Allosteric modulation of the muscarinic M4 receptor as an approach to treating schizophrenia.

Current antipsychotics provide symptomatic relief for patients suffering from schizophrenia and related psychoses; however, their effectiveness is variable and many patients discontinue treatment due to side effects. Although the etiology of schizophrenia is still unclear, a leading hypothesis implicates an imbalanced dopaminergic system. Muscarinic acetylcholine (ACh) receptors regulate dopamine levels in key areas of the brain involved in psychosis, with the M(4) subtype emerging as a key regulator of dopaminergic hyperactivity. Unfortunately, no selective small molecule tools exist to provide pharmacological validation of this hypothesis. Here, we describe the discovery of a small molecule modulator, LY2033298, that is highly selective for human M(4) receptors by virtue of targeting an allosteric site on this receptor. Pharmacological assays confirmed the selectivity of LY2033298 for the M(4) receptor and revealed the highest degree of positive allosteric enhancement of ACh potency thus far identified. Radioligand binding assays also show this compound to directly potentiate agonist binding while having minimal effects on antagonist binding. Mutational analysis identified a key amino acid (D(432)) in the third extracellular loop of the human M(4) receptor to be critical for selectivity and agonist potentiation by LY2033298. Importantly, LY2033298 was active in animal models predictive of clinical antipsychotic drug efficacy indicating its potential use as a first-in-class, selective, allosteric muscarinic antipsychotic agent.

Pharmacologic effects in man of a specific serotonin-reuptake inhibitor.

Fluoxetine (Li-ly 110140) caused a 63 percent inhibition of [3H]serotonin uptake into platelets obtained from normal volunteers to whom the drug was administered daily for 7 days. This dose had no effect on the usual pressor response produced by injections of norepinephrine or tyramine.

Olanzapine treatment of psychotic and behavioral symptoms in patients with Alzheimer disease in nursing care facilities: a double-blind, randomized, placebo-controlled trial. The HGEU Study Group.

BACKGROUND: Patients with Alzheimer disease (AD) commonly exhibit psychosis and behavioral disturbances that impair patient functioning, create caregiver distress, and lead to institutionalization. This study was conducted to assess the efficacy and safety of olanzapine in treating psychosis and/or agitation/aggression in patients with AD. METHODS: A multicenter, double-blind, placebo-controlled, 6-week study was conducted in 206 elderly US nursing home residents with AD who exhibited psychotic and/or behavioral symptoms. Patients were randomly assigned to placebo or a fixed dose of 5, 10, or 15 mg/d of olanzapine. The primary efficacy measure was the sum of the Agitation/Aggression, Hallucinations, and Delusions items (Core Total) of the Neuropsychiatric Inventory-Nursing Home version. RESULTS: Low-dose olanzapine (5 and 10 mg/d) produced significant improvement compared with placebo on the Core Total (-7.6 vs -3.7 [P<.001] and -6.1 vs -3. 7 [P =.006], respectively). Core Total improvement with olanzapine, 15 mg/d, was not significantly greater than placebo. The Occupational Disruptiveness score, reflecting the impact of patients' psychosis and behavioral disturbances on the caregiver, was significantly reduced in the 5-mg/d olanzapine group compared with placebo (-2.7 vs -1.5; P =.008). Somnolence was significantly more common among patients receiving olanzapine (25.0%-35.8%), and gait disturbance occurred in those receiving 5 or 15 mg/d (19.6% and 17.0%, respectively). No significant cognitive impairment, increase in extrapyramidal symptoms, or central anticholinergic effects were found at any olanzapine dose relative to placebo. CONCLUSION: Low-dose olanzapine (5 and 10 mg/d) was significantly superior to placebo and well tolerated in treating agitation/aggression and psychosis in this population of patients with AD.

Comparison of second-messenger responses to muscarinic receptor stimulation in M1-transfected A9 L cells.

