A literature review of recent advances in gastric cancer treatment: exploring the cross-talk between targeted therapies

Abstract

Background

Gastric cancer (GC) ranks fourth in global mortality rates and fifth in prevalence, making it one of the most common cancers worldwide. Recent clinical studies have highlighted the potential of immunotherapies as a promising approach to treating GC. This study aims to shed light on the most impactful therapeutic strategies in the context of GC immunotherapy, highlighting both established and emerging approaches.

Main body

This review examines over 160 clinical studies conducted globally, focusing on the effectiveness of various immunotherapy modalities, including cancer vaccines, adoptive cell therapy, immune checkpoint inhibitors (ICIs), and monoclonal antibodies (mAbs). A comprehensive search of peer-reviewed literature was performed using databases such as Web of Science, PubMed, and Scopus. The selection criteria included peer-reviewed articles published primarily within the last 10 years, with a focus on studies that provided insights into targeted therapies and their mechanisms of action, clinical efficacy, and safety profiles. The findings indicate that these immunotherapy strategies can enhance treatment outcomes for GC, aligning with current treatment guidelines. ICIs like pembrolizumab and nivolumab have shown significant survival benefits in specific GC subgroups. Cancer vaccines and CAR-T cell therapies demonstrate potential, while mAbs targeting HER2 and VEGFR pathways enhance outcomes in combination regimens. We discuss the latest advancements and challenges in targeted therapy and immunotherapy for GC. Given the evolving nature of this field, this research emphasizes significant evidence-based therapies and those currently under evaluation rather than providing an exhaustive overview. Challenges include resistance mechanisms, immunosuppressive tumor environments, and inconsistent results from combination therapies. Biomarker-driven approaches and further research into emerging modalities like CAR-T cells and cancer vaccines are critical for optimizing treatments.

Conclusions

Immunotherapy is reshaping GC management by improving survival and quality of life. Ongoing research and clinical evaluations are crucial for refining personalized and effective therapies.

Introduction

Gastric Cancer (GC) represents a significant global health burden, ranking as the fifth most commonly diagnosed malignancy and the fourth leading cause of cancer-related mortality worldwide. According to GLOBOCAN 2022 data, GC accounts for nearly one million new cases annually, resulting in approximately 659,000 deaths [1]. Despite advancements in detection and treatment, the prognosis for GC remains poor, particularly for advanced and metastatic cases, where therapeutic options are primarily palliative. Currently, complete surgical resection with lymph node dissection offers the only curative treatment for GC. However, most patients present with advanced-stage disease at diagnosis, rendering them ineligible for surgery. Conventional treatment modalities, including radiation therapy and cytotoxic chemotherapy, have demonstrated limited efficacy due to inherent challenges such as radioresistance, chemoresistance, and the high proportion of unresectable tumors [2, 3]. These limitations underscore the urgent need for novel therapeutic strategies to improve patient outcomes.

The advent of precision medicine has significantly enhanced the understanding of GC pathogenesis and molecular heterogeneity, paving the way for the development of targeted therapies tailored to specific tumor characteristics. Agents such as trastuzumab and ramucirumab, designed for HER-2-positive and VEGFR2-positive GC subtypes, respectively, represent notable advancements in this domain. However, the modest survival benefits observed with these agents highlight the necessity for further innovation [4, 5].

In recent years, immunotherapy has emerged as a promising strategy in GC treatment, offering the potential to address the limitations of traditional approaches. Immune-based interventions, including ICIs, cancer vaccines, adoptive cell therapy, and mAbs, have demonstrated encouraging clinical outcomes. Drugs such as pembrolizumab have already been incorporated into treatment guidelines for advanced GC, and numerous other immunotherapeutic agents are currently under investigation in clinical trials [5, 6].

The evolving landscape of GC treatment necessitates a comprehensive understanding of the opportunities and challenges of novel therapeutic approaches. Although immunotherapies and targeted medicines have shown great promise, information gaps about their best use, resistance mechanisms, and suitability for various patient populations make it difficult to incorporate them into clinical practice. A deeper comprehension of the molecular composition of GC is necessary to address these issues and direct the creation of more potent solutions. In this article, we provide a detailed review of recent advances in GC treatment, focusing on updates in targeted therapy and immunotherapy. By synthesizing evidence from the latest clinical trials and preclinical studies, we aim to highlight the most promising therapies, critically evaluate their limitations, and identify areas for further research. While not exhaustive—given the rapidly evolving nature of this field—this study seeks to emphasize proven therapies and those under active evaluation. Ultimately, our goal is to contribute to developing personalized treatment paradigms that improve survival outcomes and quality of life for GC patients.

Heterogeneity of GC microenvironment

Cancer cells progress in complex tissue environments and their development is associated with desmoplastic reactions involving immune cells and different stromal cell types in the local environment, which is called tumor microenvironment (TME). Components of this environment and interactions among them ultimately determine the growth, invasion, and metastasis of tumors and more importantly predict therapeutic efficacy [7, 8]. There is growing data that the inflammatory environment within GC involves a variety of cell types, including natural killer cells (NK cells), dendritic cells (DCs), tumor-associated mast cells, tumor-associated macrophages (TAMs), cancer-associated fibroblasts (CAFs), tumor-infiltrating neutrophils and lymphocytes and (TINs and TILs), and mesenchymal stem cells [8, 9]. It has been shown that all of the cells listed as well as their products (cytokines) together with changes in the extracellular matrix, microRNAs [10, 11], and deregulated cellular signaling contribute to a TME in GC that enables tumor cells to evade host immunity and develop resistance to multiple treatments [7]. In Fig. 1, the interactions between immune cells within TME are illustrated.

Fig. 1

An overview of the interactions of the immune cells in TME; Different components of this environment and interactions among them which ultimately determine the growth, invasion, and metastasis of tumors is presented

Tumor-associated macrophages (TAMs)

A subset of macrophages known as TAMs or tumor-associated macrophages are vital to immune cell infiltration of tumors. TAMs originate from inflammatory monocytes in the blood that express the chemokine receptor type 2 (CCR2) [12]. In addition to accelerating tumor growth, TAMs release pro-inflammatory cytokines, proteolytic enzymes, and growth factors, that cause tumor progression and resistance to treatment [7, 8]. It is known that up to 50% of the entire mass of the tumor may be made up of infiltrating macrophages, which tends to be linked with poor prognosis regarding solid tumors [13]. There are two main categories of macrophages, classified based on their responses to the microenvironment as M1 (Classical) and M2 (Alternative). Macrophages of type M1 perform inflammatory functions and aid in pathogen clearance and anti-tumor immunity. M1 macrophages, through the creation of chemokines such as IL-1α, TNF-α, IL-6, IL-1β, and IL-12 drive the polarization and recruitment of consequently amplifying the type 1 immune response and type 1 T helper cells (Th1) [14]. Alternatively, M2 macrophages, which compose an important part of TAM accumulation, possess anti-inflammatory functions, are stimulated by type 2 T helper cell cytokines (IL-10, IL-13, and IL-4), and represent pro-tumorigenic functions, which include tumor progression [15]. This frame shows a direct relationship between tumor cells and TAMs. Numerous cytokines and growth factors, including prostaglandin E2 (PGE2), IL-6, and colony-stimulating factor 1 (CSF-1), are released by tumor cells and cause M2 polarization in TAMs. Through the production of vascular endothelial growth factor (VEGF), platelet-derived growth factor (PDGF), fibroblast growth factor (FGF), hepatocyte growth factor (HGF), and epidermal growth factor (EGF), TAMs can stimulate tumor growth directly. Disruption of these signaling pathways can convert M2 macrophages into M1 macrophages, resulting in an alteration in the immune microenvironment [12]. In a recent investigation, Che et al. showed that polarization of M2 macrophages in TME induced by IL-33 successfully impeded the evolution of the tumor and GC’s peritoneal metastasis, improving the immunotherapy’s ultimate outcomes. Hence, IL-33 injection created a unique inflammatory environment locally, which can be used in conjunction with TAM reprogramming to modify TME to improve therapy effectiveness for peritoneal metastases [16].

