Synthesis of [(125)I]iodoDPA-713: a new probe for imaging inflammation

Selective Imaging of Lung Macrophages Using [11C]PBR28-Based Positron Emission Tomography

PURPOSE

We tested whether the translocator protein (TSPO)-targeted positron emission tomography (PET) tracer, N-acetyl-N-(2-[¹¹C]methoxybenzyl)-2-phenoxy-5-pyridinamine ([¹¹C]PBR28), could distinguish macrophage dominant from neutrophilic inflammation better than 2-deoxy-2-[¹⁸F]fluoro-D-glucose ([¹⁸F]FDG) in mouse models of lung inflammation and assessed TSPO association with macrophages in lung tissue from the mouse models and in patients with chronic obstructive pulmonary disease (COPD).

PROCEDURES

MicroPET imaging quantified [¹¹C]PBR28 and [¹⁸F]FDG lung uptake in wild-type (Wt) C57BL/6J or heterozygous transgenic monocyte-deficient Wt/opT mice at 49 days after Sendai virus (SeV) infection, during macrophage-dominant inflammation, and in Wt mice at 3 days after SeV infection or 24 h after endotoxin instillation during neutrophilic inflammation. Immunohistochemical staining for TSPO in macrophages and neutrophils was performed using Mac3 and Ly6G for cell identification in mouse lung sections and CD68 and neutrophil elastase (NE) in human lung sections taken from explanted lungs from patients with COPD undergoing lung transplantation and donor lungs rejected for transplantation. Differences in tracer uptake among SeV-infected, endotoxin-treated, and uninfected/untreated control mice and in TSPO staining between neutrophils and macrophage populations in human lung sections were tested using analysis of variance.

RESULTS

In Wt mice, [¹¹C]PBR28 uptake (% injected dose/ml lung tissue) increased significantly with macrophage-dominant inflammation at 49 days (D49) after SeV infection compared to controls (p = <0.001) but not at 3 days (D49) after SeV infection (p = 0.167). [¹¹C]PBR28 uptake was unchanged at 24 h after endotoxin instillation (p = 0.958). [¹⁸F]FDG uptake increased to a similar degree in D3 and D49 SeV-infected and endotoxin-treated Wt mice compared to controls with no significant difference in the degree of increase among the tested conditions. [¹¹C]PBR28 but not [¹⁸F]FDG lung uptake at D49 post-SeV infection was attenuated in Wt/opT mice compared to Wt mice. TSPO localized predominantly to macrophages in mouse lung tissue by immunostaining, and TSPO staining intensity was significantly higher in CD68+ cells compared to neutrophils in the human lung sections.

CONCLUSIONS

PET imaging with [¹¹C]PBR28 can specifically detect macrophages versus neutrophils during lung inflammation and may be a useful biomarker of macrophage accumulation in lung disease.

GMP-compliant fully automated radiosynthesis of [18F]FEPPA for PET/MRI imaging of regional brain TSPO expression

Background

Expression of translocator protein (TSPO) on the outer mitochondrial membrane of activated microglia is strongly associated with neuroinflammation. The second-generation PET ligand [¹⁸F]FEPPA specifically binds TSPO to enable in vivo visualization and quantification of neuroinflammation. We optimized a fully automated radiosynthesis method and evaluated the utility of [¹⁸F]FEPPA, the second-generation PET ligand specifically binds TSPO, in a mouse model of systemic LPS challenge to detect TSPO-associated signals of central and peripheral inflammation. In vivo dynamic PET/MR imaging was performed in LPS-induced and control mice after [¹⁸F]FEPPA administration. The relationship between the [¹⁸F]FEPPA signal and the dose of LPS was assessed. The cytokine levels (i.e., TNF-α, Il-1β, Il-6) in LPS-induced mice were measured by RT-PCR. Standard uptake value (SUV), total volume of distribution (VT) and area under the curve (AUC) were determined based on the metabolite-uncorrected plasma input function. Western blotting and immunostaining were used to measure TSPO expression in the brain.

