Low-dimensional compounds containing bioactive ligands. Part XVII: Synthesis, structural, spectral and biological properties of hybrid organic-inorganic complexes based on [PdCl4]2- with derivatives of 8-hydroxyquinolinium.

In this study, four hybrid organic-inorganic compounds (8-H2Q)2[PdCl4] (1), (H2ClQ)2[PdCl4] (2), (H2NQ)2[PdCl4] (3) and (H2MeQ)2[PdCl4]·2H2O (4) (where 8-H2Q = 8-hydroxyquinolinium, H2ClQ = 5-chloro-8-hydroxyquinolinium, H2NQ = 5-nitro-8-hydroxyquinolinium and H2MeQ = 2-methyl-8-hydroxyquinolinium) were synthesized through organic cation modulation. Single-crystal X-ray structure analysis of compounds 1 and 3 indicates that their structures are planar and consist of [PdCl4]2- anions and 8-H2Q or H2NQ cations, respectively. Both ionic components are held together through ionic interactions and hydrogen bonds forming infinite chains linked through π-π interactions to form 2D structures. Furthermore, NMR spectroscopy, UV-Vis spectroscopy, elemental analysis, and FT-IR spectroscopy were used to explore the synthesized compounds. The DNA interaction, antimicrobial activity, antiproliferative activity, and radical scavenging effect of the compounds were evaluated. The hybrid compounds and their free ligands can interact with the calf thymus DNA via an intercalation mode involving the insertion of the aromatic chromophore between the base pairs of DNA; compound 1 has the highest binding affinity. Moreover, they have high antimicrobial efficacy against the tested 14 strains of microorganisms with minimum inhibitory concentration values ranging from <1.95 to 250 µg/mL. The antiproliferative activity of the compounds was investigated against three different cancer cell lines, and their selectivity was verified on mesenchymal stem cells. Compounds 1 and 2 displayed selective and high cytotoxicity against human lung and breast cancer cells and showed moderate cytotoxicity against colon cancer cells. Accordingly, they might be auspicious candidates for future pharmacological investigations in lung and breast cancer research.

O-prenylated carbostyrils as a novel class of 15-lipoxygenase inhibitors: Synthesis, characterization, and inhibitory assessment.

Catalyzed peroxidation of unsaturated lipid in animals and plants intimately is linked to the activity of 15-Lipoxygenase enzymes. Lipoxygenases (LOXs) are well known to play an important role in many acute and chronic syndromes such as inflammation, asthma, cancer, and allergy. In this study, a series of mono prenyloxycarbostyrils were synthesized and evaluated as potential inhibitors of soybean 15-Lipoxygenase (SLO) and their inhibitory potencies were compared to mono prenyloxycoumarins which had been reported in the previous works. The synthetic compounds inhibit lipoxygenase enzyme by competitive mechanism like the prenyloxy coumarins. The results showed that position and length of the prenyl moiety play the important role in lipoxygenase inhibitory activity. Among all of the synthetic compounds (coumarin and carbostyril derivatives), 5-farnesyloxycoumarin and 8-farnesyloxycarbostyril demonstrated the best inhibitory activity by IC50  values of 1.1 µM and 0.53 µM, respectively.

High pressure assisted synthetic approach for novel 6,7-dihydro-5H-benzo[6,7]cyclohepta[1,2-b]pyridine and 5,6-dihydrobenzo[h]quinoline derivatives and their assessment as anticancer agents.

