Identification of bis-quindolines as new antiinfective agents

Indoloquinoline Alkaloids as Antimalarials: Advances, Challenges, and Opportunities

Malaria infections affect almost half of the world's population, with over 200 million cases reported annually. Cryptolepis sanguinolenta, a plant native to West Africa, has long been used across various regions of Africa for malaria treatment. Chemical analysis has revealed that the plant is abundant in indoloquinolines, which have been shown to possess antimalarial properties. Cryptolepine, neocryptolepine, and isocryptolepine are well-studied indoloquinoline alkaloids known for their potent antimalarial activity. However, their structural rigidity and associated cellular toxicity are major drawbacks for preclinical development. This review focuses on the potential of indoloquinoline alkaloids (cryptolepine, neocryptolepine, and isocryptolepine) as scaffolds in drug discovery. The article delves into their antimalarial effects in vitro and in vivo, as well as their proposed mechanisms of action and structure-activity relationship studies. Several studies aim to improve these leads by reducing cytotoxicity while preserving or enhancing antimalarial activity and gaining insights into their mechanisms of action. These investigations highlight the potential of indoloquinolines as a scaffold for developing new antimalarial drugs.

Synthetic account on indoles and their analogues as potential anti-plasmodial agents

Malaria caused by P. falciparum, has been recognized as one of the major infectious diseases causing the death of several patients as per the reports from the World Health Organization. In search of effective therapeutic agents against malaria, several research groups have started working on the design and development of novel heterocycles as anti-malarial agents. Heterocycles have been recognized as the pharmacophoric features for the different types of medicinally important activities. Among all these heterocycles, nitrogen containing aza-heterocycles should not be underestimated owing to their wide therapeutic window. Amongst the aza-heterocycles, indoles and fused indoles such as marinoquinolines, isocryptolepines and their regioisomers, manzamines, neocryptolenines, and indolones have been recognized as anti-malarial agents active against P. falciparum. The present work unleashes the synthetic attempts of anti-malarial indoles and fused indoles through cyclocondensation, Fischer-indole synthesis, etc. along with the brief discussions on structure-activity relationships, in vitro or in vivo studies for the broader interest of these medicinal chemists, working on their design and development as potential anti-malarial agents.

Systematic Photoassisted Access to Designer Polyheterocycles via Modular Blocks and Scaffolding

Diverse polyheterocycles are accessed via scaffolded photoassisted synthesis involving decarboxylative aromatization of the primary photoproducts from intramolecular cycloadditions of azaxylylenes and tethered heteroaromatic unsaturated pendants.

A review on quinoline derivatives as anti-methicillin resistant Staphylococcus aureus (MRSA) agents

Methicillin Resistant Staphylococcus aureus (MRSA) consists of strains of S. aureus which are resistant to methicillin. The resistance is due to the acquisition of mecA gene which encodes PBP2a unlike of any PBPs normally produced by S. aureus. PBP2a shows unusually low β-Lactam affinity and remains active to allow cell wall synthesis at normally lethal β-Lactam concentrations. MRSA can cause different types of infections like Healthcare associated MRSA, Community associated MRSA and Livestock associated MRSA infections. It causes skin lesions, osteomyelitis, endocarditis and furunculosis. To treat MRSA infections, only a few options are available like vancomycin, clindamycin, co-trimoxazole, fluoroquinolones or minocycline and there is a dire need of discovering new antibacterial agents that can effectively treat MRSA infections. In the current review, an attempt has been made to compile the data of quinoline derivatives possessing anti-MRSA potential reported to date.

A concise Friedländer/Buchwald–Hartwig approach to the total synthesis of quindoline, a bioactive natural indoloquinoline alkaloid, and toward the unnatural 10-methylquindoline

Friedländer and Buchwald–Hartwig reactions were used to achieve an efficient total synthesis of quindoline and a new synthesis of the unnatural 10-methylquindoline. DFT calculations were employed to explain the outcome of a failed key transformation.

Efficient total synthesis of neocryptolepine and synthetic access to 6-methylquinindoline from a common intermediate

The total synthesis of neocryptolepine and the synthesis of its non-natural isomer 6-methyl quinindoline were efficiently achieved in a few steps from a common intermediate.

Synthesis, analysis and biological evaluation of novel indolquinonecryptolepine analogues as potential anti-tumour agents

A small library of cryptolepine analogues were synthesised incorporating halogens and/or nitrogen containing side chains to optimise their interaction with the sugar–phosphate backbone of DNA to give improved binding, interfering with topoisomerase II hence enhancing cytotoxicity.

