December 1, 2023

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What are Challenges in CAR-T Cell Therapy for Cancer treatment?

What are Challenges in CAR-T Cell Therapy for Cancer treatment?

What are Challenges in CAR-T Cell Therapy for Cancer treatment?

Chemotherapy, radiation therapy, and surgery have been the traditional methods for cancer treatment, but their effectiveness is limited, and they often come with severe side effects.

Consequently, in the past decade, researchers have developed new strategies to achieve complete remission of diseases.

Currently, immunotherapy has emerged as a revolutionary approach to cancer treatment, with several types of immunotherapies available, including Adoptive Cell Therapy (ACT).

Chimeric Antigen Receptor (CAR) T-cell therapy is a form of ACT in which a patient’s own T cells are genetically engineered to express CARs to specifically target and kill cancer cells.

CAR-T cell therapy offers a chance for patients who do not respond to other first-line cancer treatments and has shown superior anti-tumor effects in the treatment of hematologic malignancies.

However, there are still many challenges to be overcome for these therapies to become first-line clinical treatments.

From a pharmaceutical perspective, this emerging technology is still classified as advanced therapy, which means that certain regulatory requirements must be met before it can be applied.

Therefore, it is essential to analyze the elements and challenges of CAR-T cell technology and consider factors from basic, clinical, and practical aspects in order to develop strategies that can make CAR-T technology an affordable treatment modality.

A Brief History of CAR-T Cell Development

Looking back at the development of CAR-T cells, we must first mention the pioneering work in 1957 when Thomas and colleagues reported the first bone marrow transplant in a leukemia patient and subsequent discoveries by Miller and others regarding the origin of T cells. However, it was not until 1986 when Steven Rosenberg reported a study on tumor-infiltrating lymphocytes (TILs) that the concept of “patient’s own immune cells can combat their own cancer” gained prominence.

In 1992, Sadelain and others successfully established a method for retrovirus-mediated gene transfer into T lymphocytes, enabling genetic modification as a means to control immunity in experimental or therapeutic settings. Almost concurrently, Zelig Eshhar and colleagues used antibody binding domains and the immunoglobulin γ or ζ subunits on T cells to engineer cytotoxic lymphocytes with specificity, leading to the development of first-generation CAR-T cells.

Five years later, Dr. Sadelain’s research group demonstrated that integrating co-stimulatory signals like CD28 into CAR-T cells enhanced their survival, proliferation, and maintenance of activity, resulting in the development of second-generation CARs. Subsequently, CAR-T cells targeting CD19 were developed and initiated Phase I clinical trials in chronic lymphocytic leukemia (CLL) and acute lymphoblastic leukemia (ALL). The trial results demonstrated the efficacy of CAR-T therapy in inducing remission in adults with chemotherapy-resistant ALL, leading to the scaling up of bioproduction.

In 2017, the FDA approved CD19 CAR-T cell therapy (Tisagenlecleucel) for children and young adults with ALL. To date, the FDA has approved six CAR-T cell therapy drugs for cancer treatment.

What are Challenges in CAR-T Cell Therapy for Cancer treatment?

Clinical Challenges of CAR-T Cell Therapy

The challenges faced by CAR-T cell therapy are primarily related to side effects, toxicity, T cell exhaustion, and the tumor microenvironment (TME).

Additionally, the manufacturing process in large-scale production is currently time-consuming and costly, making it a significant challenge to make CAR-T cell immunotherapy accessible to as many patients as possible.

What are Challenges in CAR-T Cell Therapy for Cancer treatment?

Side Effects and Toxicity

CAR-T cell therapy may have potential life-threatening toxicities following infusion. Reported side effects include fever, inflammation, elevated liver enzymes, difficulty breathing, chills, confusion, dizziness, severe nausea, vomiting, and diarrhea.

