Combined-modality oncotherapy with cyclophosphamide (NSC-26271) and radiotherapy: control of murine mastocytoma.

We investigated the effects of cyclophosphamide, alone and in combination with a 1,000-R/week radiotherapy schedule, on the growth of solid P815X2 tumors in 12-week-old male DBA/2 mice. Single-dose treatments of 150 mg cyclophosphamide/kg were given to animals bearing tumors of different ages. Such treatment of young tumors resulted in proportionately greater degrees of regression and steeper regrowth curves than did treatment of older tumors. Although slopes of regrowth curves differed greatly, time to regrowth (to pretreatment size) was the same for all age classes of tumors. Graded weekly exposures of 50-250 mg/kg for 4 weeks resulted in dose-dependent increases in incidence of complete remission, duration of remission (time to regrowth), and mean animal life-spans. The combination of radiotherapy to the tumor and 75, 150, or 225 mg cyclophosphamide/kg/week resulted in better local tumor control than occurred with radiotherapy or the drug alone. However, a dose-dependent increase in radiosensitivity of the gastrointestinal mucosa included in radiotherapy fields was observed. A 3-week course of radiotherapy plus 75 mg cyclophosphamide/kg/week (which is tolerated by the mucosa) increased animal lifespans to 165% of those of controls.