Challenges of Expanding CAR-T Cell Therapy into Solid Tumors

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The development of CAR-T therapies for solid tumor cancers presents a myriad of challenges.

Since the first FDA approval in 2017, chimeric antigen receptor T-cell (CAR-T) therapy has garnered excitement as a potentially curative option for hard-to-treat blood cancers (1). In this therapy, T cells are genetically engineered to recognize specific tumor-surface antigens. When the engineered T cells encounter the target antigen, they proliferate and coordinate an immune response to destroy the cells expressing that antigen. There are six approved CAR-T therapies in the United States that target blood cancers. These “autologous” treatments are manufactured from T cells obtained from the patient and are genetically engineered to express a CAR that recognizes an antigen specific to the patient’s blood cancer.

When CAR-T therapy works well, it puts patients with malignant, lethal blood cancers into long-term remission. However, approved CAR-T therapies remain ineffective for many patients, and no CAR-T therapies have been successfully developed for solid tumors (2). The high cost, clinical complexity, and dangerous side effects of existing CAR-T therapies have further limited the application of an otherwise exciting and novel treatment.

Significant obstacles remain before CAR-T therapies can be adopted to treat solid tumors. Persistent efforts to expand CAR-T therapies to solid tumors are focused on identifying solid tumor-specific antigens for CAR T to target, enabling the therapy to penetrate solid tumors and ensuring that the therapy persists and functions in the immunosuppressive microenvironment of the solid tumor.

Selecting the right target

Because the immune system uses antigens to identify targets, antigens are a key factor in the development of CAR-T cell therapies. To minimize off-target effects, an appropriate antigen target for a CAR-T therapy must be present on cancer cells but not significantly on healthy cells. If a patient receives CAR-T therapy that targets an antigen present on both tumor cells and healthy tissue, the subsequent immune response may attack normal cells and has the potential to generate difficult-to-manage serious side effects.

All FDA-approved CAR-T therapies target either CD19 or B-cell maturation antigen, which are antigens associated with cell development that are highly expressed on blood cancer cells but are only limitedly expressed on healthy cells. While CAR-T therapy for blood cancers has a few clear antigen targets, identifying antigen targets for solid tumors has proved much more complicated.

Solid tumors have a wider array of potential targets than blood cancers, and the antigens expressed on solid tumors are often also found at low levels on normal cells. The overlap in antigen expression between tumor cells and healthy tissue has led to severe toxicity in early solid-tumor CAR-T therapy clinical trials, despite promising results in earlier animal models. Along with the judicious selection of antigen targets, an important parameter for the clinical success of CAR-T therapy will be the development of animal models that better represent the human immune system and the differential expression of the target antigen.

The challenge of selecting a target antigen is compounded by the heterogeneity of antigen expression in solid tumors. This heterogeneity is likely one important reason for the diminished initial and durable CAR T response (3). Heterogeneous expression means that some tumor cells may express an antigen more strongly than others, and some may not express that antigen at all. A CAR-T therapy targeting a cancer-specific antigen that was heterogeneously expressed would miss tumor cells not expressing that antigen and would be less effective on cells weakly expressing that target.

One proposed approach to circumvent the lack of cancer-specific, consistently expressed antigens on solid tumors is engineering CAR T that can identify multiple targets. This could be achieved by genetically engineering multiple CARs into the same T cell or engineering a receptor that would recognize more than one target antigen. However, multi-target CAR Ts can carry an elevated risk of severe side effects from off-target effects (4).

Individual antigens for targeting solid tumors are also under investigation. Cancers sometimes express embryonic genes, such as CLDN6, which are silenced after birth (5). CLDN6, which enables cancers to metastasize, is present in most testicular cancers, as well as in some ovarian, non-small cell lung, gastric, breast, and endometrial cancers. Still, targeting CLDN6 carries some danger of off-target effects, as the protein is expressed at low levels in the pancreas and liver, and a protein with a similar structure, CLDN9, is expressed throughout the body.


