Recognizing and managing on toxicities in cancer immunotherapy

Interferon

In the 1980s, interferon-a (IFN-a) had been demonstrated to play a pivotal role in the course of antitumor activity in hairy cell leukemia, melanoma, renal cell carcinoma (RCC), and other solid tumors. Four years later, recombinant human IFN-a2 (Intron-A (Merck, Whitehouse Station, NJ); Roferon-A (Roche, Nutley, NJ)) was approved as a therapy for hairy cell leukemia, and in 1995, it became the first immunotherapy approved as the adjuvant treatment for the stage IIB/III of melanoma after treating with the FDA. ⁵

Contrary to initial expectations, IFN has not proven to be an innocuous therapy, despite it being a natural substance. The most common systemic toxicities of IFN therapy were fatigue and fever (ranged from 60% to 70%) followed by headache and myalgia, which were known as flu-like symptoms, in spite of affecting numerous organ systems. ⁶ Based on data from 143 patients treated with high-dose IFN-a in Eastern Cooperative Oncology Group (ECOG) Trial E1684, 66 patients experienced nausea and vomiting after IFN, 2/3 of the patients experienced early satiety, anorexia, and weight loss. These symptoms can be controlled by non-steroidal anti-inflammatory drugs, and severe fatigue often needs to decrease the dosage of IFN. Patients with fever >39°C more than 1 day should be assessed, because it may indicate systemic infection, especially who complained about cough, dyspnea, or respiratory symptoms should undergo radiologic evaluation. ⁷

Myelosuppression is the main adverse effect of dose-limited as high-dose interferon-a-2b inhibits the proliferation and activation of megakaryocytic progenitor cells in a non-lineage specific manner. Although neutropenia is reported in 65% patients (92/143) treated with high-dose IFN-a in E1684 trial, IFN does not result into neutropenic fever or sepsis and is rarely a cause for discontinuation of therapy. Thrombocytopenia and anemia were observed in about 10% of IFN therapy patients. Myelosuppression could be controlled by suspending or reducing the dose, but anyway, complete blood count should be monitored weekly during induction phase, monthly during the first 3 months of maintenance phase, and every 3 months thereafter. Thrombocytopenic purpura and progressive anemia are rare, but permanent withdrawal from IFN is required.^8,9

In a preliminary study of IFN adjuvant therapy for high-risk melanoma patients, hepatotoxicity resulting in two deaths was observed. ¹⁰ Liver function should be assessed at baseline, especially in the presence of hepatitis B or C, weekly during induction, monthly during the first 3 months of maintenance, and then every 3 months during the remainder therapy, although mild elevations in hepatic enzymes without clinical symptoms are common. It is widely considered that IFN should be suspended in patients with grade 3 liver toxicity, until the transaminase concentration returned to within 1.5 times of normal level, and if reused, IFN should be reduced by at least 30%.

Some side effects from IFN are completely independent of dose and duration, such as autoimmune diseases, a biomarker related to good prognosis. There is a prevalence of 10% to 15% of thyroid dysfunction (hyperthyroidism or hypothyroidism) in patients treated with IFN. ¹¹ After a long period of hyperthyroidism, hypothyroidism usually appears, thus T3 and T4 levels, and thyroid autoantibodies should be measured in all patients. The rare Sarcoidosis is most likely to have an autoimmune basis and presents skin lesions similar to subcutaneous metastases or mediastinal lymph nodes with Fluorine-18 deoxyglucose (FDG) high-uptake in positron emission tomography. ¹² Also, vitiligo, lupus, rheumatoid arthritis, polymyalgia rheumatica, and psoriasis are reported. In patients with pre-existing autoimmune disease, IFN should be used with caution, because the following IFN therapy generally induces more serious side effects, possibly involved in the memory function of immunity system. ¹³

The most common neuropsychiatric side effects are depression and irritability with a frequency of as many as 24% patients; however, an acute confusional state, anhedonia, fatigue, apathy, sleep disturbances, sexual dysfunction, impaired memory, cognitive dysfunction, and suicidal ideation induced by IFN-a are also reported. Neuropsychiatric symptoms generally are mild to moderate and resolve within 2–3 weeks of IFN discontinuation. ¹⁴ However, in some patients, neuropsychiatric toxicities can persist for months or even years. Antidepressants appear to be effective. For example, paroxetine is prophylactically administered to 20 patients with malignant melanoma, resulting in decreased symptoms of major depression with 11% of aroxetine-treated patients versus 45% of placebo patients. Patients with history of severe depression are prohibited from IFN, although prophylactic use of antidepressant drugs is able to reduce the risk of depression. Nevertheless it is also notable that the use of antidepressants may worsen depression symptom. ¹⁵

