Biol. Pharm. Bull. 29(4) 709—712 (2006)

First, tumor cells are released from the primary tumor and invade the surrounding tissues, enter the vascular or lymphatic circulation, and transit there. Next, the cells arrest in the capillary bed of a distant organ and extravasate from the circulation to organ parenchyma. Finally, tumor cells grow at apparently selective sites which are distant from the original site. Neovascularization toward and into a tumor is crucial for the delivery of nutrition and oxygen to that tumor, and also functions as the metastatic pathway to distant organs. The invasion by tumor cells and endothelial cells has a similar process involving cell adhesion, motility and the degradation of the extracellular matrix (ECM) and tissues. Aminopeptidase N (APN)/CD13 is a Zn (2 )-dependent exopeptidase that is anchored to the plasma membrane by its N-terminal segment and acts as an ectoenzyme. We have reported that APN plays an important role in tumor-cell invasion, degradation of the ECM by tumor cells and tumor metastasis and angiogenesis in vitro and in vivo. Functional studies using tumor invasion assays revealed that APN on the tumor cell surface cleaves certain ECM proteins, including type IV collagen and some components of Matrigel. In addition, transfection of the APN gene into A375 human melanoma (A375) cells significantly enhanced the metastatic potential of tumor cells on their intravenous inoculation into nude mice. Bestatin is an inhibitor of APN and aminopeptidase B. The immunomodulatory properties of bestatin have been examined in both preclinical and clinical studies. The pharmacokinetics and biotransformation of bestatin have also been examined in detail. Bestatin has been shown to augment the humoral and cellular immune responses in experimental animals, and can activate murine macrophages to become cytotoxic against tumor cells in vivo and in vitro, possibly through a T-dependent mechanism. Talmadge et al. reported significant therapeutic potential of bestatin against pre-existing experimental and spontaneous metastases when it was orally administered at high doses per animal. We have shown that Bestatin inhibited tumor cell invasion and the formation of capillary structure by HUVECs in vitro and angiogenesis induced by B16-BL6 in syngeneic mice. We also found that the inhibition of tumor invasion and angiogenesis by bestatin is mainly mediated by inhibition of the degradation of the ECM and basement membrane (BM) by tumor and endothelial cells. In the present study, we investigated the effects of bestatin derivatives on the invasive activity and motility of tumor and endothelial cells in vitro and capillary formation of endothelial cells in vitro.