Structural basis for the activity of the RSK-specific inhibitor, SL0101

Unified Strategy for the Concise Total Syntheses of All Six 3″-O-Acyl Quercitrins Based on Regioselective Acylation Catalyzed by Boronic Acid

The naturally occurring 3″-O-acylquercitrin family exhibits a range of biological activities with significant potential health and medical benefits. Herein, we present a unified strategy for concise total syntheses of all six known 3″-O-acylquercitrin natural products─namely, 3″-O-galloylquercitrin, 3″-O-(E)-cinnamoylquercitrin, 3″-O-(E)-coumaroylquercitrin, 3″-O-(E)-feruloylquercitrin, 3″-O-acetylquercitrin, and 3″-O-tigloylquercitrin─based on regioselective acylation of carbohydrates catalyzed by N-methylimidazole-containing boronic acid. The core advancement in this approach is a late-stage catalytic regioselective functionalization of a common synthetic intermediate, enabling efficient access to the natural products.

Therapeutic targeting of p90 ribosomal S6 kinase

The Serine/Threonine protein kinase family, p90 ribosomal S6 kinases (RSK) are downstream effectors of extracellular signal regulated kinase 1/2 (ERK1/2) and are activated in response to tyrosine kinase receptor or G-protein coupled receptor signaling. RSK contains two distinct kinase domains, an N-terminal kinase (NTKD) and a C-terminal kinase (CTKD). The sole function of the CTKD is to aid in the activation of the NTKD, which is responsible for substrate phosphorylation. RSK regulates various homeostatic processes including those involved in transcription, translation and ribosome biogenesis, proliferation and survival, cytoskeleton, nutrient sensing, excitation and inflammation. RSK also acts as a major negative regulator of ERK1/2 signaling. RSK is associated with numerous cancers and has been primarily studied in the context of transformation and metastasis. The development of specific RSK inhibitors as cancer therapeutics has lagged behind that of other members of the mitogen-activated protein kinase signaling pathway. Importantly, a pan-RSK inhibitor, PMD-026, is currently in phase I/1b clinical trials for metastatic breast cancer. However, there are four members of the RSK family, which have overlapping and distinct functions that can vary in a tissue specific manner. Thus, a problem for transitioning a RSK inhibitor to the clinic may be the necessity to develop isoform specific inhibitors, which will be challenging as the NTKDs are very similar to each other. CTKD inhibitors have limited use as therapeutics as they are not able to inhibit the activity of the NTKD but could be used in the development of proteolysis-targeting chimeras.

P90 ribosomal S6 kinases: A bona fide target for novel targeted anticancer therapies?

The 90 kDa ribosomal S6 kinase (RSK) family of proteins is a group of highly conserved Ser/Thr kinases. They are downstream effectors of the Ras/ERK/MAPK signaling cascade. ERK1/2 activation directly results in the phosphorylation of RSKs, which further, through interaction with a variety of different downstream substrates, activate various signaling events. In this context, they have been shown to mediate diverse cellular processes like cell survival, growth, proliferation, EMT, invasion, and metastasis. Interestingly, increased expression of RSKs has also been demonstrated in various cancers, such as breast, prostate, and lung cancer. This review aims to present the most recent advances in the field of RSK signaling that have occurred, such as biological insights, function, and mechanisms associated with carcinogenesis. We additionally present and discuss the recent advances but also the limitations in the development of pharmacological inhibitors of RSKs, in the context of the use of these kinases as putative, more efficient targets for novel anticancer therapeutic approaches.

RSK inhibitors as potential anticancer agents: Discovery, optimization, and challenges

Ribosomal S6 kinase (RSK) family is a group of serine/threonine kinases, including four isoforms (RSK1/2/3/4). As a downstream effector of the Ras-mitogen-activated protein kinase (Ras-MAPK) pathway, RSK participates in many physiological activities such as cell growth, proliferation, and migration, and is intimately involved in tumor occurrence and development. As a result, it is recognized as a potential target for anti-cancer and anti-resistance therapies. There have been several RSK inhibitors discovered or designed in recent decades, but only two have entered clinical trials. Low specificity, low selectivity, and poor pharmacokinetic properties in vivo limit their clinical translation. Published studies performed structure optimization by increasing interaction with RSK, avoiding hydrolysis of pharmacophores, eliminating chirality, adapting to binding site shape, and becoming prodrugs. Besides enhancing efficacy, the focus of further design will move towards selectivity since there are functional differences among RSK isoforms. This review summarized the types of cancers associated with RSK, along with the structural characteristics and optimization process of the reported RSK inhibitors. Furthermore, we addressed the importance of RSK inhibitors' selectivity and discussed future drug development directions. This review is expected to shed light on the emergence of RSK inhibitors with high potency, specificity, and selectivity.

