18-kDa translocator protein ligand 18F-FEMPA: Biodistribution and uptake into atherosclerotic plaques in mice.

BACKGROUND: Radioligands of 18-kDa translocator protein (TSPO) expressed on activated macrophages are a potential approach for imaging of inflammation in atherosclerosis. We evaluated a novel TSPO-targeted tracer 18F-FEMPA for the detection of atherosclerotic plaque inflammation in mice. METHODS AND RESULTS: The distribution kinetics of 18F-FEMPA was evaluated by in vivo PET/CT imaging. 18F-FEMPA uptake was compared in atherosclerotic (LDLR-/-ApoB100/100, n = 10) and healthy mice (C57BL/6 N, n = 7) ex vivo at twenty minutes post-injection. Biodistribution was analyzed from harvested tissue samples, and aortas were sectioned for autoradiography. Aortas of LDLR-/-ApoB100/100 mice showed large, macrophage-rich atherosclerotic plaques. In vivo, 18F-FEMPA showed rapid blood clearance but no difference in aortic uptake between atherosclerotic and healthy mice. In the mice studied ex vivo at 20 minutes post-injection, quantification of radioactivity in the whole aorta showed 1.3-fold higher 18F-FEMPA accumulation in atherosclerotic than healthy mice (P = .028). Autoradiography showed higher tracer uptake in plaque areas with high macrophage content as compared with areas of no macrophages (count densities 190 ± 54 vs 40 ± 13 PSL/mm2, P < .001), but the uptake in the plaques was not higher than in the normal vessel wall (230 ± 78 PSL/mm2). In vitro blocking showed specific accumulation in mouse and human atherosclerotic plaques. Immunohistochemistry confirmed co-localization of TSPO and macrophages. CONCLUSIONS: 18F-FEMPA shows rapid blood clearance and uptake in the mouse aorta. Uptake in atherosclerotic plaques correlated with the amount of macrophages, but did not exceed that in the normal vessel wall.

Positron emission tomography imaging of the 18-kDa translocator protein (TSPO) with [18F]FEMPA in Alzheimer's disease patients and control subjects.

PURPOSE: Imaging of the 18-kDa translocator protein (TSPO) is a potential tool for examining microglial activation and neuroinflammation in early Alzheimer's disease (AD). [(18)F]FEMPA is a novel high-affinity second-generation TSPO radioligand that has displayed suitable pharmacokinetic properties in preclinical studies. The aims of this study were to quantify the binding of [(18)F]FEMPA to TSPO in AD patients and controls and to investigate whether higher [(18)F]FEMPA binding in AD patients than in controls could be detected in vivo. METHODS: Ten AD patients (five men, five women; age 66.9 ± 7.3 years; MMSE score 25.5 ± 2.5) and seven controls (three men, four women; age 63.7 ± 7.2 years, MMSE score 29.3 ± 1.0) were studied using [(18)F]FEMPA at Turku (13 subjects) and at Karolinska Institutet (4 subjects). The in vitro binding affinity for TSPO was assessed using PBR28 in a competition assay with [(3)H]PK11195 in seven controls and eight AD patients. Cortical and subcortical regions of interest were examined. Quantification was performed using a two-tissue compartment model (2TCM) and Logan graphical analysis (GA). The outcome measure was the total distribution volume (V T). Repeated measures analysis of variance was used to assess the effect of group and TSPO binding status on V T. RESULTS: Five AD patients and four controls were high-affinity binders (HABs). Three AD patients and three controls were mixed-affinity binders. V T estimated with Logan GA was significantly correlated with V T estimated with the 2TCM in both controls (r = 0.97) and AD patients (r = 0.98) and was selected for the final analysis. Significantly higher V T was found in the medial temporal cortex in AD patients than in controls (p = 0.044) if the TSPO binding status was entered as a covariate. If only HABs were included, significantly higher V T was found in the medial and lateral temporal cortex, posterior cingulate, caudate, putamen, thalamus and cerebellum in AD patients than in controls (p < 0.05). CONCLUSION: [(18)F]FEMPA seems to be a suitable radioligand for detecting increased TSPO binding in AD patients if their binding status is taken into account.

