Blocking the programmed cell death 1 (PD-1) pathway with monoclonal antibodies has shown promising antitumor responses in clinical trials, with less toxicity than has been seen with prior immune therapies such as interleukin 2 and ipilimumab. Pembrolizumab, an anti–PD-1 antibody, recently gained US Food and Drug Administration (FDA) accelerated approval for the treatment of patients with ipilimumab-refractory melanoma, while nivolumab, another anti–PD-1 antibody, and MPDL3280A, an anti–programmed cell death 1 ligand (PD-L1) antibody, have been granted FDA “breakthrough designation” for treatment of subsets of patients with refractory Hodgkin lymphoma and metastatic bladder cancer, respectively. Encouraging antitumor activity has also been seen with these agents in patients with other malignancies, including non–small-cell lung cancer and head and neck cancer, tumors not previously thought to be immune-responsive. PD-L1 expression has emerged as a potential predictive biomarker for PD-1–directed therapy. Multiple, distinct, companion assays for PD-L1 positivity have been developed, but there is as yet no comparison, standardization, or prospective validation of these assays. PD-L1 expression on tumor cells and/or the tumor-immune infiltrate is likely only part of the predictive model necessary for selecting patients predisposed to respond to monotherapy. Additional predictive biomarkers are necessary to identify patients most likely to benefit from PD-1–based combination therapy, since tumor cell PD-L1 expression appears to have limited predictive value in this setting.

Historically, active immune therapies such as interleukin 2 (IL-2) produced durable responses in only a small minority of patients while being associated with significant, nearly universal toxicity. This has limited their applicability to selected patients, usually with either melanoma or renal cell cancer (RCC) treated at experienced centers. The immune checkpoint inhibitor ipilimumab, while being better tolerated than high-dose (HD) IL-2, also exposes patients to a risk of significant toxicity. Therefore, there has been considerable effort over the past decade to identify patients most likely to benefit from these agents in order to enhance their therapeutic index. The discovery and availability of alternative treatment options for patients with advanced melanoma (BRAF inhibitor therapy) and RCC (vascular endothelial growth factor [VEGF]-targeted therapy) in the last decade have intensified the need to identify those patients who will not benefit from immunotherapy, so as not to delay the use of these more generally accessible alternative therapies, particularly in symptomatic patients. Programmed cell death 1 (PD-1) pathway–blocking antibodies appear to exhibit an improved therapeutic index, relative to HD IL-2 and ipilimumab, and efficacy against an expanded array of tumor types (eg, non–small-cell lung cancer (NSCLC), bladder cancer, head and neck cancer), each of which has a distinct array of standard treatments. This has further highlighted the need for clinically useful biomarkers that can help determine how best to incorporate these new agents into treatment algorithms for patients with specific diseases. Finally, identifying factors that predict the subpopulations of patients and tumor types that respond to immunotherapy is critical to both understanding the mechanism of action of immunotherapy and the efficient development of novel immunotherapy combinations.

A biomarker is a biologic molecule, such as a protein or gene, that is measureable in tissue, blood, or other body fluids, and is an indicator of some clinically significant condition. Biomarkers can be diagnostic, surrogate, prognostic, or predictive. While the gold standard of diagnosis in oncology is a pathologic tissue review, a highly elevated prostate-specific antigen (PSA) level in the right clinical setting can be diagnostic of prostate cancer. Although the value of PSA level as a diagnostic biomarker is limited by its sensitivity and specificity, it can be an excellent surrogate biomarker for monitoring prostate cancer response to treatment. Prognostic biomarkers refer to markers that correlate with the natural progression or aggressiveness of a disease. Prognostic biomarkers are useful for informing patients about the risk of recurrence or median survival for their particular type of malignancy and for minimizing confounding factors when analyzing clinical trial cohorts or when prospectively stratifying patients in randomized clinical trials. Predictive biomarkers are defined by their role in predicting a response to a given treatment. Therefore, these are most useful if they can be assessed before the initiation of treatment. This review will focus on predictive biomarkers, as these are most relevant to decision making regarding immunotherapy selection; we will only mention prognostic or surrogate markers to the extent that such discussion sheds potential light on the mechanisms underlying the value of a predictive biomarker.

The immune system is controlled by a delicate balance of immune-stimulatory and immune-inhibitory forces that enable the rapid destruction of cells expressing foreign antigens while preventing uncontrolled destruction of normal tissues. The inhibitory forces include regulatory T cells (Tregs), immunosuppressive cytokines, and immune checkpoints, such as the cytotoxic T-lymphocyte antigen 4 (CTLA-4)/B7 and the PD-1/programmed cell death 1 ligand (PD-L1) pathways. For example, IL-2 not only activates CD8+ effector T cells and subsets of natural killer cells, but also CD4+CD25+ and CD4+Foxp3+ Tregs,[1] which constitutively express the IL-2 receptor alpha chain. Studies in viral models of acute and chronic infections shed light on this phenomenon. PD-1 expression is induced by activation of T lymphocytes, and after a successful immune response has eliminated the foreign antigen, PD-1 expression decreases.[2,3] If the immune response is unsuccessful in eliminating the antigen-expressing cells, as in cancer or chronic infections, prolonged antigen stimulation leads to elevated PD-1 expression and induction of the expression of immune-inhibitory ligands, such as PD-L1 or PD-L2, on tumor cells and infiltrating immune cells. The binding of PD-1–expressing T cells to PD-L1 or PD-L2 leads to T-cell dysfunction and “exhaustion,” and results in eventual escape from immune elimination. In addition to this model of adaptive immune inhibition or resistance, as illustrated in melanoma,[4] other tumors may evade immune surveillance by overexpression of PD-L1 secondary to genetic amplification[5] or as a result of aberrant signaling.[6]

Some tumors have an inflamed phenotype, while others have only a sparse immune infiltrate. While the extent of the immune infiltrate can be a good prognostic indicator in some cancers,[7,8] the antitumor response is clearly insufficient to prevent disease progression. In inflamed tumors, negative immune-regulatory factors tend to dominate due to the chronic nature of the immune infiltrate, as illustrated in Gajewski et al (Figure 1A).[9] Therefore, it is hypothesized that patients with tumors containing T-cell infiltrates might be induced to respond to immunotherapy if the immune cells within the tumor microenviroment can be reactivated.[10]

Predictive Biomarkers for Response to Cytokine Therapy-a Proof of Principle

IL-2 received US Food and Drug Administration (FDA) approval for the treatment of patients with RCC or melanoma in the 1990s based on its ability to produce durable antitumor responses in a small minority of patients, an effect that was largely restricted to the high-dose regimen (HD IL-2). While tumor responses were seen in only 15% to 25% of patients, the 6% to 10% of patients who exhibited tumor responses lasting more than 30 months were likely cured. In retrospective analyses of patients with advanced melanoma, IL-2 response has been associated with normal serum lactate dehydrogenase levels, or low plasma VEGF and fibronectin levels,[11] as well as with tumors containing mutations in 

. Prospective validation of these largely clinical features of response is ongoing. In patients with RCC, tumor response to IL-2 was largely restricted to patients with clear-cell tumors.[12,13] Further, while initial studies suggested that high levels of carbonic anhydrase 9 (CA9) expression by RCC cells predicted for response to IL-2, a prospective clinical trial failed to confirm this finding, supporting a modicum of caution regarding the value of inherent tumor features in predicting response to immunotherapy.

One of the more impressive surrogate markers for the benefit of cytokine therapy is induced or increased autoimmunity. IL-2 responses and the benefit of adjuvant interferon (IFN) appeared more common in patients who exhibited autoimmune phenomena such as thyroiditis or vitiligo. These entities frequently were accompanied by enhanced titers of autoantibodies, suggesting that responses were both autoimmune in nature and more common in patients who were predisposed to develop autoimmunity.[14,15] More recent studies have suggested that response to IL-2 is associated with enhancement of a preexisting gene expression pattern within the tumor that is associated with immune-mediated, tissue-specific destruction under the control of the T helper 1 (Th1)-type cytokine, IFN-Î³.[16] Benefit from vaccination has also been linked to tumors expressing an IFN-driven chemokine signature.[9,17] In addition, preliminary results suggest that IL-2 response in patients with RCC may be correlated with tumor cell surface expression of PD-L1,[18] which, as mentioned previously, is frequently associated spatially with tumor-reactive CD8+ T cells in the microenvironment.[4] Taken together, these data suggest that effective immunotherapy may require the preexistence of tumor-specific immunity within the microenvironment and the use of agents, such as HD IL-2 or IFN, in a subset of patients in whom these agents drive effector T-cell function more potently than immune-regulatory signals. While prospective validation of this hypothesis is necessary, it suggests that approaches that could block immunoregulatory signals might also alter the balance within the tumor microenvironment in such predisposed patients, leading to a shift from immune tolerance of a tumor to an active antitumor immune response. This concept has foreshadowed the effectiveness of checkpoint blockade and informed the predictive biomarker work with such agents.

