Test-retest reliability of central [11C]raclopride binding at high D2 receptor occupancy. A PET study in haloperidol-treated patients

Improving SUVR quantification by correcting for radiotracer clearance in tissue

Standardized Uptake Value Ratio (SUVR) is a widely reported semi-quantitative positron emission tomography (PET) outcome measure, partly because of its ease of measurement from short scan durations. However, in brain, SUVR is often a biased estimator of the gold-standard distribution volume ratio (DVR) due to non-equilibrium conditions, i.e., clearance of the radiotracer in relevant tissues. Factors that affect radiotracer metabolism and clearance such as medication or subject groups could lead to artificial differences in SUVR. This work developed a correction that reduces the bias in SUVR (estimated from a short 15-30 min PET imaging session) by accounting for the effects of tracer clearance observed during the late SUVR time window. The proposed correction takes the form of a one-step non-linear algebraic transform of SUVR that is a function of radiotracer dependent parameters such as clearance rates from the reference and target tissues, and population averaged reference region clearance rate (k 2 , ref ). An important observation was the need for accurate estimation of radiotracer clearance rate in target tissue, which was addressed with a regression based model. Simulations and human data from two different radiotracers (healthy controls for [11C]LSN3172176, healthy controls and Parkinson's disease subjects for [18F]FE-PE2I) were used to validate the correction and evaluate its benefits and limitations. SUVR correction in human data significantly reduced mean SUVR bias across brain regions and subjects (from ∼25% for SUVR to <10% for corrected SUVR). This correction also significantly reduced the variability of this bias across brain regions for both tracers (approximately 50% for [11C]LSN3172176, 20% for [18F]FE-PE2I). Future work should investigate the benefits of using corrected SUVR in other populations and with different tracers.

Reading abilities and dopamine D2/D3 receptor availability: An inverted U-shaped association in subjects with schizophrenia

Reading impairments are prominent trait-like features of cognitive deficits in schizophrenia, predictive of overall cognitive functioning and presumably linked to dopaminergic abnormalities. To evaluate this, we used ¹⁸F-fallypride PET in 19 healthy and 21 antipsychotic-naïve schizophrenia subjects and correlated dopamine receptor binding potentials in relevant AFNI-derived regions and voxelwise with group performance on WRAT4 single-word reading subtest. Healthy subjects' scores were positively and linearly associated with D₂/D₃ receptor availability in the rectus, orbital and superior frontal gyri, fusiform and middle temporal gyri, as well as middle occipital gyrus and precuneus, all predominantly in the left hemisphere and previously implicated in reading, hence suggesting that higher dopamine receptor density is cognitively advantageous. This relationship was weakened in schizophrenia subjects and in contrast to healthy participants followed an inverted U-shaped curve both in the cortex and dorsal striatum, indicating restricted optimal range of dopamine D₂/D₃ receptor availability for cognitive performance in schizophrenia.

High long-term test-retest reliability for extrastriatal 11C-raclopride binding in healthy older adults

In vivo dopamine D2-receptor availability is frequently assessed with ¹¹C-raclopride and positron emission tomography. Due to low signal-to-noise ratios for ¹¹C-raclopride in areas with low D2 receptor densities, the ligand has been considered unreliable for measurements outside the dopamine-dense striatum. Intriguingly, recent studies show that extrastriatal ¹¹C-raclopride binding potential (BP_ND) values are (i) reliably higher than in the cerebellum (where D2-receptor levels are negligible), (ii) correlate with behavior in the expected direction, and (iii) showed good test-retest reliability in a sample of younger adults. The present work demonstrates high seven-month test-retest reliability of striatal and extrastriatal ¹¹C-raclopride BP_ND values in healthy, older adults (n = 27, age: 64-78 years). Mean ¹¹C-raclopride BP_ND values were stable between test sessions in subcortical nuclei, and in frontal and temporal cortices (p > 0.05). Across all structures analyzed, intraclass correlation coefficients were high (0.85-0.96), absolute variability was low (mean: 4-8%), and coefficients of variance ranged between 9 and 25%. Furthermore, regional ¹¹C-raclopride BP_ND values correlated with previously determined ¹⁸F-fallypride BP_ND values (ρ = 0.97 and 0.92 in correlations with and without striatal values, respectively, p < 0.01) and postmortem determined D2-receptor densities (including striatum: ρ = 0.92; p < 0.001; excluding striatum: ρ = 0.75; p = 0.067). These observations suggest that extrastriatal ¹¹C-raclopride measurements represent a true D2 signal.

