Mitigation and Management of Common Toxicities Associated with the Administration of CAR-T Therapies in Oncology Patients

Abstract

Chimeric antigen receptor T-cell (CAR-T) therapies are one of the main approaches among targeted cellular therapies. Despite the potential benefit and durable responses observed in some patients receiving CAR-T therapies, serious and potentially fatal toxicities remain a major challenge. The most common CAR-T-associated toxicities include cytokine release syndrome (CRS), neurotoxicity, cytopenias, and infections. While CRS and neurotoxicity are generally managed with tocilizumab and corticosteroids, respectively, high-grade toxicities can be life-threatening. Close postinfusion monitoring and assessment of clinical laboratory parameters, patient-related and clinical risk factors (e.g., age, tumor burden, comorbidities, baseline laboratory parameters, and underlying abnormalities), and therapy-related risk factors (e.g., CAR-T type, dose, and CAR-T-induced toxicity) are effective strategies to mitigate the toxicities. Clinical laboratory parameters, including various cytokines, have been identified for CRS (interleukin [IL]-1, IL-2, IL-5, IL-6, IL-8, IL-10, C-reactive protein [CRP], interferon [IFN]-γ, ferritin, granulocyte-macrophage colony-stimulating factor [GM-CSF], and monocyte chemoattractant protein-1), neurotoxicity (IL-1, IL-2, IL-6, IL-15, tumor necrosis factor [TNF]-α, GM-CSF, and IFN-γ), cytopenias (IL-2, IL-4, IL-6, IL-10, IFN-γ, ferritin, and CRP), and infections (IL-8, IL-1β, CRP, IFN-γ, and procalcitonin). CAR-T-associated toxicities can be monitored and treated to mitigate the risk to patients. Assessment of alterations in clinical laboratory parameter values that are correlated with CAR-T-associated toxicities may predict development and/or severity of a given toxicity, which can improve patient management strategies and ultimately enable the patients to better tolerate these therapies.

Supplementary Information

The online version contains supplementary material available at 10.1007/s40264-025-01538-5.

Plain Language Summary

Chimeric antigen receptor T-cell (CAR-T) therapies are used in the treatment of various aggressive blood cancers. These therapies use a patient’s immune cells (T cells) that are genetically modified to fight cancer. In this article, we focus on the adverse effects associated with CAR-T therapies and discuss how they can be managed. The most common CAR-T-associated adverse effects include cytokine release syndrome (a rapid release of signaling proteins [cytokines] from affected immune cells), neurotoxicity (toxic effects on the nervous system), cytopenias (lower-than-normal blood cell levels), and infections. Patients receiving CAR-T therapies need to be closely monitored for signs of any adverse effects. Some of these effects can be prevented or treated with medications. However, current efforts focus on making the adverse effects less severe, and on identifying risk factors that may predict the likelihood and onset of a potential adverse effect. When an adverse effect occurs, the levels of certain molecules in the blood change. These changes can help physicians determine the type of adverse effect and select the best treatment to combat it. Some patient features (e.g., age, medical conditions, the size and spread of the tumor, and levels of certain molecules in the blood) and treatment-related factors (e.g., therapy type and dose) should be considered before starting a CAR-T therapy. Adverse effects are monitored and treated to reduce the risk to patients. Evaluating the levels of certain parameters in the blood can improve patient management strategies and help patients better tolerate CAR-T therapies.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s40264-025-01538-5.

Key Points

Potentially life-threatening toxicities associated with chimeric antigen receptor T-cell (CAR-T) therapies can be monitored and treated to minimize the risks to oncology patients.

We focus on management and mitigation strategies for the most common CAR-T-associated toxicities: cytokine release syndrome, neurotoxicity, cytopenias, and infections.

Assessment of clinical laboratory parameter values that are correlated with CAR-T-associated toxicities may improve patient management during and after CAR-T infusion and help the patients better tolerate these lifesaving therapies.

Introduction

Immunotherapy has significantly improved the care of cancer patients. Among immunotherapies, adoptive cell therapies (ACTs) rely on the transfer of ex vivo-expanded autologous or allogeneic immune cells, mostly conventional alpha beta T cells [1, 2]. ACT using T cells that are genetically modified and ex vivo-activated to suppress T-cell inhibition in the tumor microenvironment and eradicate tumor cells remains the current focus of research [3–6]. Within these therapies, chimeric antigen receptor T-cell (CAR-T) therapy, consisting of genetically engineered T cells modified to express a receptor directed against a tumor antigen, represents one of the main approaches in the current development of targeted cellular therapies. CAR-T therapy utilizes genetically engineered artificial receptors that recognize tumor cell surface antigens independently of the major histocompatibility complex [7]. CAR-T therapy can potentially be used for the treatment of cancer, infectious diseases, autoimmune diseases, cardiac diseases, and others [8, 9]. Furthermore, CAR technology has recently been used to develop novel ACTs with other types of immune cells, including natural killer cells and their subtypes, macrophages, or gamma delta T cells [10–12].

