Annett Rexin

Angiopoietin-2 Promotes Disease Progression of Neuroendocrine Tumors

Purpose: Inhibition of angiogenesis represents a promising therapeutic strategy in neuroendocrine tumors. Angiopoietin-2 (Ang-2), a ligand of the endothelial tyrosine kinase Tie-2, is emerging as a key regulator of vascular remodeling during tumorangiogenesis. We therefore addressed the expression and biological significance of Ang-2 in human neuroendocrine tumors. Experimental Design: Surgical specimens and serum from neuroendocrine tumor patients were used to determine Ang-2 expression by in situ hybridization or ELISA (circulating Ang-2). Ang-2 biological effects were evaluated following stable transfection into BON human pancreatic neuroendocrine tumor cells. BON clones were grown as orthotopic xenografts in nude mice to determine tumor growth and abdominal metastatic spread. Further analyses included microvessel density, lymphatic vessel density, and nodal invasion. Results: Specimens from pancreatic neuroendocrine tumors and nontransformed pancreatic tissue revealed uniform expression of Ang-2 mRNA in endothelial cells. In contrast, epithelial expression of Ang-2 mRNA occurred exclusively in neuroendocrine tumors. Overexpression of Ang-2 in BON orthotopic xenografts did not affect primary tumor growth, although successful Ang-2 induction was confirmed from elevated serum levels. However, increased microvessel density and enhanced lymphatic metastasis were evident in Ang-2–expressing tumors, indicating a functional role of Ang-2 in experimental neuroendocrine tumors. Consistent with this notion, circulating Ang-2 was significantly elevated in neuroendocrine tumor patients compared with healthy controls. Circulating Ang-2 furthermore correlated with metastatic versus localized disease. The highest Ang-2 concentrations occurred in patients with liver metastasis, and concentrations ≥75th percentile predicted shorter survival (P = 0.0003). Conclusion: Induction of Ang-2 in neuroendocrine tumors represents a clinically relevant pathomechanism of disease progression and constitutes an adverse prognostic marker. Clin Cancer Res; 16(2); 420–9.

An Orthotopic Model of Pancreatic Somatostatin Receptor (SSTR)-Positive Tumors Allows Bimodal Imaging Studies Using 3T MRI and Animal PET-Based Molecular Imaging of SSTR Expression

Somatostatin receptor (SSTR) scintigraphy is currently used as one standard imaging modality in neuroendocrine tumors (NETs). However, future optimization of NET imaging may be achieved with positron emission tomography based methods utilizing more sensitive and specific tracers in combination with computed tomography or magnetic resonance imaging. Here we established an orthotopic mouse model that reflects relevant aspects of human pancreatic NETs such as SSTR expression, dense vascularization and metastatic disease. This model was then utilized to test the feasibility of combined magnetic resonance imaging and animal positron emission tomography. Orthotopic implantation of amphicrine, SSTR-positive pancreatic AR42J cells resulted in rapidly growing tumors, with concomitant metastatic spread into abdominal lymph nodes and peritoneal cavity. Primary tumors as well as their metastases expressed the neuroendocrine markers chromogranin A and synaptophysin. For imaging experiments, the SSTR ligands 68Ga-DOTATOC or 68Ga-DOTANOC were injected intravenously, and animals were subsequently examined in an animal positron emission tomography scanner and a clinical 3T (tesla) magnetic resonance imager. All animals showed radionuclide accumulation in the primary tumor. Definite anatomical correlation was achieved using digital image fusion of the positron emission tomography and magnetic resonance imaging data. 68Ga-DOTANOC strongly accumulated in the tumor tissue (mean 6.6-fold vs. control tissues) when compared to 68Ga-DOTATOC, which showed a higher renal clearance. In good agreement with the biodistribution data, the kidney-to-tumor ratio was higher for 68Ga-DOTATOC (2.43-fold vs. 1.75-fold). Consequently, 68Ga-DOTANOC achieved better signal enhancement in the primary tumor and allowed for detection of metastatic lesions. In summary, we established a novel orthotopic pancreatic SSTR-positive tumor model and used this model to provide proof of principle for the diagnostic combination of SSTR-based molecular imaging and magnetic resonance imaging. Specifically, the animal model allowed the comparative evaluation of 68Ga-DOTANOC and 68Ga-DOTATOC, with 68Ga-DOTANOC providing better tumor-specific accumulation and renal activity. We conclude that this animal model will be of innovative value for further investigation in the imaging of NETs.