Gastric Cancer: New Drugs – New Strategies

Abstract

Background

Gastric cancer is the second most common cause of cancer-related deaths worldwide. There are large geographic variations in the incidence of these tumors, with 60% occurring in East Asia. For patients with resectable disease, surgery and perioperative treatment can be effective. For patients with advanced gastric cancer, chemotherapy regimens result in a median survival of 9-11 months. In general, the prognosis for advanced disease is poor and 5-year overall survival rates are around 15%. Combination therapies yield better survival rates, albeit with increased toxicity. Therefore, more effective and less toxic treatment regimens are needed.

Summary

The molecular aberrations that characterize the different subgroups of gastric cancer have been used as therapeutic targets. However, the heterogeneity and complexity of gastric cancers is a major challenge for the development of effective targeted therapies. This review examines the main molecular targets in the treatment of gastric cancer, namely the vascular endothelial growth factor (VEGF), human epidermal growth factor receptor 2 (HER2), hepatocyte growth factor (HGF)/c-Met, epidermal growth factor receptor (EGFR) and phosphoinositide 3-kinase (PI3K)/Akt pathways.

Key Message

The molecular aberrations characteristic of gastric cancer are being explored for the development of targeted therapies, including the VEGF, HER2, HGF/c-Met, EGFR and PI3K/Akt signaling pathways.

Practical Implications

Trastuzumab, an antibody which targets HER2, is the first approved targeted therapy for the treatment of gastric cancer. However, trastuzumab is only effective in HER2-positive tumors (about 10-20% of all gastric cancers). Ramucirumab, which targets the VEGF receptor 2, has yielded benefits with respect to overall survival in a phase III trial and is an effective treatment for advanced gastric cancer with approval in second-line treatment. Apatinib and rilotumumab are another two promising new agents currently under development.

Introduction

Gastric cancer is the fourth most common cancer worldwide. About 600,000 men and 330,000 women were diagnosed with gastric cancer in the year 2002 worldwide [1]. Furthermore about 700,000 deaths occur due to gastric cancer, making it the second most common cause of cancer death worldwide. There are high geographic differences in prevalence, with 60% of gastric cancers arising in East Asia [2]. Apart from the decreasing incidence of gastric cancer in Western countries, the frequency of adenocarcinomas of the gastroesophageal junction (GEJ) is increasing [2,3]. Surgery and perioperative treatment are potentially curative for patients with resectable cancer. The prognosis for advanced gastric cancer remains poor, the 5-year overall survival (OS) rates being approximate 15% [4,5]. Chemotherapy remains the main treatment for patients with advanced disease and targeted therapy might be an option to improve the prognosis [6].

Targeted therapy depends on the presence of targets and is influenced by the heterogeneity of gastric cancer, which can be divided into several subgroups (histological, anatomical, epidemiological and molecular classifications) [7,8,9,10]. The overexpression of human epidermal growth factor receptor 2 (HER2) is more prevalent in proximal gastric cancer [11], similar to epidermal growth factor receptor (EGFR) expression (30-60% of proximal tumors) [12]. Also c-Met amplification occurs more frequently in gastroesophageal cancer [13]. Distal non-diffuse tumors are often related to chronic Helicobacter pylori infection [9]. This subtype expresses high vascular endothelial growth factor (VEGF) level [14]. Further molecular aberrations, including fibroblastic growth factor receptor 2 (FGFR2) signaling and phosphoinositide 3-kinase-Akt-mammalian target of rapamycin (PI3K/Akt/mTOR) pathway, have been described [15,16,17]. These multiple molecular alterations can therefore be considered as potential targets for specific biomolecular treatments. Recent data divided gastric carcinoma into five subgroups based on genomic amplifications: FGFR2 (9.3%), KRAS (8.8%), EGFR (7.7%), ERBB2 (7.2%) and c-Met (4%). These subgroups suggest that at least 37% of gastric cancer patients may be treatable by receptor tyrosine kinase/RAS-associated therapies [18].

Monoclonal antibodies as well as tyrosine kinase inhibitors and mTOR inhibitors have been administered to patients with gastric cancer in various clinical trials. However, molecular targeting therapy is actually less effective in gastric cancer compared to other cancers such as colorectal or breast cancer.

The ToGA (Trastuzumab for Gastric Cancer) trial confirmed that in HER2-positive inoperable gastric and GEJ cancers, trastuzumab plus cisplatin and either capecitabine or fluorouracil resulted in improved OS compared with chemotherapy alone [19]. This strategy has been approved as the standard regimen in HER2-positive patients. Ramucirumab was recently approved in gastric cancer based on these data in second-line setting. However, the approval of further targeted agents has been a challenge.

Anti-VEGF/VEGFR Agents

Angiogenesis is an important aspect of tumorigenesis. Vascular endothelial growth factor A (VEGF-A) plays a central role in angiogenesis [20]. The activity of VEGF-A is mediated by two tyrosine kinase receptors, VEGFR-1 and VEGFR-2. VEGF enhances the permeability of tumor vessels [21], induces serine protease or metalloproteases [22,23], inhibits apoptosis in endothelial cells [24,25] and inhibits dendritic cell maturation [26].

