Synthesis and preclinical characterization of 1-(6′-deoxy-6′-[18F]fluoro-β-d-allofuranosyl)-2-nitroimidazole (β-6′-[18F]FAZAL) as a positron emission tomography radiotracer to assess tumor hypoxia

Positron emission tomography (PET) using fluorine-18 (¹⁸F)-labeled 2-nitroimidazole radiotracers has proven useful for assessment of tumor oxygenation. However, the passive diffusion-driven cellular uptake of currently available radiotracers results in slow kinetics and low tumor-to-background ratios. With the aim to develop a compound that is actively transported into cells, 1-(6′-deoxy-6′-[¹⁸F]fluoro-β-d-allofuranosyl)-2-nitroimidazole (β-[¹⁸F]1), a putative nucleoside transporter substrate, was synthetized by nucleophilic [¹⁸F]fluoride substitution of an acetyl protected labeling precursor with a tosylate leaving group (β-6) in a final radiochemical yield of 12 ± 8% (n = 10, based on [¹⁸F]fluoride starting activity) in a total synthesis time of 60 min with a specific activity at end of synthesis of 218 ± 58 GBq/μmol (n = 10). Both radiolabeling precursor β-6 and unlabeled reference compound β-1 were prepared in multistep syntheses starting from 1,2:5,6-di-O-isopropylidene-α-d-allofuranose. In vitro experiments demonstrated an interaction of β-1 with SLC29A1 and SLC28A1/2/3 nucleoside transporter as well as hypoxia specific retention of β-[¹⁸F]1 in tumor cell lines. In biodistribution studies in healthy mice β-[¹⁸F]1 showed homogenous tissue distribution and excellent metabolic stability, which was unaffected by tissue oxygenation. PET studies in tumor bearing mice showed tumor-to-muscle ratios of 2.13 ± 0.22 (n = 4) at 2 h after administration of β-[¹⁸F]1. In ex vivo autoradiography experiments β-[¹⁸F]1 distribution closely matched staining with the hypoxia marker pimonidazole. In conclusion, β-[¹⁸F]1 shows potential as PET hypoxia radiotracer which merits further investigation.     

Synthesis and biological evaluation of radioiodinated 2NUBTA as a cerebral ischemia marker

N-[4-(Benzothiazol-2-yl)phenyl]-11-(2-nitroimidazole-1-yl)undecanamide (2NUBTA) was synthesized and radiolabeled with iodine-131. In vitro evaluation of the [¹³¹I]2NUBTA using murine sarcoma S180 cells showed increase in radioactivity in hypoxic cells up to 4 h, while it was not in aerobic cells. Its potential as a cerebral ischemia marker was evaluated using gerbil stroke models that had been subjected to right common carotid artery ligation to produce cerebral ischemia. The uptake in the right cerebral hemisphere decreased slowly than that of the left and the right/left hemisphere uptake ratios increased with time going on. It indicated that [¹³¹I]2NUBTA might be a possible cerebral ischemia marker.     

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.     

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.     

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.     

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.     

Design of Ga–DOTA-based bifunctional radiopharmaceuticals: Two functional moieties can be conjugated to radiogallium–DOTA without reducing the complex stability

From the X-ray crystal structures of Ga–DOTA chelates, we were able to deduce that two free carboxylate groups of the radiogallium–DOTA complex may be utilized for coupling to functional moieties that recognize molecular targets for in vivo imaging without reducing the radiogallium-complex stability. Thus, we designed 2,2′-[4,10-bis(2-{[2-(2-methyl-5-nitro-1H-imidazol-1-yl)ethyl]amino}-2-oxoethyl)-1,4,7,10-tetraazacyclododecane-1,7-diyl]diacetic acid (DOTA-MN2) (7), employing a metronidazole moiety as the recognition site of hypoxic lesions, based on the drug design concept of bifunctional radiopharmaceuticals. Coupling of DOTA-bis(tert-butyl)ester 5 with 1-(2-aminoethyl)-2-methyl-5-nitroimidazole dihydrochloride, followed by deprotection, afforded the required 7 (DOTA-MN2). ⁶⁷Ga-labeling was carried out by reaction of DOTA-MN2 with ⁶⁷Ga-citrate. When ⁶⁷Ga–DOTA-MN2 was incubated in phosphate-buffered saline or mouse plasma, no measurable decomposition occurred over a 24-h period. In biodistribution experiments in NFSa tumor-bearing mice, ⁶⁷Ga–DOTA-MN2 displayed not only a significant tumor uptake, but also rapid blood clearance and low accumulations in nontarget tissues, resulting in high target-to-nontarget ratios of radioactivity. These results indicate the potential benefits of the drug design of ⁶⁷Ga–DOTA-MN2. The present findings provide helpful information for the development of radiogallium-labeled radiopharmaceuticals for SPECT and PET studies.     

