1'-Benzyl-3,4-dihydrospiro[2H-1- benzothiopyran-2,4'-piperidine] (spipethiane), a potent and highly selective sigma1 ligand

The Benzoylpiperidine Fragment as a Privileged Structure in Medicinal Chemistry: A Comprehensive Review

The phenyl(piperidin-4-yl)methanone fragment (here referred to as the benzoylpiperidine fragment) is a privileged structure in the development of new drugs considering its presence in many bioactive small molecules with both therapeutic (such as anti-cancer, anti-psychotic, anti-thrombotic, anti-arrhythmic, anti-tubercular, anti-parasitic, anti-diabetic, and neuroprotective agents) and diagnostic properties. The benzoylpiperidine fragment is metabolically stable, and it is also considered a potential bioisostere of the piperazine ring, thus making it a feasible and reliable chemical frame to be exploited in drug design. Herein, we discuss the main therapeutic and diagnostic agents presenting the benzoylpiperidine motif in their structure, covering articles reported in the literature since 2000. A specific section is focused on the synthetic strategies adopted to obtain this versatile chemical portion.

3,3-Disubstituted 3,4-Dihydro-1,2,4-benzotriazines: Chemistry, Biological Activity, and Affinity to Sigma Receptors

By reducing the 2-nitrophenylhydrazone of cyclohexanone with sodium dithionite, an unexpected yellow compound was obtained instead of the corresponding colorless amino derivative. Many years later, the structure of this compound, namely, cyclohexane-3-spiro-3,4-dihydro-1,2,4-benzotriazine, was demonstrated. From that time, the reduction of 2-nitrophenylhydrazones of different kinds of ketones, followed by air oxidation of the initially formed amino compounds, has represented a general way to synthesize a variety of 3,3-disubstituted 3,4-dihydro-1,2,4-benzotriazines. Many derivatives have been obtained so far by a single research group, and most of them have demonstrated interesting pharmacological activities, mainly antihypertensive, anti-inflammatory, and diuretic effects and other activities with lower diffusion. Moreover, 3,3-disubstituted 3,4-dihydro-1,2,4-benzotriazines represent a novel class of ligands for sigma receptors, with nanomolar affinity to the σ1 subtype. This property might promote the development of agents for cardiovascular, neurodegenerative, and proliferative pathologies. The present commentary, by collecting compounds and biological results obtained so far, intends to celebrate the centennial of the discovery of the first member of this class of compounds and to promote further investigation in the field.

Cu(I)-Catalyzed Enantioselective Alkynylation of Thiochromones

A highly efficient asymmetric synthesis of chiral thiochromanones is developed via Cu(I)/phosphoramidite catalyzed asymmetric alkynylation of thiochromones under mild reaction conditions. The catalyst system is tolerant of various thiochromone precursors and terminal alkynes. The established asymmetric transformation provides different enatiomeric-enriched thiochromanones with more molecular complexity and enables access to chiral thioflavanones, a subgroup of flavonoid by further functionalization.

A Review of the Human Sigma-1 Receptor Structure

The Sigma-1 Receptor (S1R) is a small, ligand-regulated integral membrane protein involved in cell homeostasis and the cellular stress response. The receptor has a multitude of protein and small molecule interaction partners with therapeutic potential. Newly reported structures of the human S1R in ligand-bound states provides essential insights into small molecule binding in the context of the overall protein structure. The structure also raises many interesting questions and provides an excellent starting point for understanding the molecular tricks employed by this small membrane receptor to modulate a large number of signaling events. Here, we review insights from the structures of ligand-bound S1R in the context of previous biochemical studies and propose, from a structural viewpoint, a set of important future directions.

Synthesis, Characterization, and Biological Studies of a Piperidinyl Appended Dipicolylamine Ligand and Its Rhenium Tricarbonyl Complex as Potential Therapeutic Agents for Human Breast Cancer

