Brain kinetics and specific binding of [11C]PK 11195 to omega 3 sites in baboons: positron emission tomography study

Translocator protein 18 kDa (TSPO): molecular sensor of brain injury and repair

For over 15 years, the peripheral benzodiazepine receptor (PBR), recently named translocator protein 18 kDa (TSPO) has been studied as a biomarker of reactive gliosis and inflammation associated with a variety of neuropathological conditions. Early studies documented that in the brain parenchyma, TSPO is exclusively localized in glial cells. Under normal physiological conditions, TSPO levels are low in the brain neuropil but they markedly increase at sites of brain injury and inflammation making it uniquely suited for assessing active gliosis. This research has generated significant efforts from multiple research groups throughout the world to apply TSPO as a marker of "active" brain pathology using in vivo imaging modalities such as Positron Emission Tomography (PET) in experimental animals and humans. Further, in the last few years, there has been an increased interest in understanding the molecular and cellular function(s) of TSPO in glial cells. The latest evidence suggests that TSPO may not only serve as a biomarker of active brain disease but also the use of TSPO-specific ligands may have therapeutic implications in brain injury and repair. This review presents an overview of the history and function of TSPO focusing on studies related to its use as a sensor of active brain disease in experimental animals and in human studies.

The high affinity peripheral benzodiazepine receptor ligand DAA1106 binds to activated and infected brain macrophages in areas of synaptic degeneration: implications for PET imaging of neuroinflammation in lentiviral encephalitis

HIV encephalitis (HIVE) is characterized by neurodegeneration mediated by toxins derived from infected and activated brain macrophages. Since the peripheral benzodiazepine receptor (PBR) is abundant on brain macrophages, we hypothesized that [(3)H]DAA1106, a new PBR ligand, can label infected and activated brain macrophages in HIVE. Using cell culture and postmortem brain tissues from HIVE and a macaque model of HIVE, we show that [(3)H]DAA1106 binds with high affinity to activated and infected macrophages in regions of synaptic damage. Further, binding affinity reflected by lower K(D) (dissociation constant) values and the B(max) (total number of binding sites) to K(D) ratios reflective of ligand-binding potential was significantly higher with [(3)H]DAA1106 compared to the extensively characterized PBR ligand [(3)H](R)-PK11195. These data suggest that DAA1106 binds with high affinity to activated and infected brain macrophages and possesses binding characteristics beneficial for in vivo use in the detection and clinical monitoring of HIVE using positron emission tomography.

The high affinity peripheral benzodiazepine receptor ligand DAA1106 binds specifically to microglia in a rat model of traumatic brain injury: implications for PET imaging

Traumatic brain injury (TBI) is a significant cause of mortality, morbidity, and disability. Microglial activation is commonly observed in response to neuronal injury which, when prolonged, is thought to be detrimental to neuronal survival. Activated microglia can be labeled using PK11195, a ligand that binds the peripheral benzodiazepine receptor (PBR), receptors which are increased in activated microglia and sparse in the resting brain. We compared the binding properties of two PBR ligands PK11195 and DAA1106 in rats using the controlled cortical impact (CCI) model of experimental TBI. While both ligands showed relative increases with specific binding in the cortex ipsilateral to injury compared to the contralateral side, [(3)H]DAA1106 showed higher binding affinity compared with [(3)H](R)-PK11195. Combined immunohistochemistry and autoradiography in brain tissues near the injury site showed that [(3)H]DAA1106 binding co-registered with activated microglia more than astrocytes. Further, increased [(3)H]DAA1106-specific binding positively correlated with the degree of microglial activation, and to a lesser degree with reactive astrocytosis. Finally, in vivo administration of each ligand in rats with TBI showed greater retention of [(11)C]DAA1106 compared to [(11)C](R)-PK11195 at the site of the contusion as assessed by ex vivo autoradiography. These results in a rat model of TBI indicate that [(11)C]DAA1106 binds with higher affinity to microglia when compared with PK11195, suggesting that [(11)C]DAA1106 may represent a better ligand than [(11)C](R)-PK11195 for in vivo PET imaging of activated microglia in TBI.

