Preclinical Therapeutic Effects of Quercetin on Gastrointestinal Cancers

Abstract

Gastrointestinal (GI) cancers were included in the top 10 most common cancers in 2020. Future incidences are expected to rise due to the varying risk factors and aetiologies. With high incidences and mortality rates, current cancer treatments fail to reduce mortality and morbidity in GI cancer patients. A large part of cancer research has been geared towards targeted and personalized medicine, although effective, it may not be the most cost-friendly and feasible option to treat patients from varying socioeconomic backgrounds. Hence, natural compounds may present as an attractive alternative treatment in the management of GI cancers. Quercetin is a well-known flavonoid compound, found in almost all fruits and vegetables. It has also been widely studied for its anticancer properties, such as anti-oxidative, anti-inflammatory, anti-proliferative and anti-angiogenic properties. In this chapter, the authors discuss the potential of quercetin in treating GI cancers, which includes the biosafety and toxicity of quercetin, applications of quercetin in common GI cancers, such as gastric, hepatic, colorectal, pancreatic and oesophageal cancers, along with the corresponding molecular mechanisms. The authors also present evidences of quercetin as an adjuvant therapeutic agent with other anticancer drugs.

1. Introduction

Gastrointestinal (GI) cancers constitute a formidable health challenge globally, exhibit varying prevalence, epidemiological patterns and mortality rates. Colorectal cancer, with an estimated 1.9 million new cases in 2020, ranks among the most prevalent type of cancer, particularly in developed nations, which is largely influenced by lifestyle factors [1]. Gastric cancer accounts for 1.1 million new cases annually and displays diverse epidemiology linked to regional variations and risk factors such as Helicobacter pylori infection and poor dietary practices [2]. Liver cancer, which was responsible for over 830,000 new cases, is notably prevalent in East Asia and sub-Saharan Africa, primarily associated with chronic hepatitis B and C infections [3]. Oesophageal cancer, with approximately 604,000 new cases reported in 2020, exhibits distinct geographic disparities, emphasizing the impact of local risk factors [3]. High mortality rates characterize GI cancers collectively, with liver cancer ranking as the third leading cause of cancer-related deaths globally [3]. These statistics highlight the urgency of evidence-based interventions and targeted research initiatives to address the complex interplay of factors contributing to the prevalence and mortality rates of GI cancers.

The journey towards identifying novel therapeutic strategies for GI cancers begins with understanding current treatments’ shortcomings. Despite advancements in medical science, such as surgical interventions, chemotherapy and targeted therapies, current treatment modalities have not yielded a substantial reduction in mortality and morbidity associated with these malignancies [4]. One glaring example is colorectal cancer, a prevalent GI malignancy that continues to exert a significant toll on global health. Despite extensive research and therapeutic interventions, colorectal cancer remains a leading cause of cancer-related deaths worldwide [5].

The limitations of existing treatments highlight the pressing need for innovative and more effective therapeutic strategies. Responding to this imperative, attention has turned towards natural compounds with potential therapeutic benefits [6]. One compound that has emerged in the spotlight is quercetin, a polyphenolic flavonoid abundantly found in fruits, vegetables and certain beverages [7]. Quercetin has garnered attention not only for its antioxidant and anti-inflammatory properties but also for its intriguing anticancer potential, making it a subject of intense scientific investigation [8]. Extensive preclinical research has explored the potential of quercetin in inhibiting the growth of cancer cells, inducing apoptosis and suppressing the development of new blood vessels that support tumour growth [9]. Importantly, quercetin has been reported to enhance the efficacy of conventional chemotherapy agents when combined, potentially overcoming resistance mechanisms [10].

The appeal of quercetin lies not only in its anticancer properties but also in its favourable safety profile. Unlike some conventional cancer treatments associated with significant toxicities, quercetin has demonstrated low toxicity in various preclinical and clinical studies [11]. This characteristic is particularly crucial in cancer treatment where minimizing adverse effects on normal tissues is paramount for improving patients’ quality of life during and after therapy [11]. In addition to its direct effects on cancer cells, quercetin has been investigated for its potential to modulate the tumour microenvironment, which plays a critical role in cancer progression. Interventions that can modify this milieu hold promise for disrupting the supportive conditions for tumour growth and metastasis [12]. Quercetin’s ability to influence the tumour microenvironment further expands its potential therapeutic impact. Its molecular structure, depicted in Figure 1, lays the foundation for understanding its multifaceted interactions within biological systems. Its chemical composition contributes to its ability to modulate cellular pathways, making it an intriguing candidate for cancer therapy [13].

Figure 1

Chemical structure of quercetin.

This chapter aims to comprehensively review and analyse the preclinical therapeutic effects of quercetin against GI cancers, shedding light on its potential as a therapeutic agent. We seek to provide a thorough overview of quercetin’s biosafety and toxicity profiles, ensuring a nuanced understanding of its safety considerations, essential for assessing its potential as a therapeutic agent. Then, exploring quercetin’s pharmacokinetics will elucidate its journey from laboratory studies to potential clinical applications. Understanding the pharmacokinetics of quercetin is crucial, and this section will encompass in vitro and in vivo studies, providing a holistic perspective on its absorption, distribution, metabolism and excretion. Finally, the core of the chapter details the preclinical therapeutic effects and molecular mechanisms underlying quercetin’s efficacy against GI cancers, namely gastric, liver, colorectal, pancreatic and oesophageal cancers. Each cancer type is comprehensively examined in in vitro and in vivo studies, emphasizing key findings related to cell viability, proliferation, migration/invasion, chemoresistance, angiogenesis and other relevant parameters. Through this detailed analysis, it aims to provide a comprehensive resource for researchers, clinicians and policymakers interested in the potential therapeutic role of quercetin in the management of GI cancers.

Pursuing novel and effective therapeutic strategies for GI cancers is imperative, given the persistent challenges associated with current treatments. With its multifaceted properties and promising preclinical data, quercetin represents a beacon of hope in the quest for innovative cancer therapies. As research advances and clinical trials progress, the true potential of quercetin in reshaping the landscape of GI cancer treatment may unfold, offering new avenues for improving patient outcomes and addressing the unmet needs in cancer care.

2. Biosafety and toxicity profiles of quercetin

Quercetin possesses a variety of pharmacological activities, and for further medical application, it is important to evaluate its biosafety and toxicity profiles in experimental and clinical studies. With oral administration to mice and rats, quercetin consistently did not induce any significant changes in several mutagenicity/genotoxicity endpoints, such as micronuclei, chromosomal aberrations, sister chromatid exchange, unscheduled DNA synthesis and alkali-labile DNA damage in somatic cells [14]. Furthermore, based on the Ames test and combined in vivo micronucleus and comet assay, there was no induction of in vivo genotoxic potential or indication of any oxidative DNA damage, suggesting the absence of mutagenicity and genotoxicity potential of the isoquercitrin-γ-cyclodextrin (IQC-γCD) inclusion complex in rat liver tissues [15].

