A semisynthetic formulation of the cytotoxic plant alkaloid homoharringtonine isolated from the evergreen tree Cephalotaxus with potential antineoplastic activity. Omacetaxine mepesuccinate binds to the 80S ribosome in eukaryotic cells and inhibits protein synthesis by interfering with chain elongation. This agent also induces differentiation and apoptosis in some cancer cell types.