Diminished catalepsy and dopamine metabolism distinguish aripiprazole from haloperidol or risperidone

Evaluating lumateperone for its use in treating depressive episodes associated with bipolar I or II disorder in adults

INTRODUCTION

Lumateperone is a novel antipsychotic medication that has recently received approval by the United States Food and Drug Administration for treatment of major depressive episodes of type I and II bipolar disorder. It is approved for use as monotherapy or as an adjunctive treatment to lithium or valproic acid.

AREAS COVERED

Clinical trials performed with lumateperone for bipolar disorder were reviewed. Additionally, pharmacodynamic actions of lumateperone are reviewed. Lumateperone is superior to placebo whether used alone or in combination with a mood stabilizer in patients with type I or type II bipolar disorder. It achieves this effect with minimal dopamine blockade-related side effects due to less than 50% dopamine D2 receptor occupancy. While the pharmacodynamic profile of lumateperone is unique, the mechanism of action in bipolar depression remains obscure.

EXPERT OPINION

Lumateperone is an antipsychotic with full antagonist effects at the post-synaptic D2, and partial agonist effects at the presynaptic D2. This unique profile allows for both antipsychotic and antidepressant effects at the same dose, which does not produce dopamine-related side effects. Consequently, lumateperone is exceptionally well tolerated compared to other antidepressant-acting antipsychotic agents. It is now the only agent approved as an adjunct to the mood stabilizer for bipolar II depression.

Sedative effect of Clozapine is a function of 5-HT2A and environmental novelty

Antipsychotic drugs are the mainstay in the treatment of schizophrenia and bipolar disorder. However, antipsychotics often exhibit sedation or activity suppression among many other side effects, and the factors that influence them remain poorly understood. We now show, using a 5-HT_2A knockout (Htr2a^-/-) mouse, that environmental circumstances can affect suppression of activity induced by the atypical antipsychotic- Clozapine. We observed that Htr2a^-/- mice were more resistant to Clozapine-induced suppression of activity (CISA) and this behaviour was dependent on the environment being 'novel'. In their 'home' environment, at identical doses the mice exhibited CISA. Interestingly, the effect of genotype and environmental novelty on CISA could not be extended to the other antipsychotics that were tested, i.e. Haloperidol and Risperidone. Haloperidol-induced activity suppression was independent of context and genotype. Whereas context affected Risperidone-induced activity suppression only in the Htr2a^+/+ mice. Furthermore, we observed that caffeine, a stimulant, elicited resistance to CISA similar to that seen in the 'novel' context. Our study establishes a previously unknown interaction between the environmental context, 5-HT_2A and CISA and emphasises the role of non-pharmacological factors such as environment on the effects of the drug, which seem antipsychotic-specific. Our findings should advance the understanding of the side effects of individual antipsychotics and the role of environment to overcome side effects such as sedation.

Dopamine Targeting Drugs for the Treatment of Schizophrenia: Past, Present and Future

Schizophrenia is a chronic and debilitating neuropsychiatric disorder affecting approximately 1% of the world's population. This disease is associated with considerable morbidity placing a major financial burden on society. Antipsychotics have been the mainstay of the pharmacological treatment of schizophrenia for decades. The traditional typical and atypical antipsychotics demonstrate clinical efficacy in treating positive symptoms, such as hallucinations and delusions, while are largely ineffective and may worsen negative symptoms, such as blunted affect and social withdrawal, as well as cognitive function. The inability to treat these latter symptoms may contribute to social function impairment associated with schizophrenia. The dysfunction of multiple neurotransmitter systems in schizophrenia suggests that drugs selectively targeting one neurotransmission pathway are unlikely to meet all the therapeutic needs of this heterogeneous disorder. Often, however, the unintentional engagement of multiple pharmacological targets or even the excessive engagement of intended pharmacological targets can lead to undesired consequences and poor tolerability. In this article, we will review marketed typical and atypical antipsychotics and new therapeutic agents targeting dopamine receptors and other neurotransmitters for the treatment of schizophrenia. Representative typical and atypical antipsychotic drugs and new investigational drug candidates will be systematically reviewed and compared by reviewing structure-activity relationships, pharmacokinetic properties, drug metabolism and safety, pharmacological properties, preclinical data in animal models, clinical outcomes and associated side effects.

Effects of aripiprazole on caffeine-induced hyperlocomotion and neural activation in the striatum

Aripiprazole is an antipsychotic that acts as a partial agonist at dopamine D2 receptors. In addition to its antipsychotic activity, this compound blocks the effects of some psychostimulant drugs. It has not been verified, however, if aripiprazole interferes with the effects of caffeine. Hence, this study tested the hypothesis that aripiprazole prevents caffeine-induced hyperlocomotion and investigated the effects of these drugs on neural activity in the striatum. Male Swiss mice received injections of vehicle or antipsychotic drugs followed by vehicle or caffeine. Locomotion was analyzed in a circular arena and c-Fos protein expression was quantified in the dorsolateral, dorsomedial, and ventrolateral striatum, and in the core and shell regions of nucleus accumbens. Aripiprazole (0.1, 1, and 10 mg/kg) prevented caffeine (10 mg/kg)-induced hyperlocomotion at doses that do not change basal locomotion. Haloperidol (0.01, 0.03, and 0.1 mg/kg) also decreased caffeine-induced hyperlocomotion at all doses, although at the two higher doses, this compound reduced basal locomotion. Immunohistochemistry analysis showed that aripiprazole increases c-Fos protein expression in all regions studied, whereas caffeine did not alter c-Fos protein expression. Combined treatment of aripiprazole and caffeine resulted in a decrease in the number of c-Fos positive cells as compared to the group receiving aripiprazole alone. In conclusion, aripiprazole prevents caffeine-induced hyperlocomotion and increases neural activation in the striatum. This latter effect is reduced by subsequent administration of caffeine. These results advance our understanding on the pharmacological profile of aripiprazole.

