In vivo labelling of granulocytes using 123I-tagged anti-granulocyte antibodies

On the basis of previous work with various monoclonal antibodies (Mab) raised against carcino-embryonic antigen (CEA), the anti-CEA Mab 47 was identified which selectively reacted with a surface glycoprotein (95 kDa; NCA 95) of normal human granulocytes. This new tracer was quality tested and radioiodinated with 123I (123I Mab 47) for clinical use according to established procedures. Extended in vitro studies revealed a high selectivity for granulocytes without inhibiting their vital functions. In vivo cell binding to the granulocyte pool was completed very rapidly and remained unchanged over 24 h. For clinical use one dose consisting of 120 mcg of Mab was labelled with 4-5 mCi of 123I. Clinical interest was mainly concentrated on cases of osteomyelitis, infected allografts and abdominal and brain abscesses. After injection of 123I Mab 47, infectious lesions were usually seen after 3-5 h or could be excluded after 24 h. Because of high counting rates the image quality was excellent and single photon emission computerized tomography (SPECT) could be performed for an exact topographical localization of the lesions. No adverse reactions have been seen. It is concluded that there are distinct advantages of the new method compared with scanning of 111In-labelled leucocytes. However, despite this and the low dose of antibodies administered, we recommend restriction of immunoscintigraphy of infectious lesions before a clinically relevant immunization can be excluded.     

The Advantages and Challenges of Using FDG PET/CT for Response Assessment in Melanoma in the Era of Targeted Agents and Immunotherapy

The treatment of melanoma has been revolutionised in recent years by advances in the understanding of the genomic landscape of this disease, which has led to the development of new targeted therapeutic agents, and the ability to therapeutically manipulate the immune system through inhibition of cancer cell-T-cell interactions that prevent an adaptive immune response. While these therapeutic interventions have dramatically improved the prospects of survival for patients with advanced melanoma, they bring significant complexity to the interpretation of therapeutic response because their mechanisms and temporal profile of response vary considerably. In this review, we discuss the mode of action of these emerging therapies and their toxicities to provide a framework for the use of FDG PET/CT in therapeutic response assessment. We propose that the greatest utility of PET in assessment of response to agents that abrogate signalling related to BRAF mutation is for early assessment of resistance, while in anti-CTLA4 therapy, immunological flare can compromise early assessment of response but can identify potentially life-threatening autoimmune reactions. For anti-PD1/PDL1 therapy, the role of FDG PET/CT is more akin to its use in other solid malignancies undergoing treatment with conventional chemotherapy. However, further research is required to optimise the timing of scans and response criteria in this disease.     

PET tracers based on (86)Y

Positron emission tomography (PET) has become a powerful tool for probing biochemical processes in living subjects. PET imaging depends largely on the development of novel PET tracers labeled with positron-emitting radionuclides. Since the four traditional PET isotopes (18F, 11C, 13N, and 15O) are produced in a cyclotron and are short-lived, their use for long-term observation of biological processes in vivo is limited. In the last decades, extensive research in the development of other unconventional radionuclides (such as 64Cu, 68Ga, 89Zr, 86Y, and 124I) labeled tracers with half-lives complementary to the biological properties of their targeting agents has been conducted. Among these tracers, 86Y-based PET tracers have gained increasing attention since they are ideal surrogates for in vivo determination of biodistribution and dosimetry of therapeutic 90Y (pure β - emitter) pharmaceuticals. In this review article, we will brief introduce the physical characteristics, production, and radiochemistry of 86Y, and will summarize the current 86Y-based PET tracers used for molecular imaging and cancer detection in animal studies and in clinical trials.     