On the other hand, a clinical study [16] indicated that total sperm motility was significantly inhibited following exposure to 100, 200 and 400 μM quercetin for 6 and 12 hours in a dose-dependent manner, as compared to the controls (p < 0.05), suggesting that quercetin inhibits sperm functions. Another clinical study showed that extra quercetin consumption was able to lower blood pressure in adult hypertensive patients after receiving quercetin at dosages ranging from 162 to 1095 mg/day for a period of 7 days to 6 weeks [17]. A study also indicated that, after 1 week of treatment, 16 patients of the quercetin-treated group tested negative for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), of which 12 patients had all their symptoms diminished; in the control group, two patients tested negative and four patients had their symptoms partially improved. These findings suggest that quercetin significantly shortens the timing of molecular test conversion from positive to negative (p < 0.05), at the same time reducing symptom severity and negative predictors of COVID-19 [18]. Moreover, it was observed that disease biomarkers, including lactate dehydrogenase (LDH) (−35.5%), ferritin (−40%), C-reactive protein (CRP) (−54.8%) and D-dimer (−11.9%), were reduced, indicating SARS-CoV-2 viral clearance. According to a study on patients with chronic obstructive pulmonary disease (COPD) who had mild-to-severe lung disease and a first second of forced expiration (FVE1) ranging between >35% and <80%, no patients experienced any drug-related severe adverse events after supplemented with quercetin at 500, 1000 or 2000 mg/day in a dose-escalation manner for 1 week [19], suggesting that quercetin was safely tolerated up to 2000 mg/day, as assessed by lung function and blood profile. These studies provided sufficient data on quercetin biosafety and tolerance in patients while effectively administering therapeutic effects.

3. Pharmacokinetics of quercetin

Quercetin exhibits established pharmacokinetic characteristics in both laboratory animals and humans. The previous research finding suggested that unmodified quercetin material has limited bioavailability when administered orally, restricting its practical application in clinical settings. For example, quercetin is consumed in the glycoside form, with glycosyl groups being liberated during chewing, digestion and absorption. Subsequently, the enzymes known as β-glycosidases transform quercetin glycosides into aglycone in the intestines, prior to their absorption into enterocytes [20, 21]. Prior research has also indicated the involvement of oral and intestinal microorganisms in this enzymatic hydrolysis. Since quercetin is a lipophilic substance, it is thought to be able to pass through intestinal membranes by simple diffusion. In general, this absorption is preferable to those of its glycoside counterparts that enter the intestines unaltered [20, 21].

As of now, the bioavailability of quercetin glycosides has been investigated in several human studies. For example, a previous clinical study reported pharmacokinetic characteristics of corvitin (quercetin) in healthy participants. The study shows that the primary metabolites of quercetin are its methoxy derivatives, specifically isorhamnetin and tamarixetin, together with their conjugates of glucurone and sulfatide. About 20% of quercetin dosages are subject to metabolic changes in 20 minutes, free quercetin conjugates by 33% and isorhamnetin by 70% in 20–30 minutes. Corvitin is eliminated with urine as conjugates of quercetin and isorhamnetin, with an average maximal clearance rate of 1.39 mg/h occurring 1.63 hours after drug administration [22]. Hollman et al. reported that patients with ileostomy exhibited a higher absorption percentage of quercetin glycosides from onions compared to the pure aglycone form [23].

On the other hand, Mansour et al. reported that individuals who had eaten an onion-containing lunch had ileostomy fluid that included considerable levels of aglycone. A significant amount of quercetin aglycone and a small amount of quercetin glycosides were found in the fluid. One conceivable reason is that β-glycosidase enzymes hydrolyse quercetin glycosides, converting them to aglycone. Most of these enzymes are found in the GI tract, where the enzymes are then absorbed [24]. Another study looked at the excretion of quercetin metabolites in young people after eating gently fried onions. It was determined that the fraction of quercetin intermediates discharged in urine following dietary quercetin consumption is approximately 4.7% of the whole intake, which is 12.9 mol. The makeup of metabolites excreted in urine differed significantly from that of plasma. The majority of the quercetin intermediates in the urine were quercetin glucuronide sulphates, quercetin diglucuronides, with additional significant metabolites including isorhamnetin-3-glucuronide and quercetin 3′-glucuronide [25].

Moreover, a study reported the pharmacokinetic characteristics of intravenous administration of quercetin in cancer patients at dosages ranging from 60 to 2000 mg/m². The researchers determined that the safe amount of quercetin was 945 mg/m², but the hazardous level was reported to produce nephrotoxicity, hypertension, vomiting and a drop in blood potassium. Intravenous quercetin was discovered to have the following half-lives: 3.8–86 minutes for its elimination half-life and 0.7–7.8 minutes for its distribution; 0.23–0.84 L/min/m² for its clearance and 3.7 L/m² for its distribution volume [26]. Following that, experimental research conducted on rabbits showed that the oral and parenteral treatment of quercetin along with pectin enhanced GI absorption by about 11 times and bioavailability by about 10 times compared to quercetin alone. According to the study, quercetin’s pharmacokinetic characteristics and solubility are improved when combined with pectin for medicinal purposes [20]. Subsequent research conducted by Salehi et al. revealed quercetin’s pharmacokinetic properties, indicating that its T_max and C_max were 0.7 ± 0.3 hour and 2.3 ± 1.5 μg/ml, respectively, at a dose of up to 200 mg [8].

4. Preclinical therapeutic effects and associated molecular mechanisms of quercetin against gastrointestinal cancers

Gastrointestinal (GI) cancers refer to cancers occurring in the GI tract, including liver and pancreatic cancers. The most common types of GI cancers are colorectal, gastric, liver, oesophageal and pancreatic cancers [27]. According to a report in 2020, GI cancers were among the top 10 most common cancers occurring worldwide. Colorectal cancer ranks 3rd in occurrences and 2nd in mortality, gastric and liver cancers ranked 5th in incidences, while oesophageal cancers ranked 7th in incidences [3].

Gastric cancers arise from malignancies of the stomach; there are two classifications of gastric cancers depending on the topographical subsite of the tumour, which are cardia and non-cardia gastric cancers, referring to the upper stomach and the lower stomach, respectively [3, 27]. Both of these subtypes present with different risk factors, carcinogenesis and epidemiology. For example, non-cardia gastric cancers are primarily attributed to H. pylori infections [28, 29]. Cardia gastric cancers, on the other hand, have similar risk factors as that of oesophageal adenocarcinoma (gastroesophageal reflux disease and obesity) [27].

Liver cancers were reported to be more prevalent in men than in women, with rates of both incidences and mortality being two and three times higher in men than in women [3]. Several more common types of liver cancers include hepatocellular carcinoma (HCC) and intrahepatic cholangiocarcinoma (ICC). Common risk factors of HCC include Hepatitis B virus (HBV) and Hepatitis C virus (HCV) infections, aflatoxins, heavy alcohol intake, obesity, type 2 diabetes and use of tobacco [3, 27]. Meanwhile, incidences of ICC have been linked to primary sclerosing cholangitis (PSC), liver fluke infections, fibropolycystic liver disease and biliary stones [27, 30].

Colorectal cancer was observed to be one of the most common cancers in incidences, ranking 3rd overall, with 2nd in terms of mortality. One observed trend in colorectal cancer is the shift in socioeconomic development, with incidence rates higher in transitioned countries than transitioning countries [3]. Changes in lifestyle, such as increased meat consumption, reduced physical activity and excess body weight, may be among the risk factors for colorectal cancer [31]. Other risk factors include increased alcohol intake, heavy tobacco use and increased consumption of processed or red meats [3].

Pancreatic cancer was reported to be the 7th leading cause of cancer death in males and females, largely due to its poor prognosis. Incidence rates were up to 4- to 5-fold higher in developed countries, with the highest incidences recorded in Europe, North America and Australia/New Zealand [3]. Risk factors of pancreatic cancer include obesity, diabetes and heavy alcohol consumption [27].