Functional profile of a novel modulator of serotonin, dopamine, and glutamate neurotransmission

RATIONALE

Schizophrenia remains among the most prevalent neuropsychiatric disorders, and current treatment options are accompanied by unwanted side effects. New treatments that better address core features of the disease with minimal side effects are needed.

OBJECTIVES

As a new therapeutic approach, 1-(4-fluoro-phenyl)-4-((6bR, 10aS)-3-methyl-2,3,6b,9,10,10a-hexahydro-1H,7H-pyrido[3',4':4,5]pyrrolo[1,2,3-de]quinoxalin-8-yl)-butan-1-one (ITI-007) is currently in human clinical trials for the treatment of schizophrenia. Here, we characterize the preclinical functional activity of ITI-007.

RESULTS

ITI-007 is a potent 5-HT2A receptor ligand (K i  = 0.5 nM) with strong affinity for dopamine (DA) D2 receptors (K i  = 32 nM) and the serotonin transporter (SERT) (K i  = 62 nM) but negligible binding to receptors (e.g., H1 histaminergic, 5-HT2C, and muscarinic) associated with cognitive and metabolic side effects of antipsychotic drugs. In vivo it is a 5-HT2A antagonist, blocking (±)-2,5-dimethoxy-4-iodoamphetamine hydrochloride (DOI)-induced headtwitch in mice with an inhibitory dose 50 (ID50) = 0.09 mg/kg, per oral (p.o.), and has dual properties at D2 receptors, acting as a postsynaptic D2 receptor antagonist to block D-amphetamine hydrochloride (D-AMPH) hyperlocomotion (ID50 = 0.95 mg/kg, p.o.), yet acting as a partial agonist at presynaptic striatal D2 receptors in assays measuring striatal DA neurotransmission. Further, in microdialysis studies, this compound significantly and preferentially enhances mesocortical DA release. At doses relevant for antipsychotic activity in rodents, ITI-007 has no demonstrable cataleptogenic activity. ITI-007 indirectly modulates glutamatergic neurotransmission by increasing phosphorylation of GluN2B-type N-methyl-D-aspartate (NMDA) receptors and preferentially increases phosphorylation of glycogen synthase kinase 3β (GSK-3β) in mesolimbic/mesocortical dopamine systems.

CONCLUSION

The combination of in vitro and in vivo activities of this compound support its development for the treatment of schizophrenia and other psychiatric and neurologic disorders.

Haloperidol treatment downregulates DCC expression in the ventral tegmental area

A core feature in the pathophysiology of schizophrenia is abnormal development and function of mesocorticolimbic dopamine (DA) circuitry. We have previously shown that variations in the function of the netrin-1 receptor, deleted in colorectal cancer (DCC), result in changes to the development, organization and ongoing plasticity of DA circuitry. In rodents, repeated exposure to the indirect DA-agonist, amphetamine upregulates DCC expression in the ventral tegmental area (VTA), but not in DA terminal regions. This elevation in DCC expression is associated with increased vulnerability to developing and maintaining sensitized mesolimbic DA function. Antipsychotic medications remain the best treatment option for managing the symptoms in schizophrenia. The peak effects of these medications are gradual, suggesting that a therapeutic component of antipsychotic treatment involves structural reorganization. Here we assessed whether repeated exposure to typical and atypical antipsychotics could also regulate DCC. Adult mice were orally administered haloperidol, clozapine, or risperidone via their drinking water for 4 weeks. Levels of DCC were measured by Western blot analysis of tissue punches of the VTA, medial prefrontal cortex, nucleus accumbens, and dorsal striatum. Haloperidol decreased DCC levels by approximately 50% in the VTA, but not in DA targets. Furthermore, haloperidol did not alter UNC-5 homologue levels, another family of netrin-1 receptors, confirming that its effects target DCC-mediated netrin-1 signaling specifically. The atypical antipsychotics did not alter DCC expression. These results suggest that typical antipsychotics induce selective functional reorganization in the VTA via DCC-mediated netrin-1 signaling.

Aripiprazole: pharmacology, efficacy, safety and tolerability

Aripiprazole is a recently released antipsychotic medication which differs from other atypical antipsychotic agents by its partial agonist activity at postsynaptic D2 receptors. It is administered orally and is distinguished by a long elimination phase half-life relative to other antipsychotic medications. Randomized studies have demonstrated the efficacy of aripiprazole relative to placebo in the treatment of acute relapse of schizophrenia and schizoaffective disorder, maintenance treatment of schizophrenia, and treatment of acute bipolar mania. Aripiprazole is generally well tolerated relative to other antipsychotic medications, although commonly reported side effects include worsening extrapyramidal symptoms and motoric activation similar to akathisia. Further studies and postmarketing data will be helpful in providing additional information about the comparative safety, efficacy and tolerability of aripiprazole.

Peptide hormone ghrelin enhances neuronal excitability by inhibition of Kv7/KCNQ channels

The gut-derived orexigenic peptide hormone ghrelin enhances neuronal firing in the substantia nigra pars compacta, where dopaminergic neurons modulate the function of the nigrostriatal system for motor coordination. Here we describe a novel mechanism by which ghrelin enhances firing of nigral dopaminergic neurons by inhibiting voltage-gated potassium Kv7/KCNQ/M-channels through its receptor GHS-R1a and activation of the PLC-PKC pathway. Brain slice recordings of substantia nigra pars compacta neurons reveal that ghrelin inhibits native Kv7/KCNQ/M-currents. This effect is abolished by selective inhibitors of GHS-R1a, PLC and PKC. Transgenic suppression of native Kv7/KCNQ/M-channels in mice or channel blockade with XE991 abolishes ghrelin-induced hyperexcitability. In vivo, intracerebroventricular ghrelin administration causes increased dopamine release and turnover in the striatum. Microinjection of ghrelin or XE991 into substantia nigra pars compacta results in contralateral dystonic posturing, and attenuation of catalepsy elicited by systemic administration of the D2 receptor antagonist haloperidol. Our findings indicate that the ghrelin/KCNQ signalling is likely a common pathway utilized by the nervous system.

