Design and synthesis of 4-heteroaryl 1,2,3,4-tetrahydroisoquinolines as triple reuptake inhibitors

A series of 4-bicyclic heteroaryl 1,2,3,4-tetrahydroisoquinoline inhibitors of the serotonin transporter (SERT), norepinephrine transporter (NET), and dopamine transporter (DAT) was discovered. The synthesis and structure-activity relationship (SAR) of these triple reuptake inhibitors (TRIs) will be discussed. Compound 10i (AMR-2), a very potent inhibitor of SERT, NET, and DAT, showed efficacy in the rat forced-swim and mouse tail suspension models with minimum effective doses of 0.3 and 1 mg/kg (po), respectively. At efficacious doses in these assays, 10i exhibited substantial occupancy levels at the three transporters in both rat and mouse brain. The study of the metabolism of 10i revealed the formation of a significant active metabolite, compound 13.

Discovery of a cyclopentylamine as an orally active dual NK1 receptor antagonist-serotonin reuptake transporter inhibitor

Cyclopentylamine 4 was identified as a potent dual NK1R antagonist-SERT inhibitor. This compound demonstrated significant oral activity in the gerbil forced swimming test, suggesting that dual NK1R antagonists-SERT inhibitors may be useful in treating depression disorders.

Design, optimization, and in vivo evaluation of a series of pyridine derivatives with dual NK1 antagonism and SERT inhibition for the treatment of depression

A series of substituted pyridines, ether linked to a phenylpiperidine core were optimized for dual NK(1)/SERT affinity. Optimization based on NK(1)/SERT binding affinities, and minimization of off-target ion channel activity lead to the discovery of compound 44. In vivo evaluation of 44 in the gerbil forced swim test (a depression model), and ex-vivo NK(1)/SERT receptor occupancy data support the potential of a dual acting compound for the treatment of depression.

Pharmacological and behavioral characterization of the novel CRF1 antagonist BMS-763534

BMS-763534 is a potent (CRF(1) IC(50) = 0.4 nM) and selective (>1000-fold selectivity vs. all other sites tested) CRF(1) receptor antagonist (pA2 = 9.47 vs. CRF(1)-mediated cAMP production in Y79 cells). BMS-763534 accelerated the dissociation of (125)I-o-CRF from rat frontal cortex membrane CRF(1) receptors consistent with a negative allosteric modulation of CRF binding. BMS-763534 produced dose-dependent increases in CRF(1) receptor occupancy and anxiolytic efficacy; lowest effective anxiolytic dose = 0.56 mg/kg, PO, which was associated with 71 ± 5% CRF(1) receptor occupancy of frontoparietal CRF(1) receptors. Sedative/ataxic effects of BMS-763534 were only observed at high dose multiples (54-179×) relative to the lowest dose required for anxiolytic efficacy. At doses of 5- to 18-fold higher than the lowest efficacious dose in the anxiety assay, BMS-763534 shared subjective effects with the benzodiazepine chlordiazepoxide. Interestingly BMS-790318, the O-demethylated metabolite of BMS-763534, showed weak affinity for the TBOB site of the GABA(A) receptor (67% inhibition at 10 μM) and augmented GABA evoked currents (EC(50) = 1.6 μM). Thus, the unanticipated signal in the drug discrimination assay may have resulted from an interaction of the metabolite BMS-790318 with the TBOB site on the GABA(A) channel where it appears to behave as an allosteric potentiator of GABA evoked currents.

Synthesis, structure-activity relationships, and in vivo evaluation of N3-phenylpyrazinones as novel corticotropin-releasing factor-1 (CRF1) receptor antagonists

Evidence suggests that corticotropin-releasing factor-1 (CRF(1)) receptor antagonists may offer therapeutic potential for the treatment of diseases associated with elevated levels of CRF such as anxiety and depression. A pyrazinone-based chemotype of CRF(1) receptor antagonists was discovered. Structure-activity relationship studies led to the identification of numerous potent analogues including 12p, a highly potent and selective CRF(1) receptor antagonist with an IC(50) value of 0.26 nM. The pharmacokinetic properties of 12p were assessed in rats and Cynomolgus monkeys. Compound 12p was efficacious in the defensive withdrawal test (an animal model of anxiety) in rats. The synthesis, structure-activity relationships and in vivo properties of compounds within the pyrazinone chemotype are described.

Synthesis and structure-activity relationships of 8-(pyrid-3-yl)pyrazolo[1,5-a]-1,3,5-triazines: potent, orally bioavailable corticotropin releasing factor receptor-1 (CRF1) antagonists

This report describes the syntheses and structure-activity relationships of 8-(substituted pyridyl)pyrazolo[1,5-a]-1,3,5-triazine corticotropin releasing factor receptor-1 (CRF(1)) receptor antagonists. These CRF(1) receptor antagonists may be potential anxiolytic or antidepressant drugs. This research resulted in the discovery of compound 13-15, which is a potent, selective CRF(1) antagonist (hCRF(1) IC(50) = 6.1 +/- 0.6 nM) with weak affinity for the CRF-binding protein and biogenic amine receptors. This compound also has a good pharmacokinetic profile in dogs. Analogue 13-15 is orally effective in two rat models of anxiety: the defensive withdrawal (situational anxiety) model and the elevated plus maze test. Analogue 13-15 has been advanced to clinical trials.

