Molecular MR imaging of human thrombi in a swine model of pulmonary embolism using a fibrin-specific contrast agent

Molecular MR Contrast Agents

Molecular magnetic resonance (MR) imaging utilizes molecular probes to provide added biochemical or cellular information to what can already be achieved with anatomical and functional MR imaging. This review provides an overview of molecular MR and focuses specifically on molecular MR contrast agents that provide contrast by shortening the T1 time. We describe the requirements for a successful molecular MR contrast agent and the challenges for clinical translation. The review highlights work from the last 5 years and places an emphasis on new contrast agents that have been validated in multiple preclinical models. Applications of molecular MR include imaging of inflammation, fibrosis, fibrogenesis, thromboembolic disease, and cancers. Molecular MR is positioned to move beyond detection of disease to the quantitative staging of disease and measurement of treatment response.

An unmet clinical need: The history of thrombus imaging

Robust thrombus imaging is an unresolved clinical unmet need dating back to the mid 1970s. While early molecular imaging approaches began with nuclear SPECT imaging, contrast agents for virtually all biomedical imaging modalities have been demonstrated in vivo with unique strengths and common weaknesses. Two primary molecular imaging targets have been pursued for thrombus imaging: platelets and fibrin. Some common issues noted over 40 years ago persist today. Acute thrombus is readily imaged with all probes and modalities, but aged thrombus remains a challenge. Similarly, anti-coagulation continues to interfere with and often negate thrombus imaging efficacy, but heparin is clinically required in patients suspected of pulmonary embolism, deep venous thrombosis or coronary ruptured plaque prior to confirmatory diagnostic studies have been executed and interpreted. These fundamental issues can be overcome, but an innovative departure from the prior approaches will be needed.

Advocating the Development of Next-Generation High-Relaxivity Gadolinium Chelates for Clinical Magnetic Resonance

The question of improved relaxivity, and potential efficacy therein, for a next-generation of magnetic resonance gadolinium chelates with extracellular distribution and renal excretion, which could also be viewed from the perspective of dose, is addressed on the basis of historical development, animal experimentation, and human trials. There was no systematic evaluation that preceded the choice of 0.1 mmol/kg as the standard dose for human imaging with the gadolinium chelates. In part, this dose was chosen owing to bloodwork abnormalities seen in phase I and phase II studies. Animal investigations and early clinical trials demonstrated improved lesion detectability at higher doses in the brain, liver, and heart. By designing an agent with substantially improved relaxivity, higher enhancement equivalent to that provided with the conventional gadolinium agents at high dose could be achieved, translating to improved diagnosis and, thus, clinical care. Implicit in the development of such high-relaxivity agents would be stability equivalent to or exceeding that of the currently approved macrocyclic agents, given current concern regarding dechelation and gadolinium deposition in the brain, skin, and bone with the linear agents that were initially approved. Development of such next-generation agents with a substantial improvement in relaxivity, in comparison with the current group of approved agents, with a 2-fold increase likely achievable, could lead to improved lesion enhancement, characterization, diagnosis, and, thus, clinical efficacy.

Activatable magnetic resonance nanosensor as a potential imaging agent for detecting and discriminating thrombosis

The early detection and accurate characterization of life-threatening diseases such as cardiovascular disease and cancer are critical to the design of treatment. Knowing whether or not a thrombus in a blood vessel is new (fresh) or old (constituted) is very important for physicians to decide a treatment protocol. We have designed smart MRI nano-sensors that can detect, sense and report the stage or progression of cardiovascular diseases such as thrombosis. The nanosensors were functionalized with fibrin-binding peptide to specifically target thrombus and were also labelled with fluorescent dye to enable optical imaging. We have demonstrated that our nanosensors were able to switch between the T1 and T2 signal depending on thrombus age or the presence or absence of thrombin at the thrombus site. The developed nanosensors appeared to be non-toxic when tested with Chinese Hamster Ovarian cells within the tested concentrations. The working principle demonstrated in this study can be applied to many other diseases such as cancer.

In Vivo Molecular Characterization of Abdominal Aortic Aneurysms Using Fibrin-Specific Magnetic Resonance Imaging

BACKGROUND

The incidence of abdominal aortic aneurysms (AAAs) will significantly increase during the next decade. Novel biomarkers, besides diameter, are needed for a better characterization of aneurysms and the estimation of the risk of rupture. Fibrin is a key protein in the formation of focal hematoma associated with the dissection of the aortic wall and the development of larger thrombi during the progression of AAAs. This study evaluated the potential of a fibrin-specific magnetic resonance (MR) probe for the in vivo characterization of the different stages of AAAs.

METHODS AND RESULTS

AAAs spontaneously developed in ApoE-/- mice following the infusion of angiotensin-II (Ang-II, 1 μg/kg-1·per minute). An established fibrin-specific molecular MR probe (EP2104R, 10 μmol/kg-1) was administered after 1 to 4 weeks following Ang-II infusion (n=8 per group). All imaging experiments were performed on a clinical 3T Achieva MR system with a microscopy coil (Philips Healthcare, Netherlands). The development of AAA-associated fibrin-rich hematoma and thrombi was assessed. The high signal generated by the fibrin probe enabled high-resolution MR imaging for an accurate assessment and quantification of the relative fibrin composition of focal hematoma and thrombi. Contrast-to-noise-ratios (CNRs) and R1-relaxation rates following the administration of the fibrin probe were in good agreement with ex vivo immunohistomorphometry (R2=0.83 and 0.85) and gadolinium concentrations determined by inductively coupled plasma mass spectroscopy (R2=0.78 and 0.72).

