Synthesis and preliminary biological evaluation of new radioiodinated MMP inhibitors for imaging MMP activity in vivo

Towards Optimized Bioavailability of 99mTc-Labeled Barbiturates for Non-invasive Imaging of Matrix Metalloproteinase Activity

Introduction

Dysregulated activity of matrix metalloproteinases (MMPs) drives a variety of pathophysiological conditions. Non-invasive imaging of MMP activity in vivo promises diagnostic and prognostic value. However, current targeting strategies by small molecules are typically limited with respect to the bioavailability of the labeled MMP binders in vivo. To this end, we here introduce and compare three chemical modifications of a recently developed barbiturate-based radiotracer with respect to bioavailability and potential to image MMP activity in vivo.

Methods

Barbiturate-based MMP inhibitors with an identical targeting unit but varying hydrophilicity were synthesized, labeled with technetium-99m, and evaluated in vitro and in vivo. Biodistribution and radiotracer elimination were determined in C57/BL6 mice by serial SPECT imaging. MMP activity was imaged in a MMP-positive subcutaneous xenograft model of human K1 papillary thyroid tumors. In vivo data were validated by scintillation counting, autoradiography, and MMP immunohistochemistry.

Results

We prepared three new ^99mTc‐labeled MMP inhibitors, bearing either a glycine ([^99mTc]MEA39), lysine ([^99mTc]MEA61), or the ligand HYNIC with the ionic co-ligand TPPTS ([^99mTc]MEA223) yielding gradually increasing hydrophilicity. [^99mTc]MEA39 and [^99mTc]MEA61 were rapidly eliminated via hepatobiliary pathways. In contrast, [^99mTc]MEA223 showed delayed in vivo clearance and primary renal elimination. In a thyroid tumor xenograft model, only [^99mTc]MEA223 exhibited a high tumor-to-blood ratio that could easily be delineated in SPECT images.

Conclusion

Introduction of HYNIC/TPPTS into the barbiturate lead structure ([^99mTc]MEA223) results in delayed renal elimination and allows non-invasive MMP imaging with high signal-to-noise ratios in a papillary thyroid tumor xenograft model.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11307-021-01668-z.

Molecular Imaging Probes Based on Matrix Metalloproteinase Inhibitors (MMPIs)

Matrix metalloproteinases (MMPs) are a family of zinc- and calcium-dependent endopeptidases which are secreted or anchored in the cell membrane and are capable of degrading the multiple components of the extracellular matrix (ECM). MMPs are frequently overexpressed or highly activated in numerous human diseases. Owing to the important role of MMPs in human diseases, many MMP inhibitors (MMPIs) have been developed as novel therapeutics, and some of them have entered clinical trials. However, so far, only one MMPI (doxycycline) has been approved by the FDA. Therefore, the evaluation of the activity of a specific subset of MMPs in human diseases using clinically relevant imaging techniques would be a powerful tool for the early diagnosis and assessment of the efficacy of therapy. In recent years, numerous MMPIs labeled imaging agents have emerged. This article begins by providing an overview of the MMP subfamily and its structure and function. The latest advances in the design of subtype selective MMPIs and their biological evaluation are then summarized. Subsequently, the potential use of MMPI-labeled diagnostic agents in clinical imaging techniques are discussed, including positron emission tomography (PET), single-photon emission computed tomography (SPECT) and optical imaging (OI). Finally, this article concludes with future perspectives and clinical utility.

Strategies to Target Matrix Metalloproteinases as Therapeutic Approach in Cancer

Matrix metalloproteinases (MMPs) are a family of zinc-dependent endopeptidases that are capable of degrading numerous extracellular matrix (ECM) components thus participating in physiological and pathological processes. Apart from the remodeling of ECM, they affect cell-cell and cell-matrix interactions and are implicated in the development and progression of various diseases such as cancer. Numerous studies have demonstrated that MMPs evoke epithelial to mesenchymal transition (EMT) of cancer cells and affect their signaling, adhesion, migration and invasion to promote cancer cell aggressiveness. Various studies have suggested MMPs as suitable targets for treatment of malignancies, and several MMP inhibitors (MMPIs) have been developed. Although initial trials have failed to establish MMPIs as anticancer agents due to lack of specificity and side effects, new MMPIs have been developed with improved action that are currently being investigated. Furthermore, novel strategies that target MMPs for improving drug delivery and regulating their activity in tumors are presented. This review summarizes the implication of MMPs in cancer progression and discusses the advancements in their targeting.

Radiolabeled hydroxamate-based matrix metalloproteinase inhibitors: How chemical modifications affect pharmacokinetics and metabolic stability

INTRODUCTION

Dysregulated MMP expression or activation is associated with several diseases. To study MMP activity in vivo by means of PET a radiolabeled MMP inhibitor (MMPI) functioning as radiotracer has been developed by our group based on the lead structure CGS 25966.

MATERIALS AND METHODS

Aiming at the modification of the pharmacokinetics of this lipophilic model tracer a new class of MMPIs has been discovered, consisting of additional fluorinated hydrophilic substructures, such as mini-PEG and/or 1,2,3-triazole units. To identify the best candidate for further clinical applications, radiofluorinated compounds of each subgroup have been (radio) synthesized and evaluated regarding their biodistribution behavior and their metabolic stability.