A9 L cells stable transfected with m1 muscarinic receptors were stimulated with the full agonist carbachol and with the partial agonist pilocarpine. The EC50 values and maximal activation from PI hydrolysis, calcium mobilization, membrane hyperpolarization, cell proliferation and arachidonic acid release were compared. Pilocarpine was approximately half as effective in eliciting PI hydrolysis, calcium mobilization and arachidonic acid release, but was almost as effective as carbachol in the other assays. These findings suggest that the intracellular signals leading to receptor-mediated cell proliferation and to membrane hyperpolarization are amplified, and that therefore these assays are not suitable for determining whether a compound is a partial agonist.

Arachidonic acid release in cell lines transfected with muscarinic receptors: a simple functional assay to determine response of agonists.

Muscarinic agonists stimulated arachidonic acid release from 10- to 32-fold in Chinese hamster ovary (CHO) cells transfected with muscarinic M1, M3 and M5 receptor subtypes. Muscarinic agonists liberated arachidonic acid from the cAMP-coupled M2 and M4 cells only in the presence of ATP. Partial agonists were less efficacious at liberating arachidonic acid than full agonists. The ability of muscarinic agonists to liberate arachidonic acid and stimulate phosphoinositide hydrolysis in the same CHO M1, M3 and M5 cells was well correlated; however, partial agonists were more efficacious at stimulating phosphoinositide hydrolysis than arachidonic acid release. The efficacy and potency of 13 muscarinic agonists to liberate arachidonic acid was characterised. Influx of external calcium was required for arachidonic acid release even after initiation of agonist-induced release. It is concluded that arachidonic acid release is a simple assay suitable for evaluation of muscarinic agonists, antagonists and the flux of external calcium into cells.

Muscarinic receptors in schizophrenia.

An increasing body of evidence suggests that the muscarinic receptors may present a potential therapeutic target for the treatment of schizophrenia. This argument is supported by studies using postmortem CNS tissue and a neuroimaging study that have shown there are regionally specific decreases in selective muscarinic receptors in the CNS of subjects with schizophrenia. This raises the possibility that drugs specific to individual muscarinic receptors could have beneficial effects on the symptoms of schizophrenia, a posit supported by studies in receptor knockout/knockdown mice where it has been shown that specific behaviours affected by schizophrenia are also abnormal in mice lacking a single muscarinic receptor. Moreover, drugs have been synthesised that are partial agonists at muscarinic receptors and these drugs have been shown to improve the behavioural deficits in humans which are modulated by the muscarinic receptor family. The widespread distribution of muscarinic receptors in the human CNS and the receptor specific changes identified in postmortem CNS from subjects with schizophrenia would suggest that drugs targeting specific muscarinic receptors would also need to partition into selected CNS regions to achieve optimal responses. Some existing compounds show regional selectivity for the same muscarinic receptor in different CNS regions, suggesting that this characteristic could be engineered into muscarinic receptor targeting drugs. This review presents data from diverse areas of research to argue that it is now imperative that the therapeutic potential of manipulating the activity of muscarinic receptors for the treatment of schizophrenia is fully explored.

The muscarinic agonist xanomeline increases monoamine release and immediate early gene expression in the rat prefrontal cortex.

BACKGROUND: The muscarinic agonist xanomeline has been shown to reduce antipsychotic-like behaviors in patients with Alzheimer's disease. Because atypical antipsychotic agents increase dopamine release in prefrontal cortex and induce immediate early gene expression in prefrontal cortex and nucleus accumbens, the effect of xanomeline was determined on these indices. METHODS: The effect of xanomeline on extracellular levels of monoamines in brain regions was determined using a microdialysis technique, and changes in expression of the immediate early genes c-fos and zif/268 in brain regions were evaluated using in situ hybridization histochemistry. RESULTS: Xanomeline increased extracellular levels of dopamine in prefrontal cortex and nucleus accumbens but not in striatum. Xanomeline increased expression of c-fos and zif/268 in prefrontal cortex and nucleus accumbens. There was no change in immediate early gene expression in striatum. CONCLUSIONS: Xanomeline increased extracellular levels of dopamine, which is similar to the effects of the atypical antipsychotics clozapine and olanzapine. The regional pattern of immediate early gene expression induced by xanomeline resembled that of atypical antipsychotic agents. Based on the antipsychotic-like activity of xanomeline in Alzheimer's patients and the similarity to atypical antipsychotic agents, we suggest that xanomeline may be a novel antipsychotic agent.

