Biological and structural investigation of tetrahydro-ß-carboline-based selective HDAC6 inhibitors with improved stability.

Our previously reported HDAC6 inhibitor (HDAC6i) Marbostat-100 (4) has provided many arguments for further clinical evaluation. By the substitution of the acidic hydrogen of 4 for different carbon residues, we were able to generate an all-carbon stereocenter, which significantly improves the hydrolytic stability of the inhibitor. Further asymmetric synthesis has shown that the (S)-configured inhibitors preferentially bind to HDAC6. This led to the highly selective and potent methyl-substituted derivative S-29b, which elicited a long-lasting tubulin hyperacetylation in MV4-11 cells. Finally, a crystal structure of the HDAC6/S-29b complex provided mechanistic explanation for the high potency and stereoselectivity of synthesized compound series.

Pentafluorophenol (C6 F5 OH) Catalyzed Pictet-Spengler Reaction: A Facile and Metal-Free Approach Towards Tetrahydro-ß-Carbolines.

Herein, we have disclosed pentafluorophenol as an operative catalyst for synthesizing (spirocyclic) tetrahydro-ß-carbolines via the Pictet-Spengler reaction. This straightforward catalytic protocol works under mild conditions resulting indole alkaloids in excellent yield with remarkable functional group tolerance, including late-stage modifications. This transformation demonstrates a practical and adaptable approach to produce a highly effective gram-scale synthesis of the natural alkaloid Komavine and enables the synthesis of the commercial drug Tadalafil.

Discovery of Novel Tetrahydro-ß-carboline Containing Aminopeptidase N Inhibitors as Cancer Chemosensitizers.

Aminopeptidase N (APN, CD13) is closely associated with the development and progression of cancer. Previous studies suggested APN as a biomarker for cancer stem cells. APN inhibitors have been intensively evaluated as chemosensitizers for cancer treatments. In the present study, tetrahydro-ß-carboline scaffold was introduced to the structure of APN inhibitors. The synthesized compounds showed potent enzyme inhibitory activities compared with Bestatin, an approved APN inhibitor, in cell-based enzymatic assay. In combination with chemotherapeutic drugs, representative APN inhibitor molecules D12, D14 and D16 significantly improved the antiproliferative potency of anticancer drugs in the in vitro tests. Further mechanistic studies revealed that the anticancer effects of these drug combinations are correlated with decreased APN expression, increased ROS level, and induction of cell apoptosis. The spheroid-formation assay and colony-formation assay results showed effectiveness of Paclitaxel-APN inhibitor combination against breast cancer stem cell growth. The combined drug treatment led to reduced mRNA expression of OCT-4, SOX-2 and Nanog in the cancer stem cells tested, suggesting the reduced stemness of the cells. In the in vivo study, the selected APN inhibitors, especially D12, exhibited improved anticancer activity in combination with Paclitaxel compared with Bestatin. Collectively, potent APN inhibitors were discovered, which could be used as lead compounds for tumor chemo-sensitization and cancer stem cell-based therapies.

Cinchona-alkaloid-based zwitterionic chiral stationary phases as potential tools for high-performance liquid chromatographic enantioseparation of cationic compounds of pharmaceutical relevance.

Enantiomers of cationic compounds of pharmaceutical relevance, namely tetrahydro-ß-carboline and 1,2,3,4-tetrahydroisoquinoline analogs, were separated by high-performance liquid chromatography. Separations were performed on Cinchona-alkaloid-based zwitterionic ion exchanger type chiral stationary phases applied as cation exchangers using mixtures of methanol and acetonitrile or tetrahydrofuran as bulk solvent components containing triethylammonium acetate or ammonium acetate as organic salt additives. On the zwitterionic ZWIX(+) and ZWIX(-) columns investigated, retention and enantioseparation of the studied basic analytes were influenced by the nature and concentration of the organic components of the mobile phase. The effect of organic salt additives on the retention behavior of the studied analytes can be described by the stoichiometric displacement model related to the counterion concentration. Investigations on the structure-retention relationships were performed applying different mobile phase systems for the two types of cationic analytes. For the thermodynamic characterization, parameters such as changes in standard enthalpy (Δ(ΔH°)), entropy (Δ(ΔS°)), and free energy (Δ(ΔG°)) were calculated on the basis of van't Hoff plots derived from the ln α versus 1/T curves. In most cases, enthalpy-driven enantioseparations were observed, with a consistent dependence of the calculated thermodynamic parameters on the mobile phase composition. Elution sequences of the studied compounds were determined in all cases.

