Abstract Mutations in KRAS represent the most common oncogenic event in human cancer and occur in approximately 30% of lung adenocarcinomas. The mechanisms by which lung tumours evade apoptosis induced by oncogenic KRAS-driven stress remain incompletely understood. Here, we identify the anti-apoptotic regulator FLIP ( CFLAR ) as a critical dependency in KRAS-mutant lung cancers. We demonstrate that KRAS-mutant human lung cancer cell lines exhibit elevated FLIP expression and enhanced dependence on FLIP for survival compared to KRAS wild-type counterparts. Subsequently, using genetically engineered mouse models (GEMMs), we show that FLIP is essential for Kras -driven lung tumour development in vivo . In vitro, FLIP-deficient lung cancer cells display spontaneous, caspase-8- dependent apoptosis and hyper-sensitivity to the immune/inflammatory cytokines TNFα and TRAIL. Strikingly, FLIP-null lung cancer cells fail to engraft even in highly immunodeficient orthotopic models that lack TRAIL-expressing immune cells but retain TNFα-expressing monocytes. Moreover, silencing of TNFR1 or TNFα but not TRAIL-R2 rescued constitutive caspase-8-dependent apoptosis in FLIP null lung cancer cells, implicating TNFα/TNFR1 in mediating this apoptotic response. Mechanistically, we find that mutant KRAS sustains FLIP expression via ERK1/2 signalling, thereby protecting cells from caspase-8 activation. Notably, KRAS inhibition downregulates FLIP, sensitising cells to TNFα- and TRAIL-induced apoptosis. These findings uncover a novel KRAS–ERK–FLIP axis that protects tumour cells from caspase-8-mediated apoptosis and reveal FLIP as a key survival factor co-opted by KRAS -mutant lung cancers. Beyond identifying FLIP as a promising therapeutic target in KRAS mutant lung cancer, our work also provides mechanistic insight into the pro-apoptotic effects of KRAS inhibitors and suggests that FLIP expression may serve as a predictive biomarker to enhance patient stratification and the therapeutic efficacy of these agents in lung cancer.

Figure legends for Supplementary Figures

TRAIL and FasL are potent inducers of apoptosis but can also promote inflammation through assembly of cytoplasmic caspase-8/FADD/RIPK1 (FADDosome) complexes, wherein caspase-8 acts as a scaffold to drive FADD/RIPK1-mediated nuclear factor KB (NF-KB) activation. cFLIP is also recruited to FADDosomes and restricts caspase-8 activity and apoptosis, but whether cFLIP also regulates death receptor-initiated inflammation is unclear. Here, we show that silencing or deletion of cFLIP leads to robustly enhanced Fas-, TRAIL-, or TLR3-induced inflammatory cytokine production, which can be uncoupled from the effects of cFLIP on caspase-8 activation and apoptosis. Mechanistically, cFLIPL suppresses Fasor TRAIL-initiated NF-KB activation through inhibiting the assembly of caspase-8/FADD/RIPK1 FADDosome complexes, due to the low affinity of cFLIPL for FADD. Consequently, increased cFLIPL occupancy of FADDosomes diminishes recruitment of FADD/RIPK1 to caspase-8, thereby suppressing NF-KB activation and inflammatory cytokine production downstream. Thus, cFLIP acts as a dual suppressor of apoptosis and inflammation via distinct modes of action.

Response of H460 and A549 cells to IR

Combination of SAHA with IR in H460 and A549 cells using different treatment schedules

IR-induced changes in cell surface expression of death receptors

Caspase-8 dependent apoptosis induced by IR

There was an error in the original publication [...].

Therapeutic targeting of the apoptotic pathways for the treatment of cancer is emerging as a valid and exciting approach in anti-cancer therapeutics. Accumulating evidence demonstrates that cancer cells are typically “addicted” to a small number of anti-apoptotic proteins for their survival, and direct targeting of these proteins could provide valuable approaches for directly killing cancer cells. Several approaches and agents are in clinical development targeting either the intrinsic mitochondrial apoptotic pathway or the extrinsic death receptor mediated pathways. In this review, we discuss the main apoptosis pathways and the key molecular targets which are the subject of several drug development approaches, the clinical development of these agents and the emerging resistance factors and combinatorial treatment approaches for this class of agents with existing and emerging novel targeted anti-cancer therapeutics.

