Therapeutic role of allicin in gastrointestinal cancers: mechanisms and safety aspects

Abstract

Allicin, a bioactive compound found in garlic (Allium sativum), has shown significant potential in treating gastrointestinal cancers, including gastric, colorectal, liver, esophageal, and pancreatic cancers. This review focuses on the recent progress in the therapeutic application of this compound for the induction of apoptosis, inhibition of proliferation, and disruption of cancer cell signaling pathways, including the MAPK, PI3K/AKT, and NF-κB pathways. Taking these mechanisms into consideration, although challenging because of rapid metabolism and inherent instability, allicin could be a very potent candidate for cancer treatment. Allicin and its other derivatives, such as diallyl disulfide (DADS) and ajoene, have been found to have strong anticancer potential both in vitro and in vivo. In addition, the effectiveness of allicin in augmenting conventional chemotherapy and retarding tumor growth proves that allicin is one of the most efficient complementary therapies. Epidemiological studies have also demonstrated the role of garlic consumption in reducing the risk of developing cancer. This review examines the potential role of allicin in the treatment of gastrointestinal cancers and emphasizes the importance of this compound for future cancer biology research.

Introduction

Gastrointestinal (GI) cancers, including esophageal, gastric, colorectal, pancreatic, and liver cancers, remain a significant global health burden. These malignancies are often diagnosed at an advanced stage, contributing to their high mortality rates. Despite significant advancements in conventional treatments such as surgery, chemotherapy, and radiation therapy, the prognosis for GI cancer patients remains poor due to inherent challenges, including drug resistance, systemic toxicity, and tumor recurrence [26]. As a result, there is an increasing need for alternative therapeutic strategies, including natural compounds with potent anticancer properties.

Among these, garlic (Allium sativum) has been extensively studied for its pharmacological benefits, including antibacterial, anti-inflammatory, hypolipidemic, and anticancer activities [74]. The anticancer potential of garlic is primarily attributed to its organosulfur compounds, such as diallyl trisulfide, allyl mercaptan, diallyl disulfide, diallyl sulfide, and most notably, allicin [81]. Allicin, a sulfur-containing bioactive compound, is formed when garlic is crushed or chopped, catalyzed by the enzyme alliinase. It exhibits a range of biological activities, including antimicrobial, anti-inflammatory, and anticancer effects [43]. Its anticancer properties are attributed to its ability to induce apoptosis, inhibit cell proliferation, and modulate multiple signaling pathways associated with tumor progression [53, 111].

The pharmacokinetics of allicin indicate that it is efficiently absorbed and metabolized, making it a promising drug candidate. However, its clinical application is hindered by its instability and strong odor, necessitating the development of novel delivery systems to enhance its therapeutic efficacy [13]. Recent studies have focused on allicin-based nanotechnology approaches to improve its bioavailability and stability. Encapsulation of allicin in nanoparticles has shown promise in targeted delivery, increasing its anticancer effects while minimizing adverse reactions [15]. These formulations enhance the solubility, permeability, and controlled release of allicin, providing new avenues for its clinical application [74].

Preclinical and epidemiological studies have demonstrated the potential of allicin against various GI cancers. In esophageal cancer, allicin has been found to induce apoptosis by activating caspases and generating reactive oxygen species (ROS) [84]. In gastric cancer, allicin suppresses tumor growth by modulating key signaling pathways such as NF-κB and PI3K/Akt. Studies on colorectal cancer highlight allicin’s ability to regulate β-catenin signaling and induce oxidative stress, contributing to its antiproliferative and proapoptotic effects [30]. Allicin also exhibits potential in treating pancreatic cancer, one of the most lethal GI malignancies, by repressing cancer cell migration and invasion through the downregulation of MMP-9 and the suppression of epithelial–mesenchymal transition [56, 86]. In liver cancer, allicin has been shown to mediate cell cycle arrest and apoptosis through modulation of oncogenic pathways [8, 107].

Recent studies have expanded the scope of allicin’s therapeutic potential beyond its direct anticancer effects. Synergistic interactions between allicin and conventional chemotherapeutic agents are being explored to enhance treatment efficacy and overcome drug resistance [74]. Additionally, epidemiological evidence suggests that regular consumption of garlic and allicin-containing foods may reduce the risk of developing GI malignancies [81]. Given its multi-targeted mechanism and minimal toxicity to normal cells, allicin represents a promising candidate for integrative cancer therapy. However, detailed clinical studies are required to establish its safety profile, optimal dosage, and long-term therapeutic outcomes.

This review aims to provide a comprehensive update on the clinical potential of allicin in treating major GI cancers. It highlights recent advancements in nanoformulations and combination therapies, offering insights into the future direction of allicin-based cancer management strategies. With continued research, allicin may emerge as an effective, natural therapeutic agent, improving patient outcomes in GI oncology.

Methodology

Search strategy

To identify relevant studies for this review, we comprehensively searched electronic databases, including Scopus, PubMed, SpringerLink, ScienceDirect, Wiley Online, and Web of Science. We used the following search terms and combinations:

• “Allicin” paired with “Gastrointestinal Cancer,” “Gastric Cancer,” “Colorectal Cancer,” “Liver Cancer,” “Esophageal Cancer,” “Pancreatic Cancer,” “Apoptosis,” “Cancer Cell Signalling Pathways,” “MAPK,” “PI3K/AKT,” “NF-κB,” “Synergism,” “Nanotechnology,” “Complementary Therapy,” “Chemotherapy,” “Pharmacokinetics,” “Safety.”