Molecular Imaging of Biological Gene Delivery Vehicles for Targeted Cancer Therapy: Beyond Viral Vectors

Cancer persists as one of the most devastating diseases in the world. Problems including metastasis and tumor resistance to chemotherapy and radiotherapy have seriously limited the therapeutic effects of present clinical treatments. To overcome these limitations, cancer gene therapy has been developed over the last two decades for a broad spectrum of applications, from gene replacement and knockdown to vaccination, each with different requirements for gene delivery. So far, a number of genes and delivery vectors have been investigated, and significant progress has been made with several gene therapy modalities in clinical trials. Viral vectors and synthetic liposomes have emerged as the vehicles of choice for many applications. However, both have limitations and risks that restrict gene therapy applications, including the complexity of production, limited packaging capacity, and unfavorable immunological features. While continuing to improve these vectors, it is important to investigate other options, particularly nonviral biological agents such as bacteria, bacteriophages, and bacteria-like particles. Recently, many molecular imaging techniques for safe, repeated, and high-resolution in vivo imaging of gene expression have been employed to assess vector-mediated gene expression in living subjects. In this review, molecular imaging techniques for monitoring biological gene delivery vehicles are described, and the specific use of these methods at different steps is illustrated. Linking molecular imaging to gene therapy will eventually help to develop novel gene delivery vehicles for preclinical study and support the development of future human applications.     

Liposomes containing paramagnetic macromolecules as MRI contrast agents

The use of paramagnetic ions, bound to macromolecules and trapped in liposomes, as MRI contrast agents is suggested. As an example, the system of Mn2+ bound to serum proteins was tested. The binding of the metal ions to the macromolecules enhances their relaxation and at the same time decreases their rate of diffusion out of the liposomes. The use of liposomes is expected to reduce the potential toxicity of the paramagnetic ions and allows their targeting toward specific tissues.     

Local delivery of magnetic resonance (MR) contrast agent in kidney using thermosensitive liposomes and MR imaging-guided local hyperthermia: a feasibility study in vivo

PURPOSE: To investigate the feasibility of local delivery of a magnetic resonance (MR) contrast agent in vivo using paramagnetic thermosensitive liposomes and infrared (IR) laser-induced local hyperthermia under real-time MR thermometry on rabbit kidney. MATERIALS AND METHODS: Respiratory gated, radio frequency (RF)-spoiled gradient-echo sequences were used for precise MR temperature mapping (SD = 1 degrees C). In vivo heating experiments confirmed local release of MR contrast agent from liposomes. RESULTS: T1 decreased from 800 msec to about 500 msec, as measured after tissue cooling, in those locations where the renal parenchyma was heated above the phase transition temperature of the liposome membrane. CONCLUSION: The release of MR contrast agent has been demonstrated in rabbit kidney in vivo. This may be used as a reporter for simultaneous release of therapeutic agents.     

Use of (99m)Tc-labeled liposomes encapsulating blue dye for identification of the sentinel lymph node.

Colloidal radiopharmaceuticals are commonly used in combination with blue dye for localization of the sentinel node. Liposomes are colloidal particles composed of spontaneously forming lipid spheres that can carry a wide variety of diagnostic and therapeutic agents. Conventional liposomes are poorly retained in lymph nodes (<2% of the subcutaneously injected dose). We have previously described a system for increasing the retention of liposomes in the lymph nodes by approximately 7-fold. This system is comprised of subcutaneously injected biotin-coated liposomes, followed by an adjacent injection of avidin. When the avidin moves into the lymphatic vessels, it causes aggregation of the biotin-coated liposomes that are also in the process of migrating through the lymphatic vessels. These aggregated liposomes become entrapped in the next encountered lymph node. In this study, we use this novel lymph node delivery system with liposomes that encapsulate blue dye, resulting in intense blue staining of the sentinel node. These liposomes can also be labeled with (99m)Tc, permitting scintigraphic imaging and radioguided probe localization of the sentinel node. METHODS: Liposomes coated with biotin and coencapsulating blue dye and glutathione were labeled with (99m)Tc using hexamethylpropyleneamine oxime. Rabbits were subcutaneously administered 0.3 mL (99m)Tc-biotin-liposomes containing blue dye in both hind feet, followed by a subcutaneous injection (0.3 mL) of 5 mg avidin in only one hind foot (experimental). The other hind foot served as a control. RESULTS: Labeling efficiencies (mean +/- SEM) for liposomes encapsulating blue dye were 92.1% +/- 1.9%. Necropsy at 24 h revealed that the popliteal node on the experimental leg receiving the avidin was intensely blue stained compared with virtually no blue coloration of the control node. Tissue counts of these nodes were 12.2 +/- 1.5 percentage injected dose (%ID) in the experimental node compared with 1.2 +/- 0.1 %ID in the control nodes (P<0.0001). CONCLUSION: Biotin-liposomes encapsulating blue dye can be successfully labeled with (99m)Tc, providing a convenient option for the visualization and radiolocalization of the sentinel node. This biotin-liposome/avidin system may also have potential for the delivery of therapeutic drugs and radiopharmaceuticals to lymph nodes.     

