Novel cytotoxic and antitumor agents. IV. Withaferin A: relation of its structure to the in vitro cytotoxic effects on P388 cells.

In vitro effects of withaferin A and its 9 new derivatives on P388 cells have been studied. The cytotoxicity was calculated from the utilization of precursors in protein and nucleic acid (NA) synthesis and from capacity to suppress cell proliferation. The most potent agents proved to be 4-dehydrowithaferin A and withaferin A diacetate exhibited an equal inhibitory effect on thymidine, uridine, and L-valine incorporation. They stopped cell proliferation and, at the same time, killed the cells. Cytotoxicity was found to be due to a double bond at position C2-3, by dissociating this bond the cytotoxicity markedly decreased in all derivatives. A dissociation of the double bond at C24-25 or a removal of OH group from C27 did not cause any significant changes in the biological effects of the derivatives. An addition of a carbonyl group at C4 increased the effects of the agent. An addition of OH groups to the molecule of withaferin A resulted chiefly in a qualitative change in the action of derivatives manifested by a significant decrease in L-valine inhibition. As withaferin A promptly reacted with L-cysteine, it was presumed that one of the possible target sites in the cell might be the SH groups of enzymes which react with the lactone and epoxide groups of the agent.