Quantification and reliability of [11C]VC - 002 binding to muscarinic acetylcholine receptors in the human lung - a test-retest PET study in control subjects

BACKGROUND

The radioligand [11C]VC-002 was introduced in a small initial study long ago for imaging of muscarinic acetylcholine receptors (mAChRs) in human lungs using positron emission tomography (PET). The objectives of the present study in control subjects were to advance the methodology for quantification of [11C]VC-002 binding in lung and to examine the reliability using a test-retest paradigm. This work constituted a self-standing preparatory step in a larger clinical trial aiming at estimating mAChR occupancy in the human lungs following inhalation of mAChR antagonists.

METHODS

PET measurements using [11C]VC-002 and the GE Discovery 710 PET/CT system were performed in seven control subjects at two separate occasions, 2-19 days apart. One subject discontinued the study after the first measurement. Radioligand binding to mAChRs in lung was quantified using an image-derived arterial input function. The total distribution volume (VT) values were obtained on a regional and voxel-by-voxel basis. Kinetic one-tissue and two-tissue compartment models (1TCM, 2TCM), analysis based on linearization of the compartment models (multilinear Logan) and image analysis by data-driven estimation of parametric images based on compartmental theory (DEPICT) were applied. The test-retest repeatability of VT estimates was evaluated by absolute variability (VAR) and intraclass correlation coefficients (ICCs).

RESULTS

The 1TCM was the statistically preferred model for description of [11C]VC-002 binding in the lungs. Low VAR (< 10%) across analysis methods indicated good reliability of the PET measurements. The VT estimates were stable after 60 min.

CONCLUSIONS

The kinetic behaviour and good repeatability of [11C]VC-002 as well as the novel lung image analysis methodology support its application in applied studies on drug-induced mAChR receptor occupancy and the pathophysiology of pulmonary disorders.

TRIAL REGISTRATION

ClinicalTrials.gov identifier: NCT03097380, registered: 31 March 2017.

[11C]SCH23390 binding to the D1-dopamine receptor in the human brain-a comparison of manual and automated methods for image analysis

Background

The D₁-dopamine receptor radioligand [¹¹C]SCH23390 has been frequently used in PET studies. In drug-naïve patients with schizophrenia, the findings have been inconsistent, with decreases, increases, and no change in the frontal cortex D₁-dopamine receptors. While these discrepancies are likely primarily due to a lack of statistical power in these studies, we speculated that an additional explanation may be the differences due to methods of image analysis between studies, affecting reliability as well as bias between groups.

Methods

Fifteen healthy subjects underwent two PET measurements with [¹¹C]SCH23390 on the same day. The binding potential (BP_ND) was compared using a 95% confidence interval following manual and automated delineation of a region of interest (ROI) as well as with and without frame-by-frame realignment.

Results

Automated target region delineation produced lower BP_ND values, while automated delineation of the reference region yielded higher BP_ND values. However, no significant differences were observed for repeatability using automated and manual delineation methods. Frame-by-frame realignment generated higher BP_ND values and improved repeatability.

Conclusions

The results suggest that the choice of ROI delineation method is not an important factor for reliability, whereas the improved results following movement correction confirm its importance in PET image analysis. Realignment is therefore especially important for measurements in patient populations such as schizophrenia or Parkinson’s disease, where motion artifacts may be more prevalent.

Electronic supplementary material

The online version of this article (10.1186/s13550-018-0416-2) contains supplementary material, which is available to authorized users.

