MAO-A inhibition in brain after dosing with esuprone, moclobemide and placebo in healthy volunteers: in vivo studies with positron emission tomography

Do We Build Similar Molecules for Comorbid Diseases? Tevarud in Drug Design, an Analysis for Depression and Inflammation

Tevarud designates two poets coincidently writing a same verse in the Ottoman Divan literature. This study aims to analyze the structural similarity of molecules independently designed for inflammation and depression to determine if coincidentally we are building similar molecules for comorbid diseases. For this purpose, a molecule library was first constituted with structures that were developed as anti-inflammatory (AI) and antidepressant (AD) agents these last decades. Then, the similarity of the structures was determined by calculating the Tanimoto and Cosine similarity coefficients for each AD/AI pair. The highest scores were obtained for two theophylline derivatives: AD17 (for which some AI activity was found to be mentioned) and AI42. The study also pointed out the similarity of some AD coumarins with some AI flavonoids interestingly found to be highly similar to some AI coumarins and AD flavonoids, respectively. Thus, our investigation demonstrated that structures independently developed as AD and AI derivatives can present extremely high structural similarity, a finding that can suggest mechanistic interconnection for these comorbid diseases and also guide for the design of novel bioactive compounds.

Structure-Based Specific Detection and Inhibition of Monoamine Oxidases and Their Applications in Central Nervous System Diseases

Monoamine oxidases (MAOs) are the enzymes that catalyze the oxidation of monoamines, such as dopamine, norepinephrine, and serotonin, which serve as key neurotransmitters in the central nervous system (CNS). MAOs play important roles in maintaining the homeostasis of monoamines, and the aberrant expression or activation of MAOs underlies the pathogenesis of monoamine neurotransmitter disorders, including neuropsychiatric and neurodegenerative diseases. Clearly, detecting and inhibiting the activities of MAOs is of great value for the diagnosis and therapeutics of these diseases. Accordingly, many specific detection probes and inhibitors have been developed and substantially contributed to basic and clinical studies of these diseases. In this review, progress in the detecting and inhibiting of MAOs and their applications in mechanism exploration and treatment of neurotransmitter-related disorders is summarized. Notably, how the detection probes and inhibitors of MAOs were developed has been specifically addressed. It is hoped that this review will benefit the design of more effective and sensitive probes and inhibitors for MAOs, and eventually the treatment of monoamine neurotransmitter disorders.

Molecular docking studies of coumarin hybrids as potential acetylcholinesterase, butyrylcholinesterase, monoamine oxidase A/B and β-amyloid inhibitors for Alzheimer's disease

Coumarins are the phytochemicals, which belong to the family of benzopyrone, that display interesting pharmacological properties. Several natural, synthetic and semisynthetic coumarin derivatives have been discovered in decades for their applicability as lead structures as drugs. Coumarin based conjugates have been described as potential AChE, BuChE, MAO and β-amyloid inhibitors. Therefore, the objective of this review is to focus on the construction of these pharmacologically important coumarin analogues with anti-Alzheimer’s activities, highlight their docking studies and structure–activity relationships based on their substitution pattern with respect to the selected positions on the chromen ring by emphasising on the research reports conducted in between year 1968 to 2017.

Monoamine oxidase: radiotracer chemistry and human studies

Monoamine oxidase (MAO) oxidizes amines from both endogenous and exogenous sources thereby regulating the concentration of neurotransmitter amines such as serotonin, norepinephrine, and dopamine as well as many xenobiotics. MAO inhibitor drugs are used in the treatment of Parkinson's disease and in depression stimulating the development of radiotracer tools to probe the role of MAO in normal human biology and in disease. Over the past 30 years since the first radiotracers were developed and the first positron emission tomography (PET) images of MAO in humans were carried out, PET studies of brain MAO in healthy volunteers and in patients have identified different variables that have contributed to different MAO levels in brain and in peripheral organs. MAO radiotracers and PET have also been used to study the current and developing MAO inhibitor drugs including the selection of doses for clinical trials. In this article, we describe the following: (1) the development of MAO radiotracers; (2) human studies including the relationship of brain MAO levels to genotype, personality, neurological, and psychiatric disorders; and (3) examples of the use of MAO radiotracers in drug research and development. We will conclude with outstanding needs to improve the radiotracers that are currently used and possible new applications.

