mTOR in radiotherapy of lung cancer: Mechanisms of radiation resistance and therapeutic implications (Review).

1.
Introduction
Lung cancer is a prevalent malignancy worldwide; according to the latest GLOBOCAN report, ~2.48 million new lung cancer cases are diagnosed each year and it accounts for 12.4% of global cancer incidence, thus ranking as the most common cancer globally. In addition, lung cancer is the leading cause of cancer-related deaths, with an estimated 1.8 million fatalities representing 18.7% of total cancer mortality. Notably, China bears the highest lung cancer incidence and mortality rates worldwide (1).
Radiotherapy serves as a primary treatment for lung cancer across different stages and pathological types, markedly improving local control rates and survival (2,3). However, intrinsic or acquired radioresistance remains a clinical challenge to treatment outcomes and patient prognosis (4,5).
mTOR serves pivotal roles in cell proliferation, metabolism and survival (6). In lung cancer, abnormal activation of mTOR is closely associated with tumor development (7). In addition, accumulating evidence (8,9) has demonstrated that mTOR not only promotes tumor cell proliferation but also mediates radioresistance. Furthermore, mTOR signaling exhibits crosstalk with other key pathways, particularly PI3K/AKT. This crosstalk further complicates the mechanisms of radiotherapy resistance in lung cancer (10). These findings position mTOR inhibitors as promising agents to overcome radioresistance. Notably, rapamycin and its derivatives have demonstrated therapeutic potential across various types of cancer, including lung, breast, liver and colorectal cancer (11-14).
The present review focuses on the role of mTOR in lung cancer radioresistance, explores associated molecular mechanisms and evaluates the clinical prospects of mTOR inhibitors for enhancing radiotherapy efficacy. To achieve this, a systematic literature search was conducted. PubMed (https://pubmed.ncbi.nlm.nih.gov/), Web of Science (https://www.webofscience.com/) and Scopus (https://www.scopus.com/) were searched up to May 2024 using key words such as 'mTOR', 'lung cancer', 'radiotherapy resistance', 'mTOR inhibitors', 'PI3K/AKT', 'SCLC' and 'immunotherapy'. Findings from original research, including in vitro cellular experiments and in vivo animal models, were subsequently assessed, as well as relevant clinical trials, meta-analyses and other review articles. Notably, non-peer-reviewed materials were excluded unless critical original data were required. By synthesizing these findings, the present review aims to elucidate how mTOR signaling mediates radioresistance and to propose optimized therapeutic strategies to improve outcomes for patients with lung cancer.

2.
mTOR protein
mTOR is a highly conserved serine/threonine kinase belonging to the phosphatidylinositol kinase-related kinase family. The soil bacterium Streptomyces hygroscopicus, which produces rapamycin, was initially identified in 1931. Vézina et al (15) then discovered the molecular target of rapamycin, mTOR, in 1975 on Easter Island (Rapa Nui, Chile). The mature mTOR protein comprises 2,549 amino acid residues (molecular weight: ~289 kDa) and is encoded by the mTOR gene situated on chromosome 1p36.2. Following transcription, splicing, translation and post-translational modifications, the functional mTOR protein localizes to multiple intracellular compartments, including the endoplasmic reticulum, Golgi apparatus, mitochondrial outer membrane, lysosomes, cytoplasm and nucleus (16-18). Structurally, mTOR has two clusters of repeats at its N-terminus. Each cluster has 20 tandem HEAT repeats; HEAT stands for Huntingtin, EF3, the A subunit of PP2A, and TOR1, and these repeats form hydrophobic surfaces, which are critical for protein-protein interactions, membrane anchoring and cytoplasmic trafficking. Adjacent to these, the focal adhesion targeting (FAT) domain stabilizes protein complexes through scaffold-like interactions, while the FKBP12-rapamycin binding domain serves as the binding site for the FKBP12-rapamycin complex. The kinase domain, functioning as the catalytic core, phosphorylates serine/threonine residues in substrate proteins to regulate downstream signaling. Finally, the C-terminal FATC domain interacts spatially with the FAT domain to expose the catalytic site, a configuration essential for the enzymatic activity of mTOR (19) (Fig. 1A).
