Striatal D2-dopamine receptor occupancy during treatment with typical and atypical neuroleptics

Neurochemical dysfunction in treated and nontreated schizophrenia - a retrospective analysis of in vivo imaging studies

To evaluate the contribution of individual synaptic constituents, all available in vivo imaging studies on schizophrenic patients were subjected to a retrospective analysis. For the pool of drug-naïve, drug-free, and acutely medicated patients, major findings were increases in neostriatal dopamine (DA) synthesis and release and decreases in neostriatal DA transporters and D1 receptors, neostriatal, thalamic, frontal, and parietal D2 receptors, mesencephalic/pontine and temporal 5-HT1A receptors, frontal and temporal HT2A and μ-amino butyric acid (GABA)A receptors. Based on the findings on drug-naïve and drug-free patients, it may be hypothesized that schizophrenia initially is characterized by an impaired mechanism of D2 autoreceptor and heteroreceptor sensitization leading to sensitization instead of desensitization in response to increased levels of neostriatal DA. Neuroleptic medication blocks neostriatal D2 autoreceptor and heteroreceptors, reducing neostriatal DA and disinhibiting DA action mediated by D2 heteroreceptor binding sites. Ultimately, this may result in a restitution of GABA function, leading to a recovery of inhibitory input to the target regions of the descending corticothalamostriatal efferents. Furthermore, a blockade of inhibitory and excitatory neocortical 5-HT function may be inferred, which is likely to reduce (excitatory) DAergic input to the mesolimbic target regions of corticothalamostriatal projections.

Neuroprotection of paliperidone on SH-SY5Y cells against β-amyloid peptide(25-35), N-methyl-4-phenylpyridinium ion, and hydrogen peroxide-induced cell death

RATIONALE

Antipsychotic drugs (APDs) were widely used in treating schizophrenia. Some APDs were reported to have neuroprotective effects against neurotoxicants in the cell level.

OBJECTIVES

Thus, one typical APD (haloperidol) and three atypical APDs (paliperidone, olanzapine, and risperidone) were tested whether they provide neuroprotection against stressor-induced cell death of SH-SY5Y.

METHODS

Hydrogen peroxide, N-methyl-4-phenylpyridinium ion, and β-amyloid peptide were used to treat cells with or without preconditioning by APDs; cell survival and indicators of oxidative stress were measured, respectively.

RESULTS

Paliperidone has the lowest baseline cytotoxicity compared with other APDs at 24 h; in addition, the paliperidone group showed a better survival than the other APD groups (P < 0.05). In stressor challenging, with a fixed concentration of stressors, olanzapine provided the best neuroprotection at 100 μM against Aβ(25-35) and MPP(+) (P < 0.05). In contrast, paliperidone works finely at low concentrations (10 and 50 μM) against Aβ(25-35) and MPP(+) and solely protected SH-SY5Y from hydrogen peroxide. At 100 μM, paliperidone completely diminished cell reduction induced by different stressors, regardless of their dosages. Paliperidone was demonstrated with a higher oxidative stress-scavenging properties than other APDs in several aspects, such as generated bulk glutathione, low HNE, and protein carbonyl productions. Contradictorily, olanzapine, at 24 h, also enhanced HNE and protein carbonyl productions, which may underlie its induced cytotoxicity.

CONCLUSIONS

Different APDs exhibit variations against different stressors. Paliperidone might be useful not only in alleviating oxidative stress induced by Aβ(25-35) and MPP(+) but also in providing neuroprotection against hydrogen peroxide.

Pharmacokinetics and time-course of D(2) receptor occupancy induced by atypical antipsychotics in stabilized schizophrenic patients

