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A variety of microtubule-stabilizing cytotoxic agents (MSA) with diverse chemical scaffolds have been discovered from marine sponges, microorganisms, and plants. Two MSAs, docetaxel and cabazitaxel, are the exclusive chemotherapeutics that convey a survival benefit in patients with castration-resistant prostate cancer (CRPC). Additional MSAs have been investigated for their potential in treating prostate cancer in both clinical and preclinical settings. Independent of promoting mitotic arrest, MSAs can suppress the nuclear accumulation of androgen receptor (AR), which is the driving force for prostate cancer cell growth and progression. The alternative mechanism not only helps to better understand the clinical efficacy of docetaxel and cabazitaxel for AR-driven CRPC but also provides an avenue to seek better treatments for various forms of prostate cancer. The dual mechanisms of action enable MSAs to suppress AR-null prostate cancer cell proliferation by cell mitosis pathway and to interfere with the AR signaling pathway in AR positive cells. MSA chemotherapeutics, being administered alone or in combination with other therapeutics, may serve as the optimal therapeutic option for patients with either castration-sensitive or castration-resistant prostate cancer. This review provides an overview of the anti-prostate cancer profiles (including preclinical and clinical studies, and clinical use) of diverse MSAs, as well as the mechanism of action.

Androgen receptor (AR) is a viable therapeutic target for lethal castration-resistant prostate cancer (CRPC), because the continued progression of CRPC is mainly driven by the reactivation of AR transcriptional activity. The current FDA-approved AR antagonists binding to ligand binding domain (LBD) become ineffective in CRPC with AR gene amplification, LBD mutation, and the evolution of LBD-truncated AR splice variants. Encouraged by the fact that tricyclic aromatic diterpenoid QW07 has recently been established as a potential N-terminal AR antagonist, this study aims to explore the structure–activity relationship of tricyclic diterpenoids and their potential to suppress AR-positive cell proliferation. Dehydroabietylamine, abietic acid, dehydroabietic acid, and their derivatives were selected, since they have a similar core structure as QW07. Twenty diterpenoids were prepared for the evaluation of their antiproliferative potency on AR-positive prostate cancer cell models (LNCaP and 22Rv1) using AR-null cell models (PC-3 and DU145) as comparisons. Our data indicated that six tricyclic diterpenoids possess greater potency than enzalutamide (FDA-approved AR antagonist) towards LNCaP and 22Rv1 AR-positive cells, and four diterpenoids are more potent than enzalutamide against 22Rv1 AR-positive cells. The optimal derivative possesses greater potency (IC50 = 0.27 µM) and selectivity than QW07 towards AR-positive 22Rv1 cells.

Silibinin, also known as silybin, is isolated from milk thistle (Silybum marianum). Silibinin has been demonstrated to be a good lead compound due to its potential to prevent and treat prostate cancer. Its moderate potency and poor pharmacokinetic profile hindered it from moving forward to therapeutic use. Our research group has been working on optimizing silibinin for the potential treatment of castration-resistant prostate cancer. Our previous studies established 5,7,20-O-trimethylsilybins as promising lead compounds as they can selectively suppress androgen receptor (AR)-positive LNCaP cell proliferation. Encouraged by the promising data, the present study aims to investigate the relationships between the core structure of 5,7,20-O-trimethylsilybin and their antiproliferative activities towards AR-positive (LNCaP) and AR-negative prostate cancer cell lines (PC-3 and DU145). The structure–activity relationships among the four different core structures (including flavanonol-type flavonolignan (silibinin), flavone-type flavonolignan (hydnocarpin D), chalcone-type flavonolignan, and taxifolin (a flavonolignan precursor) indicate that 5,7,20-O-trimethylsilybins are the most promising scaffold to selectively suppress AR-positive LNCaP prostate cancer cell proliferation. Further investigation on the antiproliferative potency of their optically enriched versions of the most promising 5,7,20-O-trimethylsilybins led to the conclusion that (10R,11R) derivatives (silybin A series) are more potent than (10S,11S) derivatives (silybin B series) in suppressing AR positive LNCaP cell proliferation.

