(1S,2R)-1-Phenyl-2-[(S)-1-aminopropyl]-N,N-diethylcyclopropanecarboxamide (PPDC), a new class of NMDA-receptor antagonist: molecular design by a novel conformational restriction strategy

Subunit-specific effects of poricoic acid A on NMDA receptors

BACKGROUND

N-methyl-D-aspartate (NMDA) receptor is a tetrameric protein complex composed of glycine-linked NR1 subunits and glutamate-linked NR2 subunits. There are four NR2 subunits (A-D) that differ in development, anatomy, and function profiles. They play various roles in normal and neuropathologic conditions. Specific agonists, antagonists, and modulators of subunits for selective NMDA receptors may be precious mediational tools and potent agents for treating diseases. The objective of this study was to determine the effect of poricoic acid A on NMDA receptor known to mediate excitatory synaptic transmission factors and cause changes in synaptic strength. Inhibitory effect of poricoic acid A on NR1a combined with NR2A, NR2B, NR2C, or NR2D receptor was evaluated.

METHODS

Glutamate-mediated currents for each NR1a and NR2 subunits were investigated using two-electrode voltage-clamp techniques. Molecular modeling and molecular dynamics simulation studies were carried out with Autodock Tools. Poricoic acid A and NMDA receptor protein complex were examined with Ligplot and Pymol docking program. Ligplot shows binding activity at the protein and the ligand.

RESULTS

The inhibitory effect of poricoic acid A on glutamate-induced inward current in a concentration-dependent manner that was reversible. Half inhibitory concentrations of glutamate on NR1a/NR2A, NR1a/NR2B, NR1a/NR2C, and NR1a/NR2D receptors were 9.6 ± 1.2, 5.7 ± 0.4, 46.1 ± 21.5, and 21.5 ± 8.2 μM, respectively. This corresponded to the order of inhibitory effect of oocyte expressing NR1a and NR2s subunit of NR1a/NR2B > NR1a/NR2A > NR1a/NR2C > NR1a/NR2D.

CONCLUSIONS

Taken together, these results indicate that poricoic acid A can modulate the expression of NMDA receptor. In addition, the regulation of excitatory ligand-gating ion channel by poricoic acid A may have pharmaceutical functions on excitatory synaptic transmission of neuronal system.

Antidepressants are a rational complementary therapy for the treatment of Alzheimer's disease

There is a high prevalence rate (30-50%) of Alzheimer's disease (AD) and depression comorbidity. Depression can be a risk factor for the development of AD or it can be developed secondary to the neurodegenerative process. There are numerous documented diagnosis and treatment challenges for the patients who suffer comorbidity between these two diseases. Meta analysis studies have provided evidence for the safety and efficacy of antidepressants in treatment of depression in AD patients. Preclinical and clinical studies show the positive role of chronic administration of selective serotonin reuptake inhibitor (SSRI) antidepressants in hindering the progression of the AD and improving patient performance. A number of clinical studies suggest a beneficial role of combinatorial therapies that pair antidepressants with FDA approved AD drugs. Preclinical studies also demonstrate a favorable effect of natural antidepressants for AD patients. Based on the preclinical studies there are a number of plausible antidepressants effects that may modulate the progression of AD. These effects include an increase in neurogenesis, improvement in learning and memory, elevation in the levels of neurotrophic factors and pCREB and a reduction of amyloid peptide burden. Based on this preclinical and clinical evidence, antidepressants represent a rational complimentary strategy for the treatment of AD patients with depression comorbidity.

