Central D2-dopamine receptor occupancy in relation to antipsychotic drug effects: a double-blind PET study of schizophrenic patients

Differential effects of clozapine and haloperidol on interval timing in the supraseconds range

The effects of clozapine (0.6, 1.2, and 2.4 mg/kg) and haloperidol (0.03, 0.06, and 0.12 mg/kg) on the timing of 10, 30, and 90-s intervals were characterized in rats. Each drug's effect on timing behavior was assessed following intraperitoneal injections using a variant of the peak-interval procedure. Although haloperidol proportionately shifted peak times rightward in a manner consistent with a decrease in clock speed, clozapine exerted the opposite effect and proportionately shifted peak times leftward in a manner consistent with an increase in clock speed. These results support the proposal that typical antipsychotic drugs such as haloperidol and atypical antipsychotic drugs such as clozapine exert differential effects on dopaminergic, serotonergic, and glutamatergic systems within the cortex and striatum, two brain regions shown to be crucial for interval timing.

Pharmacokinetic profile and clinical efficacy of long-acting risperidone: potential benefits of combining an atypical antipsychotic and a new delivery system

Continuous long-term antipsychotic therapy is required for patients with schizophrenia to optimise treatment benefits. The use of long-acting antipsychotic preparations can help to ensure compliance with therapy and has been shown to improve efficacy in relapse prevention when compared with oral agents. How- ever, the use of long-acting agents has been limited, since this approach to patient care has only been available with conventional drugs. The atypical antipsychotic agents have provided a new option for the treatment of schizophrenia. However, entwined with health system limitations, partial or non-compliance remains a problem with oral atypical antipsychotic agents. Combining the attributes of the atypical antipsychotic class with the pharmacokinetic profile and compliance advantages of a long-acting formulation could potentially be an important advance in the management of patients requiring continuous anti- psychotic therapy. This review considers the available clinical data supporting possible advantages for the only long-acting atypical agent currently available, long-acting risperidone, as a microsphere formulation. The drug-delivery technology employed provides a sustained therapeutic plasma level, with administration once every 2 weeks, and this is translated into improved symptom control and improved quality of life, even in patients already deemed clinically stable on an oral agent or on a conventional depot antipsychotic.

The influence of chronic exposure to antipsychotic medications on brain size before and after tissue fixation: a comparison of haloperidol and olanzapine in macaque monkeys

It is unclear to what degree antipsychotic therapy confounds longitudinal imaging studies and post-mortem studies of subjects with schizophrenia. To investigate this problem, we developed a non-human primate model of chronic antipsychotic exposure. Three groups of six macaque monkeys each were exposed to oral haloperidol, olanzapine or sham for a 17-27 month period. The resulting plasma drug levels were comparable to those seen in subjects with schizophrenia treated with these medications. After the exposure, we observed an 8-11% reduction in mean fresh brain weights as well as left cerebrum fresh weights and volumes in both drug-treated groups compared to sham animals. The differences were observed across all major brain regions (frontal, parietal, temporal, occipital, and cerebellum), but appeared most robust in the frontal and parietal regions. Stereological analysis of the parietal region using Cavalieri's principle revealed similar volume reductions in both gray and white matter. In addition, we assessed the subsequent tissue shrinkage due to standard histological processing and found no evidence of differential shrinkage due to drug exposure. However, we observed a pronounced general shrinkage effect of approximately 20% and a highly significant variation in shrinkage across brain regions. In conclusion, chronic exposure of non-human primates to antipsychotics was associated with reduced brain volume. Antipsychotic medication may confound post-mortem studies and longitudinal imaging studies of subjects with schizophrenia that depend upon volumetric measures.

The potential role of lamotrigine in schizophrenia

RATIONALE

Atypical antipsychotic drugs are the drugs of choice for the treatment of schizophrenia. However, despite advances, no treatments have been established for patients who fail to improve with the most effective of these, clozapine. The inhibition of dopamine transmission through blockade of dopamine D2 receptors is considered to be essential for antipsychotic efficacy, but it is postulated that modulation of glutamate transmission may be equally important. In support of this, symptoms similar to schizophrenia can be induced in healthy volunteers using N-methyl-D-aspartate (NMDA) antagonist drugs that are also known to enhance glutamate transmission. Furthermore, lamotrigine, which can modulate glutamate release, may add to or synergise with atypical antipsychotic drugs, some of which may themselves modulate glutamate transmission.

OBJECTIVES

We examine the evidence for the efficacy of lamotrigine. We consider how this fits with a glutamate neuron dysregulation hypothesis of the disorder. We discuss mechanisms by which lamotrigine might influence neuronal activity and glutamate transmission, and possible ways in which the drug might interact with antipsychotic medications.

RESULTS

Data from four clinical studies support the efficacy of adjunctive lamotrigine in the treatment of schizophrenia. In addition, and consistent with a glutamate neuron dysregulation hypothesis of schizophrenia, lamotrigine can prevent the psychotic symptoms or behavioural disruption induced by NMDA receptor antagonists in healthy volunteers or rodents.

CONCLUSIONS

The efficacy of lamotrigine is most likely explained within the framework of a glutamate neuron dysregulation hypothesis, and may arise primarily through the drugs ability to influence glutamate transmission and neural activity in the cortex. The drug is likely to act through inhibition of voltage-gated sodium channels, though other molecular interactions cannot be ruled out. Lamotrigine may add to or synergise with some atypical antipsychotic drugs acting on glutamate transmission; alternatively, they may act independently on glutamate and dopamine systems to bring about a combined therapeutic effect. We propose new strategies for the treatment of schizophrenia using a combination of anti-dopaminergic and anti-glutamatergic drugs.

Mechanism-Based Pharmacokinetic–Pharmacodynamic Modeling—A New Classification of Biomarkers

In recent years, pharmacokinetic/pharmacodynamic (PK/PD) modeling has developed from an empirical descriptive discipline into a mechanistic science that can be applied at all stages of drug development. Mechanism-based PK/PD models differ from empirical descriptive models in that they contain specific expressions to characterize processes on the causal path between drug administration and effect. Mechanism-based PK/PD models have much improved properties for extrapolation and prediction. As such, they constitute a scientific basis for rational drug discovery and development. In this report, a novel classification of biomarkers is proposed. Within the context of mechanism-based PK/PD modeling, a biomarker is defined as a measure that characterizes, in a strictly quantitative manner, a process, which is on the causal path between drug administration and effect. The new classification system distinguishes seven types of biomarkers: type 0, genotype/phenotype determining drug response; type 1, concentration of drug or drug metabolite; type 2, molecular target occupancy; type 3, molecular target activation; type 4, physiological measures; type 5, pathophysiological measures; and type 6, clinical ratings. In this paper, the use of the new biomarker classification is discussed in the context of the application of mechanism-based PK/PD analysis in drug discovery and development.

Does clozapine work by blocking spikes and sparing bursts?

Clozapine works better and produces fewer side effects than other antipsychotics. Existing hypotheses fail to explain why. A new hypothesis, single spike suppression, supposes that psychotic symptoms are mediated by the single spikes of neurons at the D2 receptor. All antipsychotics block these spikes. Clozapine, according to the hypothesis, blocks these spikes but, unlike other antipsychotics, spares the spike bursts that mediate movement, cognition and affect. This study explores the mathematical feasibility of single spike suppression. Could an antipsychotic with the right receptor kinetics selectively block single spikes? Could this selectivity have clinical consequences? To develop the hypothesis, the author made a mathematical model of the receptor occupancy of a synapse, and performed five simulations, varying input data within the range established by research. The effects of hypothetical antipsychotics on single spikes and bursts were compared. The author confirmed that a drug with the right dissociation rate constant (k off) would dissociate slowly enough to block single spikes, but rapidly enough to spare longer bursts. If the hypothesis is correct, this spike-selective, burst-sparing drug would work at relatively low D2 occupancies, and cause minimal D2-related side effects. Single spike suppression may explain the superior properties of clozapine better than competing hypotheses. If so, it would provide a better model for a new generation of safe, effective antipsychotics.

Treatment response to olanzapine and haloperidol and its association with dopamine D receptor occupancy in first-episode psychosis

OBJECTIVE

Response to typical antipsychotic medication has been associated with achieving a level of striatal dopamine D2 receptor occupancy in the range of 65% to 70%. We undertook this study to determine whether response to the atypical antipsychotic olanzapine occurs at lower levels of D2 receptor occupancy.

METHOD

Eighteen patients who presented with a first episode of psychosis were randomized to receive olanzapine 5 mg daily or haloperidol 2 mg daily in a double-blind design. We acquired positron emission tomography (PET) scans using the D2 ligand [11C]raclopride within the first 15 days of treatment to determine the percentage of D2 receptors occupied by the medication. According to response, dosage was then adjusted to a maximum dosage of 20 mg daily of either drug. PET scans were repeated after 10 to 12 weeks of treatment.

