We have synthesized two gamma-emitting, 125I-labeled
steroids, E- and Z-7 alpha-methyl-17 alpha-(2'-[125I]iodovinyl)-19-
nortestosterone [125I](E- and Z-
MIVNT) for specific labeling of
androgen receptors. [125I]E- and [125I]Z-
MIVNT were synthesized stereospecifically from E- and Z-7 alpha-methyl-17 alpha-(2'-tri-n-butylstannyl-vinyl)-19-nortestosterone. The
tin adducts were prepared by addition of
tri-n-butyltin hydride to 7 alpha-methyl-17 alpha-ethynyl-19-nortestosterone, and after purification they were converted in high yield to the [125I]
MIVNT isomers by reaction with 125I (generated in situ by oxidation of [125I]
iodide with
chloramine T). The 125I-labeled products were purified by high-performance liquid chromatography, and their mass determined with an ultraviolet detector (specific activity of both, approximately 2,200 Ci/mmol). In rat prostate cytosol, [125I]E-
MIVNT bound with high affinity to a single class of binding sites. Nonspecific binding in the presence of
5 alpha-dihydrotestosterone was relatively low, and compared favorably with that obtained in parallel studies with [3H]
methyltrienolone (
R1881). The E-isomer bound prostate cytosol with at least twice the affinity of the Z-isomer; therefore, the interaction of the E-isomer with the
androgen receptor as well as other
steroid receptors was studied in greater detail. Complexes of the
androgen receptor with [125I]E-
MIVNT as well as [3H]
R1881 dissociate very slowly at 4C (kdiss for both = 0.04 h-1). Displacement studies showed that the interaction of [125I]E-
MIVNT with the
androgen receptor is highly specific. Competition studies showed that unlabeled E-
MIVNT binds poorly to other
steroid receptors in rat tissue cytosols. These binding properties make [125I]E-
MIVNT a promising
ligand for study of the
androgen receptor, and [123I]E-
MIVNT a potential imaging agent for the detection of
androgen-dependent
tumors, such as
prostate cancer.