Carbon-11-labeled estrogens as potential imaging agents for breast tumors

We have prepared two estrogens labeled with carbon-11, 17 alpha-[11C] methylestradiol and 11 beta-ethyl-17 alpha-[11C]methylestradiol, at a specific activity of 300-1000 Ci/mmol (11.1-37 TBq/mmol), and we have determined their in vivo biodistribution in immature female rats. Both compounds accumulated selectively in two target tissues, the uterus and ovaries, reaching levels of 3.5-4.9%ID/g at 20 min and 4.6-6.6%ID/g at 40 min; uterus-to-blood ratios reached 12-23. Uterine uptake showed a saturation dependence with the amount of injected mass, and was displaced by unlabeled estradiol, indicating that this uptake was receptor mediated. These results suggest that these compounds may be useful in estrogen receptor-based imaging of breast tumors.