A 2022 Phase I/IIa trial of CLDN6-directed CAR-T therapy, called BNT211-01, displayed one of the first examples of CAR T efficacy in solid tumors (6). Initial trial results reported tumor shrinkage in 33% of 21 patients with seven different tumor types, including one testicular cancer patient who went into complete remission. Two additional solid-tumor targets have also shown preliminary promise in clinical trials: a study treating glioblastoma with interleukin 13 receptor subunit alpha 2 (IL13Ra2)-directed CAR-T cells and CAR-T cells targeting prostate stem cell antigen for prostate cancer (7,8).

CAR-T strategies in solid tumors

To attack solid tumors, CAR-T cells must first migrate from blood vessels to the tumor tissue. Then, they must be able to infiltrate and proliferate in the tumor microenvironment. This cell movement is challenging for a number of reasons. For instance, physical barriers, including the extracellular matrix and the vasculature surrounding a solid tumor, make it difficult for CAR-T therapies to penetrate the tumor. Once CAR-T cells penetrate the tumor, the therapy must overcome an immunosuppressive tumor microenvironment that impairs its function and efficacy.

Many strategies for migrating to and functioning within the solid tumor microenvironment are in development, and many more have been proposed. Several of these strategies aim to augment CAR-T function by arming CAR-T cells with additional molecules or combining them with an additional therapy.

For example, boosting expression of some cytokines could make it easier for CAR-T cells to infiltrate solid tumors (9). Cytokines—small proteins secreted by certain immune cells—can enhance tumor-destroying activities of other immune cells, including T cells. “Armored” CAR-T cells, engineered with specific cytokine receptors or engineered to express stimulatory cytokines, have the potential to improve CAR-T therapy’s efficacy (10).

Other proposed strategies for enhancing CAR-T function include engineering CAR-T cells to be insensitive to immunosuppressive conditions and combining CAR-T therapy with immune checkpoint inhibitors (2). However, many of these approaches raise the risk of cytokine release syndrome, a toxic and potentially lethal side effect of CAR-T therapy resulting from excessive inflammatory cytokines in the circulatory system.

One combination therapy approach that has already demonstrated promise in early clinical trials is a messenger RNA (mRNA) vaccine that enhances the proliferation of CAR-T cells. The mRNA vaccine leads to target antigen expression on T-cell-activating immune cells (i.e., antigen-presenting cells). The resulting cell proliferation helps the therapy persist in the immunosuppressive solid-tumor environment and allows the therapy to be administered at lower initial doses. The clinical trial investigating CLDN6-directed CAR-T therapy, mentioned previously, used this strategy in a subset of patients (6). In the preliminary study, 43% of patients treated with a combination mRNA vaccine and CAR-T therapy experienced tumor shrinkage. Still, the efficacy and safety of this method need to be tested and validated in additional trials—approximately 40% of patients treated in the CLDN6 study developed manageable cytokine release syndrome.

History repeats

Many of the challenges facing CAR-T therapy for solid tumors remain unresolved. However, if history is a guide, these obstacles are not insurmountable. Blood cancers are often selected for development of novel therapies because tumor cells circulating in the blood are easier to access than tumor cells in tissue, and treatments are easier to deliver. Treatment options, such as chemotherapy and stem-cell transplant, were first developed for blood-based cancers and then were modified to treat solid tumors. A number of recent advancements—including the identification of solid-tumor-specific antigens, the development of armored CAR-T cells, and mRNA combination therapies—are bringing this targeted immunotherapy for solid tumors closer to reality.


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About the authors

Brian Huber is vice-president of Therapeutic Areas, Drug Development, and Consulting, and Tamie Joeckel is global business lead of the Cell and Gene Therapy Group; both at ICON.

Article details

Pharmaceutical Technology
Vol. 47, No. 5
May 2023
Page: 26–27,31


When referring to this article, please cite it as Huber, B.; Joeckel, T. Challenges of Expanding CAR-T Cell Therapy into Solid Tumors. Pharmaceutical Technology 2023, 47 (5), 26–27,31.