Antimalarial Activity Of Artemisinin Research Articles

Total Synthesis and Biological Investigation of (-)-Artemisinin: The Antimalarial Activity of Artemisinin Is not Stereospecific.

Here, we describe an efficient and diversity-oriented entry to both (-)-artemisinin (1) and its natural antipode (+)-artemisinin, starting from commercially and readily available S-(+)- and R-(-)-citronellene, respectively. Subsequently, we answered the still open question regarding the specificity of artemisinins action. By using a drug-sensitive Plasmodium falciparum NF54 strain, we showed that the antimalarial activity of artemisinin is not stereospecific. Our straightforward and biomimetic approach to this natural endoperoxide enables the synthesis of artemisinin derivatives that are not accessible through applying current methods and may help to address the problem of emerging resistance of Plasmodium falciparum towards artemisinin.

In vitro selection of Plasmodium falciparum Pfcrt and Pfmdr1 variants by artemisinin.

BackgroundAnti-malarial drugs are the major focus in the prevention and treatment of malaria. Artemisinin-based combination therapy (ACT) is the WHO recommended first-line treatment for Plasmodium falciparum malaria across the endemic world. Also ACT is increasingly relied upon in treating Plasmodium vivax malaria where chloroquine is failing. The emergence of artemisinin drug-resistant parasites is a serious threat faced by global malaria control programmes. Therefore, the success of treatment and intervention strategies is highly pegged on understanding the genetic basis of resistance.MethodsHere, resistance in P. falciparum was generated in vitro for artemisinin to produce levels above clinically relevant concentrations in vivo, and the molecular haplotypes investigated. Genomic DNA was extracted using the QIAamp mini DNA kit. DNA sequences of Pfk13, Pfcrt and Pfmdr1 genes were amplified by PCR and the amplicons were successfully sequenced. Single nucleotide polymorphisms were traced by standard bidirectional sequencing and reading the transcripts against wild-type sequences in Codon code Aligner Version 5.1 and NCBI blast.ResultsExposure of parasite strains D6 and W2 to artemisinin resulted in a decrease in parasite susceptibility to artemisinin (W2 and D6) and lumefantrine (D6 only). The parasites exhibited elevated IC50s to multiple artemisinins, with >twofold resistance to artemisinin; however, the resistance index obtained with standard methods was noticeably less than expected for parasite lines recovered from 50 µg/ml 48 h drug pressure. The change in parasite susceptibility was associated with Pfmdr-185K mutation, a mutation never reported before. The Pfcrt-CVMNK genotype (Pfcrt codons 72–76) was retained and notably, the study did not detect any polymorphisms reported to reduce P. falciparum susceptibility in vivo in the coding sequences of the Pfk13 gene.DiscussionThis data demonstrate that P. falciparum has the capacity to develop resistance to artemisinin derivatives in vitro and that this phenotype is achieved by mutations in Pfmdr1, the genetic changes that are also underpinning lumefantrine resistance. This finding is of practical importance, because artemisinin drugs in Kenya are used in combination with lumefantrine for the treatment of malaria.ConclusionArtemisinin resistance phenotype as has been shown in this work, is a decrease in parasites susceptibility to artemisinin derivatives together with the parasite’s ability to recover from drug-induced dormancy after exposure to drug dosage above the in vivo clinical concentrations. The study surmises that Pfmdr1 may play a role in the anti-malarial activity of artemisinin.

Quantitative structure-activity relationship for antimalarial activity of artemisinin

The increase in resistance to older drugs and the emergence of new types of infection have created an urgent need for discovery and development of new compounds with antimalarial activity. Quantitative-Structure Activity Relationship (QSAR) methodology has been performed to develop models that correlate antimalarial activity of artemisinin analogs and their molecular structures. In this study, the data set consisted of 197 compounds with their activities expressed as log RA (relative activity). These compounds were randomly divided into training set (n=157) and test set (n=40). The initial stage of the study was the generation of a series of descriptors from three-dimensional representations of the compounds in the data set. Several types of descriptors which include topological, connectivity indices, geometrical, physical properties and charge descriptors have been generated. The number of descriptors was then reduced to a set of relevant descriptors by performing a systematic variable selection procedure which includes zero test, pairwisecorrelation analysis and genetic algorithm (GA). Several models were developed using different combinations of modelling techniques such as multiple linear regression (MLR) and partial least square (PLS) regression. Statistical significance of the final model was characterized by correlation coefficient, r2 and root-mean-square error calibration, RMSEC. The results obtained were comparable to those from previous study on the same data set with r2 values greater than 0.8. Both internal and external validations were carried out to verify that the models have good stability, robustness and predictive ability. The cross-validated regression coefficient (r2cv) and prediction regression coefficient (r2 test) for the external test set were consistently greater than 0.7. The QSAR models developed in this study should facilitate the search for new compounds with antimalarial activity.

