Non-linear mixed effects modelling of positron emission tomography data for simultaneous estimation of radioligand kinetics and occupancy in healthy volunteers

Evaluation of [18F]FMTEB in Sprague Dawley rats as a PET tracer for metabotropic glutamate receptor 5

INTRODUCTION

[¹⁸F]FMTEB, along with other tracers, was developed as a promising PET radioligand for imaging metabotropic glutamate receptor subtype 5 (mGluR5). Despite favorable preliminary results, it has not been used further for studies of mGluR5. This paper presents an in-depth preclinical evaluation of [¹⁸F]FMTEB in healthy Sprague Dawley rats.

METHODS

[¹⁸F]FMTEB was synthesized from a boronic ester precursor using copper-mediated fluorination. In vivo PET imaging was performed on six rats, of which three were pre-treated with a high affinity mGluR5 receptor antagonist. An additional 18 rats were used for ex vivo experiments for metabolite analyses in plasma, brain and urine, and for biodistribution and ex vivo brain autoradiography at different time points.

RESULTS

[¹⁸F]FMTEB was synthesized in adequate radiochemical yield and a molar activity of 154 ± 64 GBq/μmol. Both in vivo imaging and ex vivo brain autoradiography showed high specificity for mGluR5, and the blocking experiments showed a clear decrease in radioactivity in mGluR5-rich brain areas. Metabolite analyses confirmed fast metabolism of the tracer in plasma. The percentage of parent compound in brain tissue exceeded 90 % up to 90 min after injection.

CONCLUSION

[¹⁸F]FMTEB produced via copper-mediated ¹⁸F-fluorination fulfilled the requirements for preclinical evaluation in rats. The absence of specific uptake in cerebellum and absence of defluorination of the tracer allowed cerebellum to be used as a reference tissue. Due to the fast kinetics in rats, the region-to-cerebellum ratios equilibrated within 30 min. These results prove [¹⁸F]FMTEB to be a good candidate for mapping mGluR5 in rat brain and a suitable alternative to [¹⁸F]FPEB.

Design and Measurement of Drug Tissue Concentration Asymmetry and Tissue Exposure-Effect (Tissue PK-PD) Evaluation

The "free drug hypothesis" assumes that, in the absence of transporters, the steady state free plasma concentrations equal to that at the site of action that elicit pharmacologic effects. While it is important to utilize the free drug hypothesis, exceptions exist that the free plasma exposures, either at C_max, C_trough, and C_average, or at other time points, cannot represent the corresponding free tissue concentrations. This "drug concentration asymmetry" in both total and free form can influence drug disposition and pharmacological effects. In this review, we first discuss options to assess total and free drug concentrations in tissues. Then various drug design strategies to achieve concentration asymmetry are presented. Last, the utilities of tissue concentrations in understanding exposure-effect relationships and translational projections to humans are discussed for several therapeutic areas and modalities. A thorough understanding in plasma and tissue exposures correlation with pharmacologic effects can provide insightful guidance to aid drug discovery.

Simultaneous multifactor Bayesian analysis (SiMBA) of PET time activity curve data

Positron emission tomography (PET) is an in vivo imaging method essential for studying the neurochemical pathophysiology of psychiatric and neurological disease. However, its high cost and exposure of participants to radiation make it unfeasible to employ large sample sizes. The major shortcoming of PET imaging is therefore its lack of power for studying clinically-relevant research questions. Here, we introduce a new method for performing PET quantification and analysis called SiMBA, which helps to alleviate these issues by improving the efficiency of PET analysis by exploiting similarities between both individuals and regions within individuals. In simulated [¹¹C]WAY100635 data, SiMBA greatly improves both statistical power and the consistency of effect size estimation without affecting the false positive rate. This approach makes use of hierarchical, multifactor, multivariate Bayesian modelling to effectively borrow strength across the whole dataset to improve stability and robustness to measurement error. In so doing, parameter identifiability and estimation are improved, without sacrificing model interpretability. This comes at the cost of increased computational overhead, however this is practically negligible relative to the time taken to collect PET data. This method has the potential to make it possible to test clinically-relevant hypotheses which could never be studied before given the practical constraints. Furthermore, because this method does not require any additional information over and above that required for traditional analysis, it makes it possible to re-examine data which has already previously been collected at great expense. In the absence of dramatic advancements in PET image data quality, radiotracer development, or data sharing, PET imaging has been fundamentally limited in the scope of research hypotheses which could be studied. This method, especially combined with the recent steps taken by the PET imaging community to embrace data sharing, will make it possible to greatly improve the research possibilities and clinical relevance of PET neuroimaging.

