Research Progress on Traditional Chinese Medicines Reversing Multidrug Resistance and Mechanisms in Lung Cancer

The abnormal expression of cell membrane transport proteins

The expression levels of cell membrane drug transporters, including P-glycoprotein (P-gp), MDR-associated protein (MRP), and lung resistance-related protein (LRP), play a critical role in the development of MDR during cancer treatment. ^26,27 P-gp, a transmembrane protein encoded by the MDR1 gene, uses ATP hydrolysis to actively transport chemotherapy drugs out of cells, thus impeding their accumulation within tumor cells and leading to drug resistance in lung cancer. ^{28 –30} Individuals with high P-gp expression are particularly susceptible to resistance against hydrophobic lipophilic anticancer drugs, including doxorubicin, vinblastine, VP-16, paclitaxel, and docetaxel. Another ATP-dependent transporter, MRP, shares structural homology with P-gp and mediates drug resistance by sequestering drugs into intracellular vesicles or directly expelling cytotoxic agents from the cell, thereby preventing drug binding to their intracellular targets. ^{31 –33} In the context of lung cancer, the expression of MRP reaches its peak in adenocarcinomas, with squamous cell carcinomas following behind. Small-cell lung cancer demonstrates lower expression levels in comparison with nonsmall cell lung cancer (NSCLC). Contrary to P-gp, elevated MRP expression in adenocarcinomas is mainly noted in tumors that are poorly differentiated.

Although studies on cell membrane transport proteins (e.g., P-gp, MRP, LRP) provide critical targets for reversing MDR, ³⁴ clinical translation remains challenging. For instance, P-gp inhibitors (e.g., verapamil) failed in early clinical trials due to systemic toxicity or pharmacokinetic issues. ³⁵ However, novel nanodelivery systems (e.g., liposomes) can enhance the targeting of TCM monomers and reduce off-target effects, offering new opportunities for clinical combination with chemotherapy. ³⁶ Future research should focus on biomarker-guided personalized regimens to optimize the balance between efficacy and safety.

LRP is implicated in MDR through two primary mechanisms as follows: (1) the shielding target mechanism, which prevents drugs from accessing nuclear pores and binding to nuclear targets; even if drugs enter the nucleus, they are actively pumped out before exerting therapeutic effects; and (2) the sequestration of cytoplasmic drugs into vesicles followed by expulsion via exocytosis. Berger and colleagues examined the expression levels of LRP in 16 NSCLC cell lines, embryonic lung fibroblasts, and immortalized lung bronchial epithelial cells. They observed comparable levels of LRP mRNA expression among these three types of cells. ^{37 –40} Furthermore, treatment with conventional chemotherapy drugs cisplatin and vinblastine revealed that while vinblastine had no enhancing effect on LRP, cisplatin exhibited a modest enhancement, confirming a significant correlation between LRP expression and cisplatin resistance. ^{41 –43}

Abnormal intracellular enzyme system

DNA topoisomerases (Topo) are essential enzymes involved in the processes of cellular DNA replication and transcription. Eukaryotic cells possess two main types of topoisomerases as follows: Topo I and Topo II. ^{44 –46} Although both are vital for preserving genomic integrity, Topo II holds a more significant role in DNA replication, transcription, and mitosis compared with Topo I. Topo II is a recognized target for various anticancer medications, such as doxorubicin, etoposide (VP-16), and mitoxantrone. The expression levels of Topo II exhibit a positive correlation with resistance to VP-16, but a negative correlation with resistance to doxorubicin. ^47,48

Glutathione S-transferase (GST) constitutes a group of isoenzymes that serve as a key detoxification mechanism within cells. ^49,50 Among these isoenzymes, GST-π is most closely associated with malignant tumors and their drug resistance mechanisms. GST-π catalyzes the binding of electrophilic substances and lipophilic cytotoxins to glutathione, thereby increasing the water solubility of these compounds and enhancing their metabolism within tumor cells. ^51,52 This mechanism results in a reduced ability to induce cytotoxicity in tumor cells and plays a part in the development of drug resistance. Increased expression of GST-π shows a positive association with resistance to cisplatin. Differences in the inherent expression levels of GST-π and Topo II have been noted among four human lung cancer cell lines with distinct histological characteristics, indicating that these variations might affect the responsiveness to cisplatin-based chemotherapy. ⁵³ Among the related proteins, GST-π may serve as a potential biomarker for predicting cisplatin resistance.