The (123)I-IBZM SPECT measured D(2) receptor occupancy (D(2)RO) in chronically dosed, stabilized schizophrenic patients and its relationship with antipsychotic (AP) pharmacokinetics (PK) over time is still unclear. The aims of this study were: 1) To define the relationship between striatal D(2) receptor occupancy (D( 2)RO) and plasma concentration (C(P)) in stabilized schizophrenic patients on clinically relevant doses using (123)I-IBZM SPECT; 2) To investigate the time course of AP-induced D(2)RO and corresponding C(P). Forty-six schizophrenic patients on their clinically required doses of risperidone, olanzapine, clozapine or quetiapine were included. D( 2)RO and C(P) were measured over time following a sparse-sampling experimental design, and individual PK and D(2)RO-time profiles were estimated using a population approach. Observed striatal D(2)RO and C(P) ranges were 28-75% and 9.4-60.5 ng/mL for risperidone, 22-84% and 8.6-89.5 ng/mL for olanzapine, 5-53% and 41.6-818.2 ng/mL for clozapine and 0-64% and 37.9-719.6 ng/mL for quetiapine. A PK-D(2)RO relationship was found for the four APs. D(2)RO pattern over time was stable for risperidone, olanzapine and clozapine but fluctuating for quetiapine. Stabilized schizophrenic patients show a wide range of both D(2)RO and C(P) at clinically effective doses of the four AP, suggesting that clinical response to these AP may be maintained with D(2)RO below 65%. D(2)RO patterns over time differ between AP. These results should be considered for accurate interpretation of D(2)RO measurements, proper design of studies and optimization of drug regimens for patients on AP treatment.

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.

Imaging of the dopaminergic system in parkinsonism with SPET

Parkinsonism is a feature of various neurodegenerative diseases. Since it is well known that the dopaminergic system plays a major role in parkinsonism, numerous reports have focused on the assessment of the dopaminergic system in vivo. While the use of positron emission tomography (PET) for routine application is still hampered by its limited availability, single photon emission tomography (SPET) has been shown to deliver comparable results to PET in clinical settings. In recent years SPET investigations of the dopamine transporter and of the dopamine D2-like receptors have provided valuable information about the pre- and postsynaptic dopaminergic system in parkinsonism. This article briefly reviews the literature dealing with SPET imaging in parkinsonism and specifically addresses the ligands used, the methodology applied, and the current major clinical applications.

D2 receptor occupancy under recommended and high doses of olanzapine: an iodine-123-iodobenzamide SPECT study

The aim of this study was to compare the degree of striatal dopamine D2 receptor availability in patients treated with recommended (5-20 mg, mean dose 11.9 +/- 6.3 mg daily) and higher doses (25-40 mg, mean 32.1 +/- 5.6 mg daily) of the novel antipsychotic drug olanzapine by means of [123I] IBZM Single photon emission computed tomography (SPECT). The results were compared to those of a group of 10 untreated, healthy, age- and sex-matched controls. The degree of dopamine D2 receptor occupancy in the patient group was correlated with the presence of extrapyramidal symptoms (EPS). A total of 20 patients who met the DSM III R criteria for schizophrenia or schizoaffective disorder received a clinically effective antipsychotic monotherapy with olanzapine. The mean daily dose of olanzapine ranged from 0.05-0.6 mg/kg body weight. The dopamine D2 receptor binding was reduced in all patients treated with olanzapine. Specific IBZM binding expressed as the [STR-BKG]/BKG ratio ranged from 0.13-0.61 (healthy controls 0.95). The D2 receptor availability revealed an exponential dose-response relationship (r = - 0.85, p < 0.001). The frequency of EPS induced by olanzapine was considerably lower. Only one patient, treated with 40 mg olanzapine, suffered from severe EPS symptoms and had to be given biperiden. There were no significant differences in the presence of EPS symptoms between patients with recommended doses and those with higher doses of olanzapine.

The cerebral cortex: a case for a common site of action of antipsychotics

Recent evidence from studies of receptor occupancy and regulation in post-mortem brains of patients with neuropsychiatric disorders and in non-human primates is providing new leads in the ongoing quest to understand the pathophysiology and causes of schizophrenia and to develop more effective methods of treatment. These studies suggest that the cerebral cortex may harbour the elusive common sites of action of antipsychotic medications and indicate that chronic treatment with these drugs differentially regulates both families of dopamine receptors in this structure. Upregulation of the cortical dopamine D2 receptors is accompanied by a downregulation of the D1 sites. Balancing the opposing actions of dopamine D1 and D2 receptor regulation may hold the key to optimal drug therapy and to understanding the pathophysiology of schizophrenia. In this article, Michael Lidow, Graham Williams and Patricia Goldman-Rakic review the evidence supporting the cerebral cortex as a pivotal site for these mechanisms underlying the action of antipsychotics.

