Endocrine malignancies are tumors driven from endocrine or neuroendocrine cells throughout the body resulting in disruptions in hormone production, that are heterogeneous in histology and biological features. They encompass tumors originating in traditionally endocrine organs, such as thyroid, adrenal, pancreatic, ovarian, mammary, and prostate glands, as well as neuroendocrine tumors (NETs) from non-endocrine organs, including gastrointestinal tract and bronchial tubes. Due to their low proliferative capacity, surgery to eradicate tumors is considered the standard therapy, and for some tumors like differentiated thyroid cancer (DTC), specific radiation therapy follows to decrease or eliminate tumor size and extend patients’ lives. Frustratingly, some cases are diagnosed at advanced stages due to lack of early symptoms, and curative therapy for metastatic endocrine tumors has been reported to be ineffective. Moreover, aggressive tumors, containing anaplastic thyroid cancer, small cell lung cancers (SCLC) and so on, are included among endocrine neoplasms owing to their heterogeneity. Generally, considering limited efficacy and potentially deleterious effects of conventional treatment methods, it is imperative that novel and efficacious alternatives be promptly investigated (1, 2).

In recent decades, immunotherapy has achieved significant advances in the field of cancer treatment, with an emphasis on inhibition of tumor progression through the alteration of relationship between tumor cells and immune system. Chimeric antigen receptor (CAR)-T cell is an adoptive cell-based therapy, that modifies autologous, patient-derived T cells to express a CAR and redirects their cytotoxic activity independent of major histocompatibility complex (MHC) (3, 4). Structurally, CARs are artificial receptors that are composed of an extracellular antigen-recognition domain, antibody-derived single-chain variable fragments (scFv), a transmembrane region, and intracellular co-stimulatory moiety placed upstream of CD3ζ and most frequently generated from 4–1BB or CD28 (5). Recognizing cognate tumor antigen by CARs ignites downstream signaling activities leading to activation of T-cells and targeted tumor lysis.

Despite effective clinical application in hematological malignancies, CAR-T therapy has demonstrated limited efficacy against solid tumors. The major hindrances against endocrine tumors manifest in the following aspects (3, 6): 1) paucity of target tumor antigens while limited on healthy tissues to avoid on-target off-tumor toxicity, 2) insufficiency of engineered cells homing and infiltration, 3) immunosuppressive tumor microenvironment (TME) impeding persistence and efficacy of CAR-T cells. Herein, this narrative review sought to summarize suitable novel specific targets of endocrine cancer for CAR therapy and combination therapy with alternative strategies to surmount these obstacles.

Identifying specific antigens to target endocrine cancers is the first step in establishing a successful CAR-T therapy. Theoretically, any antigen on tumor surface, such as proteins, glycolipids, and gangliosides, could potentially be targeted by CAR. However, expression of autoantigens in normal tissues results in non-targeted toxicity. Therefore, the ideal tumor target should be tumor-specific antigens (TSA) or tumor-associated antigens (TAA), and CAR-T could elicit a targeted immune response against tumor tissues while sparing surrounding normal tissues. The following section presents a comprehensive summary and detailed analysis of emerging potential CAR-T cell targets in endocrine-related malignancies.

Endocrine tumors are predominantly indolent, and for foreseeable future, monotherapy of CAR-T cells is unlikely to supplant conventional therapeutic strategies, but would be integrated into combination therapies. Therefore, it is insufficient to focus solely on optimization of CAR structure. In addition, immunotherapy is mostly used as salvage therapy, so it is anticipated to be confronted with more aggressive and rare subtypes, and the use of combination treatments targeting multiple mechanisms simultaneously has proven potential as a strategy for cancer treatment ( Figure 1 ).

Radiotherapy is integral to many endocrine tumors’ standard treatment and could be implemented as a CAR conditioning regimen. Enhanced cytotoxicity of engineered T cells by radiotherapy might be achieved through the following (49–52): 1) The DNA-damaging properties of ionizing radiation kill tumor cells and trigger pressure signals; 2) Sensitivity to apoptosis of radiation-treated cancer cells was enhanced; 3) A favorable tumor microenvironment is provided by upregulation of MHC class I, which enables CAR-T cells infiltration and increases IFNγ production. In a recent study, treatment of fractionated irradiation significantly upregulates B7-H3 expression in prostate cancer cells, and is more effective in controlling xenograft growth in mice after B7-H3 CAR-T-cell administration than monotherapies (26). The approach presented provides a sound basis for multimodality CAR therapy with radiotherapy conditioning for eliminating clonally heterogeneous tumors. However, dose and frequency of radiotherapy and sequence of this unique combinatorial model warranted further clinical examination and optimization for improving efficacy while minimizing toxicity.

Additionally, considerable research has also been done to investigate pharmaceutical methods for stimulating expression of targeted antigens (53). In neuroblastoma GD2, while exhibiting uniform expression, could be further elevated after treatment with histone deacetylase inhibitor vorinostat in vitro ( 54). Reppel L et al. delineated that GD2 could be upregulated in GD2 SCLC cell lines after exposure to Enhancer of Zeste Homolog (EZH2) inhibitor tazemetostat, rendering superior sensitivity to GD2-CAR-T cells, which demonstrated that CAR-T cell therapy with selected epigenetic modulators deserve further preclinical investigation to minimize antigen escape (32).

