Discovery of Novel Second Mitochondria-Derived Activator of Caspase Mimetics as Selective Inhibitor of Apoptosis Protein Inhibitors

Abstract

Inhibitor of apoptosis (IAP) proteins are widely considered as promising cancer drug targets, especially for drug-resistant tumors. Mimicking the IAP-binding motif of second mitochondriaderived activator of caspases (SMAC) is a rational strategy to design potential IAP inhibitors. In this report, we used the bioactive conformation of AVPI tetrapeptide in the N terminus of SMAC as a template and performed a shape-based virtual screening against a drug-like compound library to identify novel IAP inhibitors. Top hits were subsequently docked to available IAP crystal structures as a secondary screening followed by validation using in vitro biologic assays. Four novel hit compounds were identified to potently inhibit cell growth in two human melanoma (A375 and M14) and two human prostate (PC-3 and DU145) cancer cell lines. The best compound, UC-112[5-((benzyloxy)methyl)-7-(pyrrolidin-1-ylmethyl)quinolin-8-ol], has IC 50 values ranging from 0.7 to 3.4 mM. UC-112 also potently inhibits the growth of P-glycoprotein (P-gp)-overexpressed multidrug-resistant cancer cells, strongly activates caspase-3/7 and caspase-9 activities, and selectively downregulates survivin level at a concentration as low as 1 mM. Coincubation of UC-112 with a known proteasome inhibitor Z-Leu-Leu-Leu-CHO (MG-132) rescued survivin inhibition, consistent with the anticipated mechanism of action for UC-112. As a single agent, UC-112 strongly inhibits tumor growth and reduces both X chromosomelinked IAP and survivin levels in an A375 human melanoma xenograft model in vivo. Overall, our study identified novel scaffolds, especially UC-112, as new platforms on which potent and selective IAP antagonists can be developed.

Discovery of Novel Tubulin Polymerization Inhibitors and Survivin Inhibitors as Potential Anticancer Agents

“…Small-molecule PPI inhibitors can either interact with those hot spots or bind to allosteric sites to cause conformational change that will prevent protein-ligand binding, which will result to the desired disruption of protein-protein interactions. In the case of developing small-molecule survivin inhibitors, the Ala-Val-Pro-Ile (AVPI) peptide binding region has been proposed as a hot spot for small molecule inhibition [180][181]. Therefore, targeting survivin is tractable for cancer treatment in spite of the challenges.…”

“…In our recently report, [180] UC-112 induces apoptosis by selectively inhibiting the expression of survivin in cancer cells. In order to determine whether the new UC-112 analogs maintained the same mechanism of action, we performed the caspases activation and Western blotting assay for two potent new analogs 4f and 4g in two cancer cell lines (Figure 5-9).…”

“…Interestingly, preliminary studies showed that UC-112 selectively downregulated survivin level with minimal effects on other members of IAPs. [180] This compound may represent a unique scaffold for the development of new selective survivin inhibitors. Further modifications of this compound are currently being carried out.…”

“…[180] Mechanistic studies indicated that UC-112 selectively inhibits survivin expression and induces strong cancer cell apoptosis. In order to further optimize this unique oxyquinoline scaffold and explore its SAR, we designed several focused sets of new UC-112 analogs (Figure 5-1).…”

“…UC-112 was recently reported by our group as a potent and selective survivin inhibitor [180]. It was identified through a similarity-based virtual screening in which the three-dimensional shape of the AVPI peptide in the survivin-Smac crystal complex was used as the template for searching hit structures, followed by biological validation.…”