Development and characterization of a 99m Tc-tricarbonyl-labelled estradiol derivative obtained by "Click Chemistry" with potential application in estrogen receptors imaging

Click Chemistry and Radiochemistry: An Update

The term "click chemistry" describes a class of organic transformations that were developed to make chemical synthesis simpler and easier, in essence allowing chemists to combine molecular subunits as if they were puzzle pieces. Over the last 25 years, the click chemistry toolbox has swelled from the canonical copper-catalyzed azide-alkyne cycloaddition to encompass an array of ligations, including bioorthogonal variants, such as the strain-promoted azide-alkyne cycloaddition and the inverse electron-demand Diels-Alder reaction. Without question, the rise of click chemistry has impacted all areas of chemical and biological science. Yet the unique traits of radiopharmaceutical chemistry have made it particularly fertile ground for this technology. In this update, we seek to provide a comprehensive guide to recent developments at the intersection of click chemistry and radiopharmaceutical chemistry and to illuminate several exciting trends in the field, including the use of emergent click transformations in radiosynthesis, the clinical translation of novel probes synthesized using click chemistry, and the advent of click-based in vivo pretargeting.

Copper-free click bioconjugation of technetium-99m complexes using strained cyclononyne derivatives

Click chemistry, in particular copper-free click reactions, has gained growing interest for radiolabelling purposes in the field of radiopharmaceutical sciences. [^99mTc][Tc(CO)₃(H₂O)₃]⁺ works as an excellent starting complex for the radiolabelling of biomolecules under mild conditions. A new chelator, investigated for the copper-free strain-promoted cycloaddition (SPAAC), was synthesised containing the 2,2'-dipicolylamine (DPA) moiety for the ^99mTc-tricarbonyl core and compared with a DPA chelator based on activated esters for conventional radiolabelling. For the copper-free click labelling procedure, a DPA containing 4,8-diazacyclononyne moiety was prepared from a sulfonyl-modified diamide (four steps, 64% yield) followed by the Nicholas reaction with butyne-1,3-diol. The ^99mTc-DPA-DACN-complex was prepared with a radiochemical conversion (RCC) of 89% after 30 min. The following SPAAC reaction with an azide-functionalised PSMA molecule was performed within 4-5 hours at 100 °C to obtain the PSMA (prostate-specific membrane antigen) targeting ^99mTc-complex with 79% RCC and without side products. For comparison, a second DPA-chelator based on a tetrafluorophenyl (TFP) ester was prepared (three steps, 64% yield) and was successfully radiolabelled with [[^99mTc]Tc(CO)₃(H₂O)₃]⁺ with 89% RCC after 20 min and >99% radiochemical purity after separation using an RP18 cartridge. The subsequent conjugation of an amine-functionalised PSMA targeting molecule was performed with 23% RCC after 150 min. Two other unknown side products were observed indicating the decomposition of the TFP ester during the labelling. All nonradioactive Re(CO)₃ complexes were synthesised from (Et₄N)₂[ReBr₃(CO)₃] (91% yield for the ^natRe-DPA-TFP ester, 76% yield for ^natRe-DPA-DACN) and characterised to confirm the identity of the ^99mTc-complexes.

Development and Evaluation of 99mTc Tricarbonyl Complexes Derived from Flutamide with Affinity for Androgen Receptor

With the objective to develop a potential ^99mTc radiopharmaceutical for imaging the androgen receptor (AR) in prostate cancer, four ligands bearing the same pharmacophore derived from the AR antagonist flutamide were prepared, labeled with ^99mTc, and their structures corroborated via comparison with the corresponding stable rhenium analogs. All complexes were obtained with high radiochemical purity. Three of the complexes were highly stable, and, due to their favorable physicochemical properties, were further evaluated using AR-positive and AR-negative cells in culture. All complexes exhibited considerable uptake in AR-positive cells, which could be blocked by an excess of flutamide. The efflux from the cells was moderate. They also showed significantly lower uptakes in AR-negative cells, indicating interactions with the AR receptor. However, the binding affinities were considerably reduced by the coordination to ^99mTc, and the complex that exhibited the best biological behavior did not show sufficient specificity towards AR-positive cells.

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.

Recent advances in the synthesis of (99mTechnetium) based radio-pharmaceuticals

Technetium radionuclide (99mTc) has excellent extent of disintegration properties and occupies a special place in the field of nuclear medicinal chemistry and other health disciplines. Current review describes recent approaches of synthesis in detailed ways for radio-pharmaceuticals of technetium which have been developed to treat and diagnose the biotic disorders. These technetium labeled radio-pharmaceuticals have been established to apply in the field of diagnostic nuclear medicine especially for imaging of different body parts such as brain, heart, kidney, bones and so on, through single photon emission computed tomography (SPECT) that is thought to be difficult to image such organs by using common X-ray and MRI (Magnetic Resonance Imaging) techniques. This review highlights and accounts an inclusive study on the various synthetic routes of technetium labeled radio-pharmaceuticals using ligands with various donor atoms such as carbon, nitrogen, sulphur, phosphorus etc. These compounds can be utilized as next generation radio-pharmaceuticals.

Development and evaluation of a 99mTc(V)-nitrido complex derived from estradiol for breast cancer imaging

Estrogen receptors are overexpressed in 70% of breast cancer and identification of their presence is important to select the appropriate treatment. This work proposes the preparation and evaluation of an estradiol derived as potential ER imaging agent. Ethinylestradiol was derivatized to introduce a dithiocarbamate function for Tc coordination. Labeling was achieved through the formation of a symmetric Tc(V)-nitrido complex with a radiochemical purity (RCP) > 95%. Physicochemical evaluation, cell uptake, biodistribution in normal animals and in nude mice bearing induced ER + breast tumors showed promising results.