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CAR T cell therapy research has developed rapidly in recent years, largely based on the remarkable efficacy of CAR T therapy in hematological malignancies, and scientists are moving quickly to apply this innovative immunotherapy to the treatment of solid tumors, including sarcomas, carcinomas, and lymphomas. Herein, we review the background of CAR T cell therapy in solid tumor treatment, common research targets, CAR T optimization strategy, and clinical research progress.


Research Background of CAR T Therapy

Chimeric antigen receptor T (CAR T) cell therapy refers to genetically modified T Cells that can recognize specific antigens on the surface of tumor cells and kill tumor cells by releasing a variety of immune factors to achieve the purpose of tumor treatment. Developing CAR T cell therapy involves the transfer of genetic material with the tumor-specific antigen recognition domain and T cell activation signal into T cells through gene modification technology.

An increasing number of clinical trials of CAR T therapy targeting solid tumor antigen targets have been carried out in recent years. According to the data provided by, there were 1,800 cell-based clinical trials in progress as of April 2022, an increase of 33% compared to last year. Among these cell-based clinical trials, those targeting solid tumors accounted for 43%, an increase of 44% compared to last year, which actually exceeded the increase in hematological malignancies (which only increased 25% from 2021). In the last year, CAR T therapies for solid tumors developed more rapidly than even for hematological malignancies - demonstrating the growing potential of the CAR T cell therapy approach.


Common Targets of CAR T Therapy for Solid Tumor Treatment

In recent years, CAR T studies targeting various solid tumor antigens have gradually increased and a considerable number have entered the early stage of clinical trials.

  • For example, CAR T therapy targeting ganglioside 2 (GD2) has achieved significant efficacy in neuroblastoma.
  • CAR T cells targeting glypican-3 (GPC3) have achieved good results in the treatment of hepatocellular carcinoma (HCC).
  • Human epidermal growth factor receptor-2 (HER2) is highly expressed in various tumor tissues such as gastric cancer and pancreatic cancer; CAR T cells designed for this target can treat advanced biliary tract cancers and pancreatic cancers.
  • Mesothelin (MSLN) is also a popular research target for immunotherapy. This glycoprotein anchored on the membrane surface is highly expressed in a variety of tumors and is closely related to tumor proliferation, invasion and poor prognosis. The results of the phase I clinical trial of CAR T cells targeting MSLN have shown significant efficacy in treated patients with pancreatic cancer.
  • The claudin (Claudin18.2, CLDN18.2), a gastric-specific membrane protein, is considered a potential therapeutic target for gastric cancer and other types.
  • It has been reported that the receptor tyrosine kinase-like orphan receptor 1 (ROR1) is specifically expressed in lung cancer and plays an important role in tumor proliferation, metastasis, and drug resistance. ROR1 is a potential therapeutic target for lung cancer based on CAR T cells.
  • The carcinoembryonic antigen (CEA) is being targeted for the treatment of lung, colon, gastric, breast, and pancreatic cancers.
  • Researchers have discovered several other antigen targets for solid tumors, including:  fibroblast activation protein (FAP) for the treatment of malignant pleural mesothelioma, prostate-specific membrane antigen (PSMA) for the treatment of prostate cancer, and more.


Challenges of Applying CAR T Therapy for Treatment of Solid Tumors

1. Lack of tumor-specific antigens

The killing activity of CAR T cells against tumor cells mainly depends on the recognition of tumor-associated antigens (TAAs), which are often highly expressed on the surface of tumor cells, but also expressed to a certain extent on other normal tissue cells. Therefore, while removing tumor cells, CAR T cells may also mistakenly attack normal tissue cells. These kinds of off-target effects may result in serious side effects from killing non-tumor cells, making a non-specific CAR T therapy unable to achieve the desired therapeutic effect.

2. Homing and infiltration of CAR T cells

Compared with hematological malignancies, it is more difficult for CAR T cells to penetrate solid tumor tissue through the blood system. On the one hand, solid tumors can perform immune evasion by interfering with T cell trafficking; on the other hand, due to chemokine-chemokine receptor mismatch, downregulation of adhesion molecules, and abnormal tumor neovasculature, it is difficult for CAR T cells to precisely navigate to the tumor. In addition, another characteristic of solid tumors is the presence of large numbers of tumor-associated fibroblasts in stromal cells. By secreting collagen, these cells form a dense fibrotic matrix that acts as a physical barrier to trap and prevent CAR T cell infiltration and attenuate their killing activity.

3. Immunosuppressive tumor microenvironment

The tumor microenvironment (TME) is a microenvironment redesigned by cancer cells to promote their growth, which is characterized by low oxygen, low nutrition, low pH, and high permeability. This environment can promote tumor proliferation but is very unfavorable for the survival of CAR T cells. In addition, the TME also contains a variety of immunosuppressive cells such as tumor-associated macrophages (TAM), regulatory T cells (Tregs), myeloid-derived suppressor cells, MDSC), etc.; these immunosuppressive cells inhibit CAR T cells through multiple pathways, significantly reducing their killing efficacy.


CAR T Cell Therapy Optimization Strategy

To address these challenges, researchers have already performed a significant amount of preparatory and forward-looking work. The main coping strategies proposed thus far include:

1. Change the structure of CAR

In order to allow CAR T cells to simultaneously express pro-inflammatory cytokines that can change the characteristics of the tumor microenvironment and to prevent the continuous expression of these factors in the body from causing toxicity, some studies have designed CAR T cells to drive cytokine production after binding to the target expression, thereby limiting the action of cytokines to the vicinity of the tumor. Other strategies, such as expressing inhibitory CAR molecules, activating CAR molecules with dual targets, and increasing the affinity of scFv to change the specificity of CAR T cells, optimize their anti-tumor activity and safety.