Phytochemical Characterization of Pterocephalus frutescens with In-Silico Evaluation as Chemotherapeutic Medicine and Oral Pharmacokinetics Prediction Study

Virtual screening of the potential lead chemotherapeutic phytochemicals from medicinal plants has useful application in the field of in-silico modelling and computer-based drug design by orienting and scoring ligands in the active binding site of a target protein. The phytochemical investigation of the Pterocephalus frutescens extract in n-butanol resulted in the isolation and structure elucidation of three iridoids and four flavonoids which were identified as Geniposide (1), Geniposidic acid (2), Nepetanudoside C (3), Isovitexin (4), Luteolin-7-O-glucoside (5) Isoorientin (6) and Orientin (7), respectively. Molecular docking studies were used to compare the binding energies of the isolated phytochemicals at four biological cancer-relevant targets; namely, aromatase, carbonic anhydrase IX, fatty acid synthase, and topoisomerase II-DNA complex. The docking study concluded that the isolated compounds have promising cytotoxic activities, in particular, Luteolin-7-O-glucoside (5) and Orientin (7) which exhibited high binding affinities among the isolated compounds at the active sites of the target enzymes; Aromatase (−8.73 Kcal/mol), and Carbonic anhydrase IX (−8.92 Kcal/mol), respectively, surpassing the corresponding binding scores of the co-crystallized ligands and the reference drugs at these target enzymes. Additionally, among the isolated compounds, Luteolin-7-O-glucoside (5) showed the most outstanding binding affinities at the active sites of the target enzymes; Fatty acid synthase, and Topisomerase II-DNA complex with binding scores of −6.82, and −7.99 Kcal/mol, respectively. Finally, the SwissADME online web tool predicted that most of these compounds possessed acceptable oral bioavailability and drug likeness characteristics.

Guanidine Derivatives Containing the Chalcone Skeleton Are Potent Antiproliferative Compounds against Human Leukemia Cells

In this study, we investigated the effects of eleven synthetic guanidines containing the 1,3-diphenylpropenone core on the viabilities of six human cancer cells. The most cytotoxic compound against human cancer cells of this series contains a N-tosyl group and a N-methylpiperazine moiety 6f. It was cytotoxic against leukemia cells (U-937, HL-60, MOLT-3, and NALM-6) with significant effects against Bcl-2-overexpressing U-937/Bcl-2 cells as well as the human melanoma SK-MEL-1 cell line. It exhibited low cytotoxicity against quiescent or proliferating human peripheral blood mononuclear cells. The IC₅₀ value for the leukemia U-937 cells was 1.6 ± 0.6 µM, a similar value to that in the antineoplastic agent etoposide. The guanidine containing a N-phenyl substituent 6i was also as cytotoxic as the guanidine containing the N-tosyl substituent and the N-methylpiperazine group 6f against human U-937 leukemia cells and both synthetic guanidines were potent apoptotic inducers. Cell death was mediated by the activation of the initiator caspase-9 and the executioner caspase-3, and associated with the release of cytochrome c. These synthetic guanidines are potent cytotoxic compounds against several human leukemia cells and even the human melanoma cell line SK-MEL-1 and might be useful in the development of new strategies in the fight against cancer.

Discovery, Optimization, and Structure-Activity Relationship Study of Novel and Potent RSK4 Inhibitors as Promising Agents for the Treatment of Esophageal Squamous Cell Carcinoma

Ribosomal S6 protein kinase 4 (RSK4) was identified to be a promising target for the treatment of esophageal squamous cell carcinoma (ESCC) in our previous research, whose current treatments are primarily chemotherapy and radiotherapy due to the lack of targeted therapy. However, few potent and specific RSK4 inhibitors are reported. In this study, a series of 1,4-dihydro-2H-pyrimido[4,5-d][1,3]oxazin-2-ones derivatives were designed and synthesized as novel and potent RSK4 inhibitors. Compound 14f was identified with potent RSK4 inhibitory activity both in vitro and in vivo. 14f significantly inhibited the proliferation and invasion of ESCC cells in vitro with IC50 values of 0.57 and 0.98 μM, respectively. It dose dependently inhibited the phosphorylation of RSK4 downstream substrates while exerting little effect on the substrates of RSK1-3 in ESCC cells. The markedly suppressed tumor growth and no observed toxicity to main organs in the ESCC xenograft mouse model suggested 14f to be a promising RSK4-targeting agent for ESCC treatment.

Identifying requirements for RSK2 specific inhibitors

Identifying isoform-specific inhibitors for closely related kinase family members remains a substantial challenge. The necessity for achieving this specificity is exemplified by the RSK family, downstream effectors of ERK1/2, which have divergent physiological effects. The natural product, SL0101, a flavonoid glycoside, binds specifically to RSK1/2 through a binding pocket generated by an extensive conformational rearrangement within the RSK N-terminal kinase domain (NTKD). In modelling experiments a single amino acid that is divergent in RSK3/4 most likely prevents the required conformational rearrangement necessary for SL0101 binding. Kinetic analysis of RSK2 association with SL0101 and its derivatives identified that regions outside of the NTKD contribute to stable inhibitor binding. An analogue with an n-propyl-carbamate at the 4” position on the rhamnose moiety was identified that forms a highly stable inhibitor complex with RSK2 but not with RSK1. These results identify a SL0101 modification that will aid the identification of RSK2 specific inhibitors.

