Separation of from using a macroporous organic polymer containing N-methylglucamine groups

[Development of Molecular Probes for Live Imaging of Cancer and Infectious Diseases]

Positron emission tomography (PET) and single photon emission computed tomography (SPECT) are powerful molecular imaging methods for examining disease-related factors in the whole body using specific imaging probes. Recently, we tried to develop molecular imaging probes that specifically visualize pathological factors associated with cancers and infectious diseases. Although survivin is highly expressed in several cancers, its expression is undetectable in non-dividing tissues. Thus, we developed several small molecular imaging probes that target survivin. These ligands not only showed high affinity for survivin protein, but also showed consistent cellular accumulation with respect to survivin expression levels, thereby indicating the feasibility of their backbones as scaffolds for tumor-specific imaging agents that target survivin. Prion diseases are fatal neurodegenerative diseases characterized by the deposition of amyloid plaques containing abnormal prion protein aggregates (PrP^Sc). Thus, we developed flavonoids, acridines, and benzofurans as PrP^Sc-imaging probes. A styrylchromone derivative ([¹²³I]SC-OMe) appears to be a particularly promising SPECT radioligand for monitoring prion deposit levels in living brains. Gallium-68 is a positron emitter in clinical PET applications that can be produced by a ⁶⁸Ge/⁶⁸Ga generator without a cyclotron. Notably, we developed new adsorbents for ⁶⁸Ge by introducing N-methylglucamine groups into the Sephadex series to serve as a hydrophilic polymer matrix. We also demonstrated that generator-eluted ⁶⁸Ga-citrate could be used for PET imaging of infectious mouse models. Our polysaccharide-based ⁶⁸Ge/⁶⁸Ga generators were shown to be prospectively cost-effective production systems for ⁶⁸Ga radiopharmaceuticals. This Review describes the major findings of these three studies and the future prospect of these fields.

Development of a 68Ge/68Ga Generator System Using Polysaccharide Polymers and Its Application in PET Imaging of Tropical Infectious Diseases

Gallium-68 (⁶⁸Ga) is a positron emitter for clinical positron emission tomography (PET) applications that can be produced by a ⁶⁸Ge/⁶⁸Ga generator without cyclotron. However, commercially available ⁶⁸Ge/⁶⁸Ga generator systems require multiple steps for the preparation of ⁶⁸Ga radiopharmaceuticals and are sometimes plagued by metallic impurities in the ⁶⁸Ga eluent. We developed a ⁶⁸Ge/⁶⁸Ga generator system using polysaccharide-based adsorbents and direct application of the generator-eluted ⁶⁸Ga-citrate to PET imaging of tropical infectious diseases. N-Methylglucamine (MG) as a ⁶⁸Ge-adsorbing unit (Sepha-MGs) was introduced to a series of Sephadex G-10, G-15, G-25, G-50, and G-75. In the batch method, over 97% of the ⁶⁸Ge in the solution was adsorbed onto the Sepha-MG series within 15 min. In particular, ⁶⁸Ge was effectively adsorbed on the Sepha(15)-MG packed columns and 70-80% of the ⁶⁸Ga was eluted by 1 mL of 0.1 M trisodium citrate with low ⁶⁸Ge contamination (<0.001%). The chemical form of the generator-eluted ⁶⁸Ga solution was identified as ⁶⁸Ga-citrate. In PET studies, affected regions in mice infected with Leishmania and severe fever with thrombocytopenia syndrome virus were clearly visualized using the ⁶⁸Ga-citrate. Sepha-MGs are useful adsorbents for ⁶⁸Ge/⁶⁸Ga generator systems with high ⁶⁸Ga elution efficiency and minimal ⁶⁸Ge breakthrough. These results indicated that eluted ⁶⁸Ga-citrate can be directly used for PET imaging of infectious sites in mice. This novel generator system may be useful for straightforward PET imaging of infection in clinical practice.