To search for novel androgen receptor (AR) modulators for the potential treatment of castration-resistant prostate cancer (CRPC), naturally occurring silibinin was sought after as a lead compound because it possesses a moderate potency towards AR-positive prostate cancer cells and its chemical scaffold is dissimilar to all currently marketed AR antagonists. On the basis of the structure–activity relationships that we have explored, this study aims to incorporate carbamoyl groups to the alcoholic hydroxyl groups of silibinin to improve its capability in selectively suppressing AR-positive prostate cancer cell proliferation together with water solubility. To this end, a feasible approach was developed to regioselectively introduce a carbamoyl group to the secondary alcoholic hydroxyl group at C-3 without causing the undesired oxidation at C2–C3, providing an avenue for achieving 3-O-carbamoyl-5,7,20-O-trimethylsilybins. The application of the synthetic method can be extended to the synthesis of 3-O-carbamoyl-3′,4′,5,7-O-tetramethyltaxifolins. The antiproliferative potency of 5,7,20-O-trimethylsilybin and its nine 3-carbamoyl derivatives were assessed in an AR-positive LNCaP prostate cancer cell line and two AR-null prostate cancer cell lines (PC-3 and DU145). Our preliminary bioassay data imply that 5,7,20-O-trimethylsilybin and four 3-O-carbamoyl-5,7,20-O-trimethylsilybins emerge as very promising lead compounds due to the fact that they can selectively suppress AR-positive LNCaP cell proliferation. The IC50 values of these five 5,7,20-O-trimethylsilybins against the LNCaP cells fall into the range of 0.11–0.83 µM, which exhibit up to 660 times greater in vitro antiproliferative potency than silibinin. Our findings suggest that carbamoylated 5,7,20-O-trimethylsilybins could serve as a natural product-based scaffold for new antiandrogens for lethal castration-resistant prostate cancer.

Simple Summary: Prostate cancer is one of topmost health concerns, and metastatic castration-resistant prostate cancer is the fatal form. Two taxanes, docetaxel and cabazitaxel, are the only two FDA-approved chemotherapeutics that can provide survival benefits to patients with the lethal version of prostate cancer. Taxanes, alongside numerous other naturally occurring products, can promote the assembly and stability of microtubules to halt cell division and promote various cancer cell deaths. Additionally, this group of compounds, named microtubule stabilizing agents, can impede the androgen–androgen receptor complex from moving into the cell nucleus, conquering androgen receptor-containing prostate cancer cell proliferation and metastasis. This review aims to overview the preclinical and clinical studies, clinical uses, and the mechanisms of action of microtubule-stabilizing agents as anti-prostate cancer agents. A variety of microtubule-stabilizing cytotoxic agents (MSA) with diverse chemical scaffolds have been discovered from marine sponges, microorganisms, and plants. Two MSAs, docetaxel and cabazitaxel, are the exclusive chemotherapeutics that convey a survival benefit in patients with castration-resistant prostate cancer (CRPC). Additional MSAs have been investigated for their potential in treating prostate cancer in both clinical and preclinical settings. Independent of promoting mitotic arrest, MSAs can suppress the nuclear accumulation of androgen receptor (AR), which is the driving force for prostate cancer cell growth and progression. The alternative mechanism not only helps to better understand the clinical efficacy of docetaxel and cabazitaxel for AR-driven CRPC but also provides an avenue to seek better treatments for various forms of prostate cancer. The dual mechanisms of action enable MSAs to suppress AR-null prostate cancer cell proliferation by cell mitosis pathway and to interfere with the AR signaling pathway in AR positive cells. MSA chemotherapeutics, being administered alone or in combination with other therapeutics, may serve as the optimal therapeutic option for patients with either castration-sensitive or castration-resistant prostate cancer. This review provides an overview of the anti-prostate cancer profiles (including preclinical and clinical studies, and clinical use) of diverse MSAs, as well as the mechanism of action. [ABSTRACT FROM AUTHOR]

Androgen receptor (AR) is a viable therapeutic target for lethal castration-resistant prostate cancer (CRPC), because the continued progression of CRPC is mainly driven by the reactivation of AR transcriptional activity. The current FDA-approved AR antagonists binding to ligand binding domain (LBD) become ineffective in CRPC with AR gene amplification, LBD mutation, and the evolution of LBD-truncated AR splice variants. Encouraged by the fact that tricyclic aromatic diterpenoid QW07 has recently been established as a potential N-terminal AR antagonist, this study aims to explore the structure–activity relationship of tricyclic diterpenoids and their potential to suppress AR-positive cell proliferation. Dehydroabietylamine, abietic acid, dehydroabietic acid, and their derivatives were selected, since they have a similar core structure as QW07. Twenty diterpenoids were prepared for the evaluation of their antiproliferative potency on AR-positive prostate cancer cell models (LNCaP and 22Rv1) using AR-null cell models (PC-3 and DU145) as comparisons. Our data indicated that six tricyclic diterpenoids possess greater potency than enzalutamide (FDA-approved AR antagonist) towards LNCaP and 22Rv1 AR-positive cells, and four diterpenoids are more potent than enzalutamide against 22Rv1 AR-positive cells. The optimal derivative possesses greater potency (IC50 = 0.27 µM) and selectivity than QW07 towards AR-positive 22Rv1 cells. [ABSTRACT FROM AUTHOR]

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.

