The first direct small molecule activator of procaspase-3

MEDI 284

Karson S Putt, Department of Biochemistry, University of Illinois Urbana-Champaign, 600 S. Mathews, 264 RAL, Urbana, IL 61801 and Paul J. Hergenrother, hergenro@uiuc.edu, Department of Chemistry, University of Illinois at Urbana-Champaign, 600 South Mathews Ave, Urbana, IL 61801.
Mutation or aberrant expression of proteins in the apoptotic cascade is a hallmark of cancer. These changes prevent proapoptotic signals from being transmitted to the executioner caspases, thus preventing apoptotic cell death and allowing cellular proliferation. Caspase-3 and caspase-7 are the key executioner caspases, existing as inactive zymogens that are activated by upstream signals. Importantly, levels of procaspase-3 are significantly higher in certain cancerous cells relative to non-cancerous controls. Here we report the identification of a small molecule (PAC-1) that directly activates procaspase-3 to active caspase-3 in vitro with an EC50 of 220 nanomolar, and induces apoptosis in a variety of cancer cell lines. Cancerous cells isolated from primary colon tumors are considerably more sensitive to apoptotic induction by PAC-1 than the cells from adjacent non-cancerous tissue from the same patient; these cancerous cells contain on average ~7-fold more procaspase-3 than the cells from the adjacent non-cancerous primary tissue. In addition, the sensitivity to PAC-1 of the primary cells from the colon cancer tumors strongly correlates with the level of the procaspase-3 target. Finally, PAC-1 as a single entity was shown to retard the growth of tumors in three different mouse models, including two models where PAC-1 was administered orally. Thus PAC-1 directly activates cleavage of procaspase-3 to caspase-3 in vivo, thereby allowing this compound to induce apoptosis even in cells that have defective apoptotic machinery. PAC-1 is the first small molecule known to directly activate procaspase-3; the direct activation of executioner caspases is an anti-cancer strategy that may prove beneficial in the many cancers in which procaspase-3 levels are elevated.