Protecting future antimalarials from the trap of resistance: Lessons from artemisinin-based combination therapy (ACT) failures

Unlocking nitrogen compounds' promise against malaria: A comprehensive review

Plasmodium parasites are the primary cause of malaria, leading to high mortality rates, which require clinical attention. Many of the medications used in the treatment have resulted in resistance over time. Artemisinin combination therapy (ACT) has shown significant results for the treatment. However, mutations in the parasite have resulted in resistance, leading to decreased efficiency of the medications that are currently being used. Therefore, there is a critical need to find novel scaffolds that are safe, effective, and of economic advantage. Literature has reported several potent molecules with diverse scaffolds designed, synthesized, and evaluated against different strains of Plasmodium. With this growing list of compounds, it is essential to collect the data in one place to gain a concise overview of the emerging scaffolds in recent years. For this purpose, nitrogen-containing heterocycles such as β-carboline, imidazole, quinazoline, quinoline, thiazole, and thiophene have been highly explored due to their wide biological applications. Besides these, another scaffold, benzodiazepine, which is majorly used as a central nervous system depressant, is emerging as an anti-malarial agent. Hence, this review centers on the latest medication advancements designed to combat malaria, emphasizing special attention to 1,4-benzodiazepines as a novel scaffold for antimalarial drug discovery.

In vitro and in vivo antiplasmodial activity of a synthetic dihydroartemisinin-eosin B hybrid

With the inexorable prevalence and spread of drug-resistant malaria strains, many efforts have been made to find alternative chemotherapeutic agents. In this regard, scientists have developed the concept of hybridization of two or more active pharmacophores into a single chemical compound, resulting in "antimalarial hybrids." The aim of this study was planned based on the highly synergistic effect of the physical hybrid of dihydroartemisinin (DHA) with eosin B (EB). Therefore, a chemical hybrid of the two compounds (DHA-EB) was synthesized, and its antimalarial activity was investigated in vitro and in vivo. The drug hybrid was fabricated through a propionyl ester linker between DHA and EB. The antiplasmodial activity of the new hybrid was tested in vitro on the blood stages of Plasmodium falciparum (chloroquine-sensitive, 3D7 strain) and also evaluated in vivo by Peters' standard test in mice infected with Plasmodium berghei. The hybrid compound was also assessed for in vivo toxicity. Among all the compounds studied, a DHA-EB hybrid showed an appropriate inhibition percentage (53%) was at a very low dose (0.65 nM). The highest in vivo antimalarial activity until the 9th day was related to DHA-EB in a low dose (0.5 mg/kg). Also, the most survival rate was observed in the test group of hybrid compound at a dose of 1.5 mg/kg for 22 days. No external changes were identified in the toxicity assay. The weight of internal organs of treated animals and that of controls indicated nontoxicity of DHA-EB even after 60 days of consumption. In vitro and in vivo studies substantiated that DHA-EB hybrid has the potential for developing as a safe antimalarial drug.

Novel quinolinepiperazinyl-aryltetrazoles targeting the blood stage of Plasmodium falciparum

The emergence of drug resistance against the frontline antimalarials is a major challenge in the treatment of malaria. In view of emerging reports on drug-resistant strains of Plasmodium against artemisinin combination therapy, a dire need is felt for the discovery of novel compounds acting against novel targets in the parasite. In this study, we identified a novel series of quinolinepiperazinyl-aryltetrazoles (QPTs) targeting the blood stage of Plasmodium. In vitro anti-plasmodial activity screening revealed that most of the compounds showed IC50 < 10 μM against chloroquine-resistant PfINDO strain, with the most promising lead compounds 66 and 75 showing IC50 values of 2.25 and 1.79 μM, respectively. Further, compounds 64-66, 68, 75-77 and 84 were found to be selective (selectivity index >50) in their action against Pf over a mammalian cell line, with compounds 66 and 75 offering the highest selectivity indexes of 178 and 223, respectively. Explorations into the action of lead compounds 66 and 75 revealed their selective cidal activity towards trophozoites and schizonts. In a ring-stage survival assay, 75 showed cidal activity against the early rings of artemisinin-resistant PfCam3.1R539T. Further, 66 and 75 in combination with artemisinin and pyrimethamine showed additive to weak synergistic interactions. Of these two in vitro lead molecules, only 66 restricted rise in the percentage of parasitemia to about 10% in P. berghei-infected mice with a median survival time of 28 days as compared to the untreated control, which showed the percentage of parasitemia >30%, and a median survival of 20 days. Promising antimalarial activity, high selectivity, and additive interaction with artemisinin and pyrimethamine indicate the potential of these compounds to be further optimized chemically as future drug candidates against malaria.

