Dose relationship of limbic-cortical D2-dopamine receptor occupancy with risperidone

Current Understanding of the Neurobiology of Agitation

Introduction

Managing agitation in the clinical setting is a challenge that many practitioners face regularly. Our evolving understanding of the etiological factors involved in aggressive acts has better informed our interventions through pharmacologic and behavioral strategies. This paper reviews the literature on the neurobiological underpinnings of aggressive behaviors, linking psychopathology with proposed mechanisms of action of psychiatric medications shown to be effective in mitigating agitation.

Methods

We performed a review of the extant literature using PubMed as a primary database. Investigation focused on neurobiology of agitation and its relation to the current evidence base for particular interventions.

Results

There are well-established pathways that can lead to increased autonomic response and the potential for violence. Psychopathology and substance-induced perceptual distortions may lead to magnification and overestimation of environmental threat, heightening the potential for aggression. Additional challenges have arisen with the advent of several novel drugs of abuse, many of which lead to atypical clinical presentations and which can elude standard drug screens. Our interventions still lean on the evidence base found in Project BETA (Best Practices in Evaluation and Treatment of Agitation). Although not a new drug and not included in the Project BETA guidelines, ketamine and its use are also discussed, given its unique pharmacology and potential benefits when other protocoled interventions have failed.

Conclusion

Aggression can occur due to manifold reasons in the clinical setting. Having an informed understanding of the possible determinants of agitation can help with more tailored responses to individual patients, limiting the unnecessary use of medications or of interventions that could be deemed forceful.

Changes in plasma concentrations of risperidone and 9-hydroxyrisperidone and the associated clinical effects during the switch from oral risperidone to extended-release paliperidone tablets in patients with schizophrenia

This study aimed to investigate changes in plasma concentrations of risperidone and 9-hydroxy-risperidone (9-OHR) and the associated clinical effects when switching from oral risperidone to extended-release (ER) paliperidone in patients with schizophrenia. This study included 25 patients with schizophrenia. Following a one-week screening period with a stable dose of risperidone, a six-week open-label switch study from risperidone to extended-release paliperidone (paliperidone ER) was conducted. Efficacy and safety assessments were performed on Day 1 and at Weeks 1, 2, 4, and 6. Plasma levels of the active fractions of oral risperidone and paliperidone ER were measured on Day 1 and at Week 1, respectively. Plasma levels of the active moiety (risperidone plus 9-OHR) while taking risperidone (mean dose: 4.0 mg) were significantly higher than plasma levels of 9-OHR while taking 6 mg of paliperidone ER. For 12 subjects taking only 3 mg of risperidone, plasma concentrations of the active moiety of risperidone were also significantly higher than those of 9-OHR while taking 6 mg of paliperidone ER. The amount of reduction in plasma levels was correlated with a temporal deterioration of clinical symptoms. These findings suggest that for patients with schizophrenia taking 3 mg or more of risperidone, an initial switching dose of 6 mg of paliperidone ER may be relatively low in terms of subsequent plasma concentrations and the associated clinical response.

Striatal and extrastriatal dopamine D2 receptor occupancy by a novel antipsychotic, blonanserin: a PET study with [11C]raclopride and [11C]FLB 457 in schizophrenia

Blonanserin is a novel antipsychotic with high affinities for dopamine D(2) and 5-HT(2A) receptors, and it was recently approved for the treatment of schizophrenia in Japan and Korea. Although double-blind clinical trials have demonstrated that blonanserin has equal efficacy to risperidone, and with a better profile especially with respect to prolactin elevation, its profile of in vivo receptor binding has not been investigated in patients with schizophrenia. Using positron emission tomography (PET), we measured striatal and extrastriatal dopamine D(2) receptor occupancy by blonanserin in 15 patients with schizophrenia treated with fixed doses of blonanserin (ie, 8, 16, and 24 mg/d) for at least 4 weeks before PET scans, and in 15 healthy volunteers. Two PET scans, 1 with [(11)C]raclopride for the striatum and 1 with [(11)C]FLB 457 for the temporal cortex and pituitary, were performed on the same day. Striatal dopamine D(2) receptor occupancy by blonanserin was 60.8% (3.0%) [mean (SD)] at 8 mg, 73.4% (4.9%) at 16 mg, and 79.7% (2.3%) at 24 mg. The brain/plasma concentration ratio calculated from D(2) receptor occupancy in the temporal cortex and pituitary was 3.38, indicating good blood-brain barrier permeability. This was the first study to show clinical daily dose amounts of blonanserin occupying dopamine D(2) receptors in patients with schizophrenia. The clinical implications obtained in this study were the optimal therapeutic dose range of 12.9 to 22.1 mg/d of blonanserin required for 70% to 80% dopamine D(2) receptor occupancy in the striatum, and the good blood-brain barrier permeability that suggested a relatively lower risk of hyperprolactinemia.

