Optical and multimodal peptide-based probes for in vivo molecular imaging

Molecular imaging consists of non-invasive monitoring of spatial-temporal distribution of molecular or cellular processes, and may be used for early disease detection and real-time monitoring of therapeutic responses. Several strategies have been developed over the last two decades. Early attempts used monoclonal antibodies or antibody fragments and, although specific targeting was achieved, these probes was largely unsuccessful. In the quest for better agents, labeled peptides were then used. Peptides are easier to synthesize, less likely to be immunogenic, and have rapid blood clearance, which results in adequate target-to-background ratios in a short period of time. This review discusses state-of-the-art cancer imaging by means of labeled peptides, the radionuclide, optical and nanoplatform-based imaging techniques which can provide functional information of the disease and track biochemical processes in vivo. The advantages and disadvantages of each technique are discussed. Lastly, the emphasis of this paper is on the new multimodal probes which can overcome individual limitations and exploit the individual strengths of the latest molecular imaging techniques.     

Targeted multimodal imaging modalities

Molecular imaging non-invasively visualizes and characterizes the biologic functions and mechanisms in living organisms at a molecular level. In recent years, advances in imaging instruments, imaging probes, assay methods, and quantification techniques have enabled more refined and reliable images for more accurate diagnoses. Multimodal imaging combines two or more imaging modalities into one system to produce details in clinical diagnostic imaging that are more precise than conventional imaging. Multimodal imaging offers complementary advantages: high spatial resolution, soft tissue contrast, and biological information on the molecular level with high sensitivity. However, combining all modalities into a single imaging probe involves problems yet to be solved due to the requirement of high dose contrast agents for a component of imaging modality with low sensitivity. The introduction of targeting moieties into the probes enhances the specific binding of targeted multimodal imaging modalities and selective accumulation of the imaging agents at a disease site to provide more accurate diagnoses. An extensive list of prior reports on the targeted multimodal imaging probes categorized by each modality is presented and discussed. In addition to accurate diagnosis, targeted multimodal imaging agents carrying therapeutic medications make it possible to visualize the theranostic effect and the progress of disease. This will facilitate the development of an imaging-guided therapy, which will widen the application of the targeted multimodal imaging field to experiments in vivo.     

Liposomes and their applications in molecular imaging

Molecular imaging is a relatively new discipline with a crucial role in diagnosis and treatment tracing of diseases through characterization and quantification of biological processes at cellular and sub-cellular levels of living organisms. These molecular targeted systems can be conjugated with contrast agents or radioligands to obtain specific molecular probes for the purpose of diagnosis of diseases more accurately by different imaging modalities. Nowadays, an interesting new approach to molecular imaging is the use of stealth nanosized drug delivery systems such as liposomes having convenient properties such as biodegradability, biocompatibility and non-toxicity and they can specifically be targeted to desired disease tissues by combining with specific targeting ligands and probes. The targeted liposomes as molecular probes in molecular imaging have been evaluated in this review. Therefore, the essential point is detection of molecular target of the disease which is different from normal conditions such as increase or decrease of a receptor, transporter, hormone, enzyme etc, or formation of a novel target. Transport of the diagnostic probe specifically to targeted cellular, sub-cellular or even to molecular entities can be performed by molecular imaging probes. This may lead to produce personalized medicine for imaging and/or therapy of diseases at earlier stages.     

Liposomes and their applications in molecular imaging

Molecular imaging is a relatively new discipline with a crucial role in diagnosis and treatment tracing of diseases through characterization and quantification of biological processes at cellular and sub-cellular levels of living organisms. These molecular targeted systems can be conjugated with contrast agents or radioligands to obtain specific molecular probes for the purpose of diagnosis of diseases more accurately by different imaging modalities. Nowadays, an interesting new approach to molecular imaging is the use of stealth nanosized drug delivery systems such as liposomes having convenient properties such as biodegradability, biocompatibility and non-toxicity and they can specifically be targeted to desired disease tissues by combining with specific targeting ligands and probes. The targeted liposomes as molecular probes in molecular imaging have been evaluated in this review. Therefore, the essential point is detection of molecular target of the disease which is different from normal conditions such as increase or decrease of a receptor, transporter, hormone, enzyme etc, or formation of a novel target. Transport of the diagnostic probe specifically to targeted cellular, sub-cellular or even to molecular entities can be performed by molecular imaging probes. This may lead to produce personalized medicine for imaging and/or therapy of diseases at earlier stages.     

