Development of 68Ga labeled human serum albumin for blood pool imaging: a comparison between two ligands

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.

Radiolabeling Strategies of Micron- and Submicron-Sized Core-Shell Carriers for In Vivo Studies

Core-shell particles made of calcium carbonate and coated with biocompatible polymers using the Layer-by-Layer technique can be considered as a unique drug-delivery platform that enables us to load different therapeutic compounds, exhibits a high biocompatibility, and can integrate several stimuli-responsive mechanisms for drug release. However, before implementation for diagnostic or therapeutic purposes, such core-shell particles require a comprehensive in vivo evaluation in terms of physicochemical and pharmacokinetic properties. Positron emission tomography (PET) is an advanced imaging technique for the evaluation of in vivo biodistribution of drug carriers; nevertheless, an incorporation of positron emitters in these carriers is needed. Here, for the first time, we demonstrate the radiolabeling approaches of calcium carbonate core-shell particles with different sizes (CaCO₃ micron-sized core-shell particles (MicCSPs) and CaCO₃ submicron-sized core-shell particles (SubCSPs)) to precisely determine their in vivo biodistribution after intravenous administration in rats. For this, several methods of radiolabeling have been developed, where the positron emitter (⁶⁸Ga) was incorporated into the particle's core (co-precipitation approach) or onto the surface of the shell (either layer coating or adsorption approaches). According to the obtained data, radiochemical bounding and stability of ⁶⁸Ga strongly depend on the used radiolabeling approach, and the co-precipitation method has shown the best radiochemical stability in human serum (96-98.5% for both types of core-shell particles). Finally, we demonstrate the size-dependent effect of core-shell particles' distribution on the specific organ uptake, using a combination of imaging techniques, PET, and computerized tomography (CT), as well as radiometry of separate organs. Thus, our findings open up new perspectives of CaCO₃-radiolabeled core-shell particles for their further implementation into clinical practice.

Design of immunogens: The effect of bifunctional chelator on immunological response to chelated copper

To produce specific antibodies for the detection and quantification of copper ions, bifunctional chelators (BFCs) are commonly applied in the preparation of copper conjugates. However, some copper-chelator complexes exhibit limited stability under in vivo conditions. In this study, Cu²⁺ was coupled with carrier proteins via three different macrocyclic BFCs: p-SCN-Bn-DOTA, p-SCN-Bn-NOTA, and p-SCN-Bn-TETA. The stability in plasma and the immunogenicity of three copper immunoconjugates were compared. The chelators other than p-SCN-Bn-DOTA were very stable in plasma, with <9% dissociation of Cu²⁺ over 96 h. The immune response varied depending on the choice of chelator; notably, antisera from the Cu²⁺-NOTA-KLH conjugate demonstrated the best reactivity toward chelated Cu²⁺. p-SCN-Bn-NOTA, which showed significant advantages over the other chelators, was used for antibody production. The efficiency of immune-positive hybridoma production was satisfactory, and the resultant monoclonal antibodies (McAbs) 4B7 showed sensitivity (half-maximal inhibitory concentration (IC₅₀) of 8.9 ng/mL) to chelated Cu²⁺, with a working range from 1.21 to 48.9 ng/mL. The recovery of Cu²⁺ from water samples was 85.7-108%, and the intra- and inter-assay coefficients of variation were 4.0-10.1% and 7.1-11.4%, respectively. Compared with previously reported McAb specific to Cu²⁺, DF4, the sensitivity of the newly developed assay was improved 100-fold. The results of this study indicate the utility of NOTA for the efficient generation of highly sensitive McAbs against Cu²⁺.

