Effect of single dose irradiation on the proliferation kinetics in a human malignant melanoma in athymic nude mice

Measurement of proliferation activity in human melanoma xenografts by magnetic resonance imaging

Tumor proliferation may be predictive for malignant progression and response to fractionated therapy of cancer. The purpose of the present work was to investigate whether the proliferation activity of solid tumors can be assessed in vivo from the proton relaxation times, T1 and T2. Tumors of four amelanotic human melanoma xenograft lines were studied. Three parameters were used to represent tumor proliferation activity; the volume doubling time, Tvol, the potential doubling time, Tpot, and the fraction of cells in S-phase. Tvol was determined from volumetric growth data. Tpot and S-phase fraction were determined by flow cytometric analysis of tumor cells after bromodeoxyuridine (BrdU) incorporation in vivo. T1 and T2 were measured by 1H-MRI in vivo, using spin-echo pulse sequences. The proliferation parameters and relaxation times differed considerably among the tumor lines. Significant correlations were found between the proliferation parameters and the relaxation times, regardless of whether Tvol, Tpot, or S-phase fraction was considered. Tumors with short Tvol and Tpot and high S-phase fraction had long T1 and T2 compared to tumors with long Tvol and Tpot and low S-phase fraction. The elongated T1 and T2 of fast growing tumors were probably due to increased interstitial and/or intravascular water content. The present results suggest that in vivo spin-echo 1H-MRI can be used to discriminate between tumors of high and low proliferation activity.

Tumour necrotisation in nude mice xenografts by the reversible protein synthesis inhibitor zilascorb(2H)

The deuterated benzaldehyde derivative zilascorb(2H), 5,6-O-benzylidene-d-L-ascorbic acid, was administered once daily by i.v. injection in nude mice with grafted tumours of a human malignant melanoma (E.E.) and ovarian carcinoma (OVCAR-3) origins. Like benzaldehyde, zilascorb(2H) has been shown to induce protein synthesis inhibition at otherwise non-toxic doses in cells grown in vitro, and acts reversibly in the sense that protein synthesis returns to normal shortly after removal of the drug. The present data indicate that daily injections with zilascorb(2H) induce a tumour volume growth inhibitory effect in both tumour xenografts studied. Furthermore, from histological examinations of each single tumour it was found that tumours of drug-treated animals, although smaller than those of placebo-treated (i.e. control) animals, had, on average, a higher necrotic fraction than control tumours. Thus, it is concluded that zilascorb(2H) induces tumour necrotisation and not just inhibition of the rate of tumour cell production. Continued measurement of tumour volume after ended treatment with zilascorb(2H) indicated that surviving tumour cells resumed their normal growth rate immediately. The reversibility of the effect induced by this compound, earlier observed in vitro only, is therefore here confirmed to be valid also in two different tumour xenografts in vivo. The present data accords well with the assumption that protein synthesis inhibition is the primary cellular effect of zilascorb(2H) in vivo. We therefore conclude that zilascorb(2H)-induced cancer cell lethality in tumour xenografts probably comes as a secondary consequence of prolonged protein synthesis inhibition.

Repopulation between radiation fractions in human melanoma xenografts

Rate of tumor repopulation between radiation fractions was studied using two human melanoma xenograft lines (E.F. and V.N.). Tumors were given five radiation fractions under hypoxic conditions in vivo and clonogenic cells survival was measured in vitro after the last radiation fraction. Dose-response curves were established for interfraction times of 12, 24, 36, and 48 hr by varying dose per fraction from 4.2 to 11.2 Gy. Assuming an oxygen enhancement ratio of 2.8, these doses corresponded to doses of 1.5 to 4.0 Gy under aerobic conditions, that is, clinically relevant doses per fraction were used. The dose-response curves were nearly parallel and were shifted to the right with increasing interfraction time, demonstrating significant repopulation between the radiation fractions. Iso-effect analyses showed that additional radiation doses of 2.0 +/- 0.6 Gy/day (E.F.) and 2.2 +/- 0.6 Gy/day (V.N.), corresponding to doses of 0.7 +/- 0.2 Gy/day (E.F.) and 0.8 +/- 0.2 Gy/day under aerobic conditions, would be required to compensate for the repopulation. These doses were equivalent to the surviving clonogenic cells showing doubling times of 40-50 hr (E.F.) and 30-40 hr (V.N.) during the treatment period. The radioresponsiveness of the two melanoma xenograft lines was also measured. Tumors in air-breathing mice were given from 5 to 15 daily fractions of 2.0 Gy and cell survival curves were established in vitro. Theoretical survival curves, calculated from SF2 in vitro and rate of repopulation during fractionated irradiation in vivo, agreed fairly well with the measured survival curves. This suggested that the radioresponsiveness of the melanoma xenograft lines was governed by two main parameters: (a) cellular radiation sensitivity and repair capacity and (b) rate of repopulation between radiation fractions.

