Abstract

Diarylsulfonylurea (DSU) antitumor agents represent a new class of oncolytic compounds with an unknown, potentially novel, mechanism of action. At high concentrations of several of these agents, cytotoxicity appears to be a consequence of uncoupling of mitochondria. However, the mechanism of action at pharmacologically achievable concentrations is unknown. To further study these agents a subline of human colon carcinoma, GC3/c1, was selected for resistance to N-(5-indanylsulfonyl)-N'-(4-chlorophenyl)urea (ISCU) (Sulofenur). This clone (designated LYC5) was stably resistant for 2 years in the absence of selection pressure and was characterized for cross-resistance to other antitumor DSU and therapeutically used oncolytic agents. LYC5 was cross-resistant to six of seven DSU analogues examined when cells were exposed to drugs for 7 days. However, the degree of resistance was inversely related to the potency of the individual DSU against the parental GC3/c1 clone. Consequently, against LYC5 cells there was a relatively narrow range for concentrations inhibiting colony formation by 50% (4-fold), compared with that in GC3/c1 cells (12-fold range). With a single exception, each DSU examined caused uncoupling of oxidative phosphorylation in isolated mitochondria at 50 microM, and data suggest that cytotoxicity in LYC5 cells may be a consequence of mitochondrial impairment. In contrast, LYC5 cells were collaterally sensitive to the mitochondrial toxins rotenone, antimycin, and oligomycin, by 11.4-, 7.2-, and 36.9-fold respectively. LYC5 cells were also collaterally sensitive to vincristine (7.7-fold), Actinomycin D (5.9-fold), and rhodamine-123 (10.5-fold), agents associated with P-glycoprotein (Pgp)-mediated multidrug resistance (MDR). LYC5 cells were slightly more sensitive to Melphalan and doxorubicin (2.8- and 2.3-fold, respectively) but not to cisplatin or dideazatetrahydrofolic acid. Collateral sensitivity to vincristine and Actinomycin D was consistent with decreased Pgp levels in LYC5 cells. Immunohistochemical staining and Western blotting with anti-Pgp antibodies indicated an 8-fold reduction in Pgp levels in LYC5 cells, relative to expression in parental GC3/c1 cells. Consequently, association of mitochondrial toxins with resistance in MDR KB8-5 cells was examined in the presence or absence of the MDR-reversing agent verapamil. KB8-5 cells had equal or greater sensitivity, compared with parental KB3-1 cells, to rotenone, antimycin, and oligomycin and also to each DSU analogue examined. In addition, verapamil tended to have a protective effect against these mitochondrial toxins.(ABSTRACT TRUNCATED AT 400 WORDS)