A second-generation antisense oligonucleotide targeting heat shock protein 27 (Hsp27) with potential antitumor and chemosensitizing activities. Apatorsen suppresses tumor cell expression of Hsp27, which may induce tumor cell apoptosis and enhance tumor cell sensitivity to cytotoxic agents. Hsp27, a chaperone belonging to the small heat shock protein (sHsp) group of proteins, is a cytoprotective protein that supports cell survival under conditions of stress; it has been found to be over-expressed in a variety of human cancers.