Extrapyramidal side effects with risperidone and haloperidol at comparable D2 receptor occupancy levels

Risperidone is an antipsychotic drug with high affinity at dopamine D2 and serotonin 5-HT2 receptors. Previous clinical studies have proposed that risperidone's pharmacologic profile may produce improved efficacy for negative psychotic symptoms and decreased propensity for extrapyramidal side effects; features shared by so-called 'atypical' neuroleptics. To determine if routine risperidone treatment is associated with a unique degree of D2 receptor occupancy and pattern of clinical effects, we used [123I]IBZM SPECT to determine D2 occupancy in subjects treated with routine clinical doses of risperidone (n = 12) or haloperidol (n = 7). Both risperidone and haloperidol produced D2 occupancy levels between approximately 60 and 90% at standard clinical doses. There was no significant difference between occupancy levels obtained with haloperidol or risperidone. Drug-induced parkinsonism was observed in subjects treated with risperidone (42%) and haloperidol (29%) and was observed at occupancy levels above 60%. Based on these observations, it is concluded that 5-HT2 blockade obtained with risperidone at D2 occupancy rates of 60% and above does not appear to protect against the risk for extrapyramidal side effects.

[123I]IBZM SPECT in patients treated with typical and atypical neuroleptics: relationship to drug plasma levels and extrapyramidal side effects

[123I]Iodobenzamide (IBZM) is an iodine-labeled dopamine receptor ligand and can be used to visualize brain D2 receptors in humans with single photon emission computerized tomography (SPECT). The ratio of striatal IBZM uptake to uptake in frontal cortex (ST/FC ratio) represents a semiquantitative measure of D2 receptor binding in the striatum. Our study sample included six patients treated with haloperidol (3.0-8.0 mg/day orally; one patient with an average of 0.9 mg/day intramuscularly), five patients with benperidol (9.0-15.0 mg/day orally) and nine patients treated with clozapine (200.0-600.0 mg/day orally). Typical neuroleptics (TNs) and atypical neuroleptics (ANs) were significantly different in their ST/FC ratios. The ST/FC ratios indicated that patients treated with benperidol exhibited the lowest ST/FC ratios, with increasingly higher ratios in patients on haloperidol or clozapine. We found a curvilinear relationship between the ST/FC ratios and the dose/kg body wt. of TNs and ANs on the basis of a dose-normalization according to Ki-values of the neuroleptic at D2 receptors and a weaker, but also curvilinear relationship between ST/FC ratios and normalized dosages according to clinically defined chlorpromazine equivalents. The specific uptake of IBZM did not correlate with the plasma levels of the TN haloperidol at the present dose range (0-12.4 ng/ml). For clozapine, a meaningful negative correlation between plasma levels and ST/FC ratio could be established. There was a negative continuous correlation between uptake of IBZM and extrapyramidal side effects, which is different from the threshold-based relationship between extrapyramidal side effects and IBZM uptake reported previously.

D2-dopamine-receptor occupancy during treatment with haloperidol decanoate

We investigated in an open, explanatory study a total of 24 patients meeting DSM-III-R criteria for schizophrenia. Eighteen patients were treated for at least 4 weeks with a fixed dose of orally administered haloperidol for at least 4 weeks (mean daily dosage ranging from 0.07 to 0.35 mg/kg b.w.), and 6 patients received haloperidol decanoate with a fixed dose for at least 4 months (dosage range 50-150 mg/4 weeks; calculated mean daily dosage ranging from 0.02 to 0.09 mg/kg b.w.). One week after injection of haloperidol decanoate, the single photon emission computed tomography examination was performed. Our data suggest that D2-dopamine-receptor occupancy of 50 mg/4 weeks haloperidol decanoate 1 week after injection corresponds to an oral dose of 4.5 mg/day haloperidol.

Striatal dopamine-2 receptor occupancy in psychotic patients treated with risperidone

Seventeen psychiatric patients (11 with schizophrenia, 5 with other psychotic disorders, and 1 with obsessive-compulsive disorder) were examined by single photon emission computed tomography with 123I-iodobenzamide (IBZM) as tracer. Patients were treated with risperidone in two different dosage groups (3 mg and 8 mg) and haloperidol (10-20 mg) and compared with eight healthy control subjects. There was a statistically significant difference in basal ganglia/frontal cortex ratios of IBZM binding between controls and all treatment groups. A statistically significant difference was also found concerning these ratios and percentage of dopamine D2 receptor occupancy rates between the treatment groups with lowest ratios and highest percentage of D2 receptor occupancy in the group of patients treated with haloperidol, followed by the group treated with 8 mg of risperidone and the group treated with 3 mg of risperidone.