Target-specific delivery of peptide-based probes for PET imaging

Positron emission tomography (PET) is one of the most rapidly growing areas of medical imaging, with many applications in the clinical management of patients with various diseases. The principal goal of PET imaging is to visualize, characterize, and measure biological processes at the cellular, subcellular, and molecular level in living subjects with non-invasive procedures. PET imaging takes advantage of the traditional diagnostic imaging techniques and introduces positron-emitting probes to determine the expression of indicative molecular targets at different stages of disease. During the last decade, advances in molecular biology have revealed an increasing number of potential molecular targets, including peptide receptors and peptide-related biomolecules. With the help of sophisticated bioconjugation and radiolabeling techniques, numerous peptide-based agents have been developed and evaluated for delivery of PET radionuclides to the specific molecular targets in preclinical and clinical studies. As compared to macromolecules, such as proteins or antibodies, low-molecular-weight peptides have their distinctive advantages and predominantly demonstrate their favorable pharmacokinetics for in vivo PET applications. This review summarizes the criteria of peptide-based PET probes design, the selection of radioisotopes, labeling methods, and provides an overview of the current status and trends in the development of target-specific peptide-based probes with respect to their unique PET imaging applications.     

Molecular imaging with nucleic acid aptamers

With many desirable properties such as ease of synthesis, small size, lack of immunogenicity, and versatile chemistry, aptamers represent a class of targeting ligands that possess tremendous potential in molecular imaging applications. Non-invasive imaging of various disease markers with aptamer-based probes has many potential clinical applications such as lesion detection, patient stratification, treatment monitoring, etc. In this review, we will summarize the current status of molecular imaging with aptamer-based probes. First, fluorescence imaging will be described which include both direct targeting and activatable probes. Next, we discuss molecular magnetic resonance imaging and targeted ultrasound investigations using aptamer-based agents. Radionuclide-based imaging techniques (single-photon emission computed tomography and positron emission tomography) will be summarized as well. In addition, aptamers have also been labeled with various tags for computed tomography, surface plasmon resonance, dark-field light scattering microscopy, transmission electron microscopy, and surface-enhanced Raman spectroscopy imaging. Among all molecular imaging modalities, no single modality is perfect and sufficient to obtain all the necessary information for a particular question. Thus, a multimodality probe has also been constructed for concurrent fluorescence, gamma camera, and magnetic resonance imaging in vivo. Although the future of aptamer-based molecular imaging is becoming increasingly bright and many proof-of-principle studies have already been reported, much future effort needs to be directed towards the development of clinically translatable aptamer-based imaging agents which will eventually benefit patients.     

Human serum albumin conjugated biomolecules for cancer molecular imaging

Molecular imaging is a fast growing field in biomedical research. The discovery, development and continual improvement of molecular probes are important for ongoing research efforts in molecular imaging. Human serum albumin (HSA) offers favorable characteristics and opportunities as a platform protein for molecular imaging probe discovery and optimization. It has many advantages, including alternation of biodistribution and pharmacokinetic properties of molecular imaging probes, enhancing the blood half-life of bio-molecules, and making these molecules multivalent, all of which make HSA a promising carrier for cancer-targeted imaging and therapy. Numerous studies have focused on the development and application of HSA-based cancer imaging and treatment. This review gives a brief account of albumin-based molecular probes, focusing on their applications in cancer molecular imaging, such as PET/SPECT, MRI and optical imaging.     

Advances in imaging probes and optical microendoscopic imaging techniques for early in vivo cancer assessment

A new chapter in the history of medical diagnosis happened when the first X-ray technology was invented in the late 1800s. Since then, many non-invasive and minimally invasive imaging techniques have been invented for clinical diagnosis to research in cellular biology, drug discovery, and disease monitoring. These imaging modalities have leveraged the benefits of significant advances in computer, electronics, and information technology and, more recently, targeted molecular imaging. The development of targeted contrast agents such as fluorescent and nanoparticle probes coupled with optical imaging techniques has made it possible to selectively view specific biological events and processes in both in vivo and ex vivo systems with great sensitivity and selectivity. Thus, the combination of targeted molecular imaging probes and optical imaging techniques have become a mainstay in modern medicinal and biological research. Many promising results have demonstrated great potentials to translate to clinical applications. In this review, we describe a discussion of employing imaging probes and optical microendoscopic imaging techniques for cancer diagnosis.     

