Positron Emission Tomography Imaging of Staphylococcus aureus Infection Using a Nitro-Prodrug Analogue of 2-[18F]F-p-Aminobenzoic Acid

Recently developed radiopharmaceuticals for bacterial infection imaging

BACKGROUND

Infection remains a major cause of morbidity and mortality, regardless of advances in antimicrobial therapy and improved knowledge of microorganisms. With the major global threat posed by antimicrobial resistance, fast and accurate diagnosis of infections, and the reliable identification of intractable infection, are becoming more crucial for effective treatment and the application of antibiotic stewardship. Molecular imaging with the use of nuclear medicine allows early detection and localisation of infection and inflammatory processes, as well as accurate monitoring of treatment response. There has been a continuous search for more specific radiopharmaceuticals to be utilised for infection imaging. This review summarises the most prominent discoveries in specifically bacterial infection imaging agents over the last five years, since 2019.

MAIN BODY

Some promising new radiopharmaceuticals evaluated in patient studies are reported here, including radiolabelled bacterial siderophores like [68Ga]Ga-DFO-B, radiolabelled antimicrobial peptide/peptide fragments like [68Ga]Ga-NOTA-UBI29-41, and agents that target bacterial synthesis pathways (folic acid and peptidoglycan) like [11C]para-aminobenzoic acid and D-methyl-[11C]-methionine, with clinical trials underway for [18F]fluorodeoxy-sorbitol, as well as for 11C- and 18F-labelled trimethoprim.

CONCLUSION

It is evident that a great deal of effort has gone into the development of new radiopharmaceuticals for infection imaging over the last few years, with remarkable progress in preclinical investigations. However, translation to clinical trials, and eventually clinical Nuclear Medicine practice, is apparently slow. It is the authors' opinion that a more structured and harmonised preclinical setting and well-designed clinical investigations are the key to reliably evaluate the true potential of the newly proposed infection imaging agents.

In Vivo Detection of Staphylococcus aureus Infections Using Radiolabeled Antibodies Specific for Bacterial Toxins

Purpose

The Gram-positive Staphylococcus aureus bacterium is one of the leading causes of infection in humans. The lack of specific noninvasive techniques for diagnosis of staphylococcal infection together with the severity of its associated complications support the need for new specific and selective diagnostic tools. This work presents the successful synthesis of an immunotracer that targets the α-toxin released by S. aureus.

Methods

[89Zr]Zr-DFO-ToxAb was synthesized based on radiolabeling an anti-α-toxin antibody with zirconium-89. The physicochemical characterization of the immunotracer was performed by high-performance liquid chromatography (HPLC), radio-thin layer chromatography (radio-TLC), and electrophoretic analysis. Its diagnostic ability was evaluated in vivo by positron emission tomography/computed tomography (PET/CT) imaging in an animal model of local infection-inflammation (active S. aureus vs. heat-killed S. aureus) and infective osteoarthritis.

Results

Chemical characterization of the tracer established the high radiochemical yield and purity of the tracer while maintaining antibody integrity. In vivo PET/CT image confirmed the ability of the tracer to detect active foci of S. aureus. Those results were supported by ex vivo biodistribution studies, autoradiography, and histology, which confirmed the ability of [89Zr]Zr-DFO-ToxAb to detect staphylococcal infectious foci, avoiding false-positives derived from inflammatory processes.

Conclusions

We have developed an immuno-PET tracer capable of detecting S. aureus infections based on a radiolabeled antibody specific for the staphylococcal alpha toxins. The in vivo assessment of [89Zr]Zr-DFO-ToxAb confirmed its ability to selectively detect staphylococcal infectious foci, allowing us to discern between infectious and inflammatory processes.