RESULTS

At the first PET scan, the 8 olanzapine-treated patients had significantly lower D2 receptor occupancies (mean 63.4%, SD 7.3) than those observed in the 10 patients treated with haloperidol (mean 73.0%, SD 6.1). When patients were rescanned following dosage adjustment, mean D2 receptor occupancies were greater than 70% in both groups. D2 receptor occupancies did not differ significantly between the olanzapine-treated group (mean 72.0%, SD 5.7) and the haloperidol-treated group (mean 78.7%, SD 7.6).

CONCLUSIONS

These results suggest that, in patients being treated for a first episode of psychosis, olanzapine has its antipsychotic effect at approximately the same levels of D2 receptor occupancy as are achieved with low dosages of haloperidol.

A positron emission tomography (PET) study of cerebral dopamine D2 and serotonine 5-HT2A receptor occupancy in patients treated with cyamemazine (Tercian)

RATIONALE

Cyamemazine (Tercian) is an antipsychotic drug with anxiolytic properties. Recently, an in vitro study showed that cyamemazine possesses high affinity for serotonin 5-HT(2A) receptors, which was fourfold higher than its affinity for dopamine D(2) receptors (Hameg et al. 2003).

OBJECTIVES

The aim of this study is to confirm these previous data in vivo in patients treated with clinically relevant doses of Tercian.

METHODS

Eight patients received 37.5, 75, 150 or 300 mg/day of Tercian depending on their symptomatology. Dopamine D(2) and serotonin 5-HT(2A) receptor occupancies (RO) were assessed at steady-state plasma levels of cyamemazine with positron emission tomography (PET), using [(11)C]raclopride and [(11)C]N-methyl-spiperone, respectively. The effective plasma level of the drug leading to 50% of receptor occupancy was estimated by fitting RO with plasma levels of cyamemazine at the time of the PET scan.

RESULTS

Cyamemazine induced near saturation of 5-HT(2A) receptors (RO=62.1-98.2%) in the frontal cortex even at low plasma levels of the drug. On the contrary, occupancy of striatal D(2) receptors increased with plasma levels, and no saturation was obtained even at high plasma levels (RO=25.2-74.9%). The effective plasma level of cyamemazine leading to 50% of D(2) receptor occupancy was fourfold higher than that for 5-HT(2A) receptors. Accordingly, individual 5-HT(2A)/D(2) RO ratios ranged from 1.26 to 2.68. No patients presented relevant increased prolactin levels, and only mild extrapyramidal side effects were noticed on Simpson and Angus Scale.

CONCLUSION

This in vivo binding study conducted in patients confirms previous in vitro findings indicating that cyamemazine has a higher affinity for serotonin 5-HT(2A) receptors compared to dopamine D(2) receptors. In the dose range 37.5-300 mg, levels of dopamine D(2) occupancy remained below the level for motor side effects observed with typical antipsychotics and is likely to explain the low propensity of the drug to induce extrapyramidal side effects.

Mechanisms of action of second generation antipsychotic drugs in schizophrenia: insights from brain imaging studies

Multiple lines of evidence including recent imaging studies suggest that schizophrenia is associated with an imbalance of the dopaminergic system, entailing hyperstimulation of striatal dopamine (DA) D2 receptors and understimulation of cortical DA D1 receptors. This DA endophenotype presumably emerges from the background of a more general synaptic dysconnectivity, involving alterations in N-methyl-d-aspartate (NMDA) and glutamatergic (GLU) functions. Equally important is the fact that this DA dysregulation might further impair NMDA transmission. The first generation antipsychotic (FGA) drugs are characterized by high affinity to and generally high occupancy of D2 receptors. The efficacy of FGAs is limited by a high incidence of extrapyramidal side-effects (EPS). Second generation antipsychotic (SGA) drugs display reduced EPS liability and modest but clinically significant enhanced therapeutic efficacy. Compared to FGAs, the improved therapeutic action of SGAs probably derives from a more moderate D2 receptor blockade. We will review the effects of SGAs on other neurotransmitter systems and conclude by highlighting the importance of therapeutic strategies aimed at directly increasing prefrontal DA, D1 receptor transmission or NMDA transmission to enhance the therapeutic effect of moderate D2 receptor antagonism.

How antipsychotics work: the patients' perspective

BACKGROUND

While much is known about the neuropharmacology and objective efficacy of antipsychotics, little is known about how these drugs act on psychosis from the patients' perspective. Most previous studies of the patient's perspective have focused on drug tolerability and acceptability-rather than their effects on psychosis per se.

METHODS

The authors examined how antipsychotics work from a patient's perspective by analyzing their responses to a subjective questionnaire. Ninety-one patients with schizophrenia (cross-sectional component) and eight neuroleptic naïve patients (before and after treatment, longitudinal component) participated. The patients' responses to the questionnaire were analyzed using Principal Component Analysis (PCA) and general linear models.

RESULTS

Analysis of the patients' responses showed that from their perspective the drugs were substantially more effective in: "help deal, help stop thinking, and make the symptoms not bother" rather than "take away" or "change my mind". This differentiation was clear in the raw data and was supported by a formal PCA. Two underlying factors-the first termed detachment and second eradication-explained 71% of the variance in the patients' perspective on how antipsychotics work for them. Neuroleptic naïve patients, who had no prior exposure, expected drugs to help with both detachment and eradication, but, changed their mind with just 6 weeks of experience with the medications.

CONCLUSIONS

From the patients' perspective the action of antipsychotics is best characterized by a detachment from symptoms-rather than an eradication or elimination of symptoms. They have more wide-ranging expectations prior to antipsychotic exposure, but, even 6 weeks of exposure is sufficient to change their mind in favor of detachment. This finding is consistent with some of the very earliest ideas that antipsychotics produced a state of "indifference" and is also consistent with the more recent, neurobiologically informed notions that antipsychotics work by dampening the salience of psychotic symptoms.

Intramuscular olanzapine: a review of its use in the management of acute agitation

Intramuscular olanzapine (Zyprexa) is a rapid-acting atypical antipsychotic drug that is also indicated for use in patients with agitation associated with schizophrenia or bipolar mania, the focus of this review. Evidence from three well designed trials indicates that this formulation of olanzapine is at least as effective as intramuscular haloperidol or lorazepam in the treatment of patients with acute agitation associated with schizophrenia or bipolar mania, and has a faster onset of action. Although transient reductions in blood pressure and heart rate may occur in some patients administered intramuscular olanzapine, preliminary evidence of a general lack of clinical effect on the corrected QT (QTc) interval and a low incidence of extrapyramidal symptoms (EPS) is promising. The parenteral formulation of olanzapine appears to offer an effective, fast-acting and generally well tolerated alternative in the treatment of this significant behavioural problem.

Decreased levels of dopamine D3 receptor mRNA in schizophrenic and bipolar patients

Previous studies found an elevation of the dopamine D3 receptor (DRD3) mRNA as determined in peripheral lymphocytes in schizophrenic patients. The aim of this study was to test the hypothesis of elevated DRD3 mRNA in schizophrenia compared to bipolar disorder. Twenty-four patients, 13 schizophrenic and 11 bipolar, were included according to DSM-IV criteria. Psychometric measures were conducted using the Scale for the Assessment of Positive Symptoms, Scale for the Assessment of Negative Symptoms, Brief Psychiatric Rating Scale, Montgomery-Asberg Depression Rating Scale and Young Mania Rating Scale. mRNA was isolated from lymphocytes of venous blood samples and DRD3 mRNA was quantified using real-time reverse transcription PCR. We found a decrease in DRD3 mRNA in 13 schizophrenic (p = 0.009) and 11 bipolar (p = 0.023) patients as compared to controls. Medication history and severity of positive symptoms did not significantly influence DRD3 expression. Higher levels of DRD3 mRNA were correlated with negative schizophrenic symptoms. Interestingly, after treatment of patients with antipsychotics, DRD3 mRNA levels increased to similar levels as those of healthy controls. Bipolar patients, however, showed a slower increase in DRD3 mRNA levels after 3 weeks of therapy. Our findings suggest that the expression of DRD3 mRNA is reduced in schizophrenia and bipolar disorder, supporting the hypothesis of distorted homeostasis of dopamine receptor subtypes in psychotic disorder. The observed diminution was not specific for schizophrenia but also for bipolar disorder requiring further analysis of the regulatory factors involved in dopamine receptor subtype expression.

Striatal dopaminergic D2 receptor occupancy and clinical efficacy in psychosis exacerbation: a 123I-IBZM study with ziprasidone and haloperidol

OBJECTIVE

The aim of this study was to compare striatal dopaminergic D2 receptor occupancy (D2 RO) induced by ziprasidone and haloperidol and its relationship with clinical response and extrapyramidal side effects (EPS) in patients with acute psychosis exacerbation.

METHOD

Twenty patients hospitalized with an acute psychosis exacerbation were randomised in a single-blind study to receive either ziprasidone (80-120 mg/day) or haloperidol (5-20 mg/day) for more than 2 weeks. When stable doses were achieved, data on 123I-IBZM single-photon emission computed tomography (SPECT), as well as data on clinical efficacy (positive and negative symptoms scale [PANSS]) and EPS (Simpson Angus scale [SAS]), were compared between the two groups of patients. Clinical response was defined as a percentage of change of >30% in PANSS. Striatal D2 RO and clinical data were also compared between responders and nonresponders on each treatment group.

