R16, a novel amonafide analogue, induces apoptosis and G₂-M arrest via poisoning topoisomerase II

Amonafide, a naphthalimide derivative, although selected for exploratory clinical trials for its potent anticancer activity, has long been challenged by its unpredictable side effects. In the present study, a novel amonafide analogue, 2-(2-dimethylamino)-6-thia-2-aza-benzo-[def]-chrysene-1.3-diones (R16) was synthesized by substituting 5′-NH₂ of the naphthyl with a heterocyclic group to amonafide, with additional introduction of a thiol group. In a panel of various human tumor cell lines, R16 was more cytotoxic than its parent compound amonafide. It was also effective against multidrug-resistant cells. Importantly, the i.p. administration of R16 inhibited tumor growth in mice implanted with S-180 sarcoma and H₂₂ hepatoma. The molecular and cellular machinery studies showed that the R16 functions as a topoisomerase II (topo II) poison via binding to the ATPase domain of human topo IIα. The superior cytotoxicity of R16 to amonafide was ascribed to its potent effects on trapping topo II-DNA cleavage complexes. Moreover, using a topo II catalytic inhibitor aclarubicin, ataxia-telangiectasia-mutated (ATM)/ATM-and Rad3-related (ATR) kinase inhibitor caffeine and topo II-deficient HL-60/MX2 cells, we further showed that R16-triggered DNA double-strand breaks, tumor cell cycle arrest, and apoptosis were in a topo II-dependent manner. Taken together, R16 stood out by its improved anticancer activity, appreciable anti-multidrug resistance activities, and well-defined topo II poisoning mechanisms, as comparable with the parent compound amonafide. All these collectively promise the potential value of R16 as an anticancer drug candidate, which deserves further development.