Clinical Results of Checkpoint Blockade-Efficacy and Toxicity

CTLA-4 was the first of the immune checkpoints to be therapeutically targeted in cancer. Ipilimumab, a fully human immunoglobulin (Ig) G-1 monoclonal antibody blocking CTLA-4, improved overall survival in patients with metastatic melanoma and received FDA approval in 2011.[19,20] As with HD IL-2, the great majority of responders alive at 2 years were still alive long after therapy was completed. Although acute side effects from ipilimumab administration were minimal, patients did sometimes develop immune-related adverse events weeks into therapy that required aggressive immunosuppression in order to control.[21] Similar to the autoimmunity described with IL-2 therapy, baseline presence and induction of NY-ESO-1 antibodies have been associated with clinical benefit from ipilimumab.[22] Specific ipilimumab-associated, immune-mediated adverse effects, such as hypophysitis, have been associated with the development of autoantibodies targeting the affected organ.[23]

PD-1 and its ligands, PD-L1 and PD-L2, are also members of the B7-CD28 superfamily and form an immune-checkpoint pathway. Blocking the binding site of the PD-1 receptor and the ligand PD-L1 with monoclonal antibodies also has shown impressive antitumor effects in patients in multiple tumor types,[24-26] including those not previously thought to be immune-responsive. The PD-1/PD-L1 pathway appears to induce peripheral tolerance of the adaptive immune system.[27] Unlike in CTLA-4 knockout mice, which exhibit lethal autoimmunity within weeks of birth,[28] mice in which PD-1 or PD-L1 is knocked out require some additional initiator to develop an autoimmune phenotype.[27,29-31] Clinically, blockade of the PD-1/PD-L1 pathway appears to produce fewer immune-related adverse events than ipilimumab. This improved side-effect profile and broad span of antitumor efficacy have resulted in extremely active drug development in recent years. Pembrolizumab is the first of this anti–PD-1 pathway family of checkpoint inhibitors to gain FDA–
accelerated approval for the treatment of ipilimumab-refractory melanoma. The phase III clinical trial comparing dacarbazine with nivolumab, an anti–PD-1 monoclonal antibody, recently was stopped early due to improved overall survival for the patients who received anti–PD-1 therapy, and nivolumab has already been approved in Japan. In the United States, nivolumab has received breakthrough status for Hodgkin lymphoma. MPDL3280A, an anti–PD-L1 monoclonal antibody, has received breakthrough status for the treatment of bladder cancer. Development of PD-1/PD-L1 pathway–blocking agents in many other tumors is underway, given the responses seen in early trials.

Initial single-arm trials of combination checkpoint blockade (ipilimumab and nivolumab) have shown potential to significantly improve response rates.[32,33] However, they also appear to increase the number of grade 3 and 4 side effects. The ipilimumab + nivolumab combination is currently being further investigated, including in phase III trials in which the efficacy and toxicity of the combination can be compared with either nivolumab monotherapy or standard of care. While the increased antitumor efficacy of combination immune-modulatory therapies is encouraging, such combinations may not only significantly increase the toxicity but also the cost of therapy. Thus, the development of predictive biomarkers for the routing of patients to the most appropriate therapies is essential to maximize clinical benefit and minimize toxicity. Predictive biomarkers may also facilitate use of checkpoint inhibitors in patients with high-risk resected disease, since a percentage of patients treated in the adjuvant setting may already be cured, and thus the toxicity and treatment expense may be harder to justify.

Clearer insight into the biology of a therapeutic target facilitates its clinical development. Therefore, it may prove easier to define predictive biomarkers for individual immune-checkpoint inhibitors than was the case for pleiotropic cytokines such as IL-2 or IFNs. Despite the role of autoimmunity as a surrogate for response to cytokine therapy, its role in CTLA-4 and PD-1 immune-checkpoint pathway–blocking therapy is less clear. Both CTLA-4 and PD-1/PD-L1 play important roles in regulating T-lymphocyte tolerance, and both have been implicated in human autoimmune disease.[34] In addition, both ipilimumab and, to a lesser extent, PD-1–pathway inhibitors induce immune-related toxicities that appear to be autoimmune in nature. The relationship of these toxicities to tumor response with these agents is unclear, and because trials to date have generally excluded patients with a history of autoimmune disease, the extent to which a predisposition to autoimmunity predicts for antitumor activity remains to be established. Studies are actively investigating not only predictive biomarkers for efficacy but also for toxicity. Moving forward, a patient’s germline genetics also may play a critical role in predicting the clinical effects of various immune therapies, as has already been proposed in the case of ipilimumab.[35,36]

While pretreatment gene expression profiling of tumors has suggested that those with high baseline expression levels of immune-related genes are more likely to respond to ipilimumab,[37] few biomarkers in ipilimumab studies have been truly predictive in nature. For example, the appearance of ICOS+CD4+ T lymphocytes after ipilimumab treatment has been proposed as a pharmacodynamic biomarker that can be used to assess adequate biologic response to treatment.[38] While early reports suggested that an absolute lymphocyte count > 1,000/Î¼L after two doses of ipilimumab[39] correlated with clinical benefit and overall survival,[39] Postow et al reported that nearly all patients who received ipilimumab had a significant increase in their absolute lymphocyte count, potentially mitigating the value of this observation.[40] In addition, investigators have reported that absence of tumor-infiltrating lymphocytes at 4 weeks into therapy or an increase in myeloid-derived suppressor cell (MDSC) numbers in the peripheral blood over the first 24 weeks were both negatively associated with clinical benefit from ipilimumab therapy.[41] In any event, since these studies measured pharmacodynamic markers of treatment effect, they do not inform the choice of patients for ipilimumab therapy.[38] Of note, in a small cohort of 26 patients receiving ipilimumab at 10 mg/kg, a lower number of MDSCs in the baseline peripheral blood was predictive of overall survival.[41] However, much like the negative association of pretreatment serum VEGF level with clinical benefit and overall survival after ipilimumab,[42] baseline MDSC count and serum VEGF level may be functioning as prognostic rather than predictive markers.[37]

Some uncertainty about the mechanism of action of ipilimumab in the tumor microenvironment may be confounding the ability to develop predictive biomarkers of benefit. Given that tumor-infiltrating lymphocytes likely express many immune checkpoints and are already deactivated, it has been postulated that the mechanism of action of ipilimumab in tumors is to target Tregs, as suggested by mouse studies.[43] Thus, ipilimumab’s primary antitumor effect may not be unblocking T-cell activation at initial tumor-antigen presentation but targeting Tregs for destruction within the tumor microenvironment. In support of this concept, Hamid et al reported that elevated Foxp3 and IDO expression in baseline tumor biopsies was associated with clinical benefit from ipilimumab.[44] This and other reports suggest that, as with IL-2, tumors with an inflamed microenvironment may respond better to ipilimumab, since the presence of immunosuppressive factors such as Tregs may not be as much of an obstacle to its efficacy.[10]

Biomarkers for the PD-1/PD-L1–Pathway Inhibitors

PD-L1 expression in tumors is one of a number of different immunosuppressive factors proposed in the Gajewski model (Figure 1) of chronically inflamed tumors with an immunosuppressed microenvironment. In preclinical models, expression of PD-L1 allows tumors to evade tumor-specific effector T-cell cytotoxicity.[45] Clinically, PD-L1 is expressed by many tumor types and is associated with worse prognosis in several, including lung adenocarcinoma and RCC.[46,47] PD-L1 expression is IFN-Î³–inducible and can be on either the immune infiltrate or the tumor cells (Figure 2). Patients whose kidney tumors had ≥ 10% tumor cell expression with the murine anti–human PD-L1 antibody (clone 5H1) had a threefold increased risk of dying of their disease. By contrast, PD-L1 expression on some tumors, most notably melanoma, appears to be associated with a good prognosis.[48] The mechanisms underlying this prognostic diversity and the relative importance of PD-L1 expression in various cell populations within the tumor have yet to be fully elucidated. Nonetheless, the critical role played by PD-L1 expression in tumor immunobiology has clearly identified this pathway as an important target for checkpoint-inhibitor therapy.

Early clinical trials with PD-1 pathway–blocking agents have produced exciting clinical responses in multiple tumor types, including some previously believed to be unresponsive to systemic immunotherapy (Figure 3).[49] Tumor PD-L1 expression was initially explored as a potential predictive biomarker in the single-dose pilot phase I study of nivolumab.[24] This exploratory analysis consisted of tumor specimens from nine patients. PD-L1 expression on tumors by immunohistochemical analysis was performed with clone 5H1 mentioned above.[50] While three of four patients with PD-L1 expression on the tumor cell surface responded to nivolumab (75% response rate), all of the five tumors that did not express any membranous PD-L1 did not respond to treatment. In a follow-up study performed in conjunction with the multidose, phase I nivolumab trial, 9 of 25 patients with PD-L1–expressing tumors responded to treatment, while none of 18 patients with PD-L1–nonexpressing tumors responded (Figure 3). These preliminary data suggest that tumor cell expression of PD-L1, using the clone 5H1 assay, could be a strong predictor of response to nivolumab treatment.

Unfortunately, several subsequent observations have suggested that the predictive value of PD-L1 expression may not be as clear-cut as initially reported. As previously mentioned, PD-L1 could also be expressed on infiltrating lymphocytes, monocytes, and macrophages (Figure 2).[47] Taube et al explored the predictive function of the expression of PD-L1 and PD-L2 on tumor cells and of PD-1 on the immune infiltrate.[51] They found that the expression of PD-L1 on tumor cells and immune cells was highly associated with PD-1 expression on infiltrating lymphocytes-yet PD-L1 expression on tumor cells had the strongest association with response to nivolumab. Based on these data, Bristol-Meyers Squibb developed an automated assay (using the 28-8 clone) that defined > 5% tumor cell membranous expression of PD-L1 to be positive.[47,52,53] However, the other two companies (Genentech/Roche and Merck) leading in the development of PD-1–pathway blockers have developed distinct companion assays for PD-L1 expression, each with its own anti–PD-L1 antibody. The Genentech/Roche assay measures PD-L1 expression on immune-infiltrating cells, which investigators claim best enriches for responders to MPDL3280A.[54-57] This assay predicted for benefit from MPDL3280A in patients with bladder cancer, contributing to this antibody receiving breakthrough status for study in patients with PD-L1–positive metastatic bladder cancers. The Merck PD-L1 assay, developed in conjunction with pembrolizumab, defined PD-L1 positivity in melanoma specimens as tumor surface expression (> 1%).[58,59] However, variations in this assay and its application have emerged across tumor types. For example, in NSCLC patients, both immune and tumor cells were included in the cut-off (Figure 3, Gandh et al),[60] while in the head and neck cancer study, only patients with some PD-L1 expression on tumor cells were enrolled (Figure 3, Seiwert et al).[61] Since these assays have not been compared, it is impossible to ascertain whether these differences in definitions are a function of the assay (antibody specificity), of the biologic differences between anti–PD-1 and anti–PD-L1 therapy, or of the nature of the patient samples analyzed. Thus, it is hard to determine whether there is any consistency in the tumors that are declared to be PD-L1–positive. Clearly, some sort of standardized definition of PD-L1 positivity that links these various assays is necessary to facilitate the study of PD-L1 as a predictive biomarker for PD-1–pathway blockade. Lacking this, comparing clinical results across studies in biomarker-defined subsets of patients will be especially hazardous.