High-performance liquid chromatographic evaluation of strong cation exchanger-based chiral stationary phases focusing on stationary phase characteristics and mobile phase effects employing enantiomers of tetrahydro-ß-carboline and 1,2,3,4-tetrahydroisoquinoline analogs.

In this study, we present results obtained on the enantioseparation of some cationic compounds of pharmaceutical relevance, namely tetrahydro-ß-carboline and 1,2,3,4-tetrahydroisoquinoline analogs. In high-performance liquid chromatography, chiral stationary phases (CSPs) based on strong cation exchanger were employed using mixtures of methanol and acetonitrile or tetrahydrofuran as mobile phase systems with organic salt additives. Through the variation of the applied chromatographic conditions, the focus has been placed on the study of retention and enantioselectivity characteristics as well as elution order. Retention behavior of the studied analytes could be described by the stoichiometric displacement model related to the counter-ion effect of ammonium salts as mobile phase additives. For the thermodynamic characterization parameters, such as changes in standard enthalpy Δ(ΔH°), entropy Δ(ΔS°), and free energy Δ(ΔG°), were calculated on the basis of van't Hoff plots derived from the ln α vs. 1/T curves. In all cases, enthalpy-driven enantioseparations were observed with a slight, but consistent dependence of the calculated thermodynamic parameters on the eluent composition. Elution sequences of the studied compounds were determined in all cases. They were found to be opposite on the enantiomeric stationary phases and they were not affected by either the temperature or the eluent composition.

Novel deoxyvasicinone and tetrahydro-beta-carboline hybrids as inhibitors of acetylcholinesterase and amyloid beta aggregation.

A novel series of deoxyvasicinone-tetrahydro-beta-carboline hybrids were synthesized and evaluated as acetylcholinesterase (AChE) and ß-amyloid peptide (Aß) aggregation inhibitors for the treatment of Alzheimer's disease. The results revealed that the derivatives had multifunctional profiles, including AChE inhibition, Aß1-42 aggregation inhibition, and neuroprotective properties. Inspiringly, hybrids 8b and 8d displayed excellent inhibitory activities against hAChE (IC50 = 0.93 and 1.08 nM, respectively) and Aß1-42 self-aggregation (IC50 = 19.71 and 2.05 µM, respectively). In addition, 8b and 8d showed low cytotoxicity and good neuroprotective activity against Aß1-42-induced damage in SH-SY5Y cells.

Antibacterial Functions and Proposed Modes of Action of Novel 1,2,3,4-Tetrahydro-ß-carboline Derivatives that Possess an Attractive 1,3-Diaminopropan-2-ol Pattern against Rice Bacterial Blight, Kiwifruit Bacterial Canker, and Citrus Bacterial Canker.

In recent years, naturally occurring tetrahydro-ß-carboline (THC) alkaloids and their derivatives have been of biological interest. However, few studies and developments have reported the use of such structures in managing plant bacterial diseases. Herein, an array of novel THC derivatives containing an attractive 1,3-diaminopropan-2-ol pattern were prepared to evaluate the antiphytopathogen activity in vitro and in vivo and explore innovative antibacterial frameworks. Notably, target compounds exhibited excellent activities against three rebellious phytopathogens, namely, Pseudomonas syringae pv. actinidiae (Psa), Xanthomonas axonopodis pv. citri, and Xanthomonas oryzae pv. oryzae, at related optimal EC50 values of 2.39 (II9), 2.06 (I23), and 1.69 (II9) µg/mL, respectively. These effects were superior to those of the parent structure 1,2,3,4-THC and positive controls. In vivo assays showed that II9 exhibited excellent control efficiencies of 51.89 and 65.45% at 200 µg/mL against rice bacterial blight and kiwifruit bacterial canker, respectively, and I23 substantially relieved the citrus canker on the leaves. Antibacterial mechanisms indicated that these THC compounds could induce the increment of reactive oxygen species and subsequently endow the tested bacteria with distinct apoptotic behavior. In addition, II9 could alleviate the hypersensitive response and pathogenicity of Psa. Overall, these simple THC derivatives can be further developed as versatile antibacterial agents.