Monoamine oxidase A and B substrates: probing the pathway for drug development

Drug-discovery and -development efforts focused on the MAOs have increased at an accelerated rate over the past decade. Since the first crystal structure of human MAO-B was solved in 2002, over 40 additional structures have been reported and have helped define new, or confirm speculative, binding modes of inhibitors. The detailed mechanism of the MAO-catalyzed oxidation of amine substrates has not been fully elucidated, but its significance is central in the development of new mechanism-based inactivators. Novel fungal MAO-N variants derived from directed evolution strategies are enabling the production of new chiral amine products. Robust assays have been established for measuring MAO status in tissue and cells, while improved MAO radioligands are being deployed for PET imaging studies. This review will attempt to highlight the more recent and salient aspects of MAO research in drug discovery and development, with emphasis on substrates 'probing the pathway'.

Update on the pharmacology of selective inhibitors of MAO-A and MAO-B: focus on modulation of CNS monoamine neurotransmitter release

Inhibitors of monoamine oxidase (MAO) were initially used in medicine following the discovery of their antidepressant action. Subsequently their ability to potentiate the effects of an indirectly-acting sympathomimetic amine such as tyramine was discovered, leading to their limitation in clinical use, except for cases of treatment-resistant depression. More recently, the understanding that: a) potentiation of indirectly-acting sympathomimetic amines is caused by inhibitors of MAO-A but not by inhibitors of MAO-B, and b) that reversible inhibitors of MAO-A cause minimal tyramine potentiation, has led to their re-introduction to clinical use for treatment of depression (reversible MAO-A inhibitors and new dose form MAO-B inhibitor) and treatment of Parkinson's disease (MAO-B inhibitors). The profound neuroprotective properties of propargyl-based inhibitors of MAO-B in preclinical experiments have drawn attention to the possibility of employing these drugs for their neuroprotective effect in neurodegenerative diseases, and have raised the question of the involvement of the MAO-mediated reaction as a source of reactive free radicals. Despite the long-standing history of MAO inhibitors in medicine, the way in which they affect neuronal release of monoamine neurotransmitters is still poorly understood. In recent years, the detailed chemical structure of MAO-B and MAO-A has become available, providing new possibilities for synthesis of mechanism-based inhibitors. This review describes the latest advances in understanding the way in which MAO inhibitors affect the release of the monoamine neurotransmitters dopamine, noradrenaline and serotonin (5-HT) in the CNS, with an accent on the importance of these effects for the clinical actions of the drugs.

Peripheral and central nervous system inhibition of 11β-hydroxysteroid dehydrogenase type 1 in man by the novel inhibitor ABT-384

ABT-384 is a potent, selective inhibitor of 11-beta-hydroxysteroid dehydrogenase type 1 (HSD-1). One milligram of ABT-384 daily fully inhibited hepatic HSD-1. Establishing the dose that fully inhibits central nervous system (CNS) HSD-1 would enable definitive clinical studies in potential CNS indications. [9,11,12,12-(2)H4] cortisol (D4 cortisol), a stable labeled tracer, was used to characterize HSD-1 inhibition by ABT-384. D4 cortisol and its products were measured in the plasma and cerebrospinal fluid (CSF) of healthy male volunteers during D4 cortisol infusions, for up to 40 h after five daily doses of 1-50 mg ABT-384. Similar procedures were conducted in control subjects who received no ABT-384. Peripheral HSD-1 inhibition was calculated from plasma levels of D4 cortisol and its products. CNS HSD-1 inhibition was characterized from plasma and CSF levels of D4 cortisol and its products. ABT-384 regimens ≥2 mg daily maintained peripheral HSD-1 inhibition ≥88%. ABT-384 1 mg daily maintained peripheral HSD-1 inhibition ≥81%. No CNS formation of D3 cortisol (the mass-labeled product of HSD-1) was detected following ABT-384 ≥2 mg daily, indicating full CNS HSD-1 inhibition by these regimens. Partial CNS HSD-1 inhibition was associated with 1 mg ABT-384 daily. CNS HSD-1 inhibition was characterized by strong hysteresis and increased with maximum post-dose plasma concentration of ABT-384 and its active metabolites. ABT-384 has a wide potential therapeutic window for potential indications including Alzheimer's disease and major depressive disorder. Stable labeled substrates may be viable tools for measuring CNS effect during new drug development for other enzyme targets.

