Inhibitors of tumor angiogenesis (new blood vessel formation induced by cancer) have become the new frontier in the fight against cancer. Some of these drugs are now in clinical trials. Tumor angiogenesis occurs through the response of blood vessels to growth factors, such as vascular endothelial growth factor (VEGF) and basic fibroblast growth factor, which are largely produced by cancer cells. The vascular endothelial cells (VECs), which form the internal lining of the blood vessels, begin to migrate toward the tumor, eventually providing the tumor with its own blood supply. Most angiogenesis inhibitors act at the level of the blood vessel. We have discovered novel agents that act directly on the tumor by inhibiting the synthesis and secretion of the angiogenic factors. We found that malignant invasive breast cancers aberrantly produce tissue factor (TF) and VEGF. TF is a signaling protein with components inside and outside the cell that interacts with the factor VIIa (fVIIa) normally present in the outer layer but not the inner layer (VECs) of the blood vessels. Upon injury of the blood vessels, fVIIa in the blood meets TF outside the blood vessels. When fVIIa binds to the extracellular domain of TF, the fVIIa-TF complex initiates blood clotting to prevent excessive bleeding. We found that the intracellular domain of TF in cancer cells mediates a signal occurring within the cells to produce VEGF. This is the mechanism by which breast cancer cells produce both TF and VEGF. In addition, TF is also expressed on the inner layer (VECs) of the blood vessels that are attracted into invasive breast tumors. Expression of TF on cancer cells and VECs within the tumors gives cancer cells several advantages. First, VEGF production and release by cancer cells attract blood vessels to the tumor. Second, when cancer cells enter into blood circulation and metastasize, aberrant expression of TF on cancer cell surfaces triggers blood clotting, which in turn protects cancer cells from attack by a variety of host defense mechanisms.
Curcumin, a major chemical component of the turmeric, has been shown to have antiangiogenic and antitumor properties by inhibiting expression of TF on VECs. We have demonstrated that curcumin inhibited TF and VEGF synthesis in human cancer cell lines. We also have synthesized more potent analogs of curcumin that exhibit similar properties. Some of the novel analogs, especially, EF15, A279L, and A279U, selectively inhibit VEGF synthesis without affecting cell viability. Thus, these analogs are potentially good candidates as antiangiogenic agents because they are less likely to be toxic to normal cells. The problem with antiangiogenic drugs, as with most cancer drugs, is that these agents are free to act anywhere in the body and are not selective for the blood vessels or the tumor itself. Therefore, unwanted side effects can occur due to the effect of these drugs on normal processes.
In this research proposal, we have devised a way to deliver antiangiogenic agents directly to the intended site of action. The method will achieve a therapeutic drug concentration with much less total dose compared to systemic, nontargeted drug administration, and thereby minimize side effects. Because TF is essentially expressed on malignant breast cancer cells and VECs within breast tumors but not on benign breast tumors, it is an ideal target for attacking only malignant breast cancers. Our strategy is to utilize specific inhibitors of fVIIa, such as FFR-ck-VIIa, as vehicles for delivering novel antiangiogenic compounds to the malignant tumor, since TF is the specific receptor for VIIa and FFR-ck-VIIa. FFR-ck-VIIa can bind five times more strongly than fVIIa. Upon binding of FFR-ck-VIIa to TF, a drug-carrier protein complex is internalized within cancer cells to deliver drugs directly. Thus, antiangiogenic agents, such as curcumin and its analogs, can be linked to a fVIIa inhibitor, such as FFR-ck-VIIa, and delivered to malignant breast cancer cells. This is the purpose and rationale of our research proposal.