3-(2,4)-Dimethoxybenzilidine Anabaseine (DMXB-A; Also Known as GTS-21)
While there are a number of nicotinic receptor agonists known to be selective for the α4β2 subtype, there are some agonists which bind the α7 nAChRs selectively over other subtypes [13
]. Anabaseine (2-(3-pyridyl)-3,4,5,6-tetrahydropyridine) (Fig.
), a naturally occurring substance in nemertines, is an agonist at the neuromuscular junction [95
] and is structurally related to nicotine. The better known compound anabasine (neonicotine; 3-(2-piperidinyl)pyridine) (Fig.
) is a weak nicotinic alkaloid found in tobacco that lacks the imine double bond present in anabaseine. Three analogues of anabaseine, 3-(2,4)-dimethoxybenzilidine anabaseine (DMXB-A; also known as GTS-21), 3-(4)-dimethylaminobenzylidine anabaseine (DMAB), and 3-(4)-dimethylaminocinnamylidine (DMAC), have been reported to be functionally selective for the α7 nAChRs [96
). Compared with anabaseine and the other derivatives, DMAC was found to be the most potent at displacing [125
I](-bungarotoxin binding (putative α7 subtype) and the least potent at displacing [3
H]cytisine binding (putative α4β2 subtype) to brain membranes. These anabaseine derivatives were partial agonists at α7 nAChRs [96
]. Furthermore, DMXB-A bound to human α4β2 nAChRs (K
i=20 nM) 100-fold more potently than to human α7 nAChRs, and was 18- and 2-fold less potent than (–)-nicotine at human α4β2 and α7 nAChRs, respectively [97
]. The primary human metabolite, 3-(4-hydroxy-2-methoxybenzylidine) anabaseine (4-OH DMXB-A; Fig.
), of DMXB-A exhibited a similar level of efficacy for human α7 nAChRs [98
Chemical structures of anabaseine, anabasine, DMXB-A, DMAB, DMAC and 4-OH DMXB-A.
Initial (phase I) clinical studies on DMXB-A have been reported [100
]. DMXB-A was administered to 87 healthy volunteers. Initially, the effects of single doses (range, 1-250 mg) were assessed. The elimination half-life ranged between 0.5 and 1.0 h for DMXB-A and its major phase I metabolite, 4-OH DMXB-A. No serious adverse effects were reported at these doses. At twice daily doses of 75 and 150 mg for 5 days, DMXB-A improved the cognitive function of young adult volunteers. Furthermore, DMXB-A improved long-term memory as well as working memory and attention, as measured by the Cognitive Drug Research test battery [100
]. A randomized, placebo-controlled, multiple dose study of the safety, pharmacokinetics and cognitive effects of DMXB-A in healthy volunteers was also reported [101
]. A total of 18 subjects were randomized to receive DMXB-A (25, 75, and 150 mg) or a placebo administered three times daily for 5 days with a 10 day washout period between drug-taking periods. DMXB-A was well tolerated up to doses of 450 mg/day, with no clinically significant safety findings. Peak plasma levels (Cmax
) were achieved at 1-1.4 h after the first dose and 1-1.2 h after 5 days of dosing. Cmax
and the area under the plasma concentration-time curve (AUC) of DMXB-A and metabolite 4-OH-DMXB-A increased in a dose-related manner. DMXB-A showed statistically significant enhancement of three measures of cognitive function (attention, working memory, and episodic second memory) compared to placebo. A relationship between exposure to DMBX-A and the magnitude of the cognitive response was apparent, with a maximal effect observed for doses between 75 and 150 mg three times a day.
Because DMXB-A appeared safe and promising for enhancing cognition, this drug was studied in schizophrenic patients to determined whether the α7 nAChRs activation is responsible for the normalization of the P50 auditory evoked potential deficits in schizophrenia [102
]. Additionally, the safety and effects of DMXB-A on neurocognition in schizophrenia patients were also evaluated. DMXB-A was administered in a double-blind, placebo-controlled cross-over design to 12 male and female non-smokers with schizophrenia. DMXB-A was administered orally (150 or 75 mg) followed 2 h later by a half dose (75 or 37.5 mg). DMXB-A improved performance on both the repeatable battery for assessment for neuropsychological status (RBANS) total scale score and the attention scale. DMXB-A also normalized the P50 ratio as well as the test wave amplitude, a more specific measure of inhibition.