68Ga-Based radiopharmaceuticals: production and application relationship

The contribution of ⁶⁸Ga to the promotion and expansion of clinical research and routine positron emission tomography (PET) for earlier better diagnostics and individualized medicine is considerable. The potential applications of ⁶⁸Ga-comprising imaging agents include targeted, pre-targeted and non-targeted imaging. This review discusses the key aspects of the production of ⁶⁸Ga and ⁶⁸Ga-based radiopharmaceuticals in the light of the impact of regulatory requirements and endpoint pre-clinical and clinical applications.

68Ga-PET radiopharmacy: A generator-based alternative to 18F-radiopharmacy

Positron emission tomography (PET) is becoming a dominating method in the field of molecular imaging. Most commonly used radionuclides are accelerator produced 11C and 18F. An alternative method to label biomolecules is the use of metallic positron emitters; among them 68Ga is the most promising as it can be produced from a generator system consisting of an inorganic or organic matrix immobilizing the parent radionuclide 68Ge. Germanium-68 has a long half-life of 271 days which allows the production of long-lived, potentially very cost-effective generator systems. A commercial generator from Obninsk, Russia, is available which uses TiO2 as an inorganic matrix to immobilize 68Ge in the oxidation state IV+. 68Ge(IV) is chemically sufficiently different to allow efficient separation from 68Ga(III). Ga3+ is redox-inert; its coordination chemistry is dominated by its hard acid character. A variety of mono- and bifunctional chelators were developed which allow immobilization of 68Ga3+ and convenient coupling to biomolecules. Especially peptides targeting G-protein coupled receptors overexpressed on human tumour cells have been studied preclinically and in patient studies showing high and specific tumour uptake and specific localization. 68Ga-radiopharmacy may indeed be an alternative to 18F-based radiopharmacy. Freeze-dried, kit-formulated precursors along with the generator may be provided, similar to the 99Mo/99mTc-based radiopharmacy, still the mainstay of nuclear medicine.

Microwave-Supported Preparation of68Ga Bioconjugates with High Specific Radioactivity

The generator-produced positron-emitting (68)Ga (T(1/2) = 68 min) is of potential interest for clinical PET. (68)Ga as a metallic cation is suitable for complexation reactions with chelators, naked or conjugated, with peptides or other macromolecules. Large (68)Ga generator eluate volumes, metal traces from the generator column material, or reaction reagents, however, disturb a fast, reliable, and quantitative labeling procedure. In this paper we describe a simple technique, based on anion exchange, aiming first, to increase the (68)Ga concentration, second to purify it from competing impurities, and third to obtain a fast and quantitative (68)Ga-labeled peptide conjugate that can be applied in humans without further purification. Within 5 min one can obtain from the original 6 mL generator eluate a 200 microL (68)Ga preparation (volume reduction by a factor 30) that is suitable for direct and quantitative labeling of peptide conjugates. DOTATOC (DOTA-D-Phe(1)-Tyr(3)-octreotide, DOTA = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) was used as a test tracer for comparing the labeling properties of the different (68)Ga preparations. In combination with microwave heating, peptide conjugates of 0.5-1 nmol quantities could be labeled within 10 min with the full (68)Ga activity of a generator. Further purification of the (68)Ga-labeled peptide conjugate was no longer required since the nuclide incorporation was quantitative. The specific radioactivity (with respect to the peptide) was improved by a factor approximately 100 compared to the previously applied techniques using the original generator eluate. The commercial (68)Ge/(68)Ga generator from Obninsk in combination with this system for purification and concentration with an integrated microwave-supported labeling technology resulted in a kitlike technology for (68)Ga-tracer production. The first automated prototype using this technology is being tested.

A new 68Ge/68Ga generator system using an organic polymer containing N-methylglucamine groups as adsorbent for 68Ge

A macroporous styrene-divinylbenzene copolymer containing N-methylglucamine groups was selected for a new 68Ge/68Ga generator system. This resin packed into a column effectively adsorbed the parent nuclide 68Ge. The daughter 68Ga was eluted from the resin with a solution of a low-affinity gallium chelating ligand such as citric or phosphoric acid. The 68Ge leakage was less than 0.0004% of the 68Ge adsorbed on the resin. By simple mixing of transferrin and desferoxamine conjugated HSA and IgG with the eluate from the column, 68Ga-labeling was completed in high yield.