Light sheet fluorescence microscopy versus confocal microscopy: in quest of a suitable tool to assess drug and nanomedicine penetration into multicellular tumor spheroids.

We recently constructed a multicellular spheroid model of pancreatic tumor based on a triple co-culture of cancer cells, fibroblasts and endothelial cells and characterized by the presence of fibronectin, an important component of the tumor extracellular matrix. By combining cancer cells and stromal components, this model recreates in vitro the three-dimensional (3D) architecture of solid tumors. In this study, we used these hetero-type spheroids as a tool to assess the penetration of doxorubicin (used as a model drug) through the whole tumor mass either in a free form or loaded into polymer nanoparticles (NPs), and we investigated whether microscopy images, acquired by Confocal Laser Scanning Microscopy (CLSM) and Light Sheet Fluorescence Microscopy (LSFM), would be best to provide reliable information on this process. Results clearly demonstrated that CLSM was not suitable to accurately monitor the diffusion of small molecules such as the doxorubicin. Indeed, it only allowed to scan a layer of 100 µm depth and no information on deeper layers could be available because of a progressive loss of the fluorescence signal. On the contrary, a complete 3D tomography of the hetero-type multicellular tumor spheroids (MCTS) was obtained by LSFM and multi-view image fusion which revealed that the fluorescent molecule was able to reach the core of spheroids as large as 1 mm in diameter. However, no doxorubicin-loaded polymer nanoparticles were detected in the spheroids, highlighting the challenge of nanomedicine delivery through biological barriers. Overall, the combination of hetero-type MCTS and LSFM allowed to carry out a highly informative microscopic assessment and represents a suitable approach to precisely follow up the drug penetration in tumors. Accordingly, it could provide useful support in the preclinical investigation and optimization of nanoscale systems for drug delivery to solid tumors.