Synthesis and evaluation of novel benzothiazole derivatives based on the bithiophene structure as potential radiotracers for β-amyloid plaques in Alzheimer’s disease

In this study, six novel benzothiazole derivatives based on the bithiophene structure were developed as potential β-amyloid probes. In vitro binding studies using Aβ aggregates showed that all of them demonstrated high binding affinities with K_i values ranged from 0.11 to 4.64 nM. In vitro fluorescent staining results showed that these compounds can intensely stained Aβ plaques within brain sections of APP/PS1 transgenic mice, animal model for AD. Two radioiodinated compounds [¹²⁵I]-2-(5′-iodo-2,2′-bithiophen-5-yl)-6-methoxybenzo[d]thiazole [¹²⁵I]10 and [¹²⁵I]-2-(2,2′-bithiophen-5-yl)-6-iodobenzo[d]thiazole [¹²⁵I]13 were successfully prepared through an iododestannylation reaction. Furthermore, in vitro autoradiography of the AD model mice brain sections showed that both [¹²⁵I]10 and [¹²⁵I]13 labeled the Aβ plaques specifically with low background. In vivo biodistribution studies in normal mice indicated that [¹²⁵I]13 exhibited high brain uptake (3.42% ID/g at 2 min) and rapid clearance from the brain (0.53% ID/g at 60 min), while [¹²⁵I]10 showed lower brain uptake (0.87% ID/g at 2 min). In conclusion, these preliminary results of this study suggest that the novel radioiodinated benzothiazole derivative [¹²⁵I]13 may be a candidate as an in vivo imaging agent for detecting β-amyloid plaques in the brain of AD patients.     

Synthesis and biological evaluation of radio-iodinated benzimidazoles as SPECT imaging agents for NR2B subtype of NMDA receptor

In this study, the benzimidazole derivatives were synthesized and evaluated as imaging agents for the NR2B subtype of NMDA receptor. Among these ligands, 2-{[4-(4-iodobenzyl)piperidin-1-yl]methyl}benzimidazol-5-ol (8) and N-{2-[4-(4-iodobenzyl)-piperidin-1-ylmethyl]benzoimidazol-5-yl}-methanesulfonamide (9) exhibited high affinity for the NR2B subunit (K_i values; 7.28 nM for 8 and 5.75 nM for 9). In vitro autoradiography experiments demonstrated high accumulation in the forebrain regions but low in the cerebellum for both [¹²⁵I]8 and [¹²⁵I]9. These regional distributions of the radioligands correlated with the expression of the NR2B subunit. The in vitro binding of these ligands was inhibited by NR2B antagonist but not by other site ligands, which suggested the high selectivity of [¹²⁵I]8 and [¹²⁵I]9 for the NR2B subunit. In mice, the regional brain uptakes of [¹²⁵I]8 and [¹²⁵I]9 at 5–180 min after administration were 0.42–0.56% and 0.44–0.67% dose/g, respectively. The brain-to-blood ratio of [¹²⁵I]8 at 180 min was reduced by 34% in the presence of non-radioactive ligands and by 59% in the presence of the NR2B ligand Ro-25,6981. These results indicated that [¹²⁵I]8 could be partially bound to the NR2B subunit in vivo. Although the brain uptake of these benzimidazole derivatives was too low to allow for in vivo SPECT imaging, these compounds might be useful scaffolds for the development of imaging probes specific for the NMDA receptors.     

Synthesis and evaluation of 2-amino-dihydrotetrabenzine derivatives as probes for imaging vesicular monoamine transporter-2

A novel series of analogs of 2-amino-dihydrotetrabenazine derivatives, 4–6, targeting the vesicular monoamine transporter have been prepared. In vitro binding was carried out in tissue homogenates prepared from rat striatal tissue homogenates with both [¹²⁵I]-iodovinyl-TBZ and [³H]DTBZ. There was a good correlation (r² = 0.925) between the affinities of the different compounds for [¹²⁵I]-iodovinyl-TBZ and [³H]-DTBZ binding. Compound 5 exhibited a better affinity for the vesicular monoamine transporter (K_i = 8.68 ± 1.26 nM and 7.01 ± 0.07 nM, respectively), which may be a good lead compound for further structural modification to develop useful probes for VMAT2.     

