Immune checkpoint inhibitors for the treatment of non-small cell lung cancer brain metastases

Abstract

Lung cancer has the highest risk of brain metastasis (BM) among all solid carcinomas. The emergence of BM has a significant impact on the selection of oncologic treatment for patients. Immune checkpoint inhibitors (ICIs) are the most promising treatment option for patients without druggable mutations and have been shown to improve survival in patients with non-small cell lung cancer (NSCLC) BM in clinical trials with good safety. Moreover, ICI has shown certain effects in NSCLC BM, and the overall intracranial efficacy is comparable to extracranial efficacy. However, a proportion of patients showed discordant responses in primary and metastatic lesions, suggesting that multiple mechanisms may exist underlying ICI activity in BM. According to studies pertaining to tumor immune microenvironments, ICIs may be capable of provoking immunity in situ. Meanwhile, systematic immune cells activated by ICIs can migrate into the central nervous system and exert antitumor effects. This review summarizes the present evidence for ICI treatment efficacy in NSCLC BM and proposes the possible mechanisms of ICI treatment for NSCLC BMs based on existing evidence.

Introduction

According to global cancer statistics 2020 reported by the International Agency for Research on Cancer, the incidence of lung cancer in China ranks first in males and second in females, while the mortality ranks first in both sexes.^[1] Recently, due to the emergence of targeted therapy and immune checkpoint inhibitors (ICIs), the survival of patients with lung cancer, specifically non-small cell lung cancer (NSCLC), has been strikingly prolonged. Lung cancer has the highest risk of brain metastasis (BM) among all solid carcinomas. Moreover, 10%–30% of patients with NSCLC have brain involvement during the course of their disease.^[2] The median time for patients with lung cancer to develop BM was 11 months.^[3] The emergence of BM has a significant impact on the clinical evaluation and oncologic treatment selection of patients.^[4] Chemotherapy is often ranked behind local therapy for BM, such as radiotherapy, due to its limited penetration through the blood-brain barrier/blood-tumor barrier (BBB/BTB) and the existence of specialized efflux pumps.^[3] With the application of lung cancer molecular testing, targeted therapy has been proven effective against NSCLC BM. The third-generation tyrosine kinase inhibitor osimertinib targeted epidermal growth factor receptor mutation, which is noted in 35%–50% of Asian patients with lung cancer, and the third-generation anaplastic lymphoma kinase (ALK) inhibitor lorlatinib targeted ALK rearrangement, which is reported in 4%–7% of patients with lung cancer, have better BBB penetration and greater intracranial objective response rate (ORR).^[2] ICI remains the most promising treatment for a substantial proportion of patients without druggable mutations. An increasing amount of clinical evidence has shown that ICI has a certain effect on lung cancer BM with good safety and tolerance. Patients with NSCLC BM are more often studied as a subgroup in clinical trials. In clinical trials of all phases, the intracranial ORR of ICI in patients with NSCLC BM ranges from 9% to 70%^[5] with great heterogeneity, mostly around 30%. Overall, the intracranial efficacy of ICI is shown to be durable and comparable to extracranial efficacy,^[6] facilitating research on the tumor immune microenvironment of BMs, which drives advances in understanding the ICI activity in BMs.

Therefore, this review aimed to summarize the evidence for ICI treatment efficacy for NSCLC BMs and compare the tumor immune microenvironments of BMs and primary lesions. Accordingly, we proposed the possible mechanisms of ICI treatment for NSCLC BMs based on existing evidence.

