Nonpeptidic and potent small-molecule inhibitors of cIAP-1/2 and XIAP proteins

Characterization of a Potent and Orally Bioavailable Lys-Covalent Inhibitor of Apoptosis Protein (IAP) Antagonist

We have recently reported on the use of aryl-fluorosulfates in designing water- and plasma-stable agents that covalently target Lys, Tyr, or His residues in the BIR3 domain of the inhibitor of the apoptosis protein (IAP) family. Here, we report further structural, cellular, and pharmacological characterizations of this agent, including the high-resolution structure of the complex between the Lys-covalent agent and its target, the BIR3 domain of X-linked IAP (XIAP). We also compared the cellular efficacy of the agent in two-dimensional (2D) and three-dimensional (3D) cell cultures, side by side with the clinical candidate reversible IAP inhibitor LCL161. Finally, in vivo pharmacokinetic studies indicated that the agent was long-lived and orally bioavailable. Collectively our data further corroborate that aryl-fluorosulfates, when incorporated correctly in a ligand, can result in Lys-covalent agents with pharmacodynamic and pharmacokinetic properties that warrant their use in the design of pharmacological probes or even therapeutics.

Stability and Cell Permeability of Sulfonyl Fluorides in the Design of Lys-Covalent Antagonists of Protein-Protein Interactions

Recently we reported on aryl-fluorosulfates as possible stable and effective electrophiles for the design of lysine covalent, cell permeable antagonists of protein-protein interactions (PPIs). Here we revisit the use of aryl-sulfonyl fluorides as Lys-targeting moieties, incorporating these electrophiles in XIAP (X-linked inhibitor of apoptosis protein) targeting agents. We evaluated stability in buffer and reactivity with Lys311 of XIAP of various aryl-sulfonyl fluorides using biochemical and biophysical approaches, including displacement assays, mass spectrometry, SDS gel electrophoresis, and denaturation thermal shift measurements. To assess whether these modified electrophilic "warheads" can also react with Tyr, we repeated these evaluations with a Lys311Tyr XIAP mutant. Using a direct cellular assay, we could demonstrate that selected agents are cell permeable and interact covalently with their intended target in cell. These results suggest that certain substituted aryl-sulfonyl fluorides can be useful Lys- or Tyr-targeting electrophiles for the design of covalent pharmacological tools or even future therapeutics targeting protein-protein interactions.

Snapshots and ensembles of BTK and cIAP1 protein degrader ternary complexes

Heterobifunctional chimeric degraders are a class of ligands that recruit target proteins to E3 ubiquitin ligases to drive compound-dependent protein degradation. Advancing from initial chemical tools, protein degraders represent a mechanism of growing interest in drug discovery. Critical to the mechanism of action is the formation of a ternary complex between the target, degrader and E3 ligase to promote ubiquitination and subsequent degradation. However, limited insights into ternary complex structures exist, including a near absence of studies on one of the most widely co-opted E3s, cellular inhibitor of apoptosis 1 (cIAP1). In this work, we use a combination of biochemical, biophysical and structural studies to characterize degrader-mediated ternary complexes of Bruton's tyrosine kinase and cIAP1. Our results reveal new insights from unique ternary complex structures and show that increased ternary complex stability or rigidity need not always correlate with increased degradation efficiency.

Potency and Selectivity of SMAC/DIABLO Mimetics in Solid Tumor Therapy

Aiming to promote cancer cell apoptosis is a mainstream strategy of cancer therapy. The second mitochondria-derived activator of caspase (SMAC)/direct inhibitor of apoptosis protein (IAP)-binding protein with low pI (DIABLO) protein is an essential and endogenous antagonist of inhibitor of apoptosis proteins (IAPs). SMAC mimetics (SMs) are a series of synthetically chemical compounds. Via database analysis and literature searching, we summarize the potential mechanisms of endogenous SMAC inefficiency, degradation, mutation, releasing blockage, and depression. We review the development of SMs, as well as preclinical and clinical outcomes of SMs in solid tumor treatment, and we analyze their strengths, weaknesses, opportunities, and threats from our point of view. We also highlight several questions in need of further investigation.

