Preparation and biological evaluation of 99mTc-CO-MIBI as myocardial perfusion imaging agent

Preparation, biological evaluation and radiolabeling of [99mTc]-technetium tricarbonyl procainamide as a tracer for heart imaging in mice

This study focuses on the synthesis and preliminary bio-evaluation of [99mTc]-technetium tricarbonyl procainamide ([99mTc]-technetium tricarbony PA) as a viable cardiac imaging agent. The compound, [99mTc]-technetium tricarbony PA, was synthesized by labelling procainamide with a [99mTc]-technetium tricarbonyl core, yielding a high radiochemical yield and radiochemical purity of 98%. Under optimal circumstances, high radiochemical yield and purity were obtained utilizing [99mTc]-technetium tricarbonyl core within 30 min of incubation at pH 9, 200 µg substrate concentration, and 100 °C reaction temperature. The heart showed a high absorption of 32.39 ± 0.88% of the injected dose/g organ (ID/g), confirming the suitability of [99mTc]-technetium tricarbonyl PA as a viable complex for heart imaging.

Synthesis and Biological Evaluation of Novel 99mTc-Labeled Palbociclib Derivatives Targeting Cyclin-Dependent Kinase 4/6 (CDK4/6) as Potential Cancer Imaging Agents

Cancer results from cell proliferation that exceeds normal growth control. There are various specific proteins that control and regulate the cell cycle, such as cyclin-dependent kinases (CDKs), cyclins, and retinoblastoma protein (pRb). The aberration of the cyclin D-CDK4/6-INK4-pRb pathway occurs frequently in cancers; thus, CDK4/6 is an attractive target for the development of radiopharmaceuticals for tumor imaging. In this study, we chose palbociclib, which was approved by the FDA for treating ER+/HER2- advanced breast cancer as the target vector and the isonitrile group, which can coordinate strongly with the [^99mTc(CO)₃]⁺ core as the bifunctional chelator, to develop four novel ^99mTc-labeled radiotracers for tumor imaging. The ligands (L2, L3, L4, and L5) were synthesized by reacting palbociclib with isocyanide-containing active esters and then radiolabeling with a [^99mTc(CO)₃]⁺ core to produce radiotracers (^99mTc-L2, ^99mTc-L3, ^99mTc-L4, and ^99mTc-L5) with high radiochemical purity (>95%) and good stability in vitro. The structures of the ^99mTc complexes were identified by preparation and characterization of the corresponding stable rhenium complexes. Partition coefficient results indicated that these complexes were lipophilic. A kinase inhibition assay demonstrated the high affinity of the stable Re complexes for CDK4. A cell study showed that all four complexes had substantial uptake by MCF-7 cells and could be significantly inhibited by palbociclib and nonradiolabeled ligand, indicating a CDK4/6-specific uptake mechanism. Biodistribution studies in nude mice bearing MCF-7 tumors showed that the complexes had obvious accumulation in tumors at 2 h postinjection. ^99mTc-L2 exhibited the highest tumor uptake and tumor/blood ratio, whereas ^99mTc-L4 showed the highest tumor/muscle ratio. The micro-SPECT/CT study showed that complex ^99mTc-L4 had visible uptake at the tumor site, and the accumulation was clearly reduced in the image after pretreatment with palbociclib, further indicating CDK4/6 specificity. All the results showed that the ^99mTc-labeled complexes in this work have the potential for tumor imaging.

The synthesis of a 99mTc-labeled tetravalent targeting probe upon isonitrile coordination to 99mTcI for enhanced target uptake in saturable systems

The presence of excess unlabeled ligands in the injectate hinders the target uptake of 99mTc-labeled targeting vectors. To address the issue, we previously developed a chemical design which provides a 99mTc-labeled trivalent RGD probe upon CN-βAla-Gly-Gly-c(RGDfK) (Lβ) coordination to [99mTc][Tc(CO)3]+ core at pH 6.0. In this study, we extended our coordination mediated synthesis of the trivalent RGD probe to that of a tetravalent one. Our initial attempts reacting Lβ with [99mTc][Tc(CO)3]+ core at pH 8.0 failed to provide [99mTc][Tc(CO)2(Lβ)4]+ due to the formation of multiple side products. A γ-aminobutylic acid (GABA) based isonitrile ligand CN-GABA-Gly-Gly-c(RGDfK) (LG), on the other hand, avoided the side reaction and selectively provided [99mTc][Tc(CO)2(LG)4]+ (99mTc-[LG]4) at pH 8.0. 99mTc-[LG]4 exhibited higher binding affinity to integrin αvβ3 than its unlabeled ligand, and visualized U87MG tumor without tedious post-labeling purification. These results indicate that the metal coordination-mediated syntheses of 99mTc-labeled multivalent probes have been successfully applied to a tetravalent one, which would allow a wider range of choices for designing novel 99mTc-labeled multivalent probes of high in vivo target uptake.

