What is the CAR-T (Chimeric Antigen Receptor T-cell) product and its use in treating blood cancers, such as acute lymphoblastic leukemia (ALL) or diffuse large B-cell lymphoma (DLBCL)?

CAR-T Cell Therapy: Overview and Clinical Applications

What is CAR-T Cell Therapy?

CAR-T (Chimeric Antigen Receptor T-cell) therapy is a revolutionary "living drug" that genetically engineers a patient's own T lymphocytes to express synthetic receptors targeting specific tumor surface antigens, creating a powerful immunotherapy that has transformed outcomes in relapsed/refractory B-cell malignancies. 1, 2

Mechanism and Structure

CAR-T cells are autologous T lymphocytes genetically modified to recognize and kill tumor cells expressing specific antigens in an HLA-independent manner 1. The CAR construct comprises four essential components 1, 3:

• Extracellular antigen recognition domain: A single-chain fragment variable that mimics an antibody's antigen-binding region 1
• Hinge region: Typically derived from IgG4, CD28, or CD8a, separating the recognition domain from the transmembrane 1
• Transmembrane domain: Anchors the CAR to the T-cell membrane 1
• Intracellular signaling domains: Include CD3 zeta for T-cell activation plus costimulatory domains (CD28 or 4-1BB) that enhance activation and proliferation 1, 2

The most successful designs incorporate either CD28 (19-28z construct) or 4-1BB (CTL019) costimulatory domains, with 4-1BB potentially conferring longer CAR-T persistence 1, 2.

FDA-Approved Indications

Six CAR-T products are currently FDA-approved for hematologic malignancies 4:

B-Cell Acute Lymphoblastic Leukemia (B-ALL)

• Tisagenlecleucel (Kymriah) was the first FDA-approved CAR-T product in 2017 for pediatric and young adult B-ALL 2
• The ELIANA trial demonstrated 83% complete remission rates with all responses being MRD-negative in relapsed/refractory B-ALL 2, 5
• At 3-year follow-up, 48% of patients remained alive and relapse-free 4

Large B-Cell Lymphoma (LBCL)

• CAR-T therapy improved 4-year overall survival to 54.6% versus 46.0% with standard chemotherapy followed by stem cell transplant 4
• Approved for relapsed/refractory disease after ≥2 prior lines of therapy 5

Additional Approved Indications

• Follicular lymphoma: After ≥2 prior systemic therapies, particularly for transformed disease 5
• Mantle cell lymphoma: After prior chemoimmunotherapy 1, 5
• Chronic lymphocytic leukemia 4
• Multiple myeloma: CAR-T targeting BCMA prolonged progression-free survival to 13.3 months versus 4.4 months with standard therapy 4

Target Antigens

CD19

CD19 emerged as the ideal initial target because it is expressed on all B cells throughout development, retained on neoplastic B cells, and increased in expression on most B-cell tumors including B-ALL, chronic lymphocytic leukemia, and B-cell lymphomas 2. Currently approved CD19-targeted products recognize this antigen for B-ALL and B-cell lymphomas 1.

BCMA (B-Cell Maturation Antigen)

BCMA is expressed on mature B cells and plasma cells but not naïve B cells or other hematopoietic cells, and is elevated in multiple myeloma patients 2. Some agents like ciltacabtagene autoleucel use two llama-derived single variable domains that bind two distinct BCMA epitopes 1.

Manufacturing Process and Logistics

The CAR-T production process requires 2, 3, 5:

1. Leukapheresis: Collection of white blood cells from the patient
2. T-cell isolation and activation: Separation and stimulation of T lymphocytes
3. Genetic modification: Using viral vectors to introduce the CAR construct
4. Ex vivo expansion: Growing sufficient cell numbers over several days to weeks
5. Formulation and cryopreservation: Preparing the final product for storage and shipment

Pre-Leukapheresis Requirements

An absolute lymphocyte count (ALC) threshold of 0.2 × 10⁹/L is generally recommended, though emerging evidence supports leukapheresis in patients with lower ALC 1. Infectious disease markers must be tested on peripheral blood within 30 days of leukapheresis with results available on the day of shipment 1.

Most manufacturers require storage of fresh leukapheresis at 2-8°C before shipping, though Novartis additionally accepts locally cryopreserved starting material within 30 months 1.

Lymphodepletion Chemotherapy

Prior to CAR-T infusion, patients must undergo lymphodepletion chemotherapy—typically fludarabine and cyclophosphamide—to prevent immunologic rejection and maximize CAR-T expansion and persistence 2, 3. Omission of fludarabine is associated with inferior outcomes in B-ALL and higher risk of cell-mediated CAR-T rejection 1.

