Radiotracers for fungal infection imaging

Biocompatible carbon quantum dots as versatile imaging nanotrackers of fungal pathogen - Candida albicans

Aim: The development of carbon quantum dots (C-QDs) as nanotrackers to understand drug-pathogen interactions, virulence and multidrug resistance. Methods: Microwave synthesis of C-QDs was performed using citric acid and polyethylene glycol. Further, in vitro toxicity was evaluated and imaging applications were demonstrated in Candida albicans isolates. Results: Well-dispersed, ultra small C-QDs exhibited no cyto/microbial/reactive oxygen species-mediated toxicity and internalized effectively in Candida yeast and hyphal cells. C-QDs were employed for confocal imaging of drug-sensitive and -resistant cells, and a study of the yeast-to-hyphal transition using atomic force microscopy in Candida was conducted for the first time. Conclusion: These biocompatible C-QDs have promising potential as next-generation nanotrackers for in vitro and in vivo targeted cellular and live imaging, after functionalization with biomolecules and drugs.

Radionuclide Imaging of Invasive Fungal Disease in Immunocompromised Hosts

Invasive fungal disease (IFD) leads to increased mortality, morbidity, and costs of treatment in patients with immunosuppressive conditions. The definitive diagnosis of IFD relies on the isolation of the causative fungal agents through microscopy, culture, or nucleic acid testing in tissue samples obtained from the sites of the disease. Biopsy is not always feasible or safe to be undertaken in immunocompromised hosts at risk of IFD. Noninvasive diagnostic techniques are, therefore, needed for the diagnosis and treatment response assessment of IFD. The available techniques that identify fungal-specific antigens in biological samples for diagnosing IFD have variable sensitivity and specificity. They also have limited utility in response assessment. Imaging has, therefore, been applied for the noninvasive detection of IFD. Morphologic imaging with computed tomography (CT) and magnetic resonance imaging (MRI) is the most applied technique. These techniques are neither sufficiently sensitive nor specific for the early diagnosis of IFD. Morphologic changes evaluated by CT and MRI occur later in the disease course and during recovery after successful treatment. These modalities may, therefore, not be ideal for early diagnosis and early response to therapy determination. Radionuclide imaging allows for targeting the host response to pathogenic fungi or specific structures of the pathogen itself. This makes radionuclide imaging techniques suitable for the early diagnosis and treatment response assessment of IFD. In this review, we aimed to discuss the interplay of host immunity, immunosuppression, and the occurrence of IFD. We also discuss the currently available radionuclide probes that have been evaluated in preclinical and clinical studies for their ability to detect IFD.

PET/MRI in paediatric disease

Nuclear medicine and molecular imaging have a small but growing role in the management of paediatric and neonatal diseases. During the past decade, combined PET/MRI has emerged as a clinically important hybrid imaging modality in paediatric medicine due to diagnostic advantages and reduced radiation exposure compared to alternative techniques. The applications for nuclear medicine, radiopharmaceuticals and combined PET/MRI in paediatric diagnosis is broadly similar to adults, however there are some key differences. There are a variety of clinical applications for PET/MRI imaging in children including, but not limited to, oncology, neurology, cardiovascular, infection and chronic inflammatory diseases, and in renal-urological disorders. In this article, we review the applications of PET/MRI in paediatric and neonatal imaging, its current role, advantages and disadvantages over other hybrid imaging techniques such as PET/CT, and its future applications. Overall, PET/MRI is a powerful imaging technology in diagnostic medicine and paediatric diseases. Higher soft tissue contrasts and lower radiation dose of the MRI makes it the superior technology compared to other conventional techniques such as PET/CT or scintigraphy. However, this relatively new hybrid imaging has also some limitations. MRI based attenuation correction remains a challenge and although methodologies have improved significantly in the last decades, most remain under development.

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.

