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GluN2B subunit-containing N-methyl-d-aspartate (NMDA) receptors have been implicated in various neurological disorders. Nonetheless, a validated fluorine-18 labeled positron emission tomography (PET) ligand for GluN2B imaging in the living human brain is currently lacking. The aim of this study was to develop a novel synthetic approach that allows an enantiomerically pure radiosynthesis of the previously reported PET radioligands (R)-[18F]OF-NB1 and (S)-[18F]OF-NB1 as well as to assess their in vitroand in vivoperformance characteristics for imaging the GluN2B subunit-containing NMDA receptor in rodents. A novel synthetic approach was successfully developed, which allows for the enantiomerically pure radiosynthesis of (R)-[18F]OF-NB1 and (S)-[18F]OF-NB1 and the translation of the probe to the clinic. While both enantiomers were selective over sigma2 receptors in vitroand in vivo, (R)-[18F]OF-NB1 showed superior GluN2B subunit specificity by in vitroautoradiography and higher volumes of distribution in the rodent brain by small animal PET studies.

Cannabinoid type 2 receptor (CB2R), belonging to the endocannabinoid system, is overexpressed in pathologies characterized by inflammation, and its activation counteracts inflammatory states. Fatty acid amide hydrolase (FAAH) is an enzyme responsible for the degradation of the main endocannabinoid anandamide; thus, the simultaneous CB2R activation and FAAH inhibition may be a synergistic anti-inflammatory strategy. Encouraged by principal component analysis (PCA) data identifying a wide chemical space shared by CB2R and FAAH ligands, we designed a small library of adamantyl-benzamides, as potential dual agents, CB2R agonists, and FAAH inhibitors. The new compounds were tested for their CB2R affinity/selectivity and CB2R and FAAH activity. Derivatives 13, 26, and 27, displaying the best pharmacodynamic profile as CB2R full agonists and FAAH inhibitors, decreased pro-inflammatory and increased anti-inflammatory cytokines production. Molecular docking simulations complemented the experimental findings by providing a molecular rationale behind the observed activities. These multitarget ligands constitute promising anti-inflammatory agents.

Background: Multifunctional thiosemicarbazones (TSCs) able to bind sigma receptors and chelate metals are considered as a promising avenue for the treatment of pancreatic cancer due to the encouraging results obtained on in vitro and in vivo models. Here, we assessed the biochemical mechanism of these TSCs also on lung (A549) and breast (MCF7) cancer cells. Methods: The density of sigma-2 receptors in normal (BEAS-2B and MCF10A) and in lung and breast (A549 and MCF7) cancer cells was evaluated by flow cytometry. In these cells, cytotoxicity (MTT assay) and activation of ER- and mitochondria-dependent cell death pathways (by spectrofluorimetric assays to measure Caspases 3/7/9; qRT-PCR detection of GRP78, ATF6, IRE1, PERK; MitoSOX, DCFDA-AM and JC-1 staining), induced by the TSCs FA4, MLP44, PS3 and ACThio1, were evaluated. Results: FA4 and PS3 exerted more potent cytotoxicity than MLP44 and ACThio1 in all cancer cell lines, where the density of sigma-2 receptors was higher than in normal cells. Remarkably, FA4 promoted ER- and mitochondria-dependent cell death pathways in both cell models, whereas the other TSCs had variable, cell-dependent effects on the activation of the two proapoptotic pathways. Conclusions: Our data suggest that FA4 is a promising compound that deserves to be further studied for lung and breast cancer treatment. However, the other multifunctional TSCs also hold promise for the development of therapies towards a personalized medicine approach. Indeed, the presence of the sigma-2 receptor-targeting moiety would lead to a more specific tumor delivery embracing the characteristics of individual tumor types.

The cannabinoid receptor subtype 2 (CB2R) represents an interesting and new therapeutic target for its involvement in the first steps of neurodegeneration as well as in cancer onset and progression. Several studies, focused on different types of tumors, report a promising anticancer activity induced by CB2R agonists due to their ability to reduce inflammation and cell proliferation. Moreover, in neuroinflammation, the stimulation of CB2R, overexpressed in microglial cells, exerts beneficial effects in neurodegenerative disorders. With the aim to overcome current treatment limitations, new drugs can be developed by specifically modulating, together with CB2R, other targets involved in such multifactorial disorders. Building on successful case studies of already developed multitarget strategies involving CB2R, in this Perspective we aim at prompting the scientific community to consider new promising target associations involving HDACs (histone deacetylases) and σ receptors by employing modern approaches based on molecular hybridization, computational polypharmacology, and machine learning algorithms.