Relationship between dose, drug levels, and D2 receptor occupancy for the atypical antipsychotics risperidone and paliperidone

Blockade of D2 family dopamine receptors (D2Rs) is a fundamental property of antipsychotics, and the degree of striatal D2R occupancy has been related to antipsychotic and motor effects of these drugs. Recent studies suggest the D2R occupancy of antipsychotics may differ in extrastriatal regions compared with the dorsal striatum. We studied this issue in macaque monkeys by using a within-subjects design. [(18)F]fallypride positron emission tomography scans were obtained on four different doses of risperidone and paliperidone (the 9-OH metabolite of risperidone) and compared with multiple off-drug scans in each animal. The half-life of the two drugs in these monkeys was determined to be between 3 and 4 h, and drug was administered by a constant infusion through an intragastric catheter. The D2R occupancy of antipsychotic was determined in the caudate, putamen, ventral striatum, and four prefrontal and temporal cortical regions and was related to serum and cerebrospinal fluid drug levels. Repeated 2-week treatment with risperidone or paliperidone did not produce lasting changes in D2R binding potential in any region examined. As expected, D2R binding potential was highest in the caudate and putamen and was approximately one-third that level in the ventral striatum and 2% of that level in the cortical regions. We found dose-dependent D2R occupancy for both risperidone and paliperidone in both basal ganglia and cortical regions of interest. We could not find evidence of regional variation in D2R occupancy of either drug. Comparison of D2R occupancy and serum drug levels supports a target of 40 to 80 ng/ml active drug for these two atypical antipsychotics.

A double-blind, randomized, controlled trial of fluoxetine plus quetiapine or clomipramine versus fluoxetine plus placebo for obsessive-compulsive disorder

Obsessive-compulsive disorder patients who do not improve sufficiently after treatment with a selective serotonin reuptake inhibitor might improve further if other drugs were added to the treatment regimen. The authors present a double-blind, placebo-controlled trial comparing the efficacy of adding quetiapine or clomipramine to a treatment regimen consisting of fluoxetine. Between May 2007 and March 2010, a total of 54 patients with a primary diagnosis of obsessive-compulsive disorder, as defined by Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Text Revision, and a current Yale-Brown Obsessive-Compulsive Scale (Y-BOCS) score of at least 16, the score having dropped by less than 35% after fluoxetine monotherapy, were allocated to 1 of 3 arms (n = 18 per arm): quetiapine + fluoxetine (≤200 and ≤40 mg/d, respectively), clomipramine + fluoxetine (≤75 and ≤40 mg/d, respectively), or placebo + fluoxetine (≤80 mg/d of fluoxetine). Follow-up was 12 weeks. The Y-BOCS scores were the main outcome measure. No severe adverse events occurred during the trial, and 40 patients (74%) completed the 12-week protocol. The Y-BOCS scores (mean [SD]) were significantly better in the placebo + fluoxetine and clomipramine + fluoxetine groups than in the quetiapine + fluoxetine group (final: 18 [7] and 18 [7], respectively, vs 25 [6], P < 0.001) (reduction from baseline: -6.7 [confidence interval {CI}, -9.6 to -3.8; and -6.5 [CI, -9.0 to -3.9], respectively, vs -0.1 [CI, -2.9 to 2.7], P < 0.001; number needed to treat = 2.4). The clomipramine-fluoxetine combination is a safe and effective treatment for fluoxetine nonresponders, especially those who cannot tolerate high doses of fluoxetine. However, the period of monotherapy with the maximum dose of fluoxetine should be extended before a combination treatment strategy is applied.