Abstract In cancer, evasion of cell death is a fundamental cause of resistance to therapy, prompting the development of therapeutics that reactivate cell death pathways such as Bcl-2 family inhibitors and IAP antagonists. The apoptosis modulator Cellular FLICE-like inhibitory protein (FLIP) is a non-redundant inhibitor of caspase-8 activation and is the only human pseudo-caspase. Caspase-8 is the initiator caspase for the extrinsic apoptotic pathway and is now recognized as the molecular “switch” that controls the 3 major forms of programmed cell death: apoptosis, necroptosis and pyroptosis. As such, methods to selectively activate caspase-8 in appropriate disease contexts represent an exciting new therapeutic paradigm. FLIP is frequently overexpressed in solid and haematological cancers where it is associated with poor prognosis and chemo- and radio-resistance. Moreover, by regulating caspase-8 activity, FLIP is a key determinant of cell death induced by death ligands such as TRAIL expressed by immune effector cells. Thus, targeting FLIP's interaction with caspase-8 represents a unique therapeutic opportunity for enhancing standard-of-care anti-cancer therapies and promoting anti-tumor immunity.We report the discovery and characterisation of small molecule first-in-class selective inhibitors capable of disrupting FLIP's interaction with procaspase-8 in human cancer cells. These small molecule inhibitors induce caspase-8-dependent cell death as single agents and dramatically enhance apoptosis induced by recombinant TRAIL and 2nd generation multivalent TRAIL-R2 agonists in the nM concentration range. KRAS mutant non-small cell lung cancer (NSCLC) was identified as a major sensitive disease setting for FLIP inhibitors with single digit nM activity in vitro in several models and single agent in vivo efficacy. FLIP inhibitors also demonstrated in vitro efficacy in combination with KRAS G12C inhibitors in KRAS G12C mutant NSCLC and in vitro and in vivo efficacy in combination with the 3rd generation EGFR inhibitor Osimertinib in EGFR mutant NSCLC. We have also observed single agency nM efficacy in lymphoma and leukemia models and significant efficacy in combination with standard-of-care chemotherapeutics in KRAS mutant colorectal and pancreatic cancers. In summary, we have identified novel FLIP-targeted activators of caspase-8 with a unique mechanism-of-action and the potential for use in the treatment of several human cancers and leukemias either as a single agent, or in combination with standard-of-care chemotherapeutics and other clinically-relevant targeted agents. AcknowledgementsThis work was supported by a Seeding Drug Discovery award from the Wellcome Trust. Citation Format: Catherine A. Higgins, Jennifer Fox, Jamie Roberts, Declan Doherty, Trevor Perrior, Ray Boffey, Tim Harrison, Daniel B. Longley. Development and preclinical evaluation of unique first-in-class small molecule inhibitors of the anti-apoptotic protein FLIP [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1342.

TRAIL-R2 (DR5) is a clinically-relevant therapeutic target and a key target for immune effector cells. Herein, we identify a novel interaction between TRAIL-R2 and the Skp1-Cullin-1-F-box (SCF) Cullin-Ring E3 Ubiquitin Ligase complex containing Skp2 (SCFSkp2). We find that SCFSkp2 can interact with both TRAIL-R2's pre-ligand association complex (PLAC) and ligand-activated death-inducing signalling complex (DISC). Moreover, Cullin-1 interacts with TRAIL-R2 in its active NEDDylated form. Inhibiting Cullin-1's DISC recruitment using the NEDDylation inhibitor MLN4924 (Pevonedistat) or siRNA increased apoptosis induction in response to TRAIL. This correlated with enhanced levels of the caspase-8 regulator FLIP at the TRAIL-R2 DISC, particularly the long splice form, FLIP(L). We subsequently found that FLIP(L) (but not FLIP(S), caspase-8, nor the other core DISC component FADD) interacts with Cullin-1 and Skp2. Importantly, this interaction is enhanced when FLIP(L) is in its DISC-associated, C-terminally truncated p43-form. Prevention of FLIP(L) processing to its p43-form stabilises the protein, suggesting that by enhancing its interaction with SCFSkp2, cleavage to the p43-form is a critical step in FLIP(L) turnover. In support of this, we found that silencing any of the components of the SCFSkp2 complex inhibits FLIP ubiquitination, while overexpressing Cullin-1/Skp2 enhances its ubiquitination in a NEDDylation-dependent manner. DISC recruitment of TRAF2, previously identified as an E3 ligase for caspase-8 at the DISC, was also enhanced when Cullin-1's recruitment was inhibited, although its interaction with Cullin-1 was found to be mediated indirectly via FLIP(L). Notably, the interaction of p43-FLIP(L) with Cullin-1 disrupts its ability to interact with FADD, caspase-8 and TRAF2. Collectively, our results suggest that processing of FLIP(L) to p43-FLIP(L) at the TRAIL-R2 DISC enhances its interaction with co-localised SCFSkp2, leading to disruption of p43-FLIP(L)'s interactions with other DISC components and promoting its ubiquitination and degradation, thereby modulating TRAIL-R2-mediated apoptosis.