RESULTS

All patients on haloperidol and four patients on ziprasidone showed EPS. Mean D2 RO was significantly higher in the haloperidol (74.7+/-3.5) than in the ziprasidone (60.2+/-14.4) group (Mann Whitney U-test [M-W U-test] 8.50; p=0.002). Five patients were responders, and five were nonresponders on each group of treatment. Haloperidol responders and nonresponders did not differ in D2 RO, duration of treatment, doses or EPS. Ziprasidone responders were on higher doses than nonresponders and showed higher D2 RO although below 74%. A positive correlation of ziprasidone D2 RO was found with dose (r Spearman 0.87; p=0.001) and with SAS scores (r Spearman 0.88; p=0.001).

CONCLUSIONS

Ziprasidone induces lower D2 RO and EPS than haloperidol, which is consistent with an atypical antipsychotic profile. A direct relationship of ziprasidone D2 RO with dose, clinical efficacy and EPS has been found in this study. These data suggest that high ziprasidone doses might be more beneficial in patients with psychosis exacerbation and claim for caution regarding EPS appearance with such high dosages.

Treatments for schizophrenia: a critical review of pharmacology and mechanisms of action of antipsychotic drugs

The treatment of schizophrenia has evolved over the past half century primarily in the context of antipsychotic drug development. Although there has been significant progress resulting in the availability and use of numerous medications, these reflect three basic classes of medications (conventional (typical), atypical and dopamine partial agonist antipsychotics) all of which, despite working by varying mechanisms of actions, act principally on dopamine systems. Many of the second-generation (atypical and dopamine partial agonist) antipsychotics are believed to offer advantages over first-generation agents in the treatment for schizophrenia. However, the pharmacological properties that confer the different therapeutic effects of the new generation of antipsychotic drugs have remained elusive, and certain side effects can still impact patient health and quality of life. Moreover, the efficacy of antipsychotic drugs is limited prompting the clinical use of adjunctive pharmacy to augment the effects of treatment. In addition, the search for novel and nondopaminergic antipsychotic drugs has not been successful to date, though numerous development strategies continue to be pursued, guided by various pathophysiologic hypotheses. This article provides a brief review and critique of the current therapeutic armamentarium for treating schizophrenia and drug development strategies and theories of mechanisms of action of antipsychotics, and focuses on novel targets for therapeutic agents for future drug development.

Changes in extracellular dopamine in the rat globus pallidus induced by typical and atypical antipsychotic drugs

Typical antipsychotic drugs with a high extrapyramidal motor side-effects liability markedly increase extracellular dopamine in the caudate-putamen, while atypical antipsychotic drugs with a low incidence of extrapyramidal motor side-effects have less pronounced stimulating actions on striatal dopamine. Therefore, it has been suggested that the extrapyramidal motor side-effects liability of antipsychotic drugs (APD) is correlated with their ability to increase extracellular dopamine in the caudate-putamen. The globus pallidus (GP) is another basal ganglia structure probably mediating extrapyramidal motor side-effects of typical antipsychotic drugs. Therefore, the present study sought to determine whether extracellular dopamine in the globus pallidus might be a further indicator to differentiate neurochemical actions of typical and atypical antipsychotic drugs. Using in vivo microdialysis we compared effects on pallidal dopamine induced by typical and atypical antipsychotic drugs in rats. Experiment I demonstrated that systemic administration of haloperidol (1 mg/kg; i.p.) and clozapine (20 mg/kg; i.p.) induced a significant pallidal dopamine release to about 160 and 180% of baseline, respectively. Experiment II revealed that reverse microdialysis of raclopride and clozapine using a cumulative dosing regimen did not stimulate extracellular dopamine in the globus pallidus if low (1microM) or intermediate (10 and 100 microM) concentrations were used. Only at a high concentration (1,000 microM), raclopride and clozapine induced a significant pallidal dopamine release to about 130 and 300% of baseline values, respectively. Thus, effects of typical and atypical antipsychotic drugs on pallidal dopamine were similar and thus, may not be related to their differential extrapyramidal motor side-effects liability. Furthermore, the finding that reverse microdialysis of raclopride over a wide range of concentrations did not stimulate pallidal dopamine concentrations tentatively suggests that pallidal dopamine release under basal conditions is not regulated by D2 autoreceptors.

A cross-validation study on the relationship between central D2 receptor occupancy and serum perphenazine concentration

RATIONALE

There is a need for laboratory measures to guide clinical treatment with antipsychotic drugs. For serum concentration of the classical antipsychotic drug perphenzine an optimal therapeutic interval has been identified between 2 and 6 nmol/l. Positron emission tomography (PET) studies have suggested an optimal interval in central dopamine D2 receptor occupancy of between 65 and 80%.

OBJECTIVES

The aim of the present cross-validation study in clinically stable schizophrenic patients was to examine the relationship between the optimal interval in central D2 receptor occupancy and the therapeutic window for serum perphenazine concentration.

METHODS

Six patients who had responded to maintenance treatment with perphenazine decanoate were examined with PET and [11C]raclopride during steady-state conditions. Blood sampling was carried out for minimum serum perphenazine concentration and during the PET examination. RESULTS. The serum perphenazine concentration was between 1.8 and 9 nmol/l and the D2 receptor occupancy varied between 66 and 82%. The relationship between central receptor occupancy and serum drug concentration was curvilinear. Mild extrapyramidal symptoms were present in the patient with the highest D2 receptor occupancy. CONCLUSIONS. The previously suggested therapeutic window in serum perphenazine concentration is in good agreement with the optimal interval suggested for central D2 receptor occupancy. Serum concentrations at low dose levels may therefore serve as a useful tool in clinical monitoring of antipsychotic drug treatment.

Prolactinemia is uncoupled from central D2/D3 dopamine receptor occupancy in amisulpride treated patients

RATIONALE

Atypical antipsychotic drugs are classically associated with lower propensity to extrapyramidal symptoms (EPS) and hyperprolactinemia than typical antipsychotic drugs. It has not been clarified why some atypical antipsychotic drugs, such as amisulpride, induce prolactin plasma concentration (PRL) elevation, but little EPS. Previous studies have found an association between striatal D2/D3 receptor occupancy and PRL in typical antipsychotic treated patients suggesting that PRL is a marker of central D2/D3 receptors blockade.

OBJECTIVE

We have evaluated the relationship between PRL and central (striatum, temporal cortex and thalamus) D2/D3 receptor occupancy in amisulpride treated schizophrenic patients.

METHODS

Single photon emission tomography (SPET) and [123I]-epidepride were used to determine D2/D3 receptor occupancy in eight amisulpride treated patients. PRL was measured concurrently with the scans.

RESULTS

The mean PRL was 1166 (range 499-1892 mIU/l) for a mean amisulpride dose of 406 mg/day (range 150-600 mg/day). Amisulpride plasma concentration and central D2/D3 receptor occupancy were positively correlated (r=0.83-0.89, df=4, P<0.05). No significant correlations were observed between PRL and amisulpride (daily dose or plasma concentration, P>0.05), or between PRL and central D2/D3 receptor occupancy (P>0.05).

CONCLUSIONS

Our findings show that amisulpride-induced hyperprolactinemia is uncoupled from central D2/D3 receptor occupancy. Amisulpride has poor blood-brain barrier penetration and reaches much higher concentration at the pituitary, which is outside the blood-brain barrier. Higher D2/D3 receptor occupancy at the pituitary gland than at central regions is a possible explanation for amisulpride PRL elevation with low EPS. Further studies evaluating pituitary D2/D3 receptor occupancy in vivo are necessary to confirm this hypothesis.

Cerebral D2 and 5-HT2 receptor occupancy in Schizophrenic patients treated with olanzapine or clozapine

We report the results of a double-blind, randomized prospective trial on D2 and 5-HT2 receptor occupancy and the clinical effects of olanzapine versus clozapine in a sample of neuroleptic-refractory schizophrenic patients. Receptor occupancy was evaluated in different cortical areas and in basal ganglia using [18F] fluoro-ethyl-spiperone ([18F] FESP) and positron emission tomography (PET). A total of 15 neuroleptic-free patients completed the study undergoing a baseline and a post-treatment PET scan (olanzapine, nine patients, one female; clozapine, six patients, three female) 8 weeks after starting treatment. PET data were analysed both by regions of interest and on a voxel-by-voxel basis using Statistical Parametric Mapping (SPM96). Olanzapine and clozapine induced a similar and significant inhibition of [18F] FESP binding index in the cortex. In the basal ganglia, receptor occupancy was significantly higher with olanzapine than with clozapine (p=0.0018). By contrast, no differences in receptor occupancy were detected at the level of the pituitary gland. Clinical outcomes, in particular a full extra pyramidal tolerability, were similar. In this sample of neuroleptic-refractory schizophrenic patients, olanzapine and clozapine showed a different pattern of occupancy of D2-like receptor despite a common lack of extrapyramidal side-effects.

