Nanoparticle-based theragnostics: Integrating diagnostic and therapeutic potentials in nanomedicine.

Theragnostics is a strategy that integrates therapeutics with diagnostics to develop new personalized treatments with enhanced efficacy and safety. One key component of theragnostics is diagnostic imaging with high sensitivity and molecular specificity. The advent of molecular imaging has brought revolutionary changes to conventional imaging by enabling characterization of biological processes at the cellular and molecular levels spatially and temporally [1]. Depending on the molecular imaging modality and contrast agent used, theragnostics could be achieved by applying imaging throughout treatment planning, drug and dosage selection, and therapeutic response monitoring. Nanotechnology has led the advances in theragnostics, as a result of the development of a variety of fine particulate materials with size dimensions in the range of 1–200 nm, and the discovery of their unique physicochemical properties that are not found in their bulk counterparts. These properties include quantum confinement in semiconductor nanoparticles (also known as quantum dots), superparamagnetism in certain oxide nanoparticles, and surface-enhanced Raman scattering (SERS) in metallic nanoparticles, among others. These unique physical properties have substantially expanded the potential of molecular imaging, and led to the development of highly sensitive, and cost-effective novel molecular imaging agents. These new imaging agents can be broadly categorized as optical (fluorescent, SERS, photoacoustic, etc.), magnetic, radioactive (positron or γ-ray emitting), X-ray opaque (nanoparticles with high electron density) and ultrasound-sensitive (nanobubbles) agents. Nanoparticles used in medicine are typically coated with a polymer that bears ample functional groups providing flexibility to integrate multiple functionalities. With this flexibility, nanoparticles can serve as diagnostic tools, or therapeutic carriers, or both. In fact, the rapid emerging of theragnostics in nanomedicine is largely dependent upon this flexibility. Various biomolecules have been conjugated on nanoparticles through surface functional groups for intended function, such as targeting ligand for cell specific binding, drugs for chemotherapy, genes for cell transfection, or combination of them. With size dimensions at the nanoscale, these particles can navigate through microvasculature and across various biological barriers to reach target tissue. The size, surface charge, and hydrophobicity of nanoparticles can be controlled to minimize renal and hepatic clearance [2], and thus extend their blood circulation time and reduce potential immunogenicity. Most of theragnostic nanoparticles, such as liposomes, micelles, and nanocomposites, have complex nanostructures that consist of building blocks with diverse chemical compositions [3-6].