Structure-affinity relationships of a unique nicotinic ligand: N(1)-dimethyl-N(4)-phenylpiperazinium iodide (DMPP)

Applications of Isosteres of Piperazine in the Design of Biologically Active Compounds: Part 2

Applications of piperazine and homopiperazine in drug design are well-established, and these heterocycles have found use as both scaffolding and terminal elements and also as a means of introducing a water-solubilizing element into a molecule. In the accompanying review (10.1021/acs.jafc.2c00726), we summarized applications of piperazine and homopiperazine and their fused ring homologues in bioactive compound design along with illustrations of the use of 4-substituted piperidines and a sulfoximine-based mimetic. In this review, we discuss applications of pyrrolidine- and fused-pyrrolidine-based mimetics of piperazine and homopiperazine and illustrate derivatives of azetidine that include stretched and spirocyclic motifs, along with applications of a series of diaminocycloalkanes.

The piperazine scaffold for novel drug discovery efforts: the evidence to date

INTRODUCTION

. Piperazine is a structural element present in drugs belonging to various chemical classes and used for numerous different therapeutic applications; it has been considered a privileged scaffold for drug design.

AREAS COVERED

The authors have searched examples of piperazine-containing compounds among drugs recently approved by the FDA, and in some research fields (nicotinic receptor modulators, compounds acting against cancer and bacterial multi-drug resistance), looking in particular to the design behind the insertion of this moiety.

EXPERT OPINION

Piperazine is widely used due to its peculiar characteristics, such as solubility, basicity, chemical reactivity, and conformational properties. This moiety has represented an important tool to modulate pharmacokinetic and pharmacodynamic properties of drugs.

Cholinergic and Noradrenergic Modulation of Corticothalamic Synaptic Input From Layer 6 to the Posteromedial Thalamic Nucleus in the Rat

Cholinergic and noradrenergic neuromodulation of the synaptic transmission from cortical layer 6 of the primary somatosensory cortex to neurons in the posteromedial thalamic nucleus (PoM) was studied using an in vitro slice preparation from young rats. Cholinergic agonist carbachol substantially decreased the amplitudes of consecutive excitatory postsynaptic potentials (EPSPs) evoked by a 20 Hz five pulse train. The decreased amplitude effect was counteracted by a parallel increase of synaptic frequency-dependent facilitation. We found this modulation to be mediated by muscarinic acetylcholine receptors. In the presence of carbachol the amplitudes of the postsynaptic potentials showed a higher trial-to-trial coefficient of variation (CV), which suggested a presynaptic site of action for the modulation. To substantiate this finding, we measured the failure rate of the excitatory postsynaptic currents in PoM cells evoked by “pseudominimal” stimulation of corticothalamic input. A higher failure-rate in the presence of carbachol indicated decreased probability of transmitter release at the synapse. Activation of the noradrenergic modulatory system that was mimicked by application of norepinephrine did not affect the amplitude of the first EPSP evoked in the five-pulse train, but later EPSPs were diminished. This indicated a decrease of the synaptic frequency-dependent facilitation. Treatment with noradrenergic α-2 agonist clonidine, α-1 agonist phenylephrine, or β-receptor agonist isoproterenol showed that the modulation may partly rely on α-2 adrenergic receptors. CV analysis did not suggest a presynaptic action of norepinephrine. We conclude that cholinergic and noradrenergic modulation act as different variable dynamic controls for the corticothalamic mechanism of the frequency-dependent facilitation in PoM.

Metal-free reductive coupling of aliphatic aldehydes/ketones with 4-cyanopyridines: expanded scope and mechanistic studies

A practical B₂pin₂ mediated reductive coupling of 4-cyanopyridine with aliphatic aldehydes/ketones has been established. This metal-free protocol provides a convenient route to construct a wide range of C₄-pyridine-functionalized alcohols.