Molecular Imaging of Invasive Pulmonary Aspergillosis Using ImmunoPET/MRI: The Future Looks Bright

Invasive pulmonary aspergillosis (IPA) is a life-threatening lung disease of immuno-compromised humans caused by the ubiquitous environmental mold Aspergillus. Biomarker tests for the disease lack sensitivity and specificity, and culture of the fungus from invasive lung biopsy is slow, insensitive, and undesirable in critically ill patients. A computed tomogram (CT) of the chest offers a simple non-intrusive diagnostic procedure for rapid decision making, and so is used in many hematology units to drive antifungal treatment. However, radiological indicators that raise the suspicion of IPA are either transient signs in the early stages of the disease or not specific for Aspergillus infection, with other angio-invasive molds or bacterial pathogens producing comparable radiological manifestations in a chest CT. Improvements to the specificity of radiographic imaging of IPA have been attempted by coupling CT and positron emission tomography (PET) with [¹⁸F]fluorodeoxyglucose ([¹⁸F]FDG), a marker of metabolic activity well suited to cancer imaging, but with limited use in invasive fungal disease diagnostics due to its inability to differentiate between infectious etiologies, cancer, and inflammation. Bioluminescence imaging using single genetically modified strains of Aspergillus fumigatus has enabled in vivo monitoring of IPA in animal models of disease. For in vivo detection of Aspergillus lung infections in humans, radiolabeled Aspergillus-specific monoclonal antibodies, and iron siderophores, hold enormous potential for clinical diagnosis. This review examines the different experimental technologies used to image IPA, and recent advances in state-of-the-art molecular imaging of IPA using antibody-guided PET/magnetic resonance imaging (immunoPET/MRI).

Currently Available Radiopharmaceuticals for Imaging Infection and the Holy Grail

Infection is ubiquitous. However, its management is challenging for both the patients and the health-care providers. Scintigraphic imaging of infection dates back nearly half a century. The advances in our understanding of the pathophysiology of disease at cellular and molecular levels have paved the way to the development of a large number of radiopharmaceuticals for scintigraphic imaging of infection. These include radiolabeling of blood elements such as serum proteins, white blood cells (WBCs), and cytokines, to name a few. Infectious foci have also been imaged using a radiolabeled sugar molecule by taking advantage of increased metabolic activity in the infectious lesions. Literature over the years has well documented that none of the radiopharmaceuticals and associated procedures that facilitate imaging infection are flawless and acceptable without a compromise. As a result, only a few compounds such as 99mTc-hexamethylpropyleneamineoxime, 18F-FDG, the oldest but still considered as a gold standard 111In-oxine, and, yes, even 67Ga-citrate in some countries, have remained in routine clinical practice. Nonetheless, the interest of scientists and physicians to improve the approaches to imaging and to the management of infection is noteworthy. These approaches have paved the way for the development of numerous, innovative radiopharmaceuticals to label autologous WBCs ex vivo or even those that could be injected directly to image infection or inflammation without direct involvement of WBCs. In this review, we briefly describe these agents with their pros and cons and place them together for future reference.

Monitoring Response to Therapy

Monitoring response to treatment is a key element in the management of infectious diseases, yet controversies still persist on reliable biomarkers for noninvasive response evaluation. Considering the limitations of invasiveness of most diagnostic procedures and the issue of expression heterogeneity of pathology, molecular imaging is better able to assay in vivo biologic processes noninvasively and quantitatively. The usefulness of ¹⁸F-FDG-PET/CT in assessing treatment response in infectious diseases is more promising than for conventional imaging. However, there are currently no clinical criteria or recommended imaging modalities to objectively evaluate the effectiveness of antimicrobial treatment. Therapeutic effectiveness is currently gauged by the patient's subjective clinical response. In this review, we present the current studies for monitoring treatment response, with a focus on Mycobacterium tuberculosis, as it remains a major worldwide cause of morbidity and mortality. The role of molecular imaging in monitoring other infections including spondylodiscitis, infected prosthetic vascular grafts, invasive fungal infections, and a parasitic disease is highlighted. The role of functional imaging in monitoring lipodystrophy associated with highly active antiretroviral therapy for human immunodeficiency virus is considered. We also discuss the key challenges and emerging data in optimizing noninvasive response evaluation.