Following isolation of T cells and their genetic modification and expansion, the cells are injected back into the patient, where they exert their cytotoxic activity and help to mount a sustained immune response (Fig. 1).

Fig. 1.

T-cell therapy patient journey. (1) Before receiving T-cell therapy, patient eligibility for treatment is assessed by a multi-disciplinary team; initial evaluations include medical history, performance status, screening laboratory tests, and imaging. (2) Following leukapheresis, T cells are shipped to a laboratory for processing. (3) T cells are isolated and activated. Activated T cells are genetically engineered, expanded, and then cryopreserved before being reintroduced into the patient. (4) Prior to infusion, the patient undergoes lymphodepletion to ensure effective CAR-T expansion and cytotoxic impact. Lymphodepletion is routinely performed with a combination of fludarabine and cyclophosphamide. (5) Modified T cells are injected back into the patient via infusion. (6) Following the infusion, patients are closely monitored for 2–4 weeks for potential toxicities with regular medical assessments thereafter

To date, six CAR-T therapies have been approved by the US Food and Drug Administration (FDA) for the treatment of various hematologic cancers (Table 1).

Table 1.

Food and Drug Administration-approved CAR-T therapies [7, 13]

Product name (brand name, manufacturer)	Indication
Tisagenlecleucel (Kymriah, Novartis Pharmaceuticals Corporation)	Acute lymphoblastic leukemia [14], B-cell lymphoma [15], and follicular lymphoma [16]
Axicabtagene ciloleucel (Yescarta, Kite Pharma Inc.)	B-cell lymphoma [17] and follicular lymphoma [18]
Brexucabtagene autoleucel (Tecartus, Kite Pharma Inc.)	Mantle cell lymphoma [19] and acute lymphoblastic leukemia [20]
Lisocabtagene maraleucel (Breyanzi, Juno Therapeutics, Inc., a Bristol-Myers Squibb Company)	Large B-cell lymphoma [21]
Idecabtagene vicleucel (Abecma, Celgene Corporation, a Bristol-Myers Squibb Company)	Multiple myeloma [22]
Ciltacabtagene autoleucel (Carvykti, Janssen Biotech, Inc.)	Multiple myeloma [23]

CAR-T, chimeric antigen receptor T cell

Long-term efficacy and remission potential with CAR-T therapy has been observed in clinical trials [24–35], as well as in postmarketing studies [36–41]. For instance, data from a postmarketing study with axicabtagene ciloleucel indicate an overall response rate of 70% and a complete response rate of 52% in patients with large B-cell lymphoma [37].

Despite the potential benefit of CAR-T therapies, they are associated with life-threatening toxicities [42, 43]. In addition, the US FDA has recently highlighted the risk of T-cell malignancies based on data from clinical trials and/or postmarketing studies with CAR-T therapies [44]. Effective mitigation and clinical management of these toxicities can reduce life-threatening risks to patients and afford them the potentially life-sustaining benefits of these therapies.

Initial preclinical models with CAR-T therapies primarily aimed to assess the efficacy of these therapies but failed to predict toxic complications in humans [43]. Toxicities associated with these therapies pose safety risks and are major barriers for their wider clinical application. Thus, the adoption of a benefit–risk-based approach in clinical development of these therapies is of crucial importance to assure the safety of patients. Furthermore, given the risk of toxicities associated with CAR-T therapies, effective mitigation and management strategies are needed to enable a broader use of these therapies to benefit patients who have limited choices for effective therapies. In this paper, we discuss the use of clinical laboratory assessments to better predict toxicities, as well as management and mitigation strategies for CAR-T-related toxicities in the clinic.

CAR-T-Associated Toxicities

The most common CAR-T-related toxicities include cytokine release syndrome (CRS) resulting from immune activation; neurotoxicity, also referred to as immune effector cell-associated neurotoxicity syndrome (ICANS); cytopenias; and infections (Fig. 2). Less common toxicities include secondary T-cell malignancies and “on-target/off-tumor” (i.e., recognition of the target antigen on normal cells) and “off-target/off-tumor” (i.e., recognition of an unrelated antigen; cross-reactivity) toxicities [43, 45–48].

Fig. 2.

Overview of the main toxicities associated with CAR-T therapies. CAR-T; chimeric antigen receptor T cell; CRS, cytokine release syndrome; ICANS, immune effector cell-associated neurotoxicity syndrome

CAR-T infusion can lead to CRS, ICANS, cytopenias, and infections. CAR-T causes inflammation that can lead to CRS and ICANS. CRS may lead to ICANS, cytopenias, and infections. In addition, lymphodepletion may lead to cytopenias and infections as well.

