Naphthylisoquinoline alkaloids: novel agents against the causative pathogens of eumycetoma and actinomycetoma-en route to broad-spectrum antimycetomal drugs.

Mycetoma is a devastating neglected tropical infection of the subcutaneous tissues. It is caused by fungal and bacterial pathogens recognized as eumycetoma and actinomycetoma, respectively. Mycetoma treatment involves diagnosing the causative microorganism as a prerequisite to prescribing a proper medication. Current therapy of fungal eumycetoma causative agents, such as Madurella mycetomatis, consists of long-term antifungal medication with itraconazole followed by surgery, yet with usually unsatisfactory clinical outcomes. Actinomycetoma, on the contrary, usually responds to treatment with co-trimoxazole and amikacin. Therefore, there is a pressing need to discover novel broad-spectrum antimicrobial agents to circumvent the time-consuming and costly diagnosis. Using the resazurin assay, a series of 23 naphthylisoquinoline (NIQ) alkaloids and related naphthoquinones were subjected to in vitro screening against two fungal strains of M. mycetomatis and three bacterial strains of Actinomadura madurae and A. syzygii. Seven NIQs, mostly dimers, showed promising in vitro activities against at least one strain of the mycetoma-causative pathogens, while the naphthoquinones did not show any activity. A synthetic NIQ dimer, 8,8'''-O,O-dimethylmichellamine A (18), inhibited all tested fungal and bacterial strains (IC50 = 2.81-12.07 µg/mL). One of the dimeric NIQs, michellamine B (14), inhibited a strain of M. mycetomatis and significantly enhanced the survival rate of Galleria mellonella larvae infected with M. mycetomatis at concentrations of 1 and 4 µg/mL, without being toxic to the uninfected larvae. As a result, broad-spectrum dimeric NIQs like 14 and 18 with antimicrobial activity are considered hit compounds that could be worth further optimization to develop novel lead antimycetomal agents.

Ancistrobrevinium A, the first N-methylated, cationic naphthylisoquinoline alkaloid, from the tropical liana Ancistrocladus abbreviatus (Ancistrocladaceae).

Ancistrobrevinium A (1) is the first N-methylated and non-hydrogenated, and thus cationic naphthylisoquinoline alkaloid. It was discovered in the root bark extract of the phytochemically productive West African liana Ancistrocladus abbreviatus (Ancistrocladaceae). Its constitution was elucidated by HR-ESI-MS and 1D and 2D NMR. Due to the steric hindrance in the proximity of the linkage between the naphthalene and isoquinoline parts, the biaryl axis is rotationally hindered. It thus constitutes a stable element of chirality - the only one in the new alkaloid since, different from most other naphthylisoquinoline alkaloids, it has no stereogenic centers. The axial configuration of 1 was assigned by electronic circular dichroism (ECD) investigations, which gave a positive couplet, indicating a 'positive chirality', here corresponding to a P-configuration. Ancistrobrevinium A (1) showed a weak cytotoxic activity against A549 lung cancer cells (IC50 = 50.6 µM).

Jozibrevine D from Ancistrocladus ileboensis, the fifth alkaloid in a series of six possible atropo-diastereomeric naphthylisoquinoline dimers, showing antiparasitic and antileukemic activities.

A new dimeric naphthylisoquinoline alkaloid, jozibrevine D (4e), was isolated from the Central-African liana Ancistrocladus ileboensis. It is a Dioncophyllaceae-type metabolite, being R-configured at C-3 and lacking an oxygen function at C-6 in both isoquinoline moieties. The two identical monomers of jozibrevine D are symmetrically linked via the sterically constrained 3',3''-positions of the naphthalene units so that the central biaryl linkage is rotationally hindered and the alkaloid is, thus, C2-symmetric. With the two outer biaryl bonds being chiral, too, 4e possesses three consecutive stereogenic axes. The absolute stereostructure of the new compound was assigned by 1D and 2D NMR, ruthenium-mediated oxidative degradation, and electronic circular dichroism (ECD) spectroscopy. Jozibrevine D (4e) is the fifth discovered isomer in a series of six possible natural atropo-diastereomeric dimers. It shows potent, and selective, antiprotozoal activity against P. falciparum (IC50 = 0.14 µM), and it also exhibits good cytotoxic activities against drug-sensitive acute lymphoblastic CCRF-CEM leukemia cells (IC50 = 11.47 µM) and their multidrug-resistant CEM/ADR5000 subline (IC50 = 16.61 µM).

