Current Pharmaceutical Biotechnology - Volume 13, Issue 4, 2012

During recent years there has been tremendous progress in understanding the molecular mechanisms of diseases and many new therapeutics has been developed that directly interfere with molecular pathways. The use of these disease-specific treatments can improve therapy outcome and significantly decrease side effects, which leads to a better life quality of the patient. On the other hand, there is increasing risk of exposing the patient to a non optimal therapy and also resistance to the treatment may develop more rapidly, which requires adaptation of the therapy regimen. As a consequence molecular treatments should be individualized and tools should be developed allowing to track the disease course at pathophysiological and molecular level. The aim of this special issue is to make the reader aware of different disease specific and theranostic imaging concepts that may gain increasing importance in clinical routine in future. The first two articles introduce optical [1] and photoacoustic imaging [2] as new molecular imaging techniques that currently are translated to the clinics. Their physical principles, current use, and potential clinical indications are discussed in detail. The next two articles continue the discussion about optical imaging and summarize findings on new enzymatically activatable optical probes [3] as well as strategies to track genetically modified cells expressing fluorescent proteins [4]. In the article of Slabu and colleagues a new concept of labelling stent material with iron oxide nanoparticles is presented, which may help to improve stent placement in interventional MRI procedures and to control the correct stent localization and its interaction with the neighboured tissue [5]. The articles from Gaertner [6] and De Saint-Hubert [7] review the current strategies of imaging hypoxia and apoptosis in tumors with nuclear imaging. The reliable assessment and monitoring of these two characteristics can significantly help to better understand tumor response to treatment and to improve individualization of anti-neoplastic therapy. This is particularly true for radiation treatment, where tumor resistance often is associated with hypoxia and decreased apoptosis rate. The following articles focus on monitoring tumor response to systemic treatments: Bone metastases occur in many tumor diseases and to date their imaging is mostly restricted to quantifying lesion size. However, lesion size has shown to be an uncertain measure for predicting and monitoring therapy response. Therefore, new functional and molecular imaging strategies to monitor bone metastases response to antiangiogenic and other tumor therapies are discussed in the article of Bauerle et al. [8]. Not only for bone metastases formation but also for many other solid tumors angiogenesis is one of the key processes for invasive and systemic tumor growth. Several antiangiogenic substances have already entered clinical use and others are currently evaluated in clinical trials. How tumor angiogenesis can be characterized and how antiangiogenic therapy effects can be assessed by multimodal functional and molecular imaging, the reader will find in the article of Lederle and colleagues [9]. For antiangiogenic drugs as well as for every other systemically applied anti-tumor agent optimal delivery to the tumor tissue and considerably low accumulation in healthy tissues are important preconditions for a successful therapy outcome. Here polymeric and other nano-carriers have shown to be capable of improving therapy efficacy. In this context, Kunjachan and coworkers review the use of imaging for monitoring drug release from polymeric and other carriers [10]. In summary, it is the idea of this issue to inspire the reader to use novel non invasive (molecular) imaging tools to improve and to individualize therapy concepts. Please also note that disease-specific and theranostic imaging can improve molecular therapy in more applications than can be covered in this issue and that even more indications still have to be discovered. REFERENCES [1] Keereweer, S.; Hutteman, M.; Kerrebijn, J.D.F.; van de Velde, C.J.H.; Vahrmeijer, A.L.; Lowik, C.W.G.M. Translational optical imaging in diagnosis and treatment of cancer. Curr. Pharm. Bioetechnol., 2012, 13 (3), 498-503. [2] Razansky, D.; Deliolanis, N.; Vinegoni, C.; Ntziachristos, V. Deep Tissue Optical and Optoacoustic Molecular Imaging - technologies for small animal research and drug discovery. Curr. Pharm. Bioetechnol., 2012, 13 (3), 504-522. [3] Thorek, D.L.J.; Grimm, J. Enzymatically Activatable Diagnostic Probes. Curr. Pharm. Bioetechnol., 2012, 13 (3), 523-536. [4] Hoffman, R.M. Cellular and subcellular imaging in live mice using fluorescent proteins. Curr. Pharm. Bioetechnol., 2012, 13(3), 537-544. [5] Slabu, I.; Guntherodt, G.; Schmitz-Rode, T.; Hodenius, M.; Kramer, N.; Donker, H.; Krombach, G.A.; Otto, J.; Klinge, U.; Baumann, M. Investigation of superparamagnetic iron oxide nanoparticles for MR-visualization of surgical implants. Curr. Pharm. Bioetechnol., 2012, 13(3), 454-551. [6] Gaertner, F.C.; Souvatzoglou, M.; Brix, G.; Beer, A.J. Imaging of hypoxia using PET and MRI. Curr. Pharm. Bioetechnol., 2012, 13 (3), 552-570. [7] De Saint-Hubert, M.; Bauwens M, Verbruggen A, Mottaghy FM. Apoptosis imaging to monitor cancer therapy: The road to fast treatment evaluation? Curr. Pharm. Bioetechnol., 2012, 13 (3), 571-583. [8] Bauerle T, Komljenovic D, Semmler W. Monitoring molecular, functional and morphologic aspects of bone metastases using non-invasive imaging. Curr. Pharm. Bioetechnol., 2012, 13 (3), 584-594. [9] Lederle W, Palmowski M, Kiessling F. Monitoring of anti-angiogenic treatments. Curr. Pharm. Bioetechnol., 2012, 13(3), 595-608. [10] Kunjachan S, Jabadurai J, Mertens ME, Storm G, Kiessling F, Lammers T. Theranostic systems and strategies for monitoring nanomedicine-mediated drug targeting. Curr. Pharm. Bioetechnol., 2012, 13(3), 609-622.

