7 alpha-iodine-125-iodo-5 alpha-dihydrotestosterone: a radiolabeled ligand for the androgen receptor.
We describe the preparation of 7 alpha-[125I]iodo-5 alpha-dihydrotestosterone (7 alpha-[125I]IDHT) and its characterization as a ligand for the androgen receptor.


METHODS
We designed a route to prepare the radioiodine-labeled androgen on microscale through treatment of the 7 beta-tosylate of 7 beta-hydroxy-5 alpha-dihydrotestosterone-17 beta-p-nitro-benzoate with Na125I, followed by alkaline hydrolysis. The radiolabeled steroid was tested as a ligand for the androgen receptor in cytosol from MCF-7 cells, and for its in vivo tissue distribution in the rat. In addition, we tested 7 alpha-[125I]IDHT as a ligand in a novel assay for the detection and quantification of the ligand activated androgen receptor by in vitro autoradiography.


RESULTS
The above synthetic route produced the 17 beta-p-nitrobenzoate of 7 alpha-[125I]IDHT in carrier-free form and in good yield. The 17 beta-ester was removed with alkali and the resulting 7 alpha-[125I]IDHT was purified by HPLC. 7 alpha-[125I]IDHT bound with high affinity, Kd = 0.26 nM, to the androgen receptor and showed low nonspecific binding. Since the ligand was carrier free and thus of very high specific activity, approximately 2,200 Ci/mmole, the sensitivity of the assay was much greater than with [3H]R1881, the classical androgen receptor ligand with which it was compared. When tested as a ligand for in vitro autoradiography, 7 alpha-[125I]IDHT produced excellent autoradiograms of the activated receptor with very low nonspecific binding and with only overnight exposure of the film.


CONCLUSIONS
These studies demonstrate that 7 alpha-[125I]IDHT is an excellent ligand for the androgen receptor.
Labaree
D C
DC
Department of Obstetrics and Gynecology, Yale University School of Medicine, New Haven, Connecticut 06510, USA.
Brown
T J
TJ
Hoyte
R M
RM
Hochberg
R B
RB
D000345:Affinity Labels; D000704:Analysis of Variance; D000818:Animals; D001665:Binding Sites; D001667:Binding, Competitive; D001943:Breast Neoplasms / Q000000981:diagnostic imaging / Q000378:metabolism; D013196:Dihydrotestosterone / Q000031:analogs & derivatives* / Q000138:chemical synthesis / Q000493:pharmacokinetics; D005260:Female; D006801:Humans; D007457:Iodine Radioisotopes / Q000493:pharmacokinetics*; D008099:Liver / Q000378:metabolism; D008297:Male; D018482:Muscle, Skeletal / Q000378:metabolism; D011467:Prostate / Q000378:metabolism; D011877:Radionuclide Imaging; D051381:Rats; D017207:Rats, Sprague-Dawley; D011944:Receptors, Androgen / Q000378:metabolism*; D013961:Thyroid Gland / Q000378:metabolism; D014018:Tissue Distribution; D014407:Tumor Cells, Cultured
C086913:7-iodo-17-hydroxy-3-androstanone; D000345:Affinity Labels; D007457:Iodine Radioisotopes; D011944:Receptors, Androgen; D013196:Dihydrotestosterone