Drug resistance in malaria parasites of animals and man

Design, synthesis and molecular docking study of novel triazole-quinazolinone hybrids as antimalarial and antitubercular agents

In a quest to discover new antimalarial and antitubercular drugs, we have designed and synthesized a series of novel triazole-quinazolinone hybrids. The in vitro screening of the triazole-quinazolinone hybrid entities against the plasmodium species P. falciparum offered potent antimalarial molecules 6c, 6d, 6f, 6g, 6j & 6k owing comparable activity to the reference drugs. Furthermore, the target compounds were evaluated in vitro against Mycobacterium tuberculosis (MTB) H37Rv strain. Among the screened compounds, 6c, 6d and 6l were found to be the most active molecules with a MIC values of 19.57-40.68 μM. The cytotoxicity of the most active compounds was studied against RAW 264.7 cell line by MTT assay and no toxicity was observed. The computational study including drug likeness and ADMET profiling, DFT, and molecular docking study was done to explore the features of target molecules. The compounds 6a, 6g, and 6k exhibited highest binding affinity of -10.3 kcal/mol with docked molecular targets from M. tuberculosis. Molecular docking study indicates that all the molecules are binding to the falcipain 2 protease (PDB: 6SSZ) of the P. falciparum. Our findings indicated that these new triazole-quinazolinone hybrids may be considered hit molecules for further optimization studies.

Antimalarial Drug Predictions Using Molecular Descriptors and Machine Learning against Plasmodium Falciparum

Malaria remains by far one of the most threatening and dangerous illnesses caused by the plasmodium falciparum parasite. Chloroquine (CQ) and first-line artemisinin-based combination treatment (ACT) have long been the drug of choice for the treatment and controlling of malaria; however, the emergence of CQ-resistant and artemisinin resistance parasites is now present in most areas where malaria is endemic. In this work, we developed five machine learning models to predict antimalarial bioactivities of a drug against plasmodium falciparum from the features (i.e., molecular descriptors values) obtained from PaDEL software from SMILES of compounds and compare the machine learning models by experiments with our collected data of 4794 instances. As a consequence, we found that three models amongst the five, namely artificial neural network (ANN), extreme gradient boost (XGB), and random forest (RF), outperform the others in terms of accuracy while observing that, using roughly a quarter of the promising descriptors picked by the feature selection algorithm, the five models achieved equivalent and comparable performance. Nevertheless, the contribution of all molecular descriptors in the models was investigated through the comparison of their rank values by the feature selection algorithm and found that the most potent and relevant descriptors which come from the ‘Autocorrelation’ module contributed more while the ‘Atom type electrotopological state’ contributed the least to the model.

Bridelia ferruginea Benth leaves attenuates diabetes nephropathy in STZ-induced rats via targeting NGAL/KIM-1/cystatin c gene

Background

Renal failure is among the main complications of diabetes disorders and free radical from hyperglycemia is the major cause of this nephrotic complication. This study investigated the therapeutic effects of Bridelia ferruginea Benth on nephrotic damage in streptozotocin (STZ) induced diabetic rats. Diabetes mellitus was induced by injection of 50 mg/kg b. w. of STZ (intraperitoneal) in rats fasted overnight. Effect on nephropathy was assessed using biochemical, histological indices and gene expression of kidney tissue.

Results

The total flavonoids and phenolic content of petroleum ether fraction of B. ferruginea (PEFBF) was observed to be higher compared to diethyl ether fraction of B. ferruginea (DEFBF). The 1,1-diphenyl-2-picryl-hydrazyl scavenging activity of the fractions were significantly (P < 0.05) reduced across the concentrations compared to the standard (gallic acid). The STZ-induced diabetic animals treated with 100 and 200 mg/kg b.w. of PEFBF and DEFBF showed significant (P < 0.05) reduction in blood glucose level on days 7, 11 and 14 compared to untreated diabetic rats. STZ-induced diabetic rats significantly (P < 0.05) exhibited increase plasma urea, creatinine, protein and albumin level while treatment with both fractions of B. ferruginea reduced the level of these parameters in treated diabetic rats. B. ferruginea also caused down-regulation of neutrophil gelatinase-associated lipocalin, kidney injury molecule-1 and cystatin c genes.

Conclusion

These results revealed that, the plant has hypoglycemic activity therefore provides a pharmacological basis for its folkloric use in the management of hyperglycemia and its associated renal dysfunction.

Polymorphisms in Plasmodium vivax antifolate resistance markers in Afghanistan between 2007 and 2017

BACKGROUND

Plasmodium vivax is the predominant Plasmodium species in Afghanistan. National guidelines recommend the combination of chloroquine and primaquine (CQ-PQ) for radical treatment of P. vivax malaria. Artesunate in combination with the antifolates sulfadoxine-pyrimethamine (SP) has been first-line treatment for uncomplicated falciparum malaria until 2016. Although SP has been the recommended treatment for falciparum and not vivax malaria, exposure of the P. vivax parasite population to SP might still have been quite extensive because of community based management of malaria. The change in the P. vivax antifolate resistance markers between 2007 and 2017 were investigated.