Structure-activity relationship of indoloquinoline analogs anti-MRSA

Indolo[3,2-b]quinoline analogs (3a-3s), 4-(acridin-9-ylamino) phenol hydrochloride (4), benzofuro[3,2-b]quinoline (3t), indeno[1,2-b]quinolines (3u and 3v) have been synthesized. Those compounds were found to exhibit anti-bacterial activity towards Methicillin-resistant Staphylococcus aureus (anti-MRSA activity). Structure-activity relationship studies were conducted that indoloquinoline ring, benzofuroquinoline ring and 4-aminophenol group are essential structure for anti-MRSA activity.

Determination of antibiotic EC50 using a zero-flow microfluidic chip based growth phenotype assay

Current existing assay systems for evaluating antimicrobial activity suffer from several limitations including excess reagent consumption and inaccurate concentration gradient preparation. Recently, microfluidic systems have been developed to provide miniaturized platforms for antimicrobial susceptibility assays. However, some of current microfluidic based assays require continuous flows of reagents or elaborate preparation steps during concentration preparation. In this study, we introduce a novel microfluidic chip based growth phenotype assay that automatically generates a logarithmic concentration gradient and allows observing the growth of pathogenic bacteria under different concentrations of antibiotics in nanoliter batch culture reactors. We chose pathogen bacterium Pseudomonas aeruginosa as a model strain and evaluated the inhibitory effects of gentamicin and ciprofloxacin. We determined the EC50 values and confirmed the validity of the present system by comparing the EC50 values obtained through conventional test tube method. We demonstrated that the EC50 values acquired from present assay are comparable to those obtained from conventional test tube cultures. The potential application of present assay system for investigating combinatorial effects of antibiotics on multidrug resistant pathogenic bacteria is discussed and it can be further used for systematic evaluation of antifungal or antiviral agents.

CoMFA studies and in vitro evaluation of some 3-substituted benzylthio quinolinium salts as anticryptococcal agents

The 3-dimensional quantitative structure-activity relationship (3D-QSAR) molecular modeling technique or comparative molecular field analysis (CoMFA) has been used to design analogs of the natural product cryptolepine (1). Twenty-three compounds with their in vitro biological activities (IC50 values) against Crytococcus neoformans were used to generate the training set database of compounds for the CoMFA studies. The cross-validated q(2), noncross-validated r(2), and partial least squares (PLS) analysis results were used to predict the biological activity of 11 newly designed test set compounds. The best CoMFA model produced a q(2) of 0.815 and an r(2) of 0.976 indicating high statistical significance as a predictive model. The steric and electrostatic contributions from the contour map were interpreted from the color-coded contour plots generated from the PLS model and the active structural components for potency against C. neoformans were determined and validated in the test set compounds. The 3-substituted benzylthio quinolinium salts (4) that make up the test set were synthesized and evaluated based on the predicted activity from the CoMFA model and the results produced a good correlation between the predicted and experimental activity (R=0.82). Thus, CoMFA has served as an effective tool to aid the design of new analogs and in this case, it has aided the identification of compounds equipotent with amphotericin B, the gold standard in antifungal drug design.

δ-Carbolines and their ring-opened analogs: synthesis and evaluation against fungal and bacterial opportunistic pathogens

Previous studies have indicated that the δ-carboline (2) ring system derived from the natural product cryptolepine (1) may represent a pharmacophore for anti-infective activity. This paper describes the design and synthesis of a small library of substituted δ-carbolines and the evaluation of the anti-fungal and anti-bacterial activities. An evaluation of the anti-bacterial activity of a previously reported library of ring-opened analogs was also conducted to provide an opportunity to test the hypothesis that both group of compounds may have the same biological target. Results indicate that against a selected group of fungal pathogens, substituted δ-carbolinium analogs displayed higher potency and several fold lower cytotoxicity than cryptolepine the parent natural product. Both the δ-carbolinium compounds and their ring-opened analogs, exhibited equally high anti-bacterial activity against the selected pathogens and especially against the gram positive bacteria evaluated.

Optimization of 3-(phenylthio)quinolinium compounds against opportunistic fungal pathogens

Ring-opened benzothieno[3,2-b]quinolinium salts (3) were designed and synthesized with substitution on the thiophene moiety. In vitro screenings were carried out against fungal pathogens including Cryptococcus neoformans, Candida albicans, Candida glabrata, Candida krusei and Aspergillus fumigatus. In all, by replacing the N-methyl group (2) with N-ω-phenylpentyl or ω-cyclohexylpentyl group to form substituted 3-(phenylthio)quinolinium compounds produced remarkable potencies, as high as 300-fold (cf, cryptolepine (1)=250 μg/mL vs 11p=0.8 μg/mL for C. albicans) over the starting tetracyclic parent. In addition, all the N-ω-cyclohexylpentyl analogs produced superior activity against all the microorganisms tested than the N-ω-phenylpentyl substituted compounds. The potential of these compounds to induce toxicity in Vero cells was also investigated and the majority of them showed lower or no cytotoxicity at 10 μg/mL than amphotericin B, the gold standard in antifungal drug development. For instance, the trifluoromethyl substituted analogs (11n-p) have selectivity indices over 2-fold better than those of amphotericin B in C. neoformans. Overall, this ring-opened scafford of benzothienoquinolines, with substitution on the thiophenyl moiety, serves as a new lead for further development.

