ANTIPROLIFERATIVE ACTIVITY OF THE TOPOISOMERASE-I INHIBITORS TOPOTECAN AND CAMPTOTHECIN, ON SUBCONFLUENT AND POSTCONFLUENT TUMOR-CELL CULTURES

We have assessed the antiproliferative effects of a 24-hr exposure to the topoisomerase I inhibitors, topotecan and camptothecin, on two col on and one ovarian human tumor cell lines, cultured as subconfluent an d as multilayered postconfluent cultures. Chemosensitivity was measure d by the sulforhodamine B assay. In general, postconfluent cultures we re less sensitive to these agents, yielding GI(50)S (drug concentratio ns inhibiting growth by 50%) from 1.2 to more than 6000 times higher t han those of subconfluent cultures. Both compounds displayed similar e ffects on subconfluent cells, inducing complete growth inhibition at c oncentrations ranging from 0.03 to 0.5 mu M Topotecan, however, was mo re potent than camptothecin in two out of the three cell lines tested as multilayered postconfluent cultures. Topoisomerase I mRNA expressio n on postconfluent cultures was 50% lower than on subconfluent culture s in the three cell lines studied. However, we did not detect any repr oducible differences in topoisomerase I protein expression and in rela xation activity of supercoiled DNA between the two types of cultures. From accumulation experiments it appeared that the peak concentration of the lactone form of topotecan as well as the area under the concent ration-time curve (AUC) were 2-fold higher in the monolayer than in th e multilayer cultures. Therefore, the differences in the activity of t opoisomerase I inhibitors under our experimental conditions were likel y due to a decreased rate of proliferation of postconfluent cells, ass ociated with a reduction in drug uptake.