By Flávia Sousa, Mitra Mosharraf, Yahya Choonara, Vaibhav Patel
What is CAR-T cell therapy?
In recent years, engineering a patient's own T cells to express a chimeric antigen receptor (CAR) specific to a tumor antigen (TA) has emerged as a groundbreaking approach for treating previously untreatable hematologic cancers[1]. This therapy is based on the CAR binding to antigens triggering a robust T cell activation and a potent anti-tumor response. This promising therapy was first introduced to the market in 2017 with the approval of tisagenlecleucel, the first CAR T cell therapy approved by the FDA, for the treatment of pediatric and young adult acute lymphoblastic leukemia [2].
The primary advantage of CAR-T cell therapy lies in its customization for each individual patient. T cells are harvested from the patient, genetically modified in the laboratory to express CARs on their surface, and then reintroduced into the patient to target and destroy cancer cells (Figure 1). Due to its high efficacy in hematological malignancies, CAR-T cell-based therapies have generated significant interest for application in solid tumors.
Figure 1. Summary of the CAR-T manufacturing and treatment process. CAR=chimeric antigen receptor; CRS=cytokine release syndrome; ICANS=immune effector cell associated neurotoxicity syndrome; WBC=white blood cells. Reprinted with permission from [3].
However, CAR-T cell therapies for the treatment of glioblastoma multiforme (GBM) remain ineffective. The limitations include: (i) the immunosuppressive tumor microenvironment (TME), (ii) insufficient trafficking and infiltration of CAR-T cells into the tumor, and (iii) associated toxicities such as cytokine release syndrome [4]. Further research is required to overcome these challenges and extend the success of CAR-T cell therapies to a broader range of cancers.
The market and current players
Currently, six FDA-approved CAR-T cell therapies are available in the U.S. market, all of which are autologous and used for treating various blood cancers. Table 1 summarizes these therapies.
Table 1. FDA Approved CAR-T Cell Immunotherapy Products
These products are manufactured by major market players; Novartis AG (manufacturer of Kymriah (tisagenlecleucel), the first FDA approved CAR-T cell therapy), Bristol-Myers Squibb (manufacturer of Breyanzi (lisocabtagene maraleucel) and Abecma (idecabtagene vicleucel) for treating large B-cell lymphoma and multiple myeloma respectively. Gilead Sciences (Kite Pharma), producer of Yescarta (axicabtagene ciloleucel) and Tecartus (brexucabtagene autoleucel), both target various forms of B-cell lymphoma. Johnson & Johnson and its subsidiary Janssen, which developed Carvykti (ciltacabtagene autoleucel) for multiple myeloma.
In addition to these marketed products, there are 1,439 global clinical trials investigating CAR-T cell therapies [5]. Using the data extracted from ClinicalTrials.Gov, the percentages of CART-Cell Therapies in various stages of development ( Early Phase 1 (Early-P1),Phase 1 (P1), Phase 2 (P2), Phase 3 (P3), and Phase 4 (P4) were calculated and compared in Figure 2. These values were 11%, 53%, 28%, 2% and 0.2%, respectively in addition to 6% that were classified under “others”.
Figure 2. The breakdown of global clinical trials on CAR-T cell therapies in 2024 (It was generated using the data extracted from ClinicalTrials.gov on Aug/1/2024 and determining the percentage of CAR-T-Cell therapies in clinical trials at each stage of development, Early Phase 1 (Early-P1), Phase 2 (P2), Phase 3 (P3), Phase 4 (P4) and other studies (Others). The global market for CAR-T cell therapies is experiencing robust growth, with a compound annual growth rate (CAGR) of 29.8%. It is projected to reach USD 80.52 billion by 2032, up from USD 8.44 billion in 2023 [6]. CAR-T cell therapies are known for their high cost (about USD 400,000 per treatment) and lengthy manufacturing time [7]. Emerging trends aim to transition to allogeneic cells and minimally synthetic cells to reduce both manufacturing time and costs.