Bioorthogonal Radiolabeling of Azide-Modified Bacteria Using [18F]FB-sulfo-DBCO

Purpose: This study was motivated by the need for better positron emission tomography (PET)-compatible tools to image bacterial infection. Our previous efforts have targeted bacteria-specific metabolism via assimilation of carbon-11 labeled d-amino acids into the bacterial cell wall. Since the chemical determinants of this incorporation are not fully understood, we sought a high-throughput method to label d-amino acid derived structures with fluorine-18. Our strategy employed a chemical biology approach, whereby an azide (-N3) bearing d-amino acid is incorporated into peptidoglycan muropeptides, with subsequent "click" cycloaddition with an 18F-labeled strained cyclooctyne partner. Procedures: A water-soluble, 18F-labeled and dibenzocyclooctyne (DBCO)-derived radiotracer ([18F]FB-sulfo-DBCO) was synthesized. This tracer was incubated with pathogenic bacteria treated with azide-bearing d-amino acids, and incorporated 18F was determined via gamma counting. In vitro uptake in bacteria previously treated with azide-modified d-amino acids was compared to that in cultures treated with amino acid controls. The biodistribution of [18F]FB-sulfo-DBCO was studied in a cohort of healthy mice with implications for future in vivo imaging. Results: The new strain-promoted azide-alkyne cycloaddition (SPAAC) radiotracer [18F]FB-sulfo-DBCO was synthesized with high radiochemical yield and purity via N-succinimidyl 4-[18F]fluorobenzoate ([18F]SFB). Accumulation of [18F]FB-sulfo-DBCO was significantly higher in several bacteria treated with azide-modified d-amino acids than in controls; for example, we observed 7 times greater [18F]FB-sulfo-DBCO ligation in Staphylococcus aureus cultures incubated with 3-azido-d-alanine versus those incubated with d-alanine. Conclusions: The SPAAC radiotracer [18F]FB-sulfo-DBCO was validated in vitro via metabolic labeling of azide-bearing peptidoglycan muropeptides. d-Amino acid-derived PET radiotracers may be more efficiently screened via [18F]FB-sulfo-DBCO modification.

Infection Imaging: Focus on New Tracers?

Infections account for relevant morbidity and mortality, especially if the cardiovascular system is affected. Clinical manifestations are often unspecific, resulting in a challenging diagnostic work-up. The use of molecular imaging methods, namely [18F]FDG PET and leukocyte scintigraphy, is increasingly recognized in recently published international guidelines. However, these 2 established methods focus on the host's immune response to the pathogen and are therefore virtually unable to differentiate infection from inflammation. Targeting the microorganism responsible for the infection directly with novel imaging agents is a promising strategy to overcome these limitations. In this review, we discuss clinically approved [18F]FDG PET with its advantages and limitations in cardiovascular infections, followed by new PET-based approaches for the detection of cardiovascular infections by bacteria-specific molecular imaging methods. A multitude of different targeting options has already been preclinically evaluated, but most still lack clinical translation. We give an overview not only on promising tracer candidates for noninvasive molecular imaging of infections but also on issues hampering clinical translation.

Evaluating the Performance of Pathogen-Targeted Positron Emission Tomography Radiotracers in a Rat Model of Vertebral Discitis-Osteomyelitis

BACKGROUND

Vertebral discitis-osteomyelitis (VDO) is a devastating infection of the spine that is challenging to distinguish from noninfectious mimics using computed tomography and magnetic resonance imaging. We and others have developed novel metabolism-targeted positron emission tomography (PET) radiotracers for detecting living Staphylococcus aureus and other bacteria in vivo, but their head-to-head performance in a well-validated VDO animal model has not been reported.

METHODS

We compared the performance of several PET radiotracers in a rat model of VDO. [11C]PABA and [18F]FDS were assessed for their ability to distinguish S aureus, the most common non-tuberculous pathogen VDO, from Escherichia coli.

RESULTS

In the rat S aureus VDO model, [11C]PABA could detect as few as 103 bacteria and exhibited the highest signal-to-background ratio, with a 20-fold increased signal in VDO compared to uninfected tissues. In a proof-of-concept experiment, detection of bacterial infection and discrimination between S aureus and E coli was possible using a combination of [11C]PABA and [18F]FDS.

CONCLUSIONS

Our work reveals that several bacteria-targeted PET radiotracers had sufficient signal to background in a rat model of S aureus VDO to be potentially clinically useful. [11C]PABA was the most promising tracer investigated and warrants further investigation in human VDO.

Infection-specific PET imaging with 18F-fluorodeoxysorbitol and 2-[18F]F-ρ-aminobenzoic acid: An extended diagnostic tool for bacterial and fungal diseases

Introduction

Suspected infectious diseases located in difficult-to-access sites can be challenging due to the need for invasive procedures to isolate the etiological agent. Positron emission tomography (PET) is a non-invasive imaging technology that can help locate the infection site. The most widely used radiotracer for PET imaging (2-deoxy-2[¹⁸F] fluoro-D-glucose: [¹⁸F]FDG) shows uptake in both infected and sterile inflammation. Therefore, there is a need to develop new radiotracers able to specifically detect microorganisms.