Results

The fully automated [¹⁸F]FEPPA radiosynthesis produced an uncorrected radiochemical yield of 30 ± 2% within 80 min, with a radiochemical purity greater than 99% and specific activity of 148.9‒216.8 GBq/µmol. Significant differences were observed in the brain after [¹⁸F]FEPPA administration: SUV, VT and AUC were 1.61 ± 0.1, 1.25 ± 0.12 and 1.58 ± 0.09-fold higher in LPS-injected mice than controls. TNF-α, Il-1β and Il-6 mRNA levels were also elevated in the brains of LPS-injected mice. Western blotting revealed TSPO (p < 0.05) and Iba-1 (p < 0.01) were upregulated in the brain after LPS administration. In LPS-injected mice, TSPO immunoactivity colocalized with Iba-1 in the cerebrum and TSPO was significantly overexpressed in the hippocampus and cerebellum. The peripheral organs (heart, lung) of LPS-injected mice had higher [¹⁸F]FEPPA signal-to-noise ratios than control mice.

Conclusions

Based on the current data on ligand specificity and selectivity in central tissues using 7 T PET/MR imaging, we demonstrate that [¹⁸F]FEPPA accumulations significant increased in the specific brain regions of systemic LPS-induced neuroinflammation (5 mg/kg). Future investigations are needed to determine the sensitivity of [¹⁸F]FEPPA as a biomarker of neuroinflammation as well as the correlation between the PET signal intensity and the expression levels of TSPO.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13550-021-00768-9.

Dementia, Depression, and Associated Brain Inflammatory Mechanisms after Spinal Cord Injury

Evaluation of the chronic effects of spinal cord injury (SCI) has long focused on sensorimotor deficits, neuropathic pain, bladder/bowel dysfunction, loss of sexual function, and emotional distress. Although not well appreciated clinically, SCI can cause cognitive impairment including deficits in learning and memory, executive function, attention, and processing speed; it also commonly leads to depression. Recent large-scale longitudinal population-based studies indicate that patients with isolated SCI (without concurrent brain injury) are at a high risk of dementia associated with substantial cognitive impairments. Yet, little basic research has addressed potential mechanisms for cognitive impairment and depression after injury. In addition to contributing to disability in their own right, these changes can adversely affect rehabilitation and recovery and reduce quality of life. Here, we review clinical and experimental work on the complex and varied responses in the brain following SCI. We also discuss potential mechanisms responsible for these less well-examined, important SCI consequences. In addition, we outline the existing and developing therapeutic options aimed at reducing SCI-induced brain neuroinflammation and post-injury cognitive and emotional impairments.

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.

Translocator protein (TSPO) and stress cascades in mouse models of psychosis with inflammatory disturbances

Changes in inflammatory cascades have been implicated in the underlying pathophysiology of psychosis. Translocator protein 18 kDa (TSPO) has been used to assess neuroinflammatory processes in psychotic disorders. Nonetheless, it is unclear whether TSPO, a mitochondrial protein, can be interpreted as a general marker for inflammation in diseases involving psychosis. To address this question, we investigated TSPO signaling in representative mouse models for psychosis with inflammatory disturbances. The maternal immune activation and cuprizone short-term exposure models show different TSPO signaling. Furthermore, we observed similarities and differences in their respective stress pathways including stress hormone signaling and oxidative stress that are functionally interconnected with the inflammatory responses. We propose that more careful studies of TSPO distribution in neuroinflammation and other stress cascades associated with psychotic symptoms will allow us to understand the biological mechanisms underlying psychosis-related behaviors.

[125 I]IodoDPA-713 Binding to 18 kDa Translocator Protein (TSPO) in a Mouse Model of Intracerebral Hemorrhage: Implications for Neuroimaging

Intracerebral hemorrhage (ICH) is a fatal stroke subtype with significant public health impact. Although neuroinflammation is a leading cause of neurological deficits after ICH, no imaging tool is currently available to monitor brain inflammation in ICH patients. Given the role of TSPO in neuroinflammation, herein we investigate whether a second-generation TSPO ligand, [¹²⁵ I]IodoDPA-713 can be used to monitor the changes in TSPO expression in a preclinical model of intracerebral hemorrhage. Male CD1 mice were subjected to ICH/Sham. The brain sections, collected at different time points were incubated with [¹²⁵ I]IodoDPA-713 and the brain uptake of [¹²⁵ I]IodoDPA-713 was estimated using autoradiography. The specificity of [¹²⁵ I]IodoDPA-713 binding was confirmed by a competitive displacement study with an unlabeled TSPO ligand, PK11195. [¹²⁵ I]IodoDPA-713 binding was higher in the ipsilateral striatum with an enhanced binding observed in the peri-hematomal brain region after ICH, whereas the brain sections from sham as well as contralateral brain areas of ICH exhibited marginal binding of [¹²⁵ I]IodoDPA-713. PK11195 completely reversed the [¹²⁵ I] IodoDPA-713 binding to brain sections suggesting a specific TSPO-dependent binding of [¹²⁵ I]IodoDPA-713 after ICH. This was further confirmed with immunohistochemistry analysis of adjacent sections, which revealed a remarkable expression of TSPO in the areas of high [¹²⁵ I]IodoDPA-713 binding after ICH. The specific as well as enhanced binding of [¹²⁵ I]IodoDPA-713 to the ipsilateral brain areas after ICH as assessed by autoradiography analysis provides a strong rationale for testing the applicability of [¹²⁵ I]IodoDPA-713 for non-invasive neuroimaging in preclinical models of ICH.

