Optimization of a Collagen-Targeted PET Probe for Molecular Imaging of Pulmonary Fibrosis

A nitroreductase responsive probe for early diagnosis of pulmonary fibrosis disease

Idiopathic pulmonary fibrosis (IPF) is a serious interstitial lung disease. However, the definitive diagnosis of IPF is impeded by the limited capabilities of current diagnostic methods, which may fail to capture the optimal timing for treatment. The main goal of this study is to determine the feasibility of a nitroreductase (NTR) responsive probe, 18F-NCRP, for early detection and deterioration monitoring of IPF. 18F-NCRP was obtained with high radiochemical purity (>95 %). BLM-injured mice were established by intratracheal instillation with bleomycin (BLM) and characterized through histological analysis. Longitudinal PET/CT imaging, biodistribution study and in vitro autoradiography were performed. The correlations between the uptake of 18F-NCRP and mean lung density (tested by CT), as well as histopathological characteristics were analyzed. In PET imaging study, 18F-NCRP exhibited promising efficacy in monitoring the progression of IPF, which was earlier than CT. The ratio of uptake in BLM-injured lung to control lung increased from 1.4-fold on D15 to 2.2-fold on D22. Biodistribution data showed a significant lung uptake of 18F-NCRP in BLM-injured mice. There was a strong positive correlation between the 18F-NCRP uptake in the BLM-injured lungs and the histopathological characteristics. Given that, 18F-NCRP PET imaging of NTR, a promising biomarker for investigating the underlying pathogenic mechanism of IPF, is attainable as well as desirable, which might lay the foundation for establishing an NTR-targeted imaging evaluation system of IPF.

[18F]AlF-CBP imaging of type I collagen for non-invasive monitoring of pulmonary fibrosis in preclinical models

PURPOSE

Pulmonary fibrosis is an irreversible scar-forming condition for which there is a lack of non-invasive and specific methods for monitoring its progression and therapy efficacy. However, the disease is known to be accompanied by collagen accumulation. Here, we developed a novel positron emission tomography (PET) probe targeting type I collagen to evaluate its utility for the non-invasive assessment of pulmonary fibrosis.

METHODS

We designed a 18F-labeled PET probe ([18F]AlF-CBP) to target type I collagen and evaluated its binding affinity, specificity and stability in vitro. PET with [18F]AlF-CBP, CT, histopathology, immunofluorescence, and biochemical indice were performed to assess and quantify type I collagen levels and pulmonary fibrosis progression and treatment in murine models. Dynamic PET/CT studies of [18F]AlF-CBP were conducted to assess lung fibrosis in non-human primate models.

RESULTS

[18F]AlF-CBP was successfully prepared, and in vitro and in vivo tests showed high stability (> 95%) and type I collagen specificity (IC50 = 0.36 µM). The lungs of the fibrotic murine model showed more elevated probe uptake and retention compared to the control group, and there was a positive correlation between the radioactivity uptake signals and the degree of fibrosis (CT: R2 = 0.89, P < 0.0001; hydroxyproline levels: R2 = 0.89, P < 0.0001). PET signals also correlated well with mean lung density in non-human primate models of pulmonary fibrosis (R2 = 0.84, P < 0.0001).

CONCLUSION

[18F]AlF-CBP PET imaging is a promising non-invasive method for specific monitoring of lung fibrosis progression and therapy efficacy.

Simultaneous Positron Emission Tomography and Molecular Magnetic Resonance Imaging of Cardiopulmonary Fibrosis in a Mouse Model of Left Ventricular Dysfunction

BACKGROUND

Aging-associated left ventricular dysfunction promotes cardiopulmonary fibrogenic remodeling, Group 2 pulmonary hypertension (PH), and right ventricular failure. At the time of diagnosis, cardiac function has declined, and cardiopulmonary fibrosis has often developed. Here, we sought to develop a molecular positron emission tomography (PET)-magnetic resonance imaging (MRI) protocol to detect both cardiopulmonary fibrosis and fibrotic disease activity in a left ventricular dysfunction model.

METHODS AND RESULTS

Left ventricular dysfunction was induced by transverse aortic constriction (TAC) in 6-month-old senescence-accelerated prone mice, a subset of mice that received sham surgery. Three weeks after surgery, mice underwent simultaneous PET-MRI at 4.7 T. Collagen-targeted PET and fibrogenesis magnetic resonance (MR) probes were intravenously administered. PET signal was computed as myocardium- or lung-to-muscle ratio. Percent signal intensity increase and Δ lung-to-muscle ratio were computed from the pre-/postinjection magnetic resonance images. Elevated allysine in the heart (P=0.02) and lungs (P=0.17) of TAC mice corresponded to an increase in myocardial magnetic resonance imaging percent signal intensity increase (P<0.0001) and Δlung-to-muscle ratio (P<0.0001). Hydroxyproline in the heart (P<0.0001) and lungs (P<0.01) were elevated in TAC mice, which corresponded to an increase in heart (myocardium-to-muscle ratio, P=0.02) and lung (lung-to-muscle ratio, P<0.001) PET measurements. Pressure-volume loop and echocardiography demonstrated adverse left ventricular remodeling, function, and increased right ventricular systolic pressure in TAC mice.

CONCLUSIONS

Administration of collagen-targeted PET and allysine-targeted MR probes led to elevated PET-magnetic resonance imaging signals in the myocardium and lungs of TAC mice. The study demonstrates the potential to detect fibrosis and fibrogenesis in cardiopulmonary disease through a dual molecular PET-magnetic resonance imaging protocol.

[68Ga]FAPI PET for Imaging and Treatment Monitoring in a Preclinical Model of Pulmonary Fibrosis: Comparison to [18F]FDG PET and CT

PURPOSE

This study aimed to evaluate the feasibility of using [68Ga]-fibroblast-activating protein inhibitor (FAPI) positron emission tomography (PET) imaging for diagnosing pulmonary fibrosis in a mouse model. We also examined its value in monitoring treatment response and compared it with traditional [18F]-fluorodeoxyglucose (FDG) PET and computed tomography (CT) imaging.

METHODS

A model of idiopathic pulmonary fibrosis was established using intratracheal injection of bleomycin (BLM, 2 mg/kg) into C57BL/6 male mice. For the treatment of IPF, a daily oral dose of 400 mg/kg/day of pirfenidone was administered from 9 to 28 days after the establishment of the model. Disease progression and treatment efficacy were assessed at different stages of the disease every week for four weeks using CT, [18F]FDG PET, and [68Ga]FAPI PET (baseline imaging performed at week 0). Mice were sacrificed and lung tissues were harvested for hematoxylin-eosin staining, picrosirius red staining, and immunohistochemical staining for glucose transporter 1 (GLUT1) and FAP. Expression levels of GLUT1 and FAP in pathological sections were quantified. Correlations between imaging parameters and pathological quantitative values were analyzed.

RESULTS

CT, [18F]FDG PET and [68Ga]FAPI PET revealed anatomical and functional changes in the lung that reflected progression of pulmonary fibrosis. In untreated mice with pulmonary fibrosis, lung uptake of [18F]FDG peaked on day 14, while [68Ga]FAPI uptake and mean lung density peaked on day 21. In mice treated with pirfenidone, mean lung density and lung uptake of both PET tracers decreased. Mean lung density, [18F]FDG uptake, and [68Ga]FAPI uptake correlated well with quantitative values of picrosirius red staining, GLUT1 expression, and FAP expression, respectively. Conclusions: Although traditional CT and [18F]FDG PET reflect anatomical and metabolic status in fibrotic lung, [68Ga]FAPI PET provides a means of evaluating fibrosis progression and monitoring treatment response.