All patients experience long-term B-cell depletion, which can be mitigated by administering immunoglobulin. Toxicities mainly fall into two categories: Cytokine Release Syndrome (CRS) and Neurotoxicity (NTX) or CAR-T cell-associated Encephalopathy Syndrome (CRES).

CRS, or “cytokine storm,” is a systemic inflammatory response caused by the activation of a large number of lymphocytes (B cells, T cells, and natural killer cells) as well as myeloid cells (macrophages, dendritic cells, and monocytes), resulting in widespread clinical symptoms including fever, fatigue, headache, rash, joint pain, and muscle pain. CRS is the most common adverse reaction occurring within days of the first CAR-T cell infusion (observed in 85% of patients to any degree). Severe CRS cases are characterized by tachycardia, hypotension, pulmonary edema, cardiac dysfunction, high fever, hypoxia, renal impairment, liver failure, coagulation disorders, and irreversible organ damage. Fortunately, the effects of CRS can be mitigated by reducing the number of infused T cells and/or administering anti-IL-6 receptor monoclonal antibodies and steroids.

NTX is another common complication of CAR-T cell immunotherapy, occurring in over 40% of patients. It typically manifests within 1 to 3 weeks after CAR-T cell infusion and is often associated with CRS. Patients exhibit various symptoms such as confusion, lethargy, tremors, delirium, and difficulty finding words; other symptoms like aphasia, cranial nerve abnormalities, and seizures have also been reported.

Timely management of toxicity is crucial for reducing mortality associated with immunotherapy, and researchers have developed various safety strategies to address and prevent CAR-T cell toxicity, including the design of next-generation CARs. Toxicity management has become a key step for the successful implementation of CAR-T cell immunotherapy.

T Cell Exhaustion

Despite the high complete remission rates achieved with CAR-T cell therapy, most patients who achieve remission experience disease relapse within a few years, with relapse rates ranging from 21% to 45% in B-ALL and increasing with longer follow-up. Part of the reason for treatment failure is CAR-T cell exhaustion caused by the tumor microenvironment (TME).

CAR-T cell exhaustion refers to a dysfunctional state characterized by the loss of antigen-specific T cells due to continuous antigen stimulation, increased expression of inhibitory receptors, and inhibition of the PI3K/AKT pathway through CTLA-4 during CAR-T cell exhaustion. Cytokines also play a crucial role in this process, as exhausted CAR-T cells have reduced expression of IL-2, TNF-α, and IFN-γ secretion capabilities. Other factors such as transcription factors, metabolism, and epigenetic modifications also play a significant role in the development of CAR-T cell exhaustion.

One potential approach to mitigate exhaustion is the construction of anti-exhaustion CAR-T cells. Recent reports suggest that the discovery of certain transcription factors like TOX and NR4A, as well as the loss or overexpression of the AP-1 family transcription factor c-Jun, increases the resistance of CAR-T cells to exhaustion. Recently, CRISPR/Cas9-mediated gene editing to knock out PD-1 or the use of PD-1 blockade antibodies has been employed to enhance CAR-T therapy effectiveness and prevent exhaustion.


Tumor Microenvironment

CAR T cell immunotherapy has not yet been successful in solid tumors. One possible reason is the immunosuppressive nature of the TME, which affects the efficacy of adoptive immunotherapy. Solid tumors are highly infiltrated by stromal cells such as cancer-associated fibroblasts (CAFs) and immunosuppressive immune cells including myeloid-derived suppressor cells (MDSCs), tumor-associated macrophages (TAMs), and regulatory T cells (Tregs). These cells secrete cytokines like TGF-β and IL-10 and express surface markers like PD-L1, which inhibit the activity of effector T cells and promote the recruitment of immunosuppressive cells.

In addition to the inhibitory factors, hypoxia, nutrient deprivation, and pH imbalances in the TME also hinder CAR-T cell expansion and function. This lack of efficacy in solid tumors necessitates the development of strategies that can overcome the immunosuppressive TME.