A novel, expedient and effective methodology for the synthesis of distinctly substituted 6,7-dihydro-5H-benzo[6,7]cyclohepta[1,2-b]pyridine and 5,6-dihydrobenzo[h]quinoline systems has been developed with a new synthetic platform. This process includes ammonium acetate-mediated cyclocondensation reactions of 3-oxo-2-arylhydrazonopropanals with benzosuberone and tetralone precursors, respectively, using the high-pressure Q-tube reactor, which has been found to be superior to both conventional heating and microwave irradiation. The novel protocol benefits from its high atom efficiency, economy, ease of workup, broad substrate scope and is also applicable to gram-scale synthesis. To identify and confirm the newly synthesized targeted compounds, the X-ray single-crystal as well as all possible spectroscopic methods were utilized. The cytotoxicity of the newly synthesized 6,7-dihydro-5H-benzo[6,7]cyclohepta[1,2-b]pyridine 4a-j and 5,6-dihydrobenzo-[h]quinolines derivatives 6a-e were preliminary examined toward three cell lines of human cancer; lung cancer (A549), breast cancer (MCF-7) and colon cancer (HCT-116), by applying the MTT colorimetric assay. The achieved results reflected the promising profile of the prepared compounds in this study against cancer cells and have shown that members from the synthesized 6,7-dihydro-5H-benzo[6,7]cyclohepta[1,2-b]pyridine 4a-j exhibited promising cytotoxicity's against MCF-7, and A549 cancer cells respectively, while the HCT-116 (colon) cancer cells were inhibited by certain examples of 5,6-dihydrobenzo[h]quinoline derivatives 6c,d. These promising results could serve as a good primary base for further research into the design of anticancer drugs.

Enantioselective Synthesis of 8-Hydroxyquinoline Derivative, Q134 as a Hypoxic Adaptation Inducing Agent.

Hypoxia is a common feature of neurodegenerative diseases, including Alzheimer's disease that may be responsible for disease pathogenesis and progression. Therefore, the hypoxia-inducible factor (HIF)1 system, responsible for hypoxic adaptation, is a potential therapeutic target to combat these diseases by activators of cytoprotective protein induction. We have selected a candidate molecule from our cytoprotective hydroxyquinoline library and developed a novel enantioselective synthesis for the production of its enantiomers. The use of quinidine or quinine as a catalyst enabled the preparation of enantiomer-pure products. We have utilized in vitro assays to evaluate cytoprotective activity, a fluorescence-activated cell sorting (FACS) based assay measuring mitochondrial membrane potential changes, and gene and protein expression analysis. Our data showed that the enantiomers of Q134 showed potent and similar activity in all tested assays. We have concluded that the enantiomers exert their cytoprotective activity via the HIF1 system through HIF1A protein stabilization.

8-Hydroxyquinolines are bactericidal against Mycobacterium tuberculosis.

There is an urgent need for new treatments effective against Mycobacterium tuberculosis, the causative agent of tuberculosis. The 8-hydroxyquinoline series is a privileged scaffold with anticancer, antifungal, and antibacterial activities. We conducted a structure-activity relationship study of the series regarding its antitubercular activity using 26 analogs. The 8-hydroxyquinolines showed good activity against M. tuberculosis, with minimum inhibitory concentrations (MIC90) of <5 µM for some analogs. Small substitutions at C5 resulted in the most potent activity. Substitutions at C2 generally decreased potency, although a sub-family of 2-styryl-substituted analogs retained activity. Representative compounds demonstrated bactericidal activity against replicating M. tuberculosis with >4 log kill at 10× MIC over 14 days. The majority of the compounds demonstrated cytotoxicity (IC50 of <100 µM). Further development of this series as antitubercular agents should address the cytotoxicity liability. However, the 8-hydroxyquinoline series represents a useful tool for chemical genomics to identify novel targets in M. tuberculosis.

Recent Synthetic Approaches and Biological Evaluations of Amino Hexahydroquinolines and Their Spirocyclic Structures.

In this review, the recent synthetic approaches of amino hexahydroquinolines and their spirocyclic structures were highlighted. The synthetic routes include, two-components, three-components or fourcomponents reactions. The two-component [3+3] atom combination reaction represents the simplest method. It involves Michael addition of the electron rich ß-carbon of ß-enaminones to the activated double bond of cinnamonitriles followed by cyclization to yield hexahydroquinoline compounds. The bioactivity profiles and SAR studies of these compounds were also reviewed with emphasis to the utility of these substances as antimicrobial, anticancer and antitubercular agents, as well as calcium channel modulators.

Development of Potent Inhibitors of the Mycobacterium tuberculosis Virulence Factor Zmp1 and Evaluation of Their Effect on Mycobacterial Survival inside Macrophages.

