Benzo[b]Tryptanthrin Inhibits MDR1, Topoisomerase Activity, and Reverses Adriamycin Resistance in Breast Cancer Cells

Tryptanthrin is an indoloquinazoline alkaloid isolated from indigo. Tryptanthrin and its benzo-annulated derivative, benzo[b]tryptanthrin, inhibit both topoisomerasesI (topoI) and II (topoII) and cause cytotoxicity in several human cancer cell lines. From diverse assessment methods, including cleavage complex stabilization, comet, DNA unwinding/intercalation, topoII ATPase inhibition, ATP competition for topoII, and wound-healing assays, we determined that the mode of action of benzo[b]tryptanthrin is as a DNA non-intercalative and ATP-competitive topoI and II dual catalytic inhibitor. Benzo[b]tryptanthrin induced apoptosis through the cleavage of caspase-3 and PARP in HCT15 colon cancer cells. Additionally, benzo[b]tryptanthrin reversed adriamycin resistance by down-regulation of multidrug resistance protein1 (MDR1) in adriamycin-resistant MCF7 breast cancer cells (MCF7adr) with more potent inhibitory activity than tryptanthrin. Taken together, derivatization by benzo-annulation of tryptanthrin ameliorated the MDR-reversing effect of tryptanthrin and may pave the way to the discovery of a novel potent adjuvant agent for chemotherapy.