Risk and incidence of fatal adverse events associated with immune checkpoint inhibitors: a systematic review and meta-analysis

Introduction

With the realization that overexpression of immune checkpoint molecules in the tumor microenvironment performs a significant function in antitumor immunity evasion, immune checkpoint inhibitors (ICIs) have expanded the scope of cancer treatment.^1,2 Monoclonal antibodies against cytotoxic T-lymphocyte-associated protein-4 (CTLA-4) and programmed cell death protein 1 (PD-1)/programmed cell death protein 1 ligand 1 (PD-L1) have shown a clinically meaningful survival benefit in a sizable subset of patients with solid cancers.^3–14 In fact, six types of drugs have been approved by the Food and Drug Administration (FDA) for the treatment of lung cancer, melanoma, renal cell carcinoma, and other tumors, including 1) CTLA-4 inhibitor, ipilimumab; 2) PD-1 inhibitors, nivolumab and pembrolizumab; and 3) PD-L1 inhibitors, atezolizumab, avelumab, and durvalumab. In addition, clinical trials of a combination of ICIs are also ongoing.^15,16

Given the increasing use of ICIs, a concomitant rise in adverse events (AEs) is inevitable. Since CTLA-4 and PD-1 play a key role in maintaining autoantigen immunity and preventing autoimmune disorders,^17,18 ICIs can trigger autoimmune-like manifestations in different organ systems, commonly known as immune-related adverse events (irAEs).¹⁹ Unlike the toxicities caused by cytotoxic or molecularly targeted agents, these irAEs were wide ranging in terms of organs affected including dermatologic, endocrine, neurologic, gastrointestinal, respiratory, and musculoskeletal toxicities, which may occur alone or in constellation, and severe adverse reactions can be life-threatening.¹⁹ In general, the overall incidence of drug-related fatal adverse events (FAEs) was low and reported to be 0.3% in hospitalized patients in the United States.²⁰ However, in a recent Phase III trial of ICIs vs placebo in patients with metastatic castration-resistant prostate cancer, nine (2%) patients in the ICI arm died as a result of treatment-related AEs, compared with no treatment-related AEs reported in the placebo arm.⁴ The gap was staggering; however, the risk of FAEs in ICIs was not clear.

To address the risks of FAEs associated with each ICI regimen, we performed a systematic review and meta-analysis of clinical trials with FDA-approved ICI regimens in patients with advanced solid tumors.

Methods

Search methods

We performed a systematic search of the literature using PubMed to identify relevant clinical trials of ICI that reported FAEs before April 15, 2018, including prospective trials of anti-CTLA-4, anti-PD-1, and anti-PD-L1 therapy in solid cancer patients using ipilimumab, nivolumab, pembrolizumab, or atezolizumab either in single-agent or combination therapies. For searching, the following keywords or corresponding medical subject heading terms were used: “ipilimumab,” “MDX-010,” “nivolumab,” “BMS-963558,” “pembrolizumab,” “MK-3475,” “atezolizumab,” “MPDL3280A,” “randomized controlled trial,” and “phase.”

Study selection and data extraction

Our main aim was to directly explore the risk of ICI mortality; hence, our selection criteria included all clinical trials that (1) were randomized controlled trials; (2) directly comparing between the study ICI either alone or in combination with other antitumor therapies to a control arm; (3) investigated the usage of the previously mentioned ICI in advanced solid tumors; and (4) clearly reported a FAE in their safety data. We excluded trials that (1) were published in the form of news, meeting abstracts, letters, or commentaries; (2) were not published in English language; and (3) were Phase I trials. The AE leading to death is defined as FAE, the grade V AE. All treatment-related AEs were under analysis among the patients who received at least one dose of study drug and were evaluated continuously starting from the first dose of the study drug to a minimum of 70 or 90 days after the last dose. The types and grades of AEs were defined according to the National Cancer Institute Common Terminology Criteria for Adverse Events (CTCAE), version 3.0 or 4.0. All FAEs should be attributed to a particular type of AE as much as possible. Any discrepancy in study selection was discussed until a consensus agreement.

We mainly extracted the following data from each selected study: numbers of FAEs in both study group and placebo group, numbers of total evaluable patients in both study group and placebo group. It is significant to note that the evaluable patients for adverse reactions mean that patients received at least one cycle of treatment, rather than randomization. We also extracted the following data: first author’s name, year of publication, phase of the trial, types of tumor, treatment regimen in both arms, and types of FAEs.

