The many roads to infection imaging

The Development and Validation of Radiopharmaceuticals Targeting Bacterial Infection

The International Atomic Energy Agency organized a technical meeting at its headquarters in Vienna, Austria, in 2022 that included 17 experts representing 12 countries, whose research spanned the development and use of radiolabeled agents for imaging infection. The meeting focused largely on bacterial pathogens. The group discussed and evaluated the advantages and disadvantages of several radiopharmaceuticals, as well as the science driving various imaging approaches. The main objective was to understand why few infection-targeted radiotracers are used in clinical practice despite the urgent need to better characterize bacterial infections. This article summarizes the resulting consensus, at least among the included scientists and countries, on the current status of radiopharmaceutical development for infection imaging. Also included are opinions and recommendations regarding current research standards in this area. This and future International Atomic Energy Agency-sponsored collaborations will advance the goal of providing the medical community with innovative, practical tools for the specific image-based diagnosis of infection.

Radiolabeling and evaluation of a novel [99mTcN]2+ complex with deferoxamine dithiocarbamate as a potential agent for bacterial infection imaging

In order to find a ^99mTc-labeled deferoxamine radiotracer for bacterial infection imaging, deferoxamine dithiocarbamate (DFODTC) was successfully synthesized and it was radiolabeled with [^99mTcN]²⁺ core to prepare the ^99mTcN(DFODTC)₂ complex. ^99mTcN(DFODTC)₂ was obtained with high radiochemical purity without further purification. The complex was lipophilic and exhibited good in vitro stability. According to the result of bacterial binding study, the binding of ^99mTcN(DFODTC)₂ to bacteria was specific. Biodistribution in mice study indicated that ^99mTcN(DFODTC)₂ had a higher uptake in bacterial infection tissues than in turpentine-induced abscesses at 120 min after injection, which showed that the radiotracer could differentiate between bacterial infection and sterile inflammation. SPECT/CT images showed that there was a clear accumulation in infection sites, suggesting that ^99mTcN(DFODTC)₂ could be a potential bacterial infection imaging radiotracer.

Recent Progress in the Molecular Imaging of Tumor-Treating Bacteria

Bacterial cancer therapy (BCT) approaches have been extensively investigated because bacteria can show unique features of strong tropism for cancer, proliferation inside tumors, and antitumor immunity, while bacteria are also possible agents for drug delivery. Despite the rapidly increasing number of preclinical studies using BCT to overcome the limitations of conventional cancer treatments, very few BCT studies have advanced to clinical trials. In patients undergoing BCT, the precise localization and quantification of bacterial density in different body locations is important; however, most clinical trials have used subjective clinical signs and invasive sampling to confirm bacterial colonization. There is therefore a need to improve the visualization of bacterial densities using noninvasive and repetitive in vivo imaging techniques that can facilitate the clinical translation of BCT. In vivo optical imaging techniques using bioluminescence and fluorescence, which are extensively employed to image the therapeutic process of BCT in small animal research, are hard to apply to the human body because of their low penetrative power. Thus, new imaging techniques need to be developed for clinical trials. In this review, we provide an overview of the various in vivo bacteria-specific imaging techniques available for visualizing tumor-treating bacteria in BCT studies.

Radiotracer Development for Bacterial Imaging

Bacterial infections remain a major threat to humanity and are a leading cause of death and disability. Antimicrobial resistance has been declared as one of the top ten threats to human health by the World Health Organization, and new technologies are urgently needed for the early diagnosis and monitoring of deep-seated and complicated infections in hospitalized patients. This review summarizes the radiotracers as applied to imaging of bacterial infections. We summarize the recent progress in the development of pathogen-specific imaging and the application of radiotracers in understanding drug pharmacokinetics as well as the local biology at the infection sites. We also highlight the opportunities for medicinal chemists in radiotracer development for bacterial infections, with an emphasis on target selection and radiosynthetic approaches. Imaging of infections is an emerging field. Beyond clinical applications, these technologies could provide unique insights into disease pathogenesis and expedite bench-to-bedside translation of new therapeutics.

Molecular Imaging of Diabetic Foot Infections: New Tools for Old Questions

Diabetic foot infections (DFIs) are a common, complex, and costly medical problem with increasing prevalence. Diagnosing DFIs is a clinical challenge due to the poor specificity of the available methods to accurately determine the presence of infection in these patients. However, failure to perform an opportune diagnosis and provide optimal antibiotic therapy can lead to higher morbidity for the patient, unnecessary amputations, and increased healthcare costs. Novel developments in bacteria-specific molecular imaging can provide a non-invasive assessment of the infection site to support diagnosis, determine the extension and location of the infection, guide the selection of antibiotics, and monitor the response to treatment. This is a review of recent research in molecular imaging of infections in the context of DFI. We summarize different clinical and preclinical methods and the translational implications aimed to improve the care of patients with DFI.