Silibinin, also known as silybin, is isolated from milk thistle (Silybum marianum). Silibinin has been demonstrated to be a good lead compound due to its potential to prevent and treat prostate cancer. Its moderate potency and poor pharmacokinetic profile hindered it from moving forward to therapeutic use. Our research group has been working on optimizing silibinin for the potential treatment of castration-resistant prostate cancer. Our previous studies established 5,7,20-O-trimethylsilybins as promising lead compounds as they can selectively suppress androgen receptor (AR)-positive LNCaP cell proliferation. Encouraged by the promising data, the present study aims to investigate the relationships between the core structure of 5,7,20-O-trimethylsilybin and their antiproliferative activities towards AR-positive (LNCaP) and AR-negative prostate cancer cell lines (PC-3 and DU145). The structure–activity relationships among the four different core structures (including flavanonol-type flavonolignan (silibinin), flavone-type flavonolignan (hydnocarpin D), chalcone-type flavonolignan, and taxifolin (a flavonolignan precursor) indicate that 5,7,20-O-trimethylsilybins are the most promising scaffold to selectively suppress AR-positive LNCaP prostate cancer cell proliferation. Further investigation on the antiproliferative potency of their optically enriched versions of the most promising 5,7,20-O-trimethylsilybins led to the conclusion that (10R,11R) derivatives (silybin A series) are more potent than (10S,11S) derivatives (silybin B series) in suppressing AR positive LNCaP cell proliferation. [ABSTRACT FROM AUTHOR]

As part of our ongoing silybin project, this study aims to introduce a basic nitrogen-containing group to 7-OH of 3,5,20-O-trimethyl-2,3-dehydrosilybin or 3-OH of 5,7,20-O-trimethyl-2,3-dehydrosilybin via an appropriate linker for in vitro evaluation as potential anti-prostate cancer agents. The synthetic approaches to 7-O-substituted-3,5,20-O-trimethyl-2,3-dehydrosilybins through a five-step procedure and to 3-O-substituted-5,7,20-O-trimethyl-2,3- dehydrosilybins via a four-step transformation have been developed. Thirty-two nitrogen-containing derivatives of silybin have been achieved through these synthetic methods for the evaluation of their antiproliferative activities towards both androgen-sensitive (LNCaP) and androgen-insensitive prostate cancer cell lines (PC-3 and DU145) using the WST-1 cell proliferation assay. These derivatives exhibited greater in vitro antiproliferative potency than silibinin. Among them, 11, 29, 31, 37, and 40 were identified as five optimal derivatives with IC50 values in the range of 1.40⁻3.06 µM, representing a 17- to 52-fold improvement in potency compared to silibinin. All these five optimal derivatives can arrest the PC-3 cell cycle in the G0/G1 phase and promote PC-3 cell apoptosis. Derivatives 11, 37, and 40 are more effective than 29 and 31 in activating PC-3 cell apoptosis.

Inspired by the synergistic effects of dietary natural products with different scaffolds on the inhibition of cancer cell proliferation, incorporation of central (1 E ,4 E )-1,4-penta-dien-3-one linker (an optimal substitute for the central metabolically unstable diketone linker of curcumin), 1-alkyl-1 H -imidazol-2-yl (a promising bioisostere of terminal aryl group in curcumin), and chromone (the common pharmacophore in genistein and quercetin) into one chemical entity resulted in ten new hybrid molecules, 3-((1 E ,4 E )-5-(1-alkyl-1 H -imidazol-2-yl)-3-oxopenta-1,4-dien-1-yl)-4 H -chromen-4-ones. They were synthesized through a three-step transformation using acid-catalyzed aldol condensation as key step. The WST-1 cell proliferation assay showed that they have greater anti-proliferative potency than curcumin, quercetin, and genistein on both androgen-dependent and androgen-independent human prostate cancer cells. [ABSTRACT FROM AUTHOR]

To search for novel androgen receptor (AR) modulators for the potential treatment of castration-resistant prostate cancer (CRPC), naturally occurring silibinin was sought after as a lead compound because it possesses a moderate potency towards AR-positive prostate cancer cells and its chemical scaffold is dissimilar to all currently marketed AR antagonists. On the basis of the structure–activity relationships that we have explored, this study aims to incorporate carbamoyl groups to the alcoholic hydroxyl groups of silibinin to improve its capability in selectively suppressing AR-positive prostate cancer cell proliferation together with water solubility. To this end, a feasible approach was developed to regioselectively introduce a carbamoyl group to the secondary alcoholic hydroxyl group at C-3 without causing the undesired oxidation at C2–C3, providing an avenue for achieving 3-O-carbamoyl-5,7,20-O-trimethylsilybins. The application of the synthetic method can be extended to the synthesis of 3-O-carbamoyl-3′,4′,5,7-O-tetramethyltaxifolins. The antiproliferative potency of 5,7,20-O-trimethylsilybin and its nine 3-carbamoyl derivatives were assessed in an AR-positive LNCaP prostate cancer cell line and two AR-null prostate cancer cell lines (PC-3 and DU145). Our preliminary bioassay data imply that 5,7,20-O-trimethylsilybin and four 3-O-carbamoyl-5,7,20-O-trimethylsilybins emerge as very promising lead compounds due to the fact that they can selectively suppress AR-positive LNCaP cell proliferation. The IC50 values of these five 5,7,20-O-trimethylsilybins against the LNCaP cells fall into the range of 0.11–0.83 µM, which exhibit up to 660 times greater in vitro antiproliferative potency than silibinin. Our findings suggest that carbamoylated 5,7,20-O-trimethylsilybins could serve as a natural product-based scaffold for new antiandrogens for lethal castration-resistant prostate cancer. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Twenty-seven L-shaped ortho-quinone analogs were designed and synthesized using a one pot double-radical synthetic strategy followed by removing methyl at C-3 of the furan ring and introducing a diverse side chain at C-2 of the furan ring. The synthetic derivatives were investigated for their cytotoxicity activities against human leukemia cells K562, prostate cancer cells PC3, and melanoma cells WM9. Compounds TB1, TB3, TB4, TB6, TC1, TC3, TC5, TC9, TC11, TC12, TC14, TC15, TC16, and TC17 exhibited a better broad-spectrum cytotoxicity on three cancer cells. TB7 and TC7 selectively displayed potent inhibitory activities on leukemia cells K562 and prostate cancer cells PC3, respectively. Further studies indicated that TB3, TC1, TC3, TC7, and TC17 could significantly induce the apoptosis of PC3 cells. TC1 and TC17 significantly induced apoptosis of K562 cells. TC1, TC11, and TC14 induced significant apoptosis of WM9 cells. The structure-activity relationships evaluation showed that removing methyl at C-3 of the furan ring and introducing diverse side chains at C-2 of the furan ring is an effective strategy for improving the anticancer activity of L-shaped ortho-quinone analogs. [ABSTRACT FROM AUTHOR]