Berberine promotes XIAP-mediated cells apoptosis by upregulation of miR-24-3p in acute lymphoblastic leukemia

BACKGROUND

Berberine (BBR) has gained considerable attention because of its anti-tumor activity. BBR can induce apoptosis of acute lymphoblastic leukemia (ALL) cells through the MDM2/p53 pathway. However, the effects of BBR on those ALL patients with p53 deficiency remain unclear.

RESULTS

We found that BBR reduced ALL cell viability and induced apoptosis in p53-null EU-4 and p53-mutant EU-6 cells by downregulating X-linked inhibitor of apoptosis protein (XIAP), which is increased in ALL tissues and cells. BBR-induced cell apoptosis was attenuated by inhibition of XIAP that was controlled by PIM-2. Mechanistic studies showed that BBR treatment induced an enhancement of miR-24-3p. PIM-2 is a direct target of miR-24-3p. Blockade of PIM-2 or miR-24-3p reversed BBR-induced cell apoptosis. In vivo studies, BBR remarkably alleviated leukemia conditions in a EU4 xenograft mouse model, whereas inhibition of miR-24-3p significantly reversed the effects of BBR in the leukemia condition.

CONCLUSIONS

miR-24-3p/PIM-2/XIAP signaling contributes to BBR-mediated leukemia mitigation in p53-defect ALL, which should be further developed as a treatment strategy in ALL patients with p53 deficiency.

METHODS

Cell viability and apoptosis were determined using CCK-8 and TUNEL assays, respectively. The dual-luciferase reporter gene system was used to determine the interaction between miR-24-3p and 3'-untranslated regions (UTRs) of PIM-2.

Aryl-fluorosulfate-based Lysine Covalent Pan-Inhibitors of Apoptosis Protein (IAP) Antagonists with Cellular Efficacy

We have recently investigated the reactivity of aryl-fluorosulfates as warheads to form covalent adducts with Lys, Tyr, and His residues. However, the rate of reaction of aryl-fluorosulfates seemed relatively slow, putting into question their effectiveness to form covalent adducts in cell. Unlike the previously reported agents that targeted a relatively remote Lys residue with respect to the target's binding site, the current agents were designed to more directly juxtapose an aryl-fluorosulfate with a Lys residue that is located within the binding pocket of the BIR3 domain of X-linked inhibitor of apoptosis protein (XIAP). We found that such new agents can effectively and rapidly form a covalent adduct with XIAP-BIR3 in vitro and in cell, approaching the rate of reaction, cellular permeability, and stability that are similar to what attained by acrylamides when targeting Cys residues. Our studies further validate aryl-fluorosulfates as valuable Lys-targeting electrophiles, for the design of inhibitors of both enzymes and protein-protein interactions.

Covalent Inhibitors of Protein-Protein Interactions Targeting Lysine, Tyrosine, or Histidine Residues

We have recently reported a series of Lys-covalent agents targeting the BIR3 domain of the X-linked inhibitor of apoptosis protein (XIAP) using a benzamide-sulfonyl fluoride warhead. Using XIAP as a model system, we further investigated a variety of additional warheads that can be easily incorporated into binding peptides and analyzed their ability to form covalent adducts with lysine and other amino acids, including tyrosine, histidine, serine, and threonine, using biochemical and biophysical assays. Moreover, we tested aqueous, plasma stability, cell permeability, and cellular efficacy of the most effective agents. These studies identified aryl-fluoro sulfates as likely the most suitable electrophiles to effectively form covalent adducts with Lys, Tyr, and His residues, given that these agents were cell permeable and stable in aqueous buffer and in plasma. Our studies contain a number of general findings that open new possible avenues for the design of potent covalent protein-protein interaction antagonists.

