Radioiodinated DPA-713 imaging correlates with bactericidal activity of tuberculosis treatments in mice

Distributable, metabolic PET reporting of tuberculosis

Tuberculosis remains a large global disease burden for which treatment regimens are protracted and monitoring of disease activity difficult. Existing detection methods rely almost exclusively on bacterial culture from sputum which limits sampling to organisms on the pulmonary surface. Advances in monitoring tuberculous lesions have utilized the common glucoside [18F]FDG, yet lack specificity to the causative pathogen Mycobacterium tuberculosis (Mtb) and so do not directly correlate with pathogen viability. Here we show that a close mimic that is also positron-emitting of the non-mammalian Mtb disaccharide trehalose - 2-[18F]fluoro-2-deoxytrehalose ([18F]FDT) - is a mechanism-based reporter of Mycobacteria-selective enzyme activity in vivo. Use of [18F]FDT in the imaging of Mtb in diverse models of disease, including non-human primates, successfully co-opts Mtb-mediated processing of trehalose to allow the specific imaging of TB-associated lesions and to monitor the effects of treatment. A pyrogen-free, direct enzyme-catalyzed process for its radiochemical synthesis allows the ready production of [18F]FDT from the most globally-abundant organic 18F-containing molecule, [18F]FDG. The full, pre-clinical validation of both production method and [18F]FDT now creates a new, bacterium-selective candidate for clinical evaluation. We anticipate that this distributable technology to generate clinical-grade [18F]FDT directly from the widely-available clinical reagent [18F]FDG, without need for either custom-made radioisotope generation or specialist chemical methods and/or facilities, could now usher in global, democratized access to a TB-specific PET tracer.

Does PET-CT Have a Role in the Evaluation of Tuberculosis Treatment in Phase 2 Clinical Trials?

Positron emission tomography-computed tomography (PET-CT) has the potential to revolutionize research in infectious diseases, as it has done with cancer. There is growing interest in it as a biomarker in the setting of early-phase tuberculosis clinical trials, particularly given the limitations of current biomarkers as adequate predictors of sterilizing cure for tuberculosis. PET-CT is a real-time tool that provides a 3-dimensional view of the spatial distribution of tuberculosis within the lung parenchyma and the nature of lesions with uptake (ie, whether nodular, consolidative, or cavitary). Its ability to provide functional data on changes in metabolism, drug penetration, and immune control of tuberculous lesions has the potential to facilitate drug development and regimen selection for advancement to phase 3 trials in tuberculosis. In this narrative review, we discuss the role that PET-CT may have in evaluating responses to drug therapy in active tuberculosis treatment and the challenges in taking PET-CT forward as predictive biomarker of relapse-free cure in the setting of phase 2 clinical trials.

Dynamic PET Reveals Compartmentalized Brain and Lung Tissue Antibiotic Exposures

Tuberculosis (TB) remains a leading cause of death, but antibiotic treatments for tuberculous meningitis, the deadliest form of TB, are based on those developed for pulmonary TB and not optimized for brain penetration. Here, we performed first-in-human dynamic 18F-pretomanid positron emission tomography (PET) studies in eight human subjects for three-dimensional, multi-compartmental in situ visualization of antibiotic concentration-time exposures (area under the curve - AUC), demonstrating preferential brain (AUCtissue/plasma 2.25) versus lung (AUCtissue/plasma 0.97) tissue partitioning. Preferential, antibiotic-specific partitioning into brain or lung tissues of antibiotics active against MDR strains were confirmed in experimentally-infected mice and rabbits, using dynamic PET with chemically identical antibiotic radioanalogs, and postmortem mass spectrometry measurements. PET-facilitated pharmacokinetic modeling predicted human dosing necessary to attain therapeutic brain exposures in human subjects. These data were used to design optimized, pretomanid-based regimens which were evaluated at human equipotent dosing in a mouse model of TB meningitis, demonstrating excellent bactericidal activity without an increase in intracerebral inflammation or brain injury. Importantly, several antibiotic regimens demonstrated discordant activities in brain and lung tissues in the same animal, correlating with the compartmentalized tissue exposures of the component antibiotics. These data provide a mechanistic basis for the compartmentalized activities of antibiotic regimens, with important implications for the development of antimicrobial regimens for meningitis and other infections in compartments with unique antibiotic penetration.

Radiotracers for Imaging of Inflammatory Biomarkers TSPO and COX-2 in the Brain and in the Periphery

Inflammation involves the activation of innate immune cells and is believed to play an important role in the development and progression of both infectious and non-infectious diseases such as neurodegeneration, autoimmune diseases, pulmonary and cancer. Inflammation in the brain is marked by the upregulation of translocator protein (TSPO) in microglia. High TSPO levels are also found, for example, in macrophages in cases of rheumatoid arthritis and in malignant tumor cells compared to their relatively low physiological expression. The same applies for cyclooxgenase-2 (COX-2), which is constitutively expressed in the kidney, brain, thymus and gastrointestinal tract, but induced in microglia, macrophages and synoviocytes during inflammation. This puts TSPO and COX-2 in the spotlight as important targets for the diagnosis of inflammation. Imaging modalities, such as positron emission tomography and single-photon emission tomography, can be used to localize inflammatory processes and to track their progression over time. They could also enable the monitoring of the efficacy of therapy and predict its outcome. This review focuses on the current development of PET and SPECT tracers, not only for the detection of neuroinflammation, but also for emerging diagnostic measures in infectious and other non-infectious diseases such as rheumatic arthritis, cancer, cardiac inflammation and in lung diseases.

Imaging-Selected Host Responses in the Context of Infections

Recently developed molecular imaging approaches can be used to visualize specific host responses and pathology in a quest to image infections where few microbe-specific tracers have been developed and in recognition that host responses contribute to morbidity and mortality in their own right. Here we highlight several recent examples of these imaging approaches adapted for imaging infections. The early successes and new avenues described here encompass diverse imaging modalities and leverage diverse aspects of the host response to infection-including inflammation, tissue injury and healing, and key nutrients during host-pathogen interactions. Clearly, these approaches merit further preclinical and clinical study as they are complementary and orthogonal to the pathogen-focused imaging modalities currently under investigation.

IL-10 Modulation Increases Pyrazinamide's Antimycobacterial Efficacy against Mycobacterium tuberculosis Infection in Mice

Mechanisms to shorten the duration of tuberculosis (TB) treatment include new drug formulations or schedules and the development of host-directed therapies (HDTs) that better enable the host immune system to eliminate Mycobacterium tuberculosis. Previous studies have shown that pyrazinamide, a first-line antibiotic, can also modulate immune function, making it an attractive target for combinatorial HDT/antibiotic therapy, with the goal to accelerate clearance of M. tuberculosis. In this study, we assessed the value of anti-IL-10R1 as an HDT along with pyrazinamide and show that short-term anti-IL-10R1 blockade during pyrazinamide treatment enhanced the antimycobacterial efficacy of pyrazinamide, resulting in faster clearance of M. tuberculosis in mice. Furthermore, 45 d of pyrazinamide treatment in a functionally IL-10-deficient environment resulted in sterilizing clearance of M. tuberculosis. Our data suggest that short-term IL-10 blockade with standard TB drugs has the potential to improve clinical outcome by reducing the treatment duration.