CONCLUSIONS

The fibrin-specific molecular MR probe allowed the delineation and quantification of changes in fibrin content in early and advanced AAAs. Fibrin MRI could provide a novel in vivo biomarker to improve the risk stratification of patients with aortic aneurysms.

Study of Cardiac Arrest Caused by Acute Pulmonary Thromboembolism and Thrombolytic Resuscitation in a Porcine Model

Background:

The success rate of resuscitation in cardiac arrest (CA) caused by pulmonary thromboembolism (PTE) is low. Furthermore, there are no large animal models that simulate clinical CA. The aim of this study was to establish a porcine CA model caused by PTE and to investigate the pathophysiology of CA and postresuscitation.

Methods:

This model was induced in castrated male pigs (30 ± 2 kg; n = 21) by injecting thrombi (10–15 ml) via the left external jugular vein. Computed tomographic pulmonary angiography (CTPA) was performed at baseline, CA, and return of spontaneous circulation (ROSC). After CTPA during CA, cardiopulmonary resuscitation (CPR) with thrombolysis (recombinant tissue plasminogen activator 50 mg) was initiated. Hemodynamic, respiratory, and blood gas data were monitored. Cardiac troponins T, cardiac troponin I, creatine kinase-MB, myoglobin, and brain natriuretic peptide (BNP) were measured by enzyme-linked immunosorbent assay. Data were compared between baseline and CA with paired-sample t-test and compared among different time points for survival animals with repeated measures analysis of variance.

Results:

Seventeen animals achieved CA after emboli injection, while four achieved CA after 5–8 ml more thrombi. Nine animals survived 6 h after CPR. CTPA showed obstruction of the pulmonary arteries. Mean aortic pressure data showed occurrence of CA caused by PTE (Z = −2.803, P = 0.002). The maximal rate of mean increase of left ventricular pressure (dp/dt_max) was statistically decreased (t = 6.315, P = 0.000, variation coefficient = 0.25), and end-tidal carbon dioxide partial pressure (PetCO₂) decreased to the lowest value (t = 27.240, P = 0.000). After ROSC (n = 9), heart rate (HR) and mean right ventricular pressure (MRVP) remained different versus baseline until 2 h after ROSC (HR, P = 0.036; MRVP, P = 0.027). Myoglobin was statistically increased from CA to 1 h after ROSC (P = 0.036, 0.026, 0.009, respectively), and BNP was increased from 2 h to 6 h after ROSC (P = 0.012, 0.014, 0.039, respectively).

Conclusions:

We established a porcine model of CA caused by PTE. The dp/dt_max and PetCO₂ may be important for the occurrence of CA, while MRVP may be more important in postresuscitation.

Molecular Imaging of Activated Platelets Allows the Detection of Pulmonary Embolism with Magnetic Resonance Imaging

Early and reliable detection of pulmonary embolism (PE) is critical for improving patient morbidity and mortality. The desire for low-threshold screening for pulmonary embolism is contradicted by unfavorable radiation of currently used computed tomography or nuclear techniques, while standard magnetic resonance imaging still struggles to provide sufficient diagnostic sensitivity in the lung. In this study we evaluate a molecular-targeted contrast agent against activated platelets for non-invasive detection of murine pulmonary thromboembolism using magnetic resonance imaging. By intravenous injection of human thrombin, pulmonary thromboembolism were consistently induced as confirmed by immunohistochemistry of the lung. Magnetic resonance imaging after thrombin injection showed local tissue edema in weighted images which co-localized with the histological presence of pulmonary thromboembolism. Furthermore, injection of a functionalized contrast agent targeting activated platelets provided sensitive evidence of focal accumulation of activated platelets within the edematous area, which, ex vivo, correlated well with the size of the pulmonary embolism. In summary, we here show delivery and specific binding of a functionalized molecular contrast agent against activated platelets for targeting pulmonary thromboembolism. Going forward, molecular imaging may provide new opportunities to increase sensitivity of magnetic resonance imaging for detection of pulmonary embolism.

25 Years of Contrast-Enhanced MRI: Developments, Current Challenges and Future Perspectives

In 1988, the first contrast agent specifically designed for magnetic resonance imaging (MRI), gadopentetate dimeglumine (Magnevist(®)), became available for clinical use. Since then, a plethora of studies have investigated the potential of MRI contrast agents for diagnostic imaging across the body, including the central nervous system, heart and circulation, breast, lungs, the gastrointestinal, genitourinary, musculoskeletal and lymphatic systems, and even the skin. Today, after 25 years of contrast-enhanced (CE-) MRI in clinical practice, the utility of this diagnostic imaging modality has expanded beyond initial expectations to become an essential tool for disease diagnosis and management worldwide. CE-MRI continues to evolve, with new techniques, advanced technologies, and novel contrast agents bringing exciting opportunities for more sensitive, targeted imaging and improved patient management, along with associated clinical challenges. This review aims to provide an overview on the history of MRI and contrast media development, to highlight certain key advances in the clinical development of CE-MRI, to outline current technical trends and clinical challenges, and to suggest some important future perspectives.

