Biodistribution and catabolism of 18F-labeled N-epsilon-fructoselysine as a model of Amadori products.

Amadori products are formed in the early stage of the so-called Maillard reaction between reducing sugars and amino acids or proteins. Such nonenzymatic glycosylation may occur during the heating or storage of foods, but also under physiological conditions. N-epsilon-fructoselysine is formed via this reaction between the epsilon-amino group of peptide-bound lysine and glucose. Despite the fact that, in certain heated foods, up to 50% of lysyl moieties may be modified to such lysine derivatives, up to now, very little is known about the metabolic fate of alimentary administered Amadori compounds. In the present study, N-succinimidyl-4-[18F]fluorobenzoate was used to modify N-epsilon-fructoselysine at the alpha-amino group of the lysyl moiety. The in vitro stability of the resulting 4-[18F]fluorobenzoylated derivative was tested in different tissue homogenates. Furthermore, the 4-[18F]fluorobenzoylated N-epsilon-fructoselysine was used in positron emission tomography studies, as well as in studies concerning biodistribution and catabolism. The results show that the 4-[18F]fluorobenzoylated N-epsilon-fructoselysine is phosphorylated in vitro, as well as in vivo. This phosphorylation is caused by fructosamine 3-kinases and occurs in vivo, particularly in the kidneys. Despite the action of these enzymes, it was shown that a large part of the intravenously applied radiolabeled N-epsilon-fructoselysine was excreted nearly unchanged in the urine. Therefore, it was concluded that the predominant part of peptide-bound lysine that was fructosylated during food processing is not available for nutrition.

Synthesis and evaluation of novel multimeric neurotensin(8-13) analogs.

Neurotensin(8-13) is a hexapeptide with subnanomolar affinity to the neurotensin receptor 1 which is expressed with high incidence in several human tumor entities. Thus, radiolabeled neurotensin(8-13) might be used for tumor targeting. However, its application is limited by insufficient metabolic stability. The present study aims at improving metabolic stability by the synthesis of multimeric neurotensin(8-13) derivatives rather than commonly employed chemical modifications of the peptide itself. Thus, different dimeric and tetrameric peptides carrying C- or N-terminal attached neurotensin(8-13) moieties have been synthesized and their binding affinity toward the neurotensin receptor has been determined. The results demonstrate that branched compounds containing neurotensin(8-13) attached via its C-terminus only show low receptor affinities, whilst derivatives with neurotensin(8-13) attached via the N-terminus show IC50 values in the nanomolar range. Moreover, within the multimeric neurotensin(8-13) derivatives with neurotensin(8-13) attached via the N-terminus an increasing number of branching units lead to higher binding affinities toward the neurotensin receptor.

[11C]SMe-ADAM, an imaging agent for the brain serotonin transporter: synthesis, pharmacological characterization and microPET studies in rats.

N,N-Dimethyl-2-(2-amino-4-methylthiophenylthio)benzylamine (SMe-ADAM, 1) is a highly potent and selective inhibitor of the serotonin transporter (SERT). This compound was labeled with carbon-11 by methylation of the S-desmethyl precursor 10 with [(11)C]methyl iodide to obtain the potential positron emission tomography (PET) radioligand [(11)C]SMe-ADAM. The radiochemical yield was 27 +/- 5%, and the specific radioactivity was 26-40 GBq/micromol at the end of synthesis. Ex vivo and in vivo biodistribution experiments in rats demonstrated a rapid accumulation of the radiotracer in brain regions known to be rich in SERT, such as the thalamus/hypothalamus region (3.59 +/- 0.41%ID/g at 5 min after injection). The specific uptake reached a thalamus to cerebellum ratio of 6.74 +/- 0.95 at 60 min postinjection. The [(11)C]SMe-ADAM uptake in the thalamus was significantly decreased by pretreatment with fluoxetine to 38 +/- 11% of the control value. Furthermore, no metabolites of [(11)C]SMe-ADAM could be detected in the SERT-rich regions of the rat brain. It is concluded that [(11)C]SMe-ADAM may be a suitable PET ligand for SERT imaging in the living brain.