Dynamic 18F-Pretomanid PET imaging in animal models of TB meningitis and human studies

Pretomanid is a nitroimidazole antimicrobial active against drug-resistant Mycobacterium tuberculosis and approved in combination with bedaquiline and linezolid (BPaL) to treat multidrug-resistant (MDR) pulmonary tuberculosis (TB). However, the penetration of these antibiotics into the central nervous system (CNS), and the efficacy of the BPaL regimen for TB meningitis, are not well established. Importantly, there is a lack of efficacious treatments for TB meningitis due to MDR strains, resulting in high mortality. We have developed new methods to synthesize ¹⁸F-pretomanid (chemically identical to the antibiotic) and performed cross-species positron emission tomography (PET) imaging to noninvasively measure pretomanid concentration-time profiles. Dynamic PET in mouse and rabbit models of TB meningitis demonstrates excellent CNS penetration of pretomanid but cerebrospinal fluid (CSF) levels does not correlate with those in the brain parenchyma. The bactericidal activity of the BPaL regimen in the mouse model of TB meningitis is substantially inferior to the standard TB regimen, likely due to restricted penetration of bedaquiline and linezolid into the brain parenchyma. Finally, first-in-human dynamic ¹⁸F-pretomanid PET in six healthy volunteers demonstrates excellent CNS penetration of pretomanid, with significantly higher levels in the brain parenchyma than in CSF. These data have important implications for developing new antibiotic treatments for TB meningitis.

Overview of positron emission tomography in functional imaging of the lungs for diffuse lung diseases

Positron emission tomography (PET) is a quantitative molecular imaging modality increasingly used to study pulmonary disease processes and drug effects on those processes. The wide range of drugs and other entities that can be radiolabeled to study molecularly targeted processes is a major strength of PET, thus providing a noninvasive approach for obtaining molecular phenotyping information. The use of PET to monitor disease progression and treatment outcomes in DLD has been limited in clinical practice, with most of such applications occurring in the context of research investigations under clinical trials. Given the high costs and failure rates for lung drug development efforts, molecular imaging lung biomarkers are needed not only to aid these efforts but also to improve clinical characterization of these diseases beyond canonical anatomic classifications based on computed tomography. The purpose of this review article is to provide an overview of PET applications in characterizing lung disease, focusing on novel tracers that are in clinical development for DLD molecular phenotyping, and briefly address considerations for accurately quantifying lung PET signals.

124I-Iodo-DPA-713 Positron Emission Tomography in a Hamster Model of SARS-CoV-2 Infection

PURPOSE

Molecular imaging has provided unparalleled opportunities to monitor disease processes, although tools for evaluating infection remain limited. Coronavirus disease (COVID-19) caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is mediated by lung injury that we sought to model. Activated macrophages/phagocytes have an important role in lung injury, which is responsible for subsequent respiratory failure and death. We performed pulmonary PET/CT with ¹²⁴I-iodo-DPA-713, a low-molecular-weight pyrazolopyrimidine ligand selectively trapped by activated macrophages cells, to evaluate the local immune response in a hamster model of SARS-CoV-2 infection.

PROCEDURES

Pulmonary ¹²⁴I-iodo-DPA-713 PET/CT was performed in SARS-CoV-2-infected golden Syrian hamsters. CT images were quantified using a custom-built lung segmentation tool. Studies with DPA-713-IRDye680LT and a fluorescent analog of DPA-713 as well as histopathology and flow cytometry were performed on post-mortem tissues.

RESULTS

Infected hamsters were imaged at the peak of inflammatory lung disease (7 days post-infection). Quantitative CT analysis was successful for all scans and demonstrated worse pulmonary disease in male versus female animals (P < 0.01). Increased ¹²⁴I-iodo-DPA-713 PET activity co-localized with the pneumonic lesions. Additionally, higher pulmonary ¹²⁴I-iodo-DPA-713 PET activity was noted in male versus female hamsters (P = 0.02). DPA-713-IRDye680LT also localized to the pneumonic lesions. Flow cytometry demonstrated a higher percentage of myeloid and CD11b + cells (macrophages, phagocytes) in male versus female lung tissues (P = 0.02).

CONCLUSION

¹²⁴I-Iodo-DPA-713 accumulates within pneumonic lesions in a hamster model of SARS-CoV-2 infection. As a novel molecular imaging tool, ¹²⁴I-Iodo-DPA-713 PET could serve as a noninvasive, clinically translatable approach to monitor SARS-CoV-2-associated pulmonary inflammation and expedite the development of novel therapeutics for COVID-19.

High-dose rifampin improves bactericidal activity without increased intracerebral inflammation in animal models of tuberculous meningitis

Tuberculous meningitis (TB meningitis) is the most severe form of tuberculosis (TB), requiring 12 months of multidrug treatment for cure, and is associated with high morbidity and mortality. High-dose rifampin (35 mg/kg/d) is safe and improves the bactericidal activity of the standard-dose (10 mg/kg/d) rifampin-containing TB regimen in pulmonary TB. However, there are conflicting clinical data regarding its benefit for TB meningitis, where outcomes may also be associated with intracerebral inflammation. We conducted cross-species studies in mice and rabbits, demonstrating that an intensified high-dose rifampin-containing regimen has significantly improved bactericidal activity for TB meningitis over the first-line, standard-dose rifampin regimen, without an increase in intracerebral inflammation. Positron emission tomography in live animals demonstrated spatially compartmentalized, lesion-specific pathology, with postmortem analyses showing discordant brain tissue and cerebrospinal fluid rifampin levels and inflammatory markers. Longitudinal multimodal imaging in the same cohort of animals during TB treatment as well as imaging studies in two cohorts of TB patients demonstrated that spatiotemporal changes in localized blood-brain barrier disruption in TB meningitis are an important driver of rifampin brain exposure. These data provide unique insights into the mechanisms underlying high-dose rifampin in TB meningitis with important implications for developing new antibiotic treatments for infections.

Tuberculosis: Role of Nuclear Medicine and Molecular Imaging With Potential Impact of Neutrophil-Specific Tracers

With Tuberculosis (TB) affecting millions of people worldwide, novel imaging modalities and tools, particularly nuclear medicine and molecular imaging, have grown with greater interest to assess the biology of the tuberculous granuloma and evolution thereof. Much early work has been performed at the pre-clinical level using gamma single photon emission computed tomography (SPECT) agents exploiting certain characteristics of Mycobacterium tuberculosis (MTb). Both antituberculous SPECT and positron emission tomography (PET) agents have been utilised to characterise MTb. Other PET tracers have been utilised to help to characterise the biology of MTb (including Gallium-68-labelled radiopharmaceuticals). Of all the tracers, 2-[¹⁸F]FDG has been studied extensively over the last two decades in many aspects of the treatment paradigm of TB: at diagnosis, staging, response assessment, restaging, and in potentially predicting the outcome of patients with latent TB infection. Its lower specificity in being able to distinguish different inflammatory cell types in the granuloma has garnered interest in reviewing more specific agents that can portend prognostic implications in the management of MTb. With the neutrophil being a cell type that portends this poorer prognosis, imaging this cell type may be able to answer more accurately questions relating to the tuberculous granuloma transmissivity and may help in characterising patients who may be at risk of developing active TB. The formyl peptide receptor 1(FPR1) expressed by neutrophils is a key marker in this process and is a potential target to characterise these areas. The pre-clinical work regarding the role of radiolabelled N-cinnamoyl –F-(D) L – F – (D) –L F (cFLFLF) (which is an antagonist for FPR1) using Technetium 99m-labelled conjugates and more recently radiolabelled with Gallium-68 and Copper 64 is discussed. It is the hope that further work with this tracer may accelerate its potential to be utilised in responding to many of the current diagnostic dilemmas and challenges in TB management, thereby making the tracer a translatable option in routine clinical care.