Prognostic Value of XIAP Level in Patients with Various Cancers: A Systematic Review and Meta-Analysis

Background: X-linked inhibitor of apoptosis protein (XIAP) plays an important role in cancer pathogenesis, which has been found to be overexpressed in multiple human cancers and associated with survival rates. Herein, we performed a meta-analysis to explore the predictive value of XIAP level in patients with various solid tumors. Methods: Relevant articles exploring the relationship between XIAP expression and survival of cancer patients were retrieved in PubMed, PMC, EMBASE and Web of Science published from 2001 to 2018. The combined hazard ratios (HRs) and 95% confidence intervals (CIs) were calculated to evaluate the significance. Results: A total of 6554 patients from 40 articles were included in this meta-analysis. It was shown in 37 studies with 4864 cases that the over-expression of XIAP was associated with poorer overall survival (OS) (combined HR=1.61, 95% CI: 1.33-1.96). Meanwhile, 8 studies with 1862 cases revealed that elevated XIAP level predicted shorter disease-free survival (DFS) (HR=2.17, 95% CI: 1.03-4.59). Subgroup analyses showed that higher XIAP detection was related to worse OS in gastric cancer (HR=1.42, 95% CI: 1.18-1.72) and head and neck cancer (HNC) (HR=2.97, 95% CI: 1.97-4.47). Conclusion: Our results suggested that elevated XIAP level seemed to represent an unfavorable prognostic factor for clinical outcomes in cancer patients. However, there were limited studies describing the association between XIAP expression and clinical prognosis in each different type of tumors. Therefore, concrete roles of XIAP in various cancers need to be further explored.

Design of Potent pan-IAP and Lys-Covalent XIAP Selective Inhibitors Using a Thermodynamics Driven Approach

Recently we reported that rapid determination of enthalpy of binding can be achieved for a large number of congeneric agents or in combinatorial libraries fairly efficiently. We show that using a thermodynamic Craig plot can be very useful in dissecting the enthalpy and entropy contribution of different substituents on a common scaffold, in order to design potent, selective, or pan-active compounds. In our implementation, the approach identified a critical Lys residue in the BIR3 domain of XIAP. We report for the first time that it is possible to target such residue covalently to attain potent and selective agents. Preliminary cellular studies in various models of leukemia, multiple myeloma, and pancreatic cancers suggest that the derived agents possess a potentially intriguing pattern of activity, especially for cell lines that are resistant to the pan-IAP antagonist and clinical candidate LCL161.

Theoretical study of the intermolecular recognition mechanism between Survivin and substrate based on conserved binding mode analysis

Survivin is the smallest member of IAP (inhibitor of apoptosis protein) family, which plays important roles in both mitosis and apoptosis. It has become an attractive drug target due to its overexpression in many human cancers. Survivin has been proven to bind to Smac/DIABLO protein that indirectly inhibits apoptosis. Meanwhile, it is the key subunit of chromosome passenger complex (CPC) which bind to the N-terminal tail of phosphorylated histone H3T3ph during mitosis. Up to now, Survivin directly targeting inhibitor has yet to merge since the difficulty of disrupting the protein-protein interactions (PPIs) between Survivin and its substrate proteins. Nevertheless, currently known binding partners of Survivin provide crucial information about conserved recognition mechanism, which can assist in the detection of some uncharted substrates and also the Survivin inhibitors. Herein, we adopted a method that using four substrates to analyze the common binding mode of Survivin. To accomplish this, conventional molecular dynamics (MD) simulations, molecular mechanics/generalized born surface area (MM-GBSA) binding free energy calculations and energy decomposition were carried out to assess the binding affinity and per-residue contributions. We found that there are two anchor sites of Survivin responsible for maintaining the binding conformation and one sub-pocket for intermolecular recognition. The results of this study synthetically describe the binding mechanism and provide valuable guidance for rational drug design of PPI inhibitor.

Protein-protein interactions as drug targets

Modulation of protein-protein interactions (PPIs) is becoming increasingly important in drug discovery and chemical biology. While a few years ago this 'target class' was deemed to be largely undruggable an impressing number of publications and success stories now show that targeting PPIs with small, drug-like molecules indeed is a feasible approach. Here, we summarize the current state of small-molecule inhibition and stabilization of PPIs and review the active molecules from a structural and medicinal chemistry angle, especially focusing on the key examples of iNOS, LFA-1 and 14-3-3.