As part of our ongoing silybin project, this study aims to introduce a basic nitrogen-containing group to 7-OH of 3,5,20-O-trimethyl-2,3-dehydrosilybin or 3-OH of 5,7,20-O-trimethyl-2,3-dehydrosilybin via an appropriate linker for in vitro evaluation as potential anti-prostate cancer agents. The synthetic approaches to 7-O-substituted-3,5,20-O-trimethyl-2,3-dehydrosilybins through a five-step procedure and to 3-O-substituted-5,7,20-O-trimethyl-2,3- dehydrosilybins via a four-step transformation have been developed. Thirty-two nitrogen-containing derivatives of silybin have been achieved through these synthetic methods for the evaluation of their antiproliferative activities towards both androgen-sensitive (LNCaP) and androgen-insensitive prostate cancer cell lines (PC-3 and DU145) using the WST-1 cell proliferation assay. These derivatives exhibited greater in vitro antiproliferative potency than silibinin. Among them, 11, 29, 31, 37, and 40 were identified as five optimal derivatives with IC50 values in the range of 1.40–3.06 µM, representing a 17- to 52-fold improvement in potency compared to silibinin. All these five optimal derivatives can arrest the PC-3 cell cycle in the G0/G1 phase and promote PC-3 cell apoptosis. Derivatives 11, 37, and 40 are more effective than 29 and 31 in activating PC-3 cell apoptosis. [ABSTRACT FROM AUTHOR]

Our earlier studies indicate that (1 E ,4 E )-1,5-bis(1-alkyl-1 H -imidazol-2-yl)penta-1,4-diene-3-ones and (1 E ,4 E )-1,5-bis(1-alkyl-1 H -benzo[ d ]imidazol-2-yl)penta-1,4-diene-3-ones exhibit up to 121-fold greater antiproliferative potency than curcumin in human prostate cancer cell models, but only 2–10 fold increase in mouse plasma concentrations. The present study aims to further optimize them as anti-prostate cancer agents with both good potency and bioavailability. (1 E ,4 E )-1,5-Bis(1 H -imidazol-2-yl)penta-1,4-diene-3-one, the potential metabolic product of (1 E ,4 E )-1,5-bis(1-alkyl-1 H -imidazol-2-yl)penta-1,4-diene-3-ones, was synthesized and evaluated for its anti-proliferative activity. The promising potency of 1,5-bis(1-alkyl-1 H -imidazol-2-yl)penta-1,4-diene-3-ones was completely abolished by removing the 1-alkyl group, suggesting the critical role of an appropriate group on the N1 position. We then envisioned that N -aryl substitution to exclude the C–H bond on the carbon adjacent to the N 1 position ( α -H) may increase the metabolic stability. Consequently, seven (1 E ,4 E )-1,5-bis(1-aryl-1 H -imidazol-2-yl)penta-1,4-dien-3-ones and three (1 E ,4 E )-1,5-bis(1-aryl-1 H -benzo[ d ]imidazol-2-yl)penta-1,4-dien-3-ones, as well as three (1 E ,4 E )-1,5-bis(1-aryl-1 H -pyrrolo[3,2- b ]pyridine-2-yl)penta-1,4-dien-3-ones, were synthesized through a three-step transformation, including N -arylation via Ullmann condensation, formylation, and Horner-Wadsworth-Emmons reaction. Six optimal (1 E ,4 E )-1,5-bis(1-aryl-1 H -imidazol-2-yl)penta-1,4-dien-3-ones exhibit 24- to 375-fold improved potency as compared with curcumin. Replacement of the imidazole with bulkier benzoimidazole and 4-azaindole results in a substantial decrease in the potency. (1 E ,4 E )-1,5-Bis(1-(2-methoxyphenyl)-1 H -imidazol-2-yl)penta-1,4-dien-3-one ( 17d ) was established as an optimal compound with both superior potency and good bioavailability that is sufficient to provide the therapeutic efficacy necessary to suppress in vivo tumor growth. [ABSTRACT FROM AUTHOR]