New Rigid Nicotine Analogues, Carrying a Norbornane Moiety, Are Potent Agonists of α7 and α3* Nicotinic Receptors

A three-dimensional database search has been applied to design a series of endo- and exo-3-(pyridin-3-yl)bicyclo[2.2.1]heptan-2-amines as nicotinic receptor ligands. The synthesized compounds were tested in radioligand binding assay on rat cortex against [³H]-cytisine and [³H]-methyllycaconitine to measure their affinity for α4β2* and α7* nicotinic receptors. The new derivatives showed some preference for the α4β2* over the α7* subtype, with their affinity being dependent on the endo/exo isomerism and on the methylation degree of the basic nitrogen. The endo primary amines displayed the lowest K_i values on both receptor subtypes. Selected compounds (1a, 2a, 3a, and 6a) were tested on heterologously expressed α4β2, α7, and α3β2 receptors and on SHSY-5Y cells. Compounds 1a and 2a showed α4β2 antagonistic properties while behaved as full agonists on recombinant α7 and on SHSY5Y cells. On the α3β2 subtype, only the chloro derivative 2a showed full agonist activity and submicromolar potency (EC₅₀ = 0.43 μM). The primary amines described here represent new chemotypes for the α7 and α3* receptor subtypes.

The twin drug approach for novel nicotinic acetylcholine receptor ligands

The association of two pharmacophoric entities generates so-called 'twin drugs' or dimer derivatives. We applied this approach for the design of a small compound library for the interaction with α4β2(∗) nicotinic acetylcholine receptors (nAChRs). In this compound series, the nAChR ligand N,N-dimethyl-2-(pyridin-3-yloxy)ethan-1-amine 9 served as one pharmacological entity and it was initially kept constant as one part of the 'twin' compound. 'Twin' compounds with identical or non-identical entities using the 'no linker mode' or 'overlap' mode were synthesized and evaluated for their nAChR affinities. Compound 17a showed the highest affinity for the α4β2(∗) nAChR subtype (Ki=0.188 nM) and its (di)fluoro analogs could retain nanomolar affinities, when replacing pyridine as the hydrogen bond acceptor system by mono- or difluoro-phenyls. The 'twin drug' approach proved to provide compounds with high affinity and subtype selectivity for α4β2(∗) nAChRs.

Discovery of a novel series of phenyl pyrazole inner salts based on fipronil as potential dual-target insecticides

A series of novel phenyl pyrazole inner salt derivatives based on fipronil were designed and synthesized in the search for dual-target insecticides. These compounds were designed to target two families of nicotinic acetylcholine receptors and γ-aminobutyric acid receptors. The insecticidal activities of the new compounds against diamondback moth (Plutella xylostella) were evaluated. The results of bioassays indicated that most of the inner salts showed moderate to high activities, of which the phenyl pyrazole inner salts containing quinoline had excellent biological activity. Previous structure-activity relationship studies revealed that a suitable structure of the quaternary ammonium salts was critical for the bioactivity of phenyl pyrazole inner salts, which contribute to exposing the cationic nitrogen to bind to the receptor (for instance, nicotinic acetylcholine receptors) and possibly interact with the receptor via hydrogen bonding and cooperative π-π interaction. The present work demonstrates that the insecticidal potency of phenyl pyrazole inner salts holds promise for the development new dual-target phenyl pyrazole insecticides.

Unique spirocyclopiperazinium salt III: Further investigation of monospirocyclopiperazinium (MSPZ) salts as potential analgesics

Two novel classes of monospirocyclopiperazinium salts were designed, synthesized, and evaluated for their in vivo analgesic activities. Some interesting structure-activity relationships are revealed: (1) Spirocyclopiperazinium moiety is favorable to improve the analgesic activity; (2) The size and conformation of spirocyclopiperazinium moiety significantly affects the analgesic activity; (3) Phenylethyl group of 3d is a crucial pharmacophore. Among the compounds synthesized, 3d exhibited the most potent activity with low toxicity. Further antinociceptive mechanism studies of 3d showed that these compounds will be a new kind of analgesics.

