Table 1

Cytotoxic potency, DNA binding affinity constants, and in vitro topoisomerase II cleavage of the anthracycline derivatives studied

Compound	Cytotoxicity IC⁵⁰	DNA binding-affinity constants		In vitro DNA cleavage
Compound	Cytotoxicity IC⁵⁰	DNA binding-affinity constants		Activity	Reference
	μm	m ⁻¹ K _app × 10⁶	n
III	0.02	2.4	0.170	+++	32
V	0.05	n.d.	n.d.	+++	27
IV	0.17	1.0	0.072	+++	27, 32
II	0.20	4.8	0.160	++	27
I	0.80	6.5	0.179	++	32
VI	1.37	0.86	0.215	+	27
VII	2.60	0.32	0.149	+	27

The compounds are listed in order of decreasing cytotoxic potency. IC₅₀ values were determined from dose-response curves from at least three independent experiments (see Fig. 2 for other details).K _app was determined in 0.1 m NaCl at 25°. n represents the apparent number of drug binding sites per nucleotide. Drug DNA cleavage activity is expressed in a semiquantitative manner as determined previously using an in vitro assay and murine topoisomerase II.

n.d., not determined.
++, doxorubicin activity; +, less than doxorubicin activity; +++, greater than doxorubicin activity.