Zalypsis: a novel marine-derived compound with potent antimyeloma activity that reveals high sensitivity of malignant plasma cells to DNA double-strand breaks.

Multiple myeloma (MM) remains incurable, and new drugs with novel mechanisms of action are still needed. In this report, we have analyzed the action of Zalypsis, an alkaloid analogous to certain natural marine compounds, in MM. Zalypsis turned out to be the most potent antimyeloma agent we have tested so far, with IC(50) values from picomolar to low nanomolar ranges. It also showed remarkable ex vivo potency in plasma cells from patients and in MM cells in vivo xenografted in mice. Besides the induction of apoptosis and cell cycle arrest, Zalypsis provoked DNA double-strand breaks (DSBs), evidenced by an increase in phospho-histone-H2AX and phospho-CHK2, followed by a striking overexpression of p53 in p53 wild-type cell lines. In addition, in those cell lines in which p53 was mutated, Zalypsis also provoked DSBs and induced cell death, although higher concentrations were required. Immunohistochemical studies in tumors also demonstrated histone-H2AX phosphorylation and p53 overexpression. Gene expression profile studies were concordant with these results, revealing an important deregulation of genes involved in DNA damage response. The potent in vitro and in vivo antimyeloma activity of Zalypsis uncovers the high sensitivity of tumor plasma cells to DSBs and strongly supports the use of this compound in MM patients.