A Pooled Analysis of Diagnostic Value of 99mTc-Ubiquicidin (UBI) Scintigraphy in Detection of an Infectious Process

PET/CT and SPECT/CT for Infection in Joints and Bones: An Overview and Future Directions

Infections of the bones and joints, if misdiagnosed, may result in serious morbidity and even mortality. A prompt diagnosis followed by appropriate management may reduce the socioeconomic impact of bone and joint infections. Morphologic imaging such as ultrasound and plain radiographs form the first line investigations, however, in early infections findings may be negative or nonspecific. Nuclear medicine imaging techniques play a complementary role to morphologic imaging in the diagnosis of bone and joint infections. The availability of hybrid systems (SPECT/CT, SPECT/MRI, PET/CT or PET/MRI) offers improved specificity with ability to assess the extent of infection. Bone scans are useful as a gatekeeper wherein negative scans rule out sepsis with a good accuracy, however positive scans are nondiagnostic and more specific tracers should be considered. These include the use of labeled white blood cells and antigranulocyte antibodies. Various qualitative and quantitative interpretation criteria have been suggested to improve the specificity of the scans. PET has better image resolution and 18F-FDG is the major tracer for PET imaging with applications in oncology and inflammatory/infective disorders. It has demonstrated improved sensitivity over the SPECT based tracers, however, still suffers from lack of specificity. 18F-FDG PET has been used to monitor therapy in bone and joint infections. Other less studied, noncommercialized SPECT and PET tracers such as 111In-Biotin, 99mTc-Ubiquicidin, 18F-Na-Fluoride, 18F-labeled white blood cells and 124I-Fialuridine to name a few have shown great promise, however, their role in various bone and joint infections has not been established. Hybrid imaging with PET or PET/MRI offers huge potential for improving diagnostics in infections of the joints and bones.

Radiosynthesis Standardization and Preclinical Assessment of the [68Ga]Ga-DOTA-Ubiquicidin29-41: A Translational Study Targeting Differential Diagnosis of Infectious Processes

Human bacterial infections significantly contribute to the increase in healthcare-related burdens. This scenario drives the study of novel techniques for the early and precise diagnosis of infectious processes. Some alternatives include Nuclear Medicine- and Molecular Imaging-based strategies. However, radiopharmaceuticals that are available for routine assessments are not specific to differentiating infectious from aseptic inflammatory processes. In this context, [68Ga]Ga-DOTA-Ubiquicidin29-41 was synthesized using an automated module and radiochemical; in vivo and in vitro studies were performed. The radiopharmaceutical remained stable in saline (up to 180 min) and in rodent serum (up to 120 min) with radiochemical purities > 99 and 95%, respectively. Partition coefficient and serum protein binding at 60 min were determined (-3.63 ± 0.17 and 44.06 ± 1.88%, respectively). Ex vivo biodistribution, as well as in vivo microPET/CT images in mice, showed rapid blood clearance with renal excretion and reduced uptake in other organs in Staphylococcus aureus-infected animals. Higher uptake was observed in the target as compared to the non-target tissue (p < 0.0001) at 60 min post administration. The presented in-human clinical case demonstrates uptake of the radiopharmaceutical by Staphyloccocus aureus bacteria. These results indicate the potential of [68Ga]Ga-DOTA-Ubiquicidin29-41 as a radiopharmaceutical that can be obtained in a hospital radiopharmacy for the diagnosis of infectious processes using PET/CT.

A Brief History of Nuclear Medicine Imaging of Infection

For nearly 50 years, nuclear medicine has played an important role in the diagnosis of infection. Gallium citrate Ga 67 was one of the first, if not the first, radionuclide used for this purpose. Unfavorable imaging characteristics, a lack of specificity, and the long interval (2-3 days) between administration and imaging spurred the search for alternatives. At the present time, gallium 67 citrate is used primarily for differentiating acute tubular necrosis from interstitial nephritis and as an alternative for indications including sarcoid, spondylodiscitis, and fever of unknown origin, when 18F-fluorodeoxyglucose (18F-FDG) is not available. The approval, in the mid-1980s, of techniques for in vitro labeling of leukocytes with indium-111 and technetium-99m that subsequently migrate to foci of infection was a significant advance in nuclear medicine imaging of infection and labeled leukocyte imaging still plays an important role in imaging of infection. There are significant disadvantages to in vitro labeled leukocyte imaging. Unfortunately, efforts devoted to developing in vivo leukocyte labeling methods have met with only limited success. Over the past 20 years 18F-FDG has established itself as a valuable imaging agent for musculoskeletal and cardiovascular infections, as well as sarcoidosis and fever of unknown origin. As useful as these agents are, their uptake is based on the host response to infection, not infection itself. Previous attempts at developing infection-specific agents, including radiolabeled antibiotics and vitamins, were limited by poor results and/or lack of availability, so investigators continue to focus on developing infection-specific nuclear medicine imaging agents.

