Recent Advances in the Search for Newer Antimalarial Agents

Oral acute toxicity study and in vivo antimalarial activity of Strychnos lucida R. Br. tablet

ETHNOPHARMACOLOGICAL RELEVANCE

Malaria eradication has been a major goal of the Indonesian government since 2020. Medicinal plants, such as Strychnos lucida R. Br., are empirically used to treat malaria through traditional preparation methods. However, the safety and efficacy of these plants have not yet been confirmed. Therefore, further investigations are necessary to confirm the safety and efficacy of S. lucida as an antimalarial agent.

AIMS OF THE STUDY

To quantify the concentration of brucine in the S. lucida extract, determine the acute oral toxicity of the standardized extract, and evaluate the in vivo antimalarial potency of S. lucida tablet (SLT).

MATERIALS AND METHODS

Acute oral toxicity of S.lucida extract was determined using the Organization for Economic Co-operation and Development 420 procedure, and the analytical method for brucine quantification was validated using high-performance liquid chromatography. In addition, antimalarial activity was determined using the Peter's four-day suppressive method.

RESULTS

Acute toxicity analysis revealed S. lucida as a low-toxicity compound with a cut-off median lethal dose of 2000-5000 mg/kg body weight [BW], which was supported by the hematological and biochemical profiles of the kidneys, liver, and pancreas (p > 0.05). Extract standardization revealed that S. lucida contained 3.91 ± 0.074% w/w brucine, adhering to the limit specified in the Indonesian Herbal Pharmacopeia. Antimalarial test revealed that SLT inhibited the growth of Plasmodium berghei by 27.74-45.27%. Moreover, SLT improved the hemoglobin and hematocrit levels. White blood cell and lymphocyte counts were lower in the SLT-treated group than in the K (+) group (p < 0.05).

CONCLUSION

Histopathological and biochemical evaluations revealed that S. lucida extract was safe at a dose of 2000 mg/kg BW with low toxicity. SLT inhibited Plasmodium growth and improved the hemoglobin, hematocrit, and red blood cell profiles. Additionally, SLT reduced the lymphocyte and WBC counts and increased the monocyte and thrombocyte counts as part of the immune system response against Plasmodium infection.

Facile synthesis of vanillin-based novel bischalcones identifies one that induces apoptosis and displays synergy with Artemisinin in killing chloroquine resistant Plasmodium falciparum

The inherent affinity of natural compounds for biological receptors has been comprehensively exploited with great success for the development of many drugs, including antimalarials. Here the natural flavoring compound vanillin has been used as an economical precursor for the synthesis of a series of novel bischalcones whose in vitro antiplasmodial activities have been evaluated against erythrocytic stages of Plasmodium falciparum. Bischalcones 9, 11 and 13 showed promising antiplasmodial activity {Chloroquine (CQ) sensitive Pf3D7 IC₅₀ (μM): 2.0, 1.5 and 2.5 respectively}but only 13 displayed potent activities also against CQ resistant PfDd2 and PfIndo strains exhibiting resistance indices of 1.4 and 1.5 respectively. IC₉₀ (8 μM) of 13 showed killing activity against ring, trophozoite and schizont stages. Further, 13 initiated the cascade of reactions that culminates in programmed cell death of parasites including translocation of phosphatidylserine from inner to outer membrane leaflet, loss of mitochondrial membrane potential, activation of caspase like enzyme, DNA fragmentation and chromatin condensation. The combinations of 13 + Artemisinin (ART) exhibited strong synergy (ΣFIC₅₀:0.46 to 0.58) while 13 + CQ exhibited mild synergy (ΣFIC₅₀: 0.7 to 0.98) to mild antagonism (ΣFIC₅₀: 1.08 to 1.23) against PfIndo. In contrast, both combinations showed marked antagonism against Pf3D7(ΣFIC₅₀: 1.33 to 3.34). These features of apoptosis and strong synergy with Artemisinin suggest that bischalcones possess promising antimalarial drug-like properties and may also act as a good partner drugs for artemisinin based combination therapies (ACTs) against Chloroquine resistant P. falciparum.

Crystal structures of two N-phenylated tricyclic benzothiazines with antimalarial activity

The crystal structures of two N-phenylated tricyclic benzothiazines were determined 3-amino-7-chloro-9-phenyl-1,9H-pyrazolo-[4,3-b]benzothiazine 4,4-dioxide, C15H11ClN4O2S crystallises in P21/c with a = 11.436 (4)Å, b = 10.894 (3)Å, c = 11.858 (4)Å, β = 95.297 (9)°, V = 1471.0 (8) Å3 and Z = 4, while 2,4-diamino-8-chloro-10H-phenylpyrimido-[5,4-b]benzothiazine 5,5-dioxide, C16H12ClN5O2S crystallises in P21/c with a = 7.496 (2)Å, b = 17.7 28 (4)Å, c = 11.889 (2)Å, (3 = 91.524 (5)°, V = 1579.4 (5)Å3 and Z = 4. Both molecules are essentially planar, including the exocyclic groups. These compounds were promising as inhibitors of hemoglobin hydrolysis, however, their effect as inhibitors of β-hematin formation was marginal. The most active compound to emerge from the in vitro and in vivo murine studies was the pyrimidobezothaizine, suggesting an antimalarial activity via inhibition of haemoglobin hydrolysis, but it was not as efficient as chloroquine.

Synthesis and Crystal Structure of Ethyl 7-Chloro-3-Amino-9-(4-Bromophenyl)Thieno-[3,2-b]Benzothiazine 4,4-Dioxide 2-Carboxylate

The X-ray crystal structure of 7-chloro-3-amino-9-(4-bromophenyl)thieno-[3,2-b]benzothiazine 4,4-dioxide 2-carboxylate 1 is determined. The thiophene ring and the exocyclic ester group adopt a co-planar conformation, which is stabilised by an intramolecular hydrogen bond. The moderate antimalarial activity shown by this compound appears to be related to the formation of such an intramolecular N–H···O =C hydrogen bond.