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Uterine serous papillary carcinoma (USPC) represents a highly aggressive variant of endometrial cancer. Using gene expression profiling, we recently identified high expression of the claudin‐3 and claudin‐4 receptors in a limited set of USPC. These tight junction proteins represent the low‐ and high‐affinity receptors, respectively, for the cytotoxic Clostridium perfringens enterotoxin (CPE) and are sufficient to mediate CPE binding and trigger subsequent toxin‐mediated cytolysis. The potential for targeting this pathway in the treatment of USPC was explored.Claudin‐3 and claudin‐4 receptor expression was analyzed at the mRNA and protein levels in flash‐frozen and formalin‐fixed, paraffin‐embedded tissue from 20 consecutive USPC patients. The potential of recombinant CPE as a novel therapy against primary, metastatic, and chemotherapy‐resistant USPC cell lines was also investigated in vitro. Finally, the in vivo therapeutic effect of sublethal doses of CPE was studied in SCID mouse xenografts harboring subcutaneous and intraperitoneal USPC that expressed claudin‐3 and claudin‐4.In all, 100% (20 out of 20) of the primary flash‐frozen USPC tested overexpressed 1 or both CPE receptors by quantitative reverse‐transcriptase polymerase chain reaction (RT‐PCR). Membranous immunoreactivity for claudin‐4 protein expression was documented in the majority of USPC specimens tested by immunohistochemistry, whereas only a low level of membranous staining was found in normal endometrial control tissue samples. When primary and metastatic short‐term USPC cell lines were incubated with different concentrations of CPE in vitro, a dose‐dependent cytotoxic effect was demonstrated. In vivo, intratumoral injections of well‐tolerated doses of CPE in large subcutaneous USPC xenografts led to large areas of tumor cell necrosis and tumor disappearance in all the treated animals, whereas sublethal intraperitoneal injections of CPE had a significant inhibitory effect on tumor progression, with extended survival of animals harboring chemotherapy‐resistant intra‐abdominal USPC carcinomatosis.Claudin‐3 and claudin‐4 receptors may offer promising targets for the use of CPE as a novel type‐specific therapy against this highly aggressive and chemotherapy‐resistant variant of endometrial cancer. Cancer 2007. © 2007 American Cancer Society. [ABSTRACT FROM AUTHOR]