Noninvasive Quantification of Radiation-Induced Lung Injury Using a Targeted Molecular Imaging Probe

PURPOSE

Radiation-induced lung injury (RILI) is a progressive inflammatory process seen after irradiation for lung cancer. The disease can be insidious, often characterized by acute pneumonitis followed by chronic fibrosis with significant associated morbidity. No therapies are approved for RILI, and accurate disease quantification is a major barrier to improved management. Here, we sought to noninvasively quantify RILI using a molecular imaging probe that specifically targets type 1 collagen in mouse models and patients with confirmed RILI.

METHODS AND MATERIALS

Using a murine model of lung radiation, mice were imaged with EP-3533, a type 1 collagen probe, to characterize the development of RILI and to assess disease mitigation after losartan treatment. The human analog probe 68Ga-CBP8, targeting type 1 collagen, was tested on excised human lung tissue containing RILI and was quantified via autoradiography. 68Ga-CBP8 positron emission tomography was used to assess RILI in vivo in 6 human subjects.

RESULTS

Murine models demonstrated that probe signal correlated with progressive RILI severity over 6 months. The probe was sensitive to mitigation of RILI by losartan. Excised human lung tissue with RILI had increased binding versus unirradiated control tissue, and 68Ga-CBP8 uptake correlated with collagen proportional area. Human imaging revealed significant 68Ga-CBP8 uptake in areas of RILI and minimal background uptake.

CONCLUSIONS

These findings support the ability of a molecular imaging probe targeted at type 1 collagen to detect RILI in preclinical models and human disease, suggesting a role for targeted molecular imaging of collagen in the assessment of RILI.

Early, non-invasive detection of radiation-induced lung injury using PET/CT by targeting CXCR4

PURPOSE

Radiation-induced lung injury (RILI) is a severe side effect of radiotherapy (RT) for thoracic malignancies and we currently lack established methods for the early detection of RILI. In this study, we synthesized a new tracer, [18F]AlF-NOTA-QHY-04, targeting C-X-C-chemokine-receptor-type-4 (CXCR4) and investigated its feasibility to detect RILI.

METHODS

An RILI rat model was constructed and scanned with [18F]AlF-NOTA-QHY-04 PET/CT and [18F]FDG PET/CT periodically after RT. Dynamic, blocking, autoradiography, and histopathological studies were performed on the day of peak uptake. Fourteen patients with radiation pneumonia, developed during or after thoracic RT, were subjected to PET scan using [18F]AlF-NOTA-QHY-04.

RESULTS

The yield of [18F]AlF-NOTA-QHY-04 was 28.5-43.2%, and the specific activity was 27-33 GBq/μmol. [18F]AlF-NOTA-QHY-04 was mainly excreted through the kidney. Significant increased [18F]AlF-NOTA-QHY-04 uptake in the irradiated lung compared with that in the normal lung in the RILI model was observed on day 6 post-RT and peaked on day 14 post-RT, whereas no apparent uptake of [18F]FDG was shown on days 7 and 15 post-RT. MicroCT imaging did not show pneumonia until 42 days post-RT. Significant intense [18F]AlF-NOTA-QHY-04 uptake was confirmed by autoradiography. Immunofluorescence staining demonstrated expression of CXCR4 was significantly increased in the irradiated lung tissue, which correlated with results obtained from hematoxylin-eosin and Masson's trichrome staining. In 14 patients with radiation pneumonia, maximum standardized uptake values (SUVmax) were significantly higher in the irradiated lung compared with those in the normal lung. SUVmax of patients with grade 2 RILI was significantly higher than that of patients with grade 1 RILI.

CONCLUSION

This study indicated that [18F]AlF-NOTA-QHY-04 PET/CT imaging can detect RILI non-invasively and earlier than [18F]FDG PET/CT in a rat model. Clinical studies verified its feasibility, suggesting the clinical potential of [18F]AlF-NOTA-QHY-04 as a PET/CT tracer for early monitoring of RILI.

Detection of Pulmonary Fibrosis with a Collagen-Mimetic Peptide

Idiopathic pulmonary fibrosis (IPF) is a disease of unknown etiology that is characterized by excessive deposition and abnormal remodeling of collagen. IPF has a mean survival time of only 2-5 years from diagnosis, creating a need to detect IPF at an earlier stage when treatments might be more effective. We sought to develop a minimally invasive probe that could detect molecular changes in IPF-associated collagen. Here, we describe the design, synthesis, and performance of [68Ga]Ga·DOTA-CMP, which comprises a positron-emitting radioisotope linked to a collagen-mimetic peptide (CMP). This peptide mimics the natural structure of collagen and detects irregular collagen matrices by annealing to damaged collagen triple helices. We assessed the ability of the peptide to detect aberrant lung collagen selectively in a bleomycin-induced mouse model of pulmonary fibrosis using positron emission tomography (PET). [68Ga]Ga·DOTA-CMP PET demonstrated higher and selective uptake in a fibrotic mouse lung compared to controls, minimal background signal in adjacent organs, and rapid clearance via the renal system. These studies suggest that [68Ga]Ga·DOTA-CMP identifies fibrotic lungs and could be useful in the early diagnosis of IPF.

Radiotracers for Imaging of Fibrosis: Advances during the Last Two Decades and Future Directions

Fibrosis accompanies various pathologies, and there is thus an unmet medical need for non-invasive, sensitive, and quantitative methods for the assessment of fibrotic processes. Currently, needle biopsy with subsequent histological analysis is routinely used for the diagnosis along with morphological imaging techniques, such as computed tomography (CT), magnetic resonance imaging (MRI), and ultrasound (US). However, none of these imaging techniques are sufficiently sensitive and accurate to detect minor changes in fibrosis. More importantly, they do not provide information on fibrotic activity on the molecular level, which is critical for fundamental understanding of the underlying biology and disease course. Molecular imaging technology using positron emission tomography (PET) offers the possibility of imaging not only physiological real-time activity, but also high-sensitivity and accurate quantification. This diagnostic tool is well established in oncology and has exhibited exponential development during the last two decades. However, PET diagnostics has only recently been widely applied in the area of fibrosis. This review presents the progress of development of radiopharmaceuticals for non-invasive detection of fibrotic processes, including the fibrotic scar itself, the deposition of new fibrotic components (fibrogenesis), or the degradation of existing fibrosis (fibrolysis).

Optimization of an Allysine-Targeted PET Probe for Quantifying Fibrogenesis in a Mouse Model of Pulmonary Fibrosis

PURPOSE

Idiopathic pulmonary fibrosis (IPF) is a destructive lung disease with a poor prognosis, an unpredictable clinical course, and inadequate therapies. There are currently no measures of disease activity to guide clinicians making treatment decisions. The aim of this study was to develop a PET probe to identify lung fibrogenesis using a pre-clinical model of pulmonary fibrosis, with potential for translation into clinical use to predict disease progression and inform treatment decisions.