Design, Synthesis, and Preliminary Pharmacological Evaluation of New Quinoline Derivatives as Nicotinic Ligands

A series of nicotinic ligands, carrying a quinoline nucleus, and characterized by a pharmacophoric distance between the quinoline nitrogen (H-bond acceptor) and the cationic nitrogen atoms higher than that proposed in the classical pharmacophoric models, have been synthesized and tested for their affinity for the central nicotinic receptor. The enantiomers of the nicotine analogue 1-methyl-2-pyrrolidinyl-6-quinoline and of its methiodide display enantioselectivity in binding studies, but not when tested in vivo; on alpha7* nicotinic receptor enantioselectivity is inverted with respect to the alpha4beta2* subtype. N,N,N-Trimethyl-4-(quinolin-6-yl)but-3-yn-1-ammonium iodide (3c) and trans-N,N,N-trimethyl-4-(quinolin-6-yl)but-3-en-1-ammonium iodide (4c), showing pharmacophoric distances in the range 8.5-10.4 A, interact with the alpha4beta2* nicotinic receptor with Ki in the microM range; compound 3c shows preference for the alpha7* subtype.

Unique spirocyclopiperazinium salt. Part 4: Modification of dispirocyclopiperazinium (DSPZ) salts as analgesics

In order to improve the analgesic activity of lead compound 7a, two series of dispirocyclopiperazinium (DSPZ) salts 9a-h, 10a-e and compounds 14, 15 were synthesized and evaluated for their in vivo analgesic activity both by acetic acid induced writhing test and hot plate test. Compounds 9h, 14, and 15 exhibited better analgesic activities than 7a. Several important structure-activity relationships were revealed from this study: (1) the introduction of aryl group would obviously improve the activity; (2) it was favorable to enhance the analgesic activity and reduce the toxicity to incorporate alkyl group with suitable length in the molecule; (3) carbamate analogues displayed lower toxicity than carboxylic ester analogues; (4) hydroxylation and chlorination of lead compound could increase the analgesic activity in hot plate test.

Central Nicotinic Receptors: Structure, Function, Ligands, and Therapeutic Potential

The growing interest in nicotinic receptors, because of their wide expression in neuronal and non-neuronal tissues and their involvement in several important CNS pathologies, has stimulated the synthesis of a high number of ligands able to modulate their function. These membrane proteins appear to be highly heterogeneous, and still only incomplete information is available on their structure, subunit composition, and stoichiometry. This is due to the lack of selective ligands to study the role of nAChR under physiological or pathological conditions; so far, only compounds showing selectivity between alpha4beta2 and alpha7 receptors have been obtained. The nicotinic receptor ligands have been designed starting from lead compounds from natural sources such as nicotine, cytisine, or epibatidine, and, more recently, through the high-throughput screening of chemical libraries. This review focuses on the structure of the new agonists, antagonists, and allosteric ligands of nicotinic receptors, it highlights the current knowledge on the binding site models as a molecular modeling approach to design new compounds, and it discusses the nAChR modulators which have entered clinical trials.

Antinociceptive effects of the novel spirocyclopiperazinium salt compound LXM-10 in mice

The compound LXM-10 (2,4-dimethyl-9-beta-phenylethyl-3-oxo-6, 9-diazaspiro [5.5]undecane chloride) is a new spirocyclopiperazinium salt compound. This is the first article to evaluate its antinociceptive effect in the abdominal constriction test induced by acetic acid and the hot-plate test. In the abdominal constriction test, LXM-10 had a significant dose-response effect, and the maximal inhibition ratio was 79.2%. In the hot-plate test, LXM-10 had significant dose-response and time-response effects. The antinociceptive effect began at 1.0 h, peaked at 2.0 h, and persisted 3.0 h after s.c. administration. The hot-plate latency was increased by 126.8% at the dose of 12.0 mg/kg. The antinociceptive effect of LXM-10 was blocked by mecamylamine (a central and peripheral neuronal nicotinic acetylcholine receptor antagonist, 0.25, 0.5, 1.0 mg/kg, i.p.), hexamethonium (a peripheral neuronal nicotinic acetylcholine receptor antagonist, 0.2, 1.0, 5.0 mg/kg, i.p.), atropine (a central and peripheral muscarinic acetylcholine receptor antagonist, 0.2, 1.0, 5.0 mg/kg, i.p.), and atropine methylnitrate (a peripheral muscarinic acetylcholine receptor antagonist, 0.2, 1.0, 5.0 mg/kg, i.p.) in a dose-dependent fashion. In contrast, the effect was not blocked by naloxone (a non-selective opioid receptor antagonist, 2.0 mg/kg, i.p.) or yohimbine (a alpha(2)-adrenergic receptor antagonist, 1.0, 2.5, 5.0 mg/kg, i.p.) in the hot-plate test. Therefore, the antinociceptive effects of LXM-10 involve the peripheral neuronal nicotinic and muscarinic acetylcholine receptors; they are not related to opioid receptors or alpha(2)-adrenergic receptors. LXM-10 did not affect motor coordination, spontaneous activity, or body temperature. These findings with LXM-10 suggest that spirocyclopiperazinium derivatives could provide insight on new analgesics.