One-step radiosynthesis and initial evaluation of a small molecule PET tracer for PD-L1 imaging

Programmed cell death protein-ligand 1 (PD-L1) is a crucial biomarker in immunotherapy and its expression level plays a key role in the guidance of anti-PD-L1 therapy. It had been reported that PD-L1 was quantified by noninvasive imaging with more developed radiotracers. In our study, a novel [¹⁸F]fluoride labeled small molecule inhibitor, [¹⁸F]LN was designed for positron emission tomography (PET) imaging in both PD-L1 transfected (A375-hPD-L1) and non-transfected (A375) melanoma-bearing mice. LN showed the specificity (IC₅₀ = 50.39 ± 2.65 nM) to PD-L1 confirmed by competitive combination and cell flow cytometry (FACS) analysis. The radiotracer [¹⁸F]LN was obtained via ¹⁸F-¹⁹F isotope exchange from precursor LN. After radiosynthesis, [¹⁸F]LN was achieved with a high radiochemical purity (RCP) above 95% and got a favorable molar activity of 36.34 ± 5.73 GBq/μmol. [¹⁸F]LN displayed the moderate affinity (K_d = 65.27 ± 3.47 nM) to PD-L1 by specific binding assay. And it showed 1.3-fold higher uptake in A375-hPD-L1 cells than that in A375 cells. PET imaging revealed that [¹⁸F]LN could enter into PD-L1 expressing tumor site and visualize the outline of tumor. And tumor uptake (1.96 ± 0.27 %ID/g) reached the maximum at 15 min in the positive group, showed 2.2-fold higher than the negative (0.89 ± 0.31 %ID/g) or the blocked (1.07 ± 0.26 %ID/g) groups. Meanwhile, biodistribution could slightly distinguish the positive from the negative. The results indicated [¹⁸F]LN would become an efficient tool for evaluating PD-L1 expression with further optimization.     

Uptake of [18F] EF5 as a Tracer for Hypoxic and Aggressive Phenotype in Experimental Head and Neck Squamous Cell Carcinoma

PURPOSE: This study aims to investigate whether the uptake of 2-(2-nitro-1H-imidazol-1-yl)-N-(2,2,3,3,3-pentafluoropropyl)-acetamide ([¹⁸F]EF5) and 2-deoxy-2-[¹⁸F]fluoro-d-glucose ([¹⁸F]FDG) is associated with a hypoxia-driven adverse phenotype in head and neck squamous cell carcinoma cell lines and tumor xenografts. METHODS: Xenografts were imaged in vivo, and tumor sections were stained for hypoxia-inducible factor 1α (Hif-1α), carbonic anhydrase IX (CA IX), and glucose transporter 1 (Glut-1). Tracer uptakes and the expression of Hif-1α were determined in cell lines under 1% hypoxia. RESULTS: High [¹⁸F]EF5 uptake was seen in xenografts expressing high levels of CA IX, Glut-1, and Hif-1α, whereas low [¹⁸F]EF5 uptake was detected in xenografts expressing low amounts of CA IX and Hif-1α. The uptake of [¹⁸F]EF5 between cell lines varied extensively under normoxic conditions. A clear correlation was found between the expression of Hif-1α and the uptake of [¹⁸F]FDG during hypoxia. CONCLUSIONS: The UT-SCC cell lines studied differed with respect to their hypoxic phenotypes, and these variations were detectable with [¹⁸F]EF5. Acute hypoxia increases [¹⁸F]FDG uptake in vitro, whereas a high [¹⁸F]EF5 uptake reflects a more complex phenotype associated with hypoxia and an aggressive growth pattern.     

