Cytotoxic effects of SMAC-mimetic compound LCL161 in head and neck cancer cell lines

Targeting inhibitor of apoptosis proteins (IAPs) enhances susceptibility of oral squamous carcinoma cells to cisplatin

PURPOSE

Oral Squamous Cell Carcinoma (OSCC) is the 6th most common cancer worldwide. It is generally aggressive and closely associated with chemoresistance and poor survival. There is accumulating evidence for the involvement of inhibitors of apoptosis proteins (IAPs), including IAP1 and XIAP, in mediating chemotherapy resistance in OSCC. Various strategies for targeting IAPs have been designed and tested in recent years and several small molecule IAP inhibitors are in clinical trials as monotherapies as well as in combination with radiotherapy and chemotherapy. The purpose of this study was to evaluate and compare the efficacy and biological activity of three IAP inhibitors both as stand-alone and sensitising agents to cisplatin in a preclinical model of squamous cell carcinoma of the tongue.

METHODS

Cisplatin-sensitive SCC4 and -resistant SCC4cisR cells were utilised in this study. Apoptosis was evaluated by flow cytometric analysis of Annexin V/Propidium Iodide-stained cells. Expression of IAP proteins was determined by western blotting and knockdown of cIAP1, livin and XIAP was conducted by transfection of cells with siRNA.

RESULTS

We establish for the first time the therapeutic efficacy of the Smac mimetic, BV6 and the XIAP inhibitor Embelin, for OSCC. Both of these IAP targeting agents synergistically enhanced cisplatin-mediated apoptotic cell death in resistant cells which was mediated in part by depletion of XIAP. In addition, knockdown of XIAP using siRNA enhanced cisplatin-mediated cell death, demonstrating the importance of targeting XIAP in this sensitisation.

CONCLUSION

These findings provide pre-clinical evidence that IAP inhibition may be a valuable therapeutic option in OSCC.

Clinical Trial Development in TP53-Mutated Locally Advanced and Recurrent and/or Metastatic Head and Neck Squamous Cell Carcinoma

TP53 mutation is the most frequent genetic event in head and neck squamous cell carcinoma (HNSCC), found in more than 80% of patients with human papillomavirus-negative disease. As mutations in the TP53 gene are associated with worse outcomes in HNSCC, novel therapeutic approaches are needed for patients with TP53-mutated tumors. The National Cancer Institute sponsored a Clinical Trials Planning Meeting to address the issues of identifying and developing clinical trials for patients with TP53 mutations. Subcommittees, or breakout groups, were tasked with developing clinical studies in both the locally advanced and recurrent and/or metastatic (R/M) disease settings as well as considering signal-seeking trial designs. A fourth breakout group was focused on identifying and standardizing biomarker integration into trial design; this information was provided to the other breakout groups prior to the meeting to aid in study development. A total of 4 concepts were prioritized to move forward for further development and implementation. This article summarizes the proceedings of the Clinical Trials Planning Meeting with the goal of developing clinical trials for patients with TP53-mutant HNSCC that can be conducted within the National Clinical Trials Network.

BH3-mimetics: recent developments in cancer therapy

The hopeful outcomes from 30 years of research in BH3-mimetics have indeed served a number of solid paradigms for targeting intermediates from the apoptosis pathway in a variety of diseased states. Not only have such rational approaches in drug design yielded several key therapeutics, such outputs have also offered insights into the integrated mechanistic aspects of basic and clinical research at the genetics level for the future. In no other area of medical research have the effects of such work been felt, than in cancer research, through targeting the BAX-Bcl-2 protein-protein interactions. With these promising outputs in mind, several mimetics, and their potential therapeutic applications, have also been developed for several other pathological conditions, such as cardiovascular disease and tissue fibrosis, thus highlighting the universal importance of the intrinsic arm of the apoptosis pathway and its input to general tissue homeostasis. Considering such recent developments, and in a field that has generated so much scientific interest, we take stock of how the broadening area of BH3-mimetics has developed and diversified, with a focus on their uses in single and combined cancer treatment regimens and recently explored therapeutic delivery methods that may aid the development of future therapeutics of this nature.