The incidence and severity of toxicities associated with CAR-T therapies vary largely between studies, likely due to various CAR-T types, the infusion time and dose, and co-administration of other therapies (Fig. 3).

Fig. 3.

Incidence and severity of the most common toxicities associated with CAR-T therapies. The circles represent the incidence reported in individual studies. Data sources are listed in Electronic Supplementary Material 1; the individual studies were published between 2014 and 2023 for CRS and ICANS, and between 2017 and 2023 for cytopenias and infections. CAR-T; chimeric antigen receptor T cell; CRS, cytokine release syndrome; ICANS, immune effector cell-associated neurotoxicity syndrome

Figure 4 provides an overview of clinical laboratory parameters that are associated with the risk of developing each of these toxicities.

Fig. 4.

Clinical laboratory parameters correlated with the risk of developing CAR-T-associated toxicities. Ang, angiopoietin; CA, catecholamines; CAR-T, chimeric antigen receptor T cell; CRP, C-reactive protein; CRS, cytokine release syndrome; F, factor; GM-CSF, granulocyte-macrophage colony-stimulating factor; ICANS, immune effector cell-associated neurotoxicity syndrome; IFN, interferon; IL, interleukin; MCP, monocyte chemoattractant protein; MIP, macrophage inflammatory protein; pTF, plasma tissue factor; sE-selectin, soluble E-selectin; sICAM, soluble intercellular adhesion molecule; TNF, tumor necrosis factor; VCAM, vascular-cell adhesion molecule; vWF, von Willebrand factor

CRS

CRS is one of the most common life-threatening toxicities associated with engineered CAR-T therapies that was not observed in preclinical models, but first noted in subsequent clinical studies [43]. CRS is a systemic inflammatory response that can be induced by the binding of CAR-T cells to a specific antigen on the surface of target cells, inducing the release of cytokines such as interferon (IFN)-γ or tumor necrosis factor (TNF)-α. Subsequent activation of bystander immune and non-immune cells, such as monocytes, macrophages, dendritic cells, and endothelial cells, results in the hypersecretion of proinflammatory cytokines, initiating a cascade of events leading to CRS (Fig. 5) [49, 50].

Fig. 5.

Sequence of events leading to cytokine release syndrome, adapted from Cosenza et al. Int J Mol Sci. 2021;22:7652 [49]. CAR-T cells bind to the tumor cells and induce the release of cytokines such as IFN-γ or TNF-α, leading to the activation of bystander immune and non-immune cells, which further release proinflammatory cytokines, triggering a cascade reaction in which high levels of released IL-6 activate T cells and other immune cells, leading to a cytokine storm. CAR-T, chimeric antigen receptor T cell; DC, dendritic cell; GM-CSF, granulocyte-macrophage colony-stimulating factor; IFN, interferon; IL, interleukin; MCP, monocyte chemoattractant protein; NK, natural killer; TNF, tumor necrosis factor

The incidence and severity of CRS vary depending on the CAR-T product, dose, and disease burden at the time of infusion [51, 52]. Symptoms of CRS may occur immediately after the administration of CAR-T cells or may be delayed until days or weeks after treatment, with a median time of onset of 2–3 days following CAR-T infusion [51, 53, 54]. The severity of CRS ranges from mild, flu-like symptoms, including fever and chills, to severe and life-threatening symptoms, including hypotension, tachycardia, pleural effusion, pulmonary edema, capillary leak syndrome, and hypoxia, which may ultimately lead to multisystem organ failure and death [51, 55].

Management and Mitigation Strategies

When treated effectively, CRS is manageable. Nevertheless, following CAR-T infusion, patients should be closely monitored at the hospital for at least 7 days, including daily assessments of biochemistry and blood counts, review of organ systems and physical exam, and assessment of vital signs at least every 4 h [56–58]. The Society for Immunotherapy of Cancer recommends daily monitoring of clinical laboratory values associated with CRS for a postinfusion period related to a specific CAR-T product (typically several weeks); in addition, patients with a high disease burden require specific attention, including cardiac function monitoring [59].