Molecular Imaging: a Novel Tool To Visualize Pathogenesis of Infections In Situ

Molecular imaging is an emerging technology that enables the noninvasive visualization, characterization, and quantification of molecular events within living subjects. Positron emission tomography (PET) is a clinically available molecular imaging tool with significant potential to study pathogenesis of infections in humans.

Molecular imaging is an emerging technology that enables the noninvasive visualization, characterization, and quantification of molecular events within living subjects. Positron emission tomography (PET) is a clinically available molecular imaging tool with significant potential to study pathogenesis of infections in humans. PET enables dynamic assessment of infectious processes within the same subject with high temporal and spatial resolution and obviates the need for invasive tissue sampling, which is difficult in patients and generally limited to a single time point, even in animal models. This review presents current state-of-the-art concepts on the application of molecular imaging for infectious diseases and details how PET imaging can facilitate novel insights into infectious processes, ongoing development of pathogen-specific imaging, and simultaneous in situ measurements of intralesional antimicrobial pharmacokinetics in multiple compartments, including privileged sites. Finally, the potential clinical applications of this promising technology are also discussed.

Molecular imaging of bacterial infections: Overcoming the barriers to clinical translation

Clinical diagnostic tools requiring direct sample testing cannot be applied to infections deep within the body, and clinically available imaging tools lack specificity. New approaches are needed for early diagnosis and monitoring of bacterial infections and rapid detection of drug-resistant organisms. Molecular imaging allows for longitudinal, noninvasive assessments and can provide key information about infectious processes deep within the body.

The role of Technetium-99m-Ethambutol scintigraphy in the management of spinal tuberculosis

Spines is a common site of extrapulmonary Mycobacterium tuberculosis infection (MTI). Spine destruction due to MTI can mimic other etiologies. Treatment of choice for spinal tuberculosis (STB) is anti-TB drugs while surgery could be needed in other causes. The gold standard for STB diagnosis is histopathology examination from biopsy tissue. Technetium-99m-ethambutol (^99mTc-EMB) scintigraphy can be used to detect and localize of TB. The aim of this study was to evaluate the role of ^99mTc-EMB scintigraphy in STB management. Retrospective study was carried out from 2006 to 2014. Subject STB were patient STB with suspected of STB and underwent ^99mTc-EMB scintigraphy. The histopathologic result was used as gold standard. Whole body planar acquisition was taken at 1 and 3 h postinjection of 370 MBq. Single-photon emission computed tomography/computed tomography acquisition was performed on suspected area. ^99mTc-EMB image were analyzed by two nuclear medicine specialis. The 93 subject STB were included in this study. Histopathologic data were available in 40/93 subject STB. Positive and negative ^99mTc-EMB scintigraphy were 32 and 8 subject STB. ^99mTc-EMB scintigraphy result STB was concordance with the histopathologic finding in 37 subject STB. Sensitivity, specivicity, positive- and negative-predictive value, and accuracy of ^99mTc-EMB scintigraphy 90.91%, 71.43%, 93.75%, 62.5%, and 87.5%, respectively. This study showed that patient STB with suspected ^99mTc-EMB scintigraphy result could be directly treated with anti-TB. ^99mTc-EMB scintigraphy has significant role in the management of STB.

Pathogen-Specific Bacterial Imaging in Nuclear Medicine

When serious infections are suspected, patients are often treated empirically with broad-spectrum antibiotics while awaiting results that provide information on the bacterial class and species causing the infection, as well as drug susceptibilities. For deep-seated infections, these traditional diagnostic techniques often rely on tissue biopsies to obtain clinical samples which can be expensive, dangerous, and has the potential of sampling bias. Moreover, these procedures and results can take several days and may not always provide reliable information. This combination of time and effort required for proper antibiotic selection has become a barrier leading to indiscriminate broad-spectrum antibiotic use. Exposure to nosocomial infections and indiscriminate use of broad-spectrum antibiotics are responsible for promoting bacterial drug-resistance leading to substantial morbidity and mortality, especially in hospitalized and immunosuppressed patients. Therefore, early diagnosis of infection and targeted antibiotic treatments are urgently needed to reduce morbidity and mortality caused by bacterial infections worldwide. Reliable pathogen-specific bacterial imaging techniques have the potential to provide early diagnosis and guide antibiotic treatments.