METHODS

Eight novel allysine-targeting chelators, PIF-1, PIF-2, …, PIF-8, with different aldehyde-reactive moieties were designed, synthesized, and radiolabeled with gallium-68 or copper-64. PET probe performance was assessed in C57BL/6J male mice 2 weeks after intratracheal bleomycin challenge and in naïve mice by dynamic PET/MR imaging and with biodistribution at 90 min post injection. Lung hydroxyproline and allysine were quantified ex vivo and histological staining for fibrosis and aldehyde was performed.

RESULTS

In vivo screening of probes identified 68GaPIF-3 and 68GaPIF-7 as probes with high uptake in injured lung, high uptake in injured lung versus normal lung, and high uptake in injured lung versus adjacent liver and heart tissue. A crossover, intra-animal PET/MR imaging study of 68GaPIF-3 and 68GaPIF-7 confirmed 68GaPIF-7 as the superior probe. Specificity for fibrogenesis was confirmed in a crossover, intra-animal PET/MR imaging study with 68GaPIF-7 and a non-binding control compound, 68GaPIF-Ctrl. Substituting copper-64 for gallium-68 did not affect lung uptake or specificity indicating that either isotope could be used.

CONCLUSION

A series of allysine-reactive PET probes with variations in the aldehyde-reactive moiety were evaluated in a pre-clinical model of lung fibrosis. The hydrazine-bearing probe, 68GaPIF-7, exhibited the highest uptake in fibrogenic lung, low uptake in surrounding liver or heart tissue, and low lung uptake in healthy mice and should be considered for further clinical translation.

Tailored Chemical Reactivity Probes for Systemic Imaging of Aldehydes in Fibroproliferative Diseases

During fibroproliferation, protein-associated extracellular aldehydes are formed by the oxidation of lysine residues on extracellular matrix proteins to form the aldehyde allysine. Here we report three Mn(II)-based, small-molecule magnetic resonance probes that contain α-effect nucleophiles to target allysine in vivo and report on tissue fibrogenesis. We used a rational design approach to develop turn-on probes with a 4-fold increase in relaxivity upon targeting. The effects of aldehyde condensation rate and hydrolysis kinetics on the performance of the probes to detect tissue fibrogenesis non-invasively in mouse models were evaluated by a systemic aldehyde tracking approach. We showed that, for highly reversible ligations, off-rate was a stronger predictor of in vivo efficiency, enabling histologically validated, three-dimensional characterization of pulmonary fibrogenesis throughout the entire lung. The exclusive renal elimination of these probes allowed for rapid imaging of liver fibrosis. Reducing the hydrolysis rate by forming an oxime bond with allysine enabled delayed phase imaging of kidney fibrogenesis. The imaging efficacy of these probes, coupled with their rapid and complete elimination from the body, makes them strong candidates for clinical translation.

Noninvasive Quantification of Radiation-Induced Lung Injury using a Targeted Molecular Imaging Probe

Rationale

Radiation-induced lung injury (RILI) is a progressive inflammatory process commonly seen following irradiation for lung cancer. The disease can be insidious, often characterized by acute pneumonitis followed by chronic fibrosis with significant associated morbidity. No therapies are approved for RILI, and accurate disease quantification is a major barrier to improved management.

Objective

To noninvasively quantify RILI, utilizing a molecular imaging probe that specifically targets type 1 collagen in mouse models and patients with confirmed RILI.

Methods

Using a murine model of lung radiation, mice were imaged with EP-3533, a type 1 collagen probe to characterize the development of RILI and to assess disease mitigation following losartan treatment. The human analog probe targeted against type 1 collagen, 68Ga-CBP8, was tested on excised human lung tissue containing RILI and quantified via autoradiography. Finally, 68Ga-CBP8 PET was used to assess RILI in vivo in six human subjects.

Results

Murine models demonstrated that probe signal correlated with progressive RILI severity over six-months. The probe was sensitive to mitigation of RILI by losartan. Excised human lung tissue with RILI had increased binding vs unirradiated control tissue and 68Ga-CBP8 uptake correlated with collagen proportional area. Human imaging revealed significant 68Ga-CBP8 uptake in areas of RILI and minimal background uptake.

Conclusions

These findings support the ability of a molecular imaging probe targeted at type 1 collagen to detect RILI in preclinical models and human disease, suggesting a role for targeted molecular imaging of collagen in the assessment of RILI.Clinical trial registered with www.clinicaltrials.gov (NCT04485286, NCT03535545).

Targeted imaging of very late antigen-4 for noninvasive assessment of lung inflammation-fibrosis axis

BACKGROUND

The lack of noninvasive methods for assessment of dysregulated inflammation as a major driver of fibrosis (i.e., inflammation-fibrosis axis) has been a major challenge to precision management of fibrotic lung diseases. Here, we determined the potential of very late antigen-4 (VLA-4)-targeted positron emission tomography (PET) to detect inflammation in a mouse model of bleomycin-induced fibrotic lung injury.

METHOD

Single time-point and longitudinal VLA-4-targeted PET was performed using a high-affinity peptidomimetic radiotracer, 64Cu-LLP2A, at weeks 1, 2, and 4 after bleomycin-induced (2.5 units/kg) lung injury in C57BL/6J mice. The severity of fibrosis was determined by measuring the hydroxyproline content of the lungs and expression of markers of extracellular matrix remodeling. Flow cytometry and histology was performed to determine VLA-4 expression across different leukocyte subsets and their spatial distribution.

RESULTS

Lung uptake of 64Cu-LLP2A was significantly elevated throughout different stages of the progression of bleomycin-induced injury. High lung uptake of 64Cu-LLP2A at week-1 post-bleomycin was a predictor of poor survival over the 4-week follow up, supporting the prognostic potential of 64Cu-LLP2A PET during the early stage of the disease. Additionally, the progressive increase in 64Cu-LLP2A uptake from week-1 to week-4 post-bleomycin correlated with the ultimate extent of lung fibrosis and ECM remodeling. Flow cytometry revealed that LLP2A binding was restricted to leukocytes. A combination of increased expression of VLA-4 by alveolar macrophages and accumulation of VLA-4-expressing interstitial and monocyte-derived macrophages as well as dendritic cells was noted in bleomycin-injured, compared to control, lungs. Histology confirmed the increased expression of VLA-4 in bleomycin-injured lungs, particularly in inflamed and fibrotic regions.

CONCLUSIONS

VLA-4-targeted PET allows for assessment of the inflammation-fibrosis axis and prediction of disease progression in a murine model. The potential of 64Cu-LLP2A PET for assessment of the inflammation-fibrosis axis in human fibrotic lung diseases needs to be further investigated.