Predicting dopamine D₂ receptor occupancy from plasma levels of antipsychotic drugs: a systematic review and pooled analysis

Measuring dopamine D₂ receptor occupancy levels using positron emission tomography (PET) is still widely unavailable. The objective of this study was to evaluate the accuracy of predicting D2 occupancy from the antipsychotic plasma level in patients with schizophrenia. Positron emission tomographic studies that measured plasma levels of antipsychotics and their corresponding D₂ occupancy levels were identified, using MEDLINE and EMBASE (last search: March 2010). Antipsychotics that were investigated in a total of 20 subjects or more were included. All data points for each antipsychotic were fit to a one-site binding model to estimate the total plasma concentration of each antipsychotic associated with a 50% occupancy (ED₅₀) of brain D₂ receptors. The mean prediction error and the root mean squared prediction error were used to measure the predictive performance of individual D₂ receptor occupancies from plasma drug levels derived from a one-site occupancy model using an ED₅₀ value calculated for each data point. A total of 34 treatment arms from 23 studies involving 281 subjects were included. The mean (95% confidence interval) prediction errors and root squared prediction errors were as low as 0.0 (-1.8 to 1.8) and 8.9 (7.6-10.2) for risperidone (n = 98); 0.0 (-3.5 to 3.5) and 15.1 (12.9-17.3) for clozapine (n = 75); -0.1 (-1.2 to 1.2), 0.0 (-1.9 to 1.9), and 4.6 (3.5-5.8) for olanzapine (n = 42); 0.1 (-3.4 to 3.5) and 9.9 (7.3-12.5) for haloperidol (n = 35); and -0.1 (-3.3 to 3.1) and 12.3 (8.8-15.7) for ziprasidone (n = 31), respectively. These findings suggest that D₂ occupancy of antipsychotics could be estimated with a high degree of accuracy using widely available plasma levels.

Central nervous system drug evaluation using positron emission tomography

In conventional pharmacological research in the field of mental disorders, pharmacological effect and dose have been estimated by ethological approach and in vitro data of affinity to the site of action. In addition, the frequency of administration has been estimated from drug kinetics in blood. However, there is a problem regarding an objective index of drug effects in the living body. Furthermore, the possibility that the concentration of drug in blood does not necessarily reflect the drug kinetics in target organs has been pointed out. Positron emission tomography (PET) techniques have made progress for more than 20 years, and made it possible to measure the distribution and kinetics of small molecule components in living brain. In this article, we focused on rational drug dosing using receptor occupancy and proof-of-concept of drugs in the drug development process using PET.

Antipsychotic occupancy of dopamine receptors in schizophrenia

Antipsychotic drugs were introduced in the early 50s on the basis of clinical observations in patients with schizophrenia. Experimental studies later revealed that antagonism at the D(2) dopamine receptor is a common characteristic of all antipsychotic drugs. In the 80s, the advent of brain imaging technologies such as positron emission tomography (PET) allowed for direct noninvasive studies of drug binding in treated patients. The concept receptor occupancy is defined as the fraction (%) of a receptor population that is occupied during treatment with an unlabelled drug. With regard to antipsychotic drugs, the radioligand [(11) C]-raclopride has been the most widely used for binding to the D(2) /D(3) -dopamine receptors. The present review discusses the contribution from molecular imaging to the current understanding of mechanism of action (MoA) of antipsychotic drugs. Consistent initial PET-findings of high D2-receptor occupancy in the striatum of patients responding to different antipsychotic drug treatments provided clinical support for the dopamine hypothesis of antipsychotic drug action. It has subsequently been demonstrated that patients with extrapyramidal syndromes (EPS) have higher occupancy (above 80%) than patients with good response but no EPS (65-80%). The PET-defined interval for optimal antipsychotic drug treatment has been implemented in the evolvement of dose recommendations for classical as well as more recently developed drugs. Another consistent finding is lower D(2) -occupancy during treatment with the prototype atypical antipsychotic clozapine. The MoA of clozapine remains to be fully understood and may include nondopaminergic mechanisms. A general limitation is that currently available PET-radioligands are not selective for any of the five dopamine receptor subtypes. Current attempts at developing such ligands may provide the tools required to refine further the MoA of antipsychotic drugs.

