oa Editorial [Hot Topic: Recent Advance in the Development of Radiolabeled In Vivo Probes for Diagnosis and Therapy (Executive Guest Editor: Junbo Zhang)]

Nuclear medicine is an intrinsically molecular technology because it often needs the use of radiolabelled molecular probes to collect information from a special biological target. In contrast to morphological imaging procedures like computed tomography (CT) and magnetic resonance imaging (MRI), the use of ‘magic’ radiopharmaceuticals for the imaging of the biochemical changes that come with any disease has allowed nuclear medicine technique as a functional imaging. Radiolabeled compounds of biological interest are required for the preparation of novel radiopharmaceuticals, which is one of the pillars on which nuclear medicine rests. Radiopharmaceuticals, along with imaging instrumentations (PET: positron emission tomography; SPECT: single photon emission computed tomography), play an important role in facilitating the progress of nuclear medicine. The topics chosen for the issue mainly focus on recent advance in the development of radiolabeled in vivo probes for diagnosis and therapy, especially dedicated to heart imaging, tumor imaging, infection and inflammation imaging and tumor therapy. The themed issue highlights an overview of significant radiopharmaceuticals for which radioligands have recently been developed. Human serum albumin (HSA) is a soluble monomeric protein representing about 60% of the overall plasma content. It has been regarded as a ligand-binding and transport protein for different exogenous and endogenous compounds, such as drugs, fluorescent molecules and radioisotopes. HSA has been an ideal carrier for targeted molecular imaging and therapy due to its many advantages. Dr. Meng Yang and coworkers [1] from Stanford University outline the detailed aspects of HSA and the recent development of many HSA-based molecular probes for their applications in cancer molecular imaging including PET/SPECT imaging, MRI and optical imaging. They conclude HSA should be a very promising platform for molecular imaging probe development and biomedical applications. Angiogenesis, the process by which new blood vessels are formed, is a necessary element at the basis of both tumor growth and metastases. Angiogenesis of tumors has gained great attention in the research community for many years. Noninvasive detection of angiogenic activities is of great importance in cancer diagnosis as well as evaluation of cancer therapeutic responses. Many angiogenesisrelated molecular targets have been identified and used in tumor vasculature targeting and imaging. Herein, Dr. Xia Lu and Rongfu Wang [2] from Peking University First Hospital summarize the current status on the research of peptide Arg-Gly-Asp (RGD) and Arg-Arg-Leu (RRL) as well as its derivatives applied in tumor angiogenesis imaging and targeted therapy with nuclear medicine modalities (SPECT and PET). Coronary artery disease (CAD) remains a primary health problem worldwide. Myocardial perfusion imaging is a technique in which radionuclide tracers are used to evaluate myocardial blood flow, as well as myocardial scarring or infarction, in order to diagnose and assess the significance of CAD. Dr. Xiao Lin and co-workers [3] from Key Laboratory of Radiopharmaceuticals, Beijing Normal University, summarize the development of various SPECT and PET radiotracers for myocardial perfusion imaging and illustrate their chemical structures and biological profiles in this review. In addition, the present status and future aspects in myocardial perfusion imaging agents are also discussed. Folate receptor ( FR), which is highly expressed on many tumor types and activated macrophages, is a promising target for imaging and therapy of cancer and inflammation. Folic acid is an important target-specific delivery molecule with high binding affinity for FR. Therefore, the development of folic acid-targeted radioconjugates for imaging and therapy of cancer and inflammation has become a hot subject in radiopharmaceutical field. Dr. Cristina Muller [4] from Paul Scherrer Institute (Switzerland) outline a variety of folic acid radioconjugates and their applications for imaging and therapy of cancer and inflammation. The existing problems, design strategy and the trend for the development of the FR-targeting radiopharmaceuticals are also discussed in this review. Tissue hypoxia results from an insufficient supply of oxygen that compromises biological functions. It is a pathophysiological aspect that is characteristic for many advanced solid tumors. As a clinical interest, to measure tumor hypoxia in assessing the aggressiveness of tumor and predicting the outcome of therapy has become more and more important. Tumor hypoxia can be detected by non-invasive and invasive techniques. Clinical study using polarographic oxygen electrodes to be a tool to measure hypoxia was the first to demonstrate the presence of hypoxia in human tumor and its association with poor prognosis. However, its invasive nature prevents frequent and repeated use. Over the years, functional imaging as a noninvasive method has attracted much attention and many radiolabelled PET and SPECT tracers have been developed for measuring tumor hypoxia. Dr. Zejun Li and Taiwei Chu [5] from Radiochemistry and Radiation Chemistry Key Laboratory of Fundamental Science, Peking University, summarize most of the hypoxia-directed radiopharmaceuticals that have been developed in the past decade. In addition, they mention the possible retention mechanism and the existing problems in the related research field. Infection is an important problem that needs accurate and prompt diagnosis for early management to avoid serious complications. The great impact of infection on daily clinical practice has promoted research into more and more accurate diagnostic and therapeutic. Nuclear medicine techniques do not rely on morphologic changes but are based on physiochemical processes in tissues thus making it suitable to visualize infectious foci in early phases. Imaging infection and inflammation is one of the most challenging areas in nuclear medicine. Nowadays, there is a great interest in the development of new radiopharmaceuticals for infection imaging. Dr. Guillermina Ferro-Flores from Instituto Nacional de Investigaciones Nucleares, Mexico and coworkers [6] summarize the recent advances and applications of radiolabeled specific agents to visualize infectious foci by targeting viruses, fungi or bacteria. They conclude until now the radiolabeled antimicrobial peptide (99mTc-UBI 29-41) has been the most promising infection imaging agent in clinical studies. Overall, the area of radiopharmaceutical in nuclear medicine is one rich with ongoing research. More and more novel specific radiolabeled probes for diagnosis and therapy are expected to be used for clinical applications in the future. In closing, it has been a great honor to be a guest editor for this special issue within the field of radiopharmaceutical sciences and assemble six major manuscripts from senior investigators in university as well as in research institute. I would like to thank all contributors and reviewers for their kind cooperation to accomplish this themed issue. REFERENCES [1] Yang M, Hoppmann SS, Chen LX, Cheng Z. Human serum albumin conjugated biomolecules for cancer molecular imaging. Curr Pharm Des 2012; 18(8): 1023-31. [2] Lu X, Wang RF. A concise review of current radiopharmaceuticals in tumor angiogenesis imaging. Curr Pharm Des 2012; 18(8): 1032-40. [3] Lin X, Zhang JB, Wang XB, Tang ZG, Zhang XZ, Lu J. Development of radiolabeled compounds for myocardial perfusion imaging. Curr Pharm Des 2012; 18(8): 1041-57. [4] Müller C. Folate Based Radiopharmaceuticals for imaging and therapy of cancer and inflammation. Curr Pharm Des 2012; 18(8): 1058-83. [5] Li ZJ, Chu TW. Recent advances on radionuclide labeled hypoxia-imaging agents. Curr Pharm Des 2012; 18(8): 1084-97. [6] Ferro-Flores G, Ocampo-García BE, Melendez-Alafort L. Development of specific radiopharmaceuticals for infection imaging by targeting infectious micro-organisms. Curr Pharm Des 2012; 18(8): 1098-1106.