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

Technetium-99m-labeled human serum albumin (^99mTc-HSA) has been utilized as a blood pool imaging agent in the clinic for several decades. However, ^99mTc-HSA has a short circulation time, which is a critical shortcoming for a blood pool imaging agent. Herein, we developed a novel ^99mTc-labeled HSA with a long circulation time using click chemistry and a chelator, 2,2'-dipicolylamine (DPA), (^99mTc-DPA-HSA). Specifically, we examined the feasibility of copper-free strain-promoted alkyne-azide cycloaddition (SPAAC) for the incorporation of HSA to the [^99mTc (CO)₃(H₂O)₃]⁺ system by adopting a chelate-then-click approach. In this strategy, a potent chelate system, azide-functionalized DPA, was first complexed with [^99mTc (CO)₃(H₂O)₃]⁺, followed by the SPAAC click reaction with azadibenzocyclooctyne-functionalized HSA (ADIBO-HSA) under biocompatible conditions. Radiolabeling efficiency of azide-functionalized DPA (^99mTc-DPA) was >98%. Click conjugation efficiency of ^99mTc-DPA with ADIBO-HSA was between 76 and 99% depending on the number of ADIBO moieties attached to HSA. In whole-body in vivo single photon emission computed tomography images, the blood pool uptakes of ^99mTc-DPA-HSA were significantly enhanced compared to those of ^99mTc-HSA at 10 min, 2, and 6 h after the injection ( P < 0.001, 0.025, and 0.003, respectively). Furthermore, the blood activities of ^99mTc-DPA-HSA were 8 times higher at 30 min and 10 times higher at 3 h after the injection compared to those of conventional ^99mTc-HSA in ex vivo biodistribution experiment. The results exhibit the potential of ^99mTc-DPA-HSA as a blood pool imaging agent and further illustrate the promise of the pre-labeling SPAAC approach for conjugation of heat-sensitive biological targeting vectors with [^99mTc (CO)₃(H₂O)₃]⁺.

Gallium-68: methodology and novel radiotracers for positron emission tomography (2012–2017)

Commercial 68Ge/68Ga generators provide a means to produce positron emission tomography agents on site without use of a cyclotron. This development has led to a rapid growth of academic literature and patents ongallium-68 (68Ga). As 68Ga positron emission tomography agents usually involve a targeting moiety attached to a metal chelator, the development lends itself to the investigation of theragnostic applications; the 68Ga-based diagnostic is utilized to determine if the biological target is present and, if so, a therapeutic isotope (e.g., 177Lu, 225Ac) can be complexed with the same scaffold to generate a corresponding radiotherapeutic. This review considers patents issued between 2012 and 2017 that contain a 68Ga-labeled molecule indexed by Chemical Abstract Services (a division of the American Chemical Society).

Automated synthesis of [68Ga]oxine, improved preparation of 68Ga-labeled erythrocytes for blood-pool imaging, and preclinical evaluation in rodents

Radiolabeled erythrocytes have multiple applications in nuclear medicine, including blood pool imaging. Historically they have been labeled with SPECT radionuclides. A PET blood pool imaging agent is highly desirable as it would improve clinical applications with better image quality and resolution, higher sensitivity, and dynamic scanning capabilities. With the coming of age of modern 68Ge/68Ga generator systems, gallium-68 is now widely accessible. In this paper we describe an updated method for the preparation of 68Ga-labeled erythrocytes and their preliminary use in rodent blood pool imaging. A novel automated synthesis of [68Ga]oxine using a 68Ga/68Ge generator and automated synthesis module is reported. [68Ga]Oxine was synthesized in 50 ± 5% (n = 3) non-decay corrected radiochemical yield and >99% radiochemical purity. Rat and human erythrocytes were successfully labeled with the complex in 42% RCY, and the 68Ga-labeled erythrocytes have been shown to clearly image the blood pool in a healthy rat. Human erythrocytes labelled with [68Ga]oxine were shown to be viable up to 2 hours post-labelling, and washout of the radiolabel was minimal up to 1 hour post-labelling. Further optimization of the labeling method to translate for use in human cardiac and oncologic blood pool PET imaging studies, is underway.