Modulation of eosinophil activation in vitro by a nicotinic receptor agonist

Nicotinic receptor agonists decreased the infiltration of eosinophils into the lung and airways in a mouse model of asthma. To better understand the mechanisms implicated in this anti-inflammatory phenomenon, the expression of nicotinic acetylcholine receptors (nAChRs) and the effect of dimethylphenylpiperazinium (DMPP), a nonselective nAChR agonist, on human blood eosinophils were studied. The expression of alpha-3, -4, and -7 nAChR subunits on human blood eosinophils was measured by cell ELISA and immunocytochemistry. mRNA expression for all three subunits was evaluated by quantitative RT-PCR. The effect of DMPP on leukotriene C4 (LTC4) and matrix metalloproteinase-9 (MMP-9) production, eosinophil migration, and intracellular calcium mobilization was measured. The results show that the alpha-3, -4, and -7 nAChR subunits and mRNAs are expressed by blood eosinophils. In vitro treatment of these cells with various concentrations of DMPP reduced platelet-activating factor (PAF)-induced LTC4 production significantly. DMPP (160 microM) decreased eotaxin, and 5-oxo-6,8,11,14-eicosatetranoic acid induced eosinophil migration through Matrigel by 40.9% and 55.5%, respectively. This effect was reversed by the nAChR antagonist mecamylamine. In addition, DMPP reduced MMP-9 release and the inositol 1,4,5-triphosphate-dependent intracellular calcium increase provoked by PAF. Taken together, these results indicate that functional nAChRs are expressed on eosinophils and that nAChR agonists down-regulate eosinophil function in vitro. These anti-inflammatory effects could be of interest in the treatment of allergic asthma.

Nicotinic agonists, antagonists, and modulators from natural sources

1. Acetylcholine receptors were initially defined as nicotinic or muscarinic, based on selective activation by two natural products, nicotine and muscarine. Several further nicotinic agonists have been discovered from natural sources, including cytisine, anatoxin, ferruginine, anabaseine, epibatidine, and epiquinamide. These have provided lead structures for the design of a wide range of synthetic agents. 2. Natural sources have also provided competitive nicotinic antagonists, such as the Erythrina alkaloids, the tubocurarines, and methyllycaconitine. Noncompetitive antagonists, such as the histrionicotoxins, various izidines, decahydroquinolines, spiropyrrolizidine oximes, pseudophrynamines, ibogaine, strychnine, cocaine, and sparteine have come from natural sources. Finally, galanthamine, codeine, and ivermectin represent positive modulators of nicotinic function, derived from natural sources. 3. Clearly, research on acetylcholine receptors and functions has been dependent on key natural products and the synthetic agents that they inspired.

Design of novel nicotinic ligands through 3D database searching

A series of quinoline derivatives have been designed on the basis of results from a 3D search of the Cambridge Structural Database using the nicotinic pharmacophore as a query and further modified using molecular modeling. Some of the synthesized compounds show nanomolar affinity for the central nicotinic receptor on rat cerebral cortex.

Rigid analogs of DMPP as probes for the nicotinic receptors

Chemical manipulation of the nicotinic agonist DMPP, endowed with modest activity on the central receptors, definitely improved its affinity and pharmacokinetic properties. Although their pharmacophore is somehow different from that of classical nicotinic ligands, some DMPP derivatives show low nanomolar affinity for the central nicotinic receptors. Introduction of rigidity in the structure of DMPP and in that of its analogue 1-(3-pyridyl)piperazine, resulted in molecules with lower or null affinity for the central nicotinic receptors. This suggests that the frozen structures chosen either do not represent the bioactive conformation, or their volume is not compatible with the space available within the interaction site.

