SUMMARYThe direct induction of apoptosis has emerged as a powerful anti-cancer strategy, and small molecules that either inhibit or activate certain proteins in the apoptotic pathway have great potential as novel chemotherapeutic agents. Central to apoptosis is the activation of the zymogen procaspase-3 to caspase-3. Caspase-3 is the key "executioner" caspase, catalyzing the hydrolysis of a multitude of protein substrates within the cell. Interestingly, procaspase-3 levels are often elevated in cancer cells, suggesting a compound that directly stimulates the activation of procaspase-3 to caspase-3 could selectively induce apoptosis in cancer cells. We recently reported the discovery of a compound, PAC-1, which enhances procaspase-3 activity in vitro and induces apoptotic death in cancer cells in culture and in mouse xenograft models. Described herein is the mechanism by which PAC-1 activates procaspase-3 in vitro. We show that zinc inhibits the enzymatic activity of procaspase-3, and that PAC-1 strongly activates procaspase-3 in buffers that contain zinc. PAC-1 and zinc form a tight complex with one another, with a dissociation constant of approximately 42 nM. The combined data indicate that PAC-1 activates procaspase-3 in vitro by sequestering inhibitory zinc ions, thus allowing procaspase-3 to autoactivate itself to caspase-3. The small molecule mediated activation of procaspases has great therapeutic potential and thus this discovery of the in vitro mechanism of action of PAC-1 is critical to the development and optimization of other procaspase-activating compounds.

A goal of personalized medicine as applied to oncology is to identify compounds that exploit a defined molecular defect in a cancerous cell. A compound called procaspase-activating compound 1 (PAC-1) was reported that enhances the activity of procaspase-3 in vitro and induces apoptotic death in cancer cells in culture and in mouse xenograft models. Experimental evidence indicates that PAC-1 activates procaspase-3 in vitro through chelation of inhibitory zinc ions. Described herein is the synthesis and biological activity of a family of PAC-1 derivatives where key functional groups have been systematically altered. Analysis of these compounds reveals a strong correlation between the in vitro procaspase-3 activating effect and their ability to induce death in cancer cells in culture. Importantly, we also show that a fluorescently-labeled version of PAC-1 co-localizes with sites of caspase-3 activity in cancer cells. The data presented herein further bolster the hypothesis that PAC-1 induces apoptosis in cancer cells through the direct activation of procaspase-3, has implications for the design and discovery of next-generation procaspase-3 activating compounds, and sheds light on the anti-apoptotic role of cellular zinc.

It is recognized that high-throughput enzyme inhibition screens often return nonspecific inhibitors as "hits". Recently, high-throughput screens for enzyme activators have led to the identification of several compounds with novel and potent biological activity. Here, we show that enzyme activation screens can also uncover compounds that activate multiple enzymes in a nonspecific fashion. Described herein are the general structural features of such compounds and methods to differentiate between specific and general enzyme activation.

The caspase enzymes are integral to cellular inflammation and apoptotic cascades, and are commonly studied by cell biologists, medicinal chemists, and chemical biologists. In particular, the assessment of caspase enzymatic activity is a standard method to evaluate cell death pathways and novel apoptosis-modulating agents. Caspase enzymatic activity can be conveniently monitored with peptidic chromogenic or fluorogenic substrates, with certain peptide sequences imparting selectivity for certain caspases. The synthesis of these peptide substrates is typically performed via solid-phase synthesis, a method that is not ideal for production of the gram quantities needed for high-throughput screening. Described herein is a facile method for the synthesis of the Ac-DEVD-pNA caspase-3 substrate using solution-phase peptide synthesis. This protocol, involving iterative PyBOP-mediated couplings and Fmoc deprotections, is rapid, operationally simple, and can be used to generate over one gram of product.

Fifteen ketone-containing peptides were designed, synthesized, and used to probe the effect of substitution at the P1' position on caspase-3 and -7 inhibition. Even with the large bias of Ac-Asp-Glu-Val-Asp at the P4-P1 positions, certain peptides with cyclic functionality in the P1' position show a dramatically reduced ability to inhibit these caspases. Additionally, trends toward isozyme selectivity were also uncovered for particular P1' substituents. The data indicate that substitution in the P1' position can drastically affect both caspase inhibition and selectivity. [structure: see text]