• Additionally, various combinations of the following terms were used: “Diallyl Disulfide (DADS),” “Ajoene,” “SAMC,” “SAC,” “ROS,” “Mitochondrial Pathways,” “Tumour Growth Inhibition,” “Cell Cycle Arrest,” and “Anticancer Efficacy.”

Only studies published in English were considered. The papers were reviewed for relevance based on the criteria below (Fig. 1).

Fig. 1.

PRISMA flow diagram depicting the selection process of studies included in the systematic review

Inclusion and exclusion criteria

Inclusion criteria

1. Studies focusing on the pharmacological and biological activities of allicin and its derivatives in the context of gastrointestinal cancers, including gastric, colorectal, liver, esophageal, and pancreatic cancers.

2. Research involving in vitro and in vivo models that examine the effects of allicin, diallyl disulfide (DADS), ajoene, SAMC, SAC, and other related compounds on cancer cell proliferation, apoptosis, and signaling pathways.

3. Investigations into the chemical properties of allicin and its derivatives, including their impact on apoptosis, cell cycle arrest, and the inhibition of cancer cell migration.

4. Studies evaluating the role of allicin and its derivatives in augmenting conventional chemotherapy and their potential as complementary therapies.

5. Research exploring the synergistic effects of allicin and its derivatives with other therapeutic agents and their safety profiles.

Exclusion criteria

• I) Studies focused on compounds derived from sources other than garlic or related to non-allium plants.

• ii) In vitro studies with duplicated data or whose titles and abstracts do not align with the inclusion criteria.

• iii) Research on topics unrelated to allicin or its derivatives in gastrointestinal cancers.

• iv) Articles published in languages other than English.

• v) Studies for which the full text is not available.

• vi) Case reports, letters, editorials, and other non-primary research articles.

Chemistry and pharmacokinetics

Allicin, chemically known as diallyl thiosulfinate, is the major bioactive compound produced from garlic (Allium sativum). The compound is not found in fresh intact garlic cloves but is rapidly generated following crushing or chopping of the garlic, which activates the enzyme alliinase [51]. Alliinase activates the comparatively stable sulfur-containing amino acid derivative alliin (usually cysteine sulfoxide) into allyl sulfenic acid. After that, two molecules of allyl sulfenic acid naturally condense and separate from one another to generate allicin [21], Fig. 2). The reaction takes place almost instantaneously and liberates the typical pungent smell of garlic, which is responsible for giving allicin many of its medicinal properties [10]. Allicin is characterized by having a sulfur‒sulfur bond (thiosulfinate group) and two allyl groups. Its molecular formula is C₆H₁₀OS₂, and its structure features a reactive sulfenic acid intermediate [47]. The inherent instability of allicin is a critical consideration,it is highly reactive and degrades quickly, transforming into a variety of sulfur-containing compounds such as ajoene, dithiin, and diallyl disulfide [81]. This instability poses a challenge for its therapeutic application, as it necessitates the development of stable delivery methods to ensure bioavailability and efficacy.

Fig. 2.

Diagram showing the biosynthesis of allicin from alliin. Alliin is broken down by the enzyme alliinase to allyl sulfenic acid and dehydroalanine. Two molecules of allyl sulfenic acid condense spontaneously to give a molecule of allicin

Pharmacokinetically, allicin undergoes very fast absorption and metabolism. Oral ingestion therefore results in the absorption of allicin in the gastrointestinal tract, although its high reactivity results in very quick metabolism [5]. There is evidence that allicin is capable of diffusing across cell membranes and eliciting its biological activities intracellularly [17]. However, very few reports of accurate methods for direct pharmacokinetic studies of allicin in human plasma exist because of its decomposition rate. Allicin is rapidly converted into transformation products such as vinyldithiins, ajoene, diallyl disulfide (DADS), and diallyl trisulfide (DATS), which possess bioactivity similar to allicin itself [32]. Studies in rats have shown that vinyldithiins are detectable in serum, kidney, and fat tissue for up to 24 h, with 1,2-vinyldithiin accumulating more in fat tissue due to its higher lipophilicity, while 1,3-vinyldithiin is eliminated more rapidly [32].

Once metabolized, allicin derivatives such as diallyl disulfide and allyl methyl sulfide circulate in the bloodstream [103]. These metabolites undergo further hepatic metabolism via oxidation, reduction, and conjugation pathways before being excreted primarily through urine and breath [52]. Whole-body autoradiographic studies have shown that sulfur-containing metabolites distribute to various organs, including the liver, lungs, and mucosa of the airways, with preferential deposition in cartilage tissue [52].

Studies have demonstrated that allicin interacts with drug metabolism pathways, particularly cytochrome P450 enzymes. Allicin was found to inhibit CYP2C19 activity, leading to increased plasma concentrations of omeprazole in individuals carrying CYP2C19*1 alleles [103]. However, it did not significantly affect CYP3A4 activity, suggesting selective modulation of drug metabolism enzymes [103]. This finding is critical for understanding potential herb-drug interactions, especially in patients undergoing pharmacological treatments that involve CYP2C19-dependent metabolism.

In human clinical studies, bioavailability of allicin from garlic supplements varies widely depending on formulation and dietary factors. Enteric-coated garlic tablets exhibit bioavailability ranging from 36 to 104%, which is reduced when taken with high-protein meals due to delayed gastric emptying [54]. The primary marker for systemic distribution of allicin is allyl methyl sulfide (AMS), which is excreted through breath and urine, reflecting the body’s metabolism and clearance of allicin-related compounds [54].

