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Studies with 17ß(16-[¹²⁵I]Iodo)-Estradiol, an Estrogen Receptor-Binding Radiopharmaceutical, in Rats Bearing Mammary Tumors

University of Wisconsin Hospital, Madison, Wisconsin

Correspondence: For reprints contact: S. John Gatley, PhD, Section of Nuclear Medicine, University Hospital, 600 Highland Ave., Madison, WI 53792.

ABSTRACT

We have studied the distribution of 17ß(16-[¹²⁵I]Iodo)-estradiol (I-E₂) in tumor-bearing and normal rats. High early adrenal-to-blood ratios (up to 22 at 5 min) were seen in all groups,but this fell to six at 1 hr. Uterus-to-blood ratios of 15 were found,and these were fairly constant up to 2 hr after administration. Uptake of label in the uterus, but not in the adrenals, was sensitive to excess diethylstilbestrol, which competes with I-E₂ for estrogen receptors. Mean tumor-to-blood ratios of 1.4, 5.5, and 8.7 were seen at 1 hr in rats with transplanted, spontaneous, and N-nitrosomethylurea-induced tumors, respectively. Diethylstilbestrol was shown to reduce uptake of label by spontaneous tumors. Most of the radioactivity was excreted in the bile by 1 hr. Better estrogen-receptor-binding radiopharmaceuticals can probably be designed.