In vitro antagonistic properties of a new angiotensin type 1 receptor blocker, azilsartan, in receptor binding and function studies

The angiotensin II (AII) antagonistic action of azilsartan (AZL) [2-ethoxy-1-{[2'-(5-oxo-4,5-dihydro-1,2,4-oxadiazol-3-yl)biphenyl-4-yl]methyl}-1H-benzimidazole-7-carboxylic acid] was investigated in radioligand binding and function studies. AZL inhibited the specific binding of ¹²⁵I-Sar¹-Ile⁸-AII to human angiotensin type 1 receptors with an IC₅₀ of 2.6 nM. The inhibitory effect of AZL persisted after washout of the free compound (IC(50) value of 7.4 nM). Olmesartan, telmisartan, valsartan, and irbesartan also inhibited the specific binding with IC₅₀ values of 6.7, 5.1, 44.9, and 15.8 nM, respectively. However, their inhibitory effects were markedly attenuated with washout (IC₅₀ values of 242.5, 191.6, >10,000, and >10,000 nM). AZL also inhibited the accumulation of AII-induced inositol 1-phosphate (IP1) in the cell-based assay with an IC₅₀ value of 9.2 nmol; this effect was resistant to washout (IC₅₀ value of 81.3 nM). Olmesartan and valsartan inhibited IP1 accumulation with IC₅₀ values of 12.2 and 59.8 nM, respectively. The activities of these compounds were markedly reduced after washout (IC₅₀ value of 908.5 and 22,664.4 nM). AZL was defined as an inverse agonist in an experiment by using a constitutively active mutant of human angiotensin type 1 receptors. In isolated rabbit aortic strips, AZL reduced the maximal contractile response to AII with a pD'₂ value of 9.9. The inhibitory effects of AZL on contractile responses induced by AII persisted after the strips were washed; these inhibitory effects were more potent than those of olmesartan. These results suggest that AZL is a highly potent and slowly dissociating AII receptor blocker. Its tight receptor binding might be expected to produce potent and long-lasting antihypertensive effects in preclinical and clinical settings.

Design and synthesis of 6-chloro-3,4-dihydro-4-methyl-2H-1,4-benzoxazine-8-carboxamide derivatives as potent serotonin-3 (5-HT3) receptor antagonists

Several 3-substituted 5-chloro-2-methoxybenzamides were synthesized and evaluated for serotonin-3 (5-HT3) receptor binding affinity. The 5-HT3 receptor antagonistic activity of zacopride, a representative 5-HT3 receptor antagonist, was unchanged by the replacement of the 4-amino substituent on the aromatic moiety by a 3-dimethyl-amino substituent. This finding prompted a structural modification of azasetron, another 5-HT3 receptor antagonist. Consequently, a new series of 3,4-dihydro-2H-1,4-benzoxazine-8-carboxamides was obtained and these compounds were found to be more potent than 3,4-dihydro-3-oxo-2H-1,4-benzoxazine-8-carboxamides. In particular, (S)-N-(1-azabicyclo[2.2.2]oct-3-yl)-6-chloro-3,4-dihydro-4-methyl-2H-1,4 -benzoxazine-8-carboxamide showed a high affinity for 5-HT3 receptors K(i) = 0.051 nM) and especially potent antagonistic activity against the von Bezold-Jarisch reflex (ED50 = 0.089 micrograms/kg i.v.) in rats.

Characterization of the binding properties of T-773 as a PET radioligand for phosphodiesterase 10A

INTRODUCTION

Phosphodiesterase 10A (PDE10A) is a dual-substrate PDE that hydrolyzes both cAMP and cGMP and is selectively expressed in striatal medium spiny neurons. Recent studies have suggested that PDE10A inhibition is a novel approach for the treatment of disorders such as schizophrenia and Huntington's disease. A positron emission tomography (PET) occupancy study can provide useful information for the development of PDE10A inhibitors. We discovered T-773 as a candidate PET radioligand for PDE10A and investigated its properties by in vitro autoradiography and a PET study in a monkey.

METHODS

Profiling of T-773 as a PET radioligand for PDE10A was conducted by in vitro enzyme inhibitory assay, in vitro autoradiography, and PET study in a monkey.

