Anti-addictive properties of COR659 - Additional pharmacological evidence and comparison with a series of novel analogues

Positive allosteric modulators of the GABAB receptor: a new class of ligands with therapeutic potential for alcohol use disorder

BACKGROUND

Positive allosteric modulators (PAMs) of the GABAB receptor constitute a new class of GABAB-receptor ligands. GABAB PAMs reproduce several pharmacological effects of the orthosteric GABAB receptor agonist, baclofen, although displaying a better safety profile.

AIMS

This paper reviews the reducing or, frequently, even suppressing effects of all GABAB PAMs tested to date on multiple alcohol-related behaviours in laboratory rodents exposed to validated experimental models of human alcohol use disorder.

RESULTS

Acute or repeated treatment with CGP7930, GS39783, BHF177, rac-BHFF, ADX71441, CMPPE, COR659, ASP8062, KK-92A, and ORM-27669 reduced excessive alcohol drinking, relapse- and binge-like drinking, operant alcohol self-administration, reinstatement of alcohol seeking, and alcohol-induced conditioned place preference in rats and mice.

CONCLUSIONS

These effects closely mirrored those of baclofen; notably, they were associated to remarkably lower levels of tolerance and toxicity. The recent transition of ASP8062 to clinical testing will soon prove whether these highly consistent preclinical data translate to AUD patients.

Anorectic effect of COR659 in a rat model of overeating

COR659 is a new compound, the action of which is exerted via a dual mechanism: positive allosteric modulation of the GABAB receptor; antagonism or inverse agonism at the cannabinoid CB1 receptor. Recent lines of experimental evidence have indicated that COR659 potently and effectively reduced operant self-administration of and reinstatement of seeking behaviour for a chocolate-flavoured beverage. The present study was designed to assess whether the ability of COR659 to diminish these addictive-like, food-motivated behaviours extended to a rat model of overeating palatable food. To this end, rats were habituated to feed on a standard rat chow for 3 h/day; every 4 days, the 3-hour chow-feeding session was followed by a 1-hour feeding session with highly palatable, calorie-rich Danish butter cookies. Even though satiated, rats overconsumed cookies. COR659 (0, 2.5, 5, and 10 mg/kg, i.p.) was administered before the start of the cookie-feeding session. Treatment with all 3 doses of COR659 produced a substantial decrease in intake of cookies and calories from cookies. These results extend the anorectic profile of COR659 to overconsumption of a highly palatable food and intake of large amounts of unnecessary calories.

Development of tolerance upon repeated administration with the GABAB receptor positive allosteric modulator, COR659, on alcohol drinking in rodents

Background: Recent work has demonstrated that acute administration of the novel positive allosteric modulator of the GABAB receptor, COR659, reduces several alcohol-related behaviors in rodents.Objective: To assess whether COR659 continues to lessen alcohol intake after repeated administration, a fundamental feature of drugs with therapeutic potential.Methods: Male C57BL/6J mice (n = 40) were exposed to daily 2-hour drinking sessions (20% (v/v) alcohol) under the 1-bottle "drinking in the dark" protocol and male Sardinian alcohol-preferring rats (n = 40) were exposed to daily 1-hour drinking sessions under the 2-bottle "alcohol (10%, v/v) vs water" choice regimen. COR659 (0, 10, 20, and 40 mg/kg in the mouse experiment; 0, 5, 10, and 20 mg/kg in the rat experiment) was administered intraperitoneally before 7 consecutive drinking sessions.Results: Alcohol intake in vehicle-treated mice and rats averaged 2.5-3.0 and 1.5-1.6 g/kg/session, respectively, indicative of high basal levels. In both experiments, treatment with COR659 resulted in an initial, dose-related suppression of alcohol intake (up to 70-80% compared to vehicle treatment; P < .0005 and P < .0001 in mouse and rat experiments, respectively). The magnitude of the reducing effect of COR659 on alcohol drinking diminished progressively, until vanishing over the subsequent 2-4 drinking sessions.Conclusion: COR659 effectively reduced alcohol intake in two different rodent models of excessive alcohol drinking. However, tolerance to the anti-alcohol effects of COR659 developed rapidly. If theoretically transposed to humans, these data would represent a possible limitation to the clinical use of COR659.