Designing a New Molecular Probe: The Potential Role for Tilmanocept (Lymphoseek®) in the Assessment of Patients with Painful Hip and Knee Joint Prostheses

BACKGROUND

There is a long history of nuclear medicine developments in orthopaedics beginning in the early 20th century. Technetium-99m (99mTc) has a short half-life of six hours, emits 140 keV gamma rays and is the most widely used isotope, imaged with the Anger (gamma) camera. Gamma image quality and test sensitivity in painful prosthetic joints can be improved with single photon emission computed tomography (SPECT) and SPECT/CT. Positron Emission Tomography-Computed Tomography (PET-CT) with Sodium Fluoride (18F-NaF) and 18Fluorine-fluorodeoxyglucose (18F-FDG) PET have promising and limited roles respectively in the investigation of painful prosthetic joints. New SPECT/CT and PET-CT isotopes targeting activated macrophages with 99mTc Tilmanocept (Lymphoseek®) and 68Gallium labelled Tilmanocept respectively show potential as agents to demonstrate wear particles ingested by macrophages and multinucleated giant cells. An imaging algorithm using SPECT and/or PET agents is proffered as a cost effective way of speedily and accurately arriving a diagnosis.

METHODS

Review of the historical role of nuclear medicine in orthopaedics and research into the potential role of new radiopharmaceutical agents was undertaken. Guidelines and algorithms for the imaging of complicated joint prosthesis are provided.

RESULTS

There is an established role for nuclear medicine in orthopaedics and particularly in the investigation of complicated joint prostheses. Imaging with Tilmanocept provides new opportunities to shorten the time to diagnose loosened and infected joint prostheses.

CONCLUSION

There is a potential new role for Tilmanocept, which can be utilised with both PET-CT and SPECT-CT technologies. Tilmanocept is a relatively new radiopharmaceutical which has a potential role in the imaging assessment of painful joint prosthesis.

A Systematic Approach for Developing Bacteria-Specific Imaging Tracers

The modern patient is increasingly susceptible to bacterial infections including those due to multidrug-resistant organisms (MDROs). Noninvasive whole-body analysis with pathogen-specific imaging technologies can significantly improve patient outcomes by rapidly identifying a source of infection and monitoring the response to treatment, but no such technology exists clinically.

METHODS

We systematically screened 961 random radiolabeled molecules in silico as substrates for essential metabolic pathways in bacteria, followed by in vitro uptake in representative bacteria-Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, and mycobacteria. Fluorine-labeled analogs, that could be developed as PET-based imaging tracers, were evaluated in a murine myositis model.

RESULTS

We identified 3 novel, nontoxic molecules demonstrating selective bacterial uptake: para-aminobenzoic acid (PABA), with uptake in all representative bacteria including Mycobacterium tuberculosis; mannitol, with selective uptake in S. aureus and E. coli; and sorbitol, accumulating only in E. coli None accumulated in mammalian cells or heat-killed bacteria, suggesting metabolism-derived specificity. In addition to an extended bacterial panel of laboratory strains, all 3 molecules rapidly accumulated in respective clinical isolates of interest including MDROs such as methicillin-resistant S. aureus, extended-spectrum β-lactamase-producing, and carbapenem-resistant Enterobacteriaceae. In a murine myositis model, fluorine-labeled analogs of all 3 molecules could rapidly detect and differentiate infection sites from sterile inflammation in mice (P = 0.03). Finally, 2-deoxy-2-[F-18]fluoro-d-sorbitol (¹⁸F-FDS) can be easily synthesized from ¹⁸F-FDG. PET, with ¹⁸F-FDS synthesized using current good manufacturing practice, could rapidly differentiate true infection from sterile inflammation to selectively localize E. coli infection in mice.

CONCLUSION

We have developed a systematic approach that exploits unique biochemical pathways in bacteria to develop novel pathogen-specific imaging tracers. These tracers have significant potential for clinical translation to specifically detect and localize a broad range of bacteria, including MDROs.

Imaging Enterobacteriaceae infection in vivo with 18F-fluorodeoxysorbitol positron emission tomography

The Enterobacteriaceae are a family of rod-shaped Gram-negative bacteria that normally inhabit the gastrointestinal tract and are the most common cause of Gram-negative bacterial infections in humans. In addition to causing serious multidrug-resistant, hospital-acquired infections, a number of Enterobacteriaceae species are also recognized as biothreat pathogens. As a consequence, new tools are urgently needed to specifically identify and localize infections due to Enterobacteriaceae and to monitor antimicrobial efficacy. In this report, we used commercially available 2-[(18)F]-fluorodeoxyglucose ((18)F-FDG) to produce 2-[(18)F]-fluorodeoxysorbitol ((18)F-FDS), a radioactive probe for Enterobacteriaceae, in 30 min. (18)F-FDS selectively accumulated in Enterobacteriaceae, but not in Gram-positive bacteria or healthy mammalian or cancer cells in vitro. In a murine myositis model, (18)F-FDS positron emission tomography (PET) rapidly differentiated true infection from sterile inflammation with a limit of detection of 6.2 ± 0.2 log10 colony-forming units (CFU) for Escherichia coli. Our findings were extended to models of mixed Gram-positive and Gram-negative thigh co-infections, brain infection, Klebsiella pneumonia, and mice undergoing immunosuppressive chemotherapy. This technique rapidly and specifically localized infections due to Enterobacteriaceae, providing a three-dimensional holistic view within the animal. Last, (18)F-FDS PET monitored the efficacy of antimicrobial treatment, demonstrating a PET signal proportionate to the bacterial burden. Therapeutic failures associated with multidrug-resistant, extended-spectrum β-lactamase (ESBL)-producing E. coli infections were detected in real time. Together, these data show that (18)F-FDS is a candidate imaging probe for translation to human clinical cases of known or suspected infections owing to Enterobacteriaceae.