In cancer imaging, many different modalities are used that each have their specific features, leading to the combined use of different techniques for the detection, staging and treatment evaluation of cancer. Optical imaging using near-infrared fluorescence light is a new imaging modality that has recently emerged in the field of cancer imaging. After extensive preclinical research, the first steps of translation to the clinical practice are currently being made. In this article, we discuss the preclinical and clinical results of near-infrared optical imaging for non-invasive detection and classification of tumors, therapy monitoring, sentinel lymph node procedures, and image-guided cancer surgery. Widespread availability of imaging systems and optical contrast agents will enable larger studies on their clinical benefit and can help establish a definitive role in clinical practice.

For centuries, biological discoveries were based on optical imaging, in particular microscopy but also several chromophoric assays and photographic approaches. With the recent emergence of methods appropriate for bio-marker in vivo staining, such as bioluminescence, fluorescent molecular probes and proteins, as well as nanoparticle-based targeted agents, significant attention has been shifted toward in vivo interrogations of different dynamic biological processes at the molecular level. This progress has been largely supported by the development of advanced tomographic imaging technologies suitable for obtaining volumetric visualization of bio-marker distributions in small animals at a whole-body or whole-organ scale, an imaging frontier that is not accessible by the existing tissue-sectioning microscopic techniques due to intensive light scattering beyond the depth of a few hundred microns. Major examples of such recently developed optical imaging modalities are reviewed here, including bioluminescence tomography (BLT), fluorescence molecular tomography (FMT), and optical projection tomography (OPT). The pharmaceutical imaging community has quickly appropriated itself of these novel forms of optical imaging, since they come with very compelling advantages, such as quantitative three-dimensional capabilities, direct correlation to the biological cultures, easiness and cost-effectiveness of use, and the use of safe non-ionizing radiation. Some multi-modality approaches, combining light with other imaging modalities such as X-Ray CT or MRI, giving the ability to acquire both an optical contrast reconstruction along with a hi-fidelity anatomical images, are also reviewed. A separate section is devoted to the hybrid imaging techniques based on the optoacoustic phenomenon, such as multispectral optoacoustic tomography (MSOT), which are poised to leverage the traditional contrast and specificity advantages of optical spectrum by delivering an ever powerful set of capabilities, including real-time operation and high spatial resolution, not affected by the scattering nature of biological tissues.

