Mattias Ögren

Receptor imaging technique with 11C-labeled receptor ligands in living brain slices: its application to time-resolved imaging and saturation analysis of benzodiazepine receptor using [11C]Ro15-1788

Recently we developed a novel imaging technique using positron emitter-labeled compounds as probes and a storage phosphor screen as a detector. This approach makes it possible to follow a variety of biochemical processes with spatial information in living brain slices. Further technical development is reported here in terms of time-resolved imaging and receptor characterization in a real equilibrium state. The method was validated by use of [11C]Ro15-1788, a benzodiazepine receptor antagonist. Fresh brain slices were incubated with [11C]Ro15-1788 in oxygenated Krebs-Ringer solution at 37 degrees C, in a specially designed chamber. By placing the chamber on a storage phosphor screen, we could obtain two-dimensional images of radioactivity in the slices. Time-resolved imaging was made at 5 min intervals, revealing that it took 60 min to reach equilibrium binding. The dissociation process was observed by adding an excess amount of unlabeled Ro15-1788 to the chamber, 25 min was required for the full dissociation. In the equilibrium state, i.e. in the presence of free radio-ligand, Scatchard plot analysis was performed on the cerebral cortex (Kd = 7.4 nM, Bmax = 146 fmol/mg tissue) and striatum (Kd = 7.5 nM, Bmax = 107 fmol/mg tissue), suggesting the presence of a single component of binding site in these two regions. The present method, for the first time, made it possible to study a ligand-receptor interaction in living brain slices with temporal and spatial resolutions. This technique should prove useful for studies of receptor function under physiological conditions.

In Vitro Evaluation of 11C-Labeled (S)-Nicotine, (S)-3-Methyl-5-(1-Methyl-2-Pyrrolidinyl)isoxazole, and (R,S)-1-Methyl-2-(3-Pyridyl)azetidine as Nicotinic Receptor Ligands for Positron Emission Tomography Studies

Abstract: The binding characteristics of the novel 11C‐labeled nicotinic ligands (R,S)‐1‐methyl‐2‐(3‐pyridyl) azetidine (MPA) and (S)‐3‐methyl‐5‐(1‐methyl‐2‐pyrrolidinyl)isoxazole (ABT‐418) were investigated in comparison with those of (S)‐[11C]nicotine in vitro in the rat brain to be able to predict the binding properties of the new ligands for positron emission tomography studies in vivo. The data from time‐resolved experiments for all ligands indicated fast binding kinetics, with the exception of a slower dissociation of [11C]MPA in comparison with (S)‐[11C]nicotine and [11C]ABT‐418. Saturation experiments revealed for all ligands two nicotinic receptor binding sites with affinity constants (KD values) of 2.4 and 560 nM and binding site densities (Bmax values) of 65.5 and 223 fmol/mg of protein for (S)‐[11C]nicotine, KD values of 0.011 and 2.2 nM and Bmax values of 4.4 and 70.7 fmol/mg of protein for [11C]MPA, and KD values of 1.3 and 33.4 nM and Bmax values of 8.8 and 69.2 fmol/mg of protein for [11C]ABT‐418. In competing with the 11C‐ligands, epibatidine was most potent, followed by cytisine. A different rank order of potencies was found for (−)‐nicotine, (+)‐nicotine, MPA, and ABT‐418 displacing each of the 11C‐ligands. Autoradiograms displayed a similar pattern of receptor binding for all ligands, whereby [11C]MPA showed the most distinct binding pattern and the lowest nonspecific binding. We conclude that the three 11C‐labeled nicotinic ligands were suitable for characterizing nicotinic receptors in vitro. The very high affinity of [11C]MPA to nicotinic acetylcholine receptors, its low nonspecific binding, and especially the slower dissociation kinetics of the [11C]MPA from the putative high‐affinity nicotinic acetylcholine receptor binding site compared with (S)‐[11C]nicotine and [11C]ABT‐418 raise the level of interest in [11C]MPA for application in positron emission tomography.

Quantification and imaging of mannitol transport through Caco-2 cell monolayers using a positron-emitting tracer

Quantification and imaging of mannitol transport through Caco-2 cell monolayers using a positron-emitting tracer

11C-LABELLED POLYMER-BOUND WITTIG REAGENTS IN THE SYNTHESIS OF 11C-LABELLED AROMATIC ALKENES

Abstract The synthesis of the polymer-supported (polystyrene) 11 C-labelled precursors [ 11 C]methyltriphenyl-phosphonium iodide and [ 11 C]cyanoalkyltriphenylphosphonium halides is presented. The polymer-supported precursors were used in Wittig reactions for the synthesis of 11 C-labelled terminal and branched alkenes. 11 C-labelled alkenes were formed in 2–98% decay corrected radiochemical yield, counted from [ 11 C]methyl iodide or hydrogen [ 11 C]cyanide, respectively, in 9–13 min. In the case of the branched alkenes, exclusive formation of the Z-isomer was observed.