METHODS

Dried blood spots were collected (n = 185) from confirmed P. vivax patients in five malaria-endemic areas of Afghanistan bordering Tajikistan, Turkmenistan and Pakistan, including Takhar, Faryab, Laghman, Nangarhar, and Kunar, in 2007, 2010 and 2017. Semi-nested PCR, RFLP and nucleotide sequencing were used to assess the pyrimethamine resistant related mutations in P. vivax dihydrofolate reductase (pvdhfr I13L, P33L, N50I, F57L, S58R, T61I, S93H, S117N, I173L) and the sulfonamide resistance related mutations in P. vivax dihydropteroate synthase (pvdhps A383G, A553G).

RESULTS

In the 185 samples genotyped for pvdhfr and pvdhps mutations, 11 distinct haplotypes were observed, which evolved over time. In 2007, wild type pvdhfr and pvdhps were the most frequent haplotype in all study sites (81%, 80/99). However, in 2017, the frequency of the wild-type was reduced to 36%, (21/58; p value ≤ 0.001), with an increase in frequency of the double mutant pvdhfr and pvdhps haplotype S58RS117N (21%, 12/58), and the single pvdhfr mutant haplotype S117N (14%, 8/58). Triple and quadruple mutations were not found. In addition, pvdhfr mutations at position N50I (7%, 13/185) and the novel mutation S93H (6%, 11/185) were observed. Based on in silico protein modelling and molecular docking, the pvdhfr N50I mutation is expected to affect only moderately pyrimethamine binding, whereas the S93H mutation does not.

CONCLUSIONS

In the course of ten years, there has been a strong increase in the frequency pyrimethamine resistance related mutations in pvdhfr in the P. vivax population in Afghanistan, although triple and quadruple mutations conferring high grade resistance were not observed. This suggests relatively low drug pressure from SP on the P. vivax parasite population in the study areas. The impact of two newly identified mutations in the pvdhfr gene on pyrimethamine resistance needs further investigation.

Plasmodium vivax drug resistance markers: Genetic polymorphisms and mutation patterns in isolates from Malaysia

Transmission of Plasmodium vivax still persist in Malaysia despite the government's aim to eliminate malaria in 2020. High treatment failure rate of chloroquine monotherapy was reported recently. Hence, parasite drug susceptibility should be kept under close monitoring. Mutation analysis of the drug resistance markers is useful for reconnaissance of anti-malarial drug resistance. Hitherto, information on P. vivax drug resistance marker in Malaysia are limited. This study aims to evaluate the mutations in four P. vivax drug resistance markers pvcrt-o (putative), pvmdr1 (putative), pvdhfr and pvdhps in 44 isolates from Malaysia. Finding indicates that 27.3%, 100%, 47.7%, and 27.3% of the isolates were carrying mutant allele in pvcrt-o, pvmdr1, pvdhfr and pvdhps genes, respectively. Most of the mutant isolates had multiple point mutations rather than single point mutation in pvmdr1 (41/44) and pvdhfr (19/21). One novel point mutation V111I was detected in pvdhfr. Allelic combination analysis shows significant strong association between mutations in pvcrt-o and pvmdr1 (X² = 9.521, P < 0.05). In the present study, 65.9% of the patients are non-Malaysians, with few of them arrived in Malaysia 1-2 weeks before the onset of clinical manifestations, or had previous history of malaria infection. Besides, few Malaysian patients had travel history to vivax-endemic countries, suggesting that these patients might have acquired the infections during their travel. All these possible imported cases could have placed Malaysia in a risk to have local transmission or outbreak of malaria. Six isolates were found to have mutations in all four drug resistance markers, suggesting that the multiple-drugs resistant P. vivax strains are circulating in Malaysia.

Design and synthesis of leucine-linked quinazoline-4(3H)-one-sulphonamide molecules distorting malarial reductase activity in the folate pathway

Optimization of a modified Grimmel's method for N-heterocyclization of a leucine-linked sulfonamide side-arm at position 2 leading to 2,3-disustituted-4-quinazolin-(3H)-ones was accomplished. Further, 22 hybrid quinazolinone motifs (4a-v) were synthesized by N-heterocyclization reaction under microwave irradiation using the ionic liquid [Bmim][BF₄ ]-H₂ O as green solvent as well as the catalyst. The in vitro screening of the hybrid entities against the malarial species Plasmodium falciparum yielded five potent molecules 4l, 4n, 4o, 4t, and 4u owning antimalarial activity comparable to those of the reference drugs. In continuation, an in silico study was carried out to obtain a pharmacophoric model and quantitative structure-activity relationship. We also built a 3D-QSAR model to procure more information that could be applied to design new molecules with more potent Pf-DHFR inhibitory activity. The designed pharmacophore was recognized to be more potent for the selected molecules, exhibiting five pharmacophoric features. The active scaffolds were further evaluated for enzyme inhibition efficacy against alleged receptor Pf-DHFR computationally and in vitro, proving their candidature as lead dihydrofolate reductase inhibitors, and the selectivity of the test candidates was ascertained by toxicity study against Vero cells. Good oral bioavailability was also proved by studying pharmacokinetic properties.