Extra domain B fibronectin as a target for near-infrared fluorescence imaging of rheumatoid arthritis affected joints in vivo.

Abstract We investigated a molecular imaging approach for the detection of collagen-induced arthritis in rats by targeting the extra domain B (ED-B) of the extracellular matrix protein fibronectin. ED-B is a highly conserved domain (identical in human and rats) that is produced by alternative splicing during embryonic development and during vascular remodeling such as angiogenesis. The hallmark of rheumatoid arthritis is synovitis leading to both angiogenesis in the synovium and the promotion of cartilage and bone disruption. For in vivo diagnostics, the ED-B-binding single-chain antibody fragment AP39 was used as a targeting probe. It was covalently linked to the near-infrared dye tetrasulfocyanine (TSC) to be visualized by near-infrared fluorescence imaging. The resulting AP39-TSC conjugate was intravenously administered to rats with collagen-induced arthritis and the respective controls. Ovalbumin-TSC was used as control conjugate. Optical imaging over a time period of 24 hours using a planar imaging setup resulted in a clear enhancement of fluorescence intensity in joints with moderate to severe arthritis compared with control joints between 3 and 8 hours postinjection. Given that AP39 is a fully human antibody fragment, this molecular imaging approach for arthritis detection might be translated to humans.

Optical molecular imaging of lymph nodes using a targeted vascular contrast agent.

We develop a highly specific antibody-dye conjugate for optical imaging of peripheral lymph nodes. The contrast agent consists of the monoclonal antibody recognizing endothelial ligands for the lymphocyte homing receptor L-selectin, MECA-79, and a near-infrared (near-IR) fluorescent indotricarbocyanine dye. The targeting and biodistribution behavior of MECA-79 is studied after radio-iodination and intravenous injection into mice demonstrating specific uptake in lymph nodes and accumulation in high endothelial venules (HEV). After conjugation of MECA-79 with indotricarbocyanine dye, the fluorescence imaging properties of the MECA-79 dye conjugate are examined by intravenous injection in nude mice and laser-induced fluorescence whole-body imaging in vivo. The MECA-79 antibody-dye conjugate accumulates in peripheral lymph nodes, whereas an isotype antibody-dye conjugate does not. Specific lymph node near-IR fluorescent signals become detectable within minutes after injection, and stable imaging persists for more than 24 h. The results demonstrate that vascular targeting of endothelial expression of glyocproteins is feasible to visualize the accumulation of near-IR fluorescent MECA-79 in lymph nodes, making this technology potentially useful to characterize processes of inflammation.

In vivo therapy monitoring of experimental rheumatoid arthritis in rats using near-infrared fluorescence imaging.

An in vivo near-infrared fluorescence (NIRF) imaging technique is described for therapy monitoring of ankle joints affected by collagen-induced arthritis, a model of human rheumatoid arthritis. Arthritis was induced in rats by intradermal injections of collagen and Freund's incomplete adjuvant. For in vivo imaging, the nonspecific NIR dye tetrasulfocyanine (TSC) was used. Prior to and after treatment with a nonsteroidal anti-inflammatory drug, meloxicam, or analgesic drug, tramadol hydrochloride (which served as no-therapy control), normalized fluorescence intensities of each ankle joint were measured. Additionally, each ankle joint was characterized by clinical arthritis scoring and histopathology. Over a 3-week treatment period, a significant difference in disease progression between animals treated with meloxicam and tramadol hydrochloride was detected. A statistically significant improvement in ankle joint pathology from high- or moderate-grade to moderate- or low-grade upon meloxicam therapy, as determined by clinical evaluation, translated into a significant decrease in fluorescence intensity. In contrast, all arthritic joints of the no-therapy control group deteriorated to high-grade arthritis with high-fluorescence intensities in NIRF imaging.

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 tumor angiogenesis. 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 >or=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.