Lung cancer is one of the most common cancers, with a 5-year survival rate of only 27%, and non-small cell lung cancer (NSCLC) is the most common subtype (1). Of note, ~57% of patients with NSCLC are initially diagnosed at an advanced stage and 40–50% of patients develop brain metastases (BMs) during the disease progression due to the limited screening methods (2,3). Currently, the primary treatment of patients with NSCLC with BMs is systemic therapy coupled with local therapy, such as surgery and radiotherapy (RT) (4). For instance, the 2022 American Society of Clinical Oncology-Society for Neuro-Oncology-American Society for Radiation Oncology guidelines (5) recommend that patients with asymptomatic BMs from NSCLC receive stereoscopic radiosurgery (SRS) or stereoscopic body RT alone for treating one to four lesions. RT should be provided after surgery in patients with one or two BMs, because it is indispensable in the treatment of brain metastases. Furthermore, targeted therapy is preferred for patients with BMs that are vital for driver genes, such as Epidermal Growth Factor Receptor gene mutations can be treated with 1–3 generation targeted drugs. But there is insufficient evidence to recommend any systemic therapy, specifically for intracranial tumour control in patients who are negative for driver genes (4–6). In summary, the current treatments for BMs are limited, which markedly affects the overall prognosis of patients with NSCLC with BMs (7).

Angiogenesis and immune escape are key elements in tumour formation and the growth of metastatic brain tumours in NSCLC is critically dependent on angiogenesis (8). Anti-angiogenic drugs (AADs) normalise blood vessels, improving access to the tumours, which is useful when combined with immunotherapy (IT) methods. This combined treatment may have a synergistic effect on the tumour immune microenvironment (TIM) to inhibit the occurrence of tumours (9–11). In addition, anti-angiogenic therapy (AAT) can modulate the tumor immunosuppressive microenvironment and help to reverse IT resistance (ITR) (12). Previous studies have reported that IT combined with AAT has tolerable toxicity and efficient antitumour activity in patients with advanced NSCLC receiving later-line treatment (13). Furthermore, AADs cause dehydration, which creates conditions for RT to further control tumour progression and effectively improve radiation-induced brain injury (14).

The emergence of IT has become a milestone in antitumour therapy. However, the frequency of ITR is also increasing with the use of immunosuppressants (15). Although the TIM is an abundant source of therapeutic targets (16), the understanding of the TIM in the brain lacks comprehensive and integrated analyses. Nevertheless, the importance of the TIM cannot be ignored in the whole treatment process of patients with NSCLC with BMs (17). Research into the mechanisms will provide a more comprehensive understanding of the treatment of diseases. Accordingly, the present article aimed to elaborate on the TIM and clinical application of AAT, IT and RT in patients with NSCLC with BMs, in order to broaden the ideas for subsequent clinical research and treatment strategy design.

The occurrence of BMs from NSCLC is a complex process and the TIM serves an indispensable role (18). Studies have demonstrated that monocyte-macrophages, neutrophils, CD4⁺ T cells and CD8⁺ T cells serve a decisive role in the TIM of BMs from lung cancer (9,19). When disseminated tumour cells meet these immune cells, complex interactions occur, forming tumour-promoting and tumour-suppressing environments (20). Due to differences in the unique composition of the extracellular matrix in terms of collagen, hyaluronic acid and enzymes, the TIM of the brain has unique characteristics, which are radically different from those of the TIM of primary lung cancer (21). In NSCLC BM, tumour cells change the function of cell junctions after penetrating the blood-brain barrier, which leads to an altered intracranial TIM (22). For instance, patients with lung adenocarcinoma show a more distinctive pattern of low infiltration of CD3⁺ and CD8⁺ lymphocytes, suggesting that most patients lack a meaningful local immune response associated with BMs (23,24). An analysis of immunohistochemical results by Kim et al (25) showed that the number of programmed cell death protein 1 (PD-1) tumour-infiltrating lymphocytes was markedly lower in BMs compared with primary lung cancer. As shown in Fig. 1, another study revealed that PD-1-positive tumour-infiltrating lymphocytes had low intracranial expression and high primary lung sites in patients with NSCLC with BMs due to their lower permeability. Programmed cell death protein ligand 1 (PD-L1) is generally highly expressed in metastatic sites and is highly expressed in the brain, which is one of the reasons for the application of IT (26). These findings reveal that the efficacy of immune checkpoint inhibitor (ICI) agents is different. In other words, tumors with high PD-L1 expression are more likely to respond to ICI. Besides, chemotherapy, targeted therapy, RT and other antitumour therapies also affect the TIM (27–29).

