Sigma-1 receptor (S1R) radioligands have the potential to detect and monitor various neurological diseases. Herein we report the synthesis, radiofluorination and evaluation of a new S1R ligand 6-(3-fluoropropyl)-3-(2-(azepan-1-yl)ethyl)benzo[d]thiazol-2(3H)-one ([18F]FTC-146, [18F]13). [18F]13 was synthesized by nucleophilic fluorination, affording a product with >99% radiochemical purity (RCP) and specific activity (SA) of 2.6 ± 1.2 Ci/Amol (n = 13) at end of synthesis (EOS). Positron emission tomography (PET) and ex vivo autoradiography studies of [18F]13 in mice showed high uptake of the radioligand in S1R rich regions of the brain. Pre treatment with 1 mg/kg haloperidol (2), non radioactive 13, or BD1047 (18) reduced the binding of [18F]13 in the brain at 60 min by 80%, 82% and 81% respectively, suggesting that [18F]13 accumulation in mouse brain represents specific binding to S1Rs. These results indicate that [18F]13 is a promising candidate radiotracer for further evaluation as a tool for studying S1Rs in living subjects.

Background Two subtypes of sigma (σ) receptors, σ1 and σ2, can be pharmacologically distinguished, and each may be involved in substance-abuse disorders. σ-receptor antagonists block cocaine place conditioning and σ-receptor agonists are self-administered in rats that previously self-administered cocaine. Self-administration has been related to increased dopamine (DA) neurotransmission for different drug classes. Actions of σ-receptor agonists on mesolimbic DA have not been fully characterized. Methods Receptor-binding studies assessed affinities of different σ-receptor ligands for σ-receptor subtypes, and for the DA transporter; effects on DA transmission in the rat nucleus accumbens shell were assessed using in-vivo microdialysis. Results Cocaine (0.1–1.0 mg/kg i.v.), the non-selective σ1/2-receptor agonist DTG (1.0–5.6 mg/kg i.v.), and the selective σ1-receptor agonist PRE-084 (0.32–10 mg/kg i.v.) dose-dependently increased DA, with maxima of about 275, 150, and 160%, respectively. DTG-induced stimulation of DA was antagonized by the nonselective σ1/2-receptor antagonist, BD 1008 (10 mg/kg i.p.), and by the preferential σ2-receptor antagonist SN79 (1–3 mg/kg i.p.), but not by the preferential σ1-receptor antagonist, BD 1063 (10–30 mg/kg i.p.). Neither PRE-084 nor cocaine was antagonized by either BD1063 or BD1008. Conclusions Stimulation of DA by σ-receptor agonists in a brain area involved in the reinforcing effects of cocaine was demonstrated. The effects appear to be mediated by σ2-receptors rather than σ1-receptors. However σ-receptors are not likely involved in mediating the acute cocaine- and PRE-084-induced stimulation of DA transmission. Different mechanisms might underlie the dopaminergic and reinforcing effects of σ-receptor agonists suggesting a dopamine-independent reinforcing pathway that may contribute to substance-abuse disorders.

The sigma-2 receptor (S2R) is a potential therapeutic target for cancer and neuronal diseases. However, the identity of the S2R has remained a matter of debate. Historically, the S2R has been defined as (1) a binding site with high affinity to 1,3-di-o-tolylguanidine (DTG) and haloperidol but not to the selective sigma-1 receptor ligand (+)-pentazocine, and (2) a protein of 18–21 kDa, as shown by specific photolabeling with [3H]-Azido-DTG and [125I]-iodoazido-fenpropimorph ([125I]-IAF). Recently, the progesterone receptor membrane component 1 (PGRMC1), a 25 kDa protein, was reported to be the S2R (Nature Communications, 2011, 2:380). To confirm this identification, we created PGRMC1 knockout NSC34 cell lines using the CRISPR/Cas9 technology. We found that in NSC34 cells devoid of or overexpressing PGRMC1, the maximum [3H]-DTG binding to the S2R (Bmax) as well as the DTG-protectable [125I]-IAF photolabeling of the S2R were similar to those of wild-type control cells. Furthermore, the affinities of DTG and haloperidol for PGRMC1 (KI = 472 μM and 350 μM, respectively), as determined in competition with [3H]-progesterone, were more than 3 orders of magnitude lower than those reported for the S2R (20–80 nM). These results clarify that PGRMC1 and the S2R are distinct binding sites expressed by different genes.

The noninvasive imaging of σ-1 receptors (S1Rs) could provide insight into their role in different diseases and lead to novel diagnostic/treatment strategies. The main objective of this study was to assess the S1R radiotracer 18F-FTC-146 in rats. Preliminary squirrel monkey imaging and human serum/liver microsome studies were performed to gain information about the potential of 18F-FTC-146 for eventual clinical translation. Methods The distribution and stability of 18F-FTC-146 in rats were assessed via PET/CT, autoradiography, γ counting, and high-performance liquid chromatography (HPLC). Preliminary PET/MRI of squirrel monkey brain was conducted along with HPLC assessment of 18F-FTC-146 stability in monkey plasma and human serum. Results Biodistribution studies showed that 18F-FTC-146 accumulated in S1R-rich rat organs, including the lungs, pancreas, spleen, and brain. Pretreatment with known S1R compounds, haloperidol, or BD1047, before radioligand administration, significantly attenuated 18F-FTC-146 accumulation in all rat brain regions by approximately 85% (P < 0.001), suggesting radiotracer specificity for S1Rs. Similarly, PET/CT and autoradiography results demonstrated accumulation of 18F-FTC-146 in rat brain regions known to contain S1Rs and that this uptake could be blocked by BD1047 pretreatment. Ex vivo analysis of 18F-FTC-146 in the brain showed that only intact radiotracer was present at 15, 30, and 60 min, whereas rapid metabolism of residual 18F-FTC-146 was observed in rat plasma. Preliminary monkey PET/MRI studies demonstrated specific accumulation of 18F-FTC-146 in the brain (mainly in cortical structures, cerebellum, and vermis) that could be attenuated by pretreatment with haloperidol. HPLC of monkey plasma suggested radioligand metabolism, whereas 18F-FTC-146 appeared to be stable in human serum. Finally, liver microsome studies revealed that 18F-FTC-146 has a longer half-life in human microsomes, compared with rodents. Conclusion Together, these results indicate that 18F-FTC-146 is a promising tool for visualizing S1Rs in pre-clinical studies and that it has potential for mapping these sites in the human brain.

Cocaine is a highly abused drug without effective pharmacotherapies to treat it. It interacts with sigma () receptors, providing logical targets for the development of medications to counteract its actions. Cocaine causes toxic and stimulant effects that can be categorized as acute effects such as convulsions and locomotor hyperactivity and subchronic effects including sensitization and place conditioning. In the present study, 3-(4-(4-cyclohexylpiperazin-1-yl)butyl)benzo[d]thiazole-2(3H)-thione (CM156), a novel compound, was developed and tested for interactions with receptors using radioligand binding studies. It was also evaluated against cocaine-induced effects in behavioral studies. The results showed that CM156 has nanomolar affinities for each of the receptor subtypes in the brain and much weaker affinities for non-binding sites. Pretreatment of male Swiss-Webster mice with CM156, before administering either a convulsive or locomotor stimulant dose of cocaine, led to a significant attenuation of these acute effects. CM156 also significantly reduced the expression of behavioral sensitization and place conditioning evoked by subchronic exposure to cocaine. The protective effects of CM156 are consistent with receptor-mediated actions. Together with previously reported findings, the data from CM156 and related compounds indicate that receptors can be targeted to alleviate deleterious actions of cocaine.