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

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.

Therapeutic utility of non-peptidic CRF1 receptor antagonists in anxiety, depression, and stress-related disorders: evidence from animal models

Adaptive responding to threatening stressors is of fundamental importance for survival. Dysfunctional hyperactivation of corticotropin releasing factor type-1 (CRF(1)) receptors in stress response system pathways is linked to stress-related psychopathology and CRF(1) receptor antagonists (CRAs) have been proposed as novel therapeutic agents. CRA effects in diverse animal models of stress that detect anxiolytics and/or antidepressants are reviewed, with the goal of evaluating their potential therapeutic utility in depression, anxiety, and other stress-related disorders. CRAs have a distinct phenotype in animals that has similarities to, and differences from, those of classic antidepressants and anxiolytics. CRAs are generally behaviorally silent, indicating that CRF(1) receptors are normally in a state of low basal activation. CRAs reduce stressor-induced HPA axis activation by blocking pituitary and possibly brain CRF(1) receptors which may ameliorate chronic stress-induced pathology. In animal models sensitive to anxiolytics and/or antidepressants, CRAs are generally more active in those with high stress levels, conditions which may maximize CRF(1) receptor hyperactivation. Clinically, CRAs have demonstrated good tolerability and safety, but have thus far lacked compelling efficacy in major depressive disorder, generalized anxiety disorder, or irritable bowel syndrome. CRAs may be best suited for disorders in which stressors clearly contribute to the underlying pathology (e.g. posttraumatic stress disorder, early life trauma, withdrawal/abstinence from addictive substances), though much work is needed to explore these possibilities. An evolving literature exploring the genetic, developmental and environmental factors linking CRF(1) receptor dysfunction to stress-related psychopathology is discussed in the context of improving the translational value of current animal models.

Comparative biotransformation of pyrazinone-containing corticotropin-releasing factor receptor-1 antagonists: minimizing the reactive metabolite formation

(S)-5-Chloro-1-(1-cyclopropylethyl)-3-(2,6-dichloro-4-(trifluoromethyl)phenylamino)pyrazin-2(1H)-one (BMS-665053), a pyrazinone-containing compound, is a potent and selective antagonist of corticotropin-releasing factor receptor-1 (CRF-R1) that showed efficacy in the defensive withdrawal model for anxiety in rats, suggesting its use as a potential treatment for anxiety and depression. In vitro metabolism studies of BMS-665053 in rat and human liver microsomes revealed cytochrome P450-mediated oxidation of the pyrazinone moiety, followed by ring opening, as the primary metabolic pathway. Detection of a series of GSH adducts in trapping experiments suggested the formation of a reactive intermediate, probably as a result of epoxidation of the pyrazinone moiety. In addition, BMS-665053 (20 mg/kg i.v.) underwent extensive metabolism in bile duct-cannulated (BDC) rats. The major drug-related materials in rat plasma were the pyrazinone oxidation products. In rat bile and urine (0-7 h), only a trace amount of the parent drug was recovered, whereas significant levels of the pyrazinone epoxide-derived metabolites and GSH-related conjugates were detected. Further evidence suggested that GSH-related conjugates also formed at the dichloroarylamine moiety possibly via an epoxide or a quinone imine intermediate. Other major metabolites in BDC rat bile and urine included glucuronide conjugates. To reduce potential liability due to metabolic activation of BMS-665053, a number of pyrazinone analogs with different substituents were synthesized and investigated for reactive metabolite formation, leading to the discovery of a CRF-R1 antagonist with diminished in vitro metabolic activation.

Drug-dependent requirement of hippocampal neurogenesis in a model of depression and of antidepressant reversal

BACKGROUND

Depression and anxiety disorders have been linked to dysfunction of the hypothalamo-pituitary-adrenal (HPA) axis and structural changes within the hippocampus. Unpredictable chronic mild stress (UCMS) can recapitulate these effects in a mouse model, and UCMS-induced changes, including downregulation of hippocampal neurogenesis, can be reversed by antidepressant (AD) treatment. We investigated causality between changes in hippocampal neurogenesis and the effects of both chronic stress and chronic ADs.

METHODS

Mice were treated with either a sham procedure or focal hippocampal irradiation to disrupt cell proliferation before being confronted with 5 weeks of UCMS. From the third week onward, we administered monoaminergic ADs (imipramine, fluoxetine), the corticotropin-releasing factor 1 (CRF(1)) antagonist SSR125543, or the vasopressin 1b (V(1b)) antagonist SSR149415 daily. The effects of UCMS regimen, AD treatments, and irradiation were assessed by physical measures (coat state, weight), behavioral testing (Splash test, Novelty-Suppressed feeding test, locomotor activity), and hippocampal BrdU labeling.