Gene Alterations

Some reports indicate that patients treated with CAR-T cell therapy lack efficacy and experience relapses due to gene alterations. Orlando et al. integrated whole exome DNA-seq and RNA-seq to study the extent of CD19 mutations leading to relapse. They found new gene alterations in exons 2-5 of the CD19 gene in all 12 patient samples, with 8 out of 9 patients showing heterozygous deletions. The conclusion is that CD19 homozygous mutations are the primary cause of acquired resistance to CAR-T cell therapy.

Asnani et al. reported similar findings, describing exon 2 and exons 5-6 skipping in relapsed leukemia patients after CAR-T cell therapy. Exon 2 is crucial for the integrity of the CAR-T CD19 epitope, while exons 5-6 are responsible for the CD19 transmembrane domain. However, further research is needed to explore the impact of genomic analysis.

Manufacturing process challenges for CAR-T cells

Manufacturing Challenges of CAR-T Cells Traditional techniques for manufacturing CAR-T cells include:

  1. Isolating the patient’s T cells (autologous).
  2. Shipping the recovered cells to a central production facility.
  3. Genetically modifying them to express CAR.
  4. Expanding them in the lab.
  5. Sending the CAR-T cells back to the hospital for infusion into the patient.

What are Challenges in CAR-T Cell Therapy for Cancer treatment?

This traditional autologous CAR-T cell manufacturing and treatment approach increases the complexity for clinical practitioners and patients. Today, this therapy brings several major manufacturing challenges, including:

Packaging, Transport, and Storage of CAR-T Cells

Clinical CAR-T cell manufacturing is currently a complex process involving multiple steps, spanning different geographical locations, using various technologies and logistics. Any errors in timing, transport methods, cold chain, or storage can lead to cell damage, directly affecting treatment efficacy.

Therefore, every step requires careful management, precise sample tracking, and sufficient preservation techniques for freezing patient samples. Different temperatures are required for transportation throughout the CAR-T cell manufacturing process, so low-temperature storage during production must ensure quality control.

Good Manufacturing Practice (GMP)

CAR-T cell production is a complex preparation process, and cGMP is crucial and a bottleneck for CAR-T cell manufacturing. cGMP’s purpose is to provide a framework that ensures trained and regularly trained personnel carry out high-quality production in well-controlled facilities and equipment. It also provides rigorous documentation processes covering all operational aspects to demonstrate continuous and full compliance.

According to the International Organization for Standardization (ISO), CAR-T cell manufacturing requires GMP facilities as cell processing cleanrooms and must be equipped with 1) facility systems (such as air handlers, 24/7 alarm monitoring systems); 2) environmental monitoring equipment (such as particle counters); 3) manufacturing process equipment (such as cell washers, bioreactors); and 4) analytical equipment (such as automated cell counters, flow cytometers).

Another key factor in maintaining a GMP-compliant production environment is highly skilled personnel with extensive knowledge of GMP production, quality control, and quality assurance.

Production of Lentiviral Vectors (LVs)

The production of LVs faces various challenges, including their inherent cytotoxicity, low stability, and dependence on transient transfection for impact. In addition, upstream and downstream processes have low yields and high cost-effectiveness. Successful commercial products in this area have partly established standardized and stable cell lines to produce GMP-compliant LVs, making amplification, reproducibility, biosafety, and cost-effectiveness more accessible.

Staffing and Training

Given the complexity of the therapy and its associated high-risk side effects, the use of CAR-T cells is highly regulated and can only be used in certified centers and managed by trained personnel. All employees involved in CAR-T cell manufacturing, from T cell collection to manufacturing, and to the clinical unit, require extensive training to reach satisfactory skill levels. This capability enables the management of complex situations that may arise during the process, ensuring product delivery.

Today, there are only a few qualified professionals in this field, requiring multidisciplinary collaboration and communication to generate more knowledge in this area. Academic involvement is also crucial.