Oesophageal cancer ranks 7th in terms of incidence and 6th in mortality worldwide. The most common subtypes of oesophageal cancer are oesophageal squamous cell carcinoma (ESCC) and oesophageal adenocarcinoma (EAC). The geographical distribution of the subtypes may signify the corresponding risk factors associated with the type of cancer. ESCC was more prevalent in Eastern Asia (largely in China), Southern Africa and Eastern Africa. Possible risk factors may include nutritional deficiencies and exposure to nitrosamines [32]. However, other risk factors include heavy tobacco and alcohol use. While EAC was reported to be more prevalent in Western countries. Incidences in high-income countries were attributed to excess body weight and incidences of gastroesophageal reflux disorder [3].

With the common occurrence of GI cancers, developing new strategies to combat and manage GI cancers can be advantageous. Using natural products in medicine has also been a well-known strategy in drug discovery. Examples of common natural products in cancer treatment are taxanes, vinca alkaloids and anthracyclines [33]. Quercetin is a flavonoid found in most fruits and vegetables, possibly one of the most common dietary flavonols [34]. Quercetin can exert antioxidant effects at low doses, which provide a chemopreventive effect, yet also exert a pro-oxidant effect at higher doses, providing a chemotherapeutic effect [35].

4.1 Gastric cancer

Quercetin’s chemopreventive effects, demonstrated in both in vitro and in vivo models, make it a flavonoid with great potential in oncology. The effects of quercetin are biphasic and dose-dependent. Quercetin exhibits chemopreventive effects when present in low concentrations, owing to its antioxidant properties. However, when found in high concentrations, quercetin serves as a pro-oxidant and might potentially induce chemotherapeutic effects [35]. Quercetin’s anticancer properties stem from its capacity to modify cyclins, pro-apoptotic, phosphatidylinositol 3‑kinase (PI3K)/protein kinase B (Akt) and mitogen-activated protein kinase (MAPK) molecular pathways, thereby reducing proliferation, inducing apoptosis, causing cell cycle arrest and inhibiting mitotic processes. Figure 2 shows signalling pathways in gastric cancer, which are commonly targeted by therapeutic intervention. Quercetin’s anticancer effects on gastric cancer have been the subject of numerous studies, which will be covered in detail in this section.

Figure 2

Signalling pathway in gastric cancer.

Zeng et al. conducted a bioinformatics analysis to assess the impact of quercetin on gastric cancer. The study aimed to discover the specific targets of quercetin in NCI-N87 gastric cancer cells exposed to this compound. The researchers exposed human gastric cancer cells (NCI-N87) to a concentration of 15 μM quercetin for 48 hours, using dimethyl sulphoxide (DMSO) as a control. The HiSeq 2500 DNA sequence data were utilized to compare differentially expressed genes (DEGs) between groups. A sophisticated technique was employed to evaluate the protein-protein interaction (PPI) network. The regulatory network of transcription factors (TFs)-DEGs was determined using Cytoscape. The DEGs identified in the study were Fos proto-oncogene (FOS, degree = 12), aryl hydrocarbon receptor (AHR, degree = 12), Jun proto-oncogene (JUN, degree = 11) and cytochrome P450 family 1 subfamily A member 1 (CYP1A1, degree = 11). These DEGs showed significant associations with other proteins in the protein-protein interaction (PPI) network, particularly with proteins having higher degrees. The expression levels of Early growth response 1 (EGR1), FOS like 1 (FOSL1), FOS and JUN were elevated among the five TFs-DEGs, while AHR was downregulated. The Wnt signalling pathway exhibited an abundance of FOSL1, JUN and Wnt family member 7B (WNT7B). CYP1A1 exhibited a strong association with AHR in the PPI network. Thus, quercetin may have specifically affected FOS, AHR, JUN, CYP1A1, EGR1, FOSL1 and WNT7B in gastric cancer [36].

Apart from that, quercetin has also been investigated in combination studies. In a study conducted by Lei et al., the effectiveness of a combination of quercetin and irinotecan in mitigating the spread of gastric cancer was examined. This investigation was done by assessing the expression of genes and proteins [37]. This study compared the effects on β-catenin expression, cell viability and apoptosis of low-dose SN-38 (irinotecan metabolite) in combination with quercetin and high-dose SN-38 alone. In addition, the effects of quercetin and low-dose irinotecan on gastric cancer metastasis were examined using in vivo xenograft animal models and in vitro investigations. The results showed that the human gastric adenocarcinoma (AGS) cells treated with a combination of quercetin and a low dose of SN-38 exhibited reduced levels of the β-catenin protein compared to the treatment with quercetin alone. The expression of integrin β6 (ITG-β6) and Twist-1 genes which are two markers of epithelial-mesenchymal transition (EMT), as well as the expression of cyclooxygenase-2 (COX-2) gene, was shown to be higher in cells treated with a high dose of irinotecan compared to cells treated with combination therapy. Following combination therapy, the AGS mouse model exhibited a notable decrease in VEGF-A (vascular endothelial growth factor A), VEGF-receptor 2 (VEGFR-2) and the proportion of Tek tyrosine-protein kinase receptor (Tie2)-expressing monocytes (TEMs). According to the results, the effectiveness of irinotecan in treating gastric cancer (GC) could be enhanced by combining it with quercetin [37].

A distinct investigation was conducted to assess the mechanism and impact of quercetin on the spread of gastric cancer. Additionally, the study aimed to determine if the involvement of urokinase plasminogen activator and urokinase plasminogen receptor (uPA/uPAR) played a role in this process [38]. The purpose of the study was to determine whether quercetin could have an impact on the uPA/uPAR system, which is crucial to gastric cancer metastasis. Precancerous tissues’ uPA and uPAR activity levels were measured, and the results were compared to the migration and invasion patterns of various gastric cancer cell lines [37]. According to the data, uPA and uPAR activities in precancerous tissues were lower than those in gastric cancer cells, and there was a correlation between uPAR expression and gastric cancer cell line migration and invasion. After gastric cancer BGC823 and AGS cells were treated with quercetin (10 μM for 72 hours), uPA and uPAR protein expression levels were decreased along with migration and invasion. The combination of quercetin and uPAR knockdown reduced in matrix metalloproteinase-9 (MMP-9) and matrix metalloproteinase-2 (MMP-2) activities, effectively inhibiting p21-activated kinase 1 (Pak1)-LIM kinase 1 (Limk1)-cofilin signalling. Treatment with quercetin suppressed the activation of adenosine monophosphate (AMP)-activated protein kinase α (AMPKα), nuclear factor-kappa B (NF-κB), extracellular signal-regulated kinase 1/2 (ERK1/2) and protein kinase C-delta (PKC-δ), decreasing uPA and uPAR expression. Ultimately, quercetin can potentially turn out to be an emerging factor in the treatment of gastric cancer, effectively diminishing metastasis and invasion [38].

Furthermore, in vitro experiments were conducted to investigate the ability of quercetin to promote cell death, specifically in human gastric cancer cells. These experiments also examined the impact of quercetin on apoptosis and gene expression. Analysis of their results demonstrated that quercetin can trigger apoptosis in GC cells and modulate gene expression [39]. According to flow cytometry, quercetin exacerbated AGS cells to undergo apoptosis, reduced certain protein levels that prevented the mitochondrial membrane from being intact and elevated reactive oxygen species (ROS). By using Western blotting, it was possible to observe that quercetin increased the levels of pro-apoptotic proteins like B-cell lymphoma protein 2 (Bcl-2)-associated X protein (Bax), Bcl-2-associated death promoter (Bad) and Bcl-2-interacting domain death agonist (Bid) while decreasing the levels of anti-apoptotic proteins like Bcl-x (B-cell lymphoma x protein), B-cell lymphoma 2 (Bcl-2) and myeloid cell leukaemia-1 (Mcl-1). Moreover, quercetin elicited diverse impacts on gene expression. For example, quercetin reduced the levels of KDELC2F (KDEL [Lys-Asp-Glu-Leu] containing 2), vascular endothelial growth factor B (VEGF-B) and cyclin-dependent kinase 10 (CDK10) while increasing the levels of tumour protein p53-induced nuclear protein 1 (TP53INP1), tumour necrosis factor receptor superfamily member 10D (TNFRSF10D) and JUN-B. Eventually, their research demonstrated the molecular process, gene expression and signalling pathway entailed in quercetin’s capacity to cause human gastric cancer cells to undergo apoptosis [39].

