Longitudinal far red gene-reporter imaging of cancer metastasis in preclinical models: a tool for accelerating drug discovery

A Review of Image Sensors Used in Near-Infrared and Shortwave Infrared Fluorescence Imaging

To translate near-infrared (NIR) and shortwave infrared (SWIR) fluorescence imaging into the clinic, the paired imaging device needs to detect trace doses of fluorescent imaging agents. Except for the filtration scheme and excitation light source, the image sensor used will finally determine the detection limitations of NIR and SWIR fluorescence imaging systems. In this review, we investigate the current state-of-the-art image sensors used in NIR and SWIR fluorescence imaging systems and discuss the advantages and limitations of their characteristics, such as readout architecture and noise factors. Finally, the imaging performance of these image sensors is evaluated and compared.

Fluorescent nanotechnology for in vivo imaging

Fluorescent imaging in living animals gives an intuitive picture of the dynamic processes in the complex environment within a living being. However, animal tissues present a substantial barrier and are opaque to most wavelengths of visible light. Fluorescent nanoparticles (NPs) with new photophysical characteristics have shown excellent performance for in vivo imaging. Hence, fluorescent NPs have been widely studied and applied for the detection of molecular and biological processes in living animals. In addition, developments in the area of nanotechnology have allowed materials to be used in intact animals for disease detection, diagnosis, drug delivery, and treatment. This review provides information on the different types of fluorescent particles based on nanotechnology, describing their unique individual properties and applications for detecting vital processes in vivo. The development and application of new fluorescent NPs will provide opportunities for in vivo imaging with better penetration, sensitivity, and resolution. This article is categorized under: Diagnostic Tools > in vivo Nanodiagnostics and Imaging.

Near-Infrared Fluorescent Proteins and Their Applications

High transparency, low light-scattering, and low autofluorescence of mammalian tissues in the near-infrared (NIR) spectral range (~650-900 nm) open a possibility for in vivo imaging of biological processes at the micro- and macroscales to address basic and applied problems in biology and biomedicine. Recently, probes that absorb and fluoresce in the NIR optical range have been engineered using bacterial phytochromes - natural NIR light-absorbing photoreceptors that regulate metabolism in bacteria. Since the chromophore in all these proteins is biliverdin, a natural product of heme catabolism in mammalian cells, they can be used as genetically encoded fluorescent probes, similarly to GFP-like fluorescent proteins. In this review, we discuss photophysical and biochemical properties of NIR fluorescent proteins, reporters, and biosensors and analyze their characteristics required for expression of these molecules in mammalian cells. Structural features and molecular engineering of NIR fluorescent probes are discussed. Applications of NIR fluorescent proteins and biosensors for studies of molecular processes in cells, as well as for tissue and organ visualization in whole-body imaging in vivo, are described. We specifically focus on the use of NIR fluorescent probes in advanced imaging technologies that combine fluorescence and bioluminescence methods with photoacoustic tomography.

Near-infrared fluorescence lymphatic imaging in vascular endothelial growth factor-C overexpressing murine melanoma

In this study we employ a near-infrared fluorescence lymphatic imaging (NIRFLI) technique to longitudinally image spatial and temporal changes in the lymphatics in mice bearing vascular endothelial growth factor (VEGF)-C overexpressing B16F10 (VEGF-C-B16F10) or mock-transduced B16F10 (mock-B16F10) melanoma tumors. Our NIRFLI data show that ICG-laden lymph accumulates into a VEGF-C-B16F10 tumor compared to mock-B16F10 at 3 days post implantation, presumably due to increased lymphatic vessel permeability. Quantification shows a significantly greater percentage of ICG-perfused area in VEGF-C-B16F10 (7.6 ± 2) as compared to MOCK-B16F10 (1 ± 0.5; p = 0.02), which is also confirmed by quantification of the lymphatic leakage of evans blue dye (optical density at 610nm; VEGF-C-B16F10, 10.5 ± 2; mock-B16F10, 5.1 ± 0.5; p = 0.009); thereafter, lymphatic leakage is visualized only in the peritumoral region. Our imaging data also show that anti-VEGF-C treatment in VEGF-C-B16F10 restores normal lymphatic vessel integrity and reduces dye extravasation. Because NIRFLI technology can be used to non-invasively detect lymphatic changes associated with cancer, it may provide a new diagnostic to assess the lack of lymphatic vessel integrity that promotes lymphovascular invasion and to assess therapies that could arrest invasion through normalization of the lymphatic vasculature.

Bacterial Phytochromes, Cyanobacteriochromes and Allophycocyanins as a Source of Near-Infrared Fluorescent Probes

Bacterial photoreceptors absorb light energy and transform it into intracellular signals that regulate metabolism. Bacterial phytochrome photoreceptors (BphPs), some cyanobacteriochromes (CBCRs) and allophycocyanins (APCs) possess the near-infrared (NIR) absorbance spectra that make them promising molecular templates to design NIR fluorescent proteins (FPs) and biosensors for studies in mammalian cells and whole animals. Here, we review structures, photochemical properties and molecular functions of several families of bacterial photoreceptors. We next analyze molecular evolution approaches to develop NIR FPs and biosensors. We then discuss phenotypes of current BphP-based NIR FPs and compare them with FPs derived from CBCRs and APCs. Lastly, we overview imaging applications of NIR FPs in live cells and in vivo. Our review provides guidelines for selection of existing NIR FPs, as well as engineering approaches to develop NIR FPs from the novel natural templates such as CBCRs.

Near-infrared bioluminescent proteins for two-color multimodal imaging

Bioluminescence imaging became a widely used technique for noninvasive study of biological processes in small animals. Bioluminescent probes with emission in near-infrared (NIR) spectral region confer the advantage of having deep tissue penetration capacity. However, there are a very limited number of currently available luciferases that exhibit NIR bioluminescence. Here, we engineered two novel chimeric probes based on RLuc8 luciferase fused with iRFP670 and iRFP720 NIR fluorescent proteins. Due to an intramolecular bioluminescence resonance energy transfer (BRET) between RLuc8 and iRFPs, the chimeric luciferases exhibit NIR bioluminescence with maxima at 670 nm and 720 nm, respectively. The 50 nm spectral shift between emissions of the two iRFP chimeras enables combined multicolor bioluminescence imaging (BLI) and the respective multicolor fluorescence imaging (FLI) of the iRFPs. We show that for subcutaneously implanted cells, NIR bioluminescence provided a 10-fold increase in sensitivity compared to NIR FLI. In deep tissues, NIR BLI enabled detection of as low as 10⁴ cells. Both BLI and FLI allowed monitoring of tumor growth and metastasis from early to late stages. Multimodal imaging, which combines concurrent BLI and FLI, provides continuous spatiotemporal analysis of metastatic cells in animals, including their localization and quantification.