Synthesis of [isopropyl-11C]nimodipine for in vivo studies of dihydropyridine binding in man using positron emission tomography.

Nimodipine, an antagonist of the L-type calcium ion channel, was labelled with 11C for in vivo positron emission tomography studies of dihydropyridine binding in the human brain. The synthesis was based on esterification of the corresponding acid (W2100) using [2-11C]isopropyl iodide as the labelling precursor. The effects of different bases, solvent mixtures and reaction temperatures on radiochemical yields were investigated. The synthesis including purification by semi-preparative reversed-phase HPLC, required 60-65 min. Conversion of [2-11C]isopropyl iodide to [isopropyl-11C] nimodipine was of the order of 60-80%. The radiochemical yield (isolated) was 20-25%, based on [11C]carbon dioxide. The specific activity of the isolated product varied from 4-40 GBq/mumol (end-of-synthesis).