Impact of amyloid imaging on drug development in Alzheimer's disease

Imaging agents capable of assessing amyloid-beta (Abeta) content in vivo in the brains of Alzheimer's disease (AD) subjects likely will be important as diagnostic agents to detect Abeta plaques in the brain as well as to help test the amyloid cascade hypothesis of AD and as an aid to assess the efficacy of anti-amyloid therapeutics currently under development and in clinical trials. Positron emission tomography (PET) imaging studies of amyloid deposition in human subjects with several Abeta imaging agents are currently underway. We reported the first PET studies of the carbon 11-labeled thioflavin-T derivative Pittsburgh Compound B in 2004, and this work has subsequently been extended to include a variety of subject groups, including AD patients, mild cognitive impairment patients and healthy controls. The ability to quantify regional Abeta plaque load in the brains of living human subjects has provided a means to begin to apply this technology as a diagnostic agent to detect regional concentrations of Abeta plaques and as a surrogate marker of therapeutic efficacy in anti-amyloid drug trials.     

Gadolinium-DTPA liposomes as a potential MRI contrast agent. Work in progress

To evaluate the use of liposomes containing Gadolinium-DTPA (Gd-DTPA) as potential intravascular contrast agents, we synthesized and tested Gd-DTPA liposomes. A freeze-thaw extrusion process was used to synthesize neutral unilamellar vesicles. Using this technique, we prepared 0.4 micron vesicles with encapsulation efficiency as high as 39% for Gd-DTPA. In vitro dialysis showed that essentially 100% of the Gd-DTPA was retained with the liposomes after 72 hours of dialysis. MR imaging of in vitro samples showed concentration-dependent increase in signal intensity with Gd-DTPA liposomes. Imaging of rats after intravenous injection of Gd-DTPA liposomes showed sustained intravascular contrast enhancement of vascular structures and liver greater than free Gd-DTPA. There was no evidence of acute toxicity in rats during the imaging experiments or on follow-up of two months. Paramagnetic liposomes may be useful to enhance the vasculature, liver, and spleen.     

Biodistribution and clearance of liposomal gadolinium-DTPA

Gadolinium-DTPA (Gd-DTPA) liposomes have been studied previously as liver contrast agents and have been shown to improve the detection of hepatic metastases in rats. We synthesized 100-nm and 50-nm liposomes that encapsulated Gd-DTPA and did biodistribution and clearance studies in rats. Parallel magnetic resonance imaging (MRI) studies were also done. Biodistribution showed a prolonged blood pool phase for Gd-DTPA liposomes with a blood pool half-life of approximately 4 hours for the 100-nm liposomes. The highest uptake per gram of tissue was achieved by the spleen. Clearance of gadolinium from the liver and spleen showed a half-life of 3 to 4 days. The smaller 50-nm Gd-DTPA liposomes resulted in a longer blood pool phase and a higher delivery of gadolinium to the liver, bone marrow, and spleen. Imaging studies after intravenous (IV) administration of liposomal Gd-DTPA showed organ enhancement that paralleled the data on biodistribution studies, with appreciable hepatic enhancement at doses as low as 0.025 mm/kg of liposomal Gd-DTPA.     

