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.

Rabbit model of Staphylococcus aureus implant-associated spinal infection

Post-surgical implant-associated spinal infection is a devastating complication commonly caused by Staphylococcus aureus. Biofilm formation is thought to reduce penetration of antibiotics and immune cells, contributing to chronic and difficult-to-treat infections. A rabbit model of a posterior-approach spinal surgery was created, in which bilateral titanium pedicle screws were interconnected by a plate at the level of lumbar vertebra L6 and inoculated with a methicillin-resistant S.aureus (MRSA) bioluminescent strain. In vivo whole-animal bioluminescence imaging (BLI) and ex vivo bacterial cultures demonstrated a peak in bacterial burden by day 14, when wound dehiscence occurred. Structures suggestive of biofilm, visualized by scanning electron microscopy, were evident up to 56 days following infection. Infection-induced inflammation and bone remodeling were also monitored using ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) positron emission tomography (PET) and computed tomography (CT). PET imaging signals were noted in the soft tissue and bone surrounding the implanted materials. CT imaging demonstrated marked bone remodeling and a decrease in dense bone at the infection sites. This rabbit model of implant-associated spinal infection provides a valuable preclinical in vivo approach to investigate the pathogenesis of implant-associated spinal infections and to evaluate novel therapeutics.

Summary: A model of post-surgical methicillin-resistant Staphylococcus aureus implant-associated spinal infection was created in rabbits, recapitulating acute infection as well as chronic low-burden infection, with structures suggestive of biofilm formation and bone remodeling.

Radiosynthesis and Biodistribution of 18F-Linezolid in Mycobacterium tuberculosis-Infected Mice Using Positron Emission Tomography

Oxazolidinones are a novel class of antibacterials with excellent activity against resistant Gram-positive bacteria including strains causing multidrug-resistant tuberculosis (TB). Despite their excellent efficacy, optimal dosing strategies to limit their toxicities are still under development. Here, we developed a novel synthetic strategy for fluorine-18-radiolabeled oxazolidinones. As proof-of-concept, we performed whole-body ¹⁸F-linezolid positron emission tomography (PET) in a mouse model of pulmonary TB for noninvasive in situ measurements of time-activity curves in multiple compartments with subsequent confirmation by ex vivo tissue gamma counting. After intravenous injection, ¹⁸F-linezolid rapidly distributed to all organs with excellent penetration into Mycobacterium tuberculosis-infected lungs. Drug biodistribution studies with PET can provide unbiased, in situ drug measurements, which could boost efforts to optimize antibiotic dosing strategies.

11C-PABA as a PET Radiotracer for Functional Renal Imaging: Preclinical and First-in-Human Study

para-Aminobenzoic acid (PABA) has been previously used as an exogenous marker to verify completion of 24-h urine sampling. Therefore, we hypothesized that PABA radiolabeled with ¹¹C might allow high-quality dynamic PET of the kidneys with less radiation exposure than other agents because of its shorter biologic and physical half-life. We evaluated if ¹¹C-PABA can visualize renal anatomy and quantify function in healthy rats and rabbits and in a first-in-humans study on healthy volunteers. Methods: Healthy rats and rabbits were injected with ¹¹C-PABA intravenously. Subsequently, dynamic PET was performed, followed by postmortem tissue-biodistribution studies. ¹¹C-PABA PET was directly compared with the current standard, ^99mTc-mercaptoacetyltriglycin, in rats. Three healthy human subjects also underwent dynamic PET after intravenous injection of ¹¹C-PABA. Results: In healthy rats and rabbits, dynamic PET demonstrated a rapid accumulation of ¹¹C-PABA in the renal cortex, followed by rapid excretion through the pelvicalyceal system. In humans, ¹¹C-PABA PET was safe and well tolerated. There were no adverse or clinically detectable pharmacologic effects in any subject. The cortex was delineated on PET, and the activity gradually transited to the medulla and then pelvis with high spatiotemporal resolution. Conclusion: ¹¹C-PABA demonstrated fast renal excretion with a very low background signal in animals and humans. These results suggest that ¹¹C-PABA might be used as a novel radiotracer for functional renal imaging, providing high-quality spatiotemporal images with low radiation exposure.