Quality Control

As a living “drug,” CAR-T cells have a complex preparation process that requires “end-to-end quality control.” During the production process, well-controlled cold chain transport and storage play a vital role in ensuring cell product quality and preventing bacterial and mycoplasma contamination. CAR-T cell quality control requirements include checking whether the T cells transduced in vitro exhibit virus replication and residual production materials.

Additionally, considering CAR-T cells as biologic, cell, and gene therapy products, final product release tests should be included to confirm their characteristics, purity, safety, and efficacy.

Moreover, stability studies are needed to validate storage conditions and shelf life. The generation of CAR-T cells requires further research to evaluate the quality of T cells for relapse and re-infusion of patients. These studies should provide data on lymphocyte subset distribution. In summary, quality control is crucial for the success of CAR-T therapy.

Production Scaling

The manufacturing of CAR-T cells should be scalable, meaning multiple single bioreactors are available per patient to expand the reach to patients without sacrificing product quality and repeatability. Personalized therapies, such as autologous cell therapy, require more sophisticated scaling than conventional biopharmaceuticals. This means having multiple bioreactors to scale up CAR-T cells for each patient. Additionally, this relies on the ability to parallelize multiple independent products.

Production Time and Repeat Dosing

CAR-T cell manufacturing can take up to four weeks, during which time patients are at risk of disease progression and death. Furthermore, CAR-T cell manufacturing does not allow for volume scaling, meaning cells must be prepared as single batches, limiting the quantity of available products. In such cases, patients may not have the opportunity for quick and convenient CAR-T cell infusions.

Pricing and Accessibility

Pricing and patient accessibility are the primary constraints to the widespread use of CAR-T cell therapy worldwide. The current CAR-T cell manufacturing model is highly centralized, and each step of the process is complex, resulting in average costs ranging from $373,000 to $475,000 per treatment (hospital-associated costs related to therapy not included in these average costs). Both patients and healthcare systems cannot afford this prohibitive cost.

This daunting cost limits patients’ access to treatment, especially in economically underdeveloped countries where it is unsustainable. This further restricts the widespread application of CAR-T cell therapy. Until CAR-T cell therapy becomes economically viable, its full treatment potential cannot be realized.

Regulatory Requirements

Another critical bottleneck for cell products is regulation. CAR-T cells are considered advanced therapy medicinal products (ATMPs) worldwide, which require licensing. Regulatory agencies are highly aligned with standard therapies, but cell products have specific requirements. US or EU regulatory agencies are working to define optimal guidelines for coordinating ATMP clinical manufacturing requirements worldwide. Meanwhile, less developed countries face greater challenges because the clinical use of CAR-T therapy is highly restricted, leading to a lack of understanding of regulatory requirements among authorities.

Strategies to Enhance CAR-T Cell Technology Discovery of New Biomarkers

Biomarkers are of great significance in clinical cancer treatment as they can be used to identify patients suitable for CAR-T therapy, predict prognosis, predict treatment response, and monitor disease progression. The first biomarker for CAR-T therapy was CD19, a B-cell surface protein predominantly expressed on malignant B-cells.

Currently, different biomarkers are being sought at various stages of immunotherapy development. T-cell activation markers, cytokine and chemokine profiles, and immune checkpoint marker expressions are some of the studied markers. These markers can help identify the immune response to CAR-T cell infusion and track the persistence of CAR-T cells.

Among these markers, soluble interleukin-2 receptor (sIL-2R) and serum ferritin levels are also being investigated as potential predictors of CAR-T cell expansion and cytokine release syndrome (CRS) severity.

Research into new biomarkers is ongoing, with the goal of improving the selectivity of CAR-T cells and their safety and efficacy, as well as minimizing adverse effects. The ultimate goal is to develop personalized and targeted treatment regimens based on the unique characteristics of each patient’s cancer.