Statistical analyses

We assessed the risk of fatal adverse reactions associated with ICIs on Pooled Peto OR and associated 95% CI based on the fixed-effect model. The range of changes in the incidence of FAE in both study group and placebo group was also collected. Subgroup analyses were conducted by classifying the kinds of ICIs, tumors, the dose of ICI, and the types of FAEs. Statistical heterogeneity between RCTs was evaluated using the Cochran Q statistic and I² statistics, with values greater than 50% regarded as being indicative of moderate to high heterogeneity. Risks of bias of RCTs included were assessed by using The Cochrane Collaboration tool. We evaluated the possibility of publication bias by constructing funnel plots which were assessed by using Begg’s and Egger’s tests, and a two-sided P-value cutoff of 0.05 was considered statistically significant. All statistical analyses were performed using Review Manager software (version 5.3) and Stata 12.0.

Results

Search results

A total of 609 records were identified in the PubMed search. After screening and eligibility assessment, the specific screening process was described in Figure 1, we identified a total of 12 clinical trials. These include eight Phase III studies and four Phase II studies. Sorting by treatment, ten trials evaluated CTLA-4 inhibitor (ipilimumab, n=10), and four trials evaluated PD-1 inhibitor (nivolumab, n=3; pembrolizumab, n=1). Two trials included three cohorts, so a pairwise analysis was conducted to compare the risk of ICI drugs. Given the control group was limited to be a placebo or a blank control group, no PD-L1 or combination therapy clinical trials were selected. Sorting by cancer types, six trials evaluated melanoma, four trials evaluated non-small-cell lung cancer (NSCLC), two trials evaluated prostate cancer, and one trial evaluated gastric or gastroesophageal junction cancer, one trial evaluated small-cell lung cancer. A total of 6,390 patients were eligible for analyzing, including 3,568 in the experimental group and 2,822 in the control group. All the patients in these 12 trials had an Eastern Cooperative Oncology Group performance status of 0 or 1, and adequate hematologic, cardiac, and renal functions. All the trials included in the analysis used CTCAE, version 3.0 (n=8) or 4.0 (n=4), to uniformly assess toxicity parameters (Table 1).

Figure 1 Flow diagram of study selection. Note: *Two trials were excluded because FAEs did not occur in either study group or control group. Abbreviation: FAE, fatal adverse event.

Table 1 Baseline characteristics of the included studies Notes: *In the annotated study, Arm B was compared with the control group; $In the annotated study, Arm B was compared with the control group. Abbreviations: FAE, fatal adverse event; G/GJC, gastric or gastroesophageal junction cancer; NSCLC, non-small-cell lung cancer; SCLC, small-cell lung cancer.

OR of FAEs

OR value of FADs in all ICIs was 2.01 (95% CI: 1.17, 3.44; P=0.01). Results were further classified according to the different treatment methods used. OR value of FADs in CTLA-4 inhibitors was 2.47 (95% CI: 1.31, 4.66; P=0.005), and OR value of FADs in PD-1 inhibitors was 1.06 (95% CI: 0.37, 3.05; P=0.92) (Figure 2).

Figure 2 Forest plots of odds ratios of FAEs with ICIs compared with controls (subgrouped by the types of ICIs). Notes: *In the annotated study, Arm B was compared with the control group; $In the annotated study, Arm B was compared with the control group. Abbreviations: FAE, fatal adverse event; ICI, immune checkpoint inhibitor; CTLA-4, cytotoxic T-lymphocyte-associated protein-4; PD-1, programmed cell death protein 1.

Incidence of FAEs

For the analysis of overall incidence of ICI treatment-related death, we considered only study arms receiving ICIs. The incidence of FAE in ICI was reported in all the studies and ranged from 0% to 3.2%. The incidence of FAE in CTLA-4 trials ranged from 0% to 2.8%. The incidence of FAE in PD-1 trials ranged from 0% to 3.2% (Table 1).

Analysis based on types of tumor

In order to determine whether there is a relationship between the risk of FAE and tumor type, we conducted an analysis based on tumor type. In the included six clinical trials with melanoma, the OR value of patients with melanoma was 1.57 (95% CI: 0.65, 3.77; P=0.31). In the included four clinical trials with NSCLC, the OR value of patients with NSCLC was 1.85 (95% CI: 0.65, 5.25; P=0.25). Few clinical trials of other tumor types were included; OR value of clinical trials of prostate cancer was 3.71 (95% CI: 1.12, 12.26; P=0.03), OR value of clinical trials of gastric cancer was 1.24 (95% CI: 0.24, 6.45; P=0.80), and OR value of clinical trials of small cell lung cancer was 3.22 (95% CI: 0.13, 81.19; P=0.48) (Figure 3).

