Design, synthesis, and pharmacological characterization of 4-[4, 4-dimethyl-3-(4-hydroxybutyl)-5-oxo-2-thioxo-1-imidazolidinyl]- 2-iodobenzonitrile as a high-affinity nonsteroidal androgen receptor ligand

The design, synthesis and bioactivity evaluation of novel androgen receptor degraders based on hydrophobic tagging

Prostate Cancer (PCa) easily progress to metastatic Castration-Resistant Prostate Cancer (mCRPC) that remains a significant cause of cancer-related death. Androgen receptor (AR)-dependent transcription is a major driver of prostate tumor cell proliferation. Proteolysis-targeting chimaera (PROTAC) technology based on Hydrophobic Tagging (HyT) represents an intriguing strategy to regulate the function of therapeutically androgen receptor proteins. In the present study, we have designed, synthesized, and evaluated a series of PROTAC-HyT AR degraders using AR antagonists, RU59063, which were connected with adamantane-based hydrophobic moieties by different alkyl chains. Compound D-4-6 exhibited significant AR protein degradation activity, with a degradation rate of 57 % at 5 μM and nearly 90 % at 20 μM in 24 h, and inhibited the proliferation of LNCaP cells significantly with an IC50 value of 4.77 ± 0.26 μM in a time-concentration-dependent manner. In conclusion, the present study lays the foundation for the development of a completely new class of therapeutic agents for the treatment of mCRPC, and further design and synthesis of AR-targeting degraders are currently in progress for better degradation rate.

Click Chemistry and Targeted Degradation: A Winning Combination for Medicinal Chemists?

Click chemistry is universally recognized as a powerful strategy for the fast and precise assembly of diverse building blocks. Targeted Protein Degradation (TPD) is a new therapeutic modality based on heterobifunctional small-molecule degraders that provides new opportunities to medicinal chemists dealing with undruggable targets and incurable diseases. Here, we highlight how very recently the TPD field and that of click chemistry have merged, opening up the possibility for fine-tuning the properties of a degrader, chemically assembled through a "click" synthesis. By reviewing concrete examples, we want to provide the reader with the insight that the application of click and bioorthogonal chemistry in the TDP field may be a winning combination.

Chemical Degradation of Androgen Receptor (AR) Using Bicalutamide Analog-Thalidomide PROTACs

A series of PROTACs (PROteolysis-TArgeting Chimeras) consisting of bicalutamide analogs and thalidomides were designed, synthesized, and biologically evaluated as novel androgen receptor (AR) degraders. In particular, we found that PROTAC compound 13b could successfully demonstrate a targeted degradation of AR in AR-positive cancer cells and might be a useful chemical probe for the investigation of AR-dependent cancer cells, as well as a potential therapeutic candidate for prostate cancers.

Second-Generation Androgen Receptor Antagonists as Hormonal Therapeutics for Three Forms of Prostate Cancer

Enzalutamide is the first second-generation nonsteroidal androgen receptor (AR) antagonist with a strong binding affinity to AR. Most significantly, enzalutamide can prolong not only overall survival time and metastatic free survival time for patients with lethal castration-resistant prostate cancer (CRPC), but also castration-resistant free survival time for patients with castration-sensitive prostate cancer (CSPC). Enzalutamide has thus been approved by the US Food and Drug Administration (FDA) for the treatment of both metastatic (in 2012) and non-metastatic (in 2018) CRPC, as well as CSPC (2019). This is an inspiring drug discovery story created by an amazing interdisciplinary collaboration. Equally important, the successful clinical use of enzalutamide proves the notion that the second-generation AR antagonists can serve as hormonal therapeutics for three forms of advanced prostate cancer. This has been further verified by the recent FDA approval of the other two second-generation AR antagonists, apalutamide and darolutamide, for the treatment of prostate cancer. This review focuses on the rational design and discovery of these three second-generation AR antagonists, and then highlights their syntheses, clinical studies, and use. Strategies to overcome the resistance to the second-generation AR antagonists are also reviewed.

