Keywords

1 Introduction

Cancer remains one of the leading sources of morbidity and death worldwide, and an upsurge in cancer incidence is witnessed in the recent years as well. It is second behind cardiovascular diseases as the leading source of mortality in developed nations [1]. The development of malignancy is marked by uncontrolled and sustained proliferation, notwithstanding apoptosis, which can invade tissues and angiogenesis. Genetic alterations in the cell can produce unstable genetic makeover resulting in normal cells transforming into a malignant cell. These changes consist of alterations in tumor-suppressor genes, oncogenes, and DNA repair genes, which are involved in cell growth and differentiation. Some extrinsic elements (smoking, infectious agents, and radiation) as well as intrinsic causes (immune conditions, and hormones) are responsible for these mutations.

Carcinoma of the stomach stands as the top ten leading sources of carcinoma-related deaths. Studies have shown that factors such as exposure to cancer causing chemical agents and bacteria such as Helicobacter pylori are the two main causative factors resulting in the initiation of several events causing gastric carcinoma [2]. Infection by H. pylori can result in gastric mucosal infiltration with macrophages and neutrophil cells which further results in the production of harmful free radicals called reactive oxygen species (ROS) including superoxide and nitric oxide, which go on to cause gastric mucosal injury, ulcer, and eventually carcinoma [3]. Phytochemicals with antioxidant properties may help in protecting against carcinoma.

Consumption of fruits and herbal medicine is the most suitable and productive method of taking phytochemicals on a daily basis and in a cost-effective manner. In this chapter, we review the natural phytochemicals that possess antioxidant, anti-oncogenic, or anti-inflammatory properties that function via altering the course of action of different molecules involved in gastric carcinogenesis. We also discuss the newer targeted therapies extensively being studied for gastric cancer with special reference to tyrosine inhibitors that play critical roles in gastric cancer outcomes.

2 Curcumin

Turmeric plants (Curcuma longa) contain a bright yellow pigment called curcumin. Curcumin also happens to be the major curcuminoid present in turmeric. There is vast evidence about the numerous advantages of curcumin such as anti-inflammatory, antioxidant, and antitumor properties [4, 5]. The mode of action of curcumin is mainly mediated by targeting multiple intracellular pathways [6]. Over the years, inability to clear H. pylori infection has been implicated as the main cause of gastritis, ulcers, and eventually gastric carcinoma. Recently, it has been shown that curcumin is effective in arresting the growth of H. pylori [7]. The prospective therapeutic capabilities of curcumin have been assessed through several in vitro studies. The main reason for tumors being resistant to chemotherapy is the overexpression of nuclear transcription factor NF-κB. Curcumin can suppress the NF-κB effects, thereby augmenting chemotherapeutic drug effects. Yu et al. [8] in their studies showed enhanced activity of chemotherapeutic drugs (doxorubicin and etoposide) in combination with curcumin as compared to either of the drugs alone. Studies on the gastric cancer cell cultures (SGC 7901) in humans have shown that curcumin causes downregulation of NF-κB, which further results in the downregulation cell death inhibitor genes such as Bcl-2 and Bcl-xl via inhibition of NF-κB activity [8].

It has been shown previously that curcumin inhibits the receptor tyrosine kinase, EGFR, as well as downstream regulation of EGFR tyrosine kinase, p21-activated tyrosine kinase 1 (PAK1), without affecting its expression. In addition, curcumin causes mRNA suppression, which results in decreased production of cyclin-D1 protein and eventually results in the arrest of cell cycle progression in G1 phase. Consequently, curcumin can inhibit not only the cell proliferation but also the invasion of gastric carcinoma cells [9].

3 Isothiocyanates

Isothiocyanates (ITCs) are plant phytochemicals that are abundantly present in vegetables belonging to the family Cruciferae such as broccoli, Brussels sprouts, wasabi, radish, and turnips [3]. These occur in their inactive form, glucosinolates, and become reactive after coming in contact with an enzyme called myrosinase present in the oral cavity and small intestine. Some of the ITCs consist of aliphatic ITC (AITC) and sulforaphane (SFN) which are aliphatic compounds, and phenethyl ITC (pEITC) and benzyl ITC (BITC) which are aromatic.

