Abstract

The effect of misonidazole on the cytotoxicity of 4-S-(propionic acid)-sulfidocyclophosphamide (C-2) was assessed by measuring the colony-forming ability of mouse L1210 leukemia cells. C-2 under physiological conditions spontaneously hydrolyzes to 4-hydroxycyclophosphamide. Misonidazole alone at a concentration of 2.5 mM was only slightly toxic to hypoxic L1210 cells and allowed a greater than 90% survival following a 2-hr exposure. Combined treatment of C-2 and 2.5 mM misonidazole resulted in a cell kill that was greater than the additive toxicities of C-2 and misonidazole. The synergistic toxicity of the C-2 and misonidazole (2.5 mM) combination increased with increasing C-2 concentration, and at 0.01 survival the dose-modification ratio of C-2 alone versus the combination was approximately 1.5. Similarly, when the concentration of C-2 was held constant (10 microM) and the concentration of misonidazole varied from 2.5 to 25 mM, a cell kill greater than the additive toxicities of misonidazole and C-2 alone was observed. The kinetic patterns of formation and removal of DNA interstrand cross-links following a 2-hr treatment of 10 microM C-2 or 10 microM C-2 plus 2.5 mM misonidazole were similar. However, with the exception of the 0-hr time point, cells treated with the C-2 plus misonidazole combination showed consistently greater cross-linking of DNA than did cells treated with C-2 alone. The interstrand cross-link ratio closely correlated with the cytotoxic dose-modification ratio of the combination compared with C-2 alone.