Genetic analysis in mice identifies cysteamine as a novel partner for artemisinin in the treatment of malaria

Malaria continues to be a serious threat to global health. The malaria problem is compounded by the absence of an efficacious vaccine and widespread drug resistance in the Plasmodium malarial parasite. The host factors and parasite virulence determinants that regulate early response to infection and subsequent onset of protective immunity are poorly understood. The molecular characterization of this early host:pathogen interface may identify novel targets for prophylactic or therapeutic intervention. Genetic analyses in mouse model of malaria show that inactivation of the enzyme pantetheinase (Char9 locus) causes susceptibility to blood-stage infection. The pantetheinase product cysteamine is an inexpensive and non-toxic aminothiol that is approved for lifelong clinical management of nephropathic cystinosis. In mouse models of infection, cysteamine not only displays anti-malarial activity of its own, but also dramatically potentiates the anti-malarial activity of artemisinin, at doses currently used for the clinical management of cystinosis. Therefore, the inclusion of cysteamine in current artemisinin combination therapies may significantly increase efficacy and may also prove effective against emerging artemisinin-resistant human Plasmodium parasite.

An important role of intramolecular free radical reactions in antimalarial activity of artemisinin and its analogs

The kinetic schemes of intramolecular reactions of five analogs of artemisinin were built. The method of intersecting parabolas was used for the calculation of activation energies and rate constants of each elementary step of these schemes. The competition between monomolecular and bimolecular free radicals was taken into account. It was evidenced that the intramolecular oxidation of these compounds proceeds as a cascade of consecutive free radical reactions with the formation of hydroperoxide groups. The latter decompose via reactions with the Fe(II) complexes generating free radicals. Among the radicals formed, the hydroxyl radical was proved to play the key role. A correlation between the yield of hydroxyl radicals n(OH) and antimalarial activity of compounds (IC(50)) was observed. The dependence of index IC(50) on n(OH) is linear in the logarithmic coordinates: ln[IC(50)(Artemisinin)/IC(50)(Compound)] = -14.10 + 3.85 ×n(OH). The proposed scheme explains and demonstrates a strong dependence of the antimalarial effectiveness of a drug on the chemical structure.

Iron enhances generation of free radicals by Artemisinin causing a caspase-independent, apoptotic death in Leishmania donovani promastigotes

An increasing incidence of unresponsiveness to antimonials in Leishmaniasis has led to identification of plant-derived anti-leishmanial compounds like Artemisinin. Since iron-mediated generation of free radicals sustains the anti-malarial activity of Artemisinin, this study investigated whether similar mechanisms accounted for its activity in Leishmania promastigotes. Artemisinin effectively disrupted the redox potential via an increased generation of free radicals along with a decrease in levels of non-protein thiols. Attenuation of its IC50 by a free radical scavenger N-acetyl l-cysteine and an iron chelator desferoxamine established the pivotal role of free radicals and of the potentiating effect of iron. An enhanced Fluo-4 fluorescence reflected Artemisinin-induced mobilization of intracellular calcium, which triggered apoptosis. However, the absence of any detectable caspase activity indicated that the leishmanicidal activity of Artemisinin is mediated by an iron-dependent generation of reactive intermediates, terminating in a caspase-independent, apoptotic mode of cell death.

The Role of Charge Distribution on the Antimalarial Activity of Artemisinin Analogues.