mTOR exerts its biological functions through two distinct complexes: mTOR complex (mTORC)1 and mTORC2 (Fig. 1B). mTORC1 primarily regulates cell proliferation and metabolism, with its core components including mTOR, the scaffold protein regulatory-associated protein of mTOR (Raptor), and mLST8/GβL (homologous to yeast TOR1, Kog1 and Lst8). Additional regulatory proteins such as proline-rich AKT substrate of 40 kDa and DEP domain-containing mTOR-interacting protein modulate mTORC1 activity, while Tel2 and Tti1 are involved in its assembly and stability. By sensing nutrient, energy and oxygen availability, mTORC1 controls protein synthesis, autophagy and metabolic pathways (20-22), and is potently inhibited by rapamycin, from which its name originates. By contrast, mTORC2 governs cell survival and cytoskeletal reorganization by regulating the AKT/PKB signaling pathway (23). Its core structure comprises mTOR, the scaffold protein rapamycin-insensitive companion of mTOR, and specific subunits including mammalian stress-activated protein kinase-interacting protein 1 and Protor1/2 (homologous to yeast TOR2, Avo3, Avo1 and Bit61/2). Unlike mTORC1, mTORC2 is not directly sensitive to rapamycin inhibition. However, prolonged or high-dose rapamycin treatment may indirectly suppress mTORC2 activity. The suppression of mTORC2 activity happens in hepatocytes, adipocytes, T cells and certain cancer cells, and impairs full AKT activation, subsequently reducing cell survival and proliferation (24-26).
The mTOR signaling pathway forms a complex regulatory network essential for eukaryotic cell proliferation, metabolism and survival, integrating multiple upstream and downstream effectors, as illustrated in Fig. 1A. mTORC1 activity is regulated by upstream signals including the PI3K/AKT pathway (27,28), AMP-activated protein kinase (AMPK) (29,30) and Rag GTPases (31). By sensing cellular nutrients, energy and oxygen levels, mTORC1 coordinates protein synthesis, autophagy, and metabolic processes. Its key downstream targets, S6 kinase 1 and 4EBP1, drive protein production and cell proliferation while suppressing autophagy (20). By contrast, mTORC2 acts as a PI3K signaling effector, enhancing cell survival and cytoskeletal organization through insulin, insulin-like growth factor-1 and leptin-mediated PI3K activation (32,33). It regulates the AKT/PKB pathway to influence cell proliferation and survival (34) and mediates PKC phosphorylation to support cytoskeletal remodeling and cell migration (35,36) (Fig. 1A). Collectively, mTOR critically impacts physiological and pathological processes such as apoptosis, metabolic regulation, immune responses and carcinogenesis (37), with its aberrant activation strongly associated with cancer progression (38), thus making it a prime therapeutic target.

3.
mTOR in lung cancer pathogenesis
mTOR serves a critical role in the pathogenesis of lung cancer. As a key effector of the PI3K/AKT signaling pathway, mTOR regulates tumor growth and survival by modulating cellular processes including proliferation, metabolism, apoptosis and autophagy (7,39). Recent investigations have shown that mTOR is ubiquitously activated in lung cancer, with aberrant mTOR signaling observed in both non-small cell lung cancer (NSCLC) and small cell lung cancer (SCLC) (40), closely associated with tumor aggressiveness and therapy resistance (41). Table I systematically outlines PI3K/AKT/mTOR-mediated resistance mechanisms in lung cancer radiotherapy, encompassing target mechanisms, interventions (such as pharmacological inhibitors, genetic modifications and radiotherapy combinations), their functions and corresponding references.
In NSCLC, mTOR overexpression (OE) drives tumor initiation, progression and metastasis, serving as a potential therapeutic target, and multiple mechanisms contribute to this dysregulation. Granville et al (42) revealed that tobacco-mediated carcinogenesis is dependent on mTOR activation. This previous study also showed that rapamycin effectively reduces the size and proliferation of tumors induced by NNK (a tobacco-specific carcinogen). This finding implicates mTOR in tobacco-related lung squamous carcinoma. Additionally, mTOR upregulation may arise from genetic mutations (such as EGFR, KRAS and ALK) disrupting PI3K/AKT signaling (43-47), epigenetic modifications (DNA methylation and histone acetylation) (48), and tumor microenvironment factors. Zhang et al (49) demonstrated that mTOR signaling promotes angiogenesis and metastasis in NSCLC. Inhibiting mTORC2, in turn, can suppress cell migration, metastasis and epithelial-mesenchymal transition (EMT).