On the basis of this initial positive trial, a phase 2 trial was conducted to assess whether the cognitive effects would continue during longer-term administration and whether the clinical ratings would also change [103
]. Thirty-one subjects with schizophrenia received DMXB-A at one of two different doses or a placebo for 4 weeks in a three-arm, two-site, double-blind, crossover phase 2 trial. The doses were those used in the phase 1 trial. The Measurement and Treatment Research to Improve Cognition in Schizophrenia (MATRICS) Consensus Cognitive Battery developed by the National Institute of Mental Health was used to assess cognitive effects, and the Scale for the Assessment of Negative Symptoms (SANS) and Brief Psychiatry Rating Scale (BPRS) were used to assess the clinical effects. Subjects continued their current antipsychotic drug during the trial and were nonsmokers. There were no significant differences in the MATRICS cognitive measures between DMXB-A and placebo over the three treatment arms, but patients taking the higher DMXB-A dose experienced significant improvement in the SANS total score and nearly significant improvement in the BPRS total score. Improvement was most notable on the SANS anhedonia and alogia subscales. Examination of the first treatment arm showed effects of DMXB-A on the attention/vigilance and working memory MATRICS domains, compared to baseline. There were, however, increased reports of nausea and restlessness during DMXB-A treatment. Nausea occurred in patients (45%) at the higher dose of DMXB-A, suggesting the known effects of nicotinic agonists on gastrointestinal motility [103
]. Considering the high incidence of nausea in the treatment with α7 nAchR agonists, α7 nAchR agonists with 5-hydroxytryptamine-3 (5-HT3
) receptor antagonism would be potential therapeutic drugs for cognitive impairments as well as negative symptoms in schizophrenic patients, since the 5-HT3
receptor antagonists have been used for the treatment of nausea [104
Researchers at AstraZeneca reported the profile of AR-R17779, (−)-spiro[1-zabicyclo-[2.2.2]octane-3,5'-oxazolidin -2'-one] (Fig.
), a potent full agonist of the α7 nAChRs that is highly selective for the α7 subtype over the α74β72 subtypes [105
]. AR-R17779 has been widely used as a selective full agonist of the α7 nAChRs. For example, AR-R17779 failed to stimulate locomotor activity in both nicotine-nontolerant and -sensitized rats, whereas nicotine and the putative agonist SIB1765F, [±]-5-ethynyl-3-(1-methyl-2-pyrrolidiny-l)pyridine fumarate (Fig.
], of α4β2 nAChRs increased the activity under both experimental conditions, suggesting a negligible role of α7 nAChRs in nicotine-induced hyperlocomotion and reward in the rat [106
]. Furthermore, it has been reported that chronic administration of both nicotine and SIB1765F, but not AR-R17779, resulted in an enhanced locomotor response to acute challenge with either nicotine or SIB1765F but not AR-R17779, suggesting that the α4β2 subtype plays a role in both the initiation and expression of sensitization to the psychomotor stimulant effects of nicotine [107
]. Moreover, administration of AR-R17779 improved learning in two radial-arm maze tasks and reversed working memory impairment caused by fimbriafornix section [108
]. These findings suggest that α7 nAChRs play a role in learning and memory, and that agonists of α7 nAChRs might have therapeutic potential for cognitive impairments in neuropsychiatric diseases, including schizophrenia.
Chemical structures of AR-R17779, SIB1765F, and AZD0328.
Very recently, researchers at AstraZeneca reported that a furopyridine, (2'R
]pyridine d-tartrate (AZD0328; Fig.