Figure 3 Forest plots of odds ratios of FAEs with ICIs compared with controls (subgrouped by the types of tumors). Notes: *In the annotated study, Arm B was compared with the control group; $In the annotated study, Arm B was compared with the control group. Abbreviations: FAE, fatal adverse event; ICI, immune checkpoint inhibitor; G/GJC, gastric or gastroesophageal junction cancer; NSCLC, non-small-cell lung cancer; SCLC, small-cell lung cancer.

Analysis of FAE type

The types of FAE were described and analyzed. A total of 63 FAE cases occurred in all clinical trials, including 48 in the ICI cohort and 15 in the control cohort. Among the ICI cohorts, the common FAEs were gastrointestinal toxicity (n=12, 25%), pulmonary toxicity (n=10, 20%), cardiac toxicity (n=5, 10%), and hepatic toxicity (n=5, 10%). Based on the different regimes, we have realized that CTLA-4 inhibitors have a higher risk of FAE of pulmonary toxicity (OR =3.48, 95% CI: 0.78, 15.61, P=0.10), gastrointestinal toxicity (OR =2.52, 95% CI: 0.75, 8.52, P=0.14), and hepatic toxicity (OR =2.80, 95% CI: 0.45, 17.42, P=0.27). PD-1 inhibitors had a higher risk of FAE of pulmonary toxicity (OR =1.51, 95% CI: 0.16, 14.61, P=0.72), cardiac toxicity (OR =0.88, 95% CI: 0.16, 4.79, P=0.88), and hepatic toxicity (OR =1.49, 95% CI: 0.06, 36.74, P=0.81). The difference was not significant (Table 2).

Table 2 Characteristics of FAE type Abbreviations: CTLA-4, cytotoxic T-lymphocyte-associated protein-4; FAE, fatal adverse event; PD-1, programmed cell death protein 1.

Analysis based on the dose of ipilimumab

We conducted subgroup analysis according to different doses of ipilimumab, and the results were as follows: OR value of 3 mg/kg group was 0.86 (95% CI: 0.27, 2.78; P=0.80), and the OR value of 10 mg/kg group was 3.63 (95% CI: 1.65, 7.99; P=0.001), so the subgroup difference was significant (Figure 4).

Figure 4 Forest plots of odds ratios of FAEs with ipilimumab compared with controls (subgrouped by the dose of ipilimumab). Note: *In the annotated study, Arm B was compared with the control group. Abbreviation: FAE, fatal adverse event.

Quality of the included studies

Figure 5 shows the risk of bias across all included studies. The possibility of publication bias was presented by constructing funnel plots in Figure 6. Begg’s and Egger’s tests indicated no publication bias among included articles regarding the OR value (Begg’s test: P=0.063 and Egger’s test P=0.373) (Figures 7 and 8). And we did pre-sensitivity analyses to evaluate the robustness of the risk we get by exchanging analysis model (random-effect model), exchanging alternative effect measure (relative risk, RR), and repetitive analysis after excluding pairwise analysis study. The results were reliable. Any disagreements in quality assessing were resolved by consensus.

Figure 5 Risk of bias graph: review authors’ judgments about each risk of bias item presented as percentages across all included studies. Notes: *In the annotated study, Arm B was compared with the control group; $In the annotated study, Arm B was compared with the control group.

Figure 6 Funnel plot of publication bias.

Figure 7 Begg’s funnel plot for publication bias testing.

Figure 8 Egger’s funnel plot for publication bias testing.