Diagnosis of Fungal Infection (Candida albicans) After Heart Transplantation in a Pediatric Case with Fever of Unknown Origin: Role of 99mTc-UBI SPECT/CT and 18F-FDG PET/CT

The diagnosis of patients with fever of unknown origin (FUO) in pediatric heart transplantation is a challenging medical problem. The physician should differentiate between rejections, infections, malignancy, adrenal insufficiency, and drug fever. Immunosuppressive therapy in these patients exposes them to a high risk of developing a post-transplantation fungal infection. In this case, we discuss the diagnostic contribution of the 99mTc-UBI scan and 18F-FDG PET scan for diagnosis of fungal infection causing FUO in these patients.

Tc-99 m Ubiquicidin Imaging in Orbital Aspergilloma: an Illustration

Aspergillus infection is relatively rare disease, and we present a case of orbital aspergillus infection who presented with right orbital pain and swelling. Right orbital lesion was identified on CT, MRI, and PET-CT imaging followed by confirmation of aspergillus on histopathological examination. We demonstrate that Tc-99 m ubiquicidin scan can yield positive results in aspergillosis too, enabling its differentiation from non-infective pathologies.

Metabolic fluorine labeling and hotspot imaging of dynamic gut microbiota in mice

Real-time localization and microbial activity information of indigenous gut microbiota over an extended period of time remains a challenge with existing visualizing methods. Here, we report a metabolic fluorine labeling (MEFLA)-based strategy for monitoring the dynamic gut microbiota via ¹⁹F magnetic resonance imaging (¹⁹F MRI). In situ labeling of different microbiota subgroups is achieved by using a panel of peptidoglycan-targeting MEFLA probes containing ¹⁹F atoms of different chemical shifts, and subsequent real-time in vivo imaging is accomplished by multiplexed hotspot ¹⁹F MRI with high sensitivity and unlimited penetration. Using this method, we realize extended visualization (>24 hours) of native gut microbes located at different intestinal sections and semiquantitative analysis of their metabolic dynamics modulated by various conditions, such as the host death and different β-lactam antibiotics. Our strategy holds great potential for noninvasive and real-time assessing of the metabolic activities and locations of the highly dynamic gut microbiota.

A decade of ubiquicidin development for PET imaging of infection: A systematic review

BACKGROUND

Ubiquicidin is a peptide fragment with selective binding to negatively charged bacterial cell membranes. Besides its earlier labelling with gamma emitting radionuclides, it has been labelled with Positron Emission Tomography (PET) radionuclides in the last decade for imaging infection and distinguishing infectious disease from sterile inflammation. This systematic review aims to evaluate the technology readiness level of PET based ubiquicidin radiopharmaceuticals.

METHODS

Two independent researchers reviewed all articles and abstracts pertaining ubiquicidin and PET imaging that are currently available. Scopus, Google Scholar and PubMed/Medline were used in the search. Upon completion of the literature search all articles and abstracts were evaluated and duplicates were excluded. All non-PET articles as well as review articles without new data were deemed ineligible.

RESULTS

From a total of 17 papers and 10 abstracts the studies were grouped into development, preclinical and clinical studies. Development was published in 15/17 (88%) publications and 6/10 (60%) abstracts, preclinical applications in 9/17 (53%) publications and 1/10 (10%) of abstracts. Finally, clinical studies made up 6/17 (35%) of full publications and 4/10 (40%) of the available abstracts. Development results were the most abundant. All the findings in the different areas of development of ubiquicidin as PET radiopharmaceutical are summarized in this paper.

CONCLUSION

Labelling procedures are generally uncomplicated and relatively fast and there are indications of adequate product stability. The production of PET radiopharmaceuticals based on UBI will therefore not be a barrier for clinical introduction of this technology. Systematization and unification of criteria for preclinical imaging and larger clinical trials are needed to ensure the translation of this radiopharmaceutical into the clinic. Therefore a conclusion with regards to the clinical relevance of ubiquicidin based PET is not yet possible.