Survival Benefits of ICI for Patients with NSCLC BM in Phase III Studies

To date, it remained speculative whether ICI could provide a survival benefit to small cell lung cancer (SCLC) patients with BM.^[7] According to updated results from CASPIAN (Durvalumab ± Tremelimumab in Combination With Platinum Based Chemotherapy in Untreated Extensive-Stage Small Cell Lung Cancer [CASPIAN]), the overall survival (OS) benefit with the addition of durvalumab was shown in SCLC patients with BM albeit with high deviation (hazard ratio 0.79 [95% confidence interval, 0.44–1.41]).^[8] Nonetheless, no survival benefits were found in other randomized trials for BM subgroup with SCLC like Impower 133, Keynote 604, and Reck 2016.^[9] Overall, the magnitude of benefit with ICI in SCLC with BM did not mirror that shown in NSCLC with BM which may be explained by a more immunosuppressive environment and more rapid progression in SCLC. Meanwhile, substantial clinical research has demonstrated that ICI can provide survival benefits to patients with NSCLC BM. Patients with BMs are more often studied as a subgroup in large multicenter phase III trials of ICIs. Despite the diversity of inclusion criteria and regimens of these trials, these studies obtained the same conclusion that ICI can significantly improve the survival of patients with NSCLC BM.

ICI monotherapy or dual immunotherapy

The survival outcomes of large multicenter phase III clinical trials involving patients with NSCLC BM are summarized in Table 1. CheckMate 227 found that patients with NSCLC with brain involvement may benefit even more from ICI dual immunotherapy than patients without BM and whether they benefit from ICI regimen may not depend on the expression of programed death ligand-1 (PD-L1) on tumor cells.^[10] The same conclusion was reached by CheckMate 017/057^[11] and OAK study^[12] which applied ICI monotherapy. As shown by CheckMate 017/057, nivolumab improved 5-year survival rate of patients with CNS metastases from 0% to 8%.^[11] All these studies have observed that after administration of ICI, the OS of patients with BM showed no significant difference from that of patients without BM. Another sizeable real-world study OAK confirmed that patients with BM receiving nivolumab have similar OS, progression-free survival (PFS), and ORR compared with the cohort without BM,^[13] which is consistent with the results of pembrolizumab as first-line treatment.^[14]

Table 1 - Summary of the survival outcomes of NSCLC patients with BM in phase III trials of ICI.

Trial	Total∗	ICI arm vs. comparator arm	OS (ICI arm vs. comparator) (months)	HR for OS (95% CI)	PFS (ICI arm vs. comparator) (months)	HR for PFS (95% CI)
CheckMate 227	81	Nivolumab + ipilimumab vs. chemotherapy	16.8 vs. 13.4	0.68 (0.41–1.11)
CheckMate 017/057	87	Nivolumab vs. docetaxel	7.6 vs. 6.2	0.81
OAK	118	Atezolizumab vs. docetaxel	16.1 vs. 8.6	0.59 (0.38–0.92)
CheckMate 9LA	122	Nivolumab + ipilimumab + chemotherapy vs. chemotherapy	NR (12.3-NR) vs. 7.9 (5.0–10.7)	0.38 (0.24–0.60)	9.8 (5.7–11.4) vs. 4.1 (2.8–5.0)	0.42 (0.28–0.65)
KEYNOTE-189	77	Pembrolizumab + pemetrexed-platinum vs. pemetrexed-platinum		0.43 (0.27–0.71)		0.42 (0.26–0.66)

^∗The number of patients participating in the trial. BM: Brain metastases; CI: Confidence interval; HR: Hazard ratio; ICI: Immune checkpoint inhibitor; NR: Not reached; NSCLC: Non-small cell lung cancer; OS: Overall survival; PFS: Progression-free survival.

ICI combined with chemotherapy

The improvement in OS and PFS was more profound in patients receiving ICI combined with chemotherapy for patients with BM, such as CheckMate 9LA^[15] and KEYNOTE-189 [Table 1],^[16] which may imply that the additive use of ICI can amplify the antitumor effect of chemotherapeutic drugs. Consistent with clinical trials of ICI monotherapy, the PD-L1 expression in the tumor tissue was not related to survival outcome or efficacy in ICI trials combined with chemotherapy.