Cancer-associated fibroblasts (CAFs)

Cancer-associated fibroblasts (CAFs) are activated fibroblasts within the TME that are a substantial stromal component in the tumor microenvironment and are vital to the growth of the tumor [7]. Bone marrow-derived stem cells, fibroblasts in healthy tissues, and certain cells surrounding arteries, such as pericytes, can provide these cells [17]. Studies carried out in vivo and in vitro have shown that CAFs promote tumor growth and metastasis. Platelet-derived growth factor (PDGF), TGF-β, VEGF, FGF, and interleukins are among the many chemicals secreted by activated CAFs, which are identified by α-smooth muscle actin (α-SMA). These substances regulate the formation of tumors and increase the rate at which CAFs infiltrate relative to normal fibroblasts [9]. Aside from CAF, stromal-derived factor 1 (SDR1), CXCL12 (C-X-C motif chemokine ligand 12), and interleukin 11 have also been identified as CAF-derived factors that promote GC migration and invasion [17]. Furthermore, research has demonstrated that CAFs overexpress several proteins, including FGF, IL-1β, TNF-α, TGF-β, and IL-6, which can promote the epithelial-mesenchymal transition (EMT) and hasten the invasion and metastasis of tumor cells [7, 17]. As a result of a complex reprogramming process known as EMT, epithelial cells become mesenchymal and lose their differentiation. This phenomenon occurs during embryonic development, tissue formation, wound healing, and carcinogenesis [18, 19]. There is a close relationship between the EMT phenomenon and GC invasion. In their study on GC subtypes, Cristescu et al. used exact molecular analysis to show a strong correlation between the poor prognosis of primary GC and the expression of the EMT gene signature [20]. Additionally, by activating the JAK2/STAT3 pathway in these cells, GC-derived CAFs can release high quantities of IL-6, which may speed up the migration of GC cells and enhance the EMT process [17]. It is also important to note that CAFs play an important role in maintaining the immune suppressive function of the TME. The IL-6 that CAFs produce inhibits the development of dendritic cells and instructs monocytes to become macrophages [21]. Additionally, CAFs are responsible for attracting macrophages to the TME as well as promoting their polarization towards the M2 type [17].

Tumor-infiltrating neutrophils (TINs)

TINs are neutrophils recruited to the tumor site, where they can exhibit either pro-tumor (N2) or anti-tumor (N1) phenotypes [22]. There is controversy regarding the role of neutrophils in tumor progression in the GC microenvironment. However, it is evident that this cell type contributes considerably to TME and has been related in multiple studies to poorer GC consequences [23]. High neutrophil proliferation seems to be detrimental to 5-year survival [9].

Additionally, TINs are reported to promote the migration and invasion of GC cells by activating the ERK pathway and inducing EMT, indicating they may strongly contribute to GC metastasis [24]. Neutrophils are widely recognized as a major source of IL-17a during inflammation. As a member of the IL-17 family, this cytokine plays an important role in inflammation and is prevalent in many cancers, including GC, where it promotes tumor growth and metastasis [25]. Recent research has found that IL-17a produced from TINs also activates the JAK2/STAT3 pathway, which promotes GC cell EMT [26]. Zhang et al. reported in a recent study that TOB1 expression within TME is essential to the GC’s potential for targeted immunotherapy. They reported that TOB1 may increase the survival rate of GC patients by causing neutrophils to become anti-tumor polarised, inhibiting their apoptosis, and improving the patient’s response to immunotherapy [27]. Lastly, the TIN population has been proposed as a valuable independent prognostic indicator and appears to be associated with the outcomes of GC.

Tumor-infiltrating lymphocytes (TILs)

The immune response to tumors is mediated by lymphocytes that infiltrate the tumor, including B cells, NK cells, and T cells, with T cells being indispensable for anti-tumor immunity [7, 28]. The T cell population includes several different cell types, such as helper cells (CD4⁺), cytotoxic T cells (CD8⁺), regulatory T cells (FOXP3⁺), and memory T cells [7]. A recent meta-analysis showed that among TILs, high levels of CD8⁺ lymphocytes were the most significant predictor of enhanced survival [29]. These lymphocytes can infiltrate the microenvironment of tumor cells and modulate the immune response against them, although their function can be suppressed by a variety of mechanisms such as abnormal expression of stress-inducible NKG2D (Natural Killer Group 2D) ligands, Inhibition of T cell function by gamma/delta T cells, which are immunosuppressive, decreased perforin, cytotoxins like TGF-β, and significantly, Cytotoxic T cells are affected by the activation of immune inhibitory pathways through programmed cell death protein 1 and its ligand (PD1/PDL-1) [30]. One of the most distinctive Treg markers is the transcription FOXP3, which is typified by the CD4⁺CD25⁺FOXP3⁺ phenotype. It is an essential intracellular molecule for Treg formation and activity. Due to Treg suppression of antitumor cytotoxic T cells in the tumor microenvironment and self-immune tolerance, high Treg infiltration in tumor tissues is associated with poor prognoses due to immune escape by tumor cells [29]. In GC tissue, Excessive FOXP3 expression was associated with lymph node metastases and a diminished survival rate in a prior investigation involving 100 GC specimens [31]. In addition, CTLA4 (cytotoxic T-lymphocyte-associated antigen4) has been recognized as a significant inhibitory mediator, especially in CD4 + Treg cells [30, 32]. Future studies and examining novel animal models will be required to pinpoint the precise pathways underlying Tumor immunity mediated by TIL in GCs.

Dendritic cells (DCs)

In addition to their morphological and functional similarities, dendritic cells are a diverse group of cells characterized by considerable flexibility. DCs are antigen-presenting cells responsible for initiating and regulating adaptive immune responses. They are primarily divided into two main populations: plasmacytoid dendritic cells (PDCs) and myeloid dendritic cells (MDCs) [33]. Human MDCs, not PDCs, are the main IL-12 producers, and this fact seems to show the suppressor role of MDCs for tumor neovascularization and therefore suppress tumor growth. However, research has revealed that angiogenesis-stimulatory DCs, including plasmacytoid DCs, are common in the tumor environment while angiogenesis-inhibitory myeloid DCs seem missing [34]. This may be due to CXCL12, a chemokine produced by tumors that attracts and preserves plasmacytoid DCs in the TME. In response, these cells stimulate vascularization by releasing IL-8 and TNF-α [35]. Separate studies have shown that PDCs contribute to tumor vascularization and ICOS⁺ regulatory T cells (Treg) in the peripheral blood of GC patients, creating an immunosuppressive tumor microenvironment that assists cancer in evading the immune system [36].

According to our mentions above, some of the cells involved in the TME of GC are significant oncogenic drivers and are consequently linked to a poor prognosis; nevertheless, other cells exhibit anti-tumor action. Targeting the pro-tumorigenic stroma and promoting an anti-tumorigenic microenvironment may be practical and successful cancer treatment approaches.

Targeting small molecules in GC

Recent studies highlight the expanding potential of small molecule-targeted treatments in GC, with a particular focus on natural small molecules derived from natural sources. These molecules have shown promise in slowing cancer progression by targeting specific molecular pathways involved in oncogenesis, offering a more effective and less toxic alternative to traditional therapies. The growing interest in small compounds stems from their ability to improve therapeutic efficacy while minimizing adverse effects. Numerous strategies and substances have demonstrated potential in combating GC, underscoring the need for innovative drug development methods. Ultimately, the therapeutic potential of natural small molecules in GI cancers emphasizes their role in overcoming the limitations of current treatments and offering better treatment alternatives [37].

