Toxicity and efficacy of radioimmunotherapy in carcinoembryonic antigen-producing medullary thyroid cancer xenograft: comparison of iodine 131-labeled F(ab')2 and pretargeted bivalent hapten and evaluation of repeated injections.

This study compared the toxicity and efficacy of 131I-labeled bivalent hapten pretargeted by anti-carcinoembryonic antigen (CEA)/anti-N alpha-(diethylenetriamine-N,N,N',N''-tetraacetic acid-indium(F6-734) bispecific antibody [affinity enhancement system (AES) reagents] with 131I-labeled anti-CEA F(ab')2 (131I-F6) in mice grafted with a human medullary thyroid carcinoma. Repeated injections of AES reagents were also evaluated. Mice bearing TT tumor xenografts were treated with 37, 74, or 92.5 MBq of AES reagents, two injections of 74 MBq of AES reagents 45 days apart, or 37 or 92.5 MBq of 131I-F6. Control groups were treated with nonspecific 131I-labeled F(ab')2, nonspecific AES reagents, nonradiolabeled F6, F6-734 bispecific antibody, and nonradiolabeled bivalent hapten or received no injection. For AES treatments, bispecific antibody was injected 48 h before the hapten. Animal weight, hematological toxicity, tumor volume, and serum thyrocalcitonin were monitored during 5 months. At 92.5 MBq, weight loss was significantly lower after AES than F6 treatment (P = 0.004). The percentages of leukocyte count changes were significantly lower after AES than F6 at 37 and 92.5 MBq (P = 0.01 and 0.04, respectively). The percentage of platelet count changes was significantly lower with AES at the 92.5-MBq dose level (P = 0.04). In the group injected twice with AES reagents, toxicity was not significantly increased after the second treatment. Tumor response was observed in all cases but was significantly longer with repeated treatments of 74 MBq AES reagents than with a single treatment (P = 0.004). Two complete responses were observed with repeated treatments. Changes in thyrocacitonin level paralleled those in tumor volume. These results indicate that pretargeted radioimmunotherapy was at least as efficient as one-step radioimmunotherapy and markedly less toxic. Repeated treatments with AES reagents increased efficacy without increasing toxicity.