Prolactin levels in antipsychotic treatment of patients with schizophrenia carrying the DRD2*A1 allele

BACKGROUND

Hyperprolactinaemia induced by D(2) dopamine receptor antagonist antipsychotic medication can result in significant health problems.

AIMS

To examine the role of DRD2 polymorphism on prolactin levels in patients treated with antipsychotic medication.

METHOD

Antipsychotic drugs with different degrees of D(2) receptor binding were given to 144 patients with schizophrenia. Serum prolactin levels were obtained and Taq1A DRD2 alleles were determined.

RESULTS

Prolactin levels increased across medication groups reflecting increasingly tight D(2) receptor binding (clozapine, olanzapine, typical antipsychotics and risperidone). In the combined medication group, patients with the DRD2(*)A1allele had 40% higher prolactin levels than patients without this allele. In patients treated with clozapine (the loosest D(2) receptor binding agent), patients with the DRD2(*)A1allele had prolactin levels twice those of patients without this allele.

CONCLUSIONS

Patients with the DRD2A1 allele receiving antipsychotic medications had higher prolactin levels and were overrepresented among those with hyperprolactinaemia, suggesting greater functional D(2) receptor binding in this group.

The KCNQ channel activator retigabine blocks haloperidol-induced c-Fos expression in the striatum of the rat

Retigabine activates inward potassium rectifying KCNQ channels. This stabilizes the membrane potential via hyperpolarization in vitro and retigabine has also been shown to inhibit convulsions in vivo. This study was carried out to determine whether retigabine inhibited haloperidol-dependent activation of neurons in the striatum as measured by expression of c-Fos. Groups of male rats were treated with retigabine (10 mg/kg i.p.), haloperidol (1 mg/kg i.p.), or the two in combination (at 15 min interval) and fixed 60 min after haloperidol treatment. Haloperidol produced a large increase in the number of c-Fos-positive nuclei in different degrees in all parts of the striatum. Pretreatment with retigabine completely blocked haloperidol-induced c-Fos in both the ventral and dorsal striatum suggesting that retigabine via activation of the KCNQ channel interacts with haloperidol and inhibits neuronal excitation in the striatum.

Relation between cortical dopamine D(2) receptor occupancy and suppression of conditioned avoidance response in non-human primate

Suppression of the conditioned avoidance response (CAR), a useful test for screening for antipsychotic effects, has been discussed in relation to the blockade of dopaminergic transmission. The purpose of the present paper was to investigate the relationship between cortical dopamine D(2) receptor occupancy and the suppression of CAR by haloperidol in non-human primate. The avoidance rate was measured for four different doses of haloperidol treatment in a rhesus monkey, and the cortical D(2) receptor occupancy was measured in a parallel session using positron emission tomography with [(11)C]FLB 457. The successful avoidance response rate was decreased for doses of 10 and 30 microg/kg of haloperidol, and this decrement was associated with 65-77% of D(2) receptor occupancy. It is suggested that the threshold level of cortical dopamine D(2) receptor occupancy for the suppression of CAR is demonstrated in the present study.

Survey on schizophrenia treatment in Mexico: perception and antipsychotic prescription patterns

BACKGROUND

Since the introduction of antipsychotics, especially the so called atypicals, the treatment of schizophrenia has shown important improvements. At the present time, it is preferred to label clozapine and other antipsychotics sharing similar profiles as second-generation antipsychotics (SGAs). These medications have been proposed by some experts as a first line treatment for schizophrenia. It is critical to have reliable data about antipsychotic prescription in Mexico and to create management guidelines based on expert meetings and not only on studies carried out by the pharmaceutical industry. Only this approach will help to make the right decisions for the treatment of schizophrenia.

METHODS

A translated version of Rabinowitz's survey was used to evaluate antipsychotic prescription preferences and patterns in Mexican psychiatrists. The survey questionnaire was sent by mail to 200 psychiatrists from public institutions and private practice in Mexico City and Guadalajara, Mexico.

RESULTS

Recommendations for antipsychotics daily doses at different stages of the treatment of schizophrenia varied widely. Haloperidol was considered as the first choice for the treatment of positive symptoms. On the contrary, risperidone was the first option for negative symptoms. For a patient with a high susceptibility for developing extrapyramidal symptoms (EPS), risperidone was the first choice. It was also considered that SGAs had advantages over typical antipsychotics in the management of negative symptoms, cognitive impairment and fewer EPS.Besides, there was a clear tendency for prescribing typical antipsychotics at higher doses than recommended and inadequate doses for the atypical ones.

CONCLUSIONS

Some of the obstacles for the prescription of SGAs include their high cost, deficient knowledge about their indications and dosage, the perception of their being less efficient for the treatment of positive symptoms and the resistance of some Mexican physicians to change their prescription pattern. It is necessary to reach a consensus, in order to establish and standardize the treatment of schizophrenia, based on the information reported in clinical trials and prevailing economic conditions in Mexico.

Delayed onset of oculogyric crisis and torticollis with intramuscular haloperidol

OBJECTIVE

To report a case of delayed-onset dystonic reactions, oculogyric crisis (OGC), and torticollis after treatment with intramuscular haloperidol lactate injection.

CASE SUMMARY

A 22-year-old Mexican American woman received intramuscular haloperidol lactate 7.5 mg followed 4 hours later by 10 mg. Twenty-six hours after the first injection, the patient reported that she was unable to lower her gaze and that her neck was stiff. She was immediately given intramuscular benztropine 2 mg; there was a nearly complete remission of symptoms within 15 minutes of treatment. An objective causality assessment revealed a probable relationship between the OGC/torticollis and haloperidol therapy.

DISCUSSION

Dystonic reactions have been reported in 10-60% of patients treated with neuroleptic medication, most commonly when patients just start or increase the dose of the drug. The highest frequency of dystonic reactions has occurred in patients receiving high-potency neuroleptics. It has also been suggested that haloperidol-induced dystonic reactions are a result of the toxic metabolites of that agent.

CONCLUSIONS

OGC and torticollis reactions may occur 12-24 hours after treatment with a high-potency neuroleptic, even in the absence of symptoms of extrapyramidal side effects (EPSEs). The delayed dystonic reaction may begin suddenly (no early EPSE symptomatology).

Effect of acute antipsychotic administration on dopamine synthesis in rodents and human subjects using 6-[18F]-L-m-tyrosine

Clinical effects of antipsychotic drugs are thought to be mediated primarily through antagonism of the dopamine D2 receptors. Recent studies have demonstrated increased aromatic decarboxylase activity following acute administration of dopamine D2 receptor antagonists both in vivo and ex vivo. However, this effect has never been demonstrated in human subjects. We studied the effect of acute antipsychotic administration on dopamine synthesis in rodents and healthy human subjects using 6-[18F]-L-m-tyrosine. In rats, we studied the effect of a single subcutaneous injection of haloperidol and risperidone on dopamine synthesis using 6-[18F]-L-m-tyrosine. In our human study, six healthy volunteers underwent two 6-[18F]-L-m-tyrosine PET scans, before and after 3 mg risperidone to measure the rate of accumulation of radioactivity in the striatum as an index of dopamine synthesis. The striatal/cerebellar radioactivity count ratio and the ratio of dopamine metabolites to dopamine concentration was significantly higher in all rodent treatment groups compared to controls. In the PET study we found no significant change in the rate of uptake in the striatum. Our results suggest that 6-[18F]-L-m-tyrosine PET may not be a useful tool in the study of the effect of antipsychotics on dopamine synthesis in human subjects.

Quantitative validation of an intracerebral ?-sensitive microprobe system to determine in vivo drug-induced receptor occupancy using [11C]raclopride in rats

In this study, we evaluated the potential of using a new beta-sensitive microprobe system for in vivo quantification of [11C]raclopride binding and for in vivo determination of drug-induced receptor occupancy in the rat striatum. To validate this system, an ex vivo tissue dissection method was used to corroborate in vivo beta-microprobe measurements. Our data showed that the beta-microprobe-derived [11C]raclopride binding kinetics in striatum could be quantified using a tissue compartmental model with a cerebellar reference region. Haloperidol (0.001-0.1 mg/kg; i.v.) induced a dose-dependent decrease in [11C]raclopride binding in striatum as measured using the beta-microprobe with an ED50 value of 0.013 mg/kg. Highly significant relationships (P < 0.0001) were observed, within the same animals, between in vivo and ex vivo measures of haloperidol-induced D2-receptor occupancy (r = 0.98) as well as between in vivo and ex vivo measures of [11C]raclopride binding potentials (r = 0.99). Results from pretreatment and displacement studies with unlabeled raclopride and amphetamine conformed to the effect of these drugs as observed in humans using [11C]raclopride and PET and allowed estimation of the in vivo k(off) value of raclopride to 0.025 +/- 0.004 min(-1). However, allowing the system to stabilize before measurements and shielding the photomultiplier tubes were critical for obtaining these consistent results. This study demonstrates that the beta-microprobe provides reliable measurements of [11C]raclopride binding kinetics in rodents, allows for quantitative in vivo measurements of antipsychotic drug action in brain, and represents a valid and cost-effective alternative to positron emission tomography imaging in small animals.