Nuclear Medicine Imaging in Pediatric Infection or Chronic Inflammatory Diseases

In this review article, we focus on the most recent applications of nuclear medicine techniques (mainly ^99mTc/¹¹¹In white blood cells (WBC) scan, [¹⁸F]-FDG-PET/CT, [¹⁸F]-FDG-PET/MRI, and ^99mTc-IL-2 scintigraphy) in the study of children affected by peripheral bone osteomyelitis, fungal infections, inflammatory bowel diseases, and type 1 diabetes, owing to recent important published evidences of their role in the management of these diseases. For osteomyelitis in children, both bone scintigraphy and [¹⁸F]-FDG-PET have a major advantage of assessing the whole body in one imaging session to confirm or exclude multifocal involvement, whereas WBC scan has a limited role. In children with fungal infections, [¹⁸F]-FDG-PET can help in defining the best location for biopsy and can help in evaluating the extent of the infection and organs involved (also sites that were not yet clinically apparent), although its main role is for therapy monitoring. In inflammatory bowel diseases, and Crohn disease in particular, WBC scan has been successfully used for many years, but it is now used only in case of doubtful magnetic resonance (MR) or when MR cannot be performed and endoscopy is inconclusive. By contrast, there is an accumulating evidence of the role of [¹⁸F]-FDG-PET in management of children with Crohn disease, and PET/MR could be a versatile and innovative hybrid imaging technique that combines the metabolic information of PET with the high soft tissue resolution of MR, particularly for distinguishing fibrotic from active strictures. Finally, there are several new radiopharmaceuticals that specifically target inflammatory cells involved in the pathogenesis of insulitis aiming at developing new specific immunotherapies and to select children candidates to these treatments for improving their quality of life.

Technetium-99m based small molecule radiopharmaceuticals and radiotracers targeting inflammation and infection

^99mTc-labeled antibiotics, antifungal drugs, antimicrobial peptides and COX-2 inhibitors are comprehensively reviewed.

Siderophores for molecular imaging applications

This review covers publications on siderophores applied for molecular imaging applications, mainly for radionuclide-based imaging. Siderophores are low molecular weight chelators produced by bacteria and fungi to scavenge essential iron. Research on these molecules has a continuing history over the past 50 years. Many biomedical applications have been developed, most prominently the use of the siderophore desferrioxamine (DFO) to tackle iron overload related diseases. Recent research described the upregulation of siderophore production and transport systems during infection. Replacing iron in siderophores by radionuclides, the most prominent Ga-68 for PET, opens approaches for targeted imaging of infection; the proof of principle has been reported for fungal infections using ⁶⁸Ga-triacetylfusarinine C (TAFC). Additionally, fluorescent siderophores and therapeutic conjugates have been described and may be translated to optical imaging and theranostic applications. Siderophores have also been applied as bifunctional chelators, initially DFO as chelator for Ga-67 and more recently for Zr-89 where it has become the standard chelator in Immuno-PET. Improved DFO constructs and bifunctional chelators based on cyclic siderophores have recently been developed for Ga-68 and Zr-89 and show promising properties for radiopharmaceutical development in PET. A huge potential from basic biomedical research on siderophores still awaits to be utilized for clinical and translational imaging.

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.

Imaging fungal infections in children

Fungal infections in children rarely occur, but continue to have a high morbidity and mortality despite the development of newer antifungal agents. It is essential for these infections to be diagnosed at the earliest possible stage so appropriate treatment can be initiated promptly. The addition of high-resolution computer tomography (HR CT) has helped in early diagnosis making; however, it lacks both sensitivity and specificity. Metabolic changes precede anatomical changes and hybrid imaging with positron emission tomography (PET) integrated with imaging modalities with high anatomical resolution such as CT or magnetic resonance imaging (MRI) is likely to detect these infections at an earlier stage with higher diagnostic accuracy rates. Several authors presented papers highlighting the advantages of PET/CT in imaging fungal infections. These papers, however, usually involve a limited number of patients and mostly adults. Fungal infections behave different in children than in adults, since there are differences in epidemiology, imaging findings, and response to treatment with antifungal drugs. This paper reviews the literature and explores the use of hybrid imaging for diagnosis and therapy decision making in children with fungal infections.

