Garlic (Allium sativum) is a species of bulbous plant, and has been globally used as a culinary and medicinal herb for centuries. The medicinal properties of garlic have been extensively studied, with a focus on its bioactive sulfur-containing compounds, such as allicin, ajoene, diallyl sulfide (DAS), diallyl disulfide (DADS), diallyl trisulfide (DATS), dimethyl trisulfide (DMTS), S-allyl-cysteine (SAC) and S-propargyl-cysteine (SPRC). These compounds are considered to contribute to garlic's various therapeutic effects, including anti-inflammatory, antimicrobial, antioxidant and anticancer activities (1-4). Garlic oil, powder and extract are available as supplements for culinary uses, natural pesticides and medicinal purposes. Aged garlic extract (AGE), a processed form of garlic produced by aging garlic in an ethanol-water mixture for >10 months, has gained attention for its ability to enhance immune function and reduce the risk of chronic diseases (2,3). Furthermore, AGE is rich in beneficial organic sulfur compounds, which provide numerous health advantages. Numerous previous studies have shown that AGE has tumor-suppressive effects on various cancer cells, and its use has shown benefit in patients (5-9). Pancreatic cancer is one of the most aggressive and lethal forms of cancer, characterized by a poor prognosis and high mortality with <10% of patients surviving at five-years (10,11). The development of pancreatic cancer involves complex interactions between genetic and environmental factors, making it a challenging disease to prevent, diagnose and treat. Pancreatic ductal adenocarcinoma (PDAC) is the most common form of pancreatic cancer, accounting for >90% of all pancreatic malignancies. Genetics and lifestyle factors such as smoking, alcohol consumption and obesity are known as risk factors of PDAC; however the great majority of PDAC cases develop in individuals without known risk factors (10-12). Of note, increasing evidence points to bacteria as possible contributors to PDAC development (11,12). Recent clinical and experimental investigations have demonstrated the presence of an intratumor microbiome in PDAC and revealed oncological actions of some species of bacteria (11-17). Accumulated studies have investigated the potential role of garlic, AGE, and their bioactive compounds in the prevention and treatment of PDAC and related conditions, including pancreatitis and periodontitis (1,4,8,18,19). In the present review, it was aimed to summarize and discuss the potential beneficial effects of garlic and its compounds on PDAC, and to highlight the possible role of tumor-associated bacteria in these effects.

Natural agents derived from plants are intensely studied for the development of supplements, antibiotics and anticancer drugs. Among them, garlic is used for a food source as well as in traditional medicine. Garlic and AGE contain multiple pharmacologically-active sulfur compounds and have great potential to prevent and treat various types of cancers including PDAC (1,5-9). Numerous volatile and non-volatile garlic compounds were previously reported (1,20,21). A case-control study conducted in the San Francisco Bay area has reported that garlic and onion consumption correlated with lower odds of developing pancreatic cancer, suggesting that these vegetables may help in preventing the disease (19). A randomized double-blind clinical trial by Ishikawa et al (8) has reported that daily AGE intake with capsules containing 500 mg AGE for 3 months increased both the number and activity of NK cells in patients with advanced liver, pancreatic, or colon cancer. Although not focused on pancreatic cancer, a randomized intervention trial included the use of AGE as part of a blinded and placebo-controlled study. After a follow-up period of over 22 years, the study concluded that this supplementation significantly reduced the risk of death in patients with gastric cancer (9).

Apoptosis is a form of programmed cell death that plays a vital role in maintaining cellular homeostasis by triggering a series of biochemical events. Decreased expression of anti-apoptosis genes, including B cell lymphoma 2 (Bcl-2), and elevated expression of pro-apoptosis factors such as caspase-3 and Bcl-2 associated X-protein (Bax), are commonly observed phenomena in the apoptotic process. Additionally, other factors such as p21, p53 and cyclins are well-documented contributors to this process. In cancer, survival-related genes are often dysregulated, and the evasion of apoptosis is a hallmark allowing malignant cells to survive and proliferate uncontrollably (21-25). The induction of apoptosis through cell cycle arrest and the regulation of its associated molecules is a critical strategy for controlling cancer progression, as it efficiently promotes the elimination of cancer cells. Various substances found in dietary natural products, including garlic, have been shown to modulate the expression of common genes involved in cancer cell survival, and induce apoptosis and cell cycle regulation in cancer cells (1,26-32). The activity of garlic and its compounds has been evaluated in several in vitro studies (Table I).