The current gold standard of CRS treatment is anti-cytokine therapy that is initiated as soon as symptoms of CRS begin to occur to prevent progression to severe CRS [53]. Tocilizumab, an IL-6 antagonist, has been approved by the US FDA for the treatment of CRS occurring after CAR-T therapy [14, 53] and is commonly used in the management of CRS [60, 61]. Prophylactic tocilizumab prior to administration of T-cell therapies has also been evaluated. For instance, in a recent single-center study, the use of prophylactic tocilizumab prior to the administration of another T-cell redirecting therapy, based on the bispecific antibody teclistamab, decreased the incidence and severity of CRS; CRS occurred in 26.3% of patients who received prophylactic tocilizumab versus 73.3% of patients who did not receive prophylactic tocilizumab [62]. In another study, among 20 patients with non-Hodgkin lymphoma who received prophylactic tocilizumab 1 h prior to infusion of anti-CD19 CAR-T cells, only low-grade CRS was observed in 50% of patients, indicating that prophylactic tocilizumab is a viable option to mitigate high grade CRS [63]. Another anti-IL-6 monoclonal antibody, siltuximab, has also been used, although it has not been approved by the US FDA for treatment of CRS following CAR-T therapy [53, 64, 65]. Other agents for the treatment of CRS include corticosteroids, dasatinib, A3 adenosine receptor agonists, JAK/STAT pathway inhibitors, or lenzilumab [53, 66].

CRS should be managed according to the toxicity grade [59, 67, 68]. The American Society of Clinical Oncology (ASCO) guidelines recommend mostly supportive care for grade 1 CRS and tocilizumab for higher grades; steroids may be considered earlier in treatment depending on the CAR-T product [67].

Several clinical trials focusing on CRS treatment are ongoing (Electronic Supplementary Material 2).

Clinical Laboratory Assessments

The broad use of CAR-T therapy necessitates the identification of clinical laboratory parameter values associated with the risk of developing severe CRS. Several studies showed that severe CRS following CAR-T therapy is associated with an increase in certain cytokine levels [69–73]. Such cytokine activation profiles could be established following CAR-T infusion to help mitigate the severe effects, and close monitoring of those levels is suggested to be a valuable tool in assessing the risk of CRS [43]. The main cytokines implicated in the pathogenesis of CRS include IL-1, IL-2, IL-5, IL-6, IL-8, IL-10, IFN-γ, monocyte chemoattractant protein 1 (MCP-1), macrophage inflammatory protein-1α, and granulocyte-macrophage colony-stimulating factor (GM-CSF) (Fig. 4) [51, 74–78]. Higher levels of these cytokines have been shown to be associated with severe CRS, indicating their potential as clinically relevant diagnostic predictors for high-grade CRS [70, 71, 79–81]. Among these, high serum IL-6 levels are detected in nearly all patients with CRS [49], and several studies found these levels to be associated with the severity of CRS after CAR-T therapy [69, 70, 72, 73, 79, 82–85]. Furthermore, an increase in C-reactive protein (CRP) levels correlating with increased IL-6 levels has also been detected in patients with CRS [68, 69, 82, 86–89]; elevated ferritin levels have also been shown to correlate with CRS [72, 90, 91] (Fig. 4). Surrogate markers of systemic inflammation could identify patients with CRS and potentially be used to guide intervention to either suppress cytokine release or eliminate CAR-T cells in the event of severe toxicity. While the results of a full cytokine panel may not be promptly available in many hospitals, CRP and ferritin may serve as surrogate markers of CRS. In addition, monitoring lactate dehydrogenase levels, complete blood count, coagulation, and uric acid levels is also recommended [51, 92].

Other, less common clinical laboratory parameters have also been shown to be predictive of the occurrence and severity of CRS, including coagulation parameters; levels of the plasma tissue factor, Factor X, Factor XII, and P-selectin [93]; angiopoietin (Ang)-2 and von Willebrand factor (vWF); soluble E‐selectin and soluble intercellular adhesion molecule-1 [71, 94]; or catecholamines [95]. Furthermore, a regression modeling study predicted a 3-cytokine signature (i.e., IFN-γ, IL-6, and soluble IL-2 receptor α) associated with progression to severe CRS [70].

Clinical, Patient-, and Treatment-Related Risk Factors

Clinical factors that increase the risk of developing higher-grade CRS include disease burden and marrow involvement, common lymphodepletion regimens such as fludarabine and/or cyclophosphamide, and higher CAR-T infusion doses. Patient-specific factors include age, bulky disease, comorbidities, early-onset CRS (within 3 days of cell infusion), pre-existent inflammation, and baseline thrombocytopenia [53, 56, 69, 72, 86, 96]. Of note, high tumor burden (≥ 40% lymphoblasts) in the bone marrow has been identified as the major risk factor for severe CRS [72, 97]. Although elevated levels of the abovementioned cytokines in the blood of patients following CAR-T infusion are generally considered predictors of CRS, individual patient factors might affect these levels, making it difficult to predict the grade of CRS in these individuals. Furthermore, a certain level of IL-6, for instance, might be associated with grade 3 CRS in one patient but not in another, indicating that other individual patient factors need to be considered. Such factors, as well as the factors contributing to the extent of cytokine release, e.g., the CAR-T type, dose, and their abundance in the patient, may help with predicting the effects of cytokine release.