Effects of aripiprazole, an atypical antipsychotic, on the motor alterations induced by acute ethanol administration in mice

Aripiprazole is an antipsychotic that acts as a partial agonist at dopamine receptors. As the effects of most drugs of abuse converge to enhance dopamine-mediated neurotransmission, the present study was designed to test the hypothesis that aripiprazole would inhibit the acute effects of ethanol, a widely abused substance. Male Swiss mice received acute injections and were evaluated for motor activity in three distinct tests. In the open field, ethanol (1.5, 2.5 and 3.5 g/kg) induced an increase in locomotion in a U-shaped dose-related fashion, whereas aripiprazole (0.1, 1 and 10 mg/kg) did not affect this parameter. All the doses of the antipsychotic were able to prevent the stimulant effects of 2.5 g/kg of ethanol. In the rotarod test, ethanol (2.5 and 3.5 g/kg) reduced the latency to fall from the apparatus, an effect also observed with the higher dose of aripiprazole. Contrary to what was observed in the open field, this antipsychotic did not interfere with the effects of ethanol in motor balance. Finally, we tested animals in the wire hang test, in which ethanol, but not aripiprazole, reduced latency to fall at all doses. In this test, aripiprazole did not change ethanol effects. The present data lead to the conclusion that aripiprazole prevents the stimulant effects of ethanol on locomotion, without interfering with the motor impairment induced by this drug.

Aripiprazole: a review of its use in the management of schizophrenia in adults

Oral aripiprazole (Abilify®) is an atypical antipsychotic agent that is approved worldwide for use in adult patients with schizophrenia. It is a quinolinone derivative that has a unique receptor binding profile as it exhibits both partial agonist activity at dopamine D(2) receptors and serotonin 5-HT(1A) receptors and antagonist activity at 5-HT(2A) receptors. In several well designed, randomized, clinical trials of 4-6 weeks duration, aripiprazole provided symptomatic control for patients with acute, relapsing schizophrenia or schizoaffective disorder. Furthermore, following 26 weeks' treatment, the time to relapse was significantly longer for patients with chronic, stabilized schizophrenia receiving aripiprazole compared with those receiving placebo. Using a variety of efficacy outcomes, aripiprazole showed a mixed response when evaluated against other antipsychotic agents in randomized clinical trials. Longer-term data showed that improvements in remission rates and response rates favoured aripiprazole over haloperidol, although, the time to failure to maintain a response was not significantly different between the treatment arms. On the other hand, improvements in positive and negative symptom scores mostly favoured olanzapine over aripiprazole, although, the time to all-cause treatment discontinuation was not significantly different between the two treatments. Several open-label, switching trials showed that aripiprazole provided continued control of symptoms in patients with schizophrenia or schizoaffective disorder. Using a variety of efficacy outcomes or quality-of-life scores, longer-term treatment generally favoured patients switched to receive aripiprazole compared with standard-of-care oral antipsychotics. Aripiprazole was generally well tolerated in patients with schizophrenia. In particular, its use seems to be associated with a lower incidence of extrapyramidal symptoms than haloperidol and fewer weight-gain issues than olanzapine. Aripiprazole also showed a favourable cardiovascular tolerability profile and its use was associated with a reduced risk of metabolic syndrome than placebo or olanzapine. As a consequence, aripiprazole may provide a more cost-effective treatment option compared with other atypical antipsychotics. In conclusion, oral aripiprazole provides an effective and well tolerated treatment alternative for the acute and long-term management of patients with schizophrenia.

Pharmacological blockade of dopamine D2 receptors by aripiprazole is not associated with striatal sensitization

The partial agonist profile of novel antipsychotics such as aripiprazole has hardly been demonstrated in biochemical assays on animal tissues. As it is established that responses induced by dopamine D₂ receptor agonists are increased in models of dopaminergic sensitization, this paradigm was used in order to facilitate the detection of the partial agonist properties of aripiprazole. At variance with all other partial and full agonists tested, the partial agonist properties of aripiprazole were not revealed in guanosine 5′-O-(γ-[³⁵S]thiotriphosphate ([³⁵S]GTPγS) binding assays on striatal membranes from haloperidol-treated rats. Hence,aripiprazole behaved as an antagonist, efficiently inhibiting the functional response to dopamine. Similarly, in behavioural assays, aripiprazole dose-dependently inhibited the stereotypies elicited by apomorphine. However, at variance with haloperidol, repeated administrations of aripiprazole(3 weeks) at the doses of 10 and 30 mg/kg did not induce any up-regulation or hyperfunctionality of the dopamine D₂ receptors in the striatum. These data highlight the putative involvement of other pharmacological targets for aripiprazole that would support in the prevention of secondary effects commonly associated with the blockade of striatal dopamine D₂ receptors. Hence, in additional experiments, aripiprazole was found to efficiently promote [³⁵S]GTPγS binding in hippocampal membranes through the activation of 5-HT(₁A) receptors. Further experiments investigating the second messenger cascades should be performed so as to establish the functional properties of aripiprazole and understand the mechanism underlying the prevention of dopamine receptor regulation in spite of the observed antagonism.

