Insight into Bortezomib Focusing on Its Efficacy against P-gp-Positive MDR Leukemia Cells.

In this paper, we compared the effects of bortezomib on L1210 (S) cells with its effects on P-glycoprotein (P-gp)-positive variant S cells, which expressed P-gp either after selection with vincristine (R cells) or after transfection with a human gene encoding P-gp (T cells). Bortezomib induced the death-related effects in the S, R, and T cells at concentrations not exceeding 10 nM. Bortezomib-induced cell cycle arrest in the G2/M phase was more pronounced in the S cells than in the R or T cells and was related to the expression levels of cyclins, cyclin-dependent kinases, and their inhibitors. We also observed an increase in the level of polyubiquitinated proteins (via K48-linkage) and a decrease in the gene expression of some deubiquitinases after treatment with bortezomib. Resistant cells expressed higher levels of genes encoding 26S proteasome components and the chaperone HSP90, which is involved in 26S proteasome assembly. After 4 h of preincubation, bortezomib induced a more pronounced depression of proteasome activity in S cells than in R or T cells. However, none of these changes alone or in combination sufficiently suppressed the sensitivity of R or T cells to bortezomib, which remained at a level similar to that of S cells.

Downregulation of Cell Cycle and Checkpoint Genes by Class I HDAC Inhibitors Limits Synergism with G2/M Checkpoint Inhibitor MK-1775 in Bladder Cancer Cells.

Since genes encoding epigenetic regulators are often mutated or deregulated in urothelial carcinoma (UC), they represent promising therapeutic targets. Specifically, inhibition of Class-I histone deacetylase (HDAC) isoenzymes induces cell death in UC cell lines (UCC) and, in contrast to other cancer types, cell cycle arrest in G2/M. Here, we investigated whether mutations in cell cycle genes contribute to G2/M rather than G1 arrest, identified the precise point of arrest and clarified the function of individual HDAC Class-I isoenzymes. Database analyses of UC tissues and cell lines revealed mutations in G1/S, but not G2/M checkpoint regulators. Using class I-specific HDAC inhibitors (HDACi) with different isoenzyme specificity (Romidepsin, Entinostat, RGFP966), cell cycle arrest was shown to occur at the G2/M transition and to depend on inhibition of HDAC1/2 rather than HDAC3. Since HDAC1/2 inhibition caused cell-type-specific downregulation of genes encoding G2/M regulators, the WEE1 inhibitor MK-1775 could not overcome G2/M checkpoint arrest and therefore did not synergize with Romidepsin inhibiting HDAC1/2. Instead, since DNA damage was induced by inhibition of HDAC1/2, but not of HDAC3, combinations between inhibitors of HDAC1/2 and of DNA repair should be attempted.

Brèves.

TITLE: Brèves. ABSTRACT: L'unité d'enseignement « Immunopathologie ¼ qui propose les brèves de ce numéro est suivie par des étudiants des sept parcours recherche du Master Biologie Santé de l'Université de Montpellier. On y étudie les bases physiopathologiques des maladies immunologiques, les cibles thérapeutiques et les mécanismes d'échappement des microorganismes et des tumeurs. Ce Master rassemble des étudiants issus du domaine des sciences et technologies et de celui de la santé. Les articles présentés ont été choisis par les étudiants selon leur domaine de prédilection.

The cancer-natural killer cell immunity cycle.

Immunotherapy with checkpoint blockade induces rapid and durable immune control of cancer in some patients and has driven a monumental shift in cancer treatment. Neoantigen-specific CD8+ T cells are at the forefront of current immunotherapy strategies, and the majority of drug discovery and clinical trials revolve around further harnessing these immune effectors. Yet the immune system contains a diverse range of antitumour effector cells, and these must function in a coordinated and synergistic manner to overcome the immune-evasion mechanisms used by tumours and achieve complete control with tumour eradication. A key antitumour effector is the natural killer (NK) cells, cytotoxic innate lymphocytes present at high frequency in the circulatory system and identified by their exquisite ability to spontaneously detect and lyse transformed or stressed cells. Emerging data show a role for intratumoural NK cells in driving immunotherapy response and, accordingly, there have been renewed efforts to further elucidate and target the pathways controlling NK cell antitumour function. In this Review, we discuss recent clinical evidence that NK cells are a key immune constituent in the protective antitumour immune response and highlight the major stages of the cancer-NK cell immunity cycle. We also perform a new analysis of publicly available transcriptomic data to provide an overview of the prognostic value of NK cell gene expression in 25 tumour types. Furthermore, we discuss how the role of NK cells evolves with tumour progression, presenting new opportunities to target NK cell function to enhance cancer immunotherapy response rates across a more diverse range of cancers.