A novel ligand bearing a central piperidinyl sulfonamide group, N(SO₂pip)dpa, and its corresponding Re tricarbonyl complex, [Re(CO)₃(N(SO₂pip)dpa)]⁺, have been synthesized in good yield. The methylene CH₂ signal seen as a singlet (4.54 ppm) in a ¹H NMR spectrum of the ligand in DMSO-d₆ appears as two doublets (5.39, 5.01 ppm) in a spectrum of the [Re(CO)₃(N(SO₂pip)dpa)]⁺ complex and confirms the presence of magnetically nonequivalent protons upon coordination to Re. Structural results revealed that the Re-N bond lengths fall within the normal range establishing coordination of ligand to metal. The presence of intraligand π → π^⁎ and n → π^⁎ transitions is indicated by the absorption peaks around 200-250 nm in UV-visible spectra. Absorption peaks in UV-visible spectra around 300 nm for metal complexes were identified as MLCT transitions. The S-N stretch observed as a strong peak at 923 cm^-1 for N(SO₂pip)dpa appeared at a shorter frequency, at 830 cm^-1 in an FTIR spectrum of the [Re(CO)₃(N(SO₂pip)dpa)]⁺. The intense fluorescence displayed by the N(SO₂pip)dpa ligand has quenched upon coordination to Re. Relatively low IC₅₀ values given by human breast cancer cells, MCF-7, (N(SO₂pip)dpa = 139 μM, [Re(CO)₃(N(SO₂pip)dpa)]⁺ = 360 μM) indicate that N(SO₂pip)dpa and [Re(CO)₃(N(SO₂pip)dpa)]⁺ are promising novel compounds that can be further investigated on their usage as potential anticancer agents.

4-aroylpiperidines and 4-(α-hydroxyphenyl)piperidines as selective sigma-1 receptor ligands: synthesis, preliminary pharmacological evaluation and computational studies

Background

Sigma (σ) receptors are membrane-bound proteins characterised by an unusual promiscuous ability to bind a wide variety of drugs and their high affinity for typical neuroleptic drugs, such as haloperidol, and their potential as alternative targets for antipsychotic agents. Sigma receptors display diverse biological activities and represent potential fruitful targets for therapeutic development in combating many human diseases. Therefore, they present an interesting avenue for further exploration. It was our goal to evaluate the potential of ring opened spipethiane (1) analogues as functional ligands (agonists) for σ receptors by chemical modification.

Results

Chemical modification of the core structure of the lead compound, (1), by replacement of the sulphur atom with a carbonyl group, hydroxyl group and 3-bromobenzylamine with the simultaneous presence of 4-fluorobenzoyl replacing the spirofusion afforded novel potent sigma-1 receptor ligands 7a–f, 8a–f and 9d–e. The sigma-1 receptor affinities of 7e, 8a and 8f were slightly lower than that of 1 and their selectivities for this receptor two to threefold greater than that of 1.

Conclusions

It was found that these compounds have higher selectivities for sigma-1 receptors compared to 1. Quantitatitive structure–activity relationship studies revealed that sigma-1 binding is driven by hydrophobic interactions.Graphical abstract

New positron emission tomography (PET) radioligand for imaging σ-1 receptors in living subjects

σ-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 ([(18)F]FTC-146, [(18)F]13). [(18)F]13 was synthesized by nucleophilic fluorination, affording a product with >99% radiochemical purity (RCP) and specific activity (SA) of 2.6 ± 1.2 Ci/μmol (n = 13) at end of synthesis (EOS). Positron emission tomography (PET) and ex vivo autoradiography studies of [(18)F]13 in mice showed high uptake of the radioligand in S1R rich regions of the brain. Pretreatment with 1 mg/kg haloperidol (2), nonradioactive 13, or BD1047 (18) reduced the binding of [(18)F]13 in the brain at 60 min by 80%, 82%, and 81%, respectively, suggesting that [(18)F]13 accumulation in mouse brain represents specific binding to S1Rs. These results indicate that [(18)F]13 is a promising candidate radiotracer for further evaluation as a tool for studying S1Rs in living subjects.

Early development of sigma-receptor ligands

Sigma receptors (σ-1 and σ-2) are non-opioid proteins implicated in the pathophysiology of various neurological disorders and cancer. The σ-1 subtype is a chaperon protein widely distributed in the CNS and peripheral tissues. These receptors are involved in the modulation of K+- and Ca2+-dependent signaling cascades at the endoplasmic reticulum and modulation of neurotransmitter release. σ-1 receptors are emerging targets for the treatment of neurophychiatric diseases (schizophrenia and depression) and cocaine addiction. σ-2 receptors are lipid raft proteins. They are highly expressed on many tumor cells and hence considered potential targets for anticancer drugs. σ receptors bind to a diverse class of pharmacological compounds like cocaine, methamphetamine, benzomorphans like (±)-pentazocine, (±)-SKF-10,047 and endogenous neurosteroids and sphingolipids. In this review we focus on the early development of σ receptor-specific ligands and radiolabeling agents.