Assessment of TSPO in a Rat Experimental Autoimmune Myocarditis Model: A Comparison Study between [18F]Fluoromethyl-PBR28 and [18F]CB251

Overexpression of the 18-kDa translocator protein (TSPO) is closely linked to inflammatory responses in the heart, including myocarditis, which can lead to myocardial necrosis. In vivo assessment of inflammatory responses has enabled the precise diagnosis of myocarditis to improve clinical outcomes. Here, we evaluated TSPO overexpression in a rat model of experimental autoimmune myocarditis (EAM) compared to healthy rats using two TSPO radiotracers, [¹⁸F]fluoromethyl-PBR28 ([¹⁸F]1) and [¹⁸F]CB251 ([¹⁸F]2). All radiolabeling methods were successfully applied to an automated module for the reproducible preparation of TSPO radiotracers. Both radiotracers were directly compared in an EAM rat model, as well as in healthy rats to determine whether either radiotracer provides a more promising assessment of in vivo TSPO overexpression. [¹⁸F]2 provided more specific TSPO-uptake in the heart of the EAM rats (1.32-fold that of the heart-to-lung uptake ratio versus healthy controls), while [¹⁸F]1 did not show a significant difference between the two groups. Histopathological characterization revealed that a prominent positron emission tomography (PET) signal of [¹⁸F]2 in the EAM rats corresponded to the presence of a higher density of TSPO compared to the healthy controls. These results suggest that the imidazole[1,2-a]pyridine-based radiotracer [¹⁸F]2 is a sensitive tool for noninvasively diagnosing myocarditis related to inflammation of the heart muscle by assessing abnormal TSPO expression.

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.

Radiosynthesis of SPECT tracers via a copper mediated 123I iodination of (hetero)aryl boron reagents

A general method for the copper mediated nucleophilic 123I-iodination of (hetero)aryl boronic esters and acids has been developed. The broad substrate scope of this radiosynthetic approach allows access to [123I]DPA-713, [123I]IMPY, [123I]MIBG and [123I]IPEB that are four commonly used SPECT radiotracers. Our results infer that aryl boronic reagents can now be employed as common precursors for both fluorine-18 and iodine-123 radiolabelling.

Progressive inflammation-mediated neurodegeneration after traumatic brain or spinal cord injury

Traumatic brain injury (TBI) has been linked to dementia and chronic neurodegeneration. Described initially in boxers and currently recognized across high contact sports, the association between repeated concussion (mild TBI) and progressive neuropsychiatric abnormalities has recently received widespread attention, and has been termed chronic traumatic encephalopathy. Less well appreciated are cognitive changes associated with neurodegeneration in the brain after isolated spinal cord injury. Also under-recognized is the role of sustained neuroinflammation after brain or spinal cord trauma, even though this relationship has been known since the 1950s and is supported by more recent preclinical and clinical studies. These pathological mechanisms, manifested by extensive microglial and astroglial activation and appropriately termed chronic traumatic brain inflammation or chronic traumatic inflammatory encephalopathy, may be among the most important causes of post-traumatic neurodegeneration in terms of prevalence. Importantly, emerging experimental work demonstrates that persistent neuroinflammation can cause progressive neurodegeneration that may be treatable even weeks after traumatic injury.

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.