Aripiprazole: a review of its use in the treatment of irritability associated with autistic disorder patients aged 6-17

A systematic review and meta-analysis were performed examining the efficacy of aripiprazole for the treatment of irritability associated with autistic disorder in children and adolescents. Aripiprazole was found to be more effective in reducing irritability compared with placebo at 8 weeks, SMD −0.64 [−0.90 to −0.39, P < 0.00001] as determined by the Aberrant Behaviour Checklist irritability subscale (ABC-I). Pooled data from two eight week trials show that sedation is the most commonly reported adverse event. Statistically significant weight gain was also associated with aripiprazole, but there was a decrease in serum prolactin. Most adverse effects were deemed to be mild to moderate in severity. Four open trials and three case series all show support for aripiprazole in reducing the behavioural symptoms of autism. Long-term studies are required to determine the efficacy and safety of aripiprazole in autistic disorder in children.

Effects of repeated and acute aripiprazole or haloperidol treatment on dopamine synthesis in the dorsal striatum of young rats: comparison to adult rats

The purpose of the present study was to determine whether repeated treatment with the D2 partial agonist aripiprazole or the D2 antagonist haloperidol alters dopamine (DA) synthesis characteristics in the dorsal striatum of young rats. To this end, rats received a daily pretreatment regimen of aripiprazole or haloperidol on postnatal days (PD) 10-20 and were tested 24 or 72 h later after an acute injection of vehicle, aripiprazole, haloperidol, or quinpirole (a D2 agonist). For comparison purposes, adult rats were pretreated with an 11-day regimen of saline or haloperidol on PD 70-80 and DA synthesis was measured after acute drug treatment on PD 83. Dorsal striatal DA synthesis was determined by measuring L-dihydroxyphenylalanine accumulation after NSD-1015 treatment. In a separate experiment, the ability of repeated drug treatment to up-regulate dorsal striatal D2 receptors was assessed in young and adult rats 72 h after drug discontinuation. The major findings of this study were that: (a) acute treatment with haloperidol and aripiprazole increased DA synthesis while quinpirole reduced it; (b) pretreatment with haloperidol and aripiprazole blunted the synthesis-modulating effects of acutely administered dopaminergic drugs; and (c) DA synthesis of young and adult rats was affected in a qualitatively similar manner by DA agonist, antagonist, and partial agonist drugs. In conclusion, results from the present study suggest that synthesis-modulating autoreceptors in the dorsal striatum are functionally mature by the end of the preweanling period and DA synthesis declines to near basal levels during the course of repeated aripiprazole treatment.

A subpopulation of neuronal M4 muscarinic acetylcholine receptors plays a critical role in modulating dopamine-dependent behaviors

Acetylcholine (ACh) regulates many key functions of the CNS by activating cell surface receptors referred to as muscarinic ACh receptors (M(1)-M(5) mAChRs). Like other mAChR subtypes, the M(4) mAChR is widely expressed in different regions of the forebrain. Interestingly, M(4) mAChRs are coexpressed with D(1) dopamine receptors in a specific subset of striatal projection neurons. To investigate the physiological relevance of this M(4) mAChR subpopulation in modulating dopamine-dependent behaviors, we used Cre/loxP technology to generate mutant mice that lack M(4) mAChRs only in D(1) dopamine receptor-expressing cells. The newly generated mutant mice displayed several striking behavioral phenotypes, including enhanced hyperlocomotor activity and increased behavioral sensitization following treatment with psychostimulants. These behavioral changes were accompanied by a lack of muscarinic inhibition of D(1) dopamine receptor-mediated cAMP stimulation in the striatum and an increase in dopamine efflux in the nucleus accumbens. These novel findings demonstrate that a distinct subpopulation of neuronal M(4) mAChRs plays a critical role in modulating several important dopamine-dependent behaviors. Since enhanced central dopaminergic neurotransmission is a hallmark of several severe disorders of the CNS, including schizophrenia and drug addiction, our findings have substantial clinical relevance.

Rotarod impairment: catalepsy-like screening test for antipsychotic side effects

Extrapyramidal motoric symptoms are casual side effects under antipsychotic medication. New generation antipsychotics are expected to have a reduced risk due to different receptor affinities. Here, haloperidol and the new generation antipsychotics, risperidone, amisulpride, and aripiprazole, were examined with both catalepsy test and rotarod performance test to screen for their usability in mice. Mice treated with haloperidol, risperidone, and aripiprazole showed dose and time-dependent impairment. Amisulpride-treated mice showed no signs of catalepsy. Catalepsy test and rotarod performance test were useful methods to detect side effects of both generation antipsychotics. Catalepsy test provided more specificity whereas the rotarod test provided higher degree of sensitivity to motor impairment including catalepsy.

Cariprazine (RGH-188), a dopamine D(3) receptor-preferring, D(3)/D(2) dopamine receptor antagonist-partial agonist antipsychotic candidate: in vitro and neurochemical profile

