Radioimmunotherapy of colorectal cancer in small volume disease and in an adjuvant setting:: Preclinical evaluation in comparison to equitoxic chemotherapy and initial results of an ongoing phase-I/II clinical trial

The 5-year survival of colorectal cancer patients with distant metastases is below 30%, despite the development and use of a variety of chemotherapeutic regimens. Therefore, new therapeutic strategies are warranted. Whereas radioimmunotherapy (RIT) has shown disappointing results in bulky disease, it may be a promising therapeutic alternative in limited and small volume disease. The aim of this study was, therefore, to compare, in a preclinical study, the therapeutic efficacy of RIT in colorectal cancer to equitoxic chemotherapy, as well as to evaluate, in a pilot clinical trial, its efficacy in small volume disease. Nude mice, bearing subcutaneous or metastatic human colon cancer xenografts, were injected either with the unlabeled (131)labeled monoclonal antibodies (MAbs), (CO17-1A or (which is a murine IgG(2a) directed against a 41-kD membrane glycoprotein) or F023C5 (which is an anti-CEA MAb of murine IgG(1) subtype), or were administered 5-fluorouracil / folinic acid (5-FU/LV) at equitoxic doses. In a pilot clinical study, 10 colorectal cancer patients with small volume metastatic disease tall lesions less than or equal to 3 cm) have been entered so far in an ongoing mCi/m(2)-based dose escalation study with the I-131-labeled F023C5. In the animals, the maximum tolerated activities (MTD) of I-131-labeled CO17-1A and F023C5 were 300 mu Ci and 600 mu Ci, respectively corresponding to blood doses of approximately 15 Gy each. Accordingly, myelotoxicity was dose-limiting. The MTD in the chemotherapy group was 0.6 mg 5-FU / 1.8 mg LV; given as intravenous bolus I h apart for 5 subsequent clays. Whereas no significant therapeutic effects were seen with both unlabeled MAbs or 5-FU/LV chemotherapy, tumor growth was retarded significantly with both radiolabeled antibodies. lit the metastatic model, chemotherapy prolonged life for only a few weeks, whereas RIT led to cures in 35-55% of the animals. As was the case in the animals, myelotoxicity seems to be dose-limiting in patients as well. Encouraging anti-tumor effects were observed lasting for up to more than 12 months. These data suggest that radioimmunotherapy may be a viable therapeutic option in colorectal cancer patients with limited disease. Myelotoxicity is the only dose-limiting organ toxicity. Although most patients were treated below the MTD, anti-tumor effects are encouraging. Further studies are ongoing.