Further complicating matters, analyses with these different companion assays (such as an automated PD-L1 antibody assay used in conjunction with nivolumab) have suggested that, in contrast to earlier results, many patients with PD-L1 negative tumors can respond to PD-1–pathway blockade, including some who have exhibited a complete response (Figure 3).[49] Nonetheless, despite the significant number of false negatives, it is notable that across multiple tumor types, with four different therapies and four different assays for PD-L1 expression, there was a clear trend for PD-L1–positive tumors to respond better than PD-L1–negative tumors. In addition, the degree of PD-L1 expression, by whatever assay, may be positively correlated with response. Therefore, establishing a more stringent cut-off for PD-L1 expression would better enrich for responding patients but fail to identify a larger absolute number of potential responders, while a less stringent PD-L1 expression cut-off might capture more of the responding patients but reduce the predictive power of the assay. An example of this dynamic in patients with melanoma is shown in the Table.[52] Also, tumors are heterogeneous, and PD-L1 expression is inducible on both immune cells and tumor cells. Therefore, the sample used for the assay may not be representative of the whole tumor. Further, PD-L1 expression may be discordant between the primary tumor and the metastatic lesions that are being targeted by the immune therapy. For example, preliminary data suggest moderate discordance in RCC PD-L1 expression between primary and metastatic lesions. Whether this discordance has been a result of tissue preparation and storage issues or biologic differences is uncertain. However, it is perhaps notable that higher PD-L1 expression was associated with higher Fuhrman grade, a factor that was more variable in the primary than the metastatic lesions. Not surprisingly, therefore, the metastatic lesions were typically more homogeneous for PD-L1 expression than the primary tumors.[62] Finally, tumor PD-L1 expression may be less relevant as a predictive biomarker for combination immunotherapy. For example, the combination of ipilimumab and nivolumab actually produced similar response rates for PD-L1–positive and –negative tumors.[32,33] Further, tumor objective response rates for combinations of PD-1–blocking agents with non–immune-based therapies may also be less likely to be associated with PD-L1 expression.[63] For all of these reasons, PD-L1 expression is not an optimal biomarker for patient selection, and lack of PD-L1 expression cannot be reliably used to exclude patients from treatment with PD-1–pathway blockade.

Nonetheless, PD-L1 expression can be a useful predictive biomarker in several capacities. The data in Table 1 clearly suggest that PD-L1 expression, by whatever assay, can enrich for patients who can respond to single-agent PD-1–pathway blockade. Thus, PD-L1 expression could serve as a stratification factor in randomized trials comparing different types of therapies. Further, data suggest that tumor types lacking a consistent percentage of PD-L1 expression are less likely to benefit from single-agent PD-1–pathway blockade.[45]

Therefore, the extent of PD-L1 expression in a tumor type might encourage the inclusion of such tumors in clinical trials. Such an approach has predicted the responsiveness of head and neck cancer and bladder cancer,[56,61] and has fostered investigation of PD-1–pathway blockade in other highly PD-L1–positive tumors, such as Merkel cell carcinoma and Hodgkin lymphoma.[64,65]

Selecting a stringent cut-off for PD-L1 expression might identify a subset of patients with sufficiently high benefit from PD-1–pathway blockade to justify the exploration of single-agent treatment either compared with standard therapy in randomized trials or in single-arm trials in a setting where there is no curative treatment approach. Such strategies have been employed for the development of PD-1–pathway inhibitors in bladder cancer, head and neck cancer, and NSCLC.

Finally, a low level of PD-L1 expression might identify a group of patients who would be best treated with combination immunotherapy. While a prospective trial will be necessary to validate a role for PD-L1 as a predictive biomarker, the data in patients with metastatic melanoma suggest that this may be a reasonable criterion for selecting patients who should be treated with the ipilimumab/nivolumab regimen. Optimal agents to combine with a PD-1–pathway blocker might be those capable of inducing immune infiltration and PD-L1 expression within the tumor. However, given the heterogeneity of many tumors, particularly primary specimens, and technical issues related to tissue processing and storage, if PD-L1 expression is to be used for any of these purposes, it would seem prudent to analyze pretreatment biopsy specimens from metastatic lesions whenever possible.

In addition, understanding the biology of PD-L1 expression in tumors may contribute to the development of better predictive markers and models for assessing response to checkpoint inhibitor therapy. This may be particularly useful in determining why some patients with PD-L1–positive tumors fail to respond to therapy. In some cases this could be because the PD-L1 expression is constitutive, driven by activation of a pathway within the tumor (eg, PI3-kinase), rather than induced by an adaptive response to tumor-specific CD8+ lymphocyte infiltration. Indeed, if PD-L1 is overexpressed on a tumor lacking an appropriate immune infiltrate, blockade may have no effect because the tumor is lacking the effector cells that fight the cancer. Therefore, some measure of immune infiltration (whether assessment of immune phenotypes, as suggested by the Immunoscore research, or gene expression profiles linked to chemokine expression) together with PD-L1 expression might be a better predictor of tumor response.[7,66] Such a model for a dual biomarker has been proposed by Sznol et al (Figure 4).[67]

Alternatively, resistance to treatment in this population might suggest that other immune-suppressive factors may be downregulating the immune response. Understanding the factors involved in a particular tumor type might be the key to the development of novel PD-1/PD-L1–based combination therapies, such as those targeting additional immune checkpoints (eg, TIM-3 [T-cell immunoglobuilin and mucin domain–containing protein 3] or LAG-3 [lymphocyte activation gene 3 protein]) or other immunosuppressive factors (eg, Tregs, IDO, or MDSCs).

Assessing Potentially Immune-Responsive Tumors by Genomics

Mutational burden varies greatly between tumors. Many tumors that respond to PD-1–pathway inhibitors, such as melanoma, NSCLC, and bladder cancer, are those with a high mutation load.[68] While the mutational load in melanoma was not sufficient to predict response to ipilimumab, it was significantly associated with clinical benefit (

 = .028, complete response vs no response, Fisher’s exact test).[69] It is interesting that PD-1–pathway inhibitors also have shown responses in tumors-such as bladder cancer, NSCLC, and head and neck cancer-that are associated with smoking, a known exogenous mutagen. Assaying the neoantigens created within these tumors that might be presented by the patient’s major histocompatibility complex may better identify tumors able to induce immune responses that can be restored by immune-checkpoint inhibition.[70] Further, it is conceivable that the presence of such recognizable neoantigens may also one day form a component of a predictive biomarker model for response to checkpoint blockade.

Immune-checkpoint inhibition clearly has a significant role in the future of oncology. Multiple agents in this class have shown remarkable antitumor effects in a wide variety of clinical trials. While intratumoral PD-L1 expression appears to predict better responses to monotherapy with PD-1 pathway–blocking agents, it is unclear whether this will translate into a clinically useful predictive biomarker in isolation. Characterizing tumors by PD-L1 expression, immune infiltration, chemokine signature, and tumor mutational frequency may be a means of creating an integrated model for determining which patients may benefit from which immune-checkpoint inhibitors, either alone or in combination. Further, the identification of tumors lacking such biomarkers might prove useful in selecting patients for alternative therapies, including adoptive transfer of genetically modified T cells. Taken together, the identification and application of such markers are critical to the rational development and application of immune therapies.

Dr. Atkins serves as a consultant to Amgen, Bristol-Myers Squibb, Genentech/Roche, Merck, and NeoSTem. Dr. Mahoney has no significant financial interest in or other relationship with the manufacturer of any product or provider of any service mentioned in this article

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 Yuan J, Zhou J, Dong Z, et al. Pretreatment serum VEGF is associated with clinical response and overall survival in advanced melanoma patients treated with ipilimumab. Cancer Immunol Res. 2014;2:127-32.

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Articles

Characterizing tumors by PD-L1 expression, immune infiltration, chemokine signature, and tumor mutational frequency may be a means of creating an integrated model for determining which patients may benefit from which immune-checkpoint inhibitors, either alone or in combination.

Prognostic and Predictive Markers for the New Immunotherapies  

Immune checkpoint–blocking drugs such as ipilimumab, pembrolizumab, and nivolumab have demonstrated clinical efficacy as anticancer agents. Through modulation of immunoregulatory molecules, these novel therapeutics can produce durable cancer remissions in a variety of tumor types. As these medications are administered to an increasing number of patients, clinicians must be able to recognize and treat the associated immune-related side effects. This review summarizes the unique mechanisms of toxicity associated with immune checkpoint–blocking drugs, appropriate steps in patient evaluation, and strategies for mitigating risk and optimizing patient outcomes. Although the management of each patient receiving immune checkpoint–blockade therapy must be individualized, a conceptual framework upon which to base a multidisciplinary approach to best practices will help oncology practitioners deliver safe and effective care.

Recent advances in the clinical application of agents that block immunoregulatory molecules have led to unprecedented optimism about the potential of these novel therapies for bringing about durable antitumor responses in patients with various advanced malignancies.[1] However, enhancing immune responses to cancer via modulation of these immune checkpoints is associated with drug-related toxicities that are distinct from those associated with traditional chemotherapeutic agents and molecularly targeted therapies.[2] Because the use of checkpoint blockade agents will likely be widely expanded in the near term, it is critical that healthcare practitioners caring for oncology patients have a basic familiarity with these immune-related adverse events (irAEs), their variable presentations, and recommendations regarding their evaluation and management.