Patient Eligibility and Selection

Recommended Eligibility Criteria 1

• No age limit: Decision based on physical condition rather than chronological age, though cell collection can be limiting in infants
• Performance status: ECOG <2, Karnofsky >60%, or Lansky >60%
• Life expectancy: >6-8 weeks
• Prior allogeneic HSCT: Not a contraindication when off immunosuppression, but may increase toxicity risk in ALL 1, 5

Infection Management

Active bacterial or fungal infections are contraindications and require treatment until well-controlled before proceeding 1. Active viral infections require deferral until controlled 1. For latent infections (HBV, HCV, HIV), prophylactic antiviral treatment is required when proceeding 1.

Prior Targeted Therapies

Prior treatment with bispecific antibodies or prior CAR-T is not a contraindication, but antigen-negative escape must be excluded at relapse before proceeding, especially in B-ALL 1, 5. Emerging data suggest prior inotuzumab ozogamicin (INO) may confer inferior CAR-T outcomes due to profound B-cell depletion potentially compromising CAR-T expansion 1.

A second infusion of anti-CD19 CAR-T cells can induce remission in a subset of patients, and in multiple myeloma, re-treatment with anti-BCMA CAR-T is possible 1, 3.

Major Toxicities and Management

Cytokine Release Syndrome (CRS)

CRS is the signature toxicity of CAR-T therapy, occurring in 40-95% of patients, characterized by fever, hypotension, tachycardia, hypoxia, and potentially life-threatening organ dysfunction 1, 2, 5, 4. CRS typically occurs within the first 2 days after infusion but can manifest as early as hours to 10-15 days post-infusion 2, 5.

Tocilizumab (anti-IL-6 therapy) is the cornerstone of CRS management, dosed at 8 mg/kg IV over 1 hour, repeatable every 8 hours if no improvement, with a maximum of 3 doses 2.

Neurotoxicity (ICANS)

Neurotoxicity occurs in 15-65% of patients, typically 1-2 weeks post-infusion, with late onset possible up to a month later 3, 5, 4. Manifestations include encephalopathy, delirium, aphasia, lethargy, headache, tremor, myoclonus, seizures, and potentially fatal cerebral edema 5.

Monitoring Protocol

Patients require close monitoring with hospitalization typically recommended for adults during the acute phase (first 4 weeks) after CAR-T therapy 1, 3, 5. Monitoring includes 3:

• Laboratory assessments: Complete blood count, comprehensive metabolic panel, C-reactive protein, and ferritin levels
• Vital signs: At least every 8 hours during the acute phase
• Neurological evaluations: At least twice daily

Patients should refrain from driving or hazardous activities for at least 8 weeks following infusion 1, 3.

Factors Affecting Outcomes

Disease Burden

High disease burden before CAR-T therapy is associated with inferior event-free survival and greater toxicity 1. Specifically, >5% blasts in bone marrow at baseline confers significantly worse progression-free survival, and CD19-modulated or -negative relapse is more common with high disease burden 1.

The best outcomes are observed in patients with minimal residual disease (MRD) before CAR-T therapy 1.

CAR-T Persistence

CAR-T persistence, evidenced by ongoing engraftment and B-cell aplasia, appears important for prolonged event-free survival in B-ALL 1. Design features that may enhance persistence include 4-1BB (rather than CD28) costimulatory domains, infusion of products enriched for central memory and stem cell memory T-cell populations, shorter ex vivo manufacturing, and low-affinity CD19 binders 1.

Antigen Escape

CD19 escape is associated with high, early CAR-T expansion, typically occurs within the first 6 months after infusion, and is more common in patients with high disease burden 1. Strategies to prevent CD19 escape include dual-targeting (CD19 and CD22) CAR-T products, though this has not definitively shown superiority over single antigen-targeted approaches 1.

Accreditation Requirements

To be a CAR-T delivery site, accreditation with Foundation for the Accreditation of Cellular Therapy (FACT)-JACIE is recommended 1. Pharmaceutical providers and health service commissioners may have additional requirements 1.

Current Limitations

The manufacturing process remains complex and logistically challenging, with significant barriers including 2, 3:

• Manufacturing limitations and extended production timelines
• Toxicity concerns requiring specialized management
• Substantial financial burden limiting widespread accessibility
• No FDA-approved CAR-T therapies for solid tumors 4

Future Directions

Allogeneic "off-the-shelf" CAR-T products are under development to reduce manufacturing time and costs, potentially addressing current logistical barriers 2, 3, 5. CRISPR-Cas9 enhancement of CAR-T cells is being explored to create next-generation products by knocking out endogenous TCR genes, eliminating T-cell inhibitory receptors, and decreasing histocompatibility antigen expression 5.