Comparison of zolpidem and midazolam self-administration under progressive-ratio schedules: consumer demand and labor supply analyses

Progressive ratio (PR) schedules of intravenous drug self-administration are useful for establishing the relationships between reinforcing effectiveness and pharmacological actions of abused drugs. The authors compared the reinforcing effects of 2 short-duration benzodiazepine-type drugs differing in their receptor selectivity: zolpidem (selective for gamma aminobutyric acid Type A [GABA(A)] receptors containing alpha1 subunits) and midazolam (nonselective). Reinforcing effectiveness was evaluated using a PR schedule of intravenous drug injection in rhesus monkeys in which the response requirement increased across the experimental session and the initial response requirement (IRR) was varied. Analyses based on consumer demand and labor supply models of behavioral economics revealed that the relative reinforcing effectiveness of zolpidem was greater than that of midazolam. For consumer demand analyses, the degree of difference between zolpidem and midazolam depended on whether price was calculated on the basis of different IRRs or different doses of drug. According to labor supply analysis, the reinforcing effects of midazolam were influenced by the economic concept referred to as a price effect to a greater degree than those of zolpidem. These findings suggest that a compound with selectivity for GABA(A) receptors containing alpha1 subunits has greater reinforcing effectiveness than a nonselective compound with similar pharmacokinetics, albeit under a limited range of conditions (high response costs). Differences in price effects may play a key role in determining the relative reinforcing effectiveness of selective versus nonselective benzodiazepine agonists.

Anti-conflict effects of benzodiazepines in rhesus monkeys: relationship with therapeutic doses in humans and role of GABAA receptors

RATIONALE AND OBJECTIVES

Conflict procedures are used to study mechanisms underlying the anxiolytic effects of benzodiazepines (BZs). We established a conflict procedure with rhesus monkeys in order to examine the role of GABAA receptors in the anxiolytic-like effects of BZs.

METHODS

Four rhesus monkeys responded under a two-component multiple schedule in which responding was maintained under a fixed-ratio schedule of food delivery in the absence (non-suppressed responding) and presence (suppressed responding) of response-contingent electric shock.

RESULTS

Conventional BZs (alprazolam, flunitrazepam, clonazepam, nitrazepam, lorazepam, bromazepam, diazepam, flurazepam, clorazepate, chlordiazepoxide) engendered increases in the average rates of suppressed responding at low to intermediate doses and decreased the average rates of non-suppressed responding at higher doses. Positive correlations were observed when the therapeutic potencies of BZs in humans were compared with potencies to increase the rates of suppressed responding (R2=0.83) or decrease the rates of non-suppressed responding (R2=0.60). The 5-HT1A agonist buspirone increased the rates of suppressed responding, although the effects were modest, whereas the opioid morphine lacked anti-conflict effects. The BZ antagonist flumazenil also modestly increased the rates of suppressed responding. A relatively low dose of flumazenil enhanced, while a high dose blocked, alprazolam's anti-conflict effects. Compounds selective for alpha1 subunit-containing GABAA receptors (zolpidem, zaleplon, CL218,872) engendered relatively weak increases in the rates of suppressed responding.

CONCLUSIONS

A rhesus monkey conflict procedure was established with predictive validity for therapeutic doses in people and provided evidence that anxiolytic-like effects of BZs can occur with relatively low intrinsic efficacy at GABAA receptors and are reduced by alpha1GABAA receptor selectivity.

The pharmacology of DMP696 and DMP904, non-peptidergic CRF1 receptor antagonists

CRF(1) antagonists DMP696 and DMP904 were designed as drug development candidates for the treatment of anxiety and depression. Both compounds display nanomolar affinity for human CRF(1) receptors, and exhibit >1000-fold selectivity for CRF(1) over CRF(2) receptors and over a broad panel of other proteins. DMP696 and DMP904 block CRF-stimulated adenylyl cyclase activity in cortical homogenates and cell-lines expressing CRF(1) receptors. Both compounds inhibit CRF-stimulated ACTH release from rat pituitary corticotropes. Binding and functional studies indicate that DMP696 and DMP904 behave as noncompetitive full antagonists. DMP696 and DMP904 exhibit anxiolytic-like efficacy in several rat anxiety models. In the defensive withdrawal test, both compounds reduce exit latency with lowest effective doses of 3 and 1 mg/kg, respectively. The anxiolytic-like effect is maintained over 14 days of repeated dosing. In the context of a novel environment used in this test, DMP696 and DMP904 reverse mild stress-induced increases in plasma CORT secretion but at doses 3-4-fold greater than those required for anxiolyticlike efficacy. DMP696 and DMP904 are ineffective in three depression models including the learned helplessness paradigm at doses up to 30 mg/kg. At lowest anxiolytic-like doses, DMP696 and DMP904 occupy >50% CRF(1) receptors in the brain. The in vivo IC(50) values (plasma concentrations required for occupying 50% CRF(1) receptors) estimated based upon free, but not total, plasma concentrations are an excellent correlation with the in vitro IC(50) values. Neither compound produces sedation, ataxia, chlordiazepoxide-like subjective effects or adverse effects on cognition at doses 10-fold higher than anxiolytic-like doses. Neither compound produces physiologically significant changes in cardiovascular, respiratory, gastrointestinal or renal functions at anxiolytic-like doses. DMP696 and DMP904 have favorable pharmacokinetic profiles with good oral bioavailabilities. The overall pharmacological properties suggest that both compounds may be effective anxiolytics with low behavioral side effect liabilities.