Bevacizumab

Bevacizumab is a monoclonal antibody targeting VEGF-A, which has shown activity in several solid tumors (i.e. colorectal cancer, breast cancer, non-small-cell lung cancer and glioblastoma). It binds to VEGF, preventing its interaction with VEGFR-1 and VEGFR-2. In patients with gastric cancer, VEGF expression has been linked to tumor aggressiveness [27] and poor prognosis [12].

In a multicenter phase II study, bevacizumab (15 mg/kg on day 1) plus platinum-containing chemotherapy had promising efficacy. The response rate was 65% (95% CI 46-80) and the median OS (mOS) was 12.3 months (95% CI 11.3-17.2) [28]. In a further phase II trial, bevacizumab (7.5 mg/kg) in addition to chemotherapy with docetaxel (70 mg/mq) and oxaliplatin (75 mg/mq) was administered in 38 patients. A disease control rate of 79% was reported, with a progression-free survival (PFS) of 6.6 months and an OS of 11.1 months [29].

Based on these data the AVAGAST study was initiated. 774 patients with previously untreated locally advanced or metastatic gastric cancer/GEJ cancer were included. Patients were treated with capecitabine (1,000 mg/mq twice daily for 14 days every 3 weeks) and cisplatin (80 mg/mq) in combination with either bevacizumab (7.5 mg/kg) or placebo. mOS was 12.1 months with bevacizumab and 10.1 months with placebo (hazard ratio [HR] = 0.87; 95% CI 0.73-1.03; p = 0.1002). Median PFS (mPFS) was 6.7 vs. 5.3 months, respectively (HR = 0.80; 95% CI 0.68-0.93; p = 0.0037) and overall response rate was 46.0 vs. 37.4% (p = 0.0315) [30]. Geographic differences in efficacy were observed in a subgroup analysis of AVAGAST. Patients enrolled in North and Latin America appeared to have a survival benefit with bevacizumab (median 11.5 vs. 6.8 months), whereas patients enrolled in Asia (90% from Japan and Korea) appeared to have no benefit. AVATAR, a study similar in design to AVAGAST, was initiated in Chinese patients with advanced gastric cancer. In total, 202 patients were included. The primary results did not show a difference in OS for bevacizumab compared to placebo (HR = 1.11; 95% CI 0.79-1.56; p = 0.5567). mPFS was also similar in both arms [31]. Further investigation with respect to benefit in subgroups is needed.

Ramucirumab

Ramucirumab is a fully humanized IgG1 monoclonal antibody specifically blocking the extracellular domain of VEGFR-2 [32]. A phase I/II trial showed promising clinical anti-tumor effects and tolerability. Four (15%) of 27 patients with measurable disease had partial response, and 11 (30%) of 37 patients had either partial response or stable disease lasting at least 6 months [33].

REGARD, an international randomized double-blind placebo-controlled phase III trial, showed benefit of monotherapy in patients with advanced gastric cancer. 355 patients were included with disease progression during first-line therapy. Patients were randomly assigned with a 2:1 ratio to receive best supportive care plus ramucirumab 8 mg/kg or placebo. mOS was 5.2 months (IQR 2.3-9.9) in patients in the ramucirumab group and 3.8 months (IQR 1.7-7.1) in those with placebo (HR = 0.776; 95% CI 0.603-0.998; p = 0.047). PFS was 2.1 months in the ramucirumab group vs. 1.3 months in the placebo group (HR = 0.483; p < 0.0001). The rates of serious adverse events were similar between arms [34].

The RAINBOW trial is a randomized multicenter double-blind placebo-controlled phase III study testing paclitaxel (80 mg/kg on days 1, 8, 15, every 4 weeks) with or without ramucirumab (8 mg/kg i.v. infusion on days 1 and 15 every 4 weeks) in patients with advanced gastric or GEJ adenocarcinoma after first-line chemotherapy. The study randomized 665 patients. mOS was 9.6 months for the combination vs. 7.4 months for paclitaxel alone (p = 0.0169). mPFS was 4.4 and 2.9 months, respectively (p < 0.0001) [35].

Ramucirumab was currently approved in gastric cancer based on these data in second-line setting.

Sunitinib

Sunitinib is an oral, multitargeted tyrosine kinase inhibitor of VEGF receptors, platelet-derived growth factor receptors, KIT and several other related receptor tyrosine kinases [36]. Sunitinib has received approval for the treatment of advanced imatinib-resistant/-intolerant gastrointestinal stromal tumors, advanced/metastatic renal cell carcinoma and unresectable or metastatic well-differentiated pancreatic neuroendocrine tumors.