In 2022, Li et al (30) performed ribonucleic acid sequencing on 86 samples from 43 patients with primary lung tumours and corresponding BMs to analyse the TIM. The results revealed that BMs were more immunosuppressed compared with primary lung tumours. In addition, the article by Wang et al (31) explored the mechanism of BMs in NSCLC, further highlighting the role of the TIM in the treatment of metastatic tumours. A previous study has concluded that combination therapy can increase the expression of PD-L1 in NSCLC BMs, thereby enhancing the treatment efficacy in these patients (32). In conclusion, in the era of IT, numerous treatment methods have different effects on the intracranial immune microenvironment of patients with BMs. The combined treatment has a synergistic effect and the impact on the microenvironment has become more evident (33,34).

RT is an effective local treatment for NSCLC and it can affect the intracranial immune microenvironment. It eliminates tumour cells by inducing double-stranded deoxyribonucleic acid (DNA) damage and regulating antitumour immune responses at both irradiated and non-irradiated sites (35). As a commonly used treatment for BMs, RT can promote tumour-associated anti-immunity and enhance reoxygenation in the TIM, thereby promoting the recruitment of several immune effector cells and helping them infiltrate tumours (36,37). In addition, RT can cause immunogenic cell death and promote the release of tumour associated antigens, thus serving a synergistic role with IT (38). RT combined with IT has shown notable efficacy and can control tumour progression and prolong the survival of patients with NSCLC. Notably, studies have demonstrated that PD-L1 expression is mediated by the PI3K/AKT/mTOR pathway, which controls numerous cellular processes, including PD-L1 translation and transcription (39). Stimulation to enhance the AKT/mTOR pathway increases PD-L1 expression and vice versa. Downstream elements of AKT stimulate gene transcription by binding to the PD-L1 promoter. Furthermore, NF-κB, a downstream signal transducer of AKT, also acts on the promoter to induce PD-L1 mRNA expression (40). RT activates PI3K/AKT signalling through various pathways, such as the DNA damage response, reactive oxygen species (ROS) activation, inflammatory factor expression and growth receptor expression, leading to PD-1 expression and radioresistance (41).

Immunosuppressive agents can eliminate radioresistance caused by the upregulation of PD-L1 induced by RT and enhance its efficacy (42). Ikarashi et al (43) reported that CD4+ T cells, CD8+ T cells and CD4 Foxp3+ T cells were significantly higher in the tumor and stromal regions in primary lung cancer specimens, whereas only CD204+ cells were elevated in the tumor regions of brain metastases. CD4+ and CD4 Foxp3+ T cells were significantly increased in brain metastases with radiation therapy in the cancer and stromal regions compared with patients without radiation therapy. These macrophages may be immunosuppressive and make the immune environment less reactive (44,45).

Previous studies have demonstrated that IT combined with RT, IT combined with AAT and RT combined with AAT achieve notable clinical benefits in the treatment of patients with NSCLC with BMs, and no serious grade 4–5 adverse reactions were reported in the process of combined treatment (137–141). However, no clinical trials have investigated the combination of these three treatments in patients with NSCLC with BMs. On the one hand, there is no clear indication for the combination therapy with these three treatments and most clinicians use combination therapy with two treatments (142,143). On the other hand, although there are safety reports on combination therapy, these reports are rare, and most are basic experiments and have not been translated into clinical practice. For example, combination therapy with aspirin, afatinib and vinorelbine was significantly more effective than monotherapy in NSCLC cell models (144,145). As a result, research into the simultaneous application of multiple treatments has been hindered.