Biodistribution, Dosimetry, and Pharmacokinetics of 68Ga-CBP8: A Type I Collagen-Targeted PET Probe

The 68Ga-Collagen Binding Probe #8, 68Ga-CBP8, is a peptide-based, type I collagen-targeted probe developed for imaging of tissue fibrosis. The aim of this study was to determine the biodistribution, dosimetry, and pharmacokinetics of 68Ga-CBP8 in healthy human subjects. Methods: Nine healthy volunteers (5 male and 4 female) underwent whole-body 68Ga-CBP8 PET/MRI using a Biograph mMR scanner. The subjects were imaged continuously for up to 2 h after injection of 68Ga-CBP8. A subset of subjects underwent an additional imaging session 2-3 h after probe injection. OLINDA/EXM software was used to calculate absorbed organ and effective dose estimates based on up to 17 regions of interest (16 for men) defined on T2-weighted MR images and copied to the PET images, assuming a uniform distribution of probe concentration in each region. Serial blood sampling up to 90 min after probe injection was performed to assess blood clearance and metabolic stability. Results: The mean injected activity (±SD) of 68Ga-CBP8 was 220 ± 100 MBq (range, 113-434 MBq). No adverse effects related to probe administration were detected. 68Ga-CBP8 demonstrated an extracellular distribution with predominantly rapid renal clearance. Doses on the urinary bladder were 0.15 versus 0.19 mGy/MBq for men versus women. The highest absorbed doses for the rest of the organs were measured in the kidneys (0.078 vs. 0.088 mGy/MBq) and the liver (0.032 vs. 0.041 mGy/MBq). The mean effective dose was 0.018 ± 0.0026 mSv/MBq using a 1-h voiding model. The 68Ga-CBP8 signal in the blood demonstrated biexponential pharmacokinetics with an initial distribution half-life of 4.9 min (95% CI, 2.4-9.4 min) and a 72-min elimination half-life (95% CI, 47-130 min). The only metabolite observed had a long blood plasma half-life, suggesting protein-bound 68Ga. Conclusion: 68Ga-CBP8 displays favorable in-human characteristics and dosimetry similar to that of other gallium-based probes. 68Ga-CBP8 could therefore be used for noninvasive collagen imaging across a range of human fibrotic diseases.

Early Prediction of Radiation-Induced Pulmonary Fibrosis Using Gastrin-Releasing Peptide Receptor-Targeted PET Imaging

Early diagnosis of radiation-induced pulmonary fibrosis (RIPF) in lung cancer patients after radiation therapy is important. A gastrin-releasing peptide receptor (GRPR) mediates the inflammation and fibrosis after irradiation in mice lungs. Previously, our group synthesized a GRPR-targeted positron emission tomography (PET) imaging probe, [⁶⁴Cu]Cu-NODAGA-galacto-bombesin (BBN), an analogue peptide of GRP. In this study, we evaluated the usefulness of [⁶⁴Cu]Cu-NODAGA-galacto-BBN for the early prediction of RIPF. We prepared RIPF mice and acquired PET/CT images of [¹⁸F]F-FDG and [⁶⁴Cu]Cu-NODAGA-galacto-BBN at 0, 2, 5, and 11 weeks after irradiation (n = 3-10). We confirmed that [⁶⁴Cu]Cu-NODAGA-galacto-BBN targets GRPR in irradiated RAW 264.7 cells. In addition, we examined whether [⁶⁴Cu]Cu-NODAGA-galacto-BBN monitors the therapeutic efficacy in RIPF mice (n = 4). As a result, the lung uptake ratio (irradiated-to-normal) of [⁶⁴Cu]Cu-NODAGA-galacto-BBN was the highest at 2 weeks, followed by its decrease at 5 and 11 weeks after irradiation, which matched with the expression of GRPR and was more accurately predicted than [¹⁸F]F-FDG. These uptake results were also confirmed by the cell uptake assay. Furthermore, [⁶⁴Cu]Cu-NODAGA-galacto-BBN could monitor the therapeutic efficacy of pirfenidone in RIPF mice. We conclude that [⁶⁴Cu]Cu-NODAGA-galacto-BBN is a novel PET imaging probe for the early prediction of RIPF-targeting GRPR expressed during the inflammatory response.

Novel tracers for molecular imaging of interstitial lung disease: A state of the art review

Interstitial lung disease is an overarching term for a wide range of disorders characterized by inflammation and/or fibrosis in the lungs. Most prevalent forms, among others, include idiopathic pulmonary fibrosis (IPF) and connective tissue disease associated interstitial lung disease (CTD-ILD). Currently, only disease modifying treatment options are available for IPF and progressive fibrotic CTD-ILD, leading to reduction or stabilization in the rate of lung function decline at best. Management of these patients would greatly advance if we identify new strategies to improve (1) early detection of ILD, (2) predicting ILD progression, (3) predicting response to therapy and (4) understanding pathophysiology. Over the last years, positron emission tomography (PET) and single photon emission computed tomography (SPECT) have emerged as promising molecular imaging techniques to improve ILD management. Both are non-invasive diagnostic tools to assess molecular characteristics of an individual patient with the potential to apply personalized treatment. In this review, we encompass the currently available pre-clinical and clinical studies on molecular imaging with PET and SPECT in IPF and CTD-ILD. We provide recommendations for potential future clinical applications of these tracers and directions for future research.

Cancer-associated fibroblasts: Origin, function, imaging, and therapeutic targeting

The tumor microenvironment (TME) is emerging as one of the primary barriers in cancer therapy. Cancer-associated fibroblasts (CAF) are a common inhabitant of the TME in several tumor types and play a critical role in tumor progression and drug resistance via different mechanisms such as desmoplasia, angiogenesis, immune modulation, and cancer metabolism. Due to their abundance and significance in pro-tumorigenic mechanisms, CAF are gaining attention as a diagnostic target as well as to improve the efficacy of cancer therapy by their modulation. In this review, we highlight existing imaging techniques that are used for the visualization of CAF and CAF-induced fibrosis and provide an overview of compounds that are known to modulate CAF activity. Subsequently, we also discuss CAF-targeted and CAF-modulating nanocarriers. Finally, our review addresses ongoing challenges and provides a glimpse into the prospects that can spearhead the transition of CAF-targeted therapies from opportunity to reality.

[68 Ga]Ga-FAPI-46 PET for non-invasive detection of pulmonary fibrosis disease activity

PURPOSE

The lack of effective molecular biomarkers to monitor idiopathic pulmonary fibrosis (IPF) activity or treatment response remains an unmet clinical need. Herein, we determined the utility of fibroblast activation protein inhibitor for positron emission tomography (FAPI PET) imaging in a mouse model of pulmonary fibrosis.

METHODS

Pulmonary fibrosis was induced by intratracheal administration of bleomycin (1 U/kg) while intratracheal saline was administered to control mice. Subgroups from each cohort (n = 3-5) underwent dynamic 1 h PET/CT after intravenously injecting FAPI-46 radiolabeled with gallium-68 ([68 Ga]Ga-FAPI-46) at 7 days and 14 days following disease induction. Animals were sacrificed following imaging for ex vivo gamma counting and histologic correlation. [68 Ga]Ga-FAPI-46 uptake was quantified and reported as percent injected activity per cc (%IA/cc) or percent injected activity (%IA). Lung CT density in Hounsfield units (HU) was also correlated with histologic examinations of lung fibrosis.

RESULTS

CT only detected differences in the fibrotic response at 14 days post-bleomycin administration. [68 Ga]Ga-FAPI-46 lung uptake was significantly higher in the bleomycin group than in control subjects at 7 days and 14 days. Significantly (P = 0.0012) increased [68 Ga]Ga-FAPI-46 lung uptake in the bleomycin groups at 14 days (1.01 ± 0.12%IA/cc) vs. 7 days (0.33 ± 0.09%IA/cc) at 60 min post-injection of the tracer was observed. These findings were consistent with an increase in both fibrinogenesis and FAP expression as seen in histology.