RESULTS

Radiosyntheses of different triazole based MMPIs could be realized using two step "click chemistry" procedures. Compared to lead structure [(18)F]FEtO-CGS 25966 ([(18)F]1e, log D(exp) =2.02, IC50=2-50nM) all selected candidates showed increased hydrophilicities and inhibition potencies (log D(exp) =0.23-1.25, IC50=0.006-6nM). Interestingly, despite different hydrophilicities most triazole based MMPIs showed no significant differences in their in vivo biodistribution behavior and were cleared predominantly via the hepatobiliary excretion route. Biostability and metabolism studies in vitro and in vivo revealed significant higher metabolic stability for the triazole moiety compared to the benzyl ring in the lead structure. Cleavage of ethylene glycol subunits of the mini-PEG chain led to a faster metabolism of mini-PEG containing MMPIs.

CONCLUSION

The introduction of hydrophilic groups such as mini-PEG and 1,2,3-triazole units did not lead to a significant shift of the hepatobiliary elimination towards renal clearance. Particularly the introduction of mini-PEG chains led to an intense metabolic decomposition. Substitution of the benzyl moiety in lead structure 1e by a 1,2,3-trizole ring resulted in an increased metabolic stability. Therefore, the 1,2,3-triazole-1-yl-methyl substituted MMPI [(18)F]3a was found to be the most stable candidate in this series and should be chosen for further preclinical evaluation.

A comprehensive review on controls in molecular imaging: lessons from MMP-2 imaging

Metalloproteinases (MMPs), including MMP-2, play critical roles in tissue remodeling and are involved in a large array of pathologies, including cancer, arthritis and atherosclerosis. Their prognostic value warranted a large investment or resources in the development of noninvasive detection methods, based on probes for many current clinical and pre-clinical imaging modalities. However, the potential of imaging techniques is only matched by the complexity of the data they generate. This complexity must be properly assessed and accounted for in the early steps of probe design and testing in order to accurately determine the efficacy and efficiency of an imaging strategy. This review proposes basic rules for the evaluation of novel probes by addressing the specific case of MMP targeted probes.

Vasa vasorum in atherosclerosis and clinical significance

Atherosclerosis is a chronic inflammatory disease that leads to several acute cardiovascular complications with poor prognosis. For decades, the role of the adventitial vasa vasorum (VV) in the initiation and progression of atherosclerosis has received broad attention. The presence of VV neovascularization precedes the apparent symptoms of clinical atherosclerosis. VV also mediates inflammatory cell infiltration, intimal thickening, intraplaque hemorrhage, and subsequent atherothrombosis that results in stroke or myocardial infarction. Intraplaque neovessels originating from VV can be immature and hence susceptible to leakage, and are thus regarded as the leading cause of intraplaque hemorrhage. Evidence supports VV as a new surrogate target of atherosclerosis evaluation and treatment. This review provides an overview into the relationship between VV and atherosclerosis, including the anatomy and function of VV, the stimuli of VV neovascularization, and the available underlying mechanisms that lead to poor prognosis. We also summarize translational researches on VV imaging modalities and potential therapies that target VV neovascularization or its stimuli.

Molecular imaging of the cardiac extracellular matrix

In almost all cardiac diseases, an increase in extracellular matrix (ECM) deposition or fibrosis occurs, mostly consisting of collagen I. Whereas replacement fibrosis follows cardiomyocyte loss in myocardial infarction, reactive fibrosis is triggered by myocardial stress or inflammatory mediators and often results in ventricular stiffening, functional deterioration, and development of heart failure. Given the importance of ECM deposition in cardiac disease, ECM imaging could be a valuable clinical tool. Molecular imaging of ECM may help understand pathology, evaluate impact of novel therapy, and may eventually find a role in predicting the extent of ECM expansion and development of personalized treatment. In the current review, we provide an overview of ECM imaging including the assessment of ECM volume and molecular targeting of key players involved in ECM deposition and degradation. The targets comprise myofibroblasts, intracardiac renin-angiotensin axis, matrix metalloproteinases, and matricellular proteins.

Characterization of human osteoarthritic cartilage using optical and magnetic resonance imaging

Purpose

Osteoarthritis (OA) is a degenerative disease starting with key molecular events that ultimately lead to the breakdown of the cartilage. The purpose of this study is to use two imaging methods that are sensitive to molecular and macromolecular changes in OA to better characterize the disease process in human osteoarthritic cartilage.

Procedures

Human femoral condyles were collected from patients diagnosed with severe OA during total knee replacement surgeries. T_1ρ and T₂ magnetic resonance measurements were obtained using a 3-Tesla whole body scanner to assess macromolecular changes in the damaged cartilage matrix. Optical imaging was performed on specimens treated with MMPSense 680 to assess the matrix metalloproteinase (MMP) activity. A linear regression model was used to assess the correlation of MMP optical data with T_1ρ magnetic resonance (MR) measurements. Slices from a representative specimen were removed from regions with high and low optical signals for subsequent histological analysis.

