Striatal and extrastriatal D2/D3-receptor-binding properties of ziprasidone: a positron emission tomography study with [18F]Fallypride and [11C]raclopride (D2/D3-receptor occupancy of ziprasidone)

Optimisation of pharmacotherapy in psychiatry through therapeutic drug monitoring, molecular brain imaging and pharmacogenetic tests: Focus on antipsychotics

BACKGROUND

For psychotic disorders (i.e. schizophrenia), pharmacotherapy plays a key role in controlling acute and long-term symptoms. To find the optimal individual dose and dosage strategy, specialised tools are used. Three tools have been proven useful to personalise drug treatments: therapeutic drug monitoring (TDM) of drug levels, pharmacogenetic testing (PG), and molecular neuroimaging.

METHODS

In these Guidelines, we provide an in-depth review of pharmacokinetics, pharmacodynamics, and pharmacogenetics for 45 antipsychotics. Over 30 international experts in psychiatry selected studies that have measured drug concentrations in the blood (TDM), gene polymorphisms of enzymes involved in drug metabolism, or receptor/transporter occupancies in the brain (positron emission tomography (PET)).

RESULTS

Study results strongly support the use of TDM and the cytochrome P450 (CYP) genotyping and/or phenotyping to guide drug therapies. Evidence-based target ranges are available for titrating drug doses that are often supported by PET findings.

CONCLUSION

All three tools discussed in these Guidelines are essential for drug treatment. TDM goes well beyond typical indications such as unclear compliance and polypharmacy. Despite its enormous potential to optimise treatment effects, minimise side effects and ultimately reduce the global burden of diseases, personalised drug treatment has not yet become the standard of care in psychiatry.

Estimation of target occupancy in repeated dosing design studies using positron emission tomography: Biases due to target upregulation

Positron emission tomography (PET) has become indispensable in the quantification of target engagement by brain targeting medications. The relationship between the drug plasma concentration (or drug dose administered) and target occupancy determined during a PET occupancy study has provided valuable information for the assessment of novel pharmaceuticals in the early phases of drug development. Such information is also critical for the understanding of the mechanisms of action and side-effect profile of approved medication commonly used in the clinic. Occupancy studies conducted following repeated drug dosing (RD) can produce systematic differences from those conducted following single drug dose (SD), differences that have not been adequately explored. We have hypothesised that when differences are observed between RD and SD studies, they are related to changes in target density induced by repeated drug accumulation. We have developed a modified occupancy model to account for potential changes in target density and tested it on a sample dataset. We found that target upregulation can parsimoniously explain the differences in drug affinity estimated in SD and RD studies. Our findings have implications for the interpretation of RD occupancy data in the literature and the relationship between specific target occupancy levels and drug efficacy and tolerability.

Update Lessons from Positron Emission Tomography Imaging Part I: A Systematic Critical Review on Therapeutic Plasma Concentrations of Antipsychotics

BACKGROUND

Positron emission tomography (PET) and single photon emission tomography (SPECT) of molecular drug targets (neuroreceptors and transporters) provide essential information for therapeutic drug monitoring-guided antipsychotic drug therapy. The optimal therapeutic windows for D 2 antagonists and partial agonists, as well as their proposed target ranges, are discussed based on an up-to-date literature search.

METHODS

This part I of II presents an overview of molecular neuroimaging studies in humans and primates involving the target engagement of amisulpride, haloperidol, clozapine, aripiprazole, olanzapine, quetiapine, risperidone, cariprazine, and ziprasidone. The systemic review particularly focused on dopamine D 2 -like and 5-HT 2A receptors. Target concentration ranges were estimated based on receptor occupancy ranges that relate to clinical effects or side effects (ie, extrapyramidal side effects). In addition, findings for other relevant receptor systems were included to further enrich the discussion.

RESULTS

The reported reference ranges for aripiprazole and clozapine align closely with findings from PET studies. Conversely, for haloperidol, risperidone, and olanzapine, the PET studies indicate that a lowering of the previously published upper limits would be necessary to decrease the risk of extrapyramidal side effect.

CONCLUSIONS

Molecular neuroimaging studies serve as a strong tool for defining target ranges for antipsychotic drug treatment and directing therapeutic drug monitoring.

Experimental Medicine Approaches in Early-Phase CNS Drug Development

Traditionally, Phase 1 clinical trials were largely conducted in healthy normal volunteers and focused on collection of safety, tolerability, and pharmacokinetic data. However, in the CNS therapeutic area, with more drugs failing in later phase development, Phase 1 trials have undergone an evolution that includes incorporation of novel approaches involving novel study designs, inclusion of biomarkers, and early inclusion of patients to improve the pharmacologic understanding of novel CNS-active compounds early in clinical development with the hope of improving success in later phase pivotal trials. In this chapter, the authors will discuss the changing landscape of Phase 1 clinical trials in CNS, including novel trial methodology, inclusion of pharmacodynamic biomarkers, and experimental medicine approaches to inform early decision-making in clinical development.

