HER2 in Esophagogastric Adenocarcinoma

Harry H. Yoon, MD, MHS

Assistant Professor, Division of Medical Oncology,

Mayo Clinic, Rochester, MN;yoon.harry@mayo.edu

Abstract

Determination of HER2 status has become an important part of the workup of patients with advanced esophagogastric cancer, given recent data from a phase III trial (ToGA) indicating that trastuzumab, an anti-HER2 monoclonal antibody, prolongs survival in these patients. HER2 positivity is at least as common in esophageal adenocarcinomas as in cancers of the gastroesophageal junction or stomach. It is important to utilize new disease-specific criteria for interpreting HER2 expression by immunohistochemistry, because HER2 overexpression in esophagogastric tumors is distinct from that in breast tumors. It is generally agreed that patients whose esophagogastric tumors show strong HER2 expression (IHC 3+), or weak-moderate HER2 expression (IHC 2+) with gene amplification as measured by fluorescence in situ hybridization, are candidates for trastuzumab. However, it is controversial whether patients whose tumors show only faint or absent HER2 expression (IHC 0 or 1+), despite being gene-amplified, should be trastuzumab-eligible. Concordance of HER2 results between IHC and FISH is high in the IHC 0-1+ and IHC 3+ subgroups, but not the IHC 2+ subgroup. IHC is generally recommended as the initial test to screen for HER2 positivity, with confirmatory testing by FISH for IHC 2+ cases. Given that lapatinib, a small-molecule inhibitor of EGFR and HER2, did not significantly improve overall survival in this malignancy, use of lapatinib is not recommended outside of a clinical trial. Ongoing trials in HER2-positive esophagogastric cancer are evaluating whether trastuzumab improves outcomes perioperatively or in combination with agents that block HER3/ERBB3.

Trastuzumab in Esophagogastric Adenocarcinoma

HER2/ERBB2 oncoprotein overexpression is an important therapeutic target in esophagogastric cancer. Trastuzumab, an anti-HER2 monoclonal antibody with known activity in breast cancer, was recently demonstrated to prolong survival in patients with advanced gastric or gastroesophageal junction (GEJ) adenocarcinoma.1 Because HER2 positivity is a strong predictor of trastuzumab benefit, determination of HER2 status has become integral in the workup of this malignancy worldwide, and its accurate determination is critical. HER2 overexpression in esophagogastric tumors is distinct from that in breast tumors, which influences patient selection for HER2-targeted therapy. Considerations relevant for the practicing physician are discussed here.The benefit of trastuzumab in advanced HER2-positive GEJ/gastric adenocarcinoma was demonstrated in an international phase III trial (ToGA), which compared cisplatin/fluoropyrimidine chemotherapy with and without trastuzumab.¹All tumors were screened for HER2 protein expression by immunohistochemistry (IHC), andHER2gene amplification by fluorescence in situ hybridization (FISH). Patients were eligible if their tumor was positive by IHC (ie, showing 3+ expression) or FISH (ie, showing aHER2/CEP17ratio of ≥ 2). Among enrolled patients (n = 594 of the 3807 screened), almost all tumors were FISH-positive, whereas protein expression rates by IHC varied.

At a median follow-up of more than 17 months, median overall survival (OS; the primary endpoint) was significantly better with trastuzumab (13.8 vs 11.1 months). Progression-free survival (PFS) and objective response rate (ORR) also favored trastuzumab. Trastuzumab was well tolerated.

Based on these data, trastuzumab was approved, in combination with cisplatin and a fluoropyrimidine, for the treatment of patients with metastatic HER2-overexpressing gastric or GEJ adenocarcinomas who have not received prior treatment for metastatic disease. The significance of these data are underscored in light of recent phase III trial results showing a failure of multiple biologic agents to prolong survival, including those targeting EGFR (cetuximab,²panitumumab,³gefitinib⁴), EGFR/HER2 (lapatinib^5,6), and VEGF (bevacizumab⁷).