Results

All specimens exhibit high T_1ρ and T₂ measurements in the range of 48–75 ms and 36–69 ms, respectively. They also show intense photon signals (0.376 to 7.89 × 10⁻⁴ cm²) from the activated MMPSense 680 probe, indicative of high MMP activity. The analysis of variance test of the regression model indicates a positive correlation between the MMP optical signal and T_1ρ measurements (R² = 0.8936, P = 0.0044). Histological data also confirmed that regions with high MMP optical signal and intense T_1ρ relaxation exhibit severe clefting, abnormal tidemarks, and irregular cellularity.

Conclusions

The high T_1ρ and T₂ measurements suggest that there is a severe loss of proteoglycans with high water mobility in the damaged cartilage. The intense optical signals found in these specimens indicate the presence of active MMPs, and the positive correlation with T_1ρ measurements implicates MMP’s involvement in OA progression, characterized by a severe loss of proteoglycans in the cartilage matrix. The bimodal approach using optical and MR imaging may provide key molecular and macromolecular information of the disease pathway, offering insights toward the development of new tools for the early detection, treatment, and/or prevention of OA.

Stroma targeting nuclear imaging and radiopharmaceuticals

Malignant transformation of tumor accompanies profound changes in the normal neighboring tissue, called tumor stroma. The tumor stroma provides an environment favoring local tumor growth, invasion, and metastatic spreading. Nuclear imaging (PET/SPECT) measures biochemical and physiologic functions in the human body. In oncology, PET/SPECT is particularly useful for differentiating tumors from postsurgical changes or radiation necrosis, distinguishing benign from malignant lesions, identifying the optimal site for biopsy, staging cancers, and monitoring the response to therapy. Indeed, PET/SPECT is a powerful, proven diagnostic imaging modality that displays information unobtainable through other anatomical imaging, such as CT or MRI. When combined with coregistered CT data, [(18)F]fluorodeoxyglucose ([(18)F]FDG)-PET is particularly useful. However, [(18)F]FDG is not a target-specific PET tracer. This paper will review the tumor microenvironment targeting oncologic imaging such as angiogenesis, invasion, hypoxia, growth, and homing, and also therapeutic radiopharmaceuticals to provide a roadmap for additional applications of tumor imaging and therapy.

A new class of highly potent matrix metalloproteinase inhibitors based on triazole-substituted hydroxamates: (radio)synthesis and in vitro and first in vivo evaluation

In vivo imaging of MMPs is of great (pre)clinical interest and can potentially be realized with modern three-dimensional and noninvasive in vivo molecular imaging techniques such as positron emission tomography (PET). Consequently, MMP inhibitors (MMPIs) radiolabeled with positron emitting nuclides (e.g., (18)F) represent a suitable tool for the visualization of activated MMPs with PET. On the basis of our previous work and results regarding radiolabeled and unlabeled derivatives of the nonselective MMPIs, we discovered a new class of fluorinated MMPIs with a triazole-substituted hydroxamate substructure. These novel MMPIs are characterized by an increased hydrophilicity compared with the lead structures and excellent MMP inhibition potencies for MMP-2, MMP-8, MMP-9, and MMP-13 (IC(50) = 0.006-107 nM). Therefore, one promising fluorinated triazole-substituted hydroxamate (30b) was selected and resynthesised as its (18)F-labeled version to yield the potential PET radioligand [(18)F]30b. The biodistribution behavior of this novel compound was investigated with small animal PET.

Radiosynthesis of a ⁶⁸Ga labeled matrix metalloproteinase inhibitor as a potential probe for PET imaging

A matrix metalloproteinase inhibitor based on a barbiturate scaffold was conjugated with a cyclooctyne derivative of the (radio)metal chelator DOTA via strain induced azide alkyne cycloaddition. Subsequent radiolabeling with (68)Ga yielded the corresponding radiometal labeled target compound (68)Ga-4 with a yield of 87% (decay corrected). The target molecule was also synthesized by a second synthesis route, the reaction of a pre-labeled (68)Ga-cyclooctyne-DOTA derivative (68)Ga-1 with an azide bearing barbiturate 3. This approach offers a valuable alternative for providing the desired (68)Ga-radiolabeled target compound. But, in this case, the strain induced cycloaddition of the reported pre-labeled cyclooctyne-DOTA derivative (68)Ga-1 with azides was proven to be slow at room temperature and heating was necessary for acceptable reaction times.

A new generation of radiofluorinated pyrimidine-2,4,6-triones as MMP-targeted radiotracers for positron emission tomography

Radiolabeled C-5-disubstituted pyrimidine-2,4,6-triones have recently been suggested by our group as a class of potent matrix metalloproteinase (MMP) targeted radiotracers that can noninvasively visualize activated MMPs by means of positron emission tomography (PET). MMPs belong to the zinc- and calcium-dependent endopeptidases which are involved in the proteolytic degradation of components of the extracellular matrix (ECM) but also are capable of processing and releasing bioactive molecules such as growth factors, proteinase inhibitors, and cytokines. Locally increased levels of activated MMPs modulate and contribute to the progression of various diseases, such as cancer, atherosclerosis, stroke, arthritis, and others. Therefore, activated MMPs are suitable biological targets for the specific and noninvasive visualization of aforementioned pathologies in vivo. On the basis of our recent results, we here describe a series of new fluorinated pyrimidine-2,4,6-triones of the second generation with maintained MMP inhibition potencies (IC(50) = 4-605 nM), which are fine-tuned toward more hydrophilic versions, and show the improved biodistribution behavior of one selected radiofluorinated pyrimidine-2,4,6-trione by means of small-animal PET.