Compared with NSCLC, SCLC is characterized by high aggressiveness and rapid growth, and it is often diagnosed at advanced stages. Studies have indicated that mTOR signaling is similarly hyperactivated in SCLC, and is associated with its aggressive phenotype and treatment resistance (50-52). mTOR governs SCLC growth and survival by regulating proliferation, metabolism, apoptosis and autophagy. In SCLC, MYC, a commonly amplified oncogene, activates mTOR signaling through multiple pathways, such as the PI3K/AKT and MAPK pathways, to accelerate tumor cell proliferation (53). Furthermore, eIF4E, a downstream effector of mTORC1, supports tumor growth by regulating protein synthesis. Matsumoto et al (54) identified aberrant activation of the MYC-eIF4E axis as a primary driver of resistance to the mTOR inhibitor everolimus in SCLC. This finding underscores the interplay between mTOR signaling and MYC/eIF4E pathways. While both NSCLC and SCLC may benefit from mTOR pathway modulation, their therapeutic efficacy and resistance mechanisms differ. As noted, mTOR activity in NSCLC is predominantly linked to EGFR signaling, whereas resistance in SCLC involves MYC and eIF4E pathways.

4.
mTOR and radioresistance in lung cancer
mTOR serves a critical role in cancer radioresistance. Radiotherapy eliminates cancer cells by inducing DNA damage; however, some tumor cells evade this effect through mTOR signaling activation, leading to radiation resistance (11,55). By regulating biological processes, such as cell proliferation, metabolism, autophagy and DNA repair, mTOR promotes cancer cell survival during radiotherapy. In lung cancer, abnormal mTOR pathway activation is strongly associated with radioresistance (56). Current evidence has demonstrated that mTOR promotes resistance to radiotherapy in lung cancer through multiple mechanisms. To provide a concise overview of these complex mechanisms, Tables I-IV summarize key findings from preclinical studies.

PI3K/AKT/mTOR signaling pathway and DNA repair
The PI3K/AKT/mTOR pathway is a critical intracellular signaling cascade that regulates physiological processes, including cell proliferation, apoptosis, angiogenesis and energy metabolism in normal cells. Its aberrant activation promotes tumor progression by suppressing apoptosis, accelerating cell cycle progression, enhancing angiogenesis and promoting metastasis (57-59). In lung cancer, the PI3K/AKT/mTOR pathway is frequently activated via genetic mutations, amplifications and receptor tyrosine kinase activation. This activation is associated with intrinsic radiosensitivity, tumor cell proliferation and hypoxia, all of which contribute to radiotherapy resistance (12,60).
Mechanistically, activation of this pathway enhances radioresistance by upregulating DNA repair proteins such as DNA-dependent protein kinase catalytic subunit (DNA-PKcs), which accelerates the repair of radiation-induced DNA double-strand breaks (61,62) (Fig. 2). This enhanced DNA repair is a primary escape mechanism for irradiated tumor cells. However, Toulany et al (61) further revealed limitations in radiosensitizing KRAS mutant NSCLC through PI3K inhibition alone. These limitations are attributed to compensatory MEK-ERK pathway activation. Tumor cells counteract PI3K suppression by enhancing MEK-ERK signaling, which sustains survival via AKT-dependent upregulation of DNA repair proteins (such as DNA-PKcs and Rad51), ultimately impairing radiotherapy efficacy. Dual targeting of PI3K and MEK has emerged as a promising strategy to overcome this resistance (63). Kim et al (64) demonstrated that combining the MEK inhibitor trametinib with the mTOR inhibitor temsirolimus may enhance radiosensitivity in NSCLC cells by boosting radiation-induced apoptosis and inducing prolonged DNA breaks.
