Development of 99mTc-Labeled Human Serum Albumin with Prolonged Circulation by Chelate-then-Click Approach: A Potential Blood Pool Imaging Agent

Protein click chemistry and its potential for medical applications

A revolution in chemical biology occurred with the introduction of click chemistry. Click chemistry plays an important role in protein chemistry modifications, providing specific, sensitive, rapid, and easy-to-handle methods. Under physiological conditions, click chemistry often overlaps with bioorthogonal chemistry, defined as reactions that occur rapidly and selectively without interfering with biological processes. Click chemistry is used for the posttranslational modification of proteins based on covalent bond formations. With the contribution of click reactions, selective modification of proteins would be developed, representing an alternative to other technologies in preparing new proteins or enzymes for studying specific protein functions in different biological processes. Click-modified proteins have potential in diverse applications such as imaging, labeling, sensing, drug design, and enzyme technology. Due to the promising role of proteins in disease diagnosis and therapy, this review aims to highlight the growing applications of click strategies in protein chemistry over the last two decades, with a special emphasis on medicinal applications.

Arginine-Selective Bioconjugation Reagent for Effective 18F-labeling of Native Proteins

Protein-based 18F-PET tracers offer new possibilities in early disease detection and personalized medicine. Their development relies heavily on the availability and effectiveness of 18F-prosthetic groups. We prepared and evaluated a novel arginine-selective prosthetic group, 4-[18F]fluorophenylglyoxal ([18F]FPG). [18F]FPG was radiosynthesized by a one-pot, two-step procedure with a non-decay-corrected (n.d.c.) isolated radiochemical yield (RCY) of 41 ± 8% (n = 10). [18F]FPG constitutes a generic tool for 18F-labeling of various proteins, including human serum albumin (HSA), ubiquitin, interleukin-2, and interleukin-4 in ∼30-60% n.d.c. isolated RCYs. [18F]FPG conjugation with arginine residues is highly selective, even in the presence of a large excess of lysine, cysteine, and histidine. [18F]FPG protein conjugates are able to preserve the binding affinity of the native proteins while also demonstrating excellent in vivo stability. The [18F]FPG-HSA conjugate has prolonged blood retention, which can be applied as a potential blood pool PET imaging agent. Thus, [18F]FPG is an arginine-selective bioconjugation reagent that can be effectively used for the development of 18F-labeled protein radiopharmaceuticals.

Development of finely tuned liposome nanoplatform for macrophage depletion

BACKGROUND

Immunotherapy with clodronate-encapsulated liposomes, which induce macrophage depletion, has been studied extensively. However, previously reported liposomal formulation-based drugs (Clodrosome® and m-Clodrosome®) are limited by their inconsistent size and therapeutic efficacy. Thus, we aimed to achieve consistent therapeutic effects by effectively depleting macrophages with uniform-sized liposomes.

RESULTS

We developed four types of click chemistry-based liposome nanoplatforms that were uniformly sized and encapsulated with clodronate, for effective macrophage depletion, followed by conjugation with Man-N3 and radiolabeling. Functionalization with Man-N3 improves the specific targeting of M2 macrophages, and radioisotope labeling enables in vivo imaging of the liposome nanoplatforms. The functionalized liposome nanoplatforms are stable under physiological conditions. The difference in the biodistribution of the four liposome nanoplatforms in vivo were recorded using positron emission tomography imaging. Among the four platforms, the clodronate-encapsulated mannosylated liposome effectively depleted M2 macrophages in the normal liver and tumor microenvironment ex vivo compared to that by Clodrosome® and m-Clodrosome®.

CONCLUSION

The newly-developed liposome nanoplatform, with finely tuned size control, high in vivo stability, and excellent ex vivo M2 macrophage targeting and depletion effects, is a promising macrophage-depleting agent.

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.

68Ga-Labeled Maleimide for Blood Pool and Lymph PET Imaging through Covalent Bonding to Serum Albumin In Vivo