Positron emission tomography and single photon emission CT molecular imaging in schizophrenia

We reviewed findings from PET and SPECT studies that have contributed to our understanding of the pathophysiology and treatment of schizophrenia. The most robust set of findings pertains to imaging of presynaptic dopaminergic function in the striatum. The results of these studies have been consistent in showing that schizophrenia, at least during episodes of illness exacerbation, is associated with increased activity of DA neurons; this increased presynaptic activity is associated with positive symptoms and good therapeutic response. Studies of cortical DA function are less numerous and less consistent. In the future, technical advances in PET instrumentation and radioligand development should contribute to a clarification of the role of prefrontal DA in the cognitive impairment that is presented by these patients. An important drawback of the literature in this field is the generally low number of subjects that are included in studies (typically less than 20 per group). Small samples are necessitated by the cost of these investigations, but also, in some instances, to the difficulty in recruiting appropriate clinical subjects (such as drug-free patients who have schizophrenia). In conditions that are characterized by marked heterogeneity, such as major depressive disorders, this factor is bound to yield divergent results across studies. Another source of discrepancy is the variety of technical approaches to data acquisition and analysis. For example, analytical methods range from "empirical" or "semiquantitative" (typically a region of interest to a region of reference ratio measured at one time point) to model-based methods that use an arterial input function. The limitations that are associated with empirical analytical methods might account for artifactual results, especially when the effect size of the between-group difference and the number of subjects are small [149]. In addressing these limitations it will be important to increase the availability of these techniques beyond a few academic centers, to promote multi-center studies in well-characterized populations, and to standardize analytical methods. Until recently, SPECT was the only widely available technique, and SPECT studies have provided a substantial contribution to this field. With the current increase in PET camera availability, the development of [18F]-based molecular imaging probes will provide unique opportunities for further dissemination of these techniques. The article reviewed seminal findings obtained with PET and SPECT molecular imaging of schizophrenia. These techniques do not play a major role in the diagnosis and treatment of this disorder, remain essentially research tools. The results that have been produced by this field to date suggest that PET will significantly contribute to unraveling the biologic bases of psychiatric disorders and may contribute to their clinical management. Moreover, it is foreseeable that PET will become increasingly involved in the development of new psychiatric medications. Expanding the availability of PET and the current radiopharmaceutical portfolio will be critical for these predictions to become reality.

Serotonin receptors: their key role in drugs to treat schizophrenia

Serotonin (5-HT)-receptor-based mechanisms have been postulated to play a critical role in the action of the new generation of antipsychotic drugs (APDs) that are usually referred to as atypical APDs because of their ability to achieve an antipsychotic effect with lower rates of extrapyramidal side effects (EPS) compared to first-generation APDs such as haloperidol. Specifically, it has been proposed by Meltzer et al. [J. Pharmacol. Exp. Ther. 251 (1989) 238] that potent 5-HT2A receptor antagonism together with weak dopamine (DA) D2 receptor antagonism are the principal pharmacologic features that differentiate clozapine and other apparent atypical APDs from first-generation typical APD. This hypothesis is consistent with the atypical features of quetiapine, olanzapine, risperidone, and ziprasidone, which are the most common treatments for schizophrenia in the United States and many other countries, as well as a large number of compounds in various stages of development. Subsequent research showed that 5-HT1A agonism may be an important consequence of 5-HT2A antagonism and that substitution of 5-HT1A agonism for 5-HT2A antagonism may also produce an atypical APD drug when coupled with weak D2 antagonism. Aripiprazole, the most recently introduced atypical APD, and a D2 receptor partial agonist, may also owe some of its atypical properties to its net effect of weak D2 antagonism, 5-HT2A antagonism and 5-HT1A agonism [Eur. J. Pharmacol. 441 (2002) 137]. By contrast, the alternative "fast-off" hypothesis of Kapur and Seeman [Am. J. Psychiatry 158 (2001) 360] applies only to clozapine and quetiapine and is inconsistent with the "slow" off rate of most atypical APDs, including olanzapine, risperidone and ziprasidone. 5-HT2A and 5-HT1A receptors located on glutamatergic pyramidal neurons in the cortex and hippocampus, 5-HT2A receptors on the cell bodies of DA neurons in the ventral tegmentum and substantia nigra and GABAergic interneurons in the cortex and hippocampus, and 5-HT1A receptors in the raphe nuclei are likely to be important sites of action of the atypical APDs. At the same time, evidence has accumulated for the important modulatory role of 5-HT2C and 5-HT6 receptors for some of the effects of some of the current APDs. Thus, 5-HT has joined DA as a critical target for developing effective APDs and led to the search for novel drugs with complex pharmacology, ending the exclusive search for single-receptor targets, e.g., the D3 or D4 receptor, and drugs that are selective for them.

Half a century of antipsychotics and still a central role for dopamine D2 receptors

A review of the history of antipsychotics reveals that while the therapeutic effects of chlorpromazine and reserpine were discovered and actively researched almost concurrently, subsequent drug development has been restricted to drugs acting on postsynaptic receptors rather than modulation of dopamine release. The fundamental property of atypical antipsychotics is their ability to produce an antipsychotic effect in the absence of extrapyramidal side effects (EPS) or prolactin elevation. Modulation of the dopamine D2 receptor remains both necessary and sufficient for antipsychotic drug action, with affinity to the D2-receptor being the single most important discriminator between a typical and atypical drug profile. Most antipsychotics, including atypical antipsychotics, show a dose-dependent threshold of D2 receptor occupancy for their therapeutic effects, although the precise threshold is different for different drugs. Some atypical antipsychotics do not appear to reach the threshold for EPS and prolactin elevation, possibly accounting for their atypical nature. To link the biological theories of antipsychotics to their psychological effects, a hypothesis is proposed wherein psychosis is a state of aberrant salience of stimuli and ideas, and antipsychotics, via modulation of the mesolimbic dopamine system, dampen the salience of these symptoms. Thus, antipsychotics do not excise psychosis: they provide the neurochemical platform for the resolution of symptoms. Future generations of antipsychotics may need to move away from a "one-size-fits-all polypharmacy-in-a-pill" approach to treat all the different aspects of schizophrenia. At least in theory a preferred approach would be the development of specific treatments for the different dimensions of schizophrenia (e.g., positive, negative, cognitive, and affective) that can be flexibly used and titrated in the service of patients' presenting psychopathology.

[Glutamatergic hypothesis of schizophrenia]

Schizophrenia is a devastating psychiatric disorder whose pathophysiology has not been fully clarified yet. Although dopamine dysfunction in schizophrenia is unequivocal, there are many evidences suggesting the involvement of the glutamatergic system. This paper briefly describes some basic knowledge regarding the functioning of the glutamatergic receptors with emphasis on the N-methyl-D-aspartate (NMDA) receptors. Presents evidence for glutamatergic dysfunction in schizophrenia, more specifically NMDA receptor hypofunction. Finaly the paper discusses the interaction between the dopaminergic and the glutamatergic systems; in special how hyperdopaminergic state found in schizophrenia can be associated to glutamatergic dysfunctions.

Differential effects of iloperidone, clozapine, and haloperidol on working memory of rats in the delayed non-matching-to-position paradigm

RATIONALE

Because cognitive function, particularly working memory (WM), is severely impaired in schizophrenia, evaluation of neuroleptic medication should include investigation of possible effects on cognition. Iloperidone is a promising, novel atypical neuroleptic drug (NL), for which no cognitive data is presently available.

OBJECTIVE

To investigate whether the novel atypical NL iloperidone would affect performance of rats on a WM test, using a delayed non-matching-to-position (DNMTP) paradigm, and compare its effects with those of the atypical NL clozapine and the typical NL haloperidol.

METHODS

Male Lister Hooded rats trained to criterion in an operant DNMTP task (0-64 s delay intervals) were administered vehicle, iloperidone (0.03, 0.1 mg/kg, i.p.), clozapine (0.1, 0.3 mg/kg, s.c.), haloperidol (0.003, 0.01, 0.03 mg/kg, s.c.), or scopolamine (0.05 mg/kg, s.c.). Together with choice accuracy, the motor performance of the task was measured.

RESULTS

It was found that: (1) iloperidone significantly improved choice accuracy delay-dependently while impairing task performance; (2) the atypical NL clozapine had no effect on choice accuracy and parameters related to motor function, but significantly increased the number of uncompleted trials; (3) haloperidol did not affect choice accuracy except at the longest delay with the highest dose, but in contrast to clozapine it significantly impaired task performance.

CONCLUSION

In accordance with their different pharmacological profiles, the three NLs iloperidone, clozapine, and haloperidol have different effects in this preclinical cognitive task. These results might provide important information for the development of NLs with beneficial effects on cognition.