FUNDING

Bayer HealthCare.

Noninvasive Imaging of Early Venous Thrombosis by 19 F Magnetic Resonance Imaging With Targeted Perfluorocarbon Nanoemulsions

Background—

Noninvasive detection of deep venous thrombi and subsequent pulmonary thromboembolism is a serious medical challenge, since both incidences are difficult to identify by conventional ultrasound techniques.

Methods and Results—

Here, we report a novel technique for the sensitive and specific identification of developing thrombi using background-free 19 F magnetic resonance imaging, together with α2-antiplasmin peptide (α2 AP )–targeted perfluorocarbon nanoemulsions (PFCs) as contrast agent, which is cross-linked to fibrin by active factor XIII. Ligand functionality was ensured by mild coupling conditions using the sterol-based postinsertion technique. Developing thrombi with a diameter <0.8 mm could be visualized unequivocally in the murine inferior vena cava as hot spots in vivo by simultaneous acquisition of anatomic matching 1 H and 19 F magnetic resonance images at 9.4 T with both excellent signal-to-noise and contrast-to-noise ratios (71±22 and 17±5, respectively). Furthermore, α2 AP -PFCs could be successfully applied for the diagnosis of experimentally induced pulmonary thromboembolism. In line with the reported half-life of factor XIIIa, application of α2 AP -PFCs >60 minutes after thrombus induction no longer resulted in detectable 19 F magnetic resonance imaging signals. Corresponding results were obtained in ex vivo generated human clots. Thus, α2 AP -PFCs can visualize freshly developed thrombi that might still be susceptible to pharmacological intervention.

Conclusions—

Our results demonstrate that 1 H/ 19 F magnetic resonance imaging, together with α2 AP -PFCs, is a sensitive, noninvasive technique for the diagnosis of acute deep venous thrombi and pulmonary thromboemboli. Furthermore, ligand coupling by the sterol-based postinsertion technique represents a unique platform for the specific targeting of PFCs for in vivo 19 F magnetic resonance imaging.

MR imaging probes: design and applications

This perspective outlines strategies towards the development of MR imaging probes that our lab has explored over the last 15 years. Namely, we discuss methods to enhance the signal generating capacity of MR probes and how to achieve tissue specificity through protein targeting or probe activation within the tissue microenvironment.

Molecular imaging to identify the vulnerable plaque--from basic research to clinical practice

Cardiovascular disease (CVD) is still the leading cause of death in the Western World. Adverse outcomes of CVD include stroke, myocardial infarction, and heart failure. Atherosclerosis is considered to be the major cause of CVD and is estimated to cause half of all deaths in developed countries. Atherosclerotic lesions of the vessel wall may obstruct blood flow mechanically through stenosis, but rupture of atherosclerotic plaques causing formation of occlusive thrombi is far more prevalent. Unfortunately, conventional diagnostic tools fail to assess whether a plaque is vulnerable to rupture. Research over the past decade identified the biological processes that are implicated in the course towards plaque rupture, like cell death and inflammation. Knowledge about plaque biology propelled the development of imaging techniques that target biologic processes in order to predict the vulnerable plaque. This paper discusses novel and existing molecular imaging targets and addresses advantages and disadvantages of these targets and respective imaging techniques in respect of clinical application and socio-economic impact.

Molecular imaging of fibrin in a breast cancer xenograft mouse model

RATIONALE AND OBJECTIVES

Fibrin deposition has been indicated within the stroma of a majority of solid tumors. Here we assess the feasibility of using the established fibrin-specific probe EP-2104R for noninvasive imaging of fibrin in the context of breast cancer.

METHODS

EP-2104R, untargeted gadopentetate dimeglumine (Gd-DTPA), and a newly synthesized nonfibrin binding control linear peptide (CLP) were compared using steady-state and dynamic contrast-enhanced magnetic resonance imaging in a breast cancer xenograft mouse model at 9.4 T.

RESULTS

EP-2104R transiently enhanced both tumor core and tumor periphery, but only the enhancement in the tumor periphery persisted even 90 minutes after EP-2104R administration. However, untargeted Gd-DTPA and CLP are not retained in the tumor periphery. The half-life of EP-2104R in the tumor periphery (103 ± 18 minutes) is significantly longer (P < 0.05) than that of either Gd-DTPA (29.6 ± 2.4 minutes) or CLP (42.4 ± 1.5 minutes), but the rate of clearance is similar for all the 3 probes from the tumor core. The presence of high concentrations of fibrin in the tumor periphery was corroborated using immunohistochemistry with a fibrin-specific antibody.

CONCLUSIONS

The persistent enhancement observed in the tumor periphery with EP-2104R is likely a result of its fibrin-specific binding rather than its size and demonstrates the feasibility of EP-2104R for molecular imaging of fibrin in tumor stroma.

