N-myc down-regulates activin A

The selective VEGFR1-3 inhibitor axitinib (AG-013736) shows antitumor activity in human neuroblastoma xenografts

Tumor angiogenesis in childhood neuroblastoma is an important prognostic factor suggesting a potential role for antiangiogenic agents in the treatment of high-risk disease. Within the KidsCancerKinome project, we evaluated the new oral selective pan-VEGFR tyrosine kinase inhibitor axitinib (AG-013736) against neuroblastoma cell lines and the subcutaneous and orthotopic xenograft model IGR-N91 derived from a primary bone marrow metastasis. Axitinib reduced cell proliferation in a dose-dependent manner with IC(50) doses between 274 and >10,000 nmol/l. Oral treatment with 30 mg/kg BID for 2 weeks in advanced tumors yielded significant tumor growth delay, with a median time to reach five times initial tumor volume of 11.4 days compared to controls (p = 0.0006) and resulted in significant reduction in bioluminescence. Simultaneous inhibition of VEGFR downstream effector mTOR using rapamycin 20 mg/kg q2d×5 did not statistically enhance tumor growth delay compared to single agent activities. Axitinib downregulated VEGFR-2 phosphorylation resulting in significantly decreased microvessel density (MVD) and overall surface fraction of tumor vessels (OSFV) in all xenografts as measured by CD34 immunohistochemical staining (mean MVD ± SD and OSFV at 14 days 21.27 ± 10.03 in treated tumors vs. 48.79 ± 17.27 in controls and 0.56% vs. 1.29%; p = 0.0006, respectively). We further explored the effects of axitinib on circulating mature endothelial cells (CECs) and endothelial progenitor cells (CEPs) measured by flow cytometry. While only transient modification was observed for CECs, CEP counts were significantly reduced during and up to 14 days after end of treatment. Axitinib has potent antiangiogenic properties that may warrant further evaluation in neuroblastoma.

[Potential role of antiangiogenic treatment in neuroblastoma]

Focus on new drug development over the last few years has yielded new agents that differ from unspecific classical chemotherapeutics and ionizing radiation, while still targeting the cancer cell itself. Antiangiogenesis is a totally distinct approach targeting the tumor's blood vessels. This concept has now found its eligibility for the treatment of several adult solid tumors: the human antivascular endothelial growth factor (VEGF) antibody bevacizumab, as well as the VEGF receptor tyrosine kinase inhibitors, sunitinib and sorafinib, have recently been licensed by the US Food and Drug Administration (FDA) and the European Medicines Agency (EMEA) for the treatment of colorectal, renal, and lung cancer. Other antiangiogenic drugs are under preclinical and early clinical evaluation. However, what do we know of the use of these drugs in pediatric solid tumors, such as sarcomas and embryonal and neuronal tumors? For some time now, neuroblastoma has been shown to be dependent on angiogenesis. However, the first preclinical data on antiangiogenic drugs in neuroblastoma have not been published until recently, and clinical trials with antiangiogenic agents in neuroblastoma treatment protocols are scarce. This review adresses current knowledge on the important role and mechanisms of angiogenesis in neuroblastoma and summarizes available preclinical and clinical results of antiangiogenic agents used to treat neuroblastoma. Our review clearly demonstrates that clinical trials are urgently needed to bring forward promising antiangiogenesis concepts in neuroblastoma therapy.

Prospects for therapeutic inhibition of neuroblastoma angiogenesis

Despite aggressive therapy, survival for advanced stage neuroblastoma remains poor with significant long-term morbidity in disease survivors. High-risk disease features are strongly correlated with tumor vascularity, suggesting that angiogenesis inhibitors may be a useful addition to current therapeutic strategies. However, challenges include the well-known clinical heterogeneity and embryonal origins of this disease, which suggests a complex regulation of neovascularization that may be distinct from epithelial-derived carcinomas. We will review what is understood about angiogenesis-related signaling in neuroblastoma. In particular, we will present evidence that angiogenesis-related molecules are differentially expressed in primary neuroblastomas in a pattern suggesting promotion of a pro-angiogenic phenotype in high-risk tumors and an anti-angiogenic phenotype in low-risk tumors. We will also discuss a variety of vascular inhibition strategies that have been used in neuroblastoma preclinical models including specific inhibition of vascular endothelial growth factor (VEGF) and methionine aminopeptidase 2 (MetAP2). Recent observations that the combination of angiogenesis inhibitors with conventional chemotherapy provides synergy without additive toxicity, suggests the potential use of angiogenesis inhibitors as an adjunct between cycles of conventional cytotoxic therapy. Further identification of critical angiogenic signaling pathways and evaluation of specific inhibitors in preclinical neuroblastoma models should provide justification for future selection and evaluation of angiogenesis inhibitors in clinical trials for high-risk neuroblastoma patients.

Blood and lymph vessels in embryonic tumors

Formation of embryonic blood and lymph vessels is mediated by different steps of vasculogenesis and angiogenesis. The vascular endothelial growth factor family (VEGF) as well as the VEGF-receptors on the surface of blood endothelial cells and lymph endothelial cells are responsible for both processes. In addition to the embryonic development of the vessel systems, the interactions of angiogenesis factors and receptors are also present in solid tumors. Embryonic tumors in children offer an interesting target for new substances developed for anti-angiogenesis. Some data on the role of blood vessels and anti-blood-angiogenesis are available on embryonic tumors. However, studies of lymph-angiogenesis are not found and anti-lymph-angiogenesis is not at all examined in embryonic tumors. This review gives an overview of the challenging field of angiogenesis and anti-angiogenesis of both blood and lymph vessels with a focus on embryonic tumors.

Activins and inhibins in endocrine and other tumors

Inhibin and activin are members of the TGF beta superfamily of growth and differentiation factors. They were first identified as gonadal-derived regulators of pituitary FSH and were subsequently assigned multiple actions in a wide range of tissues. More recently, the inhibin alpha subunit was considered as a tumor suppressor based on functional studies employing transgenic mouse models. This review evaluates the functional and molecular evidence that the inhibin alpha subunit is a tumor suppressor in endocrine cancers. The evaluation highlights the discrepant results from the human and mouse studies, as well as the differences between endocrine tumor types. In addition, we examine the evidence that the activin-signaling pathway is tumor suppressive and identify organ-specific differences in the actions and putative roles of this pathway in endocrine tumors. In summary, there is a considerable body of evidence to support the role of inhibins and activins in endocrine-related tumors. Future studies will define the mechanisms by which inhibins and activins contribute to the process of initiation, promotion, or progression of endocrine-related cancers.