Reliability of Early Iodine 123 Uptake for Treatment of Graves Disease in Children

ObjectiveTo determine the reliability of early radioiodine uptake (RAIU) in calculation of the radioiodine ablation dose for pediatric patients with Graves disease.MethodsThis retrospective review of medical records involved 22 pediatric patients with Graves disease, who had undergone early (4 to 8 hours) and late (24 to 26 hours) RAIU studies and were treated with iodine 131 (¹³¹I). Quantitative data are reported as mean ± standard error of the mean. Early and late RAIU and actual administered versus calculated ¹³¹I ablation doses were compared by using the paired t test. The correlation between early and late RAIU was assessed by curvilinear regression analysis. Significance was assessed at P < .05.ResultsMean early RAIU was 57.1% ± 18.2%, and mean late RAIU was 72.1% ± 14.4% (P < .05). Curvilinear regression analysis showed the following: late RAIU = 7.13 + 1.71 × (early RAIU) - 0.01 × (early RAIU)²; r² = 0.75. The mean ablation dose of ¹³¹I based on late RAIU was 9.3 ± 2.0 mCi. The calculated radioiodine dose would have been, on average, 32% higher (12.3 ± 3.8 mCi; P < .05) had early RAIU been used.ConclusionIn children, early RAIU can be much lower than late RAIU. This may be misleading for ablation dose calculations. Therefore, late RAIU should be used to avoid overtreatment in children with Graves disease.(Endocr Pract. 2011;17:541-545)     

Synthesis and in vitro evaluation of radioiodinated indolequinones targeting NAD(P)H: Quinone oxidoreductase 1 for internal radiation therapy

NAD(P)H: quinone oxidoreductase 1 (NQO1) is an obligate two-electron reductase and is highly expressed in many human solid cancers. Because NQO1 can be induced immediately after exposure to ionizing radiation, we aimed to develop an NQO1-targeted radiolabeled agent to establish a novel internal radiation therapy that amplifies the therapeutic effects when combined with external radiation therapy. We designed three NQO1-targeted radioiodinated compounds including two ether linkage compounds ([¹²⁵I]1 and [¹²⁵I]2) and a sulfide linkage compound ([¹²⁵I]3) based on the selective binding of indolequinone analogs to the active site of NQO1 by the stacking effect. These compounds were successfully prepared using an oxidative iododestannylation reaction with high radiochemical yields and purity. In NQO1-expressing tumor cells, [¹²⁵I]1 and [¹²⁵I]2 were readily metabolized to p-[¹²⁵I]iodophenol or m-[¹²⁵I]iodophenol and [¹²⁵I]I⁻, whereas over 85% of the initial radioactivity of [¹²⁵I]3 was observed as an intact form at 1 h after incubation. The cellular uptake of [¹²⁵I]3 was significantly higher than those of [¹²⁵I]1 and [¹²⁵I]2. The uptake of [¹²⁵I]3 was specific and was dependent on the expression of NQO1. These data suggest that the novel NQO1-targeted radioiodinated compound [¹²⁵I]3 could be used as a novel internal radiation agent for the treatment of cancer.     

Synthesis and in vitro evaluation of 18F labeled tyrosine derivatives as potential positron emission tomography (PET) imaging agents

Three new ¹⁸F labeled fluoroalkyl tyrosine derivatives, O-(2-[¹⁸F]fluoroethyl)-α-methyltyrosine (FEMT, [¹⁸F]2), O-(2-[¹⁸F]fluoroethyl)-2-l-azatyrosine (FEAT, [¹⁸F]3), O-(2-[¹⁸F]fluoroethyl)-l-tyrosineamide (FETA, [¹⁸F]4) have been synthesized and radiofluorinated with 5–34% decay-corrected yield. In vitro studies were carried out in U-138 MG human glioblastoma. Cellular uptake of new tracers was compared to clinically utilized imaging agent O-(2-[¹⁸F]fluoroethyl)-l-tyrosine (FET, [¹⁸F]1). The uptake of tracers followed the order of FET ([¹⁸F]1) > FEAT([¹⁸F]3) > FEMT ([¹⁸F]2) ≈ FETA ([¹⁸F]4).     

Synthesis and evaluation of novel radioiodinated PSMA targeting ligands for potential radiotherapy of prostate cancer

Radioligand therapy (RLT) using prostate-specific membrane antigen (PSMA) targeting ligands is an attractive option for the treatment of Prostate cancer (PCa) and its metastases. We report herein a series of radioiodinated glutamate-urea-lysine-phenylalanine derivatives as new PSMA ligands in which l-tyrosine and l-glutamic acid moieties were added to increase hydrophilicity concomitant with improvement of in vivo targeting properties. Compounds 8, 15, 19a/19b and 23a/23b were synthesized and radiolabeled with ¹²⁵I by iododestannylation. All iodinated compounds displayed high binding affinities toward PSMA (IC₅₀ = 1–13 nM). In vitro cell uptake studies demonstrated that compounds containing an l-tyrosine linker moiety (8, 15 and 19a/19b) showed higher internalization than MIP-1095 and 23a/23b, both without the l-tyrosine linker moiety. Biodistribution studies in mice bearing PC3-PIP and PC3 xenografts showed that [¹²⁵I]8 and [¹²⁵I]15 with higher lipophilicity exhibited higher nonspecific accumulations in the liver and intestinal tract, whereas [¹²⁵I]19a/19b and [¹²⁵I]23a/23b containing additional glutamic acid moieties showed higher accumulations in the kidney and implanted PC3-PIP (PSMA+) tumors. [¹²⁵I]23b displayed a promising biodistribution profile with favorable tumor retention, fast clearance from the kidney, and 2–3-fold lower uptake in the liver and blood than that observed for [¹²⁵I]MIP-1095. [^125/131I]23b may serve as an optimal PSMA ligand for radiotherapy treatment of prostate cancer over-expressing PSMA.     

