Chemoimmunotoxin therapy against a human colon tumor (HT-29) xenografted into nude mice

The efficacy of intracavitary chemoimmunotoxin therapy for cancer treatment was evaluated using the human colon carcinoma (HT-29) which had been xenografted i.p. into nude mice. Mice bearing HT-29 were treated with an immunotoxin consisting of the monoclonal antibody OVB3 coupled to Pseudomonas exotoxin (OVB3-PE), with cyclophosphamide (Cy), or with both OVB3-PE plus Cy. Mice given injections i.p. of 3 x 10(6) HT-29 ascites cells developed a localized disease that presented as both malignant ascites and solid tumor confined to the peritoneal cavity. All mice died within 30 to 40 days. Mice that received either three or six injections of OVB3-PE at a dose of 0.5 micrograms every other day beginning 3 days post-tumor inoculation exhibited significantly increased median survival times (MSTs) (P = 0.002) of 62 and 68 days, respectively, as compared to a MST of 33 days for the controls. OVB3 alone or an irrelevant monoclonal antibody conjugated to PE exhibited no antitumor activity. The therapeutic effects of the immunotoxin could be blocked by giving a large amount of unconjugated OVB3 at the same time. Treatment of mice with Cy alone at the maximal tolerated dose (250 mg/kg) on Days 10 and 17 after tumor inoculation increased the MST from 33 days to 54 days. The maximum tolerated dose could be increased to 300 mg/kg per injection if the Cy treatment was preceded by 100 mg/kg of S-2-(3-aminopropylamino)ethyl phosphorothioic acid (WR-2721), a sulfhydryl compound that selectively protects normal tissue against the toxicity of radiation and alkylating agents. Cy plus WR-2721 treatment on Days 10 and 17 increased the MST from 35 to 61 days (P = 0.002). Interestingly, groups of mice that received either two, four, or seven treatments of OVB3-PE following Cy plus WR-2721 therapy exhibited a further increase (P less than 0.002) in MSTs to 81, 87, and 96 days, respectively. Thus, the combination of cytoreductive chemotherapy with the OVB3-PE was significantly more effective for the intracavitary treatment of established HT-29 colon cancer xenografts than either chemotherapy or immunotoxin therapy alone.