In vivo imaging of oligonucleotides with nuclear tomography

Positron emission tomography (PET) and single photon emission tomography (SPECT) are high-resolution, sensitive, molecular and functional imaging techniques that permit repeated, noninvasive assessment and quantification of specific biological and pharmacological processes in humans. PET and SPECT are also the most advanced technologies currently available for studying in vivo molecular interactions and therefore can advantageously play a key role in both drug discovery and development of pharmaceuticals, by assessing their in vivo distribution, pharmacokinetics, and dynamics, once labeled with a positron or gamma-emitter. Recent advances in positron and gamma-emitter labeling of bioconjugates allow the design and development of complex high-molecular-weight bioactive chemical structures as radiopharmaceuticals including single-stranded oligonucleotides. Besides, the introduction of high-resolution tomographic devices for imaging the distribution of radioactivity in small animal models such as mice and rats offers a unique opportunity to study the biological behavior of labeled compounds in integrated, unperturbed living systems. The present review illustrates the current technologies for labeling oligonucleotides with various radioisotopes, and their use as in vivo probes of oligonucleotide distribution and molecular interactions in target tissues. Emphasis will be also given to the role which could be played by these technologies in the development of oligonucleotide-based drugs.     

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.     

Fundamentals of positron emission tomography and applications in preclinical drug development

Positron emission tomography (PET) is a nuclear imaging technique that can dynamically image trace amounts of positron-labeled radiopharmaceuticals in vivo. Tracer concentrations can be determined quantitatively, and by application of appropriate tracer kinetic models, the rates of a wide range of different biological processes can be measured noninvasively in humans. PET has been used as a research tool for more than 25 years and has also found clinical applications, particularly in oncology, neurological disorders, and cardiovascular disease. Recently, there has been tremendous interest in applying PET technology to in vivo small-animal imaging. Significant improvements in the imaging technology now permit a wide range of PET studies in mice and rats, using compact, relatively low-cost, dedicated small-animal PET scanners. This article reviews the fundamental basis of PET imaging and discusses the development of small-animal PET scanners and their possible application in preclinical drug development.     

Amino Acid transporter-targeted radiotracers for molecular imaging in oncology

Amino acids are required for sustenance of continuous uncontrolled growth of tumor cells, and upregulation of amino acid transporter expression has often been observed in tumor cells to facilitate their accelerated rates of amino acid uptake. Therefore, amino acid transporters have promise as ideal targets for tumor imaging. In fact, many natural and artificial amino acids have been radiolabeled for positron emission tomography (PET) and single-photon emission computed tomography (SPECT) imaging of tumor. In this article, we review the classification, molecular biology, and pharmacological relevance of amino acid transport systems. In addition, we discuss the chemistry, radiochemistry, current clinical applications, and future prospects for the use of radiolabeled amino acid-based probes for PET and SPECT imaging in oncology for each category of radionuclide.     

Positron emission tomography molecular imaging for drug development

Human in vivo molecular imaging with positron emission tomography (PET) enables a new kind of 'precision pharmacology', able to address questions central to drug development. Biodistribution studies with drug molecules carrying positron-emitting radioisotopes can test whether a new chemical entity reaches a target tissue compartment (such as the brain) in sufficient amounts to be pharmacologically active. Competition studies, using a radioligand that binds to the target of therapeutic interest with adequate specificity, enable direct assessment of the relationship between drug plasma concentration and target occupancy. Tailored radiotracers can be used to measure relative rates of biological processes, while radioligands specific for tissue markers expected to change with treatment can provide specific pharmacodynamic information. Integrated application of PET and magnetic resonance imaging (MRI) methods allows molecular interactions to be related directly to anatomical or physiological changes in a tissue. Applications of imaging in early drug development can suggest approaches to patient stratification for a personalized medicine able to deliver higher value from a drug after approval. Although imaging experimental medicine adds complexity to early drug development and costs per patient are high, appropriate use can increase returns on R and D investment by improving early decision making to reduce new drug attrition in later stages. We urge that the potential value of a translational molecular imaging strategy be considered routinely and at the earliest stages of new drug development.     