Immune checkpoint inhibitors for the treatment of MSI-H/MMR-D colorectal cancer and a perspective on resistance mechanisms.

Metastatic colorectal cancer (CRC) with a mismatch repair-deficiency (MMR-D)/microsatellite instability-high (MSI-H) phenotype carries unique characteristics such as increased tumour mutational burden and tumour-infiltrating lymphocytes. Studies have shown a sustained clinical response to immune checkpoint inhibitors with dramatic clinical improvement in patients with MSI-H/MMR-D CRC. However, the observed response rates range between 30% and 50% suggesting the existence of intrinsic resistance mechanisms. Moreover, disease progression after an initial positive response to immune checkpoint inhibitor treatment points to acquired resistance mechanisms. In this review article, we discuss the clinical trials that established the efficacy of immune checkpoint inhibitors in patients with MSI-H/MMR-D CRC, consider biomarkers of the immune response and elaborate on potential mechanisms related to intrinsic and acquired resistance. We also provide a perspective on possible future therapeutic approaches that might improve clinical outcomes, particularly in patients with actionable resistance mechanisms.

An increased cell cycle gene network determines MEK and Akt inhibitor double resistance in triple-negative breast cancer.

Triple-negative breast cancer (TNBC) is an aggressive subtype of breast cancer with poor clinical prognosis and limited targeted treatment strategies. Kinase inhibitor screening of a panel of 20 TNBC cell lines uncovered three critical TNBC subgroups: 1) sensitive to only MEK inhibitors; 2) sensitive to only Akt inhibitors; 3) resistant to both MEK/Akt inhibitors. Using genomic, transcriptomic and proteomic datasets of these TNBC cell lines we unravelled molecular features associated with the MEK and Akt drug resistance. MEK inhibitor-resistant TNBC cell lines were discriminated from Akt inhibitor-resistant lines by the presence of PIK3CA/PIK3R1/PTEN mutations, high p-Akt and low p-MEK levels, yet these features could not distinguish double-resistant cells. Gene set enrichment analyses of transcriptomic and proteomic data of the MEK and Akt inhibitor response groups revealed a set of cell cycle-related genes associated with the double-resistant phenotype; these genes were overexpressed in a subset of breast cancer patients. CDK inhibitors targeting the cell cycle programme could overcome the Akt and MEK inhibitor double-resistance. In conclusion, we uncovered molecular features and alternative treatment strategies for TNBC that are double-resistant to Akt and MEK inhibitors.

Varied phenotypes and management of immune checkpoint inhibitor-associated neuropathies.

OBJECTIVE: To describe the spectrum, clinical course, and management of neuropathies associated with immune checkpoint inhibitors (ICIs). METHODS: Patients with ICI-related neuropathy (irNeuropathy) were identified and their clinical characteristics compared to neuropathy attributed to cytotoxic agents. RESULTS: We identified 19 patients with irNeuropathies. ICIs included anti-programmed death-1 (PD1), 9; anti-cytotoxic T-lymphocyte-associated antigen-4 (CTLA4), 2; and combination of anti-CTLA4 and anti-PD1, 8. Median number of ICI doses prior to neuropathy onset was 4. Rate of neuropathies following ICI therapy was 0.7%. Underlying malignancies included melanoma (n = 15), lung adenocarcinoma (n = 3), and cholangiocarcinoma (n = 1). Neuropathy phenotypes were cranial neuropathies with or without meningitis (n = 7), nonlength-dependent polyradiculoneuropathies with and without cranial nerve involvement (n = 6), small-fiber/autonomic neuropathy (n = 2), ANCA-associated mononeuritis multiplex (n = 1), sensory neuronopathy (n = 1), length-dependent sensorimotor axonal polyneuropathy (n = 1), and neuralgic amyotrophy (n = 1). Immune-related adverse events involving other organ systems were common (58%). Corticosteroid use for management of neuropathy was associated with improvement in median modified Rankin Scale score (1 vs 0, p = 0.001) and Inflammatory Neuropathy Cause and Treatment Disability score (2 vs 0.5, p = 0.012) (Class IV). Significantly higher proportion of irNeuropathies had acute or subacute and nonlength-dependent presentations (p < 0.001) and rate of hospitalization for irNeuropathy was also higher (p = 0.002) compared to toxic neuropathy from chemotherapy. CONCLUSION: Neuropathy is a rare complication of ICIs that often responds to immunosuppression. Recognition of its wide phenotypic spectrum and distinct clinical characteristics and prompt management with corticosteroids may lead to favorable outcomes.