New Compound Classes: Protein-Protein Interactions

"Protein-protein interactions (PPIs) are one of the most promising new targets in drug discovery. With estimates between 300,000 and 650,000 in human physiology, targeted modulation of PPIs would tremendously extend the "druggable" genome. In fact, in every disease a wealth of potentially addressable PPIs can be found making pharmacological intervention based on PPI modulators in principle a generally applicable technology. An impressing number of success stories in small-molecule PPI inhibition and natural-product PPI stabilization increasingly encourage academia and industry to invest in PPI modulation. In this chapter examples of both inhibition as well as stabilization of PPIs are reviewed including some of the technologies which has been used for their identification."

Astemizole arrests the proliferation of cancer cells by disrupting the EZH2-EED interaction of polycomb repressive complex 2

Polycomb Repressive Complex 2 (PRC2) modulates the chromatin structure and transcriptional repression by trimethylation lysine 27 of histone H3 (H3K27me3), a process that necessitates the protein-protein interaction (PPI) between the catalytic subunit EZH2 and EED. Deregulated PRC2 is intimately involved in tumorigenesis and progression, making it an invaluable target for epigenetic cancer therapy. However, until now, there have been no reported small molecule compounds targeting the EZH2-EED interactions. In the present study, we identified astemizole, an FDA-approved drug, as a small molecule inhibitor of the EZH2-EED interaction of PRC2. The disruption of the EZH2-EED interaction by astemizole destabilizes the PRC2 complex and inhibits its methyltransferase activity in cancer cells. Multiple lines of evidence have demonstrated that astemizole arrests the proliferation of PRC2-driven lymphomas primarily by disabling the PRC2 complex. Our findings demonstrate the chemical tractability of the difficult PPI target by a small molecule compound, highlighting the therapeutic promise for PRC2-driven human cancers via targeted destruction of the EZH2-EED complex.

Investigation of the intermolecular recognition mechanism between the E3 ubiquitin ligase Keap1 and substrate based on multiple substrates analysis

E3 ubiquitin ligases are attractive drug targets due to their specificity to the ubiquitin machinery. However, the development of E3 ligase inhibitors has proven challenging for the fact that they must disrupt protein-protein interactions (PPIs). The E3 ligase involved in interactome provide new hope for the discovery of the E3 ligase inhibitors. These currently known natural binding partners of the E3 ligase can benefit the discovery of other unknown substrates and also the E3 ligase inhibitors. Herein, we present a novel strategy that using multiple substrates to elucidate the molecular recognition mechanism of E3 ubiquitin ligase. Molecular dynamics simulation, molecular mechanics-generalized born surface area (MM-GBSA) binding energy calculation and energy decomposition scheme were incorporated to evaluate the quantitative contributions of sub-pocket and per-residue to binding. In this case, Kelch-like ECH-associated protein-1 (Keap1), a substrate adaptor component of the Cullin-RING ubiquitin ligases complex, is applied for the investigation of how it recognize its substrates, especially Nrf2, a master regulator of the antioxidant response. By analyzing multiple substrates binding determinants, we found that both the polar sub-pockets (P1 and P2) and the nonpolar sub-pockets (P4 and P5) of Keap1 can make remarkable contributions to intermolecular interactions. This finding stresses the requirement for substrates to interact with the polar and nonpolar sub-pockets simultaneously. The results discussed in this paper not only show the binding determinants of the Keap1 substrates but also provide valuable implications for both Keap1 substrate discovery and PPI inhibitor design.

Potent and selective small-molecule inhibitors of cIAP1/2 proteins reveal that the binding of Smac mimetics to XIAP BIR3 is not required for their effective induction of cell death in tumor cells

Cellular inhibitor of apoptosis protein 1 and 2 (cIAP1/2) and X-linked inhibitor of apoptosis protein (XIAP) are key apoptosis regulators and promising new cancer therapeutic targets. This study describes a set of non-peptide, small-molecule Smac (second mitochondria-derived activator of caspases) mimetics that are selective inhibitors of cIAP1/2 over XIAP. The most potent and most selective compounds bind to cIAP1/2 with affinities in the low nanomolar range and show >1,000-fold selectivity for cIAP1 over XIAP. These selective cIAP inhibitors effectively induce degradation of the cIAP1 protein in cancer cells at low nanomolar concentrations and do not antagonize XIAP in a cell-free functional assay. They potently inhibit cell growth and effectively induce apoptosis at low nanomolar concentrations in cancer cells with a mechanism of action similar to that of other known Smac mimetics. Our study shows that binding of Smac mimetics to XIAP BIR3 is not required for effective induction of apoptosis in tumor cells by Smac mimetics. These potent and highly selective cIAP1/2 inhibitors are powerful tools in the investigation of the role of these IAP proteins in the regulation of apoptosis and other cellular processes.

