Cross-resistance of an amsacrine-resistant human leukemia line to topoisomerase II reactive DNA intercalating agents. Evidence for two topoisomerase II directed drug actions.

HL-60/AMSA is a human leukemia cell line that is 50-100-fold more resistant than its drug-sensitive HL-60 parent line to the cytotoxic actions of the DNA intercalator amsacrine (m-AMSA). HL-60/AMSA topoisomerase II is also resistant to the inhibitory actions of m-AMSA. HL-60/AMSA cells and topoisomerase II are cross-resistant to anthracycline and ellipticine intercalators but relatively sensitive to the nonintercalating topoisomerase II reactive epipodophyllotoxin etoposide. We now demonstrate that HL-60/AMSA and its topoisomerase II are cross-resistant to the DNA intercalators mitoxantrone and amonafide, thus strongly indicating that HL-60/AMSA and its topoisomerase II are resistant to topoisomerase II reactive intercalators but not to nonintercalators. At high concentrations, mitoxantrone and amonafide were also found to inhibit their own, m-AMSA's, and etoposide's abilities to stabilize topoisomerase II-DNA complexes. This appears to be due to the ability of these concentrations of mitoxantrone and amonafide to inhibit topoisomerase II mediated DNA strand passage at a point in the topoisomerization cycle prior to the acquisition of the enzyme-DNA configuration that yields DNA cleavage and topoisomerase II-DNA cross-links. In addition, amonafide can inhibit the cytotoxic actions of m-AMSA and etoposide. Taken together, these results suggest that the cytotoxicity of m-AMSA and etoposide is initiated primarily by the stabilization of the topoisomerase II-DNA complex. Other topoisomerase II reactive drugs may inhibit the enzyme at other steps in the topoisomerization cycle, particularly at elevated concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)