Methods

We tested two specific radiotracers: 2-deoxy-2-[¹⁸F]-fluoro-D-sorbitol ([¹⁸F]FDS) and 2-[¹⁸F]F-ρ-aminobenzoic acid ([¹⁸F]FPABA), and also developed a simplified alternative of the latter for automated synthesis. Clinical and reference isolates of bacterial and yeast species (19 different strains in all) were tested in vitro and in an experimental mouse model of myositis infection.

Results and discussion

Non-lactose fermenters (Pseudomonas aeruginosa and Stenotrophomonas maltophilia) were unable to take up [¹⁸F]FDG in vitro. [¹⁸F]FDS PET was able to visualize Enterobacterales myositis infection (i.e., Escherichia coli) and to differentiate between yeasts with differential assimilation of sorbitol (i.e., Candida albicans vs. Candida glabrata). All bacteria and yeasts tested were detected in vitro by [¹⁸F]FPABA. Furthermore, [¹⁸F]FPABA was able to distinguish between inflammation and infection in the myositis mouse model (E. coli and Staphylococcus aureus) and could be used as a probe for a wide variety of bacterial and fungal species.

Advances in image-guided drug delivery for antibacterial therapy

The emergence of antibiotic-resistant bacterial strains is seriously endangering the global healthcare system. There is an urgent need for combining imaging with therapies to realize the real-time monitoring of pathological condition and treatment progress. It also provides guidance on exploring new medicines and enhance treatment strategies to overcome the antibiotic resistance of existing conventional antibiotics. In this review, we provide a thorough overview of the most advanced image-guided approaches for bacterial diagnosis (e.g., computed tomography imaging, magnetic resonance imaging, photoacoustic imaging, ultrasound imaging, fluorescence imaging, positron emission tomography, single photon emission computed tomography imaging, and multiple imaging), and therapies (e.g., photothermal therapy, photodynamic therapy, chemodynamic therapy, sonodynamic therapy, immunotherapy, and multiple therapies). This review focuses on how to design and fabricate photo-responsive materials for improved image-guided bacterial theranostics applications. We present a potential application of different image-guided modalities for both bacterial diagnosis and therapies with representative examples. Finally, we highlighted the current challenges and future perspectives image-guided approaches for future clinical translation of nano-theranostics in bacterial infections therapies. We envision that this review will provide for future development in image-guided systems for bacterial theranostics applications.

Alternative Nuclear Imaging Tools for Infection Imaging

Purpose of Review

Cardiovascular infections are serious disease associated with high morbidity and mortality. Their diagnosis is challenging, requiring a proper management for a prompt recognition of the clinical manifestations, and a multidisciplinary approach involving cardiologists, cardiothoracic surgeons, infectious diseases specialist, imagers, and microbiologists. Imaging plays a central role in the diagnostic workout, including molecular imaging techniques. In this setting, two different strategies might be used to image infections: the first is based on the use of agents targeting the microorganism responsible for the infection. Alternatively, we can target the components of the pathophysiological changes of the inflammatory process and/or the host response to the infectious pathogen can be considered. Understanding the strength and limitations of each strategy is crucial to select the most appropriate imaging tool.

Recent Findings

Currently, multislice computed tomography (MSCT) and nuclear imaging (¹⁸F-fluorodeoxyglucose positron emission tomography/computed tomography, and leucocyte scintigraphy) are part of the diagnostic strategies. The main role of nuclear medicine imaging (PET/CT and SPECT/CT) is the confirmation of valve/CIED involvement and/or associated perivalvular infection and the detection of distant septic embolism. Proper patients’ preparation, imaging acquisition, and reconstruction as well as imaging reading are crucial to maximize the diagnostic information.

Summary

In this manuscript, we described the use of molecular imaging techniques, in particular WBC imaging, in patients with infective endocarditis, cardiovascular implantable electronic device infections, and infections of composite aortic graft, underlying the strength and limitations of such approached as compared to the other imaging modalities.