Chemoresistance is a major obstacle in the effective treatment of cancers, and contributes to poor patient prognosis in PDAC (39,40). The mechanisms behind chemoresistance are highly complex, often involving alterations in cell survival pathways, efflux pumps and changes in the tumor microenvironment (39-41). Based on accumulated research findings, garlic and its derived bioactive compounds may improve therapeutic efficacy and potentially overcome chemoresistance through modulating multiple cellular pathways and inducing cell death (1,7,21,23). Several studies examined the effects of garlic, and garlic derivatives, on resistance to chemotherapeutic agents including those indicated in PDAC treatment, and the potential benefit of combining garlic or its compounds with these agents (Table III).

Several previous studies have revealed the presence of a diverse microbiome within human PDAC tissues (13,14,55-57). Geller et al (55) found the presence of Gammaproteobacteria in PDAC tissue specimens obtained from patients who exhibit resistance to gemcitabine treatment. Similarly, Riquelme et al (56) conducted an analysis of the PDAC microbiome, and found a distinct intra-tumoral microbiome signature in long-term survivors, comprising Pseudoxanthomonas, Streptomyces, Saccharopolyspora and Bacillus clausii. Additionally, a multinational research group investigated the oral and gut microbiomes of patients with PDAC, and found 4 enriched species (Streptococcus anginosus, Streptococcus oralis, Veillonella parvula and Veillonella atypica) and a depleted Faecalibacterium prausnitzii in the gut signatures of patients with PDAC across 3 countries, Japan, Spain and Germany. Notably, these 4 microbial species are known to reside in the oral cavity (58). These emerging findings suggest the potential impact of the microbiome on cancer biology, including its effects on cell phenotype, immune responses, tumor progression and treatment outcomes. However, of note, several bacterial species known to colonize the oral cavity have been identified in PDAC tissues (59). These include species associated with periodontitis, a condition reported to increase PDAC risk (11,12). Periodontitis is a prevalent inflammatory disease that affects the gingival tissue and alveolar bone, and is generally initiated by a range of pro-inflammatory factors produced by the host in response to a dysbiotic microbiome (4,60,61). Epidemiological studies have shown that oral pathogens, such as Porphyromonas gingivalis (P. gingivalis), Aggregatibacter actinomycetemcomitans (A. actinomycetemcomitans) and Fusobacterium species are linked to increased risk of pancreatic cancer (60,62-64). P. gingivalis is an anaerobe closely associated with the progression of periodontitis, and it has been recently demonstrated that P. gingivalis translocates to the pancreas from the oral cavity in mice, where it induces acinar-to-ductal metaplasia, a precursor lesion to neoplasia (14,15). Moreover, repetitive administration of P. gingivalis to mice expressing oncogenic Kras in the pancreas accelerated pancreatic intraepithelial neoplasia progression to PDAC (15). P. gingivalis was also shown to promote the growth of pancreatic cancer in vivo in xenograft models (14,65). In addition, a study reported the detection rate for Fusobacterium species in pancreatic cancer tissue to be 8.8%, and found higher mortality in the Fusobacterium-positive group (64). A study by Udayasuryan et al (16) found that Fusobacterium nucleatum (F. nucleatum) infection elicited normal pancreatic and PDAC cells to secrete cytokines including granulocyte macrophage colony stimulating factor and C-X-C motif chemokine ligand 1 (CXCL1). Conditioned medium from infected cells promoted non-infected cell proliferation, and motility of non-infected and infected PDAC cells (16). Moreover, an independent study has reported that intracellular F. nucleatum promoted PDAC progression via the CXCL1 and C-X-C motif chemokine receptor 2 axis, and F. nucleatum positive patients with PDAC had larger tumor size and worse survival rate (17). Collectively, PDAC harbors a microbiome including oral pathogens, and emerging evidence indicates that these bacteria may contribute to the initiation and progression of PDAC. Although the effects of garlic or its compounds on the PDAC microbiome are as yet unknown, these findings suggest that targeting tumor bacteria is an attractive potential therapeutic strategy in PDAC prevention and treatment.