Synthesis and evaluation of 1-[2-(4-[11C]methoxyphenyl)phenyl]piperazine for imaging of the serotonin 5-HT7 receptor in the rat brain

1-[2-(4-Methoxyphenyl)phenyl]piperazine (4) is a potent serotonin 5-HT₇ receptor antagonist (K_i = 2.6 nM) with a low binding affinity for the 5-HT_1A receptor (K_i = 476 nM). As a potential positron emission tomography (PET) radiotracer for the 5-HT₇ receptor, [¹¹C]4 was synthesized at high radiochemical yield and specific activity, by O-[¹¹C]methylation of 2′-(piperazin-1-yl)-[1,1′-biphenyl]-4-ol (6) with [¹¹C]methyl iodide. Autoradiography revealed that [¹¹C]4 showed in vitro specific binding with 5-HT₇ in the rat brain regions, such as the thalamus which is a region with high 5-HT₇ expression. Metabolite analysis indicated that intact [¹¹C]4 in the brain exceeded 90% of the radioactive components at 15 min after the radiotracer injection, although two radiolabeled metabolites were found in the rat plasma. The PET study of rats showed moderated uptake of [¹¹C]4 in the brain (1.2 SUV), but no significant regional difference in radioactivity in the brain. Pretreatment with 5-HT₇-selective antagonist SB269970 (3) did not decrease the uptake of [¹¹C]4 in the rat brain. Further studies are warranted that focus on the development of PET ligand candidates with higher binding affinity for 5-HT₇ and higher in vivo stability in brain than 4.     

Synthesis and biological evaluation of a novel 99mTc cyclopentadienyl tricarbonyl complex ([(Cp-R)99mTc(CO)3]) for sigma-2 receptor tumor imaging

We report the design, synthesis and biological evaluation of a novel ^99mTc 4-(4-cyclohexylpiperazine-1-yl)-butan-1-one-1-cyclopentadienyltricarbonyl technetium ([^99mTc]5) as a potential SPECT tracer for imaging of σ₂ receptors in tumors. [^99mTc]5 was prepared in 25 ± 5% isolated radiochemical yield with radiochemical purity of >99% via double-ligand transfer (DLT) reaction from the ferrocene precursor 2b (4-(4-cyclohexylpiperazine-1-yl)-1-ferrocenylbutan-1-one). The corresponding Re-complex 4 and the ferrocenyl complex 2b showed relatively high affinity towards σ₂ receptors in in vitro competition binding assay (K_i values of 4 and 2b were 64.4 ± 18.5 nM and 43.6 ± 21.3 nM, respectively) and moderate to high selectivity versus σ₁ receptors (K_iσ₁/K_iσ₂ ratios were 12.5 and 95.5, respectively). The log D value of [^99mTc]5 was determined to be 2.52 ± 0.33. Biodistribution studies in mice revealed comparably high initial brain uptake of [^99mTc]5 and slow washout. Administration of haloperidol 5 min prior to injection of [^99mTc]5 significantly reduced the radiotracer uptake in brain, heart, lung, and spleen by 40–50% at 2 h p.i.. Moreover, [^99mTc]5 showed high uptake in C6 glioma cell lines (8.6%) after incubation for 1 h. Blocking with haloperidol to compete with [^99mTc]5 significantly reduced the cell uptake. Preliminary blocking study in C6-brain-tumor bearing rats showed that [^99mTc]5 binds to σ receptors in the brain-tumor specifically. These results are encouraging for further exploration of ^99mTc-labeled probes for σ₂ receptor tumor imaging in vivo.     