Isothiocyanates have been shown to decrease the activity of enzymes that are responsible for the biotransformation of xenobiotics. The oxidative enzyme that facilitates phase one reactions is nicotinamide adenine dinucleotide phosphate (NADPH) cytochrome P450 reductase while glutathione S transferase (GST) and uridine 5′-diphospho (UDP)-glucuronosyltransferase (UGT) enzymes are responsible for second-phase reactions. It is believed that ITCs such as sulforaphane exert its antioxidant effects by stimulating Nrf-2-dependent enzymes, such as GST, thus guarding against the free radicals [10,11,12]. Sulforaphane can maintain the effect of enzymes such as NADPH , quinone oxidoreductase NQOI, and GST in Nrf-2-deficient mice when injected with foods containing large quantities of salt and H. pylori [3]. These results demonstrate the antioxidant potential of sulforaphane on the gastric mucosa.

SFN can also increase the Nrf-2-dependent antioxidant effect. A study by Fahey et al. [10] has shown that sulforaphane can inhibit the development of gastric cancer in the Institute of Cancer Research (ICR) mice which were treated with benzo[a]pyrene. These effects are thought to be facilitated by stimulation of phase two reactions of GST and NQOI, and also by an increase in the production of antioxidant enzymes, which are abolished by the deletion of Nrf-2 gene in the mice [10]. In patients with gastritis due to H. pylori, eradication of the bacteria augmented or reinstated the GST enzyme levels, which further reinforces the significance of antioxidants in preventing H. pylori-associated gastric tumors [13].

Sulforaphane also possesses chemopreventive activities against H. pylori infection [3]. During a study on people infected with H. pylori, 48 subjects were followed who ingested 70 g/d of broccoli (precursor of 420 μmol/L sulforaphane) for 8 weeks and were compared with placebo. The study reported a decline in H. pylori markers such as stool antigen and urease enzyme when compared to placebo. A decline in gastric inflammation markers such as pepsinogen 1 and 2 was noticed in broccoli group as well compared to placebo [14]. A study in C57BL/6 female mice that were infected with H. pylori and were kept on excessive salt regime also established the potential of sulforaphane in bacterial inhibition [14]. Consumption of sulforaphane-enriched broccoli in mice resulted in low levels of interleukin (IL)-1β and tumor necrosis factor (TNF)-α adding to the improvement of inflammation, decrease of bacterial colonization, and thus prevention of gastric corpus atrophy via high-salt diet. Interestingly , there were no effects of sulforaphane on Nrf-2 gene-depleted mice, suggesting that its mechanism of action is Nrf-2 dependent [14].

4 Resveratrol

Resveratrol belongs to the family of a polyphenol commonly found in red wine and red grapes. There has been a widely reported benefit of red wine with the inverse relation of cardiovascular diseases [15]. Studies have also shown beneficial effects of resveratrol on the neuronal cell death [16]. It is also believed that resveratrol is responsible for the benefits of red wine on cardiovascular diseases [17]. This prompted for extensive research on resveratrol in malignancies during the last 20 years focusing on its anti-inflammatory, antioxidant, and anticancer potential [18]. Exposure of gastric cells to H. pylori results in elevated IL-8 production and free radical production. Furthermore, H. pylori infection resulted in stimulation of gastric motility and phenotype alterations observed in cell lines through hummingbird effect [19]. It was also established that resveratrol possesses antibacterial activity against H. pylori infection, resulting in hampering of the H. pylori proliferation [20,21,22]. Treatment with resveratrol significantly reduced IL-8 expression, decreased free radical production, and suppressed the phenotype alterations in H. pylori-infected cells. These positive results explain the potential of resveratrol in gastric cancer treatment.

