Discovery of novel second mitochondria-derived activator of caspase mimetics as selective inhibitor of apoptosis protein inhibitors

Small Molecule Inhibitors Targeting the "Undruggable" Survivin: The Past, Present, and Future from a Medicinal Chemist's Perspective

Survivin, a homodimeric protein and a member of the IAP family, plays a vital function in cell survival and cycle progression by interacting with various proteins and complexes. Its expression is upregulated in cancers but not detectable in normal tissues. Thus, it has been regarded and validated as an ideal cancer target. However, survivin is "undruggable" due to its lack of enzymatic activities or active sites for small molecules to bind/inhibit. Academic and industrial laboratories have explored different strategies to overcome this hurdle over the past two decades, with some compounds advanced into clinical testing. These strategies include inhibiting survivin expression, its interaction with binding partners and homodimerization. Here, we provide comprehensive analyses of these strategies and perspective on different small molecule survivin inhibitors to help drug discovery targeting "undruggable" proteins in general and survivin specifically with a true survivin inhibitor that will prevail in the foreseeable future.

Synthesis of Bioactive Aminomethylated 8-Hydroxyquinolines via the Modified Mannich Reaction

8-hydroxyquinoline (oxine) is a widely known and frequently used chelating agent, and the pharmacological effects of the core molecule and its derivatives have been studied since the 19th century. There are several synthetic methods to modify this core. The Mannich reaction is one of the most easily implementable examples, which requires mild reaction conditions and simple chemical reagents. The three components of the Mannich reaction are a primary or secondary amine, an aldehyde and a compound having a hydrogen with pronounced activity. In the modified Mannich reaction, naphthol or a nitrogen-containing naphthol analogue (e.g., 8-hydroxyquinoline) is utilised as the active hydrogen provider compound, thus affording the formation of aminoalkylated products. The amine component can be ammonia and primary or secondary amines. The aldehyde component is highly variable, including aliphatic and aromatic aldehydes. Based on the pharmacological relevance of aminomethylated 8-hydroxyquinolines, this review summarises their syntheses via the modified Mannich reaction starting from 8-hydroxyquinoline, formaldehyde and various amines.

Survivin Small Molecules Inhibitors: Recent Advances and Challenges

Survivin, as a member of the inhibitor of apoptosis proteins (IAPs) family, acts as a suppressor of apoptosis and plays a central role in cell division. Survivin has been considered as an important cancer drug target because it is highly expressed in many types of human cancers, while it is effectively absent from terminally differentiated normal tissues. Moreover, survivin is involved in tumor cell resistance to chemotherapy and radiation. Preclinically, downregulation of survivin expression or function reduced tumor growth induced apoptosis and sensitized tumor cells to radiation and chemotherapy in different human tumor models. This review highlights the role of survivin in promoting cellular proliferation and inhibiting apoptosis and summarizes the recent advances in and challenges of developing small-molecule survivin inhibitors.

Synthesis and biological evaluation of selective survivin inhibitors derived from the MX-106 hydroxyquinoline scaffold

The survivin (BIRC5) expression is very low in normal differentiated adult tissues, but it is one of the most widely upregulated genes in tumor cells. The overexpression of survivin in many cancer types has been positively correlated with resistance to chemotherapy, tumor metastasis, and poor patient survival. Survivin is considered to be a cancer specific biomarker and serves as a potential cancer drug target. In this report, we describe the design and syntheses of a series of novel selective survivin inhibitors based on the hydroxyquinoline scaffold from our previously reported lead compound MX-106. The best compound identified in this study is compound 12b. In vitro, 12b inhibited cancer cell proliferation with an average IC₅₀ value of 1.4 μM, using a panel of melanoma, breast, and ovarian cancer cell lines. The metabolic stability of 12b improved over MX-106 by 1.7-fold (88 vs 51 min in human microsomes). Western blot analyses demonstrated that treatments with 12b selectively decreased survivin protein levels, but negligibly affected other closely related members in the IAP family proteins, and strongly induced cancer cell apoptosis. In vivo, compound 12b effectively inhibited melanoma tumor growth when tested using a human A375 melanoma xenograft model. Further evaluation using an aggressive, orthotopic ovarian cancer mouse model showed that 12b was highly efficacious in suppressing both primary tumor growth in ovaries and tumor metastasis to multiple peritoneal organs. Collectively, results in this study strongly suggest that the hydroxyquinoline scaffold, represented by 12b and our earlier lead compound MX-106, has abilities to selectively target survivin and is promising for further preclinical development.