It is all about proteases: from drug delivery to in vivo imaging and photomedicine

Clinical studies provide overwhelming evidence for the importance of proteolytic imbalance and the upregulation of diverse protease classes in diseases such as cancer and arthritis. While the complex nature of proteolytic networks has hampered the development of protease inhibitors for these indications, aberrant enzyme activity could be successfully exploited for the development of proteasesensitive drug delivery systems and fluorescent in vivo imaging agents. More recently, these concepts have also been translated into photomedical applications to develop dual modality prodrugs for the simultaneous treatment and imaging of disease. After an introductory overview of proteases and their role in cancer, we present and discuss different strategies to exploit upregulated protease activity for the development of drug delivery systems, fluorescent in vivo reporter probes, and photosensitizer-prodrugs with respect to their potential and limitations. The main approaches used for targeting proteases in all three areas can be roughly divided into peptide-based and macromolecular strategies. Both involve the use of a short, peptide-based protease substrate, which is either directly tagged to the therapeutic agent or dye/quencher pair, or alternatively, serves as a linker between the polymeric carrier and a functional unit. In the latter case, the pharmacokinetic properties of peptide-based protease-sensitive prodrugs and imaging probes can be further ameliorated by the passive targeting capacity of macromolecular drug delivery systems for neoplastic and inflammatory lesions.     

Magnetomotive molecular nanoprobes

Tremendous developments in the field of biomedical imaging in the past two decades have resulted in the transformation of anatomical imaging to molecular-specific imaging. The main approaches towards imaging at a molecular level are the development of high resolution imaging modalities with high penetration depths and increased sensitivity, and the development of molecular probes with high specificity. The development of novel molecular contrast agents and their success in molecular optical imaging modalities have lead to the emergence of molecular optical imaging as a more versatile and capable technique for providing morphological, spatial, and functional information at the molecular level with high sensitivity and precision, compared to other imaging modalities. In this review, we discuss a new class of dynamic contrast agents called magnetomotive molecular nanoprobes for molecular-specific imaging. Magnetomotive agents are superparamagnetic nanoparticles, typically iron-oxide, that are physically displaced by the application of a small modulating external magnetic field. Dynamic phase-sensitive position measurements are performed using any high resolution imaging modality, including optical coherence tomography (OCT), ultrasonography, or magnetic resonance imaging (MRI). The dynamics of the magnetomotive agents can be used to extract the biomechanical tissue properties in which the nanoparticles are bound, and the agents can be used to deliver therapy via magnetomotive displacements to modulate or disrupt cell function, or hyperthermia to kill cells. These agents can be targeted via conjugation to antibodies, and in vivo targeted imaging has been shown in a carcinogeninduced rat mammary tumor model. The iron-oxide nanoparticles also exhibit negative T2 contrast in MRI, and modulations can produce ultrasound imaging contrast for multimodal imaging applications.     

Molecular approaches to the treatment, prophylaxis, and diagnosis of Alzheimer's disease: novel PET/SPECT imaging probes for diagnosis of Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disease of the brain associated with irreversible cognitive decline, memory impairment, and behavioral changes. Postmortem brains of AD patients reveal neuropathologic features, in particular the presence of senile plaques (SPs) and neurofibrillary tangles (NFTs), which contain β-amyloid peptides and highly phosphorylated tau proteins. Currently, AD can only be definitively confirmed by postmortem histopathologic examination of SPs and NFTs in the brain. Therefore, SPs and NFTs in the brain may be useful as biomarkers for the differential diagnosis of AD; the detection of individual SPs and NFTs in vivo by positron-emission tomography (PET) or single-photon emission computed tomography (SPECT) should improve diagnosis and also accelerate discovery of effective therapeutic agents for AD. Many PET/SPECT imaging probes for SPs have already been developed. Several of the PET probes have been shown in clinical trials to be useful for the imaging of β-amyloid plaques in living brain tissue. More recently, the development of PET/SPECT probes for in vivo imaging of NFTs is an active area of study in the field of molecular imaging because the appearance of NFT pathology correlates well with clinical severity of dementia. We will review current research on the development of PET/SPECT imaging probes for in vivo detection of SPs and NFTs and their application to diagnosis and therapy of AD.     

