In Vitro and In Vivo Antitumor Activity of Indolo[2,3-b] Quinolines, Natural Product Analogs from Neocryptolepine Alkaloid

Novel access to α-carbolines with biological applications

In recent years, the 9H-pyrido[2,3-b]indole nuclei, also named α-carboline which is found in many organic compounds such as natural products, pharmaceuticals, and materials, have intensively stimulated the research of new synthetic pathways. After a brief report published in 2015 describing novel accesses and biological applications of α-carbolines, this update reports between 2015 and 2023 on the emergence of original syntheses to this heterocyclic nucleus. Examples representing these processes are described and the biological activities of α-carbolines are mentioned when they have been prepared for therapeutic purposes.

Cascade Synthesis of New Indole-Containing Pentacyclic Scaffolds Mediated by Aryl and Iminyl Radicals

A five-step approach, starting from simple 1,5-disubstituted indoles, has been implemented for the synthesis of diversely substituted indole-pyrido-indene pentacyclic compounds up to 54 % yield via domino radical-mediated processes in the presence of the radical reagents DLP/TTMSS and AIBN/TTMSS. Reactions proceeded with diverse key starting radical cyano-precursors strategically synthesized which were subsequently transformed into the target pentacyclic compounds through an aryl/iminyl radical-mediated domino reactions sequence. In addition to the routine spectroscopic techniques, the structure of radical precursors, as well as, the target pentacyclic products were unequivocally established by single crystal X-ray diffraction, confirming the effectiveness of the proposed synthetic sequence.

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.

Impact of Synthesized Indoloquinoline Analog to Isolates from Cryptolepis sanguinolenta on Tumor Growth Inhibition and Hepatotoxicity in Ehrlich Solid Tumor-Bearing Female Mice

The study evaluated the antitumor efficacy of APAN, “synthesized indoloquinoline analog derived from the parent neocryptolepine isolated from the roots of Cryptolepis sanguinolenta”, versus the chemotherapeutic drug etoposide (ETO) in Ehrlich solid tumor (EST)-bearing female mice as well as its protective effect against etoposide-triggered hepatic disorders. APAN showed an ameliorative activity against Ehrlich solid tumor and hepatic toxicity, and the greatest improvement was found in the combined treatment of APAN with ETO. The results indicated that EST altered the levels of tumor markers (AFP, CEA, and anti-dsDNA) and liver biomarker function (ALT, AST, ALP, ALB, and T. protein). Furthermore, EST elevated CD68 and anti-survivin proteins immuno-expressions in the solid tumor and liver tissue. Molecular docking studies were demonstrated to investigate their affinity for both TNF-α and topoisomerase II as target proteins, as etoposide is based on the inhibition of topoisomerase II, and TNF-α is quite highly expressed in the solid tumor and liver tissues of EST-bearing animals, which prompted the authors’ interest to explore APAN affinity to its binding site. Treatment of mice bearing EST with APAN and ETO nearly regularized serum levels of the altered parameters and ameliorated the impact of EST on the tissue structure of the liver better than that by treatment with each of them separately.

Design and cytotoxic evaluation via apoptotic and antiproliferative activity for novel 11(4-aminophenylamino)neocryptolepine on hepatocellular and colorectal cancer cells

