ATP assay: ability to distinguish cytostatic from cytocidal anticancer drug effects

A bioluminescence assay for ATP was adapted to human cancer cell lines and used to study the effect of anticancer drugs on malignant cell growth by following serial ATP measurements. Eleven drugs were tested against a colon cancer cell line (WiDR). Excellent correlation was observed between simultaneously performed soft-agar colony-forming assays and the ATP assay. In addition, cytostatic (growth inhibitory) drug effects could be distinguished from cytocidal (lethal) effects by using the ATP assay. Cytocidal drugs resulted in a reduction of ATP level below baseline levels, whereas cytostatic drugs merely yielded a reduction in the rate of increase in ATP level, i.e., slower growth. Such characterizations are not possible in colony-forming assays. Changes in ATP were correlated with the number of viable cells present. Drug concentration and duration of exposure both were important. Some drugs became cytocidal only when exposures longer than the customary 1 hour were used. The ATP assay has excellent potential as a simple, inexpensive, and rapid technique for new drug screening in cell lines, with classification of drug effects as cytostatic or cytocidal.