A novel augmentation strategy for treating resistant major depression.

OBJECTIVE: Treatment-resistant depression is a significant public health concern; drug switching or augmentation often produce limited results. The authors hypothesized that fluoxetine could be augmented with olanzapine to successfully treat resistant depression. METHOD: An 8-week double-blind study was conducted with 28 patients who were diagnosed with recurrent, nonbipolar, treatment-resistant depression without psychotic features. Subjects were randomly assigned to one of three groups: olanzapine plus placebo, fluoxetine plus placebo, or olanzapine plus fluoxetine. RESULTS: Fluoxetine monotherapy produced minimal improvement on various scales that rate severity of depression. The benefits of olanzapine monotherapy were modest. Olanzapine plus fluoxetine produced significantly greater improvement than either monotherapy on one measure and significantly greater improvement than olanzapine monotherapy on the other measures after 1 week. There were no significant differences between treatment groups on extrapyramidal measures nor significant adverse drug interactions. CONCLUSIONS: Olanzapine plus fluoxetine demonstrated superior efficacy for treating resistant depression compared to either agent alone.

Effects of xanomeline, a selective muscarinic receptor agonist, on cognitive function and behavioral symptoms in Alzheimer disease.

OBJECTIVE: To evaluate the therapeutic effects of selective cholinergic replacement with xanomeline tartrate, an m1 and m4 selective muscarinic receptor (mAChR) agonist in patients with probable Alzheimer disease (AD). DESIGN: A 6-month, randomized, double-blind, placebo-controlled, parallel-group trial followed by a 1-month, single-blind, placebo washout. SETTING: Outpatients at 17 centers in the United States and Canada. PARTICIPANTS: A total of 343 men and women at least 60 years of age with mild to moderate AD. INTERVENTIONS: Patients received 75, 150, or 225 mg (low, medium, and high doses) of xanomeline per day or placebo for 6 months. OUTCOME MEASURES: Scores on the cognitive subscale of the Alzheimer's Disease Assessment Scale (ADAS-Cog), the Clinician's Interview-Based Impression of Change (CIBIC+), the Alzheimer's Disease Symptomatology Scale (ADSS), and the Nurses' Observational Scale for Geriatric Patients (NOSGER). RESULTS: A significant treatment effect existed for ADAS-Cog (high dose vs placebo; P < or = .05), and CIBIC+ (high dose vs placebo; P < or = .02). Treatment Emergent Signs and Symptoms analysis of the ADSS, which assesses behavioral symptoms in patients with AD, disclosed significant (P < or = .002) dose-dependent reductions in vocal outbursts, suspiciousness, delusions, agitation, and hallucinations. On end-point analysis, NOSGER, which assesses memory, instrumental activities of daily living, self-care, mood, social behavior, and disturbing behavior in the elderly, also showed a significant dose-response relationship (P < or = .02). In the high-dose arm, 52% of patients discontinued treatment because of adverse events; dose-dependent adverse events were predominantly gastrointestinal in nature. Syncope, defined as loss of consciousness and muscle tone, occurred in 12.6% of patients in the high-dose group. CONCLUSIONS: The observed improvements in ADAS-Cog and CIBIC+ following treatment with xanomeline provide the first evidence, from a large-scale, placebo-controlled clinical trial, that a direct-acting muscarinic receptor agonist can improve cognitive function in patients with AD. Furthermore, the dramatic and favorable effects on disturbing behaviors in AD suggest a novel approach for treatment of noncognitive symptoms.

Comparative affinity of duloxetine and venlafaxine for serotonin and norepinephrine transporters in vitro and in vivo, human serotonin receptor subtypes, and other neuronal receptors.