Abstract Caspase-8 is the initiator caspase for the extrinsic apoptotic pathway and now recognized as the molecular “switch” that controls 3 major forms of programmed cell death: apoptosis, necroptosis and pyroptosis. As such, methods to selectively activate caspase-8 in appropriate disease contexts represent an exciting new therapeutic paradigm. In cancer, evasion of cell death is a fundamental cause of resistance to therapy, prompting the development of therapeutics that reactivate cell death pathways, with the first of these (targeting the intrinsic apoptotic pathway) recently clinically approved. The apoptosis modulator FLIP is a non-redundant inhibitor of caspase-8 activation in the extrinsic apoptotic pathway and is the only human pseudo-caspase. FLIP is frequently overexpressed in a number of cancers and leukemias and, through its modulation of caspase-8, has been shown to be a major mediator of resistance to standard-of-care chemotherapies, targeted therapies and radiotherapy. Moreover, by regulating caspase-8 activity, FLIP is a key determinant of cell death induced by immune effector cells. Thus, targeting FLIP's interaction with caspase-8 represents a unique therapeutic approach for enhancing standard-of-care anti-cancer therapies and promoting anti-tumor immunity. We have identified a series of potent (nM), selective small molecule inhibitors which, by disrupting the binding of FLIP to effector proteins, are able to activate caspase-8-dependent apoptosis in multiple cancer cell lines. These protein-protein interaction inhibitors, which are relatively low MW and have favorable drug-like properties, have potent in vitro activity as single agents in several clinically-relevant settings, including KRAS mutant non-small cell lung cancer (NSCLC). Moreover, these FLIP inhibitors have been shown to potentiate the effects of standard-of-care chemotherapeutics and targeted anti-cancer agents in pre-clinical models. Recently, FLIP has emerged as a key mediator of resistance to the 3rd generation EGFR inhibitor Osimertinib in EGFR mutant NSCLC. We found that Osimertinib-resistant EGFR mutant NSCLC cells express elevated levels of FLIP and are more sensitive to FLIP inhibitors. Most importantly, compared to treatment with Osimertinib alone, co-treatment with Osimertinib and FLIP inhibitors prevented tumor regrowth in vivo after treatment cessation. Similar effects have been observed in vitro with the MEK inhibitor Selumetinib in KRAS mutant NSCLC models. In summary, we have identified novel, drug-like activators of caspase-8 with a unique mechanism-of-action and successfully completed proof-of-concept in vivo efficacy studies which have strong translational relevance to the clinical situation in EGFR mutant NSCLC, where despite excellent response rates to Osimertinib, patient relapse is inevitable. Moreover, these compounds have potential for broad application in treatment of several cancers both as single agents and in combination with standard-of-care therapy and other clinically-relevant targeted agents. Citation Format: Daniel B. Longley, Catherine Higgins, Jennifer Fox, Jamie Z. Roberts, Ray Boffey, Sophie Williams, Trevor Perrior, Martin J. Page, Tim Harrison. Development of first-in-class small molecule inhibitors of FLIP which activate caspase-8, the nodal regulator of apoptosis, necroptosis and pyroptosis [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 5220.

The long FLIP splice form FLIP(L) can act as both an inhibitor and promoter of caspase-8 at death-inducing signalling complexes (DISCs) formed by death receptors such as TRAIL-R2 and related intracellular complexes such as the ripoptosome. Herein, we describe a revised DISC assembly model that explains how FLIP(L) can have these opposite effects by defining the stoichiometry (with respect to caspase-8) at which it converts from being anti- to pro-apoptotic at the DISC. We also show that in the complete absence of FLIP(L), procaspase-8 activation at the TRAIL-R2 DISC has significantly slower kinetics, although ultimately the extent of apoptosis is significantly greater. This revised model of DISC assembly also explains why FLIP's recruitment to the TRAIL-R2 DISC is impaired in the absence of caspase-8 despite showing that it can interact with the DISC adaptor protein FADD and why the short FLIP splice form FLIP(S) is the more potent inhibitor of DISC-mediated apoptosis.