Design,synthesis, and biological evaluation of radioiodinated benzo[d]imidazole-quinoline derivatives for platelet-derived growth factor receptor β (PDGFRβ) imaging

Several malignant tumors and fibrotic diseases are associated with PDGFRβ overexpression and excessive signaling, making this receptor attractive for molecular targeting and imaging approaches. A series of benzo[d]imidazole-quinoline derivatives were designed and synthesized to develop radioiodinated compounds as PDGFRβ-specific imaging probes. The structure activity relationship (SAR) evaluation of the designed compounds was performed. Among them, 2-[5-(2-methoxyethoxy)-1H-benzo[d]imidazol-1-yl]-8-(piperazin-1-yl)quinoline (5a) and 4-{2-[5-(2-methoxyethoxy)-1H-benzo[d]imidazol-1-yl]quinolin-8-yl}morpholine (5d) exhibited a relatively high PDGFRβ-TK inhibitory potency, whereas iodinated 5a derivative 5-iodo-2-[5-(2-methoxyethoxy)-1H-benzo[d]imidazol-1-yl]-8-(piperazin-1-yl)quinoline (8) exhibited a superior inhibitory potency as PDGFRβ inhibitor than iodinated 5d derivative 4-{5-iodo-2-[5-(2-methoxyethoxy)-1H-benzo[d]imidazol-1-yl]quinolin-8-yl}morpholine (11). Furthermore, [¹²⁵I]8 and [¹²⁵I]11 were synthesized and evaluated for PDGFRβ radioligand ability, both in vitro and in vivo. Cellular uptake experiments showed that [¹²⁵I]8 had a higher uptake in BxPC3-luc cells as PDGFRβ-positive cells than [¹²⁵I]11. Incubation of [¹²⁵I]8 after pretreatment of PDGFRβ ligands significantly reduced the uptake of [¹²⁵I]8. In biodistribution experiments using tumor-bearing mice, [¹²⁵I]8 accumulation in the tumor 1?h postinjection was higher than that of the benzo[d]imidazol-quinoline derivative [¹²⁵I]IIQP, used in our previous research. These results indicate that [¹²⁵I]8 could be a promising PDGFRβ imaging agent. Although its clinical application requires further structural modifications, the results obtained in this research may be useful for the development of PDGFRβ-specific radioligands.     

Effect of a second nitroimidazole redox centre on the accumulation of a hypoxia marker: synthesis and in vitro evaluation of 99mTc-labeled bisnitroimidazole propylene amine oxime complexes

Up to now, most of the hypoxia markers contain only one nitroimidazole redox centre, such as Oxo[[3,3,9,9-tetramethyl-1-(2-nitro-1H-imidazol-1-yl)-4,8-diazaundecane-2,10-dione dioximato] (3-)-N,N',N″,N″']-technetium ((99m)Tc-1, BMS181321). Introducing a second nitroimidazole redox centre may enhance the hypoxic accumulation of the markers. In the present work, four (99m)Tc-1 (BMS181321, containing one 2-nitroimidazole) analogues, that is, (99m)Tc-2 (containing two 2-nitroimidazoles), (99m)Tc-3 (containing one 4-nitroimidazole), (99m)Tc-4 (containing two 4-nitroimidazoles) and (99m)Tc-5 (containing both a 2-nitroimidazole and a 4-nitroimidazole) were synthesized, and the hypoxic accumulation was evaluated in vitro using murine sarcoma S180 cells. (99m)Tc-3 and (99m)Tc-4 displayed no significant anoxic/normoxic differentials, whereas (99m)Tc-1 (BMS181321), (99m)Tc-2 and (99m)Tc-5 showed high anoxic cellular uptakes. The anoxic uptake of (99m)Tc-2 reached up to 59.0±0.9% at 4h, which was 2.4 times as that of (99m)Tc-1. (99m)Tc-2 displayed high hypoxic accumulation, indicating that introducing a second nitroimidazole redox centre, that is, 2-nitroimidazole, affected the hypoxic accumulation. Consequently, (99m)Tc-2 may serve as a viable candidate for hypoxia marker. This finding may eventually lead to the development of compounds containing multi-redox centres as hypoxia markers.     