Cariprazine {RGH-188; trans-N-[4-[2-[4-(2,3-dichlorophenyl)piperazin-1-yl]ethyl]cyclohexyl]-N',N'-dimethylurea hydrochloride}, a novel candidate antipsychotic, demonstrated approximately 10-fold higher affinity for human D(3) versus human D(2L) and human D(2S) receptors (pKi 10.07, 9.16, and 9.31, respectively). It displayed high affinity at human serotonin (5-HT) type 2B receptors (pK(i) 9.24) with pure antagonism. Cariprazine had lower affinity at human and rat hippocampal 5-HT(1A) receptors (pK(i) 8.59 and 8.34, respectively) and demonstrated low intrinsic efficacy. Cariprazine displayed low affinity at human 5-HT(2A) receptors (pK(i) 7.73). Moderate or low affinity for histamine H(1) and 5-HT(2C) receptors (pK(i) 7.63 and 6.87, respectively) suggest cariprazine's reduced propensity for adverse events related to these receptors. Cariprazine demonstrated different functional profiles at dopamine receptors depending on the assay system. It displayed D(2) and D(3) antagonism in [(35)S]GTPgammaS binding assays, but stimulated inositol phosphate (IP) production (pEC(50) 8.50, E(max) 30%) and antagonized (+/-)-quinpirole-induced IP accumulation (pK(b) 9.22) in murine cells expressing human D(2L) receptors. It had partial agonist activity (pEC(50) 8.58, E(max) 71%) by inhibiting cAMP accumulation in Chinese hamster ovary cells expressing human D(3) receptors and potently antagonized R(+)-2-dipropylamino-7-hydroxy-1,2,3,4-tetrahydronaphtalene HBr (7-OH-DPAT)-induced suppression of cAMP formation (pK(b) 9.57). In these functional assays, cariprazine showed similar (D(2)) or higher (D(3)) antagonist-partial agonist affinity and greater (3- to 10-fold) D(3) versus D(2) selectivity compared with aripiprazole. In in vivo turnover and biosynthesis experiments, cariprazine demonstrated D(2)-related partial agonist and antagonist properties, depending on actual dopaminergic tone. The antagonist-partial agonist properties of cariprazine at D(3) and D(2) receptors, with very high and preferential affinity to D(3) receptors, make it a candidate antipsychotic with a unique pharmacological profile among known antipsychotics.

The glycine transporter-1 inhibitor SSR103800 displays a selective and specific antipsychotic-like profile in normal and transgenic mice

Schizophrenia has been initially associated with dysfunction in dopamine neurotransmission. However, the observation that antagonists of the glutamate N-methyl-D-aspartate (NMDA) receptor produce schizophrenic-like symptoms in humans has led to the idea of a dysfunctioning of the glutamatergic system via its NMDA receptor. As a result, there is a growing interest in the development of pharmacological agents with potential antipsychotic properties that enhance the activity of the glutamatergic system via a modulation of the NMDA receptor. Among them are glycine transporter-1 (GlyT1) inhibitors such as SSR103800, which indirectly enhance NMDA receptor function by increasing the glycine (a co-agonist for the NMDA receptor) levels in the synapse. This study aimed at investigating the potential antipsychotic-like properties of SSR103800, with a particular focus on models of hyperactivity, involving either drug challenge (ie, amphetamine and MK-801) or transgenic mice (ie, NMDA Nr1(neo-/-) and DAT(-/-)). Results showed that SSR103800 (10-30 mg/kg p.o.) blocked hyperactivity induced by the non-competitive NMDA receptor antagonist, MK-801 and partially reversed spontaneous hyperactivity of NMDA Nr1(neo-/-) mice. In contrast, SSR103800 failed to affect hyperactivity induced by amphetamine or naturally observed in dopamine transporter (DAT(-/-)) knockout mice (10-30 mg/kg p.o.). Importantly, both classical (haloperidol) and atypical (olanzapine, clozapine and aripiprazole) antipsychotics were effective in all these models of hyperactivity. However, unlike these latter, SSR103800 did not produce catalepsy (retention on the bar test) up to 30 mg/kg p.o. Together these findings show that the GlyT1 inhibitor, SSR103800, produces antipsychotic-like effects, which differ from those observed with compounds primarily targeting the dopaminergic system, and has a reduced side-effect potential as compared with these latter drugs.

Effects of acute and chronic aripiprazole treatment on choice between cocaine self-administration and food under a concurrent schedule of reinforcement in rats

RATIONALE

Dopamine D2-like partial agonists such as aripiprazole have received some attention as potential pharmacotherapies for the treatment of psychostimulant addiction. However, the preclinical evaluations so far have focused on acute effects of aripiprazole.

OBJECTIVES

We tested the hypothesis that aripiprazole, both as acute and as chronic treatment, would preferentially decrease cocaine self-administration while sparing behavior maintained by a natural reinforcer, resulting in a shift in the allocation of behavior from cocaine-taking towards the alternative reinforcer.

MATERIALS AND METHODS

Rats were trained to self-administer intravenous cocaine in a concurrent choice procedure, with a palatable food as the competing reinforcer, under a fixed ratio (FR) 1 FR 5 chain schedule. Aripiprazole was then administered as continuous infusion by osmotic minipumps for 5 days, during which performance in the choice procedure was assessed daily.

RESULTS

An intermediate dose of aripiprazole decreased cocaine self-administration and shifted the cocaine choice curve to the right as an acute treatment. However, as a chronic treatment, aripiprazole failed to decrease cocaine self-administration or cocaine choice, despite a dose-dependent decrease in overall response rates and food-maintained behavior.

CONCLUSIONS

Our results confirm and extend earlier findings and indicate that acute administration of aripiprazole can decrease cocaine self-administration. However, based on the present data, chronic treatment with aripiprazole does not show much promise as a potential pharmacotherapy for cocaine addiction. Both acute and chronic treatment data are in agreement with published clinical findings, suggesting that the concurrent choice procedure in rats has predictive validity of efficacy in humans.

Aripiprazole blocks acute self-administration of cocaine and is not self-administered in mice

RATIONALE

The novel antipsychotic aripiprazole in use for treatment of schizophrenia is a partial agonist at dopamine D2 receptors with actions at a variety of other receptors as well. Cocaine is believed to exert an important part of its rewarding effect by increasing extracellular levels of dopamine that subsequently act at dopamine D2 receptors.

