Menthol: An underestimated anticancer agent

Menthol, a widely used natural, active compound, has recently been shown to have anticancer activity. Moreover, it has been found to have a promising future in the treatment of various solid tumors. Therefore, using literature from PubMed, EMBASE, Web of Science, Ovid, ScienceDirect, and China National Knowledge Infrastructure databases, the present study reviewed the anticancer activity of menthol and the underlying mechanism. Menthol has a good safety profile and exerts its anticancer activity via multiple pathways and targets. As a result, it has gained popularity for significantly inhibiting different types of cancer cells by various mechanisms such as induction of apoptosis, cell cycle arrest, disruption of tubulin polymerization, and inhibition of tumor angiogenesis. Owing to the excellent anticancer activity menthol has demonstrated, further research is warranted for developing it as a novel anticancer agent. However, there are limitations and gaps in the current research on menthol, and its antitumor mechanism has not been completely elucidated. It is expected that more basic experimental and clinical studies focusing on menthol and its derivatives will eventually help in its clinical application as a novel anticancer agent.

. In addition, the financial losses incurred by patients' families owing to patient care and treatment are becoming a social issue of concern (Alzehr et al., 2022). Chemotherapy, immunotherapy, gene therapy, and radiation therapy are the mainstay of cancer treatment (Malik et al., 2021). However, these are associated with low cure rates, prohibitive costs, and numerous residual effects. Owing to these limitations of the existing treatment options, researchers have been constantly searching for new therapeutic strategies. Natural phytochemicals have been proven to play a key role in the prevention and treatment of diseases, especially various types of cancer. Paclitaxel is one such natural phytochemical that has been successfully used as an anticancer drug. Thus, owing to their rich biological origin and safety, an increasing number of researchers have been attempting to synthesize novel anticancer drugs from natural products. From 1981 to 2019, 1,881 natural phytochemicals, including 247 anticancer drugs, were approved for therapeutic purposes, and the number continues to increase (Newman and Cragg, 2020). Among these natural products, monoterpenes offer distinct advantages in cancer treatment (Zielińska-Błajet et al., 2021).
Owing to its numerous biological properties, menthol is extensively used in multiple diseases, including inflammatory diseases (Du et al., 2020;Zhang et al., 2020), cancer (de Mesquita et al., 2019;Zielińska-Błajet et al., 2021), pain disorders (Hilfiger et al., 2021), respiratory disorders (Kanezaki et al., 2021), cardiovascular diseases (Silva, 2020), and skin diseases (Chrisman, 1978;Patel and Yosipovitch, 2010). Particularly, in vitro experiments have revealed the anti-proliferative potential of menthol against various tumor cell lines (Fatima et al., 2021). Mechanistically, menthol induces apoptosis in cancer cells, indicating its role as an anticancer agent (Beck et al., 2007) against a variety of cancers, such as prostate cancer (Kim et al., 2009), colon cancer (Faridi et al., 2016), skin cancer (Fatima et al., 2021), uveal melanoma (Walcher et al., 2018), pancreatic ductal Frontiers in Pharmacology frontiersin.org 02 adenocarcinoma (Cucu et al., 2014), gastric cancer (Santo et al., 2021), liver cancer (Lin et al., 2001), leukemia (Lu et al., 2007a), and bladder cancer (Li et al., 2009) (Figure 2). Irrespective of these data, the anticancer properties of menthol have not been sufficiently implemented in clinical practice. Moreover, no study has systematically reviewed the available anticancer data on menthol. Accordingly, the present article reviews advances in research on the anticancer properties of menthol as well as the underlying mechanisms in vivo and in vitro. In addition, the current anticancer applications and limitations of menthol in clinical practice are mentioned.
Due to the disadvantages of menthol's volatility and short duration of action, there are more researchers have started to focus on the synthesis of menthol derivatives, such as menthyl lactate (Api et al., 2018), menthyl acetate (Api et al., 2017), Menthone 1, 2-glycerol ketal(FrescolatMGA), etc., Each of these derivatives builds on the strengths of menthol by being less irritating, evaporating more slowly or retaining their fragrance longer (Api et al., 2017;Api et al., 2018). These early menthol derivatives were mostly used in the household products and cosmetics industries, but as the medical value of menthol has received more attention, scientists have come to expect more from the derivatives. Attempts have been made to combine menthol with other known effective groups or substances to obtain a new compound that incorporates the advantages. For instance, menthol sulfamate and menthol carbonyl sulfamate, synthetic derivatives of sulfamate and menthol, have been shown to have a significant positive effect on Alzheimer's disease (Daryadel et al., 2018). Menthol carbonates synthesized by Clemente's team were validated to have anti-parasitic activity and have the potential to be orally active drug candidates (Clemente et al., 2022). In general, similar to menthol itself, menthol derivatives have also been found to have anti-cancer effects (Singh et al., 2022).