Novel highly potent and selective sigma 1 receptor antagonists related to spipethiane

Conservative chemical modifications of the core structure of the lead spipethiane (1) afforded novel potent sigma(1) ligands. sigma(1) affinity and sigma(1/)sigma(2) selectivity proved to be favored by the introduction of polar functions (oxygen atom or carbonyl group) in position 3 or 4 (4-6) or by the elongation of the distance between the two hydrophobic portions of the molecule with the simultaneous presence of a carbonyl group in position 4 (8 and 9). The observed cytostatic effect against the human breast cancer cell line MCF-7/ADR, highly expressing sigma(1) receptors, and not against MCF-7, as well as the enhancement of morphine analgesia highlighted the sigma(1) antagonist profile of this series of compounds. In particular, due to its high sigma(1) affinity (pK(i) = 10.28) and sigma(1)/sigma(2) selectivity ratio (29510), compound 9 might be a novel valuable tool for sigma receptor characterization and a suitable template for the rational design of potential therapeutically useful sigma(1) antagonists.

Synthesis of 99mTc-nitrido heterocomplex of piperidine and in vitro and in vivo evaluation of its affinity for sigma receptors

Sigma receptors are overexpressed in various types of cancer cells, making ligands that bind to these receptors attractive vectors for targeting radiation to specific sites for the imaging and therapy of oncologic disorders. In this paper, we report the synthesis of a dithiocarbamate derivative of 4-amino-N-benzylpiperidine and its radiolabeling with the [(99m)TcN(PNP)](2+) metal synthon. The radiolabeled tracer has been evaluated for sigma-receptor specificity. The radiochemical purity of the (99m)Tc-complex was >98%. The in vitro cell-binding and competition studies of the complex showed affinity and specificity toward fibrosarcoma and melanoma cells. In vivo studies carried out in mice bearing melanoma and fibrosarcoma tumors showed tumor uptakes of 1% and 1.9%, respectively, at 3 hours postinjection. In vivo blocking studies were carried out, using (+)-pentazocine, a sigma-receptor-specific agent where approximately 40% decrease in the tumor uptake was observed. The affinity of [(99m)TcN(PNP)Pip-DTC](+) complex for sigma-receptor sites ascertained through in vitro and in vivo studies makes it a potential agent for further investigation.

A Medicinal-Chemistry-Guided Approach to Selective and Druglike Sigma 1 Ligands

Based on a medicinal-chemistry-guided approach, three novel series of druglike cycloalkyl-annelated pyrazoles were synthesized and display high affinity (pKi>8) for the sigma1 receptor. Structure-affinity relationships were established, and the different scaffolds were optimized with respect to sigma1 binding and selectivity versus the sigma2 receptor and the hERG channel, resulting in selective compounds that have Ki values (for sigma1) in the subnanomolar range. Selected compounds were screened for cytochrome P450 inhibition (CYP1A2, CYP2A6, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1, CYP3A4), metabolic stability (rat and human liver microsomes), and cell-membrane permeability (Caco-2). They showed favorable in vitro ADME properties as well as favorable calculated druglike and experimental physicochemical properties. Furthermore, compounds 7 f and 17 a, for example, displayed high selectivity (affinity) for the sigma1 receptor against a wide range of other receptors (>60). With these valuable tool compounds in hand, we are further exploring the role of the sigma1 receptor in relevant animal models corresponding to such medicinal indications as drug abuse, pain, depression, anxiety, and psychosis.

Synthesis and structure-activity relationships of 1-aralkyl-4-benzylpiperidine and 1-aralkyl-4-benzylpiperazine derivatives as potent sigma ligands

In the attempt to define more accurately structure-affinity relationships for sigma(1) and sigma(2) ligands, we synthesized and tested on sigma subtype receptors a series of aralkyl derivatives of 4-benzylpiperidine, in which the effect of modifications on the aralkyl moiety was studied in a systematic way. The affinity of the compounds here described varied to a great extent, with a sigma(2)/sigma(1) selectivity ranging from 0.1 to 9. Thus, to confirm the ability of the piperazine derivative to bind to sigma(1) receptors in a different way than piperidines, we synthesized and tested a series of piperazine compounds; the comparison of their affinity with that of the corresponding piperidines strongly supports the possibility of a different binding mode. While the compounds here described are on the whole selective for sigma vs serotonin 5-HT(1A) and dopamine D(2) receptors, 9aa, 9ba and 9ab possess a remarkable affinity for both sigma and 5-HT(1A) receptors, with K(i) in the nanomolar range, and are selective with respect to D(2) receptors. They displayed also a partial agonist profile in a human 5-HT(1A) [(35)S]GTP gamma S binding assay, suggesting their potential use as atypical antipsychotic agents.