• 48 New N -containing derivatives of O -tetramethylquercetin were synthesized. • Most of them are more potent than quercetin in three prostate cancer cell models. • 5- O -(N , N -Dibutylamino)propyl-3,3′,4′,7- O -tetramethylquercetin is the optimal one. • The optimal derivative is over 35- to 182-fold more potent than quercetin. • The optimal derivative activates PC-3 cell apoptosis. Forty-eight nitrogen-containing quercetin derivatives were synthesized from readily available rutin or quercetin for the in vitro evaluation of their biological profiles. The WST-1 cell proliferation assay data indicate that thirty-nine out of the forty-eight derivatives possess significantly improved antiproliferative potency as compared with quercetin and fisetin, as well as the parent 3,3′,4′,7- O -tetramethylquercetin toward both androgen-sensitive (LNCaP) and androgen-insensitive (PC-3 and DU145) human prostate cancer cell lines. 5- O -Aminoalkyl-3,3′,4′,7- O -tetramethylquercetins were established as a better scaffold for further development as anti-prostate cancer agents. Among them, 5- O -(N , N -dibutylamino)propyl-3,3′,4′,7- O -tetramethylquercetin (44) was identified as the optimal derivative with IC 50 values of 0.55–2.82 µM, being over 35 – 182 times more potent than quercetin. The flow cytometry-based assays further demonstrate that 44 effectively activates PC-3 cell apoptosis. [ABSTRACT FROM AUTHOR]

Abstract Thirty-eight 3- O -substituted-3′,4′-dimethoxyflavonols and twenty-five 3- O -substituted-3′,4′,7-trimethoxyflavonols have been synthesized for systematic investigation on the structure-activity relationships of 3- O -substituted-3′,4′-dimethoxyflavonols in three human prostate cancer cell models. Our findings indicate that incorporation of an appropriate amino group to 3-OH of 3′,4′-dimethoxyflavonol and 3′,4′,7-trimethoxyflavonol through a 3- to 5-carbon linker can substantially improve the in vitro antiproliferative potency in three human prostate cancer cell models, but not in two non-neoplastic human epithelial cell models (MCF 10A and PWR-1E). 1-Methylpiperazine, pyrrolidine, and dibutylamine are optimal terminal amine groups that, in combination with a 3- to 5-carbon linker, are notably beneficial to the anti-proliferative potency of 3- O -substituted-3′,4′-dimethoxyflavonols. It is worth noting that 3- O -(4-methylpiperazin-1-yl)propyl-3′,4′,7-trimethoxyflavonol (76) induces PC-3 cell death in a completely different way from 3- O -pyrrolidinopentyl-3′,4′,7-trimethoxyflavonol (81) even though they belong to 3- O -substituted-3′,4′,7-trimethoxyflavonols and exhibit similar potency in inhibiting PC-3 cell proliferation, suggesting that the mechanism of action for each specific 3- O -substitutedflavonol varies with different amino moiety. 3- O -(N , N -Dibutylamino)propyl-3′,4′-dimethoxyflavonol (42) emerged as the most promising derivative due to its substantially improved potency in cell models, superior bioavailability in rats, and good selectivity of inhibiting prostate cancer cell proliferation over non-neoplastic human epithelial cell proliferation. Graphical abstract Image 1 Highlights • 63 3- O -substituted flavonols were evaluated in prostate epithelial cell models. • An appropriate amino group to 3-OH of flavonols did improve the potency. • The optimal carbon linkers and terminal amino groups were identified. • One promising compound exhibit improved potency and bioavailability. [ABSTRACT FROM AUTHOR]

