Tumor Control Assay Research Articles

The potential of using hyperthermia to eliminate radioresistant hypoxic cells

It is known that cells in a nutritionally deprived and acidic environment are sensitive to heat. In general these same cells are chronically hypoxic and therefore heat possesses the potential to eliminate (some) of this radioresistant population. A direct radiosensitization is observed when heat is given simultaneously with radiation. This effect occurs to the same extent in both aerobic and hypoxic cells, thus the oxygen enhancement ratio is unchanged. By giving heat several hours after radiation the direct radiosensitization is avoided and the specific heat killing of the acidic, chronically hypoxic, tumor cells may be utilized to improve the therapeutic gain. The current investigation clearly demonstrates this concept in a C3H mammary carcinoma using a local tumor control assay. This effect could be further enhanced by adding hypoxic radiosensitizers (nimorazole, misonidazole) or a blood flow modifier (nicotinamide) which can eliminate acutely hypoxic cells.

Hydralazine-induced Changes in Tissue Perfusion and Radiation Response in A C3H Mammary Carcinoma and Mouse Normal Tissues

Hydralazine has been reported to reduce blood perfusion in tumours, thereby increasing hypoxia and subsequently enhancing tumour sensitivity to certain drugs and hyperthermia. We have investigated the ability of hydralazine to induce such changes in a C3H mouse mammary carcinoma and various normal tissues. In tumours, hydralazine (5 mg/kg; i.v.) modified the radiation response, measured by a local tumour control assay, producing an effect equivalent to that seen in tumours made fully hypoxic by clamping. This effect was time-dependent and correlated with the decrease in tissue perfusion estimated by the 86-RbCl extraction procedure. Similar effects were seen in normal skin, although the changes were less dramatic and of a shorter duration. Hydralazine also reduced 86-RbCl uptake in liver, kidney, gut and spleen, but not in bladder, muscle and lung, suggesting that it may have the potential to increase the sensitivity of some normal tissues to hypoxic cell cytotoxins.

Cytotoxic effect of misonidazole and cyclophosphamide on aerobic and hypoxic cells in a C3H mammary carcinoma in vivo

The chemosensitising effect of the nitroaromatic radiosensitiser misonidazole (MISO) on the alkylating agent cyclophosphamide (CTX) has been investigated in a C3H mammary carcinoma in CDF1 mice. The selective cytotoxicity against aerobic and hypoxic cells was measured indirectly, using a local tumour control (TCD50) assay. The hypoxic fraction was calculated from the dose difference between the TCD50S for tumours irradiated either in air or under clamped conditions. The relative survival of tumour cells after drug therapy was expressed as a surviving fraction (SF). CTX (100 mg kg-1) was found to be considerably more toxic towards hypoxic than aerobic cells (SF 4% versus 52%). MISO (1000 mg kg-1) was almost exclusively toxic to hypoxic cells (SF 22%). When MISO and CTX were administered simultaneously a decrease in the surviving fraction was observed. The effect on aerated cells was found to be 10-fold more than expected from addition of toxicities, suggesting a chemosensitising effect on these cells by MISO when used in combination with CTX. No synergistic effect was found on radiobiologically hypoxic cells. The exact role of hypoxia for the development of chemosensitisation seems to be complex and requires additional research in the future.

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 TCD 50 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 TCD 50. 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.

Hypoxic Fraction Determinations with the BA1112 Rat Sarcoma: Variation within and among Assay Techniques

Measurement of the hypoxic fraction in the same tumor system by different assay techniques often gives incompatible results. The hypoxic fraction of the BA1112 sarcoma has been measured by three assay techniques: tumor control and growth delay assays in which the tumors remain in situ after irradiation, and paired survival curve assays in which the tumors are excised after irradiation. The assays were done with and without anesthesia on tumors growing in two different subcutaneous sites. Anesthesia of the hosts produced a statistically significant decrease in the calculated hypoxic fraction in a tumor control assay, but not in a paired survival assay. The excision assays gave consistently higher hypoxic fraction estimates than either the tumor control or growth delay assays. Some of the factors which could produce higher hypoxic fractions in excision assays than in in situ assays are discussed. Interpretation of the results was complicated by disagreements between the growth delay and tumor control assays, and by disparities between replicate determinations. The discrepancies among assays are caused almost entirely by variations in the response of the artificially hypoxic tumors, rather than variations in the response of aerobic tumors. These results indicate that assumptions commonly made when measuring hypoxic fractions may not be valid.

Assessment of local tumor control using censored tumor response data

When animals die in the course of tumor control assays, the loss of information need not be absolute. Actuarial survival curves may be used to estimate local tumor control fractions, and these unbiased estimates may be weighted and used to estimate the parameters of dose/cure functions. The current standard procedure of discarding from analysis animals who do not complete a pre-assigned observation period is biased, and should not be used.