Strategies targeting enzyme systems (e.g., inhibiting GST-π or Topo II) show promise in preclinical models, but clinical applications face challenges such as metabolic instability and insufficient selectivity. For example, combining TCM formulations (e.g., curcumin) may synergistically reduce resistance-related enzyme expression through multitarget effects while mitigating toxicity. ⁵⁴ Furthermore, developing real-time therapeutic adjustment protocols based on dynamic enzyme activity monitoring could improve clinical response rates.

Abnormal apoptosis

Aberrant apoptosis, a tightly regulated genetic process that oversees programmed cell death, is vital for maintaining tissue equilibrium and preventing the progression of numerous diseases. ⁵⁵ To sustain a stable internal milieu, the body must eradicate abnormal cells via apoptosis. In cancer research, apoptosis is acknowledged as a complex and multifaceted phenomenon, governed by numerous molecular pathways. Among the various regulatory systems, the influence of the B cell lymphoma (Bcl)−2 family of proteins in modulating cell death is especially notable, as it holds a key position in cellular survival and the control of apoptosis. Furthermore, mutations or deletions in the tumor suppressor gene p53, along with the abnormal expression of other tumor suppressor genes, contribute further to this intricate process. ^{56 –58} The Bcl-2 protein has a significant role in regulating apoptosis, serving as a critical element in the management of programmed cell death. Overexpression of Bcl-2 leads to resistance against various chemotherapy agents by impeding proapoptotic factors like Bax or Bak. ⁵⁹ For example, Halasova and colleagues demonstrated that NSCLC cell lines transfected with Bcl-2 display substantial resistance to apoptosis triggered by anticancer drugs. ⁶⁰ Translating apoptosis regulation mechanisms (e.g., Bcl-2/p53 pathways) into clinics requires addressing tumor heterogeneity and microenvironmental interference. For instance, curcumin’s low bioavailability limits its efficacy despite its proapoptotic effects. Nanotechnology (e.g., albumin-bound formulations) or prodrug designs may enhance the delivery efficiency of TCM monomers. ⁶¹ In addition, combining immune checkpoint inhibitors (e.g., PD-1 inhibitors) could synergistically activate apoptosis and immune responses, offering innovative strategies to overcome MDR. Another significant participant in apoptosis is the c-myc proto-oncogene, which primarily acts to trigger cell apoptosis and differentiation and oversee DNA synthesis. ⁶² In addition, c-myc has a crucial function in the G2/M phase of the cell cycle. It is essential not only for regulating the transition from the G0/G1 phase to the S phase but also for influencing various other critical stages of the cell cycle, thereby impacting cell proliferation and growth. Research has indicated that in normal lung bronchial epithelial cells, the expression levels of the c-myc protein are generally low, indicating its limited presence under typical conditions; however, in lung cancer tissues, its expression level is significantly elevated, suggesting that this upregulation may be closely associated with the development of lung cancer. ^{63 –65} Li and colleagues carried out a study using in situ hybridization to evaluate the expression levels of MRP mRNA in a group of 113 NSCLC tissue samples. Moreover, they utilized the immunohistochemistry SP method to investigate the expression of myc protein, exploring its connection with the clinical and pathological characteristics of NSCLC. ⁶⁶ Their results showed that in lung cancer, the myc protein can enhance the expression of human telomerase reverse transcriptase, MDR protein 1 (MDR1), and MRP, all of which are involved in the effectiveness of antitumor chemotherapy.