Annickia affinis (Exell) Versteegh & Sosef methanol stem bark extract, potent fractions and isolated Berberine alkaloid target both blood and liver stages of malaria parasites

ETHNOPHARMACOLOGICAL RELEVANCE

Having identified Annickia affinis as the most potent antiplasmodial plant constituent in a hepta-herbal Agbo-iba (HHA) formula commonly used to manage malaria in Benin city, Nigeria, we have in this study attempted to identify the specialized metabolites responsible for antiplasmodial activity of A. affinis through anti-blood stage malaria parasite activity guided isolation of potent molecules from its stem bark methanol extract. After that, phenotypic effects, including stage-specific kill kinetics, were investigated. Further, the crude extract, its potent fractions, and specialized metabolites were also tested against the liver-stage malaria parasite.

MATERIALS AND METHODS

A. affinis was subjected to molecular PCR-based analysis to confirm its identity. Thereafter, extraction of its stem bark with methanol was carried out. Alkaloid enriched fractions from this stem bark extract were obtained using the acid-base-solvent extraction method. These alkaloid-enriched fractions were subjected to various chromatographic techniques that led to the isolation of two protoberberine alkaloids identified as berberine and palmatine based on NMR and mass spectrometry analysis. The efficacy of crude extract, fractions and purified alkaloids was tested against the malaria parasite's blood and liver stages, respectively.

RESULTS AND DISCUSSION

Annickia affinis methanol extract, fractions, and the isolated protoberberine alkaloids showed excellent antiplasmodial activity with good selectivity against blood-stage malaria parasite. Thus, their IC50 against various strains of the parasite ranged from 0.95 to 18.65 μg/ml, while CC50 against Human embryonic kidney (HEK) and the human hepatoma (HUH-7) cell lines ranged between 10 and > 100 μg/ml. Interestingly, the crude extract and the alkaloid enriched fractions showed promising activity against the liver-stage malaria parasite. Between berberine and palmatine isolated from the potent fractions, only the former showed ∼100% and 90% inhibitions of liver stage parasite at 5 μg/ml and 1 μg/ml, respectively, while the latter showed no inhibition even at 20 μg/ml.

CONCLUSION

This study reports that the ethnomedicinal use of HHA to manage malaria can be attributed to the presence of promising antiplasmodial protoberberine alkaloids together with synergistic effects via either enhancement of bioavailability or improved pharmacokinetics by other phytoconstituent(s) coming from other HHA constituent plants. The protoberberine alkaloids isolated have been identified as fast-acting antiplasmodial agents, with activity against all erythrocytic stages of the malaria parasite. Further, A. affinis methanol stembark extract and the protoberberine alkaloid berberine isolated from it also displayed excellent activity (>90% inhibition at 1 μg/ml) against the liver-stage malaria parasite. A. affinis and HHA can thus be useful as both liver-stage prophylactic and blood-stage curative agents.

Efficacies and ADME properties of redox active methylene blue and phenoxazine analogues for use in new antimalarial triple drug combinations with amino-artemisinins

Efforts to develop new artemisinin triple combination therapies effective against artemisinin-tolerant strains of Plasmodium falciparum based on rational combinations comprising artemisone or other amino-artemisinins, a redox active drug and a third drug with a different mode of action have now been extended to evaluation of three potential redox partners. These are the diethyl analogue AD01 of methylene blue (MB), the benzo [α]phenoxazine PhX6, and the thiosemicarbazone DpNEt. IC50 values in vitro against CQ-sensitive and resistant P. falciparum strains ranged from 11.9 nM for AD01-41.8 nM for PhX6. PhX6 possessed the most favourable pharmacokinetic (PK) profile: intrinsic clearance rate CLint was 21.47 ± 1.76 mL/min/kg, bioavailability was 60% and half-life was 7.96 h. AD01 presented weaker, but manageable pharmacokinetic properties with a rapid CLint of 74.41 ± 6.68 mL/min/kg leading to a half-life of 2.51 ± 0.07 h and bioavailability of 15%. DpNEt exhibited a half-life of 1.12 h and bioavailability of 8%, data which discourage its further examination, despite a low CLint of 10.20 mL/min/kg and a high Cmax of 6.32 µM. Efficacies of AD01 and PhX6 were enhanced synergistically when each was paired with artemisone against asexual blood stages of P. falciparum NF54 in vitro. The favourable pharmacokinetics of PhX6 indicate this is the best partner among the compounds examined thus far for artemisone. Future work will focus on extending the drug combination studies to artemiside in vitro, and conducting efficacy studies in vivo for artemisone with each of PhX6 and the related benzo[α]phenoxazine SSJ-183.