Up-regulation of CB2R on activated microglial cells, the first step in neurodegeneration, has been widely demonstrated, and this finding makes the receptor a promising target in the early diagnosis and treatment of several neurodegenerative disorders such as Alzheimer’s disease (AD), Parkinson’s disease (PD), Huntington’s disease (HD), and multiple sclerosis (MS). The development of CB2R PET ligands could help demonstrate the neurodegenerative pathogenesis, thus providing useful tools for characterizing the role of neuroinflammation in the progression of these disorders. CB2R agonists and inverse agonists have emerged as neuroprotective agents, and CB2R agonists have entered several clinical trials. CB2R ligands have therefore received great attention, and different molecular scaffolds have been selected to target CB2R subtypes. This review is focused on structure–activity relationship (SAR) and structure–affinity relationship (SAfiR) studies performed on different scaffolds with the aim to identify the molecular features useful for the design of both therapeutic and diagnostic agents.

Cannabinoid receptor subtype 2 (CB2R) is emerging as a pivotal biomarker to identify the first steps of inflammation-based diseases such as cancer and neurodegeneration. There is an urgent need to find specific probes that may result in green and safe alternatives to the commonly used radiative technologies, to deepen the knowledge of the CB2R pathways impacting the onset of the above-mentioned pathologies. Therefore, based on one of the CB2R pharmacophores, we developed a class of fluorescent N-adamantyl-1-alkyl-4-oxo-1,4-dihydroquinoline-3-carboxamide derivatives spanning from the green to the near-infrared (NIR) regions of the light spectrum. Among the synthesized fluorescent ligands, the green-emitting compound 55exhibited a favorable binding profile (strong CB2R affinity and high selectivity). Notably, this ligand demonstrated versatility as its use was validated in different experimental settings such as flow cytometry saturation, competitive fluorescence assays, and in vitromicroglia cells mimicking inflammation states where CB2R are overexpressed.

P-glycoprotein (Pgp) is highly expressed on blood–brain barrier (BBB) and glioblastoma (GB) cells, particularly on cancer stem cells (SC). Pgp recognizes a broad spectrum of substrates, limiting the therapeutic efficacy of several chemotherapeutic drugs in eradicating GB SC. Finding effective and safe inhibitors of Pgp that improve drug delivery across the BBB and target GB SC is open to investigation. We previously identified a series of thiosemicarbazone compounds that inhibit Pgp with an EC50in the nanomolar range, and herein, we investigate the efficacy of three of them in bypassing Pgp-mediated drug efflux in primary human BBB and GB cells. At 10 nM, the compounds were not cytotoxic for the brain microvascular endothelial hCMEC/D3 cell line, but they markedly enhanced the permeability of the Pgp-substrate doxorubicin through the BBB. Thiosemicarbazone derivatives increased doxorubicin uptake in GB, with greater effects in the Pgp-rich SC clones than in the differentiated clones derived from the same tumor. All compounds increased intratumor doxorubicin accumulation and consequent toxicity in GB growing under competent BBB, producing significant killing of GB SC. The compounds crossed the BBB monolayer. The most stable derivative, 10a, had a half-life in serum of 4.2 h. The coadministration of doxorubicin plus 10asignificantly reduced the growth of orthotopic GB-SC xenografts, without eliciting toxic side effects. Our work suggests that the thiosemicarbazone compounds are able to transform doxorubicin, a prototype BBB-impermeable drug, into a BBB-permeable drug. Bypassing Pgp-mediated drug efflux in both BBB and GB SC, thiosemicarbazones might increase the success of chemotherapy in targeting GB SC, which represent the most aggressive and difficult components to eradicate.

