of 67A2 markedly suppressed tumor growth, and tumors completely disappeared around 2 weeks after injection except for a single mouse treated with 1.85 MBq of 67A2. Although we observed tumor growth delay in the treatment group with 0.74 MBq of 67A2 compared with the untreated group, there was no significant difference in tumor growth compared with the unlabeled IgG treatment group. The body weight of mice treated with 12A8 and 67A2 temporarily decreased by approximately 15%, but started to increase within the first week after treatment, and all mice tolerated the RIT experiment up to day 28. Statistical analysis Tumor growth data were analyzed by two-way repeatedmeasures ANOVA with the StudentNewmanKeuls method multiple comparison test. Apoptotic cell data were analyzed by Student’s t-test. A value of P,0.05 was considered statistically significant. Results In vitro characterization of 111 In-labeled antibodies From the cell binding assay, binding of 12A8 and 67A2 using 16107 SY cells was 50 and 76%, respectively. Binding of 67A2 to 23446639 SY cells was higher compared with that of 12A8, especially at low cellular concentrations. The immunoreactive fraction of 12A8 and 67A2 was 83 and 88%, respectively. From the competitive inhibition assay, the Kd of 12A8 and 67A2 was estimated to be 8.0 and 1.9 nM, respectively. We examined the temporal change of radioactivity of 67A2 and 67A2 in the subcellular fraction. Radioactivity in the cell membrane-bound fraction of 67A2 and 67A2 rapidly decreased with time. In the case of 67A2, the radioactivity of the internalized fraction increased with time, reaching approximately 55% after a 20-h incubation at 37uC. In contrast, internalized radioactivity of 67A2 temporarily increased up to 3 h but thereafter decreased gradually. Radioactivity of the non-protein-bound fraction in the culture medium increased with time, reflecting dehalogenation of the 125I-labeled antibody in cells. When cells were incubated on ice, the membrane-bound fraction did not change and internalization was not observed for at least 3 h. These results of the internalization assay were similar to those with 12A8 in our previous study. Histological analysis High MEK162 web numbers of mitotic cells were observed in H&E-stained sections of SY tumors from the untreated group, reflecting a high proliferation activity. Tumor sections obtained at day 7 after injection of 12A8 revealed areas of necrosis and fibrosis that increased in a dose-dependent manner. Necrosis and fibrosis were also observed in tumors treated with 1.85 and 3.7 MBq of 67A2, but not with unlabeled IgG alone and 0.74 MBq of 67A2. On TUNEL-stained sections, apoptotic cells were rarely observed under untreated conditions. In tumors treated with 12A8, the percentage of apoptotic cells tended to increase with an increase in the dose of radioactivity. In contrast, in tumors treated with 67A2, the same level of apoptotic cells was observed for 0.74 to 3.7 MBq without a dose-dependent increase. Data are expressed as decay-corrected % ID/g6SD normalized to a 20-g body 9346307 weight mouse.Data are expressed as decay-corrected % ID/g6SD normalized to a 20-g body weight mouse. C-Kit Targeted Radioimmunotherapy Discussion SCLC is an aggressive tumor and the 2-year survival rate for patients with 
limited-stage and extensive-stage diseases is between 20 to 40% and 2 to 5%, respectively. Therefore, additional effective anticancer therapy is required, especially for patients with extensive-stage disease. SCLC