Design, synthesis, and activity of a series of arylpyrid-3-ylmethanones as type I positive allosteric modulators of α7 nicotinic acetylcholine receptors.

A series of novel arylpyrid-3-ylmethanones (7a-aa) were designed as modulators of α7 nicotinic acetylcholine receptors (nAChRs). The methanones were found to be type I positive allosteric modulators (PAMs) of human α7 nAChRs expressed in Xenopus ooctyes. Structure-activity relationship (SAR) studies resulted in the identification of compound 7v as a potent and efficacious type I PAM with maximum modulation of a nicotine EC5 response of 1200% and EC50 = 0.18 µM. Compound 7z was active in reversing the effect of scopolamine in the novel object recognition (NOR) paradigm with a minimum effective ip dose of 1.0 mg/kg (2.7 µmol/kg). This effect was blocked by the selective α7 nAChR antagonist methyllycaconitine (MLA). These compounds are potent type I positive allosteric modulators of α7 nAChRs that may have therapeutic value in restoring impaired sensory gating and cognitive deficits in schizophrenia and Alzheimer's disease.

Allosteric modulation of related ligand-gated ion channels synergistically induces long-term potentiation in the hippocampus and enhances cognition.

α5 Subunit-containing GABA(A) receptors (GABA(A)Rs) and α7 neuronal nicotinic-acetylcholine receptors (nAChRs) are members of the Cys-loop family of ligand-gated ion channels (LGICs) that mediate cognitive and attentional processes in the hippocampus. α5 GABA(A)Rs alter network activity by tonic inhibition of CA1/CA3 pyramidal cells of the hippocampus. Postsynaptic α7 nAChRs in the hippocampus regulate inhibitory GABAergic interneuron activity required for synchronization of pyramidal neurons in the CA1, whereas presynaptic α7 nAChRs regulate glutamate release. Can simultaneous allosteric modulation of these LGICs produce synergistic effects on cognition? We show that combined transient application of two allosteric modulators that individually 1) inhibit α5 GABA(A)Rs and 2) enhance α7 nAChRs causes long-term potentiation (LTP) of mossy fiber stimulation-induced excitatory postsynaptic currents (EPSC) from CA1 pyramidal neurons of rat hippocampal slices. The LTP effect evoked by two compounds is replicated by 3-(2,5-difluorophenyl)-6-(N-ethylindol-5-yl)-1,2,4-triazolo[4,3-b]pyridazine (522-054), a compound we designed to simultaneously inhibit α5 GABA(A)Rs and enhance α7 nAChRs. Selective antagonists for either receptor block sustained EPSC potentiation produced by 522-054. In vivo, 522-054 enhances performance in the radial arm maze and facilitates attentional states in the five-choice serial reaction time trial with similar receptor antagonist sensitivity. These observations may translate into therapeutic utility of dual action compounds in diseases of hippocampal-based cognitive impairment.

Limiting activity at beta1-subunit-containing GABAA receptor subtypes reduces ataxia.

GABA(A) receptor (R) positive allosteric modulators that selectively modulate GABA(A)Rs containing beta(2)- and/or beta(3)- over beta(1)-subunits have been reported across diverse chemotypes. Examples include loreclezole, mefenamic acid, tracazolate, and etifoxine. In general,"beta(2/3)-selective" GABA(A)R positive allosteric modulators are nonbenzodiazepines (nonBZs), do not show alpha-subunit isoform selectivity, yet have anxiolytic efficacy with reduced ataxic/sedative effects in animal models and humans. Here, we report on an enantiomeric pair of nonBZ GABA(A)R positive allosteric modulators that demonstrate differential beta-subunit isoform selectivity. We have tested this enantiomeric pair along with a series of other beta(2/3)-subunit selective, alpha-subunit isoform-selective, BZ and nonBZ GABA(A) positive allosteric modulators using electrophysiological, pharmacokinetic, and behavioral assays to test the hypothesis that ataxia may be correlated with the extent of modulation at beta(1)-subunit-containing GABA(A)Rs. Our findings provide an alternative strategy for designing anxioselective allosteric modulators of the GABA(A)R with BZ-like anxiolytic efficacy by reducing or eliminating activity at beta(1)-subunit-containing GABA(A)Rs.