Fluorine-18 radiolabeling of low-density lipoproteins: a potential approach for characterization and differentiation of metabolism of native and oxidized low-density lipoproteins in vivo.

Oxidative modification of low-density lipoprotein (LDL) is regarded as a crucial event in atherogenesis. Assessing the metabolic fate of oxidized LDL (oxLDL) in vivo with radiotracer techniques is hindered by the lack of suitable sensitive and specific radiolabeling methods. We evaluated an improved methodology based on the radiolabeling of native LDL (nLDL) and oxLDL with the positron emitter fluorine-18 ((18)F) by conjugation with N-succinimidyl-4-[(18)F]fluorobenzoate ([(18)F]SFB). We investigated whether radiolabeling of LDL induces adverse structural modifications. Results suggest that radiolabeling of both nLDL and oxLDL using [(18)F]SFB causes neither additional oxidative structural modifications of LDL lipids and proteins nor alteration of their biological activity and functionality, respectively. Thus, radiolabeling of LDL using [(18)F]SFB could prove to be a promising approach for studying the kinetics of oxLDL in vivo.

3-O-methyl-6-18F-fluoro-L-dopa, a new tumor imaging agent: investigation of transport mechanism in vitro.

UNLABELLED: (18)F-Labeled amino acids represent a promising class of imaging agents in tumors, particularly brain tumors. However, the determination of their potential to image peripheral tumors, possibly depending on individual transport characteristics, still remains an area of investigation. The present study investigated the transport mechanism for 3-O-methyl-6-(18)F-fluoro-L-dopa (OMFD), a novel (18)F-labeled phenylalanine derivative, into tumor cells. METHODS: OMFD has routinely and reliably been prepared for clinical use in 20%-25% radiochemical yield (decay corrected, related to (18)F-F(2)) using 6-(18)F-fluoro-L-3,4-dihydroxyphenylalanine preparation devices with minor modifications. In vitro uptake assays with HT-29 (human colon adenocarcinoma) cells, FaDu (squamous cell carcinoma) cells, and RBE4 (immortalized rat brain endothelial) cells were performed with OMFD under physiologic amino acid concentrations without and with the competitive transport inhibitors 2-aminobicyclo-[2,2,1]-heptane-2-carboxylic acid and alpha-(methylamino)isobutyric acid plus serine and without or with Na(+). RESULTS: Transport inhibition experiments using specific competitive inhibitors demonstrated that uptake of OMFD in all cell lines tested was mediated mainly by the sodium-independent high-capacity amino acid transport systems. The highest OMFD uptake was in FaDu cells. CONCLUSION: OMFD seems to be a promising PET tracer for imaging of amino acid transport in tumors.

Positron emission tomography imaging of the serotonin transporter in the pig brain using [11C](+)-McN5652 and S-([18F]fluoromethyl)-(+)-McN5652.

S-([(18)F]fluoromethyl)-(+)-McN5652 ([(18)F](+)-FMe-McN5652) has recently been synthesized as a new potential radiotracer for positron emission tomography (PET) imaging of the 5-HT transporter. It is an analog of [(11)C](+)McN5652, which has been used in clinical PET studies for 5-HT transporter imaging. This article describes the comparison of these two radiotracers in pigs with respect to their in vivo binding characteristics. PET images revealed that the highest accumulation of both radiotracers was found in the ventral midbrain, thalamus, olfactory lobe, and pons which is consistent with the known density of 5-HT transporters. The specific binding was determined by subtracting the values of the inactive (-) enantiomers or of the occipital cortex from those obtained with [(11)C](+)McN5652 or [(18)F](+)-FMe-McN5652 in the time period between 75 and 115 min after radiotracer injection. The specific binding of the (18)F-labeled derivative was about 40% higher than that of the (11)C-labeled derivative. A strong inhibition of the specific binding was observed for both radiotracers after pretreatment with the selective 5-HT uptake inhibitor citalopram. [(18)F](+)-FMe-McN5652 showed faster kinetics than [(11)C](+)McN5652. It reached the binding equilibrium during a study length of 120 min, which was not the case for [(11)C](+)McN5652. It is concluded that [(18)F](+)-FMe-McN5652 is suitable for 5-HT transporter imaging with PET.