Design of peptide imaging agents for whole-body and intraoperative molecular imaging

Due to the growing toolkit of targeted contrast agents, molecular imaging continues to play a prominent role in the clinical care of cancer. Peptide-based imaging approaches are of particular significance due to their favorable pharmacokinetic properties, established manufacturing infrastructure, and documented clinical success in whole-body imaging. A logical extension of molecular imaging with peptides is to improve surgical outcomes in cancer through highly sensitive and specific probes which can be used intraoperatively. Advances in fluorescent imaging have resulted in various peptide labeling strategies with intraoperative indications. In this review, we focused on the evolving design of peptide imaging agents starting with the clinically used somatostatin targeting peptides. We then review the current synthetic approaches used for dual-labeled agent development and offer perspectives on optimal protection schemes that can be used for multimodal probe development.     

Has molecular and cellular imaging enhanced drug discovery and drug development?

A great many efforts have been made to accelerate the drug discovery and development process, which is extremely time and money consuming. Recently developed molecular imaging has many significant advantages over conventional methods for examining molecular pathways and obtaining pharmacokinetic, pharmacodynamic and mechanistic information. This review briefly summarizes various molecular and cellular imaging techniques and discusses several important applications of molecular and cellular imaging in drug discovery and development, which include: (i) measurement of pharmacodynamic endpoints by imaging metabolism and proliferation, imaging angiogenic parameters, and imaging a particular pathway or downstream target; (ii) evaluation of pharmacokinetics; and (iii) imaging therapeutic gene expression with relevance to gene therapy. Molecular imaging is becoming more widely used as a non-invasive tool for drug discovery and drug screening. Further refinements in imaging techniques, optimization of imaging probes and collaborative efforts will be needed to fully realise the vast potential of molecular imaging techniques in discovering and developing new drugs.     

A steroid-conjugated magnetic resonance probe enhances contrast in progesterone receptor expressing organs and tumors in vivo

Progesterone receptor (PR) is a significant biomarker in diseases such as endometriosis and breast, ovarian, and uterine cancers that is associated with disease prognosis and therapeutic efficacy. While receptor status is currently determined by immunohistochemistry assays, the development of noninvasive PR imaging agents could improve molecular characterization, treatment decisions, and disease monitoring. ProGlo, a progesterone-conjugated magnetic resonance imaging (MRI) contrast agent, was evaluated in vivo to determine whether it targets and enhances signal intensity in organs and tumors that express high PR levels. A tissue distribution study indicated that ProGlo accumulates in the PR-rich uterus, which was confirmed by in vivo imaging studies. Ex vivo images of these organs revealed that ProGlo was distributed in the substructures that express high PR levels. In xenograft tumor models, ProGlo was taken up to a greater extent than the nonfunctionalized Gd-DO3A in tumors, particularly in PR(+) tumors. The ability to accumulate and enhance signal intensity in PR(+) organs and tumors suggests that ProGlo may be a promising MRI probe for PR(+) diseases.     

Targeted multifunctional lipid-based nanocarriers for image-guided drug delivery

Lipid-based nanocarriers have proven successful in the delivery of mainly chemotherapeutic agents, and currently they are being applied clinically in the treatment of various types of cancer. These drug delivery systems achieve increased therapeutic efficacy by altering the pharmacokinetics and biodistribution of encapsulated drugs, resulting in decreased drug toxicity and enhanced accumulation in tumor tissue. This increased accumulation is due to the relatively leaky immature vasculature of a tumor. After the clinical relevance of such drug delivery systems was demonstrated, research in this area focused on optimization, both by cell specific targeting and including controlled and triggered release concepts within the carrier. These more advanced targeted nanocarriers in general have clearly shown their potential in various animal tumor models and await clinical application. The development of targeted nanocarriers in which therapeutic and imaging agents are merged into a single carrier will certainly be of importance in the near future. Indeed, scientists active in the field of imaging (e.g. nuclear and magnetic resonance imaging) have already started to exploit nanocarriers for molecular imaging. Image-guided drug delivery using these multifunctional nanocarriers, containing therapeutic and imaging agents, will ultimately allow for online monitoring of tumor location, tumor targeting levels, intratumoral localization and drug release kinetics prior and during radio- and/or chemotherapeutic treatment. This review describes the current status and challenges in the field of nanocarrier-aided drug delivery and drug targeting and discusses the opportunities of combining imaging probes with these drug carriers and the potential of these multifunctional lipid-based nanocarriers within image-guided drug delivery.     