AAT, IT and RT can affect the intracranial immune microenvironment of patients with BMs and exert antitumour effects. However, it currently remains elusive whether the simultaneous use of these three methods will synergistically change the TME and achieve stronger antitumour effects. To date, numerous studies have investigated the mechanism of AAT combined with IT and demonstrated that the combined therapies can have a stronger antitumour effect (146,147). However, relatively few studies have investigated the combination with RT. In animal experiments, RT combined with anti-PD-L1 and anlotinib therapy increased the number of tumour-infiltrating lymphocytes and reversed the immunosuppressive effect of RT on the TIM. These findings suggest that AAT may be a potential radioimmunotherapy synergistic therapy to achieve improved antitumour efficacy in patients with NSCLC by enhancing the TIM (148,149).

Previous mechanistic studies have shown that AAT promotes vascular normalisation and immune cell infiltration, RT provides antigen release and in situ vaccine effects and IT maintains T-cell activity, thereby forming a beneficial cycle (42,150). The molecular mechanisms by which these three factors regulate the TIM through multiple dimensions include the following: First, AAT serves a central role by targeting VEGF/VEGFR and other pathways to reduce tumour vessel density, inhibit abnormal angiogenesis, alleviate hypoxia, improve vascular permeability and promote T-cell penetration and drug delivery (89,151). Second, IT enhances antitumour efficacy by restoring T-cell function and promoting the antitumour immune response while synergistically activating T cells in combination with RT (89,152). Finally, RT can upregulate PD-L1 expression and enhance immune recognition while providing an antigen ‘immune’ effect, maintaining a long-term response to IT and increasing the anti-angiogenic therapeutic window (153). The combination of these three treatments not only reverses ‘immune-excluded tumours’ to ‘immune-inflamed tumours’ in the presence or absence of a strong immune response within the TME to enhance the antitumour effect but also alleviates single-drug resistance, providing long-term benefits for the treatment of advanced tumours (150).

The molecular mechanisms of RT in combination with IT and AAT for malignant tumour treatment are described in Fig. 3. RT irradiation leads to tumour cell death and the recruitment of numerous cytokines, chemokines and growth factors in the TME. In addition, RT causes the recruitment of immune cells while activating tumour antigen-presenting dendritic cells through the apoptosis of immunogenic tumour cells, the activation of innate immunity and the activation of cytotoxic T lymphocytes (CTLs). This antigenic ‘vaccine’ effect provided by RT could allow anti-PD-1 agents to further enhance CTL activation, induce immune memory and amplify systemic antitumour responses through an ‘abnormal effect’. AAT can inhibit abnormal angiogenesis, improve vascular structure and function in the short term and restore the blood vessels to normal. PD-1 IT releases interferon, which allows CTLs to have an antitumour effect through endothelial cells in structurally normalised blood vessels and cooperate with RT and IT (42,52).

The meta-analysis performed by Xian et al (154) demonstrated that in the treatment of solid tumours, ICIs combined with RT and AAT showed improved survival benefits compared with single or dual drug combination therapy, and triple therapy was tolerable and safe. However, the analysis did not identify specific cancer conditions to evaluate tolerability, and the safety of triple therapy in patients with NSCLC BMs has not been evaluated in the published literature. Therefore, all patients in Table I screened for NSCLC BMs were treated with double therapy, and the safety evaluation was acceptable. The main adverse reactions included leukopenia and other haematological toxicities, such as hypertension. The incidence of radiation-induced brain necrosis was not high, possibly because AADs increase vascular permeability, which not only improves brain oedema during RT but also reduces the occurrence of radiation-induced brain necrosis. However, AADs can markedly increase blood pressure. In clinical practice, certain patients cannot use these drugs due to difficulty in blood pressure control (155). For these patients, the dose should be reduced or used cautiously. In addition, no obvious immune-related adverse events occurred in patients receiving the two-combination therapy. The 2023 case report by Long et al (156) described a 55-year-old patient with BMs from advanced small-cell lung cancer who received combined treatment consisting of anlotinib and WBRT, with anti-PD-L1 IT in combination with anlotinib as long-term maintenance therapy. The OS was 41 months after the onset of BM and no serious adverse reactions occurred. The application of IR and TT has improved BM treatment and several studies have suggested that IT or TT is effective for patients with NSCLC with BMs (102,157). Therefore, combining the three therapies may have clinical benefits in the treatment of patients with NSCLC with BMs with no serious adverse reactions.