Molecular imaging of disease development, progression and treatment is seen as key to further advancement in the understanding and triumph over illness. The role of enzymes is to catalyze the biochemical reactions that help regulate health, and when dysregulated in complex organisms lead to or indicate disease. The ability to image the action of these proteins for diagnostic purposes opens a window for the researcher and clinician to witness specifc molecular events in vitro and in vivo. Such probes have been developed and deployed for the optical, radionuclide and magnetic resonance modalities and offer significant benefits over conventional agents. The signal of enzymatically-activated probes is regulated by the specific activity of the desired enzyme. This allows for a higher signal to background ratio over non-specific and targeted agents. It also enables the modulation of contrast agent distribution (and even cellular accumulation) following enzymatic activity. This review summarizes the strategies and probes in development and use in this emergent field of molecular imaging, with a particular focus on the research and medical relevance of these advances.

Fluorescent proteins have revolutionized in vivo biology. Due to their intrinsic brightness, multiple colors, and ease of genetic manipulation, fluorescent proteins have been demonstrated to be the reporters of choice for in vivo imaging. The present report reviews applications of fluorescent proteins for imaging cancer progression, gene expression, angiogenesis, stem cells, bacterial infection, Leishmania infection, and asthma, at the cellular and subcellular level in live mice. With fluorescent-protein-expressing cells and a highly sensitive small animal imaging system, cellular and subcellular dynamics can now be observed in live mice in real time. Such imaging possibilities can provide new visual targets for novel drug therapy. Fluorescent proteins thus enable both micro as well as macro imaging technology and thereby provide the basis for the new field of in vivo cell biology.

For the development of a surgical mesh implant that is visible in magnetic resonance imaging (MRI), superparamagnetic iron oxides (SPIOs) are integrated into the material of the mesh. In order to get a high quality mesh regarding both mechanical and imaging properties, a narrow size distribution and homogenous spatial distribution, as well as a strong magnetization of SPIOs within the filament of the mesh are required. In this work, six different samples of SPIOs composed of a magnetite core are synthesized with and without stabilizing dodecanoic acid and analyzed using a superconducting quantum interference device (SQUID), transmission electron microscope (TEM) and a magnetic force microscope (MFM) to determine the properties that are beneficial for the assembly and imaging of the implant. These analyses show the feasibility of visualization of surgical implants with incorporated SPIOs and the influence of the agglomeration of SPIOs on their magnetization and on a homogenous spatial distribution within the polymer of the mesh.

Tumor hypoxia is the result of an inadequate supply of oxygen to tumor cells which can be caused by multiple factors. It is associated with aggressive local tumor growth and invasion, increased risk of metastasis, higher resistance to radiotherapy (RT) and chemotherapy, overall resulting in a poor clinical prognosis. Many locally advanced solid tumors may exhibit hypoxic and/or anoxic tissue areas that are heterogeneously distributed within the tumor mass. As hypoxia is a negative prognostic factor concerning response to radiotherapy and chemotherapy, in vivo measurement of tumor hypoxia could be helpful to identify patients with worse prognosis or patients that could benefit from appropriate treatments such as intensity modulated radiotherapy (IMRT) that may accurately conform the dose distribution to small intratumoral regions showing differences in the oxygen level. A manifold of different methods to assess the oxygen tension (pO2) in tissues have been developed, each of them offering advantages as well as drawbacks. They range from invasive direct measurement techniques of the pO2 in tissue by using a polarographic electrode, to non-invasive imaging techniques such as positron emission tomography (PET) or magnetic resonance imaging (MRI). This article provides an overview over the various methods, with a particular emphasis on PET and MRI for imaging of hypoxia, and reviews their performance in preclinical and clinical studies.

Molecular imaging of biological processes may allow detection of therapy effects before the tumor is reduced in size. The most frequently used PET tracer in oncology, 2-[18F]fluoro-2-deoxyglucose (FDG), suffers from low specificity due to uptake in inflammatory cells. The proliferation marker, 3'-[18F]fluoro-3'-deoxy-L-thymidine (FLT), is less influenced by the inflammatory response following therapy but here disease- and drug-specific effects need to be considered. Since cancer therapy mainly intends to eliminate cancer cells, imaging of cell death offers a direct way to image therapy response. This review gives an overview of the radiopharmaceutical development and in vivo evaluation of radioligands that have emerged so far for detection and assessment of apoptosis and necrosis. Two radiopharmaceuticals that can image cell death have made it to clinical trials for follow up of tumor treatment: i) 99mTc-and 123I-labelled AnxA5 for the response to treatment of for example lymphoma and lung cancer and ii) 18F-ML10 for the evaluation of brain tumors post-radiation. Other agents need further optimization.