Mechanism underlying the anticancer activity of menthol
Over the years, both the anticancer targets and mechanisms underlying the anticancer activity of natural products have been extensively studied. Menthol exerts its anticancer activity by inducing cancer cell death via apoptosis, cell cycle arrest, inhibiting tubulin polymerization, and cell necrosis as well as by inhibiting tumor cell invasion, metastasis, angiogenesis, and proliferation ( Figure 3; Table1).

Effect of menthol on cell apoptosis
Apoptosis is an orchestrated cellular process that occurs in both physiological and pathological conditions. Aberrant apoptosis is a primary mechanism underlying the pathogenesis of many diseases, including cancer, in which apoptosis occurs too infrequently resulting in undying malignant cells. Accordingly, studies have revealed that apoptosis can be an important target of anticancer agents (Wong, 2011). As an anticancer agent, menthol has been found to induce the apoptosis of cancer cells via several mechanisms (Fatima et al., 2021). An experimental study assessed the anticancer activity of neomenthol in the skin cancer cell line A431 and the Ehrlich Ascites Carcinoma mouse model. The study found that neomenthol altered hyaluronidase structure and inhibited hyaluronidase activity, which prevented hyaluronic acid degradation and thus induced apoptosis (Fatima et al., 2021). In another study using Caco-2 cells, L-menthol was found to downregulate heat shock protein 90, which is a molecular chaperone that mediates the activation of disparate client proteins; It additionally activated caspase3 and caspase10. These alterations further inhibit AKT (plays a key role in multiple cellular processes such as apoptosis)-related pathways to induce the release of the pro-apoptotic factor BAD from the BAD and BCL2L1 complexes (Faridi et al., 2011;Faridi et al., 2016). In addition, another study has shown that menthol induces apoptosis in LNCaP cells by stimulating the transient receptor potential melastatin subtype 8 (TRPM8) channel (Beck et al., 2007). Lu et al. found that (−)-menthol induces apoptosis in liver tumor cells of BALB/c mice, in whom leukemia was induced using the mouse leukemia cells WEHI-3 (Lu et al., 2007b).

Effect of menthol on cell cycle arrest
The cell cycle regulates cell growth, replication of genetic material, and cell division. While the cell cycle is tightly controlled in normal cells, that in tumor cells is dysregulated owing to genetic alterations, which often results in tumors (Malumbres and Barbacid, 2009). Therefore, individual cell cycles and inhibition of cell cycle proteins have become targets for cancer therapy (Suski et al., 2021).
Menthol has been found to suppress the development of cancer cells by mediating cell cycle arrest. In the prostate cancer cell line DU145, menthol induced G0/G1 phase cell arrest (Wang et al., 2012). In another prostate cancer cell line, PC-3, menthol likely altered gene expression, inhibiting the G2/M phase; In addition, menthol may be associated with the downregulation of polo-like kinase 1, which plays an important role in the initiation, maintenance, and completion of mitosis (Kim et al., 2012).
Moreover, menthol retards the growth of tumor cells in the human epidermoid carcinoma A431 cells by arresting the G2/M phase and increasing the number of sub-diploid cells (Fatima et al., 2021).