Enhanced cellular repair system

Many anticancer drugs exert their cytotoxic effects by damaging tumor cell biomolecules, such as inducing DNA cross-linking. Tumor cells, however, can acquire resistance to these agents by activating mechanisms that either repair DNA damage or actively inhibit the occurrence of DNA damage altogether. ^{67 –69} A critical enzyme in this mechanism is O-methylguanine-DNA methyltransferase (MGMT), which serves an essential function in DNA repair by eliminating alkyl adducts from DNA molecules. ⁷⁰ In a study focusing on tumor cell death induced by alkylating drugs, Bleanich and colleagues emphasized the significant role of MGMT in repairing DNA damage inflicted by these alkylating substances. This investigation highlights the essential role of MGMT in maintaining genomic stability, particularly when cells are subjected to such harmful stressors. ⁷¹ This discovery emphasizes the significance of MGMT in tumor resistance to alkylating agents, as demonstrated by its activity in both labeled immune assays and clinical drug effectiveness research. Tong and colleagues further clarified that heightened resistance of tumor cells to chemotherapy drugs frequently stems from the inhibition of DNA cross-linking. ⁷² In this context, MGMT is capable of repairing alkylated guanine residues, a key factor in mitigating the cytotoxic effects of alkylating agents.

Interventions targeting DNA repair systems (e.g., MGMT) carry risks of off-target damage to normal cells in clinical settings. For example, MGMT inhibitors are more commonly used in gliomas, with limited efficacy evidence in lung cancer. TCM monomers (e.g., baicalein) may selectively inhibit tumor repair pathways via epigenetic regulation, minimizing harm to healthy tissues. ⁷³ Future studies should integrate liquid biopsy to dynamically monitor repair enzyme activity for precise dose adjustment and explore combinations with radiotherapy to enhance sensitivity.

Reversing MDR by regulating membrane transport proteins

A fundamental mechanism contributing to MDR is the excessive production of membrane transporter proteins. These proteins operate by actively pumping drugs out of the cell, which leads to a reduction in the intracellular concentration of therapeutic agents. As a consequence, their effectiveness is significantly diminished, hindering the intended therapeutic action. ^74,75 This process results in a situation where the levels of drugs within cells decrease, consequently diminishing their efficacy in treatment. Proteins involved in membrane transport, including P-gp, MRP, and LRP, play a crucial role in the emergence of MDR in lung cancer, thereby substantially contributing to the ineffectiveness of chemotherapy. ^76,77 TCM monomers, being natural compounds, exhibit multitarget and multipathway effects. In recent years, they have demonstrated significant potential in overcoming MDR in lung cancer.

Current studies indicate that TCM monomers (e.g., Rg3, β-elemene) reverse MDR by inhibiting P-gp activity, as validated in preclinical models. ⁷⁸ However, clinical translation requires addressing low bioavailability and insufficient targeting. For instance, liposome-encapsulated curcumin formulations demonstrated enhanced stability and tumor accumulation, ⁷⁹ offering new directions for combination chemotherapy.

Sun et al. explored the potential of W6, a novel tetracycline derivative, to reverse MDR by P-gp. ⁸⁰ Through the application of flow cytometry to assess the buildup of doxorubicin, a substrate of P-gp, inside cells, the researchers demonstrated that W6 notably increased the concentration of doxorubicin within KBv200 cells, which exhibit elevated expression of P-gp. This effect was dose dependent, indicating that W6 effectively inhibits P-gp function and thereby reverses MDR.

Similarly, 20(R)-Ginsenoside Rg3 (hereafter referred to as Rg3) has been demonstrated to competitively bind to P-gp, thus inhibiting the interaction between P-gp and anticancer drug substrates. ⁸¹ This mechanism reduces drug efflux from cancer cells, ultimately reversing tumor MDR. In addition, it was further revealed that Rg3 not only inhibited the epithelial–mesenchymal transition process but also significantly impaired the invasive potential of lung cancer cells. This mode of operation was primarily accomplished by reducing the activity of cell-mediated epidermal growth factor receptor (EGFR) and efficiently inhibiting MAPK and NF-κB, two signaling pathways that are essential for tumor growth and spread.

