Anti-Angiogenesis Drug Discovery and Development

Vascular endothelial growth factor (VEGF) plays a critical role in the pathogenesis of the vaso-proliferative phase of retinopathy of prematurity (ROP), being up-regulated by the adrenergic system and down-regulated by beta-blockers. Betablockers are anti-angiogenic agents and as such they decelerate the growth of skin hemangiomas and decrease the risks associated with life-threatening hemangiomas. Numerous previous studies investigated the effect of catechol amines on the angiogenesis of various cells in culture or on relevant animal models of angiogenesis. As of April 2015, two prospective randomized trials recently evaluated oral propranolol treatment for established ROP. Mahout et al. studied 20 premature infants with ROP (10-oral propranolol, 10–placebo) and showed a 50% reduction of need for invasive interventions. Filippo et al. treated 52 infants with ROP using oral propranolol and achieved 48% and 58% risk reduction of progression to stage 3 and stage 3 plus ROP, respectively, and a 52% reduction in the need for laser or bevacizumab therapy. To avoid systemic adverse effects of oral propranolol, rabbits and mice with oxygeninduced retinopathy (OIR) recently received propranolol eye drops, producing retinal concentrations similar to those measured after oral administration but with significantly lower plasma concentrations. Propranolol eye drops promoted OIR recovery in mice. Although the systemic (oral) or topical (eye drops) routes for beta-blocker therapy for ROP appear promising, questions have emerged regarding effectiveness, safety, tolerability and timing of treatment. Additionally, should beta-blockers prove safe and effective for ROP in premature infants, then the optimal therapeutic approach should be determined: Rescue Therapy for established ROP versus pre-emptive prophylactic treatment in premature infants at high risk for developing ROP, mainly those born prior to 28 weeks' gestation. In this chapter we review the literature as to the discovery of the key role of catechol amines on angiogenesis and their effects via VEGF as well as the approaches taken to implement anti-angiogenic therapy for ROP in animals and humans.

Snake venoms comprise disintegrins, proteins which target integrin receptor-dependent cell adhesion by endothelial cells. The disintegrins Obtustatin and Viperistatin, were used as lead compounds for the synthesis of linear and cyclic peptides containing the KTS binding motif. The most active linear peptide pointed to the importance of Cys19 and Cys29, and the presence of Arg24 for biological activity, and was used as the basic linear sequence for the synthesis of cyclic peptides. The most potent peptides, named Vimocin and Vidapin, showed a high potency (IC50 = 0.17 nM) and intermediate efficacy (20% and 40%) in inhibiting adhesion of α1/α2 integrinoverexpressing cells to collagen. Vimocin was more active in inhibiting wound healing and corneal micropocket vascularization, whereas Vidapin was more potent in reducing endothelial cell migration in the Matrigel tube assay. Both compounds similarly inhibited proliferation of endothelial cells and angiogenesis induced by vascular endothelial growth factor or glioma tumor cells in the chorioallantoic membrane angiogenic assay. These peptides were well tolerated by mice after intravenous injection. They showed stability in human serum between 10–30 hours. The in vitro and in vivo potency of these cyclic peptides is consistent with the computational modeling indicating conformational similarities to the parental molecules. Vidapin significantly increased the survival of mice injected with B16 melanoma cells up to 73 days, whereas the median survival time of animals in this tumor experimental model is 40 days. These findings propose that Vimocin and Vidapin can serve as dual α1β1/α2β1 integrin antagonists in angiogenesis and cancer therapy.

Increased angiogenic activity occurs frequently in lung cancer and results in biologically more aggressive disease. There has been intense research into therapeutic agents that inhibit angiogenesis and may improve treatment options for patients with lung cancer. Bevacizumab, a monoclonal antibody directed against serum VEGF, in combination with carboplatin and paclitaxel chemotherapy, has been shown to improve survival for NSCLC patients. Meta-analysis of trials of bevacizumab in combination with platinum-based chemotherapy for NSCLC, show a 10% reduction in the risk of death (HR 0.90, 95%CI 0.81 – 0.99). However, therapy with bevacizumab is limited to NSCLC patients with non-squamous histology, good performance status, no brain metastases and the absence of bleeding or thrombotic disorders. Similar efficacy has been seen also with carboplatin, pemetrexed plus maintenance pemetrexed chemotherapy. In the second-line setting, the addition of ramucirumab to docetaxel chemotherapy, resulted in a modest improvement in survival, adding a second antiangiogenic treatment option for patients with NSCLC.