Correlation of Milestone Restricted Mean Survival Time Ratio With Overall Survival Hazard Ratio in Randomized Clinical Trials of Immune Checkpoint Inhibitors: A Systematic Review and Meta-analysis.

Importance: Immune checkpoint inhibitors (ICIs) have unique patterns of response and survival that differ from conventional chemotherapies. Novel intermediate end points are urgently required to detect the early signals of ICI activity. Objective: To evaluate milestone rate (Kaplan-Meier estimates of survival probabilities at given time points) and milestone restricted mean survival time (RMST, the area under survival curves up to given time points) as potential intermediate end points for ICI trials. Data Sources: Electronic databases (pre-MEDLINE, MEDLINE, Embase, and the Cochrane Central Register of Controlled Trials) were searched for randomized clinical trials published between January 1, 2000, and December 31, 2017. Study Selection: Phase 2 and phase 3 randomized clinical trials evaluating ICIs in advanced solid tumors. Data Extraction and Synthesis: Two investigators extracted the data and reconstructed individual patient data to estimate the milestone rate or milestone RMST from the published Kaplan-Meier curves. Main Outcomes and Measures: Trial-level milestone rates or milestone RMSTs were estimated for 6-month and 9-month progression-free survival (PFS) and 9-month and 12-month overall survival (OS). A weighted linear regression model evaluated the correlations of ratios of milestone rates or milestone RMSTs with OS hazard ratios (HRs). Results: Twenty-six trials examining 8 different tumor types were identified, including 12 892 patients. Overall survival HR was correlated with the ratio of 9-month OS milestone rate (R2 = 0.45; 95% CI, 0.27-0.74), and with the ratio of 12-month OS milestone rate (R2 = 0.40; 95% CI, 0.22-0.70). The ratio of 9-month OS milestone RMST (R2 = 0.60; 95% CI, 0.28-0.74) and ratio of 12-month OS milestone RMST were correlated with OS HR (R2 = 0.64; 95% CI, 0.42-0.78). No correlations were observed between OS HR and the ratio of 6-month or 9-month PFS milestone rates or milestone RMSTs. Conclusions and Relevance: Ratios of OS milestone RMSTs had a stronger correlation with OS HRs than ratios of OS milestone rates, whereas ratios of PFS milestone rates and ratios of PFS milestone RMSTs were not correlated with OS HRs. The OS milestone RMST could be further studied as an intermediate end point in future ICI trials.

Immune Checkpoint Inhibitor Therapy: Key Principles When Educating Patients.

BACKGROUND: Immune checkpoint inhibitor (ICI) therapy is a fast-developing field within the spectrum of cancer care. ICIs are associated with distinctive immune-related adverse events (irAEs), reflecting their unique mechanisms of action. OBJECTIVES: Effective management of irAEs requires early recognition and prompt reporting of their signs and symptoms; appropriate patient education is critical to maximizing this opportunity. METHODS: A comprehensive literature search was conducted in the public domain concerning awareness, assessment, and management of irAEs associated with ICIs. FINDINGS: Educational resources should provide timely, consistent, and personalized information, using a variety of teaching strategies that consider individual patient needs. Patient education should be developed with interprofessional team input and regularly reviewed in response to emerging guidance. Key messages include timing of therapeutic response and corresponding irAEs, early identification of irAEs, and the unique ability of ICIs to influence immune responses after treatment discontinuation.

Targeted immunotherapy with a checkpoint inhibitor in combination with chemotherapy: A new clinical paradigm in the treatment of triple-negative breast cancer.

The treatment of several solid and hematologic malignancies with immune checkpoint inhibitors (against programmed death receptor-1/ligand-1 [PD-1/PD-L1]) has dramatically changed the cancer treatment paradigm. However, no checkpoint inhibitors were previously approved for the treatment of triple-negative breast cancer (TNBC), a difficult-to-treat disease with a high unmet therapeutic need. Based on IMpassion130 clinical trial (NCT02425891), the Food and Drug Administration (FDA) has recently granted an accelerated approval for atezolizumab (TECENTRIQ®), a monoclonal antibody drug targeting PD-L1, plus chemotherapy (Abraxane; nab®-Paclitaxel) for the treatment of adults with PD-L1-positive, unresectable, locally advanced or metastatic TNBC. The FDA has also approved the Ventana diagnostic antibody SP142 as a companion test for selecting TNBC patients for treatment with atezolizumab. In the present review, we briefly discuss the importance of this breakthrough as the first cancer immunotherapy regimen to be approved for the management of breast cancer.

