In vitro and in vivo antimalarial evaluation of semi-synthetic derivatives of gomphostenin

A novel series of semi-synthetic gomphostenin derivatives (1–9) were prepared utilizing C-14 hydroxyl group for the first time and studied for their antimalarial properties. In vitro antiplasmodial activity was evaluated against both the chloroquine sensitive and resistant strains of Plasmodium falciparum. Most of the compounds exhibited superior or comparable antiplasmodial activity compared to parent compound, that is, gomphostenin (GN). Based upon in vitro antiplasmodial activity, compounds with IC₅₀ values less than 10 μM were selected for in vivo antiplasmodial evaluation against Plasmodium berghei infection in mice model. GN derivatives 3 and 5 were found to have curative activity with moderate chemosuppression of 65% and 69%, respectively, at the dose level of 150 mg/kg/day.     

Glabridin induces oxidative stress mediated apoptosis like cell death of malaria parasite Plasmodium falciparum

Plants are known as the source of novel agents for developing new antimalarial drugs. Glabridin is a polyphenolic flavonoid, a main constituent in the roots of Glycyrrhiza glabra possesses various biological activities. However, its anti-plasmodial activity is unexplored. In the present work, it is for the first time demonstrated that glabridin inhibits Plasmodium falciparum growth in vitro with an IC₅₀ 23.9 ± 0.43 μM. Glabridin showed poor cytotoxicity in vitro with an IC₅₀ 246.6 ± 0.88 μM against Vero cell line and good selectivity index (9.6). In erythrocytic cycle, trophozoite stage was found to be most sensitive to glabridin. In silico study showed that glabridin inhibits Pf LDH enzyme activity by acting on NADH binding site. Glabridin induced oxidative stress by the generation of reactive oxygen and nitrogen species. Glabridin could induce apoptosis in parasite as evidenced by the depolarization of mitochondrial membrane potential (Δψ_m), activation of caspase like proteases and DNA fragmentation. These results indicate that glabridin exhibits antiplasmodial activity and is suitable for developing antimalarial agent from a cheap and sustainable source.     

Structure–activity relationship studies of antiplasmodial aminomethylthiazoles

Structure–activity relationship (SAR) studies around a previously reported antimalarial aminomethylthiazole pyrazole carboxamide 1 are reported. Several analogues were synthesised and profiled for in vitro antiplasmodial activity against the drug-sensitive Plasmodium falciparum malaria parasite strain, NF54. Although all the reported analogues exhibited inferior in vitro antiplasmodial activity (IC₅₀ = 0.125–173 μM) relative to compound 1 (IC₅₀ = 0.0203 μM), one analogue, compound 5a, retained submicromolar activity (IC₅₀ = 0.125 μM).     

Antimalarial acridines: Synthesis,in vitro activity against P. falciparum and interaction with hematin

A series of acridine derivatives were synthesised and their in vitro antimalarial activity was evaluated against one chloroquine-susceptible strain (3D7) and three chloroquine-resistant strains (W2, Bre1 and FCR3) of Plasmodium falciparum. Structure–activity relationship showed that two positives charges as well as 6-chloro and 2-methoxy substituents on the acridine ring were required to exert a good antimalarial activity. The best compounds possessing these features inhibited the growth of the chloroquine-susceptible strain with an IC₅₀ ? 0.07 μM, close to that of chloroquine itself, and that of the three chloroquine-resistant strains better than chloroquine with IC₅₀ ? 0.3 μM. These acridine derivatives inhibited the formation of β-hematin, suggesting that, like CQ, they act on the haem crystallization process. Finally, in vitro cytotoxicity was also evaluated upon human KB cells, which showed that one of them 9-(6-ammonioethylamino)-6-chloro-2-methoxyacridinium dichloride 1 displayed a promising antimalarial activity in vitro with a quite good selectivity index versus mammalian cell on the CQ-susceptible strain and promising selectivity on other strains.     