Quantitative histological changes in a human melanoma xenograft following exposure to single dose irradiation and hyperthermia

The purpose of the present paper is, in terms of quantitative histology, to explain the growth response of a human melanoma xenograft after exposure to single dose irradiation (7.5 Gy, 15.0 Gy, and 25.0 Gy) and hyperthermia (42.5 degrees C for 60 min). Data from several experiments on the mitotic activity, the occurrence of different modes of cell death and reproductively dead cells in the tumors are discussed. The tumor cell proliferation is only transiently reduced after 7.5 Gy and 15.0 Gy, and tumor regression mainly results from an increased cell loss. Cell loss through apoptosis and cell disintegration during mitosis show a dose-dependent increase after irradiation. Although the fraction of necrosis, relative to the number of tumor cells, increases after 7.5 Gy and 15.0 Gy, the cell loss through necrosis, that is, the production of necrosis, is probably reduced. Compared to the cell loss through apoptosis and mitotic death, the removal of necrosis is probably less important in determining the regression rate of the tumors after 7.5 Gy and 15.0 Gy. After 25.0 Gy the cell production is markedly reduced, and cell loss increases, partly due to radiation injury to the vascular system, resulting in necrotization of the tumor core. Thereafter, the tumor regression rate depends mainly on the rate of necrosis removal. The mitotic activity of remaining cells is not reduced after hyperthermic treatment, and the tumor growth response is a result of an increased cell loss. Although the occurrence of apoptosis and cell disintegration in mitosis increases after hyperthermia, these modes of cell loss are of minor importance for the tumor regression after treatment. The increased cell loss is mainly due to massive necrosis formation in the central tumor areas, a result of heat injury to tumor blood vessels. After necrotization of the tumor core, the regression rate depends mainly on the rate of necrosis removal.

PLD-repair in human melanoma xenografts following single dose and fractionated irradiation

PLD-repair following single dose and fractionated irradiation was studied in vivo using five human melanoma xenograft lines. Tumours given single graded radiation doses were excised immediately after or 24 h after the radiation exposure for assay of cell survival in vitro. All melanoma lines showed PLD-repair after single dose irradiation: the PLD-repair factors, i.e. the ratio of the Do values for tumours excised 24 h after and immediately after irradiation, ranged from 1.2 +/- 0.1 to 1.4 +/- 0.1. PLD-repair following fractionated irradiation was studied by giving tumours seven fractions of 2.0 Gy over 7 days and then, after an interval of 24 h, single graded radiation doses in the range 6-21 Gy. Cell survival was assayed in vitro immediately after or 24 h after the last radiation exposure. The Do values as well as the surviving fractions were approximately equal after immediate and delayed cell seeding, i.e. none of the melanoma lines showed significant PLD-repair after fractionated irradiation. The lack of PLD-repair after fractionated irradiation was possibly a consequence of radiation-induced recruitment of quiescent tumour cells into the cell cycle. Consequently, PLD-repair is probably not a major cause of failure in the radiation therapy of malignant melanoma when treated with 2.0 Gy fractions.

