Low Brain Penetration Research Articles

Potent, orally bioavailable, liver-selective stearoyl-CoA desaturase (SCD) inhibitors

Two structurally distinct series of SCD (Δ9 desaturase) inhibitors (1 and 2) have been previously reported by our group. In the present work, we merged the structural features of the two series. This led to the discovery of compound 5b (CVT-12,012) which is highly potent in a human cell-based (HEPG2) SCD assay (IC50=6nM). This compound has 78% oral bioavailability in rats and is preferentially distributed into liver (76 times vs plasma) with relatively low brain penetration. In a five-day study (sucrose fed rats) compound 5b significantly reduced SCD activity in a dose-dependent manner as determined by GC analysis of fatty acid composition in plasma and liver, and significantly reduced liver triglycerides versus the control group (∼50%).

Orally bioavailable, liver-selective stearoyl-CoA desaturase (SCD) inhibitors

We discovered a structurally novel SCD (Δ9 desaturase) inhibitor 4a (CVT-11,563) that has 119nM potency in a human cell-based (HEPG2) SCD assay and selectivity against Δ5 and Δ6 desaturases. This compound has 90% oral bioavailability (rat) and excellent plasma exposure (dAUC 935ngh/mL). Additionally, 4a shows moderately selective liver distribution (three times vs plasma and adipose tissue) and relatively low brain penetration. In a five-day study (high sucrose diet, rat) compound 4a significantly reduced SCD activity as determined by GC analysis of fatty acid composition in plasma and liver. We describe the discovery of 4a from HTS hit 1 followed by scaffold replacement and SAR studies focused on DMPK properties.

Synthesis and preliminary biological evaluations of [18F]-1-deoxy-1-fluoro-scyllo-inositol

A novel PET radiotracer, [18F]-1-deoxy-1-fluoro-scyllo-inositol, was synthesized via a one-pot reaction in 16 +/- 3% uncorrected radiochemical yield within 80 minutes; although this compound revealed low brain penetration it shows promise in rodent tumour models for breast cancer imaging.

Sodium hyaluronate as a mucoadhesive component in nasal formulation enhances delivery of molecules to brain tissue

Intranasal administration of molecules has been investigated as a non-invasive way for delivery of drugs to the brain in the last decade. Circumvention of both the blood–brain barrier and the first-pass elimination by the liver and gastrointestinal tract is considered as the main advantages of this method. Because of the rapid mucociliary clearance in the nasal cavity, bioadhesive formulations are needed for effective targeting. Our goal was to develop a formulation containing sodium hyaluronate, a well-known mucoadhesive molecule, in combination with a non-ionic surfactant to enhance the delivery of hydrophilic compounds to the brain via the olfactory route. Fluorescein isothiocyanate-labeled 4 kDa dextran (FD-4), used as a test molecule, was administered nasally in different formulations to Wistar rats, and detected in brain areas by fluorescent spectrophotometry. Hyaluronan increased the viscosity of the vehicles and slowed down the in vitro release of FD-4. Significantly higher FD-4 transport could be measured in the majority of brain areas examined, including olfactory bulb, frontal and parietal cortex, hippocampus, cerebellum, midbrain and pons, when the vehicle contained hyaluronan in combination with absorption enhancer. The highest concentrations of FD-4 could be detected in the olfactory bulbs, frontal and parietal cortex 4 h after nasal administration in the mucoadhesive formulation. Intravenous administration of a hundred times higher dose of FD-4 resulted in a lower brain penetration as compared to nasal formulations. Morphological examination of the olfactory system revealed no toxicity of the vehicles. Hyaluronan, a non-toxic biomolecule used as a mucoadhesive in a nasal formulation, increased the brain penetration of a hydrophilic compound, the size of a peptide, via the nasal route.

Strategies to assess blood–brain barrier penetration

Background: The principles and screening strategies for brain penetration in drug discovery are important in identifying drug candidates with desirable CNS properties. Objective: Define key variables and assays that are essential for determining brain penetration. Methods: This review covers issues, methods, and strategies for assessing brain penetration of small molecules in drug discovery. Results/conclusion: Brain penetration is assessed using both initial rate and extent at steady-state. Unbound drug is the active species that exerts pharmacological effects. Low brain penetration can be due to low blood–brain barrier (BBB) permeability, P-glycoprotein (Pgp) efflux, or high plasma protein binding. Successful methods include: parallel artificial membrane permeability assay (PAMPA)-BBB permeability, MDR1-MDCKII for Pgp efflux, B-P dialysis for fraction unbound, and in vivo B/P ratio to extrapolate unbound brain drug concentration.