Tyrosine kinases, Fibroblast growth factors, PIM kinases, YAP/TAZ, and mTOR, among other small molecules that control the development, and invasion of GC might be suitable sites for targeted therapies [38,39,40,41]. Because of the intricacy of downstream signaling and the difficulty of inhibiting specific molecular interactions, not all GC-related small molecules can be effectively interfered with. Next, we want to discuss a few small molecules that have been proven to be appropriate targets for GC therapy in preclinical and clinical studies. Figure 2. shows an overview of regulatory cascades of small molecules involved in GC cell survival and proliferation.

Fig. 2

An overview of regulatory cascades of small molecules involved in GC cell survival and proliferation; Tyrosine kinases, Fibroblast growth factors, PIM kinases, YAP/TAZ, and mTOR, among other small molecules that control the development, and invasion of GC cells and interaction between potential inhibitor agents are presented

Tyrosine kinase inhibitors (TKIs)

The human genome encodes a significant class of protein kinases called tyrosine kinases (TKs), which are vital for a variety of cellular functions such as cell division, differentiation, and death. Both normal cellular function and the onset of many illnesses, especially cancer and neurological disorders, depend on them. There are 90 different tyrosine kinase genes in the human genome, which are separated into 10 subfamilies of non-receptor tyrosine kinases (non-RTKs) and 20 subfamilies of receptor-type tyrosine kinases (RTKs). These enzymes have distinct structural traits that are necessary for their regulatory activities, such as a propensity to assume inactive conformations. Their significance in both physiological and pathological circumstances is highlighted by their varied roles in cellular signaling networks [42, 43].

RTKs relay the process of extracellular signal transduction into the cells by catalyzing phosphate transfer from ATP to tyrosine residues of intracellular proteins and non-RTKs function in intracellular signal transmission. Receptors of many growth factor families, such as Fibroblast growth factor (FGF), Platelet-derived growth factor (PDGF), Epidermal growth factor (EGF), and vascular endothelial growth factor (VEGF), function through RTKs. Also, RTKs have important roles in metastasis and tumor growth, hence they are among the most frequently studied targeted therapies for different kinds of cancer [44, 45]. Upregulation of several known RTKs is observed in GC, which could make them excellent therapeutic targets [38]. So, small molecule receptor tyrosine kinase inhibitors (RTKIs), could be of significance in the process of GC treatment.

Apatinib is an RTKI that suppresses tumor angiogenesis by selectively targeting VEGFR-2. By interacting with the intracellular ATP binding portion of VEGFR-2, this orally bioavailable agent prevents phosphate transfer and the continuation of the subsequent signaling pathways. Also, Apatinib reduces angiogenesis by lowering the expression of VEGF through the suppression of PKM2, which is important for tumor development and metastasis [46, 47]. Apatinib, through suppressing angiogenesis, prevented the growth and metastasis of tumors, and several studies showed promising results of Apatinib in GC treatment [46]. Following preclinical in vivo and in vitro research that validated Apatinib’s effectiveness in treating GC [48, 49], many clinical studies have been done to investigate its antitumor potential in GC patients. In combination with SOX chemotherapy, Lin et al. demonstrated that Apatinib was an effective and protective neoadjuvant for locally advanced GC [50]. According to Su et al., apatinib targets c-kit signaling and its downstream pathways to reverse pyrotinib resistance in HER2⁺ GC [51]. Additionally, as a 1st line treatment, apatinib plus paclitaxel may prolong survival [52]. Compared to a placebo, apatinib significantly increased overall survival and progression-free survival in patients with advanced GC who did not respond to treatment [53]. Despite its promising safety profile, side effects like proteinuria, hand-foot syndrome, and hypertension, have been observed in patients participating in clinical studies [46].

Foretinib is another novel small molecule RTKI which has multiple targets including VEGFR2, RON, AXL, TIE-2, and MET. Foretinib hinders cell proliferation of cells amplified in MET and FGFR2 expression in multiple GC cell lines in preclinical studies [54]. Shah et al.‘s clinical study revealed that single-agent foretinib did not significantly improve tumor regression, even though it inhibited MET. Hence, Shah et al. suggest that Foretinib may be more effective against GC, as a combination therapy with other targeted therapy agents or chemotherapeutics [55]. Grojean et al. and Awasthi et al. then assessed the anti-tumor effects of foretinib in combination with albumin-bound paclitaxel or oxaliplatin and nanoparticle paclitaxel in two different investigations. Their results revealed that Foretinib’s simultaneous hinders of VEGFR2 and c-Met significantly enhanced the therapeutic effects of these chemotherapeutics in preclinical GC models and this therapeutic combination may improve GC patients’ survival [56, 57]. Additionally, Sohn et al. discovered that foretinib targets CD44 signaling to limit GC stemness in addition to hindering GC cell proliferation by targeting c-MET [58].

Crizotinib (PF-02341066) is also an RTKI agent that exerts antitumor action against GC by targeting MET. In vivo and in vitro, Okamoto et al. revealed that crizotinib upregulates proapoptotic Bcl2 family members and suppresses GC cell growth by targeting MET [59]. In addition, Lapatinib, an RTKI that targets EGFR/HER-2, was found to have anti-tumor effects in GC patients. The combination of Lapatinib with commonly used chemotherapeutic agents like cisplatin, paclitaxel, oxaliplatin, SN-38 (Irinotecan’s active metabolite) also, and 5-FU appears to be synergistic [60, 61]. Hence, Lapatinib could serve as an excellent adjuvant to currently used GC chemotherapeutic regimens. SAR125844 is another RTKI that targets MET. Egile et al. showed that SAR125844 in MET-amplified GC xenograft models caused tumor regression in a dose-dependent manner without adverse consequences of treatment [62].

In addition to the mentioned RTKIs, many other agents in this drug category are currently under investigation to evaluate their therapeutic efficacy against GC. A number of these agents are listed in Table 1. Overall, RTKIs have shown promising therapeutic potential in GC recently. More clinical studies in this field could establish RTKIs as a novel targeted therapy in treatment guidelines for GC.

Table 1 TKI agents under clinical investigation for the treatment of GC

Fibroblast growth factor receptors inhibitors

As part of the fibroblast growth factor (FGF) family, fibroblast growth factor receptors (FGFRs) bind the FGF molecules. There are 5 main types of FGFRs -FGFR1-5, and 23 FGF ligands that bind to these receptors. Activation of each FGFR by different types of FGFs results in triggering various downstream signaling pathways [63]. FGF/FGFR signaling could be relayed in the forms of endocrine, paracrine, or autocrine signaling. FGFs by binding to and dimerizing FGFRs, are involved in various physiological activities in the process of embryogenesis, regulation of metabolism, and maintaining homeostasis [64]. Furthermore, disruption of the FGF/FGFR signaling system may be linked to oncogenesis due to the critical roles this signaling route plays in controlling cellular migration, mitosis, and apoptosis. Two significant processes that may result in disruption of FGF/FGFR signaling throughout the oncogenesis process are the secretion of FGF ligands from tumor cells, which unhinderedly activates FGFRs, or mutations in FGFR genes, which activate this pathway ligand-independently. In addition, the overexpression of FGFR in certain cancers, including GC, is implicated in the pathogenesis of those diseases [39]. The most notable components of this signaling pathway that are dysregulated in GC are FGFR2 and FGF18. Due to FGFR2’s role in maintaining healthy stomach tissue, its interruption could lead to GC progression [65]. Amplification of FGFR2, which has been seen in 3–10% of primary GCs, is associated with poor survival rates [66]. Overexpression of FGF18 is the most prominent among the FGF family and could be used as a prognostic and diagnostic biomarker. Consequently, many therapeutic agents have attempted to target the FGF/FGFR signaling pathway in GC pathogenesis. mAbs and small molecule inhibitors targeting FGF/FGFR signaling are two main categories of drugs that could be used for this purpose. Small molecule inhibitors include non-specific RTKIs and selective RTKIs which specifically target FGFR [65]. Non-specific RTKIs were discussed accordingly in the previous section of the article. Here, we review the main selective RTKIs targeting FGFR which have the advantage of fewer off-target effects.