CONCLUSION

CT was unable to assess disease activity in a murine model of IPF. Conversely, FAPI PET detected both the presence and activity of lung fibrogenesis, making it a promising tool for assessing early disease activity and evaluating the efficacy of therapeutic interventions in lung fibrosis patients.

Radiotracers to Address Unmet Clinical Needs in Cardiovascular Imaging, Part 2: Inflammation, Fibrosis, Thrombosis, Calcification, and Amyloidosis Imaging

Cardiovascular imaging is evolving in response to systemwide trends toward molecular characterization and personalized therapies. The development of new radiotracers for PET and SPECT imaging is central to addressing the numerous unmet diagnostic needs that relate to these changes. In this 2-part review, we discuss select radiotracers that may help address key unmet clinical diagnostic needs in cardiovascular medicine. Part 1 examined key technical considerations pertaining to cardiovascular radiotracer development and reviewed emerging radiotracers for perfusion and neuronal imaging. Part 2 covers radiotracers for imaging cardiovascular inflammation, thrombosis, fibrosis, calcification, and amyloidosis. These radiotracers have the potential to address several unmet needs related to the risk stratification of atheroma, detection of thrombi, and the diagnosis, characterization, and risk stratification of cardiomyopathies. In the first section, we discuss radiotracers targeting various aspects of inflammatory responses in pathologies such as myocardial infarction, myocarditis, sarcoidosis, atherosclerosis, and vasculitis. In a subsequent section, we discuss radiotracers for the detection of systemic and device-related thrombi, such as those targeting fibrin (e.g., ⁶⁴Cu-labeled fibrin-binding probe 8). We also cover emerging radiotracers for the imaging of cardiovascular fibrosis, such as those targeting fibroblast activation protein (e.g., ⁶⁸Ga-fibroblast activation protein inhibitor). Lastly, we briefly review radiotracers for imaging of cardiovascular calcification (¹⁸F-NaF) and amyloidosis (e.g., ^99mTc-pyrophosphate and ¹⁸F-florbetapir).

Reverse translation approach generates a signature of penetrating fibrosis in Crohn's disease that is associated with anti-TNF response

OBJECTIVE

Fibrosis is a common feature of Crohn's disease (CD) which can involve the mesenteric fat. However, the molecular signature of this process remains unclear. Our goal was to define the transcriptional signature of mesenteric fibrosis in CD subjects and to model mesenteric fibrosis in mice to improve our understanding of CD pathogenesis.

DESIGN

We performed histological and transcriptional analysis of fibrosis in CD samples. We modelled a CD-like fibrosis phenotype by performing repeated colonic biopsies in mice and analysed the model by histology, type I collagen-targeted positron emission tomography (PET) and global gene expression. We generated a gene set list of essential features of mesenteric fibrosis and compared it to mucosal biopsy datasets from inflammatory bowel disease patients to identify a refined gene set that correlated with clinical outcomes.

RESULTS

Mesenteric fibrosis in CD was interconnected to areas of fibrosis in all layers of the intestine, defined as penetrating fibrosis. We found a transcriptional signature of differentially expressed genes enriched in areas of the mesenteric fat of CD subjects with high levels of fibrosis. Mice subjected to repeated colonic biopsies showed penetrating fibrosis as shown by histology, PET imaging and transcriptional analysis. Finally, we composed a composite 24-gene set list that was linked to inflammatory fibroblasts and correlated with treatment response.

CONCLUSION

We linked histopathological and molecular features of CD penetrating fibrosis to a mouse model of repeated biopsy injuries. This experimental system provides an innovative approach for functional investigations of underlying profibrotic mechanisms and therapeutic concepts in CD.

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.

Direct Detection of Pulmonary Fibrosis by Near-Infrared-Responsive Biomimetic Platelets

Background

Purpose

Methods

Results

Conclusion

Interaction of macrocyclic gadolinium-based MR contrast agents with Type I collagen. Equilibrium and kinetic studies

The interactions of gadoterate meglumine, gadobutrol, gadoteridol and Gd(HB-DO3A) with bovine Type I collagen were investigated by ultrafiltration and dialysis. The affinity of the four agents to collagen is similar. However, the maximum adsorbed amount of GdIII-complexes decreases in the following order: gadoterate meglumine > gadobutrol > gadoteridol > Gd(HB-DO3A). Calculations with the open three-compartment model reveal that the structural homologs gadoteridol and Gd(HB-DO3A) have a lower adsorption onto collagen, which may explain the less prolonged in vivo retention of gadoteridol observed in soft tissues of rats.

Radiolabeling and PET-MRI microdosing of the experimental cancer therapeutic, MN-anti-miR10b, demonstrates delivery to metastatic lesions in a murine model of metastatic breast cancer

BACKGROUND

In our earlier work, we identified microRNA-10b (miR10b) as a master regulator of the viability of metastatic tumor cells. This knowledge allowed us to design a miR10b-targeted therapeutic consisting of anti-miR10b and ultrasmall iron oxide magnetic nanoparticles (MN), termed MN-anti-miR10b. In mouse models of breast cancer, we demonstrated that MN-anti-miR10b caused durable regressions of established metastases with no evidence of systemic toxicity. As a first step towards translating MN-anti-miR10b for the treatment of metastatic breast cancer, we needed to determine if MN-anti-miR10b, which is so effective in mice, will also accumulate in human metastases.

RESULTS

In this study, we devised a method to efficiently radiolabel MN-anti-miR10b with Cu-64 (64Cu) and evaluated the pharmacokinetics and biodistribution of the radiolabeled product at two different doses: a therapeutic dose, referred to as macrodose, corresponding to 64Cu-MN-anti-miR10b co-injected with non-labeled MN-anti-miR10b, and a tracer level dose of 64Cu-MN-anti-miR10b, referred to as microdose. In addition, we evaluated the uptake of 64Cu-MN-anti-miR10b by metastatic lesions using both in vivo and ex vivo positron emission tomography-magnetic resonance imaging (PET-MRI). A comparable distribution of the therapeutic was observed after administration of a microdose or macrodose. Uptake of the therapeutic by metastatic lymph nodes, lungs, and bone was also demonstrated by PET-MRI with a significantly higher PET signal than in the same organs devoid of metastatic lesions.

CONCLUSION

Our results demonstrate that PET-MRI following a microdose injection of the agent will accurately reflect the innate biodistribution of the therapeutic. The tools developed in the present study lay the groundwork for the clinical testing of MN-anti-miR10b and other similar therapeutics in patients with cancer.