A2A Adenosine Receptor and its Modulators: Overview on a Druggable GPCR and on Structure-Activity Relationship Analysis and Binding Requirements of Agonists and Antagonists

Since the discovery of the biological effects of adenosine, the development of potent and selective agonists and antagonists of adenosine receptors has been the subject of medicinal chemistry research for several decades, even if their clinical evaluation has been discontinued. Main problems include side effects due to the ubiquity of the receptors and the possibility of side effects, or to low brain penetration (in particular for the targeting of CNS diseases), short half-life of compounds, lack of effects. Furthermore, species differences in the affinity of ligands make difficult preclinical testing in animal models. Nevertheless, adenosine receptors continue to represent promising drug targets. A(2A) receptor has proved to be a promising pharmacological target for small synthetic ligands, and while A(2A) agonists are undergoing clinical trials for myocardial perfusion imaging and as anti-inflammatory agents, A(2A) antagonists represent an attractive field of research to discover new drugs for the treatment of neurodegenerative disorders, such as Parkinson's disease. Furthermore, the information coming from bioinformatics and molecular modeling studies for the A(2A) receptor has made easier the understanding of ligand-target interaction and the rational design of agonists and antagonists for this subtype. The aim of this review is to show an overview of the most significant steps and progresses in developing A(2A) adenosine receptor agonists and antagonists.

4-Acyl-1-(4-aminoalkoxyphenyl)-2-ketopiperazines as a Novel Class of Non-Brain-Penetrant Histamine H3 Receptor Antagonists

A series of ketopiperazines were prepared and evaluated for their activity as histamine H 3 antagonists. From investigation of the tertiary basic center in the aminopropyloxyphenyl template, the 2( R)-methylpyrrolidine was identified as the most potent amine. In the more rigid piperidineoxyphenyl template the N-cyclobutyl group was the most potent amine. The 4-fluorobenzyol, 4-cyanobenzoyl, and 2,4-difluorobenzoyl groups provided good pharmacokinetic profiles for the various amides. The PSA and log D values of these compounds suggested low brain penetration. The compounds had very high selectivity over other receptors and did not inhibit hepatic cytochrome P450, indicating low drug-drug interaction potential. Compound 22i was identified as the best compound of this series based on its overall profile of high potency, selectivity, low brain penetration, lack of CYP450 inhibition, high oral bioavailability, and pharmacokinetic properties.

PET-Evaluated Transport of [11C]Hydroxyurea Across the Rat Blood-Brain Barrier - Lack of Influence of Cyclosporin and Probenecid

The transport of hydroxyurea, a ribonucleoside reductase inhibitor, over biological membranes is slow and it has therefore been suggested that the substance could interact with an active efflux transporter. The transport of [(11)C]hydroxyurea into the rat brain was therefore studied after administration of the multidrug resistance protein inhibitor probenecid (50 and 150 mg/kg), the P-glycoprotein inhibitor cyclosporin A (25 mg/kg), hydroxyurea (50, 150 and 450 mg/kg) and mannitol (25%). None of the intervention drugs affected the brain uptake of [(11)C]hydroxyurea. The brain-to-plasma concentration ratios (K(p)), with or without intervention drug, were in the range 0.12-0.25 after 60 min of [(11)C]hydroxyurea infusion. [(11)C]Verapamil, a P-glycoprotein substrate with low brain penetration, was used to study the ability of hydroxyurea to inhibit P-glycoprotein. Administration of hydroxyurea (150 and 450 mg/kg) did not increase brain concentrations of [(11)C]verapamil. It is therefore unlikely that hydroxyurea is a substrate for or an inhibitor of P-glycoprotein or a substrate for a probenecid sensitive transport system. The low brain concentrations may instead be the result of slow uptake due to the hydrophilic nature of hydroxyurea.