Metabolic fate refers to allicin being transformed into a variety of sulfur-containing compounds that are easily detectable in body fluids and exhaled air, thus serving as indicators of systemic distribution [54]. Efforts to increase stability and bioavailability have led to investigations into delivery systems [7]. This places nanotechnology at the forefront in the development of allicin-loaded nanoparticles, liposomes, and other encapsulation techniques for delivery systems [76]. The delivery system is targeted to protect allicin from degradation so that it can reach tissue targets in an active form [80]. For example, the nanoencapsulation of allicin in chitosan nanocarriers results in improved stability and delayed release, which improves its in vivo anticancer activity in preclinical models [88]. Furthermore, prodrug approaches are being considered, whereby stable allicin precursors can be administered and reconstituted within the body to an active form. Such approaches not only improve the pharmacokinetic profile of allicin but also, more importantly, can reduce problems arising from its instability and pungency, thus providing a better candidate for further clinical application. Novel delivery strategies, including nanoencapsulation and controlled-release formulations, are being explored to enhance stability, bioavailability, and targeted delivery for clinical applications in GI cancer therapy [27].

Allicin as a natural therapeutic agent for major gastrointestinal cancers

According to global cancer statistics from 2020, cancers of the gastrointestinal tract, including colorectal, liver, gastric, esophageal and pancreatic cancers, remain primary reason of deaths worldwide. The incidence and mortality rates of gastrointestinal malignancies continue to rise, and this has developed a major worldwide healthiness concern that extremely endangers wellbeing [40, 111]. Currently available treatment options for cancers of the digestive system mostly include surgery, radiation, chemotherapy, and immunotherapy. These approaches have several drawbacks, including poor prognosis, drug resistance, recurrence risk, and high cost [89]. Thus, novel therapeutic approaches are needed to treat stomach cancer effectively. Numerous Study has been focused on natural products to reduce the negative possessions of important contemporary medications. Allicin (diallyl thiosulfinate) is a compound that is generated when a garlic clove is crushed [81, 93]. The antitumour characteristics of diallyl thiosulfinate and its metabolites, which are involved in the biological activities of organosulfur compounds (OSCs), have attracted much attention in regard to tumours of the gastrointestinal tract [33].

Organosulfur compounds inhibit angiogenesis, metastasis, and tumour invasion in addition to slowing tumour growth and inducing apoptosis to prevent stomach cancers [81, 111]. Moreover, these OSCs have been demonstrated in certain situations to increase the effectiveness of chemotherapeutical medications and decrease the undesirable impact of conventional pharmaceuticals [111]. According to Ommen et al. [72] and Zhou et al. [111], allicin can initiate malignance cells to generate DNA ladders and programme cell death triggering bodies. Human cells exposed to allicin may undergo redox alterations, which trigger the mitochondrial apoptotic pathway [12, 67]. Furthermore, studies have demonstrated that the apoptosis that allicin causes in colon cancer cells can be mediated by NF-E2-related factor-2 (Nrf2), which is generally considered to be an antiapoptotic factor that enhances the expression of the antiapoptotic protein Bcl-2 [70, 93].

Alliin, the precursor of allicin, has the ability to inhibit the growth of a human gastric cancer cell line. Furthermore, it has no influence on the growth of healthy intestinal cells when it causes stomach cancer cells to undergo apoptosis [63]. Furthermore, DADS (diallyl disulfide is a sulfur-based anticancer drug generated from garlic) inhibits human esophageal xenograft tumors via a number of mechanisms, including mitochondria-dependent pathways and the RAF/MEK/ERK pathway [105]. DATS prevents the progress of cancer cells, stops the cell division and prevents angiogenesis to combat tumours [64, 99]. By preventing proliferation, inducing cell death and promoting the synthesis of peroxide, caspase-3-like and caspase-8 can stop the progress of cancer cells [9]. Additionally, two crucial secondary metabolites of allicin that are anticancer agents are SAMC and SAC, which function as antitumour agents by causing cell death and preventing the progress of cancer cells [101].

The allicin-mediated apoptotic pathway in gastric cancer cells is depicted in Fig. 2. Allicin can reduce the production of TGF-β2 and its receptor after directly entering gastric cancer cells. It induces oxidative stress by generating reactive oxygen species (ROS), leading to DNA damage and activation of key apoptotic mediators such as phospho-p53 and p21 [81]. Additionally, cyclin D1, cyclin E, and cyclin-dependent kinases (CDKs) can all be inhibited by allicin. This inhibition contributes to cell cycle arrest at the G2/M phase by suppressing the CDK-4/6/cyclin D complex, as well as P21-PCNA and P21-CDK2 interactions, ultimately reducing the cdk1/cyclin B1 complex. By lowering the outer mitochondrial membrane potential (MMP), allicin raises levels of nuclear factor kappa B (NF-κB), the proapoptotic protein Bax, while decreasing the antiapoptotic protein Bcl-2, which leads to apoptosis. Figure 2 illustrating the cellular effects of allicin, including its role in inducing ER stress, modulating TGF-β2 expression, and regulating the cell cycle. Allicin-generated ROS contribute to DNA damage, which activates downstream signaling pathways such as p21 and Bax/Bcl-2, leading to apoptosis. Additionally, allicin influences MAPK signaling, enhancing caspase activation and apoptosis-inducing factor (AIF)-mediated cell death.

Furthermore, Bax/Bcl-2 can also halt the cell cycle by increasing phospho-p53 levels. Through activation of the JNK signaling pathway, allicin can promote JNK-Jun phosphorylation, which inhibits tumor cell proliferation. Allicin also induces activation of the p38 mitogen-activated protein kinase (MAPK) pathway, increasing Fas receptor expression and its binding to Fas ligand (FasL), leading to apoptosis through caspase-8 and cytochrome c activation. This dual mechanism enables allicin to initiate both intrinsic and extrinsic apoptotic pathways [81].