Quantitative Functional Evaluation of Liver Fibrosis in Mice with Dynamic Contrast-enhanced Photoacoustic Imaging

Background Dynamic contrast-enhanced (DCE) photoacoustic (PA) imaging (PAI) is a novel noninvasive imaging modality that uses the differences in optical absorption of oxyhemoglobin and deoxyhemoglobin and may be performed in a dynamic fashion to image the indocyanine green (ICG) pharmacokinetics in the liver. Purpose To determine whether DCE PAI parameters (maximum peak time [T_max] and half-life [T_1/2]) derived from the PA liver function curve correlate with fibrosis determined using histopathologic analysis. Materials and Methods Between June 2020 and October 2020, 28 male mice aged 8 weeks were intraperitoneally injected with carbon tetrachloride solution every 2 days to establish a liver fibrosis model. At the 1st, 4th, and 8th week of modeling, the changes in liver structure were monitored dynamically by using a PA technique. The T_max and T_1/2 of ICG were calculated at different pathologic stages and within a control group. Corresponding liver histopathologic results and blood biochemical data were obtained. Spearman rank correlation was used to evaluate the relationship between the DCE PAI results and histologic scores. Results The PA liver function curve showed that the T_max and T_1/2 varied among groups (mean T_max: control group, 9 seconds ± 1.8 [standard deviation]; 1 week, 51 seconds ± 4.4; 4 weeks, 73 seconds ± 5.3; 8 weeks, 104 seconds ± 6.6; P < .001) (mean T_1/2: control group, 28 seconds ± 6.5; 1 week, 201 seconds ± 12.4; 4 weeks, 285 seconds ± 11; 8 weeks, 318 seconds ± 30.5; P < .001). There was a positive correlation between the dynamic parameters (T_max and T_1/2) and the histopathologic scores; the Spearman ρ ratios for the Sirius red and α-smooth muscle actin (αSMA)-positive areas versus the T_max were 0.93 and 0.94 (P < .001 for both), and the Spearman ρ ratios for the Sirius red and αSMA-positive areas versus T_1/2 were 0.87 and 0.89 (P < .001 for both). Conclusion Dynamic contrast-enhanced photoacoustic imaging demonstrated a higher maximum peak time and half-life in mice with induced fibrosis compared with control mice without fibrosis, and these values correlated with histologic measures of fibrosis. © RSNA, 2021 Online supplemental material is available for this article.

Immune Stroma in Lung Cancer and Idiopathic Pulmonary Fibrosis: A Common Biologic Landscape?

Idiopathic pulmonary fibrosis (IPF) identifies a specific entity characterized by chronic, progressive fibrosing interstitial pneumonia of unknown cause, still lacking effective therapies. Growing evidence suggests that the biologic processes occurring in IPF recall those which orchestrate cancer onset and progression and these findings have already been exploited for therapeutic purposes. Notably, the incidence of lung cancer in patients already affected by IPF is significantly higher than expected. Recent advances in the knowledge of the cancer immune microenvironment have allowed a paradigm shift in cancer therapy. From this perspective, recent experimental reports suggest a rationale for immune checkpoint inhibition in IPF. Here, we recapitulate the most recent knowledge on lung cancer immune stroma and how it can be translated into the IPF context, with both diagnostic and therapeutic implications.

Radiolabelling and positron emission tomography imaging of a high-affinity peptide binder to collagen type 1

INTRODUCTION

Pathological formation of fibrosis, is an important feature in many diseases. Fibrosis in liver and pancreas has been associated to metabolic disease including type 1 and 2 diabetes. The current methods for detecting and diagnosing fibrosis are either invasive, or their sensitivity to detect fibrosis in early stage is limited. Therefore, it is crucial to develop non-invasive methods to detect, stage and study the molecular processes that drive the pathology of liver fibrosis. The peptide LRELHLNNN was previously identified as a selective binder to collagen type I with an affinity of 170 nM. Radiolabelled LRELHLNNN thus constitute a potential PET tracer for fibrosis.

METHOD

LRELHLNNN was conjugated to a DOTA/NOTA moiety via a PEG2-linker. DOTA-PEG2-LRELHLNNN was labelled with Gallium-68 and NOTA- PEG2-LRELHLNNN with aluminium fluoride-18. Biodistribution of [68Ga]Ga-DOTA-PEG2-LRELHLNNN and [18F]AlF-NOTA-PEG2-LRELHLNNN was performed in healthy rats ex vivo and in vivo. The 68Ga-labelled analogue was evaluated in a mouse model of liver fibrosis by PET/MRI-imaging. The human predicted dosimetry of the tracers was extrapolated from rat ex vivo biodistribution studies at 10, 20, 40, 60, 120, 180 min (only fluoride-18) post-injection.

RESULTS

The peptides were successfully radiolabelled with gallium-68 and aluminium fluoride-18, respectively. The biodistribution of [68Ga]Ga-DOTA-PEG2-LRELHLNNN and [18F]AlF-NOTA-PEG2-LRELHLNNN was favorable showing rapid clearance and low background binding in organs where fibrosis may develop. Binding of [68Ga]Ga-DOTA-PEG2-LRELHLNNN to fibrotic liver was higher than surrounding tissues in mice with induced hepatic fibrosis. However, the binding was in the range of SUV 0.3, indicating limited targeting of the tracer to liver. The extrapolated human predicted dosimetric profiles of [68Ga]Ga-DOTA-PEG2-LRELHLNNN and [18F]AlF-NOTA-PEG2-LRELHLNNN were beneficial, potentially allowing at least three PET examinations annually.

CONCLUSIONS

We describe the modification, radiolabelling and evaluation of the collagen type I binding peptide LRELHLNNN. The resulting radiotracer analogues demonstrated suitable biodistribution and dosimetry. [68Ga]Ga-DOTA-PEG2-LRELHLNNN exhibited binding to hepatic fibrotic lesions and is a promising tool for PET imaging of fibrosis.

ADVANCES IN KNOWLEDGE

Validation of a new collagen targeting PET tracer.

IMPLICATIONS FOR PATIENT CARE

Early, non-invasive diagnosis and stratification of fibrosis in order to improve the diagnosis, staging and treatment of patients with diseases involving fibrosis.

Three-Dimensional Imaging of Pulmonary Fibrotic Foci at the Alveolar Scale Using Tissue-Clearing Treatment with Staining Techniques of Extracellular Matrix

Idiopathic pulmonary fibrosis is a progressive, chronic lung disease characterized by the accumulation of extracellular matrix proteins, including collagen and elastin. Imaging of extracellular matrix in fibrotic lungs is important for evaluating its pathological condition as well as the distribution of drugs to pulmonary focus sites and their therapeutic effects. In this study, we compared techniques of staining the extracellular matrix with optical tissue-clearing treatment for developing three-dimensional imaging methods for focus sites in pulmonary fibrosis. Mouse models of pulmonary fibrosis were prepared via the intrapulmonary administration of bleomycin. Fluorescent-labeled tomato lectin, collagen I antibody, and Col-F, which is a fluorescent probe for collagen and elastin, were used to compare the imaging of fibrotic foci in intact fibrotic lungs. These lung samples were cleared using the Clear^T2 tissue-clearing technique. The cleared lungs were two dimensionally observed using laser-scanning confocal microscopy, and the images were compared with those of the lung tissue sections. Moreover, three-dimensional images were reconstructed from serial two-dimensional images. Fluorescent-labeled tomato lectin did not enable the visualization of fibrotic foci in cleared fibrotic lungs. Although collagen I in fibrotic lungs could be visualized via immunofluorescence staining, collagen I was clearly visible only until 40 μm from the lung surface. Col-F staining facilitated the visualization of collagen and elastin to a depth of 120 μm in cleared lung tissues. Furthermore, we visualized the three-dimensional extracellular matrix in cleared fibrotic lungs using Col-F, and the images provided better visualization than immunofluorescence staining. These results suggest that Clear^T2 tissue-clearing treatment combined with Col-F staining represents a simple and rapid technique for imaging fibrotic foci in intact fibrotic lungs. This study provides important information for imaging various organs with extracellular matrix-related diseases.