Role of Brain Imaging in Drug Development for Psychiatry

Background:

Over the last decades, many brain imaging studies have contributed to new insights in the pathogenesis of psychiatric disease. However, in spite of these developments, progress in the development of novel therapeutic drugs for prevalent psychiatric health conditions has been limited.

Objective:

In this review, we discuss translational, diagnostic and methodological issues that have hampered drug development in CNS disorders with a particular focus on psychiatry. The role of preclinical models is critically reviewed and opportunities for brain imaging in early stages of drug development using PET and fMRI are discussed. The role of PET and fMRI in drug development is reviewed emphasizing the need to engage in collaborations between industry, academia and phase I units.

Conclusion:

Brain imaging technology has revolutionized the study of psychiatric illnesses, and during the last decade, neuroimaging has provided valuable insights at different levels of analysis and brain organization, such as effective connectivity (anatomical), functional connectivity patterns and neurochemical information that may support both preclinical and clinical drug development. Since there is no unifying pathophysiological theory of individual psychiatric syndromes and since many symptoms cut across diagnostic boundaries, a new theoretical framework has been proposed that may help in defining new targets for treatment and thus enhance drug development in CNS diseases. In addition, it is argued that new proposals for data-mining and mathematical modelling as well as freely available databanks for neural network and neurochemical models of rodents combined with revised psychiatric classification will lead to new validated targets for drug development.

Dopamine D2 receptor occupancy of lumateperone (ITI-007): a Positron Emission Tomography Study in patients with schizophrenia

Dopamine D₂ receptor occupancy (D₂RO) is a key feature of all currently approved antipsychotic medications. However, antipsychotic efficacy associated with high D₂RO is often limited by side effects such as motor disturbances and hyperprolactinemia. Lumateperone (ITI-007) is a first-in-class selective and simultaneous modulator of serotonin, dopamine and glutamate in development for the treatment of schizophrenia and other disorders. The primary objective of the present study was to determine D₂RO at plasma steady state of 60 mg ITI-007, a dose that previously demonstrated antipsychotic efficacy in a controlled trial, administered orally open-label once daily in the morning for two weeks in patients with schizophrenia (N = 10) and after at least a two-week washout period from standard of care antipsychotics. D₂RO was determined using positron emission tomography with ¹¹C-raclopride as the radiotracer. Mean peak dorsal striatal D₂RO was 39% at 60 mg ITI-007 occurring 1 h post-dose. Lumateperone was well-tolerated with a favorable safety profile in this study. There were no clinically significant changes in vital signs, ECGs, or clinical chemistry laboratory values, including prolactin levels. There were no adverse event reports of akathisia or other extrapyramidal motor side effects; mean scores on motor function scales indicated no motor disturbances with lumateperone treatment. This level of occupancy is lower than most other antipsychotic drugs at their efficacious doses and likely contributes to the favorable safety and tolerability profile of lumateperone with reduced risk for movement disorders and hyperprolactinemia. If approved, lumateperone may provide a new and safe treatment option for individuals living with schizophrenia.

Occupancy of pramipexole (Sifrol) at cerebral dopamine D2/3 receptors in Parkinson's disease patients

Whereas positron emission tomography (PET) with the antagonist ligand [¹⁸F]fallypride reveals the composite of dopamine D2 and D3 receptors in brain, treatment of Parkinson's disease (PD) patients with the D3-prefering agonist pramipexole should result in preferential occupancy in the nucleus accumbens, where the D3-subtype is most abundant. To test this prediction we obtained pairs of [¹⁸F]fallypride PET recordings in a group of nine PD patients, first in a condition of treatment as usual with pramipexole (ON-Sifrol; 3 × 0.7 mg p.d.), and again at a later date, after withholding pramipexole 48–72 h (OFF-Sifrol); in that condition the serum pramipexole concentration had declined by 90% and prolactin levels had increased four-fold, in conjunction with a small but significant worsening of PD motor symptoms. Exploratory comparison with historical control material showed 14% higher dopamine D2/3 availability in the more-affected putamen of patients OFF medication. On-Sifrol there was significant (p ˂ 0.01) occupancy at [¹⁸F]fallypride binding sites in globus pallidus (8%) thalamus (9%) and substantia nigra (19%), as well as marginally significant occupancy in frontal and temporal cortex of patients. Contrary to expectation, comparison of ON- and OFF-Sifrol results did not reveal any discernible occupancy in nucleus accumbens, or elsewhere in the extended striatum; present methods should be sensitive to a 10% change in dopamine D2/3 receptor availability in striatum; the significant findings elsewhere in the basal ganglia and in cerebral cortex are consistent with a predominance of D3 receptors in those structures, especially in substantia nigra, and imply that therapeutic effects of pramipexole may be obtained at sites outside the extended striatum.