Chitinases from Bacteria to Human: Properties, Applications, and Future Perspectives

Chitin is the second most plenteous polysaccharide in nature after cellulose, present in cell walls of several fungi, exoskeletons of insects, and crustacean shells. Chitin does not accumulate in the environment due to presence of bacterial chitinases, despite its abundance. These enzymes are able to degrade chitin present in the cell walls of fungi as well as the exoskeletons of insect. They have shown being the potential agents for biological control of the plant diseases caused by various pathogenic fungi and insect pests and thus can be used as an alternative to chemical pesticides. There has been steady increase in demand of chitin derivatives, obtained by action of chitinases on chitin polymer for various industrial, clinical, and pharmaceutical purposes. Hence, this review focuses on properties and applications of chitinases starting from bacteria, followed by fungi, insects, plants, and vertebrates. Designing of chitinase by applying directed laboratory evolution and rational approaches for improved catalytic activity for cost-effective field applications has also been explored.

Fungal central nervous system infections: prevalence and diagnosis

Fungal infections of the central nervous system (CNS) are rare but they pose a significant challenge. Their prevalence spans a wide array of hosts including immunosuppressed and immunocompetent individuals, patients undergoing neurosurgical procedures and those carrying implantable CNS devices. Cryptococcus neoformans and Aspergillus spp. remain the most common pathogens. Magnetic resonance imaging can help localize the lesions, but diagnosis is challenging since invasive procedures may be needed for the retrieval of tissue, especially in cases of fungal abscesses. Antigen and antibody tests are available and approved for use in the cerebrospinal fluid (CSF). PCR-based techniques are promising but they are not validated for use in the CSF. This review provides an overview on the differential diagnosis of the fungal CNS disease based on the host and the clinical syndrome and suggests the optimal use of diagnostic techniques. It also summarizes the emergence of Cryptococcus gatti and an unanticipated outbreak caused by Exserohilum rostratum.

From bacteria to human: a journey into the world of chitinases

Chitinases, the enzymes responsible for the biological degradation of chitin, are found in a wide range of organisms from bacteria to higher plants and animals. They participate in numerous physiological processes such as nutrition, parasitism, morphogenesis and immunity. Many organisms, in addition to chitinases, produce inactive chitinase-like lectins that despite lacking enzymatic activity are involved in several regulatory functions. Most known chitinases belong to families 18 and 19 of glycosyl hydrolases, however a few chitinases that belong to families 23 and 48 have also been identified in recent years. In this review, different aspects of chitinases and chi-lectins from bacteria, fungi, insects, plants and mammals are discussed.

Preclinical evaluation of two 68Ga-siderophores as potential radiopharmaceuticals for Aspergillus fumigatus infection imaging

Purpose

Invasive pulmonary aspergillosis is mainly caused by Aspergillus fumigatus, and is one of the major causes of morbidity and mortality in immunocompromised patients. The mortality associated with invasive pulmonary aspergillosis remains high, mainly due to the difficulties and limitations in diagnosis. We have shown that siderophores can be labelled with ⁶⁸Ga and can be used for PET imaging of A. fumigatus infection in rats. Here we report on the further evaluation of the most promising ⁶⁸Ga-siderophore candidates, triacetylfusarinine (TAFC) and ferrioxamine E (FOXE).

Methods

Siderophores were labelled with ⁶⁸Ga using acetate buffer. Log P, protein binding and stability values were determined. Uptake by A. fumigatus was studied in vitro in cultures with high and low iron loads. In vivo biodistribution was determined in normal mice and an infection model was established using neutropenic rats inoculated with A. fumigatus. Static and dynamic μPET imaging was performed and correlated with CT images, and lung infection was evaluated ex vivo.

Results

⁶⁸Ga-siderophores were labelled with high radiochemical purity and specific activity. ⁶⁸Ga-TAFC and ⁶⁸Ga-FOXE showed high uptake by A. fumigatus in iron-deficient cultures. In normal mice, ⁶⁸Ga-TAFC and ⁶⁸Ga-FOXE showed rapid renal excretion with high metabolic stability. In the rat infection model focal lung uptake was detected by μPET with both compounds and increased with severity of the infection, correlating with abnormal CT images.

Conclusion

⁶⁸Ga-TAFC and ⁶⁸Ga-FOXE displayed excellent in vitro stability and high uptake by A. fumigatus. Both compounds showed excellent pharmacokinetics, highly selective accumulation in infected lung tissue and good correlation with severity of disease in a rat infection model, which makes them promising agents for A. fumigatus infection imaging.

Electronic supplementary material

The online version of this article (doi:10.1007/s00259-012-2110-3) contains supplementary material, which is available to authorized users.