Preclinical studies have established robust evidence implicating PI3K/AKT/mTOR pathway activation in mediating tumor cell radioresistance (Table I). For example, scutellarin combined with iodine-125 seeds targets the AKT/mTOR pathway to enhance apoptosis and inhibit proliferation in NSCLC, offering a potential therapeutic approach (65). To validate the clinical relevance of these findings, Sebastian et al (66) conducted a clinical study analyzing tumor samples from 92 patients with T1-3N0 NSCLC treated with stereotactic body radiotherapy (SBRT). This previous study revealed that elevated PI3K pathway activity was significantly associated with increased local recurrence risk (HR=1.72, 95% CI=1.40-98.0, P=0.023) and shorter disease-free survival (DFS) (HR=3.98, 95% CI=1.57-10.09, P=0.0035) in patients with early-stage NSCLC receiving SBRT. Mechanistically, AKT/mTOR signaling deregulation has been directly linked to chemoradiation resistance in lung squamous cell carcinoma. Proteomics analysis of chemoradiation-resistant patient-derived xenograft models and cell lines has identified upregulated phosphorylated (p)-AKT and p-mTOR, and mTOR kinase inhibitors have been shown to sensitize these resistant cells to radiation (67). For KRAS-mutated NSCLC, single targeting of PI3K often fails due to MEK/ERK-dependent Akt reactivation. Dual inhibition of PI3K and MEK, however, can block this compensatory signaling, impair DNA double-strand break repair via non-homologous end joining, and significantly enhance radiosensitivity (68).
Additionally, downstream effectors of PI3K/AKT/mTOR signaling contribute to radioresistance through multiple routes. For example, Akt/mTOR signaling-driven COX-2 overexpression fosters radioresistance, and combining celecoxib (a COX-2 inhibitor) with radiotherapy enhances apoptosis and prevents radioresistance (69). Furthermore, SIRT6, an epigenetic regulator, suppresses PI3K/Akt/mTOR signaling, and SIRT6 overexpression enhances radiosensitivity and inhibits tumor progression in lung cancer (70). Targeting these downstream nodes has shown promise. For example, dysregulated PI3K/mTOR/EGFR crosstalk drives resistance, whereas dual inhibitors targeting EGFR and PI3K/mTOR show selective therapeutic benefit in lung cancer models, such as A549 cells (71).
However, PI3K activity showed no significant association with overall survival (P=0.49), regional recurrence (P=0.15) or distant metastasis (P=0.85) according to a study by Sebastian et al (66). These results position PI3K activity as a potential biomarker for predicting local recurrence and DFS in SBRT-treated NSCLC; however, validation in larger multicenter cohorts remains essential to confirm clinical applicability.

Autophagy regulation
Autophagy is a key metabolic and homeostatic mechanism that allows cells to adapt to environmental stress and damage by degrading damaged organelles, misfolded proteins and cellular debris, thereby preserving normal cellular function (72). Under physiological conditions, autophagy serves as a protective process essential for clearing dysfunctional or senescent cellular components (73). In cancer cells, however, aberrant activation of the mTOR signaling pathway suppresses autophagy, allowing tumor cells to escape radiotherapy-induced damage (74). Studies have indicated that mTOR acts as a primary negative regulator of autophagy. It not only inhibits autophagic flux but also enhances cancer cell survival by promoting growth and metabolic reprogramming, thereby fostering resistance to antitumor therapies such as radiotherapy (75,76).
In lung cancer research, the role of autophagy is complex (Table II). Kim et al (77) demonstrated that mTOR inhibition restores autophagic activity and enhances radiosensitivity. The findings of this previous study in cellular and murine models revealed that mTOR targeting alone fails to fully eradicate tumor cells but synergizes with radiotherapy to markedly augment apoptotic responses. This suggests that autophagy activation during radiotherapy may be modulated by apoptosis, implying that combining apoptosis inhibition with mTOR targeting may represent a novel therapeutic strategy to improve radiotherapy efficacy in patients with lung cancer.