MicroPET imaging of bacterial infection with nitroreductase-specific responsive 18F-labelled nitrogen mustard analogues

PURPOSE

Bacterial infection and antibiotic resistance are serious threats to human health. This study aimed to develop two novel radiotracers, ¹⁸F-NTRP and ¹⁸F-NCRP, that possess a specific nitroreductase (NTR) response to image deep-seated bacterial infections using positron emission tomography (PET). This method can distinguish infection from sterile inflammation.

METHODS

¹⁸F-NTRP and ¹⁸F-NCRP were synthesized via a one-step method; all the steps usually involved in tracer radiosynthesis were successfully adapted in the All-In-One automated module. After the physiochemical properties of ¹⁸F-NTRP and ¹⁸F-NCRP were characterized, their specificity and selectivity for NTR were verified in E. coli and S. aureus. The ex vivo biodistribution of the tracers was evaluated in normal mice. MicroPET-CT imaging was performed in mouse models of bacterial infection and inflammation after the administration of ¹⁸F-NTRP or ¹⁸F-NCRP.

RESULTS

Fully automated radiosynthesis of ¹⁸F-NTRP and ¹⁸F-NCRP was achieved within 90-110 min with overall decay-uncorrected, isolated radiochemical yields of 21.24 ± 4.25% and 11.3 ± 3.78%, respectively. The molar activities of ¹⁸F-NTRP and ¹⁸F-NCRP were 320 ± 40 GBq/μmol and 275 ± 33 GBq/µmol, respectively. In addition, ¹⁸F-NTRP and ¹⁸F-NCRP exhibited high selectivity and specificity for NTR response. PET-CT imaging in bacteria-infected mouse models with ¹⁸F-NTRP or ¹⁸F-NCRP showed significant radioactivity uptake in either E. coli- or S. aureus-infected muscles. The uptake for E. coli-infected muscles, 2.4 ± 0.2%ID/g with ¹⁸F-NTRP and 4.05 ± 0.49%ID/g with ¹⁸F-NCRP, was up to three times greater than that for uninfected control muscles. Furthermore, for both ¹⁸F-NTRP and ¹⁸F-NCRP, the uptake in bacterial infection was 2.6 times higher than that in sterile inflammation, allowing an effective distinction of infection from inflammation.

CONCLUSION

¹⁸F-NTRP and ¹⁸F-NCRP are worth further investigation to verify their potential clinical application for distinguishing bacterial infection from sterile inflammation via their specific NTR responsiveness.

11C-Para-aminobenzoic acid PET imaging of S. aureus and MRSA infection in preclinical models and humans

Tools for noninvasive detection of bacterial pathogens are needed but are not currently available for clinical use. We have previously shown that para-aminobenzoic acid (PABA) rapidly accumulates in a wide range of pathogenic bacteria, motivating the development of related PET radiotracers. In this study, ¹¹C-PABA PET imaging was used to accurately detect and monitor infections due to pyogenic bacteria in multiple clinically relevant animal models. ¹¹C-PABA PET imaging selectively detected infections in muscle, intervertebral discs, and methicillin-resistant Staphylococcus aureus–infected orthopedic implants. In what we believe to be first-in-human studies in healthy participants, ¹¹C-PABA was safe, well-tolerated, and had a favorable biodistribution, with low background activity in the lungs, muscles, and brain. ¹¹C-PABA has the potential for clinical translation to detect and localize a broad range of bacteria.

Galbofloxacin: a xenometal-antibiotic with potent in vitro and in vivo efficacy against S. aureus

Siderophore-antibiotic drug conjugates are considered potent tools to deliver and potentiate the antibacterial activity of antibiotics, but only few have seen preclinical and clinical success. Here, we introduce the gallium(iii) complex of a ciprofloxacin-functionalized linear desferrichrome, Galbofloxacin, with a cleavable serine linker as a potent therapeutic for S. aureus bacterial infections. We employed characterization using in vitro inhibitory assays, radiochemical, tracer-based uptake and pharmacokinetic assessment of our lead compound, culminating in in vivo efficacy studies in a soft tissue model of infection. Galbofloxacin exhibits a minimum inhibitory concentration of (MIC₉₈) 93 nM in wt S. aureus, exceeding the potency of the parent antibiotic ciprofloxacin (0.9 μM). Galbofloxacin is a protease substrate that can release the antibiotic payload in the bacterial cytoplasm. Radiochemical experiments with wt bacterial strains reveal that ⁶⁷Galbofloxacin is taken up efficiently using siderophore mediated, active uptake. Biodistribution of ⁶⁷Galbofloxacin in a mouse model of intramuscular S. aureus infection revealed renal clearance and enhanced uptake in infected muscle when compared to ⁶⁷Ga-citrate, which showed no selectivity. A subsequent in vivo drug therapy study reveals efficient reduction in S. aureus infection burden and sustained survival with Galbofloxacin for 7 days. Ciprofloxacin had no treatment efficacy at identical molecular dose (9.3 μmol kg⁻¹) and resulted in death of all study animals in <24 hours. Taken together, the favorable bacterial growth inhibitory, pharmacokinetic and in vivo efficacy properties qualify Galbofloxacin as the first rationally designed Ga-coordination complex for the management of S. aureus bacterial infections.