Highlights on the Development of A2A Adenosine Receptor Agonists and Antagonists

Although significant progress has been made in the past few decades demonstrating that adenosine modulates a variety of physiological and pathophysiological processes through the interaction with four subtypes of a family of cell-surface G-protein-coupled receptors, clinical evaluation of some adenosine receptor ligands has been discontinued. Major problems include side effects due to the wide distribution of adenosine receptors, low brain penetration (which is important for the targeting of CNS diseases), short half-life of compounds, or a lack of effects, in some cases perhaps due to receptor desensitization or to low receptor density in the targeted tissue. Currently, three A(2A) adenosine receptor agonists have begun phase III studies. Two of them are therapeutically evaluated as pharmacologic stress agents and the third proved to be effective in the treatment of acute spinal cord injury (SCI), while avoiding the adverse effects of steroid agents. On the other hand, the great interest in the field of A(2A) adenosine receptor antagonists is related to their application in neurodegenerative disorders, in particular, Parkinson's disease, and some of them are currently in various stages of evaluation. This review presents an update of medicinal chemistry and molecular recognition of A(2A) adenosine receptor agonists and antagonists, and stresses the strong need for more selective ligands at the A(2A) human subtype.

Evidence of lowest brain penetration of an antiemetic drug, metopimazine, compared to domperidone, metoclopramide and chlorpromazine, using an in vitro model of the blood–brain barrier

Purpose The objective of the current study was to determine the ability of some antiemetic compounds to cross the blood–brain barrier (BBB) and thereby to determine possible side effects of compounds for the central nervous system (CNS). Methods We compared the brain penetration of some antiemetic compounds using an in vitro BBB model consisting in brain capillary endothelial cells co-cultured with primary rat glial cells. Results This study clearly demonstrated that the metopimazine metabolite, metopimazine acid, has a very low brain penetration, lower than metopimazine and even less than the other antiemetic compounds tested in this study. Conclusions The poor brain penetration of metopimazine acid, metopimazine biodisponible form, seems very likely related to the clinically observed difference in therapeutic and safety profile.

A Thienopyridazinone-Based Melanin-Concentrating Hormone Receptor 1 Antagonist with Potent in Vivo Anorectic Properties

Melanin-concentrating hormone receptor antagonists containing thieno- and a benzopyridazinone cores were designed and tested as potential anorectic agents. These ligands showed high affinity for the receptor, potent functional activity in vitro, and good oral bioavailabilty in rats. The thiophene analogue exhibited low iv clearance, long half-life, and high brain penetration. In obese rats, the thienopyridazinone demonstrated a dose-dependent reduction in feeding and body weight with doses between 1 and 10 mg kg-1.

Preclinical pharmacokinetics of a novel HIV‐1 attachment inhibitor BMS‐378806 and prediction of its human pharmacokinetics

BMS-378806 is a prototype of novel HIV attachment inhibitors that block the gp120 and CD4 interaction, the first step of HIV-1 entry into cells. The present work investigated the pharmacokinetics of BMS-378806 in rats, dogs and monkeys and assessed its in vitro permeability and metabolism. BMS-378806 exhibited species-dependent oral bioavailability which was 19%-24% in rats and monkeys and 77% in dogs. In rats and monkeys, absorption was prolonged, with an apparent terminal half-life of 2.1 and 6.5 h, respectively. In rats, linear pharmacokinetics was observed between i.v. doses of 1 and 5 mg/kg and between p.o. doses of 5 and 25 mg/kg. The total body clearance was intermediate in rats and low in dogs and monkeys. The steady-state volume of distribution was moderate (0.4-0.6 l/kg), contributing to a short half-life (0.3-1.2 h) after i.v. dosing. Studies in bile-duct cannulated rats together with intraportal infusion studies revealed that the renal and hepatic clearance each accounted for 30% and 70% of the total elimination in rats, with the hepatic clearance largely being oxidative metabolism. In vitro, BMS-378806 was not highly protein bound (44%-73%). The Caco-2 permeability was modest (51 nm/s) and confounded by P-glycoprotein mediated efflux transport. Both of these may contribute to the low brain penetration observed in rats (brain/plasma AUC ratio=0.06). In human liver microsomes BMS-378806 was equally metabolized by cytochrome P450 1A2, 2D6 and 3A4 and did not inhibit major drug-metabolizing enzymes to a significant extent. Based on in vitro and animal data, a mechanistic approach that factors in absorption and first-pass metabolism was employed to predict the human oral bioavailability of BMS-378806 (ca 20%). This, together with the complex Dedrick plot method, was used to simulate human oral profiles and to project an efficacious dose. These study results offer a comprehensive assessment of the developability of BMS-378806 and provide important guidance to improving absorption and half-life of future compounds in the series. The current studies also demonstrate the value and approaches of understanding pharmacokinetic properties in the early stage of drug discovery.

