Targeting Tumor Vasculature: Reality or a Dream?

In recent years, it has become clear that angiogenesis is important not only in physiological processes such as embryonic development, the female reproductive cycle, wound healing, and organ and tissue regeneration but also in pathological processes such as tumor progression and metastasis (Hanahan and Folkman, 1996). The process of angiogenesis, new capillary blood vessel growth from preexisting vasculature, is now recognized as an important control point in cancer, mainly because the hypothesis that tumors are angiogenesis-dependent has been confirmed by a variety of experiments, but especially by genetic methods. Most tumors do not start out angiogenic, but remain as small, pinpoint dormant tumors for years or a life-time. They cannot grow until they can recruit new blood vessels, i.e. switch to the angiogenic phenotype. As a result, the microvascular endothelial cell, recruited by a tumor, has become an important second target in cancer therapy, a target that unlike the tumor cells themselves, is genetically stable (Folkman, 2001a,b). Angiogenesis is a complex multicomponent process involving many growth factors and their receptors, cytokines, proteases and adhesion molecules (Carmeliet and Jain, 2000); thus multiple targets for therapeutic intervention and targeting opportunities for anti-angiogenic therapy for cancer exist. It has become feasible to propose that treating both the cancer cell and the endothelial cell in a tumor may be more effective than treating the cancer cell alone. Table I summarizes the advantages of targeting the vessels of the tumor instead of, or in addition to treating the tumor itself. As the target is the genetically normal endothelial cell, resistance to treatment due to somatic mutations in the target cell does not occur. Angiogenesis inhibitors are emerging as a new class of drugs. In the U.S. there are currently 24 angiogenesis inhibitors in various clinical trials for late stage cancer, 8 are in clinical trial Phase III (Table II). Members of this family of drugs differ by their targets and vary from low MW molecules to polypeptides and antibodies. Some are cytostatic (Endostatin, Angiostatin and TNP-470, VEGF antagonists or VEGFR inhibitors) and some, like the vascular targeting agents (VTA), are cytotoxic. VTAs allow rapid destruction of existing blood vessels in tumors containing activated endothelial cells (EC). They consist of antitubulin agents such as combretastatin (Hill et al., 2002) analogs (CA4P, CA1P, AVE 8062A, AVE 063), and colchicine analogs (i.e. ZD6126). Other drugs such as flavone acetic acid (FAA) analog and dimethyl-xanthenone-4-acetic acid (DMXAA) induce TNF-a and serotonin and inhibit blood flow. In spite of the difference between various angiogenesis inhibitors the common ground is that all can benefit from specific targeting. A proper delivery system would enable optimization of their pharmacokinetic profile. The development of biocompatible, controlled release systems for macromolecules has provided the opportunity for researchers and clinicians to target and deliver biologically active entities. Systems releasing such biologically important polypeptides, as growth factors as well as a number of important inhibitory factors or low MW drugs, are beginning to be utilized. The first in vivo screening of a peptide library binding to the human vasculature opens new possibilities for inhibiting angiogenesis and tumor growth. The hypothesis that tumor growth is angiogenesis-dependent (Folkman, 1971) and its subsequent confirmation by genetic methods (Folkman, 2001a,b; Lyden et al., 2001) provided strong incentive for scientists to try to target peptides specifically to the vascular bed of tumors. Pasqualini and Ruoslahti achieved the first step towards this goal in 1997 when they reported a novel in vivo phage display that distinguished between active proliferating microvascular EC in a tumor and quiescent nonproliferating EC elsewhere in the vasculature (Pasqualini et al., 1997). This methodology permitted