Identification of 3-phenylaminoquinolinium and 3-phenylaminopyridinium salts as new agents against opportunistic fungal pathogens

Previous studies on the indoloquinoline alkaloid, cryptolepine (2), revealed that it has antii-nfective properties among other activities. Using Structure-activity relationship (SAR) techniques, several ring-opened analogs of cryptolepine (3-phenylaminopyridinium and 3-phenylaminoquinolinium derivatives) were designed to improve the potency and lower the cytotoxicity shown by several of the precursor agents. Results indicate that these ring-opened analogs constitute new anti-infective agents with over a 100-fold potency and several fold lower cytotoxicity than cryptolepine from which they are derived.

Synthesis and antiproliferative evaluation of certain indolo[3,2-c]quinoline derivatives

The present report describes the synthesis and antiproliferative evaluation of certain indolo[3,2-c]quinoline derivatives. For the C(6) anilino-substituted derivatives, (11H-indolo[3,2-c]quinolin-6-yl)phenylamine (6a) was inactive. Structural optimization of 6a by the introduction of a hydroxyl group at the anilino-moiety resulted in the enhancement of antiproliferative activity in which the activity decreased in an order of para-OH, 7a>meta-OH, 8a>ortho-OH, 9a. For the C(6) alkylamino-substituted derivatives, 11a, 12a, 13a, 14a, and 15a exhibited comparable antiproliferative activities against all cancer cells tested and the skin Detroit 551 normal fibroblast cells. Three cancer cells, HeLa, A549, and SKHep, are very susceptible with IC(50) of less than 2.17 microM while PC-3 is relatively resistant to this group of indolo[3,2-c]quinolines. For the 2-phenylethylamino derivatives, compound 20a is active against the growth of HeLa with an IC(50) of 0.52 microM, but is less effective against the growth of Detroit 551 with an IC(50) of 19.32 microM. For the bis-indolo[3,2-c]quinolines, N,N-bis-[3-(11H-indolo[3,2-c]quinolin-6-yl)aminopropyl]amine hydrochloride (25) is more active than its N-methyl derivative 26 and the positive Doxorubicin. Mechanism studies indicated 25 can induce caspase-3 activation, gamma-H2AX phosphorylation, cleavage of poly(ADP-ribose)polymerase and DNA fragmentation. These results provide evidence that DNA, topo I, and topo II are the primary targets of indolo[3,2-c]quinoline derivatives and that consequently inhibits proliferation and causes apoptosis in cancer cells.

Indolo[3,2-b]quinolines: synthesis, biological evaluation and structure activity-relationships

The tetracyclic indolo[3,2-b]quinoline ring system constitutes an important structural moiety in natural products exhibiting numerous biological activities. In particular, indolo [3, 2-b]quinoline, commonly known as linear quindoline is of particular interest, because of its rigid structure and scope of derivatization. Although the core linear quindoline skeleton shows little or no activity in several biological systems, introduction of a methyl group on the N-5 atom leading to cryptolepine induces remarkable activity against a broad spectrum of biological targets. A number of analogs of quindoline and cryptolepine have been synthesized, incorporating various functional groups on the core quindoline skeleton leading to improved biological activities. In this review, we describe various synthetic methodologies leading to the quindoline scaffold, the biological activities and the structure activity relationships (SAR) of quindoline derivatives toward different disease states to give a better picture of the importance of this moiety in medicinal chemistry.

Synthesis and evaluation of isosteres of N-methyl indolo[3,2-b]-quinoline (cryptolepine) as new antiinfective agents

Isosteres of cryptolepine (1) were synthesized and evaluated for their antiinfective activities. Overall, the sulfur isostere, 5-methyl benzothieno[3,2-b]quinolinium salt (5b), was equipotent to 1 and has shown no cytotoxicity at 23.8 microg/mL. Compound 5b was also found to have a broad spectrum of activity. Both the carbon and oxygen isosteres were less potent than cryptolepine. A limited library of 2-substituted analogs of 5b has been synthesized and evaluated in antifungal screens but did not show increase in potency compared to the unsubstituted 5b. Similarly, evaluation of tricyclic benzothieno[3,2-b]pyridines while showing promise in individual screens did not produce an overall increase in potency. Overall, the evaluation of the activities of 5b compared with standard antifungal/anti-protozoal agents suggests that the benzothienoquinoline scaffold could serve as a lead for optimization.