alpha -Iodo and 7 alpha -Fluoro Steroids as Androgen Receptor-Mediated Imaging Agents

We have synthesized several 7 alpha -fluoro (F) and 7 alpha -iodo (I) analogues of 5 alpha -dihydrotestosterone (5 alpha -DHT) and 19-nor-5 alpha -dihydrotestosterone (5 alpha -NDHT) and tested them for binding to the androgen receptor and for their biological activity in an in vitro assay with cells that have been engineered to respond to androgens. The relative binding affinity to the androgen receptor determined in competition assays showed that in the androstane series the fluoro steroids have the highest affinity and that F-17 alpha -CH sub(3)-DHT (4) has a higher affinity than 5 alpha -DHT. All other steroids were somewhat less potent than 5 alpha -DHT with F-DHT (2) = I-17 alpha -CH sub(3)-DHT (3) greater than or equal to F-NDHT (6) > F-17 alpha -CH sub(3)-NDHT (8) = I-DHT (1) greater than or equal to I-NDHT (5) > I-17 alpha -CH sub(3)-NDHT (7). The relative biological activity in cells transfected with the androgen receptor and an androgen responsive reporter gene is 4 >> 5 alpha -DHT > 2 > 6 > 3 greater than or equal to 1 greater than or equal to 8 greater than or equal to 5 > 7. The iodinated compound, I-17 alpha -CH sub(3)-DHT (3), with the highest binding activity was synthesized labeled with super(125)I and was shown to bind with high affinity, K sub(a) = 1.9 x 10 super(10) L/mol, and low nonspecific binding to the androgen receptor in rat prostatic cytosol. However, when radiolabeled [ super(125)I]-17 alpha -CH sub(3)-DHT ([ super(125)I]3) was injected into castrated male rats, it showed very poor androgen receptor-mediated uptake into the rat prostate. This was unexpected in light of its superior receptor binding properties and its protection by the 17 alpha -methyl group from metabolic oxidation at C-17. However, the biological potency of I-17 alpha -CH sub(3)-DHT (3) was not as high as would have been expected. When I-DHT (1) and I-17 alpha -CH sub(3)-DHT (3) were incubated in aqueous media at 37 degree C they rapidly decomposed, but they were stable at 0 degree C. The fluorinated analogue 4 treated similarly at 37 degree C was completely stable. The products of the decomposition reaction of I-DHT (1) at 37 degree C were identified as iodide and principally 17 beta -hydroxy-5 alpha -androst-7-en-3-one. The temperature dependence of this elimination reaction explains the inconsistency between the high binding to the androgen receptor (measured at 0 degree C) and the low biological activity, as well as the poor androgen receptor mediated con