To understand the mechanism of checkpoint blocker–related autoimmune toxicities, it is helpful to consider their mechanism of action. At a basic level, the role of the human immune system, and of T lymphocytes in particular, is to activate against non-self–antigens (eg, viral proteins or cancer neoantigens resulting from tumoral genetic and epigenetic alterations) and to tolerate self-antigens. Antigen recognition by a T-cell receptor is followed by interactions between a tightly regulated cadre of immunoregulatory molecules that either activate or inhibit T-cell activation.[3] Some of the best studied of these checkpoints are cytotoxic T-lymphocyte antigen 4 (CTLA-4) and the pathway composed of programmed death 1 (PD-1) and one of its major ligands, programmed death ligand 1 (PD-L1).

When they are engaged with their respective binding partners, CTLA-4 and PD-1 promote immune tolerance via downregulation of T-cell activation.[4,5] The application of antibody antagonists of these immune checkpoints promotes T-cell activation, which can lead to tumor destruction as well as clinically relevant decreases in self-tolerance. This immune dysregulation is among the mechanisms underlying the autoimmune toxicities related to immune checkpoint blockers.

Incidence of Immune Checkpoint–Blockade Toxicities

The adverse event (AE) profiles of checkpoint-blocking drugs (ie, incidence of toxicities and organ systems most frequently affected) vary depending on agent and target. In clinical studies, toxicity severity is described using the Common Terminology Criteria for Adverse Events (CTCAE), which grades toxicities on a scale of 1 (mildest) to 5 (death related to that toxicity).[6]

The CTLA-4 antibody ipilimumab is the most widely studied checkpoint-blocking agent. In 2011, after extensive clinical testing, it was approved by the US Food and Drug Administration (FDA) for use in patients with advanced melanoma. One seminal study that led to its approval was a phase III trial involving patients with advanced melanoma.[7] Ipilimumab was administered at 3 mg/kg with or without a peptide vaccine to 511 patients. Grade 3 or 4 immune-related toxicities were observed in approximately 15% of patients, and 7 patient deaths were associated with irAEs. A retrospective review of safety data from 1,498 patients treated with ipilimumab on any of 14 phase I–III clinical trials found that drug-related AEs of any grade occurred in 85% of patients.[8] About 25% of patients experienced a grade 3 or 4 drug-related toxicity, and drug-related death was observed in < 1% of patients.

Clinical trials using ipilimumab in tumor types other than melanoma have demonstrated similar AE rates. For example, in a phase III study of ipilimumab administered after patients were treated with radiotherapy for metastatic prostate cancer, 63% of patients had immune-related toxicity of any grade.[9] Similarly, in a phase II study of ipilimumab plus chemotherapy for metastatic non–small-cell lung cancer, the rate of grade 3/4 irAEs was 15% to 20%, depending on the sequence of drug administration.[10]

In a phase II trial of 245 patients with advanced melanoma, study participants were randomly assigned to ipilimumab plus granulocyte macrophage colony-stimulating factor (GM-CSF) or ipilimumab alone.[11] Of note, ipilimumab was administered at 10 mg/kg, which is higher than the FDA-approved dose of 3 mg/kg. Patients who received combination therapy experienced a significantly lower rate of high-grade AEs (45% vs 58%; two-tailed 

 = .038). The lower rate of high-grade AEs was accompanied by significantly improved overall survival (OS) in the GM-CSF arm (median, 17.5 vs 12.7 months; hazard ratio, 0.64; 

 = .014), although further study is required to determine whether the increased OS is linked to improvements in AE rates.

Another CTLA-4 antibody, tremelimumab (CP-675,206; formerly ticilimumab) has been similarly studied in patients with metastatic melanoma and other advanced cancers. In a phase II clinical trial testing tremelimumab in patients with advanced melanoma, 19% of participants had a ≥ grade 3 adverse event.[12] When tested in patients with metastatic colorectal adenocarcinoma, a similar rate of high-grade toxicities was seen, with diarrhea/colitis representing the majority of cases.[13]

Although clinical data are still emerging, anti–PD-1 and anti–PD-L1 agents appear to have toxicity profiles that are different from those of CTLA-4 antibodies (Table 1).

Pembrolizumab is a humanized monoclonal antibody (mAb) blocking PD-1 that was approved by the FDA in September 2014 for use in treatment-refractory unresectable or metastatic melanoma. In a phase I clinical trial involving 135 patients with advanced melanoma, grade 3/4 AEs were reported in 13% of subjects.[14] An expansion cohort comprising patients whose disease was refractory to ipilimumab (and, if BRAF-mutant, refractory to BRAF inhibition) was reported separately.[15] In 173 patients, at a median follow-up of 8 months, toxicity rates were similar to those in previous reports.

Nivolumab is a genetically engineered, fully human immunoglobulin (Ig) G-4 mAb specific for human PD-1. In a phase I dose-escalation study conducted in 296 patients with multiple tumor types, grade 3/4 treatment-related toxicities occurred in 41 patients (14%). Treatment-related pneumonitis was observed in 9 patients (3%), 3 cases of which were fatal.[16] Longer-term follow-up of the entire study cohort (306 patients) demonstrated that exposure-adjusted toxicity rates were not cumulative.[17]

BMS-936559 (MDX-1105), a PD-L1–blocking antibody, was tested in a 207-patient phase I trial.[18] A 9% rate of grade 3/4 drug-related irAEs was reported. Similar findings were reported in 140 patients who received MPDL3280A, a human monoclonal IgG1 antibody engineered to block PD-L1 binding.[19] Drug-related grade 3/4 AEs were observed in 14% of patients.

Preclinical evidence suggests that blockade of multiple immune checkpoints can achieve synergistic antitumor activity.[20] Several clinical trials of combinatorial regimens are underway. In a trial of ipilimumab plus nivolumab in patients with metastatic melanoma, a 53% rate of treatment-related AEs of grade 3 or 4 was reported when the drugs were administered concurrently.[21] When ipilimumab was followed sequentially by nivolumab, the rate dropped to 18%, which is closer to observed rates for the single agents.

While theoretically any immune checkpoint–blockade toxicity can occur at any time, certain toxicities have been seen more frequently earlier in the treatment course, while others generally manifest as later complications. Commonly encountered irAEs include rash/dermatitis, diarrhea/colitis, hepatitis, and endocrinopathies. However, because essentially any tissue in the body can be affected, clinicians must consider patient signs and symptoms to be autoimmune in origin until proven otherwise. The timing of adverse events from ipilimumab has been well described[22]; recently released data about the onset of irAEs in 411 patients who received pembrolizumab are illustrated in Figure 1. Familiarity with these patterns is helpful in successfully anticipating and treating these toxicities.

In the case of ipilimumab, the most common early toxicity is dermatologic toxicity.[22] The window for the highest risk for rash and other skin toxicity starts approximately 3 weeks into treatment, although late and long-lasting dermatologic toxicities have been observed. Enterocolitis most frequently occurs between weeks 5 and 10 of treatment, although gastrointestinal symptoms occurring at any time after initiation of ipilimumab could represent immune-related drug toxicity. Hepatitis is often seen slightly later than colitis, with the period of highest risk occurring between weeks 6 and 14 of treatment. Endocrinopathies generally are more unpredictable, with the risk period starting around 7 weeks after initiation of therapy. Even with optimal management, the effects of autoimmune endocrinopathies can be long-lasting, and late manifestations of hypophysitis can occur.

Patients with a history of autoimmunity are potentially at greater risk for irAEs. In clinical trials of checkpoint-blocking drugs, patients with autoimmunity were typically excluded, and a history of autoimmunity is considered a relative contraindication for immune checkpoint–blockade therapy. Although post–FDA-approval experience with ipilimumab has identified instances of successful treatment in patients with autoimmunity and other high-risk comorbidities (eg, history of organ transplant[23]), close monitoring is warranted in this patient population.

One active area of investigation is the pretreatment identification of patients who are more likely to experience irAEs. Two studies have suggested a correlation between specific genetic changes detectable in the blood and the development of colitis, the most common serious side effect observed with the anti–CTLA-4 agents.[24,25] Of note, a strategy of prophylaxis with budesonide to prevent colitis in patients being treated with ipilimumab was ineffective.[26] In that study, 115 patients were randomly assigned to budesonide at 9 mg daily or placebo at the initiation of therapy. Rates of grade 2 and higher diarrhea/colitis were similar between groups.

Identification and Characterization of Toxicities

The identification of irAEs requires effective patient-provider communication, familiarity with the potential manifestations of irAEs, and a high index of suspicion that patient complaints may reflect drug toxicity. Perhaps because irAEs have been reported to be associated with antitumor effects of therapy,[27] patients often minimize their symptoms in the hope of remaining on therapy. It is important for providers to impress upon patients that toxicities can lead to significant morbidity or even death unless they are promptly recognized and treated.

