Synthesis of 131I-labeled glucose-conjugated inhibitors of O6-methylguanine-DNA methyltransferase (MGMT) and comparison with nonconjugated inhibitors as potential tools for in vivo MGMT imaging

O(6)-Substituted guanine derivatives are powerful agents used for tumor cell sensitization by inhibition of the DNA repair enzyme O(6)-methylguanine-DNA methyltransferase (MGMT). To provide targeted accumulation of MGMT inhibitors in tumor tissue as well as tools for in vivo imaging, we synthesized iodinated C(8)-alkyl-linked glucose conjugates of 2-amino-6-(5-iodothenyl)-9H-purine (O(6)-(5-iodothenyl) guanine, ITG) and 2-amino-6-(3-iodobenzyloxy)-9H-purine (O(6)-(5-iodobenzyl) guanine, IBG). These compounds have MGMT inhibitor constants (IC(50) values) of 0.8 and 0.45 microM for ITGG and IBGG, respectively, as determined in HeLa S3 cells after 2-h incubation with inhibitor. To substantiate that the (131)I-(hetero)arylmethylene group at the O(6)-position of guanine is transferred to MGMT, both the glucose conjugated inhibitors ITGG and IBGG and the corresponding nonglucose conjugated compounds ITG and IBG were labeled with iodine-131. The radioiodinations of all compounds with [(131)I]I(-) were performed with radiochemical yields of >70% for the destannylation of the corresponding tri-n-butylstannylated precursors. The binding ability of [(131)I]ITGG, [(131)]IBGG, [(131)I]ITG, and [(131)I]IBG to purified MGMT was tested. All radioactive compounds were substrates for MGMT, as demonstrated using a competitive repair assay. The newly synthesized radioactive inhibitors were utilized to study ex vivo biodistribution in mice, and the tumor-to-blood ratio of tissue uptake of [(131)I]IBG and [(131)I]IBGG was determined to be 0.24 and 0.76 after 0.5 h, respectively.     

Radiosynthesis and preliminary biodistribution in mice of 6‐deoxy‐6‐[131I]iodo‐L‐ascorbic acid

An ascorbate analog labeled with iodine‐131, 6‐deoxy‐ 6‐[¹³¹I]iodo‐L ‐ascorbic acid was prepared for evaluation as an in vivo tracer of L ‐ascorbic acid. The no‐carrier‐added radiosynthesis was conducted by nucleophilic bromine–iodine exchange between the brominated precursor and sodium [¹³¹I]iodide in 2‐pentanone at 130–140°C. HPLC purification using a reverse‐phase column gave 6‐deoxy‐6‐[¹³¹I]iodo‐L ‐ascorbic acid in radiochemical yield of 36–60% with high radiochemical purity and satisfactory‐specific radioactivity in a total preparation time of 90 min. Biodistribution studies in fibrosarcoma‐bearing mice showed a high uptake in the adrenal glands, accompanied by low activity of tumor accumulation, accumulation properties similar to previous results obtained with ¹⁴C‐labeled ascorbic acid and 6‐deoxy‐6‐[¹⁸F]fluoro‐L ‐ascorbic acid, in spite of high level of deiodination. Copyright © 2009 John Wiley & Sons, Ltd.     

Radioiodination of balsalazide,bioevaluation, and characterization as a highly selective radiotracer for imaging of ulcerative colitis in mice

This work focuses on tracking ulcerative colitis in mice. High labeling yield and radiochemical purity were achieved for the formation of a [^125/131I]balsalazide radiotracer at optimum conditions of oxidizing agent content (chloramines-T [Ch-T], 75 μg), substrate amount (100 μg), pH of reaction mixture (6), reaction time (30 min), and temperature (37°C), using radioactive iodine-125 (200–450 MBq). The radiolabeled compound, [^125/131I]balsalazide, was stable in serum and saline solution during 24 h. Balsalazide is acting as a peroxisome proliferator-activated receptor (PPARγ). Biodistribution studies were carried in normal and ulcerated colon mice. High uptake of 75 ± 1.90% injected dose/g organ (ID/g) observed in ulcerated mice confirmed the suitability of [¹³¹I]balsalazide as a novel radiotracer for ulcerative colitis imaging in mice.     