A phase II study investigated the outcome of sunitinib monotherapy in pretreated patients with advanced gastric cancer (table 1). Sunitinib monotherapy appeared to be associated with a limited tumor response [37]. A second phase II study showed slightly better results for monotherapy of sunitinib (table 1) [38].

Table 1.

Clinical trials with anti-VEGF/VEGFR agents for advanced gastric cancer and GEJ cancer

Trial	Phase	Setting	Regimen	Patients, n	OS, months	TTP/PFS, months
NCT00084604	II	1st	Iri/Cis/Bev	47	12.3	8.3
NCT00217581	II	1st	Doc/Ox/Bev	38	11.1	6.6
AVAGAST	III	1st	Cis/Cap/Bev vs.	774	12.1 vs. 10.1 (p = 0.1002)	6.7 vs. 5.3 (p = 0.0037)
			Cis/Cap/placebo
AVATAR	III	1st	Cis/Cap/Bev vs.	202	10.5 vs. 11.4 (p = 0.56)	6.3 vs. 6.0 (p = 0.47)
			Cis/Cap/placebo
REGARD	III	2nd	Ram + BSC vs. BSC	355 (2:1)	5.2 vs. 3.8 (p = 0.047)	2.1 vs. 1.3 (p < 0.0001)
RAINBOW	III	2nd	Pac + Ram vs.	665	9.6 vs. 7.4 (p = 0.017)	4.4 vs. 2.9 (p < 0.0001)
			Pac + placebo
NCT00411151	II	2nd	Sun	51 (ITT)	5.8	1.3
NCT00226811	II	2nd	Sun	78	6.8	2.3
NCT01238055	II	2nd	Doc + Sun vs. Doc	107	-	3.9 vs. 2.6 (p = 0.206)
NCT00970138	II	3rd	Apa 850 mg daily vs.	141	4.8 vs. 4.3 vs. 2.5 (p < 0.001/p = 0.0017)	3.7 vs. 3.2 vs. 1.4 (p < 0.001)
			Apa 425 mg b.i.d. vs. placebo

Apa = Apatinib; Bev = bevacizumab; BSC = best supportive care; Cap = capecitabine; Cis = cisplatin; Doc = docetaxel; Iri = irinotecan; ITT = intention to treat; Ox = oxaliplatin; Pac = paclitaxel; Ram = ramucirumab; Sun = sunitinib; TTP = time to progression.

A randomized phase II trial investigated the effects of docetaxel and sunitinib in second-line treatment after pretreatment with fluoropyridine and platinum. Patients were assigned to either docetaxel monotherapy or a combination of docetaxel and sunitinib. The time to progression was not significantly prolonged in the docetaxel plus sunitinib arm (table 1). Patients in the docetaxel plus sunitinib arm had more frequently stomatitis, diarrhea and hand-foot syndrome [39].

In advanced gastric cancer sunitinib showed poor activity as a single drug as well as in combination in second-line setting.

Apatinib

Apatinib is a tyrosine kinase inhibitor selectively targeting VEGFR-2 [40]. A randomized phase II trial investigated apatinib in third-line setting in 144 patients with advanced gastric cancer. Patients were randomly assigned to receive placebo, apatinib 850 mg once daily or apatinib 425 mg twice daily. mOS was 2.5, 4.8 and 4.3 months, respectively. mPFS was 1.4, 3.7 and 3.2 months, respectively. There were statistically significant differences between the apatinib and placebo groups for both PFS (p < 0.001) and OS (p = 0.0017). The most common adverse effects were hypertension and hand-foot syndrome. Patients who received apatinib once daily had fewer grade 3-4 adverse events. Therefore, the dosing regimen of 850 mg once daily was recommended for later studies [41].

A randomized phase III trial for third-line therapy is actually comparing apatinib (850 mg/daily) to placebo (NCT01512745). The estimated number of patients is 270. PFS and OS are the primary endpoints and recruitment is ongoing.

HER2-Targeting Agents

The HER2 protein is a transmembrane tyrosine kinase and is composed of an extracellular ligand-binding domain, a transmembrane region and an intracellular domain with tyrosine kinase activity. The HER2 protein is a member of the EGFR family [42]. The activation of HER2 does not require ligand binding and induces a receptor dimerization that initiates phosphorylation cascades and activation of the PI3K/Akt/mTOR and Ras-Raf-ERK pathways [43].

Recent studies showed the role of HER2 in the development of several types of human cancer, including gastric cancer. HER2 is overexpressed in 10-38% of gastric cancer samples, with a higher prevalence in intestinal-type and proximal tumors than in diffuse-type and distal tumors [11].

Immunohistochemistry (IHC) should be used as the primary test for analyzing HER2 status; patients with scores of 3+ would be candidates for HER2-directed therapy, those with 2+ should be re-tested using fluorescence in situ hybridization (FISH), and FISH-positive patients would be eligible for trastuzumab with chemotherapy [44].