Interleukin-2

Interleukin-2 (IL-2), a glycoprotein first described in 1976 as a T-cell growth factor, plays a central role in immune regulation and T-cell proliferation. In 1998, high-dose cytokine of IL-2 was approved by the FDA based on its long-lasting antitumor responses in patients with advanced RCC or melanoma. High dose IL-2 should be administered under the supervision of the qualified physicians well versed in anticancer agents. ¹⁶ An intensive care facility and specialists skilled in cardiopulmonary monitoring or hemodynamic support are necessary to treat severe toxicities of IL-2 such as pulmonary edema and hypotension . The rate of drug-related deaths in metastatic RCC patients who received single-agent IL-2 was 4% (11/255) and 2% (6/270) in metastatic melanoma patients.

IL-2 common toxicities include chills, fever and fatigue, nausea, vomiting, diarrhea, hypotension, elevated transaminases, dyspnea, hyperbilirubinemia, and oliguria. ¹⁷ Capillary leak syndrome (CLS) begins immediately after treatment with IL-2 at the recommended dosages, which is characterized by a loss of vascular tone and extravasation of plasma proteins and fluid into the extravascular space. ¹⁸ CLS can lead to hypotension, and the reduction of organ perfusion might be too severe to result in death. CLS may be associated with cardiac arrhythmias (supraventricular and ventricular), angina, myocardial infarction, and pleural effusion, as well as occasional pulmonary edema, gastrointestinal bleeding or infarction, prerenal azotemia, and mental status changes. Hence, IL-2 should be restricted to patients who hold with normal cardiac and pulmonary functions. Extreme caution should be given to patients who were found to have a history of cardiac or pulmonary disease by means of normal thallium stress test and normal pulmonary function test. Hypotension generally is dose-related, and can be managed through vasopressors, even outside the intensive care unit. ¹⁹ With the possible development of extravascular fluid accumulation, ascites, pleural, or pericardial effusions, management of these events depends on a careful balance of fluid shifts so that neither the consequence of hypovolemia (e.g., impaired organ perfusion) nor the consequences of fluid accumulation (e.g., pulmonary edema) exceed the patient’s tolerance. Clinical experience has shown that early administration of dopamine (1–5 µg/kg/min) to patients manifesting CLS before the onset of hypotension can help to maintain organ perfusion especially to the kidney and thus preserve urine output. If organ perfusion and blood pressure are not sustained by dopamine therapy, clinical recommendations are to increase the dose of dopamine to 6–10 µg/kg/min or add phenylephrine hydrochloride (1–5 µg/kg/min) to low dose dopamine. ²⁰ Patients receiving vasopressors should be monitored by telemetric cardiac monitoring, as adrenergic agonists can induce atrial arrhythmia. In few patients, evidence of myocardial injury, including findings compatible with myocardial infarction or myocarditis, has been reported, which usually can be resolved within a few days, and without any sequelae. Therefore, patients should be surveilled through cardiac monitoring until cardiac enzymes are down to normal level, and the subsequent IL-2 therapy should be performed cautiously.

Thrombocytopenia, anemia, and abnormal coagulation function are observed in high-dose IL-2 treatment. Impaired neutrophil function (neutrophil chemotaxis) leads to an increased incidence of disseminated infection including sepsis, bacterial endocarditis, and catheter infection, ²⁰ which could be significantly reduced through prophylactic antibiotics with oxacillin, nafcillin, ciprofloxacin, or vancomycin.

Almost all adverse events (AEs) related to IL-2 might be controlled through suspension or cessation of administration. However, clinical trials have shown that autoimmunity, neurotoxicity, and myocarditis may worsen or continue for a period of time (mostly within 4 weeks) after IL-2 withdrawl.^16,21 The cause of long-term autoimmunity reactions is unknown. Autoimmune diseases, such as hypothyroidism, need over 6–10 months to recover; in contrast, vitiligo may deteriorate or even discontinue the therapy. The neurotoxicity of IL-2 is mild, including lethargy and irritability, or psychosis florid, which can be present as an episode of psychosis. Neurotoxicity reaches to the maximum within 24 h after the last administration, and should be vigilant and early recognized.