RESULTS

T-773 showed a high binding affinity and selectivity for human recombinant PDE10A2 in vitro; the IC50 value in an enzyme inhibitory assay was 0.77nmol/L, and selectivity over other PDEs was more than 2500-fold. In autoradiography studies using mouse, rat, monkey, or human brain sections, radiolabeled T-773 selectively accumulated in the striatum. This selective accumulation was not observed in the brain sections of Pde10a-KO mice. The binding of [(3)H]T-773 to PDE10A in rat brain sections was competitively inhibited by MP-10, a selective PDE10A inhibitor. In rat brain sections, [(3)H]T-773 bound to a single high affinity site of PDE10A with Kd values of 12.2±2.2 and 4.7±1.2nmol/L in the caudate-putamen and nucleus accumbens, respectively. In a monkey PET study, [(11)C]T-773 showed good brain penetration and striatum-selective accumulation.

CONCLUSION

These results suggest that [(11)C]T-773 is a potential PET radioligand for PDE10A.

Benzoxazines. II. Synthesis, conformational analysis, and structure--activity relationships of 3,4-dihydro-2H-1,4-benzoxazine-8-carboxamide derivatives as potent and long-acting serotonin-3 (5-HT3) receptor antagonists

A series of 3,4-dihydro-2H-1,4-benzoxazine-8-carboxamide derivatives was synthesized and evaluated for serotonin-3 (5HT3) receptor antagonistic activities by means of assays of 5-HT3 receptor binding and the ability to antagonize the von Bezold-Jarisch reflex in rats. Replacement of the 1,4-benzoxazine ring with a 1,4-benzthiepine ring or seven-membered ring (i.e., 1,5-benzoxepine or 1,5-benzthiepine) resulted in decreased affinity for 5-HT3 receptor. Introduction of substituents at the 2 position of the 1,4-benzoxazine ring increased the antagonistic activities (dimethyl > methyl > dihydro > phenyl). The compounds bearing a 9-methyl-9-azabicyclo[3.3.1]non-3-yl moiety as the basic part of 3,4-dihydro-2H-1,4-benzoxazine-8-carboxamide derivatives were equipotent to those bearing 1-azabicyclo[2.2.2]oct-3-yl moiety. The 9-methyl-9-azabicyclo[3.3.1]non-3-yl moiety was confirmed to adopt a boat-chair conformation on the basis of both NMR studies and X-ray analysis. In this series, endo-6-chloro-3,4-dihydro-N-(9-methyl-9-azabicyclo[3.3.1]non-3-yl)-2,2, 4-trimethyl-2H-1,4-benzoxazine-8-carboxamide showed the highest affinity for 5-HT3 receptors (Ki = 0.019 nM), and a long-lasting 5-HT3 receptor antagonistic activity as evidenced by antagonism to the von Bezold--Jarisch reflex in rats. Such a long-lasting 5-HT3 receptor antagonism would be attributed to the introduction of both two methyl groups at the 2 position of the benzoxazine ring and the 9-methyl-9-azabicyclo[3.3.1]non-3-yl moiety, which adopts the boat-chair conformation.

Discovery of Soticlestat, a Potent and Selective Inhibitor for Cholesterol 24-Hydroxylase (CH24H)

Cholesterol 24-hydroxylase (CH24H, CYP46A1), a brain-specific cytochrome P450 (CYP) family enzyme, plays a role in the homeostasis of brain cholesterol by converting cholesterol to 24S-hydroxycholesterol (24HC). Despite a wide range of potential of CH24H as a drug target, no potent and selective inhibitors have been identified. Here, we report on the structure-based drug design (SBDD) of novel 4-arylpyridine derivatives based on the X-ray co-crystal structure of hit derivative 1b. Optimization of 4-arylpyridine derivatives led us to identify 3v ((4-benzyl-4-hydroxypiperidin-1-yl)(2,4'-bipyridin-3-yl)methanone, IC50 = 7.4 nM) as a highly potent, selective, and brain-penetrant CH24H inhibitor. Following oral administration to mice, 3v resulted in a dose-dependent reduction of 24HC levels in the brain (1, 3, and 10 mg/kg). Compound 3v (soticlestat, also known as TAK-935) is currently under clinical investigation for the treatment of Dravet syndrome and Lennox-Gastaut syndrome as a novel drug class for epilepsies.