Negative allosteric modulation of nicotinic acetylcholine receptors blocks nicotine self-administration in rats.

Drugs that antagonize nicotinic acetylcholine receptors (nAChRs) can be used to inhibit nicotine-induced behavior in both humans and animals. The aim of our experiments is to establish a proof-of-principle that antagonism of nAChRs by negative allosteric modulation can alter behavior in a relevant animal model of addiction, nicotine self-administration. We have identified a novel, negative allosteric modulator of nAChRs, UCI-30002 [N-(1,2,3,4-tetrahydro-1-naphthyl)-4-nitroaniline], with selectivity for the major neuronal nAChR subtypes over muscle-type nAChRs. After systemic administration, UCI-30002 significantly reduces nicotine self-administration in rats on both fixed ratio and progressive ratio schedules of reinforcement. The minimum effective dose that significantly alters nicotine self-administration corresponds to brain concentrations of UCI-30002 that produce at least 30% inhibition of the major neuronal nAChR subtypes measured in vitro. UCI-30002 has no effect on responding for food reinforcement in rats on either type of schedule, indicating that there is no effect on general responding or natural reward. UCI-30002 represents validation of the concept that negative allosteric modulators may have significant benefits as a strategy for treating nicotine addiction and encourages the development of subtype-selective modulators.

Enaminone amides as novel orally active GABAA receptor modulators.

A series of enaminone esters and amides have been developed as potent allosteric modulators of gamma-aminobutyric acidA (GABAA) receptors. The compounds bind to a novel modulatory site that is independent of the benzodiazepine (BZ), isosteric GABA, and neuroactive steroid binding sites. Structure-activity relationship (SAR) studies resulted in the synthesis of the c-Bu amide 16h with an in vitro potency of 7 nM based on inhibition of [35S]TBPS binding. The activity of the enaminones as positive allosteric modulators was confirmed with electrophysiological measurements in oocytes expressing alpha1beta2gamma2L GABAA receptors. The i-Pr, s-Bu, c-Pr, and c-Bu amides (16e-h) were orally active in mice with profound central nervous system depressant effects. The i-Pr amide 16e was an orally active anxiolytic in the mouse light-dark paradigm.

Nootropic alpha7 nicotinic receptor allosteric modulator derived from GABAA receptor modulators.

Activation of brain alpha7 nicotinic acetylcholine receptors (alpha7 nAChRs) has broad therapeutic potential in CNS diseases related to cognitive dysfunction, including Alzheimer's disease and schizophrenia. In contrast to direct agonist activation, positive allosteric modulation of alpha7 nAChRs would deliver the clinically validated benefits of allosterism to these indications. We have generated a selective alpha7 nAChR-positive allosteric modulator (PAM) from a library of GABAA receptor PAMs. Compound 6 (N-(4-chlorophenyl)-alpha-[[(4-chloro-phenyl)amino]methylene]-3-methyl-5-isoxazoleacet-amide) evokes robust positive modulation of agonist-induced currents at alpha7 nAChRs, while preserving the rapid native characteristics of desensitization, and has little to no efficacy at other ligand-gated ion channels. In rodent models, it corrects sensory-gating deficits and improves working memory, effects consistent with cognitive enhancement. Compound 6 represents a chemotype for allosteric activation of alpha7 nAChRs, with therapeutic potential in CNS diseases with cognitive dysfunction.

V102862 (Co 102862): a potent, broad-spectrum state-dependent blocker of mammalian voltage-gated sodium channels.