The enzyme Zmp1 is a zinc-containing peptidase that plays a critical role in the pathogenicity of Mycobacterium tuberculosis. Herein we describe the identification of a small set of Zmp1 inhibitors based on a novel 8-hydroxyquinoline-2-hydroxamate scaffold. Among the synthesized compounds, N-(benzyloxy)-8-hydroxyquinoline-2-carboxamide (1 c) was found to be the most potent Zmp1 inhibitor known to date, and its binding mode was analyzed both by kinetics studies and molecular modeling, identifying critical interactions of 1 c with the zinc ion and residues in the active site. The effect of 1 c on intracellular Mycobacterium survival was assayed in J774 murine macrophages infected with M. tuberculosis H37Rv or M. bovis BCG and human monocyte-derived macrophages infected with M. tuberculosis H37Rv. Cytotoxicity and genotoxicity were also assessed. Overall, inhibitor 1 c displays interesting in vitro antitubercular properties worthy of further investigation.

An ortho-aldehyde modified probe to image thiols in living cells with enhanced selectivity.

Developing fluorescent probes to image thiols in the living system may provide powerful tools to study the functions of thiol-containing biological molecules. In this study, we report the design and evaluation of a novel turn-on fluorescent probe NQNO for selective detection of thiols in living cells. By introducing an ortho-aldehyde group to NNO, a conventional compound representing a class of thiol-imaging strategy, we obtained NQNO with enhanced selectivity for thiols over the major interferent hydrogen sulfide (H2S). NQNO could be applied in phosphate-buffered saline (PBS), where the efficacy of NNO was usually weakened. Notably, NQNO demonstrated solid performance in imaging endogenous thiols in living cells without exerting cytotoxicity. In summary, NQNO has the potential to serve as a safe, sensitive and effective fluorescent probe for thiol imaging in biological systems.

Synthesis of a highly Mg2+-selective fluorescent probe and its application to quantifying and imaging total intracellular magnesium.

Magnesium plays a crucial role in many physiological functions and pathological states. Therefore, the evolution of specific and sensitive tools capable of detecting and quantifying this element in cells is a very desirable goal in biological and biomedical research. We developed a Mg2+-selective fluorescent dye that can be used to selectively detect and quantify the total magnesium pool in a number of cells that is two orders of magnitude smaller than that required by flame atomic absorption spectroscopy (F-AAS), the reference analytical method for the assessment of cellular total metal content. This protocol reports itemized steps for the synthesis of the fluorescent dye based on diaza-18-crown-6-hydroxyquinoline (DCHQ5). We also describe its application in the quantification of total intracellular magnesium in mammalian cells and the detection of this ion in vivo by confocal microscopy. The use of in vivo confocal microscopy enables the quantification of magnesium in different cellular compartments. As an example of the sensitivity of DCHQ5, we studied the involvement of Mg2+ in multidrug resistance in human colon adenocarcinoma cells sensitive (LoVo-S) and resistant (LoVo-R) to doxorubicin, and found that the concentration was higher in LoVo-R cells. The time frame for DCHQ5 synthesis is 1-2 d, whereas the use of this dye for total intracellular magnesium quantification takes 2.5 h and for ion bioimaging it takes 1-2 h.

Novel 5-carboxy-8-HQ based histone demethylase JMJD2A inhibitors: introduction of an additional carboxyl group at the C-2 position of quinoline.

A series of JMJD2A inhibitors had been designed by analyzing the binding mode of 5-carboxy-8-hydroxyquinoline (5-carboxy-8-HQ) with JMJD2A. The inhibitory activity of the synthesized compounds against JMJD2A was determined, followed by docking simulations to understand the structure-activity relationships. Compounds with potent JMJD2A inhibitory activity demonstrated outstanding selectivity for JMJD2A over PHD2. Several potent compounds were selected to evaluate their anti-proliferative activity on tumor cell lines. Among them, compound 6p displayed the best anti-proliferative activity. Based on these in vitro biological data, seven compounds were chosen to determine their physicochemical properties. Compound 6p displayed good aqueous solubility and better permeability than 5-carboxy-8-HQ. Our data recognized that compound 6p could be considered as a starting point for development of new JmjC inhibitors.

Paramagnetic relaxation enhancement of membrane proteins by incorporation of the metal-chelating unnatural amino acid 2-amino-3-(8-hydroxyquinolin-3-yl)propanoic acid (HQA).

