Characterization of radiation resistant hypoxic cell subpopulations in KHT sarcomas. (I). Centrifugal elutriation

Direct relationship between radiobiological hypoxia in tumors and monoclonal antibody detection of EF5 cellular adducts

While the potential importance of hypoxia in limiting the sensitivity of tumor cells to ionizing radiation has long been appreciated, methods for accurately quantifying the number of radiation-resistant hypoxic cells within tumors have been lacking. We have used the pentafluorinated derivative [2-(2-nitro-1H-imidazol-1-yl)-N-(2,2,3,3,3-pentafluoropropyl)-acet amide] of etanidazole (EF5), which binds selectively to hypoxic cells. The adducts formed between EF5 and cellular proteins in the hypoxic cells were detected using the specific monoclonal antibody (MAb), ELK3-51 conjugated to the flurochrome Cy3, and the number of hypoxic cells was quantified by flow cytometry. To verify the validity of this technique for the detection of hypoxic cells, mice bearing KHT sarcomas were treated with various agents to alter tumor oxygenation and hence vary the fraction of radiobiologically hypoxic tumor cells. The percentage of EF5 binding cells was then compared directly with the clonogenic survival of the tumor cells following radiation treatment under the various pretreatment conditions. The results showed that allowing the mice to breathe carbogen (5% CO2/95% O2) prior to irradiation reduced clonogenic cell survival approx. 6-fold and led to an absence of cells binding high levels of EF5. In contrast, pretreating the tumor-bearing animals with either hydralazine, which decreased tumor blood flow, or phenylhydrazine hydrochloride, which made the mice anemic, increased tumor cell survival following irradiation 2- to 4-fold, indicative of an increase in the fraction of hypoxic tumor cells. EF5 measurements made under identical conditions illustrated a shift in the cells in the tumor to high EF5 binding. Our results demonstrate that flow cytometric measurement by fluorescent MAb binding to EF5 adducts may relate directly to radiobiological hypoxia in KHT tumors measured by conventional methods.

The radiation response of KHT sarcomas following nicotinamide treatment and carbogen breathing

Preclinical investigations have demonstrated that both diffusion- and perfusion-limited hypoxic cells may exist in tumors. One approach to target such hypoxic cell subpopulations is through the combined application of nicotinamide (NIC) administration and carbogen (5% CO2:95% O2) breathing. Because carbogen pre-irradiation breathing time (PIBT) can markedly influence the radiosensitizing effectiveness of this gas mixture, in the present experiments the effect of localized radiation on the transplantable KHT sarcoma was investigated in mice receiving NIC while breathing carbogen for various periods of time. When mice were given carbogen prior to radiation therapy, there was a minimum in tumor cell survival for PIBTs of 2-30 min. Longer PIBTs led to a loss of the radiosensitizing effect. NIC, administered as a 1000-mg/kg dose, effectively enhanced radiation cell killing in this tumor if given 45 min to 2 h prior to radiotherapy. In experiments in which either agent was combined on its own under optimum conditions (carbogen, 10 min PIBT; or NIC, 1000 mg/kg 2 h prior to irradiation), with a range of single doses of radiation, the results showed an enhancement ratio of approximately 1.9 as determined from the ratio of the slopes of the cell survival curves obtained in the absence or presence of the radiation sensitizer. This sensitizing effect could not be increased further when NIC and carbogen breathing were combined under optimum conditions.

BW12C: effects on tumour hypoxia, tumour thermosensitivity and relative tumour and normal tissue perfusion in C3H mice

BW12C (5-[2-formyl-3-hydroxypenoxyl] pentanoic acid) is an agent which stabilises oxyhaemoglobin and thus reduces oxygen delivery to tissues. It is of interest as a possible potentiator of bioreductive agents and/or hyperthermia. The increases in radiobiological hypoxic fraction of RIF-1 and KHT tumours 30 min after 70 mg kg-1 BW12C i.v. were measured and shown to be similar; factors (+/- 2 s.e.) ranged from 3.87 (2.84-5.29) to 5.92 (1.92-18.2) despite the large variation in initial hypoxic fraction, from 0.30 (0.18-0.50) % for RIF-1 intramuscularly in the leg to 16.3 (14.7-18.1) % for subcutaneous KHT flank tumours. Thermosensitivity of intramuscular KHT leg tumours was not enhanced by 70 mg kg-1 BW12C 30 min before heating at 43 degrees C, 43.5 degrees C or 44 degrees C, assayed by regrowth delay. The effect of 70 mg kg-1 BW12C on relative tissue perfusion (RTP), assayed by 86Rb extraction, was measured from 0.5 h to 6 h after treatment. After 1 h RTP (+/- 2 s.e.) in RIF-1 tumours was reduced to 84 +/- 5.7% and 68 +/- 9.6% of control in leg and flank tumours respectively, and to 86 +/- 6.4% in leg muscle while flank skin RTP was unaltered at 109 +/- 8.6%. There were substantial increases in kidney (149 +/- 10.7%) spleen (173 +/- 22.1%) and lung (128 +/- 10.4%) at 1 h but in liver there was a decrease at 2 h to 85 +/- 8.4%. Dose response studies showed that the threshold dose for reduction of tumour RTP is between 55 and 70 mg kg-1, but perturbations in normal tissue RTP occur at lower doses, e.g. 40 mg kg-1 for spleen. BW12C had minimal effects on renal function measured by 51CrEDTA clearance. The data as a whole indicate that reduction in tumour perfusion is likely to be an important determinant in the increase in tumour hypoxia induced by BW12C.

