The effect of time between X-irradiation and chemotherapy on the growth of three solid mouse tumors. IV. Actinomycin-D

Experiments have been carried out to determine the effect of different intervals between the administration of X-radiation (1200 rod) and actinomycin-D (200 μg/kg) on the growth delay produced in three mouse tumors. The tumors used were the EMT6 tumor in BALB/c mice and the KHT and RIF-1 sarcomas in C3H mice. All tumors were grown intramuscularly in the gastrocnemius muscle, and treatment was carried out at a mean tumor weight of 450 mg. Time to reach 2 × (for KHT) or 4 × (for EMT6 and REF-1) treatment volume was used as the endpoint of response. The drug was administered intraperitoneally either 24, 6, or 2 hr before radiation, immediately before the start of radiation, or 3, 6, or 24 hr after radiation. All irradiations were carried out in unanesthetized mice. For a single administration at this dose level (close to the maximum tolerated dose) actinomycin-D did not produce a significant delay in the growth of any of the tumors. For the RIF-1 and KHT tumors, the growth delays produced by drug/radiation combinations generally were not significantly greater than that produced by irradiation alone. For the EMT6 tumor, great variability in the growth delays of combined modality groups was seen, with mean growth delays significantly longer than predicted by the radiation alone data. No consistent dependence on timing between irradiation and drug administration was seen.