Trastuzumab

Trastuzumab is a humanized monoclonal antibody that binds to the extracellular domain of the HER2 receptor, blocks dimerization, induces antibody-dependent cellular cytotoxicity and increased endocytosis of the receptor [45]. These therapeutic effects are enhanced in combination with chemotherapy (i.e. cisplatin, capecitabine, irinotecan, doxorubicin and taxanes). To pick out one of the several phase II trials, Grávalos et al. [46] included 228 patients under first-line therapy with HER2 overexpression. Treatment consisted of trastuzumab (8 mg/kg on cycle 1 day 1 as loading; 6 mg/kg in subsequent cycles) and cisplatin (75 mg/mq) both i.v. on day 1, every 21 days. Median time to progression was 5.1 months.

The ToGA trial established trastuzumab as the first biological therapy with longer OS in patients with gastric cancer. It was a randomized phase III study of trastuzumab in combination with chemotherapy for patients with HER2-positive advanced gastric cancer and cancer of the GEJ. 594 patients were randomized to receive 5-fluorouracil (5-FU) (800 mg/m²/day on days 1-5 continuous infusion) or capecitabine (1,000 mg/m²/day on days 1-14) and cisplatin (80 mg/m² on day 1) with trastuzumab (8 mg/kg loading dose on day 1 followed by 6 mg/kg) every 3 weeks for 6 cycles, or chemotherapy alone. Of all patients screened for HER2, 22.1% were HER2-positive. The addition of trastuzumab to chemotherapy showed a significantly longer PFS (6.7 vs. 5.5 months, p = 0.0002) and a significantly longer OS (13.8 vs. 11.1 months, p = 0.0046). Patients with higher levels of HER2 expression (IHC score of 3+ or 2+ and FISH-positive) had the greatest benefit with an OS reaching 16 months. The safety profiles in both groups were similar [19].

Currently the HELOISE study is recruiting. This randomized open-label multicenter phase IIIb study will compare the efficacy and safety of two different trastuzumab dose regimens in combination with cisplatin/capecitabine. Patients for first-line treatment will be randomized to receive trastuzumab either at 8 mg/kg loading dose followed by 6 mg/kg every 3 weeks or at 8 mg/kg loading dose followed by 10 mg/kg every 3 weeks. 400 patients will be enrolled in this trial [unpublished data, [47]].

Trastuzumab emtansine (T-DM1) is an antibody-drug conjugate with trastuzumab and the cytotoxic antimicrotubule DM1 (derivative of maytansine). When T-DM1 binds to HER2, the receptors are internalized by endocytosis, with a consequent intracellular release of the active form of DM1, causing cell death. T-DM1 showed high efficiency in preclinical models of HER2-positive cancer [48]. Therefore a phase II/III trial is investigating the efficacy and safety of T-DM1 compared to standard taxane treatment in patients with HER2-positive advanced gastric cancer in second-line therapy. First-line therapy must include a combination of a platinum- and a fluoropyrimidine-based treatment. Patients will be randomized to one of three treatment arms (T-DM1 3.6 mg/kg every 3 weeks, T-DM1 2.4 mg/kg every week or standard taxane therapy with docetaxel or paclitaxel) [unpublished data, [49]].

Lapatinib

Lapatinib is an oral tyrosine kinase inhibitor for both EGFR and HER2 which could be used in trastuzumab-resistant breast cancer. One of several phase II trials tested lapatinib as a first-line therapy in 47 patients with advanced gastric cancer showing moderate single-agent activity, with a median time to treatment failure of 1.9 months and an OS of 4.8 months. Only 7% of patients showed partial response and 20% stable disease [50].

Two different phase III studies analyzed the efficacy of lapatinib in combination with chemotherapy in patients with HER2-positive gastric cancer. The LOGiC study (Lapatinib Optimization Study in HER2 Positive Gastric Cancer) is a phase III study to evaluate the efficacy and safety of lapatinib in first-line treatment. Patients received chemotherapy (oxaliplatin 130 mg/mq day 1; capecitabine 850 mg/m² b.i.d. days 1-14, every 3 weeks) plus daily lapatinib (1,250 mg) or placebo. 545 patients were randomized and 487 had HER2 positivity centrally confirmed. The primary endpoint was not met. OS in all randomized patients was 11.9 months in the lapatinib group vs. 10.4 months in the placebo group (p = 0.3244). PFS was 6.0 vs. 5.4 months, respectively [unpublished data, [51]].

The TYTAN trial combined lapatinib and paclitaxel in the second-line therapy of gastric cancer. The study included 430 HER2-positive patients (table 2). mOS was 11.0 months in the lapatinib group compared with 8.9 months with paclitaxel alone (p = 0.2088). In a subgroup analysis of the HER2 status, IHC 3+ patients had an OS of 14.0 and 7.6 months, respectively (p = 0.0176). PFS was 5.6 vs. 4.2 months (p = 0.0101) [52]. Based on these results, lapatinib may be an effective treatment against HER2 (IHC 3+) gastric cancers; however, further investigation is needed.

Table 2.