Tissue microarray analysis of X-linked inhibitor of apoptosis (XIAP) expression in breast cancer patients

The goal of this study was to determine the diagnostic and prognostic potential of X-linked inhibitor of apoptosis (XIAP) expression in breast cancer. We analyzed a tissue microarray comprised of 100 breast cancer cases and 70 matched normal samples. Analysis of an online database, which included 2,977 patients, was also performed. There was a significant difference in cytoplasmic expression of XIAP (XIAP-C) between breast cancer tissue and matched normal (p<0.001). Staining of XIAP-C was defined as negative (breast cancer 8.42% vs. normal 30.91%), slight (40.0 vs. 45.45%), moderate (43.16 vs. 23.64%), or high (8.42 vs. 0%). High XIAP-C protein expression correlated with human epidermal growth factor receptor 2 (HER-2) status (p=0.010) and with human p53 mutant-type (P53) status (p=0.039). We found that XIAP expression did not correlate with disease-free survival (p=0.706) and overall survival (p=0.496) of breast cancer patients. An Internet-based system analysis confirmed our results. In the subgroup analysis, basal-like breast cancer patients with high XIAP levels in the tumor had a significantly increased risk of relapse; thus, the up-regulation of XIAP appeared to be predictive of poor relapse-free survival (p=0.013). Kaplan-Meier curves also identified a significant correlation between distant metastasis-free survival and XIAP expression in patients with lymph-node-negative disease (p=0.030). In summary, expression of XIAP-C was significantly higher in breast cancer compared to normal tissue. XIAP-C expression correlated with HER-2 status and may be considered a prognostic biomarker for basal-like breast cancer patients.

A potent bivalent Smac mimetic (SM-1200) achieving rapid, complete, and durable tumor regression in mice

We have designed, synthesized, and evaluated a series of new compounds based upon our previously reported bivalent Smac mimetics. This led to the identification of compound 12 (SM-1200), which binds to XIAP, cIAP1, and cIAP2 with Ki values of 0.5, 3.7, and 5.4 nM, respectively, inhibits cell growth in the MDA-MB-231 breast cancer and SK-OV-3 ovarian cancer cell lines with IC50 values of 11.0 and 28.2 nM, respectively. Compound 12 has a much improved pharmacokinetic profile over our previously reported bivalent Smac mimetics and is highly effective in induction of rapid and durable tumor regression in the MDA-MB-231 xenograft model. These data indicate that compound 12 is a promising Smac mimetic and warrants extensive evaluation as a potential candidate for clinical development.

Druggable protein interaction sites are more predisposed to surface pocket formation than the rest of the protein surface

Despite intense interest and considerable effort via high-throughput screening, there are few examples of small molecules that directly inhibit protein-protein interactions. This suggests that many protein interaction surfaces may not be intrinsically “druggable” by small molecules, and elevates in importance the few successful examples as model systems for improving our fundamental understanding of druggability. Here we describe an approach for exploring protein fluctuations enriched in conformations containing surface pockets suitable for small molecule binding. Starting from a set of seven unbound protein structures, we find that the presence of low-energy pocket-containing conformations is indeed a signature of druggable protein interaction sites and that analogous surface pockets are not formed elsewhere on the protein. We further find that ensembles of conformations generated with this biased approach structurally resemble known inhibitor-bound structures more closely than equivalent ensembles of unbiased conformations. Collectively these results suggest that “druggability” is a property encoded on a protein surface through its propensity to form pockets, and inspire a model in which the crude features of the predisposed pocket(s) restrict the range of complementary ligands; additional smaller conformational changes then respond to details of a particular ligand. We anticipate that the insights described here will prove useful in selecting protein targets for therapeutic intervention.