The relationship between changes in tumour volume, tumour cell density and parenchymal cord radius in a human melanoma xenograft exposed to single dose irradiation

A human melanoma xenograft, in which the viable tumour tissue formed cylindrical cuffs around blood vessels, was irradiated with single doses of 7.5 Gy and 15.0 Gy respectively. The nuclear density, the frequency of giant cells and the mean parenchymal cord radius were measured and compared to the tumour growth response. After 7.5 Gy, the growth rate was only slightly reduced and there were no significant changes in the nuclear density or the mean parenchymal cord radius. After 15.0 Gy the mean parenchymal cord radius decreased the first week. This coincided with swelling of endothelial cell nuclei and reduced density of functional capillary-like vessels. Although the number of tumour cells declined, the tumour volume increased the first days after irradiation with 15.0 Gy since the cell density was reduced.

Single high dose-large field irradiation in drug resistant non-Hodgkin's lymphoma

Single high dose-large field irradiation (SHD-LFI), also described as half-body irradiation (HBI), has previously been reported as an effective modality for the palliation of symptoms in a number of solid tumors. This report concerns the ability of SHD-LFI to produce palliation of symptoms and/or objective response in patients with drug resistant non-Hodgkin's lymphoma (NHL). From 1981 to 1984, 34 patients with advanced drug resistant NHL were treated with SHD-LFI either to the whole abdomen (24 patients) or to the upper half body (10 patients). Overall, 19 of 23 patients achieved symptomatic improvement, while objective response was noted in 23 of 30 patients. We noted subjective and objective response in all histologies, and duration of response was not significantly different. Our results suggest a beneficial role for the early and judicious use of SHD-LFI in NHL.

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.

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.

Tumour growth delay following single dose irradiation of human melanoma xenografts. Correlations with tumour growth parameters, vascular structure and cellular radiosensitivity

The radiation response of 5 different lines of human melanoma xenografts was studied. Tumours grown s.c. in the flanks of athymic mice were exposed to single doses of 5-25 Gy and subsequently analysed with respect to specific growth delay. The variation in radiation response among these melanoma lines was almost as large as that reported for human tumour xenografts differing in histological type. The most radioresistant melanomas showed longer volume-doubling times, lower growth fractions, higher cell loss factors and lower vascular density than the most radiosensitive ones. The radiation response was not correlated to the fraction of cells in S-phase or the DNA content of the tumour cells. Cell suspensions prepared from the different melanomas, irradiated under aerobic conditions and assayed in soft agar, also showed large variability in radiation response. Specific growth delay after 15 Gy was found to be correlated to the surviving fraction measured in vitro after 6 Gy, but not clearly to the Do value. It is suggested that tumour growth characteristics in vivo as well as radiation response in vitro may be of prognostic value for prediction of radioresponsiveness of melanomas.

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.

Response to heat treatment (42.5 degrees C) in vivo and in vitro of five human melanoma xenografts

The response to heat (42.5 degrees C) of five human melanoma xenografts was studied. Tumours grown subcutaneously in the hind leg of athymic mice were heated in a water-bath and specific growth delay was used as a measure of response. In other experiments, cells from the same xenografts were heated in vitro and the colony-forming ability was assayed in soft agar. The slopes of the in-vivo dose-response curves (specific growth delay versus heating time) varied within a factor of about three among the five melanomas. The Do values of the in-vitro heat survival curves ranged from 44 +/- 3 to 123 +/- 15 min. The response to heat in vivo was not positively correlated with the tumour volume-doubling time, the growth fraction, the cell loss factor or the intrinsic heat sensitivity of the tumour cells, i.e., the Do values of the in vitro heat survival curves. If the results obtained in the present work are representative for clinical practice, they indicate that the response to heat may vary considerably among tumours in different patients. This variability can probably not be predicted from measurements of cytokinetic parameters of the tumours. The lack of correlation between the response to heat in vivo and in vitro demonstrates that extrapolations of results from studies in vitro to tumours are highly speculative and, when attempted, should be executed only with extreme caution.

