[7α-18F]fluoro-17α-methyl-5α-dihydrotestosterone: a ligand for androgen receptor-mediated imaging of prostate cancer

doi:10.1016/S0969-8051(00)00172-4

Nuclear Medicine and Biology

Volume 28, Issue 1, January 2001, Pages 85-90

https://doi.org/10.1016/S0969-8051(00)00172-4 Get rights and content

Abstract

We have synthesized a ¹⁸F-labeled androgen, [7α-¹⁸F]fluoro-17α-methyl-5α-dihydrotestosterone, in a no-carrier-added radiosynthesis by exchange of ¹⁸F- (tetrabutylammonium fluoride) with the 7β-tosyloxy of 17α-methyl-5α-dihydrotestosterone. The nonradioactive steroid binds with high affinity and specificity to the androgen receptor and binds poorly, if at all, to other steroid receptors and plasma sex hormone binding globulin. The 7α-¹⁸F-androgen concentrates markedly in the prostate of rats by an androgen receptor-dependent mechanism. It is likely that [7α-¹⁸F]fluoro-17α-methyl-5α-dihydrotestosterone will be an excellent positron emission tomography imaging agent for prostate cancer.

Introduction

The androgen receptor (AR) has been a well-recognized target in the development of radiolabeled ligands suitable for receptor-mediated imaging of prostate cancer. However, it has been noted that the stereochemical requirements of the androgen receptor have made the design of radiolabeled ligands an elusive goal [1]. While there have been many attempts to develop single-photon emission computed tomography (SPECT) imaging ligands for the AR labeled with the usable isotopes of iodine, none have been successful. The requirements of high receptor affinity, low nonspecific binding and stability have proven difficult to overcome. While the development of iodinated ligands has not been successful, several ¹⁸F-androgens that have potential as positron emission tomography (PET) imaging agents have been produced [2]. These androgens, labeled at 16α, 16β and 20 (17α-sidechain) have good affinity for the receptor, and some have concentrated in the prostate in vivo in a receptor-dependent manner.

Recently, we have synthesized a series of steroids, 5α-androstanes and estranes, which were substituted with iodine and fluorine at C-7α [3]. While some of the iodinated steroids had high binding affinity for the androgen receptor, they proved to be unstable in aqueous solution at physiological temperature, and consequently they were not useful imaging agents. One of the fluorinated steroids, 7α-fluoro-17α-methyl-5α-dihydrotestosterone (7α-F-MDHT) had high affinity for the androgen receptor and exceptional androgenic activity. Moreover, this 7α-fluorosteroid was completely stable in aqueous solution under the same conditions in which the iodosteroids were rapidly eliminated. Thus, it appeared that 7α-F-MDHT synthesized with ¹⁸F could be an excellent PET imaging agents for prostate cancer.

In this study we tested specificity of the binding of 7α-F-MDHT by measuring its affinity to several steroid receptors, as well as to plasma sex hormone binding globulin (SHBG). We compared the binding results to those of a similar AR ligand, 7α-fluoro-5α-dihydrotestosterone DHT (7α-F-DHT), that we had also prepared [3]. In addition, we synthesized the ¹⁸F-labeled steroid 7α-¹⁸F-MDHT by an exchange reaction of ¹⁸F⁻ with the 7β-tosyloxy of MDHT (Fig. 1) and tested the ¹⁸F-product for in vivo concentration in the prostate of the rat. The results indicate that 7α-¹⁸F-MDHT has properties that make it an excellent candidate for a prostate cancer imaging PET agent.

Section snippets

Methods

Radioactivity levels in tissues were assessed with a Packard Cobra γ-counter. High pressure liquid chromatography (HPLC) was performed in isocratic mode with a Spectra-physics Iso-Chrom LC pump, Spectra-physics Spectra-200 variable-wavelength UV detector, and a radioisotope detector equipped with a multichannel analyzer. A MacIntegrator II software package was used for peak analysis. The HPLC system used for purification of ¹⁸F-labeled 7α-¹⁸F-MDHT was a C-18 μ-Bonda-pak reverse-phase, 3.9×300

Results

The scheme for the radiochemical synthesis of 7α-¹⁸F-MDHT is shown in Fig. 1. The ¹⁸F⁻ for tosylate exchange reaction was performed using tetrabutylammonium [¹⁸F]-fluoride. In general, 7α-¹⁸F-MDHT was obtained in 5–9% radiochemical yields with a total synthesis time of 60 min. In attempts to further improve the radiochemical yields, we also used dimethyl formamide and methyl ethyl ketone (2-butanone) as the reaction solvents with little or no improvement in the overall radiochemical yields.

Discussion

The ¹⁸F-ligand 7α-¹⁸F-MDHT was chosen for substitution at C-7α- because other androgens that are substituted at this position with a stereochemically bulky methyl group are biologically active, which indicates that there is some tolerance for the AR at this site [11]. The 17α-methyl group was incorporated as a means of protecting the essential 17β-hydroxyl from biological oxidation by 17β-hydroxysteroid dehydrogenase enzymes. In fact, 7α-F-MDHT is approximately seven times more potent as an

Acknowledgements

We thank Toni Reynolds and Ryan Wagner for their excellent technical assistance. This work was supported in part by NIH grants CA37799 (to R.B.H.) and GM08180 (to R.M.H).

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[7α-18F]fluoro-17α-methyl-5α-dihydrotestosterone: a ligand for androgen receptor-mediated imaging of prostate cancer

Abstract

Introduction

Section snippets

Methods

Results

Discussion

Acknowledgements

Steroids

Biochem. Pharmacol.

Steroids

Steroids

J. Biol. Chem.

J. Steroid Biochem.

Fluorine-18-labeled androgensRadiochemical synthesis and tissue distribution studies on six fluorine-substituted androgens, potential imaging agents for prostatic cancer

J. Nucl. Med.

7α-iodo and fluoro steroids as androgen receptor mediated imaging agents

J. Med. Chem.

>Efficient small volume O-18 water target for producing F-18 fluoride with low energy protons

Journal of Labelled Compounds and Radiopharmaceuticals

Fluorine-18 labeling of proteins

J. Nucl. Med.

Fluorine-18 labeling of monoclonal antibodies and fragments with preservation of immunoreactivity

Bioconjug. Chem.

Synthesis and evaluation of 7α-iodo-5α-dihydrotestosterone as a potential radioligand for androgen receptor

J. Med. Chem.

[7α-¹⁸F]fluoro-17α-methyl-5α-dihydrotestosterone: a ligand for androgen receptor-mediated imaging of prostate cancer