Radiobromine and radioiodine for medical applications

The halogens bromine and iodine have similar chemical properties and undergo similar reactions due to their closeness in Group 17 of the periodic chart. There are a number of bromine and iodine radionuclides that have properties useful for diagnosis and therapy of human diseases. The emission properties of radiobromine and radioiodine nuclides with half-lives longer than 1 h are summarized along with properties that make radionuclides useful in PET/SPECT imaging and β/Auger therapy, such that the reader can assess which of the radionuclides might be useful for medical applications. An overview of chemical approaches that have been used to radiolabel molecules with radiobromine and radioiodine nuclides is provided with examples. Further, references to a large variety of different organ/cancer-targeting agents utilizing the radiolabeling approaches described are provided.

Pharmacokinetic/pharmacodynamic drug evaluation of enzalutamide for treating prostate cancer

INTRODUCTION

Enzalutamide is the first approved second-generation androgen receptor (AR) antagonist in the treatment of metastatic castration-resistant prostate cancer (mCRPC) with or without docetaxel-based chemotherapy. Over the past 5 years, a number of attempts were made to determine the efficacy of enzalutamide in the different clinical settings. Areas covered: A literature search was performed at the PubMed, Embase, and Web of Science database to collect the most relevant and impactful studies, including basic science investigations, clinical trials, and reviews. This article focuses on the pharmacology, efficacy, tolerability, and future perspective of enzalutamide. Expert opinion: The treatment paradigm of CRPC has been dramatically challenged of late. Enzalutamide are in wide use because of its favorable efficacy and safety, but primary or acquired resistance to the drug will eventually develop. Further studies are thus necessary to identify appropriate patients who can achieve apparent benefits from enzalutamide alone or in combination with other drugs.

Therapeutic Rationales, Progresses, Failures, and Future Directions for Advanced Prostate Cancer

Patients with localized prostate cancer (PCa) have several therapeutic options with good prognosis. However, survival of patients with high-risk, advanced PCa is significantly less than patients with early-stage, organ-confined disease. Testosterone and other androgens have been directly linked to PCa progression since 1941. In this review, we chronicle the discoveries that led to modern therapeutic strategies for PCa. Specifically highlighted is the biology of androgen receptor (AR), the nuclear receptor transcription factor largely responsible for androgen-stimulated and castrate-recurrent (CR) PCa. Current PCa treatment paradigms can be classified into three distinct but interrelated categories: targeting AR at pre-receptor, receptor, or post-receptor signaling. The continuing challenge of disease relapse as CR and/or metastatic tumors, destined to occur within three years of the initial treatment, is also discussed. We conclude that the success of PCa therapies in the future depends on targeting molecular mechanisms underlying tumor recurrence that still may affect AR at pre-receptor, receptor, and post-receptor levels.

Enzalutamide: looking back at its preclinical discovery

INTRODUCTION

In patients with metastatic prostate cancer (PCa), castration is the standard first-line therapy. However, all patients eventually develop castration-recurrent PCa (CRPC). In these patients who fail the first-line therapy, enzalutamide has emerged as a viable alternative. Enzalutamide is a second-generation small molecule androgen receptor (AR) antagonist that blocks the AR nuclear translocation and DNA binding without any known AR agonist activity.

AREAS COVERED

This review provides an overview of the history of the oral selective AR modulator, enzalutamide, and discusses its discovery and current preclinical experimental results including resistance mechanisms. The article illustrates the history of discovery based on enzalutamide's mechanism of action. Furthermore, the authors highlight the drug's clinical development and post-launch developments.

EXPERT OPINION

The landscape of CRPC has changed dramatically over the last few years, as new agents with proven benefit in overall survival have been approved. Compared to the average time of 10 - 15 years for a new drug to go from pre-clinical discovery to registration, enzalutamide obtained FDA approval in < 6 years after its initial characterization. Enzalutamide, a targeted agent of the androgen-AR axis, has shown promising results in CRPC and is generally well tolerated. However, drug resistance inevitably emerges is a severe limitation. The authors believe that further clinical studies that look at its use as either a combinational or sequential therapy could help to address these concerns.