Micro-Computed Tomography Analysis of the Human Tuberculous Lung Reveals Remarkable Heterogeneity in Three-dimensional Granuloma Morphology

Rationale: Our current understanding of tuberculosis (TB) pathophysiology is limited by a reliance on animal models, the paucity of human TB lung tissue, and traditional histopathological analysis, a destructive two-dimensional approach that provides limited spatial insight. Determining the three-dimensional (3D) structure of the necrotic granuloma, a characteristic feature of TB, will more accurately inform preventive TB strategies.Objectives: To ascertain the 3D shape of the human tuberculous granuloma and its spatial relationship with airways and vasculature within large lung tissues.Methods: We characterized the 3D microanatomical environment of human tuberculous lungs by using micro computed tomography, histopathology, and immunohistochemistry. By using 3D segmentation software, we accurately reconstructed TB granulomas, vasculature, and airways in three dimensions and confirmed our findings by using histopathology and immunohistochemistry.Measurements and Main Results: We observed marked heterogeneity in the morphology, volume, and number of TB granulomas in human lung sections. Unlike depictions of granulomas as simple spherical structures, human necrotic granulomas exhibit complex, cylindrical, branched morphologies that are connected to the airways and shaped by the bronchi. The use of 3D imaging of human TB lung sections provides unanticipated insight into the spatial organization of TB granulomas in relation to the airways and vasculature.Conclusions: Our findings highlight the likelihood that a single, structurally complex lesion could be mistakenly viewed as multiple independent lesions when evaluated in two dimensions. In addition, the lack of vascularization within obstructed bronchi establishes a paradigm for antimycobacterial drug tolerance. Lastly, our results suggest that bronchogenic spread of Mycobacterium tuberculosis reseeds the lung.

Caspase-Based PET for Evaluating Pro-Apoptotic Treatments in a Tuberculosis Mouse Model

PURPOSE

Despite recent advances in antimicrobial treatments, tuberculosis (TB) remains a major global health threat. Mycobacterium tuberculosis proliferates in macrophages, preventing apoptosis by inducing anti-apoptotic proteins leading to necrosis of the infected cells. Necrosis then leads to increased tissue destruction, reducing the penetration of antimicrobials and immune cells to the areas where they are needed most. Pro-apoptotic drugs could be used as host-directed therapies in TB to improve antimicrobial treatments and patient outcomes.

PROCEDURE

We evaluated [¹⁸F]-ICMT-11, a caspase-3/7-specific positron emission tomography (PET) radiotracer, in macrophage cell cultures and in an animal model of pulmonary TB that closely resembles human disease.

RESULTS

Cells infected with M. tuberculosis and treated with cisplatin accumulated [¹⁸F]-ICMT-11 at significantly higher levels compared with that of controls, which correlated with levels of caspase-3/7 activity. Infected mice treated with cisplatin with increased caspase-3/7 activity also had a higher [¹⁸F]-ICMT-11 PET signal compared with that of untreated infected animals.

CONCLUSIONS

[¹⁸F]-ICMT-11 PET could be used as a noninvasive approach to measure intralesional pro-apoptotic responses in situ in pulmonary TB models and support the development of pro-apoptotic host-directed therapies for TB.

Visualizing the dynamics of tuberculosis pathology using molecular imaging

Nearly 140 years after Robert Koch discovered Mycobacterium tuberculosis, tuberculosis (TB) remains a global threat and a deadly human pathogen. M. tuberculosis is notable for complex host-pathogen interactions that lead to poorly understood disease states ranging from latent infection to active disease. Additionally, multiple pathologies with a distinct local milieu (bacterial burden, antibiotic exposure, and host response) can coexist simultaneously within the same subject and change independently over time. Current tools cannot optimally measure these distinct pathologies or the spatiotemporal changes. Next-generation molecular imaging affords unparalleled opportunities to visualize infection by providing holistic, 3D spatial characterization and noninvasive, temporal monitoring within the same subject. This rapidly evolving technology could powerfully augment TB research by advancing fundamental knowledge and accelerating the development of novel diagnostics, biomarkers, and therapeutics.

Imaging and Characterization of Macrophage Distribution in Mouse Models of Human Prostate Cancer

PURPOSE

Prostate carcinoma consists of tumor epithelium and malignant stroma. Until recently, diagnostic and therapeutic efforts have focused exclusively on targeting characteristics of the tumor epithelium, ignoring opportunities to target inflammatory infiltrate and extracellular matrix components. Prostate tumors are rich in tumor-associated macrophages (TAMs), which can be either of the cytotoxic M1 or protumorigenic M2 phenotype. We have quantified the proportion of each in seven common human prostate tumor lines grown subcutaneously in athymic nude mice and have imaged macrophage densities in vivo in xenografts derived from these lines.

PROCEDURES

A panel of seven human prostate cancer xenografts was generated in intact male athymic nude mice reflecting variable expression of the androgen receptor (AR) and prostate-specific membrane antigen (PSMA). Mice were imaged ex vivo using near-infrared fluorescence (NIRF) imaging for PSMA expression and total macrophage densities to enable direct comparison between the two. Tumors were harvested for sectioning and additional staining to delineate M1 and M2 phenotype along with vascular density.

RESULTS

Macrophage polarization analysis of sections revealed that all xenografts were > 94% M2 phenotype, and the few M1-polarized macrophages present were confined to the periphery. Xenografts displaying the fastest growth were associated with the highest densities of macrophages while the slowest growing tumors were characterized by focal, tumor-infiltrating macrophage densities. Xenograft sections displayed a strong positive spatial relationship between macrophages, vasculature, and PSMA expression.

CONCLUSIONS

Prostate TAM disposition can be imaged ex vivo and is associated with growth characteristics of a variety of tumor subtypes regardless of PSMA or AR expression.

SPECT/CT Imaging of Mycobacterium tuberculosis Infection with [125I]anti-C3d mAb

PURPOSE

Diagnosis and therapeutic monitoring of chronic bacterial infection requires methods to detect and localize sites of infection accurately. Complement C3 activation fragments are generated and covalently bound to selective bacterial pathogens during the immune response and can serve as biomarkers of ongoing bacterial infection. We have developed several probes for detecting tissue-bound C3 deposits, including a monoclonal antibody (mAb 3d29) that recognizes the tissue-bound terminal processing fragments iC3b and C3d but does not recognize native circulating C3 or tissue-bound C3b.