In Vivo Receptor Visualization and Evaluation of Receptor Occupancy with Positron Emission Tomography

Positron emission tomography (PET) is useful for noninvasive in vivo visualization of disease-related receptors, for evaluation of receptor occupancy to determine an appropriate drug dosage, and for proof-of-concept of drug candidates in translational research. For these purposes, the specificity of the PET tracer for the target receptor is critical. Here, we review work in this area, focusing on the chemical structures of reported PET tracers, their K_i/K_d values, and the physical properties relevant to target receptor selectivity. Among these physical properties, such as cLogP, cLogD, molecular weight, topological polar surface area, number of hydrogen bond donors, and pK_a, we focus especially on LogD and LogP as important physical properties that can be easily compared across a range of studies. We discuss the success of PET tracers in evaluating receptor occupancy and consider likely future developments in the field.

The pro-psychotic metabotropic glutamate receptor compounds fenobam and AZD9272 share binding sites with monoamine oxidase-B inhibitors in humans

The metabotropic glutamate receptor 5 (mGluR5) ligands fenobam and AZD9272 have been reported to induce psychosis-like adverse events and to bind at unknown, non-GluR5-related, sites. Based on similarities of the regional binding patterns for [¹¹C]AZD9272 and the monoamine oxidase-B (MAO-B) radioligand [¹¹C]L-deprenyl-D2 in PET studies of the human brain we tested the hypothesis that the unique binding of fenobam and AZD9272 may represent specific binding to the MAO-B. PET data previously acquired for subjects examined using [¹¹C]AZD9272 or [¹¹C]L-deprenyl-D2 were re-evaluated to assess the correlations between radioligand binding parameters in human brain. In addition, the pharmacology of AZD9272 binding sites was characterized using competition binding studies carried out in vivo in non-human primates (NHPs) and in vitro using autoradiography in selected human brain regions. The regional binding of [¹¹C]AZD9272 in human brain was closely correlated with that of [¹¹C]L-deprenyl-D2. In PET studies of NHP brain administration of the MAO-B ligand L-deprenyl inhibited binding of radiolabeled AZD9272 and administration of fenobam inhibited binding of [¹¹C]L-deprenyl-D2. Binding of radiolabeled AZD9272 in vitro was potently inhibited by fenobam or MAO-B compounds, and [¹¹C]L-deprenyl-D2 binding was inhibited by fenobam or AZD9272. The findings are consistent with the hypothesis that both fenobam and AZD9272 bind to the MAO-B, which may be of relevance for understanding the mechanism of the psychosis-like adverse events reported for these compounds. Such understanding may serve as a lead to generate new models for the pathophysiology of psychosis.

Nonlinear Mixed-Effects Models for PET Data

OBJECTIVE

The current state-of-the-art for compartment modeling of dynamic PET data can be described as a two-stage approach. In Stage 1, individual estimates of kinetic parameters are obtained by fitting models using standard techniques, such as nonlinear least squares, to each individual's data one subject at a time. Population-level effects, such as the difference between diagnostic groups, are analyzed in Stage 2 using standard statistical methods by treating the individual estimates as if they were observed data. While this approach is generally valid, it is possible to increase efficiency and precision of the analysis, allow more complex models to be fitted, and also to permit parameter-specific investigation by fitting data across subjects simultaneously. We explore the application of nonlinear mixed-effects (NLME) models for estimation and inference in this setting.

METHODS

In the NLME framework, subjects are modeled simultaneously through the inclusion of random effects of subjects for each kinetic parameter; meanwhile, population parameters are estimated directly in a joint model.

RESULTS

Simulation results indicate that NLME outperforms the two-stage approach in estimating group-level effects and also has improved power to detect differences across groups. We applied our NLME approach to clinical PET data and found effects not detected by the two-stage approach.

CONCLUSION

The proposed NLME approach is more accurate and correspondingly more powerful than the two-stage approach in compartment modeling of PET data.

SIGNIFICANCE

The NLME method can broaden the methodological scope of PET modeling because of its efficiency and stability.