The increasing importance of sigma-2 receptor as target for the diagnosis and therapy of tumors paves the way for the development of innovative optically traceable fluorescent probes as tumor cell contrast and therapeutic agents. Here, a novel hybrid organic–inorganic nanostructure is developed by combining the superior fluorescent properties of inorganic quantum dots (QDs), coated with a hydrophilic silica shell (QD@SiO2NPs), the versatility of the silica shell, and the high selectivity for sigma-2 receptor of the two synthetic ligands, namely, the 6-[(6-aminohexyl)oxy]-2-(3-(6,7-dimethoxy-3,4-dihydroisoquinolin-2(1H)-yl)propyl)-3,4-dihydroisoquinolin-1(2H)-one (MLP66) and 6-[1-[3-(4-cyclohexylpiperazin-1-yl)propyl]-1,2,3,4-tetrahydronaphthalen-5-yloxy]hexylamine (TA6). The proposed nanostructures represent a challenging alternative to all previously studied organic small fluorescent molecules, based on the same sigma-2 receptor affinity moieties. Flow cytometry and confocal fluorescence microscopy experiments, respectively, on fixed and living cancerous MCF7 cells, which overexpress the sigma-2 receptor, prove the ability of functionalized (QD@SiO2-TA6 and QD@SiO2-MLP66) NPs to be internalized and demonstrate their affinity to the sigma-2 receptor, ultimately validating the targeting properties conveyed to the NPs by sigma-2 ligand conjugation. The presented QD-based nanoprobes possess a great potential as in vitroselective sigma-2 receptor imaging agent and, consequently, could provide a significant impact to future theranostic applications.

The intracellular [Ca2+] is modulated by σ receptors. An important component of the cellular machinery governing the intracellular [Ca2+] is Store-Operated Calcium Entry (SOCE). Here we want to investigate whether ligands of σ receptors affect SOCE.

Background: The intracellular [Ca2+] is modulated by s receptors. An important component of the cellular machinery governing the intracellular [Ca2+] is Store-Operated Calcium Entry (SOCE). Here we want to investigate whether ligands of s receptors affect SOCE. Methods: The intracellular [Ca2+] was monitored, with the fluorescent Ca2+-sensitive probe Fura-2, in four cell lines with a different expression of s receptors, namely MCF7 (expressing s1receptors with a low density and overexpressing s2receptors), MCF7s1(overexpressing s1receptors), SK-N-SH, and HT-29. Results: When thapsigargin was used to deplete intracellular Ca2+stores, in a Ca2+-free incubation medium, the Ca2+influx (following Ca2+re-addition) was significantly increased by 1 µM (+)-pentazocine (s1receptor agonist) in MCF7s1(by 22.5%) and SK-N-SH (by 45.6%), but not in HT-29 and MCF7 cells. We have used, as a second approach, the “Mn2+quenching” protocol. In MCF7s1cells, after thapsigargin treatment, the fluorescence quenching induced by Mn2+influx (evidence of Ca2+influx) was significantly increased (by 25.8%) by 1 µM (+)-pentazocine, significantly decreased (by 18.0%) by BD1063 (s1receptor antagonist), and not affected by the presence of both ligands. These effects were not observed in MCF7 cells. Finally, in MCF7 cells, 1 µM PB28 (s2receptor agonist), did not affect both the Ca2+response after Ca2+re-addition and the fluorescence quenching induced by Mn2+influx. Conclusions: We propose that the s1receptor agonist (+)-pentazocine increases SOCE in MCF7s1and SK-N-SH cell lines. The s2receptor agonist PB28 does not affect SOCE in MCF7 cells.

Sigma (σ) receptor subtypes, σ1and σ2, are targets of wide pharmaceutical interest. The σ2receptor holds promise for the development of diagnostics and therapeutics against cancer and Alzheimer’s disease. Nevertheless, little is known about the mechanisms activated by the σ2receptor. To contribute to the exploitation of its therapeutic potential, we developed novel specific fluorescent ligands. Indole derivatives bearing the N-butyl-3H-spiro[isobenzofuran-1,4′-piperidine] portion were functionalized with fluorescent tags. Nanomolar-affinity fluorescent σ ligands, spanning from green to red to near-infrared emission, were obtained. Compounds 19(σ pan affinity) and 29(σ2selective), which displayed the best compromise between pharmacodynamic and photophysical properties, were investigated in flow cytometry, confocal, and live cell microscopy, demonstrating their specificity for the σ2receptor. To the best of our knowledge, these are the first red-emitting fluorescent σ2ligands, validated as powerful tools for the study of σ2receptors via fluorescence-based techniques.