A comparison of the high-affinity peripheral benzodiazepine receptor ligands DAA1106 and (R)-PK11195 in rat models of neuroinflammation: implications for PET imaging of microglial activation

Activated microglia are an important feature of many neurological diseases and can be imaged in vivo using 1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinolinecarboxamide (PK11195), a ligand that binds the peripheral benzodiazepine receptor (PBR). N-(2,5-dimethoxybenzyl)-N-(5-fluoro-2-phenoxyphenyl) acetamide (DAA1106) is a new PBR-specific ligand that has been reported to bind to PBR with higher affinity compared with PK11195. We hypothesized that this high-affinity binding of DAA1106 to PBR will enable better delineation of microglia in vivo using positron emission tomography. [(3)H]DAA1106 showed higher binding affinity compared with [(3)H](R)-PK11195 in brain tissue derived from normal rats and the rats injected intrastriatally with 6-hydroxydopamine or lipopolysaccharide at the site of the lesion. Immunohistochemistry combined with autoradiography in brain tissues as well as correlation analyses showed that increased [(3)H]DAA1106 binding corresponded mainly to activated microglia. Finally, ex vivo autoradiography and positron emission tomography imaging in vivo showed greater retention of [(11)C]DAA1106 compared with [(11)C](R)-PK11195 in animals injected with either lipopolysaccaride or 6-hydroxydopamine at the site of lesion. These results indicate that DAA1106 binds with higher affinity to microglia in rat models of neuroinflammation when compared with PK11195, suggesting that [(11)C]DAA1106 may represent a significant improvement over [(11)C](R)-PK11195 for in vivo imaging of activated microglia in human neuroinflammatory disorders.

Kinetic evaluation in nonhuman primates of two new PET ligands for peripheral benzodiazepine receptors in brain

Peripheral benzodiazepine receptors (PBRs) are upregulated on activated microglia and are, thereby, biomarkers of cellular inflammation in brain. We recently developed two PET ligands with an aryloxyanilide structure to image PBRs and now evaluate the kinetics of these radiotracers in monkey to determine whether they are suitable to explore in human. Baseline and receptor-blocking scans were performed with [(11)C]PBR01 and [(18)F]PBR06 in conjunction with serial measurements of the arterial plasma concentration of parent radiotracer separated from radiometabolite. We used brain and plasma data with compartmental modeling to calculate regional brain distribution volume, which is equal to the ratio at equilibrium of the concentration of radioligand in brain to that of plasma. The distribution volume of [(11)C]PBR01 was inaccurately estimated in the baseline scans, possibly because of the short half-life of (11)C or the presence of radiometabolite in brain. In contrast, the distribution volume of [(18)F]PBR06 was stably determined within 200 min of scanning, and nondisplaceable uptake was only approximately 10% of total brain uptake. [(18)F]PBR06 is promising for use in human because brain activity could be quantified with standard compartmental models and showed higher ratios ( approximately 10:1) of specific to nonspecific uptake. A critical factor for human use will be whether the tracer has adequately fast wash out from brain relative to the half-life of the radionuclide to obtain stable values of distribution volume.

In vivo imaging of brain lesions with [11C]CLINME, a new PET radioligand of peripheral benzodiazepine receptors

The peripheral benzodiazepine receptor (PBR) is expressed by microglial cells in many neuropathologies involving neuroinflammation. PK11195, the reference compound for PBR, is used for positron emission tomography (PET) imaging but has a limited capacity to quantify PBR expression. Here we describe the new PBR ligand CLINME as an alternative to PK11195. In vitro and in vivo imaging properties of [(11)C]CLINME were studied in a rat model of local acute neuroinflammation, and compared with the reference compound [(11)C]PK11195, using autoradiography and PET imaging. Immunohistochemistry study was performed to validate the imaging data. [(11)C]CLINME exhibited a higher contrast between the PBR-expressing lesion site and the intact side of the same rat brain than [(11)C]PK11195 (2.14 +/- 0.09 vs. 1.62 +/- 0.05 fold increase, respectively). The difference was due to a lower uptake for [(11)C]CLINME than for [(11)C]PK11195 in the non-inflammatory part of the brain in which PBR was not expressed, while uptake levels in the lesion were similar for both tracers. Tracer localization correlated well with that of activated microglial cells, demonstrated by immunohistochemistry and PBR expression detected by autoradiography. Modeling using the simplified tissue reference model showed that R(1) was similar for both ligands (R(1) approximately 1), with [(11)C]CLINME exhibiting a higher binding potential than [(11)C]PK11195 (1.07 +/- 0.30 vs. 0.66 +/- 0.15). The results show that [(11)C]CLINME performs better than [(11)C]PK11195 in this model. Further studies of this new compound should be carried out to better define its capacity to overcome the limitations of [(11)C]PK11195 for PBR PET imaging.

