Synthesis and evaluation of iodine-123 labelled tricyclic tropanes as radioligands for the serotonin transporter

Ex vivo and in vivo evaluation of (2R,3R)-5-[(18)F]-fluoroethoxy- and fluoropropoxy-benzovesamicol, as PET radioligands for the vesicular acetylcholine transporter

Molecular imaging of the vesicular acetylcholine transporter (VAChT) using positron emission tomography (PET) may provide insights into early diagnosis and better understanding of Alzheimer's disease. We further characterized the VAChT ligand (2R,3R)-5-FEOBV (1) and developed new fluoropropoxy analogues. Ex vivo studies of the new nonradiolabeled analogues (2R,3R)-5-FPOBV (2) (k(D) = 0.7 nM) and (2S,3S)-5-FPOBV (3) (k(D) = 8.8 nM) were performed in rat brain and showed an enantioselective inhibition of (-)-5-[(125)I]-IBVM uptake in striatum, cortex, and hippocampus (e.g., 74% for 2 and only 54% for 3 in the cortex). Radiochemical procedures were developed to produce [(18)F]1 and [(18)F]2 as potential imaging agent for the VAChT. The radiochemistry was carried out in a one step procedure, with radiolabeling yields of 17 and 2.6% (range: 1-5.4), respectively, nondecay corrected with good specific activity: 124-338 GBq/micromol. The radiochemical purity was greater than 98%. The biological (ex vivo and in vivo) properties of these radioligands were evaluated in rats and showed a low (less then 0.1% of the injected dose) and homogeneous brain uptake. The in vivo PET study of [(18)F]2 performed in baboon also revealed rapid defluorination as the main problem. Therefore [(18)F]1 and [(18)F]2 appear to be unsuitable for in vivo imaging of the VAChT using PET.

Small molecule receptors as imaging targets

The aberrant expression and function of certain receptors in tumours and other diseased tissues make them preferable targets for molecular imaging. PET and SPECT radionuclides can be used to label specific ligands with high affinity for the target receptors. The functional information obtained from imaging these receptors can be used to better understand the systems under investigation and for diagnostic and therapeutic applications. This review discusses some of the aspects of receptor imaging with small molecule tracers by PET and SPECT and reviews some of the tracers for the receptor imaging of tumours and brain, heart and lung disorders.

Synthesis and in vivo evaluation of a novel peripheral benzodiazepine receptor PET radioligand

The novel pyrazolopyrimidine ligand, N,N-diethyl-2-[2-(4-methoxyphenyl)-5,7-dimethyl-pyrazolo[1,5-a]pyrimidin-3-yl]-acetamide 1 (DPA-713), has been reported as a potent ligand for the peripheral benzodiazepine receptor (PBR) displaying an affinity of K(i)=4.7 nM. In this study, 1 was successfully synthesised and demethylated to form the phenolic derivative 6 as precursor for labelling with carbon-11 (t(1/2) = 20.4 min). [11C]1 was prepared by O-alkylation of 6 with [11C]methyl iodide. The radiochemical yield of [(11)C]1 was 9% (non-decay corrected) with a specific activity of 36 GBq/micromol at the end of synthesis. The average time of synthesis including formulation was 13.2 min with a radiochemical purity >98%. In vivo assessment of [11C]1 was performed in a healthy Papio hamadryas baboon using positron emission tomography (PET). Following iv administration of [11C]1, significant accumulation was observed in the baboon brain and peripheral organs. In the brain, the radioactivity peaked at 20 min and remained constant for the duration of the imaging experiment. Pre-treatment with the PBR-specific ligand, PK 11195 (5 mg/kg), effectively reduced the binding of [11C]1 at 60 min by 70% in the whole brain, whereas pre-treatment with the central benzodiazepine receptor ligand, flumazenil (1mg/kg), had no inhibitory effect on [11C]1 uptake. These results indicate that accumulation of [11C]1 in the baboon represents selective binding to the PBR. These exceptional in vivo binding properties suggest that [11C]1 may be useful for imaging the PBR in disease states. Furthermore, [11C]1 represents the first ligand of its pharmacological class to be labelled for PET studies and therefore has the potential to generate new information on the pathological role of the PBR in vivo.

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.