Newly designed compounds from scaffolds of known actives as inhibitors of survivin: computational analysis from the perspective of fragment-based drug design

Survivin is an apoptosis suppressing protein linked to different forms of cancer. As it stands, there are no approved drugs for the inhibition of survivin in cancer cells despite a number of promising compounds in clinical trials. This study designed a new set of compounds from fragments of active survivin inhibitors to potentiate their binding with survivin at BIR domain. Three hundred and five (305) fragments made from eight potent inhibitors of survivin were reconstructed to form a new set of compounds. The compounds were optimized using R group enumeration and bioisostere replacement after extensive docking analysis. The optimised compounds were filtered by a validated pharmacophore model to reveal how well they are aligned to the pharmacophore sites. Molecular docking of the well aligned compounds revealed the top-scoring compounds; and these compounds were compared with the eight inhibitors used as template for fragment-based design on the basis of binding affinity (rigid and flexible docking), predicted pIC50 and intermolecular interactions. The electronic behaviours (global descriptors, HOMO/LUMO, molecular electrostatic potential and Fukui functions) of newly designed compounds were calculated to investigate their reactivity and atomic sites prone to neutrophilic/electrophilic attack. The nine newly designed compounds had better rigid and flexible docking scores, free energy of binding and intermolecular interactions with survivin at BIR domain than the eight active inhibitors. Based on frontier molecular orbitals, OPE-3 was found to be the most reactive and less stable compound (0.13194 eV), followed by OPE-4 and OPE-9. The global descriptive parameters showed that OPE-3 had highest softness value (7.5245 eV) while OPE-8 recorded the maximum hardness value (0.08486 eV). The well-validated QSAR model also showed that OPE-3, OPE-7 and OPE-8 had the most significant bioactivity of all the inhibitors. This study thus provides new insight into the design of compounds capable of modulating the activity of survivin.

Supplementary Information

The online version contains supplementary material available at 10.1007/s40203-021-00108-8.

The Effects and Mechanisms of Cyanidin-3-Glucoside and Its Phenolic Metabolites in Maintaining Intestinal Integrity

Cyanidin-3-glucoside (C3G) is a well-known natural anthocyanin and possesses antioxidant and anti-inflammatory properties. The catabolism of C3G in the gastrointestinal tract could produce bioactive phenolic metabolites, such as protocatechuic acid, phloroglucinaldehyde, vanillic acid, and ferulic acid, which enhance C3G bioavailability and contribute to both mucosal barrier and microbiota. To get an overview of the function and mechanisms of C3G and its phenolic metabolites, we review the accumulated data of the absorption and catabolism of C3G in the gastrointestine, and attempt to give crosstalk between the phenolic metabolites, gut microbiota, and mucosal innate immune signaling pathways.

Cancer therapeutics using survivin BIRC5 as a target: what can we do after over two decades of study?

Survivin (also named BIRC5) is a well-known cancer therapeutic target. Since its discovery more than two decades ago, the use of survivin as a target for cancer therapeutics has remained a central goal of survivin studies in the cancer field. Many studies have provided intriguing insight into survivin's functional role in cancers, thus providing promise for survivin as a cancer therapeutic target. Despite this, moving survivin-targeting agents into and through the clinic remains a challenge. In order to address this challenge, we may need to rethink current strategies in order to develop a new mindset for targeting survivin. In this Review, we will first summarize the current survivin mechanistic studies, and then review the status of survivin cancer therapeutics, which is classified into five categories: (i) survivin-partner protein interaction inhibitors, (ii) survivin homodimerization inhibitors, (iii) survivin gene transcription inhibitors, (iv) survivin mRNA inhibitors and (v) survivin immunotherapy. We will then provide our opinions on cancer therapeutics using survivin as a target, with the goal of stimulating discussion that might facilitate translational research for discovering improved strategies and/or more effective anticancer agents that target survivin for cancer therapy.

