antagonist radioligands ͉ tumor targeting ͉ peptide hormones ͉ neuropeptides ͉ receptor internalization P eptide receptor targeting in vivo is a successful method to image and treat various types of cancers (1). The best example is somatostatin receptor targeting with 111 In-, 90 Y-, or 177 Lu-labeled somatostatin radioligands that are injected into the patients intravenously and accumulate in their somatostatin receptor-expressing tumors. For this purpose, agonists have been selected. The rationale is that agonists, after high-affinity binding to the receptor, usually trigger internalization of the ligandreceptor complex (2). This process of internalization is the basis for an efficient accumulation of the radioligand in a cell over time (1,(3)(4)(5), and it has been considered a crucial step in the process of in vivo receptor targeting with radiolabeled peptides (4-6). Recently, a highly significant correlation between the rate of ligand internalization in vitro into AR42J cells expressing somatostatin receptor subtype 2 (sst 2 ) and the in vivo uptake in the sst 2 -expressing rat tumor model has been reported
Background-Macrophages participate centrally in atherosclerosis, and macrophage markers (eg, CD68, correlate well with lesion severity and therapeutic modulation. On the basis of the avidity of lesional macrophages for polysaccharide-containing supramolecular structures such as nanoparticles, we have developed a new positron emission tomography (PET) agent with optimized pharmacokinetics to allow in vivo imaging at tracer concentrations. Methods and Results-A dextranated and DTPA-modified magnetofluorescent 20-nm nanoparticle was labeled with the PET tracer 64 Cu (1 mCi/0.1 mg nanoparticles) to yield a PET, magnetic resonance, and optically detectable imaging agent. Peak PET activity 24 hours after intravenous injection into mice deficient in apolipoprotein E with experimental atherosclerosis mapped to areas of high plaque load identified by computed tomography such as the aortic root and arch and correlated with magnetic resonance and optical imaging. Accumulated dose in apolipoprotein E-deficient aortas determined by gamma counting was 260% and in carotids 392% of respective wild-type organs (PϽ0.05 both). Autoradiography of aortas demonstrated uptake of the agent into macrophage-rich atheromata identified by Oil Red O staining of lipid deposits. The novel nanoagent accumulated predominantly in macrophages as determined by fluorescence microscopy and flow cytometry of cells dissociated from aortas. Conclusions-This report establishes the capability of a novel trimodality nanoparticle to directly detect macrophages in atherosclerotic plaques. Advantages include improved sensitivity; direct correlation of PET signal with an established biomarker (CD68); ability to readily quantify the PET signal, perform whole-body vascular surveys, and spatially localize and follow the trireporter by microscopy; and clinical translatability of the agent given similarities to magnetic resonance imaging probes in clinical trials. (Circulation. 2008;117:379-387.)
Purpose Gallium-68 is a metallic positron emitter with a half-life of 68 min that is ideal for the in vivo use of small molecules, such as [ 68 Ga-DOTA,Tyr 3 ]octreotide, in the diagnostic imaging of somatostatin receptor-positive tumours. In preclinical studies it has shown a striking superiority over its 111 In-labelled congener. The purpose of this study was to evaluate whether third-generation somatostatin-based, radiogallium-labelled peptides show the same superiority. Methods Peptides were synthesised on solid phase. The receptor affinity was determined by in vitro receptor autoradiography. The internalisation rate was studied in AR4-2J and hsst-HEK-transfected cell lines. The pharmacokinetics was studied in a rat xenograft tumour model, AR4-2J. Results All peptides showed high affinities on hsst2, with the highest affinity for the Ga III -complexed peptides. On hsst3 the situation was reversed, with a trend towards lower affinity of the Ga III peptides. A significantly increased internalisation rate was found in sst2-expressing cells for all 67
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