Effects of CRF1 receptor antagonists and benzodiazepines in the Morris water maze and delayed non-matching to position tests

RATIONALE

Benzodiazepines continue to be widely used for the treatment of anxiety, but it is well known that benzodiazepines have undesirable side effects, including sedation, ataxia, cognitive deficits and the risk of addiction and abuse. CRF(1) receptor antagonists are being developed as potential novel anxiolytics, but while CRF(1) receptor antagonists seem to have a better side-effect profile than benzodiazepines with respect to sedation and ataxia, the effects of CRF(1) receptor antagonists on cognitive function have not been well characterized. It is somewhat surprising that the potential cognitive effects of CRF(1) receptor antagonists have not been more fully characterized since there is some evidence to suggest that these compounds may impair cognitive function.

OBJECTIVE

The Morris water maze and the delayed non-matching to position test are sensitive tests of a range of cognitive functions, including spatial learning, attention and short-term memory, so the objective of the present experiments was to assess the effects of benzodiazepines and CRF(1) receptor antagonists in these tests.

RESULTS

The benzodiazepines chlordiazepoxide and alprazolam disrupted performance in the Morris water maze and delayed non-matching to position at doses close to their therapeutic, anxiolytic doses. In contrast, the CRF(1) receptor antagonists DMP-904 and DMP-696 produced little or no impairment in the Morris water maze or delayed non-matching to position test even at doses 10-fold higher than were necessary to produce anxiolytic effects.

CONCLUSIONS

The results of the present experiments suggest that, with respect to their effects on cognitive functions, CRF(1) receptor antagonists seem to have a wider therapeutic index than benzodiazepines.

Different GABAA receptor subtypes mediate the anxiolytic, abuse-related, and motor effects of benzodiazepine-like drugs in primates

Benzodiazepines exert their effects by binding to multiple subtypes of the GABAA receptor, the predominant subtypes in the brain being those that contain alpha1-, alpha2-, alpha3-, and alpha5-subunits. To understand the potentially different roles of these subtypes in the therapeutic and side effects of benzodiazepines, we evaluated GABAA receptor subtype-preferring compounds in nonhuman primate models predictive of anxiolytic, sedative, motor, subjective, and reinforcing effects of benzodiazepine-type drugs. These compounds included zolpidem, which shows preferential binding to GABAA receptors containing alpha1-subunits (alpha1GABAA receptors); L-838,417, which shows functional selectivity for alpha2GABAA, alpha3GABAA, and alpha5GABAA receptors; and nonselective conventional benzodiazepines. The results provide evidence in nonhuman primates that alpha1GABAA receptors do not play a key role in the anxiolytic and muscle-relaxant properties of benzodiazepine-type drugs; instead, these effects involve alpha2GABAA, alpha3GABAA, and/or alpha5GABAA subtypes. Our results also suggest that the alpha1GABAA receptor subtype might be critically involved in the subjective, sedative, and motor effects of benzodiazepine-type drugs. In contrast, stimulation of alpha1GABAA receptors is sufficient, but not necessary, for mediation of the abuse potential of these drugs.

Synthesis, Structure−Activity Relationships, and in Vivo Properties of 3,4-Dihydro-1H-pyrido[2,3-b]pyrazin-2-ones as Corticotropin-Releasing Factor-1 Receptor Antagonists

Corticotropin releasing factor (CRF) is the primary regulator of the hypothalamus-pituitary-adrenal (HPA) axis, coordinating the endocrine, behavioral, and autonomic responses to stress. It has been postulated that small molecules that can antagonize the binding of CRF1 to its receptor may serve as a treatment for anxiety-related and/or affective disorders. Members within a series of 3,4-dihydro-1H-pyrido[2,3-b]pyrazin-2-ones, exemplified by compound 2 (IC50 = 0.70 nM), were found to be very potent antagonists of CRF1. Compound 8w showed high CRF1 receptor binding affinity and was examined further in vivo. The compound was efficacious in a defensive withdrawal model of anxiety in rats and had a long half-life and reasonable oral bioavailability in dog pharmacokinetic studies.