The metabotropic glutamate receptor 5 radioligand [11C]AZD9272 identifies unique binding sites in primate brain

The metabotropic glutamate receptor 5 (mGluR5) is a target for drug development and for imaging studies of the glutamate system in neurological and psychiatric disorders. [¹¹C]AZD9272 is a selective mGluR5 PET radioligand that is structurally different from hitherto applied mGluR5 radioligands. In the present investigation we compared the binding patterns of radiolabeled AZD9272 and other mGluR5 radioligands in the non-human primate (NHP) brain. PET studies were undertaken using [¹¹C]AZD9272 and the commonly applied mGluR5 radioligand [¹¹C]ABP688. Autoradiography studies were performed in vitro using [³H]AZD9272 and the standard mGluR5 radioligands [³H]M-MTEP and [³H]ABP688 in NHP tissue. Competition binding studies were undertaken in vivo and in vitro using different mGluR5 selective compounds as inhibitors. In comparison to other mGluR5 radioligands radiolabeled AZD9272 displayed a distinct regional distribution pattern with high binding in ventral striatum, midbrain, thalamus and cerebellum. While the binding of [¹¹C]AZD9272 was almost completely inhibited by the structurally unique mGluR5 compound fenobam (2.0 mg/kg; 98% occupancy), it was only partially inhibited (46% and 20%, respectively) by the mGluR5 selective compounds ABP688 and MTEP, at a dose (2.0 mg/kg) expected to saturate the mGluR5. Autoradiography studies using [³H]AZD9272 confirmed a distinct pharmacologic profile characterized by preferential sensitivity to fenobam. The distinctive binding in ventral striato-pallido-thalamic circuits and shared pharmacologic profile with the pro-psychotic compound fenobam warrants further examination of [¹¹C]AZD9272 for potential application in psychiatric neuroimaging studies.

Metabotropic glutamate receptor 5 tracer [18F]-FPEB displays increased binding potential in postcentral gyrus and cerebellum of male individuals with autism: a pilot PET study

Background

Autism is a neurodevelopmental disorder that is first manifested during early childhood. Postmortem experiments have identified significantly elevated expression of metabotropic glutamate receptor 5 (mGluR5) in cerebellar vermis and prefrontal cortex of individuals with autism.

Methods

In the current study we employed the mGluR5 tracer [¹⁸F]-3-fluoro-5-[(pyridin-3-yl)ethynyl]benzonitrile ([¹⁸F]-FPEB) to quantify mGluR5 binding in vivo in adults with autism vs. healthy controls using positron emission tomography (PET).

Results

We identified significantly higher [¹⁸F]-FPEB binding potential in the postcentral gyrus and cerebellum of individuals with autism. There was a significant negative correlation between age and [¹⁸F]-FPEB binding potential in the cerebellum but not in the postcentral gyrus. In the precuneus, [¹⁸F]-FPEB binding potential correlated positively with the lethargy subscale score for the Aberrant Behavioral Checklist (ABC). In cerebellum, there were significant negative correlations between [¹⁸F]-FPEB binding potential and ABC total score, ABC hyperactivity subscale score, and the ABC inappropriate speech subscale score.

Conclusions

These novel findings demonstrate for the first time that mGluR5 binding is altered in critical brain areas of subjects with autism, suggesting abnormal glutamate signaling in these regions. Finally, the correlations between altered [¹⁸F]-FPEB binding potential in the cerebellum and precuneus suggest that some autistic symptoms may be influenced by abnormal glutamate signaling.

Target and Tissue Selectivity Prediction by Integrated Mechanistic Pharmacokinetic-Target Binding and Quantitative Structure Activity Modeling