The impact of anti-proliferative drugs (doxorubicin (DOX) and quercetin) and small interfering RNA (si-RNA) directed against CDC20 (cell division cycle protein 20 homolog) on GC was assessed by Hemati et al. The researchers examined the use of niosome-encapsulated delivery vehicles to deliver si-RNA and medicines. It was discovered that the administration of si-RNA, together with anticancer medications, resulted in the suppression of CDC20 expression, hence enhancing the effectiveness of gastric cancer treatment [40]. In order to enhance the loading capacity and improve the physicochemical properties of si-RNA, the researchers altered the amount of cationic lipid in cationic PEGylated niosomes. Quercetin, DOX and anti-CDC20 si-RNA were incorporated into the co-delivery system. The system was then evaluated for its physicochemical qualities, controlled release, thermosensitivity, rates of apoptosis and gene silencing efficacy. Interestingly, the data showed that the co-delivery system, intended explicitly for loading si-RNA, had a suitable and significant positive charge for delivering drugs. In addition, the researchers demonstrated a thermosensitive drug release pattern that effectively suppressed CDC20 expression, surpassing the individual delivery of either si-RNA or the drug alone. Additionally, their approach efficiently suppressed the proliferation of gastric cancer cells. Their findings indicated that PEGylated niosomes containing both CDC20 si-RNA and anticancer medicines could serve as an innovative approach for treating gastric cancer [40]. A few studies on quercetin’s therapeutic effects in gastric cancer are listed in Table 1.

Gastrointestinal cancer	Preclinical model (in vitro/in vivo)	Anticancer mode	Molecular mechanism	Reference
Gastric cancer	In vitro (NCI-N87 cells)	Anti-proliferative effects	EGR1, FOS-1, FOS and JUN expression levels were elevated while AHR was downregulated.	[36]
	In vitro (AGS cells) and in vivo	Inhibition on cell viability, anti-apoptosis and anti-proliferation of gastric cancer metastasis	The combination of quercetin and a low dose of SN-38 reduced β-catenin protein levels in AGS cells compared to quercetin alone.	[37]
	In vitro (BGC823 AGS cells)	Anti-proliferation of gastric cancer metastasis	Inhibition of uPA, uPAR and downstream target expression	[38]
	In vitro (AGS cells)	Apoptosis induction	Apoptosis induction via ROS increased the levels of pro-apoptotic proteins (Bax, Bad and Bid), decreasing the levels of anti-apoptotic proteins (Bcl-x, Bcl-2 and Mcl-1)	[39]
	In vitro (AGS cells)	Inhibition on gastric cancer cell growth	Suppression of CDC20 by si-RNA which inhibited cancer cell growth	[40]

Table 1.

Summary of the latest study on the therapeutic effect of quercetin in gastric cancer.

4.2 Liver cancer

Fruits, vegetables and plants all contain quercetin having anti-inflammatory, anticancer and antioxidant qualities. Its advantageous properties, including hepatoprotective benefits against liver diseases, have been established in various human illnesses. Hepatocellular carcinoma (HCC) is a primary liver tumour with a high death rate and a delayed diagnosis, which makes it a promising target for research on quercetin effects. HCC is the most prevalent type of primary liver cancer and the sixth most often occurring tumour globally, making it the fourth most deadly neoplasm [41]. Hepatitis C and B virus (HCV and HBV, respectively) and other aetiological agents cause liver damage that leads to the development of HCC through the phases of liver fibrosis and cirrhosis, which can take years or even decades to manifest. Due to its intricate aetiology and molecular heterogeneity, curative therapy for liver cancer is met with challenges [41, 42]. In the first-line scenario for advanced liver cancer, systemic treatment is employed in these instances, employing the two tyrosine kinase inhibitors (TKIs) that are currently available: lenvatinib and sorafenib [43]. After prolonged treatment, liver cancer cells can become resistant to sorafenib, regardless of its efficacy [42], where several TKIs (regorafenib and cabozantinib) and monoclonal antibody have been approved by the FDA for HCC previously treated with sorafenib. Several studies have examined the anticancer benefits of natural substances against liver cancers, such as resveratrol, curcumin and melatonin, to reduce the toxicity and adverse responses brought on by these chemotherapeutic drugs [44].

According to Yamada et al. [45], myricetin and quercetin can block the protein kinase B (AKT) signalling axis, which may subsequently impede HuH7 cell migration driven by transforming growth factor-α (TGF-α) and hepatocyte growth factor (HGF). The study examined how quercetin affected HuH7 cancer cells’ migration in response to TGF-α or HGF. In a dose-dependent manner, quercetin significantly impeded HuH7 cell migration induced by both TGF-α and HGF. Moreover, myricetin, an additional flavonol molecule, significantly inhibited the migration of cancer cells. TGF-α- and HGF-mediated receptor autophosphorylation was unaffected by myricetin or quercetin. Furthermore, quercetin did not affect the TGF-α- or HGF-induced activation of p38 MAPK. However, the growth factors-mediated AKT phosphorylation was inhibited by myricetin and quercetin. Through inhibiting the AKT signalling pathway but not p38 MAPK, their study demonstrated that quercetin might reduce growth factor-driven cell migration of liver cancer [45].

Quercetin can reduce phosphorylation of ERK1/2 and limit proteasome activity, as demonstrated by Ding et al. Proteasome chymotrypsin-like activity rose in response to increasing ERK1/2 activity, while proteasome chymotrypsin-like activity decreased in response to increased MEK1 (mitogen-activated protein kinase kinase 1) activity. The administration of quercetin reduced the amount of proteasome β subunits expressed. According to Ding et al., their investigation demonstrated that the proteasome β subunits’ expression may be modulated by the MEK1/ERK1/2 signalling pathway, resulting in a decrease in the proteasome’s chymotrypsin-like activity [46]. HepG2 cells continued to exhibit steady levels of caspase and trypsin-like protease activity as well as elevated c-Jun N-terminal kinase (JNK) and p38 MAPK activity and reduced phosphorylation of ERK1/2. The inhibition of the JNK and p38 MAPK signalling pathways may not repair the reduced proteasome activity caused by quercetin. Proteasome chymotrypsin-like activity increased or decreased depending on whether MEK1 was upregulated or downregulated. MEK1/ERK1/2 inhibition and quercetin treatment decreased the expression of the proteasome’s β subunits [46].