Triptolide (TPL) is an epoxidized diterpenoid lactone derived from the TCM Tripterygium wilfordii, which is the principal bioactive constituent responsible for the antineoplastic effects of this herb. ^{82 –84} Recent researches indicate that the TPL markedly boosts the cytotoxic impact of cisplatin on A549/DPP cells, a lung cancer cell line resistant to cisplatin, specifically, at nontoxic (2 µmol/L) and low-toxicity (10 µmol/L) concentrations. Compared with the control group, TPL enhanced cisplatin’s inhibitory effect on A549/DPP cell proliferation, resulting in reversal factors of 2.09 and 2.93, respectively. ¹ Additional investigations utilizing Western blotting and RT-PCR techniques indicated that treatment with TPL led to a reduction in the expression of genes associated with MDR in A549/DPP cells. The levels of both mRNA and protein for MDR1 and LRP were significantly reduced. For example, the expression levels of MDR1 mRNA in the groups treated with 2 µmol/L and 10 µmol/L TPL were 64.2% and 22.6% of those in the control group, respectively. Likewise, the expression levels of LRP mRNA were 54.8% and 34.7% of the control group, respectively. These results imply that TPL effectively counteracted the resistance of A549/DPP cells to cisplatin by decreasing the expression of crucial MDR genes, thus enhancing the cells’ vulnerability to the chemotherapy agent. It is worth noting that the BRCA1 protein, as a core component in the DNA homologous recombination repair process, has been found by researchers through immunoblotting technology; TPL can effectively reduce the expression level of BRCA1 protein, and subsequently, hinder the DNA damage repair process, thereby facilitating the apoptosis of lung cancer cells. ⁸⁶ This suggests that TPL may be one of the effective drugs to improve tumor cell drug resistance.

Psoralen, a key bioactive compound found in Fructus Psoraleae, plays a significant role in its medicinal properties. ^87,88 Hsieh et al. demonstrated that Psoralen, at a concentration of 20 µmol/L, effectively overcomes docetaxel resistance in A549/D16 cells. This compound enhances the cells’ sensitivity to the drug and induces cell death. ⁸⁹ This effect is concentration dependent and time correlated. Notably, psoralen itself does not inhibit cell proliferation directly. Alternatively, the way it functions includes decreasing the expression of the ABCB1 gene and its corresponding protein, impeding the activity of the ABCB1 protein and, in turn, reversing resistance to medications. Likewise, Zhebe alkaloids have been proven to combat MDR in lung cancer through several different mechanisms. These phenomena include promoting apoptosis in cancer cells, reducing the expression levels of the LRP protein, and enhancing the cells’ responsiveness to chemotherapy agents. ⁹⁰ Kim et al. investigated the effects of the phenanthrolinyllixidine alkaloid (-)-Antofine on paclitaxel-resistant A549/PA cells. ⁹¹ The researchers discovered that (-)-Antofine serves a dual purpose as follows: it reduces the expression of P-gp and suppresses P-gp activity in a manner that is contingent on the dosage administered.

Xu has been shown to effectively reverse doxorubicin resistance in GLC-82/ADR-resistant lung cancer cells. ⁹² This compound downregulates MRP protein expression in chemotherapy-resistant lung cancer cells and synergistically enhances apoptosis in drug-resistant cells when combined with doxorubicin. Wu and colleagues conducted a study to examine the effect of As₂O₃ on reversing MDR in the human lung adenocarcinoma cell line A549/DDP, utilizing a nude mouse model. ⁹³ The results indicated that As₂O₃ was capable of reversing drug resistance in A549/DDP cells within a living organism. The mechanism behind this effect is likely related to the reduction in MRP1 gene expression, which enhances the responsiveness of A549/DDP cells to cisplatin (DDP) and may trigger programmed cell death in tumor cells. Moreover, various other compounds have exhibited potential in reversing MDR in lung cancer. Examples include ginsenoside Rg₃, ^94,95 β-elemene, ^96,97 and disodium norcantharidate. ^98,99 It has been documented that these substances contribute to lowering the expression levels of P-gp and MRP, effectively counteracting MDR seen in lung cancer cells.