Positron emission tomography imaging of peripheral benzodiazepine receptor binding in human immunodeficiency virus-infected subjects with and without cognitive impairment

The pathology associated with late-stage dementia in human immunodeficiency virus (HIV) infection has been studied extensively. Neuropathological examination has demonstrated abundant activation and infection of macrophages/microglia termed HIV encephalitis. For obvious reasons, less is known regarding the neuropathology of minor cognitive impairment seen in earlier stages of HIV infection. The authors examined the utility of the peripheral benzodiazepine receptor ligand PK11195 in positron emission tomography (PET) imaging to assess microglial/macrophage activation in the brains of HIV-infected subjects with minor neurocognitive impairment in a cross-sectional study of 12 HIV infected individuals and 5 age-matched noninfected controls. Subjects were given a battery of neuropsychological tests in addition to assessing CD4 T-cell count and peripheral viremia followed by contrast enhanced magnetic resonance imaging (MRI) and PET with [15O]H2O followed by [11C](R)-PK11195. Two of the six neurocognitively impaired HIV-infected subjects demonstrated plasma viral breakthrough, whereas only one of six nonimpaired individuals demonstrated plasma viral load near the limits of detection. MRI demonstrated no abnormal enhancement and although atrophy was more prominent in impaired subjects, it was also present though to a lesser extent in nonimpaired subjects. None of the 12 HIV-infected subjects demonstrated increased retention of [11C](R)-PK11195 in the brain parenchyma compared to the 5 controls. These results suggest that either [11C](R)-PK11195 PET assessment is insensitive to the degree of macrophage activation in HIV-associated minor neurocognitive impairment or macrophage activation is not the pathological substrate of this neurological condition.

The peripheral benzodiazepine receptor (Translocator protein 18kDa) in microglia: from pathology to imaging

Microglia constitute the primary resident immune surveillance cell in the brain and are thought to play a significant role in the pathogenesis of several neurodegenerative disorders, such as Alzheimer's disease, multiple sclerosis, Parkinson's disease and HIV-associated dementia. Measuring microglial activation in vivo in patients suffering from these diseases may help chart progression of neuroinflammation as well as assess efficacy of therapies designed to modulate neuroinflammation. Recent studies suggest that activated microglia in the CNS may be detected in vivo using positron emission tomography (PET) utilizing pharmacological ligands of the mitochondrial peripheral benzodiazepine receptor (PBR (recently renamed as Translocator protein (18kDa)). Beginning with the molecular characterization of PBR and regulation in activated microglia, we examine the rationale behind using PBR ligands to image microglia with PET. Current evidence suggests these findings might be applied to the development of clinical assessments of microglial activation in neurological disorders.

How healthy is the acutely reperfused ischemic penumbra?

Because it is the main determinant of clinical recovery, early reperfusion of the ischemic penumbra has become the mainstay of acute stroke therapy. Although early permanent recanalization can be associated with spectacular and complete recovery, some patients in fact exhibit delayed or incomplete recovery, even despite small infarcts on late structural imaging. This might result from tissue inflammation and selective neuronal death/damage, two probably inter-related cellular events well described in the animal literature, precluding full functional restoration in the salvaged penumbra. However, impact of these processes on recovery may be complex because of the interplay with ongoing plasticity and the possible promoting effect of inflammation on the latter. Preliminary results from imaging studies of inflammation and selective neuronal loss after middle cerebral artery territory stroke, using radioligands of the central benzodiazepine receptor and the activated microglia, respectively, reviewed here, suggest these phenomena also exist in man, although their relationship with acute-stage hypoperfusion and their impact on clinical recovery, if any, remain poorly understood. Furthermore, their inter-relationships in the salvaged penumbra have not been addressed. Better understanding of these potentially harmful processes might help to maximize benefits from thrombolysis, and could also have implications for patients who enjoy spontaneous recanalization.