Extrastriatal dopamine D(2) receptor occupancy in olanzapine-treated patients with schizophrenia

Olanzapine is described as a multi-acting receptor-targeted antipsychotic agent. Although regional differences of dopamine D(2) receptor occupancy, i.e., limbic selectivity, were reported for olanzapine, contradictory results were also reported. We measured dopamine D(2) receptor occupancy of olanzapine in extrastriatal regions in patients with schizophrenia using positron-emission tomography with [(11)C]FLB457 and the plasma concentrations of olanzapine. Ten patients with schizophrenia taking 5-20 mg/day of olanzapine participated. Dopamine D(2) receptor occupancy in the temporal cortex ranged from 61.1 to 85.8%, and plasma concentration was from 12.7 to 115.4 ng/ml. The ED(50) value was 3.4 mg/day for dose and 10.5 ng/ml for plasma concentration. The ED(50) values obtained in this study were quite similar to those previously reported in the striatum. In conclusion, although the subjects and methods were different from previous striatal occupancy studies, these results suggest that limbic occupancy by olanzapine may not be so different from that in the striatum.

No regional difference in dopamine D2 receptor occupancy by the second-generation antipsychotic drug risperidone in humans: a positron emission tomography study

The effects of antipsychotic drugs have generally been considered to be mediated by blockade of dopamine D2 receptors. The concept of limbic and cortical selectivity of second-generation antipsychotics, i.e. higher dopamine D2 receptor occupancy in the cerebral cortices than in the striatum, has been suggested to explain their clinical efficacy with lower incidence of extrapyramidal side-effects. In this study, regional distribution of dopamine D2 receptor occupancy by risperidone was determined in order to elucidate the limbic and cortical selectivity of second-generation antipsychotics. Striatal and extrastriatal dopamine D2 receptor binding at baseline and after oral administration of 2 mg risperidone were measured in ten healthy men by positron emission tomography (PET) using different tracers with different affinity for the receptors, [11C]raclopride and [11C]FLB 457, respectively. Striatal and extrastriatal occupancies of dopamine D2 receptors were calculated for each brain region. Occupancies of dopamine D2 receptors were about 70% and 60% in the striatum and extrastriatum, respectively. A simulation study showed that non-negligible specific binding in the reference region (cerebellum), could cause systemic underestimation of occupancy in [11C]FLB 457 PET studies, indicating that occupancies in both the striatum and extrastriatum may not have differed. Among the extrastriatal regions including limbic and neocortical regions, no significant regional differences in dopamine D2 receptor occupancy were observed. Thus, limbic and cortical selectivity was not observed by one of the second-generation antipsychotics, risperidone.

Quantitative PET analysis of the dopamine D2 receptor agonist radioligand 11C-(R)-2-CH3O-N-n-propylnorapomorphine in the human brain

It has been demonstrated in vitro that the dopamine D(2) receptor has 2 interconvertible affinity states for endogenous dopamine, referred to as the high- and the low-affinity states. (11)C-(R)-2-CH(3)O-N-n-propylnorapomorphine ((11)C-MNPA) is a new agonist radioligand for in vivo imaging of the high-affinity state of dopamine D(2) receptors using PET. In the present study, the kinetics of (11)C-MNPA were examined for the first time, to our knowledge, in the human brain and analyzed using quantitative approaches with or without an arterial input function.

METHODS

A 90-min dynamic PET scan was obtained for 10 healthy men after an intravenous injection of (11)C-MNPA. The binding potential (BP(ND)) was calculated using the indirect kinetic method, a kinetic compartment analysis with a metabolite-corrected arterial input function. BP(ND) was also calculated by the simplified reference tissue model (SRTM) and transient equilibrium methods, both with the cerebellum as the reference brain region. The results of the quantitative methods were compared in a cross-validation approach.

RESULTS

The highest regional radioactivity was observed in the putamen. BP(ND) values obtained by kinetic analysis were 0.82 +/- 0.09, 0.59 +/- 0.11, and 0.28 +/- 0.06, respectively, in the putamen, caudate, and thalamus. BP(ND) values obtained by the SRTM and transient equilibrium methods were in good agreement with those obtained by the indirect kinetic method (r = 0.98 and r = 0.93, respectively). For all quantification methods, the BP(ND) values based on data acquired from 0 to 60 min were in good agreement with those based on data acquired from 0 to 90 min (r = 0.90-0.99).