Drug-induced phospholipidosis (PLD) involves the accumulation of phospholipids in cells of multiple tissues, particularly within lysosomes, and it is associated with prolonged exposure to druglike compounds, predominantly cationic amphiphilic drugs (CADs). PLD affects a significant portion of drugs currently in development and has recently been proven to be responsible for confounding antiviral data during drug repurposing for SARS-CoV-2. In these scenarios, it has become crucial to identify potential safe drug candidates in advance and distinguish them from those that may lead to false in vitro antiviral activity. In this work, we developed a series of machine learning classifiers with the aim of predicting the PLD-inducing potential of drug candidates. The models were built on a high-quality chemical collection comprising 545curated small molecules extracted from ChEMBL v30. The most effective model, obtained using the balanced random forest algorithm, achieved high performance, including an AUC value computed in validation as high as 0.90. The model was made freely available through a user-friendly web platform named AMALPHI (https://www.ba.ic.cnr.it/softwareic/amalphiportal/), which can represent a valuable tool for medicinal chemists interested in conducting an early evaluation of PLD inducer potential.

Background: A number of sreceptor ligands have been demonstrated to possess antidepressant-like effect in some experimental paradigms (e.g.forced swim test, tail suspension test, olfactory bulbectomy model, conditioned fear stress). The objective of the present study was to find out whether PB190 and PB212, new s1receptor ligands, show the effects in some models predictive of antidepressant activity. Methods: The impact of PB190 and PB212 on the immobility time in the forced swim test (FST) and tail suspension test (TST) was assessed in C57BL/6J male mice. Extracellular bradykinin triggers a transient increase in intracellular calcium concentration by activating the phospholipase C/IP3pathway. The intracellular calcium concentration was estimated with the dual wavelength ratiometric probe Fura-2. Results: In the FST model, PB190 showed a moderate antidepressant-like effect (only in the dose of 3 mg/kg) which was enhanced by joint treatment with amantadine (AMA), 10 mg/kg (inactive per se). The decrease in the immobility time induced by the combined treatment with PB190 and AMA was counteracted by PB212 and by BD1047, a s1-receptor antagonist. The in vitrostudies indicated that Ca2+-response was increased by 1 µM PB190, like by the s1-agonist (+)-pentazocine, while 1 µM PB212 behaved line s1-antagonist, BD1063. On the other hand, 100 µM PB190 negatively affected the Ca2+-response after bradykinin. Conclusions: The obtained results: 1/indicated that in the in vivoconditions PB190 behaved as a s1-receptor agonist while PB212 counteracted its effect, confirming the in vitrodata; 2/gave support to the hypothesis that s1-receptors might be one of possible mechanisms by which drugs induce antidepressant-like activity; 3/revealed that this effect may be potentiated by NMDA receptor antagonists, e.g.AMA.

Although several pieces of information are still missing about sigma-2 ( 2) receptor, the production of high affinity 2 receptor ligands allowed important acquisitions. Morphans such as CB64D and CB184 were the first truly 2-selective ligands synthesized, and their use in cell cultures highlighted the relationship between 2 receptors and cell proliferation, shedding light on important diagnostic and therapeutic potentials with which 2 ligands are endowed. The most significant classes of compounds are herein discussed. The design and Structure-Affinity Relationship studies (SAfiR) of 2 receptor ligands belonging to the classes of morphans, indoles (siramesine analogues), granatanes, flexible benzamides and N-cyclohexylpiperazines are reported, together with the biological results which these compounds provided giving a crucial contribution to the 2 receptor research. The pharmacophore models which were generated on the basis of different classes of the 2 ligands and the attempts for 2 receptor purification are briefly described.

The hyperactivity of the hypothalamic-pituitary-adrenocortical (HPA) axis is often observed in patients with major depression. It has even been implicated in the pathophysiology of this disease. Some antidepressant drugs (ADs) inhibit glucocorticoid receptor (GR) function under in vitroconditions. The d1receptor agonists reveal potential antidepressant activity in animals, moreover, igmesine is promising as an AD in humans. As already shown, d receptors are involved in stress-induced responses (e.g., conditioned fear stress in mice). The aim of the present study was to find out whether the new selective d receptor ligands, PB190 and PB212, are able to affect directly the endocrine system activity. To this end, we evaluated their influence on GR function in mouse fibroblast cells (L929), stably transfected with mouse mammary tumor virus-chloramphenicol acetyltransferase (MMTV-CAT) plasmid (LMCAT cells). Fluvoxamine, a selective serotonin reuptake inhibitor, recognized as a d1receptor agonist was used for comparison. The obtained results showed that both PB190 and PB212 (potential d1receptor agonist and antagonist, respectively) like fluvoxamine, decreased the corticosterone-induced CAT activity in a concentration-dependent manner. The significance of this fact remains ambiguous and requires further studies.