Selectivity is an important attribute of effective and safe drugs, and prediction of in vivo target and tissue selectivity would likely improve drug development success rates. However, a lack of understanding of the underlying (pharmacological) mechanisms and availability of directly applicable predictive methods complicates the prediction of selectivity. We explore the value of combining physiologically based pharmacokinetic (PBPK) modeling with quantitative structure-activity relationship (QSAR) modeling to predict the influence of the target dissociation constant (K _D) and the target dissociation rate constant on target and tissue selectivity. The K _D values of CB1 ligands in the ChEMBL database are predicted by QSAR random forest (RF) modeling for the CB1 receptor and known off-targets (TRPV1, mGlu5, 5-HT1a). Of these CB1 ligands, rimonabant, CP-55940, and Δ⁸-tetrahydrocanabinol, one of the active ingredients of cannabis, were selected for simulations of target occupancy for CB1, TRPV1, mGlu5, and 5-HT1a in three brain regions, to illustrate the principles of the combined PBPK-QSAR modeling. Our combined PBPK and target binding modeling demonstrated that the optimal values of the K _D and k _off for target and tissue selectivity were dependent on target concentration and tissue distribution kinetics. Interestingly, if the target concentration is high and the perfusion of the target site is low, the optimal K _D value is often not the lowest K _D value, suggesting that optimization towards high drug-target affinity can decrease the benefit-risk ratio. The presented integrative structure-pharmacokinetic-pharmacodynamic modeling provides an improved understanding of tissue and target selectivity.

Development of a population pharmacokinetic model to predict brain distribution and dopamine D2 receptor occupancy of raclopride in non-anesthetized rat

BACKGROUND

Raclopride is a selective antagonist of the dopamine D2 receptor. It is one of the most frequently used in vivo D2 tracers (at low doses) for assessing drug-induced receptor occupancy (RO) in animals and humans. It is also commonly used as a pharmacological blocker (at high doses) to occupy the available D2 receptors and antagonize the action of dopamine or drugs on D2 in preclinical studies. The aims of this study were to comprehensively evaluate its pharmacokinetic (PK) profiles in different brain compartments and to establish a PK-RO model that could predict the brain distribution and RO of raclopride in the freely moving rat using a LC-MS based approach.

METHODS

Rats (n=24) received a 10-min IV infusion of non-radiolabeled raclopride (1.61μmol/kg, i.e. 0.56mg/kg). Plasma and the brain tissues of striatum (with high density of D2 receptors) and cerebellum (with negligible amount of D2 receptors) were collected. Additional microdialysis experiments were performed in some rats (n=7) to measure the free drug concentration in the extracellular fluid of the striatum and cerebellum. Raclopride concentrations in all samples were analyzed by LC-MS. A population PK-RO model was constructed in NONMEM to describe the concentration-time profiles in the unbound plasma, brain extracellular fluid and brain tissue compartments and to estimate the RO based on raclopride-D2 receptor binding kinetics.

RESULTS

In plasma raclopride showed a rapid distribution phase followed by a slower elimination phase. The striatum tissue concentrations were consistently higher than that of cerebellum tissue throughout the whole experimental period (10-h) due to higher non-specific tissue binding and D2 receptor binding in the striatum. Model-based simulations accurately predicted the literature data on rat plasma PK, brain tissue PK and D2 RO at different time points after intravenous or subcutaneous administration of raclopride at tracer dose (RO <10%), sub-pharmacological dose (RO 10%-30%) and pharmacological dose (RO >30%).

CONCLUSION

For the first time a predictive model that could describe the quantitative in vivo relationship between dose, PK and D2 RO of raclopride in non-anesthetized rat was established. The PK-RO model could facilitate the selection of optimal dose and dosing time when raclopride is used as tracer or as pharmacological blocker in various rat studies. The LC-MS based approach, which doses and quantifies a non-radiolabeled tracer, could be useful in evaluating the systemic disposition and brain kinetics of tracers.

Comparison of two PET radioligands, [11C]FPEB and [11C]SP203, for quantification of metabotropic glutamate receptor 5 in human brain

Of the two ¹⁸F-labeled PET ligands currently available to image metabotropic glutamate receptor 5 (mGluR5), [¹⁸F]FPEB is reportedly superior because [¹⁸F]SP203 undergoes glutathionlyation, generating [¹⁸F]-fluoride ion that accumulates in brain and skull. To allow multiple PET studies on the same day with lower radiation exposure, we prepared [¹¹C]FPEB and [¹¹C]SP203 from [¹¹C]hydrogen cyanide and compared their abilities to accurately quantify mGluR5 in human brain, especially as regards radiometabolite accumulation. Genomic plot was used to estimate the ratio of specific-to-nondisplaceable uptake ( BP_ND) without using a receptor blocking drug. Both tracers quantified mGluR5; however [¹¹C]SP203, like [¹⁸F]SP203, had radiometabolite accumulation in brain, as evidenced by increased distribution volume ( V_T) over the scan period. Absolute V_T values were ∼30% lower for ¹¹C-labeled compared with ¹⁸F-labeled radioligands, likely caused by the lower specific activities (and high receptor occupancies) of the ¹¹C radioligands. The genomic plot indicated ∼60% specific binding in cerebellum, which makes it inappropriate as a reference region. Whole-body scans performed in healthy subjects demonstrated a low radiation burden typical for ¹¹C-ligands. Thus, the evidence suggests that [¹¹C]FPEB is superior to [¹¹C]SP203. If prepared in higher specific activity, [¹¹C]FPEB would presumably be as effective as [¹⁸F]FPEB for quantifying mGluR5 in human brain.