Effect of menthol on tubulin polymerization
Tubulin polymerization involves the assembly of the highly conserved a-and ß-tubulin heterodimers into dynamic microtubules, which perform multiple important cellular functions such as providing structural support, allowing intracellular substance transport, and generating force for cell division. Therefore, the degree of tubulin aggregation in cells is highly correlated with cell morphology and division, and alteration in this aggregation may cause cell death. Notably, tubulins are expressed in different forms by specific genes. A few tubulins are differentially expressed between normal and tumor cells, which could provide a basis for the development of anticancer drugs (Binarová and Tuszynski, 2019). Menthol has been shown to play a role in regulating tubulin in cancer cells, providing a basis for its use as an anticancer agent. Faridi et al. found that high concentrations of L-menthol (IC 50 = 12 mg/mL) promoted microtubule protein polymerization, prompting cell death in Caco-2 cells (Faridi et al., 2011). In addition to L-menthol, neomenthol (100 μm) inhibits tubulin polymerization, thus arresting the proliferation of A431 skin cancer cells (Fatima et al., 2021).

Intracellular Ca 2+ concentrationmediated cell death
Calcium ions (Ca 2+ ) are the key secondary messengers in both excitable and non-excitable cells. There is now a large body of research showing that intracellular Ca 2+ [(Ca 2+ )i] fluxes are associated with cancer progression and could be a potential target for cancer therapy, particularly targeting tumor growth Frontiers in Pharmacology frontiersin.org and metastasis (Marchi et al., 2018;Marchi et al., 2020). In addition to this, research has demonstrated the involvement of Ca 2+ in cell death (Marchi et al., 2018) as well as in the necrotic process of cancer cells (Danese et al., 2021). Menthol has been reported to induce cell necrosis primarily by upregulating (Ca 2+ )i within cancer cells via TRPM8, a non-selective, multimodal ion channel, activated at low temperatures (<28°C), pressure, and cooling compounds (e.g., menthol and icilin). Menthol additionally increases (Ca 2+ )i concentration in other ways, which are still unclear (Kim et al., 2009).
In the prostate cancer cell line PC-3, menthol increases (Ca 2+ )i concentration and induces cell death by activating the calcium influx pathway and other unclear channels (Kim et al., 2009;Asuthkar et al., 2015;Grolez et al., 2019). Similarly, in LNCaP cells (lymph nodederived prostate cancer cells), menthol induces cell death by reducing store-operated channel current (Thebault et al., 2005;Beck et al., 2007), thus releasing the small amount of calcium stored in the cell (Zhang and Barritt, 2004). Menthol can induce a dose-dependent increase in (Ca 2+ )i through TRPM8, resulting in the death of T24 cells (human bladder cancer cells) (Li et al., 2009). In HL60 (human promyelocytic leukemia) cells, menthol was found to increase (Ca 2+ )i concentration, thus inducing necrosis (Lu et al., 2006).

Inhibition of tumor invasion and metastasis
Metastatic cancer is responsible for a vast majority of cancerrelated deaths and is extremely difficult to treat. Intrinsic factors both in the tumor cells and the host contribute to tumor metastasis; thus, inhibiting metastasis involves the alteration of key pathways in multiple stages of tumor progression. In addition, scientists believe that motility is a key driver of metastasis and may be used as a target for the development of anticancer treatment and prevention of metastasis (Stoletov et al., 2020). Menthol has been found to impede tumor cell motility (Wang et al., 2012). In Panc-1 cells (pancreatic ductal adenocarcinoma cells), menthol was found to significantly reduce cell motility and motility by activating TRPM8 channels (Cucu et al., 2014).
Further studies have found that in DU145 and PC-3 prostate cancer cells, menthol inhibits cell motility by inhibiting the phosphorylation of protein tyrosine kinase 2 (PTK2) through the TRMP8 pathway Zhu et al., 2011;Wang et al., 2012). PTK2 is a non-receptor tyrosine kinase that is considered to play a key role in tumor cell migration, invasion, and metastasis (Lee et al., 2020).

Inhibition of tumor angiogenesis
In 1971, Folkman first proposed that angiogenesis is essential for the development and growth of solid tumors beyond the size of 1-2 mm (Folkman, 1971). Since then, treatments targeting angiogenesis have been considered a promising therapeutic approach for solid tumors and have been focused on by various research groups after experimental validation by several research teams (Li et al., 2018). Accordingly, researchers have focused on the anti-tumorigenic activity of menthol. Vascular endothelial growth factor (VEGF) is one of the most potent and specific angiogenic factors that is closely associated with tumor progression (Carmeliet, 2005). Zhu and Walcher's et al. found that in uveal melanoma UM92.1 cells and PC-3 cells, menthol can block the trans-activation of transient receptor potential vanilloid 1 (TRPV1;) by activating TRPM8, which further inhibits VEGFinduced angiogenesis (Zhu et al., 2011;Walcher et al., 2018).