Allicin induces apoptosis through both caspase-dependent and caspase-independent mechanisms. It promotes the release of cytochrome c from mitochondria into the cytosol, which triggers caspase-9 and caspase-3 activation, leading to poly (ADP-ribose) polymerase (PARP) cleavage and DNA fragmentation. Additionally, increased mitochondrial membrane permeability allows for translocation of apoptosis-inducing factor (AIF) and endonuclease G (Endo G) into the nucleus, leading to caspase-independent apoptosis. Furthermore, allicin elevates intracellular free Ca²⁺ levels, causing endoplasmic reticulum (ER) stress, which plays a critical role in apoptosis induction [81].

Allicin also mitigates the adverse effects of certain anticancer therapies and enhances their efficacy. For instance, by activating the Nrf2 pathway via KLF9, allicin protects against arsenic trioxide-induced liver damage, a major concern in acute promyelocytic leukemia therapy [102] (Fig. 3). Additionally, allicin has shown promise in reducing hepatotoxicity caused by tamoxifen (TAM), a commonly used treatment for hormone-dependent breast cancer [90]. Table 1 lists the benefits of allicin against gastrointestinal malignancies. Furthermore, multiple epidemiological studies have investigated the effects of Allium vegetable consumption on digestive system malignancies. Some studies indicate that high intake of these vegetables is associated with a lower risk of gastrointestinal cancers. However, further clinical research is needed to confirm these protective effects.

Fig. 3.

Schematic diagram illustrating the cellular effects of allicin, highlighting its role in inducing ER stress, leading to the activation of TGF-2, Cyclin D1 and E, which contribute to stop cell cycle. Allicin also generates ROS triggering DNA damage and trigger downstream signaling, including the stimulation of P21 and Bax/Bcl2 and the inactivation of TGF-2

Table 1.

The advantages of allicin against gastrointestinal malignancies

Cancer	Ingredient used	Amount	Advantage	Cell lines type	References
Gastric cancer	Allicin	6 mg/L and 12 mg/L	Stop cell cycle (G2/M phase), prevent cell division, and trigger apoptosis	SGC-7901 Cell lines	[92]
	Allicin	15–120 μg/ml	prevent cell division, and trigger apoptosis	SGC-7901 Cell lines	[108]
	Allicin	0.1, μg/ml and 10 μg/ml	prevent cell division, and trigger apoptosis	MGC-803, BGC-823 and SGC-7901 Cell lines	[109]
	Allicin	0.016 mg/ml	Allicin can inhibit telomerase activity and induce apoptosis of gastric cancer SGC-7901 cells. Allicin may be more effective than AZT	SGC-7901	[91]
Colorectal cancer	Allicin	25 μM	Stop the growth of tumours activation of the STAT3 pathway	HCT-116	[58]
	Allicin	0 to 256 μg/ml	Increase colon cancer cells' radiosensitivity by blocking the NF-κB signalling pathway	HCT-116 and CT26	[35]
	Allicin	0–1.2 mM	Diminish the vitality and proliferation of cells	HT-29	[38]
	Allicin	1.625–100 μM	When 5-FU and allicin are used together, they have a synergistic impact on colon cancer cells, and better outcomes can be achieved at IC50 with a lower dosage of 5-FU than when 5-FU is used alone	DLD-1	[94]
	Allicin	2.5–100 μg/ml	Increased the anticancer effects of oxaliplatin and 5-FU	Cell lines was Caco-2 and HT-29	[75]
Esophagel cancer	Allicin	0, 2.5, 5, 10, 20, 40, 80, 100 µg/ml	The apoptotic rate was significantly increased and accompanied by Cyt c release from the mitochondria to the cytosol, as well as Bax, caspase-3 and cleaved caspase-9 which were significantly increased	Eca109 and EC9706 cells	[18]
	Allicin	Na	LDH activity was decreased compared with 5-fluorouracil and cisplatin. The increased activity of caspase-3 and caspase-8 in allicin-treated cells was statistically significant, but caspase-9 activity changed without statistical significance	Eca109	[100]
	Allicin	Na	The mitochondrial membrane potential is significantly reduced, in the early apoptosis and late apoptosis percentage increased. Compared with the control group, the mRNA and protein expressions of Bax significantly increased, but the mRNA and protein expression of Bcl-2 significantly decreased	esophageal cancer EC-109	[44]
	Allicin	(0 μg/ml, 10 μg/ml, 20 μg/ml and 40 μg/mL	Conclusion Allicin prodrugs could effectively inhibit the proliferation of esophageal cancer cell line Eca9706,and control the proliferation of esophageal cancer cells by regulating the expression of apoptosis associated genes	Esophageal cancer cell line Eca9706	[19]
Liver cancer	Allicin	0–64 μg/ml	Encourage 5-FU’s antitumour action via the ROS-mediated mitochondrial mechanism	SK-Hep-1 and BEL-7402 Cell lines	[112]
	Allicin	Na	Allicin decreased the level of cytoplasmic p53, the PI3K/mTOR signaling pathway, and the level of Bcl-2 and increased the expression of AMPK/TSC2 and Beclin-1 signaling pathways in Hep G2 cells	liver cancer Hep G2 cells	[23]
	Allicin	35 μM	Induced apoptotic cell death in p53 knocked down Hep G2 cells similar to that of Hep 3B cells	HCC Hep G2	[22]
Pancreatic cancer	Allicin	10 mg/kg	Prevent the formation of tumours and lengthen their survival	BXPC-3	[96]
	Allicin	Na	Allicin-induced caspase-3 expression, DNA fragmentation, cell cycle arrest, p21Waf1/Cip1 cyclin-dependent kinase inhibitor expression, ROS generation, GSH depletion, and led to various epigenetic modifications which resulted in stimulation of apoptosis	MIA PaCa-2 cells	[20]