Quercetin targets apoptosis by upregulating Bax, caspase-3 and p21 and downregulating Akt, polo-like kinase 1 (PLK-1), cyclin-B1, cyclin-A, cell division control 2 (CDC-2), cyclin-dependent kinase 2 (CDK-2) and Bcl-2. It has been observed to decrease signal transducer and activator of transcription 3 (STAT-3) activation and promote STAT-3 protein degradation in liver cancer cells. Numerous studies have demonstrated quercetin’s anticancer properties, suggesting a possible role for the compound in chemoprevention. When quercetin was combined with other anticancer substances, it demonstrated synergistic benefits. Research has demonstrated that quercetin and 5-fluorouracil (5-FU) work synergistically in liver cancer cell lines. Compared to quercetin treatment alone, this combination resulted in increased growth inhibition in some cell lines [47]. Another research found that the anti-inflammatory, proliferative and angiogenesis-related genes TNF-α, VEGF, P53 and NF-κB were all downregulated by quercetin, either alone or in combination with sorafenib, the first medication licensed to treat advanced liver cancer. Treatment with sorafenib plus quercetin resulted in cell cycle arrest, apoptosis and necrosis, in addition to a considerable inhibition of liver tumour growth [48]. Research on the synergistic antitumour effects of quercetin and oncolytic adenovirus in liver cancer cells suggests that quercetin might enhance oncolytic adenovirus ZD55-TRAIL (tumour necrosis factor-related apoptosis-inducing ligand)-mediated growth inhibition and death in liver cancer cells. In vitro and in vivo antiliver cancer experiments have demonstrated the potential of quercetin combination therapy [49].

These findings point to a distinct anti-proliferative and pro-apoptotic action of quercetin in liver cancer as well as a possible regulating function for the tumour cell cycle progression that requires more research. Through the modification of several cellular processes and signalling pathways, this flavonoid appears to have antitumoural efficaciousness; nevertheless, more research is necessary to fully understand its mechanisms of action against liver cancer. In addition to the emerging usage of quercetin derivatives with anticancer efficaciousness, emerging tactics of combination and drug-delivery systems may enhance such features of cancer inhibition and expand treatment choices for patients with liver cancer [44]. Figure 3 illustrates the common mechanistic pathways of quercetin. A few research on quercetin’s therapeutic benefits in liver cancer are listed in Table 2.

Figure 3

Diagram of the mechanism of quercetin in the treatment of liver fibrosis. Source: Adapted from Ref. [50].

Gastrointestinal cancer	Preclinical model (in vitro/in vivo)	Anticancer mode	Molecular mechanism	Reference
Liver cancer	In vitro (SMMC-7721, BEL-7402 liver cancer cells, LO2 normal liver cells)	Inhibition of cell proliferation	Inhibition of Akt/mTOR, reduced glucose uptake and lactate production, reduced p-Akt/Akt and p-mTOR/mTOR rates, increased cell growth inhibition	[51]
	In vitro (SMMC-7721 and HepG2 liver cancer cells LO2 normal liver cells)	Cell growth inhibition, autophagy induction, apoptosis induction	Akt/mTOR inhibition and MAPK activation lead to increase in LC3A/LC3B-II and Beclin1 protein levels and decrease in p62 protein expression, increase in the percentage of apoptotic cells	[52]
	In vitro (BEL-7402 cells, multidrug-resistant cell line BEL/5-FU)	Chemosensitizing effect towards 5-FU	FZD7/β-catenin inhibition, reduction in ABCB1, ABCC1 and ABCC2 expression, as well as inhibition of ABCC2 function	[53]
	In vitro (HepG2 cells)	Apoptosis induction	MEK1/ERK1/2 inhibition, increased levels of cleaved caspase-3, cleaved PARP and Bax proteins, reduced levels of Bcl-2 protein	[46]
	In vitro (SMMC-7721, HepG2 and HuH7 cell lines)	Decrease of cell proliferation, apoptosis induction	Quercetin combined with ZD55-TRAIL inhibited NF-κB which increased apoptotic activity (nuclear fragmentation, chromatin condensation) as well as apoptotic markers	[49]
	In vitro (HepG2 cells)	Reduced cell viability	Synergistic activity of sorafenib and quercetin, with reduced IC50 of both drugs and increased uptake of both drugs	[54]
	In vivo (Nude mice subcutaneously injected with LM3 cells)	Tumour growth inhibition	Quercetin 100 mg/kg was orally administered, reduced in tumour volume, increased necrosis and TUNEL-positive cells	[51]
		In vivo (SMMC-7721 xenograft mouse model)	Quercetin 50 mg/kg administered via intraperitoneal injection, reduction in tumour size, HK2 and Ki67 protein expression and p-Akt and p-mTOR protein levels observed	[51]
	In vivo (Nude mice subcutaneously injected with SMMC-7221 cells)	Suppression of tumour growth, apoptosis and autophagy induction	Quercetin 60 mg/kg administered orally, reduction in tumour weight and volume observed, as well as apoptotic markers (increased Bax, cleaved caspase-3, reduced Bcl-2 protein expression)	[52]
	In vivo (MHCC97H xenograft mouse model)	Suppression of tumour growth and progression, apoptosis induction	Quercetin encapsulation (PLGA-loaded gold-quercetin nanoparticles) 30, 40 and 50 mg/kg via intraperitoneal injection causes inhibition of AP-2β/hTERT, p50/NF-κB/COX-2, Akt/ERK1/2 pathways, reduced tumour weight and volume and increased apoptotic activity observed	[55]
	In vivo (Mice subcutaneously injected with HuH7 cells)	Inhibition of tumour growth	Quercetin combined with dasatinib 50 mg/kg of quercetin with 5 mg/kg of dasatinib administered orally halted tumour progression	[56]

Table 2

Basic characteristics of in vitro and in vivo studies using quercetin in liver cancer.

4.3 Colorectal cancer

Quercetin has garnered significant interest in its ability to prevent and combat colorectal cancer [57]. Numerous research have examined these effects both in laboratory settings (in vitro) [57, 58, 59, 60] and in living organisms (in vivo) [61, 62, 63, 64]. This section will provide a concise overview of the anticancer effects of quercetin, followed by a summary of its potential as a novel therapeutic agent for treating colorectal cancer.

Evaluations have been conducted on the role of quercetin in apoptosis, hyperproliferation and inflammation, as well as its mechanism in 1,2-dimethylhydrazine (DMH)-induced carcinogenicity and tumour multiplicity [65]. Rats were orally given quercetin at doses of 25 or 50 mg/kg body weight and subcutaneously injected with 20 mg/kg body weight of DMH for 15 weeks. Afterwards, the rats were euthanized. The DMH generates reactive oxygen species (specifically superoxide) through the process of membrane lipid peroxidation, leading to a disruption in redox equilibrium. DMH also reduces the amount of antioxidants in tissues. The DMH-induced intestinal carcinogenicity led to an increase in proliferative and inflammatory variables due to a low Bax/Bcl-2 ratio and dysregulation of apoptosis. Pretreatment with quercetin mitigated the deleterious effects of DMH, such as maintaining the activity of detoxifying enzymes and decreasing proliferation as well as early indicators (such as mucin depletion and goblet cell disintegration) in the colonic tissue. Quercetin controlled the expression of β-catenin and adenomatous polyposis coli (APC), decreasing in the occurrence and number of tumours. The histology findings confirm the beneficial effect of quercetin in mitigating the pathogenic changes generated by DMH [65].

A prior investigation similarly assessed the impact of daily intake of microencapsulated probiotics either alone or in conjunction with microencapsulated quercetin to prevent colorectal cancer. The researchers utilized ApcMin/+ mice which exhibit spontaneous intestinal adenomas and cancer. The researchers evaluated changes in tissue structure, intestinal bleeding, fat accumulation, respiratory quotient, body weight and energy usage. In addition, gene expression related to the Wnt signalling pathway was assessed [66]. ApcMin/+ mice were given Bifidobacterium bifidum (Bf) and Lactobacillus gasseri (Lg) probiotic strains, with a concentration of 10 [57] colony-forming units (CFU)/100 g of food or a combination of both probiotic strains together with microencapsulated quercetin at a dosage of 15 mg/100 g of food, for 73 days. Subsequently, energy metabolism, alterations in organ and body weight, histology of colon tissue, composition of intestinal microbiota and gene expression related to the Wnt signalling system were also assessed. The data showed that the microencapsulated supplement, consisting of probiotics and quercetin, effectively inhibited the advancement of colorectal cancer in ApcMin/+ mice [66].