Radioiodinated progesterone derivative for progesterone receptor targeting with enhanced nucleus uptake via phenylboronic acid conjugation

A novel ¹³¹I‐radiolabeled probe with aromatic boronate motif (¹³¹I‐EIPBA) was designed to target progesterone receptor (PR)–positive breast cancer with enhanced nucleus uptake. Acetylene progesterone was conjugated with pegylated phenylboronic acid via click reaction and radiolabeled with ¹³¹I to afford ¹³¹I‐EIPBA. Meanwhile, ¹³¹I‐EIPB without boronate was prepared as control agent. After determination of the lipophilicity and stability of these tracers, in vitro cell uptake studies and in vivo biodistribution in rats were performed to verify the enhanced nucleus uptake and PR targeting ability of ¹³¹I‐EIPBA. ¹³¹I‐EIPBA was obtained with moderate radiochemical yield (40.35 ± 3.52%) and high radiochemical purity (>98%). As expected, the high binding affinity (39.58 nM) of ¹³¹I‐EIPBA for PR was determined by cell binding assay. The internalization ratio of ¹³¹I‐EIPBA was remarkably higher than that of ¹³¹I‐EIPB in PR‐positive MCF‐7 cells. Furthermore, the enhanced nucleus uptake of ¹³¹I‐EIPBA (0.59 ± 0.02%) was found to be significantly higher than that of ¹³¹I‐EIPB (0.13 ± 0.01%) in MCF‐7 cells. A novel ¹³¹I‐EIPBA compound was developed for PR targeting with improved cellular nucleus uptake. Furthermore, the introduction of aromatic boronate motif provides a worthwhile strategy for enhancing the nuclear receptor targeting of tracers.     

Preliminary evaluation of [18F]AlF-NOTA-MAL-Cys39-exendin-4 in insulinoma with PET

Background: High expression of glucagon-like peptide-1 receptor (GLP-1R) in insulinoma supplies a potential drug target for tumor imaging. Exendin-4 can specifically bind to GLP-1R as an agonist and its analogs are extensively used in receptor imaging studies.

Purpose: A new GLP-1R imaging agent, [¹⁸F]AlF-NOTA-MAL-Cys³⁹-exendin-4, was designed and prepared for insulinoma imaging.

Methods: Cys³⁹-exendin-4 was conjugated with NOTA-MAL, then the compound was radiolabeled with [¹⁸F]AlF complex to obtained [¹⁸F]AlF-NOTA-MAL-Cys³⁹-exendin-4. The tumor-targeting characters of the tracer were evaluated in INS-1 cells and BALB/c nude mice models.

Results: [¹⁸F]AlF-NOTA-MAL-Cys³⁹-exendin-4 can be efficiently produced with a yield of 17.5?±?3.2% (non-decay corrected) and radiochemical purity of >95%. The IC₅₀ value of displacement [¹⁸F]AlF-NOTA-MAL-Cys³⁹-exendin-4 with Cys³⁹-exendin-4 was 13.52?±?1.36?nM. PET images showed excellent tumor visualization with high uptake (9.15?±?1.6%ID/g at 30?min and 7.74?±?0.87%ID/g at 60?min). The tumor to muscle, pancreas and liver ratios were 63.25, 3.85 and 7.29 at 60?min after injection. GLP-1R binding specificity was demonstrated by co-injection with an excess of unlabeled Cys³⁹-exendin-4 and the tumor uptake was found to be reduced significantly.

Conclusion: [¹⁸F]AlF-NOTA-MAL-Cys³⁹-exendin-4 shows favorable characteristics for insulinoma imaging and may be translated to clinical studies.     

