Degree Of Thermotolerance Research Articles

Thermochemotherapyin vivoof a C3H mouse mammary carcinoma: Thermotolerant tumours

The ability of cyclophosphamide (CTX) and mitomycin C (MMC) to modify the expression of thermotolerance in vivo at 43.5 degrees C was investigated in a transplantable C3H mouse mammary carcinoma grown s.c. in feet of C3D2F1/Bom mice. Dose-effect curves subjected to linear regression analysis were constructed for single-fraction treatment and for a second treatment 24 h after a priming heat treatment of 43.5 degrees C for 30 min. Tumour volume doubling time during regrowth showed no significant variation among treatment groups, justifying the use of tumour growth time as effect parameter. Thermotolerance ratio for heat alone was 11.6 +/- 2.3. Drug enhancement ratio in thermotolerant tumours was 6.2 +/- 1.4 for CTX (100 mg/kg) and 3.5 +/- 0.9 for MMC (3 mg/kg). These values are about 4 and 3 times larger than the corresponding enhancement ratios found for previously untreated tumours. Thermotolerance ratio for thermochemotherapy was 2.6 +/- 0.3 for CTX and 4.4 +/- 0.7 for MMC, i.e. the degree of thermotolerance was substantially reduced by both drugs, but not completely overcome. Thermal enhancement ratio for CTX and MMC was about equally large in thermotolerant and previously untreated tumours. Thermotolerant tumours showed a tendency for increased resistance to drug treatment alone. Both CTX and MMC may be used clinically to reduce the expression of thermotolerance, particularly in situations with inhomogeneous heating and short fractionation intervals.

Thermotolerance induced by fractionated hyperthermia: dependence of the interval between fractions.

The induction of thermotolerance by fractionated hyperthermia was investigated in the mouse ear. Ears were heated at 43.5 degrees C by immersion in water. One to ten treatments of 20 min were followed by test treatments. Thermotolerance was assessed as the increase in the duration of the test treatment required for a thermal response in 50 per cent of the ears (NT50). A single treatment induced thermotolerance which reached a maximum at 24 h when the NT50 was increased by a factor of 2.4. The same maximum was observed after each fractionated treatment used in the present study. The time course of development, however, depended on the interval between fractions. (1) When the interval was too short to allow development of thermotolerance after a single fraction (4 h), thermotolerance was not induced during fractionated treatment but it developed during the first 24 h after treatment. (2) When the interval between fractions allowed the maximal development of thermotolerance (24 h), this maximum was maintained during fractionated treatment and persisted for 24 h after treatment. (3) When the interval allowed some decay of thermotolerance (72 or 168 h) there was a further increase to maximal thermotolerance after each fraction. The decay of thermotolerance from the maximum did not depend on the interval between fractions. These results indicate that the degree of thermotolerance may fluctuate during fractionated hyperthermia.

Experimental studies of thermotolerance in vivo. I. The baby rat tail model.

A detailed study of the kinetics and magnitude of thermotolerance has been made using the endpoint of loss of vertebrae in the baby rat tail. A range of different priming treatments was used and for analysis each priming treatment (PT) was given in terms of the heat sensitivity of the tissue as defined by the single treatment (Ds) required for a given effect, i.e. fractional priming treatment PT/Ds. A considerable degree of thermotolerance could be induced so that at maximum the tissue sensitivity was reduced by a factor of more than four in heating time to produce a given effect, or by more than the equivalent of 2 degrees C. The main results of the study were: (a) that the time to reach maximum thermotolerance depends primarily on PT/Ds, independent of the particular conditions used in the priming treatment; (b) that the extent of the maximum also depends primarily on PT/Ds, but the precise form of the relationship depends on the method used to define thermotolerance; and (c) that the rate of decay of thermotolerance is independent of the priming treatment. The implications of these results for clinical hyperthermia are briefly discussed.

Thermotolerance after fractionated hyperthermia: assessment of cell survival by response to X-rays.

A previous study of the mouse ear showed that daily treatment at 43.5 degrees C, either 10 X 20 min or 20 min + 9 X 70 min, induced the same resistance to further heating as was induced by a single treatment of 20 min. The results could be explained in at least two ways: (a) no cells are killed by the heat treatments but thermotolerance is induced; and (b) a proportion of cells is killed by each fraction and the degree of thermotolerance induced in survivors increases as the number of fractions is increased. These two possibilities were tested by measuring the response to X-rays at 24 h after various regimes of fractionated hyperthermia. At this time interval the enhancing effect of a single heat treatment would have decayed, so that radiosensitivity should then be related to the number of surviving cells. Up to 49 daily treatments of 20 min had little effect on radiosensitivity, suggesting that these heating regimes did not cause a significant reduction in the number of basal epidermal cells. A regime of 20 min + 4 X 70 min daily also had little effect but a treatment of 20 min + 9 X 70 min daily increased the radiation response, suggesting that the more severe heat treatment had reduced cell survival to approximately 4 per cent.

Investigation of thermotolerance in mouse testis.

