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Characterization of High Specific Activity [16-¹²³I]Iodo-17ß-estradiol as an Estrogen Receptor-specific Radioligand Capable of Imaging Estrogen Receptor-positive Tumors¹

Departments of Obstetrics and Gynecology [E. J. P., K. N., H. H. G., J. R. V. N., E. S. D.], Biochemistry [E. J. P.], Radiation Medicine [W. J. S.], and Surgery [D. E. K.], University of Kentucky College of Medicine, Lexington, Kentucky 40536; Department of Radiation Medicine, University of Kansas Medical Center, School of Medicine, Kansas City, Kansas 66103 [J. A. S., D. F. P.], and BioMedical Research Laboratories, Overland Park, Kansas 66204 [R. J. B.]

16-[¹²³I]Iodo-17ß-estradiol (16-[¹²³I]E₂) has been characterized for use as a selective radioligand for estrogen receptor (ERc) that is capable of generating in situ images of ERc-positive tumors. High specific activity 16-[¹²³I]E₂ (7,500–10,000 Ci/mmol) was used in all determinations. Radiochemical purity was determined by thin layer chromatography, and the selectivity of radioligand for ERc was evaluated using size exclusion high performance liquid chromatography on ERc prepared from rodent uteri. Efficiencies of radioiodination approaching 100% were achieved, and excellent receptor selectivity was obtained even when the efficiency of radioiodination was as low as 10%. Low radiochemical purity was always associated with poor selectivity for ERc. No new radioligand species was generated during the course of radiodecay; however, reduced binding over time, even when increased activity was used to compensate for radiodecay, indicated that the formation of a radioinert competitor does occur. 16-[¹²³I]E₂ demonstrated stable, high affinity binding to ERc and was concentrated by ERc-positive tissues. After injecting 16-[¹²³I]E₂ in vivo, images of ERc-containing tissues were obtained, including rabbit reproductive tract and dimethylbenzanthracene-induced tumors. The demonstrations of ERc selectivity and image formation both indicate that 16-[¹²³I]E₂ should have promise as a useful new radiopharmaceutical for imaging ERc-positive cancers.

¹ This work was supported by grants from NIH (HD-16087), the American Cancer Society (PDT-211C), and the Veterans Administration.

² To whom requests for reprints should be addressed, at Department of Biochemistry, University of Kentucky College of Medicine, 800 Rose Street, Lexington, KY 40536.

Received 4/10/90. Accepted 9/ 4/90.