Central muscarinic acetylcholine receptor availability in patients treated with clozapine

Clozapine is the prototypical atypical antipsychotic. In vitro, clozapine antagonizes a broad range of receptors, including dopamine, serotonin and muscarinic acetylcholine receptors. In vivo, receptor occupancy studies have shown moderate dopamine D(2) receptor blockade as well as high serotonin 5HT(2) receptor blockade for clozapine. Using [I-123]IQNB SPECT, we explored the influence of clozapine on muscarinic receptors in vivo. Eight schizophrenia patients underwent a total of 12 [I-123]IQNB SPECT scans after treatment with low to moderate doses of clozapine (mean 210 mg/day, range 50-450 mg/day). Muscarinic receptor availability was determined for basal ganglia, cortex, thalamus, and pons. A group of 12 age- and sex-matched unmedicated schizophrenia patients was used for comparison. Compared to unmedicated patients, [I-123]IQNB binding was lower in all regions in subjects treated with clozapine and decreased with increasing dose. In patients treated with a daily clozapine dose of at least 200 mg (mean 275+/-88 mg/day), these differences were highly significant (p <0.003) with mean reductions of muscarinic receptor availability of 45% for basal ganglia, 58% for cortex, 66% for pons, and 79% for thalamus. These preliminary data indicate that reduction of muscarinic receptor availability by clozapine can be measured in vivo and that moderate daily doses are associated with moderate to high reductions of muscarinic receptor availability. These results may explain, at least in part, the lack of extrapyramidal side effects as well as some side effects seen with clozapine.

Dopamine D2S and D2L receptors may differentially contribute to the actions of antipsychotic and psychotic agents in mice

Regulation of dopamine D2 receptor (D2) function plays an important role in alleviating either the motor deficits of Parkinson's disease or psychotic symptoms of schizophrenia. D2 also plays a critical role in sensorimotor gating which can be measured by monitoring the prepulse inhibition of the startle response. Alternative splicing of the D2 gene generates two isoforms, D2S and D2L. Here we investigated the role of D2S and D2L in the mechanisms of action of dopaminergic drugs, using mice lacking D2L (D2L(-/-)) but expressing D2S as a model system. We found that the typical antipsychotic raclopride was much less potent in inhibiting locomotor activity and eliciting catalepsy (or parkinsonism) in D2L(-/-) mice, whereas the atypical antipsychotic clozapine was equally effective in D2L(-/-) and wild-type mice. These suggest that the deletion of D2L diminishes drug-induced parkinsonism. Furthermore, two dopamine agonists, amphetamine and apomorphine, reduced prepulse inhibition to a similar degree in D2L(-/-) and wild-type mice. These results together suggest that D2S alone can mediate the action of clozapine and the dopamine agonist-induced disruption of prepulse inhibition. The differential binding affinities of these agents for D2S vs D2L were not sufficient to explain the divergent effects of typical vs atypical antipsychotics in D2L(-/-) mice. These findings suggest that D2S and D2L may differentially contribute to the therapeutic actions and side effects of antipsychotic agents, and may have implications for developing better antipsychotic agents.

A comparison of the effects of loxapine with ziprasidone and thioridazine on the release of dopamine and acetylcholine in the prefrontal cortex and nucleus accumbens

RATIONALE

Atypical, but not typical, antipsychotic drugs (APDs), produce preferential increases in dopamine (DA) and acetylcholine (ACh) release in rat medial prefrontal cortex (mPFC) compared to the nucleus accumbens (NAC). The increase in DA release has been attributed, in part, to their greater serotonin (5-HT)(2A) relative to D(2) receptor occupancy, while the basis for the increase in ACh has not yet been determined. Loxapine, a dibenzoxazepine congener of clozapine, is generally considered to be a typical APD because it produces significant extrapyramidal symptoms (EPS) in humans, at generally recommended clinical doses (60-100 mg/day), and catalepsy in rodents, although several studies have found it to be effective at lower doses which do not produce significant EPS. Moreover, loxapine, like its congener clozapine, has higher affinity for serotonin (5-HT)(2A) than dopamine D(2) receptors, in vitro, suggesting the possibility it could be an atypical APD with clozapine-like potential.

OBJECTIVES

The purpose of this study was to compare the effects of loxapine on DA and ACh release in the mPFC and NAC with those of ziprasidone, a novel atypical APD, and thioridazine, which is generally classified as a typical APD.

RESULTS

Loxapine, 0.03-10 mg/kg, increased prefrontal dopamine release with the magnitude of this increase exceeding that in the NAC, at all doses, other than the 10 mg/kg dose. The effect of loxapine (0.3 mg/kg) on DA release in the prefrontal cortex was attenuated by WAY 100635 (0.2 mg/kg), a 5-HT(1A) antagonist, as is the case for other atypical APDs. Ziprasidone (0.1-3 mg/kg) also preferentially increased DA release in the mPFC compared to NAC. Thioridazine (5 and 20 mg/kg) did not increase DA release in either the mPFC or NAC. Loxapine (3 mg/kg) and ziprasidone (1 and 3 mg/kg), but not thioridazine (10 and 20 mg/kg), significantly increased cortical ACh release.

CONCLUSION

Loxapine has effects on cortical and NAC DA and ACh release which are comparable to those of known atypical APDs. Ziprasidone and thioridazine have effects on cortical DA and ACh characteristic of atypical and typical APDs, respectively. It is concluded that further clinical studies of the atypical APD properties of loxapine are indicated.

Schizophrenia: from phenomenology to neurobiology

Schizophrenia is a common and debilitating illness, characterized by chronic psychotic symptoms and psychosocial impairment that exact considerable human and economic costs. The literature in electronic databases as well as citations and major articles are reviewed with respect to the phenomenology, pathology, treatment, genetics and neurobiology of schizophrenia. Although studied extensively from a clinical, psychological, biological and genetic perspective, our expanding knowledge of schizophrenia provides only an incomplete understanding of this complex disorder. Recent advances in neuroscience have allowed the confirmation or refutation of earlier findings in schizophrenia, and permit useful comparisons between the different levels of organization from which the illness has been studied. Schizophrenia is defined as a clinical syndrome that may include a collection of diseases that share a common presentation. Genetic factors are the most important in the etiology of the disease, with unknown environmental factors potentially modulating the expression of symptoms. Schizophrenia is a complex genetic disorder in which many genes may be implicated, with the possibility of gene-gene interactions and a diversity of genetic causes in different families or populations. A neurodevelopmental rather than degenerative process has received more empirical support as a general explanation of the pathophysiology, although simple dichotomies are not particularly helpful in such a complicated disease. Structural brain changes are present in vivo and post-mortem, with both histopathological and imaging studies in overall agreement that the temporal and frontal lobes of the cerebral cortex are the most affected. Functional imaging, neuropsychological testing and clinical observation are also generally consistent in demonstrating deficits in cognitive ability that correlate with abnormalities in the areas of the brain with structural abnormalities. The dopamine and other neurotransmitter systems are certainly involved in the treatment or modulation of psychotic symptoms. These broad findings represent the distillation of a large body of disparate data, but firm and specific findings are sparse, and much about schizophrenia remains unknown.

Applications of positron emission tomography in psychiatry

Blood flow, metabolism, and structural imaging studies suggest altered neural circuits in major psychiatric disorders including mood disorders, schizophrenia, and obsessive compulsive disorder. Neuroreceptor mapping studies have identified serotonergic abnormalities in mood disorders and dopaminergic abnormalities in schizophrenia. Further imaging applications have involved development of new positron emission tomography (PET) tracers that may identify abnormalities in peptide neurotransmitter systems such as corticotrophin releasing factor or substance P. Finally, PET can play an important role in quantifying the relationship between receptor occupancy, drug blood levels, oral dose and therapeutic outcome. In that way PET scanning can contribute to both therapeutics and to drug development by more rapid identification of the likely therapeutic dose range compared with conventional parallel group dose comparisons or dose ranging studies.

High-dose olanzapine for treatment-refractory schizophrenia

To date there is no common or adequate therapeutic strategy for treatment of refractory schizophrenic patients. Increasing antipsychotics' doses in treatment-resistant schizophrenic patients is the most common intervention used by clinicians. Olanzapine is an atypical antipsychotic, which in a number of double-blind, placebo-controlled studies has been found to be more effective than haloperidol for the treatment of positive and negative symptoms of schizophrenia. During the last few years there have been several reports of successful results in prescribing olanzapine at dosages of more than 25 mg/day for treatment-resistant schizophrenic and schizoaffective patients. This report presents the results from the treatment of three resistant schizophrenic patients treated successfully with high dosages of olanzapine (35, 40, and 60 mg/day). None of the patients had any side effects, including abnormal laboratory levels and weight gain. The results and literature data suggest that in clinical practice some schizophrenic patients resistant to conventional neuroleptic treatment and not responding to olanzapine at recommended dosages as high as 20 mg/day may respond to higher dosages such as 40 or 60 mg/day, and these dosages are well tolerated. Further prospective clinical studies are needed.

alpha2C-Adrenoceptor blockade by clozapine and other antipsychotic drugs

The noradrenergic system may play a role in antipsychotic modulation of schizophrenia symptoms. Therefore, the antagonistic potencies of the antipsychotics clozapine, chlorpromazine, risperidone, olanzapine, haloperidol, quetiapine, ziprasidone, iloperidone and aripiprazole were quantified using cell lines expressing the recombinant human alpha(2C)-adrenoceptor, alpha(2A)-adrenoceptor, or dopamine D(2L) receptor. The alpha(2)-adrenoceptor antagonists, yohimbine and idazoxan, were also tested. Alterations in cAMP were measured as changes in luminescence. In the alpha(2A)-adrenoceptor cell line, the agonist 5-bromo-6-(2-imidazolin-2-ylamino)quinoxaline (UK14,304) induced a concentration-dependent increase in luminescence. In cell lines expressing alpha(2C) and D(2L) receptors, agonists induced a concentration-dependent reduction in luminescence. Yohimbine and idazoxan were the most potent alpha(2A)-adrenoceptor antagonists, yohimbine and iloperidone were the most potent alpha(2C)-adrenoceptor antagonists, and haloperidol and olanzapine were the most potent dopamine D(2) receptor antagonists. Clozapine had the highest alpha(2C)/D(2) selectivity, and iloperidone the highest alpha(2C)/alpha(2A) ratio. It is hypothesised that alpha(2C)-adrenoceptor blockade contributes to improvement of cognitive function.