Several σ1 receptor ligands with sub-nanomolar affinity and excellent selectivity have been reported, but relatively few σ2-selective ligands are known. 1-Cyclohexyl-4-[3-(5-methoxy-1,2,3,4-tetrahydronaphthalen-1-yl)propyl] piperazine (PB28; 1) has been reported by us as a high-affinity σ2 receptor ligand with significant σ2 selectivity, and several analogs of (1) now have been developed. Among these are the class of cyclohexylpiperazines that display a good compromise between affinity/activity and selectivity for σ2 receptors. Very little is currently known about the nature of σ2 receptors. In the absence of structure-based receptor information, we applied a comparative molecular field analysis (CoMFA) – a three-dimensional structure-activity relationship (3D-QSAR) method – to a set of cyclohexylpiperazine σ2 ligands to develop a predictive model that might provide information about the stereoelectronic nature of the receptor binding site. Two CoMFA models were generated from two different alignments: the first used an automated FlexS algorithm, and the second used a rationally-driven manual alignment. Significantly better predictivity was obtained with the manual alignment (TSET: q2 = 0.73, r2 = 0.95; PSET: r2 = 0.55/0.73) than from the automated alignment (TSET: q2 = 0.69, r2 = 0.98; PSET: r2 = 0.13/0.16). The resulting CoMFA maps account for observed structure-affinity relationships and suggest a possible anatomy for the σ2 receptor/cyclohexylpiperazine binding site.

Herein the evolution in the development of new sigma (σ) receptor ligands since the middle & 90s by our research group is reported. In the effort to contribute to the identification of the structural features for high-affinity ligands selective versus serotonin, dopamine and other CNS-related receptors, two general classes of (naphthalene)alkylamine compounds were prepared and explored, with the aim of addressing the affinities toward the two recognized σ receptor subtypes. The common template of these compounds was mainly an unsubstituted or methoxy-substituted naphthalene or tetralin nucleus, linked by an alkyl spacer to a substituted piperazine or piperidine ring. The design of new ligands was thought keeping in mind their possible application as PET diagnostic tools and fluorescence tools. High-affinity σ2 receptor ligands were found among N-cyclohexylpiperazine derivatives, such as 1-cyclohexyl-4-[3-(5-methoxy-1,2,3,4- tetrahydronaphthalen-1-yl)propyl]piperazine (3) (PB 28), when they were assayed in radioligand binding with [3H]-DTG in rat liver. Unfortunately, these ligands were all devoid of a significant selectivity relative to σ1 receptor whose binding was assayed with (+)-[3H]-pentazocine in guinea pig brain. Nevertheless, compound 3 had previously shown to be 40-fold selective with a slightly different binding method in animals' tissues. Moreover, it demonstrated 46-fold and 59-fold σ2 versus σ1 receptor binding selectivity in MCF7 and MCF7 ADR tumor cell lines respectively. In the class of piperazines, also high-affinity σ1 receptor ligands were found, possibly due to the presence of a double N-atom and an additional reverse mode of binding. Piperidine derivatives were investigated as high-affinity and selective σ1 receptor ligands leading to some 3,3-dimethylpiperidines such as 3,3-dimethyl-1-[3-(6-methoxynaphthalen-1-yl)propyl]piperidine (69) which resulted to be highly selective relative to the σ2 receptor. For the best ligands, functional assays were conducted in order to investigate agonist/antagonist activity. The effect of chirality in the intermediate methyl-alkyl chain was explored for a class of 4-methylpiperidines linked to some (4-chlorophenoxy)alkyl moieties, and compound ( - )-(S)-92 emerged as the most selective σ1 relative to σ2 receptor ligand.

