Challenges and limitations of chimeric antigen receptor T-cell therapies in solid tumors: why are approvals restricted to hematologic malignancies?

Chimeric antigen receptor T-cell (CAR-T) therapy has revolutionized the treatment of hematologic malignancies, offering a highly personalized and potent immunotherapeutic approach. To date, the U.S. Food and Drug Administration has approved seven CAR-T therapies targeting CD19 and B-cell maturation antigen; each demonstrated remarkable clinical efficacy across various hematologic malignancies. Despite significant advancements in preclinical studies and clinical trials, no CAR-T therapy has been approved for solid tumors, which account for the majority of cancer cases worldwide. These key challenges include the lack of distinct and accessible target antigens, the immunosuppressive tumor microenvironment (TME) that impairs immune cell efficacy, the heterogeneity of solid tumors that complicates treatment uniformity, and the potential risks of off-tumor toxicity. These obstacles represent a complex array of biological and clinical obstacles, distinct from the more favorable immune environment of hematologic cancers that has been pivotal to the success of CAR-T therapy. Preclinical studies in multiple myeloma emphasize memory T-cell optimization and combinatorial strategies to enhance CAR-T efficacy in solid tumors. Our review emphasizes innovative strategies to address these key challenges in CAR-T therapy for solid tumors, including advanced multi-antigen targeting approaches, reprogramming of the TME, and the development of next-generation safety measures to mitigate toxicity risks. By addressing both scientific and clinical obstacles, this review envisions a future in which CAR-T therapy's full potential extends beyond hematologic malignancies, transforming the landscape of oncology and improving outcomes for patients with solid tumors.