FGFR1-3 is the target of the selective small-molecule FGFR inhibitor AZD4547. AZD4547 caused apoptosis in GC cell lines SNU-16 and KATO-III, which overexpressed the FGFR2 gene, according to in vivo and in vitro investigations. Inhibition of GC tumor growth induced by AZD4547 was also more significant in cell lines that harbor FGFR2 amplification compared to nonamplified models. Also in vivo studies confirmed that AZD4547 enhanced the cytotoxicity of chemotherapeutic agents against GC cells [67]. However, the phase II ‘SHINE’ clinical study showed that AZD4547 did not considerably enhance progression-free survival compared to paclitaxel in individuals with FGFR2-amplified GC and maybe another biomarker should be chosen to select patients who may show a better therapeutic response [68]. Schmidt et al. carried out research to determine the effectiveness of BGJ398, a pan-FGFR inhibitor, on three GC cell lines. Their findings demonstrated that the expression levels of FGFR1 and FGFR2IIIc in GC cells determine the cytotoxicity of BGJ398. FGFR inhibition was most effective in inhibiting the invasion and growth of GC cells in KKLS cells, which express high levels of FGFR [69]. PRN1371 is an FGFR1-4 inhibitor that is effective against various solid tumors. In vitro studies confirmed an antiproliferative role of PRN1371 against the SNU-16 GC cell line [70]. Tsimafeyeu et al. designed Alofanib (RPT835), a selective FGFR2 inhibitor, using a computational molecular modeling protocol. Then, KATO-III, a FGFR2-amplified GC cell line, was significantly suppressed by Alofanib in subsequent in vitro experiments [71]. Another specific FGFR inhibitor, ARQ087, has the ability to cause FGFR2-amplified GC cell lines to undergo apoptosis and G1 phase cell cycle arrest [72]. LY2874455 is another FGFR inhibitor that targets FGFR1-4. The in vivo and in vitro antiproliferative effects of LY2874455 were observed in KATO-III and SNU-16 cell lines in a study by Zhao et al. [73]. Studies conducted in clinical settings, the effective half-life of LY2874455 was shown to be about 12 h. A total of 12 patients with stable disease and one patient with partial response were evaluated in this study out of 15 evaluable GC patients [74]. However, a case of acquired resistance to LY2874455 has been reported in a patient who had shown a long-term response to this drug through a potential mechanism of FGFR2-ACSL5 fusion protein [75]. Additionally, preclinical research on the mechanism of GC cell lines’ resistance to FGFR inhibitors showed that drug resistance may be caused by the quick reactivation of the mitogen-activated protein kinase (MEK) pathway in response to FGFR inhibition. And combinational use of inhibitors of the MEK pathway with FGFR inhibitors could be more effective in FGFR-amplified GC [76]. Table 2 lists additional FGFR inhibitors that are presently being studied clinically for the treatment of GC.

Table 2 FGFR inhibitors under clinical investigation for the treatment of GC

PIM kinase inhibitors

The class of serine/threonine kinases known as the Proviral Integration site for Moloney murine leukemia virus (PIM) kinases was named after the chromosomal loci where they were initially discovered. The PIM-3, PIM-1, PIM-2, and isoforms are the three main members of this family. Cell proliferation, growth, survival, and migration are all regulated by PIM kinases in different cell types. The expression of PIM kinases is virtually undetectable in normal tissues. However, numerous forms of human malignancies, including GC, have shown different expression patterns [77, 78].

A lower prognosis is linked to elevated expression of PIM-1 and PIM-3 in GC. There has been a correlation between lymph node metastases and GC patients who overexpress PIM-1. Overexpression of PIM-1 causes oncogenesis primarily through three mechanisms: apoptosis inhibition, cell proliferation promotion, and genomic instability promotion. Due to its oncogenic properties and overexpression in numerous cancers, PIM-1 may be a potential therapeutic target for the development of anticancer medications. The creation of PIM inhibitors that are selective against other kinases has advanced significantly in recent years [77, 79].

To assess the therapeutic effectiveness of targeting PIM-1 in GC, Yan et al. carried out a study. A comparison between normal gastric epithelial cells and GC samples showed that PIM-1 was aberrantly expressed. Also, the ratio of cytoplasmic to nuclear expression of PIM-1, which is correlated with poor survival rate and tumor invasion, is significantly higher in GC tissue samples. It was also demonstrated that K00135, a small molecule inhibitor targeting PIM-1, has cytotoxic effects against GC cell lines harboring PIM-1 overexpression. Altogether, PIM-1 as a possible therapeutic target in the treatment of GC was validated by this investigation [80]. AZD1208 is a PIM kinase inhibitor that targets PIM-1-3. This pan-PIM kinase inhibitor through induction of autophagy inhibits tumor growth and proliferative characteristics of tumor cells. Additionally, because PIM kinases play a part in the DNA damage repair pathway, AZD1208 causes tumor cells to accumulate DNA damage by blocking PIM-1-3. As a consequence, resistance to AZD1208 may result from the reactivation of DNA damage repair. Moreover, AZD1208 showed synergistic effects when used in combination with Akt inhibitors which could reverse drug resistance. Interestingly, expression levels of different PIM isoforms in GC cells are not correlated with response to AZD1208 treatment [78].

Resveratrol is a phytochemical first extracted from the Veratrum grandiflorum plant. Kim et al. demonstrated that resveratrol could function as a PIM kinase inhibitor by directly binding to PIM-1. Through induction of apoptosis, Resveratrol prevents SNU-601 GC cells from growing and proliferating. Resveratrol administration decreases PIM-1’s catalytic activity in a dose-dependent manner even though it does not affect PIM-1 expression levels. Resveratrol is also an effective inhibitor of PIM-2, but its effectiveness against PIM-3 is much weaker [81]. Overall, though most of the studies involving PIM kinase inhibitors assessed the efficacy of these agents against hematologic malignancies, recently some preclinical studies have shown the effectiveness of PIM kinase inhibitors against GC. Further clinical studies are needed to evaluate their effectiveness in GC patients as potential anticancer drugs.

YAP/TAZ inhibitors

The primary mediators of the Hippo signaling pathway in human cells are transcriptional coactivators with PDZ-binding motif (TAZ) and yes-associated protein (YAP) [82]. The Hippo pathway, which is conserved from Drosophila to humans, controls organ growth, tissue homeostasis, and carcinogenesis [83]. In response to activation of the Hippo pathway, phosphorylation of several serine residues results in the degradation of YAP and TAZ by the proteasome. Dephosphorylated YAP/TAZ reaches the nucleus and increases the expression of invasion and proliferation mediators [84]. Aside from its role as a downstream effector of the Hippo pathway, YAP has a role in several important carcinogenic pathways, including the mTOR and WNT pathways, which enhances its influence on cancer development [40, 85]. YAP/TAZ expression is typically upregulated in several cancers, such as the lung, breast, liver, and GC. Only minor YAP expression could be seen in the normal proliferating cells of the gastric mucosa in adults, but YAP expression is elevated in both metastatic and primary GC [40].