Molecular modeling of sigma 1 receptor ligands: a model of binding conformational and electrostatic considerations

We have performed molecular modeling studies on several sigma 1 specific ligands, including PD144418, spipethiane, haloperidol, pentazocine, and others to develop a pharmacophore for sigma 1 receptor-ligand binding, under the assumption that all the compounds interact at the same receptor binding site. The modeling studies have investigated the conformational and electrostatic properties of the ligands. Superposition of active molecules gave the coordinates of the hypothetical 5-point sigma 1 pharmacophore, as follows: R1 (0.85, 7.26, 0.30); R2 (5.47, 2.40, -1.51); R3 (-2.57, 4.82, -7.10); N (-0.71, 3.29, -6.40); carbon centroid (3.16, 4.83, -0.60), where R1, R2 were constructed onto the aromatic ring of each compound to represent hydrophobic interactions with the receptor; and R3 represents a hydrogen bond between the nitrogen atom and the receptor. Additional analyses were used to describe secondary binding sites to electronegative groups such as oxygen or sulfur atom. Those coordinates are (2.34, 5.08, -4.18). The model was verified by fitting other sigma 1 receptor ligands. This model may be used to search conformational databases for other possibly active ligands. In conjunction with rational drug design techniques the model may be useful in design and synthesis of novel sigma 1 ligands of high selectivity and potency. Calculations were performed using Sybyl 6.5.

A comparative molecular field analysis (CoMFA) study using semiempirical, density functional,ab initio methods and pharmacophore derivation using DISCOtech on sigma 1 ligands

The Comparative Molecular Field Analysis (CoMFA) was developed to investigate a three-dimensional quantitative structure activity relationship (3D-QSAR) model of ligands for the sigma 1 receptor. The starting geometry of sigma-1 receptor ligands was obtained from the Tripos force field minimizations and conformations were decided from DISCOtech using the SYBYL 6.8. program. The structures of 48 molecules were fully optimized at the ab initio HF/3-21G* and semiempirical AM1 calculations using GAUSSIAN 98. The electrostatic charges were calculated using several methods such as semiempirical AM1, density functional B3LYP/3-21G*, and ab initio HF/3-21G*, MP2/3-21G* calculations within GAUSSIAN 98. Using the optimized geometries, the CoMFA results derived from the HF/3-21G method were better than those from AM1. The best CoMFA was obtained from HF/3-21G* optimized geometry and charges (R2 = 0.977). Using the optimized geometries, the CoMFA results derived from the HF/3-21G methods were better than those from AM1 calculations. The training set of 43 molecules gave higher R2 (0.989-0.977) from HF/3-21G* optimized geometries than R2 (0.966-0.911) values from AM1 optimized geometries. The test set of five molecules also suggested that HF/3-21G* optimized geometries produced good CoMFA models to predict bioactivity of sigma 1 receptor ligands but AM1 optimized geometries failed to predict reasonable bioactivity of sigma 1 receptor ligands using different calculations for atomic charges.

Spiro[1,2,4-benzotriazine-3(4H),4'-(1'-substituted)piperidines] and related compounds as ligands for sigma receptors

As analogues of some conformationally restricted spiropiperidine derivatives which are endowed with high affinity for sigma1 receptor, a set of 16 spiro[1,2,4-benzotriazine-3(4H),4'-(1'-substituted)piperidines] and congeneric compounds was prepared and tested for affinity to sigma1 receptor subtype. All N-arylalkyl substituted derivatives exhibited high affinity for the relevant receptor, with Ki in the low nanomolar range. Affinity for sigma2 subtype (assayed only for a few representative compounds) was from one to three order of magnitude lower. Spiro[1,2,4-benzotriazine-3(4H),4'-(1'-benzyl)piperidine] (2), with a ratio Ki sigma2/Ki sigma1 = 7000 should represent the most selective sigma1 ligand so far described.