ICI combined with radiotherapy

Presently, most studies on immunotherapy in combination with radiotherapy for NSCLC BM are retrospective. It is recognized that the addition of ICI to radiotherapy is safe and tolerable and provides better local control rate,^[17] PFS,^[18,19] and OS^[17-19]vs. radiotherapy alone, although whether the order and time interval between these two therapies affect the efficacy remains unclear. More research evidence has shown that synchronization of the two therapies or a time interval of <1 week can improve the remission rate^[20] and survival.^[21] Nevertheless, it seems that the delivery of programed death-1 (PD-1)/PD-L1 prior to radiation could not achieve equally good intracranial control or prolonged survival.^[22] A study claimed that concurrent use of ICI and stereotactic radiosurgery (SRS) does not lead to better local response or longer survival than concurrent use of chemotherapy and SRS.^[23] However, another study found that ICI plus baseline radiotherapy was superior to chemotherapy plus radiation when restricted to the PD-L1 ≥ 50% subgroup.^[24] Whether ICI performs better than chemotherapy in combination with radiation requires further study.

Based on the results of the abovementioned major multicenter phase III clinical trials, we concluded that ICI can effectively improve PFS and OS of patients with NSCLC BM, and the improvement is more significant when combined with chemotherapy. To determine whether to deliver immunotherapy, it is not necessary to consider PD-L1 expression levels in the tumor tissue, as patients with PD-L1 expression <1% can also benefit from immunotherapy. Moreover, for patients with symptomatic BMs, a meta-analysis showed that glucocorticoid administration has no effect on intracranial PFS, while it predicts shorter OS and systemic PFS. However, considering that most studies did not exclude other confounding factors, the effect of glucocorticoids on patients with NSCLC BMs treated with ICIs requires more in-depth research.^[25] The combination of ICI and anti-angiogenic agents also showed a potential synergistic effect with acceptable tolerability according to several phase I trials.^[26]

Comparison of the Efficacy of ICI Between Primary and Brain Metastatic Lesions

Accordingly, we confirmed that ICI can confer a survival benefit for patients with NSCLC BMs; however, the specific effects of ICI on BMs and primary lesions are still unclear. Several studies have evaluated the systemic response and intracranial response individually after ICI treatment, which more directly reflects the efficacy of ICI treatment on BMs, providing insights into the mechanism of ICI action on BMs.

The efficacy outcomes of these clinical studies are summarized in Table 2, and the responding patterns of intracerebral and paired extracerebral lesions are presented in Table 3. Because of the limitations associated with retrospective data collection, these studies included BMs under various circumstances while bringing more confounding factors. All these studies excluded patients who received ICI concurrent with local treatment, such as radiotherapy for BMs. The intracerebral ORRs of these studies in Table 2 are almost the same, which is approximately 30% and close to that of primary lesions. Two^[27,28] of them decreased to approximately 10%, which may be attributed to the inclusion of active BMs. As shown in Table 2, intracerebral disease control rates (DCRs) generally reached 50%, similar to paired systemic DCRs. Therefore, it can be implied that the curative effects of ICI on BMs and extracerebral lesions are analogous.

Table 2 - Efficacy outcomes of ICI in treatment for NSCLC patients with BM.

No.	Journal	Treatment	Type	Number	CNS ORR	CNS DCR	Systemic ORR	Systemic DCR
1	Lancet Oncol [29]	Pembrolizumab monotherapy	Phase II trial	37	29.7%	40.5%	29.7%
2	Cancers [30]	PD-1/PD-L1 inhibitor monotherapy	Retrospective	33	24.2%	48.5%		30.3%
3	J Thorac Oncol [31]	ICI monotherapy	Prospective	73/255∗	27.3%	60.3%	20.6%	43.9%
4	Clin Lung Cancer [32]	Pembrolizumab-based therapy	Retrospective	11/126†	36.4%	63.6%	27.8%
5	Onco Targets Ther [33]	Anti-PD-1/PD-L1-based treatment	Retrospective	67	37.3%	80.6%
6	Thorac Cancer [27]	PD-1/PD-L1 inhibitor monotherapy	Retrospective	24	13.3%		26.7%
7	Cancer Immunol Immunother [34]	Nivolumab monotherapy	Retrospective	32	28.1%	46.9%	25.0%	53.1%
8	Lung Cancer [35]	Nivolumab monotherapy	Retrospective	5	40%	60%	40%	40%
9	Lung Cancer [28]	Nivolumab monotherapy	Retrospective	43	9%	51%	11%	47%
10	Clin Transl Oncol [36]	Pembrolizumab monotherapy	Retrospective	9	55.5%	66.6%	55.5%	66.6%

^∗73 patients measured CNS response and 255 patients measured systemic response.