Imaging biomarkers of brain tumour margin and tumour invasion

Invasion of tumour cells into the normal brain is one of the major reasons of treatment failure for gliomas. Although there is a good understanding of the molecular and cellular processes that occur during this invasion, it is not possible to detect the extent of the tumour with conventional imaging. However, there is an understanding that the degree of invasion differs with individual tumours, and yet they are all treated the same. Newer imaging techniques that probe the pathological changes within tumours may be suitable biomarkers for invasion. Imaging methods are now available that can detect subtle changes in white matter organisation (diffusion tensor imaging), tumour metabolism and cellular proliferation (using MR spectroscopy and positron emission tomography) occurring in regions of tumour that cannot be detected by conventional imaging. The role of such biomarkers of invasion should allow better delineation of tumour margins, which should improve treatment planning (especially surgery and radiotherapy) and provide information on the invasiveness of an individual tumour to help select the most appropriate therapy and help stratify patients for clinical trials.

Non-invasive detection of vulnerable coronary plaque

Critical coronary stenoses have been shown to contribute to only a minority of acute coronary syndromes and sudden cardiac death. Autopsy studies have identified a subgroup of high-risk patients with disrupted vulnerable plaque and modest stenosis. Consequently, a clinical need exists to develop methods to identify these plaques prospectively before disruption and clinical expression of disease. Recent advances in invasive and non-invasive imaging techniques have shown the potential to identify these high-risk plaques. Non-invasive imaging with magnetic resonance imaging, computed tomography and positron emission tomography holds the potential to differentiate between low- and high-risk plaques. There have been significant technological advances in non-invasive imaging modalities, and the aim is to achieve a diagnostic sensitivity for these technologies similar to that of the invasive modalities. Molecular imaging with the use of novel targeted nanoparticles may help in detecting high-risk plaques that will ultimately cause acute myocardial infarction. Moreover, nanoparticle-based imaging may even provide non-invasive treatments for these plaques. However, at present none of these imaging modalities are able to detect vulnerable plaque nor have they been shown to definitively predict outcome. Further trials are needed to provide more information regarding the natural history of high-risk but non-flow-limiting plaque to establish patient specific targeted therapy and to refine plaque stabilizing strategies in the future.

Molecular imaging of tumor invasion and metastases: the role of MRI

The processes of tumor invasion and metastasis have been well characterized at the molecular level, and numerous biomarkers of tumor aggressiveness have been discovered. Molecular imaging offers the opportunity to depict specific cell markers relevant to tumor aggressiveness. Here, we describe the role of MRI in identifying tumor invasiveness and metastasis with reference to other methods. Target-specific molecular imaging probes for tumor invasiveness have been developed for positron emission tomography and optical imaging, but progress in MRI has been slower. For example, proteases associated with tumor invasion, such as specific matrix metalloproteinases or cathepsins, can be targeted in vivo using optical and positron emission tomography methods, but have not yet been successful with MRI. In addition, we describe the use of MRI to detect metastases. Novel MR contrast agents based on iron oxide and dendrimer nanomaterials allow for better characterization of tumor metastases. Organ-specific MR contrast agents are used to identify metastatic disease in the liver. Finally, diffusion-weighted whole-body MRI is discussed as an alternative offered by MRI that does not require the use of molecular probes to screen distant metastases.

Synthesis and evaluation of a novel hydroxamate based fluorescent photoprobe for imaging of matrix metalloproteinases

The assessment of matrix metalloproteinase (MMP) activity in vivo is highly desirable in various human diseases such as cancers. Hydroxamic acids based on CGS27023A or CGS25966 are nonpeptidyl lead structures that specifically target activated MMPs in vivo. The aim of this study was the modification and fluorescent labeling of these lead structures to develop a highly affine, nonpeptide MMP inhibitor (MMPI)-ligand for molecular optical imaging of activated MMPs. An 11 step synthesis was developed involving a PEGylated benzyl derivative as a spacer to minimize the interactions between the activated MMP and the dye of conjugate 11 with an azide as a protected amino function. After reducing the azide (Staudinger reaction) and labeling with Cy5.5, we obtained a CGS-based MMP inhibitor 11 with a fluorescent signaling flag. To evaluate the biological properties of this photoprobe, three human cancer cell lines (A-673, HT-1080 and BT-20) were characterized with respect to their MMP-2 and -9 (gelatinases) expression levels (real-time PCR) and protein levels (Western blotting). Initially, fluorogenic inhibition assays were used to assess the MMP inhibition potential. The PEGylated CGS 10 showed complete inhibition of MMP-2 and MMP-9 activities in vitro both for purified MMP-2/-9 (active and pro-forms) and MMP-2/-9 containing cell culture supernatants. To test the imaging potential in biological tissues, gelatinase activity was measured on tumor cryostat sections of the above-mentioned tumor cells using FITC-labeled dye-quenched gelatin. Gelatinase positive tumors revealed strong binding of CGS-Cy5.5 11, while gelatinase negative tumors were not targeted. In conclusion, this new CGS-based MMP photoprobe has a high affinity for MMP-2 and -9 and is thus a promising candidate for sensitive imaging of MMP activity in various diseases in patients.

