Research progress of 18F labeled small molecule positron emission tomography (PET) imaging agents

Legumain-Triggered Macrocyclization of Radiofluorinated Tracer for Enhanced PET Imaging

Enhancing the accumulation and retention of small-molecule probes in tumors is an important way to achieve accurate cancer diagnosis and therapy. Enzyme-stimulated macrocyclization of small molecules possesses great potential for enhanced positron emission tomography (PET) imaging of tumors. Herein, we reported an 18F-labeled radiotracer [18F]AlF-RSM for legumain detection in vivo. The tracer was prepared by a one-step aluminum-fluoride-restrained complexing agent ([18F]AlF-RESCA) method with high radiochemical yield (RCY) (88.35 ± 3.93%) and radiochemical purity (RCP) (>95%). More notably, the tracer can be transformed into a hydrophobic macrocyclic molecule under the joint action of legumain and reductant. Simultaneously, the tracer could target legumain-positive tumors and enhance accumulation and retention in tumors, resulting in the amplification of PET imaging signals. The enhancement of radioactivity enables PET imaging of legumain activity with high specificity. We envision that, by combining this highly efficient 18F-labeled strategy with our intramolecular macrocyclization reaction, a range of radiofluorinated tracers can be designed for tumor PET imaging and early cancer diagnosis in the future.

Exploration of radionuclide labeling of a novel scFv-Fc fusion protein targeting CLDN18.2 for tumor diagnosis and treatment

PURPOSE

Claudin 18.2 (CLDN18.2), due to its highly selective expression in tumor cells, has made breakthrough progress in clinical research and is expected to be integrated into routine tumor diagnosis and treatment.

METHODS

In this research, we obtained an scFv-Fc fusion protein (SF106) targeting CLDN18.2 through hybridoma technology. The scFv-Fc fusion protein was labeled with radioactive isotopes (124I and 177Lu) to generate the radio-probes. The targeting and specificity of the radio-probes were tested in cellular models, and its diagnostic and therapeutic potential was further evaluated in tumor-bearing models.

RESULTS

The molecular probes [124I]I-SF106 and [177Lu]Lu-DOTA-SF106 possess high radiochemical purity (RCP, 98.18 ± 0.93 % and 97.05 ± 1.1 %) and exhibit good stability in phosphate buffer saline and 5 % human serum albumin (92.44 ± 4.68 % and 91.03 ± 2.42 % at 120 h). [124I]I-SF106 uptake in cells expressing CLDN18.2 was well targeted and specific, and the dissociation constant was 17.74 nM [124I]I-SF106 micro-PET imaging showed that the maximum standardized uptake value (SUVmax) was significantly higher than CLDN18.2-negative tumors (1.83 ± 0.02 vs. 1.23 ± 0.04, p < 0.001). The maximum uptake was attained in tumors expressing CLDN18.2 at 48 h after injection. [124I]I-SF106 and [177Lu]Lu-DOTA-SF106 dosimetric study showed that the effective dose in humans complies with the medical safety standards required for their clinical application. The results of treatment experiments showed that 3 MBq of [177Lu]Lu-DOTA-SF106 in CLDN18.2-expressing tumor-bearing mice could significantly inhibit tumor growth.

CONCLUSION

These results indicate that radionuclide-labeled scFv-Fc molecular probes ([124I]I-SF106 and [177Lu]Lu-DOTA-SF106) provide a new possibility for the diagnosis and treatment of CLDN18.2-positive cancer patients in clinical practice.

[18F]fluoride Activation and 18F-Labelling in Hydrous Conditions-Towards a Microfluidic Synthesis of PET Radiopharmaceuticals

18F-labelled radiopharmaceuticals are indispensable in positron emission tomography. The critical step in the preparation of 18F-labelled tracers is the anhydrous F-18 nucleophilic substitution reaction, which involves [18F]F- anions generated in aqueous media by the cyclotron. For this, azeotropic drying by distillation is widely used in standard synthesisers, but microfluidic systems are often not compatible with such a process. To avoid this step, several methods compatible with aqueous media have been developed. We summarised the existing approaches and two of them have been studied in detail. [18F]fluoride elution efficiencies have been investigated under different conditions showing high 18F-recovery. Finally, a large scope of precursors has been assessed for radiochemical conversion, and these hydrous labelling techniques have shown their potential for tracer production using a microfluidic approach, more particularly compatible with iMiDEV™ cassette volumes.