RESULTS

Our results show that elimination of hippocampal neurogenesis has no effect on animals' sensitivity to UCMS in several behavioral assays, suggesting that reduced neurogenesis is not a cause of stress-related behavioral deficits. Second, we present evidence for both neurogenesis-dependent and -independent mechanisms for the reversal of stress-induced behaviors by AD drugs. Specifically, loss of neurogenesis completely blocked the effects of monoaminergic ADs (imipramine, fluoxetine) but did not prevent most effects of the CRF(1) and the V(1b) antagonists.

CONCLUSIONS

Hippocampal neurogenesis might thus be used by the monoaminergic ADs to counteract the effects of stress, whereas similar effects could be achieved by directly targeting the HPA axis and related neuropeptides.

Discriminative stimulus properties of the selective norepinephrine reuptake inhibitor, reboxetine, in rats: a characterization with alpha/beta-adrenoceptor subtype selective ligands, antidepressants, and antagonists at neuropeptide receptors

Although little information is available concerning discriminative stimulus (DS) properties of antidepressants, rats can be trained to recognize the selective norepinephrine (NE) reuptake inhibitor, reboxetine (2.5 mg/kg i.p.). By analogy to reboxetine (effective dose50, 1.1), 'full' (80%) substitution dose50 was obtained with the NE reuptake inhibitors, nisoxetine (4.9), nomifensine (0.5) and BW1555,U88 (1.0). Full substitution was also attained with the NE/serotonin (5-HT) reuptake inhibitors, S33005 (0.3), venlafaxine (4.8) and duloxetine (26.8), and the tricyclics, imipramine (2.5) and clomipramine (2.9). In contrast, the 5-HT reuptake inhibitors, citalopram, sertraline and paroxetine (all >2.5), and the 5-HT reuptake inhibitors/5-HT2 receptor antagonists, nefazodone and trazodone (both >10.0), did not substitute for reboxetine. The 'atypical' antidepressants, mirtazapine (>10.0) and mianserin (>2.5), similarly failed to substitute. DS properties of reboxetine were dose-dependently blocked by the alpha1-adrenoceptor (AR) antagonists, prazosin (inhibitory dose50, 0.3) and WB4101 (0.5), but resistant to the alpha2-AR antagonists, atipamezole (>0.63), idazoxan (>2.5) and RX821,002 (>0.08), and to the beta1-AR and beta2-AR antagonists, betaxolol (>2.5) and ICI118,551 (>10.0). Interestingly, the neurokinin-1 receptor antagonist, GR205,171, stereospecifically substituted for reboxetine (1.1) compared to its less active isomer, GR226,206 (>10.0). The corticotrophin-releasing factor-1 antagonists, DMP695 (>40), CP154,526 (>10.0) and SN003 (>40.0), and the melanin-concentrating hormone-1 antagonist, SNAP-7941 (>40.0), failed to substitute for reboxetine. In conclusion, DS properties of reboxetine are mimicked by antidepressants recognizing NE transporters, and require functionally intact alpha1-ARs for their expression. The neurokinin-1 antagonist, GR205,171, mimics the interoceptive properties of reboxetine, possibly reflecting its elevation of extracellular levels of NE in corticolimbic structures.

Factor analysis of Forced Swimming test, Sucrose Preference test and Open Field test on enriched, social and isolated reared rats

Developmental and social factors are known to play a crucial role in the pathogenesis of affective disorders. Although it has been demonstrated that early life aversive experiences can be a risk factor in the development of human depression, most of the investigation in animals that try to model depression do not include postnatal manipulations. Since housing represents a fundamental ethological factor which modifies behavior and brain development, this study aimed to investigate the impact of different social and structural housing conditions on the development of a depressive-like syndrome in the behavioral despair paradigm and an anxiety-like syndrome in the unconditioned anxiety paradigm. The present study uses several multivariate analyses to study the impact of housing conditions in animal models of depression and anxiety. In this study, social isolation was able to reproduce the effects found in other animals models based on stress, suggesting that only 2 months of social isolation are enough to produce effects that can be useful as behavioral model of depression. Moreover, environmental enrichment showed an antidepressive and anxiolytic like effect in animal models of depression and anxiety. This effect, which has not been reported in earlier studies, suggests that stimulation during the first stages of growth might play a "protective" role on behavior and brain development.

The therapeutic potential of CRF1 antagonists for anxiety

Preclinical studies suggest that the brain corticotropin-releasing factor (CRF) systems mediate anxiety-like behavioural and somatic responses through actions at the CRF1 receptor. CRF1 antagonists block the anxiogenic-like effects of CRF and stress in animal models. Cerebrospinal fluid levels of CRF are elevated in some anxiety disorders and normalise with effective treatment, further implicating CRF systems as a therapeutic target. Prototypical CRF1 antagonists are highly lipophilic, non-competitive antagonists of peptide ligands. Modification of the chemotype and the identification of novel pharmacophores are yielding more drug-like structures with increased hydrophilicity at physiological pHs. Newer compounds exhibit improved solubility, pharmacokinetic properties, potency and efficacy. Several clinical candidates have entered Phase I/II trials. However, unmet challenges await resolution during further discovery, clinical development and therapeutic application of CRF1 antagonists.

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.

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.