The role of positron emission tomography within the spectrum of medical imaging

The aim of the survey is to help to define the role of positron emission tomography (PET) as an investigative procedure, and to identify strategies to perfect PET and in vivo tracer studies in general so that their full potential is realised. Within this context, the sensitivity and specificity of PET are reviewed. The impact of PET through studies of regional cerebral activation based on blood flow measurements is discussed. Emphasis is placed on the role of PET in clinical research and as a means of assessing new therapeutic agents. This latter ability renders PET of much interest to pharmaceutical companies.     

RGD-targeted paramagnetic liposomes for early detection of tumor: in vitro and in vivo studies

Magnetic resonance molecular imaging has emerged as a potential approach for tumor diagnosis in the last few decades. This approach consists of the delivery of MR contrast agents to the tumor by specific targeted carriers. For this purpose, a lipopeptide was constructed by using a cyclic RGD peptide headgroup coupled to palmitic acid anchors via a KGG tripeptide spacer. Targeted paramagnetic liposomes were then prepared by the incorporation of RGD-coupled-lipopeptides into lipid bilayers for specific bounding to tumor. In vitro, study demonstrated that RGD-targeted liposomes exhibited a better binding affinity to targeted cells than non-targeted liposomes. MR imaging of mice bearing A549 tumors with the RGD-targeted paramagnetic liposomes also resulted in a greater signal enhancement of tumor compared to non-targeted liposomes and pure contrast agents groups. In addition, biodistribution study also showed specific tumor targeting of RGD-targeted paramagnetic liposomes in vivo. Therefore, RGD-targeted paramagnetic liposomes prepared in the present study may be a more promising method for early tumor diagnosis.     

Nano/microparticles and ultrasound contrast agents

Microbubbles have been used for many years now in clinical practice as contrast agents in ultrasound imaging. Recently, their therapeutic applications have also attracted more attention. However, the short circulation time (minutes) and relatively large size (two to ten micrometers) of currently used commercial microbubbles do not allow effective extravasation into tumor tissue, preventing efficient tumor targeting. Fortunately, more multifunctional and theranostic nanoparticles with some special advantages over the traditional microbubbles have been widely investigated and explored for biomedical applications. The way to synthesize an ideal ultrasound contrast agent based on nanoparticles in order to achieve an expected effect on contrast imaging is a key technique. Currently a number of nanomaterials, including liposomes, polymers, micelles, dendrimers, emulsions, quantum dots, solid nanoparticles etc., have already been applied to pre or clinical trials. Multifunctional and theranostic nanoparticles with some special advantages, such as the tumor-targeted (passive or active), multi-mode contrast agents (magnetic resonance imaging, ultrasonography or fluorescence), carrier or enhancer of drug delivery, and combined chemo or thermal therapy etc., are rapidly gaining popularity and have shown a promising application in the field of cancer treatment. In this mini review, the trends and the advances of multifunctional and theranostic nanoparticles are briefly discussed.     

Persistent contrast enhancement by sterically stabilized paramagnetic liposomes in murine melanoma.

In the present research, we investigated the use of paramagnetic liposomes as contrast agents (CAs) for the detection of solid tumors. The liposomes were sterically stabilized by a polyethylene glycol (PEG) coating, and their size was constrained to approximately 100 nm. Dimyristoyl-sn-glycero-3-phosphoethanolamine-N-diethylene-triaminepentaacetate (DMPE-DTPA) was used as the gadolinium-carrying fatty acid chain. The relaxation properties were characterized through nuclear magnetic relaxation dispersion (NMRD) measurements, and analyzed with the use of theories and computer programs that are adequate for slowly rotating systems. Their relaxivity at 1.5 T was found to be acceptable for in vivo use. We then tested the liposomes against B16-F10 murine melanomas using standard T1-weighted schemes at 1.5 T, and concentrations corresponding to 0.03 mmol/kg of gadolinium (i.e., three to six times lower than the concentration of the small gadolinium complexes in clinical use). The blood half-life was found to be 120 +/- 20 min. The experiments show a good contrast enhancement in the tumor (33% +/- 22%) 2 hr after administration, a further increase (43 +/- 27%) 20 hr after administration, and a decrease (25% +/- 14%) 54 hr after administration. High persistence of the CA was also observed in the liver and intestine, as expected in a hepatobiliar excretion pathway.     

