Non-anticoagulant heparins and inhibition of cancer

Abstract

Low-molecular-weight heparins (LMWH) appear to prolong survival of patients with cancer. Such a beneficial effect is thought to be associated with interruption of molecular mechanisms involving the heparan sulfate (HS) chains of cell surface and extracellular matrix proteoglycans (HSPGs), growth factors and their receptors, heparanase, and selectins. The beneficial effects of heparin species could also be associated with their ability to release tissue factor pathway inhibitor from endothelium. The utility of heparin and LMWH as anticancer drugs is limited due to their anticoagulant properties. Non-anticoagulant heparins can be obtained either by removing chains containing the antithrombin-binding sequence, or by inactivating critical functional groups or units of this sequence. The non-anticoagulant heparins most extensively studied are regioselectively desulfated heparins and 'glycol-split' heparins. Some modified heparins of both types are potent inhibitors of heparanase. A number of them also attenuate metastasis in experimental models. With cancer cells overexpressing selectins, heparin-mediated inhibition of tumor cells-platelets aggregation and tumor cell interaction with the vascular endothelium appears to be the prevalent mechanism of attenuation of early stages of metastasis. The structural requirements for inhibition of growth factors, heparanase, and selectins by heparin derivatives are somewhat different for the different activities. An N-acetylated, glycol-split heparin provides an example of application of a non-anticoagulant heparin that inhibits cancer in animal models without unwanted side effects. Delivery of this compound to mice bearing established myeloma tumors dramatically blocked tumor growth and progression.

Fig. 1.

Idealized representation of a heparin chain constituted of N-acetylated (NA), N-sulfated (NS), and mixed NA/NS domains also containing an AT-binding domain (AT-bs). Formulas of major disaccharidic sequences within NS, NA/NS and NA domains (1–3) and of the AT-bd (4) are shown. Symbols are as explained in Casu and Lindahl [8].

Fig. 2.

Prevalent sequences of heparin (5), N-acetylated heparin (6), and the corresponding RO (reduced oxyheparin) derivatives RO.H (7) and NA-RO.H (8), where all non-sulfated uronic acid residues of H and NA-H are glycol split; for simplicity, only GlcA and glycol-split (gs) GlcA residues are shown. Formula 9 represents a heparin derivative where additional non-sulfated uronic acid residues (generated by selective 2-O-desulfation of IdoA2SO 3 residues) were glycol split, to reach about 1: 1 ratio of split to non-split residues. The value of n varies from 1 to 5 in 5–8 and is = 1 in 9 [9].

Fig. 3.

Heparanase inhibition activity of N-acetyl heparins (a) and of glycol-split N-acetyl heparins of the RO type (b), as a function of the degree of N-acetylation. The anti-heparanase activity is expressed in terms of residual activity [17].

Fig. 4.

A modified non-anticoagulant heparin inhibits myeloma growth in vivo. Animals were treated with either PBS or 100% Nacetylated and 25% glycol-split heparin (NA-RO.H) for 28 days beginning 10 days after subcutaneous injection of heparanaseexpressing myeloma tumor cells. Bioluminescence imaging of luciferase-expressing tumor cells just prior to euthanasia of animals reveals all 5 animals treated with PBS have large tumors. In contrast, only 1/6 animals treated with NA-RO.H has clearly detectable tumor. This was the only tumor that was found in any of the 6 animals at necropsy [50].