Dual time-point imaging for post-dose binding potential estimation applied to a [11C]raclopride PET dose occupancy study

Receptor occupancy studies performed with PET often require time-consuming dynamic imaging for baseline and post-dose scans. Shorter protocol approximations based on standard uptake value ratios have been proposed. However, such methods depend on the time-point chosen for the quantification and often lead to overestimation and bias. The aim of this study was to develop a shorter protocol for the quantification of post-dose scans using a dual time-point approximation, which employs kinetic parameters from the baseline scan. Dual time-point was evaluated for a [¹¹C]raclopride PET dose occupancy study with the D2 antagonist JNJ-37822681, obtaining estimates for binding potential and receptor occupancy. Results were compared to standard simplified reference tissue model and standard uptake value ratios-based estimates. Linear regression and Bland-Altman analysis demonstrated excellent correlation and agreement between dual time-point and the standard simplified reference tissue model approach. Moreover, the stability of dual time-point-based estimates is shown to be independent of the time-point chosen for quantification. Therefore, a dual time-point imaging protocol can be applied to post-dose [¹¹C]raclopride PET scans, resulting in a significant reduction in total acquisition time while maintaining accuracy in the quantification of both the binding potential and the receptor occupancy.

Dopamine D2/3 Receptor Occupancy Following Dose Reduction Is Predictable With Minimal Plasma Antipsychotic Concentrations: An Open-Label Clinical Trial

BACKGROUND

Population pharmacokinetics can predict antipsychotic blood concentrations at a given time point prior to a dosage change. Those predicted blood concentrations could be used to estimate the corresponding dopamine D2/3 receptors (D2/3R) occupancy by antipsychotics based on the tight relationship between blood and brain pharmacokinetics. However, this 2-step prediction has never been tested.

METHODS

Two blood samples were collected at separate time points from 32 clinically stable outpatients with schizophrenia (Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition; mean ± SD age: 60.1 ± 7.3 years) to measure plasma concentrations of olanzapine or risperidone at baseline. Then, subjects underwent a dose reduction of olanzapine or risperidone and completed a [(11)C]-raclopride positron emission tomography scan to measure D2/3R occupancy in the putamen. The plasma concentration at the time of the scan was predicted with the 2 samples based on population pharmacokinetic model, using NONMEM. D2/3R occupancy was then estimated by incorporating the predicted plasma concentration in a hyperbole saturation model. The predicted occupancy was compared to the observed value.

RESULTS

The mean (95% CI) prediction errors for the prediction of D2/3R occupancy were -1.76% (-5.11 to 1.58) for olanzapine and 0.64% (-6.18 to 7.46) for risperidone. The observed and predicted D2/3R occupancy levels were highly correlated (r = 0.67, P = .001 for olanzapine; r = 0.67, P = .02 for risperidone).

CONCLUSIONS

D2/3R occupancy levels can be predicted from blood drug concentrations collected prior to dosage change. Although this 2-step model is subject to a small degree of error, it could be used to select oral doses aimed at achieving optimal D2/3R occupancy on an individual basis.

Dose and dosing frequency of long-acting injectable antipsychotics: a systematic review of PET and SPECT data and clinical implications

Brain imaging data of antipsychotics have mainly been derived from oral antipsychotic drugs, which hampers our understanding of the requirement of dose/dosing frequency of long-acting injectable (LAI) antipsychotics for the maintenance treatment of schizophrenia. A systematic literature search was performed to identify positron emission tomography (PET) and single-photon emission computed tomography (SPECT) studies that assessed dopamine D2 receptor occupancy levels with LAI antipsychotic drugs in humans, using PubMed, EMBASE, and PsycINFO (last search, February 2013). Twenty studies (15 PET and 5 SPECT studies) were identified. The most investigated drug in these PET and SPECT studies was haloperidol decanoate (44 subjects; 11 studies), followed by risperidone LAI (24 subjects; 3 studies), olanzapine pamoate (14 subject; 1 study), and fluphenazine decanoate (12 subjects; 3 studies). The data have demonstrated high and continuous D2 receptor blockade with LAIs; the effects of LAI first-generation antipsychotics on the central nervous system may persist for several months. The prospective and cross-sectional studies showed that continuous dopamine D2 receptor blockade above 65% (ie, the lower end of the established therapeutic window for the acute phase treatment) was not always necessary for maintenance treatment for at least some of the patients. In conclusion, because of the limited brain imaging data on LAI antipsychotics, we still do not know the best way to dose them. Still, the currently available brain imaging data raises a possibility that the dosing interval of LAI antipsychotics may be extended beyond the currently indicated range in some patients.