The blockade of serotonin (5-HT) and norepinephrine (NE) transporters in vitro and in vivo by the dual 5-HT/NE reuptake inhibitors duloxetine and venlafaxine was compared. Duloxetine inhibited binding to the human NE and 5-HT transporters with K(i) values of 7.5 and 0.8 nM, respectively, and with a K(i) ratio of 9. Venlafaxine inhibited binding to the human NE and 5-HT transporters with K(i) values of 2480 and 82 nM, respectively, and with a K(i) ratio of 30. Duloxetine inhibited ex vivo binding to rat 5-HT transporters and NE transporters with ED(50) values of 0.03 and 0.7 mg/kg, respectively, whereas venlafaxine had ED(50) values of 2 and 54 mg/kg, respectively. The depletion of rat brain 5-HT by p-chloramphetamine and depletion of rat hypothalamic NE by 6-hydroxydopamine was blocked by duloxetine with ED(50) values of 2.3 and 12 mg/kg, respectively. Venlafaxine had ED(50) values of 5.9 and 94 mg/kg for blocking p-chloramphetamine- and 6-hydroxydopamine-induced monoamine depletion, respectively. Thus, duloxetine more potently blocks 5-HT and NE transporters in vitro and in vivo than venlafaxine.

Radioreceptor binding profile of the atypical antipsychotic olanzapine.

The affinities of olanzapine, clozapine, haloperidol, and four potential antipsychotics were compared on binding to the neuronal receptors of a number of neurotransmitters. In both rat tissues and cell lines transfected with human receptors olanzapine had high affinity for dopamine D1, D2, D4, serotonin (5HT)2A, 5HT2C, 5HT3, alpha 1-adrenergic, histamine H1, and five muscarinic receptor subtypes. Olanzapine had lower affinity for alpha 2-adrenergic receptors and relatively low affinity for 5HT1 subtypes, GABAA, beta-adrenergic receptors, and benzodiazepine binding sites. The receptor binding affinities for olanzapine was quite similar in tissues from rat and human brain. The binding profile of olanzapine was comparable to the atypical antipsychotic clozapine, while the binding profiles for haloperidol, resperidone, remoxipride, Org 5222, and seroquel were substantially different from that of clozapine. The receptor binding profile of olanzapine is consistent with the antidopaminergic, antiserotonergic, and antimuscarinic activity observed in animal models and predicts atypical antipsychotic activity in man.

Synergistic effects of olanzapine and other antipsychotic agents in combination with fluoxetine on norepinephrine and dopamine release in rat prefrontal cortex.

To understand the mechanism of the clinical efficacy of olanzapine and fluoxetine combination therapy for treatment-resistant depression (TRD), we studied the effects of olanzapine and other antipsychotics in combination with the selective serotonin uptake inhibitors fluoxetine or sertraline on neurotransmitter release in rat prefrontal cortex (PFC) using microdialysis. The combination of olanzapine and fluoxetine produced robust, sustained increases of extracellular levels of dopamine ([DA](ex)) and norepinephrine ([NE](ex)) up to 361 +/- 28% and 272 +/- 16% of the baseline, respectively, which were significantly greater than either drug alone. This combination produced a slightly smaller increase of serotonin ([5-HT](ex)) than fluoxetine alone. The combination of clozapine or risperidone with fluoxetine produced less robust and persistent increases of [DA](ex) and [NE](ex). The combination of haloperidol or MDL 100907 with fluoxetine did not increase the monoamines more than fluoxetine alone. Olanzapine plus sertraline combination increased only [DA](ex). Therefore, the large, sustained increase of [DA](ex), [NE](ex), and [5-HT](ex) in PFC after olanzapine-fluoxetine treatment was unique and may contribute to the profound antidepressive effect of the olanzapine and fluoxetine therapy in TRD.

LY248686, a new inhibitor of serotonin and norepinephrine uptake.

