The development of radiotracers for imaging sigma (σ) receptors in the central nervous system (CNS) using positron emission tomography (PET)

Sigma (σ) receptors are unique mammalian proteins, distributed in the central nervous system and elsewhere, which are increasingly implicated in the pathophysiology of virtually all major central nervous system disorders. The heterogeneous but wide distribution of σ1 in the brain has prompted the development of selective radiotracers for imaging these sites using positron emission tomography (PET). To date, some 50 carbon-11-labelled and fluorine-18-labelled candidate PET radioligands targeting σ receptors have been reported. The historical development of selective σ1 receptor ligands as potential PET imaging agents, as well as the radiochemistry and application of the most recently developed examples, is described herein.

Oxidation of Aromatic Lithium Thiolates into Sulfinate Salts: An Attractive Entry to Aryl Sulfones Labeled with Carbon-11

[reaction: see text] Aromatic 11C-sulfones were synthesized by S alkylation of lithium arenesulfinates, which are readily available from the corresponding thiols by an oxaziridine-mediated oxidation reaction with [11C]alkyl iodides in THF/H2O (4:1) at 150 degrees C. The radiosyntheses, including purification by HPLC, were completed in an average of 35 min from the end of the bombardment with 55-76% overall radiochemical yields (decay corrected). The described procedure extends the range of accessible labeling methods.

N-(n-Benzylpiperidin-4-yl)-2-[18F]fluorobenzamide: a potential ligand for PET imaging of breast cancer

N-(N-Benzylpiperidin-4-yl)-2-[(18)F]fluorobenzamide (2), a potential ligand for PET imaging of sigma receptor, has been found to be a potential agent for detection of breast cancer. In vivo studies in severe combined immunodeficient (SCID) mice bearing MDA-MB231 tumors showed that the uptake of compound 2 in these tumors was high (3.8%/g); the ratios of tumor/muscle and tumor/blood were 6.2 and 7.0, respectively, at 1 h postinjection. Pretreatment of SCID mice with haldol increased the uptake of compound 2 in blood, muscle, and other well-perfused organs while decreasing its uptake in tumors. The ratios of tumor/muscle and tumor/blood decreased from 6.2 and 7.0 to 1.3 and 1.1, respectively, at 1 h postinjection. At 2 h postinjection, the ratios of tumor/muscle and tumor/blood decreased from 4.9 and 7.8 to 1.4 and 1.4, respectively. The tumor uptake of compound 2 in SCID mice bearing primary tumor explants from a human breast cancer patient was lower than that in MDA-MB231 tumors (1.66%/g versus 3.78%/g), and the ratios of tumor/muscle and tumor/blood were 3.5 and 3.7, respectively, at 1 h postinjection. These results suggest that compound 2 may be a potential ligand for PET imaging of breast cancer.

N-(N-benzylpiperidin-4-yl)-2-[18F]fluorobenzamide: a potential ligand for PET imaging of sigma receptors

Four nitro- and fluorobenzamides (1-4) have been synthesized in good yields from nitro- and fluoro-substituted benzoyl chloride with 4-amino-1-benzylpiperidine. In vitro studies showed that these compounds have high affinities to sigma receptors. N-(N-Benzylpiperidin-4-yl)-2-fluorobenzamide (3), in particular, bound to sigma receptors with high affinity (Ki = 3.4 nM, guinea pig brain membranes) and high selectivity (sigma-2/sigma-1 = 120). It was, therefore, labeled with 18F and evaluated as a sigma receptor radioligand. N-(N-Benzylpiperidin-4-yl)-2-[18F]fluorobenzamide (3a) was synthesized in one step by nucleophile substitution of the 2-nitro precursor (1) with [18F]fluoride in DMSO at 140 degrees C for 20 min followed by purification with HPLC in 4-10% yield (decay corrected). The synthesis time was 90 min and the specific activity was 0.4-1.0 Ci/mumol. Tissue distribution in mice revealed that the uptakes of 3a in the brain, heart, liver, lungs, spleen, kidneys and small intestine were high, and the radioactivity in these organs remained constant from 60 to 120 min post-injection. The radioactivity in the bone did not significantly increase, suggesting in vivo defluorination may not be the major route of metabolism of 3a in mice. Blocking studies with haloperidol in rats indicated that the uptake of compound 3a in the rat brain was selective to haloperidol-sensitive sigma sites. These results suggest that compound 3a is a potent sigma receptor radioligand and may be a potential ligand for PET imaging of sigma receptors in humans.

In vivo labeling of sigma receptors in mouse brain with [3H]4-phenyl-1-(4-phenylbutyl)piperidine

4-Phenyl-1-(4-phenylbutyl)piperidine (4-PPBP) is a very potent ligand for sigma (Sigma) receptors. The present study was undertaken to evaluate [3H]4-PPBP as a radioligand for in vivo labeling of cerebral sigma receptors. After intravenous administration of [3H]4-PPBP to mice, there is high uptake of radioactivity in the brain. The regional distribution of radioactivity in the brain 2 h after intravenous injection of [3H]4-PPBP parallels the in vitro binding of the radioligand in rat brain (pons/medulla > cerebellum > or = prefrontal cortex > or = parietal cortex > hypothalamus > olfactory tubercle > or = thalamus > hippocampus > striatum). Pretreatment with haloperidol (2 mg/kg) significantly decreases the radioactivity measured in the brain 30-120 min after injection of [3H]4-PPBP. Pretreatment with unlabeled 4-PPBP or ifenprodil also significantly decreases radioactivity in the brain 2 h after injection of [3H]4-PPBP, in a dose-dependent manner. The in vivo binding of [3H]4-PPBP in the brain also is significantly inhibited by SL 82.0715, BMY 14802, 1,3-di-o-tolylguanidine (DTG), and (+)-enantiomers of pentazocine, SKF 10,047, and 3-PPP, but not by the corresponding (-)-enantiomers, consistent with stereoselectivity of inhibition obtained in in vitro binding studies. In contrast, pretreatment with dizocilpine and spiperone does not inhibit in vivo binding of [3H]4-PPBP. The results indicate that [3H]4-PPBP would be a suitable radioligand for in vivo labeling of sigma receptors in brain.

(+)-[C-11]-cis-N-benzyl-normetazocine: a selective ligand for sigma receptors in vivo

The in vivo biodistribution profile of the novel sigma (sigma) receptor ligand (+)-[C-11]-cis-N-benzyl-normetazocine ([C-11]-(+)-NBnNM) in mouse brain was examined. This radioligand displayed high brain uptake and a distribution consistent with the density of sigma receptors. Brain radioactivity levels peaked at 15 min postinjection and were largely maintained (ca. 80% of maximal values) up to 90 min postinjection. Pretreatment with several different sigma ligands (haloperidol, (+)-pentazocine, DuP 734, ifenprodil) effectively inhibited [C-11]-(+)-NBnNM binding in a dose-dependent manner in all brain regions. [C-11]-(+)-NBnNM binding sites were shown to be saturable with unlabeled (+)-NBnNM (ED50 = 0.02 mg/kg) and enantioselectively inhibited by the optical isomers of pentazocine. A blocking dose of the dopamine D2 antagonist spiperone (1 mg/kg) did not significantly inhibit [C-11]-(+)-NBnNM binding. Pretreatment with the phencyclidine (PCP) blocker 1-[1-(2-thienyl)cyclohexyl] piperidine (TCP) did not significantly alter total brain tissue radioactivity. Thus, [C-11]-(+)-NBnNM binds with high specificity and selectivity to sigma receptors in vivo and offers excellent potential to study sigma receptors in living human brain via positron emission tomography.