Intravital microscopy in window chambers: a unique tool to study tumor angiogenesis and delivery of nanoparticles

Solid tumor growth is heavily dependant on angiogenesis. Tumor angiogenesis is the result of a complex interplay between tumor cells, endothelial cells, and other stromal cells. It has been found to be under strict control of a plethora of molecular factors that function as angiogenic up- and down-regulators; nevertheless, the identification of molecular and cellular players and their roles in angiogenesis is still ongoing. The microvasculature resulting from tumor angiogenesis lacks hierarchy and has a high permeability for macromolecules and nanoparticles, which offers significant potential for nanoparticulate tumor imaging and drug delivery platforms. However, improvements in the delivery to poorly vascularized regions and the distribution throughout the tumor interstitium are critical for nanoparticles to become more effective in the battle against cancer. A tool that has proven extremely valuable in both unraveling angiogenic pathways and characterizing in vivo nanoparticle behavior in solid tumors is intravital microscopy of tumors grown in window chamber preparations. In this review this technique is explained, several exciting examples illustrating its value in elucidating tumor angiogenesis are presented and the study of nanoparticle behavior in solid tumors using this approach is described. We conclude with a discussion of the potential value of intravital microscopy in window chambers in multimodality studies of tumor pathophysiology and nanoparticle dynamics.