CONCLUSION

The regional distribution of (11)C-MNPA binding was in good agreement with previous PET studies of dopamine D(2) receptors in the human brain using antagonist radioligands. The results support routine use of the SRTM and transient equilibrium methods, that is, methods that do not require an arterial input function and need a scan time of only about 60 min. (11)C-MNPA should thus be useful for clinical research on the pathophysiology of neuropsychiatric disorders and estimation of dopamine D(2) receptor occupancy by dopaminergic drugs.

Receptor reserve-dependent properties of antipsychotics at human dopamine D2 receptors

Aripiprazole is the first dopamine D(2)/D(3) receptor partial agonist approved for use in the treatment of psychiatric disorders including schizophrenia, bipolar disorder, and unipolar depression in the US. Aripiprazole has demonstrated a relatively favorable side effect profile compared to other commonly prescribed antipsychotics, including a low propensity for treatment-limiting extrapyramidal symptoms, hyperprolactinemia, and body weight gain. In an effort to elucidate aripiprazole's pharmacological activity in relation to clinically relevant fluctuation of dopamine D(2) receptor reserves, we compared the properties of aripiprazole to other antipsychotics, quetiapine, clozapine, olanzapine, ziprasidone, risperidone and haloperidol, a dopamine D(2) receptor partial agonist, bifeprunox, dopamine D(3) receptor modulators, BP897 (N-[4-[4-(2-Methoxyphenyl)-1-piperazinyl]butyl]naphthalene-2-carboxamide) and GR103691 (4'-Acetyl-N-[4-[4-(2-methoxyphenyl)piperazin-1-yl]butyl]biphenyl-4-carboxamide), and a 5-HT(1A) partial agonist, buspirone using forskolin-stimulated cAMP accumulation in clonal Chinese hamster ovary cell lines expressing low and high densities of human dopamine D(2S) receptors (hD(2S)-Low and hD(2S)-High, respectively). In hD(2S)-Low cells lacking receptor reserves for dopamine, all drugs antagonized dopamine responses, and their potencies correlated well with respective affinities. In hD(2S)-High cells possessing receptor reserves, all antipsychotics except aripiprazole antagonized dopamine responses, and their antagonist potencies were less than those in hD(2S)-Low cells treated with the equal dopamine concentration. In contrast, aripiprazole and bifeprunox acted as full agonists. BP897, GR103691 and buspirone acted as partial agonists. These data suggest that the level of receptor reserves influences antagonist potencies and side effects associated with antipsychotics. Aripiprazole's unique receptor reserve dependent properties may account for its favorable tolerability in the clinical setting.

Dose-finding study of paliperidone ER based on striatal and extrastriatal dopamine D2 receptor occupancy in patients with schizophrenia

RATIONALE

Paliperidone ER is a novel antipsychotic drug in an extended-release (ER) formulation. As with all antipsychotics, careful dose setting is necessary to avoid side effects.

OBJECTIVES

In this study, we measured striatal and extrastriatal dopamine D2 receptor occupancy during paliperidone ER treatment in patients with schizophrenia using positron emission tomography (PET) to compare regional occupancy and to estimate the optimal dose.

MATERIALS AND METHODS

Thirteen male patients with schizophrenia participated in this 6-week multiple-dose study. Six of them took 3 mg of paliperidone ER per day, four took 9 mg, and three took 15 mg. Two to 6 weeks after first drug intake, two PET scans, one with [11C]raclopride and one with [11C]FLB 457, were performed in each patient on the same day. The relationship between the dose or plasma concentration of paliperidone and dopamine D2 receptor occupancy was calculated.

RESULTS

The dopamine D2 receptor occupancies in the striatum measured with [11C]raclopride and the temporal cortex measured with [11C]FLB 457 were 54.2-85.5% and 34.5-87.3%, respectively. ED50 values of the striatum and temporal cortex were 2.38 and 2.84 mg/day, respectively. There was no significant difference in dopamine D2 receptor occupancy between the striatum and the temporal cortex.

CONCLUSIONS

The data from this study suggest that paliperidone ER at 6-9 mg provides an estimated level of dopamine D2 receptor occupancy between 70-80% and that the magnitude of dopamine D2 receptor occupancy is similar between the striatum and temporal cortex.