^†11 patients measured CNS response and 126 patients measured systemic response.BM: Brain metastases; CNS: Central nervous system; DCR: Disease control rate; ICI: Immune checkpoint inhibitor; No.: Number; NSCLC: Non-small cell lung cancer; ORR: Objective response rate; PD-1: Programed death-1; PD-L1: Programed death ligand-1.

Table 3 - Concordance of ICI response in BMs and paired extracranial lesions.

			Extracranial response∗		No extracranial response
No.	Journal	Treatment	Intracranial response	No intracranial response	Intracranial response	No intracranial response	Concordant response	Disconcordant response
1	Lancet Oncol [29]	Pembrolizumab monotherapy	8 (29.6)†	3 (11.1)	3 (11.1)	13 (48.1)	21 (77.8)	6 (22.2)
4	Clin Lung Cancer [32]	Pembrolizumab-based therapy	0 (0)	0 (0)	3 (42.9)	4 (57.1)	4 (57.1)	3 (42.9)
6	Thorac Cancer [27]	PD-1/PD-L1 inhibitor monotherapy	2 (16.7)	2 (16.7)	0 (0)	8 (66.7)	10 (83.3)	2 (16.7)
8	Lung Cancer [35]	Nivolumab monotherapy	2 (40.0)	0 (0)	0 (0)	3 (60.0)	5 (100.0)	0 (0)
9	Lung Cancer [28]	Nivolumab monotherapy	1 (3.4)	3 (10.3)	3 (10.3)	22 (76.0)	23 (79.3)	6 (20.7)
10	Clin Transl Oncol [36]	Pembrolizumab monotherapy	5 (71.4)	0 (0)	0 (0)	2 (28.6)	7 (100.0)	0 (0)

No., the ordinal number of the trial in Table 2. Data are presented as n (%).

^∗Response means CR or PR according to RECIST Version 1.1. BM: Brain metastases; CR: Complete response; ICI: Immune checkpoint inhibitors; PD-1: Programed death-1; PD-L1: Programed death ligand-1; PR: Partial response; RECIST: Response Evaluation Criteria in Solid Tumors.

Table 3 illustrates more clearly how people responded to ICI and whether the efficacy of ICI is consistent between BMs and its matched primary lesions. By searching Pubmed, we found that only six studies separately evaluated responses in the central nervous system (CNS) and lung including 87 patients. Discordant responses were shown in four studies and the rates of discordance ranged from 0% to 42.9%, which shows great divergence among different studies. The pooled concordant rate was 80.5% and the pooled discordant rate was 19.5%. Among the patients with divergent responses in CNS and lung, nine (10.3%) had intracranial lesion response only, while eight (9.2%) had extracranial lesion response only, and the ratios were nearly the same. Thus, most patients have BMs with the same sensitivity to ICI as primary lesions, while a small proportion of patients have BMs that are more sensitive to ICI, and another subset of patients of the same proportion have primary lesions that are more sensitive to ICI. Different responding patterns of ICI in NSCLC patients with BM shed light on the possible mechanisms of ICI for the treatment of NSCLC BMs.

Rationale for ICI Treatment in BMs

Because of the existence of the BBB, the brain has been considered an immune-privileged organ for a long time. Until the discovery of subdural cerebral lymphatic vessels in 2015,^[37] it was confirmed that immune cells can traffic between the neuroimmune system and peripheral immune system under physiological conditions, albeit rarely. When BMs are formed, disturbed BBB/BTB allows for activated immune cell entry into the CNS. Many clinical studies have demonstrated that BMs have been infiltrated by immune cells before receiving ICI, which emphasizes one possible mechanism that ICI may be capable of provoking immunity in situ with regard to its access to the CNS through the BBB/BTB. Conversely, expanded immune cells activated by ICI in the primary site may migrate into the CNS, exerting antitumor effects in the brain. A panoramic view of potential mechanisms underlying ICI treatment in BMs is visualized in [Figure 1].