Grade 1 and 2 dermatologic toxicities from checkpoint-blocking drugs are among the most commonly seen irAEs. Typically, mild dermatitis manifests as a maculopapular rash or erythroderma (Figure 2). Pruritus can accompany the rash or, less commonly, can present as an isolated complaint in the absence of skin findings. Rarely, these processes can be severe, manifesting as ulceration, bullous dermatitis, or Stevens-Johnson syndrome. Pathologic evaluations of biopsy specimens of affected skin often demonstrate eosinophilic infiltration or leukocytoclastic vasculitis; or, they may reveal a lymphocytic predominance characterized by CD8+ T cells, sometimes with tropism for melanin-containing cells. Specifically for patients with metastatic melanoma, the rash associated with immune-checkpoint inhibitors may be indicative of immune response to melanocytes and may progress to vitiligo in some cases.[28,29]

Autoimmune enterocolitis is common in patients receiving ipilimumab and generally manifests as watery diarrhea, abdominal pain, nausea/vomiting, or hematochezia. About 30% of 511 patients who received ipilimumab in a large phase III trial experienced diarrhea of any grade.[7] In severe cases, patients may experience significant dehydration, fever, peritoneal signs, bowel perforation, or ileus. Stool cultures are negative. Endoscopy performed in these cases may identify an inflamed mucosa with ulceration, which can involve any part of the bowel but most commonly the descending colon.[30] Biopsy specimens of the colon typically reveal an inflammatory cell infiltrate (neutrophilic, lymphocytic, or mixed) with cryptitis.[31] Even in the setting of a relatively mild colitis seen grossly during endoscopy, tissue biopsies can demonstrate significant inflammation. Deaths secondary to bowel perforation have occurred.[7]

Endocrinopathies are perhaps the most elusive irAEs, due to their nonspecific presentations. A patient may have vague symptoms yet be at risk for serious morbidity or death if the endocrinopathy is not promptly identified and treated. For example, hypophysitis, which can cause pituitary inflammation with meningeal irritation, often presents as headache refractory to nonsteroidal anti-inflammatory drugs or other analgesics. Compression of the optic nerve from an inflamed pituitary may also lead to visual complaints. However, hypophysitis may manifest more nonspecifically, with symptoms secondary to hypopituitarism, such as hypotension, electrolyte disturbances, abdominal pain, weakness, behavioral changes, loss of libido, or fatigue.

Magnetic resonance imaging (MRI) of the brain typically demonstrates enlargement of the pituitary with variable contrast enhancement characteristics.[32] Patients will generally have abnormal endocrinologic laboratory test results; assessment of basic chemistries, adrenocorticotropic hormone, cortisol, thyroid-stimulating hormone (TSH), free thyroxine (T

), prolactin, follicle-stimulating hormone, luteinizing hormone, and testosterone/estradiol is indicated.[33] Biopsy should only be considered in rare cases in which metastatic disease of the pituitary must be ruled out.

In the appropriate clinical setting, other endocrine irAEs that should be considered include thyroiditis (ie, hyper- or hypothyroidism), adrenal insufficiency, and hypogonadism. Because patients who experience irAEs often require prolonged courses of corticosteroids, resultant temporary secondary adrenal insufficiency is not uncommon.

Hepatic toxicity often manifests as asymptomatic elevated levels of hepatic transaminases. Rarely, patients may complain of nonspecific symptoms, including fever, fatigue, nausea, and abdominal pain. In these cases, one must distinguish between checkpoint drug–related toxicity and other etiologies of hepatic injury, such as neoplastic disease progression in the liver, infections, and effects of other medications or alcohol intake. Titers for antibodies associated with autoimmune hepatitis, including antinuclear antibodies, anti–smooth-muscle antibodies, and anti-mitochondrial antibodies, are nonspecific and of questionable utility. Serologic testing should be performed for viral hepatitis (including hepatitis A and B), cytomegalovirus, and Epstein-Barr virus. Ultrasonograms of the liver can appear normal or may demonstrate homogenous hepatomegaly, edema, or enlarged perihepatic lymph nodes. Biopsy of the liver most commonly shows a diffuse T-cell infiltrate. Other reported pathology patterns include inflammation focused around either hepatocytes or bile ducts.[34]

Presentations of pneumonitis range from asymptomatic lung infiltrates to a mimic of severe bacterial pneumonia. For symptomatic patients, complaints and findings may include dyspnea, cough, pleuritic chest pain, and hypoxia. Workup for these patients includes chest imaging, yet there are no characteristic radiographic findings that will rule in or rule out pneumonitis. For example, a patient who experienced pneumonitis, first related to ipilimumab and later from nivolumab therapy, was found to have interstitial, lobar, and nodular infiltrates at different time points (Figure 3). Suspicion for pneumonitis must remain high regardless of radiographic findings, as pneumonitis can have highly variable appearances on chest CT scans. Tissue and lavage samples obtained by bronchoscopy can be helpful in ruling out infectious pathogens. Cryptogenic organizing pneumonia may be identified by pathologic evaluation of lung tissue.

In patients with pulmonary metastases or cardiopulmonary comorbidities, evaluation can be particularly challenging. Tumor progression (eg, lymphangitic spread), pseudoprogression (ie, inflammation of an existing metastasis), exacerbations of chronic obstructive pulmonary disease, congestive heart failure, diffuse alveolar hemorrhage, and pulmonary embolism are often possible diagnoses. Because pneumonitis can quickly escalate and become fatal, early recognition and careful management are essential.[16]

Neuromuscular toxicity is relatively uncommon but may manifest as a mild peripheral sensory neuropathy or muscle weakness. Symptoms are typically detected through a symptom-directed interview, with physical examination findings ranging from sensory changes to loss of deep-tendon reflexes. Although they are rare, severe neuropathies and myopathies have been observed.[35] Emergent cases of neuropathy can present with Guillain-BarrÃ© syndrome, transverse myelitis, or myasthenia gravis, among other diagnoses.[7,36-38] Depending on presentation, patients may require neuroimaging, nerve conduction studies, and, potentially, nerve or muscle biopsy to arrive at the diagnosis. Of note, one case of Guillain-BarrÃ© syndrome reported in the registrational phase III trial for ipilimumab was fatal.[7]

Management of Immune Checkpoint–Blockade Toxicities

Patients treated with immune-checkpoint inhibitors should be seen clinically prior to each cycle of therapy (eg, every 3 weeks for ipilimumab and pembrolizumab). The clinical visit should include a comprehensive review of systems with specific focus on common and serious toxicities, such as skin changes, diarrhea and abdominal pain, headache, fever, shortness of breath, cough, and neurologic changes. Routine laboratory testing, including hematologic profile, comprehensive metabolic panel, and TSH level, should be obtained. Any new symptoms or abnormalities in examination or laboratory test results should be evaluated prior to administration of the next dose of drug.

The diagnosis of immune checkpoint–blockade toxicity is one of exclusion. Specifically, progression of underlying malignancy, infection, and other possible causes of a particular abnormality must be ruled out prior to identifying a given toxicity as an irAE. This workup may include symptom-focused diagnostic radiologic studies and additional laboratory tests and cultures. Clinicians should feel comfortable withholding checkpoint-blockade therapy until the workup is complete.

Understanding the severity of a toxicity is key to determining the proper approach to its management. Toxicities should be graded using the CTCAE.[6] Clinicians should consult published guidelines (eg, 

) for management strategies, keeping in mind that each toxicity is different and that approaches will need to be individualized. For example, grade 1 pneumonitis is a potentially life-threatening problem, while grade 1 dermatitis generally does not require dose interruption. Guidance with regard to temporary or permanent drug discontinuation can be found in the full prescribing information for pembrolizumab and ipilimumab.[39,40] In the case of pembrolizumab, withholding the drug is recommended for grade 2 pneumonitis, colitis, nephritis, or hepatitis; any grade 3 adverse reaction; and any symptomatic hypophysitis. Pembrolizumab can be restarted with improvement of these toxicities to grade ≤ 1. Permanent discontinuation is recommended for life-threatening adverse events, including grade 3 pneumonitis, nephritis, or hepatitis; any recurrent grade 3 adverse event; or inability to reduce prednisone dosage to 10 mg daily after 12 weeks. While the inability to wean a patient from corticosteroids may be a cause for permanent drug discontinuation, many practitioners would institute a course of infliximab in the case of an ongoing high-dose prednisone requirement. Thus, the need for infliximab or alternative immunosuppression would generally be an indication for permanent checkpoint inhibitor discontinuation as well.

Grade 1 toxicities are mild, asymptomatic, or minimally symptomatic. Generally, patients with this level of toxicity are able to continue therapy with supportive care and increased monitoring. Examples of grade 1 toxicity include a mild rash or alteration in thyroid function test results. Follow-up should be tailored to the specific abnormality (eg, more frequent for pulmonary issues, less so for dermatologic issues).

Grade 2 toxicities generally require at least temporary discontinuation of checkpoint therapy. Patients should be monitored closely, and therapy can often be restarted if the toxicity improves to grade 1. However, for patients who have persistent grade 2 symptoms for at least one week, initiation of low-dose corticosteroid therapy is indicated. In general, these patients can be managed as outpatients with oral prednisone of 0.5 to 1.0 mg/kg/day or the equivalent. Patients must be monitored carefully, with strict instructions for reporting any worsening of symptoms. For grade 2 hepatic, renal, or hematologic abnormalities, laboratory monitoring every few days is recommended. As patients improve, the corticosteroid can be tapered over at least 1 month. For prolonged steroid tapers, prophylaxis against 

Grade 3/4 toxicities have the potential for significant morbidity and mortality. Not infrequently, patients require hospitalization for evaluation and frequent monitoring. Permanent discontinuation of the checkpoint-blockade agent should be considered. Prompt diagnostic studies, including symptom-focused imaging (eg, pituitary MRI or chest CT), cultures and/or biopsy (eg, bronchoscopy with bronchoalveolar lavage and transbronchial biopsy or flexible sigmoidoscopy with colon biopsy) should be performed in consultation with an interventional pulmonologist or gastroenterologist. When a high-grade immunologic toxicity is suspected, corticosteroid therapy should be initiated. A dosage of 1 to 2 mg/kg/day of methylprednisolone or the equivalent is generally required; however, in the case of pneumonitis, some investigators have used a higher dosage of 2 to 4 mg/kg/day. Prophylaxis against PCP should be considered. In anticipation of possible need for anti–tumor necrosis factor (TNF) agents, patients should have viral hepatitis serologies and a tuberculosis test performed. Corticosteroids should be continued at high dose until improvement of the toxicity to grade 1, at which point a prolonged taper can be initiated. In general, patients should not be rechallenged with the same immune checkpoint–blockade agent.