Drug discrimination: A versatile tool for characterization of CNS safety pharmacology and potential for drug abuse

Drug discrimination studies for assessment of psychoactive properties of drugs in safety pharmacology and drug abuse and drug dependence potential evaluation have traditionally been focused on testing novel compounds against standard drugs for which drug abuse has been documented, e.g. opioids, CNS stimulants, cannabinoids etc. (e.g. Swedberg & Giarola, 2015), and results are interpreted such that the extent to which the test drug causes discriminative effects similar to those of the standard training drug, the test drug would be further characterized as a potential drug of abuse. Regulatory guidance for preclinical assessment of abuse liability by the European Medicines Agency (EMA, 2006), the U.S. Food and Drug Administration (FDA, 2010), the International Conference of Harmonization (ICH, 2009), and the Japanese Ministry of Health Education and Welfare (MHLW, 1994) detail that compounds with central nervous system (CNS) activity, whether by design or not, need abuse and dependence liability assessment. Therefore, drugs with peripheral targets and a potential to enter the CNS, as parent or metabolite, are also within scope (see Swedberg, 2013, for a recent review and strategy). Compounds with novel mechanisms of action present a special challenge due to unknown abuse potential, and should be carefully assessed against defined risk criteria. Apart from compounds sharing mechanisms of action with known drugs of abuse, compounds intended for indications currently treated with drugs with potential for abuse and or dependence are also within scope, regardless of mechanism of action. Examples of such compounds are analgesics, anxiolytics, cognition enhancers, appetite control drugs, sleep control drugs and drugs for psychiatric indications. Recent results (Swedberg et al., 2014; Swedberg & Raboisson, 2014; Swedberg, 2015) on the metabotropic glutamate receptor type 5 (mGluR5) antagonists demonstrate that compounds causing hallucinatory effects in humans did not exhibit clear discriminative effects when tested against classical drugs of abuse in drug discrimination studies, and were not self-administered by rats. However, these compounds did cause salient discriminative effects of their own in animals trained to discriminate them from no drug. Therefore, from a safety pharmacology perspective, novel compounds that do not cause discriminative effects similar to classical drugs of abuse, may still cause psychoactive effects in humans and carry the potential to maintain drug abuse, suggesting that proactive investigation of drug abuse potential is warranted (Swedberg, 2013). These and other findings will be discussed, and the application of drug discrimination procedures beyond the typical standard application of testing novel compounds against known and well characterized reference drugs will be addressed.

Improved simultaneous estimation of tracer kinetic models with artificial immune network based optimization method

Tracer kinetic modeling (TKM) is a promising quantitative method for physiological and biochemical processes in vivo. In this paper, we investigated the applications of an immune-inspired method to better address the issues of Simultaneous Estimation (SIME) of TKM with multimodal optimization. Experiments of dynamic FDG PET imaging experiments and simulation studies were carried out. The proposed artificial immune network (TKM_AIN) shows more scalable and effective when compared with the gradient-based Marquardt-Levenberg algorithm and the scholastic-based simulated annealing method.

Estimation of drug receptor occupancy when non-displaceable binding differs between brain regions – extending the simplified reference tissue model

AIM

The simplified reference tissue model (SRTM) is used for estimation of receptor occupancy assuming that the non-displaceable binding in the reference region is identical to the brain regions of interest. The aim of this work was to extend the SRTM to also account for inter-regional differences in non-displaceable concentrations, and to investigate if this model allowed estimation of receptor occupancy using white matter as reference. It was also investigated if an apparent higher affinity in caudate compared with other brain regions, could be better explained by a difference in the extent of non-displaceable binding.

