Initial single-strand DNA damage and cellular pharmacokinetics of bleomycin A2

The cellular association and fate of high specific activity [3H]bleomycin A2 (BLM A2) were examined in three previously untreated cultured cell lines. Human head and neck A-253 carcinoma cells were 10-fold more sensitive to a 1-hr exposure to BLM A2 than either murine leukemic L1210 or human ovarian SK-OV cells. Both murine and human cells displayed rapid drug association with steady-state drug levels being reached within 15-30 min. At steady state, the T1/2 of drug dissociation was slow (between 65 and 155 min), unaltered by 100-fold excess of unlabeled BLM A2, and unrelated to cellular sensitivity to BLM. Approximately 15% of the total cellular drug was found in the nuclei at steady state. In intact cells, BLM hydrolase activity appeared latent; significant BLM hydrolase activity was detected using broken cell homogenates with all cell types, but no extensive drug metabolism was evident in intact cells. Murine L1210 cells differed from both human cell lines in that they had only 50% of the steady-state drug levels, had lower nuclear drug content, and had markedly less initial single-strand DNA damage. Human SK-OV cells had 2.4-fold greater initial single-strand DNA damage despite similar nuclear content and a much lower rate of DNA repair. Thus, cellular or nuclear factors, in addition to BLM A2 content, affect initial single-strand DNA damage. Collectively, our data support the proposition that lesions other than single-strand DNA breaks contribute to the cytotoxicity of BLM.