Depending on its expression levels, the long splice form of the pseudo-caspase FLIP (FLIP(L)) can act as an inhibitor (high expression) or activator (low expression) of apoptosis induction by the TRAIL-R2 death-inducing signalling complex (DISC); its expression levels are therefore tightly regulated. Here, we demonstrate that the Skp1-Cullin-1-F-box (SCF) Cullin-Ring E3 Ubiquitin Ligase complex containing Skp2 (SCFSkp2) regulates the stability of FLIP(L) (but not the short splice form FLIP(S)), and, unusually, this is mediated by direct binding of FLIP(L) to Cullin-1 rather than via Skp2. By fine mapping the interaction of FLIP(L) with Cullin-1 to the large subunit of its pseudo-caspase domain, we found that the interaction is significantly stronger with FLIP(L)’s DISC-processed p43-form. Importantly, this interaction disrupts the ability of p43-FLIP to interact with FADD, caspase-8 and another DISC component, TRAF2. Moreover, we find that SCFSkp2 associates with TRAIL-R2 constitutively and does so independently of FLIP(L) and other canonical DISC components. Inhibition of Cullin-1 expression (using siRNA) or activity (using a NEDDylation inhibitor, MLN4924) enhanced FLIP(L) and TRAF2 levels at the TRAIL-R2 DISC and enhanced caspase-8 processing. This suggests that processing of FLIP(L) to p43-FLIP at the TRAIL-R2 DISC enhances its interaction with co-localised SCFSkp2, leading to disruption of p43-FLIP’s association with the DISC thereby altering caspase-8 processing. These findings provide important new insights into how FLIP(L) expression and TRAIL-R2 signaling is controlled.

AbstractDepending on its expression levels, the long splice form of the pseudo-caspase FLIP (FLIP(L)) can act as an inhibitor (high expression) or activator (low expression) of apoptosis induction by the TRAIL-R2 death-inducing signalling complex (DISC); its expression levels are therefore tightly regulated. Here, we demonstrate that the Skp1-Cullin-1-F-box (SCF) Cullin-Ring E3 Ubiquitin Ligase complex containing Skp2 (SCFSkp2) regulates the stability of FLIP(L) (but not the short splice form FLIP(S)), and, unusually, this is mediated by direct binding of FLIP(L) to Cullin-1 rather than via Skp2. By fine mapping the interaction of FLIP(L) with Cullin-1 to the large subunit of its pseudo-caspase domain, we found that the interaction is significantly stronger with FLIP(L)’s DISC-processed p43-form. Importantly, this interaction disrupts the ability of p43-FLIP to interact with FADD, caspase-8 and another DISC component, TRAF2. Moreover, we find that SCFSkp2associates with TRAIL-R2 constitutively and does so independently of FLIP(L) and other canonical DISC components. Inhibition of Cullin-1 expression (using siRNA) or activity (using a NEDDylation inhibitor, MLN4924) enhanced FLIP(L) and TRAF2 levels at the TRAIL-R2 DISC and enhanced caspase-8 processing. This suggests that processing of FLIP(L) to p43-FLIP at the TRAIL-R2 DISC enhances its interaction with co-localised SCFSkp2, leading to disruption of p43-FLIP’s association with the DISC thereby altering caspase-8 processing. These findings provide important new insights into how FLIP(L) expression and TRAIL-R2 signaling is controlled.