LY248686 is an inhibitor of serotonin (5-hydroxytryptamine; 5-HT) and norepinephrine (NE) uptake in synaptosomal preparations of hypothalamus and cerebral cortex, and 5-HT uptake in human blood platelets, with inhibitor constants near nanomolar concentrations. Upon administration to rats 1 hour before sacrifice, LY248686 caused dose-dependent and parallel decreases of 5-HT and NE uptake in hypothalamus homogenates ex vivo. LY248686 is a positive enantiomer and was slightly more potent than its negative isomer, LY248685, as an inhibitor of 5-HT uptake. Both isomers were only weak inhibitors of dopamine (DA) uptake in striatal synaptosomes. The inhibitory effects on 5-HT and NE uptake after a single administration of LY248686 followed similar time courses and simultaneously persisted for as long as 6 hours. LY248686 in vivo could effectively antagonize the p-chloroamphetamine-induced decreases of 5-HT uptake and levels of 5-HT and 5-hydroxyindoleacetic acid in cerebral cortex, and block the accumulation of 14C-NE in rat hearts. In food deprived rats, LY248686 suppressed food intake synergistically with 5-hydroxytryptophan, a precursor amino acid of 5-HT. Because of its lack of affinity for receptors of 5-HT, NE, DA, acetylcholine, histamine and naloxone, and its ability to inhibit 5-HT and NE uptake simultaneously, LY248686 has a favorable pharmacological profile as a potential antidepressant drug.

Norfluoxetine enantiomers as inhibitors of serotonin uptake in rat brain.

Like fluoxetine, the N-demethylated metabolite norfluoxetine exists in R- and S-enantiomeric forms. S-Norfluoxetine inhibited serotonin (5-HT) uptake and [3H]paroxetine binding to 5-HT uptake sites with a pKi of 7.86 and 8.88 or 14 and 1.3 nM, respectively, whereas R-norfluoxetine was 22 and 20 times, respectively, less potent. R- and S-Norfluoxetine were less potent than the corresponding enantiomers of fluoxetine as inhibitors of norepinephrine uptake and [3H]tomoxetine binding to norepinephrine uptake sites. Ex vivo studies showed that S-norfluoxetine inhibited 5-HT uptake with an ED50 of 3 mg/kg intraperitoneally, 4.7 mg/kg subcutaneously, and 9 mg/kg orally (7.3, 11.4 and 21.9 mumol/kg, respectively), while the ED50 for R-norfluoxetine exceeded 20 mg/kg intraperitoneally (48.6 mumol/kg). Inhibition of 5-HT uptake in cerebral cortex ex vivo and decrease in 5-HIAA levels in hypothalamus persisted for 24 hours after administration of S-norfluoxetine as demonstrated with the administration of fluoxetine. Thus, S-norfluoxetine is the active N-demethylated metabolite responsible for the persistently potent and selective inhibition of 5-HT uptake in vivo.

Novel functional M1 selective muscarinic agonists. 2. Synthesis and structure-activity relationships of 3-pyrazinyl-1,2,5,6-tetrahydro-1-methylpyridines. Construction of a molecular model for the M1 pharmacophore.

A series of 3-(3-substituted-pyrazinyl)-1,2,5,6-tetrahydro-1-methylpyridines were synthesized and found to have high affinity for central muscarinic receptors. The ability of some of these compounds to inhibit the electrically stimulated twitch of the guinea pig vas deferens indicated that the compounds were M1 agonists. M1 agonist activity was related to the length of the side chain attached to the pyrazine ring, with maximal activity being obtained with the hexyloxy side chain. The (hexyloxy)pyrazine 3f lacked M2 agonist activity as it failed to affect the guinea pig atria and was also relatively devoid of M3 agonist activity as determined by its lack of tremorogenic and sialogogic effects in mice. A comparison of the M1 agonist efficacy of these pyrazines and related 1,2,5-thiadiazoles and 1,2,5-oxadiazoles suggested that M1 efficacy was related to the magnitude of electrostatic potential located over the nitrogens of the respective heterocycles. The heteroatom directly attached to the 3 position of the pyrazine or 1,2,5-thiadiazole heterocycle markedly influenced the M1 efficacy of the compounds by determining the energetically favorably conformers for rotation about the bond connecting the tetrahydropyridyl ring and the heterocycle. A three-dimensional model for the M1-activating pharmacophore was proposed based on computational studies and the model of the muscarinic pharmacophore proposed by Schulman.

