Differences in thermosensitization among cloned cell lines isolated from a single human melanoma xenograft

Differences in thermosensitization (effect of step-down heating) among one uncloned and five cloned cell lines isolated from a single tumor of a human melanoma xenograft were studied. Cells from passages 7-12 in vitro were exposed to graded heat treatments at 41.5 degrees C immediately, 1 h, and 2 h after a conditioning treatment of 43.5 degrees C (90 min). The thermosensitization was largest immediately after the conditioning treatment and then decayed exponentially. The differences among the cell lines were reflected in the maximum magnitude as well as in the rate of decay of the thermosensitization. The maximum thermosensitization ratios (TSR), calculated as the ratio of the D0 values at 41.5 degrees C for single-heated and preheated cells, ranged from 5.3 +/- 1.5 to 14.9 +/- 5.2 and were not correlated to the surviving fractions after the conditioning treatment. The half-times for the decay of the thermosensitization ranged from 1.5 +/- 0.3 h to 3.1 +/- 0.5 h and were not correlated to the maximum TSR. Moreover, there was no correlation between the magnitude of the maximum thermosensitization at 41.5 degrees C and the magnitude of the maximum thermotolerance at 43.5 degrees C, as induced by the same treatment (43.5 degrees C for 90 min).