Radiosynthesis and PET Bioimaging of 76Br-Bedaquiline in a Murine Model of Tuberculosis

Bedaquiline is a promising drug against tuberculosis (TB), but limited data are available on its intralesional pharmacokinetics. Moreover, current techniques rely on invasive tissue resection, which is difficult in humans and generally limited even in animals. In this study, we developed a novel radiosynthesis for 76Br-bedaquiline and performed noninvasive, longitudinal whole-body positron emission tomography (PET) in live, Mycobacterium tuberculosis-infected mice over 48 h. After the intravenous injection, 76Br-bedaquiline distributed to all organs and selectively localized to adipose tissue and liver, with excellent penetration into infected lung lesions (86%) and measurable penetration into the brain parenchyma (15%). Ex vivo high resolution, two-dimensional autoradiography, and same section hematoxylin/eosin and immunofluorescence provided detailed intralesional drug biodistribution. PET bioimaging and high-resolution autoradiography are novel techniques that can provide detailed, multicompartment, and intralesional pharmacokinetics of new and existing TB drugs. These technologies can significantly advance efforts to optimize drug dosing.

Matrix Metalloproteinase Inhibition in a Murine Model of Cavitary Tuberculosis Paradoxically Worsens Pathology

Matrix metalloproteinases (MMPs) degrade extracellular matrix and are implicated in tuberculosis pathogenesis and cavitation. In particular, MMP-7 is induced by hypoxia and highly expressed around pulmonary cavities of Mycobacterium tuberculosis-infected C3HeB/FeJ mice. In this study, we evaluated whether administration of cipemastat, an orally available potent inhibitor of MMP-7, could reduce pulmonary cavitation in M. tuberculosis-infected C3HeB/FeJ mice. We demonstrate that, compared with untreated controls, cipemastat treatment paradoxically increases the frequency of cavitation (32% vs 7%; P = .029), immunopathology, and mortality. Further studies are needed to understand the role of MMP inhibitors as adjunctive treatments for pulmonary tuberculosis.

Noninvasive 11C-rifampin positron emission tomography reveals drug biodistribution in tuberculous meningitis

Tuberculous meningitis (TBM) is a devastating form of tuberculosis (TB), and key TB antimicrobials, including rifampin, have restricted brain penetration. A lack of reliable data on intralesional drug biodistribution in infected tissues has limited pharmacokinetic (PK) modeling efforts to optimize TBM treatments. Current methods to measure intralesional drug distribution rely on tissue resection, which is difficult in humans and generally limited to a single time point even in animals. In this study, we developed a multidrug treatment model in rabbits with experimentally induced TBM and performed serial noninvasive dynamic ¹¹C-rifampin positron emission tomography (PET) over 6 weeks. Area under the curve brain/plasma ratios were calculated using PET and correlated with postmortem mass spectrometry. We demonstrate that rifampin penetration into infected brain lesions is limited, spatially heterogeneous, and decreases rapidly as early as 2 weeks into treatment. Moreover, rifampin concentrations in the cerebrospinal fluid did not correlate well with those in the brain lesions. First-in-human ¹¹C-rifampin PET performed in a patient with TBM confirmed these findings. PK modeling predicted that rifampin doses (≥30 mg/kg) were required to achieve adequate intralesional concentrations in young children with TBM. These data demonstrate the proof of concept of PET as a clinically translatable tool to noninvasively measure intralesional antimicrobial distribution in infected tissues.

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.

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.

A Systematic Approach for Developing Bacteria-Specific Imaging Tracers

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

METHODS

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

RESULTS

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

CONCLUSION

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

Imaging 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.