Identifying small-molecule inhibitors of protein interactions has traditionally presented a challenge for modern screening methods, despite interest stemming from the fact that such interactions comprise the underlying mechanisms for cell proliferation, differentiation, and survival. This suggests that many protein interaction surfaces may not be intrinsically “druggable” by small molecules, and elevates in importance the few successful examples as model systems for improving our understanding of factors contributing to druggability. Here we describe a new approach for exploring protein fluctuations leading to surface pockets suitable for small molecule binding. We find that the presence of such pockets is indeed a signature of druggable protein interaction sites, suggesting that “druggability” is a property encoded on a protein surface through its propensity to form pockets. We anticipate that the insights described here will prove useful in selecting protein targets for therapeutic intervention.

Discovery of a potent small-molecule antagonist of inhibitor of apoptosis (IAP) proteins and clinical candidate for the treatment of cancer (GDC-0152)

A series of compounds were designed and synthesized as antagonists of cIAP1/2, ML-IAP, and XIAP based on the N-terminus, AVPI, of mature Smac. Compound 1 (GDC-0152) has the best profile of these compounds; it binds to the XIAP BIR3 domain, the BIR domain of ML-IAP, and the BIR3 domains of cIAP1 and cIAP2 with K(i) values of 28, 14, 17, and 43 nM, respectively. These compounds promote degradation of cIAP1, induce activation of caspase-3/7, and lead to decreased viability of breast cancer cells without affecting normal mammary epithelial cells. Compound 1 inhibits tumor growth when dosed orally in the MDA-MB-231 breast cancer xenograft model. Compound 1 was advanced to human clinical trials, and it exhibited linear pharmacokinetics over the dose range (0.049 to 1.48 mg/kg) tested. Mean plasma clearance in humans was 9 ± 3 mL/min/kg, and the volume of distribution was 0.6 ± 0.2 L/kg.

Bivalent Smac mimetics with a diazabicyclic core as highly potent antagonists of XIAP and cIAP1/2 and novel anticancer agents

Nonpeptidic, bivalent Smac mimetics designed based upon monovalent Smac mimetics with a diazabicyclic core structure bind to XIAP, cIAP1, and cIAP2 with low to subnanomolar affinities and are highly effective in antagonizing XIAP in cell-free functional assays. They efficiently induce the degradation of cIAP1 and cIAP2 in cancer cells at concentrations as low as 1 nM, activate caspase-3 and -8, and cleave PARP at 3-10 nM. The most potent compounds in the series have IC(50) of 3-5 nM in inhibition of cell growth in both MDA-MB-231 and SK-OV-3 cell lines and are promising lead compounds for the development of a new class of cancer therapy.

Solid-phase synthesis of smac peptidomimetics incorporating triazoloprolines and biarylalanines

Apoptotic induction mechanisms are of crucial importance for the general homeostasis of multicellular organisms. In cancer the apoptotic pathways are downregulated, which, at least partly, is due to an abundance of inhibitors of apoptosis proteins (IAPs) that block the apoptotic cascade by deactivating proteolytic caspases. The Smac protein has an antagonistic effect on IAPs, thus providing structural clues for the synthesis of new pro-apoptotic compounds. Herein, we report a solid-phase approach for the synthesis of Smac-derived tetrapeptide libraries. On the basis of a common (N-Me)AVPF sequence, peptides incorporating triazoloprolines and biarylalanines were synthesized by means of Cu(I)-catalyzed azide-alkyne cycloaddition and Pd-catalyzed Suzuki cross-coupling reactions. Solid-phase procedures were optimized to high efficiency, thus accessing all products in excellent crude purities and yields (both typically above 90%). The peptides were subjected to biological evaluation in a live/dead cellular assay which revealed that structural decorations on the AVPF sequence indeed are highly important for cytotoxicity toward HeLa cells.

