Cancer Immunotherapy with Chimeric Antigen Receptor (CAR) T-Cells

Updated: Oct 20, 2017
  • Author: Sameh Gaballa, MD, MS; Chief Editor: Emmanuel C Besa, MD  more...
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Overview

Overview

Chimeric antigen receptor (CAR) T-cell therapy is an emerging cancer treatment modality in which the patients’ own immune cells are collected, genetically engineered to recognize a tumor-related target, expanded in vitro, and then reinfused to produce responses and prevent progression in a variety of malignancies (ie, adoptive cell transfer). [1] Several types of adoptive cell transfer are under investigation, but CAR T-cell therapy is the first to enter clinical practice.

Currently available techniques for producing CAR-T cells include retroviral vectors, plasmids, and gene editing with the evolving CRISPR/Cas9 (clustered regularly interspaced short palindromic repeats–CRISPR-associated protein-9 nuclease) technology. In addition, considerable research continues to enhance current efforts as well as extend this approach to many tumor types.

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CAR T-Cell Structure

CARs are synthetic proteins with 2 components: an extracellular antigen-recognizing domain, composed of fragments of monoclonal antibodies recognizing a specific protein on the surface of the malignant cells (eg, CD19 on B-cells), and an intracellular activation domain that ensures the T-cell receptor (TCR) signaling necessary to activate the effector functions of the CAR T-cell. First-generation CAR T-cells utilized an intracellular domain from the CD3 ζ-chain of the TCR, which induced cytotoxicity against targeted malignant cells but failed to support CAR T-cell expansion in vivo after reinfusion.

In contrast, second- and third-generation CAR T-cells have an additional costimulatory intracellular domain (eg,  CD2841BB, CD3z-CD28-41BB, CD3z-CD28-OX40) that enhances the CAR-T cells’ ability to proliferate, expand, and persist in vivoLymphocyte depletion further enhances the ability of CAR-T cells to proliferate in vivo.

The modified T-cells are typically reinfused into the patient 2-14 days after lymphocyte depletion with conditioning chemotherapy.  Once in the patient’s body, the cells continue to expand in number and bind to cancer cells via the engineered receptor, resulting in cell death. [2]  Persistence of CAR T-cells for as long as 3 years has been reported. [3]

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Clinical Experience

The first report of CAR T-cell therapy was in 2010, in a patient with advanced follicular lymphoma. Kochenderfer et al described dramatic regression of the lymphoma after infusion of CAR T-cells engineered to target CD19. [4]

Early studies of CAR T-cell therapy selected CD19 as the target antigen because it is expressed on the surface of almost all B-cell malignancies, including acute lymphoblastic leukemia (ALL), chronic lymphocytic leukemia (CLL), and many types of non-Hodgkin lymphoma (NHL), as well as normal B cell precursors. [1]  Not unexpectedly, bone marrow examination in the case reported by Kochenderfer et al showed selective elimination of B-cell precursors, and B cells remained absent from the blood for at least 39 weeks, whereas other blood cell counts recovered promptly. [4]

Although interaction of anti-CD19 CAR T-cells with normal B cells can result in B-cell aplasia, this condition is treatable with intravenous immunoglobulin (IVIG) replacement therapy.

Acute lymphoblastic leukemia

ALL is the most common pediatric cancer, with approximately 3,100 cases diagnosed in children and adolescents younger than 20 years each year in the United States. Eighty to 85% of pediatric ALL cases originate in B cells  [5] .

Approximately 98% of children with ALL attain complete remission (CR) with standard treatment, and 85% of patients aged 1 to 18 years with newly diagnosed ALL treated with current regimens have long-term event-free survival.  [5]  However, relapsed or refractory ALL carries a poor prognosis: with current chemotherapy, fewer than 25% of patients achieve CR, and responses typically last only 4 to 9 weeks  [3] .

In patients with B-cell ALL, early studies with CAR T-cells targeting CD19 reported CR rates of 70%-90%, and remissions were often long-lasting. [3, 6, 7] . For example, in 2014, Maude et al reported a 90% CR rate in 30 children and adults with relapsed or refractory ALL. Nineteen patients had sustained remissions beyond 2 to 3 months, suggesting continued function of the infused cells. [3] .

Three patients in CR subsequently underwent allogeneic stem cell transplantation. In the patients who did not undergo transplantation, event-free survival at median follow-up of 6 months was 67%. In patients with relapse who received salvage therapy, overall survival at 6 months was 78%. [3]  However, longer follow-up is needed, since the median follow up for this study was short.