Anxiolytic-Like Effects of the Corticotropin-Releasing Factor1 (CRF1) Antagonist DMP904 [4-(3-pentylamino)-2,7-dimethyl-8-(2-methyl-4-methoxyphenyl)-pyrazolo-[1,5-a]-pyrimidine] Administered Acutely or Chronically at Doses Occupying Central CRF1 Receptors in Rats

Corticotropin-releasing factor(1) (CRF(1)) antagonists may be effective in the treatment of anxiety disorders with fewer side effects compared with classic benzodiazepines. The behavioral effects of DMP904 [4-(3-pentylamino)-2,7-dimethyl-8-(2-methyl-4-methoxyphenyl)-pyrazolo-[1,5-a]-pyrimidine] and its effects on the hypothalamic-pituitary-adrenal (HPA) axis were related to its levels in plasma and estimated occupancy of central CRF(1) receptors. DMP904 (10-30 mg/kg, p.o.) and alprazolam (10 mg/kg, p.o.) increased time spent in open arms of an elevated-plus maze. In addition, acutely or chronically (14 days) administered DMP904 (1.0-30 mg/kg, p.o.) and acute alprazolam (1.0-3.0 mg/kg, p.o.) significantly reduced exit latency in the defensive withdrawal model of anxiety in rats, suggesting that tolerance may not develop to the anxiolytic-like effects of DMP904 in this model of anxiety. Acutely, DMP904 reversed the stress-induced increase in plasma corticosterone levels in defensive withdrawal at doses of 3.0 mg/kg and higher. These doses also resulted in levels of DMP904 in plasma similar to (for anxiolytic-like effects) or 4-fold higher (for effects on the HPA axis) than the in vitro IC(50) value for binding affinity at CRF(1) receptors and greater than 50% occupancy of CRF(1) receptors. Unlike alprazolam, DMP904 did not produce sedation, ataxia, or chlordiazepoxide-like subjective effects (as measured by locomotor activity, rotorod performance, and chlordiazepoxide discrimination assays, respectively) at doses at least 3-fold higher than anxiolytic-like doses. In conclusion, anxiolytic-like effects and effects on the stress-activated HPA axis of DMP904 in the defensive withdrawal model of anxiety required 50% or greater occupancy of central CRF(1) receptors. This level of CRF(1) receptor occupancy resulted in fewer motoric side effects compared with classic benzodiazepines.

The anxiolytic CRF(1) antagonist DMP696 fails to function as a discriminative stimulus and does not substitute for chlordiazepoxide in rats

RATIONALE

Compounds with a mechanism of action different from benzodiazepines may retain the anxiolytic effects of benzodiazepines with fewer side effects. CRF(1) antagonists have anxiolytic-like effects but may have different discriminative stimulus (DS) effects compared with benzodiazepines.

OBJECTIVE

The present study evaluated the similarity of DS effects of a CRF(1) antagonist DMP696 to the benzodiazepine chlordiazepoxide and the ability of DMP696 to produce DS effects on its own using drug discrimination procedures, as well as its anxiolytic-like effects after acute or chronic administration.

METHODS

Rats were trained to discriminate chlordiazepoxide (5.0 mg/kg, IP, 30 min prior to session) from vehicle under a fixed-ratio 10 schedule of food reinforcement and drug- or vehicle-lever selection following administration of DMP696 was determined. The effects of DMP696 on latency to exit a dark chamber (defensive withdrawal model of anxiety) were used as an index of anxiolytic-like activity.

RESULTS

In chlordiazepoxide-trained rats, DMP696 (1.0-100 mg/kg, PO) resulted in most of the animals selecting the vehicle lever, as did another anxiolytic, the 5-HT(1A) partial agonist buspirone (0.3-10 mg/kg, IP). DMP696 reduced exit latency in defensive withdrawal at 10 mg/kg administered either acutely or chronically for 14 days. Thus, the doses of DMP696 studied in drug discrimination were up to 10-fold higher than those active in the anxiety model. In addition, DMP696 (10-60 mg/kg, PO) could not be established as a DS under the conditions used in this study. In a subsequent study, chlordiazepoxide was established as a DS in these same animals.

CONCLUSIONS

Lack of substitution of DMP696 for the chlordiazepoxide DS in rats and its inability to acquire DS properties suggest that the DS effects of DMP696 differ from those of benzodiazepines.

Discriminative stimulus effects of zolpidem in squirrel monkeys: role of GABA(A)/alpha1 receptors

RATIONALE

The discriminative stimulus effects of zolpidem in squirrel monkeys trained at doses greater than or equal to 3.0 mg/kg differ from those of conventional benzodiazepines (BZs), but the extent to which these effects reflect the selectivity of zolpidem for GABA(A)/alpha(1) receptors is not known.