An Inhibitor of the Interaction of Survivin with Smac in Mitochondria Promotes Apoptosis

Herein we report the first small molecule that disrupts the survivin-Smac interaction taking place in mitochondria. The inhibitor, PZ-6-QN, was identified by initially screening a phenothiazine library using a fluorescence anisotropy assay and then conducting a structure-activity relationship study. Mutagenesis and molecular docking studies suggest that PZ-6-QN binds to survivin similarly to the known Smac peptide, AVPI. The results of the effort also show that PZ-6-QN exhibits good anticancer activity against various cancer cells. Moreover, cell-based mechanistic studies provide evidence for the proposal that PZ-6-QN enters mitochondria to inhibit the survivin-Smac interaction and promotes release of Smac and cytochrome c from mitochondria into the cytosol, a process that induces apoptosis in cancer cells. Overall, the present study suggests that PZ-6-QN can serve as a novel chemical probe for study of processes associated with the mitochondrial survivin-Smac interaction and it will aid the discovery of novel anticancer agents.

Design, synthesis and biological evaluation of selective survivin inhibitors

The differential distribution between cancer cells and normal adult tissues makes survivin a very attractive cancer drug target. We have previously reported a series of novel selective survivin inhibitors with the most potent compound MX106 reaching nanomolar activity in several cancer cell lines. Further optimization of the MX106 scaffold leads to the discovery of more potent and more selective survivin inhibitors. Various structural modifications were synthesized and their anticancer activities were evaluated to determine the structure activity relationships for this MX106 scaffold. In vitro anti-proliferative assays using two human melanoma cell lines showed that several new analogs have improved potency compared to MX106. Very interestingly, these new analogs generally showed significantly higher potency against P-glycoprotein overexpressed cells compared with the corresponding parental cells, suggesting that these compounds may strongly sensitize tumors that have high expressions of the P-glycoprotein drug efflux pumps. Western blotting analysis confirmed that the new MX106 analogs maintained their mechanism of actions by selectively suppressing survivin expression level among major inhibitors of apoptotic proteins and induced strong apoptosis in melanoma tumor cells.

Therapeutic strategies involving survivin inhibition in cancer

Survivin is a small protein that belongs to the inhibitor of apoptosis protein family. It is abundantly expressed in tumors compared with adult differentiated tissues, being associated with poor prognosis in many human neoplasms. This apoptotic inhibitor has a relevant role in both the promotion of cancer cell survival and in the inhibition of cell death. Consequently, aberrant survivin expression stimulates tumor progression and confers resistance to several therapeutic strategies in a variety of tumors. In fact, efficient survivin downregulation or inhibition results in spontaneous apoptosis or sensitization to chemotherapy and radiotherapy. Therefore, all these features make survivin an attractive therapeutic target to treat cancer. Currently, there are several survivin inhibitors under clinical evaluation, although more specific and efficient survivin inhibitors are being developed. Moreover, novel combination regimens targeting survivin together with other therapeutic approaches are currently being designed and assessed. In this review, recent progress in the therapeutic options targeting survivin for cancer treatment is analyzed. Direct survivin inhibitors and their current development status are explored. Besides, the major signaling pathways implicated in survivin regulation are described and different therapeutic approaches involving survivin indirect inhibition are evaluated. Finally, promising novel inhibitors under preclinical or clinical evaluation as well as challenges of developing survivin inhibitors as a new therapy for cancer treatment are discussed.

Survivin Inhibitors Mitigate Chemotherapeutic Resistance in Breast Cancer Cells by Suppressing Genotoxic Nuclear Factor-κB Activation