Garlic and its derived products have antimicrobial properties on a wide spectrum of bacteria and fungi, suggesting that treatment with garlic products may impact PDAC indirectly through effects on the microbes associated with this cancer (Table IV) (4,66-78). Three cases will be considered to illustrate this point. First, in both human and mouse models of PDAC, a unique fungal community inhabits the pancreas (79-83). Malassezia species, including Malassezia globosa, were enriched in PDAC samples and associated with a poorer survival rate (79,80). Garlic extracts strongly inhibit the growth of Malassezia species in a dose dependent manner and with activity comparative to a known antifungal compound, ketoconazole (66). Second, garlic products were shown to be active against Helicobacter pylori (H. pylori), a spiral shaped bacterium that resides in the stomach lining and is a well-established risk factor for chronic gastritis, duodenal and gastric ulcers, and gastric cancer (84). The presence of H. pylori was also determined in the oral cavity and pancreas, and its prevalence was reported to have positive association with pancreatitis and periodontitis (84-88). Moreover, H. pylori infection is considered to increase risk of PDAC although the evidence remains debated (89-92). Garlic and garlic extract are bactericidal against H. pylori (68-70). In addition, ajoenes isolated from oil-macerated garlic extract and allicin inhibited H. pylori proliferation (68). In vivo, AGE alleviated H. pylori-induced gastritis, indicating that garlic extract could serve as an effective agent in preventing an inflammatory disorder caused by H. pylori infection (71). Thus, garlic activity against this pathogen may impact PDAC risk. Finally, evidence that garlic products are active against oral anaerobic bacterial species associated with periodontal disease may also link garlic's antimicrobial activity to inhibition of the progression of PDAC (75-78). Bachrach et al (75) reported inhibitory effects of allicin on the periopathogenic species A. actinomycetemcomitans, F. nucleatum and P. gingivalis. It was also demonstrated that allicin can reduce the activity of P. gingivalis proteases, known as major virulence factors of this bacterium (75). In addition, pharmacological studies have shown that the aqueous extract of garlic inhibited the proliferation of P. gingivalis, F. nucleatum and A. actinomycetemcomitans, and blocked the proteolytic activity of P. gingivalis proteases (76,77). The garlic compound DAS also inhibits the proliferation and biofilm formation of A. actinomycetemcomitans (78). Finally, a randomized controlled double-blind study has reported that daily intake of an AGE product containing 300 mg of AGE powder for 18 months reduced the level of probing pocket depth, indicating that AGE can prevent and improve the progression of periodontitis (18). Taken together, garlic products such as AGE have great potential to modulate microbial and inflammatory contributors to PDAC, and thereby reduce the risk of PDAC development.

PDAC remains among the most difficult of cancers and there is an urgent need for improvements in early diagnosis and effective therapy. Garlic, along with its derived compounds and products, holds great potential to contribute to the prevention and treatment of PDAC by exerting direct effects on tumor growth and indirect effects on tumor-associated microbes (summarized in Fig. 1). However, to explore the full potential of garlic in managing PDAC, further mechanistic studies, and pharmacological experiments aimed at optimizing the delivery of bioactive compounds to the pancreas, are necessary. The antimicrobial potential of garlic against intratumor microbes, and in particular intracellular bacteria, is an exciting area for exploration, along with garlic's contribution to the balance of reactive oxygen species in PDAC, and to the tumor immune microenvironment. These studies will determine how to integrate garlic products in the future treatment toolbox for PDAC.