Antia Laboratories Inc. http://www.antialabs.com Synthesis and Biological Evaluation of [ 18 F]Bicalutamide, 4-[ 76 Br]Bromobicalutamide, and 4-[ 76 Br]Bromo-thiobicalutamide as Non-Steroidal Androgens for Prostate Cancer Imaging Ephraim E. Parent, ² Carmen S. Dence, ‡ Carl Jenks, ‡ Terry L. Sharp, ‡ Michael J. Welch, ‡ and John A. Katzenellenbogen* ,² Department of Chemistry, UniVersity of Illinois, 600 South Mathews AVenue, Urbana, Illinois 61801, and Washington UniVersity School of Medicine, Saint Louis, Missouri 63110 ReceiVed July 19, 2006 Androgen receptors (AR) are overexpressed in most primary and metastatic prostate cancers. To develop a nonsteroidal AR-mediated imaging agent, we synthesized and radiolabeled several analogs of the potent antiandrogen bicalutamide: [ 18 F]bicalutamide, 4-[ 76 Br]bromobicalutamide, and [ 76 Br]bromo-thiobicalutamide. Two of these analogs, 4-[ 76 Br]bromobicalutamide and [ 76 Br]bromo-thiobicalutamide, were found to have a substantially increased affinity for the androgen receptor (AR) compared to that of bicalutamide. The synthesis of [ 18 F]bicalutamide utilized a pseudocarrier approach to effect addition of a carbanion generated from tracer-level amounts of a radiolabeled precursor to an unlabeled carbonyl precursor. 4-[ 76 Br]Bromobicaluta- mide and [ 76 Br]bromo-thiobicalutamide were labeled through electrophilic bromination of a tributylstannane precursor. The former could be prepared in high specific activity, and its tissue distribution was tested in vivo. Androgen target tissue uptake was evident in castrated adult male rats; however, in DES-treated, AR-positive, tumor-bearing male mice, tumor uptake was low. Introduction Prostate cancer is the second leading cause of cancer death in men in the United States, 1 and it has been directly linked to the androgen receptor (AR) a in most cases. Androgen ablation monotherapy, surgical or chemical castration with nonsteroidal antiandrogens such as flutamide (1) and bicalutamide (2), is the principal initial treatment for progressive prostate cancer and results in the regression of most androgen-dependent tumors. 2 Many men eventually fail androgen ablation therapy, however, and die of recurrent androgen-independent prostate cancer (AIPC). In our efforts to develop agents for the in vivo imaging of androgen receptor (AR) in prostate cancers by positron emission tomography (PET), we and others have prepared steroidal 3-5 and nonsteroidal 6,7 AR ligands labeled with fluorine-18. Several of these 18 F-labeled steroidal androgens show tissue distribution in chemically castrated rats that is consistent with their uptake by an AR-dependent process, namely, selective target tissue (prostate) accumulation that is effectively blocked by coadmin- istration of a blocking dose of unlabeled androgen. 3,5 One of the agents we have developed, 16-[ 18 F]fluoro-5R-dihydrotes- tosterone (FDHT), has proved to be an effective agent for PET imaging of AR-positive prostate tumors in humans. 8,9 Despite androgen deprivation therapy, most patients will experience disease progression to AIPC, 10 a stage at which only a small fraction of tumors respond to secondary hormonal therapies, including treatment with nonsteroidal antiandrogens. 11 In fact, at this stage treatment with an antiandrogen often leads to increased tumor growth. 12-14 Although the cause of this antiandrogen activation is not known, it is believed to be due to changes in the androgen signaling cascade. 15 Therefore, PET imaging agents that are close structural analogs of the nonste- roidal antiandrogen compounds used in second-line hormone therapy, namely, flutamide and bicalutamide, might be particu- larly useful because failure of these agents generally presages the onset of unmanageable disease. The development of effective PET imaging agents based on such nonsteroidal androgens, however, has proved to be a challenge. 16-18 The nonsteroidal antiandrogen bicalutamide is the leading antiandrogen used for the treatment of prostate cancer. Non- steroidal antiandrogens, such as flutamide and bicalutamide, are often referred to as “pure antiandrogens” because they bind exclusively to the AR and, therefore, are devoid of antigona- dotropic, antiestrogenic, and progestational effects (Figure 1). 19 Bicalutamide is a racemic mixture, 20,21 with the R enantiomer having 30-fold higher binding affinity than the S isomer. 22 In this study, we describe the syntheses of 18 F and 76 Br-labeled androgen receptor ligands based on the bicalutamide core: [ 18 F]bicalutamide, 4-[ 76 Br]bromobicalutamide, and [ 76 Br]bromo- thiobicalutamide, the last being based on thiobicalutamide, a bicalutamide analog reported to have improved AR binding affinity. 23 We determined the AR binding affinity of these compounds and related analogs, and we determined the meta- bolic stability and tissue biodistribution of 4-[ 76 Br]bromobi- calutamide in rats and tumor-bearing mice. Results Chemistry. Linear Synthesis of Bicalutamide (1). Our first attempt to develop a synthesis of bicalutamide suitable for radiolabeling involved a linear synthetic route, shown in Scheme 1. 4-Cyano-3-trifluoromethyl-aniline (5) was condensed with pyruvic acid chloride 24 to produce ketoamide 6 in 27% yield. 21 The keto moiety of 6 was attacked by the anion of amine 12, 25 which was formed using either lithium diisopropyl amide (LDA) or n-BuLi, to afford anilide 7 in 30%. Anilide 7 was protected with acetyl chloride to give the corresponding ester (8), 26 and then N-methylated with methyl trifluoromethanesulfonate 27,28 to form the ammonium salt 9 in 53% and 66% yields, respectively. * Corresponding author. Phone: 217-333-6310. E-mail: jkatzene@uiuc.edu. ² University of Illinois. ‡ Washington University School of Medicine. a Abbreviations: AR, androgen receptor; AIPC, androgen-independent prostate cancer; PET, positron emission tomography; FDHT, 16-[ 18 F]- fluoro-5R-dihydrotestosterone; LDA, lithium diisopropyl amide; TBAF, tetrabutylammonium fluoride; RBA, relative binding affinity; DHT, dihy- drotestosterone. 1028 J. Med. Chem. 2007, 50, 1028-1040 10.1021/jm060847r CCC: $37.00 © 2007 American Chemical Society Published on Web 01/20/2007