OBJECTIVES

As a partial agonist, aripiprazole may antagonize effects at D2 receptors and we accordingly tested whether aripiprazole could antagonize self-administration of cocaine. Because D2-like receptor agonists are self-administered, a D2 receptor partial agonist like aripiprazole might itself be reinforcing. Thus, we also assessed whether mice would acquire self-administration of aripiprazole.

MATERIALS AND METHODS

A single session, mouse self-administration procedure was used.

RESULTS

Oral pretreatment with aripiprazole dose-dependently decreased cocaine self-administration under a fixed ratio 1 schedule at the peak cocaine dose (0.03 mg/kg/infusion), reaching significance at 0.2 and 0.4 mg/kg of aripiprazole. Using 0.4 mg/kg, aripiprazole decreased rates of cocaine self-administration without shifting the peak of the dose-response function. There was no effect of aripiprazole per se, suggesting that its inhibitory action was due to effects on cocaine self-administration rather than non-specific motor effects. Aripiprazole was not found to be self-administered in the tested dose range (0.0003-0.3 mg/kg/infusion). The three highest doses (0.03, 0.1, and 0.3 mg/kg/infusion) even caused significant decreases in nose-poking activity, possibly due to extrapyramidal side effects.

CONCLUSIONS

These data are consistent with a potential role for aripiprazole in treatment of cocaine addiction without abuse potential per se.

Partial agonist actions of aripiprazole and the candidate antipsychotics S33592, bifeprunox, N-desmethylclozapine and preclamol at dopamine D2Lreceptors are modified by co-transfection of D3receptors: potential role of heterodimer formation

Aripiprazole and the candidate antipsychotics, S33592, bifeprunox, N-desmethylclozapine (NDMC) and preclamol, are partial agonists at D(2) receptors. Herein, we examined their actions at D(2L) and D(3) receptors expressed separately or together in COS-7 cells. In D(2L) receptor-expressing cells co-transfected with (D(3) receptor-insensitive) chimeric adenylate cyclase-V/VI, drugs reduced forskolin-stimulated cAMP production by approximately 20% versus quinpirole (48%). Further, quinpirole-induced inhibition was blunted by aripiprazole and S33592, confirming partial agonist properties. In cells co-transfected with equal amounts of D(2L)and D(3) receptors (1 : 1), efficacies of aripiprazole and S33592 were attenuated. Further, in cells co-transfected with D(2L) and an excess of D(3) receptors (1 : 3), aripiprazole and S33592 were completely inactive, and they abolished the actions of quinpirole. Likewise, bifeprunox, NDMC and preclamol lost agonist properties in cells co-transfected with D(2L)and D(3) receptors. Accordingly, at split D(2trunk)/D(3tail) and D(3trunk)/D(2tail) chimeras, agonist actions of quinpirole were blocked by aripiprazole and S33592 that, like bifeprunox, NDMC and preclamol, were inactive alone. Conversely, when a 12 amino acid sequence in the third intracellular loop of D(3) receptors was replaced by the homologous sequence of D(2L) receptors, aripiprazole, S33592, bifeprunox, NDMC and preclamol inhibited cAMP formation by approximately 20% versus quinpirole (42%). Moreover, at D(2L) receptor-expressing cells co-transfected with modified D(3i3(D2)) receptors, drugs behaved as partial agonists. To summarize, low efficacy agonist actions of aripiprazole, S33592, bifeprunox, NDMC and preclamol at D(2L) receptors are abrogated upon co-expression of D(3) receptors, probably due to physical association and weakened coupling efficacy. These findings have implications for the functional profiles of antipsychotics.

The Dopamine Stabilizers (S)-(-)-(3-Methanesulfonyl-phenyl)-1-propyl-piperidine [(-)-OSU6162] and 4-(3-Methanesulfonylphenyl)-1-propyl-piperidine (ACR16) Show High in Vivo D2Receptor Occupancy, Antipsychotic-Like Efficacy, and Low Potential for Motor Side Effects in the Rat

"Dopamine stabilizers" are a new class of compounds that have the ability to reverse both hypo- as well as hyperdopaminergia in vivo. This class, exemplified by the phenylpiperidines (S)-(-)-3-(3-methanesulfonyl-phenyl)-1-propyl-piperidine [(-)-OSU6162] and 4-(3-methanesulfonyl-phenyl)-1-propyl)-piperidine [ACR16] although lacking high in vitro binding affinity for dopamine D2 receptor [(-)-OSU6162, Ki = 447 nM; ACR16, Ki > 1 microM], shows functional actions, suggestive of their interaction. Hence, we evaluated in vivo D2 occupancy of these agents in rats and correlated it to observed effects in a series of behavioral, neurochemical, and endocrine models relevant to the dopamine system and antipsychotic effect. Both (-)-OSU6162 and ACR16 showed robust dose-dependent striatal D2 occupancy with ED50 values of 5.27 and 18.99 mg/kg s.c., respectively, and functional assays showed no partial agonism. Over an occupancy range of 37 to 87% (3-60 mg/kg) for (-)-OSU6162 and 35 to 74% (10-60 mg/kg) for ACR16, we observed both inhibitory (amphetamine-induced locomotor activity) and stimulatory effects (in habituated rats). Haloperidol, over a similar occupancy range (33-78%), potently inhibited psychostimulant activity and induced catalepsy, but it failed to activate habituated animals. In the conditioned avoidance response assay, ACR16 was clearly more efficacious than (-)-OSU6162. In addition, both these compounds demonstrated significant preferential Fos induction in the nucleus accumbens compared with the dorsolateral striatum, a strong predictor of atypical antipsychotic efficacy. The results suggest that dopamine stabilizers exhibit locomotor stabilizing as well as antipsychotic-like effects, with low motor side effect liability, in a dose range that corresponds to high D2 in vivo occupancy.

