Chimeric Antigen Receptor T-Cell Therapy in Acute Lymphoblastic Leukemia

Introduction

Acute lymphoblastic leukemia (ALL) is a heterogeneous group of hematologic malignancies with an annual age-adjusted incidence rate of 1.8 per 100,000 people in the United States, with an estimated 5690 new cases and 1580 deaths according to the most recent Surveillance, Epidemiology, and End Results data. Although remission rates for this malignancy are quite high and can reach up to 90%, relapse remains a challenge, especially in adult patients, and thus long-term survival may be as low as 20% in high-risk subsets. However, the recent U.S. Food and Drug Administration (FDA) approval of highly effective immunotherapies for B-lineage ALL (B-ALL), such as blinatumomab, a bispecific T-cell engager monoclonal antibody targeting CD19, inotuzumab ozogamicin, a monoclonal antibody bound to calicheamicin and targeting CD22, and a chimeric antigen receptor (CAR)-modified T-cell therapy targeting CD19, have dramatically changed the therapeutic landscape for patients with B-ALL. Compared to conventional chemotherapy in the salvage setting, which at best results in a transient remission in up to 30% of patients, remission rates up to 80% (~60% on in-to-treat) are reported for CAR therapies. Importantly, these are deep remissions, with the majority being minimal residual disease (MRD) negative. In this chapter, we will first review the basic manufacturing of CAR T-cell therapy and some of the unique considerations of the process. We will then review the data for two currently FDA-approved CAR product for B-ALL, tisagenlecleucel and brexucabtagene autoleucel, and their unique toxicity profiles. Finally, we will review the existing challenges and unmet needs of CAR T-cell therapy in ALL through a summary of CARs under investigation.

Chimeric Antigen Receptor T-Cell therapy or Production

CAR T-cells are generated from either patient-derived (autologous) or donor-derived (allogeneic) cytotoxic T-cells, which are engineered to express a CAR that redirects the T-cells to the chosen antigen. This allows the CAR T-cell to selectively target tumor cells. A CAR consists of at least one intracellular signaling domain of the T-cell receptor (TCR; CD3ζ) and a single-chain variable fragment (scFv) (now called first-generation CARs). Second-generation CARs have a costimulatory signaling domain (CD28 or 4-1BB, etc.) added to the body of first-generation CARs, which allows for dual signaling. Third-generation CARs contain two costimulatory domains, and fourth-generation CARs incorporate a third stimulatory signal by genetic engineering to induce expression of transgenic cytokines. Thus far, FDA approved two second-generation CD-19 targeted CAR T-cell products in B-ALL: tisagenlecleucel ciloceul (marketed as Kymriah) and brexucabtagene autoleucel (marketed as Tecartus). Despite clinical progress and continuous process improvements in CAR T-cell manufacturing over time, many challenges remain toward achieving standard and streamlined processes. In general, the production of CAR T-cells involves T-cell collection, enrichment and gene modification, ex vivo expansion, quality assessment, and eventually administration to the patient.

Currently, CAR T-cell therapy is offered to patients with active disease and who have had extensive prior therapy. It has been shown that the quantity and quality of naïve T-cells decrease with cumulative chemotherapy cycles. Therefore it is critical for patients to be referred promptly for CAR T-cell production as soon as this therapy is considered for the patient. The recovery period from prior therapy to leukapheresis varies across trials. Consensus guidelines from the European Society for Blood and Marrow Transplantation and the American Society for Blood and Marrow Transplantation recommend varied recovery times based on the particular procedure. These include: at least 12 weeks from allogeneic hematopoietic cell transplantation (HCT); 8 weeks from T-cell lytic agents or central nervous system (CNS) directed radiation; 4 weeks from pegylated-asparaginase or donor lymphocyte infusion; 2 weeks from systemic graft-versus-host disease (GVHD) treatment, immunomodulatory drugs, long-acting growth factors, or vincristine; 5 days from short-acting growth factors; and at least 72 hours from a nonphysiologic dose of systemic steroid treatment.

