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

Abstract

We have synthesized several 7alpha-fluoro (F) and 7alpha-iodo (I) analogues of 5alpha-dihydrotestosterone (5alpha-DHT) and 19-nor-5alpha-dihydrotestosterone (5alpha-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-17alpha-CH3-DHT (4) has a higher affinity than 5alpha-DHT. All other steroids were somewhat less potent than 5alpha-DHT with F-DHT (2) = I-17alpha-CH3-DHT (3) >/= F-NDHT (6) > F-17alpha-CH3-NDHT (8) = I-DHT (1) >/= I-NDHT (5) > I-17alpha-CH3-NDHT (7). The relative biological activity in cells transfected with the androgen receptor and an androgen responsive reporter gene is 4 >> 5alpha-DHT > 2 > 6 > 3 >/= 1 >/= 8 >/= 5 > 7. The iodinated compound, I-17alpha-CH3-DHT (3), with the highest binding activity was synthesized labeled with 125I and was shown to bind with high affinity, Ka = 1.9 x 10(10) L/mol, and low nonspecific binding to the androgen receptor in rat prostatic cytosol. However, when radiolabeled [125I]-17alpha-CH3-DHT ([125I]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 17alpha-methyl group from metabolic oxidation at C-17. However, the biological potency of I-17alpha-CH3-DHT (3) was not as high as would have been expected. When I-DHT (1) and I-17alpha-CH3-DHT (3) were incubated in aqueous media at 37 degrees C they rapidly decomposed, but they were stable at 0 degrees C. The fluorinated analogue 4 treated similarly at 37 degrees C was completely stable. The products of the decomposition reaction of I-DHT (1) at 37 degrees C were identified as iodide and principally 17beta-hydroxy-5alpha-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 degrees C) and the low biological activity, as well as the poor androgen receptor mediated concentration in vivo. The fluorinated analogue F-17alpha-CH3-DHT (4) has both high affinity for the androgen receptor and high stability in aqueous media. Of the compounds tested, 4 has the highest affinity for the androgen receptor as well as the highest androgenic activity. Thus it is likely that F-17alpha-CH3-DHT 4 labeled with 18F will be an excellent receptor-mediated diagnostic imaging agent.

7α-18F-Fluoromethyl-Dihydrotestosterone and 7α-18F-Fluoromethyl-Nortestosterone: Ligands to Determine the Role of Sex Hormone–Binding Globulin for Steroidal Radiopharmaceuticals

“…To maintain structural consistency, both of these agents were labeled with 18 F at the same position, on a 7a methyl group. Androgens labeled at the 7a position with halogens (fluorine and iodine) are known and have good AR binding affinity (37), but 7a-fluoromethyl androgens are novel. We chose this new site less for chemical novelty than to avoid fluorine labeling at the 16b position, a site known to be prone to defluorination (17), as well as to incorporate into both compounds a 7a-methyl group, which has been shown to slow metabolism of steroids (30).…”

A remote controlled system for the preparation of 7α-[18F]fluoro-17α-methyl 5α-dihydrotestosterone ([18F]FMDHT) using microwave

“…7-β-tosyl-17 α-methyl-5-α-dihydrotestosterone (tosylate) was used as our starting material for radio-fluorination reaction and was synthesized as previously described with minor modifications (Labaree et al, 1999) F]-fluoride was transferred from the cyclotron on to a QMA cartridge in the hot cell to recover O-18 water, followed by elution of [ 18 F]fluoride from the cartridge using kryptofix/K 2 CO 3 solution (1.5 mL). The water from the eluant was evaporated azeotropically (2 × 500 μL acetonitrile) using an oil bath maintained at 100-110°C and a gentle stream of argon as described (Garg, et al, 1994).…”

“…Earlier, Labaree et al (Labaree, et al, 1999) reported the synthesis of 7α-fluoro-17α-methyl 5α-dihydrotestosterone (FMDHT). This compound showed higher binding affinity to AR when compared to 5α-DHT and other related structural analogues.…”

“…This compound showed higher binding affinity to AR when compared to 5α-DHT and other related structural analogues. In addition, binding to other receptors was negligible for this compound, thus exhibiting high specificity towards AR (Labaree et al, 1999). To explore its utility as a PET imaging agent for AR, we radio-labeled FMDHT with fluorine-18 to generate 7α-[ 18 F]fluoro-17α-methyl 5α-dihydrotestosterone ([ 18 F]FMDHT) and assessed its binding and distribution in vivo.…”

A Structure Based, Solid Phase Synthesis Approach to the Development of Novel Selective Estrogen Receptor Modulatory Steroids

“…Hochberg's group subsequently prepared a series of 7a-iodo (and fluoro) androgens as potential imaging agents. From this series, the radiohalogen was introduced by simple nucleophilic displacement into a steroid nucleus bearing appropriate 19/17a substituents [35,36]. They evaluated the effects of dihydro testosterone vs. dihydro nortestosterone vs. 17a-methyl dihydro(nor)testosterone.…”