Synthesis and analgesic activity of hydrochlorides and quaternary ammoniums of epibatidine incorporated with amino acid ester

Hydrochloride derivatives 5a-c and quaternary ammonium derivatives 6a-c of epibatidine incorporated with amino acid ester were synthesized and evaluated for their in vivo analgesic activity and toxicity. Among all tested compounds, compound 6c has the most potent analgesic activity. The quaternary ammonium salts 6a and 6c showed better analgesic activity than the corresponding hydrochlorides 5a and 5c. Both 5a-c and 6a-c showed significantly lower toxicity than epibatidine itself.

Design and synthesis of poly ADP-ribose polymerase-1 inhibitors. 2. Biological evaluation of aza-5[H]-phenanthridin-6-ones as potent, aqueous-soluble compounds for the treatment of ischemic injuries

A series of aza-5[H]-phenanthridin-6-ones were synthesized and evaluated as inhibitors of poly ADP-ribose polymerase-1 (PARP-1). Inhibitory potency of the unsubstituted aza-5[H]-phenanthridin-6-ones (i.e., benzonaphthyridones) was dependent on the position of the nitrogen atom within the core structure. The A ring nitrogen analogues (7-, 8-, and 10-aza-5[H]-phenanthridin-6-ones) were an order of magnitude less potent than C ring nitrogen analogues (1-, 2-, 3-, and 4-aza-5[H]-phenanthridin-6-ones). Preliminary stroke results from 1- and 2-aza-5[H]-phenanthridin-6-one prompted structure-activity relationships to be established for several 2- and 3-substituted 1-aza-5[H]-phenanthridin-6-ones. The 2-substituted 1-aza-5[H]-phenanthridin-6-ones were designed to improve the solubility and pharmacokinetic profiles for this series of PARP-1 inhibitors. Most importantly, three compounds from this series demonstrated statistically significant protective effects in rat models of stroke and heart ischemia.

Cholinergic nicotinic receptors: competitive ligands, allosteric modulators, and their potential applications

Discovery of the important role played by nicotinic acetylcholine receptors (nAChRs) in several CNS disorders has called attention to these membrane proteins and to ligands able to modulate their functions. The existence of different subtypes at multiple levels has complicated the understanding of this receptor's physiological role, but at the same time has increased the efforts to discover selective compounds in order to improve the pharmacological characterization of this kind of receptor and to make the possible therapeutical use of its modulators safer. This review focuses on the structure of new ligands for nAChRs, agonists, antagonists and allosteric modulators, and on their possible applications.

Unique spirocyclopiperazinium salt. Part 2: synthesis and structure-activity relationship of dispirocyclopiperazinium salts as analgesics

Three series of spirocyclopiperazinium derivatives 5a-d, 6a-f and 17a-d were synthesized and evaluated for their in vivo analgesic activities. Compounds 5a, 17a and 17b exhibited excellent analgesic activity. Two important structure-activity relationships were observed from this study: (1) the quaternary ammonium functionality is a critical pharmacophore for analgesic activity; (2) it is important to adjust the lipophilic property of compounds to improve analgesic activity.

Unique spirocyclopiperazinium salt I: synthesis and structure-activity relationship of spirocyclopiperazinium salts as analgesics

Based on the structure of compound 3, two series of spirocyclopiperazinium derivatives 7a-n and 10a-h were synthesized and evaluated for their in vivo analgesic and sedative activities. Compounds 7f and 10c were discovered to exhibit excellent analgesic activity. Structure-activity relationships revealed that anion of the quaternary salt affected the analgesic and sedative activity significantly; the allyl group is a most effective group among the compounds 7a-n; the electron-released substitute on the aromatic ring is favorable to increase the analgesic activity.

Synthesis and pharmacological evaluation of some (pyridyl)cyclopropylmethyl amines and their methiodides as nicotinic receptor ligands

A series of 3- and (4-pyridyl)cyclopropylmethyl amines and their quaternary ammonium derivatives have been synthesized; they can be considered as rigid analogues of nicotine. The compounds have been tested on rat cerebral cortex to measure the affinity for the central nicotinic receptor. Only the methiodides show affinity in the micromolar range. The results obtained can provide useful information on the topography of the nicotinic receptor-binding site.