Therapeutic resistance developed after chemotherapy and aggressive metastasis are the major causes of cancer-related death in patients with triple-negative breast cancer (TNBC). Survivin is the smallest member of the inhibitor-of-apoptosis proteins (IAPs) family, which plays critical roles in cell division and cell survival. High expression levels of survivin have been associated with therapeutic resistance in various cancers. We recently developed a novel small-molecule survivin inhibitor mimicking the IAP-binding motif of second mitochondria-derived activator of caspase, which showed high potency in promoting survivin degradation. Here, we show that survivin inhibitor MX106/MX107 suppresses TNBC cell proliferation. Moreover, MX106/MX107 synergized with chemotherapeutic drugs or radiation and significantly enhanced tumoricidal efficacy of genotoxic treatments. Mechanistically, MX106/MX107 induced degradation of XIAP and/or cIAP1, which inhibited nuclear factor κB (NF-κB) activation by genotoxic agents. Treatment with MX106/MX107 alone did not activate alternative NF-κB signaling in breast cancer cells, which is likely attributable to their selective potency in degrading survivin in these cells. In addition, survivin degradation by MX106/MX107 dramatically increased abnormal mitotic spindle formation and cell division failure, which led to cell cycle arrest in breast cancer cells. Overall, our study suggests that combination treatment of TNBC using survivin inhibitors MX106/MX107 with cytotoxic chemotherapeutic drugs can achieve significantly improved therapeutic efficacy, which depends on MX106/MX107-mediated inhibition of genotoxic NF-κB activation.

Synthesis and biological evaluation of indole-based UC-112 analogs as potent and selective survivin inhibitors

The anti-apoptotic protein survivin is highly expressed in cancer cells but has a very low expression in fully differentiated adult cells. Overexpression of survivin is positively correlated with cancer cell resistance to chemotherapy and radiotherapy, cancer cell metastasis, and poor patient prognosis. Therefore, selective targeting survivin represents an attractive strategy for the development of anticancer therapeutics. Herein, we reported the extensive structural modification of our recently discovered selective survivin inhibitor UC-112 and the synthesis of thirty-three new analogs. The structure-activity relationship (SAR) study indicated that replacement of the benzyloxy moeity in UC-112 with an indole moiety was preferred to other moieties. Among these UC-112 analogs, 10f, 10h, 10k, 10n showed the most potent antiproliferative activities. Interestingly, they were more potent against the P-glycoprotein overexpressing cancer cell lines compared with the parental cancer cell lines. Mechanistic studies confirmed that new analogs maintained their unique selectivity against survivin among the IAP family members. In vivo study using 10f in a human A375 melanoma xenograft model revealed that it effectively inhibited melanoma tumor growth without observable acute toxicity. Collectively, this study strongly supports the further preclinical development of selective survivin inhibitors based on the UC-112 scaffold.

Targeting survivin for therapeutic discovery: past, present, and future promises

Survivin, the smallest member of the inhibitor of apoptosis protein (IAP) family, is overexpressed in cells of almost all cancers but not in most normal tissues in adults. Survivin expression is required for cancer cell survival and knocking down its expression or inhibiting its function using molecular approaches results in spontaneous apoptosis. Thus, survivin is an attractive and perhaps ideal target for cancer drug discovery. However, a US Food and Drug Administration (FDA)-approved drug targeting survivin has yet to emerge. In this Foundation Review, we examine and evaluate various strategies that have been used to target survivin and the stages of each survivin inhibitor to help understand this lack of success. We also provide future perspectives moving forward in targeting survivin for drug discovery.

Methyl Protodioscin Induces Apoptosis in Human Osteosarcoma Cells by Caspase-Dependent and MAPK Signaling Pathways

Methyl protodioscin (MPD), a furostanol saponin derived from the rhizomes of Dioscorea collettii var. hypoglauca (Dioscoreaceae), has been shown to exhibit broad bioactivities such as anti-inflammation and antitumor activities. Here, we explored the molecular mechanisms by which MPD induced apoptosis in MG-63 cells. The data showed that MPD significantly suppressed cell growth (cell viabilities: 22.5 ± 1.9% for 8 μM MPD versus 100 ± 1.4% for control, P < 0.01) and enhanced cell apoptosis. The exposure to MPD resulted in a significant induction of reactive oxygen species, loss of mitochondrial membrane potential, and activation of caspase-9 and caspase-3 (P < 0.01, all cases). Furthermore, treatment with MPD increased the levels of phosphorylated JNK and p38 MAPK and markedly decreased the levels of phosphorylated ERK in MG-63 cells. Co-administration of the JNK-specific antagonist, the p38-specific antagonist, or the caspase antagonist (P < 0.05, all cases) has reversed the apoptotic effects in MPD treatment. We also found that exposure to MPD resulted in a significant reduction in the protein level of anti-apoptotic proteins Bcl-2, survivin, and XIAP (P < 0.05, all cases). In conclusion, our results indicate that MPD induces apoptosis of human osteosarcoma MG-63 cells, at least in part, by caspase-dependent and MAPK signaling pathways.