Clinical phase III trials with anti-HER2 agents for advanced gastric cancer and GEJ cancer

Trial	Phase	Setting	Regimen	Patients, n	OS, months	TTP/PFS, months
ToGA	III	1st	5-FU or Cap + Cis + Tras vs.	594	13.8 vs. 11.1 (p = 0.0046)	6.7 vs. 5.5 (p = 0.0002)
			5-FU or Cap + Cis
LOGiC	III	1st	Cap/Ox + Lap vs.	545	11.9 vs. 10.4 (p = 0.3244)	6.0 vs. 5.4
			Cap/Ox + placebo
TYTAN	III	2nd	Pac + Lap vs. Pac	430	11.0 vs. 8.9 (p = 0.2088); subgroup: 14.0 vs. 7.6 (p = 0.0176)	5.4 vs. 4.4 (p = 0.2441); subgroup: 5.6 vs. 4.2 (p = 0.0101)
JACOB	III	1st	Tras + 5-FU/Cis + Per vs.	780 (est.)	ongoing	ongoing
			Tras + 5-FU/Cis + placebo

Cap = Capecitabine; Cis = cisplatin; Lap = lapatinib; Ox = oxaliplatin; Pac = paclitaxel; Per = pertuzumab; Tras = trastuzumab; TTP = time to progression.

Pertuzumab

Pertuzumab is a humanized anti-HER2 antibody which inhibits the dimerization of HER2 and prevents ligand-dependent HER2 signaling. Because of different mechanisms of HER2 inhibition, the combination of pertuzumab and trastuzumab may be more effective. As has already been shown, the combination of pertuzumab and trastuzumab increases antitumor activity in HER2-positive human gastric cancer xenograft models [53].

For dose finding of pertuzumab the JOSHUA phase II trial was developed, evaluating two different doses in first-line setting of gastric cancer. Patients were randomized to receive either pertuzumab 840 mg for cycle 1 and 420 mg for cycles 2-6 or pertuzumab 840 mg every 3 weeks in combination with trastuzumab and cisplatin and capecitabine or 5-FU. The mean concentration was higher in patients with 840 mg pertuzumab continuously. Based on these data, a pertuzumab dose of 840 mg every 3 weeks was recommended [54].

The ongoing phase III JACOB trial is a double-blind placebo controlled randomized multicenter study to evaluate the efficacy and safety of pertuzumab in gastric cancer. Patients will be randomized to receive either pertuzumab 840 mg or placebo in combination with trastuzumab (initial dose of 8 mg/kg i.v. followed by 6 mg/kg i.v. every 3 weeks) and cisplatin and capecitabine or 5-FU for the first 6 treatment cycles. Patients will continue to receive pertuzumab or placebo and trastuzumab until disease progression or increased toxicity [unpublished data, [55]].

Hepatocyte Growth Factor-c-Met Pathway

The receptor tyrosine kinase c-Met and its ligand, the hepatocyte growth factor (HGF), are involved in the regulation of multiple cellular processes including cell proliferation, invasion and angiogenesis [56]. High c-Met expression is associated with poor prognosis [57]. c-Met amplification was shown in approximately 4-10% of gastric tumors [58] and c-Met protein overexpression by IHC was shown in about 50% of advanced gastric cancers [59]. There are several drugs that inhibit c-Met activity, some of which we would like to highlight below.

Foretinib

Foretinib is an oral multikinase inhibitor targeting c-Met. A phase II study evaluated the safety, tolerability and response of two dosing regimens (240 mg/day for 5 days every 2 weeks or 80 mg/day) in 74 patients with gastric cancer. 93% of the patients had received prior therapy. Best response was stable disease in 10 (23%) patients receiving intermittent dosing and 5 (20%) receiving daily dosing. Stable disease duration was 1.9-7.2 months (median 3.2). Treatment-related adverse events occurred in 91% of patients. The rates of hypertension (35 vs. 15%) and elevated aspartate aminotransferase (23 vs. 8%) were higher with intermittent dosing. These results indicate that single-agent foretinib lacked efficacy in unselected patients with metastatic gastric cancer [60].

Tivantinib

Tivantinib is a selective small-molecule c-Met inhibitor. In a phase II trial the activity of tivantinib was tested in 30 gastric cancer patients in second- or third-line setting. No objective response was observed. mPFS was 43 days (95% CI 29.0-92.0). Grade 3 or 4 adverse events occurred in 13 patients (43.3%). Tivantinib as a single agent showed modest efficacy in previously treated gastric cancer [61].

Crizotinib

Crizotinib is a potent c-Met inhibitor, targeting ATP binding sites of the c-Met kinase domain. Therefore, it might represent a potential drug for the treatment of gastric cancer with c-Met amplification. Crizotinib was recently approved for treatment of non-small-cell lung cancer. Treatment with crizotinib resulted in induction of apoptosis and inhibition of Akt in gastric cancer cells with c-Met amplification, but not in those without. Crizotinib exhibited a marked antitumor effect in gastric cancer xenografts positive for c-Met amplification, whereas it had little effect on those negative for this genetic change [62]. Clinical studies to evaluate in vivo efficacy should confirm these in vitro findings.

