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.

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).

Fast detection of liver fibrosis with collagen-binding single-nanometer iron oxide nanoparticles via T1-weighted MRI

SNIO-CBP, a single-nanometer iron oxide (SNIO) nanoparticle functionalized with a type I collagen-binding peptide (CBP), was developed as a T1-weighted MRI contrast agent with only endogenous elements for fast and noninvasive detection of liver fibrosis. SNIO-CBP exhibits 6.7-fold higher relaxivity compared to a molecular gadolinium-based collagen-binding contrast agent CM-101 on a per CBP basis at 4.7 T. Unlike most iron oxide nanoparticles, SNIO-CBP exhibits fast elimination from the bloodstream with a 5.7 min half-life, high renal clearance, and low, transient liver enhancement in healthy mice. We show that a dose of SNIO-CBP that is 2.5-fold lower than that for CM-101 has comparable imaging efficacy in rapid (within 15 min following intravenous injection) detection of hepatotoxin-induced liver fibrosis using T1-weighted MRI in a carbon tetrachloride-induced mouse liver injury model. We further demonstrate the applicability of SNIO-CBP in detecting liver fibrosis in choline-deficient L-amino acid-defined high-fat diet mouse model of nonalcoholic steatohepatitis. These results provide a platform with potential for the development of high relaxivity, gadolinium-free molecular MRI probes for characterizing chronic liver disease.

Imaging High-Risk Atherothrombosis Using a Novel Fibrin-Binding Positron Emission Tomography Probe

BACKGROUND AND PURPOSE

High-risk atherosclerosis is an underlying cause of cardiovascular events, yet identifying the specific patient population at immediate risk is still challenging. Here, we used a rabbit model of atherosclerotic plaque rupture and human carotid endarterectomy specimens to describe the potential of molecular fibrin imaging as a tool to identify thrombotic plaques.

METHODS

Atherosclerotic plaques in rabbits were induced using a high-cholesterol diet and aortic balloon injury (N=13). Pharmacological triggering was used in a group of rabbits (n=9) to induce plaque disruption. Animals were grouped into thrombotic and nonthrombotic plaque groups based on gross pathology (gold standard). All animals were injected with a novel fibrin-specific probe ⁶⁸Ga-CM246 followed by positron emission tomography (PET)/magnetic resonance imaging 90 minutes later. ⁶⁸Ga-CM246 was quantified on the PET images using tissue-to-background (back muscle) ratios and standardized uptake value.

RESULTS

Both tissue-to-background (back muscle) ratios and standardized uptake value were significantly higher in the thrombotic versus nonthrombotic group (P<0.05). Ex vivo PET and autoradiography of the abdominal aorta correlated positively with in vivo PET measurements. Plaque disruption identified by ⁶⁸Ga-CM246 PET agreed with gross pathology assessment (85%). In ex vivo surgical specimens obtained from patients undergoing elective carotid endarterectomy (N=12), ⁶⁸Ga-CM246 showed significantly higher binding to carotid plaques compared to a D-cysteine nonbinding control probe.

CONCLUSIONS

We demonstrated that molecular fibrin PET imaging using ⁶⁸Ga-CM246 could be a useful tool to diagnose experimental and clinical atherothrombosis. Based on our initial results using human carotid plaque specimens, in vivo molecular imaging studies are warranted to test ⁶⁸Ga-CM246 PET as a tool to stratify risk in atherosclerotic patients.

Fibrotic Response to Neoadjuvant Therapy Predicts Survival in Pancreatic Cancer and Is Measurable with Collagen-Targeted Molecular MRI

PURPOSE

To evaluate the prognostic value of posttreatment fibrosis in human PDAC patients, and to compare a type I collagen targeted MRI probe, CM-101, to the standard contrast agent, Gd-DOTA, for their abilities to identify FOLFIRINOX-induced fibrosis in a murine model of PDAC.

EXPERIMENTAL DESIGN

Ninety-three chemoradiation-treated human PDAC samples were stained for fibrosis and outcomes evaluated. For imaging, C57BL/6 and FVB mice were orthotopically implanted with PDAC cells and FOLFIRINOX was administered. Mice were imaged with Gd-DOTA and CM-101.