Evaluation of cell subpopulations isolated from human tumor xenografts by centrifugal elutriation

Human epidermoid tumor cells (Coll2, ME180, A431, HEp3) grown as xenografts in nude mice, were dissociated into single cell suspensions using an enzyme cocktail containing 0.025% collagenase, 0.05% pronase, and 0.04% DNase. The dissociated cell suspensions were separated by centrifugal elutriation into fractions containing homogeneous cell subpopulations primarily based on the differences in the rates of sedimentation. The quality of separation was evaluated by several techniques including flow cytometry, cell volume distributions, in vitro colony forming assay and morphological examination of Wright-Giemsa stained cells. In each separated fractions, the host to neoplastic cell ratio, the DNA ploidy, the plating efficiency and the cell cycle distribution were determined. After an initial separation of non-neoplastic host cells from malignant cells, a purity of greater than 95% host cells was obtained from the four xenografts studied. DNA analysis of tumor suspensions showed that neoplastic cells of different xenografts contained aneuploid cells with a DNA index of 1.51 to 1.95. The neoplastic cells were further separated into fractions according to their positions in the cell cycle. Fractions containing greater than 95% G1, 65% S, and 72% G2M cells were obtained from HEp3 xenografts. Less efficient separation with respect to cell cycle was attained with cells derived from Coll2, ME180, and A431 xenografts. Colony forming abilities of the neoplastic cells were determined at different phases of the cell cycle and found to be similar to those of the unseparated cell suspensions after corrections for non-neoplastic host cells were made. These investigations indicate that centrifugal elutriation is an effective technique of obtaining homogeneous subpopulations of cells from human tumor xenografts for various tumor biology and cell kinetics studies.

Characterization of radiation resistant hypoxic cell subpopulations in KHT sarcomas. (II). Cell sorting

Hypoxic cells in KHT sarcomas were characterized using fluorescence activated cell sorting based on the diffusion properties of the fluorochrome Hoechst 33342. Tumour-bearing female C3H/HeJ mice were injected i.v. with 10 micrograms g-1 Hoechst 33342 and the cells derived from the tumours sorted on the basis of their staining intensities. For each sorted fraction the DNA histogram was evaluated using FCM analysis. The results indicated that the bright and dim cells were not equally distributed about the cell cycle. For example, a greater proportion of S phase cells were in the bright subpopulations whereas the dim subpopulations contained an increased proportion of cells in G1. When the tumours were irradiated with a single dose of radiation prior to cell sorting, the dim cells survived preferentially. Dose response curves for the 20% most dim and 20% most bright cells, sorted on the basis of fluorescence intensity, then were determined. The survival curves of the dim and bright cells were found to have slopes similar to those of KHT cells irradiated in situ in dead animals or in vitro under fully oxic conditions, respectively. In addition, when KHT sarcoma-bearing mice were given a 2.5 mmol kg-1 dose of misonidazole (MISO) prior to irradiation and cell sorting, the dim subpopulation was sensitized whereas the bright subpopulation was not. These findings suggest that (i) compared to well-oxygenated areas, hypoxic regions of KHT tumours contain a smaller percentage of cells actively proliferating and (ii) Hoechst 33342 sorting may allow the detailed in situ evaluation of agents acting directly against hypoxic cells in solid tumours.

Applicability of animal tumor data to cancer therapy in humans

The problem of applying experimental tumor studies to clinical cancer therapy is a complex one. The radiotherapy literature contains many examples of premature efforts to apply laboratory observations to the clinic, and many examples of failures to adequately consider animal tumor observations in the design of clinical studies. This review covers three areas: tumor hypoxia, where clinical trials based on animal tumor data have been conducted with radiosensitizers, hyperbaric oxygen, and systemic oxygen carriers; dose fractionation, where current trials of hyperfractionation are based in part on animal tumor studies; and chemo-radiotherapy, where clinical trials are only beginning to exploit concepts developed in animal tumor systems. The use of animal tumor systems extends past the screening of new agents. Animal tumor models can be used in biological, physiological, and pharmacological studies to elucidate the biological factors influencing the efficacy of therapeutic agents. Tumor studies can be combined with studies of normal tissues to predict the toxicities to be anticipated in clinical trials, and to assess the potential for therapeutic gain. Animal studies can also provide data which are useful in designing optimal clinical trials of new agents and maximizing the potential for successful clinical application of new approaches. In general, it is not possible to apply specific laboratory data directly to man. To translate, rather than transpose, information from the laboratory to the clinic, the model studies must be directed at evaluating principles, rather than merely quantifying results. Only through studies of mechanisms, by designing experiments to test or refute a hypothesis, will it be possible to apply model studies to man.