Synthesis and evaluation of ethyleneoxylated and allyloxylated chalcone derivatives for imaging of amyloid β plaques by SPECT

We report radioiodinated chalcone derivatives as new SPECT imaging probes for amyloid β (Aβ) plaques. The monoethyleneoxy derivative 2 and allyloxy derivative 8 showed a high affinity for Aβ(1–42) aggregates with K_i values of 24 and 4.5 nM, respectively. Fluorescent imaging demonstrated that 2 and 8 clearly stained thioflavin-S positive Aβ plaques in the brain sections of Tg2576 transgenic mice. In vitro autoradiography revealed that [¹²⁵I]2 displayed no clear accumulation toward Aβ plaques in the brain sections of Tg2576 mice, whereas the accumulation pattern of [¹²⁵I]8 matched with the presence of Aβ plaques both in the brain sections of Tg2576 mice and an AD patient. In biodistribution studies using normal mice, [¹²⁵I]2 showed preferable in vivo pharmacokinetics (4.82%ID/g at 2 min and 0.45%ID/g at 60 min), while [¹²⁵I]8 showed only a modest brain uptake (1.62%ID/g at 2 min) with slow clearance (0.56%ID/g at 60 min). [¹²⁵I]8 showed prospective binding properties for Aβ plaques, although further structural modifications are needed to improve the blood brain barrier permeability and washout from brain.     

Radiosynthesis,in vitro and in vivo evaluation of 123I-labeled anandamide analogues for mapping brain FAAH

Fatty acid amide hydrolase (FAAH) is one of the main enzymes responsible for terminating the signaling of endocannabinoids, including anandamide. This paper is the first report of the synthesis, [¹²³I]-labeling and in vitro and in vivo evaluation of anandamide analogues as potential metabolic trapping radioligands for in vivo evaluation of brain FAAH. N-(2-Iodoethyl)linoleoylamide (2) and N-(2-iodoethyl)arachidonylamide (4) were synthesized with good yields (75% and 86%, respectively) in a two steps procedure starting from their respective acids. In vitro analyses, performed using recombinant rat FAAH and [³H]-anandamide, demonstrated interaction of 2 and 4 with FAAH (IC₅₀ values of 5.78 μM and 3.14 μM, respectively). [¹²³I]-2 and [¹²³I]-4 were synthesized with radiochemical yields of 21% and 12%, respectively, and radiochemical purities were >90%. Biodistribution studies in mice demonstrated brain uptake for both tracers (maximum values of 1.23%ID/g at 3 min pi for [¹²³I]-2 and 0.58%ID/g at 10 min pi for [¹²³I]-4). However, stability studies demonstrated the sensitivity of both tracers to dehalogenation.     

Highly efficient method for 125I-radiolabeling of biomolecules using inverse-electron-demand Diels–Alder reaction

In this report, we present a rapid and highly efficient method for radioactive iodine labeling of trans-cyclooctene group conjugated biomolecules using inverse-electron-demand Diels–Alder reaction. Radioiodination reaction of the tetrazine structure was carried out using the stannylated precursor 2 to give ¹²⁵I-labeled product ([¹²⁵I]1) with high radiochemical yield (65 ± 8%) and radiochemical purity (>99%). For radiolabeling application of [¹²⁵I]1, trans-cyclooctene derived cRGD peptide and human serum albumin were prepared. These substrates were reacted with [¹²⁵I]1 under mild condition to provide the radiolabeled products [¹²⁵I]6 and [¹²⁵I]8, respectively, with excellent radiochemical yields. The biodistribution study of [¹²⁵I]8 in normal ICR mice showed significantly lower thyroid uptake values than that of ¹²⁵I-labeled human serum albumin prepared by a traditional radiolabeling method. Therefore [¹²⁵I]8 will be a useful radiolabeled tracer in various molecular imaging and biological studies. Those results clearly demonstrate that [¹²⁵I]1 will be used as a valuable prosthetic group for radiolabeling of biomolecules.     

A new single-photon emission computed tomography (SPECT) imaging agent for serotonin transporters: [125I]Flip-IDAM, (2-((2-((dimethylamino)methyl)-4-iodophenyl)thio)phenyl)methanol

New ligands for in vivo brain imaging of serotonin transporter (SERT) with single photon emission tomography (SPECT) were prepared and evaluated. An efficient synthesis and radiolabeling of a biphenylthiol, FLIP-IDAM, 4, was accomplished. The affinity of FLIP-IDAM was evaluated by an in vitro inhibitory binding assay using [¹²⁵I]-IDAM as radioligand in rat brain tissue homogenates (K_i = 0.03 nM). New [¹²⁵I]Flip-IDAM exhibited excellent binding affinity to SERT binding sites with a high hypothalamus to cerebellum ratio of 4 at 30 min post iv injection. The faster in vivo kinetics for brain uptake and a rapid washout from non-specific regions provide excellent signal to noise ratio. This new agent, when labeled with ¹²³I, may be a useful imaging agent for mapping SERT binding sites in the human brain.     

