Academic Report

The incidence of global Invasive Fungal Infections (IFIs) has increased over the past few decades, primarily affecting immunocompromised patients, and these infections are often associated with high mortality and morbidity rates. Aspergillus fumigatus is one of the most common and deadly IFI pathogens. The main obstacles to effective treatment of fungal infections are the lack of rapid and accurate diagnostic tools, including the frequent need for invasive procedures to obtain microbiological confirmation, and the insufficient specificity of structural imaging methods.

To address these issues, the research team focused on developing an Aspergillus-specific Positron Emission Tomography (PET) imaging agent. This study was jointly completed by Swati Shah, Jianhao Lai, and others, with research institutions including multiple departments of the National Institutes of Health (NIH), and was published in the journal Science Translational Medicine on August 14, 2024.

Research Background and Objectives

Current standard diagnostic methods include culture techniques, which often require invasive procedures such as bronchoalveolar lavage and biopsy. Non-invasive tests such as serum galactomannan or 1,3-β-D-glucan detection and computed tomography (CT) also show variability in specificity and sensitivity across different clinical situations. Moreover, widely used clinical PET tracers like 2-deoxy-2-[18F]fluoro-glucose ([18F]FDG), while adding diagnostic value in structural imaging, still have limited specificity. Therefore, the aim of this study was to develop a PET tracer based on fungal-specific metabolism to facilitate non-invasive, rapid, and accurate diagnosis of deep fungal infections and to monitor treatment efficacy.

Research Source

This study was co-authored by Swati Shah, Jianhao Lai, Falguni Basuli, and others, affiliated with multiple departments of the National Institutes of Health (NIH), including the Center for Infectious Disease Imaging, Chemistry and Synthesis Center, National Institute of Allergy and Infectious Diseases (NIAID), etc. The research was published in the journal Science Translational Medicine on August 14, 2024.

Research Process

Synthesis and Validation of [18F]FCB

The research team utilized the unique sugar metabolism pathway in fungi, radioactively labeled cellobiose known to be metabolized by Aspergillus, and synthesized 2-deoxy-2-[18F]fluoro-cellobiose ([18F]FCB) through enzymatic conversion of 2-deoxy-2-[18F]fluoro-glucose ([18F]FDG). The synthesis process included the following steps: 1. Enzymatic Conversion: 1 mCi of [18F]FDG, 1 mg of cellobiose phosphorylase (CBP), and 37 mg of glucose-1-phosphate were converted at 40°C. Over 90% of [18F]FDG was consumed within 30 minutes, reaching quantitative consumption within 60 minutes. 2. Purification: The reaction mixture was heated at 100°C for 5 minutes, filtered to remove CBP, then purified using a Ni column and FDG purification cartridge. [18F]FCB was eluted in 4 ml of saline. 3. Radiochemical Purity: The synthesis time was 1.5 hours, with a total reaction yield of 60-70% and chemical purity >98%.

In Vitro and In Vivo Experiments

1. Cell Uptake Experiments: In vitro testing of [18F]FCB uptake and retention rates with different strains (including Aspergillus, bacteria, and fungi). Results showed that Aspergillus significantly retained radioactivity within 120 minutes compared to other strains.
2. β-Glucosidase Activity Assay: Enzyme activity experiments confirmed that only Aspergillus produced β-glucosidase. No enzyme activity was detected in bacterial samples, confirming that cellobiose is specifically degraded by fungi.
3. PET/CT Imaging: In vivo uptake of [18F]FCB was tested in a mouse myositis model. Results showed that retained radioactivity was only seen locally in mice infected with active Aspergillus, while no significant radioactive signal was observed in bacterial and sterile inflammation models.

Research Results

1. Radiochemical Synthesis and Testing Demonstrate Efficient Synthesis of [18F]FCB: The synthesis process of [18F]FCB is efficient, with high radiochemical purity (>98%). The rapid reaction time and high yield make this method particularly suitable for in vitro and in vivo trials.
2. In Vitro Uptake Experiments: In vitro experiments showed that Aspergillus more readily uptakes and retains [18F]FCB compared to other strains. This demonstrates the fungal specificity of the substance, especially for Aspergillus.
3. PET/CT Imaging: In mouse models, only mice infected with active Aspergillus retained significant radioactive signals locally after injection of radioactively labeled [18F]FCB. This signal can be used for infection diagnosis and treatment monitoring.

Research Conclusions and Significance

This study demonstrates the great potential of [18F]FCB as an Aspergillus-specific PET imaging tracer with clinical translational potential. Its high specificity and signal-to-noise ratio make [18F]FCB superior to existing [18F]FDG in imaging. Moreover, due to its rapid synthesis and high efficiency, [18F]FCB can be easily translated and applied clinically. Rapid diagnosis and treatment monitoring of invasive fungal infections will greatly improve patient treatment outcomes and prognosis.

Research Highlights

1. Efficient Radiochemical Synthesis Method: The proposed synthesis method for [18F]FCB obtains high-purity and high-yield radioactively labeled products in a short time.
2. High Signal-to-Noise Ratio PET Imaging: The high signal-to-noise ratio of [18F]FCB in Aspergillus-infected mice significantly improves the specificity of imaging.
3. Potential Clinical Applications: The rapid synthesis and excellent specificity of this method provide a solid foundation for [18F]FCB to serve as a fungal-specific tracer in clinical settings.

Future Prospects

The research team plans to further test the application of [18F]FCB in other pathogenic fungi and conduct related clinical translation research. Such studies will further validate the effectiveness and safety of this radioactive tracer in clinical applications, providing new avenues for the diagnosis and treatment of invasive fungal infections.