Potent bivalent Smac mimetics: effect of the linker on binding to inhibitor of apoptosis proteins (IAPs) and anticancer activity

We have synthesized and evaluated a series of nonpeptidic, bivalent Smac mimetics as antagonists of the inhibitor of apoptosis proteins and new anticancer agents. All these bivalent Smac mimetics bind to full-length XIAP with low nanomolar affinities and function as ultrapotent antagonists of XIAP. While these Smac mimetics bind to cIAP1/2 with similar low nanomolar affinities, their potencies to induce degradation of cIAP1/2 proteins in cells differ by more than 100-fold. The most potent bivalent Smac mimetics inhibit cell growth with IC(50) from 1 to 3 nM in the MDA-MB-231 breast cancer cell line and are 100 times more potent than the least potent compounds. Determination of intracellular concentrations for several representative compounds showed that the linkers in these bivalent Smac mimetics significantly affect their intracellular concentrations and hence the overall cellular activity. Compound 27 completely inhibits tumor growth in the MDA-MB-231 xenografts while causing no signs of toxicity in the animals.

A potent and orally active antagonist (SM-406/AT-406) of multiple inhibitor of apoptosis proteins (IAPs) in clinical development for cancer treatment

We report the discovery and characterization of SM-406 (compound 2), a potent and orally bioavailable Smac mimetic and an antagonist of the inhibitor of apoptosis proteins (IAPs). This compound binds to XIAP, cIAP1, and cIAP2 proteins with K(i) of 66.4, 1.9, and 5.1 nM, respectively. Compound 2 effectively antagonizes XIAP BIR3 protein in a cell-free functional assay, induces rapid degradation of cellular cIAP1 protein, and inhibits cancer cell growth in various human cancer cell lines. It has good oral bioavailability in mice, rats, non-human primates, and dogs, is highly effective in induction of apoptosis in xenograft tumors, and is capable of complete inhibition of tumor growth. Compound 2 is currently in phase I clinical trials for the treatment of human cancer.

Mechanisms of drug sensitization to TRA-8, an agonistic death receptor 5 antibody, involve modulation of the intrinsic apoptotic pathway in human breast cancer cells

TRA-8, a monoclonal antibody to death receptor 5 induces apoptosis in various cancer cells; however, the degree of sensitivity varies from highly sensitive to resistant. We have previously shown that resistance to TRA-8 can be reversed by using chemotherapeutic agents, but the mechanism underlying this sensitization was not fully understood. Here, we examined the combination of TRA-8 with doxorubicin or bortezomib in breast cancer cells. In TRA-8-resistant BT-474 and T47D cells, both chemotherapy agents synergistically sensitized cells to TRA-8 cytotoxicity with enhanced activation of apoptosis shown by cleavage of caspases and PARP, reduced Bid, increased proapoptotic Bcl-2 proteins, and increased mitochondrial membrane depolarization. Doxorubicin or bortezomib combined with TRA-8 also reduced Bcl-XL and X-linked inhibitors of apoptosis (XIAP) in treated cells. Furthermore, targeting these proteins with pharmacologic modulators, AT-101, BH3I-2' and AT-406, produced sensitization to TRA-8. TRA-8 combined with AT-101 or BH3I-2', inhibitors of antiapoptotic Bcl-2 proteins, produced synergistic cytotoxicity against ZR-75-1, BT-474, and T47D cells. The IAP-targeting compound, AT-406, was synergistic with TRA-8 in BT-474 cells, and to a lesser extent T47D cells. Activation of the intrinsic apoptotic pathway was a common mechanism associated with sensitization of TRA-8-resistant breast cancer cell lines. Collectively, these studies show that the Bcl-2 and IAP families of proteins are involved in TRA-8 and chemotherapy resistance via their modulation of the intrinsic apoptotic pathway. Targeting these proteins with novel agents sensitized TRA-8-resistant breast cancer cells, suggesting this approach may represent a potent therapeutic strategy in the treatment of breast cancer.

Design of small-molecule Smac mimetics as IAP antagonists

Smac/DIABLO, discovered in 2000 as a protein released from mitochondria into the cytosol in response to apoptotic stimuli, functions as an endogenous antagonist of X-linked inhibitor of apoptosis protein (XIAP) and several other IAP proteins through direct binding. The interaction between Smac and IAPs involves the AVPI tetrapeptide binding motif on the N-terminus of Smac and a well-defined groove on the surface of these IAP proteins, providing an ideal site for the design of small-molecule Smac mimetics. Potent and cell-permeable small-molecule Smac mimetics have provided powerful pharmacological tools for study of the regulation of apoptosis by IAP proteins, and several such compounds are now in early clinical trials as new anticancer agents.