Special considerations for specific toxicities

This review is meant to highlight the principles underlying the evaluation and management of irAEs associated with checkpoint-blockade agents. Although an exhaustive list of possible toxicities is beyond the scope of this article, clinicians should be aware of some of the less common irAEs, which may be hematologic (eg, hemolytic anemia, thrombocytopenia), cardiovascular (eg, myocarditis, pericarditis, vasculitis), ocular (eg, blepharitis, conjunctivitis, iritis, scleritis, uveitis), renal (eg, nephritis), or pancreatic (both endocrine and exocrine pancreatitis). Management considerations regarding several of the more common or potentially serious toxicities are described in detail below.

. Autoimmune enterocolitis deserves special attention, due to its relatively high frequency in patients receiving CTLA-4 blockade and the multiple facets of its management. Patient symptoms should first be graded by combining CTCAE criteria with the overall clinical picture.

Patients with mild symptoms (eg, grade 1 abdominal pain or diarrhea) should be evaluated for infection, including 

 infection. Antidiarrheals (eg, loperamide) and supportive measures, such as increasing oral fluid intake, should be considered.

Moderate symptoms often necessitate discontinuation of therapy, an infectious workup as above, and, if symptoms do not improve within a week, initiation of low-dose oral corticosteroids (0.5 mg/kg/day of prednisone or the equivalent). If symptoms improve and the patient is requiring the equivalent of < 7.5 mg/day of prednisone, clinicians might consider cautiously restarting checkpoint-blockade therapy. Collaboration with a gastroenterologist who is familiar with the evaluation and workup of autoimmune colitis is recommended. Visual inspection of the colon, as well as pathologic evaluation of colonic biopsies, is often useful in evaluating patients with persistent or recurrent symptoms.

Patients with severe symptoms (eg, grade 3/4 diarrhea, significant hematochezia, or peritoneal signs) require discontinuation of checkpoint therapy and should be hospitalized and closely monitored. An immediate workup for infection is appropriate, and, if perforation is suspected, surgical consultation should be obtained for operative management prior to administration of high-dose systemic corticosteroids (1 to 2 mg/kg/day of intravenous methylprednisolone or equivalent). Abdominal CT scans may be instructive. In one study of patients diagnosed with ipilimumab-associated colitis, characteristic findings included mesenteric vessel engorgement, bowel wall thickening, and fluid-filled colonic distension.[41]

Patients should continue on corticosteroids until symptoms improve, at which time a steroid taper lasting ≥ 1 month should be initiated. While patients may be concerned about corticosteroids interfering with the efficacy of treatment, one small study suggests that steroids do not affect the duration of clinical response to ipilimumab.[42] Given the risk of narcotic bowel, opiates should be used with caution in patients with abdominal pain. Administration of infliximab (5 mg/kg) should be considered for patients who do not improve within 3 to 5 days of initiation of corticosteroid therapy.[31,43] Infliximab should be used with caution in patients with sepsis from enterocolitis. The use of infliximab for steroid-refractory enterocolitis can be generalized to most other severe autoimmune toxicities, with the possible exception of hepatitis.

. When patients receiving checkpoint inhibition present with liver dysfunction, a high suspicion for drug-related autoimmune hepatitis is warranted. Clinicians should rule out other causes of hepatic injury, such as viruses, metabolic or cardiovascular derangements, other medications, or tumor progression.

Grade 1 elevations in liver function laboratory values require close monitoring. For grade 2 hepatotoxicity, the immune checkpoint–blockade agent should be withheld, and corticosteroids should be promptly initiated and tapered over at least 1 month. In consultation with a hepatologist, clinicians may consider liver biopsy. Grade 3/4 hepatitis requires discontinuation of drug and initiation of high-dose corticosteroids (1 to 2 mg/kg/day of prednisone). Hospitalization should be considered, as cases of death from liver failure have been reported in patients with this level of hepatotoxicity.[38] If a high-grade liver toxicity does not improve within 48 to 72 hours of initiation of high-dose corticosteroids, consideration of alternative immunosuppression agents is warranted. The alternative immunosuppression agent of choice is mycophenolate mofetil instead of infliximab, as infliximab can be independently associated with liver function abnormalities.[44] One case report describes the use of antithymocyte globulin in a patient with severe ipilimumab-related hepatitis that was found to be refractory to mycophenolate mofetil.[45] Tacrolimus 0.10 to 0.15 mg/kg/day has also been recommended by some investigators for mycophenolate-refractory hepatitis.[46]

. In a 306-patient trial of nivolumab, 3 patients died as a result of drug-related pneumonitis.[16] Although it is relatively uncommon, pneumonitis has significant potential for morbidity and mortality, and this condition should be approached with great caution. Optimal evaluation and management of patients with suspected autoimmune pneumonitis has not been assessed in prospective clinical trials, and there is no consensus statement regarding the care of this patient population. In the case of grade 1 pneumonitis (asymptomatic radiographic changes only), immune checkpoint–blockade therapy should be withheld, and patients should be monitored closely (every 2 to 3 days) for the development of symptoms. Consultation with an interventional pulmonologist and an infectious-disease specialist is recommended. Repeat CT imaging of the chest is recommended within 3 weeks to document resolution or progression.

For patients with symptomatic pneumonitis (grade ≥ 2), patients should be monitored daily, with consideration given to hospitalization. In consultation with an interventional pulmonologist, performance of a bronchoscopy with bronchoalveolar lavage and tissue biopsies should be considered. Treatment with high-dose corticosteroids is recommended, along with empiric antibiotics, both of which can be administered while the above evaluation is underway. As with other severe toxicities, a lack of improvement after 48 to 72 hours of high-dose steroid treatment should prompt consideration of infliximab therapy. If possible, administration of infliximab should be delayed until a pulmonary infection has been reasonably excluded.

 An ipilimumab-associated rash should first be graded according to CTCAE. Macules/papules covering < 10% (grade 1) or 10% to 30% (grade 2) of body surface area with or without symptoms (eg, pruritus, burning, tightness) can often be managed with topical corticosteroids and oral over-the-counter antihistamines. In general, checkpoint-blockade therapy can be continued, although careful monitoring for progression of symptoms is warranted.

Grade 3 rash is characterized by macules/papules covering > 30% of body surface area with or without symptoms, as above. Oral corticosteroid administration (approximately 1 mg/kg/day prednisone or equivalent) frequently achieves symptom control. Temporary discontinuation of checkpoint therapy should be considered. Again, careful monitoring for progression of symptoms is warranted.

Grade 4 skin toxicity includes Stevens-Johnson syndrome; toxic epidermal necrolysis; and rash complicated by full-thickness dermal ulceration, necrosis, or hemorrhage. These symptoms require emergent administration of high-dose corticosteroids, inpatient admission, and consultation with a dermatologist.

Pruritus often accompanies a rash and can persist even after resolution of visible skin irritation. Mild, localized pruritus can be managed with moisturizing creams, topical corticosteroids, and topical antihistamines. More intense or widespread pruritus resulting in skin changes from scratching (eg, edema, papulation, excoriations, lichenification, oozing/crusts) may require low-dose oral corticosteroids or antipruritics (eg, hydroxyzine). For symptoms that limit activities of daily living or sleep or are intense or constant, clinicians could consider prescribing gabapentin, pregabalin, mirtazapine, or aprepitant.[28]

 Autoimmune endo­crinopathies can affect any organ in the hypothalamic-pituitary-adrenal axis. Patients who have hypophysitis but who are not in adrenal crisis generally require a short course of high-dose corticosteroids (eg, methylprednisolone 1 to 2 mg/kg/day or the equivalent) to address the acute inflammation; such treatment may preserve some degree of pituitary function.[47] Steroids should be tapered over at least 1 month, and patients should be followed by an endocrinologist, since many patients who develop endocrine deficiencies as a result of autoimmune hypophysitis do not experience restoration of normal hormonal function despite control of the initial inflammation with corticosteroids.[48] These patients may require long-term pituitary hormone replacement.

In rare cases, an autoimmune endocrinopathy may manifest as an adrenal crisis. These patients should receive stress-dose corticosteroids (eg, hydrocortisone 100 mg IV immediately, followed by 50 to 100 mg every 8 hours) along with fluid resuscitation as appropriate.

Patients who present with hyperthyroidism may initially require treatment (eg, beta-blockers) for alleviation of symptoms. Frequently, thyroiditis resolves over time, but then patients become hypothyroid, requiring thyroid hormone replacement. Isolated, asymptomatic hypothyroidism that can be managed with thyroid replacement alone generally does not require dose interruption or discontinuation of checkpoint-blockade drugs.

The successful clinical application of immune checkpoint–blockade therapies will continue to change the way we treat patients with advanced cancers. As novel immunomodulators increasingly play a role in oncologic therapy, practitioners must have strategies and protocols in place for identifying and treating immune-related toxicities. Indeed, the National Comprehensive Cancer Network (NCCN) consensus panel strongly recommends that physicians who prescribe ipilimumab participate in the corresponding Risk Evaluation and Mitigation Strategy (REMS) program.

While guidelines and algorithms are available to assist in the management of immune-related events, a patient’s entire treatment team-in particular, nurses, who are often a patient’s first point of contact in an oncology practice setting-must be familiar with and trained to properly manage these toxicities.[49,50] Patients, too, must be educated regarding possible side effects of therapy and the need for prompt symptom reporting. To facilitate effective patient-provider communication, some practitioners elect to use a standard list of questions to screen for possible irAEs at each patient encounter.

Ongoing studies aim to identify biomarkers to help identify patients at highest risk for irAEs. For now, practitioners must maintain a high level of suspicion for checkpoint-inhibitor toxicity, must understand the common and serious side effects, and must recognize that the potential exists for irAEs to affect any of the body’s organ systems.

Dr. Lipson has served as an advisory board member for Amgen, and has received research funding from Genentech and AstraZeneca. Dr. Teply has no significant financial interest in or other relationship with the manufacturer of any product or provider of any service mentioned in this article.

