Targeting tumor stroma

VEGF in nuclear medicine: Clinical application in cancer and future perspectives (Review)

Clinical trials using antiangiogenic drugs revealed their potential against cancer. Unfortunately, a large percentage of patients does not yet benefit from this therapeutic approach highlighting the need of diagnostic tools to non-invasively evaluate and monitor response to therapy. It would also allow to predict which kind of patient will likely benefit of antiangiogenic therapy. Reasons for treatment failure might be due to a low expression of the drug targets or prevalence of other pathways. Molecular imaging has been therefore explored as a diagnostic technique of choice. Since the vascular endothelial growth factor (VEGF/VEGFR) pathway is the main responsible of tumor angiogenesis, several new drugs targeting either the soluble ligand or its receptor to inhibit signaling leading to tumor regression could be involved. Up today, it is difficult to determine VEGF or VEGFR local levels and their non-invasive measurement in tumors might give insight into the available target for VEGF/VEGFR-dependent antiangiogenic therapies, allowing therapy decision making and monitoring of response.

Role of the VEGF/VEGFR axis in cancer biology and therapy

New vessel formation (angiogenesis) is an essential physiological process for embryologic development, normal growth, and tissue repair. Angiogenesis is tightly regulated at the molecular level; however, this process is dysregulated in several pathological conditions such as cancer. The imbalance between pro- and antiangiogenic signaling molecules within tumors creates an abnormal vascular network that is characterized by dilated, tortuous, and leaky vessels. The pathophysiological consequences of these vascular abnormalities include temporal and spatial heterogeneity in tumor blood flow, oxygenation, and increased tumor interstitial fluid pressure. The resultant microenvironment deeply impacts on tumor progression, and also leads to a reduction in therapy efficacy. The discovery of vascular endothelial growth factor (VEGF) as a major driver of tumor angiogenesis has led to efforts to develop novel therapeutics aimed at inhibiting its activity. Anti-VEGF therapy has become an important option for the management of several human malignancies; however, a significant number of patients do not respond to anti-VEGF therapy when used either as single agent or in combination with chemotherapy. In addition, the benefit of antiangiogenic therapy is relatively short lived and the majority of patients relapse and progress. An increasing amount of reports suggest several potential mechanisms of resistance to antiangiogenic therapy including, but not limited to, tumor hypoxia. This chapter discusses the role of the VEGF axis in tumor biology and highlights the clinical application of anti-VEGF therapies elaborating on pitfalls and strategies to improve clinical outcome.

Targeting lymphatic vessel functions through tyrosine kinases

The lymphatic vascular system is actively involved in tissue fluid homeostasis, immune surveillance and fatty acid transport. Pathological conditions can arise from injury to the lymphatics, or they can be recruited in the context of cancer to facilitate metastasis. Protein tyrosine kinases are central players in signal transduction networks and regulation of cell behavior. In the lymphatic endothelium, tyrosine kinases are involved in processes such as the maintenance of existing lymphatic vessels, growth and maturation of new vessels and modulation of their identity and function. As such, they are attractive targets for both existing inhibitors and the development of new inhibitors which affect lymphangiogenesis in pathological states such as cancer. RNAi screening provides an opportunity to identify the functional role of tyrosine kinases in the lymphatics. This review will discuss the role of tyrosine kinases in lymphatic biology and the potential use of inhibitors for anti-lymphangiogenic therapy.