Recent advances in positron emission tomography (PET) radiotracers for imaging phosphodiesterases

Phosphodiesterases (PDEs) are a family of enzymes that metabolically inactivate the second messengers 3',5'- cyclic adenosine monophosphate (cAMP) and/or 3',5'-cyclic guanosine monophosphate (cGMP). These two messengers regulate the extracellular signal from the plasma membrane G protein-coupled receptors (GPCRs) to the intracellular effector proteins, hence modulating a wide variety of biological processes both in the central nervous system (CNS) and peripheral tissues. Although there are many radiotracers available for positron emission tomography (PET) studies of different receptors, there are just a few tracers available for imaging studies of second messenger systems. The first reported PDE PET ligands were the 11C-labeled versions of the PDE4 inhibitors rolipram and Ro 20-1724, and, to date, PET imaging studies in human subjects have only been reported with [11C]rolipram. As a consequence of the growing interest in identifying selective PDE inhibitors as potential new therapeutic agents, new PET radiotracers for imaging specific PDEs have been described in literature as well. This article highlights these efforts on the design and evaluation of novel PET radioligands for in vivo imaging of PDEs.     

Use of positron emission tomography in anticancer drug development

Positron emission tomography (PET) is increasingly being used in anticancer drug development. The technique is applicable to studies of drug delivery, and where specific probes are available, to provide pharmacodynamic readouts noninvasively in patients. Mathematical modeling of the imaging data enhances the quality of information that is obtained from such studies. This section provides a review of the PET methodologies that have been used for the development of new cancer therapies. Other than imaging of radiolabeled drugs, PET modeling has found extensive application in studies with 2-[¹¹C]thymidine, [¹⁸F]fluorodeoxyglucose, H₂ ¹⁵O, C¹⁵O, and receptor ligands.     

Radiopharmaceutical chemistry for positron emission tomography

Molecular imaging is an emerging technology that allows the visualization of interactions between molecular probes and biological targets. Molecules that either direct or are subject to homeostatic controls in biological systems could be labeled with the appropriate radioisotopes for the quantitative measurement of selected molecular interactions during normal tissue homeostasis and again after perturbations of the normal state. In particular, positron emission tomography (PET) offers picomolar sensitivity and is a fully translational technique that requires specific probes radiolabeled with a usually short-lived positron-emitting radionuclide. PET has provided the capability of measuring biological processes at the molecular and metabolic levels in vivo by the detection of the gamma rays formed as a result of the annihilation of the positrons emitted. Despite the great wealth of information that such probes can provide, the potential of PET strongly depends on the availability of suitable PET radiotracers. However, the development of new imaging probes for PET is far from trivial and radiochemistry is a major limiting factor for the field of PET. In this review, we provided an overview of the most common chemical approaches for the synthesis of PET-labeled molecules and highlighted the most recent developments and trends. The discussed PET radionuclides include ¹¹C (t_1/2 = 20.4 min), ¹³N (t_1/2 = 9.9 min), ¹⁵O (t_1/2 = 2 min), ⁶⁸Ga (t_1/2 = 68 min), ¹⁸F (t_1/2 = 109.8 min), ⁶⁴Cu (t_1/2 = 12.7 h), and ¹²⁴I (t_1/2 = 4.12 d).     

Molecular imaging kits for hexosamine biosynthetic pathway in oncology

Noninvasive imaging assessment of tumor cell proliferation could be helpful in the evaluation of tumor growth potential, the degree of malignancy, and could provide an early assessment of treatment response prior to changes in tumor size determined by computed tomography (CT), magnetic resonance imaging (MRI), Positron emission tomography (PET), Single-Photon emission computed tomography (SPECT) or ultrasonography, respectfully. Understanding of tumor proliferative activity, in turn, could aid in the selection of optimal therapy by estimating patient prognosis and selecting the proper management. PET/CT imaging of (18)F-fluoro-2-deoxy-glucose (FDG) is recognized as a technology for diagnosing the presence and extent of several cancer types. Recently, radiolabeled glucosamine analogues were introduced as a promising SPECT agent to complement FDG imaging to increase specificity and improve the accuracy of lesion size in oncology applications. Radiolabeled glucosamine analogues were developed to localize in the nuclear components of cells primarily via the hexosamine biosynthetic pathway whereas glucose localizes in the cytoplasm of cells through the glycolytic/TCA pathway. This paper reviews novel kit-based radiolabeled glucosamine analogues for metabolic imaging of tumor lesions. The novel radiolabeled glucosamine analogues may increase the specificity in oncology applications and can influence patient diagnosis, planning and monitoring of cancer treatment.     

