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Bleomycin as a possible synchronizing agent for human tumor cells in vivo.

At low doses bleomycin (BLM) reversibly inhibits cell progression at the S-G2 boundary. Cells located in other stages of the cell cycle are essentially unaffected; therefore, when present for a complete cell cycle, BLM becomes a prospective in vivo cell-synchronizing agent. In the five trials reported here, BLM was used to synchronize human malignant melanoma cells in vivo . Tumor biopsies were pulse-labeled with tritiated thymidine and assayed by liquid scintillation and autoradiographic techniques. Following the synchrony block, cells at the S-G2 boundary progressed to S phase in a partially synchronized wave. The labeling indices indicated about 1.5 to 4 times the normal number of cells in S phase at the peak times following the first BLM treatment. Therefore, BLM partially synchronizes cells in vivo , and this technique provides a rapid and accurate means of locating the synchronized cells and for scheduling of a second drug for a maximum effect on tumor cell killing.