Discussion

This is the first meta-analysis to comprehensively probe into the ICI-related FAE. We found that the use of ICI increased the risk of FAE, among which the risk of FAE caused by CTLA-4 inhibitor was significantly higher than that caused by the PD-1 inhibitor (P=0.01). This is similar to another phenomenon observed in the inclusion trials:^3–14 the incidence of AE above grade 3 of CTLA-4 inhibitor is higher than that of PD-1 inhibitor. The exact reason for the difference is not clear and may be related to the differences in the CTLA-4 and PD-1 pathways.^21–23 This may provide some guidance for the clinical application of ICIs. Although the adverse reaction rate of ICI drugs is far lower than that of traditional treatment methods such as chemotherapy and radiotherapy, it still has a certain risk of drug-related death. Moreover, adverse reactions of ICI drugs tend to last for a long time, and they would not disappear immediately after drug withdrawal;^24,25 therefore, the management of ICI-related AEs is of great importance. In addition, although we did not include clinical trials of combined immunotherapy for comparison, we found that the incidence of AE above grade 3 in combination therapy was often higher in literature.²⁶ With the combination of immunotherapy in clinical promotion, further studies are needed to explore the risk of FAE in combination therapy.

Besides OR value as the outcome, we analyzed the incidence of FAEs. The two drug incidences of FAE were up to 2.8% (CTLA-4) and 3.2% (PD-1) of patients, respectively, among the included clinical trials. Unfortunately, we did not see a significant difference. In one of the other literature, treatment-related deaths occured in up to 2% of patients and varied by ICI.²⁴ In the included trials, most of the experimental groups used ICI in combination with other types of treatment methods, such as chemotherapy and vaccines. Therefore, the pure rate value cannot accurately reflect the risk of FAEs. In further study, we will reinclude the single-arm trials only using ICI to calculate the precise total incidence of FAE.

ICI is now approved by the FDA for eight types of tumors, including lung cancer, melanoma, renal cell cancer, and so on, and an increasing number of clinical trials of other tumor types are ongoing. We wondered whether the toxicity varied with the type of tumor. A total of five types of tumors were studied in the included literature, among which the most high-risk type of FAE was prostate cancer, significantly higher than other types of tumors (P=0.03), and there was no difference between the other four types of tumors. Searching in Medline database, we found this tumor type dependence was not properly explained, so further studies are needed to investigate the associations. The interpretation of these findings may be hampered by the low number of events and limited RCTs in some subgroups, and we must acknowledge that bias is inevitable because of the limited number of trials included.

In this meta-analysis, we collected all the FAEs that occurred in included trials. The most common FAEs are gastrointestinal toxicity, cardiotoxicity, hepatotoxicity, and pulmonary toxicity. It is not the same as the total occurrence of AE observed in clinical practice, where the most common are fatigue, pruritus, rash, endocrine adverse reactions, and so on.²⁶ The occurrence of FAE types changes with the types of ICIs. Gastrointestinal FAE has the higher risk among the trials using CTLA-4 inhibitors, and the immediate cause of death from gastrointestinal toxicity is often perforation of the stomach and intestine induced by colitis.^3–11 A large number of cases of gastrointestinal AEs have also been reported in many individual ipilimumab-related reports.²⁷ Occasionally steroids do not control immune-related gastrointestinal AEs. The NCCN guidelines recommend infliximab, which may successfully treat colitis.²⁵ In clinical trials involving PD-1 inhibitors, the highest risk of FAE was pulmonary toxicity. The direct causes of death were dyspnea and hypoxia induced by PD-1-related pneumonitis.^10,12–14 PD-1-related pneumonitis is a type of noninfectious lung inflammation characterized by interstitial and alveolar infiltration, whose presentation is always complicated and unpredictable, and the disease tends to occur later than other irAEs.²⁸ Since different kinds of pneumonia are similar in their early stages, careful multidisciplinary consultation should be conducted in each case of suspected fatal pneumonia. Workup for infections and timely discontinuation of immunotherapy are required for serious immune-related pneumonia, and high-dose steroids are advised with a taper over 6 weeks or longer. If there is no improvement after 48 hours, infliximab, mycophenolate mofetil, or intravenous immunoglobulin should be considered.²⁹

According to the results of subgroup analysis classifying dosage of CTLA-4 inhibitor, we found that the occurrence of ipilimumab-related FAE was dose related. The risk of FAE was significantly lower in the group with dosage of 3 mg/kg than in the group with dosage of 10 mg/kg (P=0.005). This dose-dependence has also been demonstrated in other trials.^30,31 Although it was not analyzed in this paper, this phenomenon does not seem to exist in PD-1 drugs. The toxicities of PD-1 blockade with nivolumab are similar at doses ranging from 0.3 to 10 mg/kg.^32–34

The limitations of this article should be taken into account. One of the limitation was that when comparing different types of tumors for subgroup analysis, the number of RCTs in each subgroup is deficient, and the conclusion may be biased. In future studies, we need more suitable included RCTs to determine the relationship between FAE occurrence and cancer types. The other limitation was that we did not include appropriate clinical trials of PD-L1 inhibitor drugs or combined immunotherapy, which may hinder us to recognize the risk of FAE of PD-L1 drugs and ICIs combined immunotherapy.