Therapeutic window for striatal dopamine D(2/3) receptor occupancy in older patients with schizophrenia: a pilot PET study

OBJECTIVE

In younger patients with schizophrenia, positron emission tomography (PET) studies have identified a therapeutic window of striatal dopamine D(2/3) receptor occupancy of 65%-80%. This type of empirical information is not available in late life. Our primary aim was to assess the effect of changes in D(2/3) relative receptor occupancy (RRO) on clinical outcomes in this population.

DESIGN

Open-label intervention.

SETTING

Centre for Addiction and Mental Health, Toronto.

PARTICIPANTS

Subjects with schizophrenia age 50 years or more who were clinically stable and previously maintained on oral risperidone for D(2/3) RRO in dorsal putamen was assessed, using the region of interest analysis of [¹¹C]raclopride PET scans, before and after the dose reduction. Clinical assessments included the Positive and Negative Syndrome Scale and the Simpson-Angus Scale.

RESULTS

Nine subjects (mean ± SD age: 58 ± 7 years; mean ± SD baseline risperidone dose: 3.4 ± 1.6 mg/day) participated in the study. Extrapyramidal symptoms (EPS) were present in six subjects and were associated with 70% or more D(2/3) RRO in the putamen (range: 70%-87%). Following the dose reduction, EPS resolved in five subjects. Two subjects experienced a clinical worsening at 52% and at less than 50% D(2/3) RRO.

CONCLUSION

EPS diminished less than 70% D(2/3) RRO, which suggests a lower therapeutic window for older patients with schizophrenia than that for younger patients. Although these findings have to be replicated in a larger sample, they have important implications for future drug development and clinical guidelines in late-life schizophrenia.

Relationship between impulsivity, prefrontal anticipatory activation, and striatal dopamine release during rewarded task performance

Impulsivity, and in particular the negative urgency aspect of this trait, is associated with poor inhibitory control when experiencing negative emotion. Individual differences in aspects of impulsivity have been correlated with striatal dopamine D2/D3 receptor availability and function. This multi-modal pilot study used both positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) to evaluate dopaminergic and neural activity, respectively, using modified versions of the monetary incentive delay task. Twelve healthy female subjects underwent both scans and completed the NEO Personality Inventory Revised to assess Impulsiveness (IMP). We examined the relationship between nucleus accumbens (NAcc) dopaminergic incentive/reward release, measured as a change in D2/D3 binding potential between neutral and incentive/reward conditions with [(11)C]raclopride PET, and blood oxygen level-dependent (BOLD) activation elicited during the anticipation of rewards, measured with fMRI. Left NAcc incentive/reward dopaminergic release correlated with anticipatory reward activation within the medial prefrontal cortex (mPFC), left angular gyrus, mammillary bodies, and left superior frontal cortex. Activation in the mPFC negatively correlated with IMP and mediated the relationship between IMP and incentive/reward dopaminergic release in left NAcc. The mPFC, with a regulatory role in learning and valuation, may influence dopamine incentive/reward release.

