Potential limitations of in vitro clonogenic drug sensitivity assays

P388 murine leukemic cells lines which were resistant (P388R) or sensitive (P388S) to adriamycin (adr) were used to evaluate the potential utility of in vitro clonogenic assays for detecting and quantitating the number of adr-resistant cells present in a cell mixture. The progeny of P388S cells that had been exposed for 1 h or continuously to adr were as sensitive to the drug as the original P388S cells. Serial passages in the presence of adr, however, resulted in the selection of adr-resistant cells, but even these cells varied in their level of resistance. When P388S and P388R cells were mixed together and exposed to adr, the clonogenic assay consistently overestimated the proportion of P388S cells present. These studies demonstrate some of the inherent limitations of clonogenic drug sensitivity assays and may provide an explanation for the reported discrepancies between in vitro drug sensitivity and clinical responses to therapy reported in the literature.