 Drake CG, Lipson EJ, Brahmer JR. Breathing new life into immunotherapy: review of melanoma, lung and kidney cancer. Nat Rev Clin Oncol. 2014;11:24-37.

 Gangadhar TC, Vonderheide RH. Mitigating the toxic effects of anticancer immunotherapy. Nat Rev Clin Oncol. 2014;11:91-9.

 Pardoll DM. The blockade of immune checkpoints in cancer immunotherapy. Nat Rev Cancer. 2012;12:252-64.

 Peggs KS, Quezada SA, Allison JP. Cell intrinsic mechanisms of T-cell inhibition and application to cancer therapy. Immunol Rev. 2008;224:141-65.

 Bour-Jordan H, Esensten JH, Martinez-Llordella M, et al. Intrinsic and extrinsic control of peripheral T-cell tolerance by costimulatory molecules of the CD28/B7 family. Immunol Rev. 2011;241:180-205.

 National Cancer Institute Common Terminology Criteria for Adverse Events v4.0. NCI, NIH, DHHS. May 29, 2009. NIH publication # 09-7473.

 Ibrahim RA, Berman DM, DePril V, et al. Ipilimumab safety profile: summary of findings from completed trials in advanced melanoma. J Clin Oncol. 2011;29:abstr 8583.

 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;15:700-12.

 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;30:2046-54.

 Hodi FS, Lee SJ, McDermott DF, et al. Multicenter, randomized phase II trial of GM-CSF (GM) plus ipilimumab (ipi) versus ipi alone in metastatic melanoma: E1608. J Clin Oncol. 2013;31(suppl):abstr CRA9007.

 Kirkwood JM, Lorigan P, Hersey P, et al. Phase II trial of tremelimumab (CP-675,206) in patients with advanced refractory or relapsed melanoma. Clin Cancer Res. 2010;16:1042-8.

 Chung KY, Gore I, Fong L, et al. Phase II study of the anti-cytotoxic T-lymphocyte-associated antigen 4 monoclonal antibody, tremelimumab, in patients with refractory metastatic colorectal cancer. J Clin Oncol. 2010;28:3485-90.

 Hamid O, Robert C, Daud A, et al. Safety and tumor responses with lambrolizumab (anti-PD-1) in melanoma. N Engl J Med. 2013;369:134-44.

 Robert C, Ribas A, Wolchok JD, et al. Anti-programmed-death-receptor-1 treatment with pembrolizumab in ipilimumab-refractory advanced melanoma: a randomised dose-comparison cohort of a phase 1 trial. Lancet 2014;384:1109-17.

 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;32:1020-30.

 Woo SR, Turnis ME, Goldberg MV, et al. Immune inhibitory molecules LAG-3 and PD-1 synergistically regulate T-cell function to promote tumoral immune escape. Cancer Res. 2012;72:917-27.

 Weber JS, Kahler KC, Hauschild A. Management of immune-related adverse events and kinetics of response with ipilimumab. J Clin Oncol. 2012;30:2691-7.

 Lipson EJ, Bodell MA, Kraus ES, Sharfman WH. Successful administration of ipilimumab to two kidney transplantation patients with metastatic melanoma. J Clin Oncol. 2014;32:e69-71.

As immune checkpoint–blocking medications are administered to an increasing number of patients, clinicians must be able to recognize and treat the associated immune-related side effects.

Identification and Management of Toxicities From Immune Checkpoint–Blocking Drugs

Immune checkpoints, such as programmed death ligand 1 (PD-L1) or its receptor, programmed death 1 (PD-1), appear to be Achilles’ heels for multiple tumor types. PD-L1 not only provides immune escape for tumor cells but also turns on the apoptosis switch on activated T cells. Therapies that block this interaction have demonstrated promising clinical activity in several tumor types. In this review, we will discuss the current status of several anti–PD-1 and anti–PD-L1 antibodies in clinical development and their direction for the future.

Programmed death 1 (PD-1) is an immune inhibitory receptor expressed on several immune cells, particularly cytotoxic T cells.[1] It interacts with two ligands, programmed death ligand 1 (PD-L1) (B7-H1, CD274) and PD-L2 (B7-DC). While PD-L2 is expressed primarily on macrophages and dendritic cells, PD-L1 is expressed on tumor cells, as well as other immune cells. The interaction of these ligands with PD-1 inhibits T-cell activation and cytokine production. Their ligation with PD-1 during infection or inflammation in normal tissue is critically important in maintaining homeostasis of immune response to prevent autoimmunity. Their interaction in tumor microenvironments, however, provides an immune escape for tumor cells by turning off cytotoxic T cells.[2] Thus, blocking these interactions may subject the tumor cells to attack from cytotoxic T cells.

How Is PD-L1 Expression Determined? What Is Its Significance?

PD-L1 expression is measured most commonly by immunohistochemistry (IHC). Tumoral PD-L1 expression status has been shown to be prognostic in multiple tumor types, including melanoma (MEL), renal cell carcinoma (RCC), and non–small-cell lung cancer (NSCLC). In addition, tumoral PD-L1 expression appears to correlate closely with response to anti–PD-1 antibodies.[3] However, no test is uniformly accepted as the standard for quantitating PD-L1 expression. The IHC tests used in clinical trials are proprietary; data on similarities between and among the antibodies used and the assay conditions, staining pattern, threshold for signal detection, and assessment of positivity are not published. The protein expression patterns of PD-L1 on tumor cells, dendritic cells, and tumor-infiltrating immune cells differ, and exact cell type and degree of expression vary between assays. A different methodology for evaluating PD-L1 messenger RNA (mRNA) expression, using an antibody-independent in situ hybridization assay coupled with quantitative fluorescence, showed that increased PD-L1 mRNA transcript was associated with elevated tumor-infiltrating lymphocytes and better clinical outcomes in patients with breast cancer[4] and NSCLC.[5] The role of PD-L1 expression as a biomarker is discussed in this supplement to ONCOLOGY, in the review “Prognostic and Predictive Markers for the New Immunotherapies,” by Drs. Kathleen M. Mahoney and Michael B. Atkins.[6]

Several PD-1 and PD-L1 antibodies are in clinical development (Table 1). Overall, they are very well tolerated; most did not reach dose-limiting toxicity in their phase I studies. As listed in Table 2, no clinically significant difference in adverse event profiles has been seen between anti–PD-1 and anti–PD-L1 antibodies. Slightly higher rates of infusion reactions (11%) were observed with BMS-936559 (anti–PD-L1) than with BMS-96558 (nivolumab). In an early stage of a nivolumab phase I study, there was concern about fatal pneumonitis.[7] It has been hypothesized that PD-1 interaction with PD-L2 expressed on the normal parenchymal cells of lung and kidney provides unique negative signaling that prevents autoimmunity.[8] Thus, anti–PD-1 antibody blockage of such an interaction may remove this inhibition, allowing autoimmune pneumonitis or nephritis. Anti–PD-L1 antibody, however, would theoretically leave PD-1–PD-L2 interaction intact, preventing the autoimmunity caused by PD-L2 blockade. With implementation of an algorithm to detect early signs of pneumonitis and other immune-related adverse events, many of these side effects have become manageable. However, it does require discerning clinical attention to detect potentially fatal side effects. In terms of antitumor activity, both anti–PD-1 and anti–PD-L1 antibodies have shown responses in overlapping multiple tumor types. Although limited to a fraction of patients, most responses, when observed, were rapid and durable.

MEL is among the first types of solid tumors in which nivolumab has shown promising antitumor activity. In a phase I study across all dose levels, nivolumab resulted in an objective response rate (ORR) of 28% (95% confidence interval [CI], 19–38) in patients with advanced MEL who had no prior ipilimumab treatment.[7] Responses were adjudicated according to Response Evaluation Criteria in Solid Tumors (RECIST), version 1.0, with modification. A recently updated analysis in 107 MEL patients confirmed these responses and suggested potential survival benefit.[9,10] The ORR was 32% (95% CI, not reported [NR]). The median duration of response was 99.4 weeks (range, 17.0+ to 117.0+). More than half (11/21) of responding patients who discontinued therapy for reasons other than progressive disease responded for more than 24 weeks. The 1-, 2-, and 3-year overall survival (OS) rates were 63% (95% CI, 53–71), 48% (95% CI, 38–57), and 41% (95% CI, 31–51), respectively. Median OS was 17.3 months (95% CI, 12.5–36.7). Survival analysis by type of response suggested that patients who had immune-related responses, such as reduction in the target lesion in the presence of new lesions or following initial progression, had similar OS to patients with RECIST responses.[9,10]

Nivolumab in patients who were pretreated with ipilimumab was tested as part of a “sequential cohort” of an ongoing phase I trial. Nivolumab was given at two dose levels (1 mg/kg and 3 mg/kg every 2 weeks). Of 33 patients, ORR by RECIST criteria was 31% (95% CI, NR). The 1-year survival rate was 70% (95% CI, NR). The residual plasma concentration of prior ipilimumab at the start of nivolumab appears to have an influence on response to nivolumab. An ipilimumab concentration ≥ 7.255 Î¼g/mL (the median) was associated with greater ORR to nivolumab (57.1% vs 14.3%).[11]

Pembrolizumab (MK-3475, lambrolizumab) is a humanized immunoglobulin G (IgG)-4 kappa antibody against PD-1. In KEYNOTE-001, the safety and antitumor activities of pembrolizumab were tested in the largest-ever phase I study of patients with metastatic MEL (N = 411).[12] This study began with ipilimumab-naive (IPI-N) and ipilimumab-treated (IPI-T) patients, and later implemented a randomized dosing comparison of 2 mg/kg vs 10 mg/kg in ipilimumab-refractory (IPI-R) and IPI-N patients. Among patients evaluable by RECIST 1.1, overall ORR was 34% (95% CI, 29–39). The ORRs were 40% (95% CI, 32–48) in IPI-N patients and 28% (95% CI, 22–35) in IPI-T patients. The 1-year OS rate was 69%; median OS has not yet been reached. A subgroup analysis suggested that a tumor size smaller than the median of 90 mm was an independent predictor of response.[13]

In the randomized comparison of 2 mg/kg vs 10 mg/kg of pembrolizumab in IPI-R and IPI-N patients,[14] there was no statistically significant difference between the ORRs for the two cohorts by either RECIST 1.1 (independent central review) or immune-related response criteria (investigator review). In the IPI-N cohort, the ORRs by RECIST 1.1 were 33% (95% CI, 20–49) vs 40% (95% CI, 26–56) in the 2-mg/kg and 10-mg/kg arms, respectively (

 = .4835). In the IPI-R cohort, the ORRs were 26% in both arms (95% CI, 17–37 in the 2-mg/kg arm and 95% CI, 17–36 in the 10-mg/kg arm). According to Kaplan-Meier estimates of OS in IPI-N patients, 1-year OS rates were 72% (95% CI, 15–NR) and 64% (95% CI, 10–NR), and in IPI-R patients, they were 58% (95% CI, 11–NR) and 63% (95% CI, NR)-in the 2-mg/kg and 10-mg/kg arms, respectively. Given favorable benefit-risk profiles at both doses, 2 mg/kg every 3 weeks was recommended for future investigation. The most common immune-mediated adverse event of any grade was hypothyroidism (~8%). There was a < 1% incidence of grade 3 or 4 pneumonitis, hepatitis, colitis, or hypophysitis reported.