RESULTS

In humans, post-chemoradiation PDAC tumor fibrosis was associated with longer overall survival (OS) and disease-free survival (DFS) on multivariable analysis (OS P = 0.028, DFS P = 0.047). CPA increased the prognostic accuracy of a multivariable logistic regression model comprised of previously established PDAC risk factors [AUC CPA (-) = 0.76, AUC CPA (+) = 0.82]. In multiple murine orthotopic PDAC models, FOLFIRINOX therapy reduced tumor weight (P < 0.05) and increased tumor fibrosis by collagen staining (P < 0.05). CM-101 MR signal was significantly increased in fibrotic tumor regions. CM-101 signal retention was also increased in the more fibrotic FOLFIRINOX-treated tumors compared with untreated controls (P = 0.027), consistent with selective probe binding to collagen. No treatment-related differences were observed with Gd-DOTA imaging.

CONCLUSIONS

In humans, post-chemoradiation tumor fibrosis is associated with OS and DFS. In mice, our MR findings indicate that translation of collagen molecular MRI with CM-101 to humans might provide a novel imaging technique to monitor fibrotic response to therapy to assist with prognostication and disease management.

Abstract WP526: Molecular Imaging of Carotid Plaques Using a Fibrin-Binding PET Probe

Objectives: We aim to develop molecular fibrin imaging as a tool to stratify high risk atherosclerotic carotid plaques. Here, we describe our results with a fibrin-specific probe [ 68 Ga]CM-246 in two different experimental settings: ex vivo imaging of carotid endarterectomy specimens and in vivo / ex vivo imaging of atherosclerotic plaque rupture in rabbits.

Methods: We incubated discarded endarterectomy specimens (n=11) from asymptomatic patients with [ 68 Ga] CM-246 or a scrambled peptide control probe and measured uptake by autoradiography and a tissue binding assay. In a rabbit plaque rupture model of high cholesterol diet followed by abdominal aorta balloon injury (n=13) and subsequent pharmacological plaque rupture, in vivo and ex vivo PET-MR with [ 68 Ga]CM-246 was performed. Tissues were processed for autoradiography and histology to verify presence of fibrin and plaque rupture.

Results: Autoradiography (A-B) and tissue binding assays (C) with [ 68 Ga]CM-246 with human carotid plaques show increased tissue binding compared with non-specific control probe. Carstair’s staining (D) verified the presence of fibrin in the specimens (red stained areas). Rabbits with plaque rupture had an increase in [ 68 Ga]CM-246 signal in the abdominal aorta compared with the control group. At 105-120 min post probe injection, there was a clear PET uptake on the aorta in plaque rupture rabbit (E) compared with control rabbit (F). MR images were used to differentiate abdominal aorta (green arrows) and the inferior vena cava (blue arrows). T2-dark blood images confirmed the plaques in rupture (G) and control rabbits (H). ToF images were used to visualize aortic lumen in plaque rupture (I) and control rabbits (J). Ex-vivo PET imaging, autoradiography, and histology confirmed in vivo results.

Conclusions: We demonstrated that [ 68 Ga]CM-246 detects high-risk plaque in human and rabbits and can be a useful tool for the stratification of atherosclerotic plaques in patients.

Abstract 324: Molecular Imaging of High Risk Atherosclerotic Plaque Using Fibrin-Binding PET Probe

Objectives: There is a need for better stratification of atherosclerotic patients to identify individuals at high risk for a cardiovascular event. Two key hallmarks of plaque at high risk for rupture are intraplaque hemorrhage and evidence of prior rupture. The protein fibrin is strongly associated with both hemorrhage and rupture but it is not present in earlier stages of plaque development, suggesting that fibrin imaging would be sensitive for high risk plaque versus stable/early disease. Here, we describe imaging properties of a fibrin-specific PET probe for atherosclerotic plaque rupture in rabbits.

Methods: Rabbits were fed with high cholesterol diet (HCD) for 8 weeks followed by 2 weeks of normal diet. Two weeks after the initiation of HCD, endothelial denudation of abdominal aorta was performed using balloon catheter. At 10 weeks, rabbits were randomly divided into plaque rupture (n=8; triggered by Russell’s viper venom and histamine) and control group (n=4), injected with fibrin-binding probe [ 68 Ga]CM-246, and scanned on a PET/MRI. Then, vessels of interest were removed for ex-vivo PET, autoradiography, and histology.