Furthermore, an investigation conducted by Liu and Zhi in 2021 employed a rat model to create constipation, a recognized risk factor for colorectal issues, by providing loperamide. Subsequently, the impact of quercetin on constipation induced by loperamide was examined. The data indicated that administration of quercetin at doses of 25 and 50 mg/kg in rats resulted in an elevation in intestinal transit rate, as well as an increase in the concentration of short-chain fatty acids and levels of gastrin, motilin and substance P. Besides, quercetin enhanced the motility of the intestines and decreased the levels of somatostatin. The levels of aquaporin 3 (AQP3), transient receptor potential vanilloid 1 (TRPV1), glial cell line-derived neurotrophic factor (GDNF), enteric nerve-related factors, nitric oxide (NO) synthase, stem cell factor (SCF) and its receptor c-Kit were measured using Western blotting and reverse transcription-quantitative polymerase chain reaction (RT-qPCR). Quercetin administration was discovered to decrease loperamide-induced constipation by enhancing the levels of interstitial cells of Cajal indicators, such as stem cell factor, its receptor c-Kit and AQP3. Ultimately, their findings demonstrated that quercetin exhibited a safeguarding impact against loperamide-induced constipation, hence potentially mitigating the likelihood of developing colorectal cancer [67].

The antiangiogenic and anticancer effects of quercetin and luteolin on colon cancer cells (HT29) were assessed by Erdoğan et al. in comparison with the conventional chemotherapy drug 5-FU and a combination of 5-FU and luteolin or quercetin. An MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide) assay, fluorescence microscopy, human VEGF ELISA (enzyme-linked immunosorbent assay), Western blotting and qRT-PCR were employed. Their method of evaluating the effects was Western blotting on the following genes: mTOR, Akt protein, PTEN, p53, Bax, Bcl-2 and P38. The MTT assay assessed cell viability, and ELISA was utilized to ascertain how the treatment affected angiogenesis. Fluorescence microscopy was employed to identify HT-29 cell apoptosis. Eight times as much apoptosis was induced in cells treated with quercetin and 5-FU as in cells treated with luteolin and 5-FU, and 10 times more in cells treated with the combination. The VEGF level exhibited a notable decrease in cells subjected to a combination treatment of quercetin or luteolin along with 5-FU. The researchers discovered that quercetin and luteolin can govern programmed cell death (apoptosis) in HT-29 cells. Additionally, it was observed that combination therapy decreased the expression of anti-apoptotic proteins, such as Bcl-2, mTOR and Akt genes, as compared to the control group. The groups treated with 5-FU and quercetin exhibited a more rapid rise in the expression of P53, P38, MPK and PTEN genes compared to those treated with 5-FU and luteolin. To summarize, the combined use of luteolin and quercetin can enhance the anticancer properties of 5-FU and mitigate its harmful side effects in colorectal cancer [68]. Figure 4 shows signalling transduction pathways in colon commonly modulated by quercetin.

Figure 4

Signalling transduction pathway in colorectal cancer that being modulated by therapeutic approaches of quercetin. Source: Adaptation by Ref. [69] under creative common license. https://creativecommons.org/licenses/by/4.0/

Al-Ghamdi et al. examined the mechanism and impact of a 200 mg dosage of quercetin and a 150 mg dose of EGCG (epigallocatechin gallate) at different ratios on the stimulation of programmed cell death and suppression of growth in the human colorectal cell line HCT-116 [70]. Quercetin suppressed cell proliferation, halted the cell cycle, induced annexin V and decreased clonogenicity. The lowest dose of the investigated medicines suppressed colony development. Moreover, a substantial rise in annexin V was observed at 150 mg of quercetin and 100 mg of EGCG. The combination therapy resulted in the cessation of cell division at the G1 phase. Ultimately, the combination of EGCG and quercetin holds promise as a potent combination of chemotherapy in the future. However, further investigations are necessary to determine the appropriate dosage and potential adverse reactions [70]. Table 3 lists some studies on the therapeutic effect of quercetin in colorectal cancer.

Gastrointestinal cancer	Preclinical model (in vitro/in vivo)	Anticancer mode	Molecular mechanism	Reference
Colorectal cancer	In vivo (rat)	Anti-apoptotic, anti-hyperproliferation	Quercetin reduced tumour occurrence and number by controlling β-catenin and APC expression. Histology confirmed quercetin’s beneficial effect in reducing DMH-induced pathogenic changes.	[65]
	In vivo (ApcMin/+ mice)	—	Probiotics and quercetin-containing microencapsulated supplementation successfully prevented colorectal cancer in ApcMin/+ mice from progressing via alteration of Wnt signalling	[66]
	In vivo (rat)	—	Results showed that quercetin had a protective effect against constipation brought on by loperamide, which may lessen the risk of colorectal cancer by increasing intestinal transit rate and motility and reducing somatostatin levels	[67]
	In vitro (HT-29 cells)	Anti-angiogenic and anti-apoptotic	Treatment with 5-FU and quercetin increased expression levels of P53, P38, MPK and PTEN genes	[68]
	In vitro (HCT-116 Cells)	Anti-proliferation, inhibition on cell cycle and suppression of growth	Halted cell division at G1 phase	[69]
	In vivo	Apoptosis, anti-angiogenesis and anti-proliferation	Induced apoptosis, anti-angiogenesis and anti-proliferation via EGFR, Akt, Cdk1, cyclin B and VEGF alteration	[63]
	In vitro (Caco-2 cells)	Apoptosis and antioxidant	Apoptosis induction via TNF-α and TNF-R1 alteration	[71]
	In vitro (DLD-1 cells)	Apoptosis	Apoptosis induction via KRAS, JNK and caspase-3 alteration	[72]
	In vitro (Caco-2 cells)	Lipid peroxidation	Inhibition of lipid peroxidation via Nrf-2 and Prx-6 protein	[73]

Table 3

Summary on the latest studies on the therapeutic effect of quercetin in colorectal cancer.

4.4 Pancreatic cancer

The plant-derived medication presents a compelling option for treating cancer. Quercetin has been demonstrated that quercetin modulates several dysregulated signalling pathways, including those linked to autophagy and apoptosis, in order to achieve its anticancer effects through a variety of methods. In particular, quercetin inhibits multiple signalling pathways, including NF-κB, P53, Wnt/β-catenin, MAPK, janus kinase/signal transducer and activator of transcription (JAK/STAT) and the Hedgehog pathway, to achieve its anticancer actions. Numerous intracellular signalling molecules, including tumour necrosis factor-α (TNF-α), Bax, Bcl-2, caspases and VEGF, are interfered by quercetin.

Numerous cancer types, including breast, prostate, ovarian, lung, colon, hepatocellular, lymphoma and pancreatic cancer, have been researched in relation to quercetin’s anticancer properties. Nevertheless, most current research on quercetin’s anticancer effects focus on cancer in laboratory animals.

