CAR T Cells in Solid Tumors: Challenges and Opportunities

Purpose: CAR T cell therapy has transformed treatment of hematologic malignancies, but its efficacy in solid tumors remains unsupported. Solid tumors present distinct barriers—antigen heterogeneity, poor T cell trafficking/infiltration, and an immunosuppressive tumor microenvironment (TME)—that limit CAR T cell function. This review addresses these barriers and evaluates emerging strategies to improve CAR T cell therapy in non-hematologic malignancies. ---

Hypothesis: The central thesis is: if the key barriers of solid tumors—antigen heterogeneity, impaired trafficking, and immunosuppressive TME—can be overcome through advanced CAR design, combination therapies, and genetic engineering, then CAR T cells can become effective treatments for solid tumors. ---

Aims: - Discuss CAR signaling and efficacy of CAR T cells in solid tumors. - Evaluate the most significant barriers in solid tumor CAR T cell therapy. - Describe the most novel therapeutic methods aimed at achieving promising outcomes in non-hematologic malignancies. - Review recent advances across specific solid tumor types (ovarian, breast, prostate, renal, gastric, pancreatic, lung, liver, colorectal). ---

Delivery system: CAR T cell platform: - T cells engineered to express chimeric antigen receptors (CARs) composed of: - Extracellular single-chain variable fragment (scFv) for antigen recognition - Transmembrane domain - Intracellular immunoreceptor tyrosine-based activation motifs (ITAMs), typically CD3ζ - Co-stimulatory domains (CD28, 4-1BB, OX40, ICOS, etc.) CAR generations: - First generation: CD3ζ only - Second generation: CD3ζ + one co-stimulatory domain (CD28 or 4-1BB) - Third generation: CD3ζ + two co-stimulatory domains Gene transfer methods: - Viral vectors: gamma-retroviral, lentiviral - Transposons: Sleeping Beauty, piggyBac - RNA electroporation (transient CAR expression, 7–14 days) - Cell surface conjugation Target antigens (solid tumors): - EGFR, HER2, EGFRvIII, mesothelin, PSCA, MUC1, Claudin 18.2, EpCAM, GD2, VEGFR2, AFP, Nectin4/FAP, CEA, Lewis Y, Glypican-3, IL-13Rα2, CD171, MUC16, PSMA, AXL, CD20, CD80/86, c-MET, DLL-3, DR5, EphA2, FR-α, gp100, MAGE-A1/3/4, LMP1, NKG2D, TAG-72, CD46, GUCY2C, CAIX, B7-H3, ROR1, DLL3, DCLK1, and others. Advanced/combination strategies: - Bispecific CARs (e.g., Trop2/PD-L1) - Multi-antigen targeting - Chemokine receptor modification (CCR2, CCR4, CXCR2) - Oncolytic viruses expressing chemokines (e.g., CCL5) - Checkpoint blockade (anti-PD-1/PD-L1, anti-CTLA-4) - CRISPR-mediated gene editing (e.g., PD-1, LAG-3 disruption) - CAR T cells secreting IL-12, IL-15, IL-21 - FAP-specific CAR T cells to target cancer-associated fibroblasts - ECM-degrading enzyme-secreting CAR T cells ---

Approach: Review of preclinical and clinical literature. Model systems include: - In vitro: Human cancer cell lines (e.g., SK-OV-3, OVCAR-3, A549, H82, H196, H446, TNBC cells, HCC cells, colorectal cancer cells). - In vivo: Xenograft, orthotopic, and metastatic mouse models; immunocompetent and human xenograft models. - Clinical: Phase I/II clinical trials across solid tumors (Table 1 lists trials for EGFR, HER2, mesothelin, MUC1, Claudin 18.2, GD2, PSMA, CEA, Glypican-3, EGFRvIII, IL-13Rα2, CD171, MUC16, and others). - Disease context: Ovarian, breast, prostate, renal, gastric, pancreatic, lung, liver, and colorectal cancers. ---