The effect of (+/-)-CP-101,606, an NMDA receptor NR2B subunit selective antagonist, in the Morris watermaze

It is well established that the NMDA receptor antagonists block hippocampal long-term potentiation and impair acquisition in the Morris watermaze task, although the role of individual NMDA receptor subtypes is largely unknown. In the present study, we compared the effects of (+/-)-CP-101,606, an antagonist selective for NMDA receptor NR1/NR2B subunit-containing receptors and the nonselective NMDA receptor antagonist MK-801, on acquisition in the Morris watermaze. Male hooded Lister rats were given 4 trials/day to find a fixed hidden platform submerged beneath the opaque water of the Morris watermaze. Twenty-four hours after the last acquisition trial, a 'probe trial' was conducted to assess the rat's spatial memory for the location of the hidden platform. Those rats treated with MK-801 (0.1 mg/kg, i.p.) 60 min prior to the acquisition and probe trials took significantly longer to find the hidden platform during training and spent significantly less time searching the platform's location during the probe trial than vehicle-treated rats. In contrast, 60-min pretreatment with (+/-)-CP-101,606 (60 mg/kg, p.o.), a dose that fully occupied hippocampal NR1/NR2B subunit-containing receptors, as determined by ex vivo NMDA receptor-specific [3H]ifenprodil binding immediately following watermaze experiments, had no effect on acquisition or the probe trial. These results suggest that antagonists selective for NR1/NR2B subunit-containing receptors may not impair spatial memory in rats in the Morris watermaze.

Cyclopropane-based conformational restriction of histamine. (1S,2S)-2-(2-aminoethyl)-1-(1H-imidazol-4-yl)cyclopropane, a highly selective agonist for the histamine H3 receptor, having a cis-cyclopropane structure

A series of cyclopropane-based conformationally restricted analogues of histamine, the "folded" cis-analogues, i.e., (1S,2R)-2-(aminomethyl)-1-(1H-imidazol-4-yl)cyclopropane (11), (1S,2S)-2-(2-aminoethyl)-1-(1H-imidazol-4-yl)cyclopropane (13), and their enantiomers ent-11 and ent-13, and the "extended" trans-analogues, i.e., (1R,2R)-2-(aminomethyl)-1-(1H-imidazol-4-yl)cyclopropane (12) and its enantiomer ent-12, were designed as histamine H(3) receptor agonists. These target compounds were synthesized from the versatile chiral cyclopropane units, (1S,2R)- and (1R,2R)-2-(tert-butyldiphenylsilyloxy)methyl-1-formylcyclopropane (14 and 15, respectively) or their enantiomers ent-14 and ent-15. Among the conformationally restricted analogues, the "folded" analogue 13 (AEIC) having the cis-cyclopropane structure was identified as a potent H(3) receptor agonist, which showed a significant binding affinity (K(i) = 1.31 +/- 0.16 nM) and had an agonist effect (EC(50) value of 10 +/- 3 nM) on the receptor. This compound owes its importance to being the first highly selective H(3) receptor agonist to have virtually no effect on the H(4) subtype receptor. These studies showed that the cis-cyclopropane structure is very effective in the conformational restriction of histamine to improve the specific binding to the histamine H(3) receptor.

Protective effect of methanol extract of Uncaria rhynchophylla against excitotoxicity induced by N-methyl-D-aspartate in rat hippocampus

Uncaria rhynchophylla is a medicinal herb used for convulsive disorders in Oriental medicine. In this study, the effect of the methanol extract of Uncaria rhynchophylla against N-methyl-D-aspartate (NMDA)-induced excitotoxicity was investigated. Pretreatment with the extract of Uncaria rhynchopylla reduced the degree of neuronal damage induced by NMDA exposure in cultured hippocampal slices. In the patch clamp study, Uncaria rhynchophylla significantly inhibited NMDA receptor-activated ion current in acutely dissociated hippocampal CA1 neurons. These results indicate that Uncaria rhynchophylla offers protection against NMDA-induced neuronal injury and inhibitory action on NMDA receptor-mediated ion current may be a mechanism behind the neuroprotective effect of Uncaria rhynchophylla.

