Potent antimalarial activity of newly synthesized substituted chalcone analogs in vitro

Abstract   Several new chalcone analogues were synthesized and evaluated as inhibitors of malaria parasite. Inhibitory activity was determined in vitro against a chloroquine-sensitive Plasmodium falciparum strain of parasites. The compound 3-(4-methoxyphenyl)-1-(4-pyrrol-1-yl-phenyl)prop-2-en-1-one was found to be the most active with 50% inhibition concentration (IC₅₀) of 1.61 μg/ml. This inhibitory concentration is comparable to a prototype phytochemical chalcone, licochalcone A, with an IC₅₀ of 1.43 μg/ml. The present study suggests that small, lipophilic nitrogen heterocyclic ring A together with small hydrophobic functionality at ring B can enhance antimalarial activity. These results suggest that chalcones are a class of compounds that provides an option of developing inexpensive, synthetic therapeutic antimalarial agents in the future. Graphical Abstract  Claisen-Schmidt condensation method was employed to synthesize various substituted chalcones. Among all, 3-(4-Methoxyphenyl)-1-(4-pyrrol-1-yl-phenyl)prop-2-en-1-one was found to be most effective with IC₅₀ value of 1.6 μg/ml in vitro against chloroquine sensitive strain (3D7) of Plasmodium falciparum.      

The safety of amodiaquine use in pregnant women

Few antimalarial drugs have been evaluated extensively in pregnancy because of fears over toxicity. However, increasing Plasmodium falciparum resistance to chloroquine and sulfadoxine–pyrimethamine makes finding alternative antimalarials that are safe and effective in pregnancy a priority. There is a renewed interest in amodiaquine as a potential candidate, particularly as a partner drug in artemisinin-based combination therapy. The available data suggest that, at standard dosages, amodiaquine is not teratogenic and that the adverse events associated with taking amodiaquine in pregnancy are not greater than those associated with falciparum malaria in pregnancy. Thus, amodiaquine in combination with other antimalarial drugs may be useful for malaria treatment in pregnancy, but inadequate data on its safety and pharmacokinetics in pregnancy limits its deployment for intermittent preventive treatment in pregnancy.     

Synthesis and antimalarial activity of dihydroperoxides and tetraoxanes conjugated with bis(benzyl)acetone derivatives

Dihydroperoxides and tetraoxanes derived from symmetrically substituted bis(arylmethyl)acetones were synthesized in modest to good yields using several methods. Three of these compounds exhibit an important in vitro antimalarial activity (1.0 μm ≤ IC₅₀ ≤ 5.0 μm ) against blood forms of the human malaria parasite Plasmodium falciparum.     

青蒿素类似物的研究——Ⅰ、还原青蒿素的醚类、羧酸酯类及碳酸酯类衍生物的合成

青蒿素是当前治疗抗恶性疟疾的首选药物。为克服它的近期复燃率高的缺点,我们以还原青蒿素为中间体,在酸或碱的催化下与各种醇、羧酸酐或酰氯、氯甲酸酯反应,合成了它的醚类,羧酸酯类和碳酸酯类衍生物47个。经鼠疟(P.berghei)抗氯喹原虫株筛选,发现其中大多数化合物的抗疟效果超过青蒿素;以SD₉₀)作比较标准,超过青蒿素十倍左右的化合物有12个。     

New Compounds Hybrids 1H‐1,2,3‐Triazole‐Quinoline Against Plasmodium falciparum

Malaria is one of the most prevalent parasitic diseases in the world. The global importance of this disease, current vector control limitations, and the absence of an effective vaccine make the use of therapeutic antimalarial drugs the main strategy to control malaria. Chloroquine is a cost‐effective antimalarial drug with a relatively robust safety profile, or therapeutic index. However, chloroquine is no longer used alone to treat patients with Plasmodium falciparum due to the emergence and spread of chloroquine‐resistant strains, which have also been reported for Plasmodium vivax. However, the activity of 1,2,3‐triazole derivatives against chloroquine‐sensitive and chloroquine‐resistant strains of P. falciparum has been reported in the literature. To enhance the anti‐P. falciparum activity of quinoline derivatives, we synthesized 11 new quinoline‐1H‐1,2,3‐triazole hybrids with different substituents in the 4‐positions of the 1H‐1,2,3‐triazole ring, which were assayed against the W2‐chloroquine‐resistant P. falciparum clone. Six compounds exhibited activity against the P. falciparum W2 clone, chloroquine‐resistant, with IC₅₀ values ranging from 1.4 to 46 μm . None of these compounds was toxic to a normal monkey kidney cell line, thus exhibiting good selectivity indexes, as high 351 for one compound ( 11 ).     

gamma-Substituted bis(pivaloyloxymethyl)ester analogues of fosmidomycin and FR900098

The synthesis and in-vitro antimalarial activity of gamma-substituted bis(pivaloyloxymethyl)ester analogues of the drug candidate fosmidomycin have been investigated. In contrast to the high antimalarial activity of alpha-aryl substituted fosmidomycin analogues like alpha-phenylfosmidomycin, gamma-substituted derivatives display only weak to moderate activity against the chloroquine-sensitive strain 3D7 of Plasmodium falciparum.     