Radiosynthesis of N-(4-chloro-3-[11C]methoxyphenyl)-2-picolinamide ([11C]ML128) as a PET radiotracer for metabotropic glutamate receptor subtype 4 (mGlu4)

N-(Chloro-3-methoxyphenyl)-2-picolinamide (3, ML128, VU0361737) is an mGlu₄ positive allosteric modulator (PAM), which is potent and centrally penetrating. 3 is also the first mGlu₄ PAM to show efficacy in a preclinical Parkinson disease model upon systemic dosing. As a noninvasive medical imaging technique and a powerful tool in neurological research, positron emission tomography (PET) offers a possibility to investigate mGlu₄ expression in vivo under physiologic and pathological conditions. We synthesized a carbon-11 labeled ML128 ([¹¹C]3) as a PET radiotracer for mGlu₄, and characterized its biological properties in Sprague Dawley rats. [¹¹C]3 was synthesized from N-(4-chloro-3-hydroxyphenyl)-2-picolinamide (2) using [¹¹C]CH₃I. Total synthesis time was 38 ± 2.2 min (n = 7) from the end of bombardment to the formulation. The radioligand [¹¹C]3 was obtained in 27.7 ± 5.3% (n = 5) decay corrected radiochemical yield based on the radioactivity of [¹¹C]CO₂. The radiochemical purity of [¹¹C]3 was >99%. Specific activity was 188.7 ± 88.8 GBq/mol (n = 4) at the end of synthesis (EOS).PET images were conducted in 20 normal male Sprague Dawley rats including 11 control studies, 6 studies blocking with an mGlu₄ modulator (4) to investigate specificity and 3 studies blocking with an mGlu₅ modulator (MTEP) to investigate selectivity. These studies showed fast accumulation of [¹¹C]3 (peak activity between 1–3 min) in several brain areas including striatum, thalamus, hippocampus, cerebellum, and olfactory bulb following with fast washout. Blocking studies with the mGlu₄ modulator 4 showed 22–28% decrease of [¹¹C]3 accumulation while studies of selectivity showed only minor decrease supporting good selectivity over mGlu₅. Biodistribution studies and blood analyses support fast metabolism. Altogether this is the first PET imaging ligand for mGlu₄, in which the labeled ML128 was used for imaging its in vivo distribution and pharmacokinetics in brain.     

Synthesis and in vivo evaluation of [11C]MPTQ: A potential PET tracer for alpha2A-adrenergic receptors

Radiosynthesis and in vivo evaluation of [N-methyl-¹¹C] 5-methyl-3-[4-(3-phenylallyl)-piperazin-1-ylmethyl]-3,3a,4,5-tetrahydroisoxazolo[4,3-c]quinoline (1), a potential PET tracer for alpha2-adrenergic receptors is described. Syntheses of nonradioactive standard 1 and corresponding desmethyl precursor 2 were achieved from 2-aminobenzaldehyde in 40% and 65% yields, respectively. Methylation using [¹¹C]CH₃I in presence of aqueous potassium hydroxide in DMSO afforded [¹¹C]1 in 25% yield (EOS) with >99% chemical and radiochemical purities with a specific activity ranged from 3–4 Ci/μmol (n = 6). The total synthesis time was 30 min from EOB. PET studies in anesthetized baboon show that [¹¹C]1 penetrates BBB and accumulates in alpha2A-AR enriched brain areas.     