Applications of in vivo imaging in the evaluation of the pathophysiology of viral and bacterial infections and in development of countermeasures to BSL3/4 pathogens

While preclinical and clinical imaging have been applied to drug discovery/development and characterization of disease pathology, few examples exist where imaging has been used to evaluate infectious agents or countermeasures to biosafety level (BSL)3/4 threat agents. Viruses engineered with reporter constructs, i.e., enzymes and receptors, which are amenable to detection by positron emission tomography (PET), single photon emission tomography (SPECT), or magnetic resonance imaging (MRI) have been used to evaluate the biodistribution of viruses containing specific therapeutic or gene transfer payloads. Bioluminescence and nuclear approaches involving engineered reporters, direct labeling of bacteria with radiotracers, or tracking bacteria through their constitutively expressed thymidine kinase have been utilized to characterize viral and bacterial pathogens post-infection. Most PET, SPECT, CT, or MRI approaches have focused on evaluating host responses to the pathogens such as inflammation, brain neurochemistry, and structural changes and on assessing the biodistribution of radiolabeled drugs. Imaging has the potential when applied preclinically to the development of countermeasures against BSL3/4 threat agents to address the following: (1) presence, biodistribution, and time course of infection in the presence or absence of drug; (2) binding of the therapeutic to the target; and (3) expression of a pharmacologic effect either related to drug mechanism, efficacy, or safety. Preclinical imaging could potentially provide real-time dynamic tools to characterize the pathogen and animal model and for developing countermeasures under the U.S. FDA Animal Rule provision with high confidence of success and clinical benefit.

Molecular imaging of inflammation in the ApoE -/- mouse model of atherosclerosis with IodoDPA

BACKGROUND

Atherosclerosis is a common and serious vascular disease predisposing individuals to myocardial infarction and stroke. Intravascular plaques, the pathologic lesions of atherosclerosis, are largely composed of cholesterol-laden luminal macrophage-rich infiltrates within a fibrous cap. The ability to detect those macrophages non-invasively within the aorta, carotid artery and other vessels would allow physicians to determine plaque burden, aiding management of patients with atherosclerosis.

METHODS AND RESULTS

We previously developed a low-molecular-weight imaging agent, [(125)I]iodo-DPA-713 (iodoDPA), which selectively targets macrophages. Here we use it to detect both intravascular macrophages and macrophage infiltrates within the myocardium in the ApoE -/- mouse model of atherosclerosis using single photon emission computed tomography (SPECT). SPECT data were confirmed by echocardiography, near-infrared fluorescence imaging and histology. SPECT images showed focal uptake of radiotracer at the aortic root in all ApoE -/- mice, while the age-matched controls were nearly devoid of radiotracer uptake. Focal radiotracer uptake along the descending aorta and within the myocardium was also observed in affected animals.

CONCLUSIONS

IodoDPA is a promising new imaging agent for atherosclerosis, with specificity for the macrophage component of the lesions involved.

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

Current tools for monitoring response to tuberculosis treatments have several limitations. Noninvasive biomarkers could accelerate tuberculosis drug development and clinical studies, but to date little progress has been made in developing new imaging technologies for this application. In this study, we developed pulmonary single-photon emission computed tomography (SPECT) using radioiodinated DPA-713 to serially monitor the activity of tuberculosis treatments in live mice, which develop necrotic granulomas and cavitary lesions. C3HeB/FeJ mice were aerosol infected with Mycobacterium tuberculosis and administered either a standard or a highly active bedaquiline-containing drug regimen. Serial (125)I-DPA-713 SPECT imaging was compared with (18)F-fluorodeoxyglucose ((18)F-FDG) positron emission tomography (PET) and standard microbiology. Ex vivo studies were performed to characterize and correlate DPA-713 imaging with cellular and cytokine responses. Pulmonary (125)I-DPA-713 SPECT, but not (18)F-FDG PET, was able to correctly identify the bactericidal activities of the two tuberculosis treatments as early as 4 weeks after the start of treatment (P < 0.03). DPA-713 readily penetrated the fibrotic rims of necrotic and cavitary lesions. A time-dependent decrease in both tumor necrosis factor alpha (TNF-α) and interferon gamma (IFN-γ) levels was observed with treatments, with (125)I-DPA-713 SPECT correlating best with tissue TNF-α levels (ρ = 0.94; P < 0.01). (124)I-DPA-713 was also evaluated as a PET probe and demonstrated a 4.0-fold-higher signal intensity in the infected tuberculous lesions than uninfected controls (P = 0.03). These studies provide proof of concept for application of a novel noninvasive imaging biomarker to monitor tuberculosis treatments, with the potential for application for humans.