Conclusion

This meta-analysis shows that ICIs are associated with a high risk of FAEs and that the CTLA-4 inhibitors have a significantly higher risk of FAE (P=0.01), whereas PD-1 inhibitors were not. Compared with other tumor types, there may be a higher correlation between prostate cancer and the occurrence of treatment-related FAE in ICIs. The most common CTLA-4-related FAE was gastrointestinal toxicity, and the most common PD-1-related FAE was pulmonary toxicity. Moreover, we have shown that ipilimumab has significant dose-dependent lethal toxicity. Additional studies are needed to get an accurate mortality incidence of ICIs and an optimal treatment plan of serious AE management.

Disclosure

The authors report no conflicts of interest in this work.

---

References

1.		Wolchok JD. PD-1 Blockers. Cell. 2015;162(5):937.
2.		Leach DR, Krummel MF, Allison JP. Enhancement of antitumor immunity by CTLA-4 blockade. Science. 1996;271(5256):1734–1736.
3.		Reck M, Bondarenko I, Luft A, et al. Ipilimumab in combination with paclitaxel and carboplatin as first-line therapy in extensive-disease-small-cell lung cancer: results from a randomized, double-blind, multicenter phase 2 trial. Ann Oncol. 2013;24(1):75–83.
4.		Beer TM, Kwon ED, Drake CG, et al. Randomized, double-blind, phase III trial of ipilimumab versus placebo in asymptomatic or minimally symptomatic patients with metastatic Chemotherapy-Naive castration-resistant prostate cancer. J Clin Oncol. 2017;35(1):40–47.
5.		Govindan R, Szczesna A, Ahn MJ, et al. Phase III trial of ipilimumab combined with paclitaxel and carboplatin in advanced squamous non-small-cell lung cancer. J Clin Oncol. 2017;35(30):3449–3457.
6.		Lynch TJ, Bondarenko I, Luft A, et al. Ipilimumab in combination with paclitaxel and carboplatin as first-line treatment in stage IIIB/IV non-small-cell lung cancer: results from a randomized, double-blind, multicenter phase II study. J Clin Oncol. 2012;30(17):2046–2054.
7.		Eggermont AM, Chiarion-Sileni V, Grob JJ, et al. Adjuvant ipilimumab versus placebo after complete resection of high-risk stage III melanoma (EORTC 18071): a randomised, double-blind, phase 3 trial. Lancet Oncol. 2015;16(5):522–530.
8.		Kwon ED, Drake CG, Scher HI, et al. Ipilimumab versus placebo after radiotherapy in patients with metastatic castration-resistant prostate cancer that had progressed after docetaxel chemotherapy (CA184-043): a multicentre, randomised, double-blind, phase 3 trial. Lancet Oncol. 2014;15(7):700–712.
9.		Hodi FS, O’Day SJ, Mcdermott DF, et al. Improved survival with ipilimumab in patients with metastatic melanoma. N Engl J Med. 2010;363(8):711–723.
10.		Larkin J, Chiarion-Sileni V, Gonzalez R, et al. Combined nivolumab and ipilimumab or monotherapy in untreated melanoma. N Engl J Med. 2015;373(1):23–34.
11.		Robert C, Thomas L, Bondarenko I, et al. Ipilimumab plus dacarbazine for previously untreated metastatic melanoma. N Engl J Med. 2011;364(26):2517–2526.
12.		Kang YK, Boku N, Satoh T, et al. Nivolumab in patients with advanced gastric or gastro-oesophageal junction cancer refractory to, or intolerant of, at least two previous chemotherapy regimens (ONO-4538-12, ATTRACTION-2): a randomised, double-blind, placebo-controlled, phase 3 trial. Lancet. 2017;390(10111):2461–2471.
13.		Langer CJ, Gadgeel SM, Borghaei H, et al. Carboplatin and pemetrexed with or without pembrolizumab for advanced, non-squamous non-small-cell lung cancer: a randomised, phase 2 cohort of the open-label KEYNOTE-021 study. Lancet Oncol. 2016;17(11):1497–1508.