HER2 Status in Esophagogastric Adenocarcinoma

Ramucirumab, an anti-VEGFR-2 monoclonal antibody, is the second biologic agent to demonstrate improved OS in this malignancy in a phase III trial (REGARD8). According to preliminary results presented at the 2013 Gastrointestinal Cancers Symposium (ASCO), 355 patients with previously treated metastatic gastroesophageal cancer were randomized (2:1) in the REGARD trial to receive ramucirumab or placebo. The study reached its primary endpoint: median OS was higher in ramucirumab-treated patients (5.2 vs 3.8 months, respectively; HR = 0.776; 95% CI, 0.603-0.998;P=.0473).⁸While ORRs were similar between arms, ramucirumab-treated patients had significantly improved median PFS (2.1 vs 1.3 months; HR = 0.483;P<.0001) and disease control rate (49% vs 23%;P<.0001). Ramucirumab was well tolerated, and no unexpected safety findings were noted. Further data from other ongoing ramucirumab trials are awaited.Patients with advanced esophagogastric cancer who are potential candidates for trastuzumab should be screened to determine HER2 status. Approximately 7% to 22% of esophagogastric cancers overexpress HER2, a similar percentage to that seen in breast cancer.^2,3,9,10-15Of note, HER2 positivity is at least as common in esophageal^9,16,17as in gastric adenocarcinomas, and has a predilection for intestinaltype (rather than diffuse-type) gastric cancers. In our study of 713 surgically resected esophageal or GEJ adenocarcinomas, HER2 overexpression and/or gene amplification was significantly more frequent in low- (vs high-) grade tumors, and in tumors with (vs without) adjacent pathologic Barrett&rsquo;s metaplasia.⁹As in breast cancer, there appears to be a high concordance between HER2 results obtained from the primary tumor and metastatic sites.¹²In contrast to breast cancer, the association between HER2 expression/ amplification and prognosis in esophagogastric cancer remains unknown.^{2,3,9,11-15,18}

HER2 status in esophagogastric cancers can be assessed using the same FDA-approved HER2 assays as for breast tumors, which typically includes IHC and FISH. Protein expression by IHC is categorized into one of four levels (0, 1+, 2+, 3+) based on a composite score that incorporates the intensity of staining and the percentage of cancer cells demonstrating that intensity (Table 1^1,19,20). However, interpretive criteria for determining the IHC score in esophagogastric cancers are distinct in two ways from breast tumors, underscoring the importance of utilizing tumor-specific criteria in clinical practice:

• HER2 protein expression in esophagogastric cancers tends to spare the digestive luminal membrane, resulting in membrane staining that is not completely circumferential.^19,21An esophagogastric cancer with only partial membrane staining (ie, basolateral or lateral) can be categorized as 2+ or 3+. By contrast, a breast tumor must have complete circumferential membrane staining to be designated as 2+ or 3+.
• A greater degree of intratumor HER2 heterogeneity (ie, HER2-positive cells comprising only a minority of the tumor) has been reported in esophagogastric compared with breast tumors.^19,21Because heterogeneity may increase the likelihood that esophagogastric tissue biopsies will miss a HER2-positive region of the tumor, the cutpoint in the percentage of cancer cells that must show HER2 expression is lower in tumor biopsies from esophagogastric cancers as compared with breast cancers (Table 1). Tumor biopsies may carry a significant HER2 falsenegative rate when compared with whole-tissue sections from corresponding surgical resection specimens.²²

Clinical Pearls

• HER2 status should be examined in all patients with advanced adenocarcinoma of the esophagus, in addition to the gastroesophageal junction or stomach.
• IHC is generally recommended as the initial test to screen for HER2 positivity. Testing forHER2gene amplification by FISH should be performed for tumors with equivocal (2+) HER2 expression.
• HER2 protein expression using IHC should be interpreted using criteria specific to gastroesophageal cancer, given potentially false-negative results when breast cancer criteria are utilized.

Definition of HER2 Positivity: Selection of Patients for Trastuzumab Therapy

Table 1. Criteria for Evaluating HER2 Protein Expression by IHC in Esophagogastric vs Breast Cancer

IHC Score

Breast^a

Surgical resection or biopsy specimen

Esophagogastric^b

Surgical resection specimen

Esophagogastric^b

Biopsy specimen

3+

Uniform intense membrane staining in >30% tumor cells

Strong complete, basolateral, or lateral membrane staining in &ge;10% cancer cells

Strong complete, basolateral, or lateral membrane staining in a tumor cluster of &ge;5 cancer cells

2+

Weak or nonuniform complete membrane staining in &ge;10% tumor cells

Weak-to-moderate complete, basolateral, or lateral membrane staining in &ge;10% cancer cells

Weak-to-moderate complete, basolateral, or lateral membrane staining in a tumor cluster of &ge;5 cancer cells

1+

Weak, incomplete membrane staining in any proportion of tumor cells

Faint membrane staining in &ge;10% cancer cells

Faint membrane staining in a tumor cluster of &ge;5 cancer cells

0

No staining

No staining or staining in <10% of cancer cells

No staining

IHC, immunohistochemistry.