Evasion of cell death is a major cause of resistance to anti-cancer therapies, making proteins that regulate cell death clinically relevant therapeutic targets. The anti-apoptotic protein FLIP is frequently overexpressed in a number of cancers and leukemias and has been shown to be a major mediator of resistance to chemo- and radio-therapies and to cell death induced by immune effector cells. FLIP and procaspase-8 form complexes with the adaptor protein FADD in response to a variety of clinically relevant stimuli, including ligation of death receptors, such as TRAIL-R1 and R2, and treatment with chemotherapeutic agents. In these complexes, FLIP modulates the activation of procaspase-8, and thereby regulates induction of apoptosis and necroptosis - two major cell death mechanisms. Herein, we report the development and pre-clinical characterization of first-in-class inhibitors of FLIP. Molecular modelling identified a putative drug-binding pocket on FLIP against which a virtual small-molecule screen was carried out. Subsequent biochemical screening of selected compounds using protein-protein interaction assays identified hits with on-target activity. Medicinal chemistry optimization of these hits identified a series of compounds that are able to disrupt FLIP’s interaction with the TRAIL-R2 DISC and display nanomolar activity in NSCLC cell-based assays in line with their binding affinity in an orthogonal biophysical assay (isothermal calorimetry). The pro-apoptotic effects of these FLIP inhibitors were enhanced upon addition of death ligands, such as TRAIL, and lead-molecules have been shown to potentiate the effects of standard-of-care chemotherapeutics, ionizing radiation and targeted anti-cancer agents. FLIP overexpression and CRISPR-mediated deletion of procaspase-8 and FADD abrogated the effects of these novel inhibitors consistent with their expected mechanism-of-action. Recent studies have also demonstrated that lead FLIP inhibitors target Treg cells, which are key immuno-suppressive, tumor-promoting immune cells. Lead molecules have also demonstrated in vivo efficacy in NSCLC cancer models both as a single agent and in combination with a multivalent TRAIL agonist. The first-in-class inhibitors of FLIP developed in this study have the potential for broad application in treatment of several cancers, including solid tumors and haematological malignancies, either as monotherapy or in combination with other agents. Acknowledgements: This work was supported by a Seeding Drug Discovery award from the Wellcome Trust. Citation Format: Catherine A. Higgins, Jennifer Fox, Joanna Majkut, Greti E. Fiedler, Jamie Roberts, Luke Humphreys, Ray J. Boffey, Trevor R. Perrior, Timothy Harrison, Daniel B. Longley. Development and pre-clinical assessment of a first-in-class small molecule inhibitor of FLIP [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 382.

Abstract Evasion of cell death is a major cause of resistance to anticancer therapies, making proteins that regulate cell death clinically relevant therapeutic targets. The anti-apoptotic protein FLIP is frequently overexpressed in a number of cancers, including non-small cell lung cancer (NSCLC) and colorectal cancer (CRC), and has been shown to be a major mediator of drug resistance. FLIP and procaspase-8 form complexes with the adaptor protein FADD in response to a variety of clinically relevant stimuli, including ligation of death receptors, such as TRAIL-R1 and R2, and treatment with chemotherapeutic agents. In these complexes, FLIP modulates the activation of procaspase-8, and thereby apoptosis and necroptosis –two major cell death mechanisms. We have found that there are important differences between FLIP and procaspase-8 in terms of their preferred modes of interaction with FADD that are potentially therapeutically exploitable (1). Herein, we report the development and preclinical characterization of first-in-class inhibitors of FLIP. Molecular modelling of the FLIP-FADD interaction identified a putative drug-binding pocket on FLIP against which a virtual small-molecule screen was carried out. Subsequent biochemical screening of selected compounds using a FLIP-FADD protein-protein interaction assay identified hits with on-target activity. Medicinal chemistry optimization of these hits identified a series of compounds that are able to disrupt FLIP’s interaction with the DISC and display nanomolar activity in NSCLC and CRC cell-based assays in line with their binding affinity in an orthogonal biophysical assay (isothermal calorimetry). The pro-apoptotic effects of these FLIP inhibitors were enhanced upon addition of death ligands, such as TRAIL, and lead molecules have been shown to potentiate the effects of standard-of-care chemotherapeutics such as, e.g., cisplatin (NSCLC). FLIP overexpression and CRISPR-mediated procaspase-8 deletion abrogated the effects of these novel inhibitors consistent with the expected mechanism of action. In addition, using peripheral blood mononuclear cells (PBMCs), we demonstrate that FLIP inhibitors have selectivity against cancer cells. Finally, we identified lead molecules with ADME profiles suitable for in vivo evaluation and using these compounds, single-agent antitumor effects have been demonstrated in xenograft models. The first-in-class inhibitors of FLIP developed in this study have the potential for broad application in treatment of cancer, either as monotherapy or in combination with other agents. Acknowledgments: This work was supported by a Seeding Drug Discovery award from the Wellcome Trust. Reference: 1. Majkut J, et al. Differential affinity of FLIP and procaspase 8 for FADD's DED binding surfaces regulates DISC assembly. Nat Commun 2014;5:3350. Citation Format: Catherine A. Higgins, Joanna Majkut, Jennifer Fox, Luke Humphreys, Greti Espona Fiedler, Ray J. Boffey, Trevor R. Perrior, David Haigh, Timothy Harrison, Daniel B. Longley. Development and preclinical assessment of a first-in-class small-molecule inhibitor of FLIP [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2017 Oct 26-30; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2018;17(1 Suppl):Abstract nr B129.