Furthermore, Fei et al (78) reported that the PI3K/mTOR dual inhibitor PF-04691502 induces a senescence-like phenotype in A549 cells. This phenotype is characterized by elevated SA-β-galactosidase activity and increased LC3-II expression. This previous study also observed that co-treatment with the autophagy inhibitor chloroquine potentiates the DNA-damaging effects of the inhibitor. This observation highlights the dual role of autophagy; while its restoration by mTOR inhibitors can be sensitizing, in some contexts, inhibiting the autophagy machinery may further amplify the cytotoxic effects of PI3K/mTOR blockade. In 2020, Yan et al (79) investigated the interplay between autophagy and mitochondrial function by combining carbon ion irradiation (IR) with tigecycline, a glycylcycline antibiotic used clinically for antimicrobial therapy and in preclinical tumor research to modulate cellular metabolism and signaling. This study demonstrated that this combination triggers severe mitochondrial dysfunction in A549 cells, marked by a notable reduction in mitochondrial ATP content and collapse of mitochondrial membrane potential. In addition, it was indicated that p-AMPK and p-AKT may antagonize mTOR signaling through cross-talk, as demonstrated by western blot analysis of these phosphorylated proteins and downstream mTOR pathway components, modulating mitochondrial translation proteins to influence autophagy and apoptosis. Co-targeting autophagy and mTOR-associated signaling pathways may offer a more effective strategy to enhance the therapeutic efficacy of radiotherapy in lung cancer. For example, combining rapamycin with the Bcl-2 inhibitor ABT-737 enhances radiosensitization in lung cancer (80). Furthermore, restoring autophagy in PTEN-deficient contexts can be achieved with mTOR inhibitors and PTEN knockdown (81). The dual PI3K/mTOR inhibitor NVP-BEZ-235 has been shown to synergize with radiotherapy to enhance radiosensitivity in cisplatin-resistant NSCLC (82). In addition, knocking down ATM/MAPK14 reduces autophagy and boosts radiosensitivity (83), and targeting STMN1 via knockdown combined with X-ray irradiation mitigates autophagy-mediated radioresistance (84). Rapamycin also inhibits the mitochondrial unfolded protein response by modulating Maf1 phosphorylation, further impacting autophagic processes in lung cancer (85).
Collectively, these studies underline the dual role of autophagy in lung cancer therapy and highlight the central regulatory role of mTOR in autophagic processes. Co-targeting autophagy and mTOR-associated signaling pathways may offer a more effective strategy to enhance the therapeutic efficacy of radiotherapy in lung cancer.

MicroRNAs (miRNAs/miRs)
miRNAs are a class of non-coding RNAs that serve pivotal roles in regulating gene expression and are frequently dysregulated in diverse types of cancer (86). Studies have revealed that miRNAs markedly contribute to tumorigenesis, progression and radioresistance by modulating cancer cell proliferation, migration, invasion and apoptosis, highlighting their potential value in early diagnosis, prognosis prediction and therapeutic intervention (87-89). In NSCLC, aberrant miRNA expression profoundly impacts mTOR signaling pathway activity, thereby influencing radiotherapy outcomes (Table III).
miRNAs frequently promote radioresistance by suppressing tumor suppressor genes that regulate the mTOR pathway. For example, miR-410, which is commonly upregulated in cancer, promotes EMT and radioresistance by targeting PTEN to activate the PI3K/mTOR axis (90-92). OE of miR-410 has been shown to be positively associated with EMT-related markers (such as E-cadherin downregulation and vimentin upregulation) and negatively associated with PTEN expression. Genetic knockdown of miR-410 can suppress EMT and enhance radiosensitivity, suggesting it may serve as a potential biomarker or therapeutic target in NSCLC radiotherapy. Restoring PTEN expression or administering mTOR inhibitors could reverse miR-410-induced EMT and radioresistance, offering novel strategies for NSCLC-targeted therapy. Similarly, miR-181a reduces NSCLC radiosensitivity by suppressing the expression of PTEN, a negative regulator of the PI3K/AKT/mTOR pathway (93). miR-21 also reduces NSCLC radiosensitivity by targeting programmed cell death 4 to activate PI3K/AKT/mTOR signaling (94). Furthermore, Huang et al (95) revealed that circular PVT1 promotes radioresistance by acting as a competing endogenous RNA to sequester miR-1208, subsequently reactivating the PI3K/AKT/mTOR pathway.
Conversely, miRNAs downregulated in lung cancer enhance NSCLC radiosensitivity by directly targeting mTOR. For example, miR-99a acts via mTOR as it is highly expressed in radiation-sensitive A549 cells compared with resistant counterparts. Its OE enhances radiosensitivity, whereas its inhibition induces resistance in vitro and in vivo (96). Similarly, miR-101-3p is commonly downregulated in NSCLC tissues and cell lines (97), and modulates radiosensitivity via mTOR. Li et al (98) showed that in radiation-resistant A549R cells, miR-101-3p upregulation can increase radiosensitivity; however, this effect is attenuated by high mTOR activity, whereas its inhibition induces resistance that can be reversed by rapamycin. This confirms that miR-101-3p downregulation drives resistance by activating mTOR, consistent with the mechanism of miR-99a. Another example is miR-208a, which promotes cell proliferation and radioresistance in NSCLC by targeting the AKT/mTOR pathway (99). Collectively, these studies highlight mTOR as a core mediator of miRNA-regulated NSCLC radiosensitivity, supporting co-targeting mTOR and these miRNAs as a promising strategy to optimize lung cancer radiotherapy.