PROCEDURES

To determine whether mAb 3d29 could be used to detect chronic Mycobacterium tuberculosis infection non-invasively, aerosol-infected female C3HeB/FeJ mice were injected with [¹²⁵I]3d29 mAb and either imaged using single-photon emission computed tomography (SPECT)/X-ray computed tomography (CT) imaging at 24 and 48 h after radiotracer injection or being subjected to biodistribution analysis.

RESULTS

Discrete lesions were detected by SPECT/CT imaging in the lungs and spleens of infected mice, consistent with the location of granulomas in the infected animals as detected by CT. Low-level signal was seen in the spleens of uninfected mice and no signal was seen in the lungs of healthy mice. Immunofluorescence microscopy revealed that 3d29 in the lungs of infected mice co-localized with aggregates of macrophages (detected with anti-CD68 antibodies). 3d29 was detected in the cytoplasm of macrophages, consistent with the location of internalized M. tuberculosis. 3d29 was also present within alveolar epithelial cells, indicating that it detected M. tuberculosis phagocytosed by other CD68-positive cells. Healthy controls showed very little retention of fluorescent or radiolabeled antibody across tissues. Radiolabeled 3d29 compared with radiolabeled isotype control showed a 3.5:1 ratio of increased uptake in infected lungs, indicating specific uptake by 3d29.

CONCLUSION

3d29 can be used to detect and localize areas of infection with M. tuberculosis non-invasively by 24 h after radiotracer injection and with high contrast.

Digital Image Analysis of Heterogeneous Tuberculosis Pulmonary Pathology in Non-Clinical Animal Models using Deep Convolutional Neural Networks

Efforts to develop effective and safe drugs for treatment of tuberculosis require preclinical evaluation in animal models. Alongside efficacy testing of novel therapies, effects on pulmonary pathology and disease progression are monitored by using histopathology images from these infected animals. To compare the severity of disease across treatment cohorts, pathologists have historically assigned a semi-quantitative histopathology score that may be subjective in terms of their training, experience, and personal bias. Manual histopathology therefore has limitations regarding reproducibility between studies and pathologists, potentially masking successful treatments. This report describes a pathologist-assistive software tool that reduces these user limitations, while providing a rapid, quantitative scoring system for digital histopathology image analysis. The software, called 'Lesion Image Recognition and Analysis' (LIRA), employs convolutional neural networks to classify seven different pathology features, including three different lesion types from pulmonary tissues of the C3HeB/FeJ tuberculosis mouse model. LIRA was developed to improve the efficiency of histopathology analysis for mouse tuberculosis infection models, this approach has also broader applications to other disease models and tissues. The full source code and documentation is available from https://Github.com/TB-imaging/LIRA.

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: 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.

Can we predict tuberculosis cure? What tools are available?

Antibiotic treatment of tuberculosis takes ≥6 months, putting a major burden on patients and health systems in large parts of the world. Treatment beyond 2 months is needed to prevent tuberculosis relapse by clearing remaining, drug-tolerant Mycobacterium tuberculosis bacilli. However, the majority of patients treated for only 2-3 months will cure without relapse and do not need prolonged treatment. Assays that can identify these patients at an early stage of treatment may significantly help reduce the treatment burden, while a test to identify those patients who will fail treatment may help target host-directed therapies.In this review we summarise the state of the art with regard to discovery of biomarkers that predict relapse-free cure for pulmonary tuberculosis. Positron emission tomography/computed tomography scanning to measure pulmonary inflammation enhances our understanding of "cure". Several microbiological and immunological markers seem promising; however, they still need a formal validation. In parallel, new research strategies are needed to generate reliable tests.

Biodistribution and Radiation Dosimetry of 124I-DPA-713, a PET Radiotracer for Macrophage-Associated Inflammation

Whole-body PET/CT was performed using 124I-DPA-713, a radioligand for the 18-kDa translocator protein (TSPO), to determine biodistribution and radiation dosimetry. Methods: Healthy subjects aged 18-65 y underwent whole-body PET/CT either at 4, 24, and 48 h or at 24, 48, and 72 h after intravenous injection of 124I-DPA-713. Time-activity curves were generated and used to calculate organ time-integrated activity coefficients for each subject. The resulting time-integrated activity coefficients provided input data for calculation of organ absorbed doses and effective dose for each subject using OLINDA. Subjects were genotyped for the TSPO polymorphism rs6971, and plasma protein binding of 124I-DPA-713 was measured. Results: Three male and 3 female adults with a mean age of 40 ± 19 y were imaged. The mean administered activity and mass were 70.5 ± 5.1 MBq (range, 62.4-78.1 MBq) and 469 ± 34 ng (range, 416-520 ng), respectively. There were no adverse or clinically detectable pharmacologic effects in any of the 6 subjects. No changes in vital signs, laboratory values, or electrocardiograms were observed. 124I-DPA-713 cleared rapidly (4 h after injection) from the lungs, with hepatic elimination and localization to the gastrointestinal tract. The mean effective dose over the 6 subjects was 0.459 ± 0.127 mSv/MBq, with the liver being the dose-limiting organ (0.924 ± 0.501 mGy/MBq). The percentage of free radiotracer in blood was approximately 30% at 30 and 60 min after injection. Conclusion:124I-DPA-713 clears rapidly from the lungs, with predominantly hepatic elimination, and is safe and well tolerated in healthy adults.

Imaging Pulmonary Foreign Body Reaction Using [125I]iodo-DPA-713 SPECT/CT in Mice

PURPOSE

Foreign body reactions elicit granulomatous inflammation composed of reactive macrophages. We hypothesized that [¹²⁵I]iodo-DPA-713 single-photon emission computed tomography (SPECT), a low-molecular-weight pyrazolopyrimidine ligand selectively trapped by phagocytes, could be used to detect foreign body reactions in a murine model.

PROCEDURES

C57BL/6 mice intratracheally inoculated with dextran beads, which developed foreign body lesions, were imaged after injection of [¹²⁵I]iodo-DPA-713 or DPA-713-IRDye800CW using SPECT and optical imaging, respectively.

RESULTS

Foreign body lesions were clearly observed in the lungs of the dextran-treated mice on computer tomography imaging and demonstrated significantly higher [¹²⁵I]iodo-DPA-713 uptake compared with control animals (p < 0.01). Ex vivo studies demonstrated granulomatous reactions in the lungs of dextran-treated mice and localization of DPA-713-IRDye800CW at the diseased sites confirming the imaging findings.

CONCLUSION

Radioiodinated DPA-713 may be used as a noninvasive biomarker for the detection of pulmonary foreign body reactions.