PET studies on the brain uptake and regional distribution of [11C]vinpocetine in human subjects

OBJECTIVES

Vinpocetine is a compound widely used in the prevention and treatment of cerebrovascular diseases. It is still not clear whether the drug has a direct and specific effect on neurotransmission or its effects are due to extracerebral actions, such as changes in cerebral blood flow. The main objective of the present investigation was to determine the global uptake and regional distribution of radiolabelled vinpocetine in the human brain in order to explore whether it may have direct central nervous system effects.

MATERIAL AND METHODS

Three healthy subjects were examined with positron emission tomography and [11C]vinpocetine. The regional uptake was determined in anatomically defined volumes-of-interest. The fractions of [11C]vinpocetine and labelled metabolites in plasma were determined using high pressure liquid chromatography.

RESULTS

The uptake of [11C]vinpocetine in brain was rapid and 3.7% (mean; n = 4) of the total radioactivity injected was in brain 2 min after radioligand administration. The uptake was heterogeneously distributed among brain regions. When compared with the cerebellum, an a priori reference region, the highest regional uptake was in the thalamus, upper brain stem, striatum and cortex. Following an initial peak, the total concentration of radioactivity in blood was relatively stable with time, whereas the concentration of the unchanged compound decreased with time in an exponential manner.

CONCLUSION

Vinpocetine, administered intravenously in humans, readily passes the blood-brain barrier and enters the brain. Its regional uptake and distribution in the brain is heterogeneous, indicating binding to specific sites. The brain regions showing increased uptake in the human brain correspond to those in which vinpocetine has been shown to induce elevated metabolism and blood flow. These observations support the hypothesis that vinpocetine has direct neuronal actions in the human brain.

Can antipsychotic drugs be classified by their effects on a particular group of dopamine neurons in the brain?

During the four decades that research has been carried out on antipsychotic drugs, a variety of methods have been used to study the effects of these compounds on dopamine neurotransmission. An important issue in this research was to find an explanation for the difference between "typical" and "atypical" antipsychotic drugs. The hypothesis that the beneficial properties and the motor side effects of antipsychotic drugs result from their effects on different groups of dopamine neurons has received considerable attention. Numerous researchers have tried to discover regiospecific actions of antipsychotic drugs in mesolimbic and in mesocortical dopamine neurons. An overview of these research attempts is presented here. Electrophysiological studies showed a selective action of atypical antipsychotic drugs on A10 dopamine neurons. It was found that chronic treatment with these compounds induced a preferential depolarisation block of the A10 neurons that project to the mesolimbic areas. The model represents certain clinical features of antipsychotic drug use and offers a possible explanation for the lack of extrapyramidal side effects of atypical antipsychotic drugs. Dopamine neurons projecting from A10 to the frontal cortex are also considered as a possible site of action of atypical antipsychotic drugs. Microdialysis studies have shown that certain atypical antipsychotic drugs selectively enhance the release of dopamine in the prefrontal cortex when compared with typical antipsychotic drugs. The finding that repeated treatment with antipsychotic drugs increased dopamine D(2) receptor binding in the frontal cortex confirms the significance of this brain area. These properties might indeed explain certain beneficial effects of atypical antipsychotic drugs such as improvement of cognitive dysfunction. However the effects of typical and atypical antipsychotic drugs in the frontal cortex could not be fully differentiated, which illustrates the difficulty of localising clinical effects of antipsychotic drugs in terms of regional dopamine neurons. Recently new insights into the mechanism of action of typical and atypical antipsychotic drugs have been published. Clinical positron emission tomography (PET) studies have indicated that a moderate dopamine D(2) receptor occupancy, probably combined with a high dissociation rate, might provide the optimal clinical conditions for an antipsychotic drug, without inducing extrapyramidal side effects. Moreover the efficacy of benzamides as atypical antipsychotic drugs suggests that low to moderate dopamine D(2) blockade is probably the most important-if not the only-criterion that determines "atypicality". Interestingly these new insights are based on PET studies of the human basal ganglia and not on the comparison of different brain areas. Apparently, according to this concept an ideal antipsychotic drug need not to act on a particular type of dopamine neurons, as it is the moderate dopamine D(2) receptor occupancy that determines the desirable clinical effects. It is concluded that both beneficial actions and side effects, of antipsychotic drugs might be dose dependently localised in A9 as well as A10 dopamine neurons.

Antipsychotic combination therapy in schizophrenia. A review of efficacy and risks of current combinations

OBJECTIVE

To review the literature on efficacy and risks of combining antipsychotics (atypical with atypical or conventional) and suggest a rationale and strategies for future clinical trials.

METHOD

A computerized Medline search supplemented by an examination of cross-references and reviews was performed.

RESULTS

Empirical evidence for the efficacy of combining antipsychotics is too limited to draw firm conclusions. The practice of augmenting clozapine with more 'tightly bound' D2 receptor antagonists as exemplified by risperidone augmentation of clozapine has some empirical and theoretical support. The risks of augmentation strategies have not been studied systematically. No study has examined the economic impact of combination treatment.

CONCLUSION

Further trials of antipsychotic combination therapies are needed before this currently unsupported practice can be recommended. Rationales for combination treatment include a broadening of the range of receptor activity or an increase in D2 receptor occupancy with certain atypical agents. Trial methodology needs to take into account subject characteristics, duration of treatment, optimization of monotherapy comparators, and appropriate outcome measures.

Prolactin responses to acute clomipramine and haloperidol of male schizophrenic patients in a drug-free state and after treatment with clozapine or with olanzapine

Atypical neuroleptics share a common feature, showing higher affinity for 5-HT2 receptors than for D2 dopamine receptors, but show considerable differences in their clinical and pharmacological properties. In clinical doses, they occupy serotonergic receptors near saturation, but show considerable differences regarding the D2 receptor occupancies, with clozapine showing the lowest degree of occupation. We assessed serotonergic and dopaminergic receptor responsiveness in two groups of male schizophrenic patients, one treated with the atypical neuroleptic clozapine (14 patients, doses 200-600 mg/d) and the other treated with olanzapine (11 patients, doses 10-30 mg/d). We measured the prolactin responses to the acute administration of a serotonergic drug, clomipramine, and a dopaminergic one, haloperidol. Tests were first performed in the drug-free state, and were repeated after the patients had been treated with stable doses of either drug for six weeks. Clomipramine administration induced significant increases of prolactin in the drug-free state. These responses were eliminated after treatment of the patients with either drug, thereby indicating a high 5-HT receptor occupancy by both clozapine and olanzapine. The prolactin responses to haloperidol were not altered after treatment with clozapine, but were significantly reduced after the olanzapine treatment. The baseline prolactin levels were not influenced by clozapine treatment, and were moderately but significantly increased after treatment with olanzapine. The results indicate that there is a difference between the two drugs in their capacity to block dopamine receptors at the hypothalamus-pituitary level, and match the results obtained by SPECT receptor binding studies for striatal dopamine receptors.

Differential effects of typical and atypical antipsychotic drugs on striosome and matrix compartments of the striatum

Administration of typical antipsychotic drugs (APDs) is often accompanied by extrapyramidal side-effects (EPS). Treatment with atypical APDs has a lower incidence of motor side-effects and atypical APDs are superior to typical APDs in treating the negative symptoms of schizophrenia. Although typical APDs strongly induce the immediate-early gene c-fos in the striatum while atypical APDs do so only weakly, it is possible that the effects of atypical APDs are more pronounced within certain regions of the striatum. The striatum contains two histochemically defined compartments, the striosome (patch) and the matrix. These compartments have been well characterized anatomically but their functional attributes are unclear. We therefore examined the effects of typical and atypical APDs on Fos expression in the striosome and matrix of the rat. Typical and atypical APDs were distinguished by the pattern of striatal compartmental activation they induced: the striosome : matrix ratio of Fos-li neurons was greater in rats treated with atypical APDs. Pretreating animals with selective antagonists of receptors that atypical APDs target with high affinity did not increase the striosome : matrix Fos ratio of typical APD-treated rats and thus did not mimic the ratio seen in response to atypical APDs. However, pretreatment with the atypical APD clozapine did recapitulate the characteristic compartmental Fos pattern seen in response to typical APDs. These data suggest that some characteristics of atypical APDs, such as the lower EPS liability and greater reduction of negative symptoms, may be linked to the coordinate regulation of the striatal striosome and matrix.