Pyrrolo [3,4-b]-quinolin-9-amine compound FZU-0038-056 suppresses triple-negative breast cancer partially through inhibiting the expression of Bcl-2.

Triple-negative breast cancer (TNBC) has a poorer prognosis than other subtypes of breast cancer; however, it lacks effective targeted therapies clinically. In this study, we found FZU-0038-056, a novel compound derived from last-stage functionalization of tetrahydro-ß-carboline scaffold, showed the most potent anti-cancer activity against TNBC cells among the 42 synthesized derivatives. We found FZU-0038-056 significantly induces apoptosis in HCC1806 and HCC1937 TNBC cells. FZU-0038-056 reduces the expression levels of several anti-apoptosis proteins, including Bcl-2, Mcl-1 and XIAP. Furthermore, we found FZU-0038-056 induces apoptosis partially through inhibiting the expression of Bcl-2. Finally, we found FZU-0038-056 significantly suppresses HCC1806 xenograft tumor growth in nude mice without affecting their body weight. Therefore, FZU-0038-056 has the potential to be a new anticancer agent for treating human TNBC.

Study on chemical modification and analgesic activity of N-(4-tert-butylphenyl)-4-(3-chloropyridin-2-yl) piperazine-1-carboxamide.

N-(4-Tert-butylphenyl)-4-(3-chloropyridin-2-yl) piperazine-1-carboxamide (BCTC) is a potent and extensively studied urea-based TRPV1 antagonist. Although BCTC was effective in alleviating chronic pain in rats, it showed obvious hyperthermia side-effect and unsatisfactory pharmacokinetic profile, therefore, it was not developed further. In order to enrich the structural types of urea-based TRPV1 antagonists, two series of novel analogs, in which the pyridine ring of BCTC was replaced with a mildly basic pyrimidine ring or 1,2,3,4-tetrahydro-ß-carboline scaffold, were designed and synthesized. Advancing the structure-activity relationship of these two series led to the discovery of N-(4-methoxyphenyl)-1,3,4,9-tetrahydro-2H-pyrido[3,4-b]indole-2-carboxamide (7o), with an improved pharmacological and tolerability profile compared with the lead compound BCTC.

The Pictet-Spengler Reaction Updates Its Habits.

The Pictet-Spengler reaction (P-S) is one of the most direct, efficient, and variable synthetic method for the construction of privileged pharmacophores such as tetrahydro-isoquinolines (THIQs), tetrahydro-ß-carbolines (THBCs), and polyheterocyclic frameworks. In the lustro (five-year period) following its centenary birthday, the P-S reaction did not exit the stage but it came up again on limelight with new features. This review focuses on the interesting results achieved in this period (2011-2015), analyzing the versatility of this reaction. Classic P-S was reported in the total synthesis of complex alkaloids, in combination with chiral catalysts as well as for the generation of libraries of compounds in medicinal chemistry. The P-S has been used also in tandem reactions, with the sequences including ring closing metathesis, isomerization, Michael addition, and Gold- or Brønsted acid-catalyzed N-acyliminium cyclization. Moreover, the combination of P-S reaction with Ugi multicomponent reaction has been exploited for the construction of highly complex polycyclic architectures in few steps and high yields. The P-S reaction has also been successfully employed in solid-phase synthesis, affording products with different structures, including peptidomimetics, synthetic heterocycles, and natural compounds. Finally, the enzymatic version of P-S has been reported for biosynthesis, biotransformations, and bioconjugations.

Separation, synthesis, and cytotoxicity of a series of mogrol derivatives.

