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.

Application and Mechanism of Cryptolepine and Neocryptolepine Derivatives as T3SS Inhibitors for Control of Bacterial Leaf Blight on Rice.

Bacterial leaf blight (BLB) caused by Xanthomonas oryzae pv oryzae (Xoo) is extremely harmful to rice production. The traditional control approach is to use bactericides that target key bacterial growth factors, but the selection pressure on the pathogen makes resistant strains the dominant bacterial strains, leading to a decline in bactericidal efficacy. Type III secretion system (T3SS) is a conserved and critical virulence factor in most Gram-negative bacteria, and its expression or absence does not affect bacterial growth, rendering it an ideal target for creating drugs against Gram-negative pathogens. In this work, we synthesized a range of derivatives from cryptolepine and neocryptolepine. We found that compound Z-8 could inhibit the expression of Xoo T3SS-related genes without affecting the growth of bacteria. an in vivo bioassay showed that compound Z-8 could effectively reduce the hypersensitive response (HR) induced by Xoo in tobacco and reduce the pathogenicity of Xoo in rice. Furthermore, it exhibited synergy in control of bacterial leaf blight when combined with the quorum quenching bacterial F20.

Regiodivergent Synthesis and Biological Activities of Indoloquinoline Based Compounds.

Cryptolepine, neocryptolepine, and isocryptolepine have remained popular synthetic targets ever since their isolation from the aqueous extracts of the West African climbing shrub Cryptolepis sanguinolenta. These natural alkaloids were found to contain significant antimalarial, antiproliferative and antimicrobial activities, making them ideal starting points for the development of novel drug candidates. As natural product synthesis is often plagued with step-heavy procedures and poor atom economy, the discovery of synthetic protocols addressing these concerns are sorely needed. In our laboratories, we have devoted our efforts into the development of regiodivergent synthesis whereby two of the indoloquinoline natural products, namely neocryptolepine and 11H-indolo[3,2-c]quinolines, could be assembled in only a few steps from a common and readily available starting material. Our synthetic endeavors to meet these goals include a cascade palladium-catalyzed Suzuki-Miyuara cross-coupling and intramolecular C-N bond formation and a photochemical nitrene insertion strategy. Furthermore, our methods also allowed for the construction of several diversely functionalized natural product derivatives which were subjected to biological evaluations.

Biological Evaluation of 8-Methoxy-2,5-dimethyl-5H-indolo[2,3-b] Quinoline as a Potential Antitumor Agent via PI3K/AKT/mTOR Signaling.

Chemotherapy is commonly used clinically to treat colorectal cancer, but it is usually prone to drug resistance, so novel drugs need to be developed continuously to treat colorectal cancer. Neocryptolepine derivatives have attracted a lot of attention because of their good cytotoxic activity; however, cytotoxicity studies on colorectal cancer cells are scarce. In this study, the cytotoxicity of 8-methoxy-2,5-dimethyl-5H-indolo[2,3-b] quinoline (MMNC) in colorectal cells was evaluated. The results showed that MMNC inhibits the proliferation of HCT116 and Caco-2 cells, blocks the cell cycle in the G2/M phase, decreases the cell mitochondrial membrane potential and induces apoptosis. In addition, the results of western blot experiments suggest that MMNC exerts cytotoxicity by inhibiting the expression of PI3K/AKT/mTOR signaling pathway-related proteins. Based on these results, MMNC is a promising lead compound for anticancer activity in the treatment of human colorectal cancer.

CFNC, a neocryptolepine derivative, inhibited the growth of gastric cancer AGS cells by inhibiting PI3K/AKT signaling pathway.

