Effective treatment of a murine model of adult T-cell leukemia using 211At-7G7/B6 and its combination with unmodified anti-Tac (daclizumab) directed toward CD25

Adult T-cell leukemia (ATL) consists of an overabundance of T cells, which express CD25. Therapeutic efficacy of astatine-211 (At-211)-labeled murine monoclonal antibody 7G7/B6 alone and in combination with daclizumab was evaluated in non-obese diabetic/severe combined immunodeficiency (NOD/SCID) mice given injections of MET-1 human T-cell leukemia cells. Daclizumab and 7G7/B6 are directed toward different epitopes of CD25. Either a single dose of 12 mu Ci (0.444 MBq) At-211-7G7/B6 per mouse given intravenously or receptor-saturating doses of daclizumab given at 100 mu g weekly for 4 weeks intravenously inhibited tumor growth as monitored by serum levels of human beta-2 microglobulin (beta 2 mu) and by prolonged survival of leukemia-bearing mice compared with the control groups (P < .001). The combination of 2 agents enhanced the antitumor effect when compared with groups treated with 12 mu Ci (0.444 MBq) of At-211-7G7/B6 (P < .05) or daclizumab alone (P < .05). The median survival duration of the PBS group was 62.6 days and 61.5 days in the radiolabeled nonspecific antibody At-211-11F11-treated group. In contrast, 91% of mice in the combination group survived through day 94. These results that demonstrate a significantly improved therapeutic efficacy by combining At-211-7G7/B6 with daclizumab support a clinical trial of this regimen in patients with ATL.