Comparison of biological properties of [177Lu]Lu-ProBOMB1 and [177Lu]Lu-NeoBOMB1 for GRPR targeting

The gastrin-releasing peptide receptor (GRPR) is overexpressed in prostate cancer and other solid malignancies. Following up on our work on [⁶⁸Ga]Ga-ProBOMB1 that had better imaging characteristics than [⁶⁸Ga]Ga-NeoBOMB1, we investigated the effects of substituting ⁶⁸Ga for ¹⁷⁷Lu to determine if the resulting radiopharmaceuticals could be used with a therapeutic aim. We radiolabeled the bombesin antagonist ProBOMB1 (DOTA-pABzA-DIG-D-Phe-Gln-Trp-Ala-Val-Gly-His-Leu-ψ-Pro-NH₂) with lutetium-177 and compared it with [¹⁷⁷Lu]Lu-NeoBOMB1 (obtained in 54.2 ± 16.5% isolated radiochemical yield with >96% radiochemical purity and 440.8 ± 165.1 GBq/μmol molar activity) for GRPR targeting. Lu-NeoBOMB1 had better binding affinity for GRPR than Lu-ProBOMB1 (K_i values: 2.26 ± 0.24 and 30.2 ± 3.23nM). [¹⁷⁷Lu]Lu-ProBOMB1 was obtained in 53.7 ± 5.4% decay-corrected radiochemical yield with 444.2 ± 193.2 GBq/μmol molar activity and >95% radiochemical purity. In PC-3 prostate cancer xenograft mice, tumor uptake of [¹⁷⁷Lu]Lu-ProBOMB1 was 3.38 ± 1.00, 1.32 ± 0.24, and 0.31 ± 0.04%ID/g at 1, 4, and 24 hours pi. However, the uptake in tumor was lower than [¹⁷⁷Lu]Lu-NeoBOMB1 at all time points. [¹⁷⁷Lu]Lu-ProBOMB1 was inferior to [¹⁷⁷Lu]Lu-NeoBOMB1, which had better therapeutic index for the organs receiving the highest doses.     

Development of novel PET probes, [18F]BCPP‐EF, [18F]BCPP‐BF,and [11C]BCPP‐EM for mitochondrial complex 1 imaging in the living brain

We developed three novel positron‐emission tomography (PET) probes, 2‐tert‐butyl‐4‐chloro‐5‐{6‐[2‐(2[¹⁸F]fluoroethoxy)‐ethoxy]‐pyridin‐3‐ylmethoxy}‐2H‐pyridazin‐3‐one ([¹⁸F]BCPP‐EF), 2‐tert‐butyl‐4‐chloro‐5‐[6‐(4‐[¹⁸F]fluorobutoxy)‐pyridin‐3‐ylmethoxy]‐2H‐pyridazin‐3‐one ([¹⁸F]BCPP‐BF), and 2‐tert‐butyl‐4‐chloro‐5‐{6‐[2‐(2‐[¹¹C]methoxy‐ethoxy)‐ethoxy]‐pyridin‐3‐ylmethoxy}‐2H‐pyridazin‐3‐one ([¹¹C]BCPP‐EM), for quantitative imaging of mitochondrial complex 1 (MC‐1) activity in vivo. These three PET probes were successfully labeled by nucleophilic [¹⁸F]fluorination or by [¹¹C]methylation of their corresponding precursor with sufficient radioactivity yield, good radiochemical purity, and sufficiently high specific radioactivity for PET measurement. The specificity of these probes for binding to MC‐1 was assessed with rotenone, a specific MC‐1 inhibitor, by a rat brain slice imaging method in vitro. Rat whole‐body imaging by small‐animal PET demonstrated that all probes showed high uptake levels in the brain as well as in the heart sufficient to image them clearly. The rank order of uptake levels in the brain and the heart just after injection was as follows: high in [¹⁸F]BCPP‐BF, intermediate in [¹¹C]BCPP‐EM, and low in [¹⁸F]BCPP‐EF. The kinetics of [¹⁸F]BCPP‐EF and [¹¹C]BCPP‐EM provided a reversible binding pattern, whereas [¹⁸F]BCPP‐BF showed nonreversible accumulation‐type kinetics in the brain and heart. Metabolite analyses indicated that these three compounds were rapidly metabolized in the plasma but relatively stable in the rat brain up to 60 min post‐injection. The present study demonstrated that [¹⁸F]BCPP‐EF could be a useful PET probe for quantitative imaging of MC‐1 activity in the living brain by PET.     