^aAdapted from 2007 ASCO/CAP guidelines.²⁰

^bAdapted from criteria developed for gastric cancer and utilized in the phase III ToGA trial.^1,19

These modified criteria for interpreting HER2 by IHC in esophagogastric cancers were developed and validated by an international oncology and pathology panel^19,21and utilized to assess eligibility for the ToGA trial. Data suggest that applying breast cancer criteria to esophagogastric tumors may result in an underestimation of HER2 expression.^19,21,23Guidelines for selecting patients with esophagogastric cancer for trastuzumab treatment based on HER2 test results are not uniform worldwide. European regulators approved trastuzumab only for tumors that show IHC 3+ expression, or IHC 2+ expression with amplification (FISH-positive). This limited approval was based on an exploratory analysis of the ToGA trial, which showed that trastuzumab was most effective in prolonging survival in the subgroup of patients whose tumors were IHC 3+ or IHC 2+ with amplification (median OS, 16.0 vs 11.8 months in trastuzumab vs non-trastuzumab arms, respectively; HR = 0.65; 95% CI, 0.51-0.83).¹By contrast, patients with FISH-positive IHC 0-1+ tumors, which represented 22% of the enrolled population, did not appear to benefit from trastuzumab (median OS, 10.0 vs 8.7 months in trastuzumab vs non-trastuzumab arms, respectively; HR = 1.0; 95% CI, 0.70-1.62). Preliminary data suggest that these FISH-positive IHC 0-1+ tumors have &ldquo;low-level&rdquo; amplification,²⁴and the potential cost-ineffectiveness in treating IHC 0-1+ FISH-positive cases has been raised.^25,26However, given the exploratory post-hoc nature of these analyses, the FDA approved trastuzumab for patients whose HER2 status meets the eligibility criteria of the ToGA trial (ie, FISH-positive or IHC 3+).²⁷

In other words, it is widely accepted that patients whose tumors are IHC 3+ or IHC 2+ with amplification are candidates for trastuzumab. However, controversy exists on whether to recommend trastuzumab in FISH-positive cases with IHC 0-1+ scores; exploratory analysis from ToGA suggests that these patients do not benefit from trastuzumab, but benefit has not been definitively ruled out.

The concordance between IHC and FISH is high in the IHC 0-1+ and 3+ subgroups, but not in the IHC 2+ subgroup. The largest studies to date are shown inTable 2.^{10,13,23,28,29}These concordance data, together with the exploratory analysis from ToGA, support the use of IHC as the initial screening test for HER2, as recommended by the European Medicines Agency and others.30 The following points integrate best available evidence, and may assist clinicians in interpreting IHC test results with regard to further testing by FISH and patient selection for treatment:

• If the IHC result is 3+, no further testing is required, and the patient is eligible for trastuzumab.While HER2 overexpression can occur through mechanisms other than gene amplification,^31,32most IHC 3+ cases are FISH-positive. IHC3+ FISH-negative cases were too few in ToGA to be able to rule out trastuzumab benefit in this subgroup.
• If the IHC result is 2+, reflex testing by FISH should be performed.Patients whose tumors are IHC 2+ and FISH-positive are candidates for trastuzumab, as this subgroup appeared to benefit in ToGA (HR = 0.75 for OS [95% CI, 0.51- 1.11]; n = 159). FISH-positive rates among IHC 2+ tumors vary (~13%-62%).^{10,12,15,17,23,28,33}Esophagogastric tumors with a HER2/CEP17 ratio of 2 or greater are generally considered HER2-amplified.
• If the IHC result is 0 or 1+, it is controversial as to whether FISH should be ordered.The FISH-positive frequency in IHC 0-1+ cases is 0.3% to 7.5% (Table 2). As described above, exploratory analysis of ToGA data indicated that these patients did not benefit from trastuzumab. Given the potential presence of intratumor heterogeneity, FISH may be considered in IHC 0-1+ cases if the available tissue is a cytologic or biopsy specimen (rather than surgical resection), or if tumor histology is low-grade or intestinaltype, or if adjacent Barrett&rsquo;s metaplasia is present, since these histologic characteristics may be associated with a higher incidence of HER2 positivity.⁹Otherwise, FISH testing in IHC 0-1+ cases is likely to be low-yield.