Tisagenlecleucel

The anti-CD19 agent tisagenlecleucel (Kymriah) is the first CAR T-cell therapy to enter clinical practice. In August 2017, the US Food and Drug Administration (FDA) approved tisagenlecleucel for the treatment of patients up to 25 years of age with B-cell precursor ALL that is refractory or in second or later relapse. [8]

Approval of tisagenlecleucel was based on the results of an open-label, multicenter single-arm trial (Study B2202). Eighty-eight children and young adults were enrolled, 68 were treated, and 63 were evaluable for efficacy. Of the 63 evaluable patients, 52 responded, including 40 patients (63%) with a CR within 3 months after infusion, and 12 (19%) with a CR but with incomplete blood count recovery. Bone marrow from all patients demonstrated minimum residual disease–negative status  [9] .

The major toxicity associated with this approach is severe and potentially life-threatening cytokine release syndrome (CRS), discussed in more detail below. In clinical trials, 69% of patients with CRS related to CAR T-cell therapy had complete resolution of CRS within 2 weeks after receiving one or two doses of the anti-IL-6 monoclonal antibody tocilizumab (Actemra). [8]

In conjunction with the approval of tisagenlecleucel, the FDA also expanded the approval of tocilizumab to include the treatment of severe or life-threatening CRS resulting from CAR T-cell therapy in patients 2 years of age or older. Because of the potential for life-threatening adverse effects, the FDA approval mandates inclusion of a risk evaluation and mitigation strategy that requires special certification for hospitals and clinics as well as additional training for physicians and support staff. [8]

As with many new therapies, cost can be a significant barrier to widespread adoption, and the cost of tisagenlecleucel has been estimated at $475,000. However, Novartis, the company that makes tisagenlecleucel, is adopting an outcomes-based approach to pricing, so that the company would be reimbursed only if the patient’s condition responds by the end of the first month of treatment. [10] .

Non-Hodgkin lymphoma

The second CAR T-cell therapy to enter clinical practice, axicabtagene ciloleucel (Yescarta), was approved by the FDA in October 2017. Axicabtagene ciloleucel is indicated for use in adults with diffuse large B-cell lymphoma (DLBCL) who have not responded to or who have relapsed after at least two other kinds of treatment. Approved uses include DLBCL, primary mediastinal large B-cell lymphoma, high-grade B-cell lymphoma and DLBCL arising from follicular lymphoma. Axicabtagene ciloleucel is not indicated for the treatment of primary central nervous system lymphoma. [11]

The ZUMA-1 trial in refractory aggressive non-Hodgkin lymphoma (DLBCL, primary mediastinal B-cell lymphoma, or transformed follicular lymphoma) enrolled 111 patients, 101 of whom received axicabtagene ciloleucel. The objective response rate (ORR)—consistent across disease subtypes and other key covariates—was 82%. At a median follow up of 8.7 months, 44% of patients had responded, and 39% were in CR. [12]

Multiple myeloma

CAR T-cells that target B-cell maturation antigen (BCMA) on multiple myeloma cells have entered early clinical trials. A Chinese trial reported durable responses in 33 of 35 patients, including stringent complete responses in 14 patients. Similar results have been reported in 27% of heavily pretreated patients with multiple myeloma in a United States phase I trial. [13]

Solid tumors

Use of CAR T-cell therapy to treat solid tumors has proved challenging because of the difficulty in identifying target antigens on the surface of solid tumors that are not also expressed on normal tissues. One promising candidate is the cancer germline antigen NY-ESO-1, which is expressed in 70%–80% of synovial cell sarcomas and approximately 25% of melanomas.

A pilot study of anti–NY-ESO-1 CAR T-cell therapy reported objective clinical responses in 11 of 18 patients with NY-ESO-1+ synovial cell sarcomas (61%) and 11 of 20 patients with NY-ESO-1+ melanomas (55%). Estimated overall 3- and 5-year survival rates for patients with synovial cell sarcoma were 38% and 14%, respectively, whereas the corresponding estimated survival rates for patients with melanoma were both 33%. [14]

Other trials in solid tumors include the following:

  • Phase I/II trial of anti-mesothelin CAR T-cells for metastatic or unresectable epithelial mesotheliomas, pancreatic cancers, and other cancers that express mesothelin [15]
  • CAR T-cells directed against epidermal growth factor receptor variant III (EGFRvIII) for treatment of recurrent glioblastoma [16]
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Adverse Effects

The most serious adverse effects of CAR T-cell therapy, which can be fatal, include cytokine release syndrome (CRS), neurological toxicity, and B-cell aplasia. Researchers at the University of Texas MD Anderson Cancer Center in Houston have developed guidelines for helping clinicians recognize and manage the unique acute toxicities associated with CAR T-cell therapy, including CRS and CAR T-cell–related encephalopathy syndrome (CRES). [17, 18]

Cytokine release syndrome (CRS)