Rilotumumab

Rilotumumab is a human IgG2 antibody targeting human HGF/scatter factor that blocks the binding of HGF/scatter factor to its receptor c-Met. It results in inhibition of the c-Met signaling pathways as shown in animal model [63]. In a phase I/II study, patients with unresectable gastric cancer were treated with rilotumumab in first-line setting (initial dose 15 mg/kg i.v. on day 1) plus ECX (epirubicin, cisplatin, capecitabine). 121 patients were randomly assigned (40 to rilotumumab 15 mg/kg, 42 to rilotumumab 7.5 mg/kg, 39 to placebo). mPFS was 5.1 months (95% CI 2.9-7.0) in the rilotumumab 15 mg/kg group, 6.8 months (95% CI 4.5-7.5) in the rilotumumab 7.5 mg/kg group, 5.7 months (95% CI 4.5-7.0) in both rilotumumab groups combined, and 4.2 months (95% CI 2.9-4.9) in the placebo group. The HR for PFS compared with placebo was 0.69 (80% CI 0.49-0.97; p = 0.164) for rilotumumab 15 mg/kg, 0.53 (80% CI 0.38-0.73; p = 0.009) for rilotumumab 7.5 mg/kg, and 0.60 (80% CI 0.45-0.79; p = 0.016) for combined rilotumumab. Rilotumumab plus ECX had no unexpected adverse events and showed greater activity than placebo plus ECX [64]. According to these results, two phase III randomized double-blind placebo-controlled studies (RILOMET-1 and RILOMET-2) are currently active. RILOMET-1 (NCT01697072) is evaluating epirubicin, cisplatin and capecitabine with rilotumumab (15 mg/kg) or placebo for untreated advanced c-Met-positive gastric cancer. RILOMET-2 (NCT02137343) is evaluating cisplatin and capecitabine with rilotumumab or placebo (table 3).

Table 3.

Phase II/III trials of c-Met inhibitors for advanced gastric cancer and GEJ cancer

Trial	Phase	Setting	Regimen	Patients, n	OS, months	TTP/PFS, months
NCT00725712	II	1st, 2nd, 3rd	For	74	–	3.2
NCT01152645	II	2nd, 3rd	Tiv	30	–	43 days
NCT00719550	II	1st	Ril 15 mg/kg + ECX vs.	121	–	5.1 (p = 0.164) vs.
			Ril 7.5 mg/kg + ECX vs.			6.8 (p = 0.009) vs. 4.2
			placebo + ECX
NCT01697072	III	1st	Ril + ECX vs. placebo + ECX	600 (est.)	ongoing	ongoing
NCT02137343	III	1st	Ril + CX vs. placebo + CX	450 (est.)	ongoing	ongoing
NCT01662869	III	1st	mFOLFOX6 + Ona vs.	800 (est.)	ongoing	ongoing
			mFOLFOX6 + placebo

CX = Cisplatin, capecitabine; ECX = epirubicin, cisplatin, capecitabine; For = foretinib; mFOLFOX6 = modified regimen with 5-FU, folinic acid and oxaliplatin; Ona = onartuzumab; Ril = rilotumumab; Tiv = tivantinib.

Onartuzumab

Onartuzumab is a monovalent, humanized anti-c-Met antibody that targets the extracellular domain of c-Met. A randomized multicenter double-blind placebo-controlled phase III study evaluating the efficacy and safety of onartuzumab in combination with mFOLFOX6 in patients with metastatic HER2-negative and c-Met positive adenocarcinoma of the stomach or GEJ is now ongoing. Patients are being randomized in a 1:1 ratio to receive onartuzumab or placebo in combination with mFOLFOX6 (NCT01662869).

Anti-EGFR Therapies

EGFR-HER1 is one of four receptors involved in the pathway of epidermal growth factor-mediated signaling. It is a transmembrane receptor composed of an extracellular binding domain, a transmembrane portion and an intracellular cytoplasmic domain with tyrosine kinase activity [65]. It is activated by epidermal growth factor and several other factors. Ligand binding induces dimerization with consecutive tyrosine kinase autophosphorylation and activation [66]. This leads to several intracellular signal cascades, including the Ras/Raf/mitogen-activated protein kinase (MAPK) or the Akt/mTOR pathway, which regulate cell proliferation and growth, inhibition of apoptosis, tumor-induced angiogenesis as well as invasive and metastatic growth [67].

The expression of EGFR in gastric cancer was examined in a large study including 511 samples via IHC and FISH. As a result it was shown that the samples were positive for IHC (2+ and 3+) in 27.4% of all cases; EGFR overexpression was associated with older age (p = 0.001), moderately or poorly differentiated histology (p = 0.001) and higher-stage disease (p = 0.046). Sixteen cases (3.1%) showed high polysomy and 12 cases (2.3%) had gene amplification by FISH. The correlation between IHC and FISH was statistically significant (p < 0.001) [68].

