Radiosensitization by misonidazole of a human malignant melanoma grown in athymic nude mice

Hypoxia and reoxygenation in human melanoma xenografts

Tumor hypoxia and reoxygenation pattern following single dose (10.0 Gy) and fractionated (7 fractions of 2.0 Gy, 1 fraction per day) irradiation were studied in five human melanoma xenograft lines using the paired survival curve method. The hypoxic fractions differed significantly among the melanoma lines; they were found to be 6 +/- 3% (E.E.), 22 +/- 8% (E.F.), 31 +/- 11% (G.E.), 45 +/- 17% (M.F.), and 15 +/- 5% (V.N.). There were no clear correlations between hypoxic fraction and tumor volume-doubling time or vascular density, suggesting that intrinsic cellular characteristics, for example, rate of oxygen consumption and ability to retain clonogenicity under hypoxic stress, also may play an important role for the magnitude of the hypoxic fractions in the melanomas. Reoxygenation was rapid and extensive in all melanoma lines; 12-24 hr after the single dose irradiation or the last fraction of the fractionated irradiation, the hypoxic fractions were similar to those in untreated tumors and stayed at that level up to at least 10 days after irradiation. The hypoxic fractions 1-10 days after irradiation tended to be higher after fractionated than after single dose irradiation, but the differences were not statistically significant. There was a positive correlation between the hypoxic fractions in untreated tumors and the hypoxic fractions after irradiation and reoxygenation, suggesting that it may be possible to predict radiation resistance caused by hypoxia from the hypoxic fractions in tumors before start of radiation therapy. However, hypoxia is probably not a major cause of failure in the radiation therapy of malignant melanoma.

Radioresponsiveness of human melanoma xenografts given fractionated irradiation in vivo--relationship to the initial slope of the cell survival curves in vitro

The radioresponsiveness of five human melanoma xenograft lines given fractionated irradiation in vivo was studied using specific growth delay and cell survival in vitro as endpoints. Superfractionation (3 fractions of 2.0 Gy with 4-h intervals each day) as well as conventional fractionation (1 fraction of 2.0 Gy each day) were used. The total dose was varied within the ranges 12 to 30 Gy and 10 to 30 Gy, respectively. The rankings of the melanomas in radioresponsiveness were almost identical, irrespective of the endpoint and the fractionation regime considered. The radioresponsiveness was found to be positively correlated to the initial slope of the in vitro cell survival curves, i.e. the alpha and the surviving fraction at 2.0 Gy (conventional dose rate; 3.0 Gy/min) and the D0 (low dose rate; 1.25 cGy/min). There was no relationship between the radioresponsiveness and any known growth or microenvironmental parameter. It is concluded that the differences in radioresponsiveness among the melanomas for the fractionation regimes studied here were governed mainly by the intrinsic repair capacity of the tumour cells and not by microenvironmental factors. The potential of in vitro cell survival curve parameters in predicting the clinical radioresponsiveness of tumours is discussed.

Response of human tumour xenografts to fractionated X-irradiation

The response of two human tumour xenografts to single dose and fractionated X-rays has been tested using regrowth delay as the assay. The tumours were line transplanted cells from a moderately well-differentiated squamous carcinoma of the tonsillar fossa (XJ) and an undifferentiated carcinoma of the floor of the mouth (XR). Comparison of the dose response curves for single doses in air, clamped, or after misonidazole administration, led to estimates of the hypoxic fraction (approximately 15%) and the sensitizer enhancement ratio (less than or equal to 1.6). When 5 daily fractions were used, the effect of misonidazole (miso) was lost and reoxygenation appeared to be effective in both tumours. Comparison of single doses and 5 fractions in clamped tumours, and in those sensitized by miso, allowed the sparing effect of fractionation to be estimated. When analysed by the linear quadratic model the alpha/beta ratios were found to be in the range of 6.4-9.2 Gy and 6.8-16.0 Gy for the two tumours. These values are in good agreement with murine tumours (assayed in vivo or in vitro), with human tumour cells assayed in vitro, and with analyses of fractionated clinical data for skin cancer.