Four metabolites of mogrol were separated, identified and characterized. Their antitumor activity was evaluated, and the results showed side chain modification would probably enhance the cytotoxicity. Therefore, three types of amines, alcohols and rigid planar derivatives were synthesized. Compounds 20 and 21 containing a tetrahydro-ß-carboline structure at the end of the side chain exhibited IC50 values around 2-9 µM against A549 and CNE1 cell comparing with 80-90 µM of mogrol. Structure analysis suggested that the perhydrocyclopentanophenanthrene moiety and the tetrahydro-ß-carboline moiety could probably enhance the activity through an intramolecular synergistic effect.[Formula: see text].

Synthesis, anti-plasmodial and cytotoxic evaluation of 1H-1,2,3-triazole/acyl hydrazide integrated tetrahydro-ß-carboline-4-aminoquinoline conjugates.

A series of 1H-1,2,3-triazole/acylhydrazide-tethered tetrahydro-ß-carboline-4-aminoquinoline conjugates was synthesized with an aim to study their anti-plasmodial structure-activity relationship. The presence of 1H-1,2,3-triazole-core along with a flexible alkyl chain length on aminoquinoline core has favourable influence on the anti-plasmodial activity against Chloroquine-resistant W2 strain of P. falciparum while the introduction of hydrazine core not only diminished the activities but also resulted in increased cytotoxicity against mammalian Vero cells.

Synthesis and Structure-Activity Relationships of Tetrahydro-ß-carboline Derivatives as Anticancer and Cancer-chemopreventive Agents.

BACKGROUND/AIM: There is an unmet clinical need to develop new anticancer and chemopreventive agents. The aim of the present study was to identify ß-carboline derivatives with cancer chemopreventive and therapeutic potential. MATERIALS AND METHODS: Forty-eight tetrahydro-ß-carboline derivatives were synthesized and evaluated for their anticancer and chemopreventive activities, through induction of quinone reductase 1 (QR1), aromatase inhibition, as well as inhibition of nitric oxide (NO) production. RESULTS: 2-((1-Bromonaphthalen-2-yl)methyl)-2,3,4,9-tetrahydro-1H-pyrido[3,4-b]indole demonstrated the most potent activity in the QR1 induction assay with an induction ratio value of 3.2 (CD=1.3 µM). The R-isomer of the amide derivative (2-((1-bromonaphthalen-2-yl)methyl)-2,3,4,9-tetrahydro-1H-pyrido[3,4-b]indol-3-yl)(4-methylpiperazin-1-yl)methanone was the most potent inhibitor of NO production with a 50% inhibitory concentration, IC50=6.54 µM and had a low cytotoxic effect (IC50=17.98 µM) on RAW 264.7 cells. Subsequent computational docking study revealed that this compound binds to the active site of inducible nitric oxide synthase with favorable interactions. CONCLUSION: our results provided promising ß-carboline leads for further optimization and development with therapeutic potential as new chemopreventive and chemotherapy agents.

Ingenine F: A New Cytotoxic Tetrahydro Carboline Alkaloid from the Indonesian Marine Sponge Acanthostrongylophora ingens.

