Experimental anti-tumor therapy in 3-D: Spheroids - old hat or new challenge?

Purpose: To give a state-of-the-art overview on the promise of three-dimensional ( 3-D) culture systems for anticancer drug development, with particular emphasis on multicellular tumor spheroids ( MCTS). Results and conclusions: Cell-based assays have become an integral component in many stages of routine anti-tumor drug testing. However, they are almost always based on homogenous monolayer or suspension cultures and thus represent a rather artificial cellular environment. 3-D cultures - such as the well established spheroid culture system - better reflect the in vivo behavior of cells in tumor tissues and are increasingly recognized as valuable advanced tools for evaluating the efficacy of therapeutic intervention. The present article summarizes past and current applications and particularly discusses technological challenges, required improvements and recent progress with the use of the spheroid model in experimental therapeutics, as a basis for sophisticated drug/therapy screening. A brief overview is given focusing on the nomenclature of spherical 3-D cultures, their potential to mimic many aspects of the pathophysiological situation in tumors, and currently available protocols for culturing and analysis. A list of spheroid-forming epithelial cancer cell lines of different origin is provided and the recent trend to use spheroids for testing combination treatment strategies is highlighted. Finally, various spheroid co-culture approaches are presented that have been established to study heterologous cell interactions in solid tumors and thereby are able to reflect the cellular tumor environment with increasing accuracy. The intriguing observation that in order to retain certain tumor initiating cell properties, some primary tumor cell populations must be maintained exclusively in 3-D culture is mentioned, adding a new but fascinating challenge for future therapeutic campaigns.