N-(5-Indanylsulfonyl)-N'-(4-chlorophenyl)urea, a Novel Agent Equally Cytotoxic to Nonproliferating Human Colon Adenocarcinoma Cells

Diarylsulfonylureas have been shown to have therapeutic activity against rodent and human tumor models, notably causing regressions in some lines of human colon adenocarcinomas in mice. At present the mechanism of cytotoxicity is unknown, although preliminary data implicate mitochondria as a potential site of action. In this study, the cytotoxicity of the diarylsulfonylurea N5-indanylsulfonyl)-N'-(4-chlorophenyl)-urea (ISCU) has been examined in GC₃/c₁ human colon adenocarcinoma cells. At cytotoxic concentrations of ISCU, in the presence of albumin as a drug binding species, there was only slight inhibition of [3H]thymidine and [3H]uridine incorporation at concentrations of ISCU up to 140 Mg/ml and no inhibition of synthesis of protein as determined by incorporation of L-leucine. In the absence of albumin, incorporation of pHJthymidine into DNA or [3H]uridine into RNA was inhibited at >70 Mg/ml and 140 Mg/ml, respectively. As ISCU is highly bound to serum albumin (>99%), it would appear that inhibition of nucleic acid synthesis occurs only at supralethal concentrations of ISCU. The cytotoxicity of ISCU in proliferating or quiescent cell populations was examined. GCj/Ci cells grown in medium containing 0.5% fetal calf serum (FCS) had a 60% reduction in rate of growth, but were more sensitive than cells exposed for 24 h in 10% FCS-medium (IC50 1.9 and 31 Mg/ml, respectively). When the albumin concentration was adjusted (240 mg/100 ml) to allow equivalent drug binding, IQo values were similar. In cultures of GC₃/c₁ cells growth rate was related to the concentration of FCS. In the absence of serum, growth rate was 2.5 to 3.2% that of exponentially growing control cultures in the presence of 10% FCS. Addition of FCS to quiescent cultures after 1 to 6 days in serum-free conditions resulted in immediate growth of cells. Clonogenic potential was also unchanged for at least 6 days under serum-free conditions. Under these conditions, where albumin concentration was adjusted to be equivalent to medium containing 10% FCS, sensitivity of proliferatively quiescent GC₃/c₁ cells was similar to that in exponentially growing control cultures in which the population doubling time was a22 h. Further, there was no recovery of clonogenic potential when cells were exposed for 24 h to ISCU and maintained in a quiescent state for up to 4 days prior to serum stimulation. These data suggest that the cytotoxic effects of ISCU are independent of the proliferative state of the cell population. Studies in which synchronized cells were stimulated by addition of serum in the presence of ISCU suggest that this drug may retard progress from the GJGt state, in a concentration-dependent manner.