Cancer molecular imaging is an evolving field in which diverse optical tools and strategies are used for early detection and management of tumors. This field arose from the merger of several pre-existing disciplines such as modern cancer molecular biology, chemistry and imaging technologies. Consequently, cancer molecular imaging has created unique opportunities to study and noninvasively monitor tumor genesis, development and metastasis in vivo
]. It is expected to provide more comprehensive anatomical, physiological and functional information of diseases in a clinical setting. Molecular imaging techniques could be powerful tools in early cancer detection, drug discovery and development as well as monitoring response to treatment [3
]. There are a variety of well-established imaging modalities, such as positron-emission tomography (PET) [3
], single photon emission computed tomography (SPECT) [7
], magnetic resonance imaging (MRI) [8
] and optical fluorescence imaging [11
], that effectively image specific tumor associated molecular targets.
Here, we describe the development of near-infrared fluorescence (NIRF) nanoprobes for cancer molecular imaging. Because NIRF imaging (650–900 nm) displays properties of low absorption and relatively low autofluorescence, it offers several advantages over other modalities for imaging living organisms. In addition, NIRF imaging has potentially high spatial resolution, high sensitivity and low risk to the living subject because it utilizes nonionizing radiation. Moreover, it is cost efficient in terms of preparation of molecular probes and the detection hardware is relatively simple to operate.
The integration of nanotechnology with molecular biology and medicine has resulted in active developments of an emerging research area, namely nanobiotechnology. This research offers exciting and abundant opportunities for discovering new materials and tools for biomedicine. Recent advancements in functional nanomaterials offer to improve detection sensitivity and specificity in molecular imaging [14
]. Functional nanomaterial-based molecular probes, namely nanoprobes, could target tumors either through the enhanced permeability and retention (EPR) effect of the tumor microvasculature or by the specific binding with tumor-associated biomarkers such as tumor cell receptors, tumor extracellular matrix and enzymes. A variety of nanoprobes have been prepared, evaluated and applied in various imaging modalities including: fluorescence, MRI, radionuclide, Raman and photoacoustic imaging. Examples of probes include gold (Au) nanoparticles (nano-shells, nano-rods and nano-cages) or single wall carbon nanotubes (SWNT) for photoacoustic imaging based on heating effects [15
]; magnetic iron oxide nanoparticles for MRI [17
]; and fluorescent nanoparticles for in vivo
fluorescence imaging [18
]. Nanoparticle-based NIRF probes can overcome several limitations of conventional NIR organic dyes, such as poor hydrophilicity and photostability, low quantum yield and detection sensitivity, insufficient stability in biological systems and weak multiplexing capability.
In this short review, we focus on the latest progress in NIRF nanoprobes for cancer molecular imaging. More specifically, we summarize the development of different types of NIRF nanoprobes, their use for molecular imaging in living subjects, the potential toxicity of NIRF nanoprobes and the combination of NIRF-nanoprobe imaging with several other imaging techniques. We also discuss future perspectives of the NIRF nanoprobes for cancer molecular imaging.