Preclinical pharmacology of FL442, a novel nonsteroidal androgen receptor modulator

The preclinical profiles of two most potent compounds of our recently published cycloalkane[d]isoxazole pharmacophore-based androgen receptor (AR) modulators, FL442 (4-(3a,4,5,6,7,7a-hexahydro-benzo[d]isoxazol-3-yl)-2-(trifluoromethyl)benzonitrile) and its nitro analog FL425 (3-(4-nitro-3-(trifluoromethyl)phenyl)-3a,4,5,6,7,7a-hexahydrobenzo[d]isoxazole), were explored to evaluate their druggability for the treatment of AR dependent prostate cancer. The studies revealed that both compounds are selective to AR over other closely related steroid hormone receptors and that FL442 exhibits equal inhibition efficiency towards the androgen-responsive LNCaP prostate cancer cell line as the most widely used antiandrogen bicalutamide and the more recently discovered enzalutamide. Notably, FL442 maintains antiandrogenic activity with enzalutamide-activated AR mutant F876L. In contrast to bicalutamide, FL442 does not stimulate the VCaP prostate cancer cells which express elevated levels of the AR. Distribution analyses showed that [(14)CN]FL442 accumulates strongly in the mouse prostate. In spite of its low plasma concentration obtained by intraperitoneal administration, FL442 significantly inhibited LNCaP xenograft tumor growth. These findings provide a preclinical proof for FL442 as a promising AR targeted candidate for a further optimization.

Structural basis for computational screening of non-steroidal androgen receptor ligands

IMPORTANCE OF THE FIELD

Deep structural and chemical understanding of the protein target and computational methods for detection of receptor-selective ligands are important for the early drug discovery in the steroid receptor field.

AREAS COVERED IN THIS REVIEW

This review focuses on the use of currently available structural information of the androgen receptor (AR) and known AR ligands to make computational strategies for the discovery of AR ligands in order to offer new chemical platforms for drug development.

WHAT THE READER WILL GAIN

AR is a challenging target for drug discovery and modeling even if there is a wealth of experimental data available. First, only the active structure of AR is currently known, which hampers the design of AR antagonists. Second, the structural similarity between the ligand-binding sites of AR and its mutated forms and closely related steroid receptors (SRs) such as progesterone receptors presents challenges for the development of drugs with receptor-selective function.

TAKE HOME MESSAGE

Research indicates that a very small chemical change in the structure of a non-steroidal ligand can cause a complete change in its activity. One source of this effect arises from binding to similar binding sites in related SRs and other proteins in the signaling pathway. Currently, computational methods are not able to predict the subtle differences between AR ligand activities but modeling does offer the possibility of generating new lead structures that might have the desired properties.

Grappling with the androgen receptor: a new approach for treating advanced prostate cancer

In this issue of Cancer Cell, Andersen et al. report on a small molecule that interacts with and blocks transactivation of the androgen receptor amino-terminal domain. This agent can overcome the shortcomings of clinically used antiandrogens, an important advance in the development of effective therapy for advanced prostate cancer.

Development of a second-generation antiandrogen for treatment of advanced prostate cancer

Metastatic prostate cancer is treated with drugs that antagonize androgen action, but most patients progress to a more aggressive form of the disease called castration-resistant prostate cancer, driven by elevated expression of the androgen receptor. Here we characterize the diarylthiohydantoins RD162 and MDV3100, two compounds optimized from a screen for nonsteroidal antiandrogens that retain activity in the setting of increased androgen receptor expression. Both compounds bind to the androgen receptor with greater relative affinity than the clinically used antiandrogen bicalutamide, reduce the efficiency of its nuclear translocation, and impair both DNA binding to androgen response elements and recruitment of coactivators. RD162 and MDV3100 are orally available and induce tumor regression in mouse models of castration-resistant human prostate cancer. Of the first 30 patients treated with MDV3100 in a Phase I/II clinical trial, 13 of 30 (43%) showed sustained declines (by >50%) in serum concentrations of prostate-specific antigen, a biomarker of prostate cancer. These compounds thus appear to be promising candidates for treatment of advanced prostate cancer.