Dioncophyllidine E: The first configurationally semi-stable, 7,3'-coupled naphthyldihydroisoquinoline alkaloid, from Ancistrocladus abbreviatus, with antiausterity activity against PANC-1 human pancreatic cancer cells.

The discovery of a new naphthylisoquinoline alkaloid, dioncophyllidine E (4), from the tropical liana Ancistrocladus abbreviatus (Ancistrocladaceae) is described. Due to its rare 7,3'-coupling type, combined with the lack of an oxygen function at C-6, it is configurationally semi-stable at the biaryl axis, and thus occurs as a pair of slowly interconverting atropo-diastereomers, 4a and 4b. Its constitution was assigned mainly by 1D and 2D NMR. The absolute configuration at the stereocenter, C-3, was elucidated by oxidative degradation. The absolute axial configuration of the individual atropo-diastereomers was established by their HPLC resolution, combined with online electronic circular dichroism (ECD) investigations, providing nearly mirror-imaged LC-ECD spectra. These were assigned to the respective atropisomers by ECD comparison with a related, but configurationally stable alkaloid, ancistrocladidine (5). Dioncophyllidine E (4a/4b) exhibits a strong preferential cytotoxicity against PANC-1 human pancreatic cancer cells under nutrient-deprived conditions, with a PC50 value of 7.4 µM, suggesting its potential as an agent against pancreatic cancer.

Phytochemical and antiplasmodial investigation of Gardenia thunbergia L. f. leaves.

Previous results indicated that the methanol extract of Gardenia thunbergia has antiplasmodial activity but no compounds have ever been isolated from the plant. Therefore, this study aimed to investigate the phytochemical and antiplasmodial properties of the plant. The methanol leaf extract of G. thunbergia inhibited Plasmodium falciparum at 50 µg/mL (> 80% inhibition) and was not cytotoxic against HeLa cells. Chromatographic purification of the extract afforded a new saponin and eight other known compounds. The saponin and two flavonoid glycosides displayed non-selective antiplasmodial activity at 50 µg/mL but the activities were diminished at 10 µg/mL. The presence of the isolated compounds in the leaf extract of G. thunbergia could account for the folkloric use of the plant in treating malaria.

Antiplasmodial and Cytotoxic Flavonoids from Pappea capensis (Eckl. & Zeyh.) Leaves.

Ethnobotanical surveys indicate that the Masai and Kikuyu in Kenya, the Venda in South Africa, and the Gumuz people of Ethiopia use Pappea capensis for the treatment of malaria. The present study aimed to investigate the phytochemical and antiplasmodial properties of the plant leaves. The bioactive compounds were isolated using chromatographic techniques. The structures were established using NMR, HRMS, and UV spectroscopy. Antiplasmodial activity of P. capensis leaf extract and isolated compounds against chloroquine-sensitive 3D7 P. falciparum was evaluated using the parasite lactate dehydrogenase assay. Cytotoxicity against HeLa (human cervix adenocarcinoma) cells was determined using the resazurin assay. The extract inhibited the viability of Plasmodium falciparum by more than 80% at 50 µg/mL, but it was also cytotoxic against HeLa cells at the same concentration. Chromatographic purification of the extract led to the isolation of four flavonoid glycosides and epicatechin. The compounds displayed a similar activity pattern with the extract against P. falciparum and HeLa cells. The results from this study suggest that the widespread use of P. capensis in traditional medicine for the treatment of malaria might have some merits. However, more selectivity studies are needed to determine whether the leaf extract is cytotoxic against noncancerous cells.

N,C-Coupled naphthylisoquinoline alkaloids: a versatile new class of axially chiral natural products.

Covering: up to April 2021During the past decades, a plethora of natural products with restricted rotation about a biaryl axis have been discovered, among them the naphthylisoquinoline (NIQ) alkaloids, mostly C,C-coupled and having remarkable bioactivities. Within this fascinating class of naturally occurring biaryl compounds, NIQ alkaloids bearing an N,C-heterobiaryl axis have attracted particular attention. They are structurally and biosynthetically unprecedented, with interesting stereochemical implications and biological activities. In contrast to existing articles and reviews about axially chiral - yet C,C-coupled - natural products, this is the first, comprehensive review on the new subclass of N,C-coupled NIQs, their isolation and structural elucidation, their N,C-axial chirality, their biosynthetic origin, their promising antiparasitic and antileukemic activities, and their total synthesis.