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Fallypride PET recordings in nine PD patients, scanned on- and off medication with pramipexole

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No occupancy in the striatum, despite improved motor symptoms

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Substantial occupancy in substantia nigra, thalamus and globus pallidus

Evaluation of the pharmacokinetics, safety and clinical efficacy of ziprasidone for the treatment of schizophrenia and bipolar disorder

INTRODUCTION

Multiple strategies exist for the pharmacological treatment of schizophrenia and related disorders. In the last 20 years, several 'new' compounds have been introduced, called 'atypical antipsychotics', which have higher efficacy and better tolerability than first-generation neuroleptics. Among them, ziprasidone (ZPR) is currently finding widespread use, and it has also been shown to be active as an augmenter in bipolar disorder therapy.

AREAS COVERED

This review aims to provide the latest information on ZPR, an 'atypical' agent for the pharmacological therapy of schizophrenia and bipolar disorder. A literature search has been carried out with the keywords 'ziprasidone', 'schizophrenia', 'psychosis', 'bipolar', 'pharmacokinetics' and 'clinical trials'. In this process, particular attention has been paid to the drug pharmacokinetic characteristics and its safety in clinical use.

EXPERT OPINION

ZPR shares most advantages and disadvantages with other atypical antipsychotics. However, it can be useful for its low tendency to cause metabolic syndrome and hyperprolactinaemia, especially in patients suffering from excess weight, hyperlipidaemia, diabetes or who have suffered from hyperprolactinaemia when using other antipsychotics. However, there are serious doubts as to whether ZPR should be administered to patients suffering from arrhythmias or QTc prolongation, and even more for administration to bipolar patients undergoing polypharmacy with antidepressants.

Towards a therapeutic window of D2/3 occupancy for treatment of psychosis in Alzheimer's disease, with [18F]fallypride positron emission tomography

OBJECTIVE

Dopamine D2/3 receptor positron emission tomography tracers have guided antipsychotic prescribing in young people with schizophrenia by establishing a 'therapeutic window' of striatal D2/3 receptor occupancy. Older people, particularly those with dementia, are highly susceptible to motor side effects and may benefit from the appropriate application of imaging techniques. The study aimed to adapt [18F]fallypride imaging for use in occupancy studies in Alzheimer's disease (AD) and to investigate whether data acquisition could be made more tolerable by piloting the protocol in a small sample.

METHODS

Six participants with AD (three men; 85.0 ± 5.6 years old; MMSE = 16.0 ± 2.4) were recruited prior to commencing amisulpride for the treatment of psychosis and associated agitation. [18F]fallypride binding potential (BPND ) was determined using an interrupted scanning protocol at baseline (n = 6) and after 27.0 ± 6.1 days of amisulpride (25-50 mg) treatment (n = 4). D2/3 occupancy was calculated by percentage reduction in BPND between scanning sessions. Image data were re-analysed after reducing individual sampling times to 20 min.

RESULTS

The protocol was tolerated well, apart from the final (40 min) session of the post-treatment scan in one participant. Higher occupancies were achieved in the striatum (caudate 47-70%, putamen 31-58%) and thalamus (54-76%) than in the inferior temporal gyrus (27-43%). There was high agreement between occupancy values derived using longer and shorter sampling times (mean absolute difference 6.1% in the inferior temporal gyrus; <2% all other regions).

CONCLUSIONS

The protocol is feasible for use in AD and represents the first step towards establishing dose-occupancy relationships across older clinical populations.

Regional differences in SERT occupancy after acute and prolonged SSRI intake investigated by brain PET

