndrome, although no specific nasal model has been tested | Controlled clinical trial | ||
| Inhibits cell accumulation, collagen destruction and angiogenesis | |||
| As for adult respiratory distress syndrome, however it has also been shown to improve lung function in experimental models | Controlled clinical trials | ||
| Inhibits tumour growth, metastasis and angiogenesis, and increases survival time in animal models | Several anecdotal reports | ||
| Effective in animal models | - | ||
| Inhibits inflammatory cell transport in general. No specific model tested | Controlled clinical trials | ||
| Effective in a human experimental model of interstitial cystitis | Related molecule now used clinically | ||
| Prolongs allograph survival in animal models | - |
- indicates no information available
As a result of heparins effect on such a wide variety of disease states a number of drugs are indeed in development whose molecular structures are identical or similar to those found within parts of the polymeric heparin chain.[17]
| Drug molecule | Effect of new drug compared to heparin | Biological activities | |
| Heparin tetrasaccharide | Non-anticoagulant, non-immunogenic, orally active | Anti-allergic | |
| Plant derived, little anticoagulant activity, Anti-inflammatory, orally active | Anti-inflammatory, anti-adhesive, anti-metastatic | ||
| Phosphomannopentanose sulfate | Potent inhibitor of heparanase activity | Anti-metastatic, anti-angiogenic, anti-inflammatory | |
| Selectively chemically O-desulphated heparin | Lacks anticoagulant activity | Anti-inflammatory, anti-allergic, anti-adhesive |
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