AbstractResistance to radiotherapy due to insufficient cancer cell death is a significant cause of treatment failure in non–small cell lung cancer (NSCLC). The endogenous caspase-8 inhibitor FLIP is a critical regulator of cell death that is frequently overexpressed in NSCLC and is an established inhibitor of apoptotic cell death induced via the extrinsic death receptor pathway. Apoptosis induced by ionizing radiation (IR) has been considered to be mediated predominantly via the intrinsic apoptotic pathway; however, we found that IR-induced apoptosis was significantly attenuated in NSCLC cells when caspase-8 was depleted using RNA interference (RNAi), suggesting involvement of the extrinsic apoptosis pathway. Moreover, overexpression of wild-type FLIP, but not a mutant form that cannot bind the critical death receptor adaptor protein FADD, also attenuated IR-induced apoptosis, confirming the importance of the extrinsic apoptotic pathway as a determinant of response to IR in NSCLC. Importantly, when FLIP protein levels were downregulated by RNAi, IR-induced cell death was significantly enhanced. The clinically relevant histone deacetylase (HDAC) inhibitors vorinostat and entinostat were subsequently found to sensitize a subset of NSCLC cell lines to IR in a manner that was dependent on their ability to suppress FLIP expression and promote activation of caspase-8. Entinostat also enhanced the antitumor activity of IR in vivo. Therefore, FLIP downregulation induced by HDAC inhibitors is a potential clinical strategy to radiosensitize NSCLC and thereby improve response to radiotherapy. Overall, this study provides the first evidence that pharmacological inhibition of FLIP may improve response of NCSLC to IR. Mol Cancer Ther; 15(10); 2432–41. ©2016 AACR.

Abstract Background Colorectal Cancer (CRC) is the second most common cause of cancer death, with 40% of patients with this disease obtaining no benefit from current chemotherapy. Novel therapeutic strategies are needed to improve CRC patient response rates and survival. FLIP is an inhibitor of the extrinsic apoptotic pathway that binds to FADD at death-inducing signalling complexes (DISCs), such as those formed by the TNF-α-related apoptosis inducing ligand (TRAIL) receptors TRAIL-R1 and TRAIL-R2, thereby blocking homodimerization and activation of procaspase-8 and inhibiting apoptosis induction. We previously reported that FLIP blocks apoptosis induced by TRAIL and standard-of-care chemotherapeutics (5-Fluorouracil, oxaliplatin and SN38) in CRC models. Moreover, FLIP is frequently overexpressed in CRC and its overexpression correlates with poor prognosis. Subsequently, we have developed novel small molecule inhibitors that target FLIP's critical protein-protein interactions, preventing its interaction with FADD and therefore promoting activation of caspase-8 and apoptosis induction. Methods A DISC recruitment assay was used to assess levels of FLIP at the TRAIL-R2 DISC. Caspase activity, cell viability and apoptosis induction were assessed in CRC models treated with FLIP inhibitors alone and in combination with TRAIL or standard-of-care chemotherapeutics. Mechanism-of-action was assessed using caspase-8-targeted siRNA and FLIP overexpressing models. Results Using CRC cell line models, it was demonstrated that FLIP recruitment to the DISC is inhibited by FLIP inhibitors from the lead series. As a result, TRAIL-induced caspase-8 and caspase-3/7 activity were enhanced and increased levels of apoptosis cells were observed. Decreased cell viability was observed that was proportionate to the levels of apoptosis induced. Cell death triggered by FLIP inhibitors was shown to be caspase-8-dependent consistent with the expected mechanism-of-action. Importantly, inhibitors of FLIP also enhanced apoptosis induction in response to 5-Fluorouracil, oxaliplatin and SN38. Conclusion We have developed inhibitors of FLIP that decrease its recruitment to the TRAIL-R2 DISC and increase TRAIL-induced caspase activation and apoptosis. Moreover, these inhibitors synergise with 5-Fluorouracil, oxaliplatin and SN38, suggesting that this novel class of agents has therapeutic potential in CRC when used in conjunction with standard-of-care chemotherapeutic agents. Acknowledgements This work was supported by a Seeding Drug Discovery award from the Wellcome Trust (reference: 099470). Citation Format: Jennifer P. Fox, Joanna Majkut, Catherine Higgins, Zsuzsanna Nemeth, Adnan Malik, Christopher J. Scott, Peter Blurton, Ray J. Boffey, Trevor R. Perrior, Timothy Harrison, Daniel B. Longley. FLIP protein-protein interaction inhibitors enhance sensitivity of colorectal cancer cells to chemotherapy and TRAIL. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr C5.