Discovery of novel tricyclic pyrido[3′,2′:4,5]thieno[3,2-d]pyrimidin-4-amine derivatives as VEGFR-2 inhibitors

In an effort to develop ATP-competitive VEGFR-2 selective inhibitors, a novel series of tricyclic pyrido[3′,2′:4,5]thieno[3,2-d]pyrimidin-4-amine derivatives were designed and synthesized. These compounds were characterized by IR, ¹H NMR, ¹³C NMR, elemental and mass spectral analyses. Docking studies have given a partial insight into the molecular determinants of the activity of this novel series in VEGFR-2 kinase active site. Moreover, these compounds were assessed at 10 μM for their selective inhibitory activities over a panel of 6 human kinases, namely VEGFR-1/Flt-1, VEGFR-2/KDR, EGFR, CDK5/p25, GSK3α and GSK3β. Compound N-(4,6-dimethylthieno[2,3-b]pyridine)-7,9-dimethylpyrido[3′,2′:4,5]thieno[3,2-d]pyrimidin-4-amine (9d) exhibited the most potent and selective inhibitory activity against VEGFR-2/KDR over the six human kinases, with an IC₅₀ value 2.6 μM. The identification of this hit candidate could aid the design of new tricyclic-based VEGFR-2 kinase modulators.     

Synthesis and preliminary biological evaluation of [11C]methyl (2-amino-5-(benzylthio)thiazolo[4,5-d]pyrimidin-7-yl)-d-leucinate for the fractalkine receptor (CX3CR1)

The reference standard methyl (2-amino-5-(benzylthio)thiazolo[4,5-d]pyrimidin-7-yl)-d-leucinate (5) and its precursor 2-amino-5-(benzylthio)thiazolo[4,5-d]pyrimidin-7-yl)-d-leucine (6) were synthesized from 6-amino-2-mercaptopyrimidin-4-ol and BnBr with overall chemical yield 7% in five steps and 4% in six steps, respectively. The target tracer [¹¹C]methyl (2-amino-5-(benzylthio)thiazolo[4,5-d]pyrimidin-7-yl)-d-leucinate ([¹¹C]5) was prepared from the acid precursor with [¹¹C]CH₃OTf through O-[¹¹C]methylation and isolated by HPLC combined with SPE in 40–50% radiochemical yield, based on [¹¹C]CO₂ and decay corrected to end of bombardment (EOB). The radiochemical purity was >99%, and the specific activity (SA) at EOB was 370–1110 GBq/μmol with a total synthesis time of ～40-min from EOB. The radioligand depletion experiment of [¹¹C]5 did not display specific binding to CX₃CR1, and the competitive binding assay of ligand 5 found much lower CX₃CR1 binding affinity.     

Novel anilinophthalimide derivatives as potential probes for β-amyloid plaque in the brain

A group of novel 4,5-dianilinophthalimide derivatives has been synthesized in this study for potential use as β-amyloid (Aβ) plaque probes. Staining of hippocampus tissue sections from Alzheimer’s disease (AD) brain with the representative compound 9 indicated selective labeling of it to Aβ plaques. The binding affinity of radioiodinated [¹²⁵I]9 for AD brain homogenates was 0.21 nM (K_d), and of other derivatives ranged from 0.9 to 19.7 nM, except for N-methyl-4,5-dianilinophthalimide (K_i > 1000 nM). [¹²⁵I]9 possessed the optimal lipophilicity with Log P value of 2.16, and its in vivo biodistribution in normal mice exhibited excellent initial brain uptake (5.16% ID/g at 2 min after injection) and a fast washout rate (0.56% ID/g at 60 min). The encouraging results suggest that this novel derivative of [¹²³I]9 may have potential as an in vivo SPECT probe for detecting amyloid plaques in the brain.     