Novel functional M1 selective muscarinic agonists. Synthesis and structure-activity relationships of 3-(1,2,5-thiadiazolyl)-1,2,5,6-tetrahydro-1-methylpyridines .

A series of novel 3-(3-substituted-1,2,5-thiadiazol-4-yl)-1,2,5,6-tetrahydro- 1-methylpyridines (substituted-TZTP; 5a-l, 7a-h, 8, 9c-n, 11, 13j) were synthesized and tested for central muscarinic cholinergic receptor affinity by using [3H]-oxotremorine-M (Oxo-M) and [3H]-pirenzepine (Pz) as ligands. The potency and efficacy of the compounds for the pharmacological defined M1 and M2 muscarinic receptors were determined on isolated electrically stimulated rabbit vas deferens and on spontaneously beating isolated guinea pig atria, respectively. Selected compounds were also tested for M3 activity in the isolated guinea pig ileum. The C1-8 alkoxy-TZTP 5a-l analogues all displaced [3H]-Oxo-M and [3H]-Pz with low nanomolar affinity. Depicting chain length against Oxo-M binding and against Pz binding the unbranched C1-8 alkoxy-TZTP (5a-h) derivatives produced U-shaped curves with butoxy- (5d) and (pentyloxy)-TZTP (5e) as the optimum chain length, respectively. This U-shaped curve was also seen in the ability of the compounds 5a-h to inhibit the twitch height in the vas deferens preparation. The (pentyloxy)- (5e) and the (hexyloxy)-TZTP (5f) analogues produced an over 90% inhibition of the twitch height with IC50 values in the low picomolar range. In both the atria and in the ileum preparations 5f had low efficacy and potency. With the (alkylthio)-TZTP (7a-h) analogues the structure-activity relationship was similar to the one observed with the alkoxy (5a-h) analogues, but generally 7a-h had higher receptor affinity and was more potent than the corresponding 5a-h. However, the C3-8 alkyl-TZTP (9c,e,g,h) analogues had 10-100 times lower affinity for the central muscarinic receptors than the corresponding alkoxy and alkylthio derivatives, and their efficacy in the vas deferens preparation was too low to obtain IC50 values. The unsubstituted TZTP (11) compound was a potent but nonselective muscarinic agonist. The two 3-(3-butoxy/(hexyloxy)-1,2,5-oxadiazol-4-yl)-1,2,5,6-tetrahy dro-1- methylpyridines (butoxy/hexyloxy)-OZTP; 19a/b) were also synthesized and tested. Both 19a and 19b had much lower affinity for the central muscarinic receptors than 5d and 5f, and the efficacy of 19a,b was too low to give IC50 values in the vas deferens preparation. Therefore, the C5-6 (alkyloxy)/(alkylthio)-TZTP's represent a unique series of potent functional M1 selective muscarinic agonists.

Conformationally constrained analogues of the muscarinic agonist 3-(4-(methylthio)-1,2,5-thiadiazol-3-yl)-1,2,5,6-tetrahydro-1-methylpyr idine. Synthesis, receptor affinity, and antinociceptive activity.

Conformationally constrained analogues of the potent muscarinic agonist 3-(4-methylthio)-1,2,5-thiadiazol-3-yl)-1,2,5,6-tetrahydro-1-methy lpyridine (methylthio-TZTP, 17) were designed and synthesized with the aim of (a) improving the antinociceptive selectivity over salivation and tremor and (b) predicting the active conformation of 17 with respect to the dihedral angle C4-C3-C3'-N2'. Using MOPAC 6.0 tricyclic analogues (7, 15, 16) with C4-C3-C3'-N2' dihedral angles close to 180 degrees and a rotation hindered analogue (9) with a C4-C3-C3'-N2' dihedral angle close to 274 degrees were designed, as these conformations had previously been suggested as being the active conformations. The analogues were tested for central muscarinic receptor binding affinity, for their antinociceptive activity in the mouse grid shock test, and, in the same assay, for their ability to induce tremor and salivation. The data showed that the tricyclic analogues (7, 15, 16) were equipotent with 17 as analgesics, but with no improved side effect profiles. The rotation-hindered analogue 9 had neither muscarinic receptor binding affinity nor antinociceptive activity. These results suggest that the active conformation of 17 has a C3-C4-C3'-N2' dihedral angle close to 180 degrees.