Figure 1: Potential mechanisms of ICIs for treatment of NSCLC brain metastases. ICIs may provoke immunity in situ after infiltration into the CNS through the BBB/BTB. Expanded immune cells activated by ICI in the primary site may also migrate into the CNS. BBB: Blood-brain barrier; BTB: Blood-tumor barrier; CNS: Central nervous system; CTLA4-Ab: Cytotoxic T lymphocyte-associated antigen 4 antibody; ICIs: Immune checkpoint inhibitors; NK: Natural killer; NSCLC: Non-small cell lung cancer; PD-(L)1-Ab: Programed death ligand-1 antibody; TAM: Tumor-associated macrophage.

ICI Intracranial Efficacy may be Dependent on BBB/BTB Penetration

BBB/BTB permeability of ICI

The BBB consists of three major components: endothelial cells, pericytes, and astrocytes. Vascular endothelial cells maintain barrier integrity by the tight junction (TJ) and specialized basement membrane forming connections with pericytes and astrocytic endfeet. The formation of BMs infers defects in paracellular connections between endothelial cells, and metastatic cancer cells can destroy TJ by secreting proteases that degrade the anchor molecule jam2.^[38] With the development of BMs, the BBB is partially remodeled by modifying endothelial cells from the transcriptome to a phenotype that deprives them of normal barrier function. Neoanginegeosis induced by tumor growth contributes to the formation of BTB, which replaces part of the BBB as a new barrier. BTB is characterized by the loss of TJs and inherent heterogeneous permeability, which disrupts intracranial homeostasis. Defects in BTB structural integrity are also embodied by aberrant pericyte distribution and decreased astrocyte endfeet.^[39] It can be implied that components of the BBB and interactions among them are destroyed in the development of BMs, which theoretically confers the ability of ICIs to penetrate brain metastatic lesions.

Drug delivery across the BBB/BTB relies on paracellular and transcytotic pathways, and antibodies as macromolecular agents prefer to penetrate the brain through transcytosis.^[40] Figueria et al^[41] found that as BMs develop, the endothelial cell TJ protein claudin-5 presented an irregular and discontinuous distribution, resulting in increased BBB permeability; conversely, a sustained increase was observed in vesicular transport protein caveolin-1 expression, which may enhance transcellular transmigration and thereby increase penetration of ICI into the BMs.

Pluim et al^[42] measured the concentration of PD-1 inhibitors in the cerebrospinal fluid (CSF) using enzyme-linked immunosorbent assay, and the serum/CSF ratios ranged from 52 to 299. The results indicated that the PD-1 inhibitor is capable of migration through the BBB/BTB into the CNS despite substantial interpatient variability. However, the detailed mechanisms underlying transportation are unclear, and more studies investigating the association between BBB permeability and efficacy are warranted.

Association between penetration of ICI across the BBB/BTB and efficacy

Mittapalli et al^[43] established a mathematical modeling approach to calculate interendothelial pore size on the BTB by two preclinical models. The pore diameter of the glioma vasculature was found to be large enough to allow monoclonal antibody (mAb) diffusion into tumors. The pore size of a BM tumor was ten-fold smaller than that of a primary brain tumor, indicating much less vascular permeability. Based on this observation, it can be inferred that PD-1 inhibitors have higher BBB penetration in glioblastoma than brain metastatic cancer. However, no existing clinical trials have validated that PD-1 inhibitors are beneficial for patients with glioblastoma.^[44] For example, in a Checkmate143 phase III randomized clinical trial, nivolumab monotherapy showed no improvement in OS (9.8 vs. 10.0 months) or ORR (7.8% vs. 23.1%) for recurrent glioblastoma compared with bevacizumab.^[45] PD-1/PD-L1 inhibitors generally fail to treat glioblastoma, which may be attributed to its relatively low tumor mutant burden and conversely significant immunosuppression. PD-1/PD-L1 inhibitors have been shown effective against BMs of other malignancies in clinical trials. Therefore, the immune microenvironment plays a crucial role in ICI activity. Considering the heterogeneity of cancer, more research is needed regarding the relationship between BBB permeability and intracranial efficacy of ICIs.