Results: Rabbits with plaque rupture had >2-fold increase in [ 68 Ga]CM-246 signal in the abdominal aorta compared with the control group (A). At 105-120 min post probe injection, there was a clear PET uptake on the aorta in plaque rupture rabbit (B) compared with control rabbit (C). MR images were used to differentiate abdominal aorta (green arrows) and the inferior vena cava (blue arrows). T2-dark blood images confirmed the atherosclerotic plaques in plaque rupture (D) and control rabbits (E). ToF images were used to visualize aortic lumen in plaque rupture (F) and control rabbits (G). Ex vivo PET imaging (H) and autoradiography confirmed in vivo results.

Conclusions: We demonstrated that [ 68 Ga]CM-246 detects atherosclerotic plaque rupture in a rabbit model and can be a useful tool for the diagnosis of high-risk atherosclerotic plaque in patients.

CM-101: Type I Collagen-targeted MR Imaging Probe for Detection of Liver Fibrosis

Purpose To evaluate the biodistribution, metabolism, and pharmacokinetics of a new type I collagen-targeted magnetic resonance (MR) probe, CM-101, and to assess its ability to help quantify liver fibrosis in animal models. Materials and Methods Biodistribution, pharmacokinetics, and stability of CM-101 in rats were measured with mass spectrometry. Bile duct-ligated (BDL) and sham-treated rats were imaged 19 days after the procedure by using a 1.5-T clinical MR imaging unit. Mice were treated with carbon tetrachloride (CCl₄) or with vehicle two times a week for 10 weeks and were imaged with a 7.0-T preclinical MR imaging unit at baseline and 1 week after the last CCl₄ treatment. Animals were imaged before and after injection of 10 µmol/kg CM-101. Change in contrast-to-noise ratio (ΔCNR) between liver and muscle tissue after CM-101 injection was used to quantify liver fibrosis. Liver tissue was analyzed for Sirius Red staining and hydroxyproline content. The institutional subcommittee for research animal care approved all in vivo procedures. Results CM-101 demonstrated rapid blood clearance (half-life = 6.8 minutes ± 2.4) and predominately renal elimination in rats. Biodistribution showed low tissue gadolinium levels at 24 hours (<3.9% injected dose [ID]/g ± 0.6) and 10-fold lower levels at 14 days (<0.33% ID/g ± 12) after CM-101 injection with negligible accumulation in bone (0.07% ID/g ± 0.02 and 0.010% ID/g ± 0.004 at 1 and 14 days, respectively). ΔCNR was significantly (P < .001) higher in BDL rats (13.6 ± 3.2) than in sham-treated rats (5.7 ± 4.2) and in the CCl₄-treated mice (18.3 ± 6.5) compared with baseline values (5.2 ± 1.0). Conclusion CM-101 demonstrated fast blood clearance and whole-body elimination, negligible accumulation of gadolinium in bone or tissue, and robust detection of fibrosis in rat BDL and mouse CCl₄ models of liver fibrosis. ^© RSNA, 2017 Online supplemental material is available for this article.

Multivalent scaffolds for affinity maturation of small molecule cell surface binders and their application to prostate tumor targeting

Adamantane scaffolds for affinity maturation of prostate cancer specific ligands of low molecular mass are described. These scaffolds are modular and can be used for conjugation of up to three ligands and an additional effector molecule by standard peptide coupling techniques. The potential of the scaffolds is demonstrated with the multimerization of GPI 1, a prostate cancer specific small molecule. A detailed study of multimerized GPI conjugates with near-infrared fluorophores and their binding properties to different prostate cancer cell lines shows the specific binding of these conjugates to cell types positive for prostate specific membrane antigen (PSMA). We demonstrate that these conjugates allow the sensitive imaging of prostate cancer cells with NIR methodology and suggest that our adamantane scaffolds might be generally useful for affinity maturation of small molecules targeting cell surface epitopes.

Real-time intraoperative assessment of the extrahepatic bile ducts in rats and pigs using invisible near-infrared fluorescent light

BACKGROUND

Currently, only x-ray fluoroscopy is available for visualization of the extrahepatic bile ducts intraoperatively. We hypothesized that with an appropriate fluorophore and imaging system, invisible near-infrared (NIR) light could be used for image-guided procedures on the extrahepatic bile ducts.

