Effects of hyperthermia on survival and progression of Chinese hamster ovary cells.

Abstract

Abstract In general, the survival of Chinese hamster ovary cells exposed to hyperthermic temperatures of 42.5–46.0° decreases exponentially as a function of duration of heat exposure in a manner quite similar to survival as a function of radiation dose. The data indicate that above 43° a 1° change in temperature requires a 2-fold change in time to achieve the same degree of killing, whereas below 43° the same 2-fold change in time requires only a 0.5° change in temperature for the same effect. An Arrhenius-type plot of the logarithm of the rate of killing as a function of reciprocal temperature exhibits linearity with a change in slope at 43°. This change in slope suggests either a change in the mechanism of cell killing below this temperature or a manifestation of thermal tolerance that is readily observed when the duration of heating exceeds 4 to 5 hr. Thermotolerance to 45.5°, as evidenced by a 3- to 4-fold increase in D0, is observed in synchronous G1 cells exposed to heat 20 hr after an initial heat dose. This thermotolerance develops, although no progression of cells into S phase occurs during this period. In addition, thermotolerance develops in both asynchronous and synchronous G1 cells exposed to single heat doses between 41.5 and 42.5° for periods exceeding 4 to 5 hr, i.e., survival decreases exponentially as a function of duration of heating up to 4 to 5 hr, after which survival decreases very little. At 42.0–42.5°, survival is extremely sensitive to changes in temperature, with as much as a 10-fold difference in survival for a 0.1° difference in temperature with heat exposures greater than 4 hr. The above data indicate the importance of careful treatment design and precise temperature control if hyperthermia is to be used for cancer therapy. No progression of synchronous G1 cells into S phase is observed for cells continuously exposed to temperatures of 42.0° and above. However, computer simulation of sequential DNA histograms from flow cytometry of synchronous cells continuously exposed to 41.5° indicates that cell cycle delays of 5.4 and 2.4 hr for G1 and S, respectively, occurred for cells for which exposure began in G1, and delays of 0.5 and 5.4 hr for S and G2 plus mitosis, respectively, occurred for cells for which exposure began in late S. Normal cell cycle phase transit times for G1, S, and G2 plus mitosis are 4.3, 7.1, and 2.4 hr, respectively. In addition, mitotic indices and increases in cell number of asynchronous populations of cells continuously exposed to 41.5° indicate that entry into mitosis is delayed for approximately 2 hr. Following this delay, cells begin to enter mitosis and accumulate from 2 to 6 hr in metaphase; after about 6 hr, they begin to progress into G1. However, comparison of flow cytometry data and mitotic index data suggests that during the initial 6 hr of heating, the majority of cells accumulating in G2 plus mitosis are actually delayed in G2.