MDM2 induces EMT via the B‑Raf signaling pathway through 14‑3‑3

MDM2 proto‑oncogene, E3 ubiquitin protein ligase (MDM2) is a well‑known oncogene and has been reported to be closely associated with epithelial‑to‑mesenchymal transition (EMT). The present study first demonstrated that the expression levels of MDM2 were markedly increased in TGF‑β‑induced EMT using quantitative PCR and western blotting. In addition, MDM2 was demonstrated to be associated with pathological grade in clinical glioma samples by immunohistochemical staining. Furthermore, overexpression of MDM2 promoted EMT in glioma, lung cancer and breast cancer cell lines using a scratch wound migration assay. Subsequently, the present study explored the mechanism by which MDM2 promoted EMT and revealed that MDM2 induced EMT by upregulating EMT‑related transcription factors via activation of the B‑Raf signaling pathway through tyrosine 3‑monooxygenase activation protein ε using RNA sequencing and western blotting. This mechanism depended on the p53 gene. Furthermore, in vivo experiments and the colony formation experiment demonstrated that MDM2 could promote tumor progression and induce EMT via the B‑Raf signaling pathway. Since EMT contributes to increased drug resistance in tumor cells, the present study also explored the relationship between MDM2 and drug sensitivity using an MTT assay, and identified that MDM2 promoted cell insensitivity to silibinin treatment in an EMT‑dependent manner. This finding is crucial for the development of cancer therapies and can also provide novel research avenues for future biological and clinical studies.

RSK Isoforms in Acute Myeloid Leukemia

Ribosomal S6 Kinases (RSKs) are a group of serine/threonine kinases that function downstream of the Ras/Raf/MEK/ERK signaling pathway. Four RSK isoforms are directly activated by ERK1/2 in response to extracellular stimuli including growth factors, hormones, and chemokines. RSKs phosphorylate many cytosolic and nuclear targets resulting in the regulation of diverse cellular processes such as cell proliferation, survival, and motility. In hematological malignancies such as acute myeloid leukemia (AML), RSK isoforms are highly expressed and aberrantly activated resulting in poor outcomes and resistance to chemotherapy. Therefore, understanding RSK function in leukemia could lead to promising therapeutic strategies. This review summarizes the current information on human RSK isoforms and discusses their potential roles in the pathogenesis of AML and mechanism of pharmacological inhibitors.

Prominent roles of ribosomal S6 kinase 4 (RSK4) in cancer

RSK4 refers to one Ser/Thr protein kinase functioning downstream pertaining to the signaling channel of protein kinase (MAPK) stimulated by Ras/mitogen. RSK4 can regulate numerous substrates impacting cells' surviving state, growing processes and proliferating process. Thus, dysregulated RSK4 active state display a relationship to several carcinoma categories, covering breast carcinoma, esophageal squamous cell carcinoma, glioma, colorectal carcinoma, lung carcinoma, ovarian carcinoma, leukemia, endometrial carcinoma, and kidney carcinoma. Whether RSK4 is a tumor suppressor gene or one oncogene remains controversial. No specific inhibiting elements for RSK4 have been found. This review briefs the existing information regarding RSK4 activating process, the function and mechanism of RSK4 in different tumors, and the research progress and limitations of existing RSK inhibitors. RSK4 may be a potential target of molecular therapy medicine in the future.

Concise synthesis and antidiabetic activity of natural flavonoid glycosides, oroxins C and D, isolated from the seeds of Oroxylum indium

The first concise synthesis of natural flavonoid glycosides, oroxins C (1) and D (2), which were isolated from the seeds of Oroxylum indicum, was efficiently achieved by a convergent strategy. The synthesized natural products 1 and 2 were evaluated for their inhibitory activities against α-glucosidase, α-amylase, and lipase. Compound 1 showed strong α-amylase and lipase inhibition, with IC50 values of 210 and 190 μM, respectively, but exhibited no inhibitory activity against α-glucosidase. Compound 2 showed strong inhibition against α-glucosidase and lipase, with the respective IC50 values of 180 and 80 μM.

A New Polyoxygenated Cyclohexene From the Rhizomes of Globba pendula Roxb

A new polyoxygenated cyclohexene, globbanol A (1), 2 other cyclohexene derivatives, crotepoxide (2) and boesenboxide (3), along with a chalcone, 2ʹ- hydroxy-4,4ʹ,6ʹ-trimethoxy-chalcone (4) were isolated from the rhizomes of Globba pendula Roxb. ( Zingiberaceae) of Vietnam. Their structures were elucidated on the basis of extensive analysis of high-resolution electrospray ionization mass spectrometry and NMR spectral data. Absolute configuration of new compound was determined by circular dichroism spectra. All these compounds are first reported from the genus Globba.