Anti-CD20 inhibits T cell-mediated pathology and microgliosis in the rat brain

OBJECTIVE

The mechanism of action of anti-B cell therapy in multiple sclerosis (MS) is not fully understood. Here, we compared the effect of anti-CD20 therapy on microglial activation in two distinct focal rat models of MS.

METHODS

The effect of anti-CD20 therapy on lesion formation and extralesional microglial activation was evaluated in the fDTH-EAE (experimental allergic encephalomyelitis) model, which is a focal demyelinating type-IV delayed-type hypersensitivity lesion. For comparison, effects were also assessed in the focal humoral MOG model induced by intracerebral injection of cytokine in myelin oligodendrocyte glycoprotein immunized rats. Microglial activation was assessed in situ and in vivo using the TSPO SPECT ligand [(125)I]DPA-713, and by immunostaining for MHCII. The effect of treatment on demyelination and lymphocyte recruitment to the brain were evaluated.

RESULTS

Anti-CD20 therapy reduced microglial activation, and lesion formation in the humoral model, but it was most effective in the antibody-independent fDTH-EAE. Immunohistochemistry for MHCII also demonstrated a reduced volume of microglial activation in the brains of anti-CD20-treated fDTH-EAE animals, which was accompanied by a reduction in T-cell recruitment and demyelination. The effect anti-CD20 therapy in the latter model was similarly strong as compared to the T-cell targeting MS compound FTY720.

INTERPRETATION

The suppression of lesion development by anti-CD20 treatment in an antibody-independent model suggests that B-cells play an important role in lesion development, independent of auto-antibody production. Thus, CD20-positive B-cell depletion has the potential to be effective in a wider population of individuals with MS than might have been predicted from our knowledge of the underlying histopathology.

Glial activation in the early stages of brain metastasis: TSPO as a diagnostic biomarker

Metastatic spread of cancer cells to the brain is associated with high mortality, primarily because current diagnostic tools identify only well-advanced metastases. Brain metastases have been shown to induce a robust glial response, including both astrocyte and microglial activation. On the basis of these findings, we hypothesized that this stromal response may provide a sensitive biomarker of tumor burden, in particular through the use of SPECT/PET imaging agents targeting the translocator protein (TSPO) that is upregulated on activated glia. Our goals, therefore, were first to determine the spatial and temporal profile of glial activation during early metastasis growth in vivo and second to assess the potential of the radiolabeled TSPO ligand (123)I-DPA-713 for early detection of brain metastases.

METHODS

Metastatic mouse mammary carcinoma 4T1-green fluorescent protein cells were injected either intracerebrally or intracardially into female BALB/c mice to induce brain metastases. Astrocyte and microglial activation was assessed immunohistochemically over a 28-d period, together with immunofluorescence detection of TSPO upregulation. Subsequently, SPECT imaging and autoradiography were used to determine in vivo binding of (123)I-DPA-713 at metastatic sites.

RESULTS

Dynamic astrocyte and microglial activation was evident throughout the early stages of tumor growth, with the extent of astrocyte activation correlating significantly with tumor size (P < 0.0001). Microglial activation appeared to increase more rapidly than astrocyte activation at the earlier time points, but by later time points the extent of activation was comparable between the glial cell types. Upregulation of TSPO expression was found on both glial populations. Both autoradiographic and in vivo SPECT data showed strong positive binding of (123)I-DPA-713 in the intracerebrally induced model of brain metastasis, which was significantly greater than that observed in controls (P < 0.05). (123)I-DPA-713 binding was also evident autoradiographically in the intracardially induced model of brain metastasis but with lower sensitivity because of smaller tumor size (∼ 100-μm diameter vs. ∼ 600-μm diameter in the intracerebral model).

CONCLUSION

These data suggest that the glial response to brain metastasis may provide a sensitive biomarker of tumor burden, with a tumor detection threshold lying between 100 and 600 μm in diameter. This approach could enable substantially earlier detection of brain metastases than the current clinical approach of gadolinium-enhanced MR imaging.