Synthesis and preliminary evaluation of a 18F‐labeled ethisterone derivative [18F]EAEF for progesterone receptor targeting

To develop a novel progesterone receptor‐targeting probe for positron emission tomography imaging, an ethisterone derivative [¹⁸F]EAEF was designed and prepared in high decay‐corrected radiochemical yield (30–35%) with good radiochemical purity (>98%). [¹⁸F]EAEF is a lipophilic tracer (logP = 0.53 ± 0.06) with very good stability in saline and serum. In the biodistribution study, high radioactivity accumulation of [¹⁸F]EAEF were found in uterus (5.73 ± 1.83% ID/g) and ovary (4.05 ± 0.73% ID/g) at 2 hr postinjection (p.i.), which have high progesterone receptor expression after treated with estradiol, while the muscle background has very low uptake (0.50 ± 0.17% ID/g). For positron emission tomography imaging, [¹⁸F]EAEF showed high uptake in progesterone receptor‐positive MCF‐7 tumor (3.15 ± 0.07% ID/g at 2 hr p.i.) with good tumor to muscle ratio (2.90), and obvious lower tumor uptakes were observed in MCF‐7 with EAEF blocking (1.84 ± 0.05% ID/g at 2 hr p.i.) or in progesterone receptor‐negative MDA‐MB‐231 tumor (1.80 ± 0.03% ID/g at 2 hr p.i.). Based on the good stability and specificity of [¹⁸F]EAEF, it may be a good candidate for imaging progesterone receptor and worth further investigation.     

Initial experience in synthesis of (2S,4R)‐4‐[18F]fluoroglutamine for clinical application

We report initial experience in synthesis of (2S,4R)‐4‐[¹⁸F]fluoroglutamine, [¹⁸F]FGln, which has been used as a tool for monitoring glutamine metabolism in cancer patients. [¹⁸F]FGln was prepared by a fully automated PET‐MF‐2V‐IT‐I synthesizer under GMP‐compliant conditions for routine clinical studies. The total radiosynthesis time was about 65 minutes, the decay‐corrected radiochemical yield was 18.0 ± 4.2% (n = 59; failure n = 15), and the radiochemical purity was greater than 90%. In some situations, the yields were low (less than 5%), and the most likely cause of this problem is the initial fluorination step; the fluoride ion might not have been fully activated. In other occasions, low final radiochemical purity was often associated with the failure of the second step—removal of protection groups by anhydrous trifluoroacetic acid. A trace amount of water led to production of undesired 4‐[¹⁸F]fluoroglutamic acid. Knowledge learned from the successes and failures of synthesis may be helpful to identify critical steps and pitfalls for preparation of this clinically useful metabolic probe, [¹⁸F]FGln, for imaging glutamine utilization in tumor of cancer patients.     