Tumour growth delay, cell inactivation and vascular damage following hyperthermic treatment of a human melanoma xenograft

The effect of hyperthermia at 42.5 degrees C on a human melanoma xenograft in athymic mice was studied. The tumours were heated in vivo in a water-bath. Tumour growth delay and single-cell survival in vitro were used as endpoints. Qualitative information regarding heat-induced vascular damage was obtained from microangiographic analysis. Tumour growth delay after a given treatment was considerably longer than that expected from the cell survival measured in vitro immediately after treatment. Experiments in which removal of the tumours was delayed revealed that tumour cells were continuously dying for at least 24 hr after heat treatment. The volume of the tumour vasculature was considerably reduced after treatment, suggesting that the delayed cell death was attributed to vascular occlusion which resulted in an insufficient supply of oxygen and nutrients and an increased tumour acidity. The present work indicates that at least two mechanisms may be involved in heat-induced cell inactivation in our xenograft: firstly, direct cytotoxic effect of heat; secondly, indirect effect following heat-induced vascular damage.

Changes of proliferation kinetics after X-irradiation of a human malignant melanoma grown in nude mice

A human malignant melanoma grown in nude mice was exposed to single-dose X-irradiation and the effect on the proliferation kinetics was investigated by two methods. Flow cytometric DNA analysis was performed on tumour tissue obtained by sequential fine-needle aspirations after the treatment to monitor the initial cell cycle distribution changes. The technique of labelled mitoses was used to examine the kinetics of the tumours during regrowth. The results showed that the treatment initially induced a partial synchronization of small fractions of cells accumulated in the G2 phase of the cell cycle and a dose-dependent decrease of the cell generation time due to a shortening of the G1 duration time during regrowth of the tumours. Furthermore, it was shown that the calculated values of growth fraction and cell loss factor became unreliable because the tumours contained a dose-related increasing proportion of radiation-inactivated tumour cells.

Proliferation kinetics of a human malignant melanoma serially grown in nude mice

The technique of labelled mitoses and flow cytometric DNA analysis were used to determine the proliferation kinetics of a human malignant melanoma grown in nude mice. The effect of tumour volume and of long-term serial transplantation on the kinetic parameters was investigated. The results showed that the cell loss factor, which was the dominant factor in the growth of this melanoma, increased from 52 to 69% with increasing tumour size, whereas the calculated growth fraction showed no systematic changes. The cell generation time increased from 34 to 44 hr with tumour size, mainly due to a prolongation of the G1 duration time, whereas no significant changes occurred in the duration of the S and G2 phases of the cell cycle. The stability of the investigated tumour characteristics indicated that the kinetics of this melanoma remained unchanged during more than sixty serial transplantations in nude mice. The methods applied are suitable for a detailed description of tumour growth kinetics, since they provide complementary results.

Radiation and heat sensitivity of cells from two slowly growing human melanoma xenografts

The radiation and heat sensitivity of cells from two melanin-rich, slowly growing human melanoma xenografts (B.E. and R.A.) were studied. The volume-doubling times of the xenografts in the volume range 200-500 mm3 were found to be 22.5 47.5 days (B.E.) and 25.3-39.2 days (R.A.). The cells were suspended in culture medium during irradiation or heating, and the colony forming ability of the cells was assayed in soft agar. The X-ray survival curve parameters were found to be: Do = 1.09 +/- 0.14 Gy, Dq = 1.99 +/- 0.58 Gy (B.E.); Do = 1.23 +/- 0.08 Gy, Dq = 2.03 +/- 0.35 Gy (R.A.). The Do-values of the heat survival curves were found to be 119.0 +/- 26.6 min (42.5 degrees C), 20.4 +/- 3.9 min (43.5 degrees C) and 9.6 +/- 1.6 min (44.5 degrees C) for the B.E. melanoma and 112.9 +/- 13.3 min (42.5 degrees C), 17.9 +/- 2.0 min (43.5 degrees C) and 7.7 +/- 0.5 min (44.5 degrees C) for the R.A. melanoma. Both the radiation and the heat sensitivities of the cells are within the range of sensitivities reported for rapidly growing melanoma xenografts, suggesting that the intrinsic radiation and heat sensitivity of tumour cells are not strongly related to the rate of tumour growth prior to treatment.