), was a novel selective partial agonist for an α7 nAChR [109
]. AZD0328 exhibited high affinity for the native rat (K
i=4.7 nM) and recombinant human (K
i=3.0 nM) α7 nAChR, respectively. ADZ0328 also exhibited high affinity for both native rat (K
i=25 nM) and recombinant human (K
i=12 nM) 5-HT3
receptors. In contrast, AZD0328 exhibited only moderate affinity for the α4β2 nAChR of the native rat (K
i=140 nM) and very low affinity for the “ganglionic” α3 subunit-containing (K
i=2,500 nM) and muscle α1β1γδ nAChR (K
i=20,000) of the mouse. Functional assay revealed the partial agonistic nature of AZD0328. The half maximal (50%) effective concentration (EC50
) and intrinsic activity of AZD0328 were 150 nM and 61% for rat α7 nAChR and 338 nM and 65% for human α7 nAChR, respectively. Administration of a low dose (0.00138 mg/kg) of AZD0328 in vivo
leads to a significant increase in the excitability of midbrain dopaminergic neurons with no independent change in the spike patterns produced by these neurons. Within the same dose range, AZD0328 led to a significant increase in cortical dopamine release and improved both conditioned response learning and memory retention in an object recognition task. These effects were blocked by the selective α7 nAChR antagonist MLA (Fig.
). AZD0328 improved novel object recognition in mice over a broad range of doses (0.00178–1.78 mg/kg) and the compound effect was found to be absent in homozygous α7 nAChR-knockout mice. Together, these data indicate that selective interaction with α7 nAChRs by AZD0328 selectively enhances midbrain dopaminergic neuronal activity, causing an enhancement of cortical dopamine levels; these neurochemical changes, in turn, likely underlie the positive behavioral responses observed in the two different animal models. These results also suggest that selective α7 nAChR agonists may have significant therapeutic utility in neurologic and psychiatric disorders in which cognitive effects and dopamine neuron dysfunction co-exist.
Macor and Wu [110
] reported some derivatives of 1-azabicyclo[2.2.2]oct-3-yl phenylcarbamate as agonists of α7 nAChRs (Fig.
). In order to develop novel agonists of α7 nAChRs, researchers at Mitsubishi Pharma Corporation (currently Mitsubishi Tanabe Pharma Corporation) hypothesized that 1-azabicyclo[2.2.2]octane derivatives, bearing an aromatic part and a spacer group at the 3-position, may exhibit agonist activity at α7 nAChRs [111
]. By examining a series of 3-substituted 1-azabicyclo[2.2.2]octane derivatives, they found that (+)-3-[2-(benzo[b
]thiophen-2-yl)-2-oxoethyl]-1- azabicycle[2.2.2]octane was a potent and partial agonist of α7 nAChRs (Fig.
]. Furthermore, they reported the structure-activity relationships and pharmacokinetic profiles of the series of compounds leading to the discovery of (R
]. This compound has potent binding affinity (K
i=9 nM for α7 nAChRs) and good selectivity toward the other nicotinic subtypes (α4β2 and α1β2γδ). Also, this compound has good oral bioavailability and brain permeability. Interestingly, this compound (10 mg/kg, p.o.) significantly improved dizocilpine (3 mg/kg)-induced auditory gating deficits in rats, suggesting that this compound has the potential to improve sensory gating deficits in schizophrenic patients [112
]. Moreover, they have developed a novel partial α7 nAChR agonist, W-56203, (R
]. W-56203 bound to α7 nAChRs with a K
i value of 3 nM. No significant binding of W-56203 was detected at α4β2 nAChRs or muscarinic receptors. Furthermore, W-56203 showed no binding to other known receptors (dopamine D1
, adrenergic α1, α2, histamine H1
) or ion channels (NMDA and α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)), although W-56203 exhibited a moderate affinity to 5-HT3
receptors. In cultured hippocampal neurons, W-56203 evoked a rapidly desensitizing inward current, which was blocked by the selective α7 nAChR antagonist MLA (1 nM) (Fig.
). Interestingly, W-56203 has been shown to significantly improve dizocilpine-induced auditory gating deficits in rats [113
]. These results suggest that W-56203 is an orally active and partial agonist of α7 nAChRs, and that W-56203 would be a potential drug for the treatment of schizophrenia.
Fig. (5) Chemical structures of 1-Azabicyclo[2.2.2]oct-3-yl phenylcarbamate derivatives, (+)-3-[2-(benzo[b]thiophen-2-yl)-2-oxoethyl]-1-
azabicycle[2.2.2]octane, (R)-3’(5-chlorothiophen-2-yl)spiro-1-azabicyclo[2,2,2]octane-3,5’-[1’,3’]oxazolidin-2’-one, (more ...)
Researchers at Targacept Inc. reported that 2-(3-pyridyl)-1-azabicyclo[3.2.2]nonane (TC-1698; Fig.