14.		Postow MA, Chesney J, Pavlick AC, et al. Nivolumab and ipilimumab versus ipilimumab in untreated melanoma. N Engl J Med. 2015;372(21):2006–2017.
15.		Topalian SL, Drake CG, Pardoll DM. Immune checkpoint blockade: a common denominator approach to cancer therapy. Cancer Cell. 2015;27(4):450–461.
16.		Pardoll DM. The blockade of immune checkpoints in cancer immunotherapy. Nat Rev Cancer. 2012;12(4):252–264.
17.		Waterhouse P, Penninger JM, Timms E, et al. Lymphoproliferative disorders with early lethality in mice deficient in CTLA-4. Science. 1995;270(5238):985–988.
18.		Nishimura H, Okazaki T, Tanaka Y, et al. Autoimmune dilated cardiomyopathy in PD-1 receptor-deficient mice. Science. 2001;291(5502):319–322.
19.		Michot JM, Bigenwald C, Champiat S, et al. Immune-related adverse events with immune checkpoint blockade: a comprehensive review. Eur J Cancer. 2016;54:139–148.
20.		Lazarou J, Pomeranz BH, Corey PN. Incidence of adverse drug reactions in hospitalized patients: a meta-analysis of prospective studies. JAMA. 1998;279(15):1200–1205.
21.		Buchbinder EI, Desai A. CTLA-4 and PD-1 pathways: similarities, differences, and implications of their inhibition. Am J Clin Oncol. 2016;39(1):98–106.
22.		Fife BT, Bluestone JA. Control of peripheral T-cell tolerance and autoimmunity via the CTLA-4 and PD-1 pathways. Immunol Rev. 2008;224:166–182.
23.		Parry RV, Chemnitz JM, Frauwirth KA, et al. CTLA-4 and PD-1 receptors inhibit T-cell activation by distinct mechanisms. Mol Cell Biol. 2005;25(21):9543–9553.
24.		Puzanov I, Diab A, Abdallah K, et al. Managing toxicities associated with immune checkpoint inhibitors: consensus recommendations from the Society for immunotherapy of cancer (SITC) toxicity management Working Group. J Immunother Cancer. 2017;5(1):95.
25.		Thompson JA. New NCCN guidelines: recognition and management of Immunotherapy-Related toxicity. J Natl Compr Canc Netw. 2018;16(5S):594–596.
26.		El Osta B, Hu F, Sadek R, Chintalapally R, Tang SC. Not all immune-checkpoint inhibitors are created equal: meta-analysis and systematic review of immune-related adverse events in cancer trials. Crit Rev Oncol Hematol. 2017;119:1–12.
27.		Wang DY, Ye F, Zhao S, Johnson DB. Incidence of immune checkpoint inhibitor-related colitis in solid tumor patients: a systematic review and meta-analysis. Oncoimmunology. 2017;6(10):e1344805.
28.		Nishino M, Sholl LM, Hodi FS, Hatabu H, Ramaiya NH. Anti-PD-1-Related pneumonitis during cancer immunotherapy. N Engl J Med. 2015;373(3):288–290.
29.		Thompson JA. New NCCN guidelines: recognition and management of immunotherapy-related toxicity. J Natl Compr Canc Netw. 2018;16(5S):594–596.
30.		O’Day SJ, Maio M, Chiarion-Sileni V, et al. Efficacy and safety of ipilimumab monotherapy in patients with pretreated advanced melanoma: a multicenter single-arm phase II study. Ann Oncol. 2010;21(8):1712–1717.
31.		Wolchok JD, Neyns B, Linette G, et al. Ipilimumab monotherapy in patients with pretreated advanced melanoma: a randomised, double-blind, multicentre, phase 2, dose-ranging study. Lancet Oncol. 2010;11(2):155–164.
32.		Topalian SL, Sznol M, McDermott DF, et al. Survival, durable tumor remission, and long-term safety in patients with advanced melanoma receiving nivolumab. J Clin Oncol. 2014;32(10):1020–1030.
33.		Weber JS, Kudchadkar RR, Yu B, et al. Safety, efficacy, and biomarkers of nivolumab with vaccine in ipilimumab-refractory or -naive melanoma. J Clin Oncol. 2013;31(34):4311–4318.
34.		Topalian SL, Hodi FS, Brahmer JR, et al. Safety, activity, and immune correlates of anti-PD-1 antibody in cancer. N Engl J Med. 2012;366(26):2443–2454.