Interventional nuclear medicine: "click" chemistry as an in vivo targeting strategy for imaging microspheres and bacteria

AIM

Pre-targeting is a proven strategy for in vivo delivery of a diagnostic or therapeutic payload. The pre-targeting concept can be realized through various conjugation strategies, one of which is based on copper-free "click" chemistry. Copper-free click reactions have shown in vivo potential for imaging and radionuclide therapy, but this conjugation strategy has not yet been explored in combination with microspheres or unicellular organisms. This study aims to evaluate the in vivo efficacy of strain-promoted azide-alkyne cycloaddition (SPAAC) reactions to achieve imaging and targeting of azide-functionalized macro-aggregated albumin (MAA) microspheres and Staphylococcus aureus bacteria.

METHODS

MAA microspheres (diameter 10-90 μm) were functionalized with a biorthogonal Cy5 fluorophore, bearing an azide functionality (N3), to generate MAA-Cy5-N3. S. aureus (diameter ∼1 μm) were functionalized with 99mTc-UBI29-41-Cy5-N3, generating S. aureus-99mTc-UBI29-41-Cy5-N3. In situ and in vitro click conjugation on the -N3 moieties was studied for 20 h using a radioactivity-based assay and fluorescence microscopy. For in vivo validation, both primary entities, radiolabeled with 99mTc, were deposited into the microvasculature of the liver via intrasplenic injections. Secondary targeting was realized following the intravenous administration of indium-111-radiolabeled diethylenetriaminepentaacetic acid-dibenzocyclooctyne (111In-DTPA-DBCO). To assess click reaction efficiency in vivo, 99mTc and 111In-biodistributions were measured (SPECT and %ID g-1). Use of 111In-DTPA-DBCO in mice without MAA deposits or mice infected with non-functionalized S. aureus served as controls. Ex vivo confocal fluorescence imaging was carried out in excised tissues to confirm the presence of functionalized MAA and bacteria.

RESULTS

In vitro data confirmed effective click reactions on both the MAA particles and the bacterial membrane. SPECT imaging and biodistribution studies revealed significantly (p < 0.05) increased accumulation of 111In-DTPA-DBCO at the sites where MAA-Cy5-N3 (7.5 ± 1.5%ID g-1vs. 3.5 ± 0.5%ID g-1 in control mice) and S. aureus-99mTc-UBI29-41-Cy5-N3 (9.3 ± 1.3%ID g-1vs. 6.0 ± 0.5%ID g-1 in control mice) resided. Ex vivo fluorescence imaging confirmed the presence of either functionalized MAA or S. aureus in excised spleens and livers of mice.

CONCLUSION

Copper-free click chemistry between a DBCO moiety and Cy5-N3-functionalized microspheres or bacterial entities in the liver can be used to realize in vivo imaging and targeting.

A magneto-fluorescence bacteria assay strategy based on dual colour sulfide fluorescent nanoparticles with high near-IR conversion efficiency

Anti-Stokes fluorescence induced by near-IR (NIR) radiation is particularly advantageous for the bioassay of complex samples, but most of the commonly used NIR-induced fluorescence nanomaterials such as up-conversion nanoparticles (UCNPs) do not exhibit satisfactory fluorescence intensity and work against achieving a highly sensitive bioassay. In this study, we a construct sensitive and specific bacteria biosensor based on the NIR-stimulated CaS: Eu, Sm, Mn and SrS: Ce, Sm, Mn nanoparticles. The fluorescent nanoparticles are conjugated with bacteria recognition fragments. In addition, the independent emission bands of these two types of fluorescent nanoparticles make it possible to detect and quantify Gram-positive strain and Gram-negative strain, simultaneously. Intense fluorescence and magnetic enrichment of magneto-fluorescence systems enable bacteria discrimination with the naked eye and improve sensitivity in trace bacteria detection (<20 CFU mL^-1). The linear relationship between the fluorescence intensity and bacterial concentration is established with a detection range of 25-10⁶ CFU mL^-1. Furthermore, this NIR-excited assay strategy demonstrates better anti-interference capability than UV/visible-excited assay methods, showing high potential and practical value for medical diagnostics and bacteria monitoring.