The potential of imaging tools as correlates of infection and disease for new TB vaccine development

The development of an improved vaccine to stimulate an effective response against Mycobacterium tuberculosis (MTB) infection and disease will be a major breakthrough in the fight against TB. A lack of tools to adequately track the progression or resolution of events in TB pathogenesis that occur at bacterial loads below the threshold for culture in human samples seriously hampers vaccine development and evaluation. In this review we discuss recent studies that use new imaging applications, modalities and analysis techniques to provide insight into the dynamic processes of MTB infection and disease that are challenging to monitor. These include early infection, the spectrum of latency and subclinical disease, the paucibacillary state induced by treatment, and events leading to recurrence, including relapse.

Adjunct antibody administration with standard treatment reduces relapse rates in a murine tuberculosis model of necrotic granulomas

Matrix metalloproteinase (MMP)-9 is a zinc-dependent protease associated with early immune responses to Mycobacterium tuberculosis infection, macrophage recruitment and granuloma formation. We evaluated whether adjunctive inhibition of MMP-9 could improve the response to standard TB treatment in a mouse model that develops necrotic lesions. Six weeks after an aerosol infection with M. tuberculosis, C3HeB/FeJ mice received standard TB treatment (12 weeks) comprising rifampin, isoniazid and pyrazinamide alone or in combination with either anti-MMP-9 antibody, etanercept (positive control) or isotype antibody (negative control) for 6 weeks. Anti-MMP-9 and the isotype control had comparable high serum exposures and expected terminal half-life. The relapse rate in mice receiving standard TB treatment was 46.6%. Compared to the standard TB treatment, relapse rates in animals that received adjunctive treatments with anti-MMP-9 antibody or etanercept were significantly decreased to 25.9% (P = 0.006) and 29.8% (P = 0.019) respectively, but were not different from the arm that received the isotype control antibody (25.9%). Immunostaining demonstrated localization of MMP-9 primarily in macrophages in both murine and human lung tissues infected with M. tuberculosis, suggesting the importance of MMP-9 in TB pathogenesis. These data suggest that the relapse rates in M. tuberculosis-infected mice may be non-specifically improved by administration of antibodies in conjunction with standard TB treatments. Future studies are needed to evaluate the mechanism(s) leading to improved outcomes with adjunctive antibody treatments.

The Promise of Molecular Imaging in the Study and Treatment of Infectious Diseases

Infectious diseases are a major threat to humanity, and it is imperative that we develop imaging tools that aid in their study, facilitate diagnosis, and guide treatment. The alarming rise of highly virulent and multi-drug-resistant pathogens, their rapid spread leading to frequent global pandemics, fears of bioterrorism, and continued life-threatening nosocomial infections in hospitals remain as major challenges to health care in the USA and worldwide. Early diagnosis and rapid monitoring are essential for appropriate management and control of infections. Tomographic molecular imaging enables rapid, noninvasive visualization, localization, and monitoring of molecular processes deep within the body and offers several advantages over traditional tools used for the study of infectious diseases. Noninvasive, longitudinal assessments could streamline animal studies, allow unique insights into disease pathogenesis, and expedite clinical translation of new therapeutics. Since molecular imaging is already in common use in the clinic, it could also become a valuable tool for clinical studies, for patient care, for public health, and for enabling precision medicine for infectious diseases.

Imaging Macrophage-associated Inflammation

Macrophages belong to the mononuclear phagocyte system comprising closely related cells of bone marrow origin. Activated macrophages are critical in several diseases such as tuberculosis, sarcoidosis, Crohn's disease, and atherosclerosis. Noninvasive imaging techniques that can specifically image activated macrophages could therefore help in differentiating various forms of inflammatory diseases and to monitor therapeutic responses.

Use of a leukocyte-targeted peptide probe as a potential tracer for imaging the tuberculosis granuloma

Granulomas are the histopathologic hallmark of tuberculosis (TB), both in latency and active disease. Diagnostic and therapeutic strategies that specifically target granulomas have not been developed. Our objective is to develop a probe for imaging relevant immune cell populations infiltrating the granuloma. We report the binding specificity of Cyanine 3 (Cy3)-labeled cFLFLFK-PEG₁₂ to human leukocytes and cellular constituents within a human in vitro granuloma model. We also report use of the probe in in vivo studies using a mouse model of lung granulomatous inflammation. We found that the probe preferentially binds human neutrophils and macrophages in human granuloma structures. Inhibition studies showed that peptide binding to human neutrophils is mediated by the receptor formyl peptide receptor 1 (FPR1). Imaging the distribution of intravenously administered cFLFLFK-PEG₁₂-Cy3 in the mouse model revealed probe accumulation within granulomatous inflammatory responses in the lung. Further characterization revealed that the probe preferentially associated with neutrophils and cells of the monocyte/macrophage lineage. As there is no current clinical diagnostic imaging tool that specifically targets granulomas, the use of this probe in the context of latent and active TB may provide a unique advantage over current clinical imaging probes. We anticipate that utilizing a FPR1-targeted radiopharmaceutical analog of cFLFLFK in preclinical imaging studies may greatly contribute to our understanding of granuloma influx patterns and the biological roles and consequences of FPR1-expressing cells in contributing to disease pathogenesis.

Tuberculosis: Just the FAQs

Tuberculosis is responsible for more deaths worldwide than any other infectious disease. For anyone looking to learn more about this persistent public health threat, this conversational "frequently asked questions" style review addresses a breadth of questions. It offers a brief, somewhat opinionated, review of what is and is not known, particularly in light of how findings in the lab do or do not help inform the understanding of human tuberculosis.

Microglia activation in a pediatric rabbit model of tuberculous meningitis

Central nervous system (CNS) tuberculosis (TB) is the most severe form of extra-pulmonary TB and disproportionately affects young children where the developing brain has a unique host response. New Zealand white rabbits were infected with Mycobacterium tuberculosis via subarachnoid inoculation at postnatal day 4-8 and evaluated until 4-6 weeks post-infection. Control and infected rabbit kits were assessed for the development of neurological deficits, bacterial burden, and postmortem microbiologic and pathologic changes. The presence of meningitis and tuberculomas was demonstrated histologically and by in vivo magnetic resonance imaging (MRI). The extent of microglial activation was quantified by in vitro immunohistochemistry as well as non-invasive in vivo imaging of activated microglia/macrophages with positron emission tomography (PET). Subarachnoid infection induced characteristic leptomeningeal and perivascular inflammation and TB lesions with central necrosis, a cellular rim and numerous bacilli on pathologic examination. Meningeal and rim enhancement was visible on MRI. An intense microglial activation was noted in M. tuberculosis-infected animals in the white matter and around the TB lesions, as evidenced by a significant increase in uptake of the tracer ¹²⁴I-DPA-713, which is specific for activated microglia/macrophages, and confirmed by quantification of Iba-1 immunohistochemistry. Neurobehavioral analyses demonstrated signs similar to those noted in children with delayed maturation and development of neurological deficits resulting in significantly worse composite behavior scores in M. tuberculosis-infected animals. We have established a rabbit model that mimics features of TB meningitis in young children. This model could provide a platform for evaluating novel therapies, including host-directed therapies, against TB meningitis relevant to a young child's developing brain.