Development and rigorous validation of antimalarial predictive models using machine learning approaches

The large collection of known and experimentally verified compounds from the ChEMBL database was used to build different classification models for predicting the antimalarial activity against Plasmodium falciparum. Four different machine learning methods, namely the support vector machine (SVM), random forest (RF), k-nearest neighbour (kNN) and XGBoost have been used for the development of models using the diverse antimalarial dataset from ChEMBL. A well-established feature selection framework was used to select the best subset from a larger pool of descriptors. Performance of the models was rigorously evaluated by evaluation of the applicability domain, Y-scrambling and AUC-ROC curve. Additionally, the predictive power of the models was also assessed using probability calibration and predictiveness curves. SVM and XGBoost showed the best performances, yielding an accuracy of ~85% on the independent test set. In term of probability prediction, SVM and XGBoost were well calibrated. Total gain (TG) from the predictiveness curve was more related to SVM (TG = 0.67) and XGBoost (TG = 0.75). These models also predict the high-affinity compounds from PubChem antimalarial bioassay (as external validation) with a high probability score. Our findings suggest that the selected models are robust and can be potentially useful for facilitating the discovery of antimalarial agents.

Green synthesis, biological evaluation, molecular docking studies and 3D-QSAR analysis of novel phenylalanine linked quinazoline-4(3H)-one-sulphonamide hybrid entities distorting the malarial reductase activity in folate pathway

A modified Grimmel's method for N-heterocyclization of phenylalanine linked sulphonamide side arm at position-2 was optimized leading to 2,3-disustituted-4-quinazolin-(3H)-ones. Further, [Bmim][BF₄]-H2O (IL) was used as green solvent as well as catalyst for the synthesis of twenty two hybrid quinazolinone motifs (4a-4v) by N-heterocyclization reaction using microwave irradiation technique. The in vitro screening of the hybrid entities against the malarial species Plasmodium falciparum yielded five potent molecules 4l, 4n, 4r, 4t & 4u owing comparable antimalarial activity to the reference drugs. In continuation, anin silicostudy was carried out to obtain a pharmacophoric model and quantitative structure activity relationship. We also built a 3D-QSAR model to procure more information that could be applied to design new molecules with more potent Pf-DHFR inhibitory activity. The designed pharmacophore was recognized to be more potent for the selected molecules, exhibiting five pharmacophoric features. The active scaffolds were further evaluated for enzyme inhibition efficacy against alleged receptor Pf-DHFR computationally and in vitro, proving their candidature as lead dihydrofolate reductase inhibitors as well as the selectivity of the test candidates was ascertained by toxicity study against vero cells. The perception of good oral bioavailability was also proved by study of pharmacokinetic properties.

A comprehensive model for assessment of liver stage therapies targeting Plasmodium vivax and Plasmodium falciparum

Malaria liver stages represent an ideal therapeutic target with a bottleneck in parasite load and reduced clinical symptoms; however, current in vitro pre-erythrocytic (PE) models for Plasmodium vivax and P. falciparum lack the efficiency necessary for rapid identification and effective evaluation of new vaccines and drugs, especially targeting late liver-stage development and hypnozoites. Herein we report the development of a 384-well plate culture system using commercially available materials, including cryopreserved primary human hepatocytes. Hepatocyte physiology is maintained for at least 30 days and supports development of P. vivax hypnozoites and complete maturation of P. vivax and P. falciparum schizonts. Our multimodal analysis in antimalarial therapeutic research identifies important PE inhibition mechanisms: immune antibodies against sporozoite surface proteins functionally inhibit liver stage development and ion homeostasis is essential for schizont and hypnozoite viability. This model can be implemented in laboratories in disease-endemic areas to accelerate vaccine and drug discovery research.

Ionic liquid mediated stereoselective synthesis of alanine linked hybrid quinazoline-4(3H)-one derivatives perturbing the malarial reductase activity in folate pathway

Grimmel's method was optimized as well as modified leading to the cyclization and incorporation of alanine linked sulphonamide in 4-quinazolin-(3H)-ones. Further, the generation of heterocyclic motif at position-3 of 4-quinazolinones was explored by synthesis of imines, which unfortunately led to an isomeric mixture of stereoisomers. The hurdle of diastereomers encountered on the path was eminently rectified by development of new rapid and reproducible methodology involving the use of imidazolium based ionic liquid as solvents as well as catalyst for cyclization as well as synthesis of imines in situ at position-3 leading to procurement of single E-isomer as the target hybrid heterocyclic molecules. The purity and presence of single isomer was also confirmed by HPLC and spectroscopic techniques. Further, the synthesized sulphonamide linked 4-quinazolin-(3H)-ones hybrids were screened for their antimalarial potency rendering potent entities (4b, 4c, 4 l, 4 t and 4u). The active hybrids were progressively screened for enzyme inhibitory efficacy against presumed receptor Pf-DHFR and h-DHFR computationally as well as in vitro, proving their potency as dihydrofolate reductase inhibitors. The ADME properties of these active molecules were also predicted to enhance the knowhow of the oral bioavailability, indicating good bioavailability of the active entities.