Fibrin-targeted PET probes for the detection of thrombi

There is an ongoing effort to develop better methods for noninvasive detection and characterization of thrombi. Here we describe the synthesis and evaluation of three new fibrin-targeted positron emission tomography (PET) probes (FBP1, FBP2, FBP3). Three fibrin-specific peptides were conjugated as 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-monoamides at the C- and N-termini and chelated with (64)CuCl2. Probes were prepared with a specific activity ranging from 10 to 130 μCi/nmol. Both the peptides and the probes exhibited nanomolar dissociation constants (Kd) for the soluble fibrin fragment DD(E), although the Cu-DOTA derivatization resulted in a 2-3 fold loss in affinity relative to the parent peptide. Biodistribution and imaging studies were performed in a rat model of carotid artery thrombosis. For FBP1 and FBP2 at 120 min post injection, the vessel containing the thrombus showed the highest concentration of radioactivity after the excretory organs, that is, the liver and kidneys. This was confirmed ex vivo by autoradiography, which showed >4-fold activity in the thrombus-containing artery compared to the contralateral artery. FBP3 showed much lower thrombus uptake, and the difference was traced to greater metabolism of this probe. Hybrid MR-PET imaging with FBP1 or FBP2 confirmed that these probes were effective for the detection of an arterial thrombus in this rat model. A thrombus was visible on PET images as a region of high activity that corresponded to a region of arterial occlusion identified by simultaneous MR angiography. FBP1 and FBP2 represent promising new probes for the molecular imaging of thrombi.

Fibrin specific peptides derived by phage display: characterization of peptides and conjugates for imaging

Peptides that bind to fibrin but not to fibrinogen or serum albumin were selected from phage display libraries as targeting moieties for thrombus molecular imaging probes. Three classes of cyclic peptides (cyclized via disulfide bond between two Cys) were identified with consensus sequences XArXCPY(G/D)LCArIX (Ar = aromatic, Tn6), X(2)CXYYGTCLX (Tn7), and NHGCYNSYGVPYCDYS (Tn10). These peptides bound to fibrin at ∼2 sites with K(d) = 4.1 μM, 4.0 μM, and 8.7 μM, respectively, whereas binding to fibrinogen was at least 100-fold weaker. The peptides also bind to the fibrin degradation product DD(E) with similar affinity to that measured for fibrin. The Tn7 and Tn10 peptides bind to the same site on fibrin, while the Tn6 peptides bind to a unique site. Alanine scanning identified the N- and C-terminal ends of the Tn6 and Tn7 peptides as most tolerant to modification. Peptide conjugates with either fluorescein or diethylenetriaminepentaaceto gadolinium(III) (GdDTPA) at the N-terminus were prepared for potential imaging applications, and these retained fibrin binding affinity and specificity in plasma. Relaxivity and binding studies on the GdDTPA derivatives revealed that an N-terminal glycyl linker had a modest effect on fibrin affinity but resulted in lower fibrin-bound relaxivity.

In vivo assessment of intraplaque and endothelial fibrin in ApoE(-/-) mice by molecular MRI

OBJECTIVE

Molecular magnetic resonance imaging (MRI) has emerged as a promising non-invasive modality to characterize atherosclerotic vessel wall changes on a morphological and molecular level. Intraplaque and endothelial fibrin has recently been recognized to play an important role in the progression of atherosclerosis. This study aimed to investigate the feasibility of intraplaque and endothelial fibrin detection using a fibrin-targeted contrast-agent, FTCA (EPIX Pharmaceuticals, Lexington, MA), in a mouse model of atherosclerosis.

METHODS

Male apolipoproteinE-knockout mice (ApoE(-/-)) were fed a high fat diet (HFD) for one to three months. MRI of the brachiocephalic artery was performed prior to and 90 min after the administration of FTCA (n=8 per group). Contrast to noise ratios (CNR) and longitudinal relaxation rates (R1) of plaques were determined and compared to ex vivo fibrin density measurements on immunohistological sections stained with a fibrin-specific antibody and gadolinium concentrations measured by inductively coupled mass spectroscopy (ICP-MS).

RESULTS

Molecular MRI after FTCA administration demonstrated a significant increase (p<0.05) in contrast agent uptake in brachiocephalic artery plaques. In vivo CNR measurements were in good agreement with ex vivo fibrin density measurements on immunohistochemistry (y=2.4x+11.3, R(2)=0.82) and ICP-MS (y=0.95x+7.1, R(2)=0.70). Late stage atherosclerotic plaques displayed the strongest increase in CNR, R1, ex vivo fibrin staining and gadolinium concentration (p<0.05).

CONCLUSION

This study demonstrated the feasibility of intraplaque and endothelial fibrin imaging using FTCA. Direct in vivo fibrin detection and quantification could be useful for characterization and staging of coronary and carotid atherosclerotic lesions, which may aid diagnosis and intervention.

Targeted probes for cardiovascular MRI

Molecular MRI plays an important role in studying molecular and cellular processes associated with heart disease. Targeted probes that recognize important biomarkers of atherosclerosis, apoptosis, necrosis, angiogenesis, thrombosis and inflammation have been developed. This review discusses the properties of chemically different contrast agents including iron oxide nanoparticles, gadolinium-based nanoparticles or micelles, discrete peptide conjugates and activatable probes. Numerous examples of contrast agents based on these approaches have been used in preclinical MRI of cardiovascular diseases. Clinical applications are still under investigation for some selected agents with highly promising initial results. Molecular MRI shows great potential for the detection and characterization of a wide range of cardiovascular diseases, as well as for monitoring response to therapy.