Imaging devices for use in small animals

Imaging devices for small animals have emerged in the past 10 years as extraordinarily useful tools in translational research and drug development. The Food and Drug Administration requires animal testing after in vitro drug discovery but before human application. Many small animal instruments have been developed in analogy to human scale devices, including positron emission tomography, single-photon emission computed tomography, computed tomography, magnetic resonance imaging, and ultrasound. Conversely, optical imaging with fluorescent and bioluminescent tracer technology, originating in single-cell in vitro studies, has been scaled up to whole-body animal imaging. Imaging that uses multiple devices permits a comparison of different aspects of function, anatomy, gene expression, and phenotype by the use of software algorithms or more recently with hybrid instruments. Animal imaging facilitates "bench-to-bedside" drug development in 2 ways. Longitudinal imaging improves the science of animal research through the benefit of paired statistics with the use of animals as their own controls while it simultaneously reduces animal sacrifice. In addition, imaging makes explicit the development of diagnostic and therapeutic agents on nearly identical molecular synthesis platforms, therefore linking drug discovery to the development of imaging tracers. This powerful paradigm, now known as diagnostic/therapeutic pairing or theranostics, is already familiar from the use of (123)I used for thyroid diagnosis and (131)I for therapy of benign and malignant thyroid conditions. Many newer examples exist, such as "cold" or "hot" octreotide and meta-iodobenzylguanidine in neuroendocrine tumors; and rituximab in pharmaceutical doses, or with beta emitter tags, for therapy of indolent non-Hodgkin's lymphoma. Theranostic agents are also rapidly emerging that use nanoparticles, aptamers, peptides, and antibodies for magnetic resonance imaging/positron emission tomography/single-photo emission computed tomography/computed tomography imaging devices in animals with subsequent therapeutic drug development for translation to human use.     

Current trends in infection imaging

Infection imaging is a field of intense interest in medicine because, despite the great advances in our understanding of microorganisms and the development of new antimicrobial agents, infection remains a major cause of patient morbidity and mortality. Nuclear Medicine for many years has an important role in this field. The significance of this role varies in different clinical situations, in some cases being complementary to other imaging modalities as in postoperative patients, and in some cases being the method of choice as in orthopaedic infections after implants have been placed. Today there are many agents in Nuclear Medicine capable in imaging infection. Each of them has its own advantages and drawbacks. In recent years there is an increased research in the field with new methods tested. Among them are labelled antibacterial agents, labelled antimicrobial peptides, monoclonal antibodies for leukocyte labelling, labelled liposomes, and (18)F-FDG-PET. In this short article there is an effort to briefly review these agents and their possible role in infection imaging.     

Gadolinium-labeled liposomes containing paramagnetic amphipathic agents: targeted MRI contrast agents for the liver

Unique paramagnetic liposomal contrast agents were synthesized and utilized for selective augmentation of T1 MR imaging of the livers of normal Balb/c mice. Amphipathic gadolinium complexes, which mimic phospholipids, were incorporated into the lamella of small unilamellar liposomes (SUV) such that they became an integral part of its surface. T1 signal enhancement of normal liver approached 150% after iv administration of the paramagnetic liposomes, determined by experiments performed on a 1.9-T, experimental whole-body MRI unit. Tracer studies utilizing gadolinium-153-tagged SUV revealed that the agents exhibited excellent in vivo stability, compared to liposomal preparations in which paramagnetic agents are simply entrapped in the aqueous core of the liposome vesicle.     