OBJECTIVES

The present study investigated the ability of GABA(A)/alpha(1)-preferring agonists to substitute for training doses of zolpidem greater than or equal to 3.0 mg/kg and the ability of GABA(A)/alpha(1)-preferring antagonists to block zolpidem's discriminative stimulus effects.

METHODS

Squirrel monkeys were trained to discriminate intravenous injections of zolpidem (3.0 or 5.6 mg/kg) from saline and tested with BZ agonists differing in selectivity and efficacy at GABA(A)/alpha(1) receptors. Antagonism of the effects of zolpidem was studied using the GABA(A)/alpha(1)-preferring antagonists beta-carboline-3-carboxylate-t-butyl ester (beta-CCT) and 3-propyloxy-beta-carboline (3-PBC).

RESULTS

Zolpidem and quazepam (GABA(A)/alpha(1)-preferring agonist) engendered full substitution for zolpidem, whereas CL 218,872 (GABA(A)/alpha(1)-preferring partial agonist) and the non-selective BZ agonists alprazolam and flunitrazepam engendered low and variable levels of zolpidem-lever responding (35-58%). Both beta-CCT and 3-PBC antagonized the discriminative stimulus effects of zolpidem in a surmountable fashion.

CONCLUSIONS

Our findings provide evidence for a key role of GABA(A)/alpha(1) receptors in the discriminative stimulus effects of zolpidem at relatively high training doses, and suggest that selectivity and relatively high efficacy at GABA(A)/alpha(1) receptors is required for BZ agonists to reproduce these discriminative stimulus effects.

The CRF(1) receptor antagonist DMP696 produces anxiolytic effects and inhibits the stress-induced hypothalamic-pituitary-adrenal axis activation without sedation or ataxia in rats

RATIONALE

CRF(1) antagonists may be effective in the treatment of anxiety disorders while having fewer side effects compared with classical benzodiazepines.

OBJECTIVES

The effects of a small molecule selective CRF(1) antagonist DMP696 on anxiety-like behaviors and stress-induced increases in corticosterone in rats exposed to a novel environment and on locomotor activity and motor coordination were determined in rats. These effects of DMP696 were compared with those produced by the classical benzodiazepine chlordiazepoxide (CDP).

METHODS

DMP696 or CDP were administered PO, 60 minutes before behavioral testing in rats. Their effects on latency to exit a dark chamber and stress-induced increase in corticosterone in the Defensive Withdrawal test (an animal model of anxiety), locomotor activity, and rotorod performance (measure of ataxia) were determined.

RESULTS

DMP696 significantly reduced exit latency and reversed the stress-induced increase in corticosterone in the Defensive Withdrawal test at doses of 3.0-10 mg/kg and higher. In contrast, CDP significantly decreased exit latency at 10 and 30 mg/kg, but not at 100 mg/kg, due to concurrent non-specific side effects. Unlike DMP696, CDP had no effect on the stress-induced increase in corticosterone at lower doses, but resulted in a significant increase at higher doses. DMP696 did not reduce locomotor activity or impair motor coordination at doses up to 30-fold higher than doses effective in the Defensive Withdrawal model. In contrast, CDP produced significant sedation and ataxia at the same doses that were effective in reducing exit latency.

CONCLUSIONS

These data suggest that the CRF(1) antagonist DMP696 might retain the therapeutic benefits of classical benzodiazepines but have fewer motoric side effects.

Role of GABAA/benzodiazepine receptors containing alpha 1 and alpha 5 subunits in the discriminative stimulus effects of triazolam in squirrel monkeys

RATIONALE

Conventional benzodiazepines (BZs), clinically used for treatment of anxiety and insomnia, bind to GABA(A) receptors containing alpha(1), alpha(2), alpha(3), or alpha(5) subunits. The role of these different GABA(A) receptor subtypes in mediating the subjective effects of BZs remains largely unknown.

OBJECTIVE

The purpose of the present study was to evaluate the role of GABA(A) receptors containing the alpha(1) or alpha(5) subunits in the discriminative stimulus (DS) effects of the conventional BZ agonist triazolam.

METHODS

Squirrel monkeys were trained to discriminate triazolam (0.03 mg/kg, i.v.) from vehicle under a fixed-ratio 10 schedule of food reinforcement.

RESULTS

The GABA(A)/alpha(1)-preferring agonists zolpidem and zaleplon engendered responses predominantly on the triazolam lever (73-80% drug-lever responding), and the GABA(A)/alpha(1) partial agonist CL 218,872 engendered an average maximum of less than 50% triazolam-lever responding. The GABA(A)/alpha(1)-preferring antagonists beta-carboline-3-carboxylate-t-butyl ester (betaCCT) and 3-(propyloxy)-beta-carboline (3-PBC) blocked the DS effects of triazolam and zolpidem in a surmountable manner. Schild analyses for betaCCT and 3-PBC in combination with triazolam and zolpidem suggest that the interactions between these compounds were competitive in nature and mediated by a common population of receptors, presumably GABA(A)/alpha(1) receptors. In contrast, the GABA(A)/alpha(5)-preferring agonist QH-ii-66 did not engender triazolam-lever responding regardless of dose and did not alter the DS effects of triazolam when administered in combination.

