Clinically Approved Nanoparticle Imaging Agents

Abstract

Nanoparticles are a new class of imaging agent used for both anatomic and molecular imaging. Nanoparticle-based imaging exploits the signal intensity, stability, and biodistribution behavior of submicron-diameter molecular imaging agents. This review focuses on nanoparticles used in human medical imaging, with an emphasis on radionuclide imaging and MRI. Newer nanoparticle platforms are also discussed in relation to theranostic and multimodal uses.Key Words: nanoparticles; medical imaging; molecular imaging; nanoparticle; imaging and diagnostics Nucl Med 2016; 57:1833 57: -1837 57: DOI: 10.2967 Medical imaging offers rapid, longitudinal, and noninvasive visualization of the interior of living subjects. There are two main approaches to medical imaging, the first being anatomic imaging, which provides information on gross structure, and the second being molecular or functional imaging, which provides information on physiology and cellular processes such as metabolism, protein expression, and DNA synthesis (1). Although exogenous imaging agents are optional for anatomic imaging (e.g., MRI or CT contrast medium agents, which help improve tissue contrast), they are virtually a requirement for molecular imaging, especially within the realm of nuclear medicine, for which radioisotopes are required for single-photon emission CT (SPECT) or PET. JThe 3 main classes of imaging agents include small molecules, proteins, and nanoparticles. Most scans use small molecules, which are agents below 2,000 kDa and measure approximately 1 nm (e.g., 18 F-FDG for PET, iodinated small molecules for CT, and chelated gadolinium for MRI). Protein imaging agents, such as radiolabeled monoclonal antibodies, are less common but offer precise molecular information and are a growing area of research. Nanoparticles are a new and exciting class of imaging agent that can be used for both anatomic and molecular imaging.Their small size and unique properties (high ratio of surface area to volume) offer, first, intense and longitudinally stable imaging signals (quantum and C dots); second, different targeting strategies (passive targeting via the mononuclear phagocyte system or active targeting to specific molecular targets as a result of functionalization with ligands); third, high avidity (a large association constant brought about by the presence of multiple ligands per particle); fourth, theranostic capabilities (use for both diagnostic purposes, by generating an imaging signal, and therapeutic purposes, by delivering a drug payload); fifth, multimodal signal capabilities (detection of one nanoparticle by more than one imaging modality, making it suitable for deep tissue imaging, screening with MRI, and intraoperative guidance using superficial imaging with optical imaging); and sixth, multiplexing (detection of multiple different molecular targets simultaneously).Although many of these features have been demonstrated only in animal models, there are several nanoparticle formulations that have been transitioned into clinical practice. This revi...

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