A large number of trials in NSCLC have been conducted evaluating oral antiangiogenic compounds, both in first-line therapy in combination with chemotherapy, or upon disease progression, either as combination, or single agent therapy. Some level of activity has been observed with most agents. No clear improvements in overall survival have been observed, although a subgroup analysis of a trial evaluating the addition of nintedanib to docetaxel showed improved survival limited to patients with adenocarcinoma. However, these findings require validation. All of the oral antiangiogenic agents result in added toxicities. Some agents have resulted in an increased risk of death, limiting their development. Currently, there is no established role for anti-angiogenic therapy in SCLC, although there is some promise for sunitinib as maintenance therapy following platinum and etoposide chemotherapy.

Despite the large number of anti-angiogenic agents evaluated in clinical trials, there is evidence supporting a limited number of agents as treatment options for patients with lung cancer. To date no biomarkers have been identified. It is unclear whether treatment effects in a subpopulation, are lost among a larger unselected population of patients. There is a need for additional translational research to identify predictive biomarkers for anti-angiogenic therapy.

Hepatocellular carcinoma (HCC) is a primary tumor of the liver and one of the most malignant of tumors, with a mortality rate ranked second worldwide. Conventional therapeutics such as liver transplants and surgical resection, are of importance in HCC treatments. However, those treatments are limited and have a high frequency of tumor recurrence and metastasis. Currently, there is only one FDA approved anti-HCC drug - Sorafenib, which is well-tolerated. Thus, there is a great need to develop more effective HCC-targeting drugs. Angiogenesis, with the involvement of various angiogenic and anti-angiogenic factors, is fundamental for tumor growth, invasion and metastasis. HCC is a tumor type having a high level of neo-angiogenesis, and widely varying vascularity during tumor development. The accumulated experimental and clinical data indicate that HCC tumor progression is strongly associated with angiogenesis, with poor HCC prognosis related to the increase in micro-vascular density. A series of angiogenic factors have been investigated in HCC and show anti-HCC potential. Presently, there are certain anti-angiogenic therapeutics that have been evaluated in animal models and clinical trials. In this chapter, we will review the advances of HCC angiogenesis, angiogenic factors, the applications of anti-angiogenic agents, and also discuss the limitation, challenge and other potential strategies in searching for anti-HCC drugs via targeting angiogenesis.

Glioblastoma multiforme (GBM) is the most aggressive malignant primary brain tumor in adults with a very poor prognosis. The standard treatment for newly diagnosed glioblastoma is surgical debulking followed by radiotherapy and temozolomide (TMZ) with additional maintenance with TMZ. However, this regimen offers modest benefits with a median survival of less than 15 months, with an inevitable recurrence.

GBM is one of the most vascularized tumors; therefore antiangiogenic therapeutic strategies are very appealing. Bevacizumab, a humanized monoclonal antibody against vascular endothelial growth factor A, was the first FDA approved angiogenesis inhibitor, based on impressive response rates in recurrent GBM. Recent trials have shown that the combination of bevacizumab with standard radiotherapy–TMZ for the treatment of newly diagnosed glioblastoma resulted in improved median progressionfree survival, without gain in overall survival. Data regarding quality of life and functional status are conflicting. Not surprisingly, there was an increase in adverse events associated with bevacizumab therapy, namely thrombosis, bleeding and hypertension. Therefore, the efficacy of antiangiogenic therapy in the management of GBM remains unclear. To improve outcomes, it has been made a huge effort to better understand the biology underlying angiogenesis and tumor survival, as well as the mechanisms of antiangiogenic resistance in GBM.