CONCLUSIONS

The results are consistent with GABA(A)/alpha(1) receptor involvement in mediating the DS effects of triazolam. In contrast, binding to GABA(A)/alpha(5) receptors may not play a critical role in mediating triazolam's DS effects.

Isobolographic analysis of chlordiazepoxide and triazolam combinations in squirrel monkeys discriminating triazolam

RATIONALE

The discriminative stimulus (DS) effects of chlordiazepoxide (CDP) differ from those of other typical benzodiazepine (BZ) agonists in that CDP does not always occasion full substitution for a BZ agonist DS.

OBJECTIVES

The present study tested the hypothesis that the unusual DS effects of CDP may result from its relatively low intrinsic efficacy by examining the combinations of CDP and triazolam using isobolographic analysis in squirrel monkeys discriminating triazolam.

METHODS AND RESULTS

Squirrel monkeys were previously trained to discriminate triazolam (0.03 mg/kg, i.v.) from vehicle under a fixed-ratio 10 (FR 10) schedule of food reinforcement. CDP occasioned partial substitution for triazolam and did not alter the DS effects of triazolam, whereas single doses of triazolam enhanced the DS effects of triazolam, and bretazenil antagonized the triazolam DS. The isobolographic analysis showed that CDP and triazolam combinations resulted in additive effects in animals in which CDP substituted for triazolam, whereas infra-additive effects were obtained in animals in which CDP did not substitute for triazolam.

CONCLUSIONS

The partial substitution of CDP for triazolam and the infra-additive effects obtained in animals in which CDP did not substitute for triazolam suggest that CDP may have lower intrinsic efficacy than triazolam. However, the lack of overall effect of CDP pretreatment and the lack of shift in animals in which CDP substituted for triazolam suggest that other factors, such as differential activity at BZ receptor subtypes, may play a role in the effects of CDP.

Triazolam discrimination in squirrel monkeys distinguishes high-efficacy agonists from other benzodiazepines and non-benzodiazepine drugs

RATIONALE

Triazolam is a high-efficacy benzodiazepine (BZ) agonist, which might be hypothesized to engender highly pharmacologically specific discriminative stimulus (DS) effects and distinguish among BZ agonists with different intrinsic efficacy.

OBJECTIVES

The pharmacological specificity of the triazolam stimulus was determined by examining the effects of conventional and atypical BZ agonists, and other ligands active at the gamma-aminobutyric acidA (GABAA) receptor complex. Receptor mechanisms underlying the DS effects of triazolam were examined further using the BZ receptor antagonist flumazenil.

METHODS AND RESULTS

Squirrel monkeys were trained to discriminate triazolam (0.03 mg/kg, i.v.) from vehicle under a fixed-ratio 10 (FR 10) schedule of food reinforcement. While the BZ agonists midazolam, diazepam, and lorazepam substituted fully for triazolam, chlordiazepoxide, oxazepam and nordiazepam produced only partial substitution, suggesting these latter compounds may have reduced intrinsic efficacy. The BZ/alpha1-preferring agonist zolpidem substituted fully for triazolam, and potencies for triazolam-like effects of BZ agonists were significantly correlated with potencies for their zolpidem-like effects (Rowlett et al. 1999). Flumazenil antagonized the DS effects of triazolam, but the slope of the Schild plot was significantly different from unity, suggesting multiple receptors may be involved in the DS effects of triazolam.

CONCLUSIONS

BZ agonists can be distinguished on the basis of substitution for triazolam and, thus, the triazolam discrimination may be a useful tool for identifying compounds of different efficacy at BZ receptors. BZ/alpha1 receptors appear to play a prominent role in the DS effects of triazolam, but the contribution of other subtypes of BZ receptors cannot be ruled out.

Using behavior to elucidate receptor mechanisms: a review of the discriminative stimulus effects of benzodiazepines

Drug discrimination procedures have been used to study receptor mechanisms of benzodiazepine (BZ) agonists with the goal of developing new therapeutic agents that retain positive effects of conventional BZ ligands yet have reduced side effects. The present review provides a synthesis of existing literature on discriminative stimulus effects of BZ agonists in order to elucidate their underlying receptor mechanisms, specifically in terms of intrinsic efficacy and receptor selectivity. The available evidence suggests that receptor selectivity is a critical determinant of the discriminative stimulus effects of BZ agonists. In particular, BZ-1 receptors appear to play a fundamental role, whereas the role of BZ-2 receptors remains elusive. In addition, data from many drug discrimination studies suggest that the conventional BZ agonist chlordiazepoxide may have reduced intrinsic efficacy compared with other BZ agonists.