Discovery of antiandrogen activity of nonsteroidal scaffolds of marketed drugs

Finding good drug leads de novo from large chemical libraries, real or virtual, is not an easy task. High-throughput screening is often plagued by low hit rates and many leads that are toxic or exhibit poor bioavailability. Exploiting the secondary activity of marketed drugs, on the other hand, may help in generating drug leads that can be optimized for the observed side-effect target, while maintaining acceptable bioavailability and toxicity profiles. Here, we describe an efficient computational methodology to discover leads to a protein target from safe marketed drugs. We applied an in silico "drug repurposing" procedure for identification of nonsteroidal antagonists against the human androgen receptor (AR), using multiple predicted models of an antagonist-bound receptor. The library of marketed oral drugs was then docked into the best-performing models, and the 11 selected compounds with the highest docking score were tested in vitro for AR binding and antagonism of dihydrotestosterone-induced AR transactivation. The phenothiazine derivatives acetophenazine, fluphenazine, and periciazine, used clinically as antipsychotic drugs, were identified as weak AR antagonists. This in vitro biological activity correlated well with endocrine side effects observed in individuals taking these medications. Further computational optimization of phenothiazines, combined with in vitro screening, led to the identification of a nonsteroidal antiandrogen with improved AR antagonism and marked reduction in affinity for dopaminergic and serotonergic receptors that are the primary target of phenothiazine antipsychotics.

Discovery of potent and muscle selective androgen receptor modulators through scaffold modifications

A novel series of imidazolin-2-ones were designed and synthesized as highly potent, orally active and muscle selective androgen receptor modulators (SARMs), with most of the compounds exhibiting low nM in vitro potency in androgen receptor (AR) binding and functional assays. Once daily oral treatment with the lead compound 11a (AR Ki = 0.9 nM, EC50 = 1.8 nM) for 14 days induced muscle growth with an ED50 of 0.09 mg/kg, providing approximately 50-fold selectivity over prostate growth in an orchidectomized rat model. Pharmacokinetic studies in rats demonstrated that the lead compound 11a had oral bioavailability of 65% and a plasma half-life of 5.5 h. On the basis of their preclinical profiles, the SARMs in this series are expected to provide beneficial anabolic effects on muscle with minimal androgenic effects on prostate tissue.

Design and synthesis of an array of selective androgen receptor modulators

We describe the design, using shape comparison and fast docking computer algorithms, and rapid parallel synthesis of a 1300 member array based on GSK7721, a 4-aminobenzonitrile androgen receptor (AR) antagonist identified by focused screening of the GSK compound collection. The array yielded 352 submicromolar and 17 subnanomolar AR agonists as measured by a cell-based reporter gene functional assay. The rapid synthesis of a large number of active compounds provided valuable information in the optimization of AR modulators, which may be useful in treating androgen deficiency in aging males.

Synthesis of irreversibly binding bicalutamide analogs for imaging studies

A new synthetic methodology for preparing radioactive androgen receptor binding compounds in order to determine receptor-ligands interactions has been developed.

The Para Substituent of S-3-(Phenoxy)-2-hydroxy-2-methyl-N-(4-nitro-3-trifluoromethyl-phenyl)-propionamides Is a Major Structural Determinant of in Vivo Disposition and Activity of Selective Androgen Receptor Modulators

