16 alpha-[77Br]bromo-11 beta-methoxyestradiol-17 beta: a gamma-emitting estrogen imaging agent with high uptake and retention by target organs.

16 alpha[77Br]Bromo-11 beta-methoxyestradiol-17 beta [MBE(Br-77)], a compound with high affinity for the estrogen receptor and with low nonspecific binding, has been prepared with an effective specific activity of 770--1450 Ci per mmole at the time of synthesis. In immature female rats, this compound is taken up selectively by the uterus and is retained for prolonged periods. This is presumably due to the binding of this compound to the estrogen receptor, as uterine uptake is blocked selectively by coadministration of an excess of unlabeled estradiol, and administration of a chase dose of unlabeled estradiol results in a rapid decrease in activity in the uterus. In double-label experiments with 16 alpha[125I]estradiol and MBE(Br-77), the two compounds showed equally selective uterine uptake at 1 hr, but the bromine-labeled compound became increasingly more selective at 3 and 6 hr. MBE(Br-77) may prove to be a more favorable agent for imaging human breast tumors than our previously described compound, 16 alpha-[77Br]bromoestradiol-17 beta.