Radiosynthesis and biodistribution of a histamine H3 receptor antagonist 4-[3-(4-piperidin-1-yl-but-1-ynyl)-[11C]benzyl]-morpholine: evaluation of a potential PET ligand.

The potent histamine H(3) receptor antagonist JNJ-10181457 (1) was successfully labeled with (11)C in a novel one-pot reaction sequence, with high chemical yield (decay-corrected yield, 28+/-8%) and high specific radioactivity (56+/-26 GBq/mumol). The binding of [(11)C]1 to H(3) receptors was studied in vitro in rat brain and in vivo in rats and mice. The in vitro binding of [(11)C]1 in rat coronal brain slices showed high binding in the striatum, and this binding was blocked by histamine and by two known H(3) antagonists, JNJ-5207852 (2) and unlabeled Compound (1), in a concentration-dependent manner. The biodistribution of [(11)C]1 in rats was measured at 5, 10, 30 and 60 min. The uptake of [(11)C]1 in regions rich in H(3) receptors was highest at 30 min, giving 0.98%, 1.41%, 1.28% and 1.72% dose/g for the olfactory bulb, hippocampus, striatum and cerebral cortex, respectively. However, the binding of [(11)C]1 in the rat brain could not be blocked by pretreatment with either Compound (2) (30 min or 24 h pretreatment) or cold Compound (1) (30-min pretreatment). The biodistribution of [(11)C]1 in a second species (Balb/c mice) showed a higher overall uptake of the radioligand with an average brain uptake of 8.9% dose/g. In C57BL/6-H(3)(-/-) knockout mice, a higher brain uptake was also observed. Analyses of metabolites and plasma protein binding were also undertaken. It appeared that [(11)C]1 could not specifically label H(3) receptors in rodent brain in vivo. Possible causes are discussed.

Synthesis and evaluation of radiolabeled analogs of the antidepressant drug zimelidine as potential SPECT-ligands for the serotonin transporter.

Z-3-(4-bromophenyl)-N,N-dimethyl-3-(3-pyridinyl)-2-propen-1-amine or zimelidine (ZIM) and its first metabolite nor-zimelidine, were radioiodinated via a nonisotopic exchange, using the Cu(I)-assisted nucleophilic labeling method. To evaluate their potential as SPECT ligands for the serotonin transporter (SERT), the biodistribution of both ligands was determined and pretreatment "blocking" studies performed. Both radioligands demonstrated a good brain penetration of 0.8-1% ID/g, stable after 60 min., p.i., and a brain/blood ratio of up to 3. In vivo brain distribution did not reveal specific binding. Blocking studies by pretreatment with a known SERT ligand, had minor influence on the uptake of [(123)I]I-ZIM, between the several isolated brain regions. It may therefore be concluded that [(123)I]I-ZIM and [(123)I]I-nor-ZIM do not appear to be promising SPECT ligands for the SERT.

Evaluation of [11C]laniquidar as a tracer of P-glycoprotein: radiosynthesis and biodistribution in rats.

At present, P-glycoprotein (P-gp) function can be studied using positron emission tomography (PET) together with a labelled P-gp substrate such as R-[11C]verapamil. Such a tracer is, however, less suitable for investigating P-gp (over)expression. Laniquidar is a third-generation P-gp inhibitor, which has been used in clinic trials for modulating multidrug resistance transporters. The purpose of the present study was to develop the radiosynthesis of [11C]laniquidar and to assess its suitability as a tracer of P-gp expression. The radiosynthesis of [11C]laniquidar was performed by methylation of the carboxylic acid precursor with [11C]CH3I. The product was purified by HPLC and reformulated over a tC18 Seppak, yielding a sterile solution of [11C]laniquidar in saline. For evaluating [11C]laniquidar, rats were injected with 20 MBq [11C]laniquidar via a tail vein and sacrificed at 5, 15, 30 and 60 min after injection. Several tissues and distinct brain regions were dissected and counted for radioactivity. In addition, uptake of [11C]laniquidar in rats pretreated with cyclosporine A and valspodar (PSC 833) was determined at 30 min after injection. Finally, the metabolic profile of [11C]laniquidar in plasma was determined. [11C]Laniquidar could be synthesized in moderate yields with high specific activity. Uptake in brain was low, but significantly increased after administration of cyclosporine A. Valspodar did not have any effect on cerebral uptake of [11C]laniquidar. In vivo rate of metabolism was relatively low. Further kinetic studies are needed to investigate the antagonistic behaviour of [11C]laniquidar at tracer level.