Selective androgen receptor modulators (SARMs) have many potential therapeutic applications, including male hypogonadism, osteoporosis, muscle-wasting diseases, sexual libido, and contraception. A series of S-3-(phenoxy)-2-hydroxy-2-methyl-N-(4-nitro-3-trifluoromethyl-phenyl)-propionamides bearing a four-halogen substituent in the B-ring that displayed in vivo activity were identified in our previous study. Interestingly, in vivo pharmacological activity was not correlated with in vitro androgen receptor (AR) binding affinity. In this study, analysis of the area under the concentration-time curve-response relationship demonstrated that the discrepancy between in vitro and in vivo pharmacological activity of these halogen-substituted SARMs was due to differences in systemic exposure rather than intrinsic pharmacological activity. Studies also suggested that two simple criteria (i.e., Ki < 10 nM and lower in vivo clearance) could be used to identify efficacious and potent SARMs. We tested this hypothesis using a series of four compounds incorporating either a nitro or cyano substituent at the para-position of the A- and B-aromatic rings. The S-3-(4-nitrophenoxy) and S-3-(4-cyanophenoxy) 2-hydroxy-2-methyl-N-(4-nitro-3-trifluromethylphenyl) propionamides (S-19 and S-20, respectively) and S-3-(4-nitrophenoxy) and S-3-(4-cyanophenoxy) 2-hydroxy-2-methyl-N-(4-cyano-3-trifluromethylphenyl) propionamides (S-21 and S-22, respectively) demonstrated high AR binding affinity, with Ki values ranging from 2.0 to 3.8 nM. Pharmacokinetic studies of selected compounds showed that the in vivo clearance of S-22 was the slowest followed sequentially by S-20, S-21, and S-19. The dose-response relationships for S-22 showed that S-22 exerted efficacious and selective activity in anabolic tissues at dose rates as low as 0.03 mg/day, indicative of the high potency of this compound in anabolic tissue (relative potency 4.41) and its potential for clinical use in androgen deficiency-related disorders.

Three-dimensional structure-activity relationships of nonsteroidal ligands in complex with androgen receptor ligand-binding domain

We studied the three-dimensional quantitative structure-activity relationships (3D QSAR) of 70 structurally and functionally diverse androgen receptor (AR) binding compounds using the comparative molecular similarity indices analysis (CoMSIA) method. The compound set contained 67 nonsteroidal analogues of flutamide, nilutamide, and bicalutamide whose binding mode to AR was unknown. Docking was used to identify the preferred binding modes for the nonsteroidal compounds within the AR ligand-binding pocket (LBP) and to generate the ligand alignment for the 3D QSAR analysis. The alignment produced a statistically significant and predictive model, validated by random group cross-validation and external test sets (q(2)(LOO) = 0.656, SDEP = 0.576, r(2) = 0.911, SEE = 0.293; q(2)(10) = 0.612, q(2)(5) = 0.571; pred-r(2) = 0.800). Additional model validation comes from the CoMSIA maps that were interpreted with respect to the LBP structure. The model takes into account and links the AR LBP structure, docked ligand structures, and the experimental binding activities. The results provide valuable information on intermolecular interactions between nonsteroidal ligands and the AR LBP.

Synthesis and structure–activity investigation of iodinated arylhydantoins and arylthiohydantoins for development as androgen receptor radioligands

A series of side-chain derivatives of the arylhydantoin RU 58841 and the arylthiohydantoin RU 59063, wherein the aromatic trifluoromethyl group was replaced with iodine, was synthesized for possible development as radioiodinated androgen receptor (AR) ligands. Derivatives containing the cyanomethyl, methoxyethyl and propenyl side-chains displayed moderately high affinity (K(i)=20-59nM) towards the rat AR. Side-chains containing bulky lipophilic groups such as, benzyl and phenylpropyl, were poorly tolerated (K(i)>219nM). Superior AR binding affinities (0.71nM<K(i)<11nM) were displayed by arylhydantoins and arylthiohydantoin derivatives containing hydroxybutyl or methyl side-chains. The latter compounds are potential candidates for development as radioiodinated AR ligands.