Novel spiropiperidines as highly potent and subtype selective sigma-receptor ligands. Part 1

A series of spiro[[2]benzopyran-1,4'-piperidines] and spiro[[2]benzofuran-1,4'-piperidines] of general structure 10 is prepared, and the affinity for sigma(1)- and sigma(2)-receptors is investigated by means of radioligand binding assays. The synthesis of the spiropiperidines 14a and 23 proceeds from bromine/lithium exchange of the bromoacetals 11 and 21, addition to piperidin-4-one 12a, and subsequent cyclization. Systematic variations of the substituent R at the nitrogen atom, the group X in position 3, and the ring size of the oxygen heterocycle are performed. The sigma(1)- and sigma(2)-receptor affinities are determined with guinea pig brain and rat liver membrane preparations using [(3)H]-labeled (+)-pentazocine and ditolylguanidine, respectively. Test results show that a benzyl residue at the piperidine nitrogen atom and a methoxy group in position 3 are advantageous for high sigma(1)-receptor affinity. In this series the 1'-benzyl-3-methoxy-3,4-dihydrospiro[[2]benzopyran-1,4'-piperidine] (14a) and the 1'-benzyl-3-methoxy-3H-spiro[[2]benzofuran-1,4'-piperidine] (23) are among the most potent sigma(1)-ligands interacting in the low nanomolar range with sigma(1)-receptors (14a, K(i) = 1.29 nM; 23, K(i) = 1.14 nM). Variation of the nitrogen substituent R from benzyl to H, alkyl, phenyl, or omega-phenylalkyl and the group X from methoxy to hydroxy, carbonyl, or alkyloxy led to reduced sigma(1)-receptor affinity. In addition to their high sigma(1)-receptor affinity, the spiropiperidines 14a and 23 display excellent selectivity toward sigma(2)-receptors (sigma(1)/sigma(2) = 2708 and 1130) and several other receptor and reuptake systems. Introduction of a polar hydroxy group in position 3 and elongation of the distance between the piperidine nitrogen atom and the phenyl moiety result in ligands with considerable sigma(2)-receptor affinity and therefore diminished sigma(1)/sigma(2)-receptor selectivity. The hemiacetalic 1'-(3-phenylpropyl)-3,4-dihydrospiro[[2]benzopyran-1,4'-piperidin]-3-ol (15e) represents the most active sigma(2)-receptor ligand in this series with a K(i) value of 83.1 nM.

Enigmatic receptors

The identification of new binding sites raises the problem of defining their role, if any. At times they are shown to be pharmacological receptors, in a strict sense, as they fulfill certain requirements, and a precise physiological role and function, and an endogenous ligand (neurotransmitter) are discovered. At other times, however, neither a clear physiological role nor an endogenous ligand are found, but the term "receptor" is still used, although it may not be a proper one in the conventional pharmacological sense. Furthermore, no clear intracellular signalling transduction pathway is defined and, as a consequence, it is not possible to determine whether drugs binding to these receptors act as agonists or antagonists. What their structure and biological function are and how they mediate the pharmacological effects of ligands may remain for a long time an enigma. The matter, in any case, is of great interest to researchers of different areas, especially to medicinal chemists who foresee novel potential targets for therapeutic interventions. In this meeting one section is dedicated to two examples of this kind of receptors: imidazoline (I) and sigma (sigma) receptors.

Synthesis and SAR of 1-alkyl-2-phenylethylamine derivatives designed from N,N-dipropyl-4-methoxy-3-(2-phenylethoxy)phenylethylamine to discover sigma(1) ligands

The synthesis and structure-activity relationships (SAR) of 1-alkyl-2-phenylethylamine derivatives 5-8 designed from N, N-dipropyl-2-[4-methoxy-3-(2-phenylethoxy)phenyl]ethylamine hydrochloride (1, NE-100) are presented. The SAR between compound 1 and 1-alkyl-2-phenylethylamine derivatives suggested that the alkyl group on the 1-position carbon of 2-[4-methoxy-3-(2-phenylethyl)phenyl]ethylamine derivatives played the role of one of the propyl groups on the aminic nitrogen of compound 1. (-)-N-Propyl-1-butyl-2-[4-methoxy-3-(2-phenylethoxy)phenyl]ethylam ine hydrochloride ((-)-6d, NE-537) and (-)-N-propyl-1-(3-methybutyl)-2-[4-methoxy-3-(2-phenylethoxy )phenyl]e thylamine hydrochloride ((-)-6i, NE-535), typical compounds in this series, have potent and selective sigma(1) affinity.