Reversing MDR by regulating autophagy

Autophagy is a biological process in which impaired organelles and other malfunctioning cellular components are broken down and reused, aiding in the preservation of cellular equilibrium. In addition, lysosomes degrade misfolded proteins to uphold cellular homeostatic balance. Within the framework of tumor cells, autophagy fulfills a dual function: on one hand, it acts as a vital mechanism for tumor suppression by eliminating damaged organelles, thereby sustaining cellular equilibrium and restraining tumor growth and advancement; on the other hand, autophagy significantly contributes to the survival of tumor cells, especially when they are subjected to the stress of chemotherapy. This process assists these cells in mitigating the adverse impacts of chemotherapeutic agents, consequently leading to the gradual emergence of drug resistance. ^{100 –102} The clinical potential of autophagy modulation has emerged in early trials, but its dual role may lead to therapeutic paradoxes. For example, resveratrol combined with gefitinib enhanced efficacy by inducing autophagy in NSCLC resistance models, yet dose optimization is critical to avoid cytoprotective effects from excessive autophagy. ¹⁰³

Recent extensive research has demonstrated that the autophagy phenomenon is pivotal in the development of MDR in lung cancer. The precise modulation of autophagy-related signaling pathways is essential for regulating the response of tumor cells to chemotherapeutic drugs. These signaling pathways function as intracellular regulatory controls, and their modification can directly influence the sensitivity of tumor cells to chemotherapy. By accurately regulating these autophagy-related pathways, there is hope for achieving new advancements in lung cancer treatment. More specifically, such regulation could potentially increase the responsiveness of cancer cells to chemotherapeutic drugs, thereby enhancing treatment effectiveness, or decrease their responsiveness, yet it also offers fresh approaches and concepts for reversing MDR through targeted manipulation. Therefore, a thorough investigation into the regulatory mechanisms of autophagy-related signaling pathways is of considerable importance for enhancing the treatment outcomes of lung cancer patients. ^104,105

Resveratrol is a natural polyphenolic compound with high biological activity. ^106,107 Recent research indicates that the combination of resveratrol and gefitinib effectively inhibits the proliferation of PC9/G cells, a subtype of NSCLC that has acquired resistance to gefitinib. ¹⁰⁸ The potential mechanism behind this effect may be linked to elevated levels of p53 and p21 expression in PC9/G cells, which subsequently initiates autophagy and cellular aging. This series of events results in cell cycle arrest at the G2/M phase and promotes programmed cell death by activating the caspase-3 pathway, thereby contributing to the overall cellular response. These results imply that resveratrol primarily stimulates autophagy and senescence in cells and works in conjunction with gefitinib to reverse MDR in PC9/G cells. Chen and colleagues reported that curcumin hinders the monoubiquitination of FANCD2 and diminishes nucleosome assembly in cisplatin-resistant A549/DDP cells. ¹⁰⁹ When curcumin is combined with cisplatin, it exhibits a synergistic cytotoxic impact on A549/DDP cells, indicating its capacity to oppose cisplatin resistance. The study disclosed that curcumin enhances the antiproliferative and proapoptotic effects of cisplatin by suppressing the DNA repair process mediated by the Fanconi anemia/BRCA pathway, which is crucial for overcoming cisplatin resistance in A549/DDP lung cancer cells.

At present, limited research exists on MDR in lung cancer using monomers from TCMs. These studies primarily explore mechanisms such as autophagy regulation, DNA repair enhancement, and suppression of excessive proliferation in cancer stem cells. Future research should focus on exploring these mechanisms to better understand the potential of TCM monomers in overcoming MDR in lung cancer.

Reversing MDR by regulating apoptosis

Programmed cell death, commonly known as apoptosis, is a fundamental biological mechanism that holds significant importance in controlling the viability of cancer cells and influencing their reaction to chemotherapeutic interventions. ^56,110,111 In reversing MDR, apoptosis plays a crucial role in aiding to overcome the problem of chemotherapeutic resistance by inducing abnormal cell death pathways, which are often found in drug-resistant cancer cells. Curcumin, a bioactive compound with significant therapeutic potential, is derived from the rhizome of turmeric and is a member of the ginger family. This naturally occurring compound has attracted substantial interest due to its numerous health advantages, especially its anti-inflammatory and antioxidant characteristics. ^112,113 Although curcumin excels in reversing MDR by regulating apoptosis pathways (e.g., Bax/Bcl-2), its clinical translation is limited by solubility and metabolic stability. Nanoparticle delivery systems (e.g., PLGA microspheres) significantly enhance its plasma concentration, with Phase II trials confirming the safety of its combination with cisplatin. ¹¹⁴