The use of paramagnetic constraints in protein NMR is an active area of research because of the benefits of long-range distance measurements (>10 Å). One of the main issues in successful execution is the incorporation of a paramagnetic metal ion into diamagnetic proteins. The most common metal ion tags are relatively long aliphatic chains attached to the side chain of a selected cysteine residue with a chelating group at the end where it can undergo substantial internal motions, decreasing the accuracy of the method. An attractive alternative approach is to incorporate an unnatural amino acid that binds metal ions at a specific site on the protein using the methods of molecular biology. Here we describe the successful incorporation of the unnatural amino acid 2-amino-3-(8-hydroxyquinolin-3-yl)propanoic acid (HQA) into two different membrane proteins by heterologous expression in E. coli. Fluorescence and NMR experiments demonstrate complete replacement of the natural amino acid with HQA and stable metal chelation by the mutated proteins. Evidence of site-specific intra- and inter-molecular PREs by NMR in micelle solutions sets the stage for the use of HQA incorporation in solid-state NMR structure determinations of membrane proteins in phospholipid bilayers.

Coordination of hydroxyquinolines to a ruthenium bis-dimethyl-phenanthroline scaffold radically improves potency for potential as antineoplastic agents.

A series of ruthenium coordination complexes containing hydroxyquinoline ligands were synthesized that exhibited radically improved potencies up to 86-fold greater than clioquinol, a known cytotoxic compound. The complexes were also >100-fold more potent than clioquinol in a tumor spheroid model, with values similar to currently used chemotherapeutics for the treatment of solid tumors. Cytotoxicity occurs through rapid processes that induce apoptosis but appear to be mediated by cell-cycle independent mechanisms. The ruthenium complexes do not inhibit the proteasome at concentrations relevant for cell death, and contrary to previous reports, clioquinol and other hydroxyquinoline compounds do not act as direct proteasome inhibitors to induce cell death.

Hydroxyquinoline derived vanadium(IV and V) and copper(II) complexes as potential anti-tuberculosis and anti-tumor agents.

Several mixed ligand vanadium and copper complexes were synthesized containing 8-hydroxyquinoline (8HQ) and a ligand such as picolinato (pic(-)), dipicolinato (dipic(2-)) or a Schiff base. The complexes were characterized by spectroscopic techniques and by single-crystal X-ray diffraction in the case of [V(V)O(L-pheolnaph-im)(5-Cl-8HQ)] and [V(V)O(OMe)(8HQ)2], which evidenced the distorted octahedral geometry of the complexes. The electronic absorption data showed the presence of strong ligand to metal charge transfer bands, significant solvent effects, and methoxido species in methanol, which was further confirmed by (51)V-NMR spectroscopy. The structures of [Cu(II)(dipic)(8HQ)]Na and [V(IV)O(pic)(8HQ)] were confirmed by EPR spectroscopy, showing only one species in solution. The biological activity of the compounds was assessed through the minimal inhibitory concentration (MIC) of the compounds against Mycobacterium tuberculosis (Mtb) and the cytotoxic activity against the cisplatin sensitive/resistant ovarian cells A2780/A2780cisR and the non-tumorigenic HEK cells (IC50 values). Almost all tested vanadium complexes were very active against Mtb and the MICs were comparable to, or better than, the MICs of drugs, such as streptomycin. The activity of the complexes against the A2780 cell line was dependent on incubation time presenting IC50 values in the 3-14 µM (at 48 h) range. In these conditions, the complexes were significantly (*P<0.05-**P<0.001) more active than cisplatin (22 µM), in the A2780 cells and even surpassing its activity in the cisplatin-resistant cells A2780cisR (2.4-8 µM vs. 75.4; **P<0.001). In the non-tumorigenic HEK cells poor selectivity toward cancer cells for most of the complexes was observed, as well as for cisplatin.

Oxidative stress induced by copper and iron complexes with 8-hydroxyquinoline derivatives causes paraptotic death of HeLa cancer cells.