Synthesis and evaluation of novel 18F‐labeled quinazoline derivatives with low lipophilicity for tumor PET imaging

Four novel ¹⁸F‐labeled quinazoline derivatives with low lipophilicity, [¹⁸F]4‐(2‐fluoroethoxy)‐6,7‐dimethoxyquinazoline ( [ ¹⁸ F]I ), [¹⁸F]4‐(3‐((4‐(2‐fluoroethoxy)‐7‐methoxyquinazolin‐6‐yl)oxy)propyl)morpholine ( [ ¹⁸ F]II ), [¹⁸F]4‐(2‐fluoroethoxy)‐7‐methoxy‐6‐(2‐methoxyethoxy)quinazoline ( [ ¹⁸ F]III ), and [¹⁸F]4‐(2‐fluoroethoxy)‐6,7‐bis(2‐methoxyethoxy)quinazoline ( [ ¹⁸ F]IV ), were synthesized via a 2‐step radiosynthesis procedure with an overall radiochemical yield of 10% to 38% (without decay correction) and radiochemical purities of >98%. The lipophilicity and stability of labeled compounds were tested in vitro. The log P values of the 4 radiotracers ranged from 0.52 to 1.07. We then performed ELISA to measure their affinities to EGFR‐TK; ELISA assay results indicated that each inhibitor was specifically bounded to EGFR‐TK in a dose‐dependent manner. The EGFR‐TK autophosphorylation IC₅₀ values of [ ¹⁸ F]I , [ ¹⁸ F]II , [ ¹⁸ F]III , and [ ¹⁸ F]IV were 7.732, 0.4698, 0.1174, and 0.1176 μM, respectively. All labeled compounds were evaluated via cellular uptake and blocking studies in HepG2 cell lines in vitro. Cellular uptake and blocking experiment results indicated that [ ¹⁸ F]I and [ ¹⁸ F]III had excellent cellular uptake at 120‐minute postinjection in HepG2 carcinoma cells (51.80 ± 3.42%ID/mg protein and 27.31 ± 1.94%ID/mg protein, respectively). Additionally, biodistribution experiments in S180 tumor‐bearing mice in vivo indicated that [ ¹⁸ F]I had a very fast clearance in blood and a relatively high uptake ratio of tumor to blood (4.76) and tumor to muscle (1.82) at 60‐minute postinjection. [ ¹⁸ F]III had a quick clearance in plasma, and its highest uptake ratio of tumor to muscle was 2.55 at 15‐minute postinjection. These experimental results and experiences were valuable for the further exploration of novel radiotracers of quinazoline derivatives.     

Initial evaluation of 99mTc-labeled anti-carcinoembryonic antigen single-chain fragment variable for micro-single-photon emission computed tomography imaging in mice with colorectal cancer

Carcinoembryonic antigen (CEA) has emerged as an important molecular target for several neoplastic diseases, including colorectal cancer with CEA over-expression. In this study, we report the production and radiolabeling of a novel anti-CEA single-chain fragment variable (scFv-96NRT, concentration for 50% of maximal effect 46 ng/ml), and evaluation of [^99mTc]Tc-scFv-96NRT in non-invasive detection of CEA expression. [^99mTc]Tc-scFv-96NRT was synthesized by one step reduction in labeling yield of >95% with radiochemical purity of >98% and molar activity of 10–11 GBq/μmol. [^99mTc]Tc-scFv-96NRT showed high stability in 0.01 M phosphate-buffered saline (PBS) and 5% human serum albumin (HSA). It exhibited elevated uptake in CEA over-expressing cells. Biodistribution studies in BALB/c mice revealed that the probe was cleared from blood rapidly, and the highest retention was observed in the kidneys. The micro-single-photon emission computed tomography (micro-SPECT) imaging of [^99mTc]Tc-scFv-96NRT showed a specific accumulation pattern, as blocking experiment with excess scFv-96NRT suppressed the tumor uptake. These preliminary results suggest that [^99mTc]Tc-scFv-96NRT is a potential non-invasive molecular probe for imaging tumors with CEA over-expression.     