Biomedical imaging in pharmacology with nuclear medical imaging methodologies: positron emission tomography (PET) and single photon emission computed tomography (SPECT)

The nuclear imaging technologies, positron emission tomography (PET) and single photon emission computed tomography (SPECT), have the power to non-invasively obtain dynamic and real-time information on the in vivo behaviors of radiolabeled molecules not only in humans but also in experimental animals. Thus, PET and SPECT can image molecular interactions of biological processes in vivo directly and reveal biological phenomena that are hidden from view. Furthermore, these imaging procedures also can be repeatedly performed before and after interventions, thereby allowing each subject to be used as its own control. In these studies, the radiolabeled compounds used as imaging probes for non-invasive assays of biochemical processes should have defined in vivo behaviors that can provide valuable information on the physiological and pharmacological processes. This paper describes the principle of the nuclear medical imaging systems, rational design of radiolabeled imaging probes, and the application to in vivo investigation of the change of various neurotransmission systems under disease and drug treatment. The efficient utilization of these nuclear medical imaging technologies will accelerate biomedical studies and drug development.     

Multifunctional ligands in medicinal inorganic chemistry--current trends and future directions

This review will highlight recent advances in ligand design for innovative applications in medicinal inorganic chemistry. Ligands that effectively bind metal ions and also include specific features to enhance targeting, reporting, and overall efficacy are driving innovation in areas of disease diagnosis and therapy. Increasing the potency of therapeutic compounds, while limiting side-effects, is a common goal in medicinal chemistry. In an effort to achieve this goal, compounds are being developed that either target a disease site, or are activated by a disease specific biological process. Metal complexes containing targeting functions and/or bioactive ligands, as well as agents that are activated by specific enzymes, or changes in pH and pO2, provide new avenues for drug development. Radiodiagnostic compounds, magnetic resonance imaging agents, and optical probes containing transition metals offer versatility unavailable to organic imaging agents. In certain cases, dual modality agents have been developed, and will be highlighted. Finally, we will discuss targeted metal binding compounds for treatment of metal overload disorders, and the recent application to neurodegenerative disease.     

Application of cardiac molecular imaging using positron emission tomography in evaluation of drug and therapeutics for cardiovascular disorders

The incidence of cardiovascular disease is increasing with the aging population. This has stimulated a need for innovative means to evaluate and develop therapeutic strategies intended to improve patient care. Positron emission tomography (PET) imaging is an advanced nuclear imaging technology. The advantage of PET over other non-invasive imaging modalities is its ability to accurately measure tissue concentrations of specific radiolabeled compounds. These radioligands can be used as molecular probes to quantify physiological processes and the effects of therapy. Molecular imaging with PET has been applied to evaluate new and established drugs and therapies, as well as their effects on physiological parameters. New radiolabeled receptor ligands will also allow in vivo pharmacokinetic studies following drug treatment, yielding insights into drug delivery, optimal drug occupancy, and mechanism of action at the receptor level. These exciting tissue pharmacokinetic data could revolutionize evaluation of drug therapies in cardiovascular diseases. In addition, serial evaluations of these processes are now possible in both animals and humans permitting sensitive means to evaluate disease progression and therapies. New tools for imaging such as PET/CT and small animal PET broaden the potential of PET in drug evaluation. This review will describe the accuracy of PET as a non-invasive modality to quantify various parameters, and the application of PET in evaluating new and established therapies. This paper will also review the application of receptor ligand imaging and the principles of using surrogate physiological end-points in early drug development and evaluation.     

Applications for site-directed molecular imaging agents coupled with drug delivery potential

Molecular imaging allows non-invasive characterization and quantification of biological processes at cellular and molecular level. Such technologies make it possible to enhance our understanding of drug activity and pharmokinetic properties, and therefore aid decisions to select candidates that are most likely to benefit from targeted drug therapy. Targeted DDSs are nanometer-sized carrier materials designed for improving the biodistribution of systemically applied (chemo-)therapeutics by strictly localizing its pharmacological activity to the site or organ of action. The parallel development of molecular imaging and targeted drug delivery offers great challenges and opportunities for a single multifunctional platform technology, combining targeted motif, therapeutic agents and imaging agents for imaging guided drug delivery. This review article summarizes the synthesis and characterization of various biomaterials that carry targeting motifs, imaging tags and therapeutic agents as theragnostics.     

Novel paramagnetic contrast agents for molecular imaging and targeted drug delivery

Molecular biology and genomic sciences are revealing the early biological signatures for many diseases. In response, the Molecular Imaging community is rapidly developing contrast agents to visualize the nascent pathological changes and to concomitantly deliver treatment directly to the site of disease. The evaluation, development and use of these new agents require a complementary understanding of contrast chemistry and imaging techniques. The fundamental issues surrounding magnetic contrast agent development, rational drug delivery, MR molecular imaging, and their interdependence are elucidated.