Applications of nanoparticles for diagnosis and therapy of cancer

During the last decades, a plethora of nanoparticles have been developed and evaluated and a real hype has been created around their potential application as diagnostic and therapeutic agents. Despite their suggestion as potential diagnostic agents, only a single diagnostic nanoparticle formulation, namely iron oxide nanoparticles, has found its way into clinical routine so far. This fact is primarily due to difficulties in achieving appropriate pharmacokinetic properties and a reproducible synthesis of monodispersed nanoparticles. Furthermore, concerns exist about their biodegradation, elimination and toxicity. The majority of nanoparticle formulations that are currently routinely used in the clinic are used for therapeutic purposes. These therapeutic nanoparticles aim to more efficiently deliver a (chemo-) therapeutic drug to the pathological site, while avoiding its accumulation in healthy organs and tissues, and are predominantly based on the “enhanced permeability and retention” (EPR) effect. Furthermore, based on their ability to integrate diagnostic and therapeutic entities within a single nanoparticle formulation, nanoparticles hold great promise for theranostic purposes and are considered to be highly useful for personalizing nanomedicine-based treatments. In this review article, we present applications of diagnostic and therapeutic nanoparticles, summarize frequently used non-invasive imaging techniques and describe the role of EPR in the accumulation of nanotheranostic formulations. In this context, the clinical potential of nanotheranostics and image-guided drug delivery for individualized and improved (chemo-) therapeutic interventions is addressed.Currently, in vivo molecular imaging comprises an important focus area of medical research. The rapidly evolving field of molecular imaging improves early disease detection and disease staging and enables image-guided therapy and treatment personalization. Furthermore, it provides essential information on the therapy efficacy. However, molecular imaging requires the use of molecular imaging probes to visualize and characterize biological processes at the cellular and molecular level.^1–5Recent advances in nanotechnology have led to the development of various nanoparticle formulations for diagnostic and therapeutic applications. Diagnostic nanoparticles aim to visualize pathologies and to improve the understanding of important (patho-) physiological principles of various diseases and disease treatments. Clinically, however, nanodiagnostics are only useful in a limited number of situations, due to the complex demands on their pharmacokinetic properties and elimination. Therefore, the majority of nanoparticle formulations currently used in the clinics is applied for therapeutic purposes. Therapeutic nanoparticles aim to improve the accumulation and release of pharmacologically active agents at the pathological site, increase therapeutic efficacy and reduce the incidence and intensity of side effects by reducing their localization in healthy tissues.^6–9 The intrinsic characteristics of nanoparticles hold great promise for integrating diagnostic and therapeutic agents into a single nanoparticle formulation, enabling their application for theranostic purposes, such as monitoring the biodistribution and target site accumulation, visualizing and quantifying drug release and longitudinally assessing the therapeutic efficacy. Such theranostic nanoparticles may be used for personalizing nanomedicine-based therapies by enabling patient preselection and by controlling therapeutic efficacy.^7–15In this review article, indications of current nanoparticle formulations for diagnostic and therapeutic applications and a brief overview of non-invasive imaging modalities will be given. In addition, the suitability of nanoparticles as molecular imaging probes and contrast agents to enhance disease diagnosis and treatment and their potential clinical application to facilitate personalized therapy interventions will be addressed and discussed.     

Long-circulating liposomal contrast agents for magnetic resonance imaging.

Contrast-enhanced magnetic resonance imaging (CE-MRI) is a dynamic technique for imaging vasculature. However, the currently used gadolinium (Gd) chelates, such as Gd-DTPA, restrict the time window for image acquisition due to their rapid elimination from blood and their rapid diffusion into the extravascular space, which prevents their use in steady-state imaging, particularly for MR angiography (MRA). The goal of this study was to prepare long-circulating polyethylene glycol-bearing ((PEG)ylated) liposomes encapsulating Gd chelate, and characterize and demonstrate their utility for MRA. The liposomes were prepared by hydrating a mixture of lipids with gadodiamide (Omniscan). The liposomes were sized down to around 100 nm by extruder and exhaustively dialysed to remove the unencapsulated gadodiamide. The Gd liposomes exhibited a significant sustained (>4 hr) contrast enhancement of the vasculature with improved spatial details in a rat model with little leakage relative to Gd-DTPA controls as shown by MRI. We suggest that such long-circulating liposomal formulations allow for high spatial resolution imaging without the confounding effects of clearance and extravascular diffusion of the agent complicating the data and image analysis.