Neuroreceptor imaging studies in schizophrenia

The ability of SPECT and PET to image specific biomolecules in the living brain provides a unique tool for clinical researchers. It is therefore not surprising that the use of neuroreceptor-imaging techniques has become more widespread over the past decade. This article reviews the application of these techniques to the study of schizophrenia. The design of neuroreceptor-imaging studies performed in the field of schizophrenia research can be broadly divided into two categories: (1) studies of pathophysiology and (2) studies of pharmacology. The former examines neuroreceptor and neurotransmitter parameters in individuals with schizophrenia compared to control subjects in order to provide a better understanding of the disease process. Studies of pharmacology seek to elucidate the mechanism of action for the treatments utilized in schizophrenia. This review will consider both studies of pathophysiology and pharmacology, with a discussion of the application of these techniques to drug development.

Positron emission tomography imaging of risperidone augmentation in serotonin reuptake inhibitor-refractory patients

We studied 15 nondepressed patients with obsessive-compulsive disorder (OCD) who were nonresponders to serotonin reuptake inhibitors with an additive trial of risperidone. Positron emission tomography with (18)F-deoxyglucose and magnetic resonance imaging was obtained at baseline and following 8 weeks of either risperidone or placebo in a double-blind parallel group design. Risperidone treatment was associated with significant increases in relative metabolic rate in the striatum, cingulate gyrus, the prefrontal cortex, especially in the orbital region, and the thalamus. Four of 9 patients who received risperidone showed clinical improvement (CGI score of 1 or 2 at 8 weeks) while none of the 6 patients who received placebo showed improvement. Patients with low relative metabolic rates in the striatum and high relative metabolic rates in the anterior cingulate gyrus were more likely to show a clinical response. These metabolic predictors of clinical response are consistent with earlier PET studies showing similar prediction when either neuroleptics or serotonin reuptake inhibitor treatments are administered individually. Our results are consistent with a frontostriatal circuit change related to both dopaminergic and serotonergic systems and with the presence of psychopharmacological subtypes within OCD.

Modulation of [18F]fluorodopa (FDOPA) kinetics in the brain of healthy volunteers after acute haloperidol challenge

In animal studies, acute antipsychotic treatment was shown to enhance striatal DOPA-decarboxylase (DDC) activity. However, this phenomenon has not been demonstrated in humans by positron emission tomography (PET). Therefore, we investigated acute haloperidol effects on DDC activity in humans using [18F]fluorodopa (FDOPA) PET. Nine healthy volunteers were scanned with FDOPA in drug-free baseline conditions and after 3 days of haloperidol treatment (5 mg/day). A continuous performance test (CPT) was administered in both conditions. The net blood-brain clearance of FDOPA (K(in)app) in striatum, mesencephalon, and medial prefrontal cortex was calculated by volume-of-interest analysis. The macroparameter K(in)app is a composite of several kinetic terms defining the distribution volume of FDOPA in brain (V(e)D) and the relative activity of DOPA decarboxylase (k3D). Therefore, compartmental kinetic analysis was used to identify the physiological basis of the observed changes in K(in)app period. The magnitude of K(in)app was significantly increased in the putamen (18%) and mesencephalon (36%). Furthermore, V(e)D in the brain was increased by 15%. Increments of k3(D) in the basal ganglia did not attain statistical significance. The significant worsening of CPT results did not correlate with changes in FDOPA utilization. The present PET results indicate potentiation of FDOPA utilization in human basal ganglia by acute haloperidol treatment, apparently due to increased availability throughout the brain. The stimulation of DDC cannot be excluded due to insufficient statistical power in the estimation of k3(D) changes.

How antipsychotics work-from receptors to reality

How does a small molecule blocking a few receptors change a patients' passionately held paranoid belief that the FBI is out to get him? To address this central puzzle of antipsychotic action, we review a framework linking dopamine neurochemistry to psychosis, and then link this framework to the mechanism of action of antipsychotics. Normal dopamine transmission has a role in predicting novel rewards and in marking and responding to motivationally salient stimuli. Abnormal dopamine transmission alters these processes and results in an aberrant sense of novelty and inappropriate assignment of salience leading to the experience of psychosis. Antipsychotics improve psychosis by diminishing this abnormal transmission by blocking the dopamine D2/3 receptor (not D1 or D4), and although several brain regions may be involved, it is suggested that the ventral striatal regions (analog of the nucleus accumbens in animals) may have a particularly critical role. Contrary to popular belief, the antipsychotic effect is not delayed in its onset, but starts within the first few days. There is more improvement in the first 2 weeks, than in any subsequent 2-week period thereafter. However, a simple organic molecule cannot target the complex phenomenology of the individual psychotic experience. Antipsychotics diminish dopamine transmission and thereby dampen the salience of the pre-occupying symptoms. Therefore, in the initial stage of an antipsychotic response, the patients experience a detachment from symptoms, a relegation of the delusions and hallucinations to the back of their minds, rather than a complete erasure of the symptoms. Only with time, and only in some, via the mediation of new learning and plasticity, is there a complete resolution of symptoms. The implications of these findings for clinical care, animal models, future target discovery and drug development are discussed.

