Current Pharmaceutical Design - Volume 10, Issue 1, 2004

The field of angiogenesis modulation is at a major crossroad. A tremendous advancement in basic science in this field is providing an excellent support for the concept, which is in contrast to a lack of strong clinical support to date. With regard to the large gap between experimental data and clinical data, the best model of human malignancy is in human cancer patients and the best model of human ocular angiogenesis-mediated disorders such as diabetic retinopathy (DR) and age related macular degeneration (AMD) is in human RD and AMD patients. Additionally, clinical outcomes should include benefit / risk ratios, hard end points (mortality and quality of life as opposed to increased microvascular density with pro-angiogenic agents or tumor size reduction with anti-angiogenesis agents) as well as cost effectiveness. Experimental models should be used to provide guidance, placebo effect, comparative data, and mechanistic understanding as opposed to being used for expected clinical efficacy. We also have to understand existing strategies and how angiogenesis modulation can add further value (i.e. not to replace existing strategy but rather improve efficacy / safety). Recent investigation defined numerous strategies in the modulation of angiogenesis. Those strategies are driven from haemostatic, fibrinolytic, cell adhesion molecules, extracellular matrix, growth factors, and other endogenous systems involved in the modulation of angiogenesis.

The induction of neoangiogenesis is a critical step already present at the early stages of tumor development and dissemination. The progressive identification of molecules playing a relevant role in neoangiogenesis has fostered the development of a wide variety of new selective agents. Antiangiogenic drugs should be integrated with conventional therapies; however, the design of the best sequence and timing for such combined treatments are still under investigation. In this review will be discussed the signal transduction mechanisms of angiogenic molecules, the development of specific inhibitors and their translation into clinical studies and, finally, the new perspectives in antiangiogenic therapy.

Angiogenesis, the formation of blood vessels from preexisting ones, plays a crucial role in tumor progression. Activation of an “angiogenic switch” allows tumor cells to invade and metastasize. The growing interest in the use of antiangiogenic agents in the treatment and prevention of cancer lies in the theoretical advantages of this molecularly targeted modality of chemotherapy. Delivery of antiangiogenic agents are not complicated by having to penetrate large bulky masses but, instead, have easy access to tumoral endothelial cells. Antiangiogenic drugs may not cause cytopenias and thus will avoid many of the unwarranted toxicities of standard chemotherapeutic agents. Because they act directly on nascent endothelial cells, antiangiogenic agents may avoid tumor resistance mechanisms. If antiangiogenic agents are successful, they might be applicable to many tumor types and not be dependent on cell type or growth fraction of cells within a tumor. However, several important obstacles remain with regards to using antiangiogenic drugs in clinical trials with which we must contend in order to determine accurately the efficacy of these agents. In this article, we review the different classes of antiangiogenic agents available, ongoing clinical trials, as well as potential pitfalls and future directions in this exciting field.

The urokinase plasminogen activator (uPA) system consists of the serine protease uPA, its glycolipid-anchored receptor, uPAR and its 2 serpin inhibitors, plasminogen activator inhibitor-1 (PAI-1) and plasminogen activator inhibitor- 2 (PAI-2). Recent findings suggest that the uPA system is causally involved at multiple steps in cancer progression. In particular, uPA has been implicated in remodelling of the extracellular matrix, enhancing both cell proliferation and migration and modulating cell adhesion. Consistent with its role in cancer progression, multiple groups have shown that high levels of uPA in primary breast cancers are independently associated with adverse outcome. Paradoxically, high levels of PAI-1 also correlate with poor prognosis in patients with breast cancer. The prognostic value of uPA / PAI-1 in axillary node-negative breast cancer patients was recently validated using both a prospective randomised trial and a pooled analysis, i.e., in 2 different Level 1 Evidence studies. Assay of uPA and PAI-1 may thus help identify low risk node-negative patients for whom adjuvant chemotherapy is unnecessary. Finally, preclinical studies show that either inhibition of uPA catalytic activity or prevention of uPA binding to its receptor reduces tumor growth, angiogenesis and metastasis.

The number of anti-angiogenic agents developed for clinical use has risen greatly over the past decade. Currently, more than 80 are in trials ranging from phase I through to phase III studies and many more are in preclinical evaluation. Much hope was envisaged for these new agents to become the panacea of anti-tumoural treatment. Unfortunately the single agent activity to date has proven to be disappointing although one trial has recently reported a survival advantage when chemotherapy was administered with anti-VEGF antibodies in the setting of advanced colorectal cancer. To an extent, this may be due to great expectations of cytostatic compounds, but recently many factors have been examined to explain the differences between clinical and experimental findings. In this review, some of the factors responsible for the discrepancy are examined, with a specific focus on inhibitors of VEGF. The key factors responsible for the lack of activity are tumour heterogeneity and redundancy in the VEGF signalling system. An increased understanding of these factors is critical to the development of effective anti-angiogenic agents and need to be taken into account as new generations of drugs emerge

The dysfunction or proliferation of lymphatic vessels (lymphangiogenesis) is linked to a number of pathological conditions including lymphedema and cancer. The recent discovery and characterisation of the lymphangiogenic growth factors vascular endothelial growth factor-C (VEGF-C) and VEGF-D and of their receptor on lymphatic endothelial cells, VEGFR-3, has provided an understanding of the molecular mechanisms controlling the growth of lymphatic vessels. In addition, other genes and protein markers have been identified with specificity for lymphatic endothelium that have enhanced the characterization and isolation of lymphatic endothelial cells. Our growing understanding of the molecules that control lymphangiogenesis allows us to design more specific drugs with which to manipulate the relevant signalling pathways. Modulating these pathways and other molecules with specificity to the lymphatic system could offer alternative treatments for a number of important clinical conditions.

Studies on the lymphatic endothelium have been hampered by the difficulty to identify lymphatic endothelial cells (LECs) and to distinguish them from blood vascular endothelial cells (BECs). The situation was greatly improved by the identification of molecules with high specificity for LECs. A great deal of progress in the field of lymphangiogenesis research has been due to the detection of lymphangiogenic growth factors and their receptors, and there is growing evidence that these molecules are also involved in tumor-induced lymphangiogenesis and lymphatic dissemination of tumor cells. There is a considerable spectrum of congenital and acquired lymphedema-lymphangiodysplasia syndromes ranging from primary aplasia, hypoplasia and hyperplasia to secondary (acquired) obstructive, obliterative and surgical hindrance of lymph drainage. Consequently, there are a number of clinical applications for therapeutics that either inhibit or induce lymphangiogenesis. Although natural lymphatic regeneration is mostly very efficient, engineering of LECs may be useful in cases of lymphatic aplasia or hypoplasia. To achieve these goals, studies on the embryonic development and differentiation of LECs will reveal the key regulatory factors that need to be targeted.