Sex differences in translocator protein 18 kDa (TSPO) in the heart: implications for imaging myocardial inflammation

Myocarditis is more severe in men than in women and difficult to diagnose due to a lack of imaging modalities that directly detect myocardial inflammation. Translocator protein 18 kDa (TSPO) is used extensively to image brain inflammation due to its presence in CD11b(+) brain microglia. In this study, we examined expression of TSPO and CD11b in mice with coxsackievirus B3 (CVB3) myocarditis and biopsy sections from myocarditis patients in order to determine if it could be used to image myocarditis. We found that male mice with CVB3 myocarditis upregulated more genes associated with TSPO activation than female mice. TSPO expression was increased in the heart of male mice and men with myocarditis compared with female subjects due to testosterone, where it was expressed predominantly in CD11b(+) immune cells. We show that TSPO ligands detect myocardial inflammation using microSPECT, with increased uptake of [(125)I]-IodoDPA-713 in male mice with CVB3 myocarditis compared with undiseased controls.

Noninvasive molecular imaging of tuberculosis-associated inflammation with radioiodinated DPA-713

BACKGROUND

Increased expression of translocator protein (TSPO) is a feature of microglial and macrophage activation. Since activated macrophages are key components of tuberculosis-associated inflammation, we evaluated radioiodinated DPA-713, a synthetic ligand of TSPO, for in vivo imaging of host response.

METHODS

Mice were infected with aerosolized Mycobacterium tuberculosis and evaluated using whole-body [(125)I]iodo-DPA-713 single-photon emission computed tomography (SPECT). Ex vivo biodistribution and correlative immunofluorescence studies were also performed.

RESULTS

[(125)I]Iodo-DPA-713 SPECT imaging clearly delineated tuberculosis-associated pulmonary inflammation in live animals. Biodistribution studies confirmed radiotracer specificity for inflamed pulmonary tissues. Immunofluorescence studies demonstrated that TSPO is highly expressed in CD68(+) macrophages and phagocytic cells within tuberculosis lesions and that [(125)I]DPA-713 specifically accumulates within these cells. Coadministration of excess unlabelled DPA-713 abrogated both the SPECT and ex vivo fluorescence signals. Lesion-specific signal-to-noise ratios were significantly higher with [(125)I]iodo-DPA-713 SPECT (4.06 ± 0.52) versus [(18)F]fluorodeoxyglucose (FDG) positron emission tomography (PET) (2.00 ± 0.28) performed in the same mice (P = .004).

CONCLUSIONS

[(125)I]Iodo-DPA-713 accumulates specifically in tuberculosis-associated inflammatory lesions by selective retention within macrophages and phagocytic cells. [(125)I]Iodo-DPA-713 SPECT provides higher lesion-specific signal-to-noise ratios than [(18)F]FDG PET and may prove to be a more specific biomarker to monitor tuberculosis in situ.

PET imaging of lung inflammation with [18F]FEDAC, a radioligand for translocator protein (18 kDa)

Purpose

The translocator protein (18 kDa) (TSPO) is highly expressed on the bronchial and bronchiole epithelium, submucosal glands in intrapulmonary bronchi, pneumocytes and alveolar macrophages in human lung. This study aimed to perform positron emission tomography (PET) imaging of lung inflammation with [¹⁸F]FEDAC, a specific TSPO radioligand, and to determine cellular sources enriching TSPO expression in the lung.

Methods

An acute lung injury model was prepared by intratracheal administration of lipopolysaccharide (LPS) to rat. Uptake of radioactivity in the rat lungs was measured with small-animal PET after injection of [¹⁸F]FEDAC. Presence of TSPO was examined in the lung tissue using Western blot and immunohistochemical assays.

Results

The uptake of [¹⁸F]FEDAC increased in the lung with the progress of inflammation by treatment with LPS. Pretreatment with a TSPO-selective ligand PK11195 showed a significant decrease in the lung uptake of [¹⁸F]FEDAC due to competitive binding to TSPO. TSPO expression was elevated in the inflamed lung section and its level responded to the [¹⁸F]FEDAC uptake and severity of inflammation. Increase of TSPO expression was mainly found in the neutrophils and macrophages of inflamed lungs.

Conclusion

From this study we conclude that PET with [¹⁸F]FEDAC may be a useful tool for imaging TSPO expression and evaluating progress of lung inflammation. Study on human lung using [¹⁸F]FEDAC-PET is promising.

New iodinated quinoline-2-carboxamides for SPECT imaging of the translocator protein

With the aim of developing new SPECT imaging agents for the translocator protein (TSPO), a small library of iodinated quinoline-2-carboxamides have been prepared and tested for binding affinity with TSPO. N,N-Diethyl-3-iodomethyl-4-phenylquinoline-2-carboxamide was found to have excellent affinity (K(i) 12.0 nM), comparable to that of the widely used TSPO imaging agent PK11195.