In vitro studies have shown that resveratrol can cause cell cycle arrest in the G0/G1 phase via inhibition of the kinase C-mediated processes and further stimulation of cell apoptosis. This cell cycle inhibition hampers the formation of RF-1 and KATO-III cells [23, 24]. Another critical mechanism by which resveratrol regulates the growth and expansion of gastric adenocarcinoma cells happens to be the MEK1/2-ERK1/2-c-Jun cascade. Studies have postulated that resveratrol causes MEK1/2-ERK1/2 phosphorylation downregulation, thereby further inhibiting c-Jun translocation into the nucleus, ultimately resulting in cell growth inhibition [25]. Furthermore, resveratrol (50–200 μmol/L) can also stimulate cell death by producing ROS in human gastric cancer SGC7901 cells. These effects of resveratrol could be overturned when tumor cells are treated with substances such as superoxide dismutase and catalase, which dilute the apoptotic process [26]. Resveratrol can cause cell death of transplanted tumor cells, most likely mediated by suppression of Bcl2 anti-apoptotic genes and cell death activation via Bax gene in an implanted gastric tumor cells in nude mice [27] (Fig. 9.1).

Fig. 9.1
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Schematic representation of the mechanisms of action of phytochemicals and pro-cancer/tumor formation agents

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5 Receptor Tyrosine Kinases (RTKs) and Tyrosine Kinase Inhibitors (TKIs)

Studies have shown that in gastric cancer , several receptor tyrosine kinases get stimulated and amplified. So, drugs aimed at controlling the RTKs can be beneficial in advanced gastric carcinoma people. RTKs occur as transmembrane glycoproteins that comprise a domain for ligand attachment extracellular, a motif for the tyrosine kinase, and another domain across the membrane [28,29,30]. The extracellular domain of the RTK helps in the identification of various subfamilies of the kinases. Binding of the corresponding ligands to the RTKs results in their activation via tyrosine molecule phosphorylation and further activation of cellular proteins [31]. Activated RTKs play a major regulatory role in a variety of cellular processes including proliferation, differentiation, migration, and survival [32]. When some of the bivalent ligands bind to two receptor molecules, it forms a dimer, which results in the activation of the kinases [33]. The activation of kinases is dependent on two key steps; the first step is augmentation of catalysis intrinsically while the second step consists of formation of protein attachment sites intracellularly, both of which are dependent on tyrosine autophosphorylation. While phosphorylation of tyrosine molecules near the enzyme’s activation loop upregulates kinase actions, phosphorylation of the enzyme adjacent to the membrane helps in the formation of anchors for the attachment of modules, which identify the phosphotyrosine molecules in precise patterns [34] (Fig. 9.2).

Fig. 9.2
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Selected tyrosine kinase inhibitors and their relationship with gastric cancer

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Tyrosine kinase receptors are classified into 21 groups, such as the epidermal growth factor receptor (EGFR), vascular endothelial growth factor receptor (VEGFR), fibroblast growth factor receptor (FGFR), and platelet-derived growth factor receptor (PDGFR) families [35]. In cancer cells, RTKs play a major role in various cellular processes such as growth, differentiation, and metabolism. Studies have also shown that monoclonal antibodies can inhibit the activation and overexpression of kinases in cancer cells. Some of the RTK inhibitors that became standard treatment options in various malignancies include trastuzumab in carcinoma of breast [36], gefitinib in lung cancer [37], and cetuximab in advanced colorectal carcinoma [38].

Studies have shown that RTKs exhibit various mutations and changes in gastric cancer patients. Mutations and overexpression of RTKs were observed in 37% of people diagnosed with gastric carcinoma [39]. The study has also reported the family of kinases that were amplified including KRAS in 8.8% of people with gastric cancer, FGFR2 in 9.3%, EGFR in 7.7%, and ErbB2 in 7.2% of the diagnosed patients. Moreover, upregulation of RTKs was found to be associated with patient prognosis; higher RTK levels correlated with inferior patient outcomes. Later, in a study by Morishita et al. [31], the levels of various RTKs (EGFR, FGFR1/2, ErbB2) were amplified in tumor cells in contrast with healthy gastric cells. The findings of these studies propose that drugs targeting kinase receptors can be beneficial in patients with gastric malignancies.

There are many monoclonal antibodies under various phases of clinical trials such as trastuzumab, cetuximab, and lapatinib, which are classified based on their ability of inhibition of various families of RTKs (Table 9.1). Below we summarize the biological and clinical applications of these monoclonal antibodies in gastric cancer.