Effects of primary and recurrent sacral chordoma on the motor and nociceptive function of hindlimbs in rats: an orthotopic spine model

OBJECTIVE Chordoma is a slow-growing, locally aggressive cancer that is minimally responsive to conventional chemotherapy and radiotherapy and has high local recurrence rates after resection. Currently, there are no rodent models of spinal chordoma. In the present study, the authors sought to develop and characterize an orthotopic model of human chordoma in an immunocompromised rat. METHODS Thirty-four immunocompromised rats were randomly allocated to 4 study groups; 22 of the 34 rats were engrafted in the lumbar spine with human chordoma. The groups were as follows: UCH1 tumor-engrafted (n = 11), JHC7 tumor-engrafted (n = 11), sham surgery (n = 6), and intact control (n = 6) rats. Neurological impairment of rats due to tumor growth was evaluated using open field and locomotion gait analysis; pain response was evaluated using mechanical or thermal paw stimulation. Cone beam CT (CBCT), MRI, and nanoScan PET/CT were performed to evaluate bony changes due to tumor growth. On Day 550, rats were killed and spines were processed for H & E-based histological examination and immunohistochemistry for brachyury, S100β, and cytokeratin. RESULTS The spine tumors displayed typical chordoma morphology, that is, physaliferous cells filled with vacuolated cytoplasm of mucoid matrix. Brachyury immunoreactivity was confirmed by immunostaining, in which samples from tumor-engrafted rats showed a strong nuclear signal. Sclerotic lesions in the vertebral body of rats in the UCH1 and JHC7 groups were observed on CBCT. Tumor growth was confirmed using contrast-enhanced MRI. In UCH1 rats, large tumors were observed growing from the vertebral body. JHC7 chordoma-engrafted rats showed smaller tumors confined to the bone periphery compared with UCH1 chordoma-engrafted rats. Locomotion analysis showed a disruption in the normal gait pattern, with an increase in the step length and duration of the gait in tumor-engrafted rats. The distance traveled and the speed of rats in the open field test was significantly reduced in the UCH1 and JHC7 tumor-engrafted rats compared with controls. Nociceptive response to a mechanical stimulus showed a significant (p < 0.001) increase in the paw withdrawal threshold (mechanical hypalgesia). In contrast, the paw withdrawal response to a thermal stimulus decreased significantly (p < 0.05) in tumor-engrafted rats. CONCLUSIONS The authors developed an orthotopic human chordoma model in rats. Rats were followed for 550 days using imaging techniques, including MRI, CBCT, and nanoScan PET/CT, to evaluate lesion progression and bony integrity. Nociceptive evaluations and locomotion analysis were performed during follow-up. This model reproduces cardinal signs, such as locomotor and sensory deficits, similar to those observed clinically in human patients. To the authors' knowledge, this is the first spine rodent model of human chordoma. Its use and further study will be essential for pathophysiology research and the development of new therapeutic strategies.

Imaging Macrophage Accumulation in a Murine Model of Chronic Pancreatitis with 125I-Iodo-DPA-713 SPECT/CT

Pancreatitis remains a diagnostic challenge in patients with mild to moderate disease, with current imaging modalities being inadequate. Given the prominent macrophage infiltration in chronic pancreatitis, we hypothesized that ¹²⁵I-iodo-DPA-713, a small-molecule radiotracer that specifically targets macrophages, could be used with SPECT/CT to image pancreatic inflammation in a relevant experimental model. Methods: Chronic pancreatitis was induced with cerulein in C57BL/6 mice, which were contrasted with saline-injected control mice. The animals were imaged at 7 wk after induction using N,N-diethyl-2-(2-(3-¹²⁵I-iodo-4-methoxyphenyl)-5,7-dimethylpyrazolo[1,5-a]pyrimidin-3-yl)acetamide (¹²⁵I-iodo-DPA-713) SPECT/CT or ¹⁸F-FDG PET/CT. The biodistribution of ¹²⁵I-iodo-DPA-713 was determined under the same conditions, and a pair of mice was imaged using a fluorescent analog of ¹²⁵I-iodo-DPA-713, DPA-713-IRDye800CW, for correlative histology. Results: Pancreatic ¹²⁵I-iodo-DPA-713 uptake was significantly higher in treated mice than control mice (5.17% ± 1.18% vs. 2.41% ± 0.34% injected dose/g, P = 0.02), as corroborated by imaging. Mice imaged with ¹⁸F-FDG PET/CT showed cerulein-enhanced pancreatic uptake in addition to a moderate signal from healthy pancreas. Near-infrared fluorescence imaging with DPA-713-IRDye800CW showed strong pancreatic uptake, focal liver uptake, and gastrointestinal uptake in the treated mice, whereas the control mice showed only urinary excretion. Ex vivo fluorescence microscopy revealed a large influx of macrophages in the pancreas colocalizing with the retained fluorescent probe in the treated but not the control mice. Conclusion: These data support the application of both ¹²⁵I-iodo-DPA-713 SPECT/CT and DPA-713-IRDye800CW near-infrared fluorescence to delineate pancreatic, liver, or intestinal inflammation in living mice.

Introduction

Infectious diseases are a major cause of morbidity and mortality worldwide and in the USA. Overall costs and morbidity are expected to continue to rise due to increasing rates of drug-resistant pathogens, use of invasive techniques such as implants, as well as immunosuppressive and cancer therapies. Tomographic molecular imaging techniques enable rapid visualization and monitoring of molecular processes noninvasively and promise unparalleled opportunities for field of infectious diseases. These technologies are an emerging field of research, overcome several fundamental limitations of current tools, and could have a broad impact on both basic research and patient care. Beyond diagnosis and monitoring disease, these technologies could also provide a uniform cross-species platform for animal studies, allow unique insights into understanding disease pathogenesis, and expedite bench-to-bedside translation of new therapeutics. Finally, since molecular imaging is readily available for humans, validated tracers could also become valuable tools for clinical applications and for enabling personalized medicine for infectious diseases.

Polymeric nanofiber coating with tunable combinatorial antibiotic delivery prevents biofilm-associated infection in vivo

Bacterial biofilm formation is a major complication of implantable medical devices that results in therapeutically challenging chronic infections, especially in cases involving antibiotic-resistant bacteria. As an approach to prevent these infections, an electrospun composite coating comprised of poly(lactic-coglycolic acid) (PLGA) nanofibers embedded in a poly(ε-caprolactone) (PCL) film was developed to locally codeliver combinatorial antibiotics from the implant surface. The release of each antibiotic could be adjusted by loading each drug into the different polymers or by varying PLGA:PCL polymer ratios. In a mouse model of biofilm-associated orthopedic-implant infection, three different combinations of antibiotic-loaded coatings were highly effective in preventing infection of the bone/joint tissue and implant biofilm formation and were biocompatible with enhanced osseointegration. This nanofiber composite-coating technology could be used to tailor the delivery of combinatorial antimicrobial agents from various metallic implantable devices or prostheses to effectively decrease biofilm-associated infections in patients.