Novel 99mTc labelled complexes with 2-nitroimidazole isocyanide: design, synthesis and evaluation as potential tumor hypoxia imaging agents

Radiolabelled 2-nitroimidazoles have been used for imaging hypoxia. With the aim of developing novel 99mTc radiotracers for imaging hypoxia, four novel 2-nitroimidazole isocyanide derivatives (2a, 2b, 2c, and 2d) were synthesized and radiolabelling was carried out for preparing their corresponding 99mTc complexes. These 99mTc complexes were stable in vitro and could exhibit good hypoxic selectivity. The partition coefficient results indicated that they were hydrophilic, and an evaluation of biodistribution in mice bearing S180 tumors indicated that all of the complexes could accumulate in the tumors. Among them, 99mTc-2c exhibited the highest tumor uptake and tumor/blood and tumor/muscle ratios at 2 h post-injection. Further, single photon emission computed tomography (SPECT) imaging studies indicated clear accumulation in tumors, suggesting that 99mTc-2c was a promising candidate for hypoxia imaging.

Novel 99mTc-labelled complexes with thymidine isocyanide: radiosynthesis and evaluation as potential tumor imaging tracers

A novel thymidine isocyanide (CN-TdR) functionalized at the N3 position of thymidine was synthesized and then radiolabelled with 99mTc(i) and [99mTc(i)(CO)3]+ cores to produce [99mTc(CN-TdR)6]+ and [99mTc(CO)3(CN-TdR)3]+, respectively. Both of them were prepared with high radiochemical purity and were stable over 6 h in saline at ambient temperature and in serum at 37 °C. The partition coefficient results demonstrated that they were hydrophilic. The cell internalization studies showed that their uptake might be mediated by nucleoside transporters. Biodistribution of these complexes in mice bearing the S180 tumor showed that they accumulated in the tumor with high uptake and cleared rapidly from blood and muscles, producing high tumor/blood and tumor/muscle ratios. Between them, [99mTc(CN-TdR)6]+ exhibited advantages concerning a higher tumor uptake, tumor/blood ratio and tumor/muscle ratio at 60 min post-injection. Single photon emission computed tomography imaging studies showed that there was a clear accumulation in tumor sites, suggesting that [99mTc(CN-TdR)6]+ could be a promising candidate for tumor imaging.

Preparation, chromatographic evaluation and biodistribution of 99mTc-procainamide as a radiopharmaceutical for heart imaging

Procainamide (4-amino-N-[2-(diethylamino) ethyl] benzamide) is a sodium channel blocker, which acts as an effective antiarrhythmic agent used in the treatment of a variety of atrial and ventricular arrhythmias. The aim of this study was to prepare 99mTc-procainamide complex, apply different chromatographic techniques for the assay of radiolabeling yield and study its biodistribution as a novel radiopharmaceutical for heart imaging. 99mTc-procainamide was obtained with a maximum labeling yield of 95.76±0.20% via direct labeling method under optimum conditions of 200 μg of procainamide, 300 μL of buffer (carbonate) at pH 11, 30 μg SnCl2·2H2O at room temperature (25°C) for 15 min. In terms of in vitro stability, the complex was stable for 3 h. Chromatographic evaluation using paper chromatography, thin layer chromatography, gel chromatography, and high performance liquid chromatography showed reliable results for measuring the radiochemical yield. Biodistribution study of 99mTc-procainamide showed ratios of heart/lung and heart/liver (6.38±1.50, 2.06±0.31, respectively at 30 min post injection) which was comparable to that of 99mTc-sestamibi (7.4±2.00, 0.97±0.10, respectively at 60 min, P<0.05).

Purification-Free Method for Preparing Technetium-99m-Labeled Multivalent Probes for Enhanced in Vivo Imaging of Saturable Systems

Metallic radionuclides provide target-specific radiolabeled probes for molecular imaging in radiochemical yields sufficient for administration to subjects without purification. However, unlabeled ligands in the injectate can compete for targeted molecules with radiolabeled probes, which eventually necessitates postlabeling purification. Herein we describe a "1 to 3" design to circumvent the issue by taking advantage of inherent coordination properties of technetium-99m ((99m)Tc). A monovalent RGD ligand possessing an isonitrile as a coordinating moiety (CN-RGD) was reacted with [(99m)Tc(CO)3(OH2)3](+) to prepare [(99m)Tc(CO)3(CN-RGD)3](+) in over 95% radiochemical yields. This complex exhibited higher integrin αvβ3 binding affinity than its unlabeled monovalent ligand, primarily due to its multivalency. This compound visualized a murine tumor without removing unlabeled ligands, while a (99m)Tc-labeled monovalent probe derived from a monovalent ligand could not. The metal coordination-mediated synthesis of radiolabeled multivalent probes thereby can be a useful approach for preparing ready-to-use target-specific probes labeled with (99m)Tc and other metallic radionuclides of interest.