A new parameter of growth inhibition for cell proliferation assays

Cell proliferation assays are performed by four decades to test the anti‐proliferative activity of natural products and synthetic compounds in cell cultures. In cancer research, they are widely employed to evaluate drug efficacy in in vitro tumor models, such as established cell lines, primary cultures, and recently developed three‐dimensional tumor organoids. In this manuscript, we demonstrated that current employed parameters used by researchers to quantify in vitro growth inhibition, IC 50 and GI 50 , lead to a misinterpretation of results based on the exponential, and not linear, proliferation of the cells in culture. Therefore, we introduce a new parameter for the analysis of growth inhibition in cell proliferation assays, termed relative population doubling capacity, that can be employed to properly quantify the anti‐proliferative activity of tested compounds and to compare drug efficacy between distinct cell models.