Atypical Antipsychotics: Mechanism of Action

Background:

Al though the prin ci pal brain tar get that all an tipsy chotic drugs at tach to is the do pamine D2 re cep tor, tra di tional or typi cal an tipsy chot ics, by at tach ing to it, in duce ex tra py r a mi dal signs and symp toms (EPS). They also, by bind ing to the D2 re cep tor, ele vate se rum pro lac tin. Atypi cal a ntipsy chot ics given in dos ages within the clini cally ef fec tive range do not bring about these adverse clini cal ef fects. To un der stand how these drugs work, it is im por tant to ex am ine the atypical an tipsychot ics' mecha nism of ac tion and how it dif fers from that of the more typi cal drugs.

Method:

This re view analy zes the af fini ties, the oc cu pan cies, and the dis so cia tion time- course of vari ous an tipsy chot ics at do pa mine D2 re cep tors and at se ro tonin (5-HT) re cep tors, both in the test tube and in live pa tients.

Results:

Of the 31 an tipsy chot ics ex am ined, the older tra di tional an tipsy chot ics such as tri fluperazine, pi mozide, chlor pro maz ine, fluphe nazine, ha loperi dol, and flupen thixol bind more tightly than dopamine it self to the do pa mine D2 re cep tor, with dis so cia tion con stants that are lower than that for do pa mine. The newer, atypi cal an tipsy chot ics such as queti apine, re moxi pride, clo zapine, ol a nzap ine, sert in dole, zi pra si done, and amisul pride all bind more loosely than do pa mine to the do pamine D 2 re cep tor and have dis so cia tion con stants higher than that for do pa mine. These tight and loose bind ing data agree with the rates of an tipsy chotic dis so cia tion from the human- cloned D2 recep tor. For in stance, ra dio ac tive ha loperi dol, chlor pro maz ine, and ra clo pride all dis s o ci ate very slowly over a 30- minute time span, while ra dio ac tive queti apine, clo zap ine, re moxi pride, and amisul pride dis so ci ate rap idly, in less than 60 sec onds. These data also match clini cal brain-imaging find ings that show ha loperi dol re main ing con stantly bound to D2 in hu mans un der go ing 2 posi tron emis sion to mo gra phy (PET) scans 24 hours apart. Con versely, the oc cu pa tion of D2 by clo zap ine or queti apine has mostly dis ap peared af ter 24 hours.

Conclusion:

Atypi cals clini cally help pa tients by tran siently oc cu py ing D2 re cep tors and then rap idly dis so ci at ing to al low nor mal do pa mine neu ro trans mis sion. This keeps pro lac tin lev els nor mal, spares cog ni tion, and ob vi ates EPS. One the ory of atypi cal ity is that the newer drugs block 5-HT2A re ceptors at the same time as they block do pa mine re cep tors and that, some how, this serotonin- dopam i n e bal ance con fers atypi cal ity. This, how ever, is not borne out by the re sults. While 5-HT2A re cep tors are read ily blocked at low dos ages of most atypi cal an tipsy chotic drugs (with the im por tant ex ceptions of re moxi pride and amisul pride, nei ther of which is avail able for use in Can ada) the do s ages at which this hap pens are be low those needed to al le vi ate psy cho sis. In fact, the an tipsy chotic thresh old oc cu pancy of D 2 for an tipsy chotic ac tion re mains at about 65% for both typi cal and atypi cal an tipsychotic drugs, re gard less of whether 5-HT2A re cep tors are blocked or not. At the same time, the a ntipsy chotic thresh old oc cu pancy of D2 for elic it ing EPS re mains at about 80% for both typi cal and atypi cal an tipsy chot ics, re gard less of the oc cu pancy of 5-HT2A re cep tors.

Relevance:

The “fast- off-D 2” the ory, on the other hand, pre dicts which an tipsy chotic com pounds will or will not pro duce EPS and hy per pro lac ti ne mia and which com pounds pres ent a rela tiv ely low risk for tar dive dyski ne sia. This the ory also ex plains why L- dopa psy cho sis re sponds t o low atypi cal an tipsy chotic dos ages, and it sug gests vari ous in di vidu al ized treat ment strategies.

Estimate the time varying brain receptor occupancy in PET imaging experiments using non-linear fixed and mixed effect modeling approach

Positron-Emission Tomography (PET) is an imaging technology currently used in drug development as a non-invasive measure of drug distribution and interaction with biochemical target system. The level of receptor occupancy achieved by a compound can be estimated by comparing time-activity measurements in an experiment done using tracer alone with the activity measured when the tracer is given following administration of unlabelled compound. The effective use of this surrogate marker as an enabling tool for drug development requires the definition of a model linking the brain receptor occupancy with the fluctuation of plasma concentrations. However, the predictive performance of such a model is strongly related to the precision on the estimate of receptor occupancy evaluated in PET scans collected at different times following drug treatment. Several methods have been proposed for the analysis and the quantification of the ligand-receptor interactions investigated from PET data. The aim of the present study is to evaluate alternative parameter estimation strategies based on the use of non-linear mixed effect models allowing to account for intra and inter-subject variability on the time-activity and for covariates potentially explaining this variability. A comparison of the different modeling approaches is presented using real data. The results of this comparison indicates that the mixed effect approach with a primary model partitioning the variance in term of Inter-Individual Variability (IIV) and Inter-Occasion Variability (IOV) and a second stage model relating the changes on binding potential to the dose of unlabelled drug is definitely the preferred approach.

Extrastriatal and striatal D(2) dopamine receptor blockade with haloperidol or new antipsychotic drugs in patients with schizophrenia

BACKGROUND

Both traditional and atypical antipsychotics have been hypothesised to be effective in schizophrenia through limbic and cortical D(2) dopamine receptor blockade.

AIMS

To investigate this hypothesis with the D(2)/D(3)-selective positron emission tomography (PET) probe [(76)Br]-FLB457.

METHOD

PET scans were performed on 6 controls and 18 patients with schizophrenia treated with haloperidol or with risperidone, clozapine, amisulpride or olanzapine.

RESULTS

The D(2) dopamine receptor blockade was high in the temporal cortex with both haloperidol and atypical antipsychotics. The atypicals, however, induced a significantly lower D(2) binding index than haloperidol in the thalamus and in the striatum.

CONCLUSIONS

Results suggest that cortical D(2) dopamine receptors are a common target of traditional and atypical antipsychotics for therapeutic action. Higher in vivo binding to the D(2) receptors in the cortex than in the basal ganglia is suggested as an indicator of favourable profile for a putative antipsychotic compound.

Dopamine D(2) receptors and their role in atypical antipsychotic action: still necessary and may even be sufficient

"Atypical" antipsychotics are associated with a much lower propensity for extrapyramidal side effects and, with some exceptions, a lack of sustained prolactin elevation. The authors propose that a low-affinity and fast dissociation (in molecular terms) from the dopamine D(2) receptor, along with administration of the drug in doses that lead to appropriate levels of dopamine D(2) receptor blockade, are the most important requirements for atypicality. Actions at other receptors (5-HT(2), D(4), etc.) may not be necessary to achieve atypicality, and while action at these receptors may have benefits on symptoms such as mood and cognition, this is as yet to be conclusively proven. Why clozapine is effective in refractory patients is still elusive and efforts to make antipsychotics that are devoid of effects on the dopamine D(2) receptors so far have been unsuccessful. In light of this, the authors provide a heuristic model linking pathophysiology and therapeutics and suggest that the ideal treatment for schizophrenia is unlikely to be single-drug with multireceptor blockade (a sort of one-size-fits-all polypharmacy) but will require several specific and targeted treatment strategies that are titrated to match the variable expression of different dimensions of schizophrenia in each patient.

The Future of Imaging in Drug Discovery

The number of new chemical entities being registered by drug companies each year is declining, while at the same time, the number of new compounds, and thereby potential therapeutics, is increasing at an exponential rate. The need to demonstrate the safety, efficacy, and the “value” of these new compounds to a sophisticated pharmaceutical market, driven in turn by the forces of healthcare economics, make drug development difficult, resulting in a very lengthy and complex series of steps in the development of a drug. Many aspects of clinical pharmacology are more art than science, and detecting pharmacological effects at the level of living integrated systems is difficult. These challenges are most evident when developing new therapeutics for neuropsychiatric illnesses. We may at last be entering a postmonoamine era, exemplified by compounds such as NK-1 antagonists and metatropic glutamate receptor agonists. Such developments hold significant promise for the treatment of severe mental illness, while at the same time being confronted with completely unknown clinical pharmacologies. Functional imaging may not only be useful for the development of new CNS compounds, but it may in fact be essential for helping to define their clinical pharmacology. Several examples will be addressed that highlight the utility of functional imaging in the development of potentially new CNS drugs.