An Updated Review of Smac Mimetics, LCL161, Birinapant, and GDC-0152 in Cancer Treatment

Inhibitor of apoptosis proteins (IAPs) are suggested as therapeutic targets for cancer treatment. Smac/DIABLO is a natural IAP antagonist in cells; therefore, Smac mimetics have been developed for cancer treatment in the past decade. In this article, we review the anti-cancer potency and novel molecular targets of LCL161, birinapant, and GDC-0152. Preclinical studies demonstrated that Smac mimetics not only induce apoptosis but also arrest cell cycle, induce necroptosis, and induce immune storm in vitro and in vivo. The safety and tolerance of Smac mimetics are evaluated in phase 1 and phase 2 clinical trials. In addition, the combination of Smac mimetics and chemotherapeutic compounds was reported to improve anti-cancer effects. Interestingly, the novel anti-cancer molecular mechanism of action of Smac mimetics was reported in recent studies, suggesting that many unknown functions of Smac mimetics still need to be revealed. Exploring these currently unknown signaling pathways is important to provide hints for the modification and combination therapy of further compounds.

Sensitization of head and neck squamous cell carcinoma to apoptosis by combinational SMAC mimetic and Fas ligand-Fc treatment in vitro

This study aimed to investigate the in vitro efficacy of three different SMAC mimetics for pro-apoptotic sensitization of HNSCC cells. We evaluated BV-6 in comparison to Birinapant and LCL161, for which pro-apoptotic sensitization effects have been demonstrated. Concentration-dependent response was measured for BV-6 in each cell line with an average IC₅₀ value 8-fold lower than of aforementioned SMAC mimetics. Combination treatment of FasL (log2) and BV-6 (IC₁₀) showed highly significant cell count reductions even in the lowest applied concentration in five cell lines (PCI-1: p = 0.0002, PCI-13: p = 0.0002, Detroit 562: p: p < 0.0001, FaDu: p < 0.0001, SCC-25: p = 0.0047). Synergistic effects (y < 1) were evident in eight out of 10 cell lines (PCI-1, PCI-9, PCI-13, PCI-68, Detroit 562, FaDu, SCC-25 and HaCaT). Annexin V assays revealed in nine cell lines very highly significant (p < 0.001) pro-apoptotic effects of BV-6. Western blots showed a heterogeneous IAP expression following SMAC mimetic treatment. Except for two cell lines, at least the cellular inhibitor of apoptosis protein 1 (cIAP1) was degraded in response to BV-6. For prospective targeted HNSCC therapy, this study identifies SMAC mimetics, particularly BV-6 as the compound with the highest pro-apoptotic potency, as promising antitumor agents.

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.

ASTX660, an antagonist of cIAP1/2 and XIAP, increases antigen processing machinery and can enhance radiation-induced immunogenic cell death in preclinical models of head and neck cancer

Inhibitor of apoptosis protein (IAP) antagonists have shown activity in preclinical models of head and neck squamous cell carcinoma (HNSCC), and work across several cancer types has demonstrated diverse immune stimulatory effects including enhancement of T cell, NK cell, and dendritic cell function. However, tumor-cell-intrinsic mechanisms for this immune upregulation have been largely unexplored. In this study, we show that ASTX660, an antagonist of cIAP1/2 and XIAP, induces expression of immunogenic cell death (ICD) markers in sensitive HNSCC cell lines in vitro. Experiments in syngeneic mouse models of HNSCC showed that ASTX660 can also enhance radiation-induced ICD in vivo. On a functional level, ASTX660 also enhanced killing of multiple murine cell lines by cytotoxic tumor-infiltrating lymphocytes, and when combined with XRT, stimulated clonal expansion of antigen-specific T lymphocytes and expression of MHC class I on the surface of tumor cells. Flow cytometry experiments in several human HNSCC cell lines showed that MHC class I (HLA-A,B,C) was reliably upregulated in response to ASTX660 + TNFα, while increases in other antigen processing machinery (APM) components were variable among different cell lines. These findings suggest that ASTX660 may enhance anti-tumor immunity both by promoting ICD and by enhancing antigen processing and presentation.