The affinity of RSK for cylitol analogues of SL0101 is critically dependent on the B-ring C-4'-hydroxy

Five cyclitol analogues of SL0101 with variable substitution at the C-4' position (i.e., OH, Cl, F, H, OMe) were synthesized. The series of analogues were evaluated for their ability to inhibit p90 ribosomal S6 kinase (RSK) activity. The study demonstrated the importance of the B-ring C-4' hydroxy group for RSK1/2 inhibition.

Regioselective Synthesis of a C-4'' Carbamate,C-6'' n-Pr Substituted Cyclitol Analogue of SL0101

An asymmetric synthesis of two analogues of SL0101 (1) has been achieved. The effort is aimed at the discovery of inhibitors of the p90 ribosomal S6 kinase (RSK) with improved bioavailability. The route relies upon the use of the Taylor catalyst to regioselectively install C-3″ acetyl or carbamate functionality. This study led to the identification of a third-generation analogue of SL0101 with a C-4″ n-Pr-carbamate and a C-3″ acetate with improved RSK inhibitory activity.

Substituted pteridinones as p90 ribosomal S6 protein kinase (RSK) inhibitors: A structure-activity study

The activity of p90 ribosomal S6 kinase 2 (RSK2) has emerged as an attractive target for cancer therapy due to its role in the regulation of diverse cellular processes, such as cell transformation and proliferation. Several pan-RSK inhibitors have been identified with BI-D1870 and the pseudo-analogs LJH685 and LJI308 being the most selective, potent, and frequently used small molecule inhibitors. We designed and synthesized a series of pteridinones and pyrimidines to evaluate the structural features of BI-D1870 that are required for RSK2 inhibition. We have identified inhibitors of RSK2 activity, evaluated their target engagement in cells, and measured their effect on cell viability and cytotoxicity in the MOLM-13 acute myeloid leukemia (AML) cell line. The results of our studies support that RSK2 inhibition can be achieved in MOLM-13 cells without potent cytotoxicity. The structure-activity data from this study will be used as a platform to develop novel RSK2 inhibitors.

6'-Benzyloxy-4-bromo-2'-hydroxychalcone is cytotoxic against human leukaemia cells and induces caspase-8- and reactive oxygen species-dependent apoptosis

In this study, we investigated the effects of synthetic 6'-benzyloxy-4-bromo-2'-hydroxychalcone on viabilities of seven human leukaemia cells. It was cytotoxic against U-937, HL-60, K-562, NALM-6, MOLT-3 cells, and also against Bcl-2-overexpressing U-937/Bcl-2 cells and P-glycoprotein-overexpressing K-562/ADR, but had no significant cytotoxic effects against quiescent or proliferating human peripheral blood mononuclear cells. This chalcone is a potent apoptotic inducer in human leukaemia U-937 cells. Cell death was (i) mediated by the activation and the cleavage of initiator and executioner caspases and poly(ADP-ribose) polymerase; (ii) prevented by the pan-caspase inhibitor z-VAD-fmk, and by the selective caspase-3/7, -6 and -8 inhibitors, and by a cathepsins B/L inhibitor; (iii) associated with the release of mitochondrial proteins, including cytochrome c and Smac/DIABLO; (iv) accompanied by dissipation of the mitochondrial membrane potential, (v) partially blocked by the inhibition of p38^MAPK and (vi) mostly abrogated by catalase. In conclusion, the synthetic chalcone is cytotoxic against several types of human leukaemia cell with apoptosis being induced by activation of the extrinsic pathway and the generation of reactive oxygen species.

Total Synthesis of the Flavonoid Natural Product Houttuynoid A

The first total synthesis of the antiviral flavonoid houttuynoid A (1) has been achieved from aryl ketone 6 and benzofuran aldehyde 5 in nine linear steps. The C₆-C₃-C₆ structure of the flavonoid was synthesized by an I₂-catalyzed oxa-Michael addition of a chalcone intermediate, generated by the Claisen-Schmidt condensation of 5 and 6. This work provides a method for the synthesis of houttuynoids and provides a reference for the synthesis of the remaining members of the houttuynoid family.