Functional Imaging in Diagnostic of Orthopedic Implant-Associated Infections

Surgeries’ sterile conditions and perioperative antibiotic therapies decrease implant associated infections rates significantly. However, up to 10% of orthopedic devices still fail due to infections. An implant infection generates a high socio-economic burden. An early diagnosis of an infection would significantly improve patients’ outcomes. There are numerous clinical tests to diagnose infections. The “Gold Standard” is a microbiological culture, which requires an invasive sampling and lasts up to several weeks. None of the existing tests in clinics alone is sufficient for a conclusive diagnosis of an infection. Meanwhile, there are functional imaging modalities, which hold the promise of a non-invasive, quick, and specific infection diagnostic. This review focuses on orthopedic implant-associated infections, their pathogenicity, diagnosis and functional imaging.

Optical imaging of bacterial infection in living mice using deep-red fluorescent squaraine rotaxane probes

Two structurally related fluorescent imaging probes allow optical imaging of bacterial leg infection models in living athymic and immunocompetent mice. Structurally, the probes are comprised of a deep-red fluorescent squaraine rotaxane scaffold with two appended bis(zinc(II)-dicolylamine) (bis(Zn-DPA)) targeting ligands. The bis(Zn-DPA) ligands have high affinity for the anionic phospholipids and related biomolecules that reside within the bacterial envelope, and they are known to selectively target bacterial cells over the nearly uncharged membrane surfaces of healthy mammalian cells. Planar, whole-animal optical imaging studies showed that intravenous dosing of either probe (10 nmol) allowed imaging of localized infections of Gram-positive Staphylococcus aureus and Gram-negative Salmonella enterica serovar typhimurium. High selectivity for the infected target leg (T) over the contralateral nontarget leg (NT) was reflected by T/NT ratios up to six. The infection imaging signal was independent of mouse humoral immune status, and there was essentially no targeting at a site of sterile inflammation induced by injection of lambda-carrageenan. Furthermore, the fluorescent probe imaging signal colocalized with the bioluminescence signal from a genetically engineered strain of S. enterica serovar typhimurium. Although not highly sensitive (the localized infection must contain at least approximately 10(6) colony forming units for fluorescence visualization), the probes are remarkably selective for bacterial cells considering their low molecular weight (<1.5 kDa) and simple structural design. The more hydrophilic of the two probes produced a higher T/NT ratio in the early stages of the imaging experiment and washed out more rapidly from the blood clearance organs (liver, kidney). Therefore, it is best suited for longitudinal studies that require repeated dosing and imaging of the same animal. The results indicate that fluorescent probes based on squaraine rotaxanes should be broadly useful for in vivo animal imaging studies, and they further validate the ability of imaging probes with bis(Zn-DPA) ligands to selectively target bacterial infections in living animals.

Expanding role of 18F-fluoro-D-deoxyglucose PET and PET/CT in spinal infections

(18)F-fluoro-D -deoxyglucose positron emission tomography ([(18)F]-FDG PET) is successfully employed as a molecular imaging technique in oncology, and has become a promising imaging modality in the field of infection. The non-invasive diagnosis of spinal infections (SI) has been a challenge for physicians for many years. Morphological imaging modalities such as conventional radiography, computed tomography (CT), and magnetic resonance imaging (MRI) are techniques frequently used in patients with SI. However, these methods are sometimes non-specific, and difficulties in differentiating infectious from degenerative end-plate abnormalities or postoperative changes can occur. Moreover, in contrast to CT and MRI, FDG uptake in PET is not hampered by metallic implant-associated artifacts. Conventional radionuclide imaging tests, such as bone scintigraphy, labeled leukocyte, and gallium scanning, suffer from relatively poor spatial resolution and lack sensitivity, specificity, or both. Initial data show that [(18)F]-FDG PET is an emerging imaging technique for diagnosing SI. [(18)F]-FDG PET appears to be especially helpful in those cases in which MRI cannot be performed or is non-diagnostic, and as an adjunct in patients in whom the diagnosis is inconclusive. The article reviews the currently available literature on [(18)F]-FDG PET and PET/CT in the diagnosis of SI.