In colorectal cancer, the presence of a KRAS mutation is usually associated with a downstream activation of the Ras/MAPK pathway, leading to cell proliferation, which in turn leads to resistance against anti-EGFR antibodies. The prevalence of KRAS mutations in 712 samples of gastric cancer was seen overall in 30 samples (4.2%) [69]. These results do not indicate that KRAS gene mutation is frequent in gastric cancer, and later studies were not restricted to patients with wild-type RAS.

Monoclonal Antibodies

Cetuximab

Cetuximab is a chimeric (mouse/human) IgG1 antibody which binds to the extracellular EGFR domain, occluding the ligand-binding region. It results in receptor internalization and degradation. Furthermore, cetuximab initiates an immune-mediated antitumor response (antibody-dependent cell-mediated cytotoxicity) [65].

In a small phase II trial conducted by Lordick et al. [70], 52 patients received cetuximab (400 mg/m² at first infusion followed by weekly infusions of 250 mg/m²) with chemotherapy (oxaliplatin 50 mg/m², 5-FU 2,000 mg/m² and folinic acid 200 mg/m² on days 1, 8, 15 and 22 every 5 weeks). Among 46 patients assessable for response, the median time to progression was 7.6 months and the mOS was 9.5 months. In another phase II study, the efficacy of cetuximab in addition to irinotecan (80 mg/m²) and folinic acid (200 mg/m²) and 5-FU (1,500 mg/m²) was tested. mPFS and OS times were 9.0 and 16.5 months, respectively [71].

According to these promising data, the EXPAND trial was initiated [72]. In this phase III trial, 904 patients with gastric cancer were randomized to receive capecitabine (1,000 mg/m² twice daily on days 1-15) and cisplatin (80 mg/m²) with or without cetuximab (400 mg/m² followed by 250 mg/m² per week) every 3 weeks. mPFS was 4.4 months (95% CI 4.2-5.5) in the cetuximab arm compared to 5.6 months (95% CI 5.1-5.7) in the chemotherapy arm (HR = 1.091; 95% CI 0.920-1.292; p = 0.3158). mOS was 9.4 months (95% CI 8.3-10.6) in the cetuximab arm and 10.7 months (95% CI 9.4-11.3) in the chemotherapy arm (HR = 1.004; 95% CI 0.866-1.165; p = 0.9547). These results suggest that cetuximab in addition to standard chemotherapy is not effective in patients with advanced gastric cancer.

Panitumumab

Panitumumab is a fully human IgG2 monoclonal antibody targeting the EGFR. The efficacy of its addition to standard treatment in advanced esophagogastric cancer has been tested in a large phase III trial, the REAL3 study [73]. 553 patients were randomized to receive either EOC (epirubicin 50 mg/m², oxaliplatin 130 mg/m² and capecitabine 1,250 mg/m²/day) or modified-dose EOC (epirubicin 50 mg/m², oxaliplatin 100 mg/m², capecitabine 1,000 mg/m²/day) and panitumumab 9 mg/kg (table 4). The median survival time was 11.3 months in the EOC arm compared to 8.8 months in the modified-dose EOC plus panitumumab arm (HR = 1.37; 95% CI 1.07-1.76; p = 0.013). The mPFS was 7.4 and 6.0 months, respectively (HR = 1.22; 95% CI 0.98-1.52; p = 0.068). According to these results, the addition of a monoclonal antibody targeting EGFR is not effective in advanced gastric cancer.

Table 4.

Phase II/III trials of anti-EGFR agents for advanced gastric cancer and GEJ cancer

Trial	Phase	Setting	Regimen	Patients, n	OS, months	TTP/PFS, months
AIO (Lordick)	II	1st	FUFOX + Cet	52	9.5	7.6
NCT01123811	II	1st	FOLFIRI + Cet	49	16.5	9.0
EXPAND	III	1st	Cis/Cap + Cet vs. Cis/Cap	904	9.4 vs. 10.7 (p = 0.9547)	4.4 vs. 5.6 (p = 0.3158)
REAL3	III	1st	EOC vs. mEOC + Pan	553	11.3 vs. 8.8	7.4 vs. 6.0
NCT01813253	II	2nd	Nim + Iri vs. Iri	83	250.5 vs. 232.0 days (p = 0.9778)	73.0 vs. 85.0 days (p = 0.5668)
MATRIX	II	1st	ECX + Mat vs. ECX	72	4.8 vs. 7.1	9.4 vs. 12.2

Cap = Capecitabine; Cet = cetuximab; Cis = cisplatin; ECX = epirubicin, cisplatin, capecitabine; EOC = epirubicin, oxaliplatin, capecitabine; FOLFIRI = 5-FU, folinic acid, irinotecan; FUFOX = 5-FU, folinic acid, oxaliplatin; Iri = irinotecan; Mat = matuzumab; mEOC = modified EOC; Nim = nimotuzumab; Pan = panitumumab; TTP = time to progression.