Glucocorticoids, rather than non-steroidal anti-inflammatories, should be avoided to concomitant administration with IL-2 due to reducing IL-2 potential benefit from antitumor effectiveness, although these agents have been shown to relieve IL-2-induced side effects including fever, hyperbilirubinemia, confusion, and dyspnea. Many agents have been developed to reduce the toxicities of IL-2 through blockade of its receptor; however, these antagonists were widely used in cancer patients so far because of their mild efficacy. ²²

Tumor necrosis factor

Since tumor necrosis factor (TNF) was found by Carswell in 1975, which acts as a pro-inflammatory and anti-inflammatory cytokine that was identified as a significant factor participating in the development of autoimmune disease and cancer, as well as the protection against infectious pathogens, ²³ numerous anti-TNF immunotherapy cases have been described. In fact, anti-TNF immunotherapy was synergic with apoptotic-cell-based therapy which is known as an efficient treatment with human cancer including colorectal cancer and breast cancer. ²⁴ Unfortunately, unexpected side effects have been described accompanying the anti-TNF therapy in clinical practice. The main toxicities of anti-TNF therapy include fever accompanied by hypotension and flu-like symptoms (chills, joints, and aches), as well as decrease of platelet and white blood cells but increase of transaminase.

Compelling evidence has reported that paradoxical adverse events (PAEs), including psoriasis, Crohn’s disease, and hidradenitis suppurativa, mainly emerge with anti-TNF-α agents. Other PAEs could be classified as borderline including uveitis, scleritis, sarcoidosis, and other granulomatous diseases (granuloma annulare, interstitial granulomatous dermatitis), vasculitis, vitiligo, and alopecia areata. Specially, it is found that the risk for paradoxical psoriasis is non-significant based on the data from registries. PAEs resulted by anti-TNF-α might be managed mainly depending on various aspects, such as the type and severity of the AEs, pre-existing treated conditions, and the possibility of alternative therapeutic options for the underlying disease. ²⁵

IL-12

IL-12 was previously called cytotoxic lymphocyte maturation factor (CLMF) or natural killer cell stimulatory factor (NKSF) composed of two subunits, which was identified as a powerful immunostimulatory cytokine with a strong antitumoural activity. It has been about 10 years since IL-12 was proved efficacious in numerous preclinical tumor immunotherapy studies. ²⁶ The study regarding comparisons of IL-12 and conventional chemotherapy in the treatment of lung cancer has revealed that IL-12 treatment suppressed lung tumor growth, resulting in the long-term survival of lung cancer–bearing mice. ²⁷ Furthermore, studies have indicated that IL-12 seems to be less toxic than other immunotherapeutics such as IFNs or IL-2. ²⁶

Despite encouraging immunostimulatory and antitumor effects of IL-12, IL-12-related toxicities should be taken into consideration. In serum liver function tests, the results suggested that common side effects mainly included fever, chills, fatigue, nausea or vomiting, and asymptomatic elevation, as well as transient neutropenia and thrombocytopenia. Fortunately, there was no patient who required platelet transfusion or had a neutropenic fever, indicating transient neutropenia and thrombocytopenia were insignificant. However, when three patients were treated with 250 ng/kg recombinant human IL-12, dose-limiting toxicities occurred in two of them, including diarrhea and asymptomatic elevation. ²⁸ Therefore, appropriate doses of IL-12 can be given safely to patients, especially to patients after autologous stem cell transplantation for high-risk hematological malignancies. Further examinations should be performed to assess the efficacy of IL-12 in the immunotherapy setting.

Toxicities related to high affinity of TCR

Toxicities induced by antigens which are derived from tumor-restricted expression proteins are related to high affinity, rather specificity of TCRs, for which they are named as on-target toxicities. The marked potency of T cells through encoding higher affinity CARs enables the recognition of minute levels of antigen expressed on normal cells. Thus, bounding normal, non-mutated antigenic targets that are expressed on normal tissues but overexpressed on tumor cells have led to severe on-target, off-tumor toxicities. It is well known that potential cancer rejection antigens, such as MART-1 melanoma-melanocyte differentiation antigen or gp100, are formed by non-mutated protein highly expressed in malignant melanoma cells but rarely expressed in normal melanocytes. To identify these antigens, TCRs were engineered into retroviral vectors and used to transduce autologous peripheral lymphocytes administered to 36 patients with metastatic melanoma in a pilot study. ⁴³ As a non-surprising result, patients exhibited destruction of normal melanocytes in the skin, eye, and ear, due to fully activated T cells, with a strong ability to recognize cognate antigens of MART-1 and gp100 expressed in their melanocytes. And sometimes in patients with severe on-target and off-tumor toxicities, steroid administration was required to treat uveitis and hearing loss, ⁴⁴ fortunately which can be controlled by application of agents. Some factors, such as the expression level and the distribution of the attacked protein in normal tissues, and whether the affected cells immediately endanger the survival of the patient, determined the severity of damage from immunotherapy.