Dactylolide and certain analogues are attractive targets for study due to their structural resemblance to zampanolide, a very promising anticancer lead compound and a unique covalent-binding microtubule stabilizing agent. The primary goal of this project is identification and synthesis of simplified analogues of dactylolide that would be easier to prepare and could be investigated for antiproliferative activity in comparison with zampanolide. Extension of Almann’s concept of a simplified zampanolide analogue to dactylolide in the form of desTHPdactylolide was attractive not only for reasons of synthetic simplification but also for the prospect that analogues of dactylolide could be prepared in both (17 S ) and (17 R ) configurations. Since Altmann’s overall yield for the six-step procedure leading to the C9–C18 fragment of desTHPdactylolide was only 8.7%, a study focused on optimized synthesis and antiproliferative evaluation of each enantiomer of desTHPdactylolide was initiated using Altmann’s route as a framework. To this end, two optimized approaches to this fragment C9–C18 were successfully developed by us using allyl iodide or allyl tosylate as the starting material for a critical Williamson ether synthesis. Both (17 S ) and (17 R ) desTHPdactylolides were readily synthesized in our laboratory using optimized methods in yields of 37–43%. Antiproliferative activity of the pair of enantiomeric desTHPdactylolides, together with their analogues, was evaluated in three docetaxel-sensitive and two docetaxel-resistant prostate cancer cell models using a WST-1 cell proliferation assay. Surprisingly, (17 R ) desTHPdactylolide was identified as the eutomer in the prostate cancer cell models. It was found that (17 S ) and (17 R ) desTHPdactylolide exhibit equivalent antiproliferative potency towards both docetaxel-sensitive (PC-3 and DU145) and docetaxel-resistant prostate cancer cell lines (PC-3/DTX and DU145/DTX). [ABSTRACT FROM AUTHOR]

To further engineer dienones with optimal combinations of potency and bioavailability, thirty-four asymmetric 1,5-diarylpenta-1,4-dien-3-ones ( 25 – 58 ) have been designed and synthesized for the evaluation of their in vitro anti-proliferative activity in three human prostate cancer cell lines and one non-neoplastic prostate epithelial cell line. All these asymmetric dienones are sufficiently more potent than curcumin and their corresponding symmetric counterparts. The optimal dienone 58 , with IC 50 values in the range of 0.03–0.12 μM, is 636-, 219-, and 454-fold more potent than curcumin in three prostate cancer cell models. Dienones 28 and 49 emerged as the most promising asymmetric dienones that warrant further preclinical studies. The two lead compounds demonstrated substantially improved potency in cell models and superior bioavailability in rats, while exhibiting no acute toxicity in the animals at the dose of 10 mg/kg. Dienones 28 and 46 can induce PC-3 cell cycle regulation at the G 0 /G 1 phase. However, dienone 28 induces PC-3 cell death in a different way from 46 even though they share the same scaffold, indicating that terminal heteroaromatic rings are critical to the action of mechanism for each specific dienone. [ABSTRACT FROM AUTHOR]

Twenty-two 3- O -substituted-3′,4′,5′-trimethoxyflavonols have been designed and synthesized for their anti-proliferative activity towards three human prostate cancer cell lines. Our results indicate that most of them are significantly more potent than the parent 3′,4′,5′-trimethoxyflavonol in inhibiting the cell proliferation in PC-3 and LNCaP prostate cancer cell models. 3- O -Substituted-3′,4′,5′-trimethoxyflavonols have generally higher potency towards PC-3 and LNCaP cell lines than the DU145 cell line. Incorporation of an ethyl group to 3-OH of 3′,4′,5′-trimethoxyflavonol leads to 3- O -ethyl-3′,4′,5′-trimethoxyflavonol as the optimal derivative with up to 36-fold enhanced potency as compared with the corresponding lead compound 3′,4′,5′-trimethoxyflavonol, but with reversed PC-3 cell apoptotic response. Introduction of a dipentylaminopropyl group to 3-OH increases not only the antiproliferative potency but also the ability in activating PC-3 cell apoptosis. Our findings imply that modification on 3-OH of trimethoxyflavonol can further enhance its in vitro anti-proliferative potency and PC-3 cell apoptosis induction. [ABSTRACT FROM AUTHOR]

To investigate the effects of alkylation at 5-OH and 20-OH of 2,3-dehydrosilybin on prostate cancer cell proliferation, the synthetic approaches to 5- or/and 20- O -alkyl-2,3-dehydrosilybins, through a multi-step sequence from commercially available silybin, have been successfully developed. The first three reactions in the syntheses were completed through a one-pot procedure by managing anaerobic and aerobic conditions. With these synthetic methods in hand, twenty-one 2,3-dehydrosilybins, including seven 20- O -alkyl, seven 5,20- O -dialkyl, and seven 5- O -alkyl-2,3-dehydrosilybins, have been achieved for the evaluation of their biological profiles. Our WST-1 cell proliferation assay data indicate that nineteen out of the twenty-one 2,3-dehydrosilybins possess significantly improved antiproliferative potency as compared with silybin toward both androgen-sensitive (LNCaP) and androgen-insensitive prostate cancer cell lines (PC-3 and DU145). 5- O -Alkyl-2,3-dehydrosilybins were identified as the optimal subgroup that can consistently inhibit cell proliferation in three prostate cancer cell models with all IC 50 values lower than 8 µM. Our flow cytometry-based assays also demonstrate that 5- O -heptyl-2,3-dehydrosilybin effectively arrests the cell cycle in the G 0 /G 1 phase and activates PC-3 cell apoptosis. [ABSTRACT FROM AUTHOR]