SGC Sichuan Gastric Cancer, MGC Metastatic Gastric Cancer, BGC Beijing Gastric Cancer, HCT Human Colorectal Tumor, HT Human Tumor, DLD-1 Duke’s Low Differentiated-1, Caco-2 Colon Adenocarcinoma-2, CT26 Colon Tumor-26, SK-Hep-1 Sloan-Kettering Hepatoma-1, BEL-7402 Beijing Liver-7402, BXPC-3 Bx Pancreatic Cancer-3

Esophageal cancer

About 456,000 new cases and 400,000 fatalities from esophageal cancer are recorded each year, making it the sixth most common cause of cancer-related deaths globally [25]. According to Arnold et al. [3], esophageal squamous cell carcinoma (ESCC) accounts for over 90% of esophageal cancer cases worldwide. Patients with ESCC have poor outcomes even when using multimodal therapy techniques such surgery, radiation, chemotherapy, and targeted therapy [97]. Therefore, in order to enhance these patients' prognosis, new treatment approaches must be investigated. Allicin pre-treatment can prevent myocardial ischemia/reperfusion injury [59]. Shi et al. [85] found that allicin can ameliorate high-fat diet-induced obesity in mice by altering their gut microbiome. However, the effectiveness of allicin in treating Esophageal Squamous Cell Carcinoma (ESCC) remains unknown. These findings may suggest new therapeutic options for ESCC.

Nelson et al. [69] conducted case–control research in China, to investigate the link between food and biliary tract cancer. The researchers analysed meal frequency questionnaires from 225 malignance cases, 190 extrahepatic bile duct cases, and 68 ampullae of Vater cases to identify 39 dietary clusters. According to Nelson et al. [69], allium food groups (onions, garlic, and shallots) are inversely correlated with gallbladder cancer (OR = 0.81, 95% CI 0.68–0.97). A study by Jin et al. [48] revealed uncooked garlic consumption may lower the risk of oesophageal cancer in China. Case–control research in India revealed that allium plants, including onion and garlic, can protect against gallbladder cancer [65].

Allicin’s effects and underlying molecular processes on esophageal squamous cell carcinoma (ESCC) are uncertain. An inquiry into the effect and mechanism of action of allicin in the ESCC cells Eca-109 and EC9706. The vitality and invasion ability of the two ESCC cells in the allicin group were considerably reduced following allicin treatment. Allicin decreased the percentage of G0/G1 and S phases while increasing the population of G2/M phase in a dose-dependent manner. Following allicin therapy, the expression levels of p53, p21, and CHK1 increased considerably while cyclinB dropped. The apoptotic rate increased dramatically, as did Cyt c release from the mitochondria into the cytoplasm, as well as Bax, caspase-3, and cleaved caspase-9. In vivo, there was also evidence of tumor growth suppression. These findings indicate that allicin may be a unique and effective anti-oesophageal cancer agent for the treatment of ESCC [18].

DADS significantly decreased cell viability in ESCC cells in a dose- and time-dependent manner, while normal liver cells showed lower toxicity, according to a study using the MTT assay. DADS experienced G2/M phase arrest, according to cell cycle studies. According to molecular analysis, the p53/p21 pathway’s activation and the decrease of cyclin B1, cdc2, p-cdc2, and cdc25c most likely caused this cell cycle arrest. Annexin V/PI staining was used to identify apoptosis. The molecular indicators demonstrated that DADS caused apoptosis through activating caspases, changing the Bax/Bcl-2 balance and inhibiting the MEK-ERK pathway. therefore DADS has the potential to be an effective and safe anticancer drug for ESCC therapy in the near future [105].

Allicin's cell apoptosis effect on human esophageal cancer EC-109 cells is investigated, as well as the molecular pathways involved. The results showed that when human esophageal cancer EC-109 cells were treated with allicin for 48 h, the mitochondrial membrane potential was dramatically lowered, and both early and late apoptosis percentages increased. Compared to the control group, Bax mRNA and protein expressions increased considerably (p < 0.05), while Bcl-2 mRNA and protein expressions fell dramatically (p < 0.05). As a result, the study proposes that allicin can cause apoptosis in human esophageal cancer EC-109 cells in a dose-dependent manner, making it a potential therapeutic option [44].

The apoptosis of EC109 cells induced by Z-VAD-FMK, allicin, allicin + Z-VAD-FMK, 5-fluorouracil, and cisplatin was investigated using flow cytometry with Annexin V-FITC and PI double labelling. The variations in enzyme activity of caspase-3, caspase-8, and caspase-9 were measured using spectrophotometry. Allicin inhibited the development of EC 109 cells and killed them in a concentration- and time-dependent manner. LDH activity was lower than with 5-fluorouracil and cisplatin. Caspase-3 and caspase-8 activity rose significantly in allicin-treated cells, whereas caspase-9 activity did not change significantly. Allicin reduces the development of EC109 cells in a concentration- and time-dependent manner via extrinsic apoptotic pathways initiated by caspase-8, with fewer adverse effects than 5-fluorouracil and cisplatin [100].