Design, synthesis, and biological characterization of metabolically stable selective androgen receptor modulators

A series of nonsteroidal ligands were synthesized as second-generation agonists for the androgen receptor (AR). These ligands were designed to eliminate metabolic sites identified in one of our first-generation AR agonists, which was inactive in vivo due to its rapid metabolism to inactive constituents. The binding affinity of these compounds was evaluated using AR isolated from rat ventral prostate. These second-generation compounds bound the AR in a high affinity and stereoselective manner, with K(i) values ranging from about 4 to 130 nM. The ability of these ligands to stimulate AR-mediated transcriptional activation was examined in cells transfected with the human AR and a hormone-dependent luciferase reporter gene. Although some compounds were unable to stimulate AR-mediated transcription, several demonstrated activity similar to that of dihydrotestosterone (DHT, an endogenous steroidal ligand for the AR). We also evaluated the in vivo pharmacologic activity of selected compounds in castrated male rats. Three compounds were identified as selective androgen receptor modulators (SARMs), exhibiting significant anabolic activity while having only moderate to minimal androgenic activity in vivo.

Rational Design and Synthesis of Androgen Receptor-Targeted Nonsteroidal Anti-Androgen Ligands for the Tumor-Specific Delivery of a Doxorubicin−Formaldehyde Conjugate

The synthesis and preliminary evaluation of a doxorubicin-formaldehyde conjugate tethered to the nonsteroidal antiandrogen, cyanonilutamide (RU 56279), for the treatment of prostate cancer are reported. The relative ability of the targeting group to bind to the human androgen receptor was studied as a function of tether. The tether served to attach the antiandrogen to the doxorubicin-formaldehyde conjugate via an N-Mannich base of a salicylamide derivative. The salicylamide was selected to serve as a trigger release mechanism to separate the doxorubicin-formaldehyde conjugate from the targeting group after it has bound to the androgen receptor. The remaining part of the tether consisted of a linear group that spanned from the 5-position of the salicylamide to the 3'-position of cyanonilutamide. The structures explored for the linear region of the tether were derivatives of di(ethylene glycol), tri(ethylene glycol), N,N'-disubstituted-piperazine, and 2-butyne-1,4-diol. Relative binding affinity of the tethers bound to the targeting group for human androgen receptor were measured using a (3)H-Mibolerone competition assay and varied from 18% of nilutamide binding for the butynediol-based linear region to less than 1% for one of the piperazine derivatives. The complete targeted drug with the butynediol-based linear region has a relative binding affinity of 10%. This relative binding affinity is encouraging in light of the cocrystal structure of human androgen receptor ligand binding domain bound to the steroid Metribolone which predicts very limited space for a tether connecting the antiandrogen on the inside to the cytotoxin on the outside.

Homology Modeling Using Multiple Molecular Dynamics Simulations and Docking Studies of the Human Androgen Receptor Ligand Binding Domain Bound to Testosterone and Nonsteroidal Ligands†

To facilitate the rational design of novel and more potent androgen receptor ligands, three-dimensional models for the human androgen receptor ligand binding domain bound to testosterone have been developed. These models of the androgen receptor were based on the crystal structure of the highly homologous human progesterone receptor ligand binding domain. The homology modeled androgen receptor was refined using unrestrained multiple molecular dynamics simulations in explicit solvent. Key H-bonding partners with the 17-hydroxy group and 3-keto group of testosterone are Asn705 and Thr877, and Gln711 and Arg752, respectively. These models show the presence of a unique unoccupied cavity within the androgen receptor binding pocket which may be valuable in the development of novel selective androgen receptor ligands. A qualitative analysis of amino acid mutations within the hAR binding pocket that affect ligand binding are consistent with these androgen receptor models. In addition to testosterone, the binding modes of several hydroxyflutamide-like nonsteroidal ligands for the androgen receptor are investigated using flexible docking with FlexX followed by refinement of the initial complexes with molecular dynamics simulations. These docking studies indicate that Asn705 is an important determinant in binding hydroxyflutamide and its derivatives by participating in H-bond interactions with the alpha-hydroxy moiety of these ligands. In addition, the nitro functionality mimics the 3-keto group of the natural ligand testosterone and is involved in H-bonding interactions with Gln711 and Arg752. From these docking studies, we suggest a mechanism for the enantioselective binding of chiral hydroxyflutamide derivatives and expand upon the previously reported structure-activity relationship for hydroxyflutamide and its derivatives.