Preclinical studies on the ICI treatment mechanism in BMs

By using anti-PD-1 mAb in medulloblastoma-bearing mice, Pham et al^[46] found that, PD-1 blockade was present only in peripheral lymphocytes but not tumor-infiltrating lymphocytes (TILs) in the brain. Nevertheless, peripheral PD-1 mAb caused an influx or expansion of PD-1 negative T cells into tumors, which further extended the median survival of treated animals. This animal model suggested that PD-1 inhibitors could increase TILs within the brain tumor microenvironment by binding to peripheral lymphocytes without BBB/BTB penetration. Taggart et al^[47] established a melanoma tumor transplantation model and found that the intracranial efficacy of ICI could be achieved only when extracranial tumors were present. ICI induced peripheral expansion of effector T cells and recruitment of CD8⁺T and natural killer (NK) cells to the brain, the subtypes of which are required for ICI intracranial efficacy. For clinical animal models, intracranial efficacy relies on systematic immune responses when administered with ICIs. Given the structural differences in the BBB/BTB between rodent models and humans, no evidence has shown that ICI can migrate through mouse BBB/BTB so far, while the concentration of PD-1 antibody has been measured in human CSF. We can conclude that rodent models cannot interpret the panorama of mechanisms underlying the ICI effect on brain metastatic tumors, which highlights the need for more investigations on craniotomy specimens or even in situ.

Comparison of Immune Microenvironments Between Primary Lung Cancer and Paired BMs

Promising biomarkers of ICI efficacy in patients with lung cancer with CNS involvement

Biomarkers predicting better efficacy could provide clues to understand the mechanisms underlying ICI treatment for BM. At present, the most studied biomarker for predicting the efficacy of ICI on lung cancer is PD-L1 expression of tumor cells or immune cells, which makes mechanistic sense. At the beginning stage to explore the efficacy of ICI for lung cancer, the clinical trials enrolled only patients with high PD-L1 expression and consequently confirmed that ICI vs. chemotherapy could prolong both PFS and OS for lung cancer patients with high PD-L1 expression.^[48] As the expansion of enrollment, survival benefit was observed in NSCLC BM patients with negative PD-L1 expression who received ICI albeit with a much lower likelihood of benefit compared to patients with high PD-L1 expression. Furthermore, this benefit was more significant in the regimen of ICI plus chemotherapy. Consequently, the combination regimen received FDA approval for the expanded patient populations regardless of PD-L1 expression.^[49] Subsequent studies on PD-L1 expression as a biomarker of ICI efficacy seem to conclude that higher PD-L1 expression of either primary or metastatic lesions predicts longer OS and PFS,^[50-52] meanwhile patients with negative expression of PD-L1 could still benefit from ICI treatment. Some evidence has shown the prognostic and predictive significance of TILs in ICI treatment. For NSCLC BMs, an abundance of total TILs,^[53] helper T cells,^[53] CD3⁺ TILs,^[53,54] CD8⁺ TILs,^[54-56] CD45RO⁺ TIL,^[53] and lower PD-1⁺ TILs^[57,58] have been correlated with improved survival, and the prognostic value of CD4⁺ T cell density was inconclusive.^[59] There have also been studies showing that higher PD-L1 expression of either primary or metastatic lesions predicts longer OS and PFS,^[50,51] excluding the intracranial control rate.^[51]

Association between PD-L1 expression and TIL in BM

PD-L1 and TIL in BM as the two most promising biomarkers of ICI efficacy on NSCLC BM have been studied for whether there is an association between them. Berghoff et al^[54] investigated TIL subsets in 116 BM specimens and found that TILs were prevalent in BM. High TIL density was most frequently observed in CD3⁺ TILs and least frequently in PD-1⁺ TILs. Of the BM specimens, 28.4% exhibited evident PD-L1 expression, whereas the expression level was unrelated to TIL density. A previous study showed that PD-L1-positive tumor cells and PD-L1-positive TILs belong to different subpopulations.^[55] Paired primary lung cancer and BM seem to have a different distribution of immune cells within and peripheral to the tumor despite similar expression of PD-L1, suggesting that BM may develop its own immune environment.^[60] In contrast, another study including 12 cases showed a positive correlation between the infiltration of PD-1⁺ TIL and PD-L1 expression on tumor cells in BM.^[57] For the wide use of P values to determine significance, some clinically meaningful but statistically insignificant associations may be omitted.