Regulation requirements
In the United States, CAR-T cell therapies are regulated by the Food and Drug Administration (FDA) under the biologics category. The regulatory process starts with preclinical testing, where the therapy's safety and efficacy are assessed in vitro and in animal models. Successful preclinical results lead to the submission of an Investigational New Drug (IND) application, which must be approved before human clinical trials can begin.
Following successful clinical trials, developers submit a Biologics License Application (BLA) to the FDA, including all relevant data demonstrating the therapy's safety, efficacy, and quality. The FDA reviews this application, and if the therapy meets the necessary standards, it is approved for market release. Phase 4 (P4) Post-market surveillance is essential to monitor the long-term safety and efficacy of CAR-T therapies. The FDA may require additional studies and implement Risk Evaluation and Mitigation Strategies (REMS) to manage specific risks.
The FDA has issued a series of guidelines specifically addressing cellular and gene therapies [8]. The FDA's recent guidance on consideration for the development of CAR-T cell product provides comprehensive and detailed recommendations on the chemistry, manufacturing, and control (CMC) aspects, as well as the nonclinical and clinical study designs required for the development of CAR-T cell products, with a particular focus on oncology applications.
Critical considerations in developing CAR-T cell products include the precise design of the CAR construct, the careful selection of the vector, and the stringent quality control of the cellular starting material. The CAR construct must be engineered to target specific antigens while minimizing immunogenicity, and the vector must efficiently deliver the gene with minimal risk of adverse events. The cellular starting material, usually derived from leukapheresis, must be meticulously handled and processed to preserve the product's integrity and quality.
Given the complexity of CAR-T cell manufacturing, which involves multiple steps with potential variability, robust process controls and in-process testing are essential to ensure consistency across product lots. Managing changes in the manufacturing process and assessing product comparability are crucial as these therapies transition from clinical development to commercialization.
Nonclinical evaluation is a critical phase in determining the safety of CAR-T cells before clinical trials can begin. This phase includes thorough assessments of potential off-target effects and the behavior of CAR-T cells in vivo. Early-phase clinical trials are designed to establish the safety of CAR-T cell products, determine optimal dosing, and gather initial efficacy data. The guidance also highlights specific considerations for autologous and allogeneic CAR-T cell products, particularly the unique challenges they pose in manufacturing and clinical application.
Expedited pathways, such as Breakthrough Therapy Designation, Accelerated Approval, RMAT and Priority Review, are available to accelerate the development and approval process for therapies addressing serious or life-threatening conditions. These frameworks ensure that CAR-T cell therapies are safe, effective, and accessible to patients in need.
Challenges and strategies to overcome
Despite the considerable advantages of CAR-T cell therapy, this treatment faces several limitations, including CAR-T cell toxicity, antigen escape, inadequate persistence, challenges in trafficking and tumor infiltration, and the presence of an immunosuppressive tumor microenvironment [9]. The most notable and frequently occurring toxicities associated with CAR-T cell therapy are cytokine release syndrome (CRS) and neurological toxicities [10]. These adverse effects typically arise due to unexpected cross-reactivity with proteins that are not expressed on tumor cells, as well as from the cytokines released by the infused CAR-T cells. Both types of toxicities pose significant challenges in the clinical application of CAR-T cell therapy and necessitate careful management and monitoring. To address these toxicities, researchers are modifying the CAR structure to mitigate toxicity by altering the CAR antigen-binding domain affinity and creating CAR “off-switches” to be used in combination with other therapies [11].
Another significant limitation is antigen escape, where tumors develop resistance to the single antigen-targeting CAR construct [12]. This phenomenon occurs when tumor cells downregulate or alter the targeted antigen, rendering the CAR-T cells ineffective. To address this challenge, researchers have been developing CAR constructs that target multiple antigens simultaneously, using dual or tandem CARs. By targeting multiple antigens, these advanced CAR constructs can mitigate the risk of antigen escape, leading to more robust and sustained antitumor responses compared to single-target therapies [13].