Biphenyls labeled with technetium 99m for imaging β-amyloid plaques in the brain

Formation and accumulation of excess aggregates of β-amyloid (Aβ) plaques in the brain are critical factors contributing to the development and progression of Alzheimer's disease (AD). There is an urgent need for in vivo imaging agents that can specifically demonstrate the location and density of Aβ plaques in the brain. The aim of this study was to develop potential technetium 99m ( 99mTc)-labeled diagnostic imaging agents specific for the detection of Aβ plaques. Based on previously obtained Aβ plaque-specific biphenyls containing a p- N, N-dimethylaminophenyl group, a series of 99mTc and Re–N 2S 2–biphenyl derivatives was prepared. The stable neutral and lipophilic 99mTc complexes, [ 99mTc] 19 and [ 99mTc] 23, A+B, were successfully obtained. As surrogates for the 99mTc complexes, the corresponding surrogates, Re complexes of 23, were also prepared. Surprisingly, it was found that the Re complexes showed distinctively different retention profiles as compared with the corresponding 99mTc complexes. Biodistribution studies indicated that [ 99mTc] 23A readily passed through the blood–brain barrier (1.18% dose/brain at 2 min) in contrast to the low brain penetration of [ 99mTc] 19 (0.29% dose/brain at 2 min). Initial results suggested that [ 99mTc] 23A showed selective binding to the Aβ plaque-like structures in the brain sections from transgenic mice but not in the postmortem human brain tissue of patients with confirmed AD. The results provide encouraging evidence that development of a 99mTc-labeled agent for imaging Aβ plaques in the brain may be feasible. Caution should be taken when comparing these 99mTc complexes with the corresponding surrogates — the Re complexes.

Dr. Strangelove, or how I learned to stop worrying and love the core competencies

We evaluated the pharmacological profiles of (2<i>R</i>)-<i>N</i>-[1-(6- aminopyridin-2-ylmethyl)piperidin-4-yl]-2-[(1<i>R</i>)-3,3-difluorocyclopentyl]-2-hydroxy-2-phenylacetamide(compound A), which is a novel muscarinic receptor antagonist with M₂-sparing antagonistic activity. Compound A inhibited [³H]NMS binding to cloned human muscarinic m1, m2, m3, m4, and m5 receptors expressed in Chinese hamster ovary cells with <i>K</i>_i values (nM) of 1.5, 540, 2.8, 15, and 7.7, respectively. In isolated rat tissues, compound A inhibited carbachol-induced responses with 540-fold selectivity for trachea (<i>K</i>_B = 1.2 nM) over atria (<i>K</i>_B = 650 nM). In in vivo rat assays, compound A inhibited acetylcholine-induced bronchoconstriction and bradycardia with intravenous ED₅₀ values of 0.022 mg/kg and ≥10 mg/kg, respectively. Furthermore, in dogs, compound A (0.1–1 mg/kg p.o.) dose dependently shifted the methacholine concentration-respiratory resistance curves. In mice, compound A (10 mg/kg i.v.) did not inhibit oxotremorine-induced tremor. The brain/plasma ratio (<i>K</i>_p) of compound A (3 mg/kg i.v.) was 0.13 in rats; this <i>K</i>_p was less than that of scopolamine (1.7) and darifenacin (0.24). The inhibition of compound A (3 mg/kg i.v.) on ex vivo binding in rat cerebral cortex was almost similar to that of NMS. These findings demonstrate that compound A has high selectivity for M₃receptors over M₂ receptors, displays a potent, oral M₃ antagonistic activity without inhibition of central muscarinic receptors because of low brain penetration. It is well known that central muscarinic antagonists may have diverse CNS effects, and M₂ receptors regulate cardiac pacing and act as autoreceptors in the lung and bladder. Thus, compound A may have fewer cardiac or CNS side effects than nonselective compounds.

