Improved synthesis of the peripheral benzodiazepine receptor ligand [ 11C]DPA-713 using [ 11C]methyl triflate

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 (CH 3OTf). 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]CH 3OTf, resulted in an improved radiochemical yield (30–38%) compared to [ 11C]methyl iodide (CH 3I) (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.