Rapid prediction of rifampin susceptibility of Mycobacterium tuberculosis

We evaluated the relationship between rifampin (RIF) susceptiblility and amino acid substitution in rpoB gene of Mycobacterium tuberculosis and the usefulness of rpoB gene sequencing in the rapid prediction of RIF susceptibility of M. tuberculosis in clinical specimens. A total of 76 genetic alterations in the 69 bp core region of rpoB gene were detected in 74 of 130 M. tuberculosis strains. Examination of the correlation between the minimum inhibitory concentrations (MICs) of RIF and amino acid substitutions in the 69 bp core region of rpoB gene revealed that all 43 strains containing amino acid substitution with Leu or Trp in codon 531 showed RIF-resistant phenotypes, with MICs > or = 64 microg/ml. In contrast, a variable level of RIF susceptibility was observed among strains containing amino acid substitutions in either codon 516 or codon 526. In the clinical study, we tested 26 sputum samples, two gastric lavages, and one synovial fluid sample obtained from patients with tuberculosis. The RIF susceptibility predicted by direct rpoB sequencing was satisfactorily compatible with the results of the RIF-susceptibility test and the MICs of RIF against isolated organisms. Our results suggest that rpoB gene sequencing is useful for the detection of M. tuberculosis in clinical samples as well as the rapid prediction of RIF susceptibility of these strains.

Discovery of imidazo[1,5-c]imidazol-3-ones: weakly basic, orally active factor Xa inhibitors

The coagulation enzyme factor Xa (FXa) has been recognized as a promising target for the development of new antithrombotic agents. We previously found compound 1 to be an orally bioavailable FXa inhibitor in fasted monkeys; however, 1 showed poor bioavailability in rats and fed monkeys. To work out the pharmacokinetic problems, we focused our synthetic efforts on the chemical conversion of the 4-(imidazo[1,2- a]pyridin-5-yl)piperazine moiety of 1 to imidazolylpiperidine derivatives (fused and nonfused), which resulted in the discovery of the weakly basic imidazo[1,5- c]imidazol-3-one 3q as a potent and selective FXa inhibitor. Compound 3q showed favorable oral bioavailability in rats and monkeys under both fasted and fed conditions and antithrombotic efficacy in a rat model of venous thrombosis after oral administration, without a significant increase in bleeding time (unlike warfarin). On the basis of these promising properties, compound 3q was selected for further evaluation.

Discovery of TAK-272: A Novel, Potent, and Orally Active Renin Inhibitor

The aspartic proteinase renin is an attractive target for the treatment of hypertension and cardiovascular/renal disease such as chronic kidney disease and heart failure. We introduced an S1' site binder into the lead compound 1 guided by structure-based drug design (SBDD), and further optimization of physicochemical properties led to the discovery of benzimidazole derivative 10 (1-(4-methoxybutyl)-N-(2-methylpropyl)-N-[(3S,5R)-5-(morpholin-4-yl)carbonylpiperidin-3-yl]-1H-benzimidazole-2-carboxamide hydrochloride, TAK-272) as a highly potent and orally active renin inhibitor. Compound 10 demonstrated good oral bioavailability (BA) and long-lasting efficacy in rats. Compound 10 is currently in clinical trials.

Development of a novel method for operating magnetic particles, Magtration Technology, and its use for automating nucleic acid purification

Magnetic particles are useful for simple and efficient nucleic acid extraction. To achieve fully automated nucleic acid extraction and purification using magnetic particles, a new method for operating magnetic particles, Magtration Technology, was developed. In this method, magnetic separation is performed in a specially designed disposable tip. This enables high recovery of magnetic particles with high reproducibility. The features of this technology are (i) a simple mechanism for process control and (ii) flexible software to enable adaptation to commercially available reagents. Automated instruments based on Magtration Technology were developed and used for nucleic acid extraction. Total DNA, total RNA and plasmids were purified by Magtration Technology at an efficiency comparable to that of manual methods.