Radiofluorinated pyrimidine-2,4,6-triones as molecular probes for noninvasive MMP-targeted imaging

Matrix metalloproteinases (MMPs) are zinc- and calcium-dependent endopeptidases. Representing a subfamily of the metzincin superfamily, MMPs are involved in the proteolytic degradation of components of the extracellular matrix. Unregulated MMP expression, MMP dysregulation and locally increased MMP activity are common features of various diseases, such as cancer, atherosclerosis, stroke, arthritis, and others. Therefore, activated MMPs are suitable biological targets for the specific visualization of such pathologies, in particular by using radiolabeled MMP inhibitors (MMPIs). The aim of this work was to develop a radiofluorinated molecular probe for noninvasive in vivo imaging for the detection of up-regulated levels of activated MMPs in the living organism. Fluorinated MMPIs (26, 31 and 38) based on the pyrimidine-2,4,6-trione lead structure RO 28-2653 (1) were synthesized, and their MMP inhibition potency was evaluated in vitro. The radiosynthesis and the in vivo biodistribution of the first (18)F-labeled prototype, MMP-targeted tracer [(18)F]26, suitable for molecular imaging by means of positron emission tomography (PET) were realized.

Targeting the Tumour: Cell Penetrating Peptides for Molecular Imaging and Radiotherapy

Over the last couple of years, the number of original papers and reviews discussing various applications of cell penetrating peptides (CPPs) has grown exponentially. This is not remarkable since CPPs are capable of transporting the most varying cargo across cell membranes which is one of the biggest problems in drug delivery and targeted therapy. In this review, we focus on the use of CPPs and related peptides for delivery of imaging contrast agents and radionuclides to cells and tissues with the ultimate goal of in vivo molecular imaging and molecular radiotherapy of intracellular and even intranuclear targets.

Radionuclide imaging of tumor angiogenesis

Angiogenesis is a multistep process regulated by pro- and antiangiogenic factors. In order to grow and metastasize, tumors need a constant supply of oxygen and nutrients. For growth beyond 1-2 mm in size, tumors are dependent on angiogenesis. Inhibition of angiogenesis is a new cancer treatment strategy that is now widely investigated clinically. Researchers have begun to search for objective measures that indicate pharmacologic responses to antiangiogenic drugs. Therefore, there is a great interest in techniques to visualize angiogenesis in growing tumors noninvasively. Several markers have been described that are preferentially expressed on newly formed blood vessels in tumors (alpha(v)beta(3) integrin, vascular endothelial growth factor, and its receptor, prostate-specific membrane antigen) and in the extracellular matrix surrounding newly formed blood vessels (extra domain B of fibronectin, Tenascin-C, matrix metalloproteinases, and Robo-4). Several ligands targeting these markers have been tested as a radiotracer for imaging angiogenesis in tumors. The potential of some of these tracers, such as radiolabeled cyclic RGD peptides and radiolabeled anti-PSMA antibodies, has already been tested in cancer patients, while for markers such as Robo-4, the ligand has not yet been identified. In this review, an overview on the currently used nuclear imaging probes for noninvasive visualization of tumor angiogenesis is given.

Mechanism-based profiling of MMPs

The recognition that the successful clinical use of MMP inhibitors will require quantitative correlation of MMP activity with disease type, and to disease progression, has stimulated intensive effort toward the development of sensitive assay methods, improved analytical methods for the determination of the structural profile for MMP-sub-type inhibition, and the development of new methods for the determination - in both quantitative and qualitative terms - of MMP activity. This chapter reviews recent progress toward these objectives, with particular emphasis on the quantitative and qualitative profiling of MMP activity in cells and tissues. Quantitative determination of MMP activity is made from the concentration of the MMP from the tissue, using immobilization of a broad-spectrum MMP inhibitor on a chromatography resin. Active MMP, to the exclusion of MMP zymogens and endogenous TIMP-inhibited MMPs, is retained on the column. Characterization of the MMP sub-type(s) follows from appropriate analysis of the active MMP eluted from the resin. Qualitative determination of MMP involvement in disease can be made using an MMP sub-type-selective inhibitor. The proof of principle, with respect to this qualitative determination of the disease involvement of the gelatinase MMP-2 and MMP-9 sub-types, is provided by the class of thiirane-based MMP mechanism-based inhibitors (SB-3CT as the prototype). Positive outcomes in animal models of disease having MMP-2 and/or -9 dependency follow administration of this MMP inhibitor, whereas this inhibitor is inactive in disease models where other MMPs (such as MMP-14) are involved.