Herein the evolution in the development of new sigma (??) receptor ligands since the middle &90s by our research group is reported. In the effort to contribute to the identification of the structural features for high-affinity ligands selective versus serotonin, dopamine and other CNS-related receptors, two general classes of (naphthalene)alkylamine compounds were prepared and explored, with the aim of addressing the affinities toward the two recognized sig receptor subtypes. The common template of these compounds was mainly an unsubstituted or methoxy-substituted naphthalene or tetralin nucleus, linked by an alkyl spacer to a substituted piperazine or piperidine ring. The design of new ligands was thought keeping in mind their possible application as PET diagnostic tools and fluorescence tools. High-affinity sig2 receptor ligands were found among N-cyclohexylpiperazine derivatives, such as 1-cyclohexyl-4-[3-(5-methoxy-1,2,3,4- tetrahydronaphthalen-1-yl)propyl]piperazine (3) (PB 28), when they were assayed in radioligand binding with [3H]-DTG in rat liver. Unfortunately, these ligands were all devoid of a significant selectivity relative to sig1 receptor whose binding was assayed with ()-[3H]-pentazocine in guinea pig brain. Nevertheless, compound 3 had previously shown to be 40-fold selective with a slightly different binding method in animals tissues. Moreover, it demonstrated 46-fold and 59-fold sig2 versus sig1 receptor binding selectivity in MCF7 and MCF7 ADR tumor cell lines respectively. In the class of piperazines, also high-affinity sig1 receptor ligands were found, possibly due to the presence of a double N-atom and an additional reverse mode of binding. Piperidine derivatives were investigated as high-affinity and selective sig1 receptor ligands leading to some 3,3-dimethylpiperidines such as 3,3-dimethyl-1-[3-(6-methoxynaphthalen-1-yl)propyl]piperidine (69) which resulted to be highly selective relative to the sig2 receptor. For the best ligands, functional assays were conducted in order to investigate agonist/antagonist activity. The effect of chirality in the intermediate methyl-alkyl chain was explored for a class of 4-methylpiperidines linked to some (4-chlorophenoxy)alkyl moieties, and compound (-)-(S)-92 emerged as the most selective sig1 relative to sig2 receptor ligand.

Several sig1 receptor ligands with sub-nanomolar affinity and excellent selectivity have been reported, but relatively few sig2-selective ligands are known. 1-Cyclohexyl-4-[3-(5-methoxy-1,2,3,4-tetrahydronaphthalen-1-yl)propyl] piperazine (PB28; 1) has been reported by us as a high-affinity sig2 receptor ligand with significant sig2 selectivity, and several analogs of (1) now have been developed. Among these are the class of cyclohexylpiperazines that display a good compromise between affinity/activity and selectivity for sig2 receptors. Very little is currently known about the nature of sig2 receptors. In the absence of structure-based receptor information, we applied a comparative molecular field analysis (CoMFA) - a three-dimensional structure-activity relationship (3D-QSAR) method - to a set of cyclohexylpiperazine sig2 ligands to develop a predictive model that might provide information about the stereoelectronic nature of the receptor binding site. Two CoMFA models were generated from two different alignments: the first used an automated FlexS algorithm, and the second used a rationally-driven manual alignment. Significantly better predictivity was obtained with the manual alignment (TSET: q2 0.73, r2 0.95; PSET: r2 0.55/0.73) than from the automated alignment (TSET: q2 0.69, r2 0.98; PSET: r2 0.13/0.16). The resulting CoMFA maps account for observed structure-affinity relationships and suggest a possible anatomy for the sig2 receptor/cyclohexylpiperazine binding site.

Starting from the high affinity σ2 receptor ligand 2, (PB28), we synthesized amino derivative 4 and coupled it to an NHS-ester activated sepharose stationary phase column to elute a crude protein prepared by lysed human SK-N-SH neuroblastoma cells. We characterized the SDS−PAGE gel electrophoresis stained bands by MALDI-MS and LC-MS-MS analysis. The MASCOT MS-MS ion search program led to the identification of the protein components. The six eluted proteins had a molecular weight ranging from 13 kDa to 26 kDa and were human histone proteins. A human 40S ribosomal protein S3 (SwissProt accession number:  P23396) was also identified as a comigrated band. The human histone proteins that were characterized were H3.3A histone (NCBI accession number:  51859376), H2B histone (NCBI accession number:  1568557), H2A.5 histone (NCBI accession number:  70686), H1 (NCBI accession number:  22770677), and H2.1 histone (SwissProt accession number:  P16403). These results disclosed a dual hypothesis about the σ2 receptor, that is, that it is formed by histones or that the σ2 ligands also bind histone proteins.