Gastric cancer is highly heterogeneous and there is still a lack of efficient, low-toxicity small molecule compounds for the treatment of gastric cancer. Natural products are important sources for the development of antitumor compounds. Therefore, it is promising strategy to find the lead compound of anti-gastric cancer agents by structural modification of natural products. The aim of this study was to synthesize a novel neocryptolepine derivative CFNC and explore its potential anti-gastric cancer effect and molecular mechanism. The MTT assay showed that the IC50 of CFNC on AGS cells reached 148 nM. CFNC arrested AGS cells in the G2/M phase of the cell cycle. Furthermore, CFNC inhibited cell proliferation and migration, leading to the loss of membrane potential by causing mitochondrial dysfunction, which induced the apoptosis of AGS cells. Western blot assay suggested that CFNC could inhibit the expression of important proteins in the PI3K/AKT/mTOR signaling pathway. These results showed that CFNC exhibited strong cytotoxic activity in gastric cancer cell lines by regulating the PI3K/AKT/mTOR signaling pathway. Taken together, CFNC could be a promising lead compound for the clinical treatment of gastric cancer.

Design, Synthesis, and Antiproliferative Activity of Novel Neocryptolepine-Rhodanine Hybrids.

A series of novel neocryptolepine-rhodanine hybrids (9a,b, 11a-d, 14, and 16a,b) have been synthesized by combining neocryptolepine core 5 modified at the C-11 position with rhodanine condensed with the appropriate aryl/hetero aryl aldehydes. Based on these findings, the structures of the hybrids were confirmed by spectral analyses. By employing the MTT assay, all hybrids were tested for their in vitro antiproliferative activity against two cancer cell lines, including MDA-MB-231 (human breast) and HepG-2 (hepatocellular carcinoma). Interestingly, the IC50 values of all hybrids except 9b and 11c showed activity comparable to the standard anticancer drug, 5-fluorouracil, against HepG-2 cancer cells. Furthermore, the cytotoxicity of all the synthesized hybrids was investigated on a normal skin human cell line (BJ-1), and the results showed that these compounds had no significant cytotoxicity toward these healthy cells at the highest concentration used in this study. This study also indicated that the active hybrids exert their cytotoxic activity via the induction of apoptosis. A molecular docking study was used to shed light on the molecular mechanism of their anticancer activity. The docking results revealed that the hybrids exert their mode of action through DNA intercalation. Furthermore, in silico assessment for pharmacokinetic properties was performed on the most potent compounds, which revealed candidates with good bioavailability, high tolerability with cell membranes, and positive drug-likeness values.

Design, Synthesis and Biological Evaluation of Neocryptolepine Derivatives as Potential Anti-Gastric Cancer Agents.

Natural products play an important role in drug development and lead compound synthesis. Neocryptolepine is a polycyclic quinoline compound isolated from Cryptolepis sanguinolent. The cytotoxicity of neocryptolepine to gastric cancer cells AGS, MKN45, HGC27, and SGC7901 was not very strong, and it also had certain toxicity to gastric mucosa cells GES-1. Therefore, a series of neocryptolepine derivatives were synthesized by the modification of the structure of neocryptolepine, and their cytotoxicity was evaluated. The results showed that compounds C5 and C8 exhibited strong cytotoxicity to AGS cells. The cell colony formation and cell migration experiments suggested that compounds C5 and C8 could inhibit the proliferation and cell migration of AGS and HGC27 cells. Cell cycle and apoptosis experiments showed that compounds C5 and C8 did not cause the apoptosis of AGS and HGC27 cells but, mainly, caused cell necrosis. Compound C5 had no significant effect on AGS and HGC27 cell cycles at low concentration. After treatment with AGS cells for 24 h at high concentration, compound C5 could significantly arrest the AGS cell cycle in the G2/M phase. Compound C8 had no significant effect on the AGS and HGC27 cell cycles. The results of molecular docking and Western blot showed that compounds C5 and C8 might induce cytotoxicity through the PI3K/AKT signaling pathway. Therefore, compounds C5 and C8 may be promising lead compounds for the treatment of gastric cancer.

Comprehensive review of α-carboline alkaloids: Natural products, updated synthesis, and biological activities.