Development of Partial Tolerance to the Suppressing Effect of the Positive Allosteric Modulator of the GABAB Receptor, KK-92A, on Alcohol Self-Administration in Rats

AIMS

A recent study reported how acute treatment with KK-92A, a newly synthesized positive allosteric modulator (PAMs) of the GABAB receptor (GABAB PAMs), suppressed a series of alcohol-related behaviors, including operant oral alcohol self-administration, in selectively bred Sardinian alcohol-preferring (sP) rats. These findings lead to the addition of KK-92A to the long list of GABAB PAMs capable of reducing, after acute treatment, alcohol self-administration in rats. As a further step toward a more complete characterization of the anti-addictive properties of KK-92A, the present study was designed to assess the effect of repeated treatment with the compound on alcohol self-administration.

METHODS

sP rats were trained to lever-respond for oral alcohol (15%, v/v) under the fixed ratio 5 (FR5) schedule of reinforcement. Once lever-responding behavior had stabilized, KK-92A (0, 5, 10 and 20 mg/kg, i.p.) was administered 30 min prior to 10 consecutive daily self-administration sessions (likewise occurring under the FR5 schedule).

RESULTS

The first injection of KK-92A produced a dose-related suppression in number of lever-responses for alcohol and amount of self-administered alcohol. Magnitude of the suppressing effect of KK-92A decreased over the following two self-administration sessions and then tended to stabilize on continuation of treatment. Statistical significance at post hoc analysis was maintained only by the highest dose tested (20 mg/kg).

CONCLUSIONS

These data suggest the development of partial tolerance to the reducing effect of repeatedly administered KK-92A on alcohol self-administration. The agonistic component of the ago-allosteric profile of KK-92A is discussed as the likely key element underlying the observed tolerance.

Reducing effect of the novel positive allosteric modulator of the GABA(B) receptor, COR659, on binge-like alcohol drinking in male mice and rats

RATIONALE

Binge drinking (BD) is a widespread drinkingpattern that may contribute to promote the development of alcohol use disorder (AUD). The comprehension of its neurobiological basis and the identification of molecules that may prevent BD are critical. Preclinical studies demonstrated that positive allosteric modulators (PAMs) of the GABA_B receptor effectively reduced, and occasionally suppressed, the reinforcing and motivational properties of alcohol in rodents, suggesting their potential use as pharmacotherapy for AUD, including BD. Recently, we demonstrated that COR659, a novel GABA_B PAM, effectively reduced (i) alcohol drinking under the 2-bottle choice regimen, (ii) alcohol self-administration under both fixed and progressive ratio schedules of reinforcement, and (iii) cue-induced reinstatement of alcohol-seeking behavior in Sardinian alcohol-preferring (sP) rats.

OBJECTIVES

The present study investigated whether the "anti-alcohol" properties of COR659 extend to binge-like drinking in rodents.

METHODS

COR659 was tested on the "drinking in the dark" (DID) paradigm in C57BL/6J mice and the 4-bottle "alcohol [10%, 20%, 30% (v/v)] versus water" choice regimen with limited and unpredictable access to alcohol in sP rats.

RESULTS

Acute administration of non-sedative doses of COR659 (10, 20, and 40 mg/kg; i.p.) effectively and selectively suppressed the intake of intoxicating amounts of alcohol (> 2 g/kg) consumed by C57BL/6J mice and sP rats exposed to these binge-like drinking experimental procedures.

CONCLUSIONS

The present data demonstrate the ability of COR659 to suppress binge-like drinking in rodents and strengthen the hypothesis that GABA_B PAMs may represent a potentially effective pharmacotherapy for alcohol misuse.

GABAB Receptors and Alcohol Use Disorders: Preclinical Studies

Preclinical research over the past several decades has demonstrated a role for the γ-aminobutyric acid_B (GABA_B) receptor in alcohol use disorder (AUD). This chapter offers an examination of preclinical evidence on the role of the GABA_B receptor on alcohol-related behaviors with a particular focus on the GABA_B receptor agonist baclofen, for which effects have been most extensively studied, and positive allosteric modulators (PAMs) of the GABA_B receptor. Studies employing rodent and non-human primate models have shown that activation of the GABA_B receptor can reduce (1) stimulating and rewarding effects of alcohol; (2) signs of alcohol withdrawal in rats made physically dependent on alcohol; (3) acquisition and maintenance of alcohol drinking under a two-bottle alcohol versus water choice procedure; (4) alcohol intake under oral operant self-administration procedures; (5) motivational properties of alcohol measured using extinction and progressive ratio procedures; (6) the increase in alcohol intake after a period of alcohol abstinence (the alcohol deprivation effect or ADE); and (7) the ability of alcohol cues and stress to reinstate alcohol seeking when alcohol is no longer available. Baclofen and GABA_B PAMs reduce the abovementioned behaviors across different preclinical models, which provides strong evidence for a significant role of the GABA_B receptor in alcohol-related behaviors and supports development of medications targeting GABA_B receptors for the treatment of AUD. This chapter highlights the value of examining mechanisms of alcohol-related behaviors across multiple animal models to increase the confidence in identification of new therapeutic targets.