Novel stereoselective 2,3-disubstituted quinazoline-4(3H)-one derivatives derived from glycine as a potent antimalarial lead

A series of 2,3-disubstituted quinazolinones derived from sulfonamide linked glycine was designed and developed owing to their potent antimalarial activity.

A Mathematical Model for the Transmission and Spread of Drug Sensitive and Resistant Malaria Strains within a Human Population

Malaria remains by far the world's most important tropical disease, killing more people than any other communicable disease. A number of preventive and control measures have been put in place and most importantly drug treatment. The emergence of drug resistance against the most common and affordable antimalarials is widespread and poses a key obstacle to malaria control. A mathematical model that incorporates evolution of drug resistance and treatment as a preventive strategy is formulated and analyzed. The qualitative analysis of the model is given in terms of the effective reproduction number, Re. The existence and stability of the disease-free and endemic equilibria of the model are studied. We establish the threshold parameters below which the burden due to malaria can be brought under control. Numerical simulations are done to determine the role played by key parameters in the model. The public health implications of the results are twofold; firstly every effort should be taken to minimize the evolution of drug resistance due to treatment failure and secondly high levels of treatment and development of immunity are essential in reducing the malaria burden.

Functional analysis of Plasmodium vivax dihydrofolate reductase-thymidylate synthase genes through stable transformation of Plasmodium falciparum

Mechanisms of drug resistance in Plasmodium vivax have been difficult to study partially because of the difficulties in culturing the parasite in vitro. This hampers monitoring drug resistance and research to develop or evaluate new drugs. There is an urgent need for a novel method to study mechanisms of P. vivax drug resistance. In this paper we report the development and application of the first Plasmodium falciparum expression system to stably express P. vivax dhfr-ts alleles. We used the piggyBac transposition system for the rapid integration of wild-type, single mutant (117N) and quadruple mutant (57L/58R/61M/117T) pvdhfr-ts alleles into the P. falciparum genome. The majority (81%) of the integrations occurred in non-coding regions of the genome; however, the levels of pvdhfr transcription driven by the P. falciparum dhfr promoter were not different between integrants of non-coding and coding regions. The integrated quadruple pvdhfr mutant allele was much less susceptible to antifolates than the wild-type and single mutant pvdhfr alleles. The resistance phenotype was stable without drug pressure. All the integrated clones were susceptible to the novel antifolate JPC-2067. Therefore, the piggyBac expression system provides a novel and important tool to investigate drug resistance mechanisms and gene functions in P. vivax.

Comparative Study on the Effects of Chloroquine and Artesunate on Histopathological Damages Caused by Plasmodium berghei in Four Vital Organs of Infected Albino Mice

The aim of the present study was to investigate the positive influence of chloroquine and artesunate on the pathological damages caused by Plasmodium berghei on vital organs of mice in an established infection. Healthy adult albino mice with average weight of 25 g were used for the study. Treated group was administered orally with 100 mg/kg of chloroquine and artesunate, respectively. Control animals were given water for the same period. Histological examination of the liver, spleen, lungs, and kidney revealed absence of accumulation of iron (haemosiderosis) in the liver, thickened alveolar wall, and interstitial mononuclear cells infiltration in the lungs of the artesunate group, while absence of emphysema in the lungs and megakaryoblast hyperplasia in the spleen was observed in the chloroquine group. Lymphoid hypoplasia in the chloroquine group and megakayoblast hyperplasia in the artesunate group were observed but not in the control group. Thus, the use of these drugs especially under the practice of self-medication should be prohibited in lands where they are still in use as antimalaria medicine.

Benzothiophene carboxamide derivatives as inhibitors of Plasmodium falciparum enoyl-ACP reductase

Benzothiophene derivatives like benzothiophene sulphonamides, biphenyls, or carboxyls have been synthesized and have found wide pharmacological usage. Here we report, bromo-benzothiophene carboxamide derivatives as potent, slow tight binding inhibitors of Plasmodium enoyl-acyl carrier protein (ACP) reductase (PfENR). 3-Bromo-N-(4-fluorobenzyl)-benzo[b]thiophene-2-carboxamide (compound 6) is the most potent inhibitor with an IC50 of 115 nM for purified PfENR. The inhibition constant (Ki) of compound 6 was 18 nM with respect to the cofactor and 91 nM with respect to crotonoyl-CoA. These inhibitors showed competitive kinetics with cofactor and uncompetitive kinetics with the substrate. Thus, these compounds hold promise for the development of potent antimalarials.