According to a recent study on the impact of quercetin on pancreatic cancer, quercetin treatment altered the expression of 105 miRNAs, including the lethal-7 (let-7), miR-194, miR-103, miR-29, miR-125 and miR-106 families. These miRNAs are essential for inhibiting cell death and promoting invasion, proliferation and metastasis. Among these, let-7c is one of the most significant miRNAs. This miRNA controls Notch inhibitor Numbl, a downstream target of let-7c, following transcription. By intensifying its hostility towards Notch, the Numbl stops the spread of cancer [74].

Quercetin is one of the natural plant extracts that have various important biological antitumour properties. Besides, quercetin has been shown to exert its anticancer properties by targeting noncoding RNAs (ncRNAs) that include long non-coding RNA (lncRNA) and microRNA (miRNA), which have been linked to the development of cancer. It inhibits the proliferation of cancer cells, encourages cell death and increases susceptibility to chemotherapy drugs by controlling the production of ncRNAs, which in turn influences the expression of genes and proteins involved in the signalling cascade. These provide insight into the molecular relationship between quercetin and non-coding RNAs, which may be useful in using quercetin as a therapeutic adjuvant. When combined, quercetin is a naturally occurring substance that may have an anticancer impact when used as a cancer adjuvant [9].

Quercetin specifically causes death in pancreatic cancer cells to have an anticancer impact. Still, new research also suggests that quercetin modifies many signal transduction pathways to slow down the growth of cancer. Quercetin may prevent EMT, invasion and metastasis by suppressing the expression of the N-cadherin, MMP-9 and STAT-3 signalling pathways. Through its inhibitory influence on Receptor for Advanced Glycation Endproducts (RAGE) expression, quercetin increases the chemosensitivity of pancreatic cancer cells to gemcitabine (GEM). Meanwhile, it is a desirable agent for the treatment of cancer due to its broad accessibility, effectiveness and low toxicity when compared to other investigated substances. Recently, quercetin has been made available and used as a potentially effective medication to treat a variety of malignancies, either by itself or in conjunction with other chemotherapeutic drugs [75].

Guo et al. employed various methods such as a nude mice tumour formation test, migration, proliferation and invasion as well as real-time cell analysis, to assess the effects of quercetin on pancreatic ductal adenocarcinoma (PDA). The development and spread of tumours as well as the in vivo and in vitro flow cytometry investigation of Sonic hedgehog (SHH) signalling in pancreatic cancer cells and colony formation was examined [76]. Through the downregulation of c-Myc, quercetin demonstrated anticancer efficacy by inhibiting pancreatic growth. Quercetin inhibited migration and invasion by lowering transforming growth factor β1 (TGF-β1) levels and suppressing the epithelial-mesenchymal transition (EMT). Through the mitochondrial and death receptor pathways, quercetin caused apoptosis. In an investigation conducted on naked mice, quercetin therapy decreased the number of metastases. Quercetin’s therapeutic actions on PDA are connected to Gli2 (GLI Family Zinc Finger 2) but not Gli1 (GLI Family Zinc Finger 1), which entails the modulation of SHH activity. Recombinant Shh protein was used to boost SHH activity, reversing quercetin’s effects on pancreatic cancer cells’ proliferation, migration and invasion. Furthermore, by upregulating Zeb2 and Snail1 expression, Shh may trigger TGF-β1/Smad2/3 signalling and induce EMT, partially reversing the inhibition of cancer cell migration and invasion that quercetin mediated. According to these results, quercetin may be utilized to treat PDA by preventing pancreatic cancer cells’ invasion, migration and metastasis as well as by causing apoptosis by blocking the SHH and TGF-β1/Smad signalling pathways [77].

Hassan et al. assessed the effectiveness of quercetin in conjunction with well-known medications such as gemcitabine (GEM) and doxorubicin (DOX) against human pancreatic cancer cells and hepatic cancer cells, respectively. The investigation revealed that quercetin combined anticancer medications produced superior outcomes than single-drug therapy. Up to 60% of the cells in two-dimensional (2D) and three-dimensional (3D) cultures were dead, with single-drug treatment only exerting ≤20% apoptosis [78]. A more thorough analysis showed that quercetin downregulated hypoxia-inducible factor (HIF)-1α and raised p53 regulator levels, which led to p53-mediated apoptosis. The impact of quercetin on hypoxia and drug efflux pump function was also assessed by investigating the functions of HIF-1α and multidrug transporters (such as multidrug resistance (MDR)) activity [78]. The investigation revealed that quercetin could reduce multidrug resistance mutation 1 (MDR-1) efflux activity, exerting similar inhibitory effects as verapamil, the positive control used in the study. Hence, quercetin might be used with GEM or DOX against multidrug-resistant malignancies in the pancreas and liver, respectively [78].

In another investigation, Liu et al. assessed quercetin’s effects and mode of action on pancreatic cancer cells resistant to GEM. Two cell lines from hepatocellular and pancreatic cancers were used in this investigation. Liver cancer cell lines used were HepG2 and Huh-7, whereas BxPC-3 and PANC-1 were the pancreatic cancer cell lines. Quercetin was reported to have a cytotoxic effect on HepG2 and PANC-1 (GEM-resistant) using a proliferation test and a pro-apoptotic effect on HepG2 and PANC-1, according to a flow cytometry analysis. Western blotting results showed that quercetin caused apoptosis by upregulating p53 protein and downregulating cyclin-D1. Additionally, it resulted in S phase cell cycle arrest. Ultimately, the findings demonstrated that quercetin can be used with established anticancer medications to treat GEM-resistant liver and pancreatic cancer [79].

Hoca et al. investigated the effects of quercetin and resveratrol on the epithelial-mesenchymal transition (EMT) in CD133-positive and CD133-negative pancreatic cancer cells [80]. After separating the CD133+ cancer stem cells from PANC-1 cells using the MiniMACS technique, quercetin and resveratrol were added in varying amounts to the three cell variants: CD133+, CD133− and PANC-1. The investigation employed MTT test and immunocytochemistry with antibodies against vimentin, TNF-α, actin alpha 2 (ACTA-2), N-cadherin and interleukin-1β (IL-1β). The intensity of N-cadherin, ACTA-2 and IL-1β staining was lower in the quercetin-treated cells compared to CD133 + cells treated with resveratrol. Hence, quercetin is effective in significantly reducing N-cadherin expression, preventing EMT which leads to metastasis [80].

In another investigation by Serri et al., gemcitabine and quercetin combination showed synergistic activity, particularly in inhibiting the migration of cancer cells. The study employs hyaluronic acid-coated nanoparticles (PPHA NPs) carrying GEM and quercetin [81]. The nanoparticle carriers were able to improve drug delivery, thereby improving cytotoxicity and cellular uptake; it was also mentioned that the PPHA NPs improved the anti-inflammatory properties of quercetin, observed by a reduction in interleukin (IL) levels in pancreatic ductal adenocarcinoma cell lines Mia-PaCa-2 and PANC-1 [81].

A few research on quercetin’s potential therapeutic benefits for pancreatic cancer are included in Table 4.

Gastrointestinal cancer	Preclinical model (in vitro/in vivo)	Anticancer mode	Molecular mechanism	Reference
Pancreatic cancer	In vivo and in vitro (PANC-1 and Patu8988)	Inhibition of metastasis	EMT suppression by reducing TGF-β1 level, inhibition of growth, invasion and migration of cells, apoptosis of cancer cells by antagonizing TGF-β/Smad and SHH signalling pathways	[76]
	In vitro (Mia-PaCa-2 and PANC-1)	Synergism between gemcitabine and quercetin	Reduced IL-6 and IL-8 expressions and enhanced cytotoxicity against Mia-PaCa-2 and PANC-1 cell lines	[81]
	In vitro (PANC-1)	Inhibition of metastasis	Reduced immunoreactivities, such as ACTA-2, IL-1β and N-cadherin, increased TNF-α and vimentin, prevention of EMT	[82]
	In vivo and in vitro (PDAC)	Halting tumour progression	Improved effects of BET inhibitors at suppressing tumour development and reduced hnRNPA1 in vivo	[83]
	In vitro (MIA Paca-2, BxPC-3, AsPC-1, HPAC and PANC-1)	Apoptosis induction, autophagy, chemosensitizing effect	Quercetin showed a RAGE silencing like effect that attenuates RAGE expression to accelerate apoptosis, autophagy and chemosensitivity of MIA Paca-2^GEMR cells	[84]

Table 4

Anticancer effects of quercetin against pancreatic cancer.

