gms | German Medical Science

Introduction: With a five-year survival rate of only 30 percent, gastric carcinomas are one of the most aggressive tumor entities. Due to poor response rates and strong side effects of conventional cytotoxic therapies, there is a strong demand for new multimodal treatment opportunities. Our findings show that human gastric carcinoma cells express the cellular receptor of bacterial Shiga Toxin, the glycosphingolipid globotriaosylceramide (Gb₃/CD77). Therefore, the non-toxic Shiga Toxin subunit B can be used as a highly specific transport vehicle for cytotoxic agents. Binding on the target cell results in a fast uptake of the STxB-drug-conjugate, and in the release of the drug within the target cell. The aim of this study is a targeted therapy of gastric carcinoma with a conjugate of STxB and SN38, the active metabolite of the topoisomerase type I inhibitor Irinotecan.

Material and methods: Cryosections of surgically resected gastric carcinoma (n=50, n=25 intestinal, n=25 diffuse type, Laurén´s classification), as well as normal gastric tissue samples, were assessed in a blinded fashion by three observers after staining with a STxB-fluorophor-conjugate. Furthermore, flow cytometry was used for the analysis of Gb₃-expression in human gastric carcinoma cell lines (n=11), and fluorescence microscopy for the investigation of intracellular uptake kinetics of STxB along the retrograde transport route. The use of the STxB-SN38-conjugate for targeted cancer therapy was assessed with a cell proliferation assay, and compared to the effects of Irinotecan.

Results: Gastric carcinomas show an extensive expression of Gb₃ (38 of 50 cases positive, 76 %) while Gb₃-expression in non-diseased gastric mucosa is restricted to chief and parietal cells at the ground of gastric glands. Treatment of a Gb₃-expressing gastric carcinoma cell line with STxB-SN38-conjugate resulted in a more than 100-fold increased cytotoxicity, as compared to the clinically applied drug Irinotecan. Moreover, no cytotoxicity was observed on cell lines lacking Gb₃ expression, demonstrating high specificity.

Conclusion: The majority of gastric carcinomas express the glycosphingolipid Gb₃, the cell surface receptor of STxB. Therefore, a STxB-SN38-conjugate is a promising candidate for specific and effective cytotoxic tumor therapy.