Dual Antagonist of cIAP/XIAP ASTX660 Sensitizes HPV- and HPV+ Head and Neck Cancers to TNFα, TRAIL, and Radiation Therapy

PURPOSE

Human papillomavirus-negative (HPV^-) head and neck squamous cell carcinomas (HNSCC) harbor frequent genomic amplification of Fas-associated death domain, with or without concurrent amplification of Baculovirus inhibitor of apoptosis repeat containing (BIRC2/3) genes encoding cellular inhibitor of apoptosis proteins 1/2 (cIAP1/2). Antagonists targeting cIAP1 have been reported to enhance sensitivity of HPV^-, but not HPV⁺ tumors, to TNF family death ligands (TNF and TRAIL) and radiation.Experimental Design: We tested a novel dual cIAP/XIAP antagonist ASTX660 in HPV⁺ and HPV^- cell lines in combination with death ligands TNFα and TRAIL, and in preclinical xenograft models with radiation, an inducer of death ligands. The dependence of activity on TNF was examined by antibody depletion.

RESULTS

ASTX660 sensitized subsets of HPV^- and HPV⁺ HNSCC cell lines to TNFα and TRAIL. These antitumor effects of ASTX660 are the result of both apoptosis and/or necroptosis among HPV^- cells, and primarily by apoptosis (caspase 3 and caspase 8 cleavage) in HPV⁺ cells. ASTX660 enhanced restoration of protein expression and inhibitory activity of proapoptotic tumor suppressor TP53 in HPV⁺ HNSCC. Furthermore, ASTX660 combined with radiotherapy, an inducer of death ligands, significantly delayed growth of both HPV^- and HPV⁺ human tumor xenografts, an effect attenuated by anti-TNFα pretreatment blockade.

CONCLUSIONS

IAP1/XIAP antagonist, ASTX660, sensitizes HPV⁺ HNSCC to TNFα via a mechanism involving restoration of TP53. These findings serve to motivate further studies of dual cIAP/XIAP antagonists and future clinical trials combining these antagonists with radiotherapy to treat both HPV⁺ and HPV^- HNSCC.

Multi-kinase inhibitors and cisplatin for head and neck cancer treatment in vitro

Multidrug resistance (MDR) remains one of the major causes of suboptimal outcome following therapy in head and neck squamous cell carcinoma (HNSCC). ATP-binding cassette (ABC) transporters are overexpressed in HNSCC, which contributes to the limited effect of chemotherapeutic treatment. In addition to their named function, tyrosine kinase inhibitors (TKIs) have been revealed to impact on ABC transporter activity and expression. Therefore, the present study aimed to investigate the effects of combination therapy using different TKIs combined with cisplatin. Reverse transcription-quantitative PCR was used to characterize ABC transporter and receptor expression in 5 HNSCC cell lines treated with 3 different TKIs (pazopanib, dovitinib, nintedanib) and cisplatin. Treatment efficacy was analyzed using a crystal violet staining assay. Analysis of ABC transporter (ABCB1, ABCC1 and ABCG2) genetic alterations was performed using The Cancer Genome Atlas. Statistical analysis was conducted to evaluate the effects of mono- and combination treatment. With the exception of ABCB1, all of the investigated ABC transporters were expressed in each cell line. The additive effects of TKI + cisplatin combination treatment were observed for pazopanib in three cell lines, nintedanib in four cell lines, and were not observed for dovitinib in any of the cell lines investigated. The combination of multi-kinase inhibitors and conventional chemotherapy in HNSCC may strengthen the use of current therapeutic strategies; nintedanib appears to be the most suitable TKI for combination therapy. Further efforts are required to classify TKI efficacy with regard to cisplatin resistance.