Compared to hematological malignancies, CAR-T cell therapy demonstrates limited efficacy for solid tumors. This limitation is primarily due to the inability of CAR-T cells to effectively penetrate and infiltrate solid tumors, which is hindered by physical tumor barriers and an immunosuppressive microenvironment [9]. To address this challenge, the most effective strategy has been the local delivery of CAR-T cells instead of systemic administration. Local delivery enhances CAR-T cell infiltration at the tumor site and minimizes off-target toxicities. However, this approach is not feasible for all tumor types. In cases where local delivery is impractical, researchers are engineering CAR-T cells to express chemokine receptors[14]. These receptors enable CAR-T cells to respond to immunostimulatory signals from tumor-derived chemokines, thereby promoting an anti-tumor response.
Lastly, the immunosuppressive microenvironment remains a significant obstacle. This challenge can be mitigated through combination therapies with other immunotherapies, such as immune checkpoint inhibitors or nanovaccines, which enhance the overall effectiveness of CAR-T cell therapy by overcoming the immunosuppressive conditions within the tumor microenvironment[15].
Combination therapy as a future perspective
Cancer vaccines, cancer nanovaccines, cytokines, immune checkpoint inhibitors, and oncolytic viruses have been used in conjunction with CAR-T cell therapies to improve anti-tumor efficacy, expand clinical applications, and limit toxicities, as discussed in the previous chapter. Cancer vaccines and nanovaccines assist CAR-T cells in precisely targeting cancer cells by presenting tumor-associated epitopes and inducing an adaptive immune response. Cytokines have been utilized to transform the tumor microenvironment from an anti-inflammatory to a pro-inflammatory state, thereby enhancing the effectiveness of CAR-T cells.
Immune checkpoint inhibitors have garnered significant attention recently due to their ability to ensure sustained T cell persistence and function. Additionally, oncolytic viruses have been shown to increase the efficacy of CAR-T cells by promoting tumor infiltration, dendritic cell maturation, and M1 macrophage polarization.
The combination of these therapies with CAR-T cell treatments represents a forward-looking approach in cancer immunotherapy. By leveraging the synergistic effects of these adjunct therapies, it is possible to develop more effective and comprehensive cancer treatment regimens, thereby maximizing therapeutic benefits and minimizing adverse effects.
Conclusion
CAR-T cell therapy has significantly changed clinical oncology practice to treat relapsed or refractory cancers with several CAR-T cell therapies approved by the FDA. Despite the positive treatment outcomes, many patients still do not benefit from CAR-T cell therapy due to varying response rates and high rates of relapse. Hence molecular engineers continue to focus on producing the next generation of CAR-T cell therapies that can overcome neurotoxicity, tumor antigen heterogeneity, poor cell trafficking, CAR-T cell exhaustion, Cytokine Release Syndrome, and reduced cytotoxicity at the tumor site. CAR-T cell therapies have also been limited clinically due to production complexity and high cost, rendering it a niche market with only a few key companies having immunotherapy programs and products marketed mainly to treat blood cancers. The complexity of developing, manufacturing, and evaluating CAR-T cell products has been acknowledged by regulatory authorities. Measures to mitigate product variability for autologous or allogeneic cell therapies are included in the latest regulatory guidelines on chemical stability, manufacturing, pharmacology, toxicology, and the design of clinical studies. Non-clinical testing of CAR-T cell products remains a challenge due to the limited availability of pre-clinical animal models that can serve as a surrogate to assess product safety and efficacy as well as the risks of graft versus host disease. Off-the-shelf CAR-T cell therapies, leaner manufacturing processes, and combination therapy with mRNA vaccines, oncolytic viruses, and insertable bio-scaffolds are promising developments to make CAR-T cell therapy more clinically meaningful, widely available, and affordable in the future.
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