9L rat brain tumor cell spheroids were treated with α-difluoromethylornithine (DFMO) and 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) alone and in combination. In contrast to results obtained with 9L monolayer culture cells, very low concentrations of DFMO killed spheroid cell within 0.5 day after the start of treatment; cell kill was maximum within 2 to 3 days. DFMO cytotoxicity could be prevented by adding putrescine (to a final concentration of 1 mm) to the culture medium. DFMO also significantly slowed and eventually stopped the growth of spheroids in a dose-dependent manner. Cells in all regions of DFMO-pretreated spheroids were sensitized to BCNU as measured by colony-forming efficiency; this sensitization was prevented when putrescine was added to culture medium before BCNU treatment. When used as single agents, either a 3-day treatment with 10 mm DFMO or a 1-hr treatment with BCNU (1.5 µg/ml) produced similar growth delay, but used in combination, the two agents produced a much longer growth delay than produced by either agent alone. Growth of spheroids treated continuously for up to 28 days with 10 mm DFMO ceased at approximately 7 times the volume at the time of treatment. When spheroid cells were treated for 1 hr with BCNU (1.5 µg/ml) and then were treated continuously with DFMO, growth plateaued at approximately 3.5 times the volume at the time of treatment; when spheroids were treated first with DFMO for 3 days, then with BCNU (1.5 µg/ml) for 1 hr, and then treated continuously with DFMO, growth plateaued at approximately 1.5 times the volume at the time of treatment. The number of clonogenic cells per spheroid that survived combination treatment also reflected the cytotoxic effects of the two drugs. Thus, the combined drug treatment was very effective in inhibiting the growth of spheroids and in preventing an increase in the number of clonogenic cells per spheroid.

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Supported by NIH Grants CA-13525 and CA-31868, American Cancer Society Grant RD-137, and the Aaron Silvera Fund.

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