Clonogenic and nonclonogenic in vitro chemosensitivity assays

We need practical laboratory methods for predicting the chemosensitivity of human neoplasms. Over the past 20 years, numerous investigators have implied or stated with increasing certitude that clonogenic assays are the most valid (or only valid) approach to predictive chemosensitivity testing. We feel that this point of view may have insufficient scientific validity and may be harmful to progress in this area. In addition to well-known technical problems, there are serious theoretical problems with clonogenic assays. These include (a) the disruption of normal cell-to-cell interactions, (b) the possibility that true tumor stem cells may be largely nondividing (G₀) cells, while cells forming colonies are exclusively dividing cells, (c) the possibility that clonogenic cells may largely represent cells which are not true stem cells, and (d) the fact that clonogenic assays have the ability to measure cell kill over a narrow log range, while meaningful clinical responses require a multiple-log cell kill. This latter fact mandates the use of unrealistically low drug concentrations to avoid excessive false-positive results. Neither theoretical concepts, direct experimental data, nor clinical correlations support the alleged superiority of clonogenic assays. Clonogenic assays may not be advantageous compared to other more practical methods of estimating the general chemosensitivity of proliferating cells. In contrast, there is a growing body of literature which indicates that early evidence of cell damage in the entire tumor cell population may accurately predict for a multiple-log stem cell kill and meaningful clinical response. Future studies should continue to develop and test assays based on alternative methods for detecting cell kill in the proliferating and total tumor cell populations.