It has always been difficult to classify the various types of heparin, a substance produced by most organisms and used as an anticoagulant. Each type has its own molecular weight and may have different, or even opposite, functions. Researchers at the Federal University of São Paulo (Unifesp) compared the structural characteristics of a low-molecular-weight-heparin (LMWH), which has been used for decades, with an ultra-low-molecular-weight heparin (ULMWH), which was introduced just a few years ago. They found that the anticoagulant properties of the ultra-low-molecular-weight heparins may vary according to the composition of their sugar chains and their molecular weight, even though they are structurally similar to those of low-molecular-weight.
“The lower the molecular weight, the more targeted the action of heparin on certain key enzymes responsible for blood coagulation,” says Helena Nader, a Unifesp professor and one of the world’s leading heparin experts. In the 1970s, her doctoral adviser and future husband, Carl Peter Dietrich, who died in 2005, isolated a low-molecular-weight heparin; his work enabled its wide-scale production as an anticoagulant. Now a Unifesp team has developed a methodology to identify the chemical structures of low- and ultra-low-molecular-weight heparins, demonstrating how to better use each group and opening up new possibilities for their use.
“We can now understand the mechanisms of action of the low- and ultra-low-molecular-weight heparins most in use today,” says Marcelo Andrade de Lima, a researcher with Unifesp’s Department of Biochemistry and lead author of an article published in the March 2013 issue of Thrombosis and Haemostasis, whose editors in March 2014 selected it as one of the most important papers published in the journal in 2013.
It is now possible to identify the chemical reactions used to produce each type of heparin, thus avoiding mistakes and falsifications. Based on this identification, experts could now develop specific chemical reactions to obtain ultra-low-molecular-weight heparins with new or more specific actions. “We could determine by which pathway we want these new compounds to act in the body and thus create new therapeutic agents,” says Lima.
Produced by cells called mast cells, which are found in different tissues, heparins in general bind to a natural coagulation inhibitor known as antithrombin, increasing up to two thousand times the speed with which antithrombin inhibits the enzymes responsible for coagulation. Therefore, they are frequently used to prevent the formation of clots, which can be fatal. The clotting process involves a sequence of enzymatic reactions. As in a cascade, the enzymes bind to one another, converting pro-enzymes into active enzymes, which eventually transform a soluble protein, fibrinogen, into an insoluble protein called fibrin, the end product of the clotting process.
“All heparins act on antithrombin in the same way,” says Nader. The lower the molecular weight, however, the more selective the action will be. Conventional heparins, known as unfractionated heparins, consisting of molecules with different molecular weights, after binding to antithrombin, inhibit the action of at least five enzymes from the start of the blood clotting process. The low-molecular-weight heparins act primarily on two enzymes, Factor Xa and thrombin (Factor IIa), which are key to the process. The ultra-low-molecular-weight heparins are even more selective and act only on Xa, thereby inhibiting its action. “By separating heparins by molecular weight and their peculiar structural characteristics, we restrict their actions, and make them more specific,” says Lima.
Traces of heparin
These reactions have been known for decades, but few researchers have focused on identifying the chemical structures responsible for their anticoagulant action. Lima and Nader, in collaboration with researchers from the Brazilian state of Paraná, the United States and France, developed a methodology to determine the differences between two drugs of the ultra-low-molecular weight heparin group: semuloparin, with a molecular weight of 2,900 daltons (unit used to express the molecular weight of proteins) produced in France; and bemiparin, with a molecular weight of 3,800 daltons, manufactured in Spain. The two drugs were compared with enoxaparin, the most widely used low-molecular-weight heparin in the world, which is also produced in France, and has a molecular weight of 4,100 daltons. All were produced by using unfractionated heparin from the intestinal mucosa of pigs, a major source of the heparin used as a drug.
By means of nuclear magnetic resonance spectroscopy (NMR) and other techniques, the Unifesp group evaluated the structural characteristics of each substance and correlated them with their molecular weight. “We developed a method that combines different techniques and mathematical analyses to evaluate these structures,” says Nader. “We then tried to understand how we could use these data to design new drugs.”
Once the chemical structures of these heparins were defined in the NMR Laboratory of the University of Campinas (Unicamp) and other analysis centers, the researchers identified the chemical reaction used to obtain low- and ultra-low-molecular-weight heparins. Each low- and ultra-low-weight heparin was obtained through a specific process. “The chemical reaction used to depolymerize heparin leaves traces. Our method identifies these traces and the reaction used to produce each substance,” says Lima.
One of the traces is the composition of heparin sugar chains, which increase the affinity for antithrombin. Take semuloparin for example. A specific chemical reaction used by Sanofi of France is able to depolymerize heparin while preserving a sequence of five sugars that bind strongly to antithrombin. “Thus, semuloparin was obtained by a specific chemical reaction in which pentasaccharides are preserved in most of the molecules, leading to a compound having anticoagulant activity directed against factor Xa and without a mechanism of action against thrombin,” says Nader. This information helped to explain recent clinical findings. Three years ago, Sanofi reported that administering semuloparin could reduce by 64% the risk of deep vein thrombosis, pulmonary embolism and venous thromboembolism-related deaths in people with cancer at the start of chemotherapy.
Brazil produces only unfractionated heparin from the mucosa of cattle and pigs. In 2012, it exported the equivalent of R$24 million of unfractionated heparin while importing the more purified forms of higher added value.
1. Mass spectrometry and nuclear magnetic resonance in the structural characterization of glycosaminoglycans and complex polysaccharides of algae and invertebrates (nº 2010/52426-3); Grant mechanism Regular Line of Research Project Award; Principal investigator Helena Bonciani Nader (Unifesp); Investment R$ 819,080.01 (FAPESP).
2. Bioactive compounds obtained from shrimp waste and chemical modifications of heparin (nº 2012/00850-1); Grant mechanism Post-doctoral research grant; Principal investigator Helena Bonciani Nader (Unifesp); Grant recipient Marcelo Andrade de Lima (Unifesp); Investment R$ 152,469.11 (FAPESP).
LIMA, M. A. et al. Ultra-low-molecular-weight heparins: precise structural features impacting specific anticoagulant activities. Thrombosis and Haemostasis. v. 109, n. 3, p. 471-8. mar. 2013.
CHRISTIAN W. e GREGORY Y. H. L.. Editors’ Choice papers in Thrombosis and Haemostasis. Thrombosis and Haemostasis. v. 111, n. 1, p. 185-8. jan. 2014.