The complex effects of heparins on cancer progression and metastasis in experimental studies

Patients with cancer are frequently treated with anticoagulants, including heparins, to treat or to prevent thrombosis. Recent randomized trials that compared low molecular weight heparin to unfractionated heparin for the treatment of deep vein thrombosis have indicated that heparins affect survival of patients with cancer. Experimental studies support the hypothesis that cancer progression can be influenced by heparins, but results of these studies are not conclusive. Heparins are negatively charged polysaccharides that can bind to a wide range of proteins and molecules and affect their activity. As a consequence, heparins have a wide variety of biological activities other than their anticoagulant effects, which may interfere with the malignant process. In the present systematic review, we critically evaluate experimental studies in which heparins have been tested as anti-cancer drugs. All animal studies, published between 1960 and 1999, that report effects of heparins on growth of subcutaneously implanted tumors, spontaneous metastasis or experimentally induced metastasis are reviewed. In addition, we discuss mechanisms by which heparins potentially exert their activity on various steps in cancer progression and malignancy related processes. It is shown that heparins can affect proliferation, migration, and invasion of cancer cells in various ways and that heparins can interfere with adherence of cancer cells to vascular endothelium. Moreover, heparins can affect the immune system and have both inhibitory and stimulatory effects on angiogenesis. Because of the wide variety of activities of heparins, it is concluded that the ultimate effect of heparin treatment on cancer progression is uncertain.