The discovery and characterization of anticancer procaspase activating compounds

One key hallmark of cancer is its ability to evade apoptotic cell death. In normal cells, the apoptotic intrinsic and extrinsic pathways converge on the activation of executioner cysteine aspartate proteases caspases-3 and -7. However, cancer cells are able to escape apoptosis through mutations which render the key apoptotic proteins inactive or aberrantly expressed. Paradoxically, procaspases are generally upregulated and rarely mutated in cancers. Therefore, a small molecule that directly activates executioner procaspase-3/-7 to caspase-3/-7 to induce apoptosis in cancer cells could circumvent the defective apoptotic machinery and serve as an effective personalized anti-cancer therapy. Described herein are the in vitro mechanisms and the in vivo anticancer application of procaspase activators PAC-1 and S-PAC-1, the identification of novel procaspase activating compounds, and the evaluation of procaspase-3 levels in human cancers. Several procaspase activating compounds have been discovered in the Hergenrother laboratory through high-throughput screening and will be discussed in this thesis. PAC-1 activates procaspase-3/-7 in vitro through the chelation and sequestration of inhibitory zinc ions. In cells, PAC-1 is capable of inducing a decreased signal of an intracellular zinc sensor, suggesting that PAC-1 chelates zinc in the cytosol. Transcript profiling of PAC-1 reveals that the compound concentration has a very dramatic effect on the gene expression signature in lymphoma and leukemia cell lines. At high concentrations, PAC-1 may act by an ER stress mechanism, evidenced by Connectivity Map analysis, decreased ER calcium levels, and cellular morphology. Also described in this thesis are the efforts to translate procaspase activating compounds PAC-1 and S-PAC-1 to the clinic, using cell culture and animal studies. PAC-1 and S-PAC-1 are potently cytotoxic in a variety of cancer cell lines. The in vivo safety, pharmacokinetics, and efficacy of PAC-1 and S-PAC-1 have been assessed in murine and canine models. S-PAC-1 has been found to be well-tolerated in mice, research dogs, as well as client-owned patient pet dogs with lymphoma. A continuous i.v. S-PAC-1 regimen led to a reduction or stabilization of tumor progression in some of the patient dogs after 4 weeks. PAC-1 has the unique ability to induce a strong cytotoxic response in cell culture with exposure times as short as 4 hours. The short exposure cytotoxicity would allow for a more attractive and simplified dosing regimen in vivo, compared to a continuous administration of the drug. However, PAC-1 is neurotoxic in vivo; therefore ~860 PAC-1 derivatives were synthesized in parallel, and compound L28R8 was identified as a compound that is safe and induces short exposure cytotoxicity. Efforts have been made to discover novel procaspase-3/-7 activating compounds through high-throughput screening. The lead compounds from a high-throughput screen for procaspase-7 activators are likely zinc chelators, like PAC-1. Another high-throughput screening campaign was developed for non-zinc chelating procaspase-3/-7 activating compounds. The campaign resulted in promising compounds that are currently being characterized. Finally, the initial studies toward the characterization of procaspase-3 levels in human cancer tissue will be presented. The immunohistochemical analysis of procaspase-3 in colon, lung, and breast tissue may lead to the identification of a human cancer patient population that would most benefit from treatment with a procaspase activating compound