In vitro models continuously evolve to more closely mimic and predict biological responses of living organisms. Just in the past years many novel three dimensional (3D) organotypic models, which resemble tissue structure, function and even disease progression, have been developed. However, application of more complex models and technologies may increase the risk of compromising assay robustness and reproducibility. Consequently, the first developmental stage of cell-based assays is to combine complex tissue models with standard assays - a combination that already provides more physiologically insightful information when compared to two-dimensional (2D) systems. The final goal should be to exploit the full potential of tissue-like in vitro models by investigating them with modern assays such as –Omics and imaging technologies. Furthermore, organotypic models will allow for a design of novel assay concepts that utilize the whole tool box of models and endpoints.
In this chapter we focus on assessment of spheroid viability by measuring intracellular ATP content. This primary assay performed on 3D cell culture system is a powerful tool to predict with high confidence health, growth and energy status of tissue of interest. The 3D spheroid model is particularly useful to mimic solid-tumors from a physiologically relevant architectural perspective, when they are grown with multiple cell types prevalent in these tumors. However, this assay is equally applicable for other non-spheroid 3D tissue models to quantify viability and toxicity.