Inhibition of tumor proliferation
In addition to the aforementioned six anti-cancer mechanisms, menthol can inhibit the proliferation of cancer cells by other mechanisms, such as by antagonizing cancer inducers.
In human liver cancer cells (J5), menthol was found to down regulate the N-acetylation of 2-aminofluorene by inhibiting N-acetyltransferase (NAT) activity in a dose-dependent manner (Lin et al., 2001). NAT has been reported to play a role in carcinogenesis induced by a few chemicals (Grant et al., 1991;Minchin et al., 1992). Another study demonstrated that menthol induces cytotoxicity in WEHI-3 cells in a dose-dependent manner. In BALB/c mice with leukemia (induced by WEHI-3 cells), menthol inhibited liver and spleen enlargement; increased the number of megakaryocytes in the spleen; and increased the blood levels of LGALS3, integrin alpha M, and CD19 antigen. This implies that menthol inhibits tumor development and promotes hematopoietic stem cell differentiation (Lu et al., 2007b).
In female ICR mice with skin cancer (induced by 9, 10dimethylbenz[a]anthracene/12-O-tetradecanoylphorbol-13acetate), menthol could inhibit tumor formation and growth in a dose-dependent manner, reducing tumor incidence and volume. Mechanistically, menthol inhibits the expression of cyclooxygenase-2 and proinflammatory cytokine nuclear factor kappa B, mitogenactivated protein kinase protein, and extracellular signal-regulated kinase, resulting in an anti-tumor effect (Liu et al., 2015). In mice, menthol demonstrated an obvious inhibitory effect on azoxymethane/dextran sulfate sodium-induced colon cancer. In another study, female Swiss mice were administered benzo(a) pyrene to induce cancer. Treatment with menthol (50 mg/kg of body weight, twice a week) at the initial stage of benzo(a)pyrene administration showed a reduced incidence of precancerous gastric lesions owing to reduced genotoxicity, reduced cell proliferation, and regulated apoptosis (Santo et al., 2021).

Critical considerations
Despite the vast evidence regarding menthol's anticancer activity, not many human clinical trials have been conducted for this agent. Therefore, researchers are currently looking at more anticancer potentials of menthol from following perspectives.

Synergistic interactions of menthol with other treatments
Many natural products have synergistic effects with other listed anticancer drugs. The main advantage of combined therapy is Frontiers in Pharmacology frontiersin.org reducing the dose and toxicity of chemotherapy while maintaining or even increasing its efficacy. Several studies have explored the advantages of combining natural compounds with classical chemotherapeutic drugs (Chen et al., 2022). For example, menthol has been found to reduce the resistance of human hepatocellular carcinoma HepG2 cells to the anticancer drugs paclitaxel and vincristine by inhibiting the expression of cytochrome P450 family 3 subfamily A member 4 (CYP3A4), thus preventing cancer. Reportedly, CYP3A4 is implicated in the metabolism of anticancer drugs. The induction of CYP3A4 increases the metabolism of paclitaxel and vincristine, thus reducing response and eventually conferring resistance against these anticancer drugs (Yao et al., 2000;Martínez et al., 2002;Nagai et al., 2019). In addition, studies have shown that some natural compounds, such as menthol, combine with non-toxic digitalis glycosides to enhance cytotoxicity and inhibit cancer cells proliferation by downregulating the expression of ATP binding cassette subfamily B member 1 (ABCB1) (Eid et al., 2012;Eid et al., 2013).
It is worth mentioning that menthol can reduce the incidence of microadenomas by increasing the concentration of butyric acid in the feces of AD mice and increasing the abundance of bacteria. This reduces intestinal inflammation and inhibits colon cancer cell proliferation .
These studies provide a new perspective on the use of menthol in combination anticancer therapy. Although reports on the synergistic effects of menthol with other drugs are currently limited to those mentioned above, more potential anticancer options are waiting to be explored.