Evaluation of genotoxic potential of peptides used in nuclear medicine (PSMA -617 and -11, and ubiquicidine 29-41) using a flow-cytometric, semi-automated analysis of micronuclei frequency in cell cultures

Assays that rely on the assessment of frequency of micronuclei are important standard techniques currently used to quantify potential genotoxic damage after exposure to chemical or physical agents, such as ionizing radiation, or in pre-clinical studies, to assessment of the genotoxic potential of drugs or its components. The experiments are usually performed using conventional microscopy, but currently the protocols are being upgraded to automated approaches based on flow cytometry protocols based on the elimination of the plasma membrane by chemical agents, allowing quantification by flow cytometry. In this work, the genotoxic potential of peptides used as components of radiopharmaceuticals (PSMA-617 and 11 and Ubiquicidine) was evaluated exposing CHO-KI cells to a wide range of concentration (0.1X and 100X the maximum allowed concentration to human adults). Incubation with PSMA-11 or UBI29-41 did not induce genotoxicity. After 24 h of incubation, PSMA-617 induced genotoxicity only in non-practical concentration (100-fold). Results corroborate the safety of the pre-drugs and the wide detection range of technique.

Evaluation of the specific uptake of radiolabeled Staphylococcus aureus aptamers in the infectious foci

The specific uptake of ^99mTc radiolabeled Staphylococcus aureus aptamers in the infectious foci was evaluated by scintigraphic imaging of infection-bearing mice. The radiotracer uptake was inhibited by non-radiolabeled aptamers in a competition assay. In addition, when a different number of bacterial cells was used to infect mice an increase in the target/non-target ratios of images correlated with the increase of CFU per gram of tissue was verified. These results confirmed that ^99mTc-aptamers were specific to bacterial focus and the level of uptake was dependent on the number of bacterial cells.

Molecular Imaging of Infection: The First 50 Years

Despite significant advances in the understanding of microorganisms and an increased availability of antimicrobial therapy, infection remains a major cause of morbidity and mortality. The diagnosis can be challenging and imaging studies often are used for confirmation and localization. For nearly 50 years, molecular imaging agents have played an important role in the diagnosis of infection. Gallium-67 citrate was perhaps the first molecular imaging agent used for diagnosing and localizing infection. Poor imaging characteristics, along with a lack of specificity, and the long (usually 48-72 hours) interval between administration and imaging motivated investigators to search for alternatives. Currently the role of ⁶⁷Ga is limited to differentiating acute tubular necrosis from interstitial nephritis and as an alternative to ¹⁸F-FDG for indications, such as sarcoid, spondylodiscitis, and fever of unknown origin, when the latter is not available. The development, in the mid-1970s, of techniques for radiolabeling leukocytes that subsequently migrate to foci of infection was a significant advance and labeled leukocyte imaging still has a preeminent role in molecular imaging of infection. There are significant disadvantages to in-vitro labeled leukocyte imaging. Efforts devoted to developing in-vivo leukocyte labeling methods, however, met with only limited success. Over the past 20 years ¹⁸F-FDG has established itself as the molecular imaging agent of choice for fever of unknown origin, vasculitis, sarcoid, and spondylodiscitis. As useful as these agents are, their uptake is based on the host response to infection, not infection itself. Previous attempts at developing infection specific agents, including radiolabeled antibiotics, antibiotics, and vitamins like biotin were limited by poor results and/or limited availability and so investigators continue to focus on developing infection specific molecular imaging agents. Initial results with radiolabeled nucleoside analogs, sugars, and amino acids, and a renewed interest in radiolabeled antibiotics for both diagnosis and monitoring treatment are exciting and hold great promise for the future.

Multimodal Tracking of Controlled Staphylococcus aureus Infections in Mice

There is a need to develop diagnostic and analytical tools that allow noninvasive monitoring of bacterial growth and dissemination in vivo. For such cell-tracking studies to hold translational value to controlled human infections, in which volunteers are experimentally colonized, they should not require genetic modification, and they should allow tracking over a number of replication cycles. To gauge if an antimicrobial peptide tracer, 99mTc-UBI29-41-Cy5, which contains both a fluorescent and a radioactive moiety, could be used for such in vivo bacterial tracking, we performed longitudinal imaging of a thigh-muscle infection with 99mTc-UBI29-41-Cy5-labeled Staphylococcus aureus. Mice were imaged using SPECT and fluorescence-imaging modalities at various intervals during a 28 h period. Biodistribution analyses were performed to quantitate radioactivity in the abscess and other tissues. SPECT and fluorescence imaging in mice showed clear retention of the 99mTc-UBI29-41-Cy5-labeled bacteria following inoculation in the thigh muscle. Despite bacterial replication, the signal intensity in the abscess only modestly decreased within a 28 h period: 52% of the total injected radioactivity per gram of tissue (%ID/g) at 4 h postinfection (pi) versus 44%ID/g at 28 h pi (15% decrease). After inoculation, a portion of the bacteria disseminated from the abscess, and S. aureus cultures were obtained from radioactive urine samples. Bacterial staining with 99mTc-UBI29-41-Cy5 allowed noninvasive bacterial-cell tracking during a 28 h period. Given the versatility of the presented bacterial-tracking method, we believe that this concept could pave the way for precise imaging capabilities during controlled-human-infection studies.