Galbofloxacin, a novel theranostic xenosiderophore antibiotic, exhibits unparalleled potency in combating S. aureus infections in vivo.

Efficient Preparation of Sophorolipids and Functionalization with Amino Acids to Furnish Potent Preservatives

The development of new preservatives is an ongoing investigation in the food industry, especially those which are safe and environmentally friendly. In this study, biosurfactant sophorolipids (SLs) functionalized with amino acids were developed as efficient preservative agents. SLs were first isolated from fermentation broth by Candida bombicola ATCC 22214, hydrolyzed, and purified by extraction. The typical recovery is around 70%, while the extracted material consists of over 90% deacetylated acidic SLs (SL-COOH). Four types of SL derivatives were then synthesized via dicyclohexylcarbodiimide amidation reactions from prepared SL-COOH. Among the derivatives produced, the arginine SL conjugates (SL-d-Arg) displayed the highest activity against Gram-positive bacteria and fungi and even inhibited the cell growth of Gram-negative bacteria and mildew. Furthermore, the arginine conjugates performed the broadest antimicrobial activity among the derivatives evaluated. The sterilization dosage of the arginine conjugates against the food-spoilage pathogen Bacillus spp. was 63-125 mg/L, in contrast to 250 mg/L for the enterotoxin producer Staphylococcus aureus and 500 mg/L for fungi. More importantly, SL-d-Arg displayed excellent biocompatibility, with a therapeutic index of over 7.94. SL-d-Arg has excellent potential as an alternative to traditional chemical preservatives.

Synthesis and Evaluation of Novel Norfloxacin Isonitrile 99mTc Complexes as Potential Bacterial Infection Imaging Agents

To develop potential technetium-99m single-photon emission computed tomography (SPECT) imaging agents for bacterial infection imaging, the novel norfloxacin isonitrile derivatives CN4NF and CN5NF were synthesized and radiolabeled with a [^99mTc][Tc(I)]⁺ core to obtain [^99mTc]Tc-CN4NF and [^99mTc]Tc-CN5NF. These compounds were produced in high radiolabeling yields and showed hydrophilicity and good stability in vitro. The bacterial binding assay indicated that [^99mTc]Tc-CN4NF and [^99mTc]Tc-CN5NF were specific to bacteria. Compared with [^99mTc]Tc-CN4NF, biodistribution studies of [^99mTc]Tc-CN5NF showed a higher uptake in bacteria-infected tissues than in turpentine-induced abscesses, indicating that [^99mTc]Tc-CN5NF could distinguish bacterial infection from sterile inflammation. In addition, [^99mTc]Tc-CN5NF had higher abscess/blood and abscess/muscle ratios. SPECT image of [^99mTc]Tc-CN5NF showed that there was a clear accumulation in the infection site, suggesting that it could be a potential bacterial infection imaging radiotracer.

Highlight selection of radiochemistry and radiopharmacy developments by editorial board

Background

The Editorial Board of EJNMMI Radiopharmacy and Chemistry releases a biyearly highlight commentary to update the readership on trends in the field of radiopharmaceutical development.

Results

This commentary of highlights has resulted in 23 different topics selected by each member of the Editorial Board addressing a variety of aspects ranging from novel radiochemistry to first in man application of novel radiopharmaceuticals.

Conclusion

Trends in radiochemistry and radiopharmacy are highlighted demonstrating the progress in the research field being the scope of EJNMMI Radiopharmacy and Chemistry.