As patients with a higher baseline disease burden are likely to have more severe CRS, imaging techniques such as positron emission tomography–computed tomography (PET-CT) can be useful to assess the tumor burden before CAR-T administration [98]. Furthermore, abnormal imaging findings based on radiography, computed tomography (CT), or magnetic resonance imaging (MRI) have been shown to correlate with higher-grade CRS following CAR-T infusion [99]. Such abnormalities included pleural effusions and pulmonary edema; therefore, thoracic imaging (chest radiography or CT) is often performed in patients experiencing CRS, especially grade 2 or higher CRS, and in patients experiencing fever, cough, and hypoxia following the CAR-T infusion [99]. Pleural effusion with atelectasis and pulmonary edema are common complications of CAR-T-cell therapy occurring during CRS that can be detected through CT; other, less common complications include organizing pneumonia patterns and CAR-T-cell infiltration [100]. Of note, patients with grade ≥ 2 CRS were more likely to have chest CT scans or radiographs with abnormal findings and had a higher incidence of pleural effusions and atelectasis [99]. While patients with CRS may not be routinely imaged, thoracic imaging may provide a useful tool to capture early signs of pulmonary toxicity and guide physicians in managing this toxicity.

Mechanistic models capturing various factors related to the drug (i.e., structure, selectivity, and binding affinity to the tumor target), target (i.e., density of target antigen on the tumor cell surface and accessibility), and disease (i.e., tumor load) have been used to predict the extent of cytokine release. These, however, do not account for individual patient factors, such as comorbidities, received treatment, or their outcomes, and are thus not useful in predicting CRS severity. Combining mechanistic modeling with real-world data and patient databases is currently being investigated [101].

Strategies to mitigate CRS include detailed assessment of individual patient factors and tumor characteristics prior to treatment, and assessments of clinical laboratory parameters and product-related factors. Prompt clinical management of symptoms related to CRS can ensure patient safety and allow patients to realize the benefit of the therapy.

Neurotoxicity

Alongside CRS, CAR-T-associated neurotoxicity or ICANS is another common toxicity observed with CAR-T therapies. ICANS presents as a toxic encephalopathy manifested with aphasia, headache, and confusion but can progress to a depressed level of consciousness, seizures, motor weakness, cerebral edema, and coma [102]. Typically, ICANS occurs following peak CRS severity, when patients begin to recover from CRS; however, delayed onset of ICANS has also been reported [103, 104]. Pathophysiology of ICANS involves peripheral immune overactivation and massive cytokine release, which induce endothelial activation and dysfunction of the blood–brain barrier with increased permeability, facilitating infiltration of CAR-T cells, as well as immune cells and cytokines, leading to central nervous system (CNS) inflammation [51, 102, 103, 105–107] (Fig. 6). The main cytokines involved in the pathophysiology of ICANS include IL-1, IL-2, IL-6, IL-15, IFN-γ, TNF-α, and GM-CSF [107, 108].

Fig. 6.

Pathophysiology of immune effector cell-associated neurotoxicity syndrome, adapted from Morris et al. Nat Rev Immunol. 2022; 22(2):85-96 [105]. Infused CAR-T cells and other activated host immune cells release proinflammatory cytokines leading to activation of endothelial cells and disruption of the BBB; increased permeability of the BBB facilitates infiltration of CAR-T cells and cytokines into the CNS, triggering inflammation and microglial activation, leading to abnormal neuronal function. BBB, blood–brain barrier; CAR-T, chimeric antigen receptor T cell; CNS, central nervous system

Management and Mitigation Strategies

Prophylactic treatment can be used to prevent and manage ICANS. According to the ASCO guidelines, the mainstay of ICANS treatment includes supportive care and corticosteroids. The choice of treatment depends on the grade of the event and the presence of concurrent CRS [67].

Unlike for CRS, in which IL-6 is one of the critical regulators, IL-6 inhibitors are less efficacious against ICANS [57], although still recommended for ICANS with concurrent CRS [67, 109]. Owing to their broad systemic anti-inflammatory function by blocking cytokine signaling, corticosteroids are commonly used to manage ICANS [51, 108]. However, to limit the undesirable effects of corticosteroids and considering IL-1 plays a central role in ICANS, blockade with an IL-1 receptor antagonist may be recommended [110, 111]. As an example, a recent study demonstrated a low incidence of ICANS (19% all grades; 9.7% severe ICANS) in patients who received a prophylactic IL-1 receptor antagonist (anakinra) from day 2 for at least 10 days post-CAR-T infusion [112].