) was a highly selective agonist of α7 nAChRs [114
]. TC-1698 exhibited a K
i of 11 nM in the binding assay of [3
H]MLA to rat hippocampal membranes, whereas TC-1698 (10 μM) had no or very low affinity for other receptors. TC-1698 exerts neuroprotective effects via
activation of the JAK2/PI3K cascade, which can be neutralized through activation of the angiotensin II receptors [114
]. These findings suggest that JAK2 plays a central role in the α7 nAChR activation of the JAK2-PI3K cascade in PC12 cells, which ultimately contribute to α7 nAChR-mediated neuroprotection.
Chemical structures of TC-1698 and TC-5619.
They further synthesized a series of 2-(arylmethyl)-3-substituted quinuclidines as α7 nAChR agonists based on a putative pharmacophore model [115
]. Quinuclidine is a well established pharmacophoric element. Its basic nitrogen, occupying a bridgehead position within an azabicyclic system, allocates the maximal electrostatic interaction combined with minimal steric demand. To further define pharmacophoric elements, they conducted virtual and synthetic diversification around a combination of 2-position and 3-position substitutions of the quinuclidine scaffold. This effort revealed the presence of three pharmacophoric elements: the cationic site (quinuclidine nitrogen), a hydrogen bond acceptor at the 3-position, and another hydrogen bond acceptor at the 2-position (the pyridine ring). In position 2 of quinuclidine, heteroarylmethyl groups are favored over benzyl, and steric constraints restrict substitution on the heteroaryl (pyridine) ring. The pharmacophoric element in position 3 was most amenable to diversification, resulting in the generation of four classes of ligands: carbamates, ureas, amides and sulfonamide. Cis
-configuration is favorable for binding to the receptor. Within the 3-position substituent, a comparison of aliphatic and aromatic groups suggested that π-interaction may play a significant role in increasing affinity. As a result, several synthesized compounds exhibited high affinity to the α7 nAChR with K
i values near or below 1 nM. None of these compounds bound to α4β2 receptors with any significant affinity (K
i>10 μM). Among these series of compounds, N
)-2-[(pyridin-3-yl)methyl]quinuclidin-3-yl}benzofuran-2-carboxamide (TC-5619; Fig.
) has potent full agonistic activity for the α7 nAChR (EC50
=33 nM, agonist activity=100), at a concentration below those that result in desensitization. Hauser et al.
] further characterized the pharmacological properties of TC-5619. TC-5619 binds with very high-affinity to the α7 nAChR of the native rat hippocampus (K
i=1 nM) and the α7 nAChR of human HEK α7/RIC3 cells (K
i=1 nM), respectively. TC-5619 has little or no activity at other nicotinic receptors, including the α4β2 (K
i=2,800 and 2,100 nM for rat cortex and human SH-EP1 cells, respectively), ganglionic (α3β4) and muscle subtypes. In a transgenic th(tk–)/th(tk–)
mouse model that reflects many of the developmental, anatomical, and multi-transmitter biochemical aspects of schizophrenia, TC-5619 acted both alone and synergistically with the antipsychotic clozapine to correct impaired pre-pulse inhibition (PPI) and social behaviors which model positive and negative symptoms, respectively. Similar to the results in the transgenic mice, TC-5619 significantly reversed the apomorphine-induced PPI deficits in rats. In a novel object-recognition paradigm in rats, TC-5619 demonstrated long-lasting enhancement of memory over a wide dose range. These findings suggest that TC-5619, either alone or in combination with antipsychotics, could offer a new approach to treating the constellation of symptoms associated with schizophrenia, including cognitive dysfunction. A phase 1 single rising dose (10–900 mg) clinical trial of TC-5619 in 66 healthy volunteers showed that TC-5619 was generally well tolerated up to doses of 600 mg*
. A multiple dose (10–300 mg) administered for 10 days in 38 young healthy volunteers achieved significant improvements in the clinical dementia rating (CDR) test. A phase 2 proof-of-concept study will start at 4Q 2009 in patients with AD and attention deficit hyperactivity disorder (ADHD).
Researchers at Pfizer reported on the selective α7 nAChR agonist PNU-282987, N
)-1-azabicyclo[2.2.2]-oct-3-yl]-4-chlorobenzamide hydrochloride (Fig.