Synthesis and in vivo evaluation of a novel peripheral benzodiazepine receptor PET radioligand

The novel pyrazolopyrimidine ligand, N,N-diethyl-2-[2-(4-methoxyphenyl)-5,7-dimethyl-pyrazolo[1,5-a]pyrimidin-3-yl]-acetamide 1 (DPA-713), has been reported as a potent ligand for the peripheral benzodiazepine receptor (PBR) displaying an affinity of K(i)=4.7 nM. In this study, 1 was successfully synthesised and demethylated to form the phenolic derivative 6 as precursor for labelling with carbon-11 (t(1/2) = 20.4 min). [11C]1 was prepared by O-alkylation of 6 with [11C]methyl iodide. The radiochemical yield of [(11)C]1 was 9% (non-decay corrected) with a specific activity of 36 GBq/micromol at the end of synthesis. The average time of synthesis including formulation was 13.2 min with a radiochemical purity >98%. In vivo assessment of [11C]1 was performed in a healthy Papio hamadryas baboon using positron emission tomography (PET). Following iv administration of [11C]1, significant accumulation was observed in the baboon brain and peripheral organs. In the brain, the radioactivity peaked at 20 min and remained constant for the duration of the imaging experiment. Pre-treatment with the PBR-specific ligand, PK 11195 (5 mg/kg), effectively reduced the binding of [11C]1 at 60 min by 70% in the whole brain, whereas pre-treatment with the central benzodiazepine receptor ligand, flumazenil (1mg/kg), had no inhibitory effect on [11C]1 uptake. These results indicate that accumulation of [11C]1 in the baboon represents selective binding to the PBR. These exceptional in vivo binding properties suggest that [11C]1 may be useful for imaging the PBR in disease states. Furthermore, [11C]1 represents the first ligand of its pharmacological class to be labelled for PET studies and therefore has the potential to generate new information on the pathological role of the PBR in vivo.

[11C]Vinpocetine: a prospective peripheral benzodiazepine receptor ligand for primate PET studies

Vinpocetine, a synthetic derivative of the Vinca minor alkaloid vincamine, is a widely used drug in neurological practice. We tested the hypothesis that vinpocetine binds to peripheral benzodiazepine binding sites (PBBS) and is therefore a potential ligand of PBBS. Positron emission tomography (PET) measurements in two cynomolgous monkeys showed that pretreatment with vinpocetine markedly reduced the brain uptake of [11C]PK11195, a known PBBS radioligand. On the other hand, whereas pretreatment with PK11195 increased the brain uptake of [11C]vinpocetine due to the blockade of PBBS in the periphery, it significantly reduced the binding potential (BP) values of [11C]vinpocetine in the whole brain and in individual brain structures to PK11195. These findings indicate that, whereas the two ligands have different affinities to PBBS, vinpocetine is a potent ligand of PBBS, which in turn suggests that the pharmacological activity of vinpocetine may involve the regulation of glial functions.

Evaluation of three quinoline-carboxamide derivatives as potential radioligands for the in vivo pet imaging of neurodegeneration

The peripheral-type benzodiazepine receptors (PBRs) are only minimally expressed in normal brain parenchyma, where they are primarily localized in glial cells. Their basal expression rises in different neurodegenerative disorders, due to the presence of infiltrating inflammatory cells and activated microglia. [11C]PK11195, a selective PBR antagonist, has been used for the in vivo PET monitoring of neurodegeneration in clinical observations. We recently developed and labeled with carbon-11 three new carboxamide derivatives: [11C]VC193M, [11C]VC195 and [11C]VC198M. Aim of this study was to evaluate these ligands for the in vivo measuring of PBRs expression in neurodegenerations and compare their kinetic behavior with that of the reference tracer [11C]PK11195. Radioligands were evaluated in a preclinical model of Huntington's disease consisting in the monolateral striatal injection of quinolinic acid (QA). Activated microglia and astrocytic gliosis was present only within the affected striatum. A concomitant increase in radioactivity accumulation was observed for all the tracers examined (P<0.01). Among the new compounds, [11C]VC195 showed higher levels of lesioned/unlesioned striatum ratios (3.28+/-0.44), in comparison with [11C]VC193M and [11C]VC198M (2.69+/-0.53 and 1.52+/-0.36, respectively), but slightly inferior to that observed for [11C]PK11195 (3.76+/-1.41).In conclusion, the results of the study indicate that [11C]VC195 is a promising candidate for in vivo PET monitoring of neurodegenerative processes but its in vivo behavior overlap that of [11C]PK11195.