Antiplasmodial benzophenone derivatives from the root barks of Symphonia globulifera (Clusiaceae)

In an effort to find antimalarial drugs, a systematic in vitro evaluation on a chloroquine-resistant strain of Plasmodium falciparum (FcB1) was undertaken on sixty plant extracts collected in French Guiana. The ethyl acetate extract obtained from the root barks of Symphonia globulifera exhibited a strong antiplasmodial activity (97% at 10 μg/ml). The phytochemical investigation of this extract led to the isolation of nine polycyclic polyprenylated acylphloroglucinol (PPAPs) compounds and two oxidized derivatives. All compounds showed antiplasmodial activity with IC₅₀s ranged from 2.1 to 10.1 μM. A LC/ESI-MSⁿ study performed on polyprenylated benzophenones previously isolated from Moronobea coccinea provided a reliable method for their detection in the extract and structural elucidation.     

Bis-alkylamine quindolone derivatives as new antimalarial leads

Quindolone derivatives, designed to target the malaria parasite digestive vacuole and heme detoxification pathway, have been synthesized by reaction with 2-chloro-N,N-diethylethanamine. This reaction gave N,O-, N,N- and O-alkylated products containing one or two basic side-chains. The compounds were evaluated for antiplasmodial activity against the chloroquine-resistant Plasmodium falciparum W2 strain and for cytotoxicity in HepG2 A16 hepatic cells. By incorporating alkylamine side chains and chlorine atoms in the quindolone nucleus we transformed the inactive tetracyclic parent quindolones into moderate or highly active and selective antimalarial compounds. The most active and selective compound, 5c, showed an IC₅₀ = 51 nM for P. falciparum and a selectivity ratio of 98.     

Synthesis and in vitro antiplasmodial evaluation of 4-anilino-2-trichloromethylquinazolines

To identify a new safe antiplasmodial molecular scaffold, an original series of 2-trichloromethylquinazolines, functionalized in position 4 by an alkyl- or arylamino substituent, was synthesized from 4-chloro-2-trichloromethylquinazoline 1, via a cheap, fast and efficient solvent-free operating procedure. Among the 40 molecules prepared, several exhibit a good profile with both a significant antiplasmodial activity on the W2 Plasmodium falciparum strain (IC₅₀ values: 0.4–2.2 μM) and a promising toxicological behavior regarding human cells (HepG2/W2 selectivity indexes: 40–83), compared to the antimalarial drug compounds chloroquine and doxycycline. The in vitro antitoxoplasmic and antileishmanial evaluations were conducted in parallel on the most active molecules, showing that these ones specifically display antiplasmodial properties.     

Dimeric cyclohexane-1,3-dione oximes inhibit wheat acetyl-CoA carboxylase and show anti-malarial activity

A series of dimeric 1,3-cyclohexanedione oxime ethers were synthesized and found to have significant antiplasmodial activity with IC₅₀’s in the range 3–12 μM. The most active dimer was tested in the Plasmodium berghei mouse model of malaria and at a dose of 48 mg/kg gave a 45% reduction in parasitaemia. Several commercial herbicides, all known to be inhibitors of maize acetyl-CoA carboxylase, were also tested for antimalarial activity, but were essentially inactive with the exception of butroxydim which gave an IC₅₀ of 10 μM.     

Synthesis and antiplasmodial activity of new heteroaryl derivatives of 7-chloro-4-aminoquinoline

With the aim to investigate the effect of different heterocyclic rings linked to the 4-aminoquinoline nucleus on the antimalarial activity, a set of 7-chloro-N-(heteroaryl)-methyl-4-aminoquinoline and 7-chloro-N-(heteroaryl)-4-aminoquinoline was synthesized and tested in vitro against D-10 (CQ-S) and W-2 (CQ-R) strains of Plasmodium falciparum. All compounds exhibited from moderate to high antiplasmodial activities. The activity was strongly influenced both by the presence of a methylenic group, as a spacer between the 4-aminoquinoline and the heterocyclic ring, and by the presence of a basic head. The most potent molecules inhibited the growth of both CQ-S and CQ-R strains of P. falciparum with IC₅₀ < 30 nM and were not toxic against human endothelial cells. These results confirm that the presence of an heteroaryl moiety in the side chain of 7-chloro-4-aminoquinoline is useful for the design and development of new powerful antimalarial agents.     