mTOR and SCLC-specific mechanisms
Although most research on mTOR and radioresistance focuses on NSCLC, this pathway is also pivotal in highly aggressive and resistant SCLC. The mechanisms underlying radioresistance in SCLC often involve unique metabolic and epigenetic vulnerabilities, as well as oncogene-driven signaling crosstalk (Table IV).
Metabolically, the PI3K/AKT/mTOR pathway actively influences glucose metabolism to drive SCLC radioresistance (100,101). In SCLC models, dual PI3K/mTOR inhibitors (such as, BEZ235 and GSK2126458) promote autophagic degradation of glucose-6-phosphate dehydrogenase (G6PD). G6PD is the rate-limiting enzyme in the pentose phosphate pathway (PPP), which is a key survival mechanism for SCLC cells. Disrupting the PPP via G6PD degradation increases reactive oxygen species accumulation and oxidative stress damage, ultimately enhancing IR cytotoxicity and overcoming radioresistance (102). Additionally, SCLC frequently harbors the MYC oncogene amplification, which drives aggressive proliferation and mediates resistance to mTOR inhibitors such as everolimus through the eIF4E axis (54).
Beyond metabolic and MYC-driven resistance, adaptive survival pathways are another critical mTOR-related resistance mechanism in SCLC. Therapy-induced DNA stress activates compensatory pathways, such as PARP inhibition and SCLC DNA repair strategies, which upregulate PI3K/mTOR. This feedback loop, possibly via reduced LKB1 signaling, limits PARP inhibitor efficacy alone (103,104). This highlights the use of mTOR targeting to overcome SCLC therapy resistance. Additionally, in lung cancer research, targeting hypoxia/HIF-1α via PI3K/Akt/mTOR with Hsp90 inhibitors combined with radiotherapy blocks radiation-induced HIF-1α stabilization and enhances antitumor effects (105). Carbon ion radiotherapy also attenuates HIF-1 signaling and mTOR induction (106). In drug-resistant NSCLC, CXCR4 inhibition eliminates cancer stem-like cells via STAT3/Akt/mTOR signaling (107). Furthermore, disrupting the NRF2-CHML-mTOR axis by CHML knockdown inhibits NSCLC progression and overcomes chemo/radioresistance (108). Moreover, rapamycin combined with irradiation enhances lung cancer radiosensitivity and protects normal lung cells (109). The activation of mTOR to counter DNA damage in SCLC supports the combination of mTOR inhibitors with DNA-damaging agents, such as radiation, highlighting the use of mTOR targeting to overcome SCLC.

mTOR and tumor immune microenvironment
The mTOR pathway acts as a key regulator of T-cell differentiation and function; therefore, its activity within tumor cells and the tumor immune microenvironment is critical for therapeutic responses.
Oncogenic activation of the PI3K/AKT/mTOR pathway is strongly associated with the transcriptional and translational regulation of programmed death-ligand 1 (PD-L1) expression on the membranes of lung cancer cells (110). This drives immune evasion, as PD-L1 binds to PD-1 on T cells to suppress antitumor immunity. Common NSCLC mutations, such as EGFR and KRAS, activate this pathway and subsequently increase PD-L1 expression, while inhibiting mTOR can downregulate PD-L1, theoretically 're-sensitizing' tumors to immune attack (111).
Although mTOR inhibitors (rapalogs) are known as immunosuppressants, they also function as immunomodulators that promote antitumor responses, notably by expanding memory CD8+ T cells. This dual role provides a strong rationale for combination therapy as preclinical lung cancer models confirm that pairing an mTOR inhibitor with a PD-1 antibody can markedly reduce tumor growth and increase tumor-infiltrating T cells (112). Targeting mTOR signaling alongside immune checkpoints may therefore offer a potent synergistic strategy for overcoming radioresistance and achieving systemic antitumor effects (113).
mTOR drives lung cancer radioresistance via the PI3K/AKT axis. It regulates key processes such as DNA repair, miRNA activity, autophagy and PD-L1-mediated immune evasion. Clear radiosensitizing targets have been identified to counter these resistance mechanisms (Fig. 3). Notably, single mTOR inhibition shows limited clinical efficacy and requires combination with other therapies (such as anti-PD-1/PD-L1 and autophagy modulators) (79,110). Future efforts should focus on validating biomarkers, which may aid the development of personalized regimens and improve radiotherapy benefits for patients with lung cancer.