Clinical Laboratory Assessments

Similar to CRS, ICANS is also associated with elevated levels of blood cytokines, including IL-1, IL-2, IL-15, TNF-α, GM-CSF, IFN-γ, and IL-6 [57, 81, 107, 108, 113–116] (Fig. 4). In addition, increased CRP and ferritin levels, as well as IL-8, IL-10, MCP-1, and vascular-cell adhesion molecule 1 were also shown to be associated with high-grade neurotoxicity following CAR-T therapy [57, 81, 117]. Among these cytokines, IL-1 plays a crucial role in the pathophysiology of ICANS [74, 107, 110]. Cytokine levels may be used as predictive markers to aid in the clinical management of symptoms associated with ICANS. For instance, serum concentrations of IL-6 and IFN-γ correlate with the severity of toxicity and have a predictive value in identifying patients who subsequently developed severe ICANS [116]. Concentrations of IL-10 and IL-15 have also been shown to correlate with a risk of developing severe ICANS [57].

Although elevated cytokine levels can be detected in both serum and cerebrospinal fluid (CSF), ICANS is generally associated with CNS-specific cytokine production [57]. Therefore, the analysis of CSF is considered to be a more suitable method for the diagnosis of ICANS [107]. Elevated levels of IL‐1, IL‐6, IL‐10, IFN‐γ, TNF‐α, MCP-1, C-X-C motif chemokine ligand 10, and granzyme B, as well as GFAP (marker of astrocyte injury) and S100b (marker of astrocyte activation), in the CSF are typically observed in patients with neurotoxicity [57, 105, 107, 108]. In addition, decreased levels of Ang1 and increased levels of Ang2, vWF, and IL-8 were suggested to be unique markers in patients with severe ICANS [105, 118, 119].

Furthermore, patients who experience ICANS following CAR-T infusion often undergo imaging analysis (head CT or MRI), especially those with altered mental status and headache [99]. While typically there are no abnormal imaging findings in most ICANS cases, T2-weighted fluid-attenuated inversion recovery (T2/FLAIR ) hyperintensity, vasogenic edema, leptomeningeal enhancement, multifocal microhemorrhages, cortical diffusion restriction, and transient corpus callosum lesions have been observed in some patients [99, 100, 120].

Risk Factors

As ICANS is significantly associated with early systemic inflammation and CRS, mitigation or prevention of severe CRS may decrease the incidence of severe ICANS. The onset of ICANS is correlated with presence and severity of CRS and elevation of CRP and ferritin levels [57, 114, 115, 121]. Early onset and higher severity of CRS, higher pretreatment tumor burden, pre-existing neurological conditions (headache, seizures, cognitive impairment, peripheral neuropathies, and intracranial hemorrhages), older age, acute postinfusion brain MRI findings or brain electroencephalogram changes, lower baseline platelet count, fever, and higher CAR-T dose are main risk factors for ICANS [57, 107, 114, 120–126].

Based on these findings, both clinical laboratory assessments and clinical factors have been used to develop a variety of predictive tools or models for a patient’s risk to develop ICANS [121, 127–131]. For instance, the endothelial activation and stress index (EASIX) score, a surrogate marker of endothelial activation defined as (creatinine × lactate dehydrogenase )/platelets, combined with the inflammatory markers ferritin and CRP, can be used to predict the risk of developing ICANS [127]. Similarly, a modified EASIX, which replaces creatinine with CRP, has been used to predict the risk of developing severe ICANS [128]. Furthermore, on the basis of predictive factors for ICANS, a prognostic score was developed, calculated from values of parameters measured during the first 5 days following a CAR-T infusion. In that model, every parameter (e.g., age, histologic subtype, CRP level, and ferritin level) was assigned a score of 0–3; the total score could range from 0 to 14, with 14 indicating the highest probability of developing ICANS [121]. Another model based on clinical predictors such as maximum daily temperature, CRP, IL-6, and procalcitonin was shown to predict with high accuracy the day of onset of ICANS, and which patients would develop ICANS and with which severity, based on data from the first few days following CAR-T infusion [130]. In addition, a recent model demonstrating a strong association between ferritin levels at day 3 after CAR-T infusion and severe ICANS was validated in an external cohort of patients, further highlighting the key role of ferritin in the development of ICANS after CAR-T infusion [131]. Altogether, these models allow physicians to identify high-risk patients and closely monitor and effectively manage patients who may experience ICANS, thereby reducing the occurrence of serious toxicities in these patients. However, these models need to be further optimized and/or validated before broader implementation in clinical practice.