Molecular Imaging of Proteases in Cancer

Proteases play important roles during tumor angiogenesis, invasion, and metastasis. Various molecular imaging techniques have been employed for protease imaging: optical (both fluorescence and bioluminescence), magnetic resonance imaging (MRI), single-photon emission computed tomography (SPECT), and positron emission tomography (PET). In this review, we will summarize the current status of imaging proteases in cancer with these techniques. Optical imaging of proteases, in particular with fluorescence, is the most intensively validated and many of the imaging probes are already commercially available. It is generally agreed that the use of activatable probes is the most accurate and appropriate means for measuring protease activity. Molecular imaging of proteases with other techniques (i.e. MRI, SPECT, and PET) has not been well-documented in the literature which certainly deserves much future effort. Optical imaging and molecular MRI of protease activity has very limited potential for clinical investigation. PET/SPECT imaging is suitable for clinical investigation; however the optimal probes for PET/SPECT imaging of proteases in cancer have yet to be developed. Successful development of protease imaging probes with optimal in vivo stability, tumor targeting efficacy, and desirable pharmacokinetics for clinical translation will eventually improve cancer patient management. Not limited to cancer, these protease-targeted imaging probes will also have broad applications in other diseases such as arthritis, atherosclerosis, and myocardial infarction.

Pre-clinical and clinical evaluation of nuclear tracers for the molecular imaging of vulnerable atherosclerosis: an overview

Cardiovascular diseases (CVD) are the leading cause of mortality worldwide. Despite major advances in the treatment of CVD, a high proportion of CVD victims die suddenly while being apparently healthy, the great majority of these accidents being due to the rupture or erosion of a vulnerable coronary atherosclerotic plaque. A non-invasive imaging methodology allowing the early detection of vulnerable atherosclerotic plaques in selected individuals prior to the occurrence of any symptom would therefore be of great public health benefit. Nuclear imaging could allow the identification of vulnerable patients by non-invasive in vivo scintigraphic imaging following administration of a radiolabeled tracer. The purpose of this review is to provide an overview of radiotracers that have been recently evaluated for the detection of vulnerable plaques together with the biological rationale that initiated their development. Radiotracers targeted at the inflammatory process seem particularly relevant and promising. Recently, macrophage targeting allowed the experimental in vivo detection of atherosclerosis using either SPECT or PET. A few tracers have also been evaluated clinically. Targeting of apoptosis and macrophage metabolism both allowed the imaging of vulnerable plaques in carotid vessels of patients. However, nuclear imaging of vulnerable plaques at the level of coronary arteries remains challenging, mostly because of their small size and their vicinity with unbound circulating tracer. The experimental and pilot clinical studies reviewed in the present paper represent a fundamental step prior to the evaluation of the efficacy of any selected tracer for the early, non-invasive detection of vulnerable patients.

Imaging matrix metalloproteinases in cancer

Early detection of both primary tumors and metastatic disease remains a major challenge in the diagnosis and staging of cancer. The recognition of the role of MMPs in both the growth and metastasis of tumors has guided the development not only of therapeutic strategies utilizing synthetic, small-molecule MMP inhibitors (MMPIs), but has also catalyzed methods to detect and image tumors in vivo by means of tumor-associated proteolytic activity. These imaging approaches target MMPs involved in cancer progression via contrast agents linked to MMPIs or to MMP selective and specific substrates with sensitivity enhanced by amplification during enzymatic processing. This review draws attention to a variety of strategies utilized to image MMP activity in vivo.

Development of an optimized activatable MMP-14 targeted SPECT imaging probe

Matrix metalloproteinase-14 (MT1-MMP or MMP-14) is a membrane-associated protease implicated in a variety of tissue remodeling processes and a molecular hallmark of select metastatic cancers. The ability to detect MMP-14 in vivo would be useful in studying its role in pathologic processes and may potentially serve as a guide for the development of targeted molecular therapies. Four MMP-14 specific probes containing a positively charged cell penetrating peptide (CPP) d-arginine octamer (r(8)) linked with a MMP-14 peptide substrate and attenuating sequences with glutamate (8e, 4e) or glutamate-glycine (4eg and 4egg) repeating units were modeled using an AMBER force field method. The probe with 4egg attenuating sequence exhibited the highest CPP/attenuator interaction, predicting minimized cellular uptake until cleaved. The in vitro MMP-14-mediated cleavage studies using the human recombinant MMP-14 catalytic domain revealed an enhanced cleavage rate that directly correlated with the linearity of the embedded peptide substrate sequence. Successful cleavage and uptake of a technetium-99m labeled version of the optimal probe was demonstrated in MMP-14 transfected human breast cancer cells. Two-fold reduction of cellular uptake was found in the presence of a broad spectrum MMP inhibitor. The combination of computational chemistry, parallel synthesis and biochemical screening, therefore, shows promise as a set of tools for developing new radiolabeled probes that are sensitive to protease activity.

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.

Novel fluorinated derivatives of the broad-spectrum MMP inhibitors N-hydroxy-2(R)-[[(4-methoxyphenyl)sulfonyl](benzyl)- and (3-picolyl)-amino]-3-methyl-butanamide as potential tools for the molecular imaging of activated MMPs with PET

An approach to the in vivo imaging of locally upregulated and activated matrix metalloproteinases (MMPs) found in many pathological processes is offered by positron emission tomography (PET). Hence, appropriate PET radioligands for MMP imaging are required. Here, we describe the syntheses of novel fluorinated MMP inhibitors (MMPIs) based on lead structures of the broad-spectrum inhibitors N-hydroxy-2(R)-[[(4-methoxyphenyl)sulfonyl](benzyl)-amino]-3-methyl-butanamide (CGS 25966) and N-hydroxy-2(R)-[[(4-methoxyphenyl)sulfonyl](3-picolyl)-amino]-3-methyl-butanamide (CGS 27023A). Additionally, tailor-made precursor compounds for radiolabeling with the positron-emitter 18F were synthesized. All prepared hydroxamate target compounds showed high in vitro MMP inhibition potencies for MMP-2, MMP-8, MMP-9, and MMP-13. As a consequence, the promising fluorinated hydroxamic acid derivative 1f was resynthesized in its 18F-labeled version via two different procedures yielding the potential PET radioligand [18F]1f. As expected, the biodistribution behavior of this novel compound and that of the more hydrophilic variant [18F]1j, also developed by our group, indicates that there was no tissue specific accumulation in wild-type (WT) mice.