Imaging Biomarkers in Animal Models of Drug-Induced Lung Injury: A Systematic Review

For drug-induced interstitial lung disease (DIILD) translational imaging biomarkers are needed to improve detection and management of lung injury and drug-toxicity. Literature was reviewed on animal models in which in vivo imaging was used to detect and assess lung lesions that resembled pathological changes found in DIILD, such as inflammation and fibrosis. A systematic search was carried out using three databases with key words “Animal models”, “Imaging”, “Lung disease”, and “Drugs”. A total of 5749 articles were found, and, based on inclusion criteria, 284 papers were selected for final data extraction, resulting in 182 out of the 284 papers, based on eligibility. Twelve different animal species occurred and nine various imaging modalities were used, with two-thirds of the studies being longitudinal. The inducing agents and exposure (dose and duration) differed from non-physiological to clinically relevant doses. The majority of studies reported other biomarkers and/or histological confirmation of the imaging results. Summary of radiotracers and examples of imaging biomarkers were summarized, and the types of animal models and the most used imaging modalities and applications are discussed in this review. Pathologies resembling DIILD, such as inflammation and fibrosis, were described in many papers, but only a few explicitly addressed drug-induced toxicity experiments.

Longitudinal Imaging Using PET/CT with Collagen-I PET-Tracer and MRI for Assessment of Fibrotic and Inflammatory Lesions in a Rat Lung Injury Model

Non-invasive imaging biomarkers (IBs) are warranted to enable improved diagnostics and follow-up monitoring of interstitial lung disease (ILD) including drug-induced ILD (DIILD). Of special interest are IB, which can characterize and differentiate acute inflammation from fibrosis. The aim of the present study was to evaluate a PET-tracer specific for Collagen-I, combined with multi-echo MRI, in a rat model of DIILD. Rats were challenged intratracheally with bleomycin, and subsequently followed by MRI and PET/CT for four weeks. PET imaging demonstrated a significantly increased uptake of the collagen tracer in the lungs of challenged rats compared to controls. This was confirmed by MRI characterization of the lesions as edema or fibrotic tissue. The uptake of tracer did not show complete spatial overlap with the lesions identified by MRI. Instead, the tracer signal appeared at the borderline between lesion and healthy tissue. Histological tissue staining, fibrosis scoring, lysyl oxidase activity measurements, and gene expression markers all confirmed establishing fibrosis over time. In conclusion, the novel PET tracer for Collagen-I combined with multi-echo MRI, were successfully able to monitor fibrotic changes in bleomycin-induced lung injury. The translational approach of using non-invasive imaging techniques show potential also from a clinical perspective.

Collagen-targeted molecular imaging in diffuse liver diseases

Liver fibrosis is a common pathway shared by all progressive chronic liver diseases (CLD) regardless of the underlying etiologies. With liver biopsy being the gold standard in assessing fibrosis degree, there is a large unmet clinical need to develop non-invasive imaging tools that can directly and repeatedly quantify fibrosis throughout the liver for a more accurate assessment of disease burden, progression, and treatment response. Type I collagen is a particularly attractive target for molecular imaging as its excessive deposition is specific to fibrosis, and it is present in concentrations suitable for many imaging modalities. Novel molecular MRI contrast agents designed to bind with collagen provide direct quantification of collagen deposition, which have been validated across animal species and liver injury models. Collagen-targeted molecular imaging probes hold great promise not only as a tool for initial staging and surveillance of fibrosis progression, but also as a marker of fibrosis regression in drug trials.

Imaging Biomarkers and Pathobiological Profiling in a Rat Model of Drug-Induced Interstitial Lung Disease Induced by Bleomycin

A large number of systemically administered drugs have the potential to cause drug-induced interstitial lung disease (DIILD). We aim to characterize a model of DIILD in the rat and develop imaging biomarkers (IBs) for detection and quantification of DIILD. In this study, Sprague-Dawley rats received one single dose of intratracheal (i.t.) bleomycin and were longitudinally imaged at day 0, 3, 7, 14, 21, and 28 post dosing, applying the imaging techniques magnetic resonance imaging (MRI) and positron emission tomography (PET)/computed tomography (CT). Bronchoalveolar lavage fluid (BALF) was analyzed for total protein and inflammatory cells. Lungs were saved for further evaluation by gene analysis using quantitative-PCR and by histology. Lung sections were stained with Masson's-Trichrome staining and evaluated by modified Ashcroft score. Gene expression profiling of inflammatory and fibrotic markers was performed on lung tissue homogenates. Bleomycin induced significant increase in total protein concentration and total cell count in bronchoalveolar lavage (BAL), peaking at day 3 (p > 0.001) and day 7 (p > 0.001) compared to control, respectively. Lesions measured by MRI and PET signal in the lungs of bleomycin challenged rats were significantly increased during days 3-14, peaking at day 7. Two subgroups of animals were identified as low- and high-responders by their different change in total lung volume. Both groups showed signs of inflammation initially, while at later time points, the low-responder group recovered toward control, and the high-responder group showed sustained lung volume increase, and significant increase of lesion volume (p < 0.001) compared to control. Lastly, important inflammatory and pro-fibrotic markers were assessed from lung tissue, linking observed imaging pathological changes to gene expression patterns. In conclusion, bleomycin-induced lung injury is an adequate animal model for DIILD studies and for translational lung injury assessment by MRI and PET imaging. The scenario comprised disease responses, with different fractions of inflammation and fibrosis. Thereby, this study improved the understanding of imaging and biological biomarkers in DIILD and lung injury.

Improving the reactivity of hydrazine-bearing MRI probes for in vivo imaging of lung fibrogenesis

Pulmonary fibrosis (PF) is the pathologic accumulation of extracellular matrix components in lung tissue that result in scarring following chronic lung injury. PF is typically diagnosed by high resolution computed tomography (HRCT) and/or invasive biopsy. However, HRCT cannot distinguish old injury from active fibrogenesis. We previously demonstrated that allysine residues on oxidized collagen represent an abundant target during lung fibrogenesis, and that magnetic resonance imaging (MRI) with a small-molecule, gadolinium-containing probe, Gd-Hyd, could specifically detect and stage fibrogenesis in a mouse model. In this work, we present an improved probe, Gd-CHyd, featuring an N,N-dialkyl hydrazine which has an order of magnitude both greater reactivity and affinity for aldehydes. In a paired study in mice with bleomycin induced lung injury we show that the improved reactivity and affinity of Gd-CHyd results in significantly higher lung-to-liver contrast, e.g. 77% higher at 45 min post injection, and slower lung clearance than Gd-Hyd. Gd-CHyd enhanced MRI is >60-fold higher in bleomycin injured mouse lungs compared to uninjured mice. Collectively, our data indicate that enhancing hydrazine reactivity and affinity towards allysine is an effective strategy to significantly improve molecular MRI probes for lung fibrogenesis.

Microenvironment-Responsive Small-Molecule Probe for Pulmonary Fibrosis Detection

Pulmonary fibrosis (PF) is a fatal disease with increasing prevalence. Nonradioactive and noninvasive diagnosis of PF at an early stage can improve the prognosis but represents a daunting challenge. Up-regulation of nitric oxide (NO) is a typical microenvironmental feature of PF. Here, we report a small-molecule probe, PNO1, that can fluorogenically sense this microenvironmental feature for PF diagnosis. We demonstrate that PNO1 fluorescence is 6-fold higher in PF-diseased mice lungs than in normal-control groups. In addition to this in vivo result, PNO1 can also be applied in vitro to detect PF-diseased cells and ex vivo to detect PF-diseased tissues from clinical patients. These results highlight PNO1 as a complement to the traditional immunostaining-based methods for PF detection to facilitate quick screening for anti-PF drug candidates.