High-relaxivity magnetic resonance imaging contrast agents. Part 2. Optimization of inner- and second-sphere relaxivity

RATIONALE AND OBJECTIVES

The observed relaxivity of gadolinium-based contrast agents has contributions from the water molecule(s) that bind directly to the gadolinium ion (inner-sphere water), long-lived water molecules and exchangeable protons that make up the second-sphere of coordination, and water molecules that diffuse near the contrast agent (outer-sphere). Inner- and second-sphere relaxivity can both be increased by optimization of the lifetimes of the water molecules and protons in these coordination spheres, the rotational motion of the complex, and the electronic relaxation of the gadolinium ion. We sought to identify new high-relaxivity contrast agents by systematically varying the donor atoms that bind directly to gadolinium to increase inner-sphere relaxivity and concurrently including substituents that influence the second-sphere relaxivity.

METHODS

Twenty gadolinium-1,4,7,10-tetraazacyclo-dodecane-N,N',N″,N'″-tetraacetato derivatives were prepared and their relaxivity determined in presence and absence of human serum albumin as a function of temperature and magnetic field. Data was analyzed to extract the underlying molecular parameters influencing relaxivity. Each compound had a common albumin-binding group and an inner-sphere donor set comprising the 4 tertiary amine N atoms from cyclen, an α-substituted acetate oxygen atom, 2 amide oxygen atoms, an inner-sphere water oxygen atom, and a variable donor group. Each amide nitrogen was substituted with different groups to promote hydrogen bonding with second-sphere water molecules.

RESULTS

Relativities at 0.47 and 1.4 T, 37°C, in serum albumin ranged from 16.0 to 58.1 mM(-1)s(-1) and from 12.3 to 34.8 mM(-1)s(-1), respectively. The reduction of inner-sphere water exchange typical of amide donor groups could be offset by incorporating a phosphonate or phenolate oxygen atom donor in the first coordination sphere, resulting in higher relaxivity. Amide nitrogen substitution with pendant phosphonate or carboxylate groups increased relaxivity by as much as 88% compared with the N-methyl amide analog. Second-sphere relaxivity contributed as much as 24 and 14 mM(-1)s(-1) at 0.47 and 1.4 T, respectively.

CONCLUSIONS

Water/proton exchange dynamics in the inner- and second-coordination sphere can be predictably tuned by choice of donor atoms and second-sphere substituents, resulting in high-relaxivity agents.

Evaluation of a fibrin-binding gadolinium chelate peptide tetramer in a brain glioma model

PURPOSE

To compare a fibrin-targeted, high relaxivity gadolinium tetramer, EP-2104R, in terms of magnitude of contrast enhancement (CE) and temporal time course, to a conventional extracellular gadolinium chelate, in a brain glioma model at 1.5-T magnetic resonance imaging.

METHODS

Six rats were evaluated, with each animal receiving (for separate studies) 0.05 mmol/kg gadopentetate dimeglumine (Gd DTPA or Magnevist) and 0.0125 mmol/kg of EP-2104R, with the 2 magnetic resonance examinations separated in each animal by 24 hours. The compound (EP-2104R) was synthesized using published methodology, being comprised of an 11 amino acid peptide derivatized at both the C- and N-termini with Gd-DOTA-like (Dotarem-like) moieties. T1-weighted scans were acquired precontrast and for 5 consecutive 2-minute intervals postcontrast, and subsequently at 15 and 20 minutes postcontrast.

RESULTS

Maximum tumor contrast-to-noise and CE both occurred at 1 minute versus at 5 minutes following administration of Gd DTPA versus EP-2104R, respectively. Utilizing an equivalent dose on a Gd ion per body weight basis, signal-to-noise, contrast-to-noise, and CE were greater for EP-2104R at all time points postcontrast, yielding overall statistically significantly greater levels of all 3 parameters with the latter. With EP-2104R, improvements in CE ranged between 87% and 391%, increasing at each measured time postcontrast with the exception of a slight decrease from 15 to 20 minutes postadministration. Histopathology confirmed, using immunofluorescence technique, abnormally increased fibrin within the tumor.

CONCLUSIONS

Statistically significantly greater brain tumor enhancement was noted with greater lesion enhancement at all observed time points postcontrast for EP-2104R utilizing an equivalent concentration to Gd DTPA on a per gadolinium ion basis. These findings together with the prolonged time course of enhancement suggest possible fibrin-binding and altered distribution kinetics.

Bimodal thrombus imaging: simultaneous PET/MR imaging with a fibrin-targeted dual PET/MR probe--feasibility study in rat model

PURPOSE

To image thrombus by using magnetic resonance (MR) imaging and positron emission tomography (PET) simultaneously in a rat arterial thrombus model with a dual PET/MR probe.

MATERIALS AND METHODS

Animal studies were approved by the institutional animal use committee. A dual PET/MR probe was synthesized by means of partial exchange of gadolinium for copper 64 ((64)Cu) in the fibrin-targeted MR probe EP-2104R. A preformed 25-mm thrombus was injected into the right internal carotid artery of a rat. Imaging was performed with a clinical 3.0-T MR imager with an MR-compatible human PET imager. Rats (n = 5) were imaged prior to and after systemic administration of the dual probe by using simultaneous PET/MR. The organ distribution of (64)Cu and gadolinium was determined ex vivo (n = 8), 2 hours after injection by using well counting and inductively coupled plasma mass spectrometry, respectively. Signal intensity ratios (SIRs) between the thrombus-containing and contralateral vessel were computed from PET images and MR data before and after probe administration.

