Synthesis and in vitro autoradiographic evaluation of a novel high-affinity radioiodinated ligand for imaging brain cannabinoid subtype-1 receptors

Abstract

There is strong interest to study the involvement of brain cannabinoid subtype-1 (CB1) receptors in neuropsychiatric disorders with single photon emission computed tomography (SPECT) and a suitable radioligand. Here we report the synthesis of a novel high-affinity radioiodinated CB1 receptor ligand ([125I]8, [125I]1-(2-iodophenyl)-4-cyano-5-(4-methoxyphenyl)-N-(piperidin-1-yl)-1H-pyrazole-3-carboxylate, [125I]SD7015). By autoradiography in vitro, [125I]8 showed selective binding to CB1 receptors on human brain postmortem cryosections and now merits labeling with iodine-123 for further evaluation as a SPECT radioligand in non-human primate.

Figure 1

Structures of [¹¹C]2, [¹⁸F]3, [¹⁸F]4, [¹¹C]5– 7, and [¹²⁵I]8. Asterisks denote positions of radiolabels.

Figure 2

Autoradiographs from whole-hemisphere cryosections (Panels A– D; 100 m thickness) and cryosections (20 μm thickness) covering thalamus and brainstem (Panels E & F) and temporal cortex (Panels G & H) incubated with [¹²⁵I]8 under baseline (Panels A, C, E & G) and blocked (rimonabant, 10 μM; Panels B, D, F & H) conditions. Abbreviations: Amg, amygdala; Ca, caudate nucleus; CG, cingulate gyrus; FC, frontal cortex; Hi, hippocampus; IGP, globus pallidus, internal segment; PC, parietal cortex; Pu, putamen; SN, substantia nigra; TC, temporal cortex; Th, thalamus.

Scheme 1

Synthesis of 8. Reagent, conditions and yields: a) concentrated HCl, NaNO₂, ethyl 2-chloroacetoacetate, NaOAc, EtOH– H₂O, 16 h, 72%; b) 4-methoxybenzoylacetonitrile, DIPEA, tert-BuOH, reflux, 16 h, 6%; c) aq-LiOH, THF, 65 °C, 4 h; d) DMF_(cat), (COCl)₂, DCM; e) 1-aminopiperidine, DIPEA, DCM, 2 h, 73%.

Scheme 2

Radiosynthesis of [¹²⁵I]8. Reagents, conditions and yields: a) hexamethylditin, Pd(PPh₃)₄, toluene, 20 h, 32%; b) [¹²⁵I]NaI, chloramine- T, aq-HCl, 5 min, 48– 59%.