METHODS

We quantified the performance of three 800 nm NIR fluorophores, differing primarily in their degree of hydrophilicity, for real-time imaging of the extrahepatic bile ducts in rats and pigs: IR-786, indocyanine green (ICG), and the carboxylic form of IRDyetrade mark 800CW (CW800-CA). The signal-to-background ratio (SBR) of the common bile duct relative to liver and pancreas was measured as a function of the dose of contrast agent, injection site, and kinetics using an intraoperative NIR fluorescence imaging system described previously. Bile samples were examined by high performance liquid chromatography tandem mass spectrometry (HPLC/MS) to determine the chemical form of fluorophores in bile.

RESULTS

Non-sulfonated (IR-786) and di-sulfonated (ICG) NIR fluorophores had poor efficiency and kinetics of excretion into bile. Tetra-sulfonated CW800-CA, however, provided sensitive, specific, and real-time visualization of the extrahepatic bile ducts after a single low-dose given either intraportally or intravenously via systemic vein. A SBR >/=2 provided sensitive assessment of extrahepatic bile duct anatomy and function for over 30 min post-injection, including the detection of millimeter-sized, radiolucent inclusions in pigs. CW800-CA remained intact chemically after secretion into bile.

CONCLUSION

The combination of invisible NIR light and an IV injection of CW800-CA provides prolonged, real-time visualization of the extrahepatic bile duct, without ionizing radiation and without changing the look of the operative field.

Real-time intraoperative ureteral guidance using invisible near-infrared fluorescence

PURPOSE

Invisible near-infrared light is safe and it penetrates relatively deeply through tissue and blood without altering the surgical field. Our hypothesis was that near-infrared fluorescence imaging would enable visualization of the ureteral anatomy and flow intraoperatively and in real time.

MATERIALS AND METHODS

CW800-CA (LI-COR, Lincoln, Nebraska), the carboxylic acid form of near-infrared fluorophore IRDye 800CW, was injected intravenously, and its renal clearance kinetics and imaging performance were quantified in 350 gm rats and 35 kg pigs. High performance liquid chromatography and electrospray time-of-flight mass spectrometry were used to characterize CW800-CA metabolism in urine. The clinically available near-infrared fluorophore indocyanine green was also used via retrograde injection into the ureter. Using the 2 near-infrared fluorophores the ureters were imaged under the conditions of steady state, intraluminal foreign bodies and injury.

RESULTS

In rat models the highest signal-to-background ratio for visualization occurred after intravenous injection of 7.5 microg/kg CW800-CA with values of 4.0 or greater and 2.3 or greater at 10 and 30 minutes, respectively. In pig models 7.5 microg/kg CW800-CA clearly visualized the normal ureter and intraluminal foreign bodies as small as 2.5 mm in diameter. Retrograde injection of 10 microM indocyanine green also permitted the detection of normal ureter and pinpointed urine leakage caused by injury. Electrospray time-of-flight mass spectrometry, and absorbance and fluorescence spectral analysis confirmed that the fluorescent material in urine was chemically identical to CW800-CA.

CONCLUSIONS

A convenient intravenous injection of CW800-CA or direct injection of indocyanine green permits high sensitivity visualization of the ureters under steady state and abnormal conditions using invisible light.

Production of multimeric prostate-specific membrane antigen small-molecule radiotracers using a solid-phase 99mTc preloading strategy

Small-molecule ligands specific for prostate-specific membrane antigen (PSMA) have the potential to improve prostate cancer imaging. However, highly charged ligands are difficult to label with 99mTc and to purify. In this study, we present an adamantane-trimerized small molecule that has nanomolar binding to PSMA and also has 12 negative charges.

METHODS

To convert this molecule into a clinically viable SPECT diagnostic, we have developed a simple, cartridge-based, solid-phase prelabeling strategy that, within 25 min, converts readily available and inexpensive 99mTc-pertechnetate into a chemically pure complex, with a reactive N-hydroxysuccinimide (NHS) ester, in neat organic solvent. This stable intermediate can label any amine-containing small molecule or peptide with 99mTc in 1 step, with high specific activity and without the need for high-performance liquid chromatography (HPLC).