RESULTS

The dual probe was synthesized with greater than 98% radiochemical purity. Thrombus enhancement was observed in all five animals at both MR (SIR([postprobe])/SIR([preprobe]) = 1.71 ± 0.35, P = .0053) and PET (SIR = 1.85 ± 0.48, P = .0087) after injection of the dual PET/MR probe. Ex vivo analysis at 2 hours after injection showed the highest (64)Cu and gadolinium concentrations, after the excretory organs (kidney and liver), to be in the thrombus.

CONCLUSION

A fibrin-targeted dual PET/MR probe enables simultaneous, direct MR and PET imaging of thrombus.

Quantitative cardiovascular magnetic resonance for molecular imaging

Cardiovascular magnetic resonance (CMR) molecular imaging aims to identify and map the expression of important biomarkers on a cellular scale utilizing contrast agents that are specifically targeted to the biochemical signatures of disease and are capable of generating sufficient image contrast. In some cases, the contrast agents may be designed to carry a drug payload or to be sensitive to important physiological factors, such as pH, temperature or oxygenation. In this review, examples will be presented that utilize a number of different molecular imaging quantification techniques, including measuring signal changes, calculating the area of contrast enhancement, mapping relaxation time changes or direct detection of contrast agents through multi-nuclear imaging or spectroscopy. The clinical application of CMR molecular imaging could offer far reaching benefits to patient populations, including early detection of therapeutic response, localizing ruptured atherosclerotic plaques, stratifying patients based on biochemical disease markers, tissue-specific drug delivery, confirmation and quantification of end-organ drug uptake, and noninvasive monitoring of disease recurrence. Eventually, such agents may play a leading role in reducing the human burden of cardiovascular disease, by providing early diagnosis, noninvasive monitoring and effective therapy with reduced side effects.

Molecular magnetic resonance imaging of deep vein thrombosis using a fibrin-targeted contrast agent: a feasibility study

PURPOSE

To evaluate the value of a fibrin-specific MR contrast agent (EP-2104R; EPIX Pharmaceuticals) for detection of deep vein thrombosis (DVT) and monitoring of percutaneous intervention for treatment.

MATERIALS AND METHODS

In 6 domestic swine, DVT was induced in an iliac/femoral vein using an occlusion-balloon catheter and subsequent injection of thrombin. The occluded vessels were recanalized by mechanical thrombectomy using a Fogarty catheter and an Arrow rotating thrombectomy device. Magnetic resonance imaging of the pelvis and lung was repeated 4 times (before and after DVT induction, after contrast agent administration, and after intervention) using a 1.5-T whole-body XMR system (ACS-NT, Philips Medical Systems, Best, NL). The visualization of the thrombi and contrast-to-noise ratio (CNR) was assessed.

RESULTS

EP-2104R allowed selective visualization of thrombi with accurate determination of the extent of DVT with high contrast (CNR: 65.3 +/- 17.2). After intervention, dislodged thrombus fragments were selectively visualized in the lung (CNR: 27.9 +/- 9.3).

CONCLUSIONS

Molecular magnetic resonance imaging using fibrin-specific MR contrast agent EP-2104R allowed for selective visualization of DVT and monitoring of percutaneous intervention.

Molecular MRI of intracranial thrombus in a rat ischemic stroke model

BACKGROUND AND PURPOSE

Intracranial thrombus is a principal feature in most ischemic stroke, and thrombus location and size may correlate with outcome and response to thrombolytic therapy. EP-2104R is a fibrin-specific molecular MR agent that has previously been shown to enhance extracranial and venous sinus thrombi in animal models and, recently, in clinical trials. In this study, we examined whether this fibrin-specific MR probe could noninvasively characterize intracranial arterial thrombi.

METHODS

Embolic stroke was induced in adult rats by occlusion of the right internal carotid artery with an aged thrombus. We used diffusion-weighted imaging, time of flight angiography, and high-resolution 3-dimensional T1-weighted MRI at 4.7 T before and after use of contrast agents EP-2104R (n=6) and gadopentetate dimeglumine (n=5).

RESULTS

In all animals, MR angiography revealed a flow deficit and diffusion-weighted imaging showed hyperintensity consistent with ischemia. Using EP-2104R-enhanced MRI, we saw occlusive thrombi and vessel wall enhancement in all 6 animals with high contrast to noise relative to blood, whereas gadopentetate dimeglumine-injected animals showed no occlusive thrombus or vessel wall enhancement. The concentration of gadolinium in the thrombus after EP-2104R was 18 times that in the blood pool.

CONCLUSIONS

EP-2104R-enhanced MRI successfully identifies intracranial thrombus in a rat embolic stroke model.

Magnetic resonance molecular imaging of thrombosis in an arachidonic acid mouse model using an activated platelet targeted probe

OBJECTIVE

Atherosclerotic plaque rupture leads to acute thrombus formation and may trigger serious clinical events such as myocardial infarction or stroke. Therefore, it would be valuable to identify atherothrombosis and vulnerable plaques before the onset of such clinical events. We sought to determine whether the noninvasive in vivo visualization of activated platelets was effective when using a target-specific MRI contrast agent to identify thrombi, hallmarks of vulnerable or high-risk atherosclerotic plaques.

