2529 Background: Peritoneal carcinomatosis (PC) from GI-tract cancer is associated with prognosis of 3–6 months. Since chemotherapy and surgical procedures showed only limited efficacy, there is no standard therapy recommended. The trifunctional antibody (trAb) catumaxomab (anti-EpCAM x anti-CD3, removab) represents a new class of intact antibodies, which are able to re-direct T-lymphocytes to tumor cells and to induce active anti tumor immunity by activating FcγRI/III+ APC/DC by their intact Fc region. In a phase I/II trial, intraperitoneal application of catumaxomab was investigated in patients with PC due to GI-tract cancer. Methods: 17 patients (mean age 55 years; 7 gastric-ca, 9 colon-ca, 1 CUP) were included after diagnosis of EpCAM+ PC. Treatment consisted of 4 escalating doses of catumaxomab within 10 days by i.p. application. MTD was defined separately for the 1st, 2nd, 3rd and 4th infusion according to a dynamic escalation schedule. Anti-tumor efficacy was evaluated by immunohistochemical analys...