The current study evaluated the cytotoxic activity of 11(4-Aminophenylamino)neocryptolepine (APAN), a novel derivative of neocryptolepine, on hepatocellular (HepG2) and colon (HCT-116) carcinoma cell lines as well as, the possible molecular mechanism through which it exerts its cytotoxic activity. The APAN was synthesized and characterized based on their spectral analyses. Scanning for anticancer target of APAN by Swiss software indicated that APAN had highest affinity for protein tyrosine kinase 6 enzyme. Furthermore, Super pred software indicated that APAN can be indicated in hepatic and colorectal cells with 92%. Molecular docking studies indicated that the binding affinity scores of APAN for protein PDB code: 6CZ4 of tyrosine kinase 6 recorded of - 6.6084 and RMSD value of 0.8891°A, while that for protein PDB: 7JL7 of caspase 3 was - 6.1712 and RMSD of 0.8490°A. Treatment of HepG2 and HCT-116 cells with APAN induced cytotoxicity with IC₅₀ of 2.6 and 1.82 μg/mL respectively. In addition, it induced injury and serious morphological changes in cells including, disappearance of microvilli, membrane blebbing, cytoplasmic condensation, and shrunken nucleus with more condensed chromatin. Moreover, APAN significantly increased protein expression of annexin V (apoptotic marker). Furthermore, APAN significantly increased protein expression of caspase 3 and P53. However, it significantly reduced secretion of VEGF protein into the medium and decreased protein expression of PCNA and Ki67 in HepG2 and HCT-116 cells. This study indicated that APAN had cytotoxic activity against HepG2 and HCT-116 cells via increasing the expression of apoptotic proteins and reducing the expression of proliferative proteins.

Palladium-catalyzed cascade cyclization of allenamide with 2-iodoanilines to access functionalized indoloquinolines

A novel and efficient palladium-catalyzed cascade cyclization to indoloquinoline derivatives in one pot has been developed by using allenamide derivatives and 2-iodoanilines as the key building blocks. The process involved two cyclizations: intramolecular cyclization/π-allylic substitution and intramolecular 6-endo Heck cyclization. Furthermore, dihydrobenzofuro[2,3-b]quinoline derivatives could also be achieved via this strategy using allenyl ethers instead of allenamides. The readily available substrates, mild conditions, high efficiency and step economy make this strategy a promising method in the synthesis of polycyclic motifs.

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.

Synthesis, Biocidal and Antibiofilm Activities of New Isatin-Quinoline Conjugates against Multidrug-Resistant Bacterial Pathogens along with Their In Silico Screening

Isatin-quinoline conjugates 10a-f and 11a-f were assembled by the reaction of N-(bromobutyl) isatin derivatives 3a, b with aminoquinolines 6a-c and their corresponding hydrazinyl 9a-c in good yields. The structures of the resulting conjugates were established by spectroscopic tools and showed data consistent with the proposed structures. In vitro antibacterial activity against different bacterial strains was evaluated. All tested conjugates showed significant biocidal activity with lower MIC than the first line drugs chloramphenicol and ampicillin. Conjugates 10a, 10b and 10f displayed the most potent activity against all clinical isolates. The antibiofilm activity for all tested conjugates was screened against the reference drug vancomycin using the MRSA strain. The results revealed that all conjugates had an inhibitory activity against biofilm formation and conjugate. Conjugate 11a showed 83.60% inhibition at 10 mg/mL. In addition, TEM studies were used to prove the mechanism of antibacterial action of conjugates 10a and 11a against (MRSA). Modeling procedures were performed on 10a-f and 11a-f and interestingly the results were nearly consistent with the biological activities. In addition, in silico pharmacokinetic evaluation was performed and revealed that the synthesized compounds 10a-f and 11a-f were considered drug-like molecules with promising bioavailability and high GI absorption. The results confirmed that the title compounds caused the disruption of bacterial cell membranes and could be used as potential leads for the further development and optimization of antibacterial agents.

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.

Indoloquinolines as scaffolds for the design of potent G-quadruplex ligands

Indoloquinolines are natural alkaloids with known affinity to DNA and antiproliferative activity against bacteria, parasites, and cancer cells. Due to their non-chiral skeleton, their total synthesis is easy to achieve and throughout the years, many derivatives have been studied for their potential as drugs. Herein we review the indoloquinolines and bioisosters that have been designed, synthesised, and evaluated for their selective binding to G-quadruplex nucleic acid structures, as well as the reported effects in cancer cells. The data collected so far strongly suggest that indoloquinolines are good scaffolds for the development of drugs and probes targeting the G-quadruplex structures, but they also show that this scaffold is still underexplored.