Discovery and development of brain-penetrant 18F-labeled radioligands for neuroimaging of the sigma-2 receptors

We have discovered and synthesized a series of indole-based derivatives as novel sigma-2 (σ₂) receptor ligands. Two ligands with high σ₂ receptor affinity and subtype selectivity were then radiolabeled with F-18 in good radiochemical yields and purities, and evaluated in rodents. In biodistribution studies in male ICR mice, radioligand [¹⁸F]9, or 1-(4-(5,6-dimethoxyisoindolin-2-yl)butyl)-4-(2-[¹⁸F]fluoroethoxy)-1H-indole, was found to display high brain uptake and high brain-to-blood ratio. Pretreatment of animals with the selective σ₂ receptor ligand CM398 led to significant reductions in both brain uptake (29%–54%) and brain-to-blood ratio (60%–88%) of the radioligand in a dose-dependent manner, indicating high and saturable specific binding of [¹⁸F]9 to σ₂ receptors in the brain. Further, ex vivo autoradiography in male ICR mice demonstrated regionally heterogeneous specific binding of [¹⁸F]9 in the brain that is consistent with the distribution pattern of σ₂ receptors. Dynamic positron emission tomography imaging confirmed regionally distinct distribution and high levels of specific binding for [¹⁸F]9 in the rat brain, along with appropriate tissue kinetics. Taken together, results from our current study indicated the novel radioligand [¹⁸F]9 as the first highly specific and promising imaging agent for σ₂ receptors in the brain.     

Synthesis and biological evaluation of a radioiodinated spiropiperidine ligand as a potential σ1 receptor imaging agent

We report the synthesis and evaluation of 1′‐(4‐[¹²⁵I]iodobenzyl)‐3H‐spiro[isobenzofuran‐1,4′‐piperidine] ([¹²⁵I]Spiro‐I) as a potential SPECT tracer for imaging of σ₁ receptors. [¹²⁵I]Spiro‐I was prepared in 55–65% isolated radiochemical yield, with radiochemical purity of >99%, via iododestannylation of the corresponding tributyltin precursor. In receptor binding studies, Spiro‐I displayed low nanomolar affinity for σ₁ receptors (σ₁: K_i=2.75±0.12 nM; σ₂: K_i=340 nM) and high subtype selectivity (σ₂/σ₁=124). Biodistribution in mice demonstrated relatively high concentration of radioactivity in organs known to contain σ₁ receptors, including the lung, kidney, heart, spleen, and brain. Administration of haloperidol 5 min prior to injection of [¹²⁵I]Spiro‐I significantly reduced the concentration of radioactivity in the above‐mentioned organs. These findings suggest that the binding of [¹²⁵I]Spiro‐I to σ₁ receptors in vivo is specific. Copyright © 2010 John Wiley & Sons, Ltd.     

Development and radiosynthesis of the first 18F‐labeled inhibitor of monocarboxylate transporters (MCTs)

Monocarboxylate transporters 1 and 4 (MCT1 and MCT4) are involved in tumor development and progression. Their expression levels are related to clinical disease prognosis. Accordingly, both MCTs are promising drug targets for treatment of a variety of human cancers. The noninvasive imaging of these MCTs in cancers is regarded to be advantageous for assessing MCT‐mediated effects on chemotherapy and radiosensitization using specific MCT inhibitors. Herein, we describe a method for the radiosynthesis of [¹⁸F] FACH ((E)‐2‐cyano‐3‐{4‐[(3‐[¹⁸F]fluoropropyl)(propyl)amino]‐2‐methoxyphenyl}acrylic acid), as a novel radiolabeled MCT1/4 inhibitor for imaging with PET. A fluorinated analog of α‐cyano‐4‐hydroxycinnamic acid ( FACH ) was synthesized, and the inhibition of MCT1 and MCT4 was measured via an L‐[¹⁴C]lactate uptake assay. Radiolabeling was performed by a two‐step protocol comprising the radiosynthesis of the intermediate (E)/(Z)‐[¹⁸F] tert‐Bu‐FACH (tert‐butyl (E)/(Z)‐2‐cyano‐3‐{4‐[(3‐[¹⁸F]fluoropropyl)(propyl)amino]‐2‐methoxyphenyl}acrylate) followed by deprotection of the tert‐butyl group. The radiofluorination was successfully implemented using either K[¹⁸F]F‐K_2.2.2‐carbonate or [¹⁸F]TBAF. The final deprotected product [¹⁸F] FACH was only obtained when [¹⁸F] tert‐Bu‐FACH was formed by the latter procedure. After optimization of the deprotection reaction, [¹⁸F] FACH was obtained in high radiochemical yields (39.6 ± 8.3%, end of bombardment (EOB) and radiochemical purity (greater than 98%).     