Dual-isotope SPECT imaging of striatal dopamine: a comparative study between never-treated and haloperidol-treated first-episode schizophrenic patients

The aim of this dual-isotope SPECT imaging study was to evaluate striatal dopamine transporter (DAT) and D2 receptor availability in first-episode never-treated and haloperidol-treated schizophrenic patients and whether the availability is associated with psychopathology. Twenty-four inpatients with a first acute schizophrenic episode were enrolled in the study; 12 of these patients were treated with haloperidol for 2 weeks before dual-isotope SPECT was performed, whereas the other 12 patients underwent the SPECT evaluation directly after enrollment. Twelve healthy control persons were also recruited and evaluated with the dual-isotope SPECT protocol. Psychopathology was assessed by the Positive and Negative Syndrome Scale and other scales. D2-radioligand binding did not differ between drug-naïve patients and the control group but was significantly lower in the haloperidol-treated group. DAT availability was also significantly lower in the haloperidol patients than in the other two groups and differed significantly between drug-naïve, positive-syndrome-type patients and healthy controls. The data obtained with the new dual-isotope SPECT technique reveal a direct effect of haloperidol at the D2 and DAT receptor level.

Comparison of D₂ dopamine receptor occupancy after oral administration of quetiapine fumarate immediate-release and extended-release formulations in healthy subjects

Quetiapine is an established drug for treatment of schizophrenia, bipolar disorder, and major depressive disorder. While initially manufactured as an immediate-release (IR) formulation, an extended-release (XR) formulation has recently been introduced. Pharmacokinetic studies show that quetiapine XR provides a lower peak and more stable plasma concentration than the IR formulation. This study investigated if the pharmacokinetic differences translate into different time curves for central D₂ dopamine receptor occupancy. Eleven control subjects were examined with positron emission tomography (PET) and the radioligand [11C]raclopride. Eight subjects underwent all of the scheduled PET measurements. After baseline examination, quetiapine XR was administered once-daily for 8 d titrated to 300 mg/d on days 5-8, followed by 300 mg/d quetiapine IR on days 9-12. PET measurements were repeated after the last doses of quetiapine XR and IR at predicted times of peak and trough plasma concentrations. Striatal D₂ receptor occupancy was calculated using the simplified reference tissue model. Peak D₂ receptor occupancy was significantly higher with quetiapine IR than XR in all subjects (50 ± 4% and 32 ± 11%, respectively), consistent with lower peak plasma concentrations for the XR formulation. Trough D₂ receptor occupancy was similarly low for both formulations (IR 7 ± 7%, XR 8 ± 6%). The lower peak receptor occupancy associated with quetiapine XR may explain observed pharmacodynamic differences between the formulations. Assuming that our findings in control subjects are valid for patients with schizophrenia, the study supports the view that quetiapine, like the prototype atypical antipsychotic clozapine, may show antipsychotic effect at lower D₂ receptor occupancy than typical antipsychotics.

How sequential studies inform drug development: evaluating the effect of food intake on optimal bioavailability of ziprasidone

Determining a drug dosing regimen that produces consistent bioavailability and patient effects is one of the goals of the drug development process. Food consumption is one factor that can significantly alter the bioavailability of some drugs. This manuscript describes a research approach to determine what recommendations to give patients regarding taking oral ziprasidone in relation to food consumption. Four pharmacokinetic studies, the first three in volunteers and the fourth in patients at steady-state on the maximum recommended daily dose of ziprasidone, investigated the relationship between food intake and ziprasidone absorption. These studies illustrate how sequential studies are used in drug development to investigate increasingly precise questions using data from one study to refine the question being addressed by the next. In the first study, the absorption of ziprasidone was shown to double when taken following a high-calorie, high-fat meal versus the fasting state. The second study showed that the difference in absorption between the fasting and fed states increased with dose. The third study suggested that calorie rather than fat content was the critical variable. This finding was confirmed in the fourth study over a wider variety of meals and under clinically relevant dosing conditions. That study also found reduced pharmacokinetic variability (i.e., more consistent absorption) when ziprasidone was administered with 500-1000 kcal meals without regard to fat content rather than under fasting or low-calorie meal conditions. These results have several clinically important implications. First, the effect of taking ziprasidone in a fasting state cannot be overcome simply by increasing the dose. Second, significant swings in ziprasidone concentration and hence efficacy and tolerability may occur on a day-to-day basis if diet is not controlled. Third, patients should be advised to take ziprasidone with a meal containing at least 500 calories (without regard to fat content) to ensure adequate ziprasidone bioavailability and thus achieve optimal efficacy. These four studies illustrate the sequential and incremental nature of drug development research and what is meant by the concept of bioequivalence.