Enhanced avidity from a multivalent fluorescent antimicrobial peptide enables pathogen detection in a human lung model

Rapid in situ detection of pathogens coupled with high resolution imaging in the distal human lung has the potential to provide new insights and diagnostic utility in patients in whom pneumonia is suspected. We have previously described an antimicrobial peptide (AMP) Ubiquicidin (fragment UBI29-41) labelled with an environmentally sensitive fluorophore that optically detected bacteria in vitro but not ex vivo. Here, we describe further chemical development of this compound and demonstrate that altering the secondary structure of the AMP to generate a tri-branched dendrimeric scaffold provides enhanced signal in vitro and ex vivo and consequently allows the rapid detection of pathogens in situ in an explanted human lung. This compound (NBD-UBIdend) demonstrates bacterial labelling specificity for a broad panel of pathogenic bacteria and Aspergillus fumigatus. NBD-UBIdend demonstrated high signal-to-noise fluorescence amplification upon target engagement, did not label host mammalian cells and was non-toxic and chemically robust within the inflamed biological environment. Intrapulmonary delivery of NBD-UBIdend, coupled with optical endomicroscopy demonstrated real-time, in situ detection of bacteria in explanted whole human Cystic Fibrosis lungs.

68Ga-NOTA-ubiquicidin fragment for PET imaging of infection: From bench to bedside

This study explores the possibility of formulation of a cold kit for fast and easy preparation of a PET radiopharmaceutical, ⁶⁸Ga-NOTA-UBI (29-41) for clinical translation. In this study, Circular dichroism (CD) spectroscopy to study conformation of NOTA-UBI (29-41) and its comparison with conformation of UBI (29-41) was done. Pharmaceutical grade cold kits of NOTA-UBI (29-41) were formulated for radiolabeling with ⁶⁸Ga and necessary quality control tests were carried out. ⁶⁸Ga-NOTA-UBI (29-41) could be prepared in >90% radiochemical yield and radiochemical purity using cold kits of NOTA-UBI (29-41). In vitro and in vivo evaluation of ⁶⁸Ga-NOTA-UBI (29-41) was done to demonstrate specificity of the agent for imaging infection. Kits were utilized for preparation of patient dose of ⁶⁸Ga-NOTA-UBI (29-41). Simple instant thin layer chromatography (ITLC) method for estimating radiolabeling yield of ⁶⁸Ga-NOTA-UBI (29-41) at hospital radiopharmacy was demonstrated. Clinical evaluation was done in patients with suspected infection. 148-185 MBq of ⁶⁸Ga-NOTA-UBI (29-41) was injected intravenously in three patients. ⁶⁸Ga-NOTA-UBI (29-41) uptake could clearly delineate infection foci from non target normal tissues. This is the first report on formulation of a cold kit of NOTA-UBI (29-41) for preparation of ⁶⁸Ga labeled NOTA-UBI(29-41) at hospital radiopharmacy for infection imaging. Initial clinical evaluation reveal it to be a prospective agent for imaging infection.

Gallium-68 labeled Ubiquicidin derived octapeptide as a potential infection imaging agent

INTRODUCTION

Gallium-68 based infection imaging agents are in demand to detect infection foci with high spatial resolution and sensitivity. In this study, Ubiquicidin derived octapeptide, UBI (31-38) conjugated with macrocyclic chelator NOTA was radiolabeled with ⁶⁸Ga to develop infection imaging agent.

METHODS

Circular dichroism (CD) spectroscopy was performed to study conformational changes in UBI (31-38) and its NOTA conjugate in a "membrane like environment". Radiolabeling of NOTA-UBI (31-38) with ⁶⁸Ga was optimized and quality control analysis was done by chromatography techniques. In vitro evaluation of ⁶⁸Ga-NOTA-UBI (31-38) in S. aureus and preliminary biological evaluation in animal model of infection was studied. Initial clinical evaluation in three patients with suspected infection was carried out.