The Novel Positive Allosteric Modulator of the GABAB Receptor, KK-92A, Suppresses Alcohol Self-Administration and Cue-Induced Reinstatement of Alcohol Seeking in Rats

Positive allosteric modulators (PAMs) of the GABA_B receptor (GABA_B PAMs) are of interest in the addiction field due to their ability to suppress several behaviors motivated by drugs of abuse. KK-92A is a novel GABA_B PAM found to attenuate intravenous self-administration of nicotine and reinstatement of nicotine seeking in rats. This present study was aimed at extending to alcohol the anti-addictive properties of KK-92A. To this end, Sardinian alcohol-preferring rats were trained to lever-respond for oral alcohol (15% v/v) or sucrose (0.7% w/v) under the fixed ratio (FR) 5 (FR5) schedule of reinforcement. Once lever-responding behavior had stabilized, rats were exposed to tests with acutely administered KK-92A under FR5 and progressive ratio schedules of reinforcement and cue-induced reinstatement of previously extinguished alcohol seeking. KK-92A effect on spontaneous locomotor activity was also evaluated. Treatment with 10 and 20 mg/kg KK-92A suppressed lever-responding for alcohol, amount of self-administered alcohol, and breakpoint for alcohol. Treatment with 20 mg/kg KK-92A reduced sucrose self-administration. Combination of per se ineffective doses of KK-92A (2.5 mg/kg) and the GABA_B receptor agonist, baclofen (1 mg/kg), reduced alcohol self-administration. Treatment with 5, 10, and 20 mg/kg KK-92A suppressed reinstatement of alcohol seeking. Only treatment with 80 mg/kg KK-92A affected spontaneous locomotor activity. These results demonstrate the ability of KK-92A to inhibit alcohol-motivated behaviors in rodents and confirm that these effects are common to the entire class of GABA_B PAMs. The remarkable efficacy of KK-92A is discussed in terms of its ago-allosteric properties.

Neurobiology of alcohol seeking behavior

Alcohol addiction is a chronic relapsing brain disease characterized by an impaired ability to stop or control alcohol use despite adverse consequences. A main challenge of addiction treatment is to prevent relapse, which occurs in more than >50% of newly abstinent patients with alcohol disorder within 3 months. In people suffering from alcohol addiction, stressful events, drug-associated cues and contexts, or re-exposure to a small amount of alcohol trigger a chain of behaviors that frequently culminates in relapse. In this review, we first present the preclinical models that were developed for the study of alcohol seeking behavior, namely the reinstatement model of alcohol relapse and compulsive alcohol seeking under a chained schedule of reinforcement. We then provide an overview of the neurobiological findings obtained using these animal models, focusing on the role of opioids systems, corticotropin-release hormone and neurokinins, followed by dopaminergic, glutamatergic, and GABAergic neurotransmissions in alcohol seeking behavior.

Suppressing effect of the novel positive allosteric modulator of the GABAB receptor, COR659, on locomotor hyperactivity induced by different drugs of abuse

COR659 is a recently synthesized positive allosteric modulator (PAM) of the GABA_B receptor. Similarly to all GABA_B PAMs tested to date, COR659 has been reported to suppress different alcohol-related behaviors in rodents. The present study was designed to assess whether the anti-addictive properties of COR659 extend to drugs of abuse other than alcohol. Specifically, it investigated the effect of COR659 on cocaine-, amphetamine-, nicotine-, and morphine-induced locomotor hyperactivity in mice. To this aim, independent groups of CD1 mice were acutely pretreated with COR659 (0, 10, and 20 mg/kg; i.p.), then acutely treated with cocaine (0 and 10 mg/kg, s.c.), amphetamine (0 and 5 mg/kg; s.c.), nicotine (0 and 0.05 mg/kg; s.c.), or morphine (0 and 20 mg/kg; s.c.), and finally exposed for 60 min to a photocell-equipped motility cage. When given alone, both doses of COR659 were ineffective on spontaneous locomotor activity. Pretreatment with COR659 reduced, or even suppressed, the increase in motility counts induced by cocaine, amphetamine, nicotine, and morphine. Since locomotor hyperactivity is an attribute common to drugs of abuse, the results of the present study constitute the first line of evidence on the extension of the preclinical, anti-addictive profile of COR659 to cocaine, amphetamine, nicotine, and morphine.