4.5 Oesophageal cancer

Yue Liu et al. evaluated the effects of quercetin on the angiogenesis and migration of oesophageal cancer cells, in addition to the underlying mechanism [85]. In their study, human oesophageal cancer cells (Eca109) received 5 or 10 μg/ml of quercetin. A scratch wound healing assay evaluated cell migration and invasion was examined using a transwell assay, and a colony formation assay was conducted. Human umbilical vein vascular endothelial cells (CRL-1730) were inoculated in Eca109 conditioned medium, and the effects of quercetin were measured by tube formation and wound healing assays. Western blotting measured MMP-2, MMP-9 and VEGF-A protein expression levels. Quercetin (10 μg/ml) decreased colony formation in Eca109 cells; however, no difference was observed between the control group and the 5 μg/ml quercetin group. The group treated with 10 μg/ml quercetin showed reduced cell migration and invasion, while 5 μg/ml only suppressed invasion. Tube formation ability and endothelial cell migration were inhibited in cells incubated in Eca109 conditioned medium. The group treated with 10 μg/ml quercetin showed reduced MMP-2, MMP-9 and VEGF-A expression levels [85].

Interesting study by Li et al. involving Yishen Qutong Granules (YSQTG) in traditional Chinese medicine showed that quercetin (the main component of YSQTQ) could inhibit proliferation, invasion and clonal formation of oesophageal carcinoma cell lines [86]. Compared with the control group, quercetin at different concentrations of 50, 100, 150 and 200 μmol/L inhibited the proliferation of KYSE30 and KYSE150 cells at 24, 48 and 72 hours. Further study on the effects of quercetin on the formation of cell clonal ability showed that the number of clones formed in KYSE30 cells in the control and 50 and 100 μmol/L quercetin groups was 346 ± 7, 203 ± 10 and 132 ± 5, respectively, while the number of clones formed in KYSE150 cells in the control and 50 and 100 μmol/L quercetin groups was 470 ± 13, 364 ± 5 and 225 ± 15, respectively, the difference was statistically significant (p < 0.05). Molecular docking of quercetin and NF-κB was carried out and it was found that the binding energy of quercetin and NF-κB was −5.82 kcal/mol, indicating that small molecular compounds such as quercetin could directly bind to NF-κB stably. The specific binding of quercetin to NF-κB was confirmed, and the regulatory relationship of quercetin to NF-κB protein was also detected. The results showed that 50 and 100 μmol/L quercetin intervention groups could significantly reduce the level of NF-κB protein in KYSE150 cells (p < 0.01), suggesting that quercetin can inhibit the proliferation of oesophageal cancer cells in a concentration-dependent manner and directly target the key protein of NF-κB [86].

According to a study by Wang et al., quercetin treatment significantly slowed down the proliferation of oesophageal cancer cells by increasing the rate of apoptosis shown in the flow cytometry detection test (p < 0.05). Moreover, quercetin induced the expression of gene controlling Bax protein while suppressing the expression of Bcl-2 protein, further confirming its apoptosis-inducing effects on oesophageal cancer cells [87]. Combined with the results of MTT assays, it was shown that the treatment of quercetin substantially impaired the survival of oesophageal cancer cells (p < 0.05). The transwell assay showed that quercetin also inhibited the invasive capacity of oesophageal cancer cells, as evidenced by substantially fewer oesophageal cancer cells with quercetin treatments penetrating the membrane in the transwell chamber (p < 0.05) compared to normal oesophageal cancer cells. However, regarding the migration of oesophageal cancer cells detected by scratch assays, neither treatment of quercetin nor treatment of cisplatin showed significant impacts demonstrated by a similar closure rate, implying that the quercetin treatment majorly influences the invasion ability of cancer cells but had little effect on the migration. At the molecular level, the expression of miR-1-3p was induced (p < 0.05), while the expression of transgelin 2 (TAGLN2) was suppressed by quercetin (p < 0.05). Moreover, the anti-oesophageal cancer effects of quercetin were blocked by miR-1-3p inhibition (p < 0.05), which was represented by the restored growth and invasion of oesophageal cancer cells, demonstrating that quercetin exerted inhibitory effects on oesophageal cancer cells by inducing miR-1-3p [87]. Table 5 summarizes the molecular mechanisms of quercetin in oesophageal cancer.

Gastrointestinal cancer	Preclinical model (in vitro/in vivo)	Anticancer mode	Molecular mechanism	Reference
Oesophageal cancer	In vitro (Eca109 cells)	Inhibition of invasion and angiogenesis	Reduction levels of MMP-2, MMP-9 and VEGF-A	[85]
	In vitro (of KYSE30 and KYSE150 cells)	Inhibition of proliferation, invasion and clonal formation	affect the metabolic pathway, MAPK signalling pathway and nuclear factor-kappa B (NF-κB) signalling pathway	[86]
	In vitro (KYSE510 and TE-7 cells)	Induction of apoptosis	Inducing miR-1-3p level and suppressing TAGLN2 level	[87]

Table 5

Molecular mechanisms of quercetin on oesophageal cancer.

5. Conclusion and future perspectives

The use of quercetin in anticancer management has been proved to have a plethora of applications. Based on the above-mentioned evidences, quercetin makes a promising anticancer agent by targeting cancer cell proliferation, metastasis, angiogenesis and apoptosis directly on cancers of the GI tract. Furthermore, quercetin also prevents cancer progression by exerting chemopreventive effects via anti-oxidative properties. Various in vitro and in vivo studies have been cited to highlight the capabilities of quercetin in exerting anticancer effects, which include apoptosis induction via ROS activation, caspase-3 activation and downstream gene alteration. Quercetin also targets various molecular targets commonly involved in cancer, such as Akt, MAPK, epidermal growth factor receptor (EGFR), NF-κB and TNF-α, which abrogate oncogenic signalling. The safety profile of quercetin has also been described to be well tolerable in patients, with good bioavailability in glycoside form. Furthermore, quercetin has also shown great potential to be used in combination with other antineoplastic drugs, such as 5-FU, gemcitabine and doxorubicin, in combating cancer progression due to the synergism demonstrated. Hence, adjuvant therapy using quercetin can be a promising look into the near future, as it can improve drug efficiency and targeting as well as reduce adverse effects caused by high doses of anticancer drugs. Other fields of study that are worth looking into include interactions of quercetin with ncRNAs such as miRNA and lncRNA, as these fields are still left largely unexplored. These goals can be achieved with further mechanistic and safety studies, as well as taking the step further to include research on other cancers such as cancers of the central nervous system (CNS) and clinical studies in patients.

Acknowledgments

This work was supported by the Deanship of Scientific Research, Vice Presidency of Graduate Studies and Scientific Research, King Faisal University, Saudi Arabia (Grant No. 3767) and Sunway University research grants (project no. GRTIN-IGS(02)-CVVR-13-2023.