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.

In vivo affinity of [18F]fallypride for striatal and extrastriatal dopamine D2 receptors in nonhuman primates

RATIONALE

[18F]Fallypride is a new and promising radiotracer, suitable for imaging D2 receptors with Positron Emission Tomography (PET) in both striatal and extrastriatal regions. The high signal to noise ratio of [18F]fallypride has been attributed to its high affinity for D2 receptors (K(D) of 0.03 nM, measured in vitro at room temperature).

OBJECTIVES

We sought to further characterize this tracer in terms of its in vivo affinity, possible affinity differences between brain regions and dependence of in vitro affinity on temperature.

METHODS

PET scans were performed in baboons over a wide range of concentrations to measure the in vivo K(D) of [18F]fallypride in striatal and extrastriatal regions. Several analytical approaches were used, including nonlinear kinetic modeling and equilibrium methods. Also, in vitro assays were performed at 22 and 37 degrees C.

RESULTS

No significant differences in the in vivo K(D) were detected between regions. In vivo K(D) of [18F]fallypride was 0.22+/-0.05 nM in striatum, 0.17+/-0.05 nM in thalamus, and 0.21+/-0.07 nM in hippocampus. These values were intermediate between in vitro K(D) measured at 22 (0.04+/-0.03 nM) and 37 degrees (2.03+/-1.07 nM).

CONCLUSION

The in vivo affinity of [18F]fallypride was not as high as previously estimated from in vitro values. This property might contribute to the favorable kinetic properties of the tracer. The in vivo affinity was similar between striatal and extrastriatal regions. This result indicates that the measured regional in vivo affinities of this tracer are not affected by putative regional differences in endogenous dopamine, and that [18F]fallypride is an appropriate tool to provide unbiased estimates of the occupancy of D2 receptors by antipsychotic drugs in striatal and extrastriatal regions.

Optimizing limbic selective D2/D3 receptor occupancy by risperidone: a [123I]-epidepride SPET study

Selective action at limbic cortical dopamine D2-like receptors is a putative mechanism of atypical antipsychotic efficacy with few extrapyramidal side effects. Although risperidone is an atypical antipsychotic with high affinity for D2 receptors, low-dose risperidone treatment is effective without inducing extrapyramidal symptoms. The objective was to test the hypothesis that treatment with low-dose risperidone results in 'limbic selective' D2/D3 receptor blockade in vivo. Dynamic single photon emission tomography (SPET) sequences were obtained over 5 hours after injection of [123I]-epidepride (approximately 150 MBq), using a high-resolution triple-headed brain scanner (Marconi Prism 3000XP). Kinetic modelling was performed using the simplified reference region model to obtain binding potential values. Estimates of receptor occupancy were made relative to a normal volunteer control group (n = 5). Six patients treated with low-dose risperidone (mean = 2.6 mg) showed moderate levels of D2/D3 occupancy in striatum (49.9%), but higher levels of D2/D3 occupancy in thalamus (70.8%) and temporal cortex (75.2%). Occupancy values in striatum were significantly different from thalamus (F (1,4) = 26.3, p < 0.01) and from temporal cortex (F (1,4) = 53.4, p < 0.01). This is the first study to evaluate striatal and extrastriatal occupancy of risperidone. Low dose treatment with risperidone achieves a similar selectivity of limbic cortical over striatal D2/D3 receptor blockade to that of atypical antipsychotics with lower D2/D3 affinity such as clozapine, olanzapine and quetiapine. This finding is consistent with the relevance of 'limbic selective' D2/D3 receptor occupancy to the therapeutic efficacy of atypical antipsychotic drugs.