Tracking Labeled Targets In Vivo
In vivo imaging is increasingly being used to track the biodistribution of labeled cells and compounds and for preclinical toxicity testing, notes Dr. De Lille. For example, PerkinElmer is developing kidney toxicity probes to be used for fast kinetic imaging with quantitative, functional outputs. The probes can be used with the company’s current imaging systems to assess toxicity of a drug in a live animal without having to do multiple blood extractions. The same type of technology could be used for detecting liver and other types of toxicity in small animal models.
In the first quarter of this year, PerkinElmer also plans to introduce a new Bombesin RSense 680, a fluorescent agent to target and quantify upregulation of bombesin receptors in vivo associated with tumor proliferation, as well as a new in vivo cell tracking dye.
When customers began modifying their standard imaging systems to do fluorescent in vivo imaging in plants and small animals, UVP decided it was time to develop its own dedicated in vivo imaging systems.
The company’s iBox® line of small animal imaging systems ranges from the Spectra introductory level instrument; to the Scientia™, designed for automated, repeatable imaging using a cooled CCD camera (either the BioChemi HR 500 or OptiChemi HR 610) and including a darkroom, warming plate, and the BioLite MultiSpectral Source; and UVP’s newest model, the Explorer™2, which is capable of macro to micro, whole animal to individual cell imaging by transitioning through a magnification range of 0.17x to 16.5x.
UVP’s imaging systems are designed for fluorescence imaging and in animal applications used primarily in cancer research. Sean Gallagher, Ph.D., vp and CTO at UVP, describes how the ability to differentially label healthy and cancerous cells makes it possible to study metastasis.
Using a long working distance and macro imaging, the researcher can fluorescently image a whole animal, identify tumor margins, study blood vessel formation, and observe tumor growth. By zooming into the micro scale in the same animal, these processes can then be imaged at the level of individual cells.
The Explorer imaging system is compatible with conventional fluorescent probes, as well as Dylight Quantum Dot and Alexa dye technology, and can capture signals from 400 nm into the near-IR range of 700–900 nm, enabling the use of fluorescently tagged antibodies or drugs for biodistribution and pharmacokinetics studies.
The industry is increasingly moving toward “visualization of fluorescence in the red and near-infrared (NIR) wavelengths,” says Dr. Gallagher, which allow for deeper penetration into the animal and more light able to emerge from the area being imaged.
As new genetically encoded and dye labeling technologies emerge in the 700–900 nm range, “we need to match that with improved detection and hardware; they have to work together,” he says. The imaging systems need to become more efficient at these wavelengths, and “so for NIR applications we changed to a highly cooled scientific camera that has a higher quantum efficiency in the NIR,” he adds.
Quantitation remains one of the biggest challenges in imaging, notes Dr. Gallagher. For example, when imaging a tumor, estimating the size of a tumor versus determining the intensity of the fluorescent signal it generates may require the use of two different imaging techniques. To determine how large a tumor is, one can visualize its margins and use that information to estimate its volume.
Measuring the intensity of a tumor may not always be a good indicator of its size, however. Some melanomas, for example, produce so much melanin, that it will quench the fluorescent signal. Specialized software and deconvolution techniques can sometimes help in generating quantitative results from imaging studies.