Blocking of the serotonin transporter (SERT) represents the initial mechanism of action of selective serotonin reuptake inhibitors (SSRIs) which can be visualized due to the technical proceedings of SERT occupancy studies. When compared to the striatum, higher SERT occupancy in the midbrain and lower values in the thalamus were reported. This indicates that occupancy might be differently distributed throughout the brain, which is supported by preclinical findings indicating a regionally varying SERT activity and antidepressant drug concentration. The present study therefore aimed to investigate regional SERT occupancies with positron emission tomography and the radioligand [(11)C]DASB in 19 depressed patients after acute and prolonged intake of oral doses of either 10mg/day escitalopram or 20mg/day citalopram. Compared to the mean occupancy across cortical and subcortical regions, we detected increased SERT occupancies in regions commonly associated with antidepressant response, such as the subgenual cingulate, amygdala and raphe nuclei. When acute and prolonged drug intake was compared, SERT occupancies increased in subcortical areas that are known to be rich in SERT. Moreover, SERT occupancy in subcortical brain areas after prolonged intake of antidepressants was predicted by plasma drug levels. Similarly, baseline SERT binding potential seems to impact SERT occupancy, as regions rich in SERT showed greater binding reduction as well as higher residual binding. These findings suggest a region-specific distribution of SERT blockage by SSRIs and relate the postulated link between treatment response and SERT occupancy to certain brain regions such as the subgenual cingulate cortex.

Translation of central nervous system occupancy from animal models: application of pharmacokinetic/pharmacodynamic modeling

Translation of central nervous system receptor occupancy from animal models to humans has been elusive for many therapeutic targets. However, this may represent a valuable link to clinical efficacy for drugs acting within the brain and spinal cord. The introduction of positron emission tomography marked a significant noninvasive advance in determination of target engagement in the central nervous system. Pharmacokinetic/pharmacodynamic (PK/PD) modeling represents a valuable tool to translate ex vivo receptor occupancy from relevant animal models to humans. Whereas PK properties usually are reasonably scaled across species using standard allometric principles, PD properties related to receptor occupancy are usually species-independent. The translational value and applicability of PK/PD approaches are more directly evident when comparable modeling assumptions and mathematical model structures are employed across experiments and analyses. The purpose of this article is to review the basic principles of PK/PD analysis of receptor occupancy determined using noninvasive positron emission tomography imaging and first principles of allometric PK scaling and PD prediction based on animal data. By use of the data from the area of pain management, we also provide a case study of PK/PD analysis showcasing the importance of PK/PD model assumptions in predicting receptor occupancy in humans based on data from animal models.

Determination of dopamine D₂ receptor occupancy by lurasidone using positron emission tomography in healthy male subjects

RATIONALE

A positron emission tomography (PET) study of dopamine D₂ receptor occupancy was conducted to support a rational dose selection for clinical efficacy studies with lurasidone, an atypical antipsychotic that was approved for the treatment of schizophrenia by the FDA in late 2010.

OBJECTIVES

To determine the dopamine D₂ receptor occupancy of lurasidone in the ventral striatum, putamen and caudate nucleus, and to characterize the relationship between lurasidone serum concentration and D₂ receptor occupancy.

METHODS

A single oral dose of lurasidone (10, 20, 40, 60, or 80 mg) was administered sequentially to healthy male subjects (n = 4 in each cohort). Two PET scans were performed. For each scan, 20 mCi of [¹¹C]raclopride was administered intravenously as a bolus injection, followed immediately by 90 min of PET scan acquisitions.

RESULTS

The D₂ receptor occupancy levels were 41-43% for 10 mg, 51-55% for 20 mg, 63-67% for 40 mg, 77-84% for 60 mg, and 73-79% for 80 mg of lurasidone. The relationship between D₂ receptor occupancy and the mean serum lurasidone concentration during the PET scan (C PET) was similar for the putamen, caudate nucleus, and ventral striatum regions. Mean D₂ receptor occupancy levels correlated well with average peak serum concentration of lurasidone.

CONCLUSIONS

In healthy volunteers, single doses of lurasidone 40-80 mg resulted in D₂ receptor occupancy levels of >60%, a level of receptor occupancy previously associated with clinical response for atypical antipsychotics.

Dopamine D₂/₃ occupancy of ziprasidone across a day: a within-subject PET study

RATIONALE

Ziprasidone is an atypical antipsychotic recommended to be administered twice daily.

OBJECTIVES

The purpose of this study was to investigate whether occupancy of the dopamine D2/3 receptors by ziprasidone is maintained across a day employing a within subject design.

METHODS

Positron emission tomography (PET) scans with [(11)C]-raclopride were performed in 12 patients with schizophrenia while treated with ziprasidone 60 mg twice daily. Each patient completed [(11)C]-raclopride PET scans at 5, 13 and 23 h after the last dose of ziprasidone. Dopamine D2/3 receptor occupancy was estimated with reference to binding potential data of 44 age- and sex-matched control subjects in the caudate, putamen and ventral striatum.