The Selection of NFκB Inhibitors to Block Inflammation and Induce Sensitisation to FasL-Induced Apoptosis in HNSCC Cell Lines Is Critical for Their Use as a Prospective Cancer Therapy

Inflammation is a central aspect of tumour biology and can contribute significantly to both the origination and progression of tumours. The NFκB pathway is one of the most important signal transduction pathways in inflammation and is, therefore, an excellent target for cancer therapy. In this work, we examined the influence of four NFκB inhibitors—Cortisol, MLN4924, QNZ and TPCA1—on proliferation, inflammation and sensitisation to apoptosis mediated by the death ligand FasL in the HNSCC cell lines PCI1, PCI9, PCI13, PCI52 and SCC25 and in the human dermal keratinocyte cell line HaCaT. We found that the selection of the inhibitor is critical to ensure that cells do not respond by inducing counteracting activities in the context of cancer therapy, e.g., the extreme IL-8 induction mediated by MLN4924 or FasL resistance mediated by Cortisol. However, TPCA1 was qualified by this in vitro study as an excellent therapeutic mediator in HNSCC by four positive qualities: (1) proliferation was inhibited at low μM-range concentrations; (2) TNFα-induced IL-8 secretion was blocked; (3) HNSCC cells were sensitized to TNFα-induced cell death; and (4) FasL-mediated apoptosis was not disrupted.

Antagonist of cIAP1/2 and XIAP enhances anti-tumor immunity when combined with radiation and PD-1 blockade in a syngeneic model of head and neck cancer

Head and neck squamous cell carcinomas (HNSCCs) frequently harbor genomic mutations in cell death pathways. Nearly 30% of HNSCCs overexpress Fas-Associated Death Domain (FADD), with or without BIRC2/3 genes encoding cellular Inhibitor of Apoptosis Proteins 1/2 (cIAP1/2), critical components of the Tumor Necrosis Factor (TNF) Receptor signaling pathways. ASTX660 is a novel non-peptidomimetic antagonist of cIAP1/2 and XIAP under evaluation in a clinical trial for advanced solid tumors and lymphomas. Herein, we show that ASTX660, at nanomolar concentrations, sensitized Murine Oral Cancer (MOC1) cells to TNFα. Using syngeneic mouse models, ASTX660 showed additive anti-tumor activity with radiation therapy (XRT), cisplatin chemotherapy, and PD-1 blockade to significantly delay or eradicate MOC1 tumors. These combinations significantly increased CD8 + T cells and dendritic cells, as well as T cell activity. ASTX660 stimulated cytotoxic T lymphocyte (CTL) killing of MOC1 cells expressing ovalbumin. Early stages of CTL killing were predominantly mediated by perforin/granzyme B, whereas later stages were mediated by death ligands TNFα, TRAIL, and FasL. Correspondingly, depletion of CD8 + T cells and NK cells in vivo revealed both types of immune cells to be important components of the complete anti-tumor response enhanced by ASTX660+XRT. These findings serve to inform future studies of IAP inhibitors and support the potential for future clinical trials investigating ASTX660 with XRT and immunotherapies like PD-1/PD-L1 blockade in HNSCC.

Apoptosis-sensitizing activity of birinapant in head and neck squamous cell carcinoma cell lines

Inhibitor of apoptosis proteins, which are overexpressed in head and neck squamous cell carcinoma (HNSCC), may cause therapeutic resistance. Using SMAC mimetic compounds, including birinapant, to degrade and/or inhibit these proteins and sensitize apoptosis may enhance therapies in HNSCC. Fas expression was analyzed in nine HNSCC cell lines and one keratinocyte cell line via flow cytometry. These cell lines were treated with Fas ligand-Fc (FasL) and birinapant, a bivalent SMAC mimetic, in mono and combination therapies. Cytotoxicity was measured using a crystal violet assay. Annexin V assay was performed for detection of apoptosis. The treatment efficacy of mono and combination therapies was statistically analyzed. Nonlinear regression analysis was performed to determine the inhibitory concentration (IC₁₀) of birinapant. Fas expression was detected in each cell line tested. Mono treatment with FasL revealed minor to no apoptotic effects in the majority of the cell lines. Crystal violet and Annexin V staining revealed increased apoptosis rates for all cell lines following incubation with birinapant in mono treatment. Combination treatment with FasL and birinapant (IC₁₀) revealed additional and synergistic effects in eight out of the ten cell lines. To the best of our knowledge, the present study provided the first evidence of the apoptosis-sensitizing activity of combination treatment with FasL and birinapant in HNSCC cell lines.