Synthesis of thiazolyl hydrazonothiazolamines and 1,3,4‐thiadiazinyl hydrazonothiazolamines as a class of antimalarial agents

Novel thiazolyl hydrazonothiazolamines and 1,3,4‐thiadiazinyl hydrazonothiazolamines were synthesized by a facile one‐pot multicomponent approach by the reaction of 2‐amino‐4‐methyl‐5‐acetylthiazole, thiosemicarbazide or thiocarbohydrazide and phenacyl bromides or 3‐(2‐bromoacetyl)‐2H‐chromen‐2‐ones in acetic acid with good to excellent yields. These new compounds were screened in vitro for their antimalarial activity; among them, four compounds, 4h, 4i, 4k, 4l , showed moderate activity with half‐maximal inhibitory concentration (IC₅₀) values of 3.2, 2.7, 2.7, and 2.8 and 3.2, 3.2, 3.1, and 3.5 μM against chloroquine‐sensitive and ‐resistant strains of Plasmodium falciparum, respectively. Compound 4l inhibited the ring stage growth of P. falciparum 3D7 at an IC₉₀ concentration of 12.5 µM in a stage‐specific assay method, where the culture is incubated with specific stages of P. falciparum for 12 hr, and no activity was found against the trophozoite and schizont stages, confirming that 4l may have potent action against the ring stage of P. falciparum.     

A medicinal chemistry perspective on 4-aminoquinoline antimalarial drugs

A broad overview is presented describing the current knowledge and the ongoing research concerning the 4-aminoquinolines (4AQ) as chemotherapeutic antimalarial agents. Included are discussions of mechanism of action, structure activity relationships (SAR), chemistry, metabolism and toxicity and parasite resistance mechanisms. In discussions of SAR, particular emphasis has been given to activity versus chloroquine resistant strains of Plasmodium falciparum. Promising new lead compounds undergoing development are described and an overview of physicochemical properties of chloroquine and amodiaquine analogues is also included.     

Design,synthesis, and antiprotozoal evaluation of new 2,9‐bis[(substituted‐aminomethyl)phenyl]‐1,10‐phenanthroline derivatives

A series of new 2,9‐bis[(substituted‐aminomethyl)phenyl]‐1,10‐phenanthroline derivatives was synthesized, and the compounds were screened in vitro against three protozoan parasites (Plasmodium falciparum, Leishmania donovani, and Trypanosoma brucei brucei). Biological results showed antiparasitic activity with IC₅₀ values in the μm range. The in vitro cytotoxicity of these molecules was assessed by incubation with human HepG2 cells; for some derivatives, cytotoxicity was observed at significantly higher concentrations than antiparasitic activity. The 2,9‐bis[(substituted‐aminomethyl)phenyl]‐1,10‐phenanthroline 1h was identified as the most potent antimalarial candidate with ratios of cytotoxic‐to‐antiparasitic activities of 107 and 39 against a chloroquine‐sensitive and a chloroquine‐resistant strain of P. falciparum, respectively. As the telomeres of the parasite P. falciparum are the likely target of this compound, we investigated stabilization of the Plasmodium telomeric G‐quadruplexes by our phenanthroline derivatives through a FRET melting assay. The ligands 1f and 1m were noticed to be more specific for FPf8T with higher stabilization for FPf8T than for the human F21T sequence.     

Antimalarial effect of N-acetyl-L-Leucyl-L-leucyl-L-norleucinal by the inhibition of Plasmodium falciparum calpain