Pidilizumab is a humanized anti–PD1 IgG1 kappa with binding affinity at 20 nM. A phase II study in MEL patients showed ORR (by investigator review) of 7% in IPI-N patients and 5% in IPI-T patients.[15] The rate of immune-related stable disease was 37%. The 12-month survival rate was 64% (95% CI, 55.6–72.0). Pidilizumab is being investigated in hematologic malignancies.

MPDL3280A is an IgG1-engineered, anti–PD-L1 antibody. A phase I study in 38 patients with metastatic MEL reported a 39% ORR (95% CI, NR), with 43% achieving a 24-week progression-free survival (PFS).[16] Both patients with cutaneous (33%) and mucosal (25%) MEL responded. Patients with PD-L1–positive status by IHC had a 27% ORR (95% CI, NR). Similarly, patients with a negative status had a 20% ORR (95% CI, NR). An additional 60% of patients with PD-L1–positive tumors had stable disease, suggesting that PD-L1–positive patients are more likely to derive clinical benefit. No grade 3 or 4 pneumonitis or treatment-related deaths occurred. Based on these findings and a potential synergy with vemurafenib, a phase Ib study is currently testing the safety and tolerability of this combination in patients with 

 V600E mutations (National Cancer Institute ClinicalTrials.gov ID NCT01656642).

BMS-936559 is a fully human, anti–PD-L1, IgG4 monoclonal antibody. In a study of 1-, 3-, and 10-mg/kg doses in metastatic MEL patients, 9 of 52 (17%; 95% CI, 8–30) achieved objective responses.[17] Three patients achieved a complete response (CR). Five of 9 responding patients had an objective response lasting beyond 1 year. In addition, 14 of 52 patients (27%; 95% CI, 16–41) had stable disease lasting at least 24 weeks.

MEDI4736 is an engineered IgG1 kappa monoclonal antibody with triple mutations in the Fc domain to remove antibody-dependent, cell-mediated cytotoxic activity. It has shown high affinity and selectivity for PD-L1 and, reportedly, no binding to PD-L2.[18] In its dose escalation study of doses ranging from 0.1 mg/kg every 2 weeks to 15 mg/kg every 3 weeks, no dose-limiting toxicity or drug-related deaths were seen. Tumor regression was seen in multiple tumor types, including MEL, NSCLC, RCC, and colorectal cancer (CRC). Segal et al reported the results of a multiarm expansion study using a dose of 10 mg/kg every 2 weeks in 44 patients with MEL (uveal and cutaneous).[19] Although an analysis of antitumor activity in MEL patients has not been reported, preliminary data suggest that several patients with both cutaneous and uveal MEL remained in the study beyond 12 weeks, and no serious adverse events have been reported.

In light of their promising activity as single agents, several combinations of anti–PD-1 and anti–PD-L1 antibodies have been studied. A preclinical study has shown that dual immune checkpoint blockade of cytotoxic T-lymphocyte antigen 4 (CTLA-4) and PD-1 resulted in more pronounced antitumor activity than either single blockade alone.[20] Wolchok et al reported the clinical activity of concurrent administration of nivolumab and ipilimumab in 53 patients with advanced MEL; the resulting ORR was 40% (95% CI, 27–55) (by modified World Health Organization [WHO] criteria), and responses were “rapid and deep.”[21] In an updated survival analysis, Sznol et al reported 1- and 2-year OS rates of 85% and 79% (95% CI, NR), respectively; median OS was 40 months, and median PFS was 27 weeks.[11] This study added another cohort of 40 patients who received nivolumab 1 mg/kg and ipilimumab 3 mg/kg every 3 weeks for 4 cycles, followed by nivolumab 1 mg/kg every 3 weeks for 4 more cycles, then 3 mg/kg every 2 weeks for up to 48 cycles. Results confirmed clinical activity, with an ORR of 43% (95% CI, NR). Responses were seen regardless of PD-L1 tumor expression status or 

Nivolumab in combination with multiple peptide vaccines in IPI-R or IPI-N MEL patients was well tolerated.[22] This study also found that increased peripheral blood regulatory T cells and decreased antigen-specific T cells were associated with progression.The clinical benefit of adding a vaccine remains to be seen.

A phase I/II study of MEDI4736 in combination with a BRAF inhibitor (dabrafenib) and a MEK inhibitor (trametinib) or with trametinib alone is ongoing (NCT02027961). Other immune modulatory agents, such as interferon (IFN) alfa-2b, are combined with various anti–PD-1 and anti–PD-L1 antibodies in ongoing clinical trials.

Metastatic RCC (mRCC) has been a target of immunotherapy for several decades; high-dose interleukin-2 has shown an ORR of approximately 20%, with long-term remission in a small subset of patients,[23] and ipilimumab has shown an ORR of 12.5% (95% CI, 4–27).[24] Over the past decade, the US Food and Drug Administration (FDA) has approved several antiangiogenic agents for mRCC on the basis of improvement in PFS; however, none have demonstrated survival benefits.

The activity of nivolumab in mRCC was reported in its first phase I study, in which 9 of 33 patients (27%; 95% CI, 13–46) achieved objective responses and 56% (95% CI, 39–73) were alive without disease progression at 24 weeks. At the time of data analysis, five of these responding patients had a response lasting for more than a year.[7] The clinical activity of nivolumab in mRCC was confirmed in a dose-ranging phase II study, in which patients were randomized to receive either 0.3, 2, or 10 mg/kg every 3 weeks.[25] Clinical activities were seen across all dose levels, with ORRs of 20%, 22%, and 20% (95% CI, NR), respectively. On survival analysis, median OS outcomes were 18.2 months (80% CI, 16.2–24.0), 25.5 months (80% CI, 19.8–28.8), and 24.7 months (80% CI, 15.3–26.0), respectively. These survival data appeared very promising in the context of other published median OS data of 12 to 16 months for mRCC in second-line settings.[26,27] In a phase III randomized controlled trial, nivolumab is being compared with everolimus in patients previously treated with antiangiogenic therapy; the primary outcome measure is OS (NCT01668784).

In patients with mRCC, 2 of 17 (12%; 95% CI, 2–36) had an objective response to 10 mg/kg of BMS-936559, with responses lasting 4 and 17 months.[17] Seven additional patients (41%; 95% CI, 18–67) had stable disease lasting at least 24 weeks.

Cho et al reported that in 55 patients with mRCC, the anti–PD-L1 antibody MPDL3280A was very well tolerated, with no dose-limiting toxicities.[28] The overall ORR was 13% (95% CI, NR). Responses were seen in both clear-cell and non–clear-cell histology and in both PD-L1–positive and –negative patients. An additional 60% of patients had stable disease. At 24 weeks, 53% of the patients were alive without disease progression. In a phase II study, MPDL3280A is being assessed either as monotherapy or in combination with antiangiogenic agents (bevacizumab or sunitinib) in untreated mRCC (NCT01984242).

To assess combination strategies, investigators have launched a large phase Ib multiarm combination study, CheckMate-016. Nivolumab plus ipiliumab was tested in a randomized study with different induction doses (nivolumab 3 mg/kg plus ipilimumab 1 mg/kg [N3+I1] or nivolumab 1 mg/kg plus ipilimumab 3 mg/kg [N1+I3], followed by nivolumab 3 mg/kg every 2 weeks for both arms. N1+I3 had a higher frequency (26%) of treatment-related adverse events leading to discontinuation, including elevations in lipase and alanine transaminase, diarrhea, and pneumonitis. The ORR was 43% (95% CI, 21.8–66.0) in the N3+I1 cohort and 48% (95% CI, 26.8–69.4) in the N1+I3 cohort[29]; these results appear to be more than additive effects. Approximately 80% of these responses were ongoing at the time of data analysis. An additional 24% of patients given N3+I1 and 36% given N1+I3 have achieved stable disease. The 24-week PFS rates were 64% to 65%. In 44 available samples, PD-L1 expression status did not correlate with response. With 1% tumor membrane staining as a cutoff, similar percentages of patients with PD-L1–positive and –negative tumors achieved objective responses (55% and 50%).

In this review, we will discuss the current status of several anti–PD-1 and anti–PD-L1 antibodies in clinical development and their direction for the future.

Oncology Vol 28 No 11_Suppl_3

Prognostic and Predictive Markers for the New Immunotherapies

Identification and Management of Toxicities From Immune Checkpoint–Blocking Drugs

Prospects for Targeting PD-1 and PD-L1 in Various Tumor Types