Here, we report the antiproliferative/cytotoxic properties of 8-hydroxyquinoline (8-HQ) derivatives on HeLa cells in the presence of transition metal ions (Cu(2+), Fe(3+), Co(2+), Ni(2+)). Two series of ligands were tested, the arylvinylquinolinic L1-L8 and the arylethylenequinolinic L9-L16, which can all interact with metal ions by virtue of the N,O donor set of 8-HQ; however, only L9-L16 are flexible enough to bind the metal in a multidentate fashion, thus exploiting the additional donor functions. L1-L16 were tested for their cytotoxicity on HeLa cancer cells, both in the absence and in the presence of copper. Among them, the symmetric L14 exhibits the highest differential activity between the ligand alone (IC50 = 23.7 µM) and its copper complex (IC50 = 1.8 µM). This latter, besides causing a significant reduction of cell viability, is associated with a considerable accumulation of the metal inside the cells. Metal accumulation is also observed when the cells are incubated with L14 complexed with other late transition metal ions (Fe(3+), Co(2+), Ni(2+)), although the biological response of HeLa cells is different. In fact, while Ni/L14 and Co/L14 exert a cytostatic effect, both Cu/L14 and Fe/L14 trigger a caspase-independent paraptotic process, which results from the induction of a severe oxidative stress and the unfolded protein response.

8-Hydroxyquinolines: a review of their metal chelating properties and medicinal applications.

Metal ions play an important role in biological processes and in metal homeostasis. Metal imbalance is the leading cause for many neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, and multiple sclerosis. 8-Hydroxyquinoline (8HQ) is a small planar molecule with a lipophilic effect and a metal chelating ability. As a result, 8HQ and its derivatives hold medicinal properties such as antineurodegenerative, anticancer, antioxidant, antimicrobial, anti-inflammatory, and antidiabetic activities. Herein, diverse bioactivities of 8HQ and newly synthesized 8HQ-based compounds are discussed together with their mechanisms of actions and structure-activity relationships.

Hydroxyquinoline-derived compounds and analoguing of selective Mcl-1 inhibitors using a functional biomarker.

Anti-apoptotic Bcl-2 family proteins are important oncology therapeutic targets. To date, BH3 mimetics that abrogate anti-apoptotic activity have largely been directed at Bcl-2 and/or Bcl-xL. One observed mechanism of resistance to these inhibitors is increased Mcl-1 levels in cells exposed to such therapeutics. For this reason, and because Mcl-1 is important in the onset of lymphoid, myeloid, and other cancers, it has become a target of great interest. However, small molecule inhibitors displaying potency and selectivity for Mcl-1 are lacking. Identifying such compounds has been challenging due to difficulties in translating the target selectivity observed at the biochemical level to the cellular level. Herein we report the results of an HTS strategy coupled with directed hit optimization. Compounds identified have selective Mcl-1 inhibitory activity with greater than 100-fold reduced affinity for Bcl-xL. The selectivity of these compounds at the cellular level was validated using BH3 profiling, a novel personalized diagnostic approach. This assay provides an important functional biomarker that allows for the characterization of cells based upon their dependencies on various anti-apoptotic Bcl-2 proteins. We demonstrate that cells dependent on Mcl-1 or Bcl-2/Bcl-xL for survival are commensurately responsive to compounds that genuinely target those proteins. The identification of compound 9 with uniquely validated and selective Mcl-1 inhibitory activity provides a valuable tool to those studying the intrinsic apoptosis pathway and highlights an important approach in the development of a first-in-class cancer therapeutic.

Preparation of Galipea officinalis Hancock type tetrahydroquinoline alkaloid analogues as anti-tumour agents.

The preparation of chiral tetrahydroquinolines using Ir-catalysed asymmetric hydrogenation and their possible cytotoxic potential anti-cancer activity were reported. Both of the in vitro cytotoxicity assay on a series of human cancer cell lines including A549 small cell lung cancer, MDA-MB-231 breast cancer, SaoS2 sacroma, SKHep-1 hepatoma and Hep3B hepatocellular carcinoma as well as in vivo animal model using Hep3B hepatocellular tumour xenograft on athymic nude mice suggest that 1,2,3,4-tetrahydroquin-8-ol is a potential anti-tumour alkaloid which may be further developed as a novel cancer chemotherapeutic agent.