Gastric (stomach) cancer

Garlic consumption lowers the danger of stomach cancer, although data from two major prospective US cohort studies are equivocal [28, 49]. Garlic consumption does not appear to reduce the risk of stomach cancer, according to Kim et al. [49]. Three therapies were evaluated for their anti-gastric cancer effects in Linqu County, Shandong Province, China, in a blinded randomized placebo-controlled trial: supplementing with vitamin C, E, and selenium for 7.3 years (1995–2003); amoxicillin and omeprazole for 2 weeks; and garlic extract and oil. The trial included 3365 inhabitants of a high-risk area for stomach cancer [57, 58]. The anticancer properties of garlic vary depending on its preparation and processing methods, including raw, cooked, age, garlic residue, garlic oil, and garlic extract [34, 77]. Therefore, epidemiological research has led to inconsistent conclusions, which is natural.

Allicin inhibits the growth of gastric cancer cells by arresting the SGC-7901 cell line [5, 92]. An analysis of DATS's effects on the SGC-7901 cell line revealed its ability to arrest the cell cycle and inhibit the growth of these cells. Treatment with 200 μM DATS for 24 h resulted in a significant increase in the proportion of cells in the G2/M phase and a corresponding decrease in the G1 phase compared to control cells [46]. Similarly, a study on the human gastric cancer cell line BGC-823 demonstrated the anticancer properties of DATS. Jiang et al. [45, 46] reported that DATS impaired cell survival in the G2 phase by significantly upregulating cyclins A2 and B1.

These two routes specifically affect the release of cytochrome c from tumor cells, the positive regulation of Bax and Fas in tumor cells, and the increased stimulation of caspase [108]. Furthermore, allicin caused tumor cells to undergo programmed cell death in MGC-803 human gastric cancer cells by upregulating P38 expression via the P38 MAPK/caspase-3 signaling pathway [109]. These investigations demonstrated that allicin can cause stomach cancer cells to undergo apoptosis and the associated signal transduction pathway, but they were unable to identify the precise mechanism by which allicin causes this process.

In addition to TDER with protein thiols, DATS can also react with the cellular thiol glutathione to create H2S gas, which can control several other cellular functions [79]. By controlling proteins linked to apoptosis, triggering MAPK, and influencing the PI3K/AKT pathways in cells, DATS can cause cancer cells to undergo apoptosis [45]. These findings demonstrated that DATS enhanced the production of cytochrome C and P53, two indicators of apoptosis, in addition to downregulating the antiapoptotic protein Bcl-2 [45]. Moreover, in SGC-7901 cells, DATS activate three MAPK pathways, along with the ERK, JNK, and P38 pathways [45]. Because there is a clear sub-G1 peak in cell cycle analysis, it has also been proposed that the development repressive effect of DATS is correlated with cell death in another gastric cancer cell line, BGC-823 [46]. DATS markedly increased the level of Bax, P53, and cytochrome c and downregulated Bcl-2 [46]. According to this study, DATS can cause BGC-823 cells to undergo apoptosis by activating the JNK and P38-MAPK pathways, attenuating Nrf2/Akt, and activating the caspase pathway [71]. Furthermore, research in tumor-bearing mice has shown that ABGE might considerably reduce tumor growth due to its antioxidant and immunomodulatory actions. Moreover, ABGE has been shown to cause dose-dependent apoptosis in SGC-7901 human gastric cancer cells [50, 98].

Colorectal cancer

The evidence for diet, nutrition, and physical activity and colon cancer risk from the World Cancer Research Fund/American Institute for Cancer Research (WCRF/AICR) suggests that eating garlic may lower the risk of colon cancer; however, the updated evidence classifies this effect as “limited-no conclusion” [24]. A new meta-analysis of four cohort studies and eight case–control studies revealed that eating more garlic was linked to a lower incidence of colorectal cancer [110]. Allicin causes dose-dependent ROS induction in cultured cancer cells [82, 83]. Nevertheless, GSH in cancer cells neutralizes the ROS generated by allicin, and the sharp decline in GSH could enable excess allicin to interact with various thiol-containing components within the cell that are typically shielded by GSH [41]. According to Gruhlke et al. [38], allicin inhibited the proliferation of the HT-29 cell line and reduced the activity of human colon cancer cells in a concentration-dependent way. It has been shown to have a time- and dose-dependent cytostatic effect on the proliferation of HCT-116, LS174T, and Caco-2 colon cancer cell lines at dosages ranging from 6.2 to 310 μM [8].

The effect of CGE on the proliferation of human cancer cell lines and a mouse macrophage line (TIB-71), comprising hepatic (Hep-G2), colon (Caco-2), prostate (PC-3), and breast (MCF-7) cell lines, was examined in a distinct study [4]. Hep-G2, MCF-7, TIB-71, and PC-3 cells treated with 0.125, 0.25, 0.5, or 1 μg/ml CGE showed 80–90% suppression of cell growth [4]. But for Caco-2 cells, the inhibition was just 40–55%. In contrast, Caco-2 cells’ rate of proliferation inhibition in coculture studies with TIB-71 cells was 90%, but in separate cultures, it ranged from 40 to 55% [4]. By suppressing the expression of VEGF, u-PAR, and HPA mRNA, allicin stops LoVo human colon cancer cells from invading and spreading. Moreover, HPA and u-PAR can both encourage tumor invasion and metastasis. One important tumor angiogenesis factor connected to tumor growth, metastasis, and vascularization is VEGF [36].

Allicin may improve the cytotoxicity of CPT-11, a topoisomerase I inhibitor used as a first-line treatment for advanced or metastatic colorectal cancers [87]. Furthermore, it has been demonstrated that allicin enhances the efficacy of standard 5-FU and oxaliplatin chemotherapy (500 μM) in decreasing colon cancer cell viability while lowering the clinical treatment cost [75].