RESULTS

Eleven scans were available at each time point, and the mean occupancies at 5-, 13- and 23-h scans were 66, 39 and 2 % in the putamen; 62, 35 and -6 % in the caudate; and 68, 47 and 11 % in the ventral striatum, respectively. The time-course of receptor occupancy across the regions indicated an occupancy half-life of 8.3 h. The serum level of ziprasidone associated with 50 % D2/3 receptors occupancy was estimated to be 204 nmol/L (84 ng/ml). Prolactin levels were highest at 5-h post-dose and none showed hyperprolactinemia at 23-h scans.

CONCLUSIONS

The absence of ziprasidone striatal D2/3 receptor binding 23 h after taking 60 mg under steady-state conditions is consistent with its peripheral half-life. The results support our earlier report that ziprasidone 60 mg administered twice daily appears to be the minimal dose expected to achieve therapeutic central dopamine D2/3 receptor occupancy (i.e. 60 %).

CLINICAL TRIALS REGISTRATION

24-Hour Time Course of Striatal Dopamine D2 Receptor Occupancy of Ziprasidone: A PET Study, www.clinicaltrials.gov/ct2/show/NCT00818298 , NCT00818298.

Surrogate markers for cerebral blood flow correlate with [¹⁸F]-fallypride binding potential at dopamine D(2/3) receptors in human striatum

Positron emission tomography (PET) with the high affinity dopamine D(2/3) receptor ligand [¹⁸F]-fallypride affords estimates of the binding potential (BP(ND) ) in extra-striatal regions of low receptor abundance, but the sufficient recording time for accurate measurements in striatum has been called into question. We have earlier argued that transient equilibrium measurements are obtained in striatum with [¹⁸F]-fallypride PET recordings of 3 h duration, which may be the practical limit for clinical investigations without interrupted scanning. However, the high extraction fraction of [¹⁸F]-fallypride predicts flow-dependence of tracer delivery to brain, which may be a source of variance of the apparent BP(ND) in regions of high binding. To test this prediction, we conducted a retrospective analysis of [¹⁸F]-fallypride PET data from a group of 50 healthy volunteers (age 18-58 years [mean ± SD: 32.6 ± 10.6), who had participated in clinical studies without arterial input measurements. We used the initial 120-s integral (AUC) of the venous confluence (VC) as a surrogate marker for cerebral blood flow (CBF) and tested for correlations between regional estimates of BP(ND) calculated by the simplified reference tissue model (SRTM) and the individual VC-AUC. The magnitude of BP(ND) in a high binding region (putamen), but not in a low binding region (thalamus) correlated positively with VC-AUC, suggesting that approximately 9% of the variance in the [¹⁸F]-fallypride BP(ND) in putamen can be attributed to individual differences in this surrogate marker for CBF, a contribution equal in magnitude to the effects of age on BP(ND) in putamen of the present healthy control group.

D₂-receptor occupancy measurement of JNJ-37822681, a novel fast off-rate D₂-receptor antagonist, in healthy subjects using positron emission tomography: single dose versus steady state and dose selection

RATIONALE

JNJ-37822681 is a highly selective, fast dissociating dopamine D₂-receptor antagonist being developed for the treatment of schizophrenia. A single dose [¹¹C]raclopride positron emission tomography (PET) imaging study had yielded an estimated clinical dose range. Receptor occupancy at steady state was explored to test the validity of the single-dose estimates during chronic treatment.

OBJECTIVES

The aims of this study are to characterize single and multiple dose pharmacokinetics and obtain striatal D₂-receptor occupancies to predict doses for efficacy studies and assess the safety and tolerability of JNJ-37822681.

METHODS

An open-label single- and multiple-dose study with 10 mg JNJ-37822681 (twice daily for 13 doses) was performed in 12 healthy men. Twenty [¹¹C]raclopride PET scans (up to 60 h after the last dose) from 11 subjects were used to estimate D₂-receptor occupancy. A direct effect O (max) model was applied to explore the relationship between JNJ-37822681 plasma concentration and striatal D₂-receptor occupancy.

RESULTS

Steady state was reached after 4-5 days of twice daily dosing. JNJ-37822681 plasma concentrations of 3.17 to 63.0 ng/mL resulted in D₂ occupancies of 0 % to 62 %. The concentration leading to 50 % occupancy was 18.5 ng/mL (coefficient of variation 3.9 %) after single dose and 26.0 ng/mL (8.2 %) at steady state. JNJ-37822681 was well tolerated.

CONCLUSIONS

Receptor occupancy after single dose and at steady state differed for JNJ-37822681 and the robustness of the estimates at steady state will be tested in phase 2 studies. Dose predictions indicated that 10, 20, and 30 mg JNJ-37822681 twice daily could be suitable for these studies.

