Targeting tumors with iodine-123 labeled deoxyuridine: Distribution and DNA binding

5-Iodo-2'-deoxyuridine (IUdR), a thymidine analog, is transported through c ell membrane and is incorporated into newly synthesized DNA during the S ph ase of mitotic cells. In rapidly growing brain tumors such as glioma, radio iodinated IUdR may be an efficient diagnostic as well as therapeutic agent and may provide a means to determine the proliferative activity of the tumo r. IUdR was labeled with I-123 (t(1/2) = 13.3 h, gamma = 159 KeV, 83%) and injected i.v. into nude mice bearing human colorectal carcinoma LS174T. At 3 and 20 h postinjection, tumor uptake was 2.6 +/- 0.9% and 0.5 +/- 0.2%, r espectively, of the injected dose per gram of tissue. Radioactivity in othe r tissues also declined as a function of time, but much more rapidly, yield ing tumor-to-blood ratios of 16.4 +/- 2.2 and tumor-to-muscle ratios of 22. 2 +/- 7.7 at 20 h postinjection. Of the radioactivity in the tumor, 12.6 +/ - 0.9% was bound to DNA at 3 h and 25.2 +/- 2% at 20 h postinjection. A hig h (7 +/- 1.1% i.d.) uptake in thyroid at 3 h postinjection indicated dehalo genation in vivo.