Pancreatic cancer

Allicin's anticancer effects were supported by an epidemiological study that linked reduced pancreatic cancer risk with increased consumption of allium plants [106]. Experimental studies demonstrated that diallyl trisulfide (DATS), a key component of garlic oil, induced apoptosis in both pancreatic cancer cells (Capan-2) and non-tumorigenic pancreatic ductal epithelial cells (H6C7). DATS was shown to arrest the cell cycle in the G2/M phase by reducing cyclin D1 levels while increasing cyclin B1 and p21 levels [62].

Additional evidence from [1] highlighted the protective effects of allicin in nitrosamines (NNK)-induced pancreatic cancer in albino rats. Allicin treatment reduced weight loss associated with NNK administration, prevented DNA damage as observed via agarose gel electrophoresis, and mitigated neoplastic hyperplasia in histological sections of pancreatic tissue. This study suggests that allicin can serve as a chemopreventive agent against NNK-induced pancreatic cancer when administered at appropriate concentrations. Furthermore, Wang et al. [96] explored the synergistic potential of allicin combined with recombinant interleukin-2 (rIL-2) in pancreatic cancer. Their study using pancreatic cancer xenograft models demonstrated that the combination therapy significantly suppressed tumor growth, prolonged survival time, and enhanced apoptosis in tumor cells compared to individual treatments. Mechanistically, the combined therapy activated immune responses by increasing CD4 + T, CD8 + T, and NK cell populations and elevating serum IFN-γ levels.

About 50% to 75% of pancreatic tumors have inactivation of the TP53 gene on chromosome 17p, and this inactivation almost usually results from an intragenic mutation coupled with the loss of the second allele. The G1-S cell cycle checkpoint, G2-M arrest maintenance, and apoptosis induction are just a few of the crucial roles that the p53 protein plays in the cell. When p53 function is lost, cells can proliferate and survive even when their DNA is damaged, which causes more genetic abnormalities to accumulate. Neoplastic hyperplasia was detected by histological assay and it was more noticeable in the histological sections of the rats' pancreas tissue that had not received allicin treatment. According to this study, at the proper dosage, allicin may be utilized to cure NNK-induced pancreatic cancer in white albino rats [1]. Several signaling pathways have been suggested as possible targets to prevent the spread of cancer, including cyclin-dependent kinases (CDKs), hypoxia-inducible factor-1 (HIF-1), NF-κBs, PI3K/Akt, insulin-like growth factor receptor (IGF-1R), and oestrogen receptor signaling cell growth [14]

A study by Namita Pandey et al. found that by blocking the ROS/MAPK pathway, allicin treatment (40 µg/ml) for NSCLC lowers the expression of HIF-1 and HIF-2 in hypoxic cells [73]. Through STAT3 signaling, a crucial transcription factor involved in proliferation, allicin (40 µM) suppresses the invasion and proliferation of Cholangiocarcinoma (CCA) cells in liver cancer. A specific allicin dose (60 µg/ml) may increase p53 expression by reducing IE2 protein levels, inhibiting glioma cell proliferation in the central nervous system, according to Zelin Yang et al. [104]. Allicin has been shown to increase the sensitivity of X-ray radiation therapy in colorectal cancer, presumably by suppressing the levels of NF-κB, IKKβ mRNA, p-NF-κB, and p-IKKβ protein expression in vitro and in vivo [42]. Additionally, allicin suppresses colorectal cancer cell proliferation by inhibiting NF-κB signaling. A study found that 10 µg/ml of allicin effectively inhibits the development and metastasis of gastric cancer cells by increasing the expression of miR-383-5p and decreasing the expression of ERBB4, p-PI3K, and p-Akt [61]. A study found that allicin at 50 µg/ml was more effective than cis-platinum at 40 µg/ml in treating oral tongue squamous cell carcinoma (OTSCC) patients in terms of cell proliferation and apoptosis [39]. Previous investigations have shown that allicin reduces cell proliferation and alters signaling pathways related to cell growth.

Liver cancer

In Jiangsu, China, a sizable population-based case–control study was carried out on the factors that can prevent liver cancer [95]. Between 2003 and 2010, 7,933 randomly chosen population controls and 2011 incident liver cancer cases were reviewed for the study. The study found that people who ate raw garlic twice a week or more had decreased incidences of liver cancer despite controlling established risk factors and possible confounders like drinking and viral infections. Furthermore, in their stratified analyses, the researchers found possible additive interactions between low intake of raw garlic and heavy drinking, hepatitis B surface antigen (HBsAg)-negative people, frequent drinkers, and patients who had eaten raw water or mold-contaminated food [60].

These findings were observed both in individuals who regularly consumed alcohol and in those with no family history of liver cancer. Liu et al. [60] proposed the use of raw garlic as a dietary intervention to reduce the incidence of liver cancer in the Chinese population. Supporting this, Chu et al. [23] demonstrated that allicin reduces the viability of the human hepatocellular carcinoma cell line HepG2 by inducing P53-mediated autophagy. P53, a well-established tumor suppressor protein, regulates multiple autophagy pathways by modulating key signaling molecules such as mTOR, AMPK, and TSC2, ultimately contributing to the suppression of tumor progression.

In HepG2 cells, allicin has been demonstrated to lower the levels of cytoplasmic P53, Bcl-2, and the PI3K/mTOR signaling pathway while increasing the expression of the AMPK/TSC2 and Beclin-1 signaling pathways. It has also been demonstrated that allicin causes mitochondria in cancer cells to degrade, suggesting that it encourages the autophagy of cancer cells [82]. As noted by Chu et al. [22] and Savairam et al. [82], allicin may control the transcription and translation of the p53 gene, causing autophagy in cancer cells or apoptosis in other cancer cells lacking p53.