RESULTS

⁶⁸Ga-NOTA-UBI (31-38) was prepared in high radiochemical yields and high radiochemical purity. In vitro evaluation of ⁶⁸Ga-NOTA-UBI (31-38) complex in S. aureus confirmed specificity of the agent for bacteria. Biodistribution studies with ⁶⁸Ga-NOTA-UBI (31-38) revealed specific uptake of the complex in infected muscle compared to inflamed muscle with T/NT ratio of 3.24 ± 0.7 at 1 h post-injection. Initial clinical evaluation in two patients with histopathologically confirmed infective foci conducted after intravenous injection of 130-185 MBq of ⁶⁸Ga-NOTA-UBI (31-38) and imaging at 45-60 min post-injection revealed specific uptake at the sites of infection and clearance from vital organs. No uptake of tracer was observed in suspected infection foci in one patient, which was proven to be aseptic and served as negative control.

CONCLUSION

This is the first report on ⁶⁸Ga labeled NOTA-UBI (31-38) fragment for prospective infection imaging.

Diagnostic value of 99mTc-ubiquicidin scintigraphy in differentiation between osteomyelitis and bone tumors

AIM

The differentiation of osteomyelitis from bone tumors is of great importance in clinical decision-making; however, the features of both osteomyelitis and bone tumors are noncontributory. Tc-ubiquicidin scintigraphy is a new promising method with the ability to specifically localize the infection site by bacterial cell membrane binding. This study aimed to evaluate the ability of this radiopeptide for the differentiation of these two entities.

PATIENTS AND METHODS

Thirty consecutive patients (mean age=20.9 years) suspected of having either osteomyelitis or bone tumor were included in this prospective study. A Tc-UBI scan was performed in both dynamic and static phases and the images were assessed qualitatively and semiquantitatively. The final diagnosis was established for 29 patients on the basis of surgical findings and microbiological and pathology assessments as well as any other clinical, laboratory, or imaging findings during patient follow-up.

RESULTS

The final diagnosis was infectious and noninfectious processes in 19 and 10 patients, respectively. Visual assessment could not distinguish between osteomyelitis and bone tumors. However, the time-activity pattern of the images proved to be promising. The sensitivity, specificity, negative and positive predictive value, and accuracy of the time-activity curve for osteomyelitis were 73.6 (54-93), 100, 66.6 (43-91), 100, and 82%, respectively. The mean±SD tumor/nontumor (T/NT) ratios for 30 min images were 2.22±0.45 and 2.02±0.51 for infectious and noninfectious processes, respectively (P=0.29). Using a cutoff value of 0.97 for the T/NT ratio, the sensitivity and specificity were calculated to be 78.9 and 50%, respectively.

CONCLUSION

Although Tc-UBI scintigraphy in the dynamic imaging format was very useful with high accuracy in differentiating between infectious and tumoral lesions, it was not useful to distinguish these two entities on the basis of visual assessment or T/NT ratio measurement on static images. The study also showed the high accuracy of this noninvasive modality in acute osteomyelitis with low diagnostic value in chronic infectious processes.

Metabolic Imaging of Infection

Metabolic imaging has come to occupy a prominent place in the diagnosis and management of microbial infection. Molecular probes available for infection imaging have undergone a rapid evolution starting with nonspecific agents that accumulate similarly in infection, sterile inflammation, and neoplastic tissue and then extending to more targeted probes that seek to identify specific microbial species. This focus review describes the metabolic and molecular imaging techniques currently available for clinical use in infection imaging and those that have demonstrated promising results in preclinical studies with the potential for clinical applications.

Detection of bacterial infection by a technetium-99m-labeled peptidoglycan aptamer

Nuclear medicine clinicians are still waiting for the optimal scintigraphic imaging agents capable of distinguishing between infection and inflammation, and between fungal and bacterial infections. Aptamers have several properties that make them suitable for molecular imaging. In the present study, a peptidoglycan aptamer (Antibac1) was labeled with ^99mTc and evaluated by biodistribution studies and scintigraphic imaging in infection-bearing mice. Labeling with ^99mTc was performed by the direct method and the complex stability was evaluated in saline, plasma and in the molar excess of cysteine. The biodistribution and scintigraphic imaging studies with the ^99mTc-Antibac1 were carried out in two different experimental infection models: Bacterial-infected mice (S. aureus) and fungal-infected mice (C. albicans). A ^99mTc radiolabeled library, consisting of oligonucleotides with random sequences, was used as a control for both models. Radiolabeling yields were superior to 90% and ^99mTc-Antibac1 was highly stable in presence of saline, plasma, and cysteine up to 6h. Scintigraphic images of S. aureus infected mice at 1.5 and 3.0h after ^99mTc-Antibac1 injection showed target to non-target ratios of 4.7±0.9 and 4.6±0.1, respectively. These values were statistically higher than those achieved for the ^99mTc-library at the same time frames (1.6±0.4 and 1.7±0.4, respectively). Noteworthy, ^99mTc-Antibac1 and ^99mTc-library showed similar low target to non-target ratios in the fungal-infected model: 2.0±0.3 and 2.0±0.6for ^99mTc-Antibac1 and 2.1±0.3 and 1.9 ± 0.6 for ^99mTc-library, at the same times. These findings suggest that the ^99mTc-Antibac1 is a feasible imaging probe to identify a bacterial infection focus. In addition, this radiolabeled aptamer seems to be suitable in distinguishing between bacterial and fungal infection.