The results of previously published meta-analyses indicate that most studies on AAT combined with IT and RT consist primarily of animal trials, retrospective analyses, a limited number of case reports and single-arm trials with small sample sizes (154,158). These studies demonstrate that triple therapy can enhance the efficacy of antitumour treatment without inducing severe adverse reactions. To date, IT plus chemotherapy combined with AAT has achieved good results in a number of randomized controlled trials (159,160), and the results of certain clinical trials have demonstrated that RT plus chemotherapy combined with IT or AAT has effective antitumour treatment effects (161,162). However, there is still a lack of large clinical randomized controlled trials of RT combined with IT and AAT. Results of a phase II trial indicated that triple therapy plus chemotherapy showed promising antitumor activity and was well tolerated in advanced solid tumours (163). Therefore, further investigation is warranted into the integration of AAT with RT and IT (164,165). The present article provides an in-depth feasibility analysis of triple therapy for patients with NSCLC with BMs, concluding that such therapy improves patient survival with controllable safety. Based on these findings, a clinical observation project has been initiated for triple therapy in patients with NSCLC with BMs. The project has received ethical approval and is currently recruiting participants, aiming to encourage more institutions to perform large-scale clinical trials, which represents one of the key directions for future research.

WBRT is currently the first choice for patients with NSCLC with extensive BMs. SRS alone can be performed for patients with 1–4 metastatic lesions, SR can be provided after craniocerebral surgery for patients with 1–2 metastatic lesions and WBRT + SRS can be performed for patients with numerous metastatic lesions but no extensive metastases (5). A previous study demonstrated that patients receiving WBRT + SRS or SRS alone have similar intracranial control rates and survival benefits; however, patients receiving WBRT + SRS have more severe cognitive impairment (166). Therefore, regardless of the number of BMs, different RT methods can result in similar therapeutic responses, but when combined with systemic therapy, RT needs to be selected. For triple therapy, patients who can undergo SRS are more inclined to be selected to reduce adverse reactions. The application of local therapy is suitable for most patients with NSCLC with BMs, and their treatment response mainly differs with respect to the choice of systemic therapy (167).

According to the current clinical guidelines, targeted therapy is the first choice of systemic therapy for patients with NSCLC with BMs and positive driver genes (5). However, due to the blood-brain barrier, there is no strong evidence-based foundation for the systemic treatment of patients with negative driver genes (5). The results from the phase III IMpower150 study (168) revealed statistically and clinically notable PRS with combination therapy regardless of PD-L1 expression or EGFR or anaplastic lymphoma kinase mutation status. However, in patients with positive driver genes, targeted therapy seems to achieve improved survival benefits compared with patients without driver genes (168). Sintilimab plus anlotinib showed durable efficacy in metastatic NSCLC with rare EGFR mutations in a phase II study (169). Similarly, in another study, benmelstobart combined with anlotinib showed promising antitumor efficacy and a tolerable safety profile in patients with EGFR-positive advanced NSCLC after the failure of EGFR TKI (170).

As previously discussed, AAT can reverse immune resistance and exert more powerful antitumour effects in combination with IT. Therefore, it is important to evaluate the factors affecting IT and AAT efficacy. Tumours with high expression of PD-L1 (such as NSCLC) may be more sensitive to PD-1/PD-L1 inhibitors, and certain PD-L1-negative patients may still benefit. Therefore, the population suitable for IT may be selected by detecting PD-L1 expression in clinical practice. By selecting patients with a tumor proportion score (TPS) ≥1%, partially negative patients can also be evaluated for IT. In addition, different drugs can be selected based on the TPS value for more precise IT (171). The VEGF expression level can affect AAT efficacy; tumours with high VEGF expression may be more sensitive to AAT, but certain tumours with low VEGF expression may also be responsive. Serum/plasma VEGF levels can be used to predict treatment efficacy, and high VEGF levels may have greater effects on patients (172). However, it is necessary to carefully select anti-VEGF drugs for patients with hypertension and closely observe blood pressure changes during their use as hypertension is the most common adverse reaction to anti-VEGF drugs (172). Therefore, triple therapy is mainly used in patients with negative driver genes but can also be used in patients with positive driver genes where targeted therapy failed.