Nimotuzumab

Nimotuzumab (h-R3) is a humanized IgG1 monoclonal antibody targeting human EGFR. Its activity in advanced gastric cancer has been studied in a randomized phase II trial. 83 patients received either nimotuzumab with irinotecan or irinotecan monotherapy in second-line setting. mPFS was 73.0 and 85.0 days, respectively (p = 0.5668), and mOS was 250.5 and 232.0 days, respectively (p = 0.9778). mPFS/mOS in the EGFR 2+/3+ subgroups were 118.5/59.0 days (HR = 0.341; 95% CI 0.080-1.457; p = 0.1293) and 358.5/229.5 days (HR = 0.369; 95% CI 0.110-1.242; p = 0.0944), respectively [74]. These results did not show a clear benefit when nimotuzumab was added to standard chemotherapy, but it might show some activity in EGFR 2+, 3+ patients.

Matuzumab

Matuzumab is a humanized IgG1 monoclonal antibody against human EGFR. It was tested in a phase II trial (MATRIX study) in gastric cancer. There was no benefit of adding matuzumab to chemotherapy with epirubicin, cisplatin and capecitabine [75].

PI3K/Akt/mTOR-Targeted Therapy

The PI3K/Akt pathway is an intracellular signaling pathway transducing signals from cell membrane receptors (i.e. VEGF, HER2, IGF) with an important role in proliferation and apoptosis as well as in protein translation, synthesis and angiogenesis [76]. PI3K/Akt/mTOR activation was described in 30-60% of all tumors, including gastric cancer [77].

Everolimus

Everolimus (RAD001) is an inhibitor of the mTOR serine-threonine kinase inhibiting the PI3K/Akt/mTOR pathway. Everolimus showed efficacy in preclinical and phase I/II studies in gastric cancer [78]. The efficacy of the drug has been tested in a further phase II study. 53 patients with gastric cancer received everolimus (10 mg orally daily) in second-line setting. mPFS was 2.7 months (95% CI 1.6-3.0), mOS was 10.1 months (95% CI 6.5-12.1) [79]. Based on these results, a global phase III trial (GRANITE-1) was conducted to compare everolimus versus placebo in 656 patients with advanced gastric cancer with progressive disease after prior treatment with first- or second-line chemotherapy. mOS was 5.4 months with everolimus and 4.3 months with placebo (HR = 0.90; 95% CI 0.75-1.08; p = 0.124). mPFS was 1.7 and 1.4 months in the everolimus and placebo arms, respectively (HR = 0.66; 95% CI 0.56-0.78) [80]. Compared with best supportive care, everolimus did not significantly improve OS for advanced gastric cancer.

The ongoing randomized double-blind multicenter phase III study (RADPAC) evaluates paclitaxel with and without everolimus in patients with gastric carcinoma who have progressed after therapy with a fluoropyrimidine-containing regimens. A total of 480 patients will be enrolled in the study (NCT01248403, ClinicalTrials.gov).

Fibroblastic Growth Factor Receptor-Targeting Therapy

Alterations in fibroblastic growth factor signaling (FGFR2 amplification) have been reported in gastric cancer in up to 9%, with a higher frequency in diffuse-type gastric cancer [18]. Various inhibitors of FGFR signaling are in development and showed efficacy in gastric cancer cell lines in vitro [81]. The ongoing SHINE study (NCT01457846) is a phase II study to compare the selective FGFR1, FGFR2 and FGFR3 inhibitor AZD4547 to paclitaxel in second-line setting of patients with FGFR polysomy or amplification in gastric cancer. Several further studies are being planned.

Conclusion

There is growing knowledge of the different molecular alterations in gastric cancer. Treatment will probably move away from the current ‘one size fits all’ cytotoxic chemotherapy regimens to a more individualized approach. The first success to this strategy was the ToGA trial, as trastuzumab is now a standard of therapy in first-line setting of HER2-positive gastric cancer.

For evaluation of new targeted therapies, selection of patients is critical as only small subgroups show benefit. The AVAGAST study did not meet its endpoint of improved OS. However, the subgroup of patients from Pan-America might have shown a benefit whereas patients from Asia do not seem to benefit from addition of bevacizumab. Therefore failure in patient selection might lead to rejection of potentially efficacious drugs. Based on new data for ramucirumab it received approval in the second-line treatment in gastric cancer. Further promising data exist for apatinib, a tyrosine kinase inhibitor selectively targeting VEGFR-2. Furthermore, there are promising data for c-Met inhibitors such as rilotumumab in phase II studies. Three phase III studies for either rilotumumab or another c-Met inhibitor, onartuzumab, are ongoing to evaluate efficacy in c-Met-positive gastric cancer. In summary, further investigation is needed to improve patient selection, predictive markers and targeted therapy in order to provide new treatment approaches to patients with advanced gastric cancer.