Table 2. Frequency ofHER2Gene Amplification by HER2 Protein Expression Score

HER2 Protein Expression (IHC Score)

Study

Tumor Site

Negative

(0, 1+)

Indeterminate

(2+)

Positive

(3+)

N

% Amp

N

% Amp

N

% Amp

ToGA 2009^{b 29}

N = 3,280

Gastric, GEJ

2519

7.5%

388

55%

373

95%

Kim et al 2011^{a 10}

N = 575

Gastric

NR

0.3%

NR

18%

NR

100%

Park et al 2012²³

N =1054

Gastric

962

4.4%

29

62%

63

100%

Terashima et al 2012¹³

N = 829

Gastric

653

NR

101

38%

75

97%

Yoon et al 2013^{b 28}

N = 673

Esophagus, GEJ

417

3.8%

167

13%

89

89%

Amp, gene amplification; GEJ, gastroesophageal junction; IHC, immunohistochemistry; NR, not reported.

^aIn this study, the number of patients by IHC category was not reported, and amplification rates refer to the percentage of tissue microarray cores that were gene-amplified.

^bData have been reported in abstract form.

Lapatinib

At the time of disease progression on a first-line trastuzumab-containing regimen for HER2-positive disease, there are no data addressing the benefit of continuing trastuzumab with the second-line regimen. However, extrapolating from the experience in breast cancer, many oncologists appear to continue trastuzumab as long as it is well tolerated. Although trastuzumab was studied with fluoropyrimidine/ cisplatin regimens in ToGA, many clinicians incorporate trastuzumab into their first-line regimen of choice (eg, 5FU/oxaliplatin).³⁴Dual EGFR/HER2 inhibition using lapatinib did not yield positive results in two recent phase III trials of patients withHER2-amplified esophagogastric adenocarcinoma. In an international trial in the first-line setting (LOGiC),5 the addition of lapatinib to capecitabine/oxaliplatin in 545 patients with advanced HER2-positive (IHC 3+ or gene-amplified) adenocarcinoma of the upper gastrointestinal tract did not improve the primary endpoint of OS (median 12.2 vs 10.5 months in lapatinib vs non-lapatinib arms; HR = 0.91; 95% CI, 0.73-1.12;P=.35). However, a significant improvement in OS was observed in Asian patients (HR = 0.68) and those under age 60 years (HR = 0.69). Notably, there was no association between IHC and OS.

In a second-line trial (TyTAN)6 conducted in Asia and restricted to patients with gastric cancer (N = 430), the addition of lapatinib to weekly paclitaxel did not improve OS at a statistically significant level (11.0 vs 8.9 months; HR = 0.84;P= .208). However, in a preplanned analysis of the subgroup of patients (n = 261) whose tumors showed strong (3+) HER2 expression, median OS was higher in the lapatinib versus non-lapatinib arm (14.0 vs 7.6 months; HR = 0.59;P= .0176). PFS (5.6 vs 4.2 months; HR = 0.54;P= .0101) and ORR (27% vs 9%) were also better with versus without lapatinib among patients in the IHC 3+ subgroup.

Conclusions and Future

Although lapatinib is not indicated outside of a clinical trial, these data suggest the possibility that some subpopulations, as yet undefined, may benefit from combined inhibition of EGFR and HER2.HER2 status should be examined in all patients with advanced adenocarcinoma of the esophagus, in addition to those with GEJ or gastric cancer. IHC is generally recommended as the initial test to screen for HER2 positivity, with confirmatory testing by FISH for IHC 2+ cases. IHC should be interpreted using criteria specific to gastroesophageal cancer. Patients with IHC 3+ or IHC 2+ with FISH-positive tumors are candidates for trastuzumab therapy, whereas it is controversial to administer trastuzumab in FISH-positive IHC 0-1+ cases. Use of lapatinib outside of a clinical trial is not recommended.

In HER2-positive disease, current research efforts include the determination of whether blockade of HER3/ERBB3, an important heterodimerization partner of HER2, adds clinical benefit when added to trastuzumab. Agents under study in esophagogastric cancer include pertuzumab (phase III, NCT01774786), which inhibits HER2/HER3 dimerization, and MM-111 (phase II, NCT01774851), which docks to HER2 and inhibits HER3 signaling. Although trastuzumab therapy is not indicated perioperatively or in combination with radiotherapy, the question of whether trastuzumab adds benefit to preoperative chemoradiotherapy in locally advanced esophageal or GEJ adenocarcinoma is being examined in an ongoing phase III trial (RTOG-1010, NCT01196390).

Author Disclosure

Dr. Yoon has received research grants from Genentech/Roche and Eli Lilly and Company.

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