[Table, N-(4-[18F]Fluorobenzyl)-4-(3-(piperidin-1-yl)indole-1-sulfonyl)benzamide].

There are two subtypes of cannabinoid receptors in mammalian tissues: CB1 and CB2 (1, 2). CB1 receptors are expressed abundantly in neuronal terminals in the central nervous system and in some peripheral tissues to inhibit neurotransmitter release. CB1 receptors are found predominantly in the striatum, hippocampus, substantia nigra, globus pallidus, and cerebellum. CB2 receptors are present mainly on immune cells to modulate cytokine release. Both receptor subtypes are coupled through Gi/o proteins to inhibit adenylate cyclase and to modulate potassium and calcium channels. CB1 receptors have been demonstrated to be involved in analgesia, regulation of food intake, and control of movement in normal subjects (3). Alteration of CB1 receptor function has been implicated in a number of human diseases such as depression, schizophrenia, and obesity (4-6).Δ9-Tetrahydrocannabinol (THC) is a major active cannabinoid that is found in marijuana and activates CB1 receptors (7). THC has a very high lipophilicity (log D7.4 value of 7), which causes imaging studies using radiolabeled THC to be unsuccessful because of slow entry into the brain and high nonspecific binding. However, a high lipophilicity is essential for binding to CB1 receptors, and an optimal lipophilicity (log D7.4 1–4) is required for crossing the blood–brain barrier (BBB). Existing radiolabeled ligands are mainly analogs of the inverse agonist rimonabant (SR141716A) and the agonist WIN 55,212-2, which also exhibit high nonspecific binding and lipophilicity, limiting their application in imaging (8). Therefore, there is a need to lower the lipophilicity of the CB1 radioligands with little effect on binding affinity and ability to cross the BBB. N-(4-[18F]Fluorobenzyl)-4-(3-(piperidin-1-yl)indole-1-sulfonyl)benzamide ([18F]PipISB) is being evaluated for use as a CB1 receptor tracer (9, 10).