Summary Receptor interacting protein kinase 3 (RIP3 or RIPK3) has emerged as a central player in necroptosis and a potential target to control inflammatory disease. Here, three selective small molecule compounds are shown to inhibit RIP3 kinase-dependent necroptosis, although their therapeutic value is undermined by a surprising, concentration-dependent induction of apoptosis. These compounds interact with RIP3 to activate caspase 8 (Casp8) via RHIM-driven recruitment of RIP1 (RIPK1) to assemble a Casp8-FADD-cFLIP complex completely independent of pro-necrotic kinase activities and MLKL. RIP3 kinase-dead D161N mutant induces spontaneous apoptosis independent of compound; whereas, D161G, D143N, and K51A mutants only trigger apoptosis when compound is present. Accordingly, RIP3-K51A mutant mice (Rip3K51A/K51A) are viable and fertile, in stark contrast to the perinatal lethality of Rip3D161N/D161N mice. RIP3 therefore holds both necroptosis and apoptosis in balance through a Ripoptosome-like platform. This work highlights a common mechanism unveiling RHIM-driven apoptosis by therapeutic or genetic perturbation of RIP3.

Background and Purpose Intracerebral hemorrhage (ICH) leads to disability or death with few established treatments. Adverse outcomes following ICH result from irreversible damage to neurons resulting from primary and secondary injury. Secondary injury has been attributed to hemoglobin and its oxidized product hemin from lysed red blood cells. The aim of this study was to identify the underlying cell death mechanisms attributable to secondary injury by hemoglobin and hemin to broaden treatment options. Methods We investigated cell death mechanisms in cultured neurons exposed to hemoglobin or hemin. Chemical inhibitors implicated in all known cell death pathways were employed. Identified cell death mechanisms were confirmed using molecular markers and electron microscopy. Results Chemical inhibitors of ferroptosis and necroptosis protected against hemoglobin- and hemin-induced toxicity. By contrast, inhibitors of caspase-dependent apoptosis, protein or mRNA synthesis, autophagy, mitophagy or parthanatos had no effect. Accordingly, molecular markers of ferroptosis and necroptosis were increased following ICH in vitro and in vivo. Electron microscopy showed that hemin induced a necrotic phenotype. Necroptosis and ferroptosis inhibitors each abrogated death by greater than 80% and had similar therapeutic windows in vitro. Conclusion Experimental ICH shares features of ferroptotic and necroptotic cell death, but not caspase-dependent apoptosis or autophagy. We propose that ferroptosis or necroptotic signaling induced by lysed blood is sufficient to reach a threshold of death that leads to neuronal necrosis and that inhibition of either one of these pathways can bring cells below that threshold to survival.

RIP1 (RIPK1) kinase is a key regulator of TNF-induced NF-κB activation, apoptosis, and necroptosis through its kinase and scaffolding activities. Dissecting the balance of RIP1 kinase activity and scaffolding function in vivo during development and TNF-dependent inflammation has been hampered by the perinatal lethality of RIP1-deficient mice. In this study, we generated RIP1 kinase–dead (Ripk1K45A) mice and showed they are viable and healthy, indicating that the kinase activity of RIP1, but not its scaffolding function, is dispensable for viability and homeostasis. After validating that the Ripk1K45A mice were specifically protected against necroptotic stimuli in vitro and in vivo, we crossed them with SHARPIN-deficient cpdm mice, which develop severe skin and multiorgan inflammation that has been hypothesized to be mediated by TNF-dependent apoptosis and/or necroptosis. Remarkably, crossing Ripk1K45A mice with the cpdm strain protected against all cpdm-related pathology. Together, these data suggest that RIP1 kinase represents an attractive therapeutic target for TNF-driven inflammatory diseases.

RIP1 regulates necroptosis and inflammation and may play an important role in contributing to a variety of human pathologies, including immune-mediated inflammatory diseases. Small-molecule inhibitors of RIP1 kinase that are suitable for advancement into the clinic have yet to be described. Herein, we report our lead optimization of a benzoxazepinone hit from a DNA-encoded library and the discovery and profile of clinical candidate GSK2982772 (compound 5), currently in phase 2a clinical studies for psoriasis, rheumatoid arthritis, and ulcerative colitis. Compound 5 potently binds to RIP1 with exquisite kinase specificity and has excellent activity in blocking many TNF-dependent cellular responses. Highlighting its potential as a novel anti-inflammatory agent, the inhibitor was also able to reduce spontaneous production of cytokines from human ulcerative colitis explants. The highly favorable physicochemical and ADMET properties of 5, combined with high potency, led to a predicted low oral dose in humans.