]. PNU-282987 binds to α7 nAChR with a K
i of 27 nM, and showed evoked whole-cell currents from cultured rat hippocampal neurons that were sensitive to the selective α7 nAChR antagonist MLA [117
]. Systemic administration of PNU-282987 (1 mg/kg, i.v.) significantly improved d-amphetamine-induced sensory gating deficits in chloral hydrate-anesthetized rats. These findings suggest that PNU-282987 may be useful for treating the cognitive and attentional deficits of schizophrenia [117
]. This compound was later found, however, to possess significant human ether
(hERG) potassium channel activity and thus did not meet Pfizer’s criteria for further development. Additional work on this template demonstrated that fused 6,5-heterocyclic analogues, such as indole (Fig.
), provided an avenue toward novel analogues with potential for improved safety profiles. Of the 6,5-fused heterocycles evaluated, N
]pyridine-5-carboxamide (PHA-543613; Fig.
) proved to be a potent, high-affinity agonist of the α7 nAChR (K
i=8.8 nM: α7-5-HT3
=65 nM) [119
]. PHA-543613 possesses antagonist activity at the 5-HT3
i=628 nM). PHA-543613 showed no detectable agonist activity (>100 μM) and negligible antagonist activity at both muscle-like nAChRs and ganglion-like nAChRs. Further, this furopyridine derivative did not significantly displace tritiated cytisine from rat brain homogenate at 1 μM, suggesting a selectivity over the α4β2 nAChR. The excellent in vitro
profile of PHA-543613 is matched by rapid brain penetration, high oral bioavailability in rats, and a favorable hERG profile. Furthermore, PHA-543613 demonstrated efficacy in two in vivo
models, the reversal of an amphetamine-induced P50 gating deficit (0.3 mg/kg, i.v.), and improved performance in a novel object-recognition test (1.0 mg/kg, s.c.).
Chemical structures of PNU-282987, N-[(R)-quinuclidin-3-yl]-1H-indole-6-carboxamide, PHA-543613, and PHA-568487.
While further evaluation of PHA-543613 was underway, researchers at Pfizer identified additional novel, potent (<50 nM), and selective α7 nAChR agonists that, like PHA-543613, possess reduced hERG activity and low first pass metabolism [120
]. To expand the SAR on the quinuclidine amide template and gain a broader understanding of the overlap with other nicotinic pharmacophores, two parallel approaches were utilized. The first focused on the acid portion of the quinuclidine template (amine part). The second approach investigated modifications to the azabicyclic ring system (acid part). The best compounds from each series are characterized by rapid brain penetration, good oral bioavailability in rats, and in vivo
efficacy in a rat P50 auditory sensory gating assay. Finally, the researchhers discovered 2,3-dihydro-N
]-[1,4]dioxine-6-carboxamide (PHA-568487; Fig.
), which showed an improved hERG safety profile over PHA-543613. However, these two phase, I clinical candidates (PHA-543613 and PHA-568487) were discontinued due to cardiovascular findings.†
Very recently, researchers at Pfizer disclosed the synthesis of (3R
)-1-azabicyclo[3.2.1]octane-3-amine dihydrochloride (Fig.
) and demonstrated that the activity of the corresponding para-chlorobenzamide (Fig.
) in a α7-5-HT3
chimera assay was equal to that of the potent α7 nAChR agonist, PNU-282987. To identify novel, potent and orally bioavailable α7 nAChR agonists with in vitro
and in vivo
potential equal or better than that of PHA-543613, they proceeded to synthesis of an expanded set of amides derived from amine [121
]. Among them, the furopyridine derivative N
]pyridine-5-carboxamide (PHA-709829; Fig.
), which is a structurally similar analog of PHA-543613, showed very similar activity in the α7-5-HT3
chimera functional assay (EC50
=81 nM) and similar affinity in the α7 binding assay (K
i=2.9 and 3.9). PHA-709829 showed stability in rat liver microsomes comparable to that of PHA-543613. PHA-709829 showed excellent CNS penetration in a brain delivery assessment screen (MDCK cell permeability, mouse brain/plasma ratio, Pgp substrate), displaying a profile very similar to PHA-543613.
Chemical structures of (3R,5R)-1-aza-bicyclo[3.2.1]octan-3-amine dihydrochloride, para-chlorobenzamide derivatives of (3R,5R)-1-aza-bicyclo[3.2.1]octan-3-amine, PHA-709829, compound 7aa, and CP-801,123.