Bone is among the most common locations of metastasis and therefore represents an important clinical target for diagnostic follow-up in cancer patients. In the pathogenesis of bone metastases, disseminated tumor cells proliferating in bone interact with the local microenvironment stimulating or inhibiting osteoclast and osteoblast activity. Non-invasive imaging methods monitor molecular, functional and morphologic changes in both compartments of these skeletal lesions - the bone and the soft tissue tumor compartment. In the bone compartment, morphologic information on skeletal destruction is assessed by computed tomography (CT) and radiography. Pathogenic processes of osteoclast and osteoblast activity, however, can be imaged using optical imaging, positron emission tomography (PET), single photon emission CT (SPECT) and skeletal scintigraphy. Accordingly, conventional magnetic resonance imaging (MRI) and CT as well as diffusion- weighted MRI and optical imaging are used to assess morphologic aspects on the macroscopic and cellular level of the soft tissue tumor compartment. Imaging methods such as PET, MR spectroscopy, dynamic contrast-enhanced techniques and vessel size imaging further elucidate on pathogenic processes in this compartment including information on metabolism and vascularization. By monitoring these aspects in bone lesions, new insights in the pathogenesis of skeletal metastases can be gained. In translation to the clinical situation, these novel methods for the monitoring of bone metastases might be applied in patients to improve follow-up of these lesions, in particular after therapeutic intervention. This review summarizes established and experimental imaging techniques for the monitoring of tumor and bone cell activity including molecular, functional and morphological aspects in bone metastases.

Angiogenesis is a complex multistep process and a crucial pre-requisite for tumor growth, invasion and metastasis. A profound knowledge of the mechanisms including the elucidation of markers for angiogenic vessels is essential for the generation of new anti-angiogenic chemotherapeutic agents and the improvement of specific imaging techniques. During the last decades, numerous angiogenesis inhibitors have been developed and some of them have shown promising results in preclinical and clinical trials. However, the response to anti-angiogenic treatment is often delayed and shows high inter-individual variations. In order to improve anti-angiogenic therapy, new specific surrogate markers are necessary that allow the characterization of different angiogenic steps, especially at the early stage. In this respect, non-invasive imaging is a potent tool for characterizing the tumor vascularization and for sensitive and longitudinal treatment monitoring. In particular, new molecular imaging techniques might ultimately improve the characterization of the angiogenic tumor phenotype and stage. This review summarizes the current status of different imaging modalities e.g. MRI, CT, US, nuclear and optical imaging with respect to the imaging of tumor angiogenesis and of anti-angiogenic treatments. It also includes new approaches in molecular imaging, which give deep insight into the tumor stage and the response of tumor vessels to anti-angiogenic therapy. Thus, this may lead to a more personalized cancer therapy in future.

Nanomedicine formulations are considered to be superior to standard low-molecular-weight drugs because of an increased drug accumulation at the pathological site and a decreased localization to healthy non-target tissues, together leading to an improved balance between the efficacy and the toxicity of (chemo-) therapeutic interventions. To better understand and further improve nanomedicine-mediated drug targeting, it is important to design systems and strategies which are able to provide real-time feedback on the localization, the release and the therapeutic efficacy of these formulations. The advances made over the past few years with regard to the development of novel imaging agents and techniques have provided a broad basis for the design of theranostic nanomedicine materials, i.e. multicomponent carrier constructs in which drugs and imaging agents are combined, and which can be used to address issues related to drug localization, drug release and drug efficacy. Here, we summarize several recent efforts in this regard, and we show that theranostic systems and strategies hold significant potential for monitoring and improving nanomedicine-mediated drug targeting.