In vivo quantification of striatal dopamine D2 receptor occupancy by JNJ-37822681 using [11C]raclopride and positron emission tomography

JNJ-37822681 is a novel, fast-dissociating dopamine D(2) receptor antagonist, currently in development as an antipsychotic drug candidate. A previous first-in-human study demonstrated mild central nervous system effects of JNJ-37822681 in healthy male volunteers. Significant but transient serum prolactin elevations were demonstrated, whereas other neurophysiological effects were relatively small. To investigate striatal dopamine D(2) receptor occupancy by variable single doses of JNJ-37822681, an open-label [(11)C]raclopride positron emission tomography study was performed in 12 healthy male volunteers, using the simplified reference tissue model with cerebellum as reference tissue. Oral administration of JNJ-37822681 resulted in dose-dependent dopamine D(2) receptor occupancy. Receptor occupancy increased from 9-19% at 2 mg doses to 60-74% at 20 mg doses of JNJ-37822681. Therefore, single oral doses of JNJ-37822681 can produce occupancy levels that are generally associated with clinical efficacy for registered antipsychotic drugs.

Receptor targets for antidepressant therapy in bipolar disorder: an overview

The treatment of bipolar depression is one of the most challenging issues in contemporary psychiatry. Currently only quetiapine and the olanzapine-fluoxetine combination are officially approved by the FDA against this condition. The neurobiology of bipolar depression and the possible targets of bipolar antidepressant therapy remain relatively elusive. We performed a complete and systematic review to identify agents with definite positive or negative results concerning efficacy followed by a second systematic review to identify the pharmacodynamic properties of these agents. The comparison of properties suggests that the stronger predictors for antidepressant efficacy in bipolar depression were norepinephrine alpha-1, dopamine D1 and histamine antagonism, followed by 5-HT2A, muscarinic and dopamine D2 and D3 antagonism and eventually by norepinephrine reuptake inhibition and 5HT-1A agonism. Serotonin reuptake which constitutes the cornerstone in unipolar depression treatment does not seem to play a significant role for bipolar depression. Our exhaustive review is compatible with a complex model with multiple levels of interaction between the major neurotransmitter systems without a single target being either necessary or sufficient to elicit the antidepressant effect in bipolar depression.

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.

Predicting brain occupancy from plasma levels using PET: superiority of combining pharmacokinetics with pharmacodynamics while modeling the relationship

Positron emission tomography (PET) studies of dopamine receptor occupancy can be used to assess dosing of antipsychotics. Typically, studies of antipsychotics have applied pharmacodynamic (PD) modeling alone to characterize the relationship between antipsychotic dose and its effect on the brain. However, a limitation of this approach is that it does not account for the discrepancy between the time courses of plasma concentration and receptor occupancy by antipsychotics. Combined pharmacokinetic-PD (PK-PD) modeling, by incorporating the time dependence of occupancy, is better suited for the reliable analysis of the concentration-occupancy relationship. To determine the effect of time on the concentration-occupancy relationship as a function of analysis approach, we measured dopamine receptor occupancy after the administration of aripiprazole using [(11)C]raclopride PET and obtained serial measurements of the plasma aripiprazole concentration in 18 volunteers. We then developed a PK-PD model for the relationship, and compared it with conventional approach (PD modeling alone). The hysteresis characteristics were observed in the competitor concentration-occupancy relationship and the value of EC(50) was different according to the analysis approach (EC(50) derived from PD modeling alone=11.1 ng/mL (95% confidence interval (CI)=10.1 to 12.1); while that derived from combined PK-PD modeling=8.63 ng/mL (95% CI=7.75 to 9.51)). This finding suggests that PK-PD modeling is required to obtain reliable prediction of brain occupancy by antipsychotics.

[18F]Fallypride PET measurement of striatal and extrastriatal dopamine D 2/3 receptor availability in recently abstinent alcoholics

Positron emission tomography (PET) shows reduced binding of the dopamine D(2/3) antagonist [(11) C]raclopride in striatum of withdrawn psychostimulant abusers, but not consistently in patients with alcohol dependence (AD). We make first use of the high affinity ligand [(18) F]fallypride to obtain serial measures of D(2/3) receptor availability in striatal and extrastriatal regions of AD patients undergoing detoxification. Seventeen patients (mean age 44 ± 5y) with AD and 14 age-matched healthy volunteers participated. Each patient underwent [(18) F]fallypride PET upon hospital admission, and again 1-2 weeks later; two patients achieving abstinence, and two with substantial harm reduction had additional PET follow-up at 1 year. Dynamic 180-minute PET recordings were used for volume of interest (VOI)-based and voxel-wise analysis of [(18) F]fallypride binding potential (BP(ND) ). Mean baseline BP(ND) in striatum of the AD patients (15.7 ± 3.6) was unaltered during short-term follow-up, and did not differ from that in healthy controls (16.8 ± 3.0); however, BP(ND) was 10-20% lower in thalamus, hippocampus, and insular and temporal cortex of the AD patients (P < 0.05). Age-dependent declines in BP(ND) were very small in controls, but more pronounced and widespread in the AD group. Striatal and thalamic BP(ND) increased by 30% in four patients with long-term abstinence or reduced alcohol consumption. VOI-based [(18) F]fallypride PET analyses revealed group differences in D(2/3) receptor availability primarily in extra-striatal regions. Age-related loss of dopamine D(2/3) receptors was more pronounced in AD patients. Receptor availability was unaltered by acute withdrawal, but increased in the subgroup of patients with long-term follow-up, suggesting reversibility of receptor changes.