In this work the calculated nuclear quadrupole coupling constants (NQCC; Χ) of 1 7 O in artemisinin and some of its derivatives and the effects of charge density due to the nature of ligands on NQCC of 1 7 O were investigated. All calculations were performed at the HF/3-21G level using the Gaussian 98 program. The results show that the O-O linkage has a characteristic role in the antimalarial activity of artemisinin. In addition, various substitutions on C4 change the charge density on these oxygens and consequently change the pharmaceutical effect of artemisinin. Our results suggest that due to a larger charge density on O1, the heme iron approaches the endoperoxide moiety at the O1 position with preference to the 02 position.

Evidence that haem iron in the malaria parasite is not needed for the antimalarial effects of artemisinin

The role of haem iron (II) and oxidative stress in the activation and antimalarial activity of artemisinin is unclear. Thus, we submitted malaria parasite to modified culture conditions: artemisinin activity increased by 20–30% under an oxygen-rich atmosphere (20% O 2 instead of “standard” 1% O 2), and by 40–50% in the presence of carboxy-haemoglobin, and 2% carbon monoxide, conditions which inhibit haem iron (II) reactivity. In all cases, parasite growth and chloroquine activity were unaffected. We conclude that in the malaria parasite artemisinin is not activated by haem iron and that free radicals are not needed for its toxicity.

MNDO/d calculations on the interaction between artemisinin and heme

The antimalarial activity of artemisinin is supposed to be mediated by its interaction with intraparasitic heme leading to free radicals which would be responsible for killing the parasite. The semi-empirical PM3 method was used to calculate the molecular electrostatic potential of artemisinin. This shows that artemisinin has a wide negative region which extends from the peroxide oxygens O1 and O2 to the carboxyl oxygens O11 and O14. MNDO/d calculations of two complexes between artemisinin and heme, where the Fe (II) ion of heme interacts with either O1 and O2 or with O11 and O14, respectively, indicate that artemisinin interacts with heme preferentially through the peroxide oxygen atoms leading, however, to a complex with low thermodynamic stability.

Potentiation of artemisinin activity against chloroquine-resistant Plasmodium falciparum strains by using heme models.

The influence of different metalloporphyrin derivatives on the antimalarial activity of artemisinin was studied with two chloroquine-resistant strains of Plasmodium falciparum (FcB1-Colombia and FcM29-Cameroon) cultured in human erythrocytes. This potentiation study indicates that the manganese complex of meso-tetrakis(4-sulfonatophenyl)porphyrin has a significant synergistic effect on the activity of artemisinin against both Plasmodium strains.

Hemin-catalyzed decomposition of artemisinin (qinghaosu)

Artemisinin (qinghaosu) and its derivatives represent an important new class of antimalarial drugs. Previous work suggests that the antimalarial activity of artemisinin may be mediated by a reaction with intraparasitic hemin. Using cyclic voltammetry, artemisinin and dihydroartemisinin were irreversibly reduced at approximately −1 V. In the presence of concentrations of hemin as low as 50 nM, the reduction took place at much lower potentials (−0.435 to −0.460 V). Both reductions took place after adsorption onto the electrode surface. The shift of the reduction potential to more positive values is indicative of a catalytic process similar to that seen with hydrogen peroxide. The catalytic decomposition of artemisinin may play a role in the antimalarial activity of artemisinin.

Is polyamine biosynthesis a possible site of action of cinmethylin and artemisinin?

The effects of artemisinin, a naturally occurring sesquiterpene lactone, and the structurally similar herbicide cinmethylin on polyamine production were examined in seedings of lettuce, corn, and pea. Although the antimalarial activity of artemisinin in animal cell cultures is associated with a dramatic reduction in the concentration of putrescine, results presented here indicate plants treated with growth-inhibiting concentrations of artemisinin or cinmethylin generally display only slightly reduced endogenous levels of putrescine and spermidine. In addition, simultaneous addition of 0.5 m M putrescine to artemisinin- or cinmethylin-treated excised corn and pea root cultures could not prevent a reduction in root growth. Similarly, the addition of putrescine to artemisinintreated lettuce seedlings did not protect against artemisinin-induced chromosome decondensation. These results suggest that the primary biochemical activity of cinmethylin and artemisinin, which leads to inhibition in root growth, does not involve a block in the polyamine biosynthetic pathway.