Targeting VEGFR and FGFR in head and neck squamous cell carcinoma in vitro

Head and neck squamous cell carcinoma (HNSCC) is a heterogeneous disease characterized by a tumor microenvironment (TME) that overexpresses vascular endothelial growth factor receptor (VEGFR) and fibroblast growth factor receptor (FGFR), which can lead to neovascularization, tumor growth and metastasis. Therapeutic strategies inhibiting these signaling pathways might lead to innovative HNSCC treatments. Five HNSCC cell lines were characterized based on VEGFR1-3 and FGFR1-4 expression by sqRT-PCR and treated with three different tyrosine kinase inhibitors (TKIs) (nintedanib, dovitinib and pazopanib), all of which are effective against VEGFR and FGFR family members. Crystal violet assays were performed to analyze the effect of the treatments on cell growth (viability). Additionally, VEGFR1-3 and FGFR1-4 expression data were retrieved from The Cancer Genome Atlas (TCGA), and statistical analyses were performed to investigate the receptor expression level in the different cell lines and the efficacy of the single-agent treatments. A correlation analysis was performed to quantify the degree of relationship between receptor expression and drug efficacy. With the exception of VEGFR2, the targeted receptors were expressed at different levels in all of the cell lines. The cell lines exhibited concentration-dependent responses with cell line-specific differences toward two of the three TKIs (nintedanib and dovitinib). Notably, all of the cell lines were resistant to pazopanib. TKIs have potential as therapeutic agents for HNSCC. Cell line-specific differences were observed in our in vitro experiments. The observed pazopanib resistance could be explained by receptor expression. Further investigation is required to determine TKI efficacy in HNSCC.

Therapeutic Small Molecules Target Inhibitor of Apoptosis Proteins in Cancers with Deregulation of Extrinsic and Intrinsic Cell Death Pathways

The Cancer Genome Atlas (TCGA) has unveiled genomic deregulation of various components of the extrinsic and intrinsic apoptotic pathways in different types of cancers. Such alterations are particularly common in head and neck squamous cell carcinomas (HNSCC), which frequently display amplification and overexpression of the Fas-associated via death domain (FADD) and inhibitor of apoptosis proteins (IAP) that complex with members of the TNF receptor family. Second mitochondria-derived activator of caspases (SMAC) mimetics, modeled after the endogenous IAP antagonist SMAC, and IAP inhibitors represent important classes of novel small molecules currently in phase I/II clinical trials. Here we review the physiologic roles of IAPs, FADD, and other components involved in cell death, cell survival, and NF-κB signaling pathways in cancers, including HNSCC. We summarize the results of targeting IAPs in preclinical models of HNSCC using SMAC mimetics. Synergistic activity of SMAC mimetics together with death agonists TNFα or TRAIL occurred in vitro, whereas their antitumor effects were augmented when combined with radiation and chemotherapeutic agents that induce TNFα in vivo In addition, clinical trials testing SMAC mimetics as single agents or together with chemo- or radiation therapies in patients with HNSCC and solid tumors are summarized. As we achieve a deeper understanding of the genomic alterations and molecular mechanisms underlying deregulated death and survival pathways in different cancers, the role of SMAC mimetics and IAP inhibitors in cancer treatment will be elucidated. Such developments could enhance precision therapeutics and improve outcomes for cancer patients. Clin Cancer Res; 23(6); 1379-87. ©2016 AACR.