Effects of novel antipsychotics, amisulpiride and aripiprazole, on maternal behavior in rats

RATIONALE

Rat maternal behavior, which entails complex motivational and social factors, is disrupted by the currently available typical and atypical antipsychotics. It is thought that this disruption reflects a side effect of antipsychotics, modeling the neuroleptic-induced negative or deficit state. Amisulpiride and aripiprazole are new atypical antipsychotics with mechanisms of action distinct from the current typical and atypical antipsychotics. The effects of these drugs on maternal behavior have not been explored.

OBJECTIVE

In the present study, we systematically examined the behavioral effects of amisulpiride and aripiprazole on maternal behavior in postpartum female rats.

METHODS

Various components of maternal behavior (pup retrieval, pup licking, nest building and pup nursing) were examined repeatedly over a period of 24 h after a single injection of three doses of amisulpiride (10, 30, and 100 mg/kg s.c.) and aripiprazole (3, 10, and 30 mg/kg).

RESULTS

Amisulpiride at the lower doses (10 and 30 mg/kg) enhanced pup licking, and only at the highest dose disrupted the active components of maternal behavior such as pup retrieval and nest building. Its effect was delayed in onset and prolonged as compared to other antipsychotics. Aripiprazole, even at the highest dose (30 mg/kg) did not impair pup retrieval or pup licking. However, it did disrupt nest building and led to enhanced pup nursing.

CONCLUSIONS

The unique effects of these two drugs may be due to their unique actions at the mesolimbic dopamine synapses. The sparing of the major components of maternal behavior by aripiprazole may be related to its partial agonist effects, whereas the enhancement of pup licking by amisulpiride may be related to its dose-dependent preferential effect on the presynaptic autoreceptors. The potential clinical implications of these findings are discussed.

Clozapine, ziprasidone and aripiprazole but not haloperidol protect against kainic acid-induced lesion of the striatum in mice, in vivo: Role of 5-HT1A receptor activation

Excessive activation of non-NMDA receptors, AMPA and kainate, contributes to neuronal degeneration in acute and progressive pathologies, possibly including schizophrenia. Because 5-HT(1A) receptor agonists have neuroprotective properties (e.g., against NMDA-induced neurotoxicity), we compared the effects of the antipsychotics, clozapine, ziprasidone and aripiprazole, that are partial agonists at 5-HT(1A) receptor, with those of haloperidol, which is devoid of 5-HT(1A) agonist properties, on kainic acid (KA)-induced striatal lesion volumes, in C57Bl/6N mice. The involvement of 5-HT(1A) receptors was determined by antagonist studies with WAY100635, and data were compared with those obtained using the potent and high efficacy 5-HT(1A) receptor agonist, F13714. Intra-striatal KA lesioning and measurement of lesion volumes using cresyl violet staining were carried out at 48 h after surgery. F13714, antipsychotics or vehicle were administered ip twice, 30 min before and 3 1/2 h after KA injection. WAY100635 (0.63 mg/kg) or vehicle were given sc 30 min before each drug injection. Clozapine (2 x 10 mg/kg), ziprasidone (2 x 20 mg/kg) and aripiprazole (2 x 10 mg/kg) decreased lesion volume by 61%, 59% and 73%, respectively. WAY100635 antagonized the effect of ziprasidone and of aripiprazole but only slightly attenuated that of clozapine. In contrast, haloperidol (2 x 0.16 mg/kg) did not affect KA-induced lesion volume. F13714 dose-dependently decreased lesion volume. The 61% decrease of lesion volume obtained with F13714 (2 x 0.63 mg/kg) was antagonized by WAY100635. WAY100635 alone did not affect lesion volume. These results show that 5-HT(1A) receptor activation protects against KA-induced striatal lesions and indicate that some atypical antipsychotic agents with 5-HT(1A) agonist properties may protect against excitotoxic injury, in vivo.

Novel antipsychotics in schizophrenia

Several atypical antipsychotics have become available for the treatment of schizophrenia that are at least as effective as conventional treatment and with fewer extrapyramidal side effects. Their presumed mechanisms of therapeutic action vary and are no longer limited to dopamine D2 receptor antagonism. Numerous novel drugs are in development, with a variety of receptor affinities and other supposed therapeutic effects. This article will review current developments in drug discovery alongside contemporary evidence for potential substrates and mechanisms of antipsychotic action. Despite many promising developments there is no ideal antipsychotic to date. Progress in drug treatment for schizophrenia is confronted by several areas of difficulty which, barring serendipity, must be resolved before real advances can be anticipated.

Aripiprazole: a novel atypical antipsychotic drug with a uniquely robust pharmacology

Aripiprazole (Abilify) is an atypical antipsychotic drug that has been recently introduced for clinical use in the treatment of schizophrenia. Aripiprazole has a unique pharmacologic profile that includes partial agonism at several G-protein coupled receptors (GPCRs) [especially dopamine (D2) and 5-HT1A] and antagonistic action at others (especially 5-HT2A). Clinical trials indicate that aripiprazole is effective in treating the positive and negative symptoms of schizophrenia. In short-term studies rapid onset of action (within one week) has been demonstrated. Preliminary data indicate that aripiprazole may also be effective in the treatment of manic symptoms of bipolar disorder. At recommended doses, aripiprazole appears to be safe and well tolerated in most adult patients with schizophrenia and schizoaffective disorder. There is only limited information available on the use of aripiprazole in children and adolescents, and pilot data suggest that a revised dosing strategy, based on weight, is indicated in this population. In the long-term studies, the use of aripiprazole was associated with continued efficacy, good compliance and increased time-to-relapse. Aripiprazole represents the first functionally selective atypical antipsychotic drug.