It is not beyond our expectation that higher affinity CARs also led to obvious AEs. Since effective clinical treatment with anti-CD19 CAR T cells was reported for the first time in 2010 after a patient with advanced-stage lymphoma treated at the National Cancer Institute (NCI) experienced a partial remission of lymphoma and long-term eradication of normal B cells, anti-CAR CD19 therapy for refractory lymphoma or leukemia has become acceptable. ⁴⁵ The CD19 antigen molecule expressed on more than 90% of B-cell malignancies at all stages of B cells differentiation but not on other normal cells. Data from an extended sample trial that enrolled 15 patients with diffuse large B-cell lymphoma, indolent lymphomas, and chronic lymphocytic leukemia showed that grade 3 and 4 toxicities experienced by patients are fever, hypotension, and infection, as well as neurologic toxicities, mostly occurred during the first 2 weeks after infusion. One patient who had chemotherapy-refractory primary mediastinal B-cell lymphoma (PMBL) with extensive fibrotic mediastinal lymphoma involvement died suddenly, without experiencing signs of cytokine-release toxicities such as fever. A likely cause of death is cardiac arrhythmia. A risk of infection that severe on-target toxicity B cells lack caused by T cells attack can be eliminated through intravenously injected immunoglobulin G or the IL-6 receptor-blocking antibody tocilizumab. ⁴⁶

When the carcinoembryonic antigen (CEA) was targeted by T cells in colorectal cancer, all three patients had a serious risk of life-threatening colitis; similarly, a low expression of CEA existed in normal colon tissue. ⁴⁷ The use of T cells with CAR specific to HER-2 may cause lethal lung injury, being manifested as acute pulmonary edema and hypoxia. The observation of death caused by acute lung toxicity indicated that it is probably because of the low-level expression of HER-2 antigen in mistakenly attacked lung epithelial cells. ⁴⁸ Some research groups have experimented to change amino acid sites by site-specific mutagenesis in TCR to enhance the affinity of TCR antigen binding. Carbon anhydride 9 is overexpressed in renal cell carcinoma patients, which is also expressed in bile duct epithelium, ⁴⁹ and it is not surprising that CAR T-cell therapy encoding anti-carbon anhydride 9 would lead to severe liver toxicity in clinical trials. When T-cell therapy leads to life-threatening toxicity (either cytokine release or autoimmunity), standard interventions are necessary, including the use of high doses of corticosteroids and alemtuzumab (an anti-CD52 antagonist), to inhibit or eliminate lymphocytes (which may lose all of the antitumor effects). A potential method is ongoing to look for TCRs or CARs against specific surface antigens expressed on tissues that are not essential for survival.

Toxicities related to weak specificity of TCR

On theoretical grounds, cancer rejection epitopes may be derived from two classes of antigens. The first class of cancer antigens is formed by non-mutated proteins with incomplete T-cell tolerance caused by their restricted tissue expression pattern. The second class of cancer antigens is formed by peptides that are derived from epigenetic mutated proteins in tumor self and thus entirely absent from the normal human genome, so-called neoantigens. ⁵⁰ Therefore, theoretically, toxicity induced by so-called neoantigens should not occur in normal tissues; unfortunately, diverse grades or types of AEs still were frequently reported. These unpredictable toxicity, related to weak specificity but high affinity of TCR or CAR, were primarily induced by a mechanism that the receptors on T cells identify cross antigen or epitope (or cross reactive) and then generate off-target and off-tumor toxicities associated with antigen independent activation. Almost certainly, TCR- or CAR-targeted non-mutated proteins also mismatched with non-aimed epitopes in normal tissues.