Defining the role of mutations in Plasmodium vivax dihydrofolate reductase-thymidylate synthase gene using an episomal Plasmodium falciparum transfection system

Plasmodium vivax resistance to antifolates is prevalent throughout Australasia and is caused by point mutations within the parasite dihydrofolate reductase (DHFR)-thymidylate synthase. Several unique mutations have been reported in P. vivax DHFR, and their roles in resistance to classic and novel antifolates are not entirely clear due, in part, to the inability to culture P. vivax in vitro. In this study, we use a homologous system to episomally express both wild-type and various mutant P. vivax dhfr (pvdhfr) alleles in an antifolate-sensitive line of P. falciparum and to assess their influences on the susceptibility of the recipient P. falciparum line to commonly used and new antifolate drugs. Although the wild-type pvdhfr-transfected P. falciparum line was as susceptible to antifolate drugs as the P. falciparum parent line, the single (117N), double (57L/117T and 58R/117T), and quadruple (57L/58R/61M/117T) mutant pvdhfr alleles conferred a marked reduction in their susceptibilities to antifolates. The resistance index increased with the number of mutations in these alleles, indicating that these mutations contribute to antifolate resistance directly. In contrast, the triple mutant allele (58R/61M/117T) significantly reversed the resistance to all antifolates, indicating that 61M may be a compensatory mutation. These findings help elucidate the mechanism of antifolate resistance and the effect of existing mutations in the parasite population on the current and new generation of antifolate drugs. It also demonstrates that the episomal transfection system has the potential to provide a rapid screening system for drug development and for studying drug resistance mechanisms in P. vivax.

Hyperparasitaemia and low dosing are an important source of anti-malarial drug resistance

Background

Preventing the emergence of anti-malarial drug resistance is critical for the success of current malaria elimination efforts. Prevention strategies have focused predominantly on qualitative factors, such as choice of drugs, use of combinations and deployment of multiple first-line treatments. The importance of anti-malarial treatment dosing has been underappreciated. Treatment recommendations are often for the lowest doses that produce "satisfactory" results.

Methods

The probability of de-novo resistant malaria parasites surviving and transmitting depends on the relationship between their degree of resistance and the blood concentration profiles of the anti-malarial drug to which they are exposed. The conditions required for the in-vivo selection of de-novo emergent resistant malaria parasites were examined and relative probabilities assessed.

Results

Recrudescence is essential for the transmission of de-novo resistance. For rapidly eliminated anti-malarials high-grade resistance can arise from a single drug exposure, but low-grade resistance can arise only from repeated inadequate treatments. Resistance to artemisinins is, therefore, unlikely to emerge with single drug exposures. Hyperparasitaemic patients are an important source of de-novo anti-malarial drug resistance. Their parasite populations are larger, their control of the infection insufficient, and their rates of recrudescence following anti-malarial treatment are high. As use of substandard drugs, poor adherence, unusual pharmacokinetics, and inadequate immune responses are host characteristics, likely to pertain to each recurrence of infection, a small subgroup of patients provides the particular circumstances conducive to de-novo resistance selection and transmission.

Conclusion

Current dosing recommendations provide a resistance selection opportunity in those patients with low drug levels and high parasite burdens (often children or pregnant women). Patients with hyperparasitaemia who receive outpatient treatments provide the greatest risk of selecting de-novo resistant parasites. This emphasizes the importance of ensuring that only quality-assured anti-malarial combinations are used, that treatment doses are optimized on the basis of pharmacodynamic and pharmacokinetic assessments in the target populations, and that patients with heavy parasite burdens are identified and receive sufficient treatment to prevent recrudescence.

Molecular markers of in vivo Plasmodium vivax resistance to amodiaquine plus sulfadoxine-pyrimethamine: mutations in pvdhfr and pvmdr1

BACKGROUND

Molecular markers for sulfadoxine-pyrimethamine (SP) resistance in Plasmodium vivax have been reported. However, data on the molecular correlates involved in the development of resistance to 4-aminoquinolines and their association with the in vivo treatment response are scarce.

METHODS

We assessed pvdhfr (F57L/I, S58R, T61M, S117T/N, and I173F/L) and pvmdr1 (Y976F and F1076L) mutations in 94 patients who received amodiaquine (AQ) plus SP in Papua New Guinea (PNG). We then investigated the association between parasite genotype and treatment response.

RESULTS

The treatment failure (TF) rate reached 13%. Polymorphisms in pvdhfr F57L, S58R, T61M, and S117T/N and in pvmdr1 Y976F were detected in 60%, 67%, 20%, 40%, and 39% of the samples, respectively. The single mutant pvdhfr 57 showed the strongest association with TF (odds ratio [OR], 9.04; P= .01). The combined presence of the quadruple mutant pvdhfr 57L+58R+61M+117T and pvmdr1 mutation 976F was the best predictor of TF (OR, 8.56; P= .01). The difference in TF rates between sites was reflected in the genetic drug-resistance profile of the respective parasites.

CONCLUSIONS

The present study identified a new molecular marker in pvmdr1 that is associated with the in vivo response to AQ+SP. We suggest suitable marker sets with which to monitor P. vivax resistance against AQ+SP in countries where these drugs are used.

