Enhancing effect of misonidazole on the response of the RIF-1 tumour to cyclophosphamide.

The effect of misonidazole (MISO) on the cytotoxicity of cyclophosphamide (CY) was investigated in the mouse. The response of the RIF-1 tumour was measured by growth delay and by cell survival in a cloning assay. MISO enhanced the cytotoxicity of CY. For single treatment, enhancement was maximal when MISO was given 30 min to 2 h before CY. The enhancement ratio (i.e. the dose of CY alone divided by the dose of CY with MISO required to cause the same response) increased with increasing dose of MISO up to 250 mg/kg, but decreased with increasing dose of CY above 50 mg/kg. For 5 daily treatments, enhancement increased with CY dose up to approximately 25 mg/kg/injection. Survival of marrow stem cells was measured using the spleen-colony assay. MISO did not enhance significantly the cytotoxicity of CY at doses under 100 mg/kg. Enhancement was seen at higher doses, but the effect was less than in tumours. CY reduced the number of circulating white blood cells. Neutrophils were most severely depleted. The WBC count was slightly lower when CY was given in combination with MISO than after CY alone, but the effect could be accounted for by direct MISO cytotoxicity. These experiences suggest that a therapeutic gain may be achieved if MISO is combined with doses of CY in the clinical range. From experiments performed to investigated the possible mechanisms involved, we conclude that for the RIF-1 tumour the major effect of MISO is to inhibit the repair from CY-induced potentially lethal damage.