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 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.     

Biology-oriented drug synthesis (BIODS) of 2-(2-methyl-5-nitro-1H-imidazol-1-yl)ethyl aryl ether derivatives,in vitro α-amylase inhibitory activity and in silico studies

A new library of 2-(2-methyl-5-nitro-1H-imidazol-1-yl)ethyl aryl ether derivatives (1 −2 3) were synthesized and characterized by EI-MS and ¹H NMR, and screened for their α-amylase inhibitory activity. Out of twenty-three derivatives, two molecules 19 (IC₅₀ = 0.38 ± 0.82 µM) and 23 (IC₅₀ = 1.66 ± 0.14 µM), showed excellent activity whereas the remaining compounds, except 10 and 17, showed good to moderate inhibition in the range of IC₅₀ = 1.77–2.98 µM when compared with the standard acarbose (IC₅₀ = 1.66 ± 0.1 µM). A plausible structure-activity relationship has also been presented. In addition, in silico studies was carried out in order to rationalize the binding interaction of compounds with the active site of enzyme.     

Radiosynthesis and in vivo evaluation of [11C]MOV as a PET imaging agent for COX-2

Radiosynthesis and in vivo evaluation of [¹¹C]4-[5-(4-methylphenyl)-3-(trifluoromethyl)-1H-pyrazol-1-yl]benzenesulfonamide (methoxy analogue of valdecoxib, [¹¹C]MOV), a COX-2 inhibitor, was conducted in rat and baboon. Synthesis of the reference standard MOV (3), and its desmethyl precursor 2 for radiolabeling were performed using 1,2-diphenylethan-1-one as the starting material in five steps with 15% overall yield. Radiosynthesis of [¹¹C]MOV was accomplished in 40?±?10% yield and?>99% radiochemical purity by reacting the precursor 2 in dimethyl formamide (DMF) with [¹¹C]CH₃I followed by removal of the dimethoxytrityl (DMT) protective group using trifluroacetic acid. PET studies in anesthetized baboon showed very low uptake and homogeneous distribution of [¹¹C]MOV in brain. The radioligand underwent rapid metabolism in baboon plasma. MicroPET studies in male Sprague Dawley rats revealed [¹¹C]MOV binding in lower thorax. The tracer binding in rats was partially blocked in heart and duodenum by the administration of 1?mg/kg oral dose of COX-2 inhibitor valdecoxib.     

3-(Cyclopropylmethyl)-7-((4-(4-[11C]methoxyphenyl)piperidin-1-yl)methyl)-8-(trifluoromethyl)-[1,2,4]triazolo[4,3-a]pyridine: Synthesis and preliminary evaluation for PET imaging of metabotropic glutamate receptor subtype 2

Selective metabotropic glutamate receptor 2 (mGluR2) inhibitors have been demonstrated to show therapeutic effects by improving alleviating symptoms of schizophrenic patients in clinical studies. Herein we report the synthesis and preliminary evaluation of a ¹¹C-labeled positron emission tomography (PET) tracer originating from a mGluR2 inhibitor, 3-(cyclopropylmethyl)-7-((4-(4-methoxyphenyl)piperidin-1-yl)methyl)-8-(trifluoromethyl)-[1,2,4]triazolo[4,3-a]pyridine (CMTP, 1a). [¹¹C]CMTP ([¹¹C]1a) was synthesized by O-[¹¹C]methylation of desmethyl precursor 1b with [¹¹C]methyl iodide in 19.7 ± 8.9% (n = 10) radiochemical yield (based on [¹¹C]CO₂) with >98% radiochemical purity and >74 GBq/μmol molar activity. Autoradiography study showed that [¹¹C]1a possessed moderate in vitro specific binding to mGluR2 in the rat brain, with a heterogeneous distribution of radioactive accumulation in the mGluR2-rich brain tissue sections, such as the cerebral cortex and striatum. PET study indicated that [¹¹C]1a was able to cross the blood–brain barrier and enter the brain, but had very low specific binding in the rat brain. Further optimization for the chemical structure of 1a is necessary to increase binding affinity to mGluR2 and then improve in vivo specific binding in brain.