Synthesis and biological evaluation of 18F labeled fluoro-oligo-ethoxylated 4-benzylpiperazine derivatives for sigma-1 receptor imaging

We report the synthesis and evaluation of a series of fluoro-oligo-ethoxylated 4-benzylpiperazine derivatives as potential σ₁ receptor ligands. In vitro competition binding assays showed that 1-(1,3-benzodioxol-5-ylmethyl)-4-(4-(2-fluoroethoxy)benzyl)piperazine (6) exhibits low nanomolar affinity for σ₁ receptors (K_i = 1.85 ± 1.59 nM) and high subtype selectivity (σ₂ receptor: K_i = 291 ± 111 nM; K_iσ₂/K_iσ₁ = 157). [¹⁸F]6 was prepared in 30–50% isolated radiochemical yield, with radiochemical purity of >99% by HPLC analysis after purification, via nucleophilic ¹⁸F^? substitution of the corresponding tosylate precursor. The log D_{pH 7.4} value of [¹⁸F]6 was found to be 2.57 ± 0.10, which is within the range expected to give high brain uptake. Biodistribution studies in mice demonstrated relatively high concentration of radiotracers in organs known to contain σ₁ receptors, including the brain, lungs, kidneys, heart, and spleen. Administration of haloperidol 5 min prior to injection of [¹⁸F]6 significantly reduced the concentration of radiotracers in the above-mentioned organs. The accumulation of radiotracers in the bone was quite low suggesting that [¹⁸F]6 is relatively stable to in vivo defluorination. The ex vivo autoradiography in rat brain showed high accumulation of radiotracers in the brain areas known to possess high expression of σ₁ receptors. These findings suggest that [¹⁸F]6 is a suitable radiotracer for imaging σ₁ receptors with PET in vivo.     

N-(3,4-Dimethylisoxazol-5-yl)piperazine-4-[4-(2-fluoro-4-[11C]methylphenyl)thiazol-2-yl]-1-carboxamide: A promising positron emission tomography ligand for fatty acid amide hydrolase

To visualize fatty acid amide hydrolase (FAAH) in brain in vivo, we developed a novel positron emission tomography (PET) ligand N-(3,4-dimethylisoxazol-5-yl)piperazine-4-[4-(2-fluoro-4-[¹¹C]methylphenyl)thiazol-2-yl]-1-carboxamide ([¹¹C]DFMC, [¹¹C]1). DFMC (1) was shown to have high binding affinity (IC₅₀: 6.1 nM) for FAAH. [¹¹C]1 was synthesized by C–¹¹C coupling reaction of arylboronic ester 2 with [¹¹C]methyl iodide in the presence of Pd catalyst. At the end of synthesis, [¹¹C]1 was obtained with a radiochemical yield of 20 ± 10% (based on [¹¹C]CO₂, decay-corrected, n = 5) and specific activity of 48–166 GBq/μmol. After the injection of [¹¹C]1 in mice, high uptake of radioactivity (>2% ID/g) was distributed in the lung, liver, kidney, and brain, organs with high FAAH expression. PET images of rat brains for [¹¹C]1 revealed high uptakes in the cerebellar nucleus (SUV = 2.4) and frontal cortex (SUV = 2.0), two known brain regions with high FAAH expression. Pretreatment with the FAAH-selective inhibitor URB597 reduced the brain uptake. Higher than 90% of the total radioactivity in the rat brain was irreversible at 30 min after the radioligand injection. The present results indicate that [¹¹C]1 is a promising PET ligand for imaging of FAAH in living brain.     

1-(4-Phenylpiperazin-1-yl)-2-(1H-pyrazol-1-yl)ethanones as novel CCR1 antagonists

A novel series of CCR1 antagonists based on the 1-(4-phenylpiperazin-1-yl)-2-(1H-pyrazol-1-yl)ethanone scaffold was identified by screening a compound library utilizing CCR1-expressing human THP-1 cells. SAR studies led to the discovery of the highly potent and selective CCR1 antagonist 14 (CCR1 binding IC₅₀ = 4 nM using [¹²⁵I]-CCL3 as the chemokine ligand). Compound 14 displayed promising pharmacokinetic and toxicological profiles in preclinical species.     