The biological understanding of malaria parasites has increased considerably over the past two decades with the discovery of many potential targets for the development of new antimalarial drugs. Calpain, a cysteine protease of Plasmodium falciparum, is believed to be a central mediator essential for parasitic activity. However, the utility of calpain as a potential anti-malarial target in P. falciparum has not been fully determined. In the present study, we determined the effect of N-acetyl-L-Leucyl-L-leucyl-L-norleucinal (ALLN)-treatment on the expression of calpain in erythrocytic stages of P. falciparum and its usefulness as an antimalarial chemotherapeutic agent. ALLN was shown to have low toxicity to HeLa cells but high toxicity to malaria. ALLN inhibited the expression of calpain in ring, trophozoite and schizont stages when treated for 48 h. Also, after 48 h, samples were characterized by 6.15% and 0% parasitemia without ALLN treatment and with ALLN treatment, respectively. Brightfield and confocal microscopy revealed that ALLN treatment affects merozoite maturation. As ALLN concentration increased from 1 μM to 100 μM, ring stage parasites did not mature into the schizont stage. When ALLN treatment was continued for 48 h, it also significantly inhibited the maturation of ring-stage parasites into trophozoite or schizont stages and survival of malarial parasites. Taken together, these findings suggest that ALLN inhibit the maturation and survival of P. falciparum and calpain expression, and thus has potential utility as an antimalarial chemotherapeutic agent. These two authors made equal contribution to this work.     

Antimalarial activity of newly synthesized chalcone derivatives in vitro

Twenty‐seven novel chalcone derivatives were synthesized using Claisen‐Schmidt condensation and their antimalarial activity against asexual blood stages of Plasmodium falciparum was determined. Antiplasmodial IC₅₀ (half‐maximal inhibitory concentration) activity of a compound against malaria parasites in vitro provides a good first screen for identifying the antimalarial potential of the compound. The most active compound was 1‐(4‐benzimidazol‐1‐yl‐phenyl)‐3‐(2, 4‐dimethoxy‐phenyl)‐propen‐1‐one with IC₅₀ of 1.1 μg/mL, while that of the natural phytochemical, licochalcone A is 1.43 μg/mL. The presence of methoxy groups at position 2 and 4 in chalcone derivatives appeared to be favorable for antimalarial activity as compared to other methoxy‐substituted chalcones. Furthermore, 3, 4, 5‐trimethoxy groups on chalcone derivative probably cause steric hindrance in binding to the active site of cysteine protease enzyme, explaining the relative lower inhibitory activity.     

Synthesis and structure-activity relationships of 4-pyridones as potential antimalarials

A series of diaryl ether substituted 4-pyridones have been identified as having potent antimalarial activity superior to that of chloroquine against Plasmodium falciparum in vitro and murine Plasmodium yoelii in vivo. These were derived from the anticoccidial drug clopidol through a systematic study of the effects of varying the side chain on activity. Relative to clopidol the most active compounds show >500-fold improvement in IC50 for inhibition of P. falciparum in vitro and about 100-fold improvement with respect to ED50 against P. yoelii in mice. These compounds have been shown elsewhere to act selectively by inhibition of mitochondrial electron transport at the cytochrome bc1 complex.     

Isoquine and related amodiaquine analogues: a new generation of improved 4-aminoquinoline antimalarials

Amodiaquine (AQ) (2) is a 4-aminoquinoline antimalarial that can cause adverse side effects including agranulocytosis and liver damage. The observed drug toxicity is believed to involve the formation of an electrophilic metabolite, amodiaquine quinoneimine (AQQI), which can bind to cellular macromolecules and initiate hypersensitivity reactions. We proposed that interchange of the 3' hydroxyl and the 4' Mannich side-chain function of amodiaquine would provide a new series of analogues that cannot form toxic quinoneimine metabolites via cytochrome P450-mediated metabolism. By a simple two-step procedure, 10 isomeric amodiaquine analogues were prepared and subsequently examined against the chloroquine resistant K1 and sensitive HB3 strains of Plasmodium falciparum in vitro. Several analogues displayed potent antimalarial activity against both strains. On the basis of the results of in vitro testing, isoquine (ISQ1 (3a)) (IC(50) = 6.01 nM +/- 8.0 versus K1 strain), the direct isomer of amodiaquine, was selected for in vivo antimalarial assessment. The potent in vitro antimalarial activity of isoquine was translated into excellent oral in vivo ED(50) activity of 1.6 and 3.7 mg/kg against the P. yoelii NS strain compared to 7.9 and 7.4 mg/kg for amodiaquine. Subsequent metabolism studies in the rat model demonstrated that isoquine does not undergo in vivo bioactivation, as evidenced by the complete lack of glutathione metabolites in bile. In sharp contrast to amodiaquine, isoquine (and Phase I metabolites) undergoes clearance by Phase II glucuronidation. On the basis of these promising initial studies, isoquine (ISQ1 (3a)) represents a new second generation lead worthy of further investigation as a cost-effective and potentially safer alternative to amodiaquine.     

