Chimeric T84.66 (cT84.66) is a high-affinity (5 x 10(10) M-1) anti-carcino-embryonic antigen (CEA) IgG1. In a recently completed pretherapy imaging trial, 111In-labeled cT84.66 demonstrated targeting of CEA-producing metastatic sites and low immunogenicity, with human antichimeric antibody (HACA) response in only 1 of 15 patients after a single administration. The purpose of the present study was to evaluate cT84.66-diethylenetriaminepentaacetic acid labeled with 90Y in a dose-escalation Phase I trial. Patients with metastatic CEA-producing malignancies received imaging doses of 5 mCi 111In-labeled cT84.66 first, followed 1-2 weeks later by 5 mg cT84.66 labeled with the therapeutic dose of 90Y. Immediately following the therapeutic infusion, diethylenetriaminepentaacetic acid was administered by continuous i.v. infusion over 3 days at 250 mg/m2 body surface area/24 h. Biodistribution, tumor targeting, absorbed radiation dose estimates, antibody clearance, and HACA response were evaluated through blood samples, 24-h urine collections, and nuclear images performed at serial time points after infusion. To date, three patients with metastatic colorectal cancer have been evaluated at the first dose level of 5 mCi/m2. No side effects were associated with antibody administration. Localization of the antibody to nonhepatic metastatic sites was observed. Size-exclusion high-performance liquid chromatography demonstrated the formation of CEA:antibody complexes in serum in all three patients. A significant variation among patients in the clearance rate of the antibody and complexes from blood to liver was seen, which resulted in a reciprocal relationship between estimated liver dose and red marrow dose. Patients who demonstrated faster clearance to liver demonstrated greater excretion of a low-molecular-weight metabolite through the urine. Two patients developed HACA response, which persisted at 4 months after therapy. At this first dose level, no tumor responses were seen and reversible grade 1 thrombocytopenia was observed in 2 patients. cT84.66 demonstrated effective localization in CEA-producing tumors. Its low immunogenicity after a single administration makes it attractive for further evaluation as a radioimmunotherapeutic agent. However, further evaluation is needed to determine whether its immunogenicity will remain low after multiple administrations. Additionally, in two of the three patients, we identified rapid clearance of the antibody to the liver. This underscores the need to identify, characterize, and understand further those factors that influence the biodistribution and clearance of anti-CEA antibodies to allow for better selection of patients for therapy and rational planning of radioimmunotherapy.