Recent studies have identified curcumin as a powerful agent for reversing MDR, with the ability to produce its effects through various mechanisms and exhibit a wide range of activity. Zhang and colleagues reported that curcumin effectively counteracts drug resistance in A549/DDP cells, underscoring its potential as a therapeutic agent for surmounting chemotherapy resistance in the treatment of lung cancer. ¹¹⁵ The underlying mechanism likely involves downregulating the expression of cell survival proteins, thereby partially alleviating their inhibitory effects on apoptosis. Chen and colleagues conducted a research study using propidium iodide staining alongside flow cytometry analysis to examine the impact of curcumin on the advancement of the cell cycle. The findings demonstrated that treatment with curcumin at a concentration of 20 µmol/L for 24 h led to a significant arrest of A549/DDP cells in the G2/M phase, highlighting its potential impact on cell cycle regulation. This result indicates that curcumin can efficiently impede the progression of the cell cycle in these cells at the G2/M phase. ¹¹⁶ These findings imply that curcumin notably inhibits the proliferation of human lung adenocarcinoma A549/DDP cells by inducing cell cycle arrest and triggering programmed cell death. This effect is both time- and concentration-dependent, highlighting curcumin’s promising role as a therapeutic agent in overcoming chemotherapy resistance in lung cancer treatment. The observed dependency emphasizes the importance of optimizing both treatment duration and curcumin concentration for maximum therapeutic benefit.

Curcumin at a nontoxic concentration of 4 µmol/L can enhance the cytotoxic effect of gefitinib. ¹¹⁷ The cell viability in the combination group of curcumin and gefitinib was reduced by 13% to 22% compared with the group treated with gefitinib alone. These findings demonstrate that curcumin exerts a notable effect in reversing resistance in the PC9/G2 cell line. The underlying mechanism for this resistance reversal may be attributed to the reduction in the expression of PI3K within the EGFR downstream signaling pathway, leading to an increased production of the apoptotic protein caspase-3.

Recent studies have identified several natural compounds along with their mechanisms of action in reversing tumor resistance and promoting apoptosis in lung cancer cell lines such as Platycodon grandiflorum sapogenin-D. The results reveal that curcumin has a substantial impact on reversing drug resistance in the PC9/G2 cell line. ¹¹⁸ The mechanism behind this reversal of resistance could be due to the decreased expression of PI3K in the EGFR downstream signaling cascade, which results in a greater synthesis of the apoptotic protein caspase-3. This dual action effectively reverses tumor resistance. Tanshinone IIA induces apoptosis in A549/CDDP cells and inhibits their proliferation. ¹¹⁹ Its molecular mechanism involves downregulation of survivin expression and upregulation of Bax expression. Meanwhile, some studies have reported that tanshinone, as a compound with potential anticancer activity, is able to induce the differentiation of human malignant tumor cells and effectively inhibit their proliferation. Tanshinone, for one, can trigger programmed cell death, thereby decreasing the population of cancerous tumor cells. In addition, it can suppress the growth of A549/CDDP cells, a process that coincides with a decrease in survivin expression and an increase in Bax expression. Consequently, it exerts a notable influence on the cell cycle, offering novel approaches and tactics for the management of malignant tumors.

Chen and colleagues showed that dihydroartemisinin can reinstate the responsiveness of A549/CDDP cells to cisplatin. ¹²⁰ This effect is mainly attributed to the heightened induction of programmed cell death, a significant decrease in Bcl-2 protein levels, an increase in Bax expression, and a reduction in the suppression of apoptosis, all of which jointly contribute to reversing drug resistance. Furthermore, β-elemene, an active component extracted from Curcuma zedoaria, when used in conjunction with cisplatin, increases the responsiveness of A549/DDP cell lines to cisplatin, effectively counteracting the acquired resistance to the medication. ¹²¹ The mechanism of action entails the disruption of the mitochondrial membrane, which facilitates the release of cytochrome c from the mitochondria into the cytoplasm. Consequently, this process initiates the activation of caspase-3, which subsequently boosts the expression of the proapoptotic protein Bad and reduces the level of the antiapoptotic protein Bcl-2. This sequence of events ultimately collaborates to induce apoptosis in the cell.