The effect of a two-fraction heat treatment on mouse testis has been assessed by measuring testis weight loss at 1 week after treatment. The rate of repair of 'sublethal' heat damage following the first treatment was dependent on the severity of the treatment. Using a primary treatment of 41.5 degrees C for 30 min, the weight loss following a test treatment of 41.5 degrees C for 30 min returned to that of the test treatment alone within an interval of 16-24 h. Using a milder primary treatment of 40.0 degrees C for 30 min, repair of sublethal heat damage appeared to be complete by 1-2 h. When a single test treatment was used, there was no evidence of heat-induced thermal resistance (thermotolerance) following primary treatments of 40.0 or 41.5 degrees C for 30 min, for periods up to 24 h between treatments. A small degree of thermotolerance could, however, be demonstrated following the most severe primary treatment used if full dose: effect curves were obtained. Thermotolerance, manifest as a decrease in slope, was maximal at approximately 4 h after the primary treatment. The results are discussed with reference to other normal tissue data.

Modification of the Thermal Response by D 2 O: II. Thermotolerance and the Specific Inhibition of Development

In a companion paper it was shown that D/sub 2/O protects cells against the lethal effects of heat and that D/sub 2/O interfers with the induction of thermotolerance. In this paper we investigate the latter phenomenon with particular reference to an operational model of thermotolerance. Plateau-phase Chinese hamster ovary cells (HA-1) were treated at elevated temperatures permissive for the development of thermotolerance (40-42.5/sup 0/C) in media prepared with H/sub 2/O or D/sub 2/O. Cells were given a more lethal dose of heat (45/sup 0/C/45 min with surviving fraction approx. =10/sup -4/) to assess the degree of thermotolerance. Cells pretreated in H/sub 2/O media expressed a two to three log increase in survival, while thermotolerance was effectively suppressed in cells pretreated in D/sub 2/O. However, cells treated in D/sub 2/O media for 4 hr at 42/sup 0/C, then permitted a 37/sup 0/C incubation period in H/sub 2/O media, rapidly recovered their thermotolerance. Interpreting results within the framework of a three-component model of thermotolerance (trigger, development, decay), we suggest the following: (1) D/sub 2/O inhibits the development component of tolerance; (2) the effect of D/sub 2/O on the trigger component is such that the cells behave as if they were treated atmore » a temperature lower by 2/sup 0/C in H/sub 2/O media; (3) the protection afforded by D/sub 2/O is mechanistically different from that of thermotolerance, at least in the sense that they augment each other.« less

Modification of the Heat Response and Thermotolerance by Cycloheximide, Hydroxyurea, and Lucanthone in CHO Cells

Exposure of Chinese hamster ovary cells to cycloheximide for 2 hr immediately prior to 45/sup 0/C hyperthemia increased cell survival by a factor of 1.8. The increase in cell survival was independent of the heating time for heat treatments longer than 10 min at 45/sup 0/C and was similar with cycloheximide concentrations of 1 and 10 ..mu..g/ml. Thermotolerance was induced by an initial treatment of 10 min at 45/sup 0/C (conditioning), developed during a 7-hr incubation period at 37/sup 0/C, and was defined by the hyperthermia dose response with a second 45/sup 0/C heat treatment. When cycloheximide (1..mu..g/ml) was added to the medium after heat conditioning and removed prior to the second heat treatment, the degree of thermotolerance was 50% less than that in medium controls. A 3-hr exposure to 10 ..mu..g/ml cycloheximide at 37/sup 0/C by itself did not result in the progressive development of thermotolerance which occurs after a conditioning heat treatment. In contrast to the effects of cycloheximide, hydroxyurea (1 mM) and lucanthone (5 ..mu..g/ml) showed little effect on the heat sensitivity and the development of thermotolerance after heat conditioning. Although the results can be interpreted that the development of thermotolerance requires the synthesis of new proteins,more » but not that of DNA and RNA, alternate interpretations are possible based on known cycloheximide effects aside from its primary inhibition of protein synthesis.« less

Recovery from Hyperthermic Damage and Development of Thermotolerance in Unfed Plateau-Phase Cells in Vitro<xref ref-type="fn" rid="fn2">2</xref>

The hyperthermic responses of unfed plateau-phase cells and exponentially growing mouse lung cell L1A2 heated to 42 degrees C were compared in vitro. No difference in sensitivity to a single treatment up to 4 hours was found. However, in unfed cells the recovery from hyperthermic damage demonstrated during two-dose hyperthermic fractionation (1.5 hr at 42 degrees C each) was reduced apoproximately 50% relative to that for cells in exponential growth. Preheating for 1.5 hours at 42 degrees C induced thermal resistance to a second heat treatment at 42 degrees C (thermotolerance). With a 10-hour interval, the degree of thermotolerance in unfed cells was reduced to approximately 55% compared to that for exponentially growing cells. Although the pH of the unfed culture was 6.8, adjustment of the pH to 7.2 either during the entire treatment, during the interval separating the two treatments, or during the second treatment resulted in a reduced development of thermotolerance similar to that observed in unfed cells treated without pH adjustment. Therefore, the reduced development of thermotolerance in unfed plateau-phase L1A2 cells was not due to an increased environmental acidity. The results in this study may indicate a way of increasing the therapeutic ratio with the use of fractionated hyperthermic schedules inasmuch as poorly vascularized parts of tumors may be characterized by nutritional depletion and a higher acidity as compared to the surrounding normal tissue.