Radiation biology of malignant melanoma

The survival curves for melanoma cells exposed to single radiation doses in vitro and the specific growth delays for melanoma xenografts irradiated with single doses in vivo were found to differ considerably among individual cell lines and tumours. In fact, the differences could be almost as large as the largest differences observed among cell lines and xenografts from tumours of different histology with very different clinical radiocurability. Moreover, radiobiologic parameters that may have significant influence on tumour response to fractionated irradiation, e.g. growth rate, hypoxic fraction, reoxygenation ability, PLD-repair capacity and contact repair capacity, were found to differ greatly in magnitude among individual melanomas. This review therefore concludes that malignant melanoma is a tumour type that is very heterogeneous in radioresponsiveness, i.e. malignant melanomas should no longer be considered to be radiation resistant in general. The values of the alpha/beta ratio derived from cell survival curves for melanoma cells irradiated in vitro and melanoma xenografts irradiated in vivo were found to cover a wide range relative to those for acutely and late responding normal tissues. Although these alpha/beta ratios are no more than estimates of the effective alpha/beta ratios in a clinical situation, they still indicated that hyperfractionation may be beneficial in the treatment of some melanomas, whereas others may be more efficiently treated by use of conventional fractionation regimes, either based on 2 Gy or higher doses per fraction. Consequently, optimum radiation therapy of malignant melanoma will probably require an individualized treatment strategy. In vitro assays for prediction of radiocurability and choice of treatment strategy for individual melanoma patients seem therefore highly warranted.

Human tumour xenografts in radiotherapeutic research

The radiation response of human tumour xenografts has been shown to vary considerably among tumours of different histological types, tumours of the same histological type and cell subpopulations of single tumours. There is encouraging evidence that the radiation response correlates with clinical responsiveness when xenografts are exposed to single radiation doses and single cell survival in vitro or growth delay in vivo is used as endpoint. If subsequent research supports this conclusion, human tumour xenografts may be useful in studies aimed at (a) elucidating the underlying mechanisms for intertumour differences in radiation response and (b) developing short-term in vitro assays for clinical radiosensitivity testing. However, there are at least three main disadvantages with xenografts as models for human cancer. Firstly, the volume-doubling time is usually shorter than for tumours in man. Secondly, the vascular system and the supporting stromal elements originate from the host. Thirdly, host defence mechanisms may be active against xenografts. The radiation response of xenografts may be influenced by these three aspects and hence fail to reflect clinical responsiveness, especially when exposed to fractionated irradiation or when local tumour control is used as endpoint.

Radiosensitivity of Nall human melanoma transplanted into nude mice: repair, reoxygenation and dose fractionation

Split and fractionated gamma-rays experimental have been performed on a human melanoma transplanted into nude mice using an in vitro colony assay. Repair of potentially lethal observed after a single dose of 20 Gy was found to no longer occur when 7 daily doses of 2.5 Gy were administered. In split-dose experiments, the increase in survival level probably can not be explained by repair of sublethal damage. When a single high dose of radiation is administered a certain reoxygenation is observed; however there is no reoxygenation when low radiation doses are delivered daily.

Broad-shouldered survival curves of a human melanoma xenograft. Implications for radiation therapy in the absence and presence of misonidazole

A human malignant melanoma (E.E.) was irradiated in vivo in athymic nude mice, with subsequent assay of single cell survival in vitro. By assuming the survival curves for the hypoxic as well as for the aerobic cells in the tumour to be of the form S = exp(-D/D1) X (1-[1-exp (-D/D2]n), theoretic survival curves were fitted to the experimental data for tumours irradiated in air-breathing mice. Assumptions were made about hypoxic fraction, oxygen enhancement ratio, sensitization by misonidazole, repair of potentially lethal damage, and reoxygenation, all based upon own experimental data on E.E. melanoma. Theoretic surviving fractions were calculated for several clinically relevant fractionation regimes, both for irradiation in the absence and in the presence of misonidazole. The results indicate that tumours with biologic parameters like those of E.E. melanoma are best treated with fractionation regimes with few fractions and high doses per fraction, whether misonidazole is present or not.