Design,synthesis and structure–activity relationships of (1H-pyridin-4-ylidene)amines as potential antimalarials

(1H-Pyridin-4-ylidene)amines containing lipophilic side chains at the imine nitrogen atom were prepared as potential clopidol isosteres in the development of antimalarials. Their antiplasmodial activity was evaluated in vitro against the Plasmodium falciparum W2 (chloroquine-resistant) and FCR3 (atovaquone-resistant) strains. The most active of these derivatives, 4m, had an IC₅₀ of 1 μM against W2 and 3 μM against FCR3. Molecular modeling studies suggest that (1H-pyridin-4-ylidene)amines may bind to the ubiquinol oxidation Q_o site of cytochrome bc₁.     

Design,synthesis and anti-P. falciparum activity of pyrazolopyridine–sulfonamide derivatives

Ten 1-phenyl-1H-pyrazolo[3,4-b]pyridine derivatives connected by a linker group to benzenesulfonamide moieties with different substituents in the 4-position were synthesized and assayed against Plasmodium falciparum. These ten compounds exhibited activity in vitro against the chloroquine-resistant clone W2 with IC₅₀ values ranging from 3.46 to 9.30 μM. The most active derivatives with substituent R₂ = Cl or CH₃ at the benzenesulfonamide moiety exhibited the lowest IC₅₀. Compounds with an R₁ = CO₂Et substituent at the 5-position of the 1H-pyrazolo[3,4-b]pyridine ring presented lower activity than those with a CN substituent. The 1H-pyrazolo[3,4-b]pyridine system appears to be promising for further studies as an antimalarial for overcoming the burden of resistance in P. falciparum.     

Voulkensin C–E,new 11-oxocassane-type diterpenoids and a steroid glycoside from Caesalpinia volkensii stem bark and their antiplasmodial activities

A bioassay guided isolation of potential antimalarial molecules from the stem bark of Caesalpinia volkensii Harms (Fabaceae) achieved three new 11-oxocassane-type diterpenoids named voulkensin C (1), D (2) and E (3) together with one steroid glycoside named 3-O-[β-glucopyranosyl(1→2)-O-β-xylopyranosyl]-stigmasterol (4) and seven other known compounds including stigmasterol (5), β-sitosterol (6), oleanolic acid (7), 3-β-acetoxyolean-12-en-28-methyl ester (8), voucap-5-ol (9), caesadekarin C (10), deoxycaesaldekarin C (11). The structures of the new compounds were determined on the basis of extensive spectroscopic data (IR, MS, ¹H and ¹³C NMR and 2D NMR) analyses. The polar extracts revealed moderate to good antiplasmodial activities against chloquine-sensitive (D6) and -resistant strains (W2) of Plasmodium falciparum. Whereas the pure isolates exhibited limited to moderate antiplasmodial activities with compound 4 showing the highest antiplasmodial activities (IC₅₀ values of 4.44 ± 0.88 and 2.74 ± 1.10 μM against D6 and W2 strains, respectively). These results suggest a possible contribution of phytochemicals from C. volkensii stem bark towards inhibition of plasmodial parasites’ growth hence potential antimalarial.     

Thiadiazole derivatives as New Class of β-glucuronidase inhibitors

Thiadiazole derivatives 1–24 were synthesized via a single step reaction and screened for in vitro β-glucuronidase inhibitory activity. All the synthetic compounds displayed good inhibitory activity in the range of IC₅₀ = 2.16 ± 0.01–58.06 ± 1.60 μM as compare to standard d-saccharic acid 1,4-lactone (IC₅₀ = 48.4 ± 1.25 μM). Molecular docking study was conducted in order to establish the structure–activity relationship (SAR) which demonstrated that thiadiazole as well as both aryl moieties (aryl and N-aryl) involved to exhibit the inhibitory potential. All the synthetic compounds were characterized by spectroscopic techniques ¹H, ¹³C NMR, and EIMS.     