YAP and TAZ are identified as attractive therapeutic targets for the treatment of cancer due to their elevated expression in malignant cells relative to normal tissues and their crucial function in several stages of cancer cell survival and drug resistance [86, 87]. Since lymphatic invasion and a significantly lower overall survival are strongly associated with overexpression of YAP, it is possible that YAP could be used as a prognostic biomarker for GC. In addition, suppressing YAP inhibits proliferation, invasion, and metastatic spread in several GC models, suggesting it could be used as a therapeutic target in the treatment of this disease [88, 89].

Many studies have been done in an attempt to treat GC targeting YAP/TAZ. Giraud et al. showed that Verteporfin, a benzoporphyrin derivative, reduced the proliferative properties of GC stem cells by regulating Hippo/YAP signaling. Verteporfin caused anti-GC effects by inhibiting YAP expression, binding to YAP selectively and changing its structure, and interfering with YAP interactions with other molecules [90]. VGLL4 is a peptide that competes with YAP in binding to its target TEAD. Jiao et al. synthesized a VGLL4 mimetic named Super-TDU which reduced YAP-TEAD interaction by competitively binding to TEAD. Super-TDU inhibited GC cell growth and proliferation by this mechanism both in vivo and in vitro [91]. Sitagliptin which is an oral anti-diabetic agent has recently shown some anti-cancer effects. Wang et al. showed that Sitagliptin inhibited YAP by inducing its phosphorylation and inhibiting its translocation to the nucleus. Additionally, via controlling YAP, sitagliptin suppressed the production of tumor-testis antigen Melanoma-associated antigen-A3. Furthermore, sitagliptin generally inhibits GC by controlling AMPK/YAP/melanoma-associated antigen-A3 signaling [92].

In addition to agents directly targeting YAP, upstream regulators of Hippo/YAP signaling by inhibiting YAP expression could be important targets in GC treatment. Tang et al. revealed that an STRN3-containing PP2A complex dephosphorylates MST1/2 kinases, activating YAP and blocking the Hippo pathway. They synthesized a peptide called STRN3-derived Hippo-activating peptide (SHAP) that suppresses YAP activation by blocking the STRN3-PP2A interaction. SHAP has shown anti-GC effects and inhibited GC progression in vivo [93]. Song et al. evaluated the efficacy of tertiary amide derivatives incorporating benzothiazole moiety against GC. Some of their developed compounds which were named F1-F14 have shown antiproliferative effects against GC cells. In particular, compound F10 suppressed YAP via activating the Hippo signaling pathway and also induced apoptosis, and inhibited GC cell proliferation in vitro [94]. According to Seeneevassen et al., the Hippo signaling pathway reduces YAP/TAZ accumulation in GC stem cells when leukemia inhibitory factor (LIF) is stimulated. Also LIF by inhibiting YAP/TAZ mitigated chemoresistance and proliferative characteristics of GC stem cells [95]. Also, Ursolic Acid (UA) has shown anti-GC effects via indirectly targeting YAP/TAZ. Kim et al. showed that UA by inducing RASSF1 expression, activated the Hippo signaling pathway and inhibited YAP accumulation in GC cells, and mitigated proliferative and invasive characteristics of GC cells [96]. Essential oils extracted from the Pinus Koraiensis (EOP) plant have also shown anti-GC effects by targeting YAP. Zhang et al. demonstrated that EOP by inhibiting YAP expression mitigated proliferative and invasive characteristics of GC cells. Apoptosis has been demonstrated to occur when EOP is applied to GC cells in vitro, as well as cell cycle arrest in the G2/M phase when EOP is applied to these cells in vitro [97]. In another similar study, Ye et al. reported that CL-6, a derivative of commonly used phytochemical curcumin, suppressed proliferation and invasion and induced apoptosis in GC cells. In vitro analysis showed that regulation of Hippo/YAP signaling could be involved in these effects [98]. Overall, YAP inhibitors offer a variety range of therapeutic applications, although further studies on their effects and toxicity in GC are required.

mTOR inhibitors

The primary effector of the mTOR/PI3K/AKT signaling pathway is the mammalian target of rapamycin (mTOR) [99]. Since a range of upstream molecules can activate it and act as a junction for multiple signaling pathways, mTOR is one of the most autonomous elements of this axis [100]. mTOR regulates angiogenesis, cell survival, cell metabolism and by enhancing the expression of anti-apoptotic proteins, nutrient transporter proteins, and HIF-1/HIF-2 [101].

The regulation of GC cells’ development, uncontrolled proliferation, and differentiation may be influenced by overactive mTOR [41]. In 60–80% of stomach adenocarcinomas, inappropriate mTOR activation has been discovered at various stages of the disease [102, 103]. Mutations in upstream regulators including PTEN, PI3K, and EGFR are primarily responsible for activating mTOR and associated signaling pathways in GC [100]; However, nothing is known about the inherent alterations of this protein kinase in GC. Genetic mutations or other effectors may be responsible for mTOR’s activation in GC patients, but regardless, mTOR phosphorylation is linked to several clinical and prognostic aspects [104, 105]. Based on the results of a cohort study on GC patients, it appears that tumor growth and 5-year survival are closely related to the phosphorylation of this protein kinase [105]. Thus, several selective inhibitors of mTOR may be effective in treating GC due to its fundamental role in GC pathogenesis.

The multi-protein complexes mTORC1 and mTORC2 are identified as two unique subtypes of mTOR. According to this classification, 2 generations of mTOR inhibitors have been developed; The first generations of mTOR inhibitors have only The capability to suppress mTORC1 and include agents like rapamycin, temsirolimus, and everolimus. However, the second-generation mTOR inhibitors are effective against both mTORC1 and mTORC2. For instance, AZD8055 and AZD2014 are two second-generation mTOR inhibitors that have shown promising results against HER-positive and TSC1/2 mutated or null GC cells, respectively [99].

Rapamycin was first used as an antifungal agent. Numerous studies have been conducted to assess Rapamycin’s effectiveness against GC due to its anticancer properties. Matsuzaki et al. showed that Rapamycin and Temsirolimus (CCI-779) could act as a chemosensitizer of GC cells to 5-FU treatment. These mTOR inhibitors by decreasing IC₅₀ of 5-FU against GC cells increased the apoptotic rate after 5-FU treatment. So, because of the synergistic effects of these agents, mTOR inhibitors could be used as an adjuvant to 5-FU in chemotherapeutic regimens of GC patients [106]. Rapamycin also by inhibition of mTOR inhibits lymphangiogenesis of gastric tumors. Chen et al. showed that GC cells’ Akt/mTOR inhibition dramatically reduced the expression levels of VEGF-C and VEGF-D. And inhibition of the Akt/mTOR-VEGF-C/VEGF-D axis by Rapamycin could be of significance in the treatment of GC [107].

A Food and Drug Administration (FDA)-approved derivative of rapamycin, everolimus (RAD001) is used to treat a variety of tumors, including breast cancer and renal cell carcinoma [108]. Despite preclinical in vitro studies confirming some anti-GC roles of Everolimus, a phase II clinical trial failed to show an Important response of patients with advanced GC to this agent as an adjuvant to the chemotherapeutic regimen [109]. In a different study, Fukamachi et al. demonstrate that everolimus and another mTOR inhibitor, temsirolimus, are both effective only against a special subset of diffuse-type GC which harbors PIK3CA mutation and may be developed from intestinal-type GCs [110]. Additionally, Park et al. reported a successful experience using everolimus as salvage treatment in a patient with metastatic GC who had PIK3CA and pS6 abnormalities. This case study also highlighted how crucial it is to use these biomarkers to forecast how well GC patients will respond to everolimus treatment [111]. Against GC, BEZ235, a dual PI3K/mTOR inhibitor, has demonstrated greater therapeutic success than everolimus. Also, the effectiveness of BEZ235 against paclitaxel-resistance GC could be important as a part of a strategy to treat drug-resistant GC targeting the PI3K/Akt/mTOR pathway [112].

