Time-dose relationships for simultaneous misonidazole and 1,3-bis(2-chloroethyl)-1-nitrosourea exposures in vitro

Abstract

The effect of incubating KHT-iv tumor cells with 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) alone or in simultaneous combination with the radiation sensitizer misonidazole (MISO) was determined for aerobic and hypoxic incubations. Relative to aerobic exposures, the cytotoxicity of BCNU was significantly enhanced when KHT-iv cells were exposed under hypoxic conditions for 1 hr. This effect was apparently related to a trypsin effect and was absent in experiments with cells trypsinized immediately prior to treatment. In contrast to 1-hr exposures, treatment for 4 hr to the same total BCNU exposure doses resulted in equivalent cell killing under aerobic and hypoxic conditions. The addition of 3 mM MISO to the 1-hr treatment protocol did not significantly modify the toxicity of BCNU toward aerobic or hypoxic cultures of KHT-iv cells. However, the cell-killing efficiency of BCNU during a 4-hr hypoxic exposure was significantly enhanced by the addition of MISO doses as small as 0.5 mM. Addition of sensitizer did not modify the 4-hr aerobic toxicity of BCNU. The decay of BCNU was not significantly altered by MISO or by low oxygen tensions, effectively eliminating the role of altered pharmacokinetics in this example of in vitro chemosensitization. These results suggest that prolonged exposure times and hypoxia are prerequisites for the expression of chemosensitization when MISO is combined simultaneously with certain chemotherapeutic agents in vitro. By comparison to the doses of MISO used in preincubation experiments, it was possible to produce enhanced cytotoxicity by relatively small MISO doses.