Heparins as an Anti-tumor Therapy
The commonly used heparins include the unfractionated heparin (UFH) sourced from animal tissues, and biochemically derived low-molecular-weight heparins (LMWHs). The primary effect of heparins on the coagulation system is related to the inhibition of thrombin and factor Xa action. The mechanisms of heparin-mediated anti-tumor effects are more complex and involve growth factor release from the endothelial cell surface, obstruction of P- and L-selectin-associated cell adhesion, heparanase protein inhibition, and reduction in the coagulation system activity. The main potential shortcoming of heparins is their non-specific interactions with proteins, such as growth factors and selectins. For instance, tissue factor pathway inhibitor (TFPI), which is both an anti-angiogenic and anti-metastatic protein, is also released by heparins from the endothelial cell surface. The absence of selectivity could restrict the utility of heparins as anti-cancer drugs. An additional major disadvantage of heparins is associated with their ability to induce bleeding, which limits the dosage to be prescribed for patients with malignancies.
Lately, a Cochrane analysis was published, evaluating the effectiveness and safety of heparins in ambulatory cancer patients. The study included 7,622 participants from 15 randomized controlled trials (RCTs) where either UFH or LMWH was employed. The meta-analysis showed an insignificant effect of the drugs on 12-month and 24-month mortality, with a decrease in VTE and an increase in minor bleeding. The authors concluded that the effect of heparins on the survival of cancer patients should be further assessed in relation to a specific type and stage of malignancy. Given the paucity of such data, at present, the judgment regarding the utility of heparin in a specific patient with cancer has to rely on an individual risk assessment, taking into account the patient’s quality of life.18
Heparanase Procoagulant Activity Inhibition
Heparanase is an endoglycosidase able to cleave heparan sulfate (HS) side chains in restricted sites, causing liberation of HS fragments of approximately 5–7 kDa, which can still enhance anti-thrombin activity.19,20
Heparanase has been shown to augment tumor cell metastasis through HS cleavage and alteration of the extracellular matrix (ECM), increasing cell dissemination.21,22
Additionally, heparanase, released from activated cells of the immune system and promoting migration of vascular endothelial cells, appears to affect neovascularization, inflammation, and autoimmunity.21–23
Furthermore, high levels of heparanase expression have been documented in most human tumors, wound healing, and diabetic nephropathy.21–23
In contrast to multiple proteases that can degrade peptides in the ECM, heparanase is currently found to be the only human protein capable of degrading HS chains.21,22
Heparanase protein is mainly expressed in the placenta, platelets, granulocytes, and monocytes, with low or no expression in connective tissues and normal epithelia.24–27
We have previously demonstrated that heparanase regulates the expression of TF28 and releases TFPI from the endothelial cell surface membrane and tumor cells by direct interaction, intensifying cell surface coagulant activity.29 Additionally, we have shown that through a direct interaction with TF, heparanase actually serves as a cofactor in TF functioning, leading to elevated factor Xa production with ensuing coagulation system activation.30 Data supporting the procoagulant effect of heparanase have been published by Baker et al.,31 who have demonstrated in arterial injury and stent occlusion models that mice over-expressing heparanase produce a larger thrombus within a shorter time period compared to control animals.
While an elevated heparanase level in the plasma and biopsies of patients with cancer had been previously reported, we have recently explored the heparanase procoagulant activity in the plasma samples drawn at presentation from 65 patients with non-small cell lung cancer and compared the findings with those of 20 controls.32 Remarkably, not only was the heparanase antigen level higher in the patient group compared to controls (P=0.05), but also the heparanase procoagulant activity appears to be significantly increased in the patient cohort (P<0.0001). Furthermore, high heparanase procoagulant activity has been shown to correlate with a short survival (P=0.001). Therefore, high heparanase procoagulant activity could be a new mechanism of coagulation system activation in cancer, which might be further evaluated as a biomarker of patient survival.
The association of the coagulation system with cancer progression and inflammation has been further confirmed in our recent study where newly described peptides derived from TF pathway inhibitor 2 (TFPI-2) first Kunitz domain have demonstrated their ability to hinder the heparanase procoagulant activity by obstructing the interaction of TF with heparanase. Notably, in vivo, these peptides appear to significantly diminish activation of the coagulation system and sepsis severity induced by lipopolysaccharides, without putting the patient at risk of substantial bleeding.33 In another study, these peptides have significantly attenuated tumor growth and vascularization.34 Additionally, increased survival rate and a delay in tumor relapse have been demonstrated in mice treated with the peptides in question compared to control animals. Remarkably, neither in these two studies,33,34 nor in an inferior vena cava ligation model,33 was the side effect of bleeding detected. These results reinforce the evidence of existing interaction between cancer, inflammation, and the hemostatic system. Recently, Bochenek et al. have shown in a model of endothelial senescence that inhibition of heparanase activity using the TFPI-2 peptides prevents the enhanced venous thrombus formation in aged mice and restores it to the thrombotic phenotype of control adult mice.35 Thus, inhibition of heparanase may also potentially affect the process of aging.