To systematically investigate the structure-activity relationships of 1,7-diheteroarylhepta-1,4,6-trien-3-ones in three human prostate cancer cell models and one human prostate non-neoplastic epithelial cell model, thirty five 1,7-diarylhepta-1,4,6-trien-3-ones with different terminal heteroaromatic rings have been designed for evaluation of their anti-proliferative potency in vitro . These target compounds have been successfully synthesized through two sequential Horner-Wadsworth-Emmons reactions starting from the appropriate aldehydes and tetraethyl (2-oxopropane-1,3-diyl)bis(phosphonate). Their anti-proliferative potency against PC-3, DU-145 and LNCaP human prostate cancer cell lines can be significantly enhanced by the manipulation of the terminal heteroaromatic rings, further demonstrating the utility of 1,7-diarylhepta-1,4,6-trien-3-one as a potential scaffold for the development of anti-prostate cancer agents. The optimal analog 40 is 82-, 67-, and 39-fold more potent than curcumin toward the three prostate cancer cell lines, respectively. The experimental data also reveal that the trienones with two different terminal aromatic rings possess greater potency toward three prostate cancer cell lines, but also have greater capability of suppressing the proliferation of PWR-1E benign human prostate epithelial cells, as compared to the corresponding counterparts with two identical terminal rings and curcumin. The terminal aromatic rings also affect the cell apoptosis perturbation. [ABSTRACT FROM AUTHOR]

In search of more effective chemotherapeutics for the treatment of castration-resistant prostate cancer and inspired by curcumin analogues, twenty five (1 E ,3 E ,6 E ,8 E )-1,9-diarylnona-1,3,6,8-tetraen-5-ones bearing two identical terminal heteroaromatic rings have been successfully synthesized through Wittig reaction followed by Horner–Wadsworth–Emmons reaction. Twenty-three of them are new compounds. The WST-1 cell proliferation assay was employed to assess their anti-proliferative effects toward both androgen-sensitive and androgen-insensitive human prostate cancer cell lines. Eighteen out of twenty-five synthesized compounds possess significantly improved potency as compared with curcumin. The optimal compound, 78 , is 14- to 23-fold more potent than curcumin in inhibiting prostate cancer cell proliferation. It can be concluded from our data that 1,9-diarylnona-1,3,6,8-tetraen-5-one can serve as a new potential scaffold for the development of anti-prostate cancer agents and that pyridine-4-yls and quinolin-4-yl act as optimal heteroaromatic rings for the enhanced potency of this scaffold. Two of the most potent compounds, 68 and 75 , effectively suppress PC-3 cell proliferation by activating cell apoptosis and by arresting cell cycle in the G 0 /G 1 phase. [ABSTRACT FROM AUTHOR]

Flavonoids are a large class of polyphenolic compounds ubiquitously distributed in dietary plants with an array of biological activities. Flavonols are a major sub-class of flavonoids featuring a hydroxyl group at C-3. Certain natural flavonols, such as quercetin and fisetin, have been shown by in vitro cell-based and in vivo animal experiments to be potential anti-prostate cancer agents. However, the Achilles’ heel of flavonols as drug candidates is their moderate potency and poor pharmacokinetic profiles. This study aims to explore the substitution effect of 3-OH in flavonols on the in vitro anti-proliferative potency against both androgen-sensitive and androgen-insensitive human prostate cancer cell lines. Our first lead flavonol (3′,4′-dimethoxyflavonol), eight 3- O -alkyl-3′,4′-dimethoxyflavonols, and six 3- O -aminoalkyl-3′,4′-dimethoxyflavonols have been synthesized through aldol condensation and the Algar–Flynn–Oyamada (AFO) reaction. The WST-1 cell proliferation assay indicates (i) that all synthesized 3- O -alkyl-3′,4′-dimethoxyflavonols and 3- O -aminoalkyl-3′,4′-dimethoxyflavonols are more potent than the parent 3′,4′-dimethoxyflavonol and the natural flavonol quercetin in suppressing prostate cancer cell proliferation; and (ii) that incorporation of a dibutylamino group to the 3-OH group through a three- to five-carbon linker leads to the optimal derivatives with up to 292-fold enhanced potency as compared with the parent flavonol. Flow cytometry analysis showed that the most potent derivative 22 can activate PC-3 cell cycle arrest at the G 2 /M phase and induce PC-3 cell apoptosis. No inhibitory ability of 22 up to 50 μM concentration was observed against PWR-1E normal human epithelial prostate cells, suggesting its in vitro safety profile. The results indicate that chemical modulation at 3-OH is a vital strategy to optimize flavonols as anti-prostate cancer agents. [ABSTRACT FROM AUTHOR]