Clinical application
As with many natural products, clinical evidence on menthol remains scarce. However, we have found that menthol continues to play an important role in some anticancer therapies (Figure 4).
Long-term treatment-related neuropathic pain due to chemotherapy-induced peripheral neuropathy (CIPN) or surgical scars is increasingly affecting the lives of patients with cancer (Cavaletti et al., 2011). A clinical study has found that the menthol receptor TRPM8 plays a role in relieving pain and may be a promising target in CIPN treatment (Singh et al., 2021). Menthol has long been used as a topical pain reliever, and modern studies have shown that it can be used as an alternative to opioid analgesics for relieving local pain (Finch and Drummond, 2015;Pergolizzi et al., 2018). Fallon et al. evaluated changes in pain after topical treatment with 1% menthol cream in 51 patients with CIPN (35/51) or scar pain due to cancer (10/51). They found that 82% (31/38) patients showed improvement in median pain scores [from 47 (interquartile range: 30-64) to 34 (6-59), p < 0.001] (Fallon et al., 2015). In another study, Cortellini et al. reported improvement in CIPN in a male patient following treatment with an aqueous menthol emulsion (Cortellini et al., 2017). These reports suggest that Frontiers in Pharmacology frontiersin.org the topical use of menthol has the potential as a new analgesic in cancer-related neuropathic pain. In addition to pain, menthol plays a special role in other cancer residual effects. For example, in an animal study of head and neck cancer, Sanz et al. used an Orabase ® platform-loading Transcutol ® (10%) and menthol (5%) for the buccal vehiculation of doxorubicin (DOX). Comparison with no menthol loading, this increased the residue of DOX in animal mucosa which helped better relieve oral mucositis-related pain following radiotherapy and chemotherapy (Sanz et al., 2017). Tourlaki et al. assessed the effect of using a detergent containing dihydroavenanthramide D 5% + 1% menthol moisturizer daily. They found that the intervention improved chronic pruritus associated with xerosis in elderly adults with Kaposi's sarcoma (Tourlaki et al., 2020).
Previous animal studies have shown that menthol can inhibit calcium influx and potassium depolarization, resulting in reduced calcium uptake (Hawthorn et al., 1988;Hills and Aaronson, 1991;Amato et al., 2014), which relaxes gastrointestinal smooth muscles (Kim et al., 2016). In addition, a few studies have described the efficacy and safety of menthol in clinical practice (Alammar et al., 2019;Ford et al., 2008;Pittler and Ernst, 1998;Li et al., 2019; National Collaborating Centre for Nursing and Supportive Care (UK), 2008). Moreover, accumulating evidence from clinical trials has shown that directly spraying L-menthol onto the gastrointestinal mucosa significantly improves spasms; it also improves the adenoma detection rate (ADR) during an endoscopic procedure (You et al., 2020). In 2011, a study proposed that during upper gastrointestinal endoscopy, direct endoscopic spraying of L-menthol inhibits gastric peristalsis with almost no adverse reactions (Hiki et al., 2011a;Hiki et al., 2011b). Since then, more research has been conducted in this regard to validate the use of L-menthol in therapeutic endoscopies, such as endoscopic submucosal dissection of gastric tumors (Fujishiro et al., 2014;Ishiyama et al., 2021). In addition, during endoscopic surgery, L-menthol was found to effectively inhibit gastrointestinal motility, thus improve the ADR (Inoue et al., 2014;Inoue et al., 2020) and endoscopic imaging in early gastric cancer (Kikuchi et al., 2021). However, other researchers have pointed out that L-menthol could indeed alleviate adverse reactions such as gastrointestinal peristalsis and even spasm during endoscopy. Thus, the correlation between L-menthol and ADR requires further investigation (Shah et al., 2019;Aziz et al., 2020;You et al., 2020).
Recently, the timely use of menthol in some cancer treatments, such as photothermal therapy (PTT) and radiofrequency, has been found to improve treatment outcomes. Mechanistically, L-mentholmediated heating triggers a tri-phase transition that produces continuous gas microbubbles, aiding drug release and enhanced ultrasound imaging (Zhang et al., 2016a;Zhang et al., 2016b;Feng et al., 2017;Ma et al., 2019;Shi et al., 2019;Yang et al., 2019;Zhang et al., 2019;Xu et al., 2020;Li et al., 2021). In addition, menthol is highly permeable, allowing it to deliver drugs to tumor lesions faster and more efficiently in nanotechnology applications (Liang et al., 2018;Gao et al., 2019;Liang et al., 2019;Otake et al., 2021). In one such application, gold nanoshell cerasome-encapsulated L-menthol (GNC-LM) has been shown to greatly prolong the ultrasound imaging time in vivo with good contrast enhancement, via the continuous release of gas microbubbles (Guan et al., 2019). Meanwhile, PTT based on the GNC-LM system could effectively ablate tumors, indicating the potential use of GNC-LM as a therapeutic nanoprobe for ultrasound imaging and in PTT (Guan et al., 2019). For glioma treatment, menthol-modified casein nanoparticles encapsulating anti-cancer drugs showed deeper tumor penetration and higher brain tumor distribution than unmodified nanoparticles. Thus, menthol excelled in overcoming the concern of blood-brain barrier permeability Liang et al., 2019).
Although menthol is a promising anticancer drug, it needs to be further developed based on its pharmacological activity. In addition, studies on menthol, such as those related to its pharmacokinetic properties, toxicity, and bioavailability, are crucial and can provide a scientific and theoretical basis for its clinical application. Thus, studies on toxicity, combination therapy, and clinical application of menthol should be the focus of future research.

