The Heparanase Inhibitor SST0001 Is a Potent Inhibitor of Myeloma Growth In Vivo

Heparanase (HPSE) is an enzyme that cleaves heparan sulfate (HS) chains of proteoglycans. Work by us and others has demonstrated that heparanase promotes the growth and metastasis of many types of tumors, including multiple myeloma (MM). Heparanase expression is rare in normal tissue but becomes evident in many human tumors, making it a viable target for cancer therapy. SST0001, a chemically modified heparin that is 100% N-acetylated and 25% glycol-split, dramatically inhibits heparanase activity. SST0001 lacks anticoagulant activity and thus can be administered at relatively high doses in vivo. We previously reported that delivery of SST0001 by Alzet osmotic pumps to SCID mice potently inhibited growth of subcutaneous tumors formed by CAG human myeloma cells. In the present studies, we further tested the effects of SST0001 against additional MM cell lines, using alternative routes of drug delivery in two different animal models. Ten days after subcutaneous injection of either MM.1S or RPMI 8226 tumor cells, mice were treated for 28 days using Alzet pumps delivering 30 mg/kg/day of SST0001. Results showed that, compared to PBS control, MM.1S and RPMI-8226 tumors in SST0001-treated mice were reduced by 50% and 51%, respectively. In a separate experiment, delivery of SST0001 by distant subcutaneous injection inhibited tumor growth by 77% in comparison to controls. In the SCID-hu model, in which CAG cells were implanted directly into human bones engrafted in SCID mice, SST0001 also significantly inhibited tumor growth as measured by human immunoglobulin kappa light chain in murine sera (1055 ± 295 ng/ml in PBS-treated mice vs 155 ± 295 ng/ml in SST0001- treated mice (P <0.003)). These data demonstrate that SST0001 is a strong inhibitor of MM growth in vivo, even when tumors grow within the bone microenvironment and that the effect of SST0001 is not cell-line specific. We did not observe any adverse side effects in animals, even at doses as high as 120 mg/kg/day. To determine the mechanism of action of SST0001, we examined several pharmacodynamic parameters. Immunohistochemistry demonstrated that SST0001 treatment significantly reduced microvessel density of tumors as compared to controls (99% in CAG and 54% in RPMI-8226 tumors). In addition, SST0001 treatment blocked HGF expression (CAG, RPMI 8226 and MM.1S tumors) and inhibited VEGF expression in CAG tumors but not RPMI 8226 and MM.1S tumors. Moreover, a series of in vitro experiments, using the CAG MM cell line and human umbilical vein endothelial cells (HUVEC), were performed. Unlike its strong antitumor effect in vivo, SST0001 only slightly inhibited CAG cell proliferation, cell cycle and growth factor signaling in vitro, suggesting that the compound does not have a direct cytotoxic effect on tumor cells. Since blood vessels are an important element of the tumor microenvironment and angiogenic endothelium in tumors also expresses high levels of heparan sulfate proteoglycans and heparanase, we assessed the effects of SST0001 on HUVEC cells. In contrast with results on CAG MM cells, SST0001 treatment showed a strong inhibition on HUVEC proliferation (46%, MTT assay), dramatically blocked the phosphorylation of ERK stimulated by HS-binding growth factors (HGF, VEGF, HDGF and EGF), blocked the Akt pathway of HGF signaling in HUVECs and inhibited HUVEC tube formation, stimulated by HGF and VEGF. Based on these results, we conclude that SST0001 strongly inhibits the growth of myeloma tumors in vivo by targeting the tumor microenvironment, including a significant inhibition of tumor angiogenesis. Because of its unique target site in the tumor microenvironment, we predict that the combination of SST0001 with conventional tumor cell-targeting chemotherapeutic drugs will greatly improve patient outcome in MM.