Synthesis of 8-Fluoroneocryptolepine and Evaluation for Cytotoxic Activity against AGS Cancer Cells

Neocryptolepine derivatives have attracted great interest because of their unique cytotoxic activity. 8-Fluoroneocryptolepine (8FNC) was synthesized, and its cytotoxicity was evaluated by MTT assay in AGS gastric cancer cells and gastric mucosa GES-1 cells. 8-Fluoroneocryptolepine showed greater selectivity and cytotoxicity to AGS cells than the cisplatin (CIS) and fluorouracil (5-Fu) commonly used in clinical treatment of gastric cancer. Most importantly, we significantly improved the cytotoxic effect of 8FNC against AGS cells by structural modification and reduced the cytotoxicity against GES-1 cells compared with neocryptolepine. We further evaluated the activity of 8FNC against AGS cells in vitro. Our results indicate that 8FNC arrests the AGS cell cycle in the G2/M phase, reduces the mitochondrial membrane potential of AGS cells, and drives the initiation of apoptotic body formation in 8FNC-induced apoptosis. Moreover, 8FNC exhibits strong inhibitory effects on AGS cell migration. Studies on the molecular mechanisms of the cytotoxic activities of 8FNC revealed that it may play a significant role in the inhibitory effect on AGS human gastric cancer cells through the PI3K/AKT signaling pathway. In conclusion, 8FNC may become a promising lead compound in the development of potential clinical drug candidates for the treatment of gastric cancer.

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.

Highly Antiproliferative Latonduine and Indolo[2,3-c]quinoline Derivatives: Complex Formation with Copper(II) Markedly Changes the Kinase Inhibitory Profile

A series of latonduine and indoloquinoline derivatives HL¹–HL⁸ and their copper(II) complexes (1–8) were synthesized and comprehensively characterized. The structures of five compounds (HL⁶, [CuCl(L¹)(DMF)]·DMF, [CuCl(L²)(CH₃OH)], [CuCl(L³)]·0.5H₂O, and [CuCl₂(H₂L⁵)]Cl·2DMF) were elucidated by single crystal X-ray diffraction. The copper(II) complexes revealed low micro- to sub-micromolar IC₅₀ values with promising selectivity toward human colon adenocarcinoma multidrug-resistant Colo320 cancer cells as compared to the doxorubicin-sensitive Colo205 cell line. The lead compounds HL⁴ and 4 as well as HL⁸ and 8 induced apoptosis efficiently in Colo320 cells. In addition, the copper(II) complexes had higher affinity to DNA than their metal-free ligands. HL⁸ showed selective inhibition for the PIM-1 enzyme, while 8 revealed strong inhibition of five other enzymes, i.e., SGK-1, PKA, CaMK-1, GSK3β, and MSK1, from a panel of 50 kinases. Furthermore, molecular modeling of the ligands and complexes showed a good fit to the binding pockets of these targets.

Role of basic aminoalkyl chains in the lead optimization of Indoloquinoline alkaloids

Indoloquinoline (IQ) is an important class of naturally occurring antimalarial alkaloids, mainly represented by cryptolepine, isocryptolepine, and neocryptolepine. The IQ structural framework consists of four isomeric ring systems differing via the linkage of indole with quinoline as [3,2-b], [3,2-c], [2,3-c], and [2,3-b]. Structurally, IQs are planar and thus they bind strongly to the DNA which largely contributes to their biological properties. The structural rigidity and associated nonspecific cellular toxicity is a key shortcoming of the IQ structural framework for preclinical development. Thus, the lead optimization efforts were aimed at improving the therapeutic window and ADME properties of IQs. The structural modifications mainly involved attaching the basic aminoalkyl chains that positively modulates the vital physicochemical and topological parameters, thereby improves biological activity. Our analysis has found that the aminoalkylation consistently improved the selectivity index and provided acceptable in-vivo antimalarial/anticancer activity. Herein, we critically review the role of aminoalkylation in deciphering the antimalarial and cytotoxic activity of IQs.