Melanoma-associated antigen A (MAGE-A) is a multigene family consisting of 12 homologous genes MAGE-A1 to MAGE-A12 located at chromosome Xq28 (11). MAGE-A3 is a cancer testis antigen not expressed in any of the normal tissues. Nine cancer patients were treated with adoptive cell therapy using autologous anti-MAGE-A3 TCR-engineered T cells. Three patients experienced mental status changes, and two patients lapsed into comas and subsequently died because of severe brain damage in white matter. The neurological toxicity observed in three cases was known to be that, T cells modified with an HLA-A*0201-restricted MAGE-A3-specific TCR recognize the different but similar epitopes of the MAGE-A12, which is expressed in human brain (and possibly MAGE-A1, MAGE-A8, and MAGE-A9) at quiet low level in the brain. ⁵¹ There are limited data that suggested toxicity was associated with T-cell dose, but it is likely to be variable in different individuals. Two patients (one multiple myeloma and another melanoma, respectively) treated with high-affinity TCR, which is specific to HLA-A1-limited MAGE-A3^a3a, experienced with the fatal cardiogenic shock, since the epitope derived from cardiac titin, ⁵² not related to MAGE-A3^a3a, is cross-recognized by the HLA-A1-restricted TCR. Therefore, strategies in order to enhance TCR affinity to neoantigens expressed in contracting normal key tissue and not be identified by typical preclinical screening analysis may lead to unanticipated toxicities. As these toxicities were difficult to relieve as soon as any essential normal organs represent potential targets, improved preclinical testing methods to better enable prediction of TCRs specificity have to be established in experimental models.

Cytokines release syndrome

One of the most common toxicity associated with the novel immunotherapies of CAR T and checkpoint inhibitors is cytokines release syndrome (CRS), a constellation of inflammatory symptoms resulting from cytokine elevations associated with T-cell engagement and proliferation. The patients experienced with CRS associated with T-cell therapies may represent a series of symptoms, including sepsis, fever, tachycardia, blood vessel leakage, less urine, and hypotension. ⁵³ Recently, multiple organ failures have emerged in severe cases and a CRS-related death after blinatumomab treatment has been reported. When treated with systemic IL-2 therapy, these symptoms appear more quickly; therefore, it suggests toxicities induced by cytokines releasing are directly mediated by IL-2. Without the use of IL-2, the onset of these symptoms may be delayed (T-cell infusion after 5–7 days). Even severe renal failure, coma, and respiratory failure can be completely reversed by supporting treatment. In a recent study of CAR CD19 in the treatment of leukemia, all patients had a certain degree of CRS, 27% of severe CRS need to maintain blood pressure treatment to compromise side effects of the cell therapy. ⁴⁶ IL-6 is an inflammatory cytokine involved in a large number of processes within the immune system, such as neutrophil trafficking, acute phase response, angiogenesis, B-cell differentiation, and autoantibody production. In a group of patients with B-cell malignancies treated by CAR modification of T cells (CD19 targeted), researchers found that IL-6 is a regulator of hemodynamic toxicity, and tocilizumab, a IL-6 receptor-blocking antibody, had obvious advantages to control early symptoms of CRS patients. ⁵⁴ Common treatment usually included intravenous fluid infusion, non-steroidal anti-inflammatory drugs, vasopressor drugs (if needed), and even cortisol hormones, and strong monitoring needs to be warranted.

Overall, compared with other types of immunotherapies, adoptive cell therapy is a more complex approach to cancer patients. Therefore, new technologies in improving the specificity of T cells should be further developed to do no or less damage to the deadly tissue in order to ensure the safety of ACT treatment. Second, lymphatic depletion dose of pretreatment should be carefully selected. Finally, further studies are needed to determine optimal toxicity surveillance and management, when serious AEs appear.

Some biomarkers to predict patients at high-risk of developing CRS related to immunotherapy, such as PRF1, MUNC13-4, STXBP2, and STX11, still necessitate new studies to establish. ⁵⁵ It has been advocated to integrate the suicide gene into the transplanted T cells, and this gene is enabled to be activated immediately once grade 3 or 4 AEs happened, ⁵⁶ but it is not clear whether it can be started in time to prevent permanent damage.

Organ-special toxicities of checkpoint protein inhibitors

One of the most common irAEs in patients who accepted checkpoint protein inhibitors treatment was the liver toxicity with elevations in serum levels of hepatic enzymes. Of 298 patients treated with ipilimumab, 39 cases had grade 3–4 toxicities. There was an increased incidence of liver test abnormalities in the melanoma group treated by nivolumab, with increases in aspartate aminotransferase (AST) (28%), alkaline phosphatase (22%), alanine aminotransferase (ALT) (16%), and total bilirubin (9%). ⁶¹ Grade 3–4 liver function abnormalities may require prolonged treatments, and it was appropriate to stop checkpoint protein antibodies and use high doses of cortisol, although no prospective trials have been conducted to specifically test whether one management strategy is superior. Standard precautions were that inhibitors have to be permanently discontinued for severe (grade 3) or life-threatening (grade 4) immune-mediated hepatitis. As among four patients for whom nivolumab in combination with ipilimumab was restarted, three had recurrence or worsening of hepatitis and one improved on corticosteroids.