Synaptically driven phosphorylation of ribosomal protein S6 is differentially regulated at active synapses versus dendrites and cell bodies by MAPK and PI3K/mTOR signaling pathways

High-frequency stimulation of the medial perforant path triggers robust phosphorylation of ribosomal protein S6 (rpS6) in activated dendritic domains and granule cell bodies. Here we dissect the signaling pathways responsible for synaptically driven rpS6 phosphorylation in the dentate gyrus using pharmacological agents to inhibit PI3-kinase/mTOR and MAPK/ERK-dependent kinases. Using phospho-specific antibodies for rpS6 at different sites (ser235/236 versus ser240/244), we show that delivery of the PI3-kinase inhibitor, wortmannin, decreased rpS6 phosphorylation throughout the somatodendritic compartment (granule cell layer, inner molecular layer, outer molecular layer), especially in granule cell bodies while sparing phosphorylation at activated synapses (middle molecular layer). In contrast, delivery of U0126, an MEK inhibitor, attenuated rpS6 phosphorylation specifically in the dendritic laminae leaving phosphorylation in the granule cell bodies intact. Delivery of the mTOR inhibitor, rapamycin, abolished activation of rpS6 phosphorylation in granule cell bodies and dendrites, whereas delivery of a selective S6K1 inhibitor, PF4708671, or RSK inhibitor, SL0101-1, attenuated rpS6 phosphorylation throughout the postsynaptic cell. These results reveal that MAPK/ERK-dependent signaling is predominately responsible for the selective induction of rpS6 phosphorylation at active synapses. In contrast, PI3-kinase/mTOR-dependent signaling induces rpS6 phosphorylation throughout the somatodendritic compartment but plays a minimal role at active synapses. Collectively, these results suggest a potential mechanism by which PI3-kinase/mTOR and MAPK/ERK pathways regulate translation at specific subcellular compartments in response to synaptic activity.

Stereoselective Synthesis and Evaluation of C6″-Substituted 5a-Carbasugar Analogues of SL0101 as Inhibitors of RSK1/2

A convergent synthesis of 5a-carbasugar analogues of the n-Pr-variant of SL0101 is described. The analogues were synthesized in an effort to find compounds with potent in vivo efficacy in the inhibition of p90 ribosomal s6 kinase (RSK1/2). The synthesis derived the desired C-4 L-rhamnose stereochemistry from quinic acid and used a highly selective cuprate addition, NaBH₄ reduction, Mitsunobu inversion, and alkene dihydroxylation to install the remaining stereochemistry. A Pd-catalyzed cyclitolization stereoselectively installed the aglycon at the anomeric position. The analogues were evaluated as RSK1/2 inhibitors and found to have 3- to 6-fold improved activity.

Selective Targeting of RSK Isoforms in Cancer

The p90 ribosomal S6 kinase family (RSK1-4) is a group of highly conserved Ser/Thr kinases that act as downstream effectors of the Ras/Raf/MEK/ERK signaling pathway. The RSKs phosphorylate a range of substrates involved in transcription, translation, cell cycle regulation, and cell survival. Although the RSKs have a high degree of sequence homology, their functional differences in cancer are of great interest. Current RSK inhibitors target more than one RSK isoform, and this may limit their efficacy as anticancer agents. Here, we review the structure and function of the RSK kinases, their role in cancer growth and survival, and their potential as modulators of chemoresistance. In addition, we summarize the development of current RSK inhibitors and their limitations.

Total Synthesis of (+)-Vicenin-2

The bis-C-glucosyl flavonoid vicenin-2 (1) has been synthesized by exploiting bis-C-glycosylation of 1,3,5-trifluorobenzene and aromatic nucleophilic substitution to transform fluorine atoms to oxygen functions in excellent yield.

Ribosomal S6 kinase (RSK) modulators: a patent review

INTRODUCTION

The p90 ribosomal S6 kinases (RSK) are a family of Ser/Thr protein kinases that are downstream effectors of MEK1/2-ERK1/2. Increased RSK activation is implicated in the etiology of multiple pathologies, including numerous types of cancers, cardiovascular disease, liver and lung fibrosis, and infections.

AREAS COVERED

The review summarizes the patent and scientific literature on small molecule modulators of RSK and their potential use as therapeutics. The patents were identified using World Intellectual Property Organization and United States Patent and Trademark Office databases. The compounds described are predominantly RSK inhibitors, but a RSK activator is also described. The majority of the inhibitors are not RSK-specific.

EXPERT OPINION

Based on the overwhelming evidence that RSK is involved in a number of diseases that have high mortalities it seems surprising that there are no RSK modulators that have pharmacokinetic properties suitable for in vivo use. MEK1/2 inhibitors are in the clinic, but the efficacy of these compounds appears to be limited by their side effects. We hypothesize that targeting the downstream effectors of MEK1/2, like RSK, are an untapped source of drug targets and that they will generate less side effects than MEK1/2 inhibitors because they regulate fewer effectors.

Practical Synthesis of Naringenin

Two routes for the synthesis of the flavanone naringenin are described. In the first, 3,5-dimethoxyphenol is converted to 2-hydroxy-4,6-dimethoxyacetophenone and then by condensation with anisaldehyde to 2′-hydroxy-4,4′,6′-trimethoxychalcone. The chalcone is then cyclised with aqueous hydrochloric acid and demethylated with pyridine hydrochloride to form naringenin in 45% overall yield. The condensation of 2-hydroxy-4,6-dimethoxyacetophenone with anisaldehyde could also directly produce 4′,5,7-trimethoxyflavanone, which was then converted into naringenin in 60% overall yield. In the second route, a single step for the preparation of the chalcone is used in which 1,3,5-trimethoxybenzene is acylated with p-methoxycinnamic acid. Although the synthesis of naringenin is achieved in a lower overall yield of 29%, the process is simpler.