5.
Clinical applications and future prospects of mTOR inhibitors
mTOR inhibitors target the mTOR pathway, which is critical for regulating cell proliferation, metabolism and survival. These inhibitors hold broad therapeutic potential across multiple diseases, including autoimmune disorders, endocrine conditions and cancer (114,115). Aberrant activation of the mTOR signaling pathway serves a notable role in radioresistance in NSCLC. By targeting this pathway, mTOR inhibitors demonstrate marked antitumor potential. Specifically, they enhance tumor cell radiosensitivity and counteract resistance caused by dysregulated mTOR signaling. Furthermore, combining mTOR inhibitors with radiotherapy generates synergistic antitumor effects, amplifying therapeutic efficacy.

Preclinical evidence and clinical status
Preclinical studies have consistently demonstrated the radiosensitizing potential of mTOR inhibitors. For example, Ushijima et al (116) assessed the impact of the mTOR inhibitor temsirolimus on radioresistance under hypoxic conditions, using the A549 human lung adenocarcinoma cell line. This previous study revealed that while the D (10) value (dose required to kill 90% of cells) for A549 cells was 14.2 Gy under hypoxia, the combination with temsirolimus reduced this to 5.4 Gy (oxygen enhancement ratio=1.1), demonstrating its ability to suppress hypoxia-inducible factor-1α expression and overcome hypoxia-induced radioresistance. These findings position temsirolimus as a radiosensitizer for hypoxic tumors. Another study (117) reported that the mTOR inhibitor everolimus exerts radiosensitizing effects by inducing G2/M phase arrest in A549 cells. However, rapamycin pretreatment was shown to abolish this arrest 8 h post-radiotherapy. Early clinical trials also support this strategy. In a phase I clinical trial (118) assessing temsirolimus combined with thoracic radiotherapy (35 Gy/14f) in patients with NSCLC, among the eight evaluable patients, three achieved partial responses and two exhibited stable disease. These results underscore the preclinical and clinical synergy between mTOR inhibitors and radiotherapy, highlighting their potential to improve treatment outcomes.

mTOR inhibitor generations and therapeutic strategies
mTOR inhibitors are classified into three generations based on their mechanisms of action. First-generation inhibitors consist of antibiotic-derived allosteric inhibitors, including rapamycin and its derivatives such as temsirolimus, everolimus and ridaforolimus. Second-generation inhibitors are ATP-competitive inhibitors that selectively target the active kinase site of mTOR. These molecules, termed selective mTOR kinase inhibitors, achieve complete blockade of both mTORC1 and mTORC2, thereby preventing phosphorylation of PKB. Third-generation inhibitors, known as RapaLink, are hybrid molecules formed by conjugating the ATP-competitive inhibitor sapanisertib to rapalog macrocycles via diverse linker chains. Structurally, this design mimics a dual-inhibitor strategy, and such hybrid agents overcome resistance arising from monotherapy with rapalogs or ATP-competitive mTOR inhibitors. Furthermore, their multitarget activity enhances drug selectivity and therapeutic efficacy (119,120). By integrating rapamycin with mTOR kinase inhibitors, third-generation compounds exhibit superior antitumor potency and reduced off-target toxicity (Table V) (121).