In patients treated with ipilimumab, grade 3–4 colitis incidence rate is of 6%–14%, but which in great majority of PD-1/PDL-1 antibodies is less than 1%. However, diarrhea or colitis occurred in 21% (57/268) of melanoma and in 17% (50/287) of NSCLC receiving nivolumab. Colitis usually affects the distal colon and rectum, manifested as erythema, edema, erosion, and bleeding. In severe cases of patients who are hospitalized, microscopic examination revealed infiltration of neutrophils, and crypt damage or crypt microabscess. ⁶² Colitis usually occurs within 4–6 weeks in patients treated with PD-1/PDL-1 blockers, and in severe cases, the risk of colon perforation and obstruction (may also include the small intestine) may exist. ⁶³ The recovery period mostly was within 6 weeks, but extended in selected patients. Hospitalization should be considered for intravenous high-dose corticosteroids if symptoms were severe or refractory to oral corticosteroids therapy. Infliximab should be used unless symptoms could be cured or not recurrent after 3-day high dose glucocorticoid treatment. ⁶⁴ If intestinal perforation was suspected or diagnosed, administration of steroid hormones and surgery intervention should be considered. Infliximab is contraindicated to intestinal perforation and sepsis. There is no evidence that preventive oral steroid budesonide can prevent diarrhea occurrence. Physicians should always pay attention to the result of stool culture in order to exclude the infection due to deterioration of colitis.

Another common toxicity that occurred in ipilimumab therapy was skin side effect, and in 40% of patients, 2% are grade 3 or grade 4 toxicity. ⁶⁵ Common skin toxicities related to nivolumab also had a high prevalence ranged from 28% to 36%, although very few cases had three or four toxicities. These predominant dermatologic toxicities included maculopapular, erythematous rash and pruritus, but rare vitiligo and hair depigmentation were also reported, which were considered a positive prognostic factor in patients with melanoma. ⁶⁶ Mild skin toxicity might be relieved through using emollients, steroid cream (1% hydrocortisone), and antihistamines. Immune treatment needs to be terminated for grade 3 or 4 toxicity presented as Stevens Johnson syndrome and toxic epidermal necrolysis, which also should be evaluated by the dermatologists and treated with systemic high-dose corticosteroids (1–2 mg/kg/d prednisolone or corresponding drugs) followed by corticosteroid taper.

Almost no significant pneumonia was found in anti-CTLA-4 antibodies compared to anti-PDL-1 and PD-1 agents, suggesting that the spectrum of irAEs varies among different blocking antibodies. Grade 2–3 pneumonia resulted by nivolumab in NSCLC patients is 7%, but only 2.4% by pembrolizumab. Ipilimumab rarely causes pneumonia with symptoms with around 1% incidence rate. However, PD-1 antibody to treat patients may produce immune-mediated pneumonitis or interstitial lung disease identified as diffuse lymphocytic infiltration, and shortness of breath, sputum increasing, fever, chest pain, hemoptysis. The most frequent serious adverse reactions reported in at least 2% of patients receiving nivolumab were pulmonary embolism, dyspnea, pleural effusion, and respiratory failure. ⁶⁷ Seven deaths were due to pneumonia, pulmonary embolism, and limbic encephalitis. High dose glucocorticoid can result into remission of symptoms in most patients, but the recovery process might be prolonged.

The peripheral motor neuropathy and immune-mediated encephalitis are rarely found in patients treated with checkpoint protein inhibitors. Only <1% of patients were found to suffer from encephalitis in 8490 patients who received the treatments with nivolumab or combined with ipilimumab. ⁶⁸ However, the application of nivolumab is prohibited for patients diagnosed with moderate to severe neurologic symptoms or signs. ⁶⁹ Besides, the evaluation of treatment with nivolumab might include brain magnetic resonance imaging (MRI), lumbar puncture, and consultation with a neurologist, which is performed without the influences of infectious or other causes of moderate to severe neurologic deterioration. After the exclusion of other etiologies, corticosteroids are managed at a dose of 1–2 mg/kg/day prednisone equivalents for patients with immune-mediated encephalitis, followed by corticosteroid taper. Above all in the patients with immune-mediated encephalitis, the checkpoint protein inhibitors were permanently prohibited.