Scintigraphic imaging of Staphylococcus aureus infection using 99mTc radiolabeled aptamers

Staphylococcus aureus is a specie of great medical importance associated with many infections as bacteremia and infective endocarditis as well as osteoarticular, skin and soft tissue, pleuropulmonary, and device related infections. Early identification of infectious foci is crucial for successful treatment. Scintigraphy could contribute to this purpose since specific radiotracers were available. Aptamers due to their high specificity have great potential for radiopharmaceuticals development. In the present study scintigraphic images of S. aureus infectious foci were obtained using specific S. aureus aptamers radiolabeled with ^99mTc.

In vitro cytotoxic and genotoxic evaluation of peptides used in nuclear medicine (DOTATATE and Ubiquicidin29-41) in CHO-K1 cells

Micronucleus (MN) assay constitutes a valuable surrogate to the chromosome aberration technique for in vitro testing of the genotoxicity of substances. As test substances, two peptidic compounds (DOTATATE and Ubiquicidin_29-41) used in nuclear medicine, were tested for in vitro cytotoxicity and genotoxicity in CHO-K1 cells. None of the compounds showed detectable cytotoxicity (0.5-7.3 ng/mL for DOTATATE and 0.3-4.5 ng/mL for UBI_29-41), genotoxicity (0.72, 7.2 and 72.0 ng/ml for DOTATATE and 0.45, 4.5 and 45.0 ng/mL for UBI_29-41) or cell cycle changes as compared to untreated controls at the concentrations tested. Statistical analysis showed good concordance between two independent analysts. The results corroborate the notion of the safety of the compounds and present improvements of the in vitro MN assay when performed in a pre-clinical trial context that increase the throughput of small-to-medium testing facilities as an alternative to high content screening systems.

Preclinical studies and prospective clinical applications for bacteria-targeted imaging: the future is bright

Bacterial infections are a frequently occurring and major complication in human healthcare, in particular due to the rapid increase of antimicrobial resistance and the emergence of pan-drug-resistant microbes. Current anatomical and functional imaging modalities are insufficiently capable of distinguishing sites of bacterial infection from sterile inflammation. Therefore, definitive diagnosis of an infection can often only be obtained by tissue biopsy and subsequent culture and, occasionally, a definite diagnosis even appears to be impossible. To accurately diagnose bacterial infections early, novel imaging modalities are urgently needed. In this regard, bacteria-targeted imaging is an attractive option due to its specificity. Here, different bacteria-targeted imaging approaches are reviewed, and their promising future perspectives are discussed.

A labelled-ubiquicidin antimicrobial peptide for immediate in situ optical detection of live bacteria in human alveolar lung tissue

The in situ immediate detection of the presence of bacteria in the distal human lung is of significant clinical utility. Herein we describe the development and optimization of a bacterial binding fragment (UBI29-41) of the antimicrobial peptide, ubiquicidin (UBI), conjugated to an environmentally sensitive fluorophore to enable rapid live bacterial imaging within human lung tissue. UBI29-41 was modified for stability in the presence of human lung bronchoalveolar lavage fluid, for affinity to bacterial membranes and functionality in human lung tissue. The optimized cyclic structure yields an optical molecular Smartprobe for bacterial detection in human lung tissue.

99m-Tc-ubiquicidin scintigraphy in diagnosis of knee prosthesis infection and comparison with F-18 fluorodeoxy-glucose positron emission tomography/computed tomography

Total knee arthroplasty has witnessed a significant increase in recent years. Despite the advantages of this surgical procedure, it has some complications, the most serious of which is prosthetic infection. The discrimination of bacterial infections from sterile inflammatory processes is of great importance in the management of periprosthetic infection (PPI). Ubiquicidin (UBI) is a synthetic antimicrobial peptide fragment reported to be highly infection-specific. Tc99m-UBI has recently been reported to be a promising radiotracer for infection imaging. We report a case of left knee PPI diagnosed using 99mTc-UBI scintigraphy and compared with F-18 fluorodeoxy-glucose positron emission tomography.