α-carboline (9H-pyrido[2,3-b]indole), contains a pyridine ring fused with an indole backbone, is a promising scaffold for medicinal chemistry. In recent decades, accumulating evidence shows that α-carboline natural products and their derivatives possess diverse bioactivities. However, hitherto, there is no comprehensive review to systematically summarize this important class of alkaloids. In this perspective, this paper represents the first review to provide a comprehensive description of α-carbolines including natural products, updated literature of synthesis, and their diverse biological activities. Their biological activities including antitumor, anti-microbial, anti-Alzheimer's disease, anti-atherosclerosis, and antioxidant activities were hilighted. And the targets and the main structure activity relationships (SARs) will be presented. Finally, challenges and future directions of this class of compounds will be discussed. This review will be helpful in understanding and encouraging further exploration for this group of alkaloids.

Synthesis, Nanoformulations, and In Vitro Anticancer Activity of N-Substituted Side Chain Neocryptolepine Scaffolds.

The naturally occurring neocryptolepine (5-Methylindolo [2,3-b]quinoline) and its analogs exhibited prominent anticancer and antimalarial activity. However, the main problem of this class of compounds is their poor aqueous solubility, hampering their bioavailability and preventing their clinical development. To overcome the problem of insolubility and to improve the physicochemical and the pharmacological properties of 5-Methylindolo [2,3-b]quinoline compounds, this work was designed to encapsulate such efficient medical compounds into mesoporous silica oxide nanoemulsion (SiO2NPs). Thus, in this study, SiO2NPs was loaded with three different concentrations (0.2 g, 0.3, and 0.6 g) of 7b (denoted as NPA). The findings illustrated that the nanoparticles were formed with a spherical shape and exhibited small size (less than 500 nm) using a high concentration of the synthesized chemical compound (NPA, 0.6 g) and good stabilization against agglomeration (more than -30 mv). In addition, NPA-loaded SiO2NPs had no phase separation as observed by our naked eyes even after 30 days. The findings also revealed that the fabricated SiO2NPs could sustain the release of NPA at two different pH levels, 4.5 and 7.4. Additionally, the cell viability of the produced nanoemulsion system loaded with different concentrations of NPA was greater than SiO2NPs without loading, affirming that NPA had a positive impact on increasing the safety and cell viability of the whole nanoemulsion. Based on these obtained promising data, it can be considered that the prepared NPA-loaded SiO2NPs seem to have the potential for use as an effective anticancer drug nanosystem.

A Compilation of Synthetic Strategies to Access the Most Utilized Indoloquinoline Motifs.

Indoloquinoline alkaloids constitute an important class of aromatic heterocycles consisting of quinoline and indole fused together in various orientations. These compounds, both natural and synthetic, often display various bioactivities which have established them to be one of the interesting medicinal targets. This class of compounds have stimulated much interest among synthetic and medicinal chemists as evidenced by growth in the number of synthetic methods to prepare and study this class of alkaloids. This review compiles the synthetic strategies and methods currently known in the literature for the construction of four important indoloquinoline skeletons.

Integrated Proteomics and Transcriptomics Analyses Reveals the Possible Antifungal Mechanism of an Indoloquinoline Alkaloid Neocryptolepine against Rhizoctonia solani.

Rhizoctonia solani causes serious plant diseases. Neocryptolepine presented the significant antifungal activity against R. solani, however the mode of action is unclear. In this paper, we investigated the potential mode of action of neocryptolepine against R. solani integrated the proteomics and transcriptomics. Results showed that after treatment with neocryptolepine, 1012 differentially expressed proteins and 10 920 differentially expressed genes of R. solani were found, most of them were enriched in mitochondrial respiratory chain. It affected oxidative phosphorylation led to the enrichment of ROS and the decrease of MMP, and inhibited complex III activity with the inhibition rate of 63.51% at 10 µg/mL. The mitochondrial structural and function were damaged. Cytochrome b-c1 complex subunit Rieske (UQCRFS1) with the high binding score to neocryptolepine was found as a potential target. In addition, it inhibited the sclerotia formation and presented antifungal efficacy by decreasing the diameter of a wound in potato in a concentration-dependent manner. Above results indicated that neocryptolepine inhibited the complex III activity by binding UQCRFS1 and blocked the ion transfer to cause the death of R. solani mycelia. This study laid the foundation for the future development of neocryptolepine as an alternative biofungicide.