The use of healthy volunteers instead of patients to inform drug dosing studies: a [¹¹C]raclopride PET study

RATIONALE

Receptor occupancy study has been performed to evaluate pharmacokinetic profiles in new antipsychotic drug development. While these findings highlight the value of positron emission tomography (PET) for dose-finding study, what is unclear is if it is necessary to conduct these studies in patients with schizophrenia or whether studies in healthy volunteers are adequate.

OBJECTIVES

To determine if it is necessary to conduct dopamine receptor occupancy studies in patients with schizophrenia or whether studies in healthy volunteers are adequate for dose-finding study, we compared the concentration-occupancy relationship in terms of EC(50) between patients and healthy volunteers.

METHODS

Ten healthy volunteers and eight patients with schizophrenia participated in the study. We measured dopamine receptor occupancy using [(11)C]raclopride PET and plasma concentration of YKP1358, a novel antipsychotic drug under clinical development, at a number of time points after the administration of YKP1358. Pharmacokinetic data including area under the plasma concentration versus time curve, elimination half-life, maximum observed plasma concentration, and the time to reach the maximum observed plasma concentration were obtained. We explored the relationship between plasma concentration and dopamine D(2) receptor occupancy using E (max) model and calculated EC(50).

RESULTS

The elimination half-life was longer in healthy volunteers than in patients. Other pharmacokinetic parameters were not significantly different between two groups. The EC(50) was 7.6 ng/ml (95% confidence interval (CI) 6.2-9.0) in healthy volunteers and 8.6 (95% CI 7.4-9.9) in patients.

CONCLUSIONS

The antipsychotic concentration-occupancy relationship in patients can be estimated from the EC(50) data of healthy volunteers.

The applicability of SRTM in [(18)F]fallypride PET investigations: impact of scan durations

The high-affinity radioligand [(18)F]fallypride (FP) is frequently used for quantification of striatal/extrastriatal D(2/3) receptors and the receptor occupancies of antipsychotics (APs). Its 110 minutes half-life allows long scan durations. However, the optimum scan duration is a matter of debate. This investigation focuses on scan-duration-related effects on simplified reference tissue model (SRTM) results and the time point of transient equilibrium in a large sample of dynamic FP positron emission tomography (PET) scans. Fifty drug-free and 50 AP-treated subjects underwent FP-PET scans (180 minutes scan duration). The binding potential (BP(ND)) of the putamen, thalamus, and temporal cortex were calculated using the SRTM and the transient equilibrium model. Furthermore, receptor occupancies were calculated for AP-treated patients. Transient equilibrium in the unblocked putamen occurred after 121±29.6 minutes. The transient equilibrium occurred much earlier in the extrastriatal regions or under AP treatment. Stepwise scan shortening caused BP(ND) underestimations of 0.58% for the first 10-minute reduction (putamen, SRTM), finally reaching 5.76% after 1 hour scan-time reduction. We observed preferential extrastriatal AP binding irrespective of the analytical method. [(18)F]fallypride scan durations of 180 minutes reliably reach equilibrium even in D(2/3)-receptor-rich regions. Moderate reductions in FP scan durations only caused small changes to SRTM results even in receptor-rich regions. Apparently, the D(2/3) receptor occupancy results of APs, especially preferential extrastriatal binding observations, are not relevantly biased by inappropriate scan durations.

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.