Growth and vascular structure of human melanoma xenografts

The growth and the vascular structure of five human melanomas grown in athymic nude mice were studied. Four growth parameters (tumour volume doubling time, fraction of cells in S-phase, growth fraction, cell-loss factor) were analysed against each of four vascular parameters (length of vessels with diameters in the range 5-15 micron, total vessel length, total vessel surface, total vessel volume--all per unit of histologically intact tumour volume). Statistically significant linear correlations between the parameters were found for any of the combinations. However, there was a consistent trend in the data: the tumour volume doubling time and the cell-loss factor tended to decrease while the fraction of cells in S-phase and the growth fraction tended to increase with increasing vascular density, whichever vascular parameter was considered. This finding indicates that the vascular density is among the factors which are decisive for the growth rate of tumours. However, the present work does not exclude the possibility that intrinsic properties of the tumour cells may also be important.

Radiation and heat sensitivity of cells from human melanoma xenografts. Lack of correlations with tumour growth parameters

Five human malignant melanomas grown in athymic nude mice were studied. Tumour volume-doubling times were determined from Gompertzian growth curves, vascular volumes from stereological analysis of 2-microns thick tumour sections and DNA histograms by flow cytometric analysis. Single-cell suspensions prepared from the tumours were exposed to radiation or heat (42.5 degrees C; pH 7.4) under aerobic conditions in vitro and the colony-forming ability of the cells was assayed in soft agar. Tumours with short volume-doubling times tended to show higher fractions of cells in S-phase and higher vascular volumes than those with long volume-doubling times. The radiation and the heat sensitivity of the melanoma cells, i.e. the D0-values, were probably not positively correlated with the tumour volume-doubling time, the fraction of cells in S-phase or the vascular volume, or with each other either. The variation in radiation and heat sensitivity among cells from the different melanomas appears not to be due to external factors, but reflects, rather, intrinsic cellular differences.

Growth characteristics of human melanoma xenografts

The growth of twelve human malignant melanomas in athymic nude mice was studied. Gompertz curves were fitted to volumetric growth data. DNA histograms were obtained with flow cytometry. Each of the twelve melanomas exhibited a characteristic growth pattern, indicating that inherent properties of the tumours are important for the growth control. The theoretical maximum volumes (Vmax) ranged from 208 to 12,900 mm3, the volume doubling times (Td) from 2.8 to 15.3 days (V = 50 mm3) and from 3.8 to 64.6 days (V = 200 mm3), and the fraction of cells in S from 5 to 21%. Tumours with short Td were characterized by a higher growth fraction and probably by a lower cell loss factor than those with long Td. The growth was also influenced by the nude mouse host, as indicated by the values for Vmax, which were similar to those reported for mouse tumours (geometric mean = 8100 mm3), but considerably lower than the volumes of many tumours in man. Also the Td-values for the xenografts were generally lower than those reported for tumours in man, presumably due to a lower cell loss factor. During serial transplantation the growth rate of one of the melanomas increased abruptly, probably because of both an increased growth fraction and a reduced cell loss factor. The latter result demonstrates the necessity of keeping basic biological parameters of xenografts under observation during serial transplantation.

Growth kinetics of Bp8 mouse ascites sarcoma after single dose whole body irradiation. I. Analysis of the relative and total numbers of cells in various parts of the cell cycle

The effects of irradiation on the growth of the Bp8 mouse ascites sarcoma were analysed following doses of 1.75, 2.5, 5.0 and 8.0 Gy. From the total number of cells and the percentage of cells in G1, S-phase, G2 + M as measured by flow cytofluorometric DNA analysis and the mitotic index the total number of cells in the various parts of the cell cycle was estimated. After an initial delay in the increase in the cell numbers during the period of rapid growth the total number of cells shows a dose dependent decrease at the plateau stage of the ascites growth. This dose relationship is characterized by a shoulder type of curve with a D0 of about 6.5 Gy and Dq of about 2 Gy. This decrease in the total number of cells is caused by a decreasing number of G1, S-phase and mitotic cells while G2 cells generally remain at unchanged levels. This behaviour of the G2 cells reflects the preference of long-lasting blocking events in G2 of the cell cycle but may also indicate specific regulating processes linked to this type of cell. The ratio between the number of mitotic and G2 cells also decreases in a dose dependent way and is a sensitive indicator for irradiation effects even below the lowest dose (1.75 Gy) used in the present experiments.

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.