Design,synthesis, and evaluation of new chemosensitizers in multi-drug-resistant Plasmodium falciparum

A series of new chemosensitizers (modulators) against chloroquine-resistant Plasmodium falciparum were designed and synthesized in an attempt to fabricate modulators with enhancing drug-resistant reversing efficacy and minimal side effects. Four aromatic amine ring systems-phenothiazine, iminodibenzyl, iminostilbene, and diphenylamine-were examined. Various tertiary amino groups including either noncyclic or cyclic aliphatic amines were introduced to explore the steric tolerance at the end of the side chain. The new compounds showed better drug-resistant reversing activity in chloroquine-resistant than in mefloquine-resistant cell lines and were generally more effective against chloroquine-resistant P. falciparum isolates from Southeast Asian (W2 and TM91C235) than those from South America (PC49 and RCS). Structure-activity relationship studies revealed that elongation of the alkyl side chain of the molecule retained the chemosensitizing activity, and analogues with four-carbon side chains showed superior activity. Furthermore, new modulators with phenothiazine ring exhibited the best chemosensitizing activity among the four different ring systems examined. Terminal amino function has limited steric tolerance as evidenced by the dramatic lose of the modulating activity, when the size of substituent at the amino group increases. The best new modulator synthesized in this study possesses all three optimized structural features, which consist of a phenothiazine ring and a pyrrolidinyl group joined by a four-carbon alkyl bridge. The fractional inhibitory concentration (FIC) index of the best compound is 0.21, which is superior to that of verapamil (0.51), one of the best-known multi-drug-resistant reversing agents. Some of the analogues displayed moderate intrinsic in vitro antimalarial activity against a W-2 clone of P. falciparum.     

The therapeutic potential of semi-synthetic artemisinin and synthetic endoperoxide antimalarial agents

Artemisinin derivatives such as artesunate, dihydroartemisinin and artemether are playing an increasing role in the treatment of drug-resistant malaria. They are the most potent antimalarials available, rapidly killing all asexual stages of the parasite Plasmodium falciparum. This review highlights the recent developments in the area of improved second-generation semi-synthetic artemisinin derivatives and fully synthetic antimalarial endoperoxide drugs. In pursuit of synthetic analogues of the artemisinins, one of the major challenges for chemists in this area has been the non-trivial development of techniques for the introduction of the peroxide bridge into candidate drugs. Although chemical research has enabled chemists to incorporate the endoperoxide ‘warhead’ into synthetic analogues of artemisinin, significant drawbacks with many candidates have included comparatively poor antimalarial activity, non-stereoselective syntheses and chemical approaches that are not readily amenable to scale up. However, very recent progress with synthetic 1,2,4-trioxolanes provides a new benchmark for future medicinal chemistry efforts in this area.     

Anti‐African trypanocidal and antimalarial activity of natural flavonoids,dibenzoylmethanes and synthetic analogues

Objectives The known anti‐protozoal activity of flavonoids has stimulated the testing of other derivatives from natural and synthetic sources. Methods As part of our efforts to find potential lead compounds, a number of flavonoids isolated from Neoraputia paraensis, N. magnifica, Murraya paniculata, (Rutaceae), Lonchocarpus montanus, L. latifolius, L. subglaucescens, L. atropurpureus, L. campestris, Deguelia hatschbachii (Leguminosae), dibenzoylmethanes from L. subglaucescens and synthetic analogues were tested for in‐vitro activity against chloroquine‐sensitive Plasmodium falciparum and Trypanosoma brucei rhodesiense bloodstream form trypomastigotes. An assay withKB cells has been developed inorder tocompare in‐vitro cytotoxicityof flavonoids with a selective action on the parasites. Key findings Thirteen of the compounds tested had IC50 values ranging from 4.6 to 9.9 μM against T. brucei rhodesiense. In contrast, a small number of compounds showed significant activity against P. falciparum; seven of those tested had IC50 values ranging from 2.7 to 9.5 μM. Among the flavones only one had IC50 < 10 μM (7.6 μM), whereas against T. brucei rhodesiense seven had IC50 < 10 μM. Synthetic dibenzoylmethanes were the most active in terms of number (five) of compounds and the IC50 values (2.7–9.5 μM) against P. falciparum. Conclusions Dibenzoylmethanes represent a novel class of compounds tested for the first time as antimalarial and trypanocidal agents.     

Sulfur-interrupted 8-amino side chain analogues of 4-methyl-5-[m-(trifluoromethyl)phenoxy]primaquine as potential antimalarial agents

Two isomeric sulfur-interrupted 8-amino side chain analogues of 4-methyl-5-[m-(trifluoromethyl)phenoxy]primaquine (2) were prepared and tested for antimalarial activity. The compounds were evaluated for blood schizonticidal activity against Plasmodium berghei in mice and radical curative activity against Plasmodium cynomolgi in rhesus monkeys. In addition, they were evaluated for causal prophylactic activity against Plasmodium berghei yoelii in mice. Both compounds were more active and less toxic than primaquine in the P. berghei screen. One of the compounds showed radical curative activity similar to primaquine but was less active than 2. One of the compounds was active at 160 mg/kg in the P. berghei yoelii screen; the other was not active.     

