[I-125] 17 alpha-Iodovinyl 11 beta-methoxyestradiol: in vivo and in vitro properties of a high-affinity estrogen-receptor radiopharmaceutical.

17 alpha-[125I[Iodovinyl 11 beta-methoxyestradiol ([I-125]MIVE2) has been prepared with high specific activity (1500-2000 Ci/mmol) and a high affinity for the estrogen receptor (KA = 6.8 x 10(9) M/Ml). In vivo distribution studies using immature rats result in high levels of activity in the uterus (20-30% dose/g) with uterus-to-plasma ratios on the order of 68 to 100. Peak activity in the uterus is obtained between 2 and 4 hr, and by 6 hr 50% of the activity has washed out. The [I-125]MIVE2 exhibits a slower rate of washout relative to the washout of H-3 estradiol. By in vivo competition studies with nonradioactive estradiol, we found that 95% of the [I-125]MIVE2 bound in the uterus is specifically bound to estrogen receptors. The radioactive labeling of MIVE2 is sufficiently rapid so that [I-123]MIVE2 has been synthesized and is currently in clinical trials. These results suggest that MIVE2 would be an excellent agent for the study of estrogen receptors in vivo and in vitro.