Measurement of the serotonin 1A receptor availability in patients with schizophrenia during treatment with the antipsychotic medication ziprasidone

The aim of this study was to compare 5-HT(1A) availability in vivo in individuals with schizophrenia before and during treatment with the atypical antipsychotic ziprasidone. Six individuals with schizophrenia underwent two PET scans with [(11)C]WAY 100635; the first while medication-free (baseline) and the second while taking the atypical antipsychotic ziprasidone (on-medication). Regional volumes of distribution (V(T), mL g(-1)) were derived using a two-tissue compartment kinetic model. Outcome measures included binding potential relative to the plasma (BP(P), mL g(-1)) and the binding potential relative to the nonspecific distribution volume (BP(ND), unitless). No significant differences were observed in regional BP(P) or BP(ND) with ziprasidone treatment. A significant correlation was noted between BP(P) measured in the orbitofrontal cortex during the on-medication condition and degree of improvement in negative symptoms with treatment (r = 0.96, p = 0.004). Consistent with the published literature of changes in 5-HT(1A) binding during treatment with 5-HT(1A) receptor agonists, this study did not detect a significant reduction in 5-HT(1A) binding with ziprasidone. The finding of a relationship between 5-HT(1A) binding and the degree of improvement in negative symptoms provides further support for the role of the 5-HT(1A) receptor in the pathophysiology and treatment of this symptom domain.

Dopamine D2/3 receptor occupancy by quetiapine in striatal and extrastriatal areas

Quetiapine is next to clozapine an antipsychotic agent that exerts hardly any extrapyramidal side-effects at clinical efficacious doses. Some previous receptor occupancy studies reported preferential extrastriatal D2/3 receptor (D2/3R)-binding properties of second-generation antipsychotics and suggested this as possible reason for improved tolerability. This positron emission tomography (PET) investigation was designed to compare the occupancy of dopamine D2/3Rs by quetiapine in striatal and extrastriatal brain regions. Therefore, a cohort of 16 quetiapine-treated psychotic patients underwent an [18F]fallypride (FP) PET scan. Due to the high affinity of FP and its comparatively long half-life, striatal and extrastriatal binding potentials could be determined in one single scan. Receptor occupancy was calculated as percent reduction in binding potential relative to age-matched medication-free patients suffering from schizophrenia. Quetiapine occupied 44+/-18% in the temporal cortex and 26+/-17% in the putamen, a difference significant at the level of p=0.005 (Student's t test). Quetiapine showed a mean occupancy of 36+/-16% and in the thalamus. In the caudate nucleus there was an occupancy of 29+/-16% (p=0.0072). Individual occupancy levels did not exceed 59% in any of the striatal volumes of interest. The time-interval between scan and last drug ingestion did not influence the relationship between plasma concentration and central D2/3R occupancy. Taken together, quetiapine shows preferential extrastriatal binding at D2/3Rs; the extent of this difference is comparable to that previously described for clozapine. Both antipsychotics show very low affinity for D2/3Rs.

Neurochemical imaging in schizophrenia

Recent advances in the development and applications of neurochemical brain imaging methods have improved the ability to study the neurochemistry of the living brain in normal processes as well as psychiatric disorders. In particular, positron emission tomography (PET) and single photon emission computed tomography (SPECT) have been used to determine neurochemical substrates of schizophrenia and to uncover the mechanism of action of antipsychotic medications. The growing availability of radiotracers for monoaminergic neurotransmitter synthesis, transporters and receptors, has enabled the evaluation of hypotheses regarding neurotransmitter function in schizophrenia derived from preclinical and clinical observations. This chapter reviews the studies using neurochemical brain imaging methods for (1) detection of abnormalities in indices of dopamine and serotonin transmission in patients with schizophrenia compared to controls, (2) development of new tools to study other neurotransmitters systems, such as gamma-aminobutyric acid (GABA) and glutamate, and (3) characterization of target occupancy by antipsychotic drugs, as well as its relationship to efficacy and side effects. As more imaging tools become available, this knowledge will expand and will lead to better detection of disease, as well as better therapeutic approaches.

The role of imaging in proof of concept for CNS drug discovery and development

Neuroimaging, particularly that of neuroreceptor radioisotope and functional magnetic resonance imaging (fMRI), has played a fundamental role in neuropharmacology and neurophysiology. Because of the unique and pioneering role, especially of the radiolabeling of central nervous system (CNS) drugs for receptor and neurotransmitter system imaging, there is an increasingly major role to aid in CNS drug development. One component is providing evidence for proof of concept of the target for which candidate drugs are being tested for receptor occupancy mechanism of action and ultimately rational drug dosing. There is also a role for other areas of neuroimaging, including fMRI and magnetic resonance spectroscopy in other magnetic resonance-based techniques that, together with radioisotope imaging, represent 'CNS molecular imaging.' The role of these approaches and a review of the recent advances in such neuroimaging for proof-of-concept studies is the subject for this paper. Moreover, hypothetical examples and possible algorithms for early discovery/phase I development using neuroimaging provide specific working approaches. In summary, this article reviews the vital biomarker approach of neuroimaging in proof of concept studies.