Na/K-ATPase-Targeted Cytotoxicity of (+)-Digoxin and Several Semisynthetic Derivatives.

(+)-Digoxin () is a well-known cardiac glycoside long used to treat congestive heart failure and found more recently to show anticancer activity. Several known cardenolides (-) and two new analogues, (+)-8(9)-β-anhydrodigoxigenin () and (+)-17--20,22-dihydro-21α-hydroxydigoxin (), were synthesized from and evaluated for their cytotoxicity toward a small panel of human cancer cell lines. A preliminary structure-activity relationship investigation conducted indicated that the C-12 and C-14 hydroxy groups and the C-17 unsaturated lactone unit are important for to mediate its cytotoxicity toward human cancer cells, but the C-3 glycosyl residue seems to be less critical for such an effect. Molecular docking profiles showed that the cytotoxic and the noncytotoxic derivative bind differentially to Na/K-ATPase. The HO-12β, HO-14β, and HO-3'aα hydroxy groups of (+)-digoxin () may form hydrogen bonds with the side-chains of Asp121 and Asn122, Thr797, and Arg880 of Na/K-ATPase, respectively, but the altered lactone unit of results in a rotation of its steroid core, which depotentiates the binding between this compound and Na/K-ATPase. Thus, was found to inhibit Na/K-ATPase, but did not. In addition, the cytotoxic did not affect glucose uptake in human cancer cells, indicating that this cardiac glycoside mediates its cytotoxicity by targeting Na/K-ATPase but not by interacting with glucose transporters.