Changes in antioxidant status and biochemical indices after acute administration of artemether, artemether-lumefantrine and halofantrine in rats

Artemether, artemether-lumefantrine, or coartem and halofantrine are alternative antimalarial drugs to chloroquine. Their efficacy and potential to delay drug resistance in falciparum malaria had led to their increased use. Although these drugs have proven to be well tolerated, there are adverse effects associated with them. This study was designed to examine the toxic potential of acute administration of these drugs in rats. Twenty-four rats were divided into four groups: group I (control) received distilled water; group II received artemether for 5 days with an initial dose of 3.2 g/kg body weight on day 1 and 1.6 mg/kg body weight on days 2-5; group III received coartem (27 mg/kg body weight/day) for 3 days, which was divided into two equal portions per day; and group IV received halofantrine (24 mg/kg body weight/day) in three equal portions. Administration of artemether, coartem and halofantrine caused significant decrease (P < 0.05) in reduced glutathione levels in the liver by 29%, 21% and 26%, respectively. In contrast, there were no significant differences (P > 0.05) in the kidney glutathione levels. Furthermore, artemether, coartem and halofantrine decreased the liver- and kidney-enzymatic antioxidant status of the animals. Precisely, artemether, coartem and halofantrine decreased liver superoxide dismutase and catalase activities by 45%, 50% and 57%; and 20%, 29% and 23%, respectively. While the kidney catalase activities were decreased by 41%, 28% and 30%, respectively, the drugs however did not produce significant effect (P > 0.05) on the kidney superoxide dismutase activities. In addition, artemether, coartem and halofantrine decreased the hepatic levels of glutathione S-transferase by 64%, 51% and 53%, respectively. Administration of artemether, coartem and halofantrine significantly increased (P < 0.05) liver and kidney lipid peroxidation levels by 67%, 50% and 81%; and 58%, 43% and 31%, respectively. This indicates that the liver is considerably more affected than the kidneys. Similarly, halofantrine treatment caused significant elevation (P < 0.05) in the levels of serum creatinine, aspartate and alanine aminotransferases and blood urea nitrogen by 73%, 66%, 61% and 63%, respectively. These data indicate that oral administration of artemether, coartem and halofantrine has adverse effects on both enzymic and non-enzymatic antioxidant status of the animals.

Plasmodium berghei: In vitro and in vivo activity of dequalinium

Bisquinoline compounds have exhibited remarkable activity in vitro and in vivo against Plasmodium parasites by inhibition of heme detoxification. We have tested the ability of dequalinium 1,1'-(1,10-decanediyl)bis(4-amino-2-methylquinoline), a known antimicrobial agent, to inhibit beta-hematin synthesis using a non-emzymatic colorimetric assay and globin proteolysis by electrophoretic analysis (SDS-PAGE-15%). Dequalinium was able to inhibit both processes in vitro with close correlation to a murine malaria model, reducing parasitemia levels, prolonging the survival time post-infection and curing 40% of infected mice using a combination therapy with a loading dose of chloroquine. These results confirm that dequalinium is a promising lead for antimalarial drug development.

Evaluation of the activities of pyrimethamine analogs against Plasmodium vivax and Plasmodium falciparum dihydrofolate reductase-thymidylate synthase using in vitro enzyme inhibition and bacterial complementation assays

Pyrimethamine analogs were examined as potential agents against vivax malaria using a bacterial surrogate system carrying Plasmodium vivax dihydrofolate reductase-thymidylate synthase (PvDHFR-TS), in which the PvDHFR complemented chemically knocked out host dihydrofolate reductase. The system was initially tested with P. falciparum dihydrofolate reductase-thymidylate synthase and was found to have good correlation with the parasite-based system. The 50% inhibitory concentrations derived from PvDHFR-TS-dependent bacteria were correlated with their corresponding inhibition constants (Ki) from an enzyme inhibition assay, pointing to the likelihood that the potent enzyme inhibitors will also have potent antimalarial activities. Active compounds against both wild-type and S58R S117N (SP21) double-mutant P. vivax include analogs with structures which can avert a steric clash with the asparagine (S117N) side chain of the mutant, similar to those found for homologous Plasmodium falciparum mutants, raising the possibility that the same compounds can be developed against both types of antifolate-resistant malaria. This rapid and convenient drug screening system should be useful for development of new antifolates against P. vivax, for which a continuous culture system is not yet available.

Crystal structure of dihydrofolate reductase from Plasmodium vivax: pyrimethamine displacement linked with mutation-induced resistance

Pyrimethamine (Pyr) targets dihydrofolate reductase of Plasmodium vivax (PvDHFR) as well as other malarial parasites, but its use as antimalarial is hampered by the widespread high resistance. Comparison of the crystal structures of PvDHFR from wild-type and the Pyr-resistant (SP21, Ser-58 --> Arg + Ser-117 --> Asn) strain as complexes with NADPH and Pyr or its analog lacking p-Cl (Pyr20) clearly shows that the steric conflict arising from the side chain of Asn-117 in the mutant enzyme, accompanied by the loss of binding to Ser-120, is mainly responsible for the reduction in binding of Pyr. Pyr20 still effectively inhibits both the wild-type and SP21 proteins, and the x-ray structures of these complexes show how Pyr20 fits into both active sites without steric strain. These structural insights suggest a general approach for developing new generations of antimalarial DHFR inhibitors that, by only occupying substrate space of the active site, would retain binding affinity with the mutant enzymes.