Novel peripheral benzodiazepine receptor ligand [11C]DAA1106 for PET: An imaging tool for glial cells in the brain

Peripheral benzodiazepine receptor (PBR) is expressed in most organs and its expression is reported to be increased in activated microglia in the brain. [(11)C]PK11195 has been widely used for the in vivo imaging of PBRs, but its signal in the brain was not high enough for stable quantitative analysis. We synthesized a novel positron emission tomography (PET) ligand, [(11)C]DAA1106, for PBR and investigated its in vivo properties in rat and monkey brain. High uptake of [(11)C]DAA1106 was observed in the olfactory bulb and choroid plexus area, followed by the pons/medulla and cerebellum by in vivo autoradiography of rat brain, correlating with the binding in vitro. [(11)C]DAA1106 binding was increased in the dorsal hippocampus with neural destruction, suggesting glial reaction. [(11)C]DAA1106 binding was both inhibited and displaced by 1.0 mg/kg of DAA1106 and 5 mg/kg of PK11195 by 80% and 70%, respectively. Specific binding was estimated as 80% of total binding. [(11)C]DAA1106 binding was four times higher compared to the binding of [(11)C]PK11195 in the monkey occipital cortex. These results indicated that [(11)C]DAA1106 might be a good ligand for in vivo imaging of PBR.

The peripheral benzodiazepine binding site in the brain in multiple sclerosis: quantitative in vivo imaging of microglia as a measure of disease activity

This study identifies by microautoradiography activated microglia/macrophages as the main cell type expressing the peripheral benzodiazepine binding site (PBBS) at sites of active CNS pathology. Quantitative measurements of PBBS expression in vivo obtained by PET and [(11)C](R)-PK11195 are shown to correspond to animal experimental and human post-mortem data on the distribution pattern of activated microglia in inflammatory brain disease. Film autoradiography with [(3)H](R)-PK11195, a specific ligand for the PBBS, showed minimal binding in normal control CNS, whereas maximal binding to mononuclear cells was found in multiple sclerosis plaques. However, there was also significantly increased [(3)H](R)-PK11195 binding on activated microglia outside the histopathologically defined borders of multiple sclerosis plaques and in areas, such as the cerebral central grey matter, that are not normally reported as sites of pathology in multiple sclerosis. A similar pattern of [(3)H](R)-PK11195 binding in areas containing activated microglia was seen in the CNS of animals with experimental allergic encephalomyelitis (EAE). In areas without identifiable focal pathology, immunocytochemical staining combined with high-resolution emulsion autoradiography demonstrated that the cellular source of [(3)H](R)-PK11195 binding is activated microglia, which frequently retains a ramified morphology. Furthermore, in vitro radioligand binding studies confirmed that microglial activation leads to a rise in the number of PBBS and not a change in binding affinity. Quantitative [(11)C](R)-PK11195 PET in multiple sclerosis patients demonstrated increased PBBS expression in areas of focal pathology identified by T(1)- and T(2)-weighted MRI and, importantly, also in normal-appearing anatomical structures, including cerebral central grey matter. The additional binding frequently delineated neuronal projection areas, such as the lateral geniculate bodies in patients with a history of optic neuritis. In summary, [(11)C](R)-PK11195 PET provides a cellular marker of disease activity in vivo in the human brain.

High-performance liquid chromatographic determination of [11C]1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinoline carboxamide in mouse plasma and tissue and in human plasma

The high-performance liquid chromatographic determination of 1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinoline carboxamide ([11C]PK 11195) is described. The method was successfully applied for plasma and tissue analysis after i.v. injection of [11C]PK 11195 in mice and for plasma analysis after administration of [11C]PK 11195 to humans. Separation is effected on a RP-C18 column, using a mixture of acetonitrile-water-triethylamine (65:35:0.5, v/v). Quantitative measurements of radioactivity are performed on a one-channel gamma-ray spectrometer equipped with a 2 x 2 in. NaI(Tl) detector. For humans rapid metabolisation of [11C]PK 11195 was observed. At 5, 20 and 35 min post injection 5%, 22% and 32%, respectively, of the plasma activity consisted of at least two more polar metabolites. Despite the extensive metabolisation rate in mice (up to 42% at 10 min post injection of [11C]PK 11195), no 11C-labelled metabolites could be detected in the extracts of brain and heart.