METHODS AND RESULTS

Inflammatory thrombi were induced in mice via topical application of arachidonic acid on the carotid. Thrombus formation was imaged with intravital fluorescence microscopy and molecular MRI. To accomplish the latter, a paramagnetic contrast agent (P975) that targets the glycoprotein alpha(IIb)beta(3), expressed on activated platelets, was investigated. The specificity of P975 for activated platelets was studied in vitro. In vivo, high spatial-resolution MRI was performed at baseline and longitudinally over 2 hours after injecting P975 or a nonspecific agent. The contralateral carotid, a sham surgery group, and a competitive inhibition experiment served as controls. P975 showed a good affinity for activated platelets, with an IC(50) (concentration of dose that produces 50% inhibition) value of 2.6 micromol/L. In thrombosed animals, P975 produced an immediate and sustained increase in MRI signal, whereas none of the control groups revealed any significant increase in MRI signal 2 hours after injection. More important, the competitive inhibition experiment with an alpha(IIb)beta(3) antagonist suppressed the MRI signal enhancement, which is indicative for the specificity of P975 for the activated platelets.

CONCLUSIONS

P975 allowed in vivo target-specific noninvasive MRI of activated platelets.

Molecular MRI of Thrombosis

This review focuses on recent approaches in using targeted MRI probes for noninvasive molecular imaging of thrombosis. Probe design strategies are discussed: choice of molecular target; nanoparticle versus small-molecule probe; and gadolinium versus iron oxide imaging reporter. Examples of these different design strategies are chosen from the recent literature. Novel contrast agents used to image direct and indirect binding to fibrin have been described as well as direct binding to activated platelets. Emphasis is placed on probes where utility has been demonstrated in animal models or in human clinical trials.

Chapter 4 - Applications of nanotechnology in molecular imaging of the brain

Rapid advances in the field of nanotechnology promise revolutionary improvements in the diagnosis and therapy of neuroinflammatory disorders. An array of iron oxide nano- and microparticle agents have been developed for in vivo molecular magnetic resonance imaging (mMRI) of cerebrovascular endothelial targets, such as vascular cell adhesion molecule-1 (VCAM-1), E-selectin, and the glycoprotein receptor GP IIb/IIIa expressed on activated platelets. Molecular markers of glioma cells, such as matrix metalloproteinase-2 (MMP-2), and markers for brain tumor angiogenesis, such as alpha (v) beta (3) integrin (alpha(v)beta(3)), have also been successfully targeted using nanoparticle imaging probes. This chapter provides an overview of targeted, iron oxide nano- and microparticles that have been applied for in vivo mMRI of the brain in experimental models of multiple sclerosis (MS), brain ischemia, cerebral malaria (CM), brain cancer, and Alzheimer's disease. The potential of targeted nanoparticle agents for application in clinical imaging is also discussed, including multimodal and therapeutic approaches.

Molecular MRI of early thrombus formation using a bimodal alpha2-antiplasmin-based contrast agent

OBJECTIVES

We aimed to investigate whether early thrombus formation can be visualized with in vivo magnetic resonance imaging (MRI) by the use of a novel bimodal alpha(2)-antiplasmin-based contrast agent (CA).

BACKGROUND

Thrombus formation plays a central role in several vascular diseases. During the early phases of thrombus formation, activated factor XIII (FXIIIa) covalently cross-links alpha(2)-antiplasmin to fibrin, indicating the potential of alpha(2)-antiplasmin-based CAs in the detection of early thrombus formation.

METHODS

A bimodal CA was synthesized by coupling gadolinium-diethylene triamine pentaacetic acid and rhodamine to an alpha(2)-antiplasmin-based peptide. For the control CA, a glutamine residue essential for cross-linking was replaced by alanine. In vitro-generated thrombi were exposed to both CAs and imaged by MRI and 2-photon laser-scanning microscopy. Immunohistochemistry was performed on human pulmonary thromboemboli sections to determine the presence of alpha(2)-antiplasmin and FXIII in different thrombus remodeling phases. In vivo feasibility of the CA in detecting early thrombus formation specifically was investigated with MRI.

RESULTS

In vitro-generated thrombi exposed to the alpha(2)-antiplasmin-based CA showed hyperintense magnetic resonance signal intensities at the thrombus edge. No hyperintense signal was observed when we used the alpha(2)-antiplasmin-based CA in the presence of FXIII inhibitor dansylcadaverine nor when we used the control CA. Two-photon laser-scanning microscopy demonstrated that the alpha(2)-antiplasmin-based CA bound to fibrin. Immunohistochemistry demonstrated substantial alpha(2)-antiplasmin staining in fresh compared with lytic and organized thrombi. The administration of CA in vivo within seconds after inducing thrombus formation increased contrast-to-noise ratios (CNRs 2.28 +/- 0.39, n=6) at the site of thrombus formation compared with the control CA (CNRs -0.14 +/- 0.55, p = 0.003, n = 6) and alpha(2)-antiplasmin-based CA administration 24 to 48 h after thrombus formation (CNRs 0.11 +/- 0.23, p = 0.006, n = 6).

CONCLUSIONS

A bimodal CA was developed, characterized, and validated. Our results showed that this bimodal CA enabled noninvasive in vivo magnetic resonance visualization of early thrombus formation.