This study aims to develop a novel ⁶⁸Ga-labeled tracer, which can covalently bind to albumin in vivo based on the maleimide-thiol strategy, and to evaluate its potential applications using positron emission tomography (PET). ⁶⁸Ga-labeled maleimide-monoamide-DOTA (denoted as [⁶⁸Ga]Ga-DM) was prepared conveniently with a high radiochemical yield (>90%) and radiochemical purity (>99%). Its molar activity was calculated as 249.60 ± 68.50 GBq/μmol, and the octanol-water partition coefficient (LogP) was -3.15 ± 0.08 with good stabilities. In vitro experiments showed that [⁶⁸Ga]Ga-DM can bind to albumin efficiently and rapidly, with a binding fraction of over 70%. High uptake and excellent retention in blood were observed with a long half-life (t _1/2Z) of 190.15 ± 24.14 min, which makes it possible for blood pool PET imaging with high contrast. The transient micro-bleeding in the rat model was detected successfully with PET imaging. In addition, the uptakes of [⁶⁸Ga]Ga-DM in the inflammatory popliteal lymph nodes depend on the severity (5.90% ID/g and 2.32% ID/g vs 1.01% ID/g for healthy lymph nodes at 0.5 h post-injection) indicating its feasibility for lymphatic imaging. In conclusion, a novel ⁶⁸Ga-labeled tracer was prepared with high efficiency and yield in mild conditions. Based on the promising properties of bonding covalently to albumin, great stability, high blood contrast with a long half-life, and well environmental tolerance, [⁶⁸Ga]Ga-DM could be developed as a potential tracer for PET imaging of blood pool, bleeding, and vascular permeability alteration diseases in the clinic.

99mTc-labeled peptide targeting interleukin 13 receptor α 2 for tumor imaging in a cervical cancer mouse model

OBJECTIVE

Pep-1 (CGEMGWVRC) can potently bind to interleukin 13 receptor α 2 (IL-13Rα2), a tumor-restricted receptor found to be expressed in various malignancies. In this study, we intended to prepare a ^99mTc-labeled probe and evaluate its in vivo tumor accumulation properties in a cervical cancer xenograft model.

METHODS

The Pep-1 was designed and radiolabeled with ^99mTc by conjugation with mercaptoacetyl-triglycine (MAG₃). The labeling yield, radiochemical purity and stability were characterized in vitro. Cell uptake assays and fluorescence imaging were conducted for qualitative and quantitative evaluation of the specificity and affinity of Pep-1. Flow cytometry and tissue immunofluorescence were used to confirm the IL-13Rα2 expression in cervical cancer. Biodistribution and in vivo imaging were performed periodically to evaluate the imaging value of ^99mTc-MAG₃-Pep-1 in cervical cancer xenograft model.

RESULTS

^99mTc-MAG₃-Pep-1 was successfully prepared with a high labeling yield and radiochemical purity (> 95%). Specific cell uptake was demonstrated by scramble control and unlabeled MAG₃-Pep-1 blockade. Flow cytometry and tissue immunofluorescence also confirmed the mild IL-13Rα2 expression of HeLa. In the gamma imaging study and biodistribution, the tumors were imaged clearly at 2-6 h after injection of ^99mTc-MAG₃-Pep-1 and the accumulation of ^99mTc-MAG₃-Pep-1 in tumor was significantly higher than that in the blocking and scramble controls, demonstrating ligand-receptor binding specificity.

CONCLUSIONS

This work demonstrated that ^99mTc-MAG₃-Pep-1 can bind to cervical cancer with high affinity and specificity. MAG₃-Pep-1 may be a prospective precursor for IL-13Rα2-expressing cancer therapy.

Feasibility of using a 99mTc-hydroxamamide complex containing an albumin binder moiety for in vivo albumin labeling-based tumor imaging

Human serum albumin (HSA), which is distributed throughout the blood, is used as a carrier for transporting drugs to tumors based on the enhanced permeability and retention (EPR) effect. To develop an agent for the in vivo radiolabeling of endogenous albumin, we designed and synthesized novel hydroxamamide (Ham)-based technetium-99m (^99mTc) complexes, which contained a monovalent or bivalent 4-(4-iodophenyl)butyric acid (IA) derivative as an albumin binder (ALB) moiety ([^99mTc]AB2 and [^99mTc]ALB2, respectively), and evaluated their utility for in vivo tumor imaging. In an in vitro HSA-binding assay, [^99mTc]AB2 and [^99mTc]ALB2 showed greater binding to HSA than [^99mTc]BHam, a ^99mTc-Ham complex without an ALB moiety. In an in vivo biodistribution assay, [^99mTc]ALB2 showed marked blood and tumor retention (25.13 and 4.61% injected dose (ID)/g, respectively, at 1 h postinjection), suggesting that the EPR effect had been induced. However, [^99mTc]AB2 showed no marked blood or tumor retention (4.16 and 0.75% ID/g, respectively, at 1 h postinjection), probably because the affinity of the monovalent IA derivative for albumin was insufficient to induce the EPR effect. These findings indicated that the multivalent interactions of [^99mTc]ALB2 had enhanced its affinity for albumin. ^99mTc-complexes containing multivalent ALB moieties may be useful for tumor 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.

Ceria-based nanotheranostic agent for rheumatoid arthritis

Rheumatoid arthritis (RA) is an autoimmune disease that affects 1-2% of the human population worldwide, and effective therapies with targeted delivery for local immune suppression have not been described. We address this problem by developing a novel theranostic nanoparticle for RA and assessed its therapeutic and targeting effects under image-guidance.