Design and Synthesis of a New Class of 4‐Aminoquinolinyl‐ and 9‐Anilinoacridinyl Schiff Base Hydrazones as Potent Antimalarial Agents

A series of novel 4‐aminoquinolinyl and 9‐anilinoacridinyl Schiff base hydrazones have been synthesized and evaluated for their antimalarial activity. All compounds were evaluated in vitro for their antimalarial activity against chloroquine‐sensitive strain 3D7 and the chloroquine‐resistant K1 strain of Plasmodium falciparum and for cytotoxicity toward Vero cells. Compounds 17 , 20 , and 21 displayed good activity against the 3D7 strain with IC₅₀ values ranging from 19.69 to 25.38 nm . Moreover, compounds 16 , 17 , 21 , 24 , 32, and 33 exhibited excellent activities (21.64–54.26 nm ) against K1 strain and several compounds displayed β‐hematin inhibitory activity, suggesting that they act on the heme crystallization process such as CQ. Compounds were also found to be non‐toxic with good selectivity index.     

Metabolites of febrifugine and its synthetic analogue by mouse liver S9 and their antimalarial activity against Plasmodium malaria parasite

Quinazolinone type alkaloids, febrifugine (1) and isofebrifugine (2), isolated from Dichroa febrifuga roots, show powerful antimalarial activity against Plasmodium falciparum. Unfortunately, their emetic effect and other undesirable side effects have precluded their clinical use for malaria. Because of their antimalarial potency, analogues were searched for, with the goal of preserving the strong antimalarial activity, while dramatically reducing side effects. We expected that compounds useful in drug development would exist in metabolites derived from 1 and Df-1 (3), the condensation product of 1 with acetone, by mouse liver S9. Feb-A and -B (4 and 5) were isolated as the major metabolites of 1. In addition to 4 and 5, feb-C and -D (6 and 7) were also purified from the metabolic mixture of 3. Compounds 4 and 5 were compounds oxidized at C-6 and C-2 of the quinazolinone ring of 1, respectively. Compounds 6 and 7, derived from 3, also bear febrifugine type structures in which the 4' '- and 6' '-positions of the piperidine ring of 1 were oxidized. In vitro antimalarial and cytotoxic tests using synthetically obtained racemic 4-6 and enantiomerically pure 7 demonstrated that 4 and 6 had antimalarial activity against P. falciparum, of similar potency to that of 1, with high selectivity. The antimalarial activity of 5 and 7, however, was dramatically decreased in the test. The in vitro antimalarial activity of analogues 22 and 43, which are stereoisomers of 4 and 6, was also evaluated, showing that 22 is active. The results suggest that basicity of both the 1- and the 1' '-nitrogen atoms of 1 is crucial in conferring powerful antimalarial activity. Racemic 4 and 6 exhibited powerful in vivo antimalarial activity against mouse malaria P. berghei, and especially, no serious side effects were observed with 4. Thus, the metabolite 4 appears to be a promising lead compound for the development of new types of antimalarial drugs.     

Synthesis and in vitro and in vivo antimalarial activity of novel 4-anilinoquinoline Mannich base derivatives

Development of resistance has severely limited the choice of available antimalarial drugs, which clearly highlights the urgent need of novel chemotherapeutic agents for the treatment of malaria. The purpose of this study was to develop new potential antimalarial agents with 4-anilinoquinoline ring. A series of novel 4-anilinoquinoline Mannich base drug molecules have been synthesized. The synthesis involves the preparation of Mannich base and these bases subsequently coupled with 4,7-dichloroquinoline to get targeted drug molecules (Burckhalter et al. in J Am Chem Soc 70:1363–1373, 1948). Their structures were confirmed by IR, NMR, and mass spectral data. The synthesized molecules were evaluated for in vitro and in vivo antimalarial activity against the chloroquine sensitive 3D7 (West Africa) and RKL-2 strain of Plasmodium falciparum and rodent malaria parasite Plasmodium yoelii (strain N-67) in Swiss mice model, respectively (Charmot et al. in Prev Med 15:889, 1986). Except one molecule (containing diphenylamine), all the tested molecules showed activity while one of them (containing morpholine) showed promising in vitro and significant in vivo antimalarial activity under given test conditions.