68Ga-NODAGA-Indole: An Allysine-Reactive Positron Emission Tomography Probe for Molecular Imaging of Pulmonary Fibrogenesis

Oxidized collagen, wherein lysine residues are converted to the aldehyde allysine, is a universal feature of fibrogenesis, i.e. actively progressive fibrosis. Here we report the small molecule, allysine-binding positron emission tomography probe, ⁶⁸Ga-NODAGA-indole, that can noninvasively detect and quantify pulmonary fibrogenesis. We demonstrate that the uptake of ⁶⁸Ga-NODAGA-indole in actively fibrotic lungs is 7-fold higher than in control groups and that uptake is linearly correlated ( R² = 0.98) with the concentration of lung allysine.

Molecular Probes for Imaging Fibrosis and Fibrogenesis

Fibrosis, or the accumulation of extracellular matrix molecules that make up scar tissue, is a common result of chronic tissue injury. Advances in the clinical management of fibrotic diseases have been hampered by the low sensitivity and specificity of noninvasive early diagnostic options, lack of surrogate end points for use in clinical trials, and a paucity of noninvasive tools to assess fibrotic disease activity longitudinally. Hence, the development of new methods to image fibrosis and fibrogenesis is a large unmet clinical need. Herein, an overview of recent and selected molecular probes for imaging of fibrosis and fibrogenesis by magnetic resonance imaging, positron emission tomography, and single photon emission computed tomography is provided.

Molecular imaging of fibrosis: recent advances and future directions

Fibrosis, the progressive accumulation of connective tissue that occurs in response to injury, causes irreparable organ damage and may result in organ failure. The few available antifibrotic treatments modify the rate of fibrosis progression, but there are no available treatments to reverse established fibrosis. Thus, more effective therapies are urgently needed. Molecular imaging is a promising biomedical methodology that enables noninvasive visualization of cellular and subcellular processes. It provides a unique means to monitor and quantify dysregulated molecular fibrotic pathways in a noninvasive manner. Molecular imaging could be used for early detection, disease staging, and prognostication, as well as for assessing disease activity and treatment response. As fibrotic diseases are often molecularly heterogeneous, molecular imaging of a specific pathway could be used for patient stratification and cohort enrichment with the goal of improving clinical trial design and feasibility and increasing the ability to detect a definitive outcome for new therapies. Here we review currently available molecular imaging probes for detecting fibrosis and fibrogenesis, the active formation of new fibrous tissue, and their application to models of fibrosis across organ systems and fibrotic processes. We provide our opinion as to the potential roles of molecular imaging in human fibrotic diseases.

Gallium-68: methodology and novel radiotracers for positron emission tomography (2012–2017)

Commercial 68Ge/68Ga generators provide a means to produce positron emission tomography agents on site without use of a cyclotron. This development has led to a rapid growth of academic literature and patents ongallium-68 (68Ga). As 68Ga positron emission tomography agents usually involve a targeting moiety attached to a metal chelator, the development lends itself to the investigation of theragnostic applications; the 68Ga-based diagnostic is utilized to determine if the biological target is present and, if so, a therapeutic isotope (e.g., 177Lu, 225Ac) can be complexed with the same scaffold to generate a corresponding radiotherapeutic. This review considers patents issued between 2012 and 2017 that contain a 68Ga-labeled molecule indexed by Chemical Abstract Services (a division of the American Chemical Society).

Hydrazo-CEST: Hydrazone-Dependent Chemical Exchange Saturation Transfer Magnetic Resonance Imaging Contrast Agents

The rapid formation of hydrazones under physiological conditions was exploited for the detection of aldehydes through chemical exchange saturation transfer magnetic resonance imaging (CEST-MRI). A metal-free, diamagnetic contrast agent derived from N-amino anthranilic acid was introduced, which selectively "turned-on" upon hydrazone formation through an effect termed Hydrazo-CEST. While the hydrazine form of the probe produced no CEST-MRI signal enhancement, the formation of the aryl hydrazone resulted in >20 % intensity decrease in the bulk water signal through the CEST effect, as measured by 300 MHz ¹ H NMR, 3 T and 7 T MRI. Both the electronic contributions of the N-amino anthranilate and the aldehyde binding partner were shown to directly impact the exchange rate of the proton on the ring-proximal nitrogen, and thus the imaging signal. Additionally, the presence of the carboxylic acid moiety ortho to the hydrazine was necessary not only for contrast production, but also for rapid hydrazone formation and prolonged hydrazone product stability under physiological conditions. This work provided the first example of an MRI-based contrast agent capable of a "turn on" response upon reaction with bioactive aldehydes, and outlined both the structural and electronic requirements to expand on Hydrazo-CEST, a novel, hydrazone-dependent subtype of diamagnetic CEST-MRI.

Imaging the Cardiac Extracellular Matrix

Cardiovascular disease is the global leading cause of death. One route to address this problem is using biomedical imaging to measure the molecules and structures that surround cardiac cells. This cellular microenvironment, known as the cardiac extracellular matrix, changes in composition and organization during most cardiac diseases and in response to many cardiac treatments. Measuring these changes with biomedical imaging can aid in understanding, diagnosing, and treating heart disease. This chapter supports those efforts by reviewing representative methods for imaging the cardiac extracellular matrix. It first describes the major biological targets of ECM imaging, including the primary imaging target of fibrillar collagen. Then it discusses the imaging methods, describing their current capabilities and limitations. It categorizes the imaging methods into two main categories: organ-scale noninvasive methods and cellular-scale invasive methods. Noninvasive methods can be used on patients, but only a few are clinically available, and others require further development to be used in the clinic. Invasive methods are the most established and can measure a variety of properties, but they cannot be used on live patients. Finally, the chapter concludes with a perspective on future directions and applications of biomedical imaging technologies.

Fibrosis imaging: Current concepts and future directions

Fibrosis plays an important role in many different pathologies. It results from tissue injury, chronic inflammation, autoimmune reactions and genetic alterations, and it is characterized by the excessive deposition of extracellular matrix components. Biopsies are routinely employed for fibrosis diagnosis, but they suffer from several drawbacks, including their invasive nature, sampling variability and limited spatial information. To overcome these limitations, multiple different imaging tools and technologies have been evaluated over the years, including X-ray imaging, computed tomography (CT), ultrasound (US), magnetic resonance imaging (MRI), positron emission tomography (PET) and single-photon emission computed tomography (SPECT). These modalities can provide anatomical, functional and molecular imaging information which is useful for fibrosis diagnosis and staging, and they may also hold potential for the longitudinal assessment of therapy responses. Here, we summarize the use of non-invasive imaging techniques for monitoring fibrosis in systemic autoimmune diseases, in parenchymal organs (such as liver, kidney, lung and heart), and in desmoplastic cancers. We also discuss how imaging biomarkers can be integrated in (pre-) clinical research to individualize and improve anti-fibrotic therapies.