Cordycepin induces apoptosis in human liver cancer HepG2 cells through extrinsic and intrinsic signaling pathways

Cordycepin, also termed 3'-deoxyadenosine, is a nucleoside analogue from Cordyceps sinensis and has been reported to demonstrate numerous biological and pharmacological properties. Our previous study illustrated that the anti-tumor effect of cordycepin may be associated with apoptosis. In the present study, the apoptotic effect of cordycepin on HepG2 cells was investigated using 4',6-diamidino-2-phenylindole, tetraethylbenzimidazolylcarbocyanine iodide and propidium iodide staining analysis and flow cytometry. The results showed that cordycepin exhibited the ability to inhibit HepG2 cells in a time- and dose-dependent manner when cells produced typical apoptotic morphological changes, including chromatin condensation, the accumulation of sub-G1 cells and change mitochondrial permeability. A potential mechanism for cordycepin-induced apoptosis of human liver cancer HepG2 cells may occur through the extrinsic signaling pathway mediated by the transmembrane Fas-associated with death domain protein. Apoptosis was also associated with Bcl-2 family protein regulation, leading to altered mitochondrial membrane permeability and resulting in the release of cytochrome c into the cytosol. The activation of the caspase cascade is responsible for the execution of apoptosis. In conclusion, cordycepin-induced apoptosis in HepG2 cells involved the extrinsic and intrinsic signaling pathway and was primarily regulated by the Bcl-2 family proteins.

Design, Synthesis and Structure-Activity Relationship Studies of Novel Survivin Inhibitors with Potent Anti-Proliferative Properties

The anti-apoptotic protein survivin is highly expressed in most human cancer cells, but has very low expression in normal differentiated cells. Thus survivin is considered as an attractive cancer drug target. Herein we report the design and synthesis of a series of novel survivin inhibitors based on the oxyquinoline scaffold from our recently identified hit compound UC-112. These new analogs were tested against a panel of cancer cell lines including one with multidrug-resistant phenotype. Eight of these new UC-112 analogs showed IC₅₀ values in the nanomole range in anti-proliferative assays. The best three compounds among them along with UC-112 were submitted for NCI-60 cancer cell line screening. The results indicated that structural modification from UC-112 to our best compound 4g has improved activity by four folds (2.2 μM for UC-112 vs. 0.5 μM for 4g, average GI₅₀ values over all cancer cell lines in the NCI-60 panel).Western blot analyses demonstrated the new compounds maintained high selectivity for survivin inhibition over other members in the inhibition of apoptosis protein family. When tested in an A375 human melanoma xenograft model, the most active compound 4g effectively suppressed tumor growth and strongly induced cancer cell apoptosis in tumor tissues. This novel scaffold is promising for the development of selective survivin inhibitors as potential anticancer agents.

Recent Advances on Small-Molecule Survivin Inhibitors

Survivin, a member of the inhibitor of apoptosisproteins family, is highly expressed in most human neoplasms, but its expression is very low or undetectable in terminally differentiated normal tissues. Survivin has been shown to inhibit cancer cell apoptosis and promote cell proliferation. The overexpression of survivin closely correlates with tumor progression and drug resistance. Because of its key role in tumor formation and maintenance, survivin is considered as an ideal target for anticancer treatment. However, the development of small-molecule survivin inhibitors has been challenging due to the requirement to disrupt the protein-protein interactions. Currently only a limited number of survivin inhibitors have been developed in recent years, and most of these inhibitors reduce survivin levels by interacting with other biomolecules instead of directly interacting with survivin protein. Despite these challenges, developing potent and selective small-molecule survivin inhibitors will be important in both basic science to better understand survivin biology and in translational research to develop potentially more effective, broad-spectrum anticancer agents. In this review, the functions of survivin and its role in cancer are summarized. Recent developments, challenges, and future direction of small-molecule survivin inhibitors are also discussed in detail.