Assessment of the peripheral benzodiazepine receptors in human gliomas by two methods

This study was designed to evaluate the density of peripheral benzodiazepine receptor (PBR) sites as a function of tumor malignancy in human gliomas, and to compare the results obtained with autoradiographic and liquid scintillation measurements performed on the same tissue specimens. In vitro binding of [3H]PK-11195[1-(2-chlorophenyl)-N-methyl-(1-methylpropyl)-3-isoguinol ine carboxamide] to human gliomas in radioligand binding studies revealed a significantly higher level (about 3 fold) of PBR binding sites in both low grade and high grade gliomas as compared to normal cortex. The Bmax (mean +/- SD) of high and low grade gliomas, when entire tissue sections were measured by autoradiography, was 5.5 +/- 0.3 pmol/mg-tissue (n = 5) and 1.8 +/- 0.9 pmol/mg-tissue (n = 6), respectively, although it was evident that there was area of hot spots in the high grade tumors. This difference was significant (p < 0.05; two-tailed t-test). Similarly, the KD values (dissociation constant; nM) between the high (KD = 20.4 +/- 1.3 nM) and low (KD = 14.3 +/- 2.1 nM) grade gliomas were significantly different. A significant difference in binding site density (Bmax) between the two types of gliomas was also obtained in liquid scintillation measurements. The hot spot areas which showed the most intense binding of [3H]PK-11195 had KD of 24.5 +/- 1.0 nM and Bmax of 6.2 +/- 0.42 pmol/mg-tissue, values significantly higher (p < 0.05, two-tailed t-test) than those obtained when the entire tissue section was measured. The data on the Bmax/KD ratios presented here suggest that it might be possible to differentiate high from low grade gliomas in human by in vivo imaging with 11C-labelled PK-11195.

Radioligands for PET studies of central benzodiazepine receptors and PK (peripheral benzodiazepine) binding sites--current status

The status of the radiochemical development and biological evaluation of radioligands for PET studies of central benzodiazepine (BZ) receptors and the so-called peripheral benzodiazepine binding sites, here discriminated and referred to as PK binding sites, is reviewed against current pharmacological knowledge, indicating those agents with present value and those with future potential. Practical recommendations are given for the preparation of two useful radioligands for PET studies, [N-methyl-11C]flumazenil for central BZ receptors, and [N-methyl-11C]PK 11195 for PK binding sites. Quality assurance and plasma metabolite analysis are also reviewed for these radioligands and practical recommendations are given on methodology for their performance.

Positron emission tomographic studies of [11C]MDL 72222, a potential 5-HT3 receptor radioligand: distribution, kinetics and binding in the brain of the baboon

The drug MDL 72222, a selective 5-HT3 receptor antagonist, was labelled with 11C and evaluated for distribution kinetics in brain and in vivo binding to 5-HT3 receptors using cold MDL 72222 challenge and positron emission tomography (PET), in three anaesthetized baboons. After tracer doses of [11C]MDL 72222 (i.v. bolus), 11C radioactivity was equally partitioned between plasma and blood cells and readily crossed the blood-brain barrier; it was distributed heterogeneously into 17 different structures of the brain. The kinetic curves for 11C in tissue showed a rapid initial uptake, followed by a slower ascending phase, up to about the twentieth minute and by a plateau, until the end of experiment (90 min). The plateau values indicated marked uptake in brain which, however, varied according to the region considered. In inhibition studies with cold MDL 72222 (1 mg.kg-1) as pretreatment, co-injection or displacement, no clear-cut effects on the kinetics of [11C] MDL 72222 in brain were detected in any region, including those known to be rich in 5-HT3 receptors. These observations suggest that specific binding to 5-HT3 receptors was not detectable in brain in vivo, because of the high lipophilicity (thus a great capacity for non-specific binding) of MDL 72222. These negative findings may also result from both the possible suboptimal affinity of MDL 72222 for 5-HT3 receptors in vivo and the relatively low density of 5-HT3 receptors present only in selected areas of the mammalian brain. This study is a step in the search of selective 5-HT3 receptor radioligands, adequate for in vivo applications. Slow clearance of [11C]MDL 72222 from brain tissue in baboons, should be accounted for in clinical pharmacokinetic investigations for optimal posology considerations.