To assure the quality control of CAR T-cells, the manufacturing process is monitored through each step according to current Good Manufacturing Practices guidelines. The production starts with a collection of unstimulated leukocytes from a patient (autologous) or donor (allogeneic) through leukapheresis to obtain mononuclear cells. Postcollection washing and T-cell enrichment are required to isolate specific T-cell subpopulations of interest and remove contaminants. Subsequently, the T-cells are activated via several mechanisms, including but not limited to anti-CD3/anti-CD28 immunomagnetic beads, culturing with an anti-CD3 antibody in the presence of recombinant interleukin (IL)-2, or artificial antigen-presenting cells (APCs). The CAR is inserted into the T-cells’ genome via nonviral or viral transduction. Despite recent progress in nonviral vectors, such as plasmid-based gene delivery (Sleeping Beauty [SB] transposon system), viral transduction is still the most common and widely used method. Subsequently, to generate enough engineered T-cells, the CAR T-cell product is propagated in cell culture for optimal ex vivo expansion. Finally, the CAR T-cells are washed, concentrated, and cryopreserved in infusible cryomedia for future use. This cryopreserved product is thawed at a clinical site and administered to the patient.

Chimeric Antigen Receptor T-Cell Administration

It has been shown that T-cell depletion can induce homeostatic T-cell proliferation, and furthermore that lymphopenia-induced homeostatic T-cell proliferation leads to effective antitumor autoimmunity. Increased efficacy of CAR T-cell therapies in clinical trials with lymphodepletion before the adoptive transfer of tumor-specific T-cells may be due to the eradication of regulatory T-cells, enhanced APC activation, and/or elimination of a cytokine sink, which is created by the endogenous lymphocyte repertoire. This increases access of the transferred T-cells to homeostatic cytokines and enhances their function and antitumor efficacy. A study with an immunocompetent mouse model of B-ALL showed that CAR T-cell persistence increases with increasing conditioning chemotherapy. A clinical study in patients with metastatic melanoma also showed an improved therapeutic response to adoptive cell therapy following the intensification of lymphodepletion therapy.

Several early trials, which did not include lymphodepletion, had short CAR T-cell persistence and limited responses. In the initial CTL019 study in pediatric patients with relapsed/refractory (r/r) B-ALL, three patients did not receive lymphodepleting chemotherapy because of persistent cytopenia. Despite the initial response, one patient died following relapse at 6 weeks after CAR T-cell infusion, and two others had undetectable CTL019 at 2 and 8 months after infusion, respectively. Similarly, in the National Cancer Institute’s allogeneic CART19 trial, which treated 20 patients with progressive B-cell malignancy (i.e., B-ALL, chronic lymphocytic leukemia [CLL], and non-Hodgkin lymphoma) without lymphodepletion, the reported overall response rate was just 40%, and persistence of CAR T-cells was limited. These findings inform the standard recommendation of lymphodepleting conditioning before CAR T-cell infusion to facilitate homeostatic expansion and persistence of the CAR T-cells. Although the antecedent lymphodepleting schedule is disease oriented, cyclophosphamide-based regimens are commonly used. It has been shown that the addition of fludarabine to cyclophosphamide-based lymphodepleting regimens in r/r B-ALL improves anti-CD19 CAR T-cell persistence and disease-free survival. The regimen used in the pivotal ELIANA trial was fludarabine (30 mg/m ² intravenously [IV] daily for 4 days) and cyclophosphamide (500 mg/m ² IV daily for 2 days starting with the first dose of fludarabine), followed by tisagenlecleucel infusion 2 to 14 days after completion of the lymphodepleting chemotherapy. Similarly, Zuma-3 trial used the same a lymphodepleting chemotherapy regimen but with a different dosing schedule, consisting of fludarabine (25 mg/m ² IV for 3 days) and cyclophosphamide (900 mg/m ² for 1 day) on the last day of fludarabine, followed by brexucabtagene autoleucel infusion 2 days after completion of lymphodepleting chemotherapy. Although there is still no randomized data available, given the supportive data from clinical studies, the “global CAR-T-cell task force” recommends lymphodepletion before CAR T-cell infusion, with consideration of omitting lymphodepletion in patients with low lymphocyte counts and/or pancytopenia from disease or prior therapy.

In addition, the task force recommends following Advisory Committee on Immunization Practices guidelines in individuals with a complete response for ≥6 months. Serologic testing to determine both the need for vaccination and assessment of response postvaccination is recommended. Live vaccines at least 6 weeks before the start of lymphodepleting chemotherapy, during treatment, and until immune recovery following treatment with CAR T-cell products is prohibited.

Chimeric Antigen Receptor T-Cell Toxicities

CAR T-cell toxicity is one of the major barriers to the widespread use of this emerging cell therapy. Toxicities comprise different categories, including on-target on-tumor, such as cytokine release syndrome (CRS) and immune effector cell-associated neurotoxicity syndrome (ICANS), and on-target off-tumor, such as B-cell aplasia. The following sections address these toxicities in more detail, and adverse effects of CAR T-cells other than CRS/ICANS are summarized in Table 16.1 .