Neuroprotection by the multitarget iron chelator M30 on age-related alterations in mice.

Based on a multimodal drug design paradigm, we have synthesized a multifunctional non-toxic, brain permeable iron chelating compound, M30, possessing the neuroprotective N-propargyl moiety of the anti-Parkinsonian drug, monoamine oxidase (MAO)-B inhibitor, rasagiline and the antioxidant-iron chelator moiety of an 8-hydroxyquinoline derivative of the iron chelator, VK28. Here, we report that a chronic systemic treatment of aged mice with M30 (1 and 5mg/kg; 4 times weekly for 6 months), had a significant positive impact on neuropsychiatry functions and cognitive age-related impairment. M30 significantly reduced cerebral iron accumulation as demonstrated by Perl's staining, accompanied by a marked decrease in cerebral ß-amyloid plaques. In addition, our results demonstrate that M30 caused a significant inhibition of both MAO-A and -B activities in the cerebellum of aged mice, compared with vehicle-treated aged control mice. In summary, the present study indicates that the novel MAO inhibitor/iron chelating drug, M30, acting against multiple brain targets could reverse age-associated memory impairment and provide a potential treatment against the progression of neurodegeneration in ageing.

Substituted oxines inhibit endothelial cell proliferation and angiogenesis.

Two substituted oxines, nitroxoline (5) and 5-chloroquinolin-8-yl phenylcarbamate (22), were identified as hits in a high-throughput screen aimed at finding new anti-angiogenic agents. In a previous study, we have elucidated the molecular mechanism of antiproliferative activity of nitroxoline in endothelial cells, which comprises of a dual inhibition of type 2 human methionine aminopeptidase (MetAP2) and sirtuin 1 (SIRT1). Structure-activity relationship study (SAR) of nitroxoline offered many surprises where minor modifications yielded oxine derivatives with increased potency against human umbilical vein endothelial cells (HUVEC), but with entirely different as yet unknown mechanisms. For example, 5-nitrosoquinolin-8-ol (33) inhibited HUVEC growth with sub-micromolar IC(50), but did not affect MetAP2 or MetAP1, and it only showed weak inhibition against SIRT1. Other sub-micromolar inhibitors were derivatives of 5-aminoquinolin-8-ol (34) and 8-sulfonamidoquinoline (32). A sulfamate derivative of nitroxoline (48) was found to be more potent than nitroxoline with the retention of activities against MetAP2 and SIRT1. The bioactivity of the second hit, micromolar HUVEC and MetAP2 inhibitor carbamate 22 was improved further with an SAR study culminating in carbamate 24 which is a nanomolar inhibitor of HUVEC and MetAP2.

5-Amino-2-aroylquinolines as highly potent tubulin polymerization inhibitors. Part 2. The impact of bridging groups at position C-2.

A variety of studies on the modification of combretastatin A-4 triggered our interest in the impact of the linkers between the 3,4,5-trimethoxyphenyl ring and 5-amino-6-methoxyquinoline on biological activity. The replacement of the carbonyl group with bond, amine, ether, sulfide, and sulfone groups was evaluated in this study. The results showed that compounds 14 and 15 containing sulfide and sulfone groups between the 3,4,5-trimethoxyphenyl ring (A-ring) and 5-amino-6-methoxyquinoline exhibited substantial antiproliferative activity against KB, HT29, and MKN45 cells with mean IC50 values of 42 and 12 nM, respectively. 15 inhibited the tubulin polymerization with an IC50 value of 2.0 µM, similar to that with CA4. The continued work on the C-5 substituents of 3',4',5'-trimethoxybenzoyl-6-methoxyquinoline derivatives demonstrated that compound 7 possessing OH at C-5 exhibited excellent antiproliferative activity with mean IC50 values of 3.4 nM and microtubule destabilizing potency with an IC50 of 1.5 µM, comparable to that of CA4 (IC50=1.9 µM). It also exhibited substantial vascular disrupting effects. Compounds 7 and 15 exhibited significant efficacy against MDR/MRP-related drug-resistant cell lines (KB-vin10, KB-S15, and KB-7D) with mean IC50 values of 6.7 and 2.6 nM, respectively.