Synthesis and characterization of a potent, selective, radiolabeled substance-P antagonist for NK1 receptor quantitation: ([18F]SPA-RQ).

PURPOSE To develop and characterize a radiolabelled Substance-P antagonist useful for quantitation of neurokinin-1 receptors in the brain via PET imaging. PROCEDURE [18F]SPA-RQ (Substance-P antagonist - receptor quantifier) was synthesized in good yield and high specific activity by alkylation of a BOC protected phenolate anion using [18F]bromofluoromethane. Removal of the BOC protecting group with trifluoroacetic acid gave [18F]SPA-RQ. RESULTS SPA-RQ has high affinity for human, rhesus monkey and guinea pig NK1 receptors (h-IC50=67 pM) and has a log P value of 1.8. Biodistribution studies in guinea pig showed that this tracer penetrates the blood-brain barrier and selectively labels NK1 receptors in the striatum and cortex. CONCLUSION [18F]SPA-RQ is a potent, high affinity Substance-P antagonist that can be conveniently labeled with high specific activity using [18F]fluoromethylbromide. This tracer is a useful tool for noninvasive imaging of central NK1 receptors.

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