An Improved Synthesis of Apigenin

Two routes for the synthesis of the flavone apigenin are described. In the first, taxicatigenin was converted to 2-hydroxy-4,6-dimethoxyacetophenone and then by condensation with anisaldehyde to 2′-hydroxy-4,4′,6′-trimethoxychalcone. The latter was cyclised with iodine and demethylated with pyridine hydrochloride to form apigenin in 53% overall yield. In the second route, a single step for the preparation of the chalcone was used in which 1,3,5-trimethoxybenzene was acylated with p-methoxycinnamic acid. Although the synthesis of apigenin was achieved in a lower overall yield of 34%, the process was simpler.

Synthesis and Structure-Activity Relationship Study of 5a-Carbasugar Analogues of SL0101

The Ser/Thr protein kinase, RSK, is associated with oncogenesis, and therefore, there are ongoing efforts to develop RSK inhibitors that are suitable for use in vivo. SL0101 is a natural product that demonstrates selectivity for RSK inhibition. However, SL0101 has a short biological half-life in vivo. To address this issue we designed a set of eight cyclitol analogues, which should be resistant to acid catalyzed anomeric bond hydrolysis. The analogues were synthesized and evaluated for their ability to selectively inhibit RSK in vitro and in cell-based assays. All the analogues were prepared using a stereodivergent palladium-catalyzed glycosylation/cyclitolization for installing the aglycon. The l-cyclitol analogues were found to inhibit RSK2 in in vitro kinase activity with a similar efficacy to that of SL0101, however, the analogues were not specific for RSK in cell-based assays. In contrast, the d-isomers showed no RSK inhibitory activity in in vitro kinase assay.

Phase Transfer Catalysis Extends The Scope of The Algar–Flynn–Oyamada Synthesis of 3-Hydroxyflavones

The Algar–Flynn–Oyamada reaction is the classical method to synthesize 3-hydroxyflavones from chalcones. Despite its relative simplicity, the reaction has several drawbacks including variable and often low product yields. We have found that phase transfer catalysis improves the yields and expands the scope of the Algar–Flynn–Oyamada reaction of a series of 4-benzyloxy-2-hydroxy chalcones.

De novo synthesis and biological evaluation of C6″-substituted C4″-amide analogues of SL0101

In an effort to improve upon the in vivo half-life of the known ribosomal s6 kinase (RSK) inhibitor SL0101, C4″-amide/C6″-alkyl substituted analogues of SL0101 were synthesized and evaluated in cell-based assays. The analogues were prepared using a de novo asymmetric synthetic approach, which featured Pd-π-allylic catalyzed glycosylation for the introduction of a C4″-azido group. Surprisingly replacement of the C4″-acetate with a C4″-amide resulted in analogues that were no longer specific for RSK in cell-based assays.

A novel Synthesis of Naringenin and Related Flavanones

Efficient methods are reported for the preparation of naringenin (4′,5,7-trihydroxyflavanone) which could be easily scaled-up. They have been applied to three other flavanones (6-hydroxyflavanone, 6,4′-dihydroxyflavanone, 6,3′,4′-trihydroxyflavanone) suitably.

Luteolin is a novel p90 ribosomal S6 kinase (RSK) inhibitor that suppresses Notch4 signaling by blocking the activation of Y-box binding protein-1 (YB-1)

Triple-negative breast cancers (TNBC) are notoriously difficult to treat because they lack hormone receptors and have limited targeted therapies. Recently, we demonstrated that p90 ribosomal S6 kinase (RSK) is essential for TNBC growth and survival indicating it as a target for therapeutic development. RSK phosphorylates Y-box binding protein-1 (YB-1), an oncogenic transcription/translation factor, highly expressed in TNBC (~70% of cases) and associated with poor prognosis, drug resistance and tumor initiation. YB-1 regulates the tumor-initiating cell markers, CD44 and CD49f however its role in Notch signaling has not been explored. We sought to identify novel chemical entities with RSK inhibitory activity. The Prestwick Chemical Library of 1120 off-patent drugs was screened for RSK inhibitors using both in vitro kinase assays and molecular docking. The lead candidate, luteolin, inhibited RSK1 and RSK2 kinase activity and suppressed growth in TNBC, including TIC-enriched populations. Combining luteolin with paclitaxel increased cell death and unlike chemotherapy alone, did not enrich for CD44⁺ cells. Luteolin’s efficacy against drug-resistant cells was further indicated in the primary x43 cell line, where it suppressed monolayer growth and mammosphere formation. We next endeavored to understand how the inhibition of RSK/YB-1 signaling by luteolin elicited an effect on TIC-enriched populations. ChIP-on-ChIP experiments in SUM149 cells revealed a 12-fold enrichment of YB-1 binding to the Notch4 promoter. We chose to pursue this because there are several reports indicating that Notch4 maintains cells in an undifferentiated, TIC state. Herein we report that silencing YB-1 with siRNA decreased Notch4 mRNA. Conversely, transient expression of Flag:YB-1^WT or the constitutively active mutant Flag:YB-1^D102 increased Notch4 mRNA. The levels of Notch4 transcript and the abundance of the Notch4 intracellular domain (N4ICD) correlated with activation of P-RSK^S221/7 and P-YB-1^S102 in a panel of TNBC cell lines. Silencing YB-1 or RSK reduced Notch4 mRNA and this corresponded with loss of N4ICD. Likewise, the RSK inhibitors, luteolin and BI-D1870, suppressed P-YB-1 ^S102 and thereby reduced Notch4. In conclusion, inhibiting the RSK/YB-1 pathway with luteolin is a novel approach to blocking Notch4 signaling and as such provides a means of inhibiting TICs.