Synthesis and Structure−Activity Relationships of Novel Histamine H1 Antagonists: Indolylpiperidinyl Benzoic Acid Derivatives

A series of indolylpiperidinyl derivatives were prepared and evaluated for their activity as histamine H(1) antagonists. Structure-activity relationship studies were directed toward improving in vivo activity and pharmacokinetic profile of our first lead (1). Substitution of fluorine in position 6 on the indolyl ring led to higher in vivo activity in the inhibition of histamine-induced cutaneous vascular permeability assay but lower selectivity toward 5HT(2) receptor. Extensive optimization was carried out within this series and a number of histamine H(1) antagonists showing potency and long duration of action in vivo and low brain penetration or cardiotoxic potential were identified. Within this novel series, indolylpiperidines 15, 20, 48,51 and 52 exhibited a long half-life in rat and have been selected for further preclinical evaluation.

Synthesis, radiosynthesis and In vivo evaluation of 5-[3-(4-Benzylpiperidin-1-yl)prop-1-ynyl]-1,3-dihydrobenzoimidazol-2-[ 11C]one, as a potent NR 1A/2B subtype selective NMDA PET radiotracer

Recently, a new series of potent and highly subtype-selective 1-(heteroarylalkynyl)-4-benzylpiperidine antagonists of the NMDA receptors has been described by Pfizer Laboratories. In this series, 5-[3-(4-benzylpiperidin-1-yl)prop-1-ynyl]-1,3-dihydrobenzoimidazol-2-one ( 1) was identified as a selective antagonist for the NR1 A/2B subtype, displaying IC 50 values for inhibition of the NMDA responses of 5.3 nM for this subtype (compared to NR1 A/2A: 35 μM and NR1 A/2C>100 μM) and was active in rat at a relatively low dosage (10 mg/kg po). Derivative 1 has been synthesized in four chemical steps in good overall yield and labelled with carbon-11 T 1 2 : 20.4 min at its benzoimidazolone ring using [ 11C]phosgene. The pharmacological profile of [ 11C]- 1 was evaluated in vivo in rats with biodistribution studies and brain radioactivity monitored with intracerebral radiosensitive β-microprobes. The brain uptake of [ 11C]- 1 was extremely low (0.07% I.D./mL on average at 30 min) and rather uniform across the different brain structures. This in vivo brain regional distribution of [ 11C]- 1 did not match with autoradiographic or binding data obtained with other NR2B subtype-selective NMDA ligands. Competition studies with ifenprodil (20 mg/kg, ip, 30 min before the radiotracer injection) failed to demonstrate specific binding of the radiotracer in the brain. In view of these results, and especially considering the low brain penetration of the radiotracer, [ 11C]- 1 does not have the required properties for imaging NMDA receptors using positron emission tomography.

In vivo evaluation of [ 11C]-3-[2-[(3-methoxyphenylamino)carbonyl]ethenyl]-4,6-dichloroindole-2-carboxylic acid ([ 11C]3MPICA) as a PET radiotracer for the glycine site of the NMDA ion channel

Alterations in normal NMDA receptor composition, densities and function have been implicated in the pathophysiology of certain neurological and neuropsychiatric disorders such as Parkinson’s Disease, Huntington’s Chorea, schizophrenia, alcoholism and stroke. In our first effort to provide PET ligands for the NMDA/glycine site, we reported the synthesis of a novel high affinity glycine site ligand, 3-[2-[(3-methoxyphenylamino)carbonyl]ethenyl]-4,6-dichloroindole-2-carboxylic acid ((3MPICA), Ki = 4.8 ± 0.9 nM) and the corresponding carbon-11 labeled PET ligand, [ 11C]3MPICA. We report here the in vivo evaluation of [ 11C]3MPICA in rats. Biodistribution analysis revealed that [ 11C]3MPICA exhibited low degree of brain penetration and high blood concentration. The average uptake at two minutes was highest in the cerebellum (0.19 ± 0.04 %ID/g) and thalamus (0.18 ± 0.05 %ID/g) and lower in the hippocampus (0.13 ± 0.03) and frontal cortex (0.11 ± 0.04 %ID/g). The radioactivity cleared quickly from all brain regions examined. Administration of unlabeled 3MPICA (1 mg/kg, i.v.) revealed at 60 minutes a small general reduction in regional brain radioactivity concentrations in treated animals versus controls, however, the blood radioactivity concentration was also lowered, confounding the assessment of the degree of saturable binding. Warfarin co-administration (100 mg/kg, i.v.) significantly lowered blood activity at 5 minutes post-injection (−27%, P < 0.01) but failed to significantly increase the brain uptake of the radiotracer. In view of these results, and especially considering the low brain penetration of this tracer, [ 11C]3MPICA does not appear to be a promising PET radiotracer for in vivo use.