Table 9.1 An updated summary of the current clinical trials studying tyrosine kinase inhibitors in gastric cancer
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5.1 HER-2 Inhibitors

5.1.1 Trastuzumab

Trastuzumab is a monoclonal antibody which targets HER-2 and causes inhibition of its downward signaling. ErbB2/HER cluster comprises four different receptors and HER-2 (ErbB2) is one among them. Studies have demonstrated that 10–38% of people with gastric malignancies present with amplification of HER-2; inhibition of HER-2 has demonstrated successful results in metastatic gastric carcinoma cases [40,41,42]. Nevertheless, the study was unable to show clear results regarding the relationship between HER-2 amplification and clinical outcome in advanced gastric cancer [43, 44].

Bang et al. [45] in the ToGA trial reported that patients who were HER-2 positive had a greater benefit [based on the immunohistochemistry (IHC) scoring system] when managed with trastuzumab. In 2006, the results of clinical trials on the effects of trastuzumab in late stages of gastric cancer got published. In the first phase II clinical trials, the combination of trastuzumab with cisplatin and docetaxel showed improved response on radiological findings in four out of five patients with advanced gastric carcinoma and cancer of gastroesophageal junction (GEJ) who are HER-2 positive [46]. Another phase II study was conducted in people who presented with metastatic gastric cancer or GEJ cancer and are HER-2 positive. During the study, the patients were put on a combination therapy consisting of trastuzumab (8 mg/kg loading dose followed by 6 mg/kg) and cisplatin (75 mg/m2) every 3 weeks until relapse. The study reported positive results in terms of patient response (35% out of 17 people) at the end of an average of two treatment cycles [47].

An open-label, global, phase III , clinical trial (the ToGA study) was conducted including many countries where the people with gastric cancer were randomly given either combination therapy with trastuzumab + chemotherapy or chemotherapy monotherapy. The study demonstrated positive results in terms of median overall survival (OS) in the combination therapy arm which was 13.8 months, whereas in the second group the median OS was only 11.1 months. The combination arm also showed an improved OS and progression-free survival (PFS); an increase in median survival by almost 2.7 months was witnessed in the trastuzumab group [45]. However, it was worth noting that trastuzumab was associated with an elevated probability of type 2 chemotherapy-related cardiac issues, which were managed by the removal of the antibody [48]. Nevertheless, the patients in the trastuzumab group did not report any positive results in terms of quality of life [49].

5.2 EGFR Inhibitors

EGFR amplification and activation happened in significant number of patients (27–64%) with gastric carcinoma, more predominantly for proximally located cancers. The EGFR amplification was correlated with poor prognosis as well [50, 51]. Other factors that were associated with EGFR overexpression were elderly people, infiltrative pathologies, and advanced cancers. Drugs that are aimed against EGFR include the following:

5.2.1 Cetuximab

Cetuximab is a monoclonal antibody (mAb) that inhibits EGFR. It also remains the most frequently studied EGFR inhibitor in people with gastric carcinoma. In gastric cancer patients, several studies were conducted to understand the impact of cetuximab. A total of six clinical studies reported that the combination therapy of cetuximab and chemotherapy resulted in positive results in terms of response rate (41–63%) and median OS (9–16.6 months) [52,53,54,55]. Contrary to this, the initial results from a phase II study in 2011 [56] showed that the combination therapy consisting of cetuximab and docetaxel + oxaliplatin failed to produce positive results. Later, a phase III clinical study (NCT00678535) [57] that examined the effects of cetuximab in combination with cisplatin + capecitabine also reported similar results in 2013. The combination therapy reported no benefit in terms of PFS (4.4 months in combination therapy vs. 5.6 months in chemotherapy alone). Furthermore, majority of the patients in the study (83% in combination therapy group and 77% in chemotherapy alone) suffered from adverse events including diarrhea, dermatitis, low potassium and magnesium levels, and hand-foot syndrome.

5.2.2 Gefitinib and Erlotinib

EGFR inhibitors, gefitinib and erlotinib, are frontline drugs used in the management of GEJ cancer. However, in patients with advanced gastric cancer, both the drugs failed to produce positive results when used as monotherapy during the phase II clinical trials [58].