2. Using CRISPR-mediated gene knockout technology

Using CRISPR-mediated gene knockout technology can reduce the sensitivity of CAR T cells to the tumor microenvironment. For example, Tmunity Therapeutics uses gene editing technology to knock out the expression of endogenous T cell receptors (TCR) and PD-1 receptors in T cells, then express TCR targeting the New York esophageal squamous cell carcinoma 1 (NY-ESO-1) antigen - enabling CAR T cells to reach the solid tumor site more efficiently, thereby enhancing the tumoricidal effect of CAR T cells in the tumor microenvironment.

3. Combination therapy

BioNTech has used liposomes to deliver the vaccine encoding CLDN6 mRNA. After injection into the body, antigen-presenting cells (APCs) in the spleen, lymph nodes, etc. can present the CLDN6 antigen to the surface. These APCs can promote the proliferation of CLDN6 CAR T cells. In addition, the combination of CAR T therapy with the immune checkpoint inhibitor PD-1 has also improved the therapeutic effect of CAR T therapy on solid tumors.


Clinical Research of Novel CAR T Therapy in the Treatment of Solid Tumors

The top medical journal Nature Medicine published the latest study from the team of Carl June, the father of CAR T, which presents  a new CAR T therapy  that has also rekindled hope for their application in treatment of solid tumors. In the microenvironment of solid tumor metastatic castration-resistant prostate cancer (mCRPC), there are a variety of high-level inhibitory factors such as transforming growth factor-β (TGF-β), which can significantly reduce the efficacy of CAR T cells. The research team uses gene editing to target PSMA to inactivate the relevant receptors of CAR T cells, thereby resisting the inhibitory effect of TGF-β by blocking TGF-β signal transduction, and enhancing the antitumor ability of CAR T cells.

On the basis of the good curative effect obtained in the previous animal experiments, the therapy has undergone a phase I clinical trial. The results showed that in 13 treated mCRPC patients, CAR T cells expanded and persisted in vivo, and were significantly enriched in tumor tissue. In terms of efficacy assessment, 4 out of 13 patients had a decrease in Prostate-specific antigen (PSA) of ≥30% (Fig. 1-a); in addition, CT assessment showed that 5 patients (38.5%) maintained stable disease at 3-month radiological evaluation, and evidence of tumor regression was observed in one of the patients after cell therapy—its PSA decreased by 36% after cell infusion (Figure 1-b).


Figure 1. Pathological and radiological evaluation after CART-PSMA-TGFβRDN cell therapy[1]


This study is also the first clinical study of CAR T in the treatment of solid tumors, and the results confirm the feasibility and safety of anti-TGF-β CAR T cell therapy targeting PSMA for the treatment of solid tumors such as mCRPC, which has provided great motivation and confidence for CAR T therapy in tackling solid tumors.


Future of CAR T Cell Therapy Research: Conclusions

There are still many difficulties in the effective treatment of solid tumors with CAR T therapy, but the efforts and exploration of scientists have never stopped. An increasing number of studies are aimed at circumventing the challenges of the tumor microenvironment and the factors that affect the anti-tumor effect of CAR T. By coupling these strategies with the potential of a combination therapy approach, we believe that we will definitely see more possibilities for CAR T therapy to conquer the clinical treatment of solid tumors.


Why Choose Cyagen?

Cyagen provides one-stop cell therapy research solutions supporting CAR T and other cell therapy development programs, including CAR virus preparation, the construction of tumor immune cells and animal models, and the entire process of in vivo/in vitro drug efficacy evaluation — accelerating the development of CAR T and cell therapy research.

1. We provide one-stop in vitro and in vivo research model services

Cyagen has been committed to the establishment of gene-edited cells and animal models for over 15 years. We have an experienced team of experts and a mature technology platform, with customers in dozens of countries and regions around the world. We can provide customers with cell models and animal models that regulate the expression of target genes, as well as a series of related services, such as phenotyping, functional verification, and pathological analysis — providing convenient services for the majority of scientific researchers.

2. Mature lentivirus packaging and CAR-T cell construction platform

We have a mature and stable virus packaging platform, using the third-generation lentivirus packaging system, with high virus titer, low cytotoxicity, and no pollution. Customers have cited our lentivirus packaging and CAR T cell construction across multiple journals. We also provide CAR molecular design services for different targets to meet the individual needs of CAR T and cell therapy researchers.

3. Extensive catalog of tumor immune mouse models

We have established a catalog with thousands of cell lines and mouse models for tumor immunology research, and also offer a variety of effective evaluation models, such as CDX models, immunodeficiency mice, humanized mice of the immune system, and mouse models of homologous tumors. We can construct various subcutaneous, in situ or metastatic tumor models, and provide customized in vivo pharmacodynamics services for the corresponding models to meet your all needs. Our comprehensive cell repositories and strict breeding management system provides one-stop services for nearly any tumor immune mouse model project.

4. Standardized and personalized pharmacodynamic evaluation services

In addition to customized virus packaging and animal model capabilities, Cyagen provides standardized and personalized drug efficacy evaluation services, covering the various phenotypic analysis required in the field of cell therapy, and one-stop pharmacodynamic evaluation for CAR T cell therapy researchers.
Request a quote now. Alternatively, you can always email to inquire about our services or obtain a quote for your project.




[1] Narayan V, Barber-Rotenberg JS, Jung IY, et al. PSMA-targeting TGFβ-insensitive armored CAR T cells in metastatic castration-resistant prostate cancer: a phase 1 trial [published online ahead of print, 2022 Mar 21]. Nat Med. 2022;10.1038/s41591-022-01726-1.



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