The antifibrotic drug pirfenidone promotes pulmonary cavitation and drug resistance in a mouse model of chronic tuberculosis

Pirfenidone is a recently approved antifibrotic drug for the treatment of idiopathic pulmonary fibrosis (IPF). Because tuberculosis (TB) is characterized by granulomatous inflammation in conjunction with parenchymal destruction and replacement fibrosis, we sought to determine whether the addition of pirfenidone as an adjunctive, host-directed therapy provides a beneficial effect during antimicrobial treatment of TB. We hypothesized that pirfenidone's antiinflammatory and antifibrotic properties would reduce inflammatory lung damage and increase antimicrobial drug penetration in granulomas to accelerate treatment response. The effectiveness of adjunctive pirfenidone during TB drug therapy was evaluated using a murine model of chronic TB. Mice treated with standard therapy 2HRZ/4HR (H, isoniazid; R, rifampin; and Z, pyrazinamide) were compared with 2 alternative regimens containing pirfenidone (Pf) (2HRZPf/4HRPf and 2HRZPf/4HR). Contrary to our hypothesis, adjunctive pirfenidone use leads to reduced bacterial clearance and increased relapse rates. This treatment failure is closely associated with the emergence of isoniazid monoresistant bacilli, increased cavitation, and significant lung pathology. While antifibrotic agents may eventually be used as part of adjunctive host-directed therapy of TB, this study clearly demonstrates that caution must be exercised. Moreover, as pirfenidone becomes more widely used in clinical practice, increased patient monitoring would be required in endemic TB settings.

Perspectives on Advances in Tuberculosis Diagnostics, Drugs, and Vaccines

Despite concerted efforts over the past 2 decades at developing new diagnostics, drugs, and vaccines with expanding pipelines, tuberculosis remains a global emergency. Several novel diagnostic technologies show promise of better point-of-care rapid tests for tuberculosis including nucleic acid-based amplification tests, imaging, and breath analysis of volatile organic compounds. Advances in new and repurposed drugs for use in multidrug-resistant (MDR) or extensively drug-resistant (XDR) tuberculosis have focused on development of several new drug regimens and their evaluation in clinical trials and now influence World Health Organization guidelines. Since the failure of the MVA85A vaccine 2 years ago, there have been no new tuberculosis vaccine candidates entering clinical testing. The current status quo of the lengthy treatment duration and poor treatment outcomes associated with MDR/XDR tuberculosis and with comorbidity of tuberculosis with human immunodeficiency virus and noncommunicable diseases is unacceptable. New innovations and political and funder commitment for early rapid diagnosis, shortening duration of therapy, improving treatment outcomes, and prevention are urgently required.

Imaging Chronic Tuberculous Lesions Using Sodium [(18)F]Fluoride Positron Emission Tomography in Mice

PURPOSE

Calcification is a hallmark of chronic tuberculosis (TB) in humans, often noted years to decades (after the initial infection) on chest radiography, but not visualized well with traditional positron emission tomography (PET). We hypothesized that sodium [(18)F]fluoride (Na[(18)F]F) PET could be used to detect microcalcifications in a chronically Mycobacterium tuberculosis-infected murine model.

PROCEDURES

C3HeB/FeJ mice, which develop necrotic and hypoxic TB lesions, were aerosol-infected with M. tuberculosis and imaged with Na[(18)F]F PET.

RESULTS

Pulmonary TB lesions from chronically infected mice demonstrated significantly higher Na[(18)F]F uptake compared with acutely infected or uninfected animals (P < 0.01), while no differences were noted in the blood or bone compartments (P > 0.08). Ex vivo biodistribution studies confirmed the imaging findings, and tissue histology demonstrated microcalcifications in TB lesions from chronically infected mice, which has not been demonstrated previously in a murine model.

CONCLUSION

Na[(18)F]F PET can be used for the detection of chronic TB lesions and could prove to be a useful noninvasive biomarker for TB studies.

Mouse model of pulmonary cavitary tuberculosis and expression of matrix metalloproteinase-9

Cavitation is a key pathological feature of human tuberculosis (TB), and is a well-recognized risk factor for transmission of infection, relapse after treatment and the emergence of drug resistance. Despite intense interest in the mechanisms underlying cavitation and its negative impact on treatment outcomes, there has been limited study of this phenomenon, owing in large part to the limitations of existing animal models. Although cavitation does not occur in conventional mouse strains after infection with Mycobacterium tuberculosis, cavitary lung lesions have occasionally been observed in C3HeB/FeJ mice. However, to date, there has been no demonstration that cavitation can be produced consistently enough to support C3HeB/FeJ mice as a new and useful model of cavitary TB. We utilized serial computed tomography (CT) imaging to detect pulmonary cavitation in C3HeB/FeJ mice after aerosol infection with M. tuberculosis Post-mortem analyses were performed to characterize lung lesions and to localize matrix metalloproteinases (MMPs) previously implicated in cavitary TB in situ A total of 47-61% of infected mice developed cavities during primary disease or relapse after non-curative treatments. Key pathological features of human TB, including simultaneous presence of multiple pathologies, were noted in lung tissues. Optical imaging demonstrated increased MMP activity in TB lesions and MMP-9 was significantly expressed in cavitary lesions. Tissue MMP-9 activity could be abrogated by specific inhibitors. In situ, three-dimensional analyses of cavitary lesions demonstrated that 22.06% of CD11b+ signal colocalized with MMP-9. C3HeB/FeJ mice represent a reliable, economical and tractable model of cavitary TB, with key similarities to human TB. This model should provide an excellent tool to better understand the pathogenesis of cavitation and its effects on TB treatments.

Mortality in HIV-infected patients with tuberculosis treated with streptomycin and a two-week intensified regimen: data from an HIV cohort study using inverse probability of treatment weighting

Background. Despite the dramatic scale-up of antiretroviral therapy in low- and middle-income countries, tuberculosis (TB) is still the main cause of death among HIV-infected patients in resource-limited settings. Previous studies in patients with TB meningitis suggest that the use of higher doses of common anti-TB drugs could reduce mortality.

Methods. Using clinical data from an HIV cohort study in India, we compared the mortality among HIV-infected patients diagnosed with TB according to the regimen received during the first two weeks of treatment: standard anti-tuberculosis therapy (ATT) (N = 847), intensified ATT (N = 322), and intensified ATT with streptomycin (N = 446). The intensified ATT comprised double dose of rifampicin and substitution of ethambutol with levofloxacin. Multivariate analysis was performed using Cox proportional hazard models and inverse probability of treatment weighting (IPTW) based on propensity scores. Patients with TB meningitis were excluded.

Results. The use of intensified ATT alone did not improve survival. However, when streptomycin was added, the use intensified ATT was associated with reduced mortality in Cox models (adjusted hazard ratio 0.72, 95% CI [0.57–0.91]) and after IPTW (hazard ratio 0.77, 95% CI [0.67–0.96]). Other factors associated with improved survival were high serum albumin concentration, high CD4 lymphocyte cell-counts, and high glomerular filtration rates. Factors associated with increased mortality were high urea concentrations, being on antiretroviral therapy at the time of ATT initiation and high BUN/creatinine ratio. In an effect modification analysis, the survival benefits of the intensified ATT with streptomycin disappeared in patients with severe hypoalbuminemia.