Synthesis and pharmacology of 3,4-dihydro-3-oxo-1,4-benzoxazine-8-carboxamide derivatives, a new class of potent serotonin-3 (5-HT3) receptor antagonists

A series of 3,4-dihydro-3-oxo-1,4-benzoxazine-8-carboxamide derivatives was synthesized and evaluated for serotonin-3 (5-HT3) receptor antagonistic activity assessed by their ability to antagonize the von Bezold-Jarish (BJ) effect in rats. Derivatives bearing 1-azabicyclo[2.2.2]oct-3-yl moiety as a basic function attached to the carboxamide at position 8 showed more potent antagonistic activity than those bearing the other three basic moieties. Structure-activity relationships of this series showed that methyl and chloro groups were more effective as substituents at positions 4 and 6, respectively. The representative compound 15 (Y-25130) in this series showed potent antagonistic activity on the BJ effect (ED50 = 1.3 micrograms/kg i.v.), high affinity for 5-HT3 receptor (Ki = 2.9 nM) and complete protection against cisplatin-induced emesis in dogs at a dose of 0.1 mg/kg i.v.

Growth stimulating activity of heat shock protein 90 alpha to lymphoid cell lines in serum-free medium

A growth stimulating factor was purified from the culture supernatant of human-human hybridoma SH-76 cells in serum-free RPMI 1640 medium by the serial use of DEAE anion and heparin affinity chromatographies. The factor was a 85 kDa protein on SDS-polyacrylamide gel electrophoresis. N-terminal amino acid sequence (PEETQTQDQPME) of the protein was coincident with that of heat shock protein 90 alpha (HSP90 alpha). The protein reacted with anti-HSP90 monoclonal antibody. These results suggest that the protein was a member of HSP90 alpha family after taking other circumstantial evidence into account. The protein stimulated the growth of some lymphoid cell lines such as human B-lymphoblastoid cell line HO-323-3 hybridomas derived from HO-323, and several other lymphoid cell lines. There were several lymphoid cell lines which did not respond to the protein. Growth stimulating activity of the protein was heat-unstable and significantly decreased at above 60 degrees C. These are the first data that describe growth-stimulating activity of heat shock protein 90 alpha.

Synthesis and pharmacological evaluation of carboxamide derivatives as selective serotoninergic 5-HT(4) receptor agonists

A number of new carboxamide derivatives were synthesized. The affinity of these compounds for the serotoninergic 5-HT(4) receptor was evaluated by use of radioligand-binding techniques. The agonistic activity was evaluated as the contractile effect of the ascending colon isolated from guinea-pigs. Among these compounds, 4-amino-5-chloro-2-methoxy-N-[1-[2-[(methylsulfonyl)amino]ethly]-4-piperidinylmethyl]benzamide (24) showed a high affinity for the 5-HT(4) receptor (Ki = 9.6 nM). Compound 24 displayed a higher affinity for 5-HT(4) receptors than the other receptors, including, 5-HT(3) and dopamine D(2) receptors. In addition, compound 24 was confirmed to be a potent 5-HT(4) receptor agonist (ED(50) = 7.0 nM). An interaction model between compound 24 and 5-HT(4) receptor was proposed.

N-[[1-(2-phenylethyl)pyrrolidin-2-yl]methyl]cyclohexanecarboxamides as selective 5-HT1A receptor agonists

A series of benzamides was synthesized as selective agonists for the 5-HT1A receptor. It was found that (S)-N-[[1-(2-phenylethyl)pyrrolidin-2-yl]methyl]cyclohexanecarb oxamide(7-(S)) has potent and selective agonistic activity for the 5-HT1A receptor (5-HT1A; Ki 0.49 nmol/L, D2; IC50 = >1000 nmol/L, 5-HT2; Ki = 240 nmol/L).

N-[1-(2-Phenylethyl)pyrrolidin-3-yl]-1-adamantanecarboxamides as novel 5-HT2 receptor antagonists

A series of 1-adamantanecarboxamides was synthesized and examined for their potency as a selective 5-HT2 receptor antagonist. We found (S)-N-[1-[2-(4-fluorophenyl)ethyl]pyrrolidin-3-yl]-1-adamantane carboxamide hydrochloride hydrate (10-(S), Y-39241) to have a high affinity and selectivity for 5-HT2 receptors, and this potent anti-platelet effect of Y-39241 was confirmed both in vitro and in vivo.