Key methods: Techniques and endpoints highlighted across cited studies: - Cytotoxicity assays (granzyme, perforin, TRAIL, FasL) - Cytokine secretion assays (IL-2, IFN-γ, TNF-α) - Flow cytometry for CAR expression, T cell phenotype, and exhaustion markers - Tumor growth inhibition and survival analysis in mouse models - Metastasis assessment - T cell infiltration and trafficking analysis - Antigen expression analysis (e.g., MSLN, MUC1, HER2, PSMA, GPC3, EGFRvIII) - CRISPR/Cas9 gene editing for checkpoint disruption - Clinical response assessment: safety, efficacy, phase I/II endpoints ---

Key results: - Ovarian cancer: MSLN-CAR NK cells significantly killed MSLN⁺ ovarian cancer cells (SK-OV-3, OVCAR-3) in vitro. MUC16-specific CAR T cells eradicated malignant cells in mouse models. TAG72-CAR T and FRα-CAR T inhibited ovarian cancer growth. - Breast cancer: MUC28z CAR T cells (targeting tMUC1) reduced TNBC tumor proliferation and survival in a xenograft model. HRG1β-based CAR T cells inhibited breast cancer via HER family receptors. Anti-HER2 CAR T cells triggered cell death in HER2-overexpressing breast cancer. - Prostate cancer: PSMA-directed CAR T cells showed strong expansion and cytotoxicity; clinical trials by Junghans et al. and Slovin et al. confirmed safety and efficacy. - Gastric cancer: Bi-specific Trop2/PD-L1 CAR T cells significantly reduced gastric cancer growth, with greater suppression than Trop2-specific CAR T cells. Mesothelin-CAR T with M28z10 signaling induced gastric cancer cell death and inhibited tumor growth. - Pancreatic cancer: CXCR2-expressing CAR T cells provoked greater antitumor activity toward αvβ6-expressing pancreatic tumor xenografts. B7-H3 CAR T cells were effective in pancreatic ductal adenocarcinoma models. - Lung cancer: ROR1-CAR T cells infiltrated and eradicated multiple layers of A549 lung cancer cells. EGFRvIII-CAR T cells killed A549-EGFRvIII cells, reduced metastasis, and extended mouse survival without side effects. - Liver cancer: GPC3/CAR T cells expressing IL-15/21 promoted antitumor responses against HCC. - Colorectal cancer: DCLK1-targeted CAR T therapy effectively eradicated primary and metastatic colon cancer cells. GUCY2C-specific CAR T cells eliminated colorectal cancer metastases. - Challenges: Antigen heterogeneity, poor trafficking/infiltration, and immunosuppressive TME (Tregs, MDSCs, M2 TAMs, CAFs, adenosine, ROS, high extracellular K⁺) remain major barriers. ---

Interpretation: The authors claim that progression of CAR T cell therapy to solid tumors requires understanding and overcoming TME-imposed barriers and tumor heterogeneity. Genetic modification of T cells to target multiple antigens, improve trafficking, resist exhaustion, and persist in hostile conditions—combined with checkpoint blockade, oncolytic viruses, and TME modulators—will be essential. They conclude that CAR T cell therapy remains a promising therapeutic option for advanced malignancies, but success in solid tumors will depend on rational, multi-pronged engineering strategies. ---

Limitations: - Efficacy in solid tumors has not yet been supported clinically. - Antigen heterogeneity and antigen loss/escape limit single-target CAR T cells. - Poor T cell trafficking and infiltration into tumor tissue. - Immunosuppressive TME interferes with T cell differentiation, activity, and persistence. - On-target off-tumor toxicity remains a risk. - Many strategies are preclinical; clinical validation is limited. - Checkpoint blockade combined with CAR T cells may increase toxicity to normal cells. - Optimal CAR signaling, co-stimulatory domain choice, and T cell subset selection are not fully defined. - As a review, the article is not a systematic review or meta-analysis.