From depression to neurodegeneration and heart failure: re-examining the potential of MAO inhibitors

Initially introduced in the 1950s for treating depression, monoamine oxidase (MAO) inhibitors were gradually abandoned, mainly owing to their potential for drug-drug and drug-food interactions, the most widely known being with tyramine-containing food (the 'cheese' effect). Since then, more selective MAO-A or MAO-B inhibitors have been developed with substantially reduced risks, and have been approved for the treatment of depression and Parkinson's disease, respectively. Recent research suggests that some of these drugs also have neuroprotective properties, while preclinical evidence expands the spectrum of potential indications to heart failure, renal diseases and multiple sclerosis. In this article, the authors review the relevance of MAO isoforms to disease, and they also outline current research and development efforts in this class of drugs, including newer multipotent compounds.

Reversible inhibitors of monoamine oxidase-A (RIMAs): robust, reversible inhibition of human brain MAO-A by CX157

Reversible inhibitors of monoamine oxidase-A (RIMA) inhibit the breakdown of three major neurotransmitters, serotonin, norepinephrine and dopamine, offering a multi-neurotransmitter strategy for the treatment of depression. CX157 (3-fluoro-7-(2,2,2-trifluoroethoxy)phenoxathiin-10,10-dioxide) is a RIMA, which is currently in development for the treatment of major depressive disorder. We examined the degree and reversibility of the inhibition of brain monoamine oxidase-A (MAO-A) and plasma CX157 levels at different times after oral dosing to establish a dosing paradigm for future clinical efficacy studies, and to determine whether plasma CX157 levels reflect the degree of brain MAO-A inhibition. Brain MAO-A levels were measured with positron emission tomography (PET) imaging and [(11)C]clorgyline in 15 normal men after oral dosing of CX157 (20-80 mg). PET imaging was conducted after single and repeated doses of CX157 over a 24-h time course. We found that 60 and 80 mg doses of CX157 produced a robust dose-related inhibition (47-72%) of [(11)C]clorgyline binding to brain MAO-A at 2 h after administration and that brain MAO-A recovered completely by 24 h post drug. Plasma CX157 concentration was highly correlated with the inhibition of brain MAO-A (EC(50): 19.3 ng/ml). Thus, CX157 is the first agent in the RIMA class with documented reversible inhibition of human brain MAO-A, supporting its classification as a RIMA, and the first RIMA with observed plasma levels that can serve as a biomarker for the degree of brain MAO-A inhibition. These data were used to establish the dosing regimen for a current clinical efficacy trial with CX157.

Applying neuroimaging ligands to study major depressive disorder

The recent increase in radioligands available for neuroimaging major depressive disorder has led to advancements in our understanding of the pathophysiology of this illness and improved antidepressant development. Major depressive disorder can be defined as an illness of recurrent major depressive episodes of persistently low mood, dysregulated sleep, appetite and weight, anhedonia, cognitive impairment, and suicidality. The main target sites investigated with radioligand neuroimaging include receptor sites that regulate in response to lowered monoamine levels, targets related to removal of monoamines, uptake of ligands related to regional brain function, and target sites of antidepressants.

Radiopharmaceuticals: molecular imaging using positron emission tomography

We describe the use of molecules labeled with short-lived emitting radionuclides for molecular imaging in combination with the positron emission tomography technique. How to use molecular probes to visualize and quantitatively determine rates of specific biochemical events such as synaptic transmission, enzymatic processes and binding to specific receptor proteins is highlighted. The sensitivity of the PET technique and the ability to measure and validate relationships between molecular events and biological functions is a key factor for the successful application of PET in biomedical research. In specific applications, the opportunity of using molecules labeled in specific positions may be critical. Molecular imaging using PET is also gaining increasing interest as a tool in drug development, especially when applied to early proof of concept studies in man. In this chapter, the concept of molecular imaging is exemplified and the use of position-specific labeling of tracer molecules as a tool to gain understanding of complex biological processes will be discussed.

Biodistribution and radiation dosimetry of 11C-harmine in baboons

OBJECTIVE

Monoamine oxidase A is a mitochondrial enzyme which is responsible for the metabolism of catecholamines such as dopamine, norepinephrine, as well as serotonin. This study describes the biodistribution and dosimetry of 11C-harmine, a tracer designed to specifically bind to monoamine oxidase A for positron emission tomography imaging.

METHODS

Three baboon studies were acquired using a Seimens ECAT camera. Dynamic whole-body emission scans were collected in two-dimensional mode over a 2 h period after 223-255 MBq of 11C-harmine was injected. Regions of interest were drawn on transmission corrected images to encompass the entire activity in visible organs at each time point. Time-activity data were used to obtain residence times and absorbed radiation dose to various organs and to the entire body.