Long-term stability of measuring D(2) receptors in schizophrenia patients treated with antipsychotics

BACKGROUND

While antipsychotic-free schizophrenia patients showed a high degree of within-subject variability in dopamine D(2) receptor density over 6-24 months, no study has examined the long-term stability of D(2) receptor measures in medicated patients.

METHODS

Four schizophrenia patients receiving a stable dose of risperidone underwent [(11)C]raclopride positron emission tomography scans on two occasions 5-14 months apart.

RESULTS

Plasma risperidone levels were found to be consistent between scans, and consistencies of nondisplaceable D(2) binding potential and D(2) occupancy were good.

CONCLUSIONS

The finding supports the validity of quantification of D(2) receptor binding in longitudinal PET studies of medicated patients with schizophrenia.

Review of longitudinal functional neuroimaging studies of drug treatments in patients with schizophrenia

We systematically reviewed twenty-one functional neuroimaging studies that used longitudinal designs to investigate the effects of medication treatments on brain functioning among patients with schizophrenia. The studies reviewed were comprised of functional magnetic resonance imaging and positron emission tomography research using a baseline and at least one follow-up. The present review summarizes the different effects of medication and disease status on brain function, with attention to functional normalization, specific drug effects, and comparisons of typical versus atypical antipsychotics. Particular emphasis is given to methodological limitations in the existing literature, including lack of reliability data, clinical heterogeneity among studies, and inadequate study designs and statistics. Suggestions are made for improving future longitudinal neuroimaging studies of treatment effects in schizophrenia.

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

RATIONALE

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

OBJECTIVES

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

METHODS

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

Differentiation of extrastriatal dopamine D2 receptor density and affinity in the human brain using PET

Dopaminergic neurotransmission in extrastriatal regions may play a crucial role in the pathophysiology and treatment of neuropsychiatric disorders. The high-affinity radioligands [(11)C]FLB 457, [(123)I]epidepride, and [(18)F]fallypride are now used in clinical studies to measure these low-density receptor populations in vivo. However, a single determination of the regional binding potential (BP) does not differentiate receptor density (B(max)) from the apparent affinity (K(D)). In this positron emission tomography (PET) study, we measured extrastriatal dopamine D2 receptor density (B(max)) and apparent affinity (K(D)) in 10 healthy subjects using an in vivo saturation approach. Each subject participated in two to three PET measurements with different specific radioactivity of [(11)C]FLB 457. The commonly used simplified reference tissue model (SRTM) was used in a comparison of BP values with the B(max) values obtained from the saturation analysis. The calculated regional receptor density values were of the same magnitude (0.33-1.68 nM) and showed the same rank order as reported from postmortem studies, that is, in descending order thalamus, lateral temporal cortex, anterior cinguli, and frontal cortex. The affinity ranged from 0.27 to 0.43 nM, that is, approximately 10-20 times the value found in vitro (20 pM). The area under the cerebellar time activity curve (TAC) was slightly lower (11 +/- 8%, mean +/- SD, P = 0.004, n = 10) after injection of low as compared with high specific radioactivity, indicating sensitivity to the minute density of dopamine D2 receptors in the this region. The results of the present study support that dopamine D2 receptor density and affinity can be differentiated in low-density regions using a saturation approach. There was a significant (P < 0.001) correlation between the binding potential calculated with SRTM and the receptor density (B(max)), which supports the use of BP in clinical studies where differentiation of B(max) and K(D) is not required. In such studies, the mass of FLB 457 has to be less than 0.5 microg injected to avoid a mass effect of the radioligand itself.