Synthesis of neocryptolepines and carbocycle-fused quinolines and evaluation of their anticancer and antiplasmodial activities.

This study reported the discovery of novel compounds containing five-membered ring fused quinoline core structures as anticancer and antimalarial agents. Two libraries containing these core structures, neocryptolepines and carbocycle-fused quinolines, were prepared and evaluated. Compound 3h was found to be much more potent than other analogs against cancer cell lines with high selectivity. Meanwhile, carbocycle-fused quinolines 5h and 5s showed moderate anticancer properties but much less cytotoxicity to normal cell than doxorubicin. In addition, compound 3h also showed much lower cytotoxic against human normal kidney cell line compared to doxorubicin standard. However, only compounds 3s and 3p provided acceptable results for antimalarial activities.

Design, Synthesis, and Antifungal Evaluation of Neocryptolepine Derivatives against Phytopathogenic Fungi.

Neocryptolepine is an alkaloid isolated from traditional African herbal medicine Cryptolepis sanguinolenta, and its broad spectrum of biological activities has been illuminated in past decades. In this study, neocryptolepine and its derivatives (1-49) were designed and synthesized from economical and readily available starting materials. Their structures were confirmed by proton nuclear magnetic resonance, carbon nuclear magnetic resonance, and mass spectrometry. The synthesized compounds were screened for their antifungal profile against six agriculturally important fungi Rhizoctonia solani, Botrytis cinerea (B. cinerea), Fusarium graminearum, Mycosphaerella melonis, Sclerotinia sclerotiorum, and Magnaporthe oryzae. The results of in vitro assay revealed that compounds 5, 21, 24, 35, 40, 45, and 47 presented remarkable antifungal activity against the fungi tested with EC50 values lower than 1 µg/mL. Significantly, compound 24 displayed the most effective inhibitory potency against B. cinerea (EC50 = 0.07 µg/mL), and the data from in vivo experiments revealed that compound 24 demonstrated comparable protective activity with the positive control boscalid. Preliminary mechanism studies indicated that compound 24 showed impressive spore germination inhibitory effectiveness and lower cytotoxicity than azoxystrobin, imparted on normal function of the cell membrane and cell wall, and arrested the normal function of the nucleus. Besides the excellent inhibitory activity against agriculturally important phytopathogenic fungi tested, the designed assemblage possesses several benefits with a high profile of variation in synthesized molecules, the ease of synthesis, and good cost-effectiveness of commercially available synthetic reagents, all of these have highlighted the potential worth of compound 24 as a new and highly efficient agricultural fungicide.

Structural Modifications of Nature-Inspired Indoloquinolines: A Mini Review of Their Potential Antiproliferative Activity.

Cryptolepine, neocryptolepine and isocryptolepine are naturally occurring indoloquinoline alkaloids with various spectrum of biological properties. Structural modification is an extremely effective means to improve their bioactivities. This review enumerates several neocryptolepine and isocryptolepine analogues with potent antiproliferative activity against MV4-11 (leukemia), A549 (lung cancer), HCT116 (colon cancer) cell lines in vitro. Its activity towards normal mouse fibroblasts BALB/3T3 was also evaluated. Furthermore, structure activity relationships (SAR) are briefly discussed. The anticancer screening of neocryptolepine derivatives was performed in order to determine their cytotoxic and growth inhibitory activities across the JFCR39 cancer cell line panel.

Synthesis and In Vitro Antiproliferative Activity of 11-Substituted Neocryptolepines with a Branched ω-Aminoalkylamino Chain.