RESULTS

Tracer uptake was greatest in the lungs, followed by kidney, small intestine, liver and brain. The largest absorbed dose based on averaged residence times was found in the lungs (reference adult/female 3.99x10(-2)/5.03x10(-2) mSv x MBq(-1)).

CONCLUSION

The lungs are the critical organs for administration of 11C-harmine. For example, in the United States, the absorbed dose to the lungs would limit a single 11C-harmine administration for a research subject with the approval of a Radioactive Drug Research Committee to 1258/999 MBq (34/27 mCi) in the adult male/female. Quantitative measurement of monoamine oxidase A activity in the brain and elsewhere may aid in understanding the pathophysiology of several disease processes including neuroendocrine neoplasms and depression.

Synthesis, molecular modeling studies, and selective inhibitory activity against monoamine oxidase of N,N'-bis[2-oxo-2H-benzopyran]-3-carboxamides

A novel series of N,N'-bis[2-oxo-2H-1-benzopyran]-3-carboxamide derivatives have been synthesized and investigated for the ability to inhibit the activity of the A and B isoforms of monoamine oxidase (MAO). Some of the synthesized compounds show good selective inhibitory activity against the MAO-A isoform. Both the MAO-A and -B isoforms, deposited in the Protein Data Bank as the 2BXR and 1GOS models, respectively, were considered in a computational study performed with docking techniques on the most active and selective inhibitors.

Positron emission tomography quantification of [11C]-harmine binding to monoamine oxidase-A in the human brain

This article describes the kinetic modeling of [(11)C]-harmine binding to monoamine oxidase A (MAO-A) binding sites in the human brain using positron emission tomography (PET). Positron emission tomography studies were performed in healthy volunteers at placebo conditions and after treatment with clinical doses of moclobemide. In either condition, a two-tissue compartment model (2CM) provided better fits to the data than a one-tissue model. Estimates of k(3)/k(4) values from an unconstrained 2CM were highly variable. In contrast, estimates of the specifically bound radioligand distribution volume (DV(B)) from an unconstrained 2CM were exceptionally stable, correlated well with the known distribution of MAO-A in the brain (cerebellum <frontal cortex approximately putamen <temporal cortex approximately cingulate <thalamus) and thus provided reliable indices of MAO-A density. Total distribution volume (DV) values were also highly stable and not different from those estimated with the Logan approach. Fixing the DV of free and nonspecifically bound radiotracer (DV(F + NS)) or coupling DV(F + NS) between brain regions enabled more stable estimates of k(3)/k(4) as compared with an unconstrained 2CM. Moclobemide treatment leads to a 64% to 79% MAO-A blockade across brain regions, a result that supports the specificity of [(11)C]-harmine binding to MAO-A. The stability and reliability of DV(B) values obtained from an unconstrained 2CM, together with the computational simplicity associated with this method, support the use of DV(B) as an appropriate outcome measure for [(11)C]-harmine. These results indicate the suitability of using [(11)C]-harmine for quantitative evaluation of MAO-A densities using PET and should enable further studies of potential MAO-A dysregulation in several psychiatric and neurologic illnesses.

Lipophilicity Plays a Major Role in Modulating the Inhibition of Monoamine Oxidase B by 7-Substituted Coumarins

A series of coumarin derivatives (1-22), bearing at the 7-position ether, ketone, ester, carbamate, or amide functions of varying size and lipophilicity, were synthesized and investigated for their in vitro monoamine oxidase-A and -B (MAO-A and -B) inhibitory activities. Most of the compounds acted preferentially as MAO-B inhibitors, with IC(50) values in the micromolar to low-nanomolar range. A structure-activity-relationship (SAR) study highlighted lipophilicity as an important property modulating the MAO-B inhibition potency of 7-substituted coumarins, as shown by a linear correlation (n=20, r(2)=0.72) between pIC(50) and calculated log P values. The stability of ester-containing coumarin derivatives in rat plasma provided information on factors that either favor (lipophilicity) or decrease (steric hindrance) esterase-catalyzed hydrolysis. Two compounds (14 and 22) were selected to investigate how lipophilicity and enzymatic stability may affect in vivo MAO activities, as assayed ex vivo in rat. The most-potent and -selective MAO-B inhibitor 22 (=7-[(3,4-difluorobenzyl)oxy]-3,4-dimethyl-1-benzopyran-2(2H)-one) within the examined series significantly inhibited (>60%) ex vivo rat-liver and striatal MAO-B activities 1 h after intraperitoneal administration of high doses (100 and 300 mumol kg(-1)), revealing its ability to cross the blood-brain barrier. At the same doses, liver and striatum MAO-A was less inhibited in vivo, somehow reflecting MAO-B selectivity, as assessed in vitro. In contrast, the metabolically less stable derivative 14, bearing an isopropyl ester in the lateral chain, had a weak effect on hepatic MAO-B activity in vivo, and none on striatal MAO-B, but, surprisingly, displayed inhibitory effects on MAO-A in both peripheral and brain tissues.