Neurotoxicity associated with cancer immunotherapy: immune checkpoint inhibitors and chimeric antigen receptor T-cell therapy.

PURPOSE OF REVIEW: Immune checkpoint inhibitors (ICPI) and chimeric antigen receptor T cells (CAR-T) represent novel therapies recently approved to treat a number of human cancers. As both approaches modulate the immune system, they can generate a number of immune-related adverse events (irAEs), including a large spectrum of novel neurological toxicities. These are of special interest given their potential severity and risk of compromising further oncologic treatment. We aim to provide a comprehensive review of the literature and discuss their optimal management. RECENT FINDINGS: In contrast to irAEs involving other organs, neurological complications of ICPI are uncommon, may present throughout the course of treatment and involve the peripheral and central nervous system, including polyneuropathy, myositis, myasthenia gravis, demyelinating polyradiculopathy, myelitis, encephalitis and others. If started early, ICPI-related neurologic irAEs are usually responsive to steroids. In contrast, as many as 40% of patients undergoing CAR-T therapy will develop neurologic complications in the form of a cytokine-release-associated encephalopathy. It includes delirium, aphasia, tremor/myoclonus, seizure and seizure-like activity. SUMMARY: irAEs associated with CAR-T and ICPI therapy constitute new entities. Early identification and treatment are essential to optimize the functional outcome and further oncologic management of the patient.

Revisiting immune escape in colorectal cancer in the era of immunotherapy.

In colorectal cancer (CRC), T-cell checkpoint blockade is only effective in patients diagnosed with mismatch repair-deficient (MMR-d) cancers. However, defects in Human Leukocyte Antigen (HLA) class I expression were reported to occur in most MMR-d CRCs, which would preclude antigen presentation in these tumours, considered essential for the clinical activity of this immunotherapeutic modality. We revisited this paradox by characterising HLA class I expression in two independent cohorts of CRC. We determined that loss of HLA class I expression occurred in the majority (73-78%) of MMR-d cases. This phenotype was rare in CRC liver metastases, irrespective of MMR status, whereas weak, inducible expression of HLA class I molecules was frequent in liver lesions. We propose that HLA class I is an important determinant of metastatic homing in CRCs. This observation is paramount to understand CRC carcinogenesis and for the application of immunotherapies in the metastatic setting.

Methylglyoxal down-regulates the expression of cell cycle associated genes and activates the p53 pathway in human umbilical vein endothelial cells.

Although methylglyoxal (MGO) has emerged as key mediator of diabetic microvascular complications, the influence of MGO on the vascular transcriptome has not thoroughly been assessed. Since diabetes is associated with low grade inflammation causing sustained nuclear factor-kappa B (NF-κB) activation, the current study addressed 1) to what extent MGO changes the transcriptome of human umbilical vein endothelial cells (HUVECs) exposed to an inflammatory milieu, 2) what are the dominant pathways by which these changes occur and 3) to what extent is this affected by carnosine, a putative scavenger of MGO. Microarray analysis revealed that exposure of HUVECs to high MGO concentrations significantly changes gene expression, characterized by prominent down-regulation of cell cycle associated genes and up-regulation of heme oxygenase-1 (HO-1). KEGG-based pathway analysis identified six significantly enriched pathways of which the p53 pathway was the most affected. No significant enrichment of inflammatory pathways was found, yet, MGO did inhibit VCAM-1 expression in Western blot analysis. Carnosine significantly counteracted MGO-mediated changes in a subset of differentially expressed genes. Collectively, our results suggest that MGO initiates distinct transcriptional changes in cell cycle/apoptosis genes, which may explain MGO toxicity at high concentrations. MGO did not augment TNF-α induced inflammation.

8:2 Fluorotelomer alcohol causes G1 cell cycle arrest and blocks granulocytic differentiation in HL-60 cells.