2-Hexadecynoic acid inhibits plasmodial FAS-II enzymes and arrests erythrocytic and liver stage Plasmodium infections

Acetylenic fatty acids are known to display several biological activities, but their antimalarial activity has remained unexplored. In this study, we synthesized the 2-, 5-, 6-, and 9-hexadecynoic acids (HDAs) and evaluated their in vitro activity against erythrocytic (blood) stages of Plasmodium falciparum and liver stages of Plasmodium yoelii infections. Since the type II fatty acid biosynthesis pathway (PfFAS-II) has recently been shown to be indispensable for liver stage malaria parasites, the inhibitory potential of the HDAs against multiple P. falciparum FAS-II (PfFAS-II) elongation enzymes was also evaluated. The highest antiplasmodial activity against blood stages of P. falciparum was displayed by 5-HDA (IC₅₀ value 6.6 μg/ml), whereas the 2-HDA was the only acid arresting the growth of liver stage P. yoelii infection, in both flow cytometric assay (IC₅₀ value 2-HDA 15.3 μg/ml, control drug atovaquone 2.5 ng/ml) and immunofluorescence analysis (IC₅₀ 2-HDA 4.88 μg/ml, control drug atovaquone 0.37 ng/ml). 2-HDA showed the best inhibitory activity against the PfFAS-II enzymes PfFabI and PfFabZ with IC₅₀ values of 0.38 and 0.58 μg/ml (IC₅₀ control drugs 14 and 30 ng/ml), respectively. Enzyme kinetics and molecular modeling studies revealed valuable insights into the binding mechanism of 2-HDA on the target enzymes. All HDAs showed in vitro activity against Trypanosoma brucei rhodesiense (IC₅₀ values 3.7–31.7 μg/ml), Trypanosoma cruzi (only 2-HDA, IC₅₀ 20.2 μg/ml), and Leishmania donovani (IC₅₀ values 4.1–13.4 μg/ml) with generally low or no significant toxicity on mammalian cells. This is the first study to indicate therapeutic potential of HDAs against various parasitic protozoa. It also points out that the malarial liver stage growth inhibitory effect of the 2-HDA may be promoted via PfFAS-II enzymes. The lack of cytotoxicity, lipophilic nature, and calculated pharmacokinetic properties suggests that 2-HDA could be a useful compound to study the interaction of fatty acids with these key P. falciparum enzymes.     

3-Oxo-14α,15α-epoxyschizozygine: A new schizozygane indoline alkaloid from Schizozygia coffaeoides

The stem bark extract of Schizozygia coffaeoides (Apocynaceae) showed moderate antiplasmodial activity (IC₅₀ = 8–12 μg/mL) against the chloroquine-sensitive (D6) and chloroquine-resistant (W2) strains of Plasmodium falciparum. Chromatographic separation of the extract led to the isolation of a new schizozygane indoline alkaloid, named 3-oxo-14α,15α-epoxyschizozygine. In addition, two dimeric anthraquinones, cassiamin A and cassiamin B, were identified for the first time in the family Apocynaceae. The structures of the isolated compounds were deduced on the basis of spectroscopic evidence. The schizozygane indole alkaloids showed good to moderate antiplasmodial activities (IC₅₀ = 13–52 μМ).     

Search for new pharmacophores for antimalarial activity. Part II: Synthesis and antimalarial activity of new 6-ureido-4-anilinoquinazolines

Synthesis of new 6-ureido-4-anilinoquinazolines have been accomplished and their in vitro antimalarial activity against chloroquine-sensitive P. falciparum have been examined. Out of 64 compounds evaluated, the IC₅₀ of 16 compounds which have displayed MIC of 0.25 μg/mL were also recorded. One of the compounds (24g) had IC₅₀ value of 2.27 ng/mL which was equipotent to the standard drug chloroquine used in the bioassay. The in vivo evaluation of a few compounds among the series led to discovery of one analog (30g) displaying 40% curative activity (28 days) against mdr P. yoeilli nigeriensis at an oral dose of 100 mg/kg × 4days.     