Some medications that are not commonly classified as anti-cancer agents and some natural products by exerting inhibitory effects against mTOR and upstream regulators of mTOR have shown anti-GC properties. Mefloquine, an FDA-approved antimalarial drug, has shown synergistic effects with paclitaxel against GC cells by inhibiting phosphorylation of mTOR and other components of the PI3K/mTOR/Akt pathway. Mefloquine and Paclitaxel together increase apoptosis and suppress the proliferation of GC cells [113]. According to Tang et al., Tigecycline, a glycylcycline class antibiotic, inhibits mTOR phosphorylation through activation of the AMPK pathway, thereby inducing autophagy in GC cells and inhibiting cell growth [114]. Salidroside is a phytochemical extracted from the Rhodiola rosea plant. Rong et al. reported that Salidroside through inhibition of expression of mTOR induced autophagic changes in AGS GC cells. In vivo and in vitro, Salidroside also induced apoptosis and inhibited cell growth in GC cells [115]. N-butylidenephthalide (NBP) is a bioactive compound extracted from the Radix Angelica Sinensis plant. In a case-control study, Liao et al. found that GC patients taking herbal medications containing NBP were more likely to survive and had a lower mortality rate. Then, the in vitro studies that have been undergone to clarify the molecular mechanism of anti-GC effects of NBP showed that NBP by enhancing the expression of REDD1 inhibited the mTOR signaling, induced apoptosis, and inhibited proliferation of GC cells [116].

Immunotherapy-based immune checkpoint inhibitors (ICIs)

GC patients now have a more comprehensive treatment landscape because of newly developed treatments like immunotherapy with ICIs [117]. Since immunotherapy’s unique and long-lasting anti-cancer effects have been extensively researched, it has been widely accepted as a successful cancer treatment approach. Immunotherapy through ICI as a promising strategy in cancer treatment can prevent the recurrence of tumors [118]. Various methods to identify immunosuppressive components that thwart anti-tumor immune responses are necessary for significant advancement in cancer immunotherapy. To prevent immunological damage, immuno-inhibitory molecules often suppress immune responses by adversely altering immune cell communication pathways. Nevertheless, Cancer immunity is suppressed during tumor formation when inhibitory immunological checkpoints are activated within immune cells [119]. Programmed cell death protein-1 (PD-1)/programmed death ligand-1 (PD-L1) and Cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) are considered as two main co-inhibitory immune checkpoints. The inhibition of the binding of ligands to checkpoint receptors is a factor contributing to the reactivation of immunity in cells [51, 120, 121]. Hence, In the sections that follow, we will concentrate on the clinical advancements of ICIs and their combination with targeted therapy for GC.

Targeting immune checkpoint in GC

To date, the use of mAbs as ICIs has received remarkable regard for cancer therapy. T-cell activation is suppressed via the inhibitory signals by different immune checkpoint molecules in interaction with their cognate receptors such as PD-L1/PD-1 axis, CD28 pathway, and CTLA-4 or cytotoxic T-lymphocyte antigen 4 [122]. Antigen-presenting cells (APCs) attach CTLA-4 to B7 ligands, preventing B7 interaction with CD28 receptors on TCD4s. As a result, CTLA-4 blocks the CD28 stimulatory signal from reaching T cells in particular, anti-CTLA-4 antibodies bind to CTLA-4 and inhibit T-cell suppression. Ipilimumab and tremelimumab are the two main antibodies used as anti-CTLA-4 antibodies [122, 123]. Ipilimumab, the first anti-CTLA-4 human monoclonal antibody (IgG1), was authorized by the FDA in early 2011 to treat patients with metastatic melanoma [124]. Additionally, By blocking the interaction between CTLA4 and its ligand, tremelimumab, a monoclonal antibody that is completely human, increases cellular immunity [125]. A phase Ib/II trial study found that 12 patients with GC and gastroesophageal junction (GEJ) cancer who received tremelimumab as a second-line treatment after chemotherapy had a median overall survival (OS) of 7.7 months and a median progression-free survival (PFS) of 1.7 months [126]. Ralph et al. carried out another trial on the use of tremelimumab as a second-line treatment for patients with metastatic gastric and esophageal adenocarcinomas in order to block CTLA-4. The results demonstrate that one patient experienced a remarkably long-lasting benefit for this poor prognosis condition, despite tremelimumab’s low response rate. Increased proliferative responses to pertinent tumor-associated antigens are also indicated by the in vitro results. In conclusion, their study showed that tremelimumab can be effective when combined with antigen-targeted therapy and warrants further investigation [127]. According to the evidence, further investigations are needed to identify the benefit of tremelimumab as monotherapy or combination therapy in GC treatment.

As a negative costimulatory immunological checkpoint, the surface receptor PD-1 contributes to the suppression of T cell anti-tumor activity and helps tumor cells evade the immune response [122]. As a PD-1 ligand, PD-L1 is expressed on APCs as well as on different types of tumor cells. PD-1 and PD-L1 interact to induce immunosuppressive signaling pathways, suppress T-cell activity, and escape tumor cells [51]. Inhibiting anti-tumor immunity and increasing chemoresistance are effects of elevated PD-L1 expression in human malignancies [118]. Noteworthy, According to the findings, patients with GC bear a significantly high level of PD-L1 expression [128]. The PD-1 antibody pembrolizumab showed promise in treating patients with advanced GC in 2016, according to the phase Ib (KEYNOTE-012) trial. According to this study, GC patients with positive PD-L1 expression can experience anti-tumor immune responses from pembrolizumab [129]. The FDA originally authorized nivolumab, an anti-PD-1, in 2014 for the treatment of melanoma patients [130]. In patients with advanced GC who have received two or more chemotherapy regimens, nivolumab may improve overall survival (ONO-4538-12, ATTRACTION-2). According to the phase III trial, the nivolumab group’s median overall survival was 5.26 months, whereas the placebo group’s was 4.14 months. These results suggest that nivolumab may be a novel therapeutic option for patients with advanced GC who did not respond to earlier chemotherapy therapies [131]. Combining anti-PD-1 antibodies and anti-CTLA-4 considerably reduced invasion, inhibited migration, epithelial-mesenchymal transition (EMT), cell proliferation, and induced apoptosis in MGC-803 and MKN-45 cells. Moreover, the activation of signaling pathways such as MAPK, β-catenin, and PI3K/AKT was inhibited by CTLA-4 and PD-1-blocking antibodies. Consequently, anti-CTLA-4 and anti-PD-1 antibody combination therapy produced encouraging outcomes in GC patients [132]. Additionally, a phase I/II CheckMate 032 study showed encouraging clinical outcomes for patients with advanced (adv)/metastatic chemotherapy-refractory (CTx-R) GEJ cancer or gastric (G), esophageal (E) cancer when nivolumab (as anti-PD-1) was used alone or in combination with ipilimumab (as anti-CTLA4) [133]. Furthermore, ICI therapy may improve some but not all survival endpoints for patients with advanced or metastatic G/GEJ cancer, suggesting a small benefit with fewer side effects, according to a systematic review and meta-analysis study. Patients with PD-L1 positivity, MSI-H, EBV positivity, or a high tumor mutational burden have been found to benefit more from anti-PD-1/PD-L1 therapy [134].