Eight 3- O -alkyl-2,3-dehydrosilibinins have been synthesized from commercially available silibinin through two synthetic approaches. A one-pot reaction, starting with aerobic oxidation of silibinin followed by direct alkylation of the phenolic hydroxyl group in the subsequent 2,3-dehydrosilibinin, furnishes the desired derivatives in 11–16% yields. The three-step procedure employing benzyl ether to protect 7-OH in silibinin generates the desired derivatives in 30–46% overall yields. The antiproliferative activity of the 2,3-dehydrosilibinin derivatives against both androgen-sensitive and androgen-insensitive prostate cancer cells have been assessed using a WST-1 cell proliferation assay. All derivatives exhibited greater antiproliferative potency than silibinin, with 2,3-dehydrosilibinins each possessing a three- to five-carbon linear alkyl group to 3-OH (IC 50 values in a range of 1.71–3.06 μM against PC-3 and LNCaP cells) as the optimal derivatives. The optimal potency was reached with three- to five-carbon alkyl groups. Our findings suggest that 3- O -propyl-2,3-dehydrosilibinin effectively inhibits the growth of PC-3 prostate cancer cells by arresting cell cycle in the G 0 / G 1 phase, but not by activating PC-3 cell apoptosis. [ABSTRACT FROM AUTHOR]

This study aims to systematically explore the alkylation effect of 7-OH in silibinin and 2,3-dehydrosilibinin on the antiproliferative potency toward three prostate cancer cell lines. Eight 7- O -alkylsilibinins, eight 7- O -alkyl-2,3-dehydrosilibinins, and eight 3,7- O -dialkyl-2,3-dehydrosilibinins have been synthesized from commercially available silibinin for the in vitro cell-based evaluation. The WST-1 cell proliferation assay indicates that nineteen out of twenty-four silibinin derivatives have significantly improved antiproliferative potency when compared with silibinin. 7- O -Methylsilibinin ( 2 ) and 7- O -ethylsilibinin ( 3 ) have been identified as the most potent compounds with 98- and 123-fold enhanced potency against LNCaP human androgen-dependent prostate cancer cell line. Among 2,3-dehydrosilibinin derivatives, 7- O -methyl-2,3-dehydrosilibinin ( 10 ) and 7- O -ethyl-2,3-dehydrosilibinin ( 11 ) have been identified as the optimal compounds with the highest potency towards both androgen-dependent LNCaP and androgen-independent PC-3 prostate cancer cell lines. 7- O -Ethyl-2,3-dehydrosilibinin ( 11 ) was demonstrated to arrest PC-3 cell cycle at the G0/G1 phase and to induce PC-3 cell apoptosis. The findings in this study suggest that antiproliferative potency of silibinin and 2,3-dehydrosilibinin can be appreciably enhanced through suitable chemical modifications on the phenolic hydroxyl group at C-7 and that introduction of a chemical moiety with the potential to improve bioavailability through a linker to 7-OH in silibinin and 2,3-dehydrosilibinin would be a feasible strategy for the development of silibinin derivatives as anti-prostate cancer agents. [ABSTRACT FROM AUTHOR]

Abstract: To improve the potential of curcumin to treat advanced hormone-refractory prostate cancer, three series (A–C) of heteroaromatic analogs (thirty two compounds) with different monoketone linkers have been synthesized and evaluated for cytotoxicity against two human androgen-independent prostate cancer cell lines (PC-3 and DU-145). Among them, thirty analogs are more potent than curcumin against PC-3 cells, and twenty one analogs are more cytotoxic towards DU-145 cells relative to curcumin. The most potent compounds (44, 45, 51, and 52) also showed impressive cytotoxicity against three other metastatic cancer cell lines (MDA-MB-231, HeLa, and A549), with IC50 values ranging from 50 nM to 390 nM. All four most potent analogs exhibited no apparent cytotoxicity towards the MCF-10A normal mammary epithelial cells. Taken together, selective enhancement of cell death in prostate cancer cell lines and other aggressive cancer cell lines suggests that nitrogen-containing heteroaromatic rings are promising bioisosteres of the substituted phenyl ring in curcumin. [Copyright &y& Elsevier]

[Display omitted] • An amide mimic of desTHPdactylolide was synthesized via a 26-step transformation. • Antiproliferative activity was evaluated against five prostate cancer cell lines. • The lactam moiety can serve as a bioisostere for the lactone in desTHPdactylodide. (−)-Zampanolide is a unique microtubule stabilizing agent (MSA) with covalent-binding mechanism and low nanomolar anitproliferative potency towards multi-drug resistant cancer cells. MSAs have a special connection with prostate cancer by inhibiting androgen receptor nuclear translocation. Zampanolide and the structurally related dactylolide have thus been sought after by us as lead compounds for development of anti-prostate cancer agents. DesTHPdactylolide is a simplified mimic of dactylolide and has previously been synthesized by us in both configurations, with the (17 R) configuration being more potent in suppressing prostate cancer cell proliferation. The current study aims to synthesize an amide mimic of (17 R) desTHPdactylolide that was anticipated to be metabolically more stable than (17 R) desTHPdactylolide. To this end, the amide mimic has been successfully synthesized through a 26-step transformation from 2-butyn-1-ol. Our WST-1 cell proliferation assay in five human prostate cancer cell models indicated that the lactam moiety can serve as a bioisostere for the lactone in desTHPdactylolide. [ABSTRACT FROM AUTHOR]