METHODS

The analysis was based on a PET study in six healthy volunteers using the 5-HT1B receptor radioligand [(11)C]-AZ10419369. The radioligand was given intravenously as a tracer dose alone and following different oral doses of the 5-HT1B receptor antagonist AZD3783. Non-linear mixed effects models were developed where differences between regions in non-specific concentrations were accounted for. The properties of the models were also evaluated by means of simulation studies.

RESULTS

The estimate (95% CI) of Ki(PL) was 10.2 ng ml(-1) (5.4, 15) and 10.4 ng ml(-1) (8.1, 13.6) based on the extended SRTM with white matter as reference and based on the SRTM using cerebellum as reference, respectively. The estimate (95% CI) of Ki(PL) for caudate relative to other brain regions was 55% (48, 62%).

CONCLUSIONS

The extended SRTM allows consideration of white matter as reference region when no suitable grey matter region exists. AZD3783 affinity appears to be higher in the caudate compared with other brain regions.

Development of (18)F-labeled radiotracers for neuroreceptor imaging with positron emission tomography

Positron emission tomography (PET) is an in vivo molecular imaging tool which is widely used in nuclear medicine for early diagnosis and treatment follow-up of many brain diseases. PET uses biomolecules as probes which are labeled with radionuclides of short half-lives, synthesized prior to the imaging studies. These probes are called radiotracers. Fluorine-18 is a radionuclide routinely used in the radiolabeling of neuroreceptor ligands for PET because of its favorable half-life of 109.8 min. The delivery of such radiotracers into the brain provides images of transport, metabolic, and neurotransmission processes on the molecular level. After a short introduction into the principles of PET, this review mainly focuses on the strategy of radiotracer development bridging from basic science to biomedical application. Successful radiotracer design as described here provides molecular probes which not only are useful for imaging of human brain diseases, but also allow molecular neuroreceptor imaging studies in various small-animal models of disease, including genetically-engineered animals. Furthermore, they provide a powerful tool for in vivo pharmacology during the process of pre-clinical drug development to identify new drug targets, to investigate pathophysiology, to discover potential drug candidates, and to evaluate the pharmacokinetics and pharmacodynamics of drugs in vivo.

A positron emission tomography study in healthy volunteers to estimate mGluR5 receptor occupancy of AZD2066 - estimating occupancy in the absence of a reference region

AZD2066 is a new chemical entity pharmacologically characterized as a selective, negative allosteric modulator of the metabotropic glutamate receptor subtype 5 (mGluR5). Antagonism of mGluR5 has been implicated in relation to various diseases such as anxiety, depression, and pain disorders. To support translation from preclinical results and previous experiences with this target in man, a positron emission tomography study was performed to estimate the relationship between AZD2066 plasma concentrations and receptor occupancy in the human brain, using the mGluR5 radioligand [(11)C]-ABP688. The study involved PET scans on 4 occasions in 6 healthy volunteers. The radioligand was given as a tracer dose alone and following oral treatment with different doses of AZD2066. The analysis was based on the total volume of distribution derived from each PET-assessment. A non-linear mixed effects model was developed where ten delineated brain regions of interest from all PET scans were included in one simultaneous fit. For comparison the analysis was also performed according to a method described previously by Lassen et al. (1995). The results of the analysis showed that the total volume of distribution decreased with increasing drug concentrations in all regions with an estimated Kipl of 1170 nM. Variability between individuals and occasions in non-displaceable volume of distribution could explain most of the variability in the total volume of distribution. The Lassen approach provided a similar estimate for Kipl, but the variability was exaggerated and difficult to interpret.

Modeling of PET data in CNS drug discovery and development

Positron emission tomography (PET) is increasingly used in drug discovery and development for evaluation of CNS drug disposition and for studies of disease biomarkers to monitor drug effects on brain pathology. The quantitative analysis of PET data is based on kinetic modeling of radioactivity concentrations in plasma and brain tissue compartments. A number of quantitative methods of analysis have been developed that allow the determination of parameters describing drug pharmacokinetics and interaction with target binding sites in the brain. The optimal method of quantification depends on the properties of the radiolabeled drug or radioligand and the binding site studied. We here review the most frequently used methods for quantification of PET data in relation to CNS drug discovery and development. The utility of PET kinetic modeling in the development of novel CNS drugs is illustrated by examples from studies of the brain kinetic properties of radiolabeled drug molecules.