Developing new metal-based therapeutics: challenges and opportunities

Unexpected side effects and problems experienced in clinical trials have created a difficult environment for those developing new pharmaceuticals and, as a consequence, the number of new chemical entities being registered has fallen to a historic low. Those developing metal-based therapeutics will face the same difficulties, but this environment also provides many opportunities for creative solutions to be applied to overcoming the problems and concerns. In this perspective, these challenges and opportunities are considered in the context of examples of a number of classes of metal-based therapeutics.

Targeting of matrix metalloproteinase activation for noninvasive detection of vulnerable atherosclerotic lesions

INTRODUCTION

Inflammation plays an important role in vulnerability of atherosclerotic plaques to rupture and hence acute coronary events. The monocyte-macrophage infiltration in plaques leads to upregulation of cytokines and metalloproteinase enzymes. Matrix metalloproteinases result in matrix dissolution and consequently expansive remodeling of the vessel. They also contribute to attenuation of fibrous cap and hence susceptibility to rupture. Assessment of metalloproteinase expression and activity should provide information about plaque instability.

Imaging probes targeting matrix metalloproteinases

During the past few years, several imaging probes targeting matrix metalloproteinases (MMPs) have been developed. Most of these probes have been validated in animal models. Overall, results derived from most of these studies have been disappointing. Whether or not this relates to shortcomings of the imaging probes used or to the set-up of the reported studies is currently unclear. Firstly, MMPs targeted in these studies, MMP-1, -2 and -9, are cell secreted and their expression is known to vary extensively within one tumor type, depending on the stage of development of the tumor and on the presence of naturally occurring TIMPs. Given the lack of data on the levels of MMP expression by incoculated tumor tissue at the time of imaging in most studies reported, it cannot be excluded that the negative results reported are, in fact, false-negative imaging results. Secondly, given that most of the agents used for imaging are intrinsically broad-spectrum agents, their higher affinity for specific subsets of MMPs does not necessarily imply that a positive imaging result also corresponds to overexpression of specific subsets of MMPs, as suggested in some papers published. Accordingly, well-characterized tumor models need to be developed for the purpose of validating currently available, as well as future, MMP-imaging probes. So far, only 111In-DTPA-N-TIMP-2 has been injected in patients, respectively suffering from Kaposi Sarcoma. Imaging results obtained with this agent proved disappointing. Imaging results obtained with other MMP-targeting probes in patients are awaited.

The aging of the heart and blood vessels: a consideration of anatomy and physiology in the era of computed tomography, magnetic resonance imaging, and positron emission tomographic imaging methods with special consideration of atherogenesis

Physicians have long told their patients that the doctor's job is to help patients "get as old as they can." As physicians, we have been aided in this objective by many other scientists in other disciplines. The entity of aging and its related changes blends imperceptibly with a variety of age-related diseases. However, these entities do appear to be separate though interrelated. Curing disease is important and a goal that we all work toward to add years to life expectancy. Here, we consider aging as it affects the heart and great vessels and as it serves to influence and support, if not cause, age-related cardiac diseases. This relationship is drawn as cardiac mechanics, hemodynamics, perfusion, metabolism and innervation, anatomy, and pathophysiology are each considered. The effects of aging are presented in 2 sections related to the early and recent "spikes" in aging related information. The latter is largely based in recent developments in chemistry, genetic engineering, molecular biology and the new imaging methods. The purpose of this manuscript is to present these new imaging methods, especially PET, and their impact on the second "spike." This is emphasized particularly in the second half of this review. As a method of demonstrating these imaging tools and their finest potential application, we decided to "showcase" atherosclerosis as the age-related disease for which these methods have made their greatest impact, for which yet more is promised, and for which the influence on longevity is most obvious. The application of positron emission tomography and other imaging methods to the characterization and image identification of atherosclerotic plaques and particularly the "vulnerable" plaque is emphasized. Yet, even with the eradication of coronary disease, the potential for very long life would not be likely. Only with the identification and eradication of the causative factors of aging can this possibility have a chance of becoming reality.

Imaging angiogenesis

Angiogenesis represents the formation of new capillaries by cellular outgrowth from existing microvessels and plays a critical role in the response to ischemia associated with peripheral arterial disease and myocardial infarction. Imaging of angiogenesis would be valuable in risk stratification of patients with arterial occlusive disease. The progress in noninvasive imaging strategies to assess angiogenesis has been made possible with the availability of many technological advances, which include dedicated hybrid SPECT-CT and PET-CT systems and agents targeted at molecular markers of the angiogenic process, involving both receptor-probe interactions and reporter gene technology. These novel targeted approaches for imaging angiogenesis will complement standard imaging of physiological parameters and will play a crucial role for evaluation of therapeutic interventions to promote angiogenesis.