Structure optimization of positive allosteric modulators of GABAB receptors led to the unexpected discovery of antagonists/potential negative allosteric modulators

Positive allosteric modulators (PAMs) of GABA_B receptor represent an interesting alternative to receptor agonists such as baclofen, as they act on the receptor in a more physiological way and thus are devoid of the side effects typically exerted by the agonists. Based on our interest in the identification of new GABA_B receptor PAMs, we followed a merging approach to design new chemotypes starting from selected active compounds, such as GS39783, rac-BHFF, and BHF177, and we ended up with the synthesis of four different classes of compounds. The new compounds were tested alone or in the presence of 10 µM GABA using [³⁵S]GTPγS binding assay to assess their functionality at the receptor. Unexpectedly, a number of them significantly inhibited GABA-stimulated GTPγS binding thus revealing a functional switch with respect to the prototype molecules. Further studies on selected compounds will clarify if they act as negative modulators of the receptor or, instead, as antagonists at the orthosteric binding site.

Building better strategies to develop new medications in Alcohol Use Disorder: Learning from past success and failure to shape a brighter future

Alcohol Use Disorder (AUD) is a chronic disease that develops over the years. The complexity of the neurobiological processes contributing to the emergence of AUD and the neuroadaptive changes occurring during disease progression make it difficult to improve treatments. On the other hand, this complexity offers researchers the possibility to explore new targets. Over years of intense research several molecules were tested in AUD; in most cases, despite promising preclinical data, the clinical efficacy appeared insufficient to justify futher development. A prototypical example is that of corticotropin releasing factor type 1 receptor (CRF1R) antagonists that showed significant effectiveness in animal models of AUD but were largely ineffective in humans. The present article attempts to analyze the most recent venues in the development of new medications in AUD with a focus on the most promising drug targets under current exploration. Moreover, we delineate the importance of using a more integrated translational framework approach to correlate preclinical findings and early clinical data to enhance the probability to validate biological targets of interest.

Potential of GABA(B) Receptor Positive Allosteric Modulators in the Treatment of Alcohol Use Disorder

The orthosteric γ-aminobutyric acid_B (GABA_B) receptor agonist baclofen is currently considered a therapeutic option for alcohol use disorder (AUD); however, the safety profile of baclofen is a concern, thus arousing interest in the positive allosteric modulators (PAMs) of the GABA_B receptor (GABA_B PAMs), a new class of ligands expected to possess a better safety profile. The present paper summarizes the several lines of experimental evidence indicating the ability of GABA_B PAMs to inhibit multiple alcohol-motivated behaviors in rodents. All GABA_B PAMs tested to date have invariably been reported to reduce, or even suppress, excessive alcohol drinking, relapse- and binge-like drinking, operant oral alcohol self-administration, reinstatement of alcohol seeking, and alcohol-induced locomotor stimulation and conditioned place preference in rats and mice. The use of validated animal models of several aspects of AUD confers translational value to these findings. The reducing effects of GABA_B PAMs on alcohol-motivated behaviors (1) occurred at doses largely lower than those inducing sedation, suggesting that GABA_B PAMs may possess, if compared with baclofen, a higher therapeutic index and a more favorable safety profile, and (2) were often not associated with reductions on other non-drug consummatory behaviors. Additional findings with therapeutic potential were (1) the lack of tolerance, after repeated treatment, to the reducing effect of GABA_B PAMs on alcohol drinking and self-administration; (2) the efficacy of GABA_B PAMs after intragastric administration; and (3) the ability of GABA_B PAMs to selectively potentiate the suppressing effect of baclofen on alcohol self-administration. The recent transition of the first GABA_B PAMs to the initial steps of clinical testing makes investigation of the efficacy of GABA_B PAMs in AUD patients a feasible option.