1,2,5-Thiadiazole analogues of aceclidine as potent m1 muscarinic agonists.

The acetyl group of the muscarinic agonist aceclidine 4 was replaced by various 1,2,5-thiadiazoles to provide a new series of potent m1 muscarinic agonists 17 and 18. Optimal m1 muscarinic agonist potency was achieved when the 1,2,5-thiadiazole substituent was either a butyloxy, 17d, or butylthio, 18d, group. Although 1,2,5-oxadiazole 37 and pyrazine 39 are iso-pi-electronic with 1,2,5-thiadiazole 17d, both analogues were substantially less active than 17d. Compounds with high muscarinic affinity and/or m1 muscarinic agonist efficacy were also obtained when the 3-oxyquinuclidine moiety of 17d or 18c was replaced by ethanolamines, hydroxypyrrolidines, hydroxyazetidine, hydroxyisotropanes, or hydroxyazanorbornanes. The structure-activity data support the participation of the oxygen or sulfur atom in the substituent on the 1,2,5-thiadiazole in the activation of the m1 receptor. Several of these new 1,2,5-thiadiazoles have m1 agonist efficacy, potency, and selectivity comparable to those of xanomeline 2 in the muscarinic tests investigated.

Functionally selective M1 muscarinic agonists. 3. Side chains and azacycles contributing to functional muscarinic selectivity among pyrazinylazacycles.

In an attempt to improve upon the M1 agonist activity of the selective M1 agonist xanomeline and related compounds, the M1 muscarinic efficacies and potencies of 3- and 6-substituted pyrazinylazacycles were varied by changing both the 3- and 6-substituents as well as the azacycle. Significant improvements in efficacy and potency over the previously prepared [3-(hexyloxy)pyrazinyl]tetrahydropyridine 19 were obtained with the [3-(hexyloxy)pyrazinyl]-quinuclidine 5i. The M1 activity of 5i showed some enantioselectivity with (S)-5i being ca. 4-fold more potent than (R)-5i. Like 19 and xanomeline, 5i was a functionally selective M1 agonist that showed greater functional selectivity than widely studied pyrazinylquinuclidine 5n (L-689,660). The improved functional selectivity of 5i over 5n could be attributed to the additional binding interactions between the hexyloxy side chain of 5i and the M1 receptor that are not available to 5n. Although 5i may show M1 functional selectivity comparable to xanomeliine, 5i is a less efficacious and potent M1 agonist than xanomeline.

Muscarinic agonists with antipsychotic-like activity: structure-activity relationships of 1,2,5-thiadiazole analogues with functional dopamine antagonist activity.

Muscarinic agonists were tested in two models indicative of clinical antipsychotic activity: conditioned avoidance responding (CAR) in rats and inhibition of apomorphine-induced climbing in mice. The standard muscarinic agonists oxotremorine and pilocarpine were both active in these tests but showed little separation between efficacy and cholinergic side effects. Structure-activity relationships of the alkylthio-1,2,5-thiadiazole azacyclic type muscarinic partial agonists are shown, revealing the exo-6-(3-propyl/butylthio-1,2, 5-thiadiazol-4-yl)-1-azabicyclo[3.2.1]octane analogues (4a,b and 9a, b) to be the most potent antipsychotic agents with large separation between efficacy and cholinergic side effects. The lack of enantiomeric selectivity suggests the pharmacophoric elements are in the mirror plane of the compounds. A model explaining the potency differences of closely related compounds is offered. The data suggest that muscarinic agonists act as functional dopamine antagonists and that they could become a novel treatment of psychotic patients.