Comparison of PD-L1 expression between primary lung cancer and paired BMs

Some studies revealed that BM has statistically equivalent levels of PD-L1 expression compared with lung primary tumors.^{[56,57,61,62]} Conversely, some other studies observed that brain metastatic tumors have higher^[55,63] or lower^[50,58] PD-L1 expression than lung primary lesions. Compared with BM from other solid tumors, BM originating from the lung seemed to have a higher expression of genes belonging to the PD-1 axis, indicating that lung cancer BM is a more favorable target for ICI therapy.^[64] Mansfield et al^[65] analyzed the immune microenvironment of 146 paired primary lung lesions and BMs and found agreement of PD-L1 expression by tumor cells in 86% of cases, most of the remaining cases with discordant expression tended to lose PD-L1 expression in BMs, and only individual cases showed the opposite.

Comparison of TILs between primary lung lesions and paired BM

Regarding TIL, total TIL,^[65-67] CD8⁺ TIL,^{[55,56,63,67]} and PD-1⁺ TIL^[57] within BM are significantly lower than paired primary lung tumors, which may be attributed to downregulated pathways, including dendritic cell maturation, leukocyte adhesion, and extravasation signaling in BMs.^[61] According to immune gene expression profiles, BMs tend to exhibit prevalent immune suppression compared to primary lesions.^[68]

It remains unclear whether genetic mutations and TILs in BM originate from primary lesions. Jiang et al^[63] observed that a median of 8.3% of genetic mutations were shared by paired lesions through phylogenetic analysis, suggesting that BM has genetically diverged from primary lesions at an early stage and experienced clonal evolution in parallel with primary tumors. Song et al^[56] investigated 33 cases and found that oligometastatic BM had a lower tumor mutation burden than primary tumors. However, Mansfield et al^[66] found a higher mutation burden and a lower number of unique T cell clones in BM than in primary lesions, which was thought to be correlated with ICI efficacy.^[69] Meanwhile, the overlap in T-cell receptor sequences of TILs between paired lesions was minor.^[66] Conversely, Kudo et al^[67] found that T cell clones were highly conserved in BM with primary tumors as a reference. Consistently, all these studies confirmed the expansion of unique T cells in BM. As an upstream signaling molecule of TIL, the incongruent expression of human leukocyte antigen (HLA) class one may explain the partial difference in T cell clones. Approximately a quarter of the patients exhibit disagreement in HLA class one positivity between paired lesions. Most of them were BM positive only, while a small proportion had positive primary tumors only.^[70] Because the median interval between acquisition of primary and metastatic specimens was approximately one year, temporal heterogeneity may bring about confounders.

Role of Innate Immunity in ICI's Effect on BMs

In addition to adaptive immunity, PD-1/PD-L1 inhibitors also regulate innate immunity. Fan et al^[71] found that pembrolizumab plus beta-glucan (an innate immune activator) could provoke innate immune activity in lung cancer BM in vitro, accompanied by tissue damage. According to the immunophenotypic investigation, the proportions of lymphocytes tended to decrease in BM compared to primary lung tumors; however, markers of NK cells and tumor-associated macrophages (TAM) and the ratio of M1 to M2 macrophages and NK to T cells were higher in BM.^[58] A study demonstrated that the presence of peritumor mononuclear cells, which is called the mononuclear ring in lung adenocarcinoma BM, indicated a high density of intratumor mononuclear cells and predicted better survival after BM surgery.^[72] Human TAMs can also express PD-1, and the majority of PD-1⁺ TAMs originate from circulating leukocytes. Among TAM, the M2 macrophage population expressed significantly more PD-1 than the M1 macrophage population. PD-1 expression on TAM attenuated its phagocytic potency against tumor cells, and PD-1/PD-L1 antagonism could in vitro relieve this type of function arrest and augment phagocytosis of PD-1⁺ TAM, further reducing tumor growth and prolonging survival.^[73] Genome-wide transcriptomic profiling showed that endothelial cells in the BM vasculature upregulated the expression of leukocyte adhesion molecules vs. primary tumors, which may result in increased NK cell infiltration in BM.^[68] In the melanoma microenvironment after administration of PD-1/PD-L1 inhibitor, high frequency of NK cells aroused activated dendritic cells, stimulating downstream cytotoxic T cells that drived better response to anti-PD-1 therapy, and increased OS of patients.^[74] It can be inferred that innate immunity also plays an important role in the treatment of lung cancer BM by PD-1/PD-L1 inhibitors.