Development of a series of novel carbon-11 labeled PDE10A inhibitors

Phosphodiesterase 10A (PDE10A) is a member of the PDE family of enzymes that degrades cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP). Our aim was to label a series of structurally related PDE10A inhibitors with carbon-11 and evaluate them as potential positron emission tomography (PET) radioligands for PDE10A using nonhuman primates. The series consisted of seven compounds based on the 3-(1H-pyrazol-5-yl)pyridazin-4(1H)-one backbone. These compounds were selected from the initial larger library based on a number of parameters such as affinity, selectivity for hPDE10A in in vitro tests, lipophilicity, and on the results of multidrug resistance protein 1 (MDR1)-LLCPK1 and the parallel artificial membrane permeability assays. Seven radioligands (KIT-1, 3, 5, 6, 7, 9, and 12) were radiolabeled with carbon-11 employing O-methylation on the hydroxyl moiety using [(11)C]methyl triflate. In vivo examination of each radioligand was performed using PET in rhesus monkeys; analysis of radiometabolites in plasma also was conducted using HPLC. All seven radioligands were labeled with high (>90%) incorporation of [(11)C]methyl triflate into their appropriate precursors and with high specific radioactivity. Carbon-11 labeled KIT-5 and KIT-6 showed high accumulation in the striatum, consistent with the known anatomical distribution of PDE10A in brain, accompanied by fast washout and high specific binding ratio. In particular [(11)C]KIT-6, named [(11)C]T-773, is a promising PET tool for further examination of PDE10A in human brain.

Discovery of benzimidazole derivatives as orally active renin inhibitors: Optimization of 3,5-disubstituted piperidine to improve pharmacokinetic profile

We previously identified 2-tert-butyl-4-[(3-methoxypropyl)amino]-N-(2-methylpropyl)-N-[(3S,5R)-5-(morpholin-4-ylcarbonyl)piperidin-3-yl]pyrimidine-5-carboxamide 3 as a potent renin inhibitor. Since 3 showed unacceptably low bioavailability (BA) in rats, structural modification, using SBDD and focused on physicochemical properties was conducted to improve its PK profile while maintaining renin inhibitory activity. Conversion of the amino group attached at the 4-position of pyrimidine to methylene group improved PK profile and decreased renin inhibitory activity. New central cores with carbon side chains were explored to improve potency. We had designed a series of 5-membered azoles and fused heterocycles that interacted with the lipophilic S3 pocket. In the course of modification, renin inhibitory activity was enhanced by the formation of an additional hydrogen bonding with the hydroxyl group of Thr77. Consequently, a series of novel benzimidazole derivatives were discovered as potent and orally bioavailable renin inhibitors. Among those, compound 13 exhibited more than five-fold of plasma renin inhibition than aliskiren in cynomolgus monkeys at dose ratio.

Synthesis and structure-activity relationships of 2,3-dihydrobenzofuran-7-carboxamide derivatives as potent serotonin-3 (5-HT3) receptor antagonists

A series of N-(azabicyclo-3-yl)-2,3-dihydrobenzofuran-7-carboxamide derivatives were synthesized and evaluated for serotonin-3 (5-HT3) receptor antagonistic activities assessed by 5-HT3 receptor binding (in vitro) and by the ability to antagonize the von Bezold-Jarisch reflex in rats (in vivo). In these compounds, 1-azabicyclo[2.2.2]oct-3-yl derivatives were more potent than 8-methyl-8-azabicyclo[3.2.1]oct-3-yl derivatives for 5-HT3 receptor antagonistic activities. The introduction of methyl groups at position 2 of the dihydrobenzofuran ring increased the pharmacological activities (dimethyl > monomethyl > dihydro). Furthermore, the stereoisomers of dimethyl-, monomethyl-, and dihydrobenzofuran derivatives were prepared to evaluate the stereoselectivity of their 5-HT3 receptor binding affinities. Concerning the basic part, the compounds bearing (S)-1-azabicyclo[2.2.2]octan-3-yl moiety were more potent than their counterparts. With respect to the methyl substituent at position 2 of the dihydrobenzofuran ring, the rank order of the potency was dimethyl > or = (2S)-methyl > (2R)-methyl > dihydro. These results suggest that the (2S)-methyl group of the dihydrobenzofuran part contributes to the enhancement of the pharmacological activity. Among these compounds, (S)-N-(1-azabicyclo[2.2.2]oct-3-yl)-5-chloro-2,3-dihydro-2,2- dimethylbenzofuran-7-carboxamide hydrochloride (24) showed the highest affinity for 5-HT3 receptors (Ki = 0.055 nM), and the most potent antagonistic activity on the von Bezold-Jarisch reflex (ED50 = 0.18 microgram/kg i.v.).