A three-dimensional structure of Plasmodium falciparum serine hydroxymethyltransferase in complex with glycine and 5-formyl-tetrahydrofolate. Homology modeling and molecular dynamics

Cytosolic Plasmodium falciparum serine hydroxymethyltransferase (pfSHMT) is a potential target for antimalarial chemotherapy. Contrasting with the other enzymes involved in the parasite folate cycle, little information is available about this enzyme, and its crystallographic structure is unknown yet. In this paper, we propose a theoretical low-resolution 3D model for pfSHMT in complex with glycine and 5-formyl tetrahydrofolate (5-FTHF) based on homology modeling by multiple alignment followed by intensive optimization, validation and dynamics simulations in water. Comparison between the active sites of our model and that of crystallographic Human SHMT (hSHMT) revealed key differences that could be useful for the design of new selective inhibitors of pfSHMT.

Assessing African medicinal plants for efficacy and safety: pharmacological screening and toxicology

This paper reviews progress in establishing the scientific rationale for and safety of traditional medicine use in Africa. Selected plants were screened for antibacterial, antifungal, anthelmintic, anti-amoebic, antischistosomal, antimalarial, anti-inflammatory and antioxidant activity, as well as psychotropic and neurotropic activity using appropriate in vitro tests. Isolation of active compounds, in almost all cases, provided scientific validation for the use of the plants in traditional medicine. Although plants used medicinally are widely assumed to be safe, many are potentially toxic. Where poisoning from traditional medicines has been reported, it is usually because the plants used have been misidentified in the form in which they are sold, or incorrectly prepared and administered by inadequately trained personnel. The issue of quality control may, in the interim, be addressed using chromatographic techniques.

Is combination therapy for malaria based on user-fees worthwhile and equitable to consumers? Assessment of costs and willingness to pay in Southeast Nigeria

OBJECTIVES

To examine the equity implications of the costs of an episode of malaria, the benefit/cost ratios of using two artemisinin-based combination therapy (CT) from the consumers' view and inequities in willingness to pay (WTP) for CT.

METHODS

A cross-sectional survey was conducted in Southeast Nigeria, where there is a moderate to high level of malaria resistance to chloroquine and sulfadoxine-pyrimethamine formulations. WTP was elicited from respondents using the bidding game (BG) and the structured haggling technique (SH). A socio-economic status (SES) index was used to examine the level of inequity in the key variables. In the benefit/cost ratios, the average cost of CT in Nigeria and price of Coartem were, respectively, used as the cost inputs while the mean WTP was the measure of benefit. Multiple regression analyses were used to determine the validity of the WTP estimates.

RESULTS

More than 90% of the respondents were willing to pay for CT. The mean WTP in the BG was 301.1 Naira while it was 438.0 Naira in the SH. People in the highest SES quartile (Q4) were more willing to pay for CT than the lowest SES quartile (Q1). In the regression models, the SES quartiles were significantly related to levels of WTP. The benefit/cost ratios were higher in the SH group, and the ratio was only more than 1 using Coartem in only the SH group. The Q1 groups had the least benefit cost-ratios but the trend of SES differentials in benefit/cost ratios were not statistically significant in the BG group but was in the SH group.

CONCLUSION

CT based on user-fees may not be worthwhile and equitable because there are economic and equity constraints to its wide-scale use. Benefit/cost ratios depend on the type of questions that were used to elicit WTP. Governments and donors should be willing to commit funds to make CT affordable to the poor consumers for the intervention to be used to significantly reduce the burden of malaria.

Identification and expression analysis of ABC genes in Plasmodium yoelii and P. berghei

The ATP-binding cassette (ABC) proteins are one of the largest evolutionarily conserved families. They are characterized by the presence of highly conserved nucleotide-binding sites (NBS). In the present study, we identified ABC genes in rodent Plasmodia. We queried the Plasmodium yoelii genome with the ABC signature motif and retrieved 15 contigs. Sequences were classified into seven ABC families by BLAST comparison. Conservation of the five signature ABC motifs in the P. yoelii contigs was examined by multi-alignment of the NBS. Expression of the ABC genes was examined during the blood stages of P. yoelii and P. berghei and the hepatocytic stages of P. yoelii. Our results with RT-PCR on total RNA from blood stages demonstrated the expression of 14 ABC genes in P. yoelii and ten in P. berghei. In P. yoelii hepatocytic stages, the expression of four ABC genes was detected.

In vitro antiplasmodial activity and cytotoxicity of ethnobotanically selected Ivorian plants

Eight extracts from four Ivorian medicinal plants, traditionally used to treat malaria, were tested for their antiplasmodial activity in vitro by assessing their ability to inhibit the uptake of [3H]hypoxanthine into the Plasmodium falciparum K1 chloroquine-resistant strain. The most active extract was the methylene chloride extract of Anogeissus leiocarpus which exhibited an IC(50) value of 3.8 micro g/ml. Inhibition of the growth of Plasmodium falciparum was also observed with the methylene chloride extract of Cochlospermum planchonii and Microdesmis keayana as well as with both methylene chloride and methanolic extracts of Hymenocardia acida.

Molecular characterisation of drug-resistant Plasmodium falciparum from Thailand

BACKGROUND

The increasing levels of Plasmodium falciparum resistance to chloroquine (CQ) in Thailand have led to the use of alternative antimalarials, which are at present also becoming ineffective. In this context, any strategies that help improve the surveillance of drug resistance, become crucial in overcoming the problem.