Conclusions and Prospects

In this review, we summarized ICI efficacy on NSCLC BM in clinical trials and reviewed ICI responses in paired NSCLC and BM lesions. The interesting response pattern of NSCLC patients with BM which may indicate the existence of multiple mechanisms underlying the activity of ICI on NSCLC BM aroused our interest. Consequently, we proposed possible mechanisms and summarized corresponding evidence, and finally mentioned innate immunity which was rarely noted in the discussion of ICI treatment. In substantial phase III clinical trials, ICI has shown therapeutic effects in patients with NSCLC BM, and the pooled intracranial ORR is comparable to that of extracranial ORR. However, since most patients with BM were analyzed as subgroups in these studies, the number of cases was restricted. In the future, the emergence of large-scale multicenter clinical trials involving patients with lung cancer BM as the main cohort may provide stronger evidence for the use of ICI as a first-line treatment for NSCLC with BM. Currently, many real-world retrospective studies have explored the efficacy of ICIs in patients with lung cancer BM. However, due to the great heterogeneity of interstudy subjects, relatively lenient inclusion criteria, and great variance in efficacy outcomes, multicenter studies with larger sample sizes are needed. Most existing studies considered PFS and OS as outcomes, and a minority of them independently assessed primary and metastatic lesions. In terms of limited clinical data, the response rate showed no difference between intracranial and extracranial lesions after ICI treatment. However, the remission rate was not completely consistent in paired lesions, roughly 80% of cases were consistent, and 10% of cases had BMs or primary lesion response alone. This phenomenon indicates that the mechanism underlying ICI activity in BMs cannot be explained merely by the extension of peripheral immune cells into the brain.

Through investigations into the immune microenvironment of BMs and primary lesions, a higher density of TILs in tumors is associated with better survival, and PD-L1 expression is not necessarily related to prognosis. Nevertheless, the mechanism by which ICI affects the tumor immune microenvironment remains unclear. There is a dilemma that BM specimens can be acquired only before or after ICI treatment. Therefore, the ICI effect on BM could not be directly presented, and mechanisms underlying ICI treatment for BM can only be deduced indirectly through clinical evidence. Considering that BMs generally have immune cell infiltration before ICI treatment, ICI is capable of penetrating BBB/BTB despite its uncertain penetrability. In theory, ICIs can directly penetrate the CNS, causing activation of local immune cells and expansion of effector cells to reduce tumor cells. Conversely, according to preclinical studies and genetic profiles, peripheral effector T cells can infiltrate the CNS and exert antitumor effects. These two mechanisms may coexist and function synergistically, or one of the two dominates to some extent. Given the great heterogeneity of BMs and the extremely complex immune microenvironment, it is possible that a minor subset of patients has primary lesions with a more active immune status, while another small subset of patients has BMs with a more active immune status, which indicates a higher sensitivity to ICI treatment. On the other hand, ICI may arouse immune responses by multiple mechanisms not only by attaching to lymphocytes. Both innate immunity and adaptive immunity participate in ICI activity in primary and metastatic tumors. Currently, an increasing number of clinical and preclinical studies have provided insights into the mechanisms underlying ICI treatment for BM, which lays the foundation for clarifying the mechanisms and identifying efficacy biomarkers.

Funding

This study was supported by the Youth Program of the National Natural Science Foundation of China (to YX) (No. 82003309).

Conflicts of interest

None.