Development of a Hybrid Tracer for SPECT and Optical Imaging of Bacterial Infections

In trauma and orthopedic surgery, infection of implants has a major impact on the outcome for patients. Infections may develop either during the initial implantation or during the lifetime of an implant. Both infections, as well as aseptic loosening of the implant, are reasons for revision of the implants. Therefore, discrimination between aseptic-mechanical-loosening and septic-bacterial-loosening of implants is critical during selection of a patient-tailored treatment policy. Specific detection and visualization of infections is a challenge because it is difficult to discriminate infections from inflammation. An imaging tracer that facilitates bacterial identification in a pre- and intraoperative setting may aid the workup for patients suspicious of bacterial infections. In this study we evaluated an antimicrobial peptide conjugated to a hybrid label, which contains both a radioisotope and a fluorescent dye. After synthesis of DTPA-Cy5-UBI29-41 and-when necessary-radiolabeling with (111)In (yield 96.3 ± 2.7%), in vitro binding to various bacterial strains was evaluated using radioactivity counting and confocal fluorescence microscopy. Intramuscular bacterial infections (S. aureus or K. pneumoniae) were also visualized in vivo using a combined nuclear and fluorescence imaging system. The indium-111 was chosen as label as it has a well-defined coordination chemistry, and in pilot studies labeling DTPA-Cy5-UBI29-41 with technetium-99m, we encountered damage to the Cy5 dye after the reduction with SnCl2. As a reference, we used the validated tracer (99m)Tc-UBI29-41. Fast renal excretion of (111)In-DTPA-Cy5-UBI29-41 was observed. Target to nontarget (T/NT) ratios were highest at 2 h post injection: radioactivity counting yielded T/NT ratios of 2.82 ± 0.32 for S. aureus and 2.37 ± 0.05 for K. pneumoniae. Comparable T/NT ratios with fluorescence imaging of 2.38 ± 0.09 for S. aureus and 3.55 ± 0.31 for K. pneumoniae were calculated. Ex vivo confocal microscopy of excised infected tissues showed specific binding of the tracer to bacteria. Using a combination of nuclear and fluorescence imaging techniques, the hybrid antimicrobial peptide conjugate DTPA-Cy5-UBI29-41 was shown to specifically accumulate in bacterial infections. This hybrid tracer may facilitate integration of noninvasive identification of infections and their extent as well as real-time fluorescence guidance during surgical resection of infected areas.

Multiagent imaging of inflammation and infection with radionuclides

Molecular imaging with single photon- and positron-emitting tracers plays an important role in the evaluation of inflammation and infection. Although supplanted by labeled leukocyte imaging for most indications, gallium-67 remains useful for opportunistic infections, pulmonary inflammation and interstitial nephritis and, when [¹⁸F]FDG is not available, spinal infection and fever of unknown origin. In vitro labeled leukocyte imaging is the radionuclide procedure of choice for most infections in immunocompetent patients. When performed for musculoskeletal infection, complementary bone marrow imaging usually is necessary. Recent data suggest that dual time point imaging might be an alternative to marrow imaging. Several methods of labeling leukocytes in vivo, with agents including antigranulocyte antibodies and antibody fragments, peptides and cytokines, have been investigated, with variable results. These agents are not widely available and none of them are available in the USA. Radiolabeled antibiotics have been investigated as “infection-specific” tracers, but the results to date have been disappointing. Conversely, radiolabeled antimicrobial peptides do hold promise as infection-specific tracers. The use of positron-emitting tracers for diagnosing inflammation and infection has generated considerable interest. [¹⁸F]FDG is useful in fever of unknown origin, spinal osteomyelitis, vasculitis and sarcoidosis. Other positron-emitting tracers that have been investigated include [¹⁸F]FDG-labeled leukocytes, copper-64-labeled leukocytes, gallium-68 citrate and iodine-124 FIAU. Although radiolabeled tracers are used primarily for diagnosis, they also offer objective biomarkers for assessing response to therapeutic interventions in inflammatory diseases. They could also potentially be used to target cells and molecules with specific receptor expression for histological characterization, select patients for receptor-targeted therapy and predict response to treatment.