Positron emission tomography and single-photon emission computed tomography in central nervous system drug development

In this review, the value of functional imaging [positron emission tomography (PET)/single-photon emission computed tomography (SPECT)] in drug development is considered. Radionuclide imaging can help establish the diagnosis of neurodegenerative disorders where this is in doubt and provides a potential biomarker for following drug effects on disease progression. PET and SPECT can help understand mechanisms of disease and determine the functional effects of therapeutic approaches on neurotransmission and metabolism. Synthesizing radiotracer analogs of novel drugs can provide proof of principle that these agents reach their enzyme or receptor targets and delineate their regional brain distribution. If such radiotracers do not prove to have ideal properties for imaging, the concept of microdosing potentially allows multiple other drug analogs to be tested with less stringent regulatory requirements than for novel medicinals. Finally, PET tracers can provide receptor and enzyme active site dose occupancy profiles, thereby guiding dosage selection for phase 1 and phase 2 trials. The eventual hope is that radiotracer imaging will provide a surrogate marker for drug efficacy, although this has yet to be realized, and progress the concept of personalized medicine where receptor/enzyme binding profiles help predict therapeutic outcome.

Inhibition of monoamine oxidases by coumarin-3-acyl derivatives: biological activity and computational study

A series of coumarin-3-acyl derivatives have been synthesized and investigated for the ability to inhibit selectively monoamine oxidases. The coumarin-3-carboxylic acids, 2a-e, proved to be reversible and selective inhibitors of the MAO-B isoform, displaying pIC(50) values of particular interest: 2a shows pIC(50) 7.76 and a selectivity index (pS.I.) 2.94 and 2b shows pIC(50) 7.72 and a pS.I. of 2.80. The coumarin-3-acyl chlorides 3a-e showed high pIC(50) values against both MAO-A and MAO-B isoforms, 3d being the highest against MAO-B with a pIC(50) value of 8.00. In order to rationalize the activity/selectivity results, molecular descriptors were generated. Further insight about enzyme-inhibitor interaction was obtained by docking experiments with the MAO-B isoform.

Death following acute poisoning by moclobemide

A fatality due to ingestion of a reversible inhibitor of monoamine-oxidase A (MAO-A) is reported. Moclobemide is generally considered as a safe drug far less toxic than tricyclic anti-depressants. However, severe intoxications may result from interactions with other drugs and food such as selective serotonin reuptake inhibitors (SSRIs), anti-Parkinsonians of the MAOI-type (e.g. selegiline) or tyramine from ripe cheese or other sources. In the present case, high levels of moclobemide were measured in peripheral blood exceeding toxic values reported so far in the scientific literature. The body fluid concentrations of moclobemide were of 498 mg/l in peripheral whole blood, 96.3 mg/l in urine while an amount of approximately 33 g could be recovered from gastric contents. The other xenobiotics were considered of little toxicological relevance. The victim (male, 48-year-old) had a past history of depression and committed one suicide attempt 2 years before death. Autopsy revealed no evidence of significant natural disease or injury. It was concluded that the manner of death was suicide and that the unique cause of death was massive ingestion of moclobemide.

Synthesis and in vivo imaging properties of [11C]befloxatone: a novel highly potent positron emission tomography ligand for mono-amine oxidase-A

Befloxatone (1, (5R)-5-(methoxymethyl)-3-[4-[(3R)-4,4,4-trifluoro-3-hydroxybutoxy]phenyl]-2-oxazolidinone) is an oxazolidinone derivative belonging to a new generation of reversible and selective mono-amine oxidase-A (MAO-A) inhibitors. In vitro and ex vivo studies have demonstrated that befloxatone is a potent, reversible and competitive MAO-A inhibitor with potential antidepressant properties. Befloxatone (1) was labelled with carbon-11 (t(12): 20.4 min) using [(11)C]phosgene as reagent. Typically, starting from a 1.2 Ci (44.4 GBq) cyclotron-produced [(11)C]CH(4) batch, 150-300 mCi (5.55-11.10 GBq) of [(11)C]befloxatone ([(11)C]-1) with a radiochemical- and chemical purity of more than 99% were routinely obtained within 20 min of radiosynthesis (including HPLC purification) with specific radioactivities of 500-2000 mCi/micromol (18.5-74.0 GBq/micromol). The results obtained in vivo with carbon-11-labelled befloxatone not only confirm the biochemical and pharmacological profile of befloxatone found in rodent and in human tissues but also point out [(11)C]befloxatone as an excellent tool for the assessment of MAO-A binding sites using positron emission tomography, a high-resolution, sensitive, non-invasive and quantitative imaging technique.