PET and SPECT imaging of the opioid system: receptors, radioligands and avenues for drug discovery and development

As we celebrate the bicentennial of the isolation of morphine by Sertürner, opioids continue to dominate major sectors of the analgesic market worldwide. The pharmaceutical industry stands to benefit greatly from molecular imaging in preclinical and early clinical trials of new or improved opioid drugs. At this juncture, it seems fitting to summarize the past twenty or so years of research on molecular imaging of the opioid system from the viewpoint of drug discovery and development. Opioid receptors were first imaged in human volunteers by positron emission tomography (PET) in 1984. Now, quantitative PET imaging of the major opioid receptor types (micro, delta , kappa) is possible in the brain and peripheral organs of healthy persons and patient populations. Radioligands are under development for single photon emission computed tomography (SPECT) of opioid receptors as well. These functional, nuclear imaging techniques can trace the fate of radiolabeled molecules directly, but non-invasively, and allow precise pharmacokinetic and pharmacodynamic measurements. Molecular imaging provides unique data that can aid in selecting the best drug candidates, determining optimal dosing regimens, clearing regulatory hurdles and lowering risks of failure. Using a historical perspective, this review touches on opioid receptors as drug targets, and focuses on the status and use of radiotracers for opioid receptor PET and SPECT. Selected studies are discussed to illustrate the power of molecular imaging for facilitating opioid drug discovery and development.     

Nuclear imaging of inflammation in neurologic and psychiatric disorders

Cerebral inflammation is a common phenomenon during the progression of neurodegenerative diseases. In general, neurodegenerative diseases have unpredictable clinical courses and timely effective treatment is not available. For effective clinical trials on new drugs, suitable surrogate markers to monitor disease progression are required. The extent of cerebral inflammation could be such a surrogate marker. Nuclear imaging techniques, like positron emission tomography (PET) and single photon emission computed tomography (SPECT), have been applied to monitor inflammatory processes in patients. Neuroinflammation is accompanied by a variety of physiological changes, such as changes in cerebral glucose metabolism and perfusion, cyclooxygenase-2 overexpression and microglia activation. Nuclear imaging has utilized these physiological changes to visualize the inflammatory process in various chronic or acute neurodegenerative diseases. Expression of the peripheral benzodiazepine receptor in activated microglia proved a suitable specific marker to detect neuroinflammation. Currently, radiolabeled COX-2 inhibitors are under investigation for this purpose. The causative of neuroinflammation is often unknown, but the herpes simplex virus (HSV), for example, has been implicated in several neurodegenerative diseases. Recently, antiviral agents and antibiotics have been prepared that might be applicable to discriminate specific viral or bacterial infections. These radiolabeled compounds could also be used to monitor the drug pharmacokinetics noninvasively with PET. This review summarizes the progress that has been made in nuclear imaging of neuroinflammation in,neuropsychiatric diseases.     

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.     

Positron emission tomography for use in microdosing studies

Positron emission tomography (PET) imaging using microdoses of radiolabeled drug tracers is gaining increasing acceptance in modern clinical drug development. This approach is unique in that it allows for direct quantitative assessment of drug concentrations in the tissues targeted for treatment, thereby bridging the gap between pharmacokinetics and pharmacodynamics. Current applications of PET in anticancer, anti-infective and central nervous system drug research are reviewed herein. Situated at the interface of preclinical and clinical drug testing, PET microdosing is a powerful and highly innovative tool for pharmaceutical development.     

Radionuclide imaging of liposomal drug delivery

ABSTRACT

Introduction: Ever since their discovery, liposomes have been radiolabeled to monitor their fate in vivo. Despite extensive preclinical studies, only a limited number of radiolabeled liposomal formulations have been examined in patients. Since they can play a crucial role in patient management, it is of importance to enable translation of radiolabeled liposomes into the clinic.

Areas covered: Liposomes have demonstrated substantial advantages as drug delivery systems and can be efficiently radiolabeled. Potentially, radiolabeled drug-loaded liposomes form an elegant theranostic system, which can be tracked in vivo using single-photon emission computed tomography (SPECT) or positron emission tomography (PET) imaging. In this review, we discuss important aspects of liposomal research with a focus on the use of radiolabeled liposomes and their potential role in drug delivery and monitoring therapeutic effects.

Expert opinion: Radiolabeled drug-loaded liposomes have been poorly investigated in patients and no radiolabeled liposomes have been approved for use in clinical practice. Evaluation of the risks, pharmacokinetics, pharmacodynamics and toxicity is necessary to meet pharmaceutical and commercial requirements. It remains to be demonstrated whether the results found in animal studies translate to humans before radiolabeled liposomes can be implemented into clinical practice.     