BACKGROUND: Marine organisms are established to be a wealthy source of bioactive compounds with diverse chemical structures and bioactivities. Acanthostrongylophora ingens is known to be rich with pyrimidine b-carboline and manzamine-type alkaloids. The goal of the present work is to isolate and identify new alkaloids from A. ingens as well as to assess the cytotoxic potential of these metabolites towards various cancer cell lines. METHODS: The crude MeOH extract of the sponge was separated by vacuum liquid chromatography (VLC), using n-hexane, EtOAc, and MeOH. The EtOAc fraction was chromatographed on VLC, SiO2, sephadex LH-20, and RP18 columns, affording four metabolites. Their structures were identified using infrared, ultraviolet, high-resolution mass spectrometry, and nuclear magnetic resonance spectroscopic techniques, as well as comparison with the published data. RESULTS: A new 1,2,3,4-tetrahydro-ß-carboline (THßCs) alkaloid, ingenine F (4) and three known compounds: Annomontine (1), acanthomine A (2), and 1-oxo-1,2,3,4-THßCs (3) were isolated and identified. Ingenine F (4) exhibited cytotoxic activity toward hormone-dependent breast carcinoma (MCF7), colon carcinoma (HCT116), and lung carcinoma (A549) cell lines with IC50 values of 2.82, 1.00, and 2.37 µM, respectively, compared to doxorubicin (IC50 0.012, 0.036, and 0.102 µM, respectively). CONCLUSION: It is the first report for the isolation of THßCs alkaloids from A. ingens. The THßCs alkaloid with N-methylbutyramide unit as found in ingenine F is very rarely encountered in nature. Ingenine F may provide new promising candidates for potential cytotoxic agent. SUMMARY: Ingenine F, a new 1,2,3,4-THßCs derivative (4) and three known alkaloids (1-3) were isolated from A. ingens. Their structures were verified by various spectroscopic analyses. Compound 4 had potent cytotoxic effect toward MCF7, HCT116, and A549 cancer cell lines. Abbreviations used: 1D: One-dimensional; 2D: Two-dimensional; CC: Column chromatography; COSY: Correlations spectroscopy; DMSO: Dimethyl sulfoxide; HMBC: Heteronuclear multiple bond correlation experiment; HRESIMS: High resolution electrospray ionization mass spectrometry; HSQC: Heteronuclear single quantum correlation; IR: Infrared; LCQ: Liquid chromatography quadrupole; LTQ: Linear trap quadropole; NMR: Nuclear magnetic resonance; RP: Reversed phase; SiO2: Silica gel; TLC: Thin-layer chromatography; UV: Ultraviolet; VLC: Vacuum liquid chromatography.

Design and biological evaluation of tetrahydro-ß-carboline derivatives as highly potent histone deacetylase 6 (HDAC6) inhibitors.

Various diseases are related to epigenetic modifications. Histone deacetylases (HDACs) and histone acetyl transferases (HATs) determine the pattern of histone acetylation, and thus are involved in the regulation of gene expression. First generation histone deacetylase inhibitors (HDACi) are unselective, hinder all different kinds of zinc dependent HDACs and additionally cause several side effects. Subsequently, selective HDACi are gaining more and more interest. Especially, selective histone deacetylase 6 inhibitors (HDAC6i) are supposed to be less toxic. Here we present a successful optimization study of tubastatin A, the synthesis and biological evaluation of new inhibitors based on hydroxamic acids linked to various tetrahydro-ß-carboline derivatives. The potency of our selective HDAC6 inhibitors, exhibiting IC50 values in a range of 1-10 nM towards HDAC6, was evaluated with the help of a recombinant human HDAC6 enzyme assay. Selectivity was proofed in cellular assays by the hyperacetylation of surrogate parameter α-tubulin in the absence of acetylated histone H3 analyzed by Western Blot. We show that all synthesized compounds, with varies modifications of the rigid cap group, were selective and potent HDAC6 inhibitors.

Application of Pictet-Spengler Reaction to Indole-Based Alkaloids Containing Tetrahydro-ß-carboline Scaffold in Combinatorial Chemistry.

Indole-based alkaloids are well-known in the literature for their diverse biological properties. Polysubstituted optically active tetrahydro-ß-carboline derivatives functionalized on C-1 position are the common structural motif in most of the indole-based alkaloids, as well as highly marketed drugs. The stereoselective Pictet-Spengler reaction is one of the currently most important synthetic techniques used for the preparation of these privileged tetrahydro-ß-carboline scaffolds. To date, there are numerous research reports that have been published on the synthesis of the tetrahydro-ß-carboline scaffold both on solid phase, as well as in solution phase. Moreover rapid growth has been observed for the enantioselective synthesis of tetrahydro-ß-carboline scaffold using chiral organocatalysts. In this Review, efforts have been taken to shed light on the latest information available on different strategies to synthesize tetrahydro-ß-carboline both on solid phase and in solution phase during the last 20 years. Furthermore, we believe that the present synthetic methodologies covered in this Review will help to improve the status of this privileged tetrahydro-ß-carboline scaffold in its use for drug discovery.