In addition, due to the increased risk of life-threatening radiation necrosis and oedema in the brainstem and cerebellum after RT, the treatment strategy should be carefully considered in triple therapy, including the RT dose, AADs, IT drug dose and duration (173,174). For BMs, both functional and non-functional areas should be considered in RT. The performance of RT in functional areas is similar to that in the brainstem, and the combination of AADs can effectively relieve oedema and reduce the side effects of RT to a certain extent, while there is no limitation in non-functional areas (175). However, the treatment effect and survival of patients with leptomeningeal metastasis are poor, and these patients are not sensitive to palliative RT. Combined therapy may increase the burden on patients, and accordingly, triple therapy is not recommended (176).

Since the approval of IT for NSCLC treatment, patients treated with ICIs have obtained notable survival benefits compared with previous treatments (177,178). To date, with the continuous application of IT, numerous patients have developed ITR, which severely affects disease treatments (179,180). Exploring the mechanisms of resistance to immunosuppressants is crucial for further improving their clinical benefit (181). A variety of IT combination therapies have prevented and reversed ITR (182,183). Currently, an increasing number of clinical trials are exploring the safety and efficacy of combination therapy in patients with NSCLC with ICI resistance (183,184). AAT, RT, IT and other treatments will affect the TIM, which may reverse ITR and improve clinical efficacy (153,185,186). However, previous studies choose two of the three treatment methods: RT, IT and TT for combined treatment and their efficacy achieved satisfactory results with relatively few side effects (187–189). However, as the disease progressed, the two treatments were not sufficient, so triple therapy was explored. The results from animal experiments demonstrate that triple therapy can notably delay tumour growth and enhance the survival rate compared with two or single treatments (148,190), which is expected to be further extended to clinical application and bring new survival benefits.

Triple therapy is currently being explored in animal models; however, only one study has been published to date (148). Although the effect of triple therapy in animal models is notable, these models still have certain limitations. First, the survival observation time and the establishment form of the animal model were insufficient, and the effect of triple therapy on survival and tumour recurrence has not been observed in the experimental process. Second, there are no animal models to investigate whether different RT dose fractionations affect the efficacy of triple therapy. Finally, the effects of RT and different dosing intervals on efficacy have not been explored. There remains a need to study triple-therapy animal models in the future. However, owing to the notable differences in gene number, expression profile and ligand binding and function between human and murine TIM proteins, the same therapeutic strategy that works well in animal models may not work well in humans (191–194). Therefore, phase II or III clinical trials should be performed under ethical conditions to assess the efficacy and safety of triple therapy. This will not only provide new treatment strategies for patients with advanced NSCLC with BMs but also provide new ideas for the treatment of other cancers.

There are still numerous directions for future research. First, based on the evidence discussed in the present review, triple therapy can achieve good efficacy and controllable safety in patients with NSCLC with BMs, but there is a lack of large-scale prospective clinical trials to verify this; therefore, further studies are needed to evaluate its clinical benefits. Second, triple therapy can be used in the treatment of other cancers, such as liver cancer or pancreatic cancer, to provide new ideas for tumour inhibition, but this also needs to be evaluated by clinical trials. Third, the mechanism by which triple therapy jointly regulates the TIM is not fully understood and changes in the TIM when the three treatments are used simultaneously need to be explored further. Finally, at present, most studies on TIM regulation by triple therapy are carried out in animal models; therefore, future studies using human tissues or blood are needed. This article also has certain limitations. There are relatively few relevant studies including triple therapy and there is a lack of prospective clinical trials to prove its effectiveness. Secondly, it only provides a new treatment idea for patients with NSCLC with BMs and does not extend to include more methods. This is expected to be addressed in future studies.

RT combined with AAT and IT has a synergistic effect in the treatment of patients with BMs from NSCLC and may reverse the ITR during treatment. Combination therapy may provide additional survival benefits to patients with NSCLC with BMs, but its specific efficacy and safety need to be explored further.