Aripiprazole: a review of its use in schizophrenia and schizoaffective disorder

Aripiprazole, a quinolinone derivative, is an atypical antipsychotic drug indicated for the treatment of adult patients with schizophrenia. Aripiprazole 10 or 15 mg once daily is effective and well tolerated in patients with schizophrenia or schizoaffective disorder. Although aripiprazole has only been directly compared with haloperidol and olanzapine in treatment-responsive patients to date, current data generally indicate that aripiprazole has a beneficial profile in terms of a low potential for bodyweight gain. Dosage titration is not necessary and the drug is effective in the first few weeks of treatment. Head-to-head comparative trials with atypical antipsychotic agents are required, as are long-term (> or =1 year) studies, to fully define the position of aripiprazole in relation to other antipsychotic drugs. Aripiprazole is a valuable new therapeutic option in the management of patients with schizophrenia. PHARMACOLOGICAL PROPERTIES: Aripiprazole is a quinolinone derivative with a high affinity for dopamine D2 and D3 receptors, and serotonin 5-HT1A, 5-HT2A and 5-HT2B receptors. The mechanism of action of aripiprazole is not yet known, but evidence suggests that its efficacy in the treatment of the positive and negative symptoms of schizophrenia and its lower propensity for extrapyramidal symptoms (EPS) may be attributable to aripiprazole's partial agonist activity at dopamine D2 receptors. At serotonin 5-HT1A receptors, in vitro studies have shown that aripiprazole acts as a partial agonist whereas at serotonin 5-HT2A receptors aripiprazole is an antagonist. The main active metabolite, dehydro-aripiprazole, has affinity for dopamine D2 receptors and thus has some pharmacological activity similar to that of the parent compound. Aripiprazole is rapidly absorbed after oral administration. The mean time to peak plasma concentration is 3 hours following multiple-dose administration of aripiprazole 10 or 15 mg and the absolute oral bioavailability of the drug is 87%. Steady-state plasma drug concentrations are achieved by 14 days; however, the drug appears to accumulate over this period, since mean peak plasma concentration and mean area under the plasma concentration-time curve values of aripiprazole 10 or 15 mg/day are 4-fold greater on day 14 than on day 1. This accumulation may be expected, since the mean elimination half-life of a single dose of aripiprazole is about 75 hours. Aripiprazole has extensive extravascular distribution and more than 99% of aripiprazole and dehydro-aripiprazole (the main active metabolite of aripiprazole) is bound to plasma protein. Elimination of the drug is primarily hepatic; the cytochrome P450 (CYP) 3A4 and CYP2D6 enzyme systems transform aripiprazole to dehydro-aripiprazole, with the latter enzyme system subject to genetic polymorphism. Thus, dosage adjustment of aripiprazole is necessary when it is coadministered with CYP3A4 and CYP2D6 inhibitors (since aripiprazole concentration is increased) and with inducers of CYP3A4 (since aripiprazole concentration is decreased). THERAPEUTIC EFFICACY: The efficacy of aripiprazole has been demonstrated in patients with schizophrenia or schizoaffective disorder. In general, significant reductions from baseline in mean Positive and Negative Syndrome Scale total, positive and negative symptom scores, and Clinical Global Impression Severity of Illness scores were observed in patients with acute relapse of chronic schizophrenia or schizoaffective disorder receiving recommended (10 or 15 mg/day) or higher-than-recommended (20 or 30 mg/day) dosages of aripiprazole versus those receiving placebo in three well controlled, short-term trials. No additional therapeutic benefit was observed at the higher-than-recommended dosages. The drug is effective as early as the first or second week of treatment. The efficacy of aripiprazole was maintained for up to 52 weeks. The drug was significantly more effective than placebo in preventing relapse in patients with stable chronic schizophrenia in a 26-week, randomised trial. In a 52-week trial in patients with acute relapse of schizophrenia, the percentage of responders maintaining a response at study end was 77% of aripiprazole versus 73% of haloperidol recipients. Aripiprazole may improve cognitive function. In a nonblind, 26-week trial, patients with chronic schizophrenia receiving aripiprazole 30 mg/day experienced similar (general cognitive function) or better (verbal learning) changes from baseline in the neurocognitive parameters evaluated compared with recipients of olanzapine 10-15 mg/day.

TOLERABILITY

Aripiprazole 10-30 mg/day was generally well tolerated. The tolerability profile of aripiprazole was broadly similar to that observed with placebo in a meta-analysis of short-term trials in patients with acute relapse of schizophrenia or schizoaffective disorder and in a 26-week trial in patients with chronic stable schizophrenia. The most frequent treatment-emergent adverse events included insomnia and anxiety, and additionally, headache and agitation (in short-term trials) or akathisia and psychosis (in a 52-week trial). In general, the drug was associated with a placebo-level incidence of EPS and EPS-related adverse events. Significantly fewer aripiprazole recipients experienced EPS-related adverse events than haloperidol recipients in a 52-week trial. Changes in severity of EPS were minimal and usually no different from those observed with placebo. Moreover, there was less severe EPS in the aripiprazole group than the haloperidol group in a long-term trial. Treatment-emergent tardive dyskinesia was reported in only 0.2% of patients receiving aripiprazole (short-term trials), an incidence similar to that seen in placebo recipients (0.2%). Aripiprazole has a low propensity to cause clinically significant bodyweight gain, hyperprolactinaemia or corrected QT interval prolongation in patients with schizophrenia or schizoaffective disorder. In addition, there were no clinically relevant differences in mean changes from baseline in measures of diabetes and dyslipidaemia between the aripiprazole or placebo groups in a 26-week, placebo-controlled trial.