Limitations and controversies
Menthol has been demonstrated to have considerable anticancer activity but has been greatly restricted in clinical use because of its poorly defined toxic side effects and pharmacokinetics, in addition to other shortcomings. Despite extensive research on the role and use of menthol in the prevention and treatment of cancer, only a few studies have assessed its toxicity and pharmacokinetics. We collected the toxicity and pharmacokinetics reports of menthol from the literature, which can provide useful data for further research and development of new drugs containing menthol. Overdose of menthol administered orally (lethal dose: 50-150 mg/kg) has been known to cause convulsions, agitation, dizziness, ataxia, and coma (Opdyke, 1976). Reportedly, the median lethal dose (LD50) of L-menthol administered to mice via oral gavage is 3.4 g/kg, whereas the LD50 of L-menthol administered intradermally to rabbits is > 5 g/kg (Bhatia et al., 2008). Currently, no acute toxicity studies of menthol in humans have been reported. However, animal studies have shown that excessive use of L-menthol results in euphoria, coma, convulsions, and alterations in the liver and kidneys, but most of these symptoms disappear after discontinuing the agent for a while (Bhatia et al., 2008). Particularly, a study reported the case of an 86-year-old American who had been receiving a menthol-based cough suppressant for 20 years. At presentation, he was in a coma and had ataxia, recurrent oral ulcers, intermittent diarrhea, chronic dizziness, heartburn, macular skin, oliguria, and dark brown urine. However, all these symptoms disappeared after 6 months of discontinuing the cough suppressant and while receiving alternate treatment (Baibars et al., 2012). Two cases of patients presenting with coma shock possibly due to an overdose of peppermint oil were reported in India. One patient improved after hospitalization (Nath et al., 2012). However, the other patient experienced recurrent convulsions and intermittent hematuria and died after 10 days of persistent coma under hospital care (Kumar et al., 2016). Pharmacokinetic studies in humans have revealed that L-menthol (100-mg capsules) has an area under the curve of 1,214 μmol/L·min/L and a plasma half-life of 56 ± 8 min (range: 45-74 min). In addition, the peak blood concentration (C max ) of menthol was 16.73 ± 5.53 μmol/L (range: 7.95-29.10 μmol/L), and the time to reach C max was 61 ± 26 min (range: 30-120 min) (Gelal et al., 1999). In rats, the half-life of Frontiers in Pharmacology frontiersin.org inhaled and intravenously administered L-menthol was 8.53 and 6.69 h, respectively. The peak blood concentration was noted at 1.0 h after inhalation, with an absolute lung bioavailability of 50.24% (Feng et al., 2019). These data suggest that menthol is rapidly metabolized.
Studies have reported increased (Ca 2+ )i, following menthol treatment in DBTRG cells by activating TRPM8 channels. This would further activate BK ion channels (large-conductance Ca 2+activated K+ channels) to promote tumor cell migration and accelerate tumor metastasis (Wondergem et al., 2008;Wondergem and Bartley, 2009). In HSC3 and HSC4 cells (oral squamous cell carcinoma cell line), menthol enhances the migration and invasion of tumor cells by inducing the activity of matrix metalloproteinase-9 (Okamoto et al., 2012). A study demonstrated that pretreatment with menthol reduces the cytotoxicity of DOX in HepG2 cells; This occurs via reduced concentrations of intracellular DOX following the upregulation of ABCB1 (Nagai et al., 2020). Owing to the beneficial effects of menthol and following the advent of menthol cigarettes, many people believe that smokers have been consuming more cigarettes and that menthol cigarettes increase the chances of lung cancer. However, a large body of research shows that smoking menthol cigarettes do not increase the risk of lung cancer; Instead, it may reduce the risk and post-lung cancer mortality (Richardson, 1997;Carpenter et al., 1999;Brooks et al., 2003;Blot et al., 2011;Lee, 2011;Rostron, 2012).
Although menthol is widely regarded as a safe and effective anticancer agent, it is essential to monitor its toxic side effects and be cautious when using it. Researchers should be careful about doses when developing drugs to prevent dose-related toxicity. More research is needed to further clarify the specific effects and mechanisms of menthol against different cancers, to optimize its clinical applications.