Synthesis and cellular uptake of p‐[123I]‐phenyl‐amino‐thiazole (123I‐PAT) as a potential agent for targeting tubulin polymerization in tumors

The phenyl‐amino‐thiazole (PAT) templates of methoxylbenzoyl‐aryl‐thiazole are potent agents against cancer by inhibiting tubulin polymerization in the nanomolar range. Herein, a radioiodinated PAT, [¹²³I]‐PAT 1, was prepared via a tributylstannyl precursor and [¹²³I]iodide through electrophilic aromatic radioiodination. Radiolabelling of [¹²³I]‐PAT 1 was achieved in less than 15 min, with a radiochemical purity of over 99%. The accumulated radioactivity in tumor cellular uptake experiments suggested that [¹²³I]‐PAT could serve as a potential radioprobe for targeting tumor cells.     

Development and Preliminary Evaluation of TFIB,a New Bimodal Prosthetic Group for Bioactive Molecule Labeling

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Synthesis of 3-[125I]-iodo-phencyclidine for biological studies

After verification of the biological activity of 3-iodo-PCP by binding studies to the N-methyl D-aspartate gated channel, 3-[¹²⁵I]iodo-PCP was prepared by reaction of sodium iodide with a triazene precursor. The radiochemical yield was optimized and after HPLC purification, 3-[¹²⁵I]iodo-PCP was obtained with a high radiochemical purity.     

Synthesis and biological evaluation of S‐[11C]methylated mercaptoimidazole piperazinyl derivatives as potential radioligands for imaging 5‐HT1A receptors by positron emission tomography (PET)

The novel 2‐mercaptoimidazole derivatives, 1‐[4‐((2‐methoxyphenyl)‐1‐piperazinyl)butyl]‐2‐mercaptoimidazole ( 3 ) and methyl[4‐((2‐methoxyphenyl)‐1‐piperazinyl))butyl] (2‐mercapto‐1‐methylimidazol‐5‐yl)methanamide ( 8 ), were efficiently labelled with ¹¹C through methylation of the thioketone function with [¹¹C]methyl iodide. The resulting radioligands 1‐[4‐((2‐methoxyphenyl)‐1‐piperazinyl))butyl]‐2‐thio[¹¹C]methylimidazole ([¹¹C] 9 ) and methyl[4‐((2‐methoxyphenyl)‐1‐piperazinyl))butyl] (2‐thio[¹¹C]methyl‐1‐methylimidazol‐5‐yl)‐methanamide ([¹¹C] 10 ) were synthesized in radiochemical yields of 20–30% (decay‐corrected, related to [¹¹C]CO₂) at a specific radioactivity of 0.2–0.4 Ci/µmol within 40–45 min including HPLC‐purification. The radiochemical purity exceeded 99%. The reference compounds 9 and 10 were tested in a competitive receptor binding assay to determine their affinity toward the 5‐HT_1A receptor. Both compounds exhibit excellent sub‐nanomolar affinities (IC₅₀=0.576±0.008 nM ( 9 ); IC₅₀=0.86±0.02 nM ( 10 )) for the 5‐HT_1A receptor while displaying a high selectivity towards the 5‐HT_2A subtype of receptors (IC₅₀>480 nM). By contrast, compound 9 also shows substantial binding for the alpha1‐adrenergic receptor (IC₅₀=3.00±0.02 nM) when compared with compound 10 (IC₅₀=54.5±0.6 nM). Preliminary biodistribution studies in rats showed an initial brain uptake of 1.14±0.11 and 0.37±0.04% ID/g after 5 min, which decreased to 0.18±0.04 and 0.16±0.01% ID/g after 60 min for compounds [¹¹C] 9 and [¹¹C] 10 , respectively. For both compounds, the cerebellum and rest of the brain uptake are very similar at the different time points. Unlike [¹¹C] 9 , the radioligand [¹¹C] 10 has significant uptake and retention in the adrenal glands. Due to their washout from the brain compounds [¹¹C] 9 and [¹¹C] 10 seem not to be good candidates as radioligands for imaging 5‐HT_1A receptors by PET. Copyright © 2005 John Wiley & Sons, Ltd.     