Severe infusion reactions have been reported in <1% of patients in clinical trials of checkpoint inhibitors as a single agent. ⁷⁰ It was recommended to discontinue treatment for patients with severe or life-threatening infusion reactions and interrupt or slow the rate of infusion for those with mild or moderate infusion reactions.

If ipilimumab trial is performed, patients with autoimmune disease or a history of viral hepatitis should be ruled out, but the latest data suggest that ipilimumab can be safely used in these patients. ⁷¹ Therefore, for the patients who recently suffered or are suffering from autoimmune diseases and have any types of inflammatory bowel disease, treatment should be carefully carried out.

Pharmacokinetic relationships of toxicities in the checkpoint protein inhibitors

The on-target or off-target toxicity caused by ipilimumab blocking CTLA-4 is related to does and efficacy. In case of administering dosage of ipilimumab from 3 mg/kg to 10 mg/kg, the rate of severe AEs associated with grade 3–4 increased from 5% to 18%, while the rate of serious AEs in 0.3 mg/kg was 0%. ⁷² However, 595 patients received 2507 doses of ipilimumab infused at either 3 mg/kg (n = 457) or 10 mg/kg (n = 138) over 90 min, and the 10-mg/kg group had no statistically significant higher incidence of infusion-related reactions than the 3 mg/kg group (4.3%vs 2.2%, P = 0.22). Similarly, out of 120 patients treated prospectively with ipilimumab 3 mg/kg infused over 30 min, seven patients (5.8%) had infusion-related reactions (p = 0.06, compared with 90-min infusions). ⁷³

For the anti-PD-1 antibody pembrolizumab, compared to 2 mg/kg (FDA-approved dose), patients who managed with 10 mg/kg every 3 weeks have much higher grade 3–4 AE. Recently, in a phase II clinical study for pembrolizumab, three or four drug-related AEs occurred in 12% patients, 5% of patients had severe AEs, and 3% of patients had to discontinue treatment because of AEs, although no drug-related deaths were reported. ⁷⁴ The most common AEs in all the grades included fatigue, pruritus, and rash. Anti-PD-1 antibody pembrolizumab and nivolumab seemed to be very similar in AE spectrum.

In contrast, the PD-1 antagonist nivolumab toxicity was similar in the different dose ranges of 0.3 mg/kg–10 mg/kg. In a randomized phase II trial, ⁷⁵ a total of 168 patients with metastatic renal cell carcinoma were randomly assigned to the different dosages of nivolumab treatment. The incidence of treatment-related AEs of any grade was similar across dose arms: 75%, 67%, and 78% in the 0.3-, 2-, and 10-mg/kg groups, respectively. The most common treatment-related AE is fatigue (24%, 22%, and 35%, respectively). In another study of 281 patients with melanoma, renal cell carcinoma, and lung cancer, patients were treated with doses of 0.3 mg/kg–10 mg/kg nivolumab, and 5% of patients experienced grade 3–4 irAEs. In a phase III trial ⁷⁶ of patients with untreated stage IV melanoma dosed by 3 mg/kg (FDA-approved dosage) nivolumab treatment, 11.7% have prevalence of grade 3–4 drug-related AEs, 6.8% suspend treatment as AEs, and 1% reported both impaired liver function and diarrhea.

Evidence has suggested that different types of cancer patients had different toxicities in PD-1 inhibitors trials. Immune-mediated hepatitis occurred in 1.1% (3/268) of melanoma patients receiving nivolumab. The time to onset of hepatitis in three patients was 97, 113, and 86 days after initiation of nivolumab. In NSCLC trial enrolled with 292 patients, however, only 0.3% patient developed immune-mediated hepatitis after 7.8 months of nivolumab exposure, resulting in permanent discontinuation. ⁶⁸ All patients received high-dose corticosteroids (at least 40 mg prednisone equivalents). Liver function tests improved to grade 1 within 4–15 days of initiation of corticosteroids. Among the patients with Hodgkin’s lymphoma who received nivolumab, 23 (22%) had grade 3 toxicity, including the most common skin rash, thrombocytopenia, fatigue, and fever, and two had transfusion reaction event for which monitoring was required. ⁷⁷ The prevalence of pneumonia caused by PD-1/PD-L1 antibody in melanoma was similar to that in NSCLC (2.9–3.4% and 3.4–3.5%, respectively), suggesting that the pre-existing lung disease may not contribute to the formation of injury. ⁷⁸ There was no evidence that toxicity profiles were associated with anti-nivolumab antibody development.