Aberrantly glycosylated alterations provide specific markers employed in immunotherapeutic strategies. In opposite scenarios, target antigens after post-translational modifications might raise CAR-T cell activation threshold. N-glycans were proven to provide multifaceted protection to pancreatic adenocarcinoma cells by lytic immune synapse formation and interfering with transcriptional activation, cytotoxicity, and cytokine production through inhibiting N-glycan synthesis by knocking out mannoside acetyl-glucosaminyltransferase 5 (MGAT5) (55). Prophylactic administration of glucose analog 2-deoxy-d-glucose (2DG), which preferentially accumulates in tumors with high metabolic demand, offsets glycan coat and restores susceptibility to CAR-T cell. Moreover, 2DG treatment impairs proper functionality of PD-1-PD-L1 axis and leads to reduced immunosuppression. The combined 2DG and CAR-T cell therapy demonstrated efficacy against a range of carcinomas, including those arising from lung, ovary, and bladder, and with diverse clinically relevant CAR targets, such as CD44v6 and CEA. Overall, deglycosylating strategies have immediate translational opportunities and proved to be a viable approach to rational design of adjunct therapy for CAR-T cells.

Immune checkpoint inhibitors (ICIs) are agents which function through impeding immune evasion mechanisms exhibited by various cancers, and programmed cell death 1 (PD-1) and its ligand (PD-L1) blockers are representative ICIs. A significant increase in expression of PD-1 on CAR-T cells was observed in a Her2⁺ mouse model which contribute to impairing efficiency (56). Therefore, a combined immunotherapeutic approach of CAR-T cell therapy and PD-1 inhibition was hypothesized to overcome PD-1/PD-L1 pathway-mediated tumor immunosuppression and restore engineered cells’ ability. However, advancing in clinical experience is in its early stages, and initial findings are promising in some respects, yet in some cases, results are less encouraging (57, 58). In a clinical trial (NCT01822652) of GD2-CAR T cells for neuroblastoma, addition of PD-1 blockade showed no measurable effect on enhancing outcomes (59). The mechanisms through which checkpoint blockade exerts its beneficial effects on CAR-T persistence remain to be elucidated. It has been demonstrated that CAR-T cell modification through knockout of the PD-1-encoding gene, PDCD1, represents an alternative strategy for blocking the pathway without systemic toxicity (60).

Another study demonstrated oncolytic virus (OV) is another attractive approach for blocking PD-L1 signaling (61). OVs are capable of selectively infecting and lysing tumour cells while sparing healthy cells. In addition, they could genetically be modified to produce a range of therapeutic agents that facilitate tumour antigen presentation, stimulate innate immune responses and thus turn immunosuppressive milieu into a stimulatory for potentiating adoptive cell therapy (62, 63). In vivo and in vitro experiments of prostate cancer revealed that coadministration of oncolytic adenovirus expressing PD-L1 mini-antibody and CAR-T cells effectively promoted effector cell proliferation, resulting in enhanced anti-tumor efficacy and extended tumor-bearing mice survival. A previous study also presented that intratumoral release of chemokine, RANTES and IL15, expressed by OVs could facilitate migration and survival of adoptively transferred GD2-CAR-T cells, which results in superior anti-tumor effects in a neuroblastoma xenograft model (64). Preclinical studies support potential for further exploration of this combinatorial approach and clinical trials for preliminary safety and efficacy evaluation are recruiting (NCT03740256 and NCT05057715).

There is an unmet medical need to development of effective treatments for heterogeneous endocrine tumors. CAR-T therapy is gradually progressing, and researchers are engaged in developing novel strategies to overcome barriers of lack of efficacy and safety. Paucity of specific and safe neoantigens is the major obstacle hindering pursuit of CAR-T development. The review presents promising targets and current clinical trials ( Table 1 ), correspondingly CAR structure could be further optimized. Targeting two or more antigens instead of one reduces possibility of antigen loss or mutation and guarantees consistent immune activation of CAR cells. In fact, there are two principal varieties of bispecific CAR-T. TAG-72 and CD47 CAR-T designed by Shu et al. which express two scFV on two CAR proteins, could inhibit the OC development, and resist drug resistance caused by OC antigens down-regulation or expression of different antigens (65). The other is to express two scFV on the same CAR protein. Liang et al. observed an increased proliferation rate of tandem CAR-T targeting FOLR1 and MSLN, demonstrating a robust anti-tumour effect and significantly prolonged survival (66). Additionally, CAR-related endocrine tumor therapy has been extended from T cells to NK cells and macrophages. The recognition of target cells by NK cells is MHC-independence indicating the low risk of NK-cell allo-transplantation. Moreover, NK cells are not prone to cause cytokine storm due to different cytokine profile from T cells and exhibit less non-targeted toxicity with limited survival time (67). Preclinical research has confirmed the potent anti-tumour efficacy of CAR-M, which targets tumour cells based on the phagocytic activity of macrophages while enhancing antigen presentation and influencing the M1 polarization of tumour-associated macrophages. Objectively, CAR-NK and CAR-M are likely to provide breakthroughs and have intrinsic benefits over CAR-T in solid malignancy treatment, while their wide application is still limited and warrants further investigation (68).

The combination of CAR-T cell therapy with other therapeutic modalities offers considerable promise and encompasses a multitude of potential avenues for exploration (69). It is challenging to determine which strategy should be prioritized over another, given the multitude of processes and advantages associated with different combinations. To fully realize potential of combinatorial CAR-T cell therapy in cancer treatment, further research must be conducted in both preclinical and clinical settings. Furthermore, additional issues require resolution, including optimization of dosage regimens, minimization of additive toxicities, and the identification of predictive biomarkers (4, 70). The future direction of research will be informed by knowledge gathered from ongoing clinical studies, which will be crucial in assessing long-term results, safety, and effectiveness of CAR-T cell combination therapies.

Taken together, continued momentum of this field would depend on engineering CAR-T cells to attain specificity, safety, and efficacy, as well as selection of optimal combination therapies to enhance clinical outcomes in patients with endocrine-related malignancy.