Researchers at Pfizer also identified 1,4-diazabicyclo[3.2.2]nonane phenyl carbamate (compound 7aa; Fig. ) as subtype selective, high affinity α7 agonist (Ki=23 nM, agonist activity =175%). The pharmacological profiles of this compound closely resembled that of 4-bromophenyl 1,4-diazabicyclo[3.2.2]nonane-4-carboxylate hydrochloride (SSR180711; Fig. ). The advantage of compound 7aa was that it showed the potential for an improved cardiosafety profile due to its low affinity for hERG.
Chemical structure of SSR180711.
Very recently, they reported the benzoxyazole derivative 2-(5-methyloxazolo[4,5-b
]pyridin-2-yl)-2,5-diaza-bicyclo[3. 2.2]nonane (CP-810,123; Fig.
) as a novel α7 nAChR agonist.‡
This compound showed low affinity for hERG (IC50
=40,000 nM) and high affinity binding to α7 nAChR. CP-810,123 has an excellent brain penetration and slow clearance from plasma and has shown marked ex vivo
binding to cerebral binding sites (ED50
=0.34 mg/kg). CP-810,123 also showed favorable preclinical findings in a number of animal models of cognition and was therefore advanced into clinical trials. However, the multiple-dose, 14-day clinical trial of CP-810,123 did not achieve any improvement of cognition at any of the doses tested, and was discontinued due to the occurrence of non-sustained ventricular tachycardia.
Researchers at Sanofi-Aventis demonstrated a novel α7 nAChR agonist, SSR180711, 4-bromophenyl 1,4-diazabicyclo[3.2.2]nonane-4-carboxylate hydrochloride (Fig.
]. Binding assays show that SSR180711 has a high and selective affinity for the human (K
i=14 nM) and rat (K
i=22 nM) α7 nAChRs. This ligand inhibits, in a dose-dependent manner, the ex vivo
H]α-bungarotoxin binding in mouse cortical homogenates after both i.p. and p.o. administration. At recombinant human α7 nAChRs, SSR180711 displays a partial agonist profile. Furthermore, it has been reported that SSR180711 improves cognitive deficits in a variety of rat models related to schizophrenia [123
]. This drug restores the selective attention deficit induced by phencyclidine (PCP) administration at the neonatal stage. This action is reversed by the α7 nAChR antagonist MLA. This drug also restores a short-term episodic memory impairment and a spatial working memory deficit induced by PCP or dizocilpine.
Hashimoto et al.
] reported that repeated administration of PCP significantly decreased the density of α7 nAChRs in the mouse brain. In addition, Hashimoto et al
] reported that SSR180711 could ameliorate cognitive deficits in mice after repeated administration of PCP, and that these effects could be blocked by the co-administration of MLA. Furthermore, Thomsen et al.
] reported that repeated co-administration of SSR180711 (3 mg/kg) with PCP prevented both changes of parvalbumin and synaptophysin, which correspond to changes seen in patients with schizophrenia, and changes in the level of Arc, a molecule involved in synaptic plasticity, mRNA expression in the prefrontal cortex, and the behavioral impairment induced by PCP. Moreover, Barak et al.
] reported that SSR180711 potentiated latent inhibition (LI) in normal rats and reversed amphetamine-induced LI disruption, two models considered predictive of activity against positive symptoms of schizophrenia [127
]. It was also reported that increase of expression of immediate early genes in the frontal cortex and nucleus accumbens has been demonstrated after administration with SSR180711 [129
]. Søderman et al.
] reported that acute systemic administration of SSR180711 (10 mg/kg) resulted in a significant increase in Fos protein levels in the shell of the nucleus accumbens in wild-type mice, but had no effect in the Aβ1-42
overexpressing transgenic mice. This result suggests that overexpression of human Aβ peptides inhibits α7 nAChR-dependent neurotransmission in vivo
, perhaps via
direct interaction with α7 nAChR, and underscores that clinical trials testing α7 nAChR agonists should be related to the content of Aβ peptides in the patient’s nervous system.
Taken together, these findings suggest that SSR180711 has the potential to improve cognitive deficits associated with schizophrenia and AD.