ABSTRACT
Recently, the pyrazolopyrimidine, [11C] N,N-Diethyl-2-[2-(4-methoxyphenyl)-5,7-dimethylpyrazolo[1,5-a]pyrimidin-3-yl]acetamide (DPA-713) has been reported as a new promising marker for the study of peripheral benzodiazepine receptors with positron emission tomography. In the present study, DPA-713 has been labelled from the corresponding nor-analogue using [11C]methyl triflate (CH3OTf). Conditions for HPLC were also modified to include physiological saline (aq. 0.9% NaCl)/ethanol:60/40 as mobile phase making it suitable for injection. The total time of radiosynthesis, including HPLC purification, was 18-20 min. This reported synthesis of [11C]DPA-713, using [11C]CH3OTf, resulted in an improved radiochemical yield (30-38%) compared to [11C]methyl iodide (CH3I) (9) with a simpler purification method. This ultimately enhances the potential of [11C]DPA-713 for further pharmacological and clinical evaluation. These improvements make this radioligand more suitable for automated synthesis which is of benefit where multi-dose preparations and repeated syntheses of radioligand are required.
Subject(s)
Acetamides/chemical synthesis , Carbon Radioisotopes/chemistry , Mesylates/chemistry , Pyrazoles/chemical synthesis , Pyrimidines/chemical synthesis , Receptors, GABA-A/metabolism , Acetamides/chemistry , Chromatography, High Pressure Liquid , Isotope Labeling/methods , Ligands , Pyrazoles/chemistry , Pyrimidines/chemistry , Spectrophotometry, UltravioletABSTRACT
EMD-95885, 6-[3-[4-(4-fluorobenzyl)piperidino]propionyl]-3H-benzoxazol-2-one (1) has been described as a selective antagonist for the NMDA receptors containing NR2B subunits, displaying an IC50 of 3.9 nM for this subtype. EMD-95885 (1) has been synthesized in good overall yield and labelled with carbon-11 ( T1/2 : 20.4 min) at its benzoxazolinone moiety using [11C]phosgene. The pharmacological profile of [11C]EMD-95885 ([11C]-1) was evaluated in vivo in rats with biodistribution studies and brain radioactivity monitored with intracerebral radiosensitive beta-microprobes. The brain uptake of [11C]-1 was homogeneous (0.4-0.6%ID/mL) across the different brain structures studied. This in vivo brain regional distribution of [11C]-1 was not consistent with the known distribution of NR2B subunits. Also as a measure of specificity the hippocampus/cerebellum ratio reached 0.8 throughout the time course of the experiment supporting the lack of specificity. Competition studies with the NR2B prototypic ligand ifenprodil and EMD-95885 (1), 30 min before the radioligand injection, displayed homogeneous reduction of [11C]-1 uptake of 40-60%. Pre-treatment of rats with DTG (sigma ligand), MDL105519 (glycine site antagonist) and MK801 (ion channel blocker) had no inhibitory effect on [11C]-1 uptake. Use of haloperidol as a blocking drug also resulted in a homogeneous inhibition of [11C]-1 uptake by 66-60%, which does not reflect binding to dopamine or sigma receptors. Due to the homogeneous radioligand uptake and inhibition and no measure of cerebral blood flow effects during these blocking studies it is uncertain whether any specific binding is observed. In view of these results, [11C]EMD-95885 ([11C]-1) does not have the required properties for imaging NR2B containing NMDA receptors using positron emission tomography.