Neocryptolepine, which is a kind of tetracyclic indoloquinoline alkaloid, exhibits the inhibition of topoisomerase II and shows antiproliferative activity. The present study describes the synthesis and antiproliferative evaluation of several neocryptolepine analogues carrying a branched, functionalized dibasic side chain at C11. These 2-substituted 5-methyl-indolo[2,3-b]quinoline derivatives were prepared by nucleophilic aromatic substitution (SNAr) of 11-chloroneocryptolepines with appropriate 1,2- and 1,3-diamines. Some of the 11-(ω-aminoalkylamino) derivatives were further transformed into 11-ureido and thioureido analogues. Many of the prepared neocryptolepine derivatives showed submicromolar antiproliferative activity against the human leukemia MV4-11 cell line. Among them, 11-(3-amino-2-hydroxy)propylamino derivatives 2h and 2k were the most cytotoxic with a mean IC50 value of 0.042 µM and 0.057 µM against the MV4-11 cell line, 0.197 µM and 0.1988 µM against the A549 cell line, and 0.138 µM and 0.117 µM against the BALB/3T3 cell line, respectively.

The synthesis of indolo[2,3-b]quinoline derivatives with a guanidine group: highly selective cytotoxic agents.

The synthesis of indolo[2,3-b]quinoline derivatives containing guanidine, amino acid or guanylamino acid substituents as well as their in vitro evaluation for the cytotoxic and antifungal activity are reported. The influence of the guanidine group on the selective cytotoxic and hemolytic properties of indolo[2,3-b]quinoline was investigated. Most of the compounds displayed a high cytotoxic activity in vitro and two of the most promising compounds (3 and 12) exhibited a high selectivity between normal and cancer cell-lines. The cytotoxic activity of compound 3 was about 600-fold lower against normal fibroblasts than against A549 and MCF-7 cancer cell lines. Novel entities acted as the DNA-intercalators when tested using a DNA-methyl green assay but demonstrated zero or low hemolytic activity in comparison to their unsubstituted analogs. The mechanism of action was studied for guanidine derivatives 3 and 12 and both compounds were found to be very effective inducers of apoptosis.

Transition metal complexes of neocryptolepine analogues. Part I: synthesis, spectroscopic characterization, and invitro anticancer activity of copper(II) complexes.

New generation of copper(II) complexes with aminoalkylaminoneocryptolepine as bidentate ligands has been synthesized and it is characterized by elemental analyses, magnetic moment, spectra (IR, UV-Vis, (1)H NMR and ESR) and thermal studies. The IR data suggest the coordination modes for ligands which behave as a bidentate with copper(II) ion. Based on the elemental analysis, magnetic studies, electronic and ESR data, binuclear square planar geometry was proposed for complexes 7a, 7b, square pyramidal for 9a, 9b and octahedral for 8a, 8b, 10a, 10b. The molar conductance in DMF solution indicates that all complexes are electrolyte except 7a and 7b. The ESR spectra of solid copper(II) complexes in powder form showed an axial symmetry with (2)B1g as a ground state and hyperfine structure. The thermal stability and degradation of the ligands and their metal complexes were studied employing DTA and TG methods. The metal-free ligands and their copper(II) complexes were tested for their in vitro anticancer activity against human colon carcinoma (HT-29). The results showed that the synthesized copper(II) complexes exhibited higher anticancer activity than their free ligands. Of all the studied copper(II) complexes, the bromo-substituted complex 9b exhibited high anticancer activity at low micromolar inhibitory concentrations (IC50=0.58µM), compared to the other complexes and the free ligands.

Searching for new derivatives of neocryptolepine: synthesis, antiproliferative, antimicrobial and antifungal activities.

A series of novel amino acid and dipeptide derivatives of neocryptolepine were synthesized and tested for their antimicrobial, antifungal and antiproliferative activity in vitro against cancer cell lines (KB, A549, MCF-7, LoVo) and normal mice fibroblast cells (BALB/3T3). Biological evaluation revealed that almost all of the new compounds displayed high antiproliferative activity against the tested cells and moderate to potent antibacterial activities. Interestingly, these compounds were active against Candida albicans biofilms at doses significantly lower than those required against free-floating planktonic fungal cells. The most promising compounds are derivatives with glycine and L-proline as a substituent both at 2 and at 9 position of 5H-indolo[2,3-b]quinoline. In general, these new compounds (2a, 3a, 6a and 7a) showed the highest dual action against cancer lines and infectious pathogenic microbes in vitro.