CRS results when activated CAR T-cells rapidly release large amounts of cytokines, including interleukin (IL)-6 and interferon γ, into the bloodstream. Onset of signs and symptoms typically occurs days after the CAR T-cell infusion (occasionally, weeks later), coinciding with the maximal expansion of the CAR T-cell population in the patient. The incidence and severity of CRS appear to be greater in patients with large tumor burdens, presumably because those patients experience higher levels of T-cell activation. [19]

Clinically, CRS typically presents as fever (which may exceed 40.0° C) and flulike illness (eg, fever, nausea, fatigue, headache, myalgias, malaise). With higher grades of CRS, patients develop hypotension and organ toxicity (eg, acute respiratory distress syndrome, renal failure, liver failure, cardiac dysfunction, disseminated intravascular coagulation, and encephalopathy). [2, 19]

Therapy for CRS begins with symptomatic treatment for mild manifestations. Supplemental oxygen is indicated for patients with respiratory symptoms; ventilator support may be necessary for the most severe cases. Hypotension is treated with fluids and vasopressors. [19]

Tocilizumab, which blocks IL-6 activity, has become standard therapy for severe CRS. Fever and hypotension often resolve within a few hours after administration of tocilizumab, allowing patients to be weaned off vasopressors and other supportive care measures. Patients with incomplete responses may require continued aggressive support for several days. In patients whose condition does not improve or stabilize within 24 hours after receiving tocilizumab, a second dose of tocilizumab or administration of an immunosuppressant (eg, a corticosteroid) should be considered. [19]

Cerebral edema and other neurologic toxicities (eg, confusion, seizure-like activity)

The mechanism resulting in neurologic toxicity after CAR T-cell therapy is poorly understood. Neurologic toxicities can be mild to moderate (eg, headache, confusion, dysmetria, ataxia, dysphasia, tremors), but can also be severe and life-threatening (eg, seizures, coma requiring intubation and mechanical ventilation for airway protection).

The incidence of neurologic toxicity in published reports varies from 0% to 50%. [3, 20, 21]  Moreover, neurologic events may occur independent of CRS, [3]  which suggests that neurologic toxicity results from a mechanism different from CRS. Several groups have found anti-CD19 CAR T-cells with elevated IL-6 levels in the CSF in patients who developed neurotoxicity. [3, 21, 22]  However, neurotoxicity is not exclusive to CAR T-cell therapy and has also been reported with other immunotherapies such as blinatumumab. [23]

B-cell aplasia

B-cell aplasia occurs when anti-CD19 CAR T-cells kill normal B lymphocytes that express CD19. These patients are typically are at high risk of developing infections, because of their hypogammaglobulinemia. However, this can be treated with intravenous immunoglobulin (IVIG) replacement therapy. [24]

Allergic reactions and tumor lysis syndrome

Severe allergic reactions to CAR T cells have been reported. A patient with pleural mesothelioma developed severe anaphylaxis and cardiac arrest after his third infusion of CAR T-cell therapy. He required cardiopulmonary resuscitation but fortunately recovered. [25]  Additionally, tumor lysis syndrome has been reported in patients with ALL receiving CAR T-cell therapy. [22]

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Investigational Therapies and Future Directions

The FDA currently has 76 active investigational new drug applications related to CAR T-cell products under review. [10]  Some of these products address limitations of anti-CD19 therapy, while others expand CAR T-cell therapy to other indications. For example, patients who have received CAR T-cell therapy for ALL may experience recurrent disease because of antigen loss—their leukemia cells no longer express CD19.

One approach to this problem has been CAR T-cell therapy directed against CD22, which ALL cells often over-express. In a phase I study of relapsed or refractory B-cell ALL in nine patients (age range, 7-22 y)—six of whom had experienced antigen loss after anti-CD19 therapy—anti-CD22 CAR T-cell therapy proved safe and feasible, and demonstrated clinical activity. Four patients attained a complete marrow remission, all of whom were minimal residual disease (MRD) negative. [26]

An alternative strategy to deal with antigen loss is the use of dual targeting as initial therapy. CAR T-cells that target both CD19 and CD22 are under development, [2]  and initial research suggests that use of CAR T-cells that target both CD19 and CD123, another antigen commonly found on leukemia cells, may prevent antigen loss. [27]

In summary, current CAR T-cell therapies represent a major advance in the treatment of B-cell malignancies as well as a paradigm for future efforts directed at other currently incurable tumor types. CAR T-cell use can be associated with severe and even life-threatening side effects, but these toxicities must be viewed in the context of the risk-benefit ratio for the very aggressive, refractory, and life-threatening malignancies for which CAR T-cells are currently indicated or being explored. Fortunately, with experience, these toxicities can be ameliorated. One may reasonably hope that over the next few years, new and effective applications of this exciting treatment modality will emerge.

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