Non-invasive radiotracer imaging as a tool for drug development

Non-Invasive Radiotracer Imaging (NIRI) uses either short-lived positron-emitting isotopes, such as 11C and 18F, for Positron Emis ion Tomography (PET) or single photon emitting nuclides, e.g., 123I, which provide images using planar imaging or Single-Photon Emission Computed Tomography (SPECT). These high-resolution imaging modalities provide anatomical distribution and localization of radiolabeled drugs, which can be used to generate real time receptor occupancy and off-rate studies in humans. This can be accomplished by either isotopically labeling a potential new drug (usually with 11C), or indirectly by studying how the unlabelled drug inhibits specific radioligand binding in vivo. Competitive blockade studies can be accomplished using a radiolabeled analogue which binds to the site of interest, rather than a radiolabeled version of the potential drug. Imaging, particularly PET imaging, can be used to demonstrate the effect of a drug through a biochemical marker of processes such as glucose metabolism or blood flow. NIRI as a development tool in the pharmaceutical industry is gaining increased acceptance as its unique ability to provide such critical information in human subjects is recognized. This section will review recent examples that illustrate the utility of NIRI, principally PET, in drug development, and the potential of imaging advances in the development of cancer drugs and gene therapy. Finally, we provide a brief overview of the design of new radiotracers for novel targets.     

Manganese in PET imaging: Opportunities and challenges

Several radionuclides of the transition metal manganese are known and accessible. Three of them, ⁵¹Mn, ^52mMn, and ^52gMn, are positron emitters that are potentially interesting for positron emission tomography (PET) applications and, thus, have caught the interest of the radiochemical/radiopharmaceutical and nuclear medicine communities. This mini‐review provides an overview of the production routes and physical properties of these radionuclides. For medical imaging, the focus is on the longer‐living ^52gMn and its application for the radiolabelling of molecules and other entities exhibiting long biological half‐lives, the imaging of manganese‐dependent biological processes, and the development of bimodal PET/magnetic resonance imaging (MRI) probes in combination with paramagnetic ^natMn as a contrast agent.     

Using positron emission tomography to facilitate CNS drug development

Positron emission tomography (PET) is a non-invasive technology of nuclear medicine that has sensitivity for tracing low picomolar concentrations of radiolabeled molecules in the human body. Radiolabeling a new drug to high specific radioactivity facilitates a detailed mapping of its distribution to crucial organs in humans after the administration of a "microdose" (< 1 microg), for which limited toxicology documentation is required. For drugs directed at the CNS, this method is particularly useful for confirming exposure to the brain. A different approach is to develop suitable radioligands for quantitative PET studies of drug binding to target proteins and subsequently to correlate receptor occupancy with pharmacodynamic responses. To follow disease progression and to monitor the outcome of new treatments, PEt also facilitates longitudinal studies of biomarkers of pathophysiology such as amyloid plaque load in Alzheimer's disease. Finally, combining genomic knowledge with PET neuroreceptor imaging is expected to facilitate the search for genetic predictors of drug response.     

The role of positron emission tomography in the discovery and development of new drugs; as studied in laboratory animals.

Drug discovery and development is time consuming and a costly procedure. The challenges for the pharmaceutical industry range from the evaluation of potential new drug candidates, the determination of drug pharmacokinetics/pharmacodynamics, the measurement of receptor occupancy as a determinant of drug efficacy, and the pharmacological characterisation of mechanisms of action. Positron emission tomography (PET) is a powerful quantitative imaging technique for looking at biochemical pathways, molecular interactions, drug pharmacokinetics and pharmacodynamics. Recent advances in emission tomography, particularly the development of small animal PET scanners, image reconstruction and animal models of disease have led to the development of extremely sensitive and specific tools for imaging biochemical processes in vivo, therefore representing a new means of providing information for drug development and evaluation. Many human genes have a related mouse gene, allowing mice to be used as a platform for mimicking human disease, using knock-out and knock-in gene technology. Consequently PET imaging of rodents is emerging as a cost effective means of screening new pharmaceuticals and decreasing the time required for new drug development.     

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.     

Spectrum of radiopharmaceuticals in nuclear oncology

A major field of interest in nuclear medicine is in vivo tumor characterization and measurement of biological processes at cellular and molecular levels by means of positron emission tomography (PET) or single photon emission computed tomography (SPECT). Functional imaging with radiopharmaceuticals represents a useful noninvasive tool to evaluate the biological status of the tumor and its progression. The properties of radiopharmaceuticals are exploited for initial staging of cancer, assessment of recurrent or residual disease and, more recently, considerable progress has been made in the field of the evaluation of tumor response to treatment. PET and SPECT can both detect changes in tumor activity caused by therapy or disease progression before any detectable change in tumor volume. Measurement of tumor response to therapy using PET and SPECT is the subject of intense investigations because it may result in individualization of treatment and may have a prognostic value for long-term outcome. This review focuses on the various methods used to monitor anticancer therapy with a variety of clinically approved or investigational tracers. We summarize the mechanisms of radiopharmaceutical uptake based on certain physiological activities affected by treatment: proliferation, apoptosis, hypoxia, angiogenesis and multidrug resistance (MDR).