Combination Therapies

CAR-T cell therapy can be used in combination with other treatments to enhance its effectiveness and reduce the risk of relapse. Some potential combination therapies include:

  1. Checkpoint Inhibitors: Immune checkpoint inhibitors, such as PD-1 inhibitors, can be combined with CAR-T cell therapy to enhance the persistence and function of CAR-T cells by blocking inhibitory signals. This combination has shown promise in preclinical and early clinical studies.

  2. Targeted Therapies: Targeted therapies that inhibit specific signaling pathways in cancer cells can be used in combination with CAR-T cell therapy to increase the susceptibility of cancer cells to CAR-T cell killing.

  3. Chemotherapy or Radiation: Conventional cancer treatments like chemotherapy or radiation therapy can be used to reduce the tumor burden before CAR-T cell infusion, making it easier for CAR-T cells to eliminate the remaining cancer cells.

  4. Cytokines: Certain cytokines, such as interleukin-2 (IL-2) and interleukin-15 (IL-15), can be administered along with CAR-T cell therapy to support CAR-T cell expansion and function.

  5. BiTEs: Bispecific T-cell engagers (BiTEs) are antibodies that can redirect a patient’s own T cells to target cancer cells. Combining CAR-T cell therapy with BiTEs can enhance the antitumor immune response.

  6. Oncolytic Viruses: Oncolytic viruses are viruses that selectively infect and kill cancer cells. They can be used in combination with CAR-T cell therapy to create a synergistic antitumor effect.

  7. Vaccines: Therapeutic cancer vaccines can stimulate the patient’s immune system to recognize and attack cancer cells. Combining CAR-T cell therapy with cancer vaccines may enhance the immune response against cancer.

The choice of combination therapy depends on the specific type of cancer, the patient’s condition, and the stage of disease. Clinical trials are ongoing to evaluate the safety and efficacy of various combination therapies with CAR-T cell therapy.

Improving CAR-T Cell Design

Researchers are continually working to improve the design of CAR-T cells to enhance their effectiveness and safety. Some strategies for improving CAR-T cell design include:

  1. Dual CARs: Dual CAR-T cells are engineered to express two different CAR molecules on their surface, each targeting a distinct antigen on cancer cells. This approach can reduce the risk of antigen escape and improve tumor targeting.

  2. Switchable CARs: Switchable CAR-T cells are designed to have their activity controlled by an external stimulus, such as a small molecule or antibody. This allows for tighter control over CAR-T cell activity and reduces the risk of off-target effects.

  3. Armored CAR-T Cells: Armored CAR-T cells are engineered to secrete specific cytokines or other molecules that can enhance their persistence and antitumor activity within the tumor microenvironment.

  4. Memory CAR-T Cells: Memory CAR-T cells are designed to have a longer-lasting presence in the body, providing sustained protection against cancer recurrence.

  5. Universal CAR-T Cells: Universal CAR-T cells are generated from healthy donors and designed to be compatible with multiple patients, reducing the need for individualized manufacturing.

  6. Safety Switches: CAR-T cells can be equipped with safety switches that allow for their rapid elimination if severe side effects occur, providing an additional layer of safety.

  7. Tumor Microenvironment Targeting: CAR-T cells can be engineered to express receptors that specifically recognize and target components of the tumor microenvironment, such as fibroblasts or immune-suppressive cells.

  8. Reducing Cytokine Release Syndrome (CRS): Strategies to mitigate CRS, such as controlling CAR-T cell activation or using drugs like tocilizumab, are being explored to improve the safety profile of CAR-T cell therapy.

These design improvements aim to make CAR-T cell therapy more effective, versatile, and adaptable to different types of cancer and patient populations.

Reducing Toxicity

One of the major challenges of CAR-T cell therapy is managing and reducing treatment-related toxicities, such as cytokine release syndrome (CRS) and neurotoxicity. Strategies to reduce toxicity include:

  1. Fine-Tuning CAR-T Cell Activation: Modulating the activation threshold of CAR-T cells to prevent excessive cytokine release and reduce CRS.