Dual-Targeting PET Tracers Enable Enzyme-Mediated Self-Assembly for the PET Imaging of Legumain Activity

Legumain, a lysosomal cysteine protease overexpressed in a variety of tumors, has been considered a promising biomarker for various cancers. Precise detection of legumain activity in the lysosome represents an important strategy for early diagnosis and prognosis of tumors. Small-molecule probes with the property of target-enabled self-assembly hold great potential for molecular imaging. In this study, we reported two dual-targeting radiotracers ([18F]SF-AAN-M and [18F]SF-AAN-HEM) with a property of legumain-mediated self-assembly for positron emission tomography (PET) imaging. Both the radiotracers were synthesized with high labeling yield (>50%) and the radiochemical purity was over 99% via one-step straightforward 18F-labeling. Both tracers were efficiently activated by the reducing agent and legumain to self-assemble into aggregates and showed enhanced retention in legumain-overexpressed MDA-MB-468 cells and tumors, indicating that the introduction of lysosome-targeting morpholine increased the tumor uptake and extended the retention of radiotracers in legumain-overexpressed tumors. In addition, [18F]SF-AAN-HEM with a hydrophilic (histidine-glutamate)3 tag displayed significantly reduced liver uptake with no conspicuous reduction in tumor uptake, affording high signal-to-noise ratios (tumor/liver and tumor/muscle). All of these results suggest that dual-targeting tracer [18F]SF-AAN-HEM could provide a promising tool for in vivo monitoring legumain activity in tumors.

Recent Developments in PET and SPECT Radiotracers as Radiopharmaceuticals for Hypoxia Tumors

Hypoxia, a deficiency in the levels of oxygen, is a common feature of most solid tumors and induces many characteristics of cancer. Hypoxia is associated with metastases and strong resistance to radio- and chemotherapy, and can decrease the accuracy of cancer prognosis. Non-invasive imaging methods such as positron emission tomography (PET) and single-photon emission computed tomography (SPECT) using hypoxia-targeting radiopharmaceuticals have been used for the detection and therapy of tumor hypoxia. Nitroimidazoles are bioreducible moieties that can be selectively reduced under hypoxic conditions covalently bind to intracellular macromolecules, and are trapped within hypoxic cells and tissues. Recently, there has been a strong motivation to develop PET and SPECT radiotracers as radiopharmaceuticals containing nitroimidazole moieties for the visualization and treatment of hypoxic tumors. In this review, we summarize the development of some novel PET and SPECT radiotracers as radiopharmaceuticals containing nitroimidazoles, as well as their physicochemical properties, in vitro cellular uptake values, in vivo biodistribution, and PET/SPECT imaging results.

Fluorine-a small magic bullet atom in the drug development: perspective to FDA approved and COVID-19 recommended drugs

During the last twenty years, organic fluorination chemistry established itself as an important tool to get a biologically active compound. This belief can be supported by the fact that every year, we are getting fluorinated drugs in the market in extremely significant numbers. Last year, also ten fluorinated drugs have been approved by FDA and during the COVID-19 pandemic, fluorinated drugs played a very crucial role to control the disease and saved many lives. In this review, we surveyed all ten fluorinated drugs approved by FDA in 2021 and all fluorinated drugs which were directly-indirectly used during the COVID-19 period, and emphasis has been given particularly to their synthesis, medicinal chemistry, and development process. Out of ten approved drugs, one drug pylarify, a radioactive diagnostic agent for cancer was approved for use in positron emission tomography imaging. Also, very briefly outlined the significance of fluorinated drugs through their physical, and chemical properties and their effect on drug development.

Graphical abstract

Isotope Exchange-Based 18F-Labeling Methods

¹⁸F-Labeling methods for the preparation of ¹⁸F-labeled molecular probes can be classified into electrophilic fluorination, nucleophilic fluorination, metal-F coordination, and ¹⁸F/¹⁹F isotope exchange. Isotope exchange-based ¹⁸F-labeling methods demonstrate mild conditions featuring water resistance and facile high-performance liquid chromatography-free purification in direct ¹⁸F-labeling of substrates. This paper systematically reviews isotope exchange-based ¹⁸F-labeling methods sorted by the adjacent atom bonding with F, i.e., carbon and noncarbon atoms (Si, B, P, S, Ga, Fe, etc.). The respective isotope exchange mechanism, radiolabeling condition, radiochemical yield, molar activity, and stability of the ¹⁸F-product are mainly discussed for each isotope exchange-based ¹⁸F-labeling method as well as the cutting-edge application of the corresponding ¹⁸F-labeled molecular probes.