The antimalarial activity of 1,2,4-trioxolane/trioxane hybrids and dimers: A review

Malaria, a mosquito-borne parasitic infection caused by protozoan parasites belonging to the genus Plasmodium, is a dangerous disease that contributes to millions of hospital visits and hundreds and thousands of deaths across the world, especially in Sub-Saharan Africa. Antimalarial agents are vital for treating malaria and controlling transmission, and 1,2,4-trioxolane/trioxane-containing agents, especially artemisinin and its derivatives, own antimalarial efficacy and low toxicity with unique mechanisms of action. Moreover, artemisinin-based combination therapies were recommended by the World Health Organization as the first-line treatment for uncomplicated malaria infection and have remained as the mainstay of the treatment of malaria, demonstrating that 1,2,4-trioxolane/trioxane derivatives are useful prototypes for the control and eradication of malaria. However, malaria parasites have already developed resistance to almost all of the currently available antimalarial agents, creating an urgent need for the search of novel pharmaceutical interventions for malaria. The purpose of this review article is to provide an emphasis on the current scenario (January 2012 to January 2022) of 1,2,4-trioxolane/trioxane hybrids and dimers with potential antimalarial activity and the structure-activity relationships are also discussed to facilitate further rational design of more effective candidates.

Specific sub fractions from Terminalia mantaly (H. Perrier) extracts potently inhibit Plasmodium falciparum rings, merozoite egress and invasion

ETHNOPHARMACOLOGICAL RELEVANCE

Terminalia mantaly (H. Perrier) and Terminalia superba (Engl. & Diels) are sources of treatment for various diseases, including malaria and/or related symptoms in parts of Southwestern Cameroon. However, there is limited information on the extent of the antiplasmodial potential of their extracts.

AIM OF THE STUDY

The present study was designed to investigate the antiplasmodial potential of chromatographic sub fractions (SFs) from promising fractions of Terminalia mantaly (Tm) [Tmsb^wChl, the chloroform fraction from water extract of Tm, IC₅₀ (μg/mL) PfINDO: 0.56, Pf3D7: 1.12; SI > 357 (HEK/PfINDO) & 178 (HEK/Pf3D7)] and Terminalia superba (Ts) [Tsr^mEA, the ethyl acetate fraction from methanolic extract of Ts, IC₅₀ (μg/mL) PfINDO: 1.82, Pf3D7: 1.65; SI > 109 (HEK/PfINDO) & 121 (HEK/Pf3D7)] obtained from previous studies. The SFs were tested against Plasmodium falciparum 3D7 (Pf3D7-chloroquine sensitive) and INDO (PfINDO-chloroquine resistant) strains in culture. Also, the phytochemical profile of potent SFs was determined and finally, the inhibition of the asexual blood stages of Plasmodium falciparum by the SFs with the highest promise was assessed.

MATERIAL AND METHODS

Selected SFs were submitted to a second bio-guided fractionation using silica gel column chromatography. The partial phytochemical composition of potent antiplasmodial SFs was determined using gas chromatography coupled to mass spectrometry (GC-MS). The SYBR Green I-based fluorescence microtiter plate assay was used to monitor the growth of Plasmodium falciparum parasites in culture in the presence or absence of extracts. Microscopy and flow cytometry counting was used to assess the Plasmodium falciparum stage-specific inhibition and post-drug exposure growth suppression by highly potent extracts.

RESULTS

Twenty-one of the 39 SFs afforded from Tmsb^wChl showed activity (IC₅₀: 0.29-4.74 μg/mL) against both Pf3D7 and PfINDO strains. Of note, eight SFs namely, Tm25, Tm28-30, Tm34-36 and Tm38, exerted highly potent antiplasmodial activity (IC₅₀ < 1 μg/mL) with IC₅₀PfINDO: 0.41-0.84 μg/mL and IC₅₀Pf3D7: 0.29-0.68 μg/mL. They also displayed very high selectivity (50 < SI_PfINDO, SI_Pf3D7 > 344) on the two Plasmodial strains. On the other hand, 7 SFs (SFs Ts03, Ts04, Ts06, Ts09, Ts10, Ts12 and Ts13) from Tsr^mEA showed promising inhibitory potential against both parasite strains (IC₅₀: 2.01-5.14 μg/mL). Sub fraction Tm36 (IC₅₀PfINDO: 0.41 μg/mL, SI_PfINDO > 243; IC₅₀Pf3D7: 0.29 μg/mL, SI_Pf3D7 > 344) showed the highest promise. The GC-MS analysis of the 8 selected SFs led to the identification of 99 phytometabolites, with D-limonene (2), benzaldehyde (12), carvone (13), caryophyllene (35), hexadecanoic acid, methyl ester (74) and 9-octadecenoic acid, methyl ester (82) being the main constituents. Sub fractions Tm28, Tm29, Tm30, Tm36 and Tm38 inhibited all the three intraerythrocytic stages of P. falciparum, with strong potency against ring stage development, merozoite egress and invasion processes.

CONCLUSIONS

This study has identified highly potent antiplasmodial SFs from Terminalia mantaly with significant activity on the intraerythrocytic development of Plasmodium falciparum. These SFs qualify as promising sources of novel antiplasmodial lead compounds. Further purification and characterization studies are expected to unravel molecular targets in rings and merozoites.