Synthesis and evaluation of a 18F-labeled spirocyclic piperidine derivative as promising σ1 receptor imaging agent

Several spirocyclic piperidine derivatives were designed and synthesized as σ₁ receptor ligands. In vitro competition binding assays showed that the fluoroalkoxy analogues with small substituents possessed high affinity towards σ₁ receptors and subtype selectivity. Particularly for ligand 1′-((6-(2-fluoroethoxy)pyridin-3-yl)methyl)-3H-spiro[2-benzofuran-1,4′-piperidine] (2), high σ₁ receptor affinity (K_i = 2.30 nM) and high σ₁/σ₂ subtype selectivity (142-fold) as well as high σ₁/VAChT selectivity (234-fold) were observed. [¹⁸F]2 was synthesized using an efficient one-pot, two-step reaction method in a home-made automated synthesis module, with an overall isolated radiochemical yield of 8–10%, a radiochemical purity of higher than 99%, and specific activity of 56–78 GBq/μmol. Biodistribution studies of [¹⁸F]2 in ICR mice indicated high initial brain uptake and a relatively fast washout. Administration of haloperidol, compound 1 and different concentrations of SA4503 (3, 5, or 10 μmol/kg) 5 min prior to injection of [¹⁸F]2 significantly decreased the accumulation of radiotracer in organs known to contain σ₁ receptors. Ex vivo autoradiography in Sprague–Dawley rats demonstrated high accumulation of radiotracer in brain areas with high expression of σ₁ receptors. These encouraging results prove that [¹⁸F]2 is a suitable candidate for σ₁ receptor imaging with PET in humans.     

Synthesis and in vitro biological evaluation of carbon-11-labeled quinoline derivatives as new candidate PET radioligands for cannabinoid CB2 receptor imaging

Cannabinoids have been recently proposed as a new family of potential antitumor agents, and cannabinoid receptor 2 (CB2) is believed to be over-expressed in tumor cells. This study was designed to develop new radioligands for imaging of CB2 receptor in cancer using biomedical imaging technique positron emission tomography (PET). Carbon-11-labeled 2-oxoquinoline and 2-chloroquinoline derivatives, [¹¹C]6a–d and [¹¹C]9a–d, were prepared by O-[¹¹C]methylation of their corresponding precursors using [¹¹C]CH₃OTf under basic conditions and isolated by a simplified solid-phase extraction (SPE) method in 40–50% radiochemical yields based on [¹¹C]CO₂ and decay corrected to end of bombardment (EOB). The overall synthesis time from EOB was 15–20 min, the radiochemical purity was >99%, and the specific activity at end of synthesis (EOS) was 111–185 GBq/μmol. Radioligand binding assays indicated compounds 6f, 6b, and 9f display potent in vitro binding affinities with nanomolar K_i values and at least 100–2000-fold selectivity for CB2.     

Radiosynthesis and evaluation of new PET ligands for peripheral cannabinoid receptor type 1 imaging

Cannabinoid receptor type 1 (CB1) is mainly expressed in the brain, as well as being expressed in functional relevant concentrations in various peripheral tissues. 1-(4-Chlorophenyl)-3-(3-(6-(pyrrolidin-1-yl)pyridin-2-yl)phenyl)urea (PSNCBAM-1, 1) was developed as a potent allosteric antagonist for CB1 and its oral administration led to reductions in the appetite and body weight of rats. Several analogs of 1 (compounds 2 and 3) were recently identified through a series of structure-activity relationship studies. Herein, we report the synthesis of radiolabeled analogs of these compounds using [¹¹C]COCl₂ and an evaluation of their potential as PET ligands for CB1 imaging using in vitro and in vivo techniques. [¹¹C]2 and [¹¹C]3 were successfully synthesized in two steps using [¹¹C]COCl₂. The radiochemical yields of [¹¹C]2 and [¹¹C]3 were 17 ± 8% and 20 ± 9% (decay-corrected to the end of bombardment, based on [¹¹C]CO₂). The specific activities of [¹¹C]2 and [¹¹C]3 were 42 ± 36 and 37 ± 13 GBq/μmol, respectively. The results of an in vitro binding assay using brown adipose tissue (BAT) homogenate showed that the binding affinity of 2 for CB1 (K_D = 15.3 µM) was much higher than that of 3 (K_D = 26.0 µM). PET studies with [¹¹C]2 showed a high uptake of radioactivity in BAT, which decreased in animals pretreated with AM281 (a selective antagonist for CB1). In conclusion, [¹¹C]2 may be a useful PET ligand for imaging peripheral CB1 in BAT.