1. 4-(4-Fluorophenoxy)benzaldehyde semicarbazone (V102862) was initially described as an orally active anticonvulsant with robust activity in a variety of rodent models of epilepsy. The mechanism of action was not known. We used whole-cell patch-clamp techniques to study the effects of V102862 on native and recombinant mammalian voltage-gated Na+ channels. 2. V102862 blocked Na+ currents (I(Na)) in acutely dissociated cultured rat hippocampal neurons. Potency increased with membrane depolarization, suggesting a state-dependent mechanism of inhibition. There was no significant effect on the voltage dependence of activation of I(Na). 3. The dissociation constant for the inactivated state (K(I)) was approximately 0.6 microM, whereas the dissociation constant for the resting state (K(R)) was >15 microM. 4. The binding to inactivated channels was slow, requiring a few seconds to reach steady state at -80 mV. 5. The mechanism of inhibition was characterized in more detail using human embryonic kidney-293 cells stably expressing rat brain type IIA Na+ (rNa(v)1.2) channels, a major Na+ channel alpha subunit in rat hippocampal neurons. Similar to hippocampal neurons, V102862 was a potent state-dependent blocker of rNa(v)1.2 channels with a K(I) of approximately 0.4 microM and K(R) approximately 30 microM. V102862 binding to inactivated channels was relatively slow (k(+) approximately = 1.7 microM(-1) s(-1)). V102862 shifted the steady-state availability curve in the hyperpolarizing direction and significantly retarded recovery of Na+ channels from inactivation. 6. These results suggest that inhibition of voltage-gated Na+ channels is a major mechanism underlying the anticonvulsant properties of V102862. Moreover, understanding the biophysics of the interaction may prove to be useful in designing a new generation of potent Na+ channel blocker therapeutics.

N-(4-tertiarybutylphenyl)-4-(3-chloropyridin-2-yl)tetrahydropyrazine -1(2H)-carbox-amide (BCTC), a novel, orally effective vanilloid receptor 1 antagonist with analgesic properties: I. in vitro characterization and pharmacokinetic properties.

Vanilloids such as capsaicin have algesic properties and seem to mediate their effects via activation of the vanilloid receptor 1 (VR1), a ligand-gated ion channel highly expressed on primary nociceptors. Although blockade of capsaicin-induced VR1 activation has been demonstrated in vitro and in vivo with the antagonist capsazepine, efficacy in rat models of chronic pain has not been observed with this compound. Here, we describe the in vitro pharmacology of a highly potent VR1 antagonist, N-(4-tertiarybutylphenyl)-4-(3-chloropyridin-2-yl)tetrahydropyrazine-1(2H)-carbox-amide (BCTC). Similar to capsazepine, this compound inhibits capsaicin-induced activation of rat VR1 with an IC50 value of 35 nM. Interestingly however, BCTC also potently inhibits acid-induced activation of rat VR1 (IC50 value of 6.0 nM), whereas capsazepine is inactive. Similarly, in the rat skin-nerve preparation both BCTC and capsazepine block capsaicin-induced activation, whereas the response to acidification is inhibited by BCTC, but not by capsazepine. Specificity for VR1 was demonstrated against 63 other receptor, enzyme, transporter, and ion channel targets. BCTC was orally bioavailable in the rat, demonstrating a plasma half-life of approximately 1 h and significant penetration into the central nervous system. Thus, BCTC is a high potency, selective VR1 antagonist that, unlike capsazepine, has potent blocking effects on low pH-induced activation of rat VR1. These properties make it a more suitable candidate than capsazepine for testing the role played by VR1 in rat models of human disease.

Synthesis and SAR of novel di- and trisubstituted 1,4-dihydroquinoxaline-2,3-diones related to licostinel (Acea 1021) as NMDA/glycine site antagonists.

A series of novel di- and trisubstituted 1,4-dihydroquinoxaline-2,3-diones (QXs) related to licostinel (Acea 1021) was synthesized and evaluated as antagonists for the glycine site of the N-methyl-D-asparate (NMDA) receptor. The in vitro potency of these antagonists was determined by displacement of the glycine site radioligand [(3)H]-5,7-dichlorokynurenic acid ([(3)H]DCKA) in rat brain cortical membranes. Structure-activity relationship studies indicate that a cyano group is a good replacement for the nitro group in the 5-position of licostinel while 5-carboxy, 5-ester, 5-ketone and 5-amide derivatives showed reduced potency. 5,6-Cyclized analogues of licostinel also showed significantly reduced potency. Among the trisubstituted QXs investigated, 5-cyano-6,7-dichloro QX and 5-cyano-7-chloro-6-methyl QX are the most potent with IC(50) values of 32 nM and 26 nM, respectively.