11CRaclopride

11C-raclopride is a radiolabeled form of raclopride used in positron emission tomography (PET) to image dopamine D2 and D3 receptors in the brain. The radiolabel is carbon-11, which has a half-life of about 20.3 minutes, necessitating on-site cyclotron production and rapid radiochemistry. It is typically prepared by methylation of a desmethyl raclopride precursor with [11C]methylating agents such as [11C]methyl iodide or [11C]methyl triflate and then purified for injection.

Upon intravenous administration, 11C-raclopride crosses the blood–brain barrier and accumulates in regions with dopaminergic innervation, with the highest uptake in striatal structures such as the caudate nucleus and putamen. The measured signal reflects available D2/D3 receptors and is sensitive to competition from endogenous dopamine, enabling pharmacodynamic assessments such as receptor occupancy by antipsychotics and measurements of dopamine release during challenges.

Applications include estimating receptor availability and occupancy, studying dopaminergic function in conditions such as schizophrenia, Parkinson’s disease, and substance use disorders, and evaluating the in vivo effects of drugs on the dopamine system. Quantitative analysis commonly uses kinetic modeling or reference-tissue models, frequently employing the cerebellum as a reference region due to its low D2/D3 density.

Limitations include sensitivity to endogenous dopamine levels, relatively higher affinity for D2 than for D3 receptors, and potential nonspecific binding. The short half-life of 11C imposes logistical constraints on production, timing, and imaging protocol. Despite these considerations, 11C-raclopride remains a widely used PET radiotracer for studying dopaminergic neurotransmission in humans and animals.