Pulmonary MR angiography techniques and applications

This article discusses the role of magnetic resonance angiography (MRA) in evaluating the pulmonary arterial system. For depiction of pulmonary arterial anatomy and morphology, MRA techniques are compared with CT angiography and digital subtraction x-ray angiography. Perfusion, flow, and function are emphasized, as the integrated MR examination offers a comprehensive assessment of vascular morphology and function. Advances in MR technology that improve spatial and temporal resolution and compensate for potential artifacts are reviewed as they pertain to pulmonary MRA. Current and emerging gadolinium contrast-enhanced and non-contrast-enhanced MRA techniques are discussed. The role of pulmonary MRA, clinical protocols, imaging findings, and interpretation pitfalls are reviewed for clinical indications.

Molecular MRI of Atherosclerotic Plaque With Targeted Contrast Agents

Molecular MRI of atherosclerosis involves the use of novel contrast agents to image cellular and molecular processes within atherosclerotic plaque. Agents to image plaque lipid content, inflammation, angiogenesis, and thrombosis have been developed and studied extensively in animal models of atherosclerosis and vascular injury. Selected agents have also been studied in humans, with highly promising initial results. In this brief review, recent advances as well as opportunities and challenges in the field are discussed.

Nanoparticles for optical molecular imaging of atherosclerosis

Molecular imaging contributes to future personalized medicine dedicated to the treatment of cardiovascular disease, the leading cause of mortality in industrialized countries. Endoscope-compatible optical imaging techniques would offer a stand-alone alternative and high spatial resolution validation technique to clinically accepted imaging techniques in the (intravascular) assessment of vulnerable atherosclerotic lesions, which are predisposed to initiate acute clinical events. Efficient optical visualization of molecular epitopes specific for vulnerable atherosclerotic lesions requires targeting of high-quality optical-contrast-enhancing particles. In this review, we provide an overview of both current optical nanoparticles and targeting ligands for optical molecular imaging of atherosclerotic lesions and speculate on their applicability in the clinical setting.

Advances in magnetic resonance (2008)

Current advances in magnetic resonance, as a diagnostic modality, are discussed in the context of publications from Investigative Radiology during 2007 and 2008. The articles relating to this topic, published during the past 2 years, are reviewed by anatomic region. The discussion concludes with a consideration of magnetic resonance contrast media, focusing on studies published in the journal, and examining in particular the potential impact of nephrogenic systemic fibrosis.

Nanotechnology in the diagnosis of atherosclerotic disease

BACKGROUND

Atherosclerosis is a chronic, inflammatory disease in which ruptured plaques can lead to serious thrombotic events, including myocardial infarction or stroke. Often these cardiovascular events occur with no previous recognition of symptoms and only moderate stenosis. New diagnostic techniques are needed for earlier diagnosis and staging of atherosclerotic disease, so appropriate treatments, interventional procedures, or lifestyle changes can begin. Recent developments in nanotechnology could advance clinical imaging of molecular biomarkers, particularly for cardiovascular diagnosis.

OBJECTIVE

In this review, selected nanotechnologies under development for early detection of atherosclerotic disease and identification of vulnerable plaques are presented.

METHOD

The scope of this review encompasses molecular imaging of atherosclerosis using nanoparticle contrast agents. Nanoparticle approaches are grouped by their corresponding diagnostic imaging modality.

RESULTS/CONCLUSION

Diagnostic imaging techniques employing nanoparticle contrast agents targeted to molecular signatures of atherosclerotic disease offer hope for improved non-invasive detection.

MR imaging of thrombi using EP-2104R, a fibrin-specific contrast agent: initial results in patients

This study was an initial phase II trial in humans of molecular magnetic resonance (MR) imaging for improved visualization of thrombi in vessel territories potentially responsible for stroke using a new fibrin-specific contrast agent (EP-2104R). Eleven patients with thrombus in the left ventricle (n = 2), left or right atrium (n = 4), thoracic aorta (n = 4) or carotid artery (n = 1) as verified by an index examination (ultrasound, computed tomograpy, or conventional MR) were enrolled. All MR imaging was performed on 1.5 T whole-body MR-system using an inversion-recovery black-blood gradient-echo sequence. The same sequence was performed before and 2-6 h after low-dose intravenous administration of 4 mumol/kg EP-2104R. Two investigators assessed image quality and signal amplification. Furthermore, contrast-to-noise ratios (CNR) between the clot and the blood pool/surrounding soft tissue before and after administration of the contrast agent were compared using Student's t-test. MR imaging and data analysis were successfully completed in 10 patients. No major adverse effects occurred. On enhanced images, thrombi demonstrated high signal amplification, typically at the clot surface, with a significantly increased contrast in comparison to the surrounding blood pool and soft tissue (CNR for clot vs. blood pool, unenhanced and enhanced: 6 +/- 8 and 29 +/- 14; CNR for clot vs. soft tissue, unenhanced and enhanced: 0 +/- 4 and 21 +/- 13; P < 0.01 for both comparisons). EP-2104R allows for molecular MR imaging of thrombi potentially responsible for stroke. High contrast between thrombus and surrounding blood and soft tissues can be achieved with enhanced imaging.

Advances in magnetic resonance (2007)

Advances in clinical magnetic resonance (MR) are discussed in this review in the context of publications from Investigative Radiology during 2006 and 2007. The articles relevant to this topic, published during this 2 year time period, are considered as organized by anatomic region. An additional final focus of discussion is in regards to those studies involving MR contrast media.