Fluorotelomer alcohols (FTOHs) are fluorinated intermediates used in manufacturing specialty polymer and surfactants, with 8:2 FTOH the homologue of largest production. FTOHs were found to pose acute toxicity, hepatotoxicity, nephrotoxicity, developmental toxicity and endocrine-disrupting risks, whereas research regarding immunotoxicity and its underlying mechanism, especially on specific immune cells is limited. Here, we investigated the immunotoxicity of 8:2 FTOH on immature immune cells in an in vitro system. We observed that exposure of HL-60 cells, a human promyelocytic leukemic cell line, to 8:2 FTOH reduced cell viability in a dose- and time-dependent manner. In addition, 8:2 FTOH exposure caused G1 cell cycle arrest in HL-60 cells, while it showed no effect on apoptosis. Exposure to 8:2 FTOH inhibited the mRNA expression of cell cycle-related genes, including CCNA1, CCNA2, CCND1, and CCNE2. Moreover, exposure to 8:2 FTOH inhibited the mRNA expression of granulocytic differentiation-related genes of CD11b, CSF3R, PU.1, and C/EPBε in HL-60 cells . Furthermore, 8:2 FTOH exhibited no effect on intracellular ROS level, while hydralazine hydrochloride (Hyd), one reactive carbonyl species (RCS) scavenger, partially blocked 8:2 FTOH-caused cytotoxicity in HL-60 cells. Overall, the results obtained in the study show that 8:2 FTOH poses immunotoxicity in immature immune cells and RCS may partially underline its mechanism.

Suppression of cell cycle progression by poly(ADP-ribose) polymerase inhibitor PJ34 in neural stem/progenitor cells.

Neural stem/progenitor cells (NSPCs) express higher levels of poly(ADP-ribose) polymerase 1 (PARP1) than mouse embryonic fibroblasts (MEFs). Inhibition of PARP induces the expression of several genes in the p53 signaling pathway, including p21, which is critical for cell cycle control at the G1/S phase, triggers apoptosis, and suppresses cell cycle progression in NSPCs. However, upon the up-regulation of p21, the cell cycle does not arrest at any specific phase. In the present study, the expression of genes specific to the G1/S and G2/M phases of the cell cycle were analyzed following treatment with PJ34 (N-[6-oxo-5,6-dihydro-phenanthridin-2-yl]-N,N-dimethylacetamide), an inhibitor of PARP. PJ34 treatment dramatically down-regulated cyclin B1 expression in NSPCs, but not in MEFs, which was confirmed by a promoter assay. Down-regulation of FoxM1 and B-MYB revealed that the down-regulation of cyclin B occurs at the transcriptional level. GADD45 was also specifically up-regulated in NSPCs. Taken together, the activation of p53 by PJ34 treatment in NSPCs induced changes in the expression of genes involved in the cell cycle. Fluorescence-activated cell sorting analysis revealed that PJ34 treatment suppressed G2/M to G1 progression in NSPCs, but not in MEFs. These data indicate that PJ34 treatment inhibits cyclin expression at the mRNA level and suppresses cell cycle progression in NSPCs.

Impact of bowel movement condition on immune checkpoint inhibitor efficacy in patients with advanced non-small cell lung cancer.

BACKGROUND: Cancer immunotherapy is under development as a promising alternative strategy for treating advanced non-small cell lung cancer (NSCLC). However, the development of novel biomarkers to optimize the use of immune checkpoint inhibitors (ICIs) is still ongoing. Gut microbiota are known to regulate a host's immunity and are associated with the response to ICIs in melanoma. Therefore, we analyzed the association between ICI treatment efficacy and bowel movement condition in patients with NSCLC. METHODS: This retrospective study analyzed patients with advanced NSCLC who were treated with ICIs between December 2015 and March 2018 at University Hospital Kyoto Prefectural University of Medicine in Kyoto, Japan. The association between stool abnormalities and ICI efficacy was investigated. We defined patients with constipation or those who used a laxative as the stool abnormality group. RESULTS: We retrospectively enrolled 40 patients with advanced NSCLC who were treated with ICIs. The median age was 69.5 years; 20 patients had a stool abnormality and 20 patients did not. The disease control rates were lower in NSCLC patients with stool abnormalities than in those without stool abnormalities (20% vs. 77.8%, respectively; P = 0.0016). The time to treatment failure with ICI treatment was shorter in NSCLC patients with stool abnormalities compared with those without stool abnormalities (P = 0.003; odds ratio, 3.09; 95% confidence interval 1.41-6.78). CONCLUSION: Stool abnormality might be a predictive biomarker for the clinical benefit of ICI treatment in patients with NSCLC. Further investigations are warranted to validate our findings.

The Effectiveness of Checkpoint Inhibitor Combinations and Administration Timing Can Be Measured by Granzyme B PET Imaging.