A few individuals with cancer may experience negative effects from using blocking antibodies to target immunological checkpoint members, which could limit their treatment options. Adverse effects are one of the challenges and restrictions in front of ICI therapies [118, 122]. IrAEs or immune-related adverse events are immune-related diseases such as endocrinopathies, pneumonitis, colitis, hepatitis, and rash. All things considered, the immune system’s capacity to fight malignancies like GC can be enhanced by antibodies that target immunological checkpoint proteins. As a result, irAEs must be managed and treated in order to decrease, and using immunosuppressive medications is a good way to do so [118].

Combination approaches of ICIs and targeting therapy in GC

Although immune checkpoint therapy generally has satisfactory results, due to limitations in tumor immunity some patients do not achieve the desired outcomes. Therefore, combining techniques designed for different targets increases the development of various mechanisms, which enhances response rates [119]. Recent strategies to use targeted therapy plus mAbs have attracted considerable regard in several cancer treatments. The primary candidates for GC therapy in conjunction with ICIs are VEGF/VEGFR inhibitors and anti-HER2 mAbs [51]. Before this research, trastuzumab, an anti-HER2-monoclonal antibody, was the first-line treatment of choice for advanced GC patients with HER2-positive tumors [51]. A study by Rha et al. at 2020 proposed that combining chemotherapy with anti-PD-1 and HER2 targeting results in a considerable reduction of GC tumor size. For patients with advanced GC who tested positive for HER2, pembrolizumab, trastuzumab, and chemotherapy were used as the 1st line triple therapy [135]. Additionally, another study looked at the efficacy and safety of pembrolizumab in combination with chemotherapy and trastuzumab for patients with HER2-positive 1st line metastatic oesophagogastric (GEJ, or G, E) cancer. According to this trial, pembrolizumab is safe when combined with trastuzumab and chemotherapy, and it shows promise in treating HER2-positive metastatic EC [136].

In addition, a phase III study (KEYNOTE-811) evaluated the combination of trastuzumab and chemotherapy for patients with unresectable or metastatic GC or GEJC who were HER2-positive (NCT03615326). The results of the trial showed that pembrolizumab plus trastuzumab and chemotherapy greatly increases the objective response rate, significantly diminishes tumor size, and causes full responses in certain patients [137]. The tolerability, safety, and anticancer efficacy of margetuximab with pembrolizumab were evaluated in a single-arm, phase Ib–II trial (CP-MGAH22–05) in patients with HER2-positive gastro-oesophageal adenocarcinoma who had received prior treatment. The results of this study demonstrated that margetuximab and pembrolizumab had synergistic anticancer efficacy [138].

The completely humanized monoclonal antibody ramucirumab blocks VEGFR-2. As a second-line treatment for GC or GEJ adenocarcinoma, ramucirumab is approved using either alone or with paclitaxel. There are many current studies examining the Ramucirumab function when combined with new agents in both metastatic and perioperative situations. In particular, the combination of ramucirumab with ICIs is being studied in several active clinical trials [139]. GC/GEJC, hepatocellular carcinoma (HCC), and non-small-cell lung cancer (NSCLC) patients were studied in an open-label, phase Ia/b study using ramucirumab (an anti-VEGFR2 IgG1) combined with durvalumab (an anti–PD-L1 IgG1). The safety of durvalumab and ramucirumab was in line with the established safety profiles of individual treatments. The combination had an antitumor effect in all cohorts, especially in patients with elevated PD-L1 expression exhibited the strongest effects. These findings suggest that VEGF/VEGFR inhibitor combinations may be potent treatment options for advanced GC [140].

CAR T cell engineering

An additional type of immunotherapy known as CAR T Cell (chimeric antigen receptor) therapy, has recently attracted considerable interest among oncologists and researchers. While the FDA has approved mainly CAR T cell treatments for hematologic cancers, including lymphomas, leukemias, and multiple myomas, they appear to be among the most effective treatments for solid tumors, and a rising number of clinical trials are now emphasizing solid tumors like GC [141]. The use of animal xenotransplantation has shown that CAR T-cells have antitumor effectiveness and prolonged activity against GC in vivo and in vitro [142].

This immunotherapy approach involves isolating the patient’s peripheral blood T cells and genetically modifying them to express chimeric antigen receptor (CAR) genes by plasmid transfection, mRNA, or viral vectors; The T cells can now identify tumor-associated antigens because of this alteration [143]. A CAR consists of three components: an antigen-binding domain located extracellularly, a transmembrane spacer, and an intracellular signaling/activation domain [144]. A recombinant polypeptide called a single-chain variable fragment (scFv) is created when the variable light (VL) and variable heavy (VH) chains of mAbs are connected by a flexible linker to produce the extracellular domain. Without the limitations of MHC restriction, this fragment enables direct binding to tumor-associated surface antigens. The transmembrane domain, which is mostly generated from immunoglobulin G4 or CD8 molecules, is essential for strengthening the stability of the chimeric antigen receptor (CAR) and creating a structural connection between the endodomain and ectodomain. In turn, the intracellular domain mediates intracellular signal transduction required for cellular activation and guarantees strong membrane attachment of the CAR [143, 144]. The Fc receptor motifs or the CD3 chain mediate the first intracellular signals for T-cell activation. Furthermore, co-stimulatory signaling domains like CD28, CD134 (OX40), CD137 (4-1BB), or DNAX-activating protein 10 (DAP10) are frequently included in the endodomain. These co-stimulatory factors boost effector functions, cytokine generation, and T-cell proliferation. Numerous generations of chimeric antigen receptors (CARs) have had their integration thoroughly studied, leading to notable improvements in CAR performance and design [145]. From its initial design, which only contained the CD3 signaling domain (referred to as a “first-generation CAR”), CARs have undergone structural changes, evolving into more sophisticated variants that include co-stimulatory endodomains. Second-generation CARs, which combine CD3 with either the 4-1BB or CD28 signaling domains, and third-generation CARs, which combine CD3 with both the 4-1BB and CD28 signaling domains, have been developed as a result of this progression and exhibit enhanced T cell proliferation and persistence [146]. An outline of CAR-T cell therapy’s development and clinical application procedure is shown in Fig. 3.

Fig. 3

A summary of CAR-T cell therapy construction and the clinical application process. (1) Obtaining Blood from the patient, (2) CAR gene insertion, (3) Antibody receptor expression, (4) CAR-T Cell Expansion, (5) Pharmaceutical interventions, (6) Injections of CAR-T cells

An overview of ongoing CAR T cell trials in GC

As previously noted, CAR-T cell therapy has received approval for the treatment of specific hematological malignancies; however, its application in the management of solid tumors remains unapproved. Target selection is one of the most critical challenges and obstacles to implementing Therapy with CAR-T cells for solid tumors. The majority of solid tumors are of epithelial origin, resulting in tumor antigens often being expressed on the surface of healthy epithelial cells. Ideally, tumor-associated antigens (TAAs) should exhibit selective expression on tumor cells to minimize the risk of off-target effects. Failure to achieve this specificity can lead to suboptimal therapeutic outcomes and potentially severe or fatal off-target toxicities [145]. For example, mesothelin, one of the antigen targets, is found not only on the surface of mesothelioma cells but also on pericardial and pleural surfaces [147]. The development of CAR T-cell immunotherapy for GC is a major problem since tumor cells can evade host immune surveillance due to the incredibly varied surface antigen expression in GC [148].

