"In‐loop" carbonylation—A simplified method for carbon‐11 labelling of drugs and radioligands

Transition‐metal mediated carbonylation with 11C‐labelled carbon monoxide ([11C]CO) is a versatile method for introducing 11C (t1/2 = 20.3 min) into drugs and radioligands for subsequent use in positron emission tomography (PET). The aim of the current study was to perform the 11C‐carbonylation reaction on the interior surface of a stainless‐steel loop used for high performance liquid chromatography (HPLC). In the experimental setup, cyclotron produced 11C-labelled carbon dioxide ([11C]CO2) was converted to [11C]CO by reduction over heated Molybdenum and swept into an HPLC loop pre‐charged with the appropriate reaction mixture. Following a 5 min reaction, the radiochemical purity (RCP) and the trapping efficiency (TE) of the reaction mixture was determined. After optimization, [11C]N‐Benzylbenzamide was obtained in quantitative radiochemical yield (RCY) following a 5 min reaction at room temperature. The methodology was further applied to label [11C]benzoic acid (RCP≥99%, TE>91%), [11C]methyl benzoate (RCP≥99%, TE>93%) and [11C]phthalide (RCP≥99%, TE>88%). A set of pharmaceuticals was finally radiolabelled using non‐optimized conditions. Excellent yields were obtained for the histamine‐3 receptor radioligand [11C]AZ13198083, the oncology drug [11C]olaparib and the dopamine D2 receptor radioligand [11C]raclopride, whereas a moderate yield was observed for the high‐affinity dopamine D2 receptor radioligand [11C]FLB457. The presented “in‐loop” process proved efficient for diverse 11C‐carbonylations, providing [11C]amides, [11C]esters and [11C]carboxylic acids in moderate to excellent RCYs. Based on the advantages associated with performing the radiolabelling step as an integrated part of the purification system, this methodology may become a valuable addition to the toolbox of methodologies used for 11C‐carbonylation of drugs and radioligands for PET