Intravital microscopy as a tool to study drug delivery in preclinical studies
Graphical abstract
Introduction
One of the key factors in the development and successful utilization of therapeutic and diagnostic agents is the thorough understanding of the mechanisms regulating their delivery to the targeted tissue combined with the ability to provide a clear readout of their effectiveness. To this aim, several imaging techniques have been employed in live animals such as magnetic resonance imaging (MRI), computed tomography (CT), positron emission tomography (PET), and whole-body imaging using bioluminescence [1], [2], [3], [4], [5]. Although some of these techniques offer a high degree of sensitivity and provide the possibility to quantitatively study drug pharmacokinetic and pharmacodynamic, they lack spatial resolution at the tissue and cellular level. Intravital microscopy (IVM), which encompasses a series of techniques based on fluorescence light microscopy, has overcome this issue by enabling the image of several biological processes in live organisms at a subcellular resolution [6], [7], [8], [9]. Although IVM has been introduced at the beginning of the last century, its major advancements have occurred in the last twenty years with the development of non-linear microscopy that has enabled performing deep tissue imaging. IVM is now a widely utilized tool that provides information on tissues such as morphology, cellular architecture, metabolic status, and accessibility to exogenously administered molecules in a dynamic fashion. Even though it is primarily used in preclinical studies and basic research [9], [10], [11], [12], [13], [14], [15], IVM has recently been applied in clinical settings either alone or in combination with other imaging modalities [16], [17], [18], [19], [20]. In this review we will provide a general overview of IVM and we will discuss how this technique has been successfully utilized to study some aspects of drug delivery in preclinical studies.
Section snippets
Non-linear microscopy
Fluorescence light microscopy is based on the generation of contrast by the excitation of the energy levels of a molecule, generally referred as fluorophore. The excitation is achieved by illuminating the specimen with a light source such as a mercury lamp or a laser, which provides photons that have wavelengths ranging from the ultraviolet (UV) to the infrared (IR). There are two main modalities of excitation: linear and non-linear. The former, which is utilized in conventional instruments
From the vasculature to the target tissue
The efficacy of molecules that are administered systemically in a live organism depends on multiple factors such as: 1) their clearance from the circulation, 2) their ability to diffuse out from the vasculature, and 3) their ability to diffuse through the interstitial space to reach the target organ. Hence, direct imaging and measurements of blood flow and vascular permeability in the area of interest are crucial. The behavior of fluorescent probes in the vasculature has been studied by most of
Conclusions and perspectives
In conclusion, IVM provides a formidable tool to study physiological and pathological processes in vivo and to provide valuable information for the design of effective therapeutic agents in preclinical studies. One of the main directions to pursue is the use of IVM in conjunction with other imaging techniques to fully exploit the power of multimodal approaches. To this aim much effort has to be devoted towards the generation of multimodal probes and the realization of multimodal platforms in
Acknowledgments
This research was supported by the Intramural Research Program of the NIH, National Institute of Dental and Craniofacial Research. (NIDCR) We apologize to those whose work could not be cited due to space limitations.
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