Cytopenias

Cytopenias are common toxicities associated with CAR-T therapy. Early cytopenias occur up to 30 days post-CAR-T transfusion, prolonged cytopenias occur from 30 to 90 days post-transfusion, and late cytopenias may persist beyond 90 days [132, 133]. Most cases of cytopenia occur immediately following the infusion and are linked to the lymphodepletion chemotherapy [132, 134]. Factors related to prolonged cytopenias are less well understood, although baseline cytopenia is likely associated with prolonged cytopenias post-CAR-T therapy [133, 135–137]. Neutropenia seems to be the most common among cytopenias following a CAR-T infusion [138]. Sustained cytopenia predisposes for severe infectious complications; thus, mitigation and early management of severe cytopenia is crucial [139].

Management and Mitigation Strategies

Management of cytopenias includes supportive care measures such as blood transfusions, prophylactic antibiotics, corticosteroids, antifungal agents, thrombopoietin receptor agonists, and growth factors [59, 67, 140].

Certain baseline clinical characteristics have been shown to have potential prognostic value for predicting severe cytopenia, including baseline hemoglobin and blood cell count, baseline ferritin, CRP and IL-6 levels, lines of prior therapy, response to pre-CAR-T treatment, and the CAR-T product [141, 142]. High-grade CRS and/or neurotoxicity are also associated with prolonged and late cytopenia [71, 132, 143]; furthermore, grade ≥ 3 cytopenia is associated with severe CRS, suggesting that CRS severity can be used as an independent predictor for cytopenia [141, 143, 144]. Consistent with this observation, CRS-associated laboratory parameters, including CRP, ferritin, IL-2, IL-4, IL-6, IL-10, and IFN-γ, have been shown to be associated with cytopenia (Fig. 4) [133, 144]. Thus, clinicians should closely monitor cytopenias following CAR-T cell infusion, especially in patients with suspected or confirmed CRS.

Risk factors associated with prolonged cytopenia include baseline cytopenia, the baseline inflammatory state (elevated serum CRP and ferritin levels), elevated IL-6 levels, pretreatment bone marrow disease burden, severe CRS, and allogeneic hematopoietic stem cell transplantation [60, 141, 145, 146].

Identifying patients at risk for cytopenias and the magnitude of the change in the blood cell levels is crucial considering the complications of cytopenias can be life-threatening. A recently described CAR-HEMATOTOX model evaluates the association of the baseline clinical markers and the risk of developing cytopenia following CAR-T therapy [146]. In addition, this model can be used to predict duration of cytopenia and duration of hospitalization [146, 147].

Infections

Patients receiving CAR-T therapy are typically immunosuppressed due to prior treatments, leading to prolonged cytopenia; moreover, the use of IL-6 blockers and corticosteroids to treat CRS and neurotoxicity further increases the risk of infection. Several studies demonstrated an association of neutropenia [148], higher grade ICANS [149], and CRS [150] with higher rates of infections. Furthermore, prolonged cytopenias can lead to a higher risk of late infections [30, 140].

Overall, most infections are bacterial or viral, while fungal infections are less common [151–155]. The majority of early infections are bacterial, while most late infections are viral and include predominantly upper respiratory tract infections [139, 154, 156].

Management and Mitigation Strategies

According to the ASCO guidelines, full blood count and blood cultures should be obtained along with other relevant diagnostic tests in the management of infections. Broad-spectrum antibiotics should be initiated according to fever and neutropenia guidelines. Antivirals should be initiated per institutional guidelines, and antifungals should be considered for high-risk patients [67]. Physicians treating patients receiving CAR-T therapy should be aware of the various risk factors and available diagnostic tests mitigate the infections early.

To prevent severe infections in patients after CAR-T therapies, determining their immune status is of utmost importance as many patients considered for CAR-T receive prior hematopoietic cell transplantation and are immunocompromised.

To mitigate the risk of infection, antiviral prophylaxis for a minimum of 60–100 days postinfusion (or longer for high-risk patients) is recommended; antibacterial and antifungal prophylaxis should be considered until complete recovery of neutrophil count [140, 157]. Furthermore, screening for certain pathogens should be done prior to CAR-T administration, as latent infections can be reactivated in patients treated with corticosteroids for CRS and ICANS. All patients should be screened for viral (e.g., human immunodeficiency virus and hepatitis B and C viruses, herpes simplex virus, varicella-zoster virus) and bacterial infections (e.g., Treponema pallidum [syphilis], Toxoplasma gondii, or Mycobacterium tuberculosis), as well as Strongyloides stercoralis, and parasites in feces; other tests can also be recommended according to the geographical area [154, 157]. In addition to prophylactic antibiotics, prophylactic granulocyte colony stimulating factor can be considered for neutropenic patients [158, 159].