Novel approach of fragment-based lead discovery applied to renin inhibitors

A novel approach was conducted for fragment-based lead discovery and applied to renin inhibitors. The biochemical screening of a fragment library against renin provided the hit fragment which showed a characteristic interaction pattern with the target protein. The hit fragment bound only to the S1, S3, and S3^SP (S3 subpocket) sites without any interactions with the catalytic aspartate residues (Asp32 and Asp215 (pepsin numbering)). Prior to making chemical modifications to the hit fragment, we first identified its essential binding sites by utilizing the hit fragment's substructures. Second, we created a new and smaller scaffold, which better occupied the identified essential S3 and S3^SP sites, by utilizing library synthesis with high-throughput chemistry. We then revisited the S1 site and efficiently explored a good building block attaching to the scaffold with library synthesis. In the library syntheses, the binding modes of each pivotal compound were determined and confirmed by X-ray crystallography and the library was strategically designed by structure-based computational approach not only to obtain a more active compound but also to obtain informative Structure Activity Relationship (SAR). As a result, we obtained a lead compound offering synthetic accessibility as well as the improved in vitro ADMET profiles. The fragments and compounds possessing a characteristic interaction pattern provided new structural insights into renin's active site and the potential to create a new generation of renin inhibitors. In addition, we demonstrated our FBDD strategy integrating highly sensitive biochemical assay, X-ray crystallography, and high-throughput synthesis and in silico library design aimed at fragment morphing at the initial stage was effective to elucidate a pocket profile and a promising lead compound.

Whole Cell Active Inhibitors of Mycobacterial Lipoamide Dehydrogenase Afford Selectivity over the Human Enzyme through Tight Binding Interactions

Tuberculosis remains a leading cause of death from a single bacterial infection worldwide. Efforts to develop new treatment options call for expansion into an unexplored target space to expand the drug pipeline and bypass resistance to current antibiotics. Lipoamide dehydrogenase is a metabolic and antioxidant enzyme critical for mycobacterial growth and survival in mice. Sulfonamide analogs were previously identified as potent and selective inhibitors of mycobacterial lipoamide dehydrogenase in vitro but lacked activity against whole mycobacteria. Here we present the development of analogs with improved permeability, potency, and selectivity, which inhibit the growth of Mycobacterium tuberculosis in axenic culture on carbohydrates and within mouse primary macrophages. They increase intrabacterial pyruvate levels, supporting their on-target activity within mycobacteria. Distinct modalities of binding between the mycobacterial and human enzymes contribute to improved potency and hence selectivity through induced-fit tight binding interactions within the mycobacterial but not human enzyme, as indicated by kinetic analysis and crystallography.

Design and synthesis of aryl-piperidine derivatives as potent and selective PET tracers for cholesterol 24-hydroxylase (CH24H)

Cholesterol 24-hydroxylase (CH24H, CYP46A1) is a cytochrome P450 family enzyme that maintains the homeostasis of brain cholesterol. Soticlestat, a potent and selective CH24H inhibitor, is in development as a therapeutic agent for Dravet syndrome and Lennox-Gastaut syndrome. Herein, we report the discovery of aryl-piperidine derivatives as potent and selective CH24H positron emission tomography (PET) tracers which can be used for dose guidance of a clinical CH24H inhibitor and as a diagnostic tool for CH24H-related pathology. Starting from compound 1 (IC₅₀ = 16 nM, logD = 1.7), which was reported as a CH24H inhibitor with lower lipophilicity, a¹⁸F-labeling site (3-fluoroazetidine) was incorporated by structure-based drug design (SBDD) utilizing the co-crystal structure of a compound 1 analog. Subsequent optimization to adjust key parameters for PET tracers, such as potency, lipophilicity, brain penetration, and unbound plasma protein binding, enabled compounds 3f (IC₅₀ = 8.8 nM) and 3g (IC₅₀ = 8.7 nM) as PET imaging candidates. Selectivity of these compounds for CH24H was validated by a brain distribution study using CH24H-WT and KO mice. In non-human primate PET imaging, [¹⁸F]3f and [¹⁸F]3g showed similar regional uptake in the brain, indicating that these tracers were specific to the CH24H-expressed regions and validated the expression of CH24H in the living brain by different tracers.