Novel artemisinin derivatives with potential usefulness against liver/colon cancer and viral hepatitis

Antitumor and antiviral properties of the antimalaria drug artemisinin from Artemisia annua have been reported. Novel artemisinin derivatives (AD1–AD8) have been synthesized and evaluated using in vitro models of liver/colon cancer and viral hepatitis B and C. Cell viability assays after treating human cell lines from hepatoblastoma (HepG2), hepatocarcinoma (SK-HEP-1), and colon adenocarcinoma (LS174T) with AD1–AD8 for a short (6 h) and long (72 h) period revealed that AD5 combined low acute toxicity together with high antiproliferative effect (IC₅₀ = 1–5 μM). Since iron-mediated activation of peroxide bond is involved in artemisinin antimalarial activity, the effect of iron(II)-glycine sulfate (ferrosanol) and iron(III)-containing protoporphyrin IX (hemin) was investigated. Ferrosanol, but not hemin, enhanced antiproliferative activity of AD5 if the cells were preloaded with AD5, but not if both compouds were added together. Five derivatives (AD1 > AD2 > AD7 > AD3 > AD8) were able to inhibit the cytopathic effect of bovine viral diarrhoea virus (BVDV), a surrogate in vitro model of hepatitis C virus (HCV), used here to evaluate the anti-Flaviviridae activity. Moreover, AD1 and AD2 inhibited the release of BVDV-RNA to the culture medium. Co-treatment with hemin or ferrosanol resulted in enhanced anti-Flaviviridae activity of AD1. In HepG2 cells permanently infected with hepatitis B virus (HBV), AD1 and AD4, at non-toxic concentrations for the host cells were able to reduce the release of HBV-DNA to the medium. In conclusion, high pharmacological interest deserving further evaluation in animal models has been identified for novel artemisinin-related drugs potentially useful for the treatment of liver cancer and viral hepatitis B and C.     

Isolation of antiplasmodial anthraquinones from Kniphofia ensifolia,and synthesis and structure–activity relationships of related compounds

Bioassay-guided separation of the South African plant Kniphofia ensifolia for antiplasmodial activity led to the isolation of two new anthraquinones, named kniphofiones A and B (3 and 4), together with three known bioactive anthraquinone monomers (1, 2 and 5), and four known bisanthraquinones (6–9). The structures of the two new compounds were elucidated based on analyses of their 1D and 2D NMR spectra and mass spectrometric data. The dimeric compounds 6 and 7 displayed the strongest antiplasmodial activity among all the isolated compounds, with IC₅₀ values of 0.4 ± 0.1 and 0.2 ± 0.1 μM, respectively. The two new compounds displayed modest activities, with IC₅₀ values of 26 ± 4 and 9 ± 1 μM, respectively. Due to the synthetic accessibility of the new compounds and the increased activity shown by the dimeric compounds, a structure–activity relationship study was conducted. As a result, one analogue of kniphofione B (4), the caffeic acid derivative of aloe-emodin, was found to have the highest activity among all the aloe-emodin derivatives, with an IC₅₀ value of 1.3 ± 0.2 μM.     

Synthesis and biological evaluation of new bis-indolone-N-oxides

A series of bis-indolone-N-oxides, 1a–f, was prepared from bis(ethynyl)benzenes and o-halonitroaryls and studied for their in vitro antiplasmodial activities against Plasmodium falciparum and representative strains of bacteria and candida as well as for their cytotoxicity against a human tumor cell line (MCF7). They did not cause any haemolysis (300 μg mL⁻¹). Of the synthesized bis-indolones, compound 1a had the most potent antiplasmodial activity (IC₅₀ = 0.763 μmol L⁻¹ on the FcB1 strain) with a selectivity index (CC₅₀ MCF7/IC₅₀ FcB1) of 35.6. No potency against the tested microbial strains was observed.     

Ramiferin,a bisphenol-sesquiterpene from the fungus Kionochaeta ramifera BCC 7585

A new bisphenol-sesquiterpene, ramiferin, together with two known sesquiterpene derivatives, pughiinin A and eupenifeldin, were isolated from the fungus Kionochaeta ramifera BCC 7585. Ramiferin displayed antimalarial activity with an IC₅₀ value of 6.3 μM, and antitubercular activity with a minimum inhibitory concentration of 12.7 μM. It was toxic against three cancer cell lines (BC, KB, and NCI-H187) and nonmalignant (Vero) cells with respective IC₅₀ values of 9.1, 12.6, 13.0, and 9.7 μM.