The p90 RSK family members: common functions and isoform specificity

The p90 ribosomal S6 kinases (RSK) are implicated in various cellular processes, including cell proliferation, survival, migration, and invasion. In cancer, RSKs modulate cell transformation, tumorigenesis, and metastasis. Indeed, changes in the expression of RSK isoforms have been reported in several malignancies, including breast, prostate, and lung cancers. Four RSK isoforms have been identified in humans on the basis of their high degree of sequence homology. Although this similarity suggests some functional redundancy between these proteins, an increasing body of evidence supports the existence of isoform-based specificity among RSKs in mediating particular cellular processes. This review briefly presents the similarities between RSK family members before focusing on the specific function of each of the isoforms and their involvement in cancer progression.

An efficient procedure for the preparation of natural products bearing the 2-(2-phenylethyl)chromone skeleton

Several 2-(2-phenylethyl)chromones have been shown to possess neuroprotective activity. However, limited synthetic methods have been disclosed to construct the 2-(2-phenylethyl)chromone skeleton. Herein we report a straightforward 3-step preparation of five naturally occurring 2-(2-phenylethyl)chromones utilizing the Claisen condensation as the key step.

Development of flavonoid-based inverse agonists of the key signaling receptor US28 of human cytomegalovirus

A series of 31 chalcone- and flavonoid-based derivatives were synthesized in good overall yields and screened for their inverse agonist activity on the US28 receptor of human cytomegalovirus (HCMV). With one exception (e.g., 2-(5-bromo-2-methoxyphenyl)-3-hydroxy-4H-chromen-4-one), halogen-substituted flavonoids were typically more potent inverse agonists than their related hydro derivatives. While toxicity could be used to partially explain the inverse agonist activity of some members of the series, 5-(benzyloxy)-2-(5-bromo-2-methoxyphenyl)-4H-chromen-4-one (11b) acted on the US28 receptor as a nontoxic, inverse agonist. The full inverse agonism (efficacy, -89%) and potency (EC50 = 3.5 μM) observed with flavonoid 11b is especially important as it provides both a new tool to study US28 signaling and a potential platform for the future development of HCMV-targeting drugs.

Bioactive flavaglines and other constituents isolated from Aglaia perviridis

Eight new compounds, including two cyclopenta[b]benzopyran derivatives (1, 2), two cyclopenta[b]benzofuran derivatives (3, 4), three cycloartane triterpenoids (5-7), and an apocarotenoid (8), together with 16 known compounds, were isolated from the chloroform-soluble partitions of separate methanol extracts of a combination of the fruits, leaves, and twigs and of the roots of Aglaia perviridis collected in Vietnam. Isolation work was monitored using human colon cancer cells (HT-29) and facilitated with an LC/MS dereplication procedure. The structures of the new compounds (1-8) were determined on the basis of spectroscopic data interpretation. The Mosher ester method was employed to determine the absolute configurations of 5-7, and the absolute configuration of the 9,10-diol unit of compound 8 was established by a dimolybdenum tetraacetate [Mo2(AcO)4] induced circular dichroism procedure. Seven known rocaglate derivatives (9-15) exhibited significant cytotoxicity against the HT-29 cell line, with rocaglaol (9) being the most potent (ED50 0.0007 μM). The new compounds 2-4 were also active against this cell line, with ED50 values ranging from 0.46 to 4.7 μM. The cytotoxic compounds were evaluated against a normal colon cell line, CCD-112CoN. In addition, the new compound perviridicin B (2), three known rocaglate derivatives (9, 11, 12), and a known sesquiterpene, 2-oxaisodauc-5-en-12-al (17), showed significant NF-κB (p65) inhibitory activity in an ELISA assay.

