PROLIFERATION-DEPENDENT AND PROLIFERATION-INDEPENDENT CYTOTOXICITY BYANTITUMOR DIARYLSULFONYLUREAS - INDICATION OF MULTIPLE MECHANISMS OF DRUG-ACTION

The mechanism(s) by which antitumor diarylsulfonylureas (DSU) cause cy totoxicity has been examined in GC3/Cl human colon adenocarcinoma cell s and a subline selected for resistance to N-(5-indanylsulfonyl)-N'-(4 -chlorophenyl)urea (ISCU). Resistance was stable in the absence of sel ection pressure. This mutant (designated LYC5) was 5.5-fold resistant to ISCU compared to parental GC3/C1 cells in serum containing medium w hen cells were exposed for 7 days. In contrast, LYC5 cells were not re sistant to a 4-hr exposure to ISCU. These data indicated two possible mechanisms of action, dependent on concentration and time of exposure to ISCU. Proliferation-dependent and -independent mechanisms of cytoto xicity were identified in wild-type and resistant clones. In serum-fre e medium containing growth factors, the Ic50 for parental cells was 0. 51 muM and for LYC5 7.0 muM (13.6-fold resistance), whereas without gr owth factors both lines were 8- to 9-fold resistant relative to condit ions of cellular proliferation. Accumulation of ISCU was similar in qu iescent and proliferating cells, and was reduced only slightly in resi stant LYC5 cells. Analysis of DNA by agarose gel electrophoresis showe d that in GC3/C1 cells nucleosomal ladders were formed only when proli ferating cells were exposed to ISCU. No nucleosomal ladders were detec ted in quiescent cells during exposure to toxic concentrations of drug (IC90), or after removal of ISCU and addition of serum to stimulate g rowth. These data indicate several mechanisms by which diarylsulfonylu rea antitumor agents may cause cell death. In serum-free medium at ver y high concentration (IC50 approximately 370 muM) for short periods of exposure (4 hr), cytotoxicity was proliferation independent, and GC3/ Cl and LYC5 cells were equally sensitive. This mechanism may relate to the uncoupling activity of ISCU. However, at pharmacological relevant concentrations, the primary mechanism was proliferation dependent and led to formation of nucleosomal DNA ladders (IC50 approximately 0.5 m uM). A possible additional mechanism occurred at higher concentration (IC50 approximately 7 muM) in quiescent cells, and was not associated with DNA degradation.