Molecular cardiovascular imaging using scintigraphic methods

Molecular cardiovascular imaging plays an increasingly important role both in basic research and in clinical diagnosis. Scintigraphic methods have long been used to study pathophysiological changes on a cellular and molecular level, and they are likely to remain important molecular imaging modalities in the foreseeable future. This article provides an overview over current developments in cardiovascular molecular imaging using scintigraphic methods. The focus lies on imaging of cardiac innervation, plaque instability, hypoxia and angiogenesis, gene expression and stem and progenitor cell migration and proliferation.

How molecular imaging is speeding up antiangiogenic drug development

Drug development is a long process that generally spans about 10 to 15 years. The shift in recent drug discovery to novel agents against specific molecular targets highlights the need for more robust molecular imaging platforms. Using molecular probes, molecular imaging can aid in many steps of the drug development process, such as providing whole body readout in an intact system, decreasing the workload and speeding up drug development/validation, and facilitating individualized anticancer treatment monitoring and dose optimization. The main focus of this review is the recent advances in tumor angiogenesis imaging, and the targets include vascular endothelial growth factor and vascular endothelial growth factor receptor, integrin alpha(v)beta(3), matrix metalloproteinase, endoglin (CD105), and E-selectin. Through tumor angiogenesis imaging, it is expected that a robust platform for understanding the mechanisms of tumor angiogenesis and evaluating the efficacy of novel antiangiogenic therapies will be developed, which can help antiangiogenic drug development in both the preclinical stage and the clinical settings. Molecular imaging has enormous potential in improving the efficiency of the drug development process, including the specific area of antiangiogenic drugs.

Radionuclide imaging for the detection of inflammation in vulnerable plaques

Imaging of atheromatous plaques has traditionally centered on assessing the degree of luminal stenosis. More recently it has become clear that the vulnerable atherosclerotic plaques responsible for the majority of life-threatening syndromes are characterized by high numbers of inflammatory cells and proteins. This has highlighted the urgent need for suitable imaging techniques that can identify and quantify levels of inflammation within atheromatous lesions. Positron emission tomography and single-photon emission computed tomography imaging hold promise in this regard. Tracer compounds capable of assessing macrophage recruitment, foam cell generation, matrix metalloproteinase production, macrophage apoptosis, and macrophage metabolism have been developed and tested in the carotid and peripheral circulation. The identification of inflamed lesions within the coronary circulation, however, remains elusive owing to small plaque size, cardiac and respiratory motion, and lack of a suitable specific nuclear tracer.

Recent advances in MMP inhibitor design

The search for an MMP inhibitor with anticancer efficacy is a nearly three-decade endeavor. This inhibitor is yet to be found. The reasons for this failure include shortcomings in the chemistry of these compounds (including broad MMP sub-type selectivity, metabolic lability, and toxicity) as well as the emerging, and arguably extraordinary, complexity of MMP cell (and cancer) biology. Together these suggest that the successful anticancer inhibitor must possess MMP selectivity against the MMP subtype whose involvement is critical, yet highly temporally (with respect to metastatic progression) and mechanistically (with respect to matrix degradation) regulated. This review summarizes the progression of chemical structure and mechanistic thinking toward these objectives, with emphasis on the disappointment, the perseverance, and the resilient optimism that such an inhibitor is there to be discovered.

[Scintigraphic molecular imaging]

Modern medicine is currently focusing its basic and clinical research towards "molecular medicine". This trend, together with the decoding of the human genome and the resulting design and use of transgenic mouse models of human diseases, demands that innovative imaging approaches are developed for man and mice. Non-invasive imaging modalities capable of quantifying molecular processes in vivo (collectively defined as "molecular imaging" techniques) are extremely interesting in this respect. This review focuses on the clinical and experimental scintigraphic molecular-imaging modalities SPECT and PET, and summarizes their actual and future impact in medicine.

C-5-Disubstituted Barbiturates as Potential Molecular Probes for Noninvasive Matrix Metalloproteinase Imaging

Studies have demonstrated a positive correlation between inflammation, metastasis, or atherosclerosis and the unbalanced or culminated expression of matrix metalloproteinases (MMPs). The molecular imaging of locally upregulated MMP activity in vivo is a clinical challenge. Actually, radioligands based on nonpeptidyl MMP inhibitors (MMPIs) are currently in development as putative radiopharmaceutical agents for the noninvasive in vivo assessment of activated MMPs. Nonpeptidyl MMPIs bind to the zinc active site of the activated enzyme via mono- (e.g. carboxylate) or bidentate (e.g. hydroxamate) complexation thereby exhibiting a broad-spectrum MMP binding potency. Thus, these mentioned endopeptidase inhibitors should be useable lead compounds for the redevelopment as diagnostic MMPI radiotracers. Recently, the non-hydroxamate C-5-disubstituted pyrimidine-2,4,6-triones were disclosed as subgroup-selective MMP inhibitors. We here describe a set of fine-tuned barbiturates as a new class of MMPI radiotracers for the noninvasive in vivo visualization of activated MMPs using scintigraphic techniques such as SPECT or PET.