Measurement of striatal D2 dopamine receptor density and affinity with [11C]-raclopride in vivo: a test-retest analysis

Subacute and long-term stability of measurements of D2 dopamine receptor density (Bmax), affinity (Kd) was studied with positron emission tomography in eight healthy male volunteers. [11C]-Raclopride and the transient equilibrium method were used to measure D2 receptor characteristics. The interval between measurements (scan pairs) was 3 to 7 weeks (subacute) for four subjects and 6 to 11 months (long-term) for four subjects. A test-retest analysis of quantitative measurements of D2 receptor Bmax and Kd was compared with that done on binding potential (BP, Bmax/Kd) measures. In addition, the effect of error in defining the transient equilibrium time (tmax) in the parameter estimation procedure was explored with simulations. The subacute test-retest indicates good reproducibility of D2 receptor density, affinity, and BP ratio measurements with intraclass correlation coefficients of 0.90, 0.96, and 0.86, respectively. The variability of the measurements after 6 to 11 months was slightly higher than that seen in a subacute testing for Kd and more clearly so for binding potential and Bmax. The absolute variability in Bmax (14.5%) measurements was consistently higher than that of Kd (8.4%) or BP (7.9%) both in subacute and long-term measurements. Simulations indicated that the Bmax and Kd estimation procedure is more sensitive to error in the tmax than that for the BP. The results indicate a good overall stability of the equilibrium method with [11C]raclopride for measuring dopamine D2 receptor binding characteristics in the striatum. The BP approach is more stable than Kd and especially Bmax measurements. Error in defining the tmax in particular in the low specific radioactivity scan may be one source of greater variability in Bmax versus BP. However, a higher intraindividual variability in measurements of the D2 receptor Bmax also may include a component of continuous regulation of this parameter over time. These methodologic aspects should be considered in the design and interpretation of longitudinal studies on D2 dopamine receptor characteristics with [11C]-raclopride.

Functional imaging of the brain in the evaluation of drug response and its application to the study of aging

Functional neuroimaging techniques including single photon emission computerised tomography (SPECT), positron emission tomography (PET) and functional magnetic resonance imaging (FMRI) can provide insight into the functional connectivity of the human brain in both health and disease, including the effects of aging and drugs on brain function. Neuroimaging measurement techniques can either be direct, using radio-specific ligands, or indirect, using the neurophysiological consequences of pharmacological interventions. Both approaches can be combined with sensorimotor or cognitive activation to examine the interaction between the targeted receptor function and the sensorimotor or cognitive process implicit in the study design. Using radionuclides, PET can provide absolute measurement of cerebral blood flow to regions of interest and can measure changes in cerebral metabolism using labelled fluorodeoxyglucose. PET offered the first opportunity to image brain activation caused by a variety of stimuli and hence to measure the effect of drugs on brain activation. PET also enables the study of drug disposition within the brain. SPECT has been used to study relative changes in cerebral blood flow associated with disease processes and also receptor occupancy. FMRI, by contrast, does not involve ionising radiation and has better spatial and temporal resolution. It is still a relatively new technique and limited by its ability to only measure haemodynamic changes through the blood oxygen level-dependent (BOLD) signal. The effects of aging on drug responsiveness and the effects of drug treatment of diseases associated with old age are relatively unexplored areas of functional neuroimaging research.

Chemical and physiologic brain imaging in schizophrenia

Technologic advances in functional brain imaging have provided exciting and informative insights into the functional neuroanatomy and neurochemistry of schizophrenia. Using MR spectroscopy, it has been possible to examine in vivo brain metabolism and to relate observed changes to physiological processes occurring at a cellular level. Positron emission tomography and single photon emission computed tomography have revealed disturbances of cerebral blood flow and glucose metabolism in patients with schizophrenia. More recently, these tools have also proved most useful in studying the relative receptor occupancy of typical and atypical antipsychotic medications.