Effect of cancer chemotherapy on the hypoxic fraction of a solid tumor measured using a local tumor control assay

The effect of mitomycin C (MMC), adriamycin (ADM), cyclophosphamide (CTX), cisplatinum (cis-DDP) and bleomycin (BLM) on the aerobic and hypoxic cells of a C3H mammary carcinoma in CDF1 mice was investigated using the tumor control assay. Hypoxic fractions (HF) were calculated by an indirect technique using the horizontal displacement of the TCD50 value from the dose-response curves of tumors irradiated under normal or clamped conditions. The HF and absolute number of tumor cells following a combined treatment was compared to that obtained with radiation alone. MMC, ADM and CTX had a significant enhancing effect on the unclamped TCD50. All three drugs caused a marked reduction in the proportion of hypoxic cells, decreasing the HF from 5.4% to about 1 % of the total cell number. The surviving proportion of hypoxic cells were 11.1, 8.9 and 6.5% respectively. Killing of aerobic cells was also observed but the effect was less than that seen on the hypoxic cells, with the survival only being reduced to between 38 and 68% of the total number of aerobic cells. In contrast, cis-DDP and BLM were shown to produce major cell killing in the aerobic compartment but actually showed no cytotoxicity towards hypoxic cells. This would explain the lack of radiation enhancement observed for these two drugs. We conclude that the ability of adjuvant drugs to improve radiation response is dependent on the hypoxic cell killing by the drugs.