Allicin and its secondary metabolites can protect cells from the damaging effects of carcinogens and chemotherapy. The antigenotoxic activity of several garlic-derived organic supercompounds (OSCs)—allicin, DAS, DADS, SAC, and AM—was assessed in the human hepatocellular carcinoma cell line HepG2 using the comet test. The genotoxicity of the direct-acting substances methyl methane sulfonate and hydrogen peroxide was reduced by all of the OSCs that were studied [11]. Allicin can also enhance the cytotoxicity that 5-fluorouracil (5-FU), an anticancer drug, induces in HCC cells via the ROS-mediated mitochondrial pathway, per a different study. This discovery may result in the creation of a brand-new chemotherapeutic treatment for HCC [112]. By suppressing STAT3 signaling, allicin has been shown to increase apoptosis in HCT116 cells, hence reducing cancer cell survival and proliferation. This effect has also been seen in the Azoxymethane Methane/Sodium Dextran Sulphate (AOM/DSS) model for colorectal cancer [57].

Safety aspects

The safety profile of allicin is critical when this compound is used therapeutically, especially for GI cancer therapy. Allicin, derived from garlic, has been part of the diet for centuries and is most likely safe for consumption. However, because of its strong biological activity, its possible toxicity and negative effects must be carefully considered. One randomized controlled trial indicated that high doses of allicin in sensitive individuals lead to sleeplessness, vomiting, heartburn, fainting, diarrhea, tachycardia, nausea, bloating, flushing, headache, mild orthostatic hypotension, sweating, offensive odor, and flatulence [29]. Garlic poisoning can occur 1 day or a few days after ingestion, depending upon the dose intake by the subject [55]. While low-dosage allicin helps increase the antioxidant status of mice, high dosages have been associated with toxicity. Indeed, according to Banerjee et al. [6], pathological changes were observed in the liver and kidney of rats that received a high dose of 1000 mg/kg/day garlic, which showed dose-related toxicity. Although high dosages may be responsible for the damage caused to the liver, therapeutic dosages might be responsible for minor gastrointestinal disorders [2, 78]. Owing to its nature as a reactive sulfur species (RSS), allicin easily traverses membranes and penetrates cells to readily oxidize cellular thiols, such as glutathione or cysteine residues in proteins [37, 66]. Such oxidation events of protein thiols can result in structural changes in proteins through the formation of disulfide bonds, which may then secondarily affect cell function and potentially contribute to cytotoxicity and toxicity in normal cells [37, 38]. Further research has revealed that prolonged intake of high doses of raw garlic is associated with considerable side effects, including weight loss and red blood cell lysis as a result of oxidative hemolysis [16]. In addition, the continuous intake of garlic powder reportedly results in the inhibition of spermatogenesis in rats [31]. Furthermore, it was reported that high doses of garlic juice caused hepatic and pulmonary toxicity in rats, together with loss of body weight. [68].

Conclusion and future perspectives

The bioactive compound allicin, derived from garlic, has been found to hold tremendous potential as a therapeutic agent in the treatment of GI cancers. Allicin induces apoptosis, inhibits cell proliferation, and modulates key signaling pathways that underpin its potential as a useful addition to the armamentarium of cancer therapies. Nanotechnology advancements and synergistic therapeutic approaches have significantly increased its clinical potential because of its instability and fast metabolism. These novel approaches increase the bioavailability of allicin, ensure targeted delivery, and reduce systemic toxicity, thereby increasing its therapeutic efficacy. Additionally, this synergistic effect has been shown to combine allicin with conventional chemotherapeutic agents in the context of drug resistance to chemotherapy and better treatment outcomes, probably with reduced side effects. Allicin, induces apoptosis by activating intrinsic and extrinsic cell death mechanisms, inhibits cell proliferation by interfering with cell cycle regulators, and suppresses angiogenesis, thereby restricting tumor growth. Additionally, allicin modulates key signaling pathways such as NF-κB, MAPK, and p53, which are crucial in cancer progression. However, its effects vary across different gastrointestinal (GI) cancers, with variations in its impact on metastatic potential, immune modulation, and oxidative stress response. These cancer-specific differences highlight allicin's potential for targeted therapeutic strategies tailored to distinct tumor types. While studies performed on animals have offered a promising foundation for the application of allicin in cancer treatment, many more clinical studies are needed. Clinical research should ultimately be performed to establish optimal dosing regimens and assess long-term safety and potential drug interactions. Such studies should further test its efficacy with other therapeutic agents to better elucidate its anticancer properties. These delivery systems based on nanotechnology could be further developed and head toward increasingly sophisticated systems to increase both the stability and bioavailability of allicin. Research on the molecular mechanisms underlying the anticancer effects of allicin could be pursued, and potential biomarkers for both sensitivity and resistance could be further investigated. Future studies could further substantiate new insights into the therapeutic potential of allicin by further elucidating its role in modulating the tumor microenvironment and immune response. In this context, allicin represents an exciting candidate for GI cancer treatment. If research continues and new delivery technologies evolve in the future, allicin will likely become standard for cancer therapy, providing hope for better outcomes and improvement in quality of life for patients.

Author contributions

A.C., S.D., V.M.P., S.S.B. and R.C. Writing—original draft; H.S.T., D.K., S.H. Writing—review & editing, Conceptualization, S.H., F.A., H.S.T. Writing—review & editing, Conceptualization.

Funding

No funding.

Data availability

No datasets were generated or analysed during the current study.

Declarations

Competing interests

The authors declare no competing interests.