[Br-76]Bromodeoxyuridine, a potential tracer for the measurement of cell proliferation by positron emission tomography, in vitro and in vivo studies in mice

5-Bromo-2'-deoxyuridine (BrUdR) labeled with Br-77 and Br-76 was compared with 5-iodo-2'-deoxyuridine (IUdR) labeled with I-125 Or I-131, first in vitro then in in vivo experiments in mice. The results showed a significantly higher incorporation of BrUdR into DNA than IUdR, which can be explained by the greater similarity (size and surface hydrophilicity of the molecules) of BrUdR to thymidine. Both tracers are dehalogenated quickly in vivo but not in vitro. Free bromide is excreted more slowly than iodide, resulting in a higher background activity level after the application of [Br-76]BrUdR and compensates for the favorable DNA incorporation. Br-76 has more favorable properties than I-124 for imaging purposes with positron emission tomography (PET) because of a very convenient half-life (16 h vs. 4.15 days) and about double the positron yield per decay, However, the more favorable physical properties are balanced by the slower excretion and thus the estimated radiation dose is higher in the case of Br-76 than I-124. Thus, both tracers, [I-124]IUdR and [Br-76]BrUdR are potentially suitable but not optimal to measure cell proliferation in vivo. The difference between the two tracers is small and the extrapolation from mice to human difficult, and thus it cannot be concluded if one of the tracers would be better than the other for imaging of cancer patients. NUCL MED BIOL 26;6:673-679, 1999. (C) 1999 Elsevier Science Inc. All rights reserved.