Identification of 3,5-dihydro-2-aryl-1H-pyrazolo[3,4-c]quinoline-1,4(2H)-diones as novel high-affinity glycine site N-methyl-D-aspartate antagonists.

Almost all of the existing known antagonists at the glycine site of the N-methyl-D-aspartate (NMDA) receptor have a low propensity for crossing the blood-brain barrier. It has been suggested that in many cases this may be due to the presence of a carboxylic acid which is a common feature of most of the potent full antagonists at this receptor. In this study, 2-aryl-1H-pyrazolo[3,4-c]quinoline-1,4(2H)-diones were found to have high-affinity binding at the glycine receptor. In particular, structure-activity studies identified 7-chloro-3,5-dihydro-2-(4-methoxyphenyl)-1H-pyrazolo[3,4-c]quinoline- 1,4(2H)-dione as the most potent of a series of analogues with an IC50 of 3.3 nM. The measured pKa values in this class of compounds (typically 4.0) indicate they are of equivalent acidity to carboxylic acids. Functional antagonism was demonstrated by inhibition of NMDA-evoked responses in rat cortical slices. Anticonvulsant activity in DBA/2 mice was achieved after dosing by direct injection into the cerebral ventricles, but no activity was seen after systemic administration, suggesting low brain penetration with this class of antagonists.

Brain distribution characteristics of xanthine derivatives and relation to their locomotor activity in mice.

The relationship between the brain distribution and motor activity in mice of the xanthines, theophylline, enprofylline, 1-methyl-3-propylxanthine (MPX) and oxpentifylline was investigated. Their plasma protein binding and hydrophobicity were also examined. When these xanthines were administered orally, enprofylline and oxpentifylline had no effect on motor activity. While theophylline increased motor activity over 10 mg kg-1, MPX caused a decrease in such activity over 10 mg kg-1. The protein-binding behaviour varied among these xanthines and was closely related to their hydrophobicity, which is represented as a logarithmic partition coefficient (log PC). MPX had the highest hydrophobicity, while oxpentifylline had the lowest. Brain distribution characteristics varied among these xanthines, with the rank order of their brain penetration ratio, calculated as the ratio of brain to unbound plasma concentrations, being theophylline > oxpentifylline > MPX > enprofylline. The inhibition constants (Ki) for adenosine A1 receptors and cyclic 3',5'-adenosine monophosphate (cAMP)-phosphodiesterase (PDE) of these xanthines were 44.6 and 134, > 1000 and 112, 26.4 and 49, and > 1000 and 111 microM for theophylline, enprofylline, MPX, and oxpentifylline, respectively. These findings suggest that the lack of effects of enprofylline and oxpentifylline on motor activity is probably due to their low brain penetration ratio or low adenosine A1 affinity in comparison with theophylline. The decrease in the motor activity by MPX may be, in part, mediated by cAMP or adenosine.

11C](+)McN5652 as a radiotracer for imaging serotonin uptake sites with pet

The in vivo behavior of the stereoisomers of [ 11C]McN5652, a highly potent serotonin (5-HT) uptake blocker, was determined to evaluate their utility as radiotracers for imaging 5-HT uptake sites by positron emission tomography (PET). After intravenous injection into mice, [ 11C](+)McN5652 showed markedly higher uptake and longer retention in regions with high density of 5-HT uptake sites than the [ 11C]-labeled racemic mixture, while [ 11C](−)McN5652 washed out rapidly. With the [ 11C](+)-enantiomer, the ratio between hypothalamus and cerebellum reached 6 at 90 minutes. The binding of [ 11C](+)McN5652 was inhibited by 45–73% by pre-injection of 5 mg/kg of paroxetine, a selective 5-HT uptake blocker, in all regions examined except cerebellum where no significant effect of the drug was observed. [ 11C](−)McN5652 showed no specific binding in any of the regions. The [ 11C]-labeled cis isomer, [ 11C]McN5655, revealed surprisingly low brain penetration and showed no significantly higher uptake in regions of interest than cerebellum. These results suggest that [ 11C](+)McN5652 is a promising candidate as a PET radiotracer for studying 5-HT uptake sites in vivo.