Synthesis of derivatives of (1S,2R)-1-phenyl-2-[(S)-1-aminopropyl]-N,N-diethylcyclopropanecarboxamide (PPDC) modified at the 1-aromatic moiety as novel NMDA receptor antagonists: the aromatic group is essential for the activity

(1S,2R)-1-Phenyl-2-[(S)-1-aminopropyl]-N,N-diethylcyclopropanecarboxamide (PPDC, 4a), which is a conformationally restricted analogue of antidepressant milnacipran [(+/-)-1], is a new class of potent noncompetitive NMDA receptor antagonists. A series of PPDC analogues modified at the 1-phenyl moiety, that is, the analogue 6 lacking 1-phenyl group, the 1-(fluorophenyl) analogues 4b,c,d, the 1-(methylphenyl) analogues 4e-g and the 1-(naphthyl) analogues 4h,i were synthesized. Analogue 6, lacking the 1-phenyl group, was completely inactive showing that the aromatic moiety is essential for the NMDA receptor binding. Among the analogues synthesized, the 1-o-fluorophenyl and 1-m-fluorophenyl analogues 4b and 4c showed potent affinities for the NMDA receptor [IC(50)=0.16+/-0.001 microM (4b), 0.15+/-0.02 microM (4c)], which were improved to some extent compared to those of the parent compound PPDC (IC(50)=0.20+/-0.02 microM). On the other hand, compounds 4b and 4c showed none of the 5-HT-uptake inhibitory effect, while PPDC turned out to be a weak 5-HT-uptake inhibitor.

Conformational analysis of the NMDA receptor antagonist (1S,2R)-1-phenyl-2-[(S)-1-aminopropyl]-N,N-diethylcyclopropanecarboxamide (PPDC) designed by a novel conformational restriction method based on the structural feature of cyclopropane ring

(1S,2R)-1-phenyl-2-[(S)-1-aminopropyl]-N,N-diethylcyclopropanecarboxamide (2b, PPDC), a new class of potent N-methyl-D-aspartic acid (NMDA) receptor antagonist, was designed based on a new method for restricting the conformation of compounds having a cyclopropane ring. The three-dimensional structures of PPDC obtained by the three different methods of X-ray crystallographic analysis, usual MM2-calculations in vacuum, and MM2 calculations based on the nuclear Overhauser effect (NOE) data in D2O are similar, which are in accord with that hypothesized. These results suggest that this conformational restriction method is particularly effective in designing novel biologically active molecules.

Development of versatile cis- and trans-dicarbon-substituted chiral cyclopropane units: synthesis of (1S,2R)- and (1R,2R)-2-aminomethyl-1-(1H-imidazol-4-yl)cyclopropanes and their enantiomers as conformationally restricted analogues of histamine

The cyclopropane ring can be used effectively in restricting the conformation of biologically active compounds to improve activity and also to investigate bioactive conformations. We designed (1S,2R)- and (1R,2R)-2-aminomethyl-1-(1H-imidazol-4-yl)cyclopropanes (1 and 2, respectively) and their enantiomers (ent-1 and ent-2) as conformationally restricted analogues of histamine. The four types of chiral cyclopropanes bearing two differentially functionalized carbon substituents in a cis or trans relationship on a cyclopropane ring, (1S,2R)-2-(tert-butyldiphenylsilyloxy)methyl-1-formylcyclopropane (7) and (1R,2R)-2-(tert-butyldiphenylsilyloxy)methyl-1-formylcyclopropane (8) and their enantiomers (ent-7 and ent-8), were developed as the key intermediates for synthesizing 1, 2, ent-1, and ent-2. The reaction between (R)-epichlorohydrin [(R)-12] and phenylsulfonylacetonitrile (13a) in the presence of NaOEt in EtOH followed by treatment with acid gave the chiral cyclopropane lactone 11a with 98% ee in 82% yield. Compound 11a was converted into both the cis- and trans-chiral cyclopropane units 7 and 8, respectively, via reductive desulfonylation with Mg/MeOH as the key step. The corresponding enantiomers, the cis-substituted ent-7 and the trans-substituted ent-8, were also prepared starting from (S)-epichlorohydrin [(S)-12]. The four conformationally restricted target histamine analogues 1, 2, ent-1, and ent-2 were successfully synthesized from 7, 8, ent-7, and ent-8, respectively. The chiral cyclopropane units 7, 8, ent-7, and ent-8 should be useful as versatile intermediates for synthesizing various compounds having an asymmetric cyclopropane structure.