Transduction of the discriminative stimulus effects of zolpidem by GABA(A)/alpha1 receptors

Zolpidem is an imidazopyridine with high affinity at gamma-aminobutyric acid(A) (GABA(A)) receptors expressing alpha1 subunits. In squirrel monkeys trained to discriminate a high dose of zolpidem (> or =3.0 mg/kg) from saline, zolpidem and another GABA(A)/alpha1 receptor-preferring agonist, zaleplon, substituted dose-dependently for zolpidem, whereas the non-selective agonists diazepam and triazolam were did not substitute at any dose tested. These findings offer the first evidence for a selective role of GABA(A)/alpha1 receptors in the interoceptive effects of high doses of zolpidem.

Antagonism of the discriminative stimulus effects of positive gamma-aminobutyric acid(A) modulators in rhesus monkeys discriminating midazolam

The extent to which individual subtypes of benzodiazepine receptors are functionally independent has not been elucidated in vivo. This study used apparent pA(2) analysis to test the hypothesis that a single receptor subtype mediates the discriminative stimulus effects of midazolam, triazolam, and diazepam, three positive gamma-aminobutyric acid(A) (GABA(A)) modulators. Four rhesus monkeys discriminated 0.56 mg/kg midazolam from vehicle under a fixed-ratio 5 schedule of stimulus-shock termination. Midazolam, triazolam, and diazepam increased responding on the midazolam-appropriate lever. The neutral GABA(A) modulator flumazenil shifted dose-effect curves for triazolam and diazepam to the right, and the negative GABA(A) modulators Ro 15-4513 and ethyl beta-carboline-3-carboxylate (beta-CCE) shifted dose-effect curves for midazolam and triazolam to the right. Slopes of Schild plots for flumazenil and Ro 15-4513 conformed to unity. The apparent pA(2) values were 7.41 and 7.69 for flumazenil in combination with triazolam and diazepam, respectively, and 7.53 and 6.88 for Ro 15-4513 in combination with midazolam and triazolam, respectively. The slope of the Schild plot for beta-CCE in combination with midazolam deviated from unity. Slopes of Schild plots obtained with flumazenil and Ro 15-4513 support the notion that a single benzodiazepine receptor subtype mediates the effects of midazolam, triazolam, or diazepam. The similarity in apparent pA(2) values for flumazenil in combination with triazolam and diazepam or for Ro 15-4513 in combination with midazolam and triazolam suggests that the same subtype mediates the effects of these positive modulators. In contrast, beta-CCE and midazolam do not appear to interact in a simple, competitive manner.

Discriminative stimulus effects of zolpidem in squirrel monkeys: comparison with conventional benzodiazepines and sedative-hypnotics

The present study examined whether zolpidem, an imidazopyridine with selectivity for benzodiazepine (BZ)/gamma-aminobutyric acid(A) receptors containing the alpha1-subunit, had discriminative stimulus effects similar to typical BZs and other sedative/hypnotic drugs in primates. Squirrel monkeys (Saimiri sciureus) were trained to discriminate zolpidem (1.0 mg/kg i.v.) from vehicle under a 10-response fixed-ratio schedule of food delivery. Under test conditions, zolpidem (0.1-3.0 mg/kg) increased responding on the drug lever to an average maximum of 90% of total responding. When pretreatment times were varied from 5 to 50 min, the discriminative stimulus effects of zolpidem were maximal at 5 min and near control levels 35 min after administration. Flumazenil antagonized both the discriminative stimulus and rate-decreasing effects of zolpidem in a dose-dependent and surmountable fashion (in vivo apparent pA(2) values of 7.3 and 6.6 for the discriminative stimulus and rate-suppressing effects, respectively). The BZs triazolam, midazolam, diazepam, and N-desmethyldiazepam engendered dose-related increases in drug-lever responding that reached zolpidem-like levels (90%) in the majority of monkeys tested. In contrast, lorazepam, chlordiazepoxide, and oxazepam engendered average maximums of 70% or less and substituted fully for zolpidem in one or two monkeys only. Representative barbiturates as well as drugs that bind to non-BZ sites (muscimol, baclofen, buspirone, cyproheptadine, diphenhydramine) engendered 0 to 45% of responses on the drug lever up to doses that markedly reduced response rate. These results support the view that zolpidem's selectivity for the alpha1-subunit of the BZ/gamma-aminobutyric acid(A) receptor complex confers a distinctive profile of interoceptive effects that overlaps partially with those of typical BZs but not with those of barbiturates.