Methods: Albumin-cerium oxide nanoparticles were synthesized by the biomineralization process and further conjugated with near-infrared, indocyanine green (ICG) dye. Enzymatic-like properties and reactive oxygen species (ROS) scavenging activities, as well as the ability to reprogram macrophages, were determined on a monocyte cell line in culture. The therapeutic effect and systemic targeting potential were evaluated in collagen-induced arthritis (CIA) mouse model using optical/optoacoustic tomographic imaging.

Results: Small nanotheranostics with narrow size distribution and high colloidal stability were fabricated and displayed high ROS scavenging and enzymatic-like activity, as well as advanced efficacy in a converting pro-inflammatory macrophage phenotype into anti-inflammatory phenotype. When administrated into affected animals, these nanoparticles accumulated in inflamed joints and revealed a therapeutic effect similar to the gold-standard therapy for RA, methotrexate.

Conclusions: The inflammation-targeting, inherent contrast and therapeutic activity of this new albumin-cerium oxide nanoparticle may make it a relevant agent for assessing severity in RA, and other inflammatory diseases, and controlling inflammation with image-guidance. The design of these nanotheranostics will enable potential clinical translation as systemic therapy for RA.

Synthesis and Evaluation of 99mTc-Tricabonyl Labeled Isonitrile Conjugates for Prostate-Specific Membrane Antigen (PSMA) Image

Prostate-specific membrane antigen (PSMA) is a biomarker expressed on the surface of prostate cancer (PCa). In an effort to improve the detection and treatment of PCa, small urea-based PSMA inhibitors have been studied extensively. In the present study, we aimed to develop 99mTc-tricabonyl labeled urea-based PSMA conjugates containing isonitrile (CN-R)-coordinating ligands ([99mTc]Tc-15 and [99mTc]Tc-16). Both the PSMA conjugates were obtained at high radiochemical efficiency (≥98.5%). High in vitro binding affinity was observed for [99mTc]Tc-15 and [99mTc]Tc-16 (Kd = 5.5 and 0.2 nM, respectively) in PSMA-expressing 22Rv1 cells. Tumor xenografts were conducted using 22Rv1 cells and rapid accumulation of [99mTc]Tc-16 (1.87 ± 0.11% ID/g) was observed at 1 h post-injection, which subsequently increased to (2.83 ± 0.26% ID/g) at 4 h post-injection. However, [99mTc]Tc-15 showed moderate tumor uptake (1.48 ± 0.18% ID/g), which decreased at 4 h post-injection (0.81 ± 0.09% ID/g). [99mTc]Tc-16 was excreted from non-targeted tissues with high tumor-to-blood (17:1) and tumor-to-muscle ratio (41:1) at 4 h post-injection at approximately 4 times higher levels than [99mTc]Tc-15. Uptakes of [99mTc]Tc-15 and [99mTc]Tc-16 to PSMA-expressing tumor and tissues were significantly blocked by co-injection of 2-(Phosphonomethyl)-pentandioic acid (2-PMPA), suggesting that their uptakes are mediated by PSMA specifically. Whole-body single photon emission computed tomography imaging of [99mTc]Tc-16 verified the ex vivo biodistribution results and demonstrated clear visualization of tumors and tissues expressing PSMA compared to [99mTc]Tc-15. In conclusion, using [99mTc]Tc-16 rather than [99mTc]Tc-15 may be the preferable because of its relatively high tumor uptake and retention.

Recent Advances in Bioorthogonal Click Chemistry for Efficient Synthesis of Radiotracers and Radiopharmaceuticals

In recent years, several catalyst-free site-specific reactions have been investigated for the efficient conjugation of biomolecules, nanomaterials, and living cells. Representative functional group pairs for these reactions include the following: (1) azide and cyclooctyne for strain-promoted cycloaddition reaction, (2) tetrazine and trans-alkene for inverse-electron-demand-Diels–Alder reaction, and (3) electrophilic heterocycles and cysteine for rapid condensation/addition reaction. Due to their excellent specificities and high reaction rates, these conjugation methods have been utilized for the labeling of radioisotopes (e.g., radiohalogens, radiometals) to various target molecules. The radiolabeled products prepared by these methods have been applied to preclinical research, such as in vivo molecular imaging, pharmacokinetic studies, and radiation therapy of cancer cells. In this review, we explain the basics of these chemical reactions and introduce their recent applications in the field of radiopharmacy and chemical biology. In addition, we discuss the significance, current challenges, and prospects of using bioorthogonal conjugation reactions.