Clinical investigations have explored these different generations of inhibitors. First-generation rapalogs have been evaluated extensively in combination strategies for NSCLC. Early phase trials assessed rapamycin with sunitinib (122) and temsirolimus with pemetrexed (123). Other studies examined ridaforolimus in patients with NSCLC harboring KRAS mutations (124) or evaluated everolimus in a translational study for resectable NSCLC (125). This combination approach continues to evolve with more rationale-driven pairings, such as the investigation of nab-sirolimus with the KRAS G12C inhibitor adagrasib (126). In parallel, second-generation dual mTORC1/mTORC2 kinase inhibitors have advanced into clinical studies. Initial first-in-human and dose-escalation studies have established the safety and tolerability of monotherapies such as AZD2014 (127) and CC-223 (128) in patients with advanced solid tumors. This development subsequently informed strategies to combine these potent dual inhibitors with other targeted agents, such as pairing the PI3K/mTOR inhibitor SAR245409 with the MEK inhibitor pimasertib (129) or combining the PI3K/mTOR inhibitor gedatolisib (PF-05212384) with the CDK4/6 inhibitor palbociclib (PD-0332991) (130). In summary, mTOR inhibitors demonstrate substantial promise in enhancing the efficacy of radiotherapy for lung cancer. By advancing the understanding of the molecular mechanisms underlying the mTOR signaling pathway and refining therapeutic strategies, more effective treatment regimens can be developed for patients with NSCLC, ultimately improving clinical outcomes and prognosis.

Controversies and limitations
Despite promising preclinical evidence for mTOR inhibition, several critical limitations and controversies must be addressed to ensure successful clinical translation. The key challenge lies in the notable disparity between the robust body of preclinical mechanistic evidence and the limited clinical outcomes achieved to date, which necessitates a realistic perspective on the field. Most current research comes from in vitro studies and xenograft models. This raises concerns about whether conclusions can be reliably generalized to heterogeneous human tumors. Furthermore, although a number of clinical trials investigating mTOR inhibitors have been completed (Table V), the results often represent preliminary phase I studies focused predominantly on determining safety and maximum tolerated dose, rather than definitive efficacy. A major therapeutic challenge stems from the complexity of the PI3K/AKT/mTOR network (131): First-generation inhibitors (rapalogs) are mostly cytostatic with modest single-agent efficacy. Their inhibition of mTORC1 fails to suppress a negative feedback loop, leading to paradoxical AKT phosphorylation and activation. This AKT activation sustains cell survival and proliferation, limiting the clinical use of rapalogs as monotherapy and requiring combination or dual-targeting agents (45,115,132-134). Compounding this, although the PI3K/AKT/mTOR pathway is frequently dysregulated in lung cancer, the utility of biomarkers for patient selection remains context-dependent; while PIK3CA mutations have been shown to predict rapalog sensitivity in breast cancer models, PTEN loss of function is not consistently associated with sensitivity, and co-occurring oncogenic drivers (such as HER2 mutations) can further obscure predictive value (135,136). The lack of standardized, broadly validated biomarkers that account for such context-specificity still presents a notable barrier to robust patient stratification and therapeutic guidance in clinical trials.
Additionally, mTOR inhibitors are associated with a unique spectrum of adverse events, including metabolic disturbances, mucositis, fatigue, and pulmonary complications such as pneumonitis and dyspnea (137-140). Critically, the long-standing use of rapalogs as immunosuppressants in transplant patients raises concerns that their immunosuppressive properties could counteract desired anticancer effects, particularly when combining mTOR inhibition with immunotherapies designed to enhance antitumor immunity (141,142). Collectively, these limitations, from pathway redundancy and biomarker gaps to toxicity and immunological conflicts, highlight the need for further research to optimize mTOR-targeted strategies and overcome barriers to effective clinical translation.

6.
Conclusion
The role of mTOR in lung cancer radioresistance has been extensively studied and well-established. The present review summarizes the mechanisms driving aberrant activation of the mTOR signaling pathway in lung cancer and its influence on radiotherapy resistance. Specifically, mTOR markedly enhances tumor cell radioresistance by regulating multiple biological processes, including cellular proliferation, autophagy, DNA repair, miRNA regulation and EMT. Furthermore, mTOR drives adaptive resistance by regulating metabolic resilience and promoting immune evasion through PD-L1 expression.
Although mTOR inhibitors hold considerable promise in lung cancer radiotherapy, several challenges remain. For example, drug resistance may arise as tumor cells evade mTOR inhibitor activity through activation of alternative signaling pathways or compensatory mechanisms. Consequently, combination therapies, such as co-administration with PI3K or MEK inhibitors, may represent effective strategies to overcome resistance. Additionally, mTOR inhibitors are associated with toxicity and side effects, including metabolic disturbances and immunosuppression, necessitating further optimization of dosing regimens and therapeutic protocols.