Sigma (σ ) receptor history started in 1976 when Martin et al. proposed the σ opioid receptors (“ σ-opioid”) to account for the psychotomimetic effects that the benzomorphan SKF-10,047 (N-allylnormetazocine) caused in the chronic spinal dog model and which were blocked by the opioid antagonist naloxone [1]. Later Su et al. identified a binding site which was labeled by [3H]-SKF-10,047 [2] but which was insensitive to naloxone and naltrexone. This finding led to the hypothesis that the protein identified by Su and co-workers was not the - opioid receptor proposed by Martin, so that the name of the binding site was changed to the ‘ receptor’ to distinguish it from the opioid receptor. Evidence that SKF-10,047 and other benzomorphans displayed high affinity for the phencyclidine (PCP) site at the NMDA receptor, and conversely that PCP displayed affinity for the receptor, led to confusion so that sites named PCP/ were introduced for a while. However, subsequent ligand binding studies dispelled this confusion and receptors were finally considered as non-opioid, non-PCP brain receptors. The availability of more ligands led in the early 1990s to the identification of two subtypes of receptors, namely 1 and 2, and soon thereafter the 1 subtype was cloned from guinea pig liver first and from mouse and human cells later [3,4]. The 1 receptors displayed high homology among species but no homology with other known mammalian proteins. Several structures have been proposed for 1 receptors and the most accepted model today shows that the 1 subtype is made of three hydrophobic regions, two of which are transmebrane-spanning segments with the NH2 and COOH termini on the same side of the cell compartment. The binding sites within this subtype have also been identified and several pharmacophoric models have been built for the 1 subtype with the early model proposed by Glennon and co-workers being among the most inspiring [5]. In recent studies two endogenous 1 ligands have been proposed: D-erythro-sphingosine [6] and N,Ndimethyltryptamine (DMT) with the latter being controversial [7,8]. Thorough efforts have been directed towards the elucidation of the signaling mechanism but more studies are still needed to clarify the roles and pathways activated by this subtype. The association of the 1 subtype with G-protein coupled receptors has been finally ruled out, whereas it has been extensively shown that 1 is involved in the regulation of Ca2 level via IP3 receptors on the endoplasmic reticulum (ER) where a chaperone function for the cross-talk between the ER and mithocondria has been suggested [9]. In addition, 1 receptor modulation of distinct K channels and NMDA receptor-coupled ion channels have been demonstrated. All the above actions, together with the involvement in cell lipid compartmentalization suggested for this subtype, give good reason for a role of 1 proteins in cell proliferation. 1 Receptors have been shown to modulate a number of central neurotransmitter systems, and this evidence together with the early findings that several antipsychotic drugs bind with high affinity to 1 subtypes have linked these proteins to important brain functions since their proposal. Increasing evidence implicates this subtype in pathologies such as anxiety, depression, schizophrenia, drug addiction, Alzheimers disease and movement disorders (such as Parkinsons disease and juvenile amyotrophic sclerosis which has been recently shown to be caused by a mutation to the 1 gene [10]). Noteworthy roles in neuroprotection and neuroplasticity have been suggested for this subtype: 1 ligands protect against brain ischemia and potentiate neurite outgrowth [11,12]. Pharmacological studies and behavioural models for certain CNS diseases have shed light to promising ligands some of which have entered clinical trials. The availability of 1 knockout mice, which are viable and fertile, but which show a difference from the wild-type animals when challenged in behavioural experiments, will likely help to better understand 1 involvement in the CNS [13]. Although initial research was focused on receptors within the CNS, it has been shown that both subtypes are present in peripheral organs at least with the same density as in the CNS, and they are over-expressed in a variety of peripheral and brain human tumors. This evidence has prompted researchers to develop radioligands for the diagnosis of tumors for PET and SPECT analyses with encouraging results which have been recently reviewed in detail [14,15]....

Compound (I) represents an interesting tool for the development of PET traces useful in tumors overexpressing P‐glycoprotein.