  2. Cytokine Blockers: The use of drugs like tocilizumab and siltuximab to block specific cytokines (e.g., IL-6) responsible for CRS.

  3. Safety Switches: Incorporating safety switches in CAR-T cells that allow for their rapid elimination in case of severe side effects.

  4. Neurotoxicity Management: Developing strategies to manage and mitigate neurotoxicity, which can include supportive care and medications.

  5. Patient Monitoring: Regular monitoring of patients receiving CAR-T cell therapy to detect and address toxicities early.

  6. Personalized Treatment: Tailoring the CAR-T cell therapy regimen to the individual patient’s risk factors and clinical presentation to minimize toxicity.

  7. Combination Therapies: Combining CAR-T cell therapy with other treatments, such as checkpoint inhibitors, to modulate the immune response and reduce toxicity.

Optimizing Manufacturing Processes

Efforts to optimize CAR-T cell manufacturing processes include:

  1. Automation: Implementing automated systems and robotics to streamline and standardize CAR-T cell production, reducing the risk of human error.

  2. Closed Systems: Developing closed manufacturing systems that minimize the risk of contamination and improve product consistency.

  3. Scalability: Designing manufacturing processes that are easily scalable to meet the growing demand for CAR-T cell therapies.

  4. Cost Reduction: Identifying cost-effective manufacturing methods to make CAR-T cell therapy more affordable and accessible.

  5. Shortening Production Time: Reducing the time required for CAR-T cell manufacturing to improve patient access and reduce disease progression.

  6. Cryopreservation Techniques: Developing improved cryopreservation methods to extend the shelf life of CAR-T cell products.

  7. Quality Control and Assurance: Implementing rigorous quality control and assurance measures throughout the manufacturing process to ensure product safety and efficacy.

Increasing Access and Reducing Costs

Addressing the high cost and limited accessibility of CAR-T cell therapy is a significant challenge. Strategies to increase access and reduce costs include:

  1. Pooled Manufacturing: Exploring the feasibility of manufacturing CAR-T cell products in batches to reduce costs and improve efficiency.

  2. Price Negotiation: Engaging in price negotiations with pharmaceutical companies to lower the cost of CAR-T cell therapies.

  3. Health Insurance Coverage: Advocating for broader health insurance coverage of CAR-T cell therapy to reduce the financial burden on patients.

  4. Government Support: Seeking government funding and support for CAR-T cell therapy research, development, and manufacturing.

  5. Academic and Industry Collaboration: Promoting collaboration between academic institutions and industry partners to accelerate research and development and reduce costs.

  6. Developing Off-the-Shelf CAR-T Cells: Researching and developing off-the-shelf CAR-T cell products that can be readily available, reducing the need for individualized manufacturing.

  7. Global Manufacturing Hubs: Establishing regional or global manufacturing hubs to centralize CAR-T cell production and reduce costs through economies of scale.

  8. Clinical Trial Expansion: Expanding clinical trials to include a broader range of patients and indications to gather more data on safety and efficacy.

  9. Value-Based Pricing: Exploring value-based pricing models that tie the cost of CAR-T cell therapy to its demonstrated clinical outcomes.

Overall, these strategies aim to make CAR-T cell therapy more accessible to a wider patient population while reducing the financial burden on patients and healthcare systems.

In conclusion, CAR-T cell therapy is a promising and rapidly evolving field with ongoing research aimed at improving its safety, efficacy, and accessibility.

Researchers are exploring various avenues, including biomarker development, combination therapies, CAR-T cell design improvements, toxicity management, manufacturing optimization, and cost reduction strategies to advance the field and provide better treatment options for patients with cancer.

Additionally, collaboration between academia, industry, healthcare providers, and policymakers will play a crucial role in shaping the future of CAR-T cell therapy



1. Chimeric Antigen Receptor-T Cells: Pharmaceutical Field. Prepharmacology. 2021;12:720692.

(source:internet, reference only)

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