Use of other agents with anti-HER2 activity in the second-line treatment of patients who have experienced progression while on trastuzumab remains unclear. In the recent T-ACT trial, patients with disease refractory to frontline therapy with combination trastuzumab and fluoropyrimidine/platinum agents were randomly assigned to receive either weekly paclitaxel (n = 45) or weekly paclitaxel plus trastuzumab (n = 44).30 Patients in the combination cohort received an initial dose of trastuzumab 8 mg/kg followed by 6 mg/kg every 3 weeks until progression. The study did not find a difference in either PFS (3.19 months versus 3.68 months; HR 0.91 [95% CI 0.67 to 1.22], P = 0.33) or OS (9.95 months versus 10.2 months; HR 1.23 [95% CI 0.75 to 1.99], P = 0.20). The study thus failed to show a benefit to continuing trastuzumab after progression in the first-line setting.
Lapatinib in combination with paclitaxel has been compared to paclitaxel alone for the treatment of advanced HER2-positive gastric cancer in an Asian population in the phase 3 TyTAN trial.31 With a primary end point of OS, the study randomly assigned 129 patients to receive paclitaxel alone and 132 patients to receive paclitaxel with lapatinib. There was a nonsignificant trend towards improvement in OS in the combination group (11 months) as compared to the paclitaxel-only group(8.9 months, P = 0.1044), with no significant difference in median PFS (5.4 months versus 4.4 months). However, it is important to note that only 15 patients in this trial had previously been exposed to trastuzumab. Another trial, the phase 3 GATSBY study, examined the efficacy of trastuzumab emtansine in the second-line setting compared to taxanes alone and failed to show any improvement in PFS or OS.32 Given these results, no alternative anti-HER2 therapy has been proven to be efficacious for patients who are trastuzumab refractory. Therefore, including anti-HER2 therapy in the second-line treatment of HER2-positive gastric cancer is not recommended.
IMMUNOTHERAPY AND OTHER TARGETED THERAPIES
Several other targeted therapies have been studied in advanced gastric cancer, without any demonstrable survival benefit. The PI3K/AKT/mTOR pathway is known to be involved in regulation of cell growth and angiogenesis, and the mTOR inhibitor everolimus is widely used to treat other malignancies, including breast cancer. The use of everolimus in the second-line setting was studied in the phase 3 GRANITE-1 trial, where it was compared to best supportive care and failed to provide any survival benefit.33 Cetuximab, a recombinant human and mouse chimeric monoclonal antibody, and panitumumab, a recombinant human antibody against the epidermal growth factor receptor (EGFR), have also been examined in gastric and GEJ cancer patients. However, the large phase 3 EXPAND and REAL-3 trials did not show a survival benefit when these agents were added to standard chemotherapy.34,35
Overexpression of MET, a proto-oncogene and tyrosine kinase receptor, has also been implicated in gastric cancer progression. The ligand for MET is the hepatocyte growth factor (HGF), and aberrant signaling of this pathway has been shown to correlate with an aggressive gastric cancer phenotype and poorer OS by promoting tumor growth and angiogenesis. However, no MET inhibitors thus far have been found to be clinically effective. RILOMET-1 and RILOMET-2 were phase 3 trials examining the efficacy of rilotumumab, a humanized anti-HGF antibody, in combination with chemotherapy (ECX and cisplatin with capecitabine, respectively) for the frontline treatment of MET-positive GEJ and gastric cancers. Both studies were discontinued due to a higher treatment-related mortality in patients receiving rilotumumab, with a higher incidence of adverse events due to disease progression being noted.36 Similarly, onartuzumab, a monovalent monoclonal antibody against the MET receptor, was investigated in the phase 3 METGastric trial in combination with modified FOLFOX6 as first-line therapy for HER2-negative, MET-positive metastatic GEJ and gastric cancers. The study did not demonstrate any significant improvements in OS or PFS.37
There has been significant interest in incorporating immunotherapy in the treatment of early and metastatic gastric cancer. Pembrolizumab is the first programmed death receptor (PD-1) inhibitor to be approved for treatment of patients with PD-L1−positive advanced gastric cancer who had previously received 2 or more lines of chemotherapy. Although earlier studies of pembrolizumab in lung cancer utilized the tumor proportion score (TPS) to determine PD-L1 positivity, this was not found to be applicable to gastric cancer. Instead, the combined positive score (CPS) is used in gastric cancer. The CPS evaluates the number of tumor cells and immune cells (macrophages and lymphocytes) that stain positive for PD-L1 relative to all viable tumor cells. Comparatively, the TPS only examines the percentage of viable tumor cells that show complete or partial positive staining for PD-L1. A CPS score of 1 or greater identifies patients who would be suitable candidates for pembrolizumab.