Inhibitors of cytochrome P450 differentially modify discriminative-stimulus and antinociceptive effects of hydrocodone and hydromorphone in rhesus monkeys

The present study was conducted to investigate the role of cytochrome P450 in the discriminative-stimulus and antinociceptive effects of hydrocodone (HC) and hydromorphone (HM) in rhesus monkeys. In morphine-deprived monkeys, morphine dose-dependently reversed naltrexone-lever responding, an effect also produced by HC and HM. HC and HM also produced antinociception in a warm-water tail withdrawal procedure. Budipine and naltrexone shifted the dose-effect curves for the discriminative-stimulus effects of HC and HM to the right. In contrast, naltrexone, but not budipine (10.0 mg/kg) or quinidine (10.0 mg/kg), dose-dependently antagonized the antinociceptive effects of HC. Budipine and quinidine decreased the concentration of HM in plasma without significantly affecting the levels of HC, suggesting that these CYP2D6 inhibitors decreased the conversion of HC HM. Thus, some behavioral effects of HC are not modified by a marked inhibition of CYP2D6, suggesting that these effects of HC are not due to its conversion to HM but, rather, that both HC and HM act directly on mu receptors.

Discriminative-stimulus effects of triazolam and midazolam in rhesus monkeys

The present study characterized the discriminative-stimulus effects of triazolam and midazolam in rhesus monkeys. Six monkeys discriminated 0.1 mg/kg of triazolam from vehicle under a fixed-ratio 5 (FR 5) schedule of stimulus-shock termination (SST). Four monkeys subsequently discriminated 0.56 mg/kg of midazolam from vehicle under the same schedule of reinforcement. Benzodiazepine (BDZ) agonists midazolam and diazepam, and the barbiturate pentobarbital, substituted for triazolam, and the non-competitive N-methyl-D-aspartate (NMDA) antagonist ketamine did not. Triazolam, diazepam, lorazepam, flunitrazepam, as well as the barbiturates amobarbital and pentobarbital, substituted for midazolam, and ketamine did not. The BDZ antagonist flumazenil antagonized both the triazolam and midazolam discriminative stimuli. Bretazenil, a low-efficacy BDZ agonist, did not substitute for the midazolam discriminative stimulus in three of the monkeys and shifted the midazolam dose-effect curve to the right; in a fourth monkey, bretazenil substituted for midazolam and shifted the midazolam dose-effect curve to the left. Schild analyses with flumazenil or bretazenil, in combination with midazolam, yielded slopes that deviated significantly from unity. While clearly supporting the notion that BDZ agonists produce stimulus effects by acting at the gamma-aminobutyric acidA (GABA(A)) receptor complex, these data also suggest that the discriminative-stimulus effects of midazolam might be mediated by more than one BDZ receptor subtype.

Pharmacological profile of a deuterium-substituted mirfentanil derivative, OHM10579, in rhesus monkeys

The discriminative-stimulus, respiratory, and antinociceptive effects of OHM10579, an isotopic isomer of mirfentanil, were characterized in rhesus monkeys. In monkeys discriminating nalbuphine, 0.32 mg/kg of OHM10579 partially substituted for nalbuphine. In monkeys treated daily with 3.2 mg/kg of morphine and discriminating 0.01 mg/kg of naltrexone, 0.32 mg/kg of OHM10579 substituted for naltrexone. In morphine-abstinent monkeys, morphine reversed naltrexone-lever responding, an effect attenuated by OHM10579. The shift to the right in the morphine dose-effect curve was greater 2 h after 0.32 mg/kg of OHM10579 compared to 0.32 mg/kg of mirfentanil, indicating that OHM10579 has a longer duration of action than mirfentanil. In a warm-water tail-withdrawal procedure, 10 and 17.8 mg/kg of OHM10579 had antinociceptive effects that were not antagonized by naltrexone. Morphine decreased breathing in air to 48%, whereas the maximal decrease with OHM10579 was to 75% of control. OHM10579 attenuated hyperventilation induced by 5% CO2 and partially antagonized the respiratory-depressant effects of morphine. OHM10579 can be classified as a low-efficacy mu-opioid agonist with some nonopioid actions. These results indicate that the pharmacology of the mirfentanil isotope OHM10579 is similar to that of mirfentanil, but that OHM10579 might have a longer duration of action.

Interaction of metabolism of aspartate and inosine and energy state of malignant cells

1. Oxidation of glutamine in Ehrlich ascites-carcinoma cells results in a large accumulation of aspartate. 2. The addition of inosine causes a marked decrease in aspartate production from glutamine. This may be related to the resynthesis of AMP from aspartate and IMP, the latter being produced from inosine via the salvage pathway for purine nucleotides. In accordance with this assumption, a significant production of lactate was observed, which comes probably from the ribose moiety of inosine. Since lactate is known to inhibit production of aspartate from glutamine, this may explain the effect of inosine. 3. Addition of glutamine together with inosine increased cellular ATP content. This was not the case if glutamine or inosine was present separately or if inosine was added together with lactate, pyruvate or glucose. The effect did not occur if amino-oxyacetate, an inhibitor of transaminases, was added. These findings suggested again that production of aspartate is important for resynthesis of ATP from IMP via the purine nucleotide cycle. 4. If the cells were exposed to prolonged anaerobic incubation, addition of glutamine and inosine markedly increased O2 uptake and [ATP], suggesting the crucial importance of aspartate production by glutamine oxidation for the recovery of energy metabolism in the cells.