Etoposide is a widely prescribed anticancer agent that stabilizes topoisomerase II-mediated DNA strand breaks. The drug contains a polycyclic ring system (rings A-D), a glycosidic moiety at C4, and a pendant ring (E-ring) at C1. A recent study that focused on yeast topoisomerase II demonstrated that the H15 geminal protons of the etoposide A-ring, the H5 and H8 protons of the B-ring, and the H2′, H6′, 3′-methoxyl, and 5′-methoxyl protons of the E-ring contact topoisomerase II in the binary enzyme-drug complex [Wilstermann et al. (2007) Biochemistry 46, 8217-8225]. No interactions with the C4 sugar were observed. The present study used DNA cleavage assays, saturation transfer difference [1H] NMR spectroscopy, and enzyme-drug binding studies to further define interactions between etoposide and human topoisomerase IIα. Etoposide and three derivatives that lacked the C4 sugar were analyzed. Except for the sugar, 4′-demethyl epipodophyllotoxin is identical to etoposide, epipodophyllotoxin contains a 4′-methoxyl group on the E-ring, and 6,7-O,O-demethylenepipodophyllotoxin replaces the A-ring with a diol. Results suggest that etoposide-topoisomerase IIα binding is driven by interactions with the A-and B-rings and potentially by stacking interactions with the E-ring. We propose that the E-ring pocket on the enzyme is confined, because the addition of bulk to this ring adversely affects drug function. The A- and E-rings do not appear to contact DNA in the enzyme-drug-DNA complex. Conversely, the sugar moiety subtly alters DNA interactions. The identification of etoposide substituents that contact topoisomerase IIα in the binary complex has predictive value for drug behavior in the enzyme-etoposide-DNA complex. © 2008 American Chemical Society.