Recent Development of Radiofluorination of Boron Agents for Boron Neutron Capture Therapy of Tumor: Creation of 18F-Labeled C-F and B-F Linkages

Boron neutron capture therapy (BNCT) is a binary therapeutic technique employing a boron agent to be delivered to the tumor site followed by the irradiation of neutrons. Biofunctional molecules/nanoparticles labeled with F-18 can provide an initial pharmacokinetic profile of patients to guide the subsequent treatment planning procedure of BNCT. Borono phenylalanine (BPA), recognized by the l-type amino acid transporter, can cross the blood-brain barrier and be accumulated in gliomas. The radiofluoro BNCT agents are reviewed by considering (1) less cytotoxicity, (2) diagnosing and therapeutic purposes, (3) aqueous solubility and extraction route, as well as (4), the trifluoroborate effect. A trifluoroborate-containing amino acid such as fluoroboronotyrosine (FBY) represents an example with both functionalities of imaging and therapeutics. Comparing with the insignificant cytotoxicity of clinical BPA with IC50 > 500 μM, FBY also shows minute toxicity with IC50 > 500 μM. [18F]FBY is a potential diagnostic agent for its tumor to normal accumulation (T/N) ratio, which ranges from 2.3 to 24.5 from positron emission tomography, whereas the T/N ratio of FBPA is greater than 2.5. Additionally, in serving as a BNCT therapeutic agent, the boron concentration of FBY accumulated in gliomas remains uncertain. The solubility of 3-BPA is better than that of BPA, as evidenced by the cerebral dose of 3.4%ID/g vs. 2.2%ID/g, respectively. While the extraction route of d-BPA differs from that of BPA, an impressive T/N ratio of 6.9 vs. 1.5 is noted. [18F]FBPA, the most common clinical boron agent, facilitates the application of BPA in clinical BNCT. In addition to [18F]FBY, [18F] trifluoroborated nucleoside analog obtained through 1,3-dipolar cycloaddition shows marked tumoral uptake of 1.5%ID/g. Other examples using electrophilic and nucleophilic fluorination on the boron compounds are also reviewed, including diboronopinacolone phenylalanine and nonsteroidal anti-inflammatory agents.

Study of Carbamoyl Fluoride: Synthesis, Properties and Applications

Carbamoyl fluoride is a fluorinated group that, to this date, remains underexplored, probably due to the lack of data concerning its properties. In this paper, a study of carbamoyl fluoride is presented. Stability studies, in particular under physiological conditions, and lipophilicity measurement were performed. A new easy, safe, inexpensive, and metal-free synthesis method is also described. Finally, a potential use in radiochemistry through a ¹⁸ F/¹⁹ F isotopic exchange is demonstrated.

Development of a Promising 18F-Radiotracer for PET Imaging Legumain Activity In Vivo

Legumain has been found overexpressed in several cancers, which serves as an important biomarker for cancer diagnosis. In this research, a novel fluorine-18 labeled radioactive tracer [18F]SF-AAN targeting legumain was designed and synthesized for positron emission tomography (PET) imaging. Nonradioactive probe [19F]SF-AAN was obtained through chemical and solid phase peptide synthesis. After a simple one-step 18F labeling, the radiotracer [18F]SF-AAN was obtained with a high radiochemical conversion rate (>85%) and radiochemical purity (99%) as well as high molar activity (12.77 ± 0.50 MBq/nmol). The targeting specificity of [18F]SF-AAN for detecting legumain activity was investigated systematically in vitro and in vivo. In vitro cellular uptake assay showed that the uptake of [18F]SF-AAN in legumain-positive MDA-MB-468 cells was twice as much as that in legumain-negative PC-3 cells at 4 h. In vivo PET imaging revealed that the tumor uptake of [18F]SF-AAN in MDA-MB-468 tumor-bearing mice was about 2.7 times of that in PC-3 tumor-bearing mice at 10 min post injection. The experimental results indicated that [18F]SF-AAN could serve as a promising PET tracer for detecting the legumain expression sensitively and specifically, which would be beneficial for the diagnosis of legumain-related diseases.

Rapid 18F-labeling via Pd-catalyzed S-arylation in aqueous medium

We report radiolabeling of thiol-containing substrates via Pd-catalyzed S-arylation with 2-[¹⁸F]fluoro-5-iodopyridine, which is readily accessible using the "minimalist" radiofluorination method. The practicality of the procedure was confirmed by preparation of a novel PSMA-specific PET-tracer as well as labeling of glutathione, Aβ oligomer-binding RD2 peptide, bovine serum albumin and PSMA I&S.