Chemosensitization of cultured human carcinoma cells to 1,3-bis(2-chloroethyl)-1-nitrosourea by difluoromethylornithine-induced polyamine depletion.

We have investigated the effect of pretreatment with the ornithine decarboxylase inhibitor alpha-difluoromethylornithine (DFMO) on the cytocidal efficacy of 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) in a series of five cultured human adenocarcinoma cell lines. Plating efficiency assays were used to generate BCNU dose-response survival curves for DFMO-treated and control cells. The cell lines varied in their sensitivity to BCNU, with A-427 (lung) and HuTu-80 (duodenum) cells being most sensitive, HT-29 (colon) and ME-180 (cervix) most resistant, and MCF-7 (breast) showing intermediate sensitivity. For all five cell lines, a 48-h pretreatment with 5 mM DFMO reduced intracellular putrescine and spermidine content to less than 10% of control levels and decreased spermine content to between 60 and 70% of controls. This pretreatment resulted in a shift of the BCNU survival curves for each of the five cell lines downward and to the left, indicating that the cells were sensitized to the lethal effects of BCNU. Dose enhancement ratios for DFMO-induced chemosensitization ranged from 1.2 (HuTu-80 cells at the 1% survival level) to 1.9 (HT-29 cells at the 10% survival level). The cell lines most resistant to BCNU appeared to give the greatest degree of potentiation by DFMO pretreatment. For four of the five cell lines, addition of 50 to 100 microM exogenous putrescine to DFMO-pretreated cultures 12 to 24 h before BCNU addition reversed the chemosensitization. ME-180 cells were the sole exception. Exogenous putrescine did not increase the surviving fraction after BCNU of any cells not pretreated with DFMO. These results suggest that DFMO-induced chemosensitization to BCNU in the four cell lines other than ME-180 is a specific consequence of the inhibition of ornithine decarboxylase by DFMO and the resulting depletion of intracellular polyamine content.