An improved kit formulation for one‐pot synthesis of [99mTc]Tc‐HYNIC‐E[c(RGDfK)]2 for routine clinical use in cancer imaging

Radiolabeled Arg‐Gly‐Asp (RGD) peptide derivatives have immense potential for non‐invasive monitoring of malignancies overexpressing integrin α_vβ₃ receptors. Easy availability of suitable radiotracers would augment the utility of this class of molecular imaging agents. Towards this, the present article describes the development of an improved lyophilized kit for the routine clinical formulation of [^99mTc]Tc complex of HYNIC‐conjugated dimeric cyclic RGD peptide derivative E‐[c(RGDfK)]₂ (E = glutamic acid, f = phenyl alanine, K = lysine) without using Sn²⁺ and systematic evaluation of its efficacy. Five batches of the kits were prepared, and [^99mTc]Tc‐HYNIC‐E[c(RGDfK)]₂ radiotracer was synthesized with high radiochemical purity (98.6 ± 0.5%) and specific activity (124.8 GBq/μmol) using the kits. Biodistribution studies in C57BL/6 mice bearing melanoma tumor exhibited significant accumulation of the radiotracer in tumor (5.32 ± 0.56 %ID/g at 60 min p.i.), and this uptake was also found to be receptor‐specific by blocking studies. Preliminary human clinical investigations carried out in 10 breast cancer patients revealed high radiotracer uptake in the tumor along with good tumor‐to‐background contrast. The developed kit formulation showed an exceptionally high shelf‐life of at least 18 months. These results demonstrated promising attributes of the developed kit formulation and warrant more extensive clinical investigations.     

Development of 6‐[18F]fluoro‐carbohydrate‐based prosthetic groups and their conjugation to peptides via click chemistry

This work describes the development of new 6‐[¹⁸F]fluoro‐carbohydrate‐based prosthetic groups equipped with an azido arm that are able to participate in copper(I)‐catalyzed cycloadditions for ¹⁸F labeling of biomolecules under mild conditions. The radiolabeling in high radiochemical yields (up to 68 ± 6%) of these different prosthetic groups is presented. The flexibility of the azido arm introduced on the carbohydrate moieties allows efficient click reactions with different alkyne functionalized peptides such as gluthation or Arg‐Gly‐Asp derivatives in order to prepare glycopeptides. The radiosyntheses of ¹⁸F‐labeled glycopeptides proceed in high radiochemical yields (up to 76%) in an automated process with excellent radiochemical purity. The addition of a sugar moiety on peptides should enhance the bioavailability, pharmacokinetic, and in vivo clearance properties of these glycopeptides, compared with the unlabeled native peptide, and these properties are highly favorable for positron emission tomography imaging. A high uptake of ¹⁸F‐β‐gluco‐c(RGDfC) is shown by positron emission tomography imaging in a subcutaneous abscess model in the rat, revealing the potential of this tracer to monitor integrin expression as a part of inflammation and/or angiogenesis processes.