Synthesis and anti-leishmanial evaluation of 1-phenyl-2,3,4,9-tetrahydro-1H-ß-carboline derivatives against Leishmania infantum.

In the present study, antileishmanial activity of sixteen novel series of tetrahydro-ß-carboline derivatives against transgenic infrared fluorescent Leishmania infantum strain has been reported. Among these reported analogues, most of the compounds exhibited potent inhibition against both promastigote (IC50 from 1.99 ± 1.40 to 20.69 ± 0.95 µM) and amastigote (IC50 from 0.67 ± 0.05 to 4.16 ± 0.008 µM) forms of L. infantum. Moreover, compound 7l, displayed most potent and selective inhibition of parasite amastigote form with IC50 0.67 ± 0.05 µM, selectivity index >298.5 and was comparable with standard drug amphotericin B. From this study, a new class of tetrahydro-ß-carboline derivatives with potent antileishmanial activity was identified and it needs further extensive study to optimize the lead molecules to win the battle against severe and neglected disease leishmaniasis.

Enzymatic Strategy for the Resolution of New 1-Hydroxymethyl Tetrahydro-ß-carboline Derivatives in Batch and Continuous-Flow Systems.

Many alkaloids containing a tetrahydro-ß-carboline skeleton have well-known therapeutic effects, leading to increased interest in the synthesis of these natural products. Enantiomers of N-Boc-protected 1-hydroxymethyl-1,2,3,4-tetrahydro-ß-carboline [(±)-7], 1-hydroxymethyl-6-methoxy-1,2,3,4-tetrahydro-ß-carboline [(±)-8], and 1-hydroxymethyl-6-fluoro-1,2,3,4-tetrahydro-ß-carboline [(±)-9] were prepared through enzymecatalyzed asymmetric acylation of their primary hydroxyl group. The preliminary experiments were performed in a continuous-flow system, while the preparative-scale resolutions were done as batch reactions. Excellent enantioselectivities (E>200) were obtained with Candida antarctica lipase B (CAL-B) and acetic anhydride in toluene at 60 °C. The recovered alcohols and the produced esters were obtained with high enantiomeric excess values (ee≥96 %). The O-acylated enantiomers [(S)-10-(S)-12)] were transformed into the corresponding amino alcohols [(S)-7-(S)-9)] with methanolysis. Microwave-assisted Boc removals were also performed and resulted in the corresponding compounds (R)-4-(R)-6 and (S)-4-(S)-6 without a drop in the enantiomeric excess values (ee≥96 %).

SAR exploration at the C-3 position of tetrahydro-ß-carboline sstr3 antagonists.

MK-4256, a tetrahydro-ß-carboline sstr3 antagonist, was discontinued due to a cardiovascular (CV) adverse effect observed in dogs. Additional investigations revealed that the CV liability (QTc prolongation) was caused by the hERG off-target activity of MK-4256 and was not due to sstr3 antagonism. In this Letter, we describe our extensive SAR effort at the C3 position of the tetrahydro-ß-carboline structure. This effort resulted in identification of 5-fluoro-pyridin-2-yl as the optimal substituent on the imidazole ring to balance sstr3 activity and the hERG off-target liability.

Discovery of MK-1421, a Potent, Selective sstr3 Antagonist, as a Development Candidate for Type 2 Diabetes.

The imidazolyl-tetrahydro-ß-carboline class of sstr3 antagonists have demonstrated efficacy in a murine model of glucose excursion and may have potential as a treatment for type 2 diabetes. The first candidate in this class caused unacceptable QTc interval prolongation in oral, telemetrized cardiovascular (CV) dogs. Herein, we describe our efforts to identify an acceptable candidate without CV effects. These efforts resulted in the identification of (1R,3R)-3-(4-(5-fluoropyridin-2-yl)-1H-imidazol-2-yl)-1-(1-ethyl-pyrazol-4-yl)-1-(3-methyl-1,3,4-oxadiazol-3H-2-one-5-yl)-2,3,4,9-tetrahydro-1H-ß-carboline (17e, MK-1421).