Abstract
61
Background: Novel targets and therapeutics are of profound importance if we are to improve outcomes in gastric cancer. This study assessed the incidence of mesothelin expression in tumors of Western patients with gastric cancer. In addition, gastric cancer cell lines were then tested for their sensitivity to SS1(dsFv)PE38 (SS1P), a mesothelin-targeted immunotoxin. Methods: Tissue specimens from 127 patients with gastric or gastro-esophageal junction (GEJ) tumors were examined by immuno-histochemistry (IHC) for intensity of staining for mesothelin. Expression of HER2-neu, e-cadherin and c-Met were also assessed. Tumors were considered positive for mesothelin if moderate cytoplasmic/membranous or luminal staining was present in at least 10% of the cells. Gastric cancer cell lines were assessed for surface mesothelin expression by flow cytometry and the viability of cells treated with SS1P was measured. Results: Gastric and GEJ cancers were mesothelin positive in 64/127 tumors (50%) while only 9 tumors (7%) were Her2-neu IHC +3 positive and 8 tumors (6%) were c-Met positive (c-Met low 5% and c-Met high 1%). Mesothelin expression rates changed from stage I tumors to stage IV tumors (39% to 56% respectively). We found that both the AGS and MKN28 gastric cancer cell lines express mesothelin and are sensitive to the immunotoxin with EC50 values in the low picomolar range (0.4 and 0.3 ng/mL, respectively). Conclusions: Mesothelin is a cell surface glycoprotein that is expressed in 50% of resected gastric cancers. Gastric cancer cell lines were exquisitely sensitive to SS1P. The high expression of mesothelin and its sensitivity to the immunotoxin SS1P makes it an attractive tumor associated antigen for therapeutic targeting. Based on these observations clinical trials involving novel mesothelin targeted drugs in gastric cancer are currently in development.