Evodiamine has been shown to inhibit the phosphorylation of IκB-α, thereby impeding the NF-κB signaling pathway. This effect can effectively advance the process of apoptosis while suppressing the proliferative activity of cells, and more importantly, it also significantly improves the responsiveness of drug-resistant cells to cisplatin (DDP), thus offering a novel approach for reversing drug resistance. ^{122 –124} Ginsenoside Rg3 counteracts MDR by increasing Nm3 expression, which hinders the invasion and metastasis of A549/DDP cells. In addition, it elevates caspase-3 levels, resulting in a decrease in cell membrane fluidity and further contributing to the reversal of MDR. ¹²⁵ Ma and colleagues demonstrated, through extensive in vitro and in vivo experiments, that artemether can counteract the resistance of drug-resistant lung cancer cells to erlotinib. This discovery underscores its potential as a therapeutic agent for overcoming resistance in cancer cases that do not respond to conventional treatments. ¹²⁶ The mechanism involves the inhibition of the PI3K/Akt pathway and the downregulation of ABCG2 expression.

In addition, related literature reports that saponins VII, quercetin, and ferulic acid can all inhibit tumor cell proliferation, promote apoptosis, and reverse MDR in lung cancer-resistant cell lines. ^{127 –129} These results underscore the significant promise of natural compounds in tackling drug resistance and amplifying the therapeutic impact on lung cancer treatment. This emphasizes their potential role as effective adjuncts in cancer therapy. Future researchers should focus on revealing these mechanisms of action in greater depth and actively exploring the prospects for their application in practical clinical settings.

Reversing MDR by regulating enzyme system

MDR often arises due to the activation of survival signaling cascades and modifications in metabolic routes within the tumor microenvironment. These signaling cascades and metabolic modifications can be precisely addressed by modulating enzymes to overcome resistance. ^{130 –132} Recent studies have demonstrated that As₂O₃ can bind to the cysteine thiol group in the active center of GST-π, thereby inhibiting the enzyme’s catalytic activity or altering its overexpression. This interaction reduces the detoxification and metabolic capacity of drug-resistant cells toward cytotoxic agents. Clinical application of enzyme system modulation requires balancing efficacy and toxicity. For instance, As₂O₃ combined with low-dose chemotherapy showed partial remission in relapsed/refractory lung cancer patients, but its long-term safety needs validation through large-scale Phase III trials. ¹³³ Feng and colleagues stated that an As₂O₃ concentration of 0.15 µmol/L can notably elevate the level of doxorubicin (ADM) inside A549/R cells, decreasing its IC₅₀ value from 0.495 µmol/L to 0.217 µmol/L and achieving a reversal factor of 2.3. ¹³⁴ This indicates that As₂O₃ can partially reverse ADM resistance in A549/R cells, likely through decreased

GST activity (p < 0.05), downregulation of GST-π mRNA expression, and concentration-dependent effects. An alkaloid compound derived from Piper longum, specifically Piper longum amide, has demonstrated efficacy in counteracting drug resistance in A549/DDP cells and facilitating programmed cell death. ¹³⁵ The underlying process could entail an increase in the expression of genes and proteins associated with resistance, such as MDR1, MRP1, Top-II, GST-π, survivin, and CDK1.

He et al. used a progressive dosing regimen to establish an acquired MDR model of Lewis lung cancer in mice. ¹³⁶ Tanshinone IIA, the chemotherapy drug CTX, and a combination of tanshinone IIA with CTX were administered separately. Results indicated that tanshinone IIA alone had no significant inhibitory effect on Lewis lung cancer cells. Nevertheless, when used in conjunction with CTX, there was a notable increase in both the tumor inhibition rate and the cell apoptosis rate in comparison with the use of CTX alone. This indicates that tanshinone IIA has the ability to counteract acquired MDR in Lewis lung cancer. Further analysis using flow cytometry and fluorescence detection showed that the expression levels of P-gp were 3.16% ± 1.32% and 3.79% ± 1.35%, respectively, and those of TOPO II were 5.62% ± 1.84% and 6.01% ± 2.12%, both significantly reduced compared with CTX intervention alone. This result suggests that the efficacy of tanshinone IIA in reversing MDR may be associated with its ability to reduce P-gp and TOPO II expression.