Risk factors for the development of infection following CAR-T therapy include patient-related factors, i.e., baseline leukopenia, acute lymphocytic leukemia, history of infection prior to CAR-T therapy, bridging chemotherapy (lasting from the decision to treat until T-cell infusion), impaired performance status, history of prior hematopoietic cell transplant, comorbidities, and CAR-T-related factors, i.e., CAR-T-induced cytopenia, CRS, and ICANS [141, 156, 160–162]. The latter two toxicities appear as the major risk factors for infections after CAR-T therapy [139, 150, 154, 160]; grade 3 or higher CRS is a predictor of subsequent infection within 6 months after CD-19-directed CAR-T infusion [150]. Furthermore, the use of tocilizumab and steroids in patients with CRS/ICANS is likely to also contribute to a higher infection risk in such patients [156].

Clinical laboratory values such as elevated CRP levels and elevated procalcitonin levels (≥ 0.4 ng/mL) [163, 164] have been identified as being predictive for the occurrence of infections following CAR-T therapy.

As CRS and infection have overlapping clinical presentations, specific clinical laboratory values allowing for differentiation of these events are particularly useful to determine the appropriate treatment. In one study, a secondary increase in IL-6 levels in patients with CRS was associated with severe infection, and a 3-cytokine signature (IL-8, IL-1β, and IFN-γ) associated with infections was identified [165]. Furthermore, in another study, elevated IFN-γ levels in combination with low IL-1β was associated with CRS, while normal to mildly elevated IFN-γ in combination with elevated IL-1β was associated with sepsis [166].

Secondary Malignancies

CAR-T products carry a potential risk of oncogenesis owing to the genetic modifications used for their generation. To date, several cases of secondary T-cell cancers have been reported, including T-cell lymphoma and leukemia [167, 168]. Thus, the US FDA currently recommends the lifetime monitoring for the development of new cancers in patients undergoing CAR-T therapies [44]. In addition, all CAR-T products are now required to include a boxed warning of the risk of secondary cancer development [169]. Nevertheless, considering the low number of currently reported cases, the risk of developing secondary cancers following CAR-T therapy remains low and the overall benefits of these products continue to outweigh their potential risks for their approved uses [168–170].

On-Target/Off-Tumor and Off-Target/Off-Tumor Toxicities

The targets recognized by CAR-T cells are tumor-associated antigens, which may be also present in normal tissue; on-target/off-tumor toxicity is a CAR-T-mediated cytotoxicity against non-malignant tissues expressing the target antigen [171]. Such effects reported with CAR-T included liver toxicity [172], lung toxicity [173–175], sustained pyrexia and dermal toxicities [176], mucosal/cutaneous toxicities including oral mucositis, oral ulcers, gastrointestinal hemorrhage, desquamation, and pruritus, as well as acute respiratory distress [177]. In patients with hematological malignancies, the most common on-target/off-tumor toxicity is B-cell aplasia [48, 87]; however, rare cases of fatal cerebral edema and movement disorder have also been reported [178, 179].

Various strategies have been explored to mitigate these on-target/off-tumor toxicities, including the incorporation of suicide gene switches, inhibitory receptors in CAR-Ts to provide a self-regulating safety switch, intra-tumoral injection of CAR-Ts, use of reduced-affinity CAR-Ts, or use of hypoxia-inducible CAR-Ts [180].

Off-target/off-tumor toxicity, i.e., cross-reactivity with an unexpected target expressed on a healthy tissue, has mostly been observed in patients following T-cell receptor T-cell therapies but, to date, has not been reported following CAR-T therapies [181].

Conclusion

Although CAR-T therapies are associated with life-threatening toxicities, their potential benefit to patients has clearly emerged. Common toxicities such as CRS, ICANS, cytopenias, and infections result in significant morbidity but can be monitored and treated to mitigate the risk to patients. Assessment of clinical laboratory parameter values, as well as patient-related, treatment-related, and clinical risk factors appears as an effective strategy to mitigate the toxicities. These risk factors and clinical laboratory assessments can be used to select the therapies with the best benefit–risk profile, improving patient management strategies and enabling them to better tolerate these therapies and thus realize their benefits.

Supplementary Information

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Declarations

Funding

This work and the related publication were sponsored by GSK.

Conflicts of Interest

All authors are employees of GSK. J.R. and H.S. hold financial equities in GSK.

Ethics Approval

Not applicable; no data were analyzed in this opinion paper.

Consent to Participate

Not applicable.

Consent for Publication

Not applicable.

Availability of Data and Material

Data sharing is not applicable to this article, as no datasets were generated or analyzed during the current study.

Code Availability

Not applicable.

Authors’ Contribution

All authors contributed to the concept of this manuscript, and all authors reviewed the manuscript and provided final approval for publication.