RESULTS

Solid-phase conversion of 99mTc-pertechnetate to 99mTc-MAS3-NHS (MAS3 is S-acetylmercaptoacetyltriserine) could be completed in 25 min, with >99% radiochemical purity and with no coligands present. This intermediate was then conjugated to adamantane-trimerized GPI (2[(3-amino-3-carboxypropyl)(hydroxy)(phosphinyl)-methyl]pentane-1,5-dioic acid) in 1 step with >95% yield and no need for HPLC purification. The final molecule bound specifically to living human tumor cells expressing PSMA on their surface. Quantitative comparison was made among GPI monomer, GPI trimer, and their 99mTc-derivatives.

CONCLUSION

Our study describes a simple cartridge-based conversion of 99mTc-pertechnetate to a useful, preloaded NHS ester intermediate that takes only 25 min to prepare and results in >99% radiochemical purity. Using this chemistry, we produced a high-specific-activity, 99mTc-labeled, PSMA-targeted small molecule and demonstrate gamma-ray radioscintigraphic imaging of living human prostate cancer cells.

An HPLC/mass spectrometry platform for the development of multimodality contrast agents and targeted therapeutics: prostate-specific membrane antigen small molecule derivatives

The production of disease-targeted agents requires the covalent conjugation of a targeting molecule with a contrast agent or therapeutic, followed by purification of the product to homogeneity. Typical targeting molecules, such as small molecules and peptides, often have high charge-to-mass ratios and/or hydrophobicity. Contrast agents and therapeutics themselves are also diverse, and include lanthanide chelates for MRI, (99m)Tc chelates for SPECT, (90)Y chelates for radiotherapy, (18)F derivatives for PET, and heptamethine indocyanines for near-infrared fluorescent optical imaging. We have constructed a general-purpose HPLC/mass spectrometry platform capable of purifying virtually any targeted agent for any modality. The analytical sub-system is composed of a single dual-head pump that directs mobile phase to either a hot cell for the purification of radioactive agents or to an ES-TOF MS for the purification of nonradioactive agents. Nonradioactive agents are also monitored during purification by ELSD, absorbance and fluorescence. The preparative sub-system is composed of columns and procedures that permit rapid scaling from the analytical system. To demonstrate the platform's utility, we describe the preparation of five small molecule derivatives specific for prostate-specific membrane antigen (PSMA): a gadolinium derivative for MRI, indium, rhenium and technetium derivatives for SPECT, and an yttrium derivative for radiotherapy. All five compounds are derived from a highly anionic targeting ligand engineered to have a single nucleophile for N-hydroxysuccinimide-based conjugation. We also describe optimized column/mobile phase combinations and mass spectrometry settings for each class of agent, and discuss strategies for purifying molecules with extreme charge and/or hydrophobicity. Taken together, our study should expedite the development of disease-targeted, multimodality diagnostic and therapeutic agents.

High-affinity Near-infrared Fluorescent Small-molecule Contrast Agents for In Vivo Imaging of Prostate-specific Membrane Antigen

Surgical resection remains a definitive treatment for prostate cancer. Yet, prostate cancer surgery is performed without image guidance for tumor margin, extension beyond the capsule and lymph node positivity, and without verification of other occult metastases in the surgical field. Recently, several imaging systems have been described that exploit near-infrared (NIR) fluorescent light for sensitive, real-time detection of disease pathology intraoperatively. In this study, we describe a high-affinity (9 nM), single nucleophile-containing, small molecule specific for the active site of the enzyme PSMA. We demonstrate production of a tetra-sulfonated heptamethine indocyanine NIR fluorescent derivative of this molecule using a high-yield LC/MS purification strategy. Interestingly, NIR fluorophore conjugation improves affinity over 20-fold, and we provide mechanistic insight into this observation. We describe the preparative production of enzymatically active PSMA using a baculovirus expression system and an adenovirus that co-expresses PSMA and GFP. We demonstrate sensitive and specific in vitro imaging of endogenous and ectopically expressed PSMA in human cells and in vivo imaging of xenograft tumors. We also discuss chemical strategies for improving performance even further. Taken together, this study describes nearly complete preclinical development of an optically based small-molecule contrast agent for image-guided surgery.