Substituted indolin-2-ones as p90 ribosomal S6 protein kinase 2 (RSK2) inhibitors: Molecular docking simulation and structure-activity relationship analysis

A series of novel indolin-2-ones inhibitors against p90 ribosomal S6 protein kinase 2 (RSK2) were designed and synthesized and their structure-activity relationship (SAR) was studied. The most potent inhibitor, compound 3s, exhibited potent inhibition against RSK2 with an IC50 value of 0.5 μM and presented a satisfactory selectivity against 23 kinases. The interactions of these inhibitors with RSK2 were investigated based on the proposed binding poses with molecular docking simulation. Four compounds and six compounds exhibited moderate anti-proliferation activities against PC 3 cells and MCF-7 cells, respectively.

Insights into the inhibition of the p90 ribosomal S6 kinase (RSK) by the flavonol glycoside SL0101 from the 1.5 Å crystal structure of the N-terminal domain of RSK2 with bound inhibitor

The p90 ribosomal S6 family of kinases (RSK) are potential drug targets, due to their involvement in cancer and other pathologies. There are currently only two known selective inhibitors of RSK, but the basis for selectivity is not known. One of these inhibitors is a naturally occurring kaempferol-α-L-diacetylrhamnoside, SL0101. Here, we report the crystal structure of the complex of the N-terminal kinase domain of the RSK2 isoform with SL0101 at 1.5 Å resolution. The refined atomic model reveals unprecedented structural reorganization of the protein moiety, as compared to the nucleotide-bound form. The entire N-lobe, the hinge region, and the αD-helix undergo dramatic conformational changes resulting in a rearrangement of the nucleotide binding site with concomitant formation of a highly hydrophobic pocket spatially suited to accommodate SL0101. These unexpected results will be invaluable in further optimization of the SL0101 scaffold as a promising lead for a novel class of kinase inhibitors.

Diterpenoids and triterpenoids with potential anti-inflammatory activity from the leaves of Aglaia odorata

Chemical investigation of the leaves of the oriental medicinal plant Aglaia odorata resulted in the isolation of five compounds: two dolabellane diterpenoids, two dammarane triterpenoids and a protostane triterpenoid, along with twenty known compounds. Their structures were elucidated on the basis of extensive spectroscopic analysis and by comparison of their NMR spectroscopic data with those reported in the literature. The anti-inflammatory activities of all compounds were evaluated as inhibitory activities against lipopolysaccharide (LPS) induced nitric oxide (NO) production in RAW264.7 cell lines. Eleven compounds possessed potent nitric oxide inhibitory activity with IC(50) values ranging from 2.1 to 14.2 μM, these being better than that of the positive control, indomethacin (IC(50)=14.5 μM). In addition, three compounds exhibited significant activity against PGE(2) release with IC(50) values of 2.6, 16.1 and 23.0 μM.

Analogs of the RSK inhibitor SL0101: optimization of in vitro biological stability

The Ser/Thr protein kinase, RSK, is important in the etiology of tumor progression including invasion and motility. The natural product kaempferol-3-O-(3″,4″-di-O-acetyl-α-l-rhamnopyranoside), called SL0101, is a highly specific RSK inhibitor. Acylation of the rhamnose moiety is necessary for high affinity binding and selectivity. However, the acetyl groups can be cleaved by esterases, which accounts for the poor in vitro biological stability of SL0101. To address this problem a series of analogs containing acetyl group replacements were synthesized and their in vitro stability evaluated. Monosubstituted carbamate analogs of SL0101 showed improved in vitro biological stability while maintaining specificity for RSK. These results should facilitate the development of RSK inhibitors derived from SL0101 as anticancer agents.

Total Synthesis of Apigenin

An efficient method for the synthesis of apigenin (4’,5,7-trihydroxyflavone, a traditional medicine) from phloroglucinol and anisaldehyde has been developed. This transformation features a green method for hydroxyl protection as methyl ethers and a different way for cyclisation using iodine in DMSO. The overall yield of 40% is satisfactory.

Improving the affinity of SL0101 for RSK using structure-based design

Enhanced activity of the Ser/Thr protein kinase, RSK, is associated with transformation and metastasis, which suggests that RSK is an attractive drug target. The natural product, SL0101 (kaempferol 3-O-(3″,4″-di-O-acetyl-α-L-rhamnopyranoside), has been shown to be a RSK selective inhibitor. However, the Ki for SL0101 is 1 μM with a half-life of less than 30 min in vivo. To identify analogues with improved efficacy we designed a set of analogues based on the crystallographic model of SL0101 in complex with the RSK2 N-terminal kinase domain. We identified an analogue with a 5″-n-propyl group on the rhamnose that has > 40-fold improved affinity for RSK relative to SL0101 in an in vitro kinase assay. This analogue preferentially inhibited the proliferation of the human breast cancer line, MCF-7, versus the normal untransformed breast line, MCF-10A, which is consistent with results using SL0101. However, the efficacy of the 5″-n-propyl analogue to inhibit MCF-7 proliferation was only two-fold better than for SL0101, which we hypothesize is due to limited membrane permeability. The improved affinity of the 5″-n-propyl analogue for RSK will aid in the design of future compounds for in vivo use.