PURPOSE: The lack of a timely and reliable measure of response to cancer immunotherapy has confounded understanding of mechanisms of resistance and subsequent therapeutic advancement. We hypothesized that PET imaging of granzyme B using a targeted peptide, GZP, could be utilized for early response assessment across many checkpoint inhibitor combinations, and that GZP uptake could be compared between therapeutic regimens and dosing schedules as an early biomarker of relative efficacy. EXPERIMENTAL DESIGN: Two models, MC38 and CT26, were treated with a series of checkpoint inhibitors. GZP PET imaging was performed to assess tumoral GZP uptake, and tumor volume changes were subsequently monitored to determine response. The average GZP PET uptake and response of each treatment group were correlated to evaluate the utility of GZP PET for comparing therapeutic efficacy. RESULTS: In both tumor models, GZP PET imaging was highly accurate for predicting response, with 93% sensitivity and 94% negative predictive value. Mean tumoral GZP signal intensity of treatment groups linearly correlated with percent response across all therapies and schedules. Moreover, GZP PET correctly predicted that sequential dose scheduling of PD-1 and CTLA-4 targeted therapies demonstrates comparative efficacy to concurrent administration. CONCLUSIONS: Granzyme B quantification is a highly sensitive and specific early measure of therapeutic efficacy for checkpoint inhibitor regimens. This work provides evidence that GZP PET imaging may be useful for rapid assessment of therapeutic efficacy in the context of clinical trials for both novel drugs as well as dosing regimens.

Fatal Toxic Effects Associated With Immune Checkpoint Inhibitors: A Systematic Review and Meta-analysis.

Importance: Immune checkpoint inhibitors (ICIs) are now a mainstay of cancer treatment. Although rare, fulminant and fatal toxic effects may complicate these otherwise transformative therapies; characterizing these events requires integration of global data. Objective: To determine the spectrum, timing, and clinical features of fatal ICI-associated toxic effects. Design, Setting, and Participants: We retrospectively queried a World Health Organization (WHO) pharmacovigilance database (Vigilyze) comprising more than 16 000 000 adverse drug reactions, and records from 7 academic centers. We performed a meta-analysis of published trials of anti-programmed death-1/ligand-1 (PD-1/PD-L1) and anti-cytotoxic T lymphocyte antigen-4 (CTLA-4) to evaluate their incidence using data from large academic medical centers, global WHO pharmacovigilance data, and all published ICI clinical trials of patients with cancer treated with ICIs internationally. Exposures: Anti-CTLA-4 (ipilimumab or tremelimumab), anti-PD-1 (nivolumab, pembrolizumab), or anti-PD-L1 (atezolizumab, avelumab, durvalumab). Main Outcomes and Measures: Timing, spectrum, outcomes, and incidence of ICI-associated toxic effects. Results: Internationally, 613 fatal ICI toxic events were reported from 2009 through January 2018 in Vigilyze. The spectrum differed widely between regimens: in a total of 193 anti-CTLA-4 deaths, most were usually from colitis (135 [70%]), whereas anti-PD-1/PD-L1-related fatalities were often from pneumonitis (333 [35%]), hepatitis (115 [22%]), and neurotoxic effects (50 [15%]). Combination PD-1/CTLA-4 deaths were frequently from colitis (32 [37%]) and myocarditis (22 [25%]). Fatal toxic effects typically occurred early after therapy initiation for combination therapy, anti-PD-1, and ipilimumab monotherapy (median 14.5, 40, and 40 days, respectively). Myocarditis had the highest fatality rate (52 [39.7%] of 131 reported cases), whereas endocrine events and colitis had only 2% to 5% reported fatalities; 10% to 17% of other organ-system toxic effects reported had fatal outcomes. Retrospective review of 3545 patients treated with ICIs from 7 academic centers revealed 0.6% fatality rates; cardiac and neurologic events were especially prominent (43%). Median time from symptom onset to death was 32 days. A meta-analysis of 112 trials involving 19 217 patients showed toxicity-related fatality rates of 0.36% (anti-PD-1), 0.38% (anti-PD-L1), 1.08% (anti-CTLA-4), and 1.23% (PD-1/PD-L1 plus CTLA-4). Conclusions and Relevance: In the largest evaluation of fatal ICI-associated toxic effects published to date to our knowledge, we observed early onset of death with varied causes and frequencies depending on therapeutic regimen. Clinicians across disciplines should be aware of these uncommon lethal complications.