Although anti-CTLA-4 and anti-PD-1/PD-L1 antibodies have not been directly compared, immune-related effects associated with anti-CTLA-4 drugs were more common and of higher grade. ⁷⁹ Thus, patients experienced grade 3/4 adverse effects induced by anti-PD-1/PDL-1 antibodies are approximately 10%, which is lower than 20%–50% by anti-CTLA-4 antibodies. Compared to ipilimumab, however, anti-PD-1/PDL-1 therapy toxicity may need to receive long-term treatment for resolution of symptoms, and long-term monitoring. The irAE frequencies of PDL-1 antibody were lower than that of PD-1 inhibitors. In a study of a total of 207 patients who received PDL-1 antibody, the overall grade 3–4 toxicities incidence rate is 9%. Retrospective data indicated that corticosteroid treatment was not associated with impaired progression-free survival or overall survival. ⁸⁰

Toxicities in the checkpoint protein inhibitors-based combined therapy

Several studies have demonstrated that the irAE spectrum and various frequencies depended on the combination patterns of different agents. Generally, toxicities in combined therapy were more severe than those in agents alone. For instance, infusion reactions occurred in 1% (3/287) of patients receiving nivolumab and in 3% (3/94) of patients receiving nivolumab with ipilimumab. ⁸¹ In combination with nivolumab and ipilimumab, the irAEs of liver or small intestine and colon were common toxicities; moreover, the incidence of toxic reactions of grade 3 or 4 was up to 8%–10%, although the response rate was also improved to 43% and 53% cases had longer response duration. In a double-blind study involving 142 patients with metastatic melanoma who had not previously received treatment, drug-related AEs of grade 3 or 4 were reported more frequently in the combination group than in the ipilimumab monotherapy group (54% vs 24%) The most common grade 3 or 4 AEs associated with the combination therapy were colitis (17%), diarrhea (11%), and an elevated ALT level (11%). When ipilimumab combined with bevacizumab, the observed AEs were generally higher than expected with ipilimumab alone, but remained manageable. Importantly, there did not appear to be an increased incidence of dermatologic or gastrointestinal side effects such as colitis which are most concerning for ipilimumab. ⁸² In a randomized clinical trial with CTLA-4 blockade by ipilimumab and granulocyte-macrophage colony-stimulating factor (GM-CSF) combination, interestingly, grade 3–5 AEs occurred in 44.9% (95% CI; 35.8%, 54.4%) of GM-CSF plus ipilimumab and 58.3% (95% CI; 49.0%, 67.2%) of ipilimumab alone (two-sided p = 0.04). ⁸³

As with any combination therapy, the combination of checkpoint blockade immunotherapy and radiation contributes to the incidence of high-grade complications. For example, the immune-related side-effects of the specific site of irradiation include increased pneumonitis with lung irradiation, increased colitis with bowel irradiation and increased hypophysitis with central nervous system (CNS). ⁸⁴ Despite these side effects in patients, evidences demonstrated that the combination of radiation and checkpoint blockade immunotherapy can be safe for certain disease sites. In pelvis, as an example, the notable colitis was not found under the treatment in the combination of ipilimumab and radiation. Furthermore, favorable tolerance was obtained when ipilimumab in combination of stereotactic radiosurgery was applied for brain metastases. However, these results may be confused due to the limited samples. Therefore, to further evaluate the risks and toxicities caused by the combination of checkpoint blockade immunotherapy and radiation, it is urgent to carry out the additional phase I and II trials by incorporating safe endpoints in the definitive or palliative setting. ⁸⁵

However, it has to be emphasized that the grade 3/4 AE rate greatly varies from 0% to 66% (the median 21%) in a systematic review for 50 clinical trials with a total of 5071 patients receiving immune checkpoint inhibitors. ⁸⁶ Furthermore, around one fifth of the articles suggested that toxicities do not persist or are readily manageable. However, this claim is without presenting any data or evidence to support. Such trials that included incomplete data collection may significantly lead to a skewed representation of the tolerability of a novel immune checkpoint inhibitor.