The high death rate of GC and the development of drug resistance in existing treatments highlight the need to develop CAR T-cell immunotherapy as a possible treatment option for specific patients. There is mounting evidence linking GC patients who overexpress human epidermal growth factor receptor 2 (HER2) to a poorer prognosis and a more severe disease profile. Research has demonstrated that CAR-T cells, which target HER2 antigens in an MHC-independent way and exhibit strong and specific cytotoxic activity, can successfully eliminate patient-derived HER2-positive GC cells [142, 149]. Not only HER-2 antigen but also in a preclinical investigation, CAR T cells that were directed against claudin18.2 (CLDN18.2) demonstrated promise efficacy against GC [150]. Additionally, The surprising effectiveness of autologous Claudin18.2-targeted CAR T cells (CT041) in treating a patient with metastatic GC that had previously advanced following four cycles of systemic chemotherapy and immunotherapy was reported in a recent case study by Botta et al. The patient demonstrated significant CAR T-cell expansion, a radiologic response, and marked reductions in tumor-informed circulating tumor DNA (ctDNA). These results underline the therapeutic potential of using CAR T-cell treatment to target Claudin18.2-positive GC and demonstrate the usefulness of ctDNA as a biomarker in CAR T-cell therapy [151]. Moreover, according to studies on CEA-specific CAR-T cell treatments, employing CAR T-CEA cells enables mice with advanced GC to survive longer and suppress tumor growth, especially recombinant therapy with IL-12 can improve CAR-T cells’ anti-cancer function [6].

Therefore, a variety of clinical trials have identified the following as critical markers for the diagnosis and management of GC: HER2, claudin 18.2 (CLDN 18.2), mesothelin (MSLN), folate receptor 1 (FOLR1), carcinoembryonic antigen (CEA) and other markers which are listed in Table 3 of ongoing clinical trials [150, 152, 153].

Table 3 ClinicalTrials.gov-registered CAR T cell treatment trials for GC

Challenges of CAR T cell therapy

Incidence of toxicity is the major obstacle to the development and progress of CAR T-cell therapy for GC patients; the most frequent and serious side effect linked to CAR T-cell therapy is cytokine release syndrome (CRS), a form of non-antigen-specific toxicity that can result in respiratory distress syndrome and multiple organ failure [145]. This toxicity arises as a result of the rapid and improper activation of several cytokines, including TNF α, IL 1, IL 8, IL 6, IFN β, and IFN γ [154]. Additional adverse effects that have been documented include cytopenia, tumor lysis syndrome, off-target/on-target toxicities that can cause multi-organ damage, and neurotoxicity, commonly referred to as CAR T-cell–related encephalopathy syndrome (CRES) [155]. These adverse events can become serious if not treated promptly, but they are manageable and reversible with the right treatment options like anti–IL–6 receptor antagonists and corticosteroids [155]. Therefore, a full awareness of the related adverse events particularly their grading and management is vital for advanced practitioners responsible for the care of patients receiving these therapies.

Antibody-drug conjugates (ADCs)

Monoclonal antibody-drug conjugates (ADCs) are biological agents composed of mAbs attached to cytotoxic payloads by chemical linkers [156]. According to the mechanism, the ADC enters tumor cells and delivers a cytotoxic payload that kills the cells. On the surface of cancer cells, specific receptors such as mesothelin, HER2, and guanyl cyclase C (GCC) are overexpressed, which leads to this effect [157]. Because of ADC’s tumor selectivity, the maximum tolerated dosage may be increased while the therapeutic dose is lowered, hence improving the therapeutic window and minimizing off-target damage [158]. While most ADCs have been developed to treat hematological malignancies, new ADCs targeting HER2, such as trastuzumab deruxtecan and trastuzumab emtansine, are showing promising results in solid tumors as well [157].

The GATSBY study was the first phase II/III study to assess an ADC’s efficacy in terms of GC. Patients with metastatic GC who had received prior treatment were evaluated by GATSBY for the effectiveness of trastuzumab emtansine. The trial’s findings indicate that trastuzumab emtansine does not provide more benefits than taxane therapy for patients with HER2-positive GC who have already had treatment [159]. An ADC consisting of a cytotoxic topoisomerase I inhibitor, a linker, and an anti-HER2 antibody, called trastuzumab deruxtecan (DS-8201), is known as a HER2-targeted ADC. Shitara et al. investigated the efficacy of this ADC in individuals with advanced GC who tested positive for HER2. When compared to patients receiving standard regimens, patients with HER2-positive GC who underwent trastuzumab deruxtecan therapy showed considerable improvements in response and long-term survival. The two most noteworthy harmful consequences were interstitial lung disease and myelosuppression [160].

Advanced gastrointestinal malignancies have also been studied in relation to ADCs that target proteins other than HER2, such as guanylyl-cyclase-c (GCC), mesothelin, and carcinoembryonic antigen associated CEACAM5 or cell adhesion molecule 5 [161, 162]. ADCs targeting targets other than HER2, however, have not yet demonstrated clinically significant efficacy and need more investigation. Table 4 lists a number of ADCs that are under active investigation for GC.

Table 4 ADCs under clinical investigation for the treatment of GC

Conclusion and future perspective

Overall, the manuscript highlights the heterogeneity of the GC microenvironment and the potential for targeted therapies and immunotherapies to improve treatment outcomes. Preclinical and clinical research has indicated promise in targeting particular molecules implicated in angiogenesis, immune evasion, and tumor-stroma interactions. Our article highlights that immunotherapy has greatly expanded treatment options for GC (GC), particularly through the use of ICIs like pembrolizumab and nivolumab. These therapies show promise in patients with high PD-L1 expression and specific biomarkers, and combination treatments with anti-PD-1 and anti-CTLA-4 antibodies have also demonstrated encouraging results. Moreover, targeted therapies addressing key molecular pathways, including RTKs, FGFR signaling, PIM kinases, and YAP/TAZ inhibitors, are emerging as additional options for GC treatment. Ongoing research is exploring other molecular targets, including the EGFR, MET, and more features of FGFR pathways, to further expand the repertoire of targeted therapies for GC. While several of these agents have shown preclinical promise, clinical trials have highlighted challenges such as limited survival benefits and the development of drug resistance, emphasizing the need for ongoing research to optimize their use.

Additionally, CAR-T cell therapy represents a promising strategy, although its application in GC faces challenges related to target selection and the risk of off-target effects. CAR-T cells targeting antigens like HER2 and claudin18.2 have shown potential in preclinical studies and clinical trials, with markers like HER2 and CEA emerging as key targets for treatment. Antibody-drug conjugates (ADCs) have also garnered attention in GC treatment. These agents combine mAbs with cytotoxic drugs to target overexpressed tumor cell receptors such as HER2, mesothelin, and guanylyl cyclase C (GCC), thereby enhancing treatment specificity. While new HER2-targeted ADCs like trastuzumab deruxtecan have shown promising results in HER2-positive GC, others targeting different markers such as GCC and mesothelin have shown limited clinical success and need further investigation. The future perspective for GC treatment lies in personalized medicine approaches that take into account the heterogeneity of the GC microenvironment and the molecular characteristics of individual tumors. Optimizing the sequencing and combination of targeted therapies and immunotherapies will be essential to maximize treatment efficacy.

The narrative nature of our review allows for a subjective selection of studies, which may lead to potential biases in the representation of the literature. Due to the broad scope of recent advances in GC treatment, it is possible that not all relevant studies were included, which may limit the comprehensiveness of our findings. The included studies vary in methodology and quality, which may affect the reliability of the conclusions drawn from the synthesized information. Unlike systematic reviews, our narrative approach does not provide quantitative assessments or meta-analyses, which could have offered more robust insights into the effectiveness of targeted therapies.

In conclusion, this manuscript offers a detailed synthesis of the current knowledge regarding the GC microenvironment, emphasizing the promise of targeted therapies and immunotherapies in enhancing treatment efficacy. Continued investigation, coupled with well-designed clinical trials, will be essential to advancing personalized therapeutic strategies and improving prognostic outcomes for GC patients.