Conclusion. The results of this study are in accordance with a previous study from our cohort involving patients with TB meningitis, and suggest that an intensified 2-week ATT with streptomycin could reduce mortality in HIV infected patients with TB. As this is an observational study, we should be cautious about our conclusions, but given the high mortality of HIV-related TB, our findings deserve further research.

Extensively drug-resistant tuberculosis in a young child after travel to India

Extensively drug-resistant (XDR) tuberculosis is becoming increasingly prevalent worldwide, but little is known about XDR tuberculosis in young children. In this Grand Round we describe a 2-year-old child from the USA who developed pneumonia after a 3 month visit to India. Symptoms resolved with empirical first-line tuberculosis treatment; however, a XDR strain of Mycobacterium tuberculosis grew in culture. In the absence of clinical or microbiological markers, low-radiation exposure pulmonary CT imaging was used to monitor treatment response, and guide an individualised drug regimen. Management was complicated by delays in diagnosis, uncertainties about drug selection, and a scarcity of child-friendly formulations. Treatment has been successful so far, and the child is in remission. This report of XDR tuberculosis in a young child in the USA highlights the risks of acquiring drug-resistant tuberculosis overseas, and the unique challenges in management of tuberculosis in this susceptible population.

TSPO in a murine model of Sandhoff disease: presymptomatic marker of neurodegeneration and disease pathophysiology

Translocator protein (18 kDa), formerly known as the peripheral benzodiazepine receptor (PBR), has been extensively used as a biomarker of active brain disease and neuroinflammation. TSPO expression increases dramatically in glial cells, particularly in microglia and astrocytes, as a result of brain injury, and this phenomenon is a component of the hallmark response of the brain to injury. In this study, we used a mouse model of Sandhoff disease (SD) to assess the longitudinal expression of TSPO as a function of disease progression and its relationship to behavioral and neuropathological endpoints. Focusing on the presymptomatic period of the disease, we used ex vivo [(3)H]DPA-713 quantitative autoradiography and in vivo [(125)I]IodoDPA-713 small animal SPECT imaging to show that brain TSPO levels markedly increase prior to physical and behavioral manifestation of disease. We further show that TSPO upregulation coincides with early neuronal GM2 ganglioside aggregation and is associated with ongoing neurodegeneration and activation of both microglia and astrocytes. In brain regions with increased TSPO levels, there is a differential pattern of glial cell activation with astrocytes being activated earlier than microglia during the progression of disease. Immunofluorescent confocal imaging confirmed that TSPO colocalizes with both microglia and astrocyte markers, but the glial source of the TSPO response differs by brain region and age in SD mice. Notably, TSPO colocalization with the astrocyte marker GFAP was greater than with the microglia marker, Mac-1. Taken together, our findings have significant implications for understanding TSPO glial cell biology and for detecting neurodegeneration prior to clinical expression of disease.

Determination of [11C]rifampin pharmacokinetics within Mycobacterium tuberculosis-infected mice by using dynamic positron emission tomography bioimaging

Information about intralesional pharmacokinetics (PK) and spatial distribution of tuberculosis (TB) drugs is limited and has not been used to optimize dosing recommendations for new or existing drugs. While new techniques can detect drugs and their metabolites within TB granulomas, they are invasive, rely on accurate resection of tissues, and do not capture dynamic drug distribution in the tissues of interest. In this study, we assessed the in situ distribution of (11)C-labeled rifampin in live, Mycobacterium tuberculosis-infected mice that develop necrotic lesions akin to human disease. Dynamic positron emission tomography (PET) imaging was performed over 60 min after injection of [(11)C]rifampin as a microdose, standardized uptake values (SUV) were calculated, and noncompartmental analysis was used to estimate PK parameters in compartments of interest. [(11)C]rifampin was rapidly distributed to all parts of the body and quickly localized to the liver. Areas under the concentration-time curve for the first 60 min (AUC0-60) in infected and uninfected mice were similar for liver, blood, and brain compartments (P > 0.53) and were uniformly low in brain (10 to 20% of blood values). However, lower concentrations were noted in necrotic lung tissues of infected mice than in healthy lungs (P = 0.03). Ex vivo two-dimensional matrix-assisted laser desorption ionization (MALDI) imaging confirmed restricted penetration of rifampin into necrotic lung lesions. Noninvasive bioimaging can be used to assess the distribution of drugs into compartments of interest, with potential applications for TB drug regimen development.

A sterilizing tuberculosis treatment regimen is associated with faster clearance of bacteria in cavitary lesions in marmosets

Shortening the lengthy treatment duration for tuberculosis patients is a major goal of current drug development efforts. The common marmoset develops human-like disease pathology and offers an attractive model to better understand the basis for relapse and test regimens for effective shorter duration therapy. We treated Mycobacterium tuberculosis-infected marmosets with two drug regimens known to differ in their relapse rates in human clinical trials: the standard four-drug combination of isoniazid, rifampin, pyrazinamide, and ethambutol (HRZE) that has very low relapse rates and the combination of isoniazid and streptomycin that is associated with higher relapse rates. As early as 2 weeks, the more sterilizing regimen significantly reduced the volume of lung disease by computed tomography (P = 0.035) and also significantly reduced uptake of [(18)F]-2-fluoro-2-deoxyglucose by positron emission tomography (P = 0.049). After 6 weeks of therapy, both treatments caused similar reductions in granuloma bacterial load, but the more sterilizing, four-drug regimen caused greater reduction in bacterial load in cavitary lesions (P = 0.009). These findings, combined with the association in humans between cavitary disease and relapse, suggest that the basis for improved sterilizing activity of the four-drug combination is both its faster disease volume resolution and its stronger sterilizing effect on cavitary lesions. Definitive data from relapse experiments are needed to support this observation.

Evaluation of moxifloxacin-containing regimens in pathologically distinct murine tuberculosis models

In the recently concluded REMox-TB trial, two 4-month moxifloxacin-containing regimens did not meet the criteria for noninferiority compared to the current 6-month first-line regimen to treat tuberculosis (TB). Despite the disappointing result, this phase 3 clinical trial provides a rare opportunity to gauge the predictive accuracy of the nonclinical models used to support regimen development. In parallel with the REMox-TB trial, we compared the efficacy of the same three regimens against chronic TB infection in the commonly used BALB/c mouse strain and in C3HeB/FeJ mice, which have attracted recent interest as a nonclinical efficacy model because they develop caseous lung lesions which may better resemble human TB. In long-term treatment experiments at two institutions, using low-dose aerosol infection models with 6- to 8-week incubation periods in both mouse strains, control mice received rifampin, isoniazid, pyrazinamide, and ethambutol (RHZE), and test mice received the same regimen with moxifloxacin replacing isoniazid (RMZE) or ethambutol (RHZM). Outcome measures were lung CFU counts during treatment and relapse after various durations of treatment. At both institutions and in both mouse strains, RMZE and RHZM reduced by approximately 1 month and 0 to 1 month, respectively, the treatment duration needed to produce the same relapse rate as RHZE. These results demonstrating generally similar treatment-shortening effects of the moxifloxacin-containing regimens in each mouse strain, with effect sizes consistent with the REMox-TB trial results, reinforce the predictive value of murine models for TB regimen development.