Inhibition of Tumor‐induced Angiogenesis by a Synthetic Lipid A Analogue with Low Endotoxicity, DT‐5461

We investigated the effect of a synthetic lipid A analogue with low endotoxicity, DT‐5461, on the neovascularization induced by B16‐BL6 melanoma in syngenelc mice. A systemic single administration of DT‐5461 caused a marked decrease in the number of vessels oriented toward the tumor mass and in the tumor size during the early phase of vasculogenesis (on day 4 after tumor inoculation), with little or no inhibition in the following phases. Multiple i.v. administrations of DT‐5461 at intervals of 4 days (an effective schedule for inhibiting tumor metastasis) significantly reduced the number of capillary vessels and tumor growth over a period of 14 days after the tumor implantation. Multiple systemic administrations of DT‐5461 on days 1, 5 and 9 after tumor inoculation caused a high production of endogenous tumor necrosis factor‐α (TNF‐α) in tumor sites although this treatment modality induced a low production in serum of tumor‐bearing mice. Tumor homogenate from mice treated with DT‐5461 suppressed the proliferation of endothelium in vitro, whereas sera from animals given DT‐5461 had little effect. Furthermore, the antiproliferative effect of the tumor homogenate from mice treated with DT‐5461, was completely abrogated by anti‐mTNF‐α monoclonal antibody (mAb). The anti‐angiogenic effect of DT‐5461 was also completely abrogated by rabbit anti‐mouse tumor necrosis factor‐α (anti‐mTNF‐α) antiserum, whereas the inhibition of tumor growth by DT‐5461 was only slightly diminished. Tumor homogenate from mice treated with DT‐5461 suppressed the proliferation of endothelium in vitro, whereas sera from animals given DT‐5461 had little effect. Furthermore, the antiproliferative effect of the tumor homogenate from mice treated with DT‐5461 was completely neutralized by anti‐mTNF‐α mAb. Multiple i.v. administrations of DT‐5461 after s.c. implantation of B16‐BL6 cells significantly inhibited the growth of primary tumors measured at the time of tumor excision on day 21, and the lung metastasis of melanoma cells as compared with the untreated control in the spontaneous metastasis model. These results suggested that the suppressive effect upon tumor‐associated angiogenesis by DT‐5461 contributes in part to the inhibition of tumor metastasis.