Conclusions and perspective
Plants have provided us with several pharmacologically active molecules, such as artemisinin and paclitaxel. Further research and optimization of these molecules will contribute to the betterment of human health. The effectiveness of menthol as an anticancer agent has been extensively documented. Mechanistically, menthol acts on several key molecular targets and mechanisms involved in cancer pathogenesis, indicating its potential as an anticancer agent. Menthol mediates its anticancer activity by exerting a combination of proliferative, invasive, and apoptotic effects on tumor cells as well as inhibiting tumor growth through multiple pathways. In addition to these, its low side effects makes menthol a suitable candidate for the treatment of cancer.
Despite the potential anticancer activity of menthol, it has not been widely used in human clinical trials. Accordingly, research in this direction is the need of the hour. In addition, as an underestimated anticancer agent, more studies are required to broaden the scope of traditional uses and provide better formulations, particularly those involving additional active molecules, which require more extensive and interdisciplinary efforts. Existing research has demonstrated that combining menthol with other agents can help reduce menthol dose and associated toxicity while maintaining or even increasing its efficacy. In addition, menthol has proven effective in cancer sequelae and can play a crucial adjunctive role in other therapeutic approaches like PTT and RF.
Further in-depth studies are required regarding pharmacokinetics in humans, toxicology, and mechanisms underlying combination therapy. In addition, research on new menthol-based biomaterials is required for reducing drug toxicity and developing structurally optimized analogs. All these will further enhance the efficacy and safety of menthol. Moreover, clinical studies are warranted to validate preclinical studies on the anticancer activity of menthol. This review has summarized the anticancer activity and the underlying mechanisms of menthol as well as its limitations and current problems. We believe that this review can help guide the clinical application of menthol as an anticancer drug in the future.

Author contributions
Statement Concepts: YZ; Literature Collection: YZ, HP, and WL; Wrote the paper: YZ, EL, YP, and HG; Designed the figures: YZ, QH, YY, and WHL; Reviewed manuscript: JG and JZ. All authors commented on the manuscript. All authors contributed to the article and approved the submitted version.

Funding
This work was supported by the National Natural Science Foundation of China (Grant No. 82074443), the Program of Science and Technology Department of Sichuan Province (Grant Nos. 2021ZYD0089 and2023NSFSC0039), "The Hundred Talents Program" of the Hospital of the Chengdu University of Traditional Chinese Medicine , "Youth Qi Huang Scholar" by State Administration of TCM (Grant No. 2022-256), Xinglin Scholar Research Promotion Project of Chengdu University of TCM (Grant No. QJJJ2022010).

Acknowledgments
Thanks the Affiliated Hospital of Chengdu University of traditional Chinese medicine and also the Chengdu University of traditional Chinese medicine for support of the article.

Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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