5.2.3 Panitumumab

There are very few study results regarding the effect of panitumumab in advanced gastric carcinoma therapy. REAL3, a phase III clinical study [59], classified and studied esophagogastric carcinoma patients based on the treatment received; the first group received combination therapy consisting of chemotherapeutic agents epirubicin, oxaliplatin, and capecitabine (EOC) and panitumumab whereas the second set of patients received chemotherapy alone. The median overall survival in the chemotherapy-alone (EOC) group was 11.3 months whereas the median OS was 8.8 months in the combination (mEOC plus P) group. The patients in the combination therapy arm also suffered from drug side effects such as diarrhea, rash, mucositis, and neutropenia.

Matuzumab and nimotuzumab are other EGFR inhibitors that were tested in combination with chemotherapy during phase II clinical studies. Unfortunately, both the drugs produced unsatisfactory clinical outcomes in terms of PFS [60, 61].

5.3 Combined EGFR and HER-2 Inhibitor

Lapatinib is a dual inhibitor of RTKs acting on HER-2 and EGFR. The efficacy of lapatinib in gastric carcinoma patients was tested in a phase II trial where the drug demonstrated positive clinical outcomes in terms of overall reduction rate (ORR), which was 7%, and disease stabilization rate of 20%. Adverse effects included grade 4 fatigue (two patients) and vomiting [62]. In patients with metastatic gastric cancer, the efficacy of lapatinib is being tested in two current phase III clinical studies . In the first study (LoG-IC trial) [63], lapatinib is being tested as a frontline drug in combination with chemotherapeutic agents oxaliplatin and capecitabine. The second clinical trial (TYTAN) [64] is being conducted on Asian population who are diagnosed with HER-2-positive gastric cancer. In this trial, paclitaxel is tested as a second-line drug with/without lapatinib combination. The results of both these studies are expected to help establish lapatinib as an option for metastatic gastric carcinoma treatment.

5.4 VEGFR Inhibitor

Bevacizumab (Avastin) is a monoclonal antibody , which suppresses angiogenesis via inhibition of vascular endothelial growth factor-A (VEGF-A). In advanced gastric cancer patients, bevacizumab has shown an ORR of 42–67% and an OS of 8.9–16.2 months, during the phase II clinical trials. Adverse events consisted of grade 3–4 thromboembolic disease (25%) and gastric perforation (8%) [65,66,67]. The phase III clinical trial (AVAGAST) [68] focused on assessing the effectiveness of bevacizumab as second-line therapy in metastatic gastric cancer patients. Patients were classified into two groups; the first set of patients were put on bevacizumab in addition to frontline therapy with capecitabine-cisplatin whereas the second group received chemotherapy alone. The overall survival drastically enhanced after bevacizumab incorporation (46% vs. 37%) and the median PFS got notably prolonged as well (6.7 vs. 5.3 months). Moreover, the results varied based on the geographical location.

An increase in the OS was also seen in all the patients of American origin whereas no significant survival benefits were observed in the Asian and European patients. However, the authors reported a prognostic benefit with bevacizumab. These results could be due to alterations in patient selection, genetic variations within populations, and intake of second-line drugs in those patients. During the AVAGAST study, Ohtsu et al. stated that the prognosis of patients with metastatic gastric cancer could be associated with the levels of angiogenic factors , such as tumor neuropilin-1 and plasma VEGF-A [68, 69]. Unfortunately, patients in both the study arms experienced side effects such as anemia, neutropenia, and anorexia [68].

5.5 Dual Inhibitors of VEGFR and PDGFR

Sorafenib suppresses a variety of RTKs such as VEGF, PDGFR , and BRAF. Sunitinib causes inhibition of VEGFR, PDGFR, c-Kit, and Flt-3. Both the antibodies did not show any significant survival benefits in phase II studies [70,71,72] (Table 9.1).

6 Conclusion

Phytochemicals are abundantly found in fruits and vegetables and have been valuable in gastric cancer. While the combination of phytochemicals could augment antitumor effects on gastric cancer through multiple prevention mechanisms, additional translational and clinical outcome researches are necessary to greatly understand their potential benefits in cancer prevention and prognosis. In addition, several receptor tyrosine kinases are stimulated in gastric malignancies; therefore identification of kinase inhibitors can be potentially beneficial in providing tailored treatment to the patients. Several clinical trials are in development and are anticipated to provide benefits in clinical practice.