Mapping the cerebral monoamine oxidase type A: positron emission tomography characterization of the reversible selective inhibitor [11C]befloxatone

Befloxatone is a competitive and reversible inhibitor of monoamine oxidase-A (MAOI-A). The aim of the study was to characterize the in vivo properties of [(11)C]befloxatone and to validate its use as a ligand for the study of MAO-A by positron emission tomography (PET). PET studies were performed in baboons after i.v. injection of [(11)C]befloxatone (551 +/- 70 MBq, i.e.14.9 +/- 1.9 mCi). [(11)C]Befloxatone enters rapidly in the brain with a maximum uptake at 30 min. Brain concentration of the tracer is high in thalamus, striatum, pons and cortical structures (1.5-1.8% of injected dose per 100 ml of tissue), and lower in cerebellum (1.07% injected dose/100 ml). Nonsaturable uptake, obtained after a pretreatment with a high dose of nonlabeled befloxatone (0.4 mg/kg), is very low and represents only 3% of the total uptake. Brain uptake of [(11)C]befloxatone is not altered by a pretreatment of a high dose with lazabemide (0.5 mg/kg i.v.), a selective MAOI-B but is completely blocked by a pretreatment with moclobemide (MAOI-A; 10 mg/kg). This confirms, in vivo, the selectivity of befloxatone for type A MAO. [(11)C]Befloxatone brain radioactivity was displaced by administration of unlabeled befloxatone (30 min after the tracer injection). The displacement of the tracer from its binding sites is dose-dependent, with an ID(50) of 0.02 mg/kg for all studied structures. These results indicate that [(11)C]befloxatone will be an excellent probe for the study of MAO-A in humans using PET.

Moclobemide: evolution, pharmacodynamic, and pharmacokinetic properties

The benzamide moclobemide is a reversible inhibitor of monoamine-oxidase-A (RIMA). It has been extensively evaluated in the treatment of a wide spectrum of depressive disorders and less extensively in anxiety disorders. While clinical aspects will be presented in a subsequent review, this article focuses primarily on moclobemide's evolution, pharmacodynamic and pharmacokinetic properties. In particular, the effects on neurotransmission and intracellular signal transduction, the neuroendocrine system, the tyramine pressure response and animal models of depression are surveyed. In addition, other CNS effects are reviewed with special respect to experimental serotonergic syndrome, anxiolytic and antinociceptive activity, sleep, cognition and driving performance, neuroprotection and seizures.

Clinical pharmacology of psychotropic drugs

No Abstract

Brain uptake and receptor binding of two [11C]labelled selective high affinity NK1-antagonists, GR203040 and GR205171--PET studies in rhesus monkey

Two high affinity and selective NK1-receptor antagonists, GR203040 and GR205171, were labelled with 11C and used in a series of experiments in rhesus monkeys. The purpose of these studies was to evaluate the brain uptake pattern and to explore the potential use of these compounds as PET ligands to characterise NK1-receptor binding. Seventeen studies were carried out with [11C]GR205171 and five experiments with [11C]GR203040, including baseline studies and studies performed after a 5 min infusion of cold compound at doses between 0.05 and 1 mg/kg. Both compounds demonstrated a significant and rapid uptake in the brain, but the uptake of [11C]GR205171 was more than double the uptake of [11C]GR203040. At tracer doses of [11C]GR205171 and all doses of [11C]GR203040 the uptake reached a plateau with no washout during the examination time, whereas [11C]GR205171 after pre-treatment with cold GR205171 showed a significant washout. Using a model with the cerebellum as reference, a method for quantitation was applied to the studies with [11C]GR205171 and the results indicated that the highest specific binding was in the striatum. The pre-treatment dose of cold GR205171 needed for 50% inhibition of binding was less than 0.04 mg/kg. The studies indicated that [11C]GR205171 could be used for the in vivo characterisation of NK1-receptor binding.