METHODS

In the present study, we have established the in vitro sensitivity to CQ, mefloquine (MF), quinine (QUIN) and amodiaquine (AMQ) of 52 P. falciparum isolates collected in Thailand, and assessed the prevalence of four putative genetic polymorphisms of drug resistance, pfcrt K76T, pfmdr1 N86Y, pfmdr1 D1042N and pfmdr1 Y1246D, by PCR-RFLP.

RESULTS

The percentage of isolates resistant to CQ, MF, and AMQ was 96% (50/52), 62% (32/52), and 58% (18/31), respectively, while all parasites were found to be sensitive to QUIN. In addition, 41 (79%) of the isolates assayed were resistant simultaneously to more than one drug; 25 to CQ and MF, 9 to CQ and AMQ, and 7 to all three drugs, CQ, MF and AMQ. There were two significant associations between drug sensitivity and presence of particular molecular markers, i) CQ resistance / pfcrt 76T (P = 0.001), and ii) MF resistance / pfmdr1 86N (P < 0.001)

CONCLUSIONS

i) In Thailand, the high levels of CQ pressure have led to strong selection of the pfcrt 76T polymorphism and ii) pfmdr1 86N appears to be a good predictor of in vitro MF resistance.

Pyrimethamine and WR99210 exert opposing selection on dihydrofolate reductase from Plasmodium vivax

Plasmodium vivax is a major public health problem in Asia and South and Central America where it is most prevalent. Until very recently, the parasite has been effectively treated with chloroquine, but resistance to this drug has now been reported in several areas. Affordable alternative treatments for vivax malaria are urgently needed. Pyrimethamine-sulfadoxine is an inhibitor of dihydrofolate reductase (DHFR) that has been widely used to treat chloroquine-resistant Plasmodium falciparum malaria. DHFR inhibitors have not been considered for treatment of vivax malaria, because initial trials showed poor efficacy against P. vivax. P. vivax cannot be grown in culture; the reason for its resistance to DHFR inhibitors is unknown. We show that, like P. falciparum, point mutations in the dhfr gene can cause resistance to pyrimethamine in P. vivax. WR99210 is a novel inhibitor of DHFR, effective even against the most pyrimethamine-resistant P. falciparum strains. We have found that it is also an extremely effective inhibitor of the P. vivax DHFR, and mutations that confer high-level resistance to pyrimethamine render the P. vivax enzyme exquisitely sensitive to WR99210. These data suggest that pyrimethamine and WR99210 would exert opposing selective forces on the P. vivax population. If used in combination, these two drugs could greatly slow the selection of parasites resistant to both drugs. If that is the case, this novel class of DHFR inhibitors could provide effective and affordable treatment for chloroquine- and pyrimethamine-resistant vivax and falciparum malaria for many years to come.

Antiplasmodial activity of naphthoquinones and one anthraquinone from Stereospermum kunthianum

A lipophilic extract of the root bark of Stereospermum kunthianum revealed antiplasmodial activity in vitro. Bioassay-guided fractionation led to the isolation of four novel naphthoquinones (sterekunthals A and B, pyranokunthones A and B) and one novel anthraquinone (anthrakunthone) together with the known naphthoquinone pinnatal. The structures of the novel compounds were determined by comprehensive analyses of their 1D and 2D NMR data. The antiplasmodial activities and toxicity against the endothelial cell line ECV-304 of the isolated compounds have been assessed.

Mechanisms of resistance of Plasmodium falciparum to antimalarial drugs

Chemotherapy and chemoprophylaxis are the principal means of combating malaria parasite infections in the human host. In the last 75 years, since the introduction of synthetic antimalarials, only a small number of compounds have been found suitable for clinical usage, and this limited armoury is now greatly compromised by the spread of drug-resistant parasite strains. Our current knowledge of the molecular mechanisms underlying resistance in the lethal species Plasmodium falciparum is reviewed here.

Molecular characterization of dihydrofolate reductase in relation to antifolate resistance in Plasmodium vivax

The genes encoding the wild-type and six (five single and one double) mutant dihydrofolate reductase (DHFR) domains of the human malaria parasite, Plasmodium vivax (Pv), were cloned and expressed in Escherichia coli. The catalytic activities and the kinetic parameters of the purified recombinant wild-type and the mutant PvDHFRs were determined. Generally, all the PvDHFR mutants yielded enzymes with poorer catalytic activities when compared to the wild type enzyme. The widely used antifolates, pyrimethamine and cycloguanil, were effective inhibitors of the wild-type PvDHFR, but were approximately 60 to >4000 times less active against the mutant enzymes. In contrast to the analogous S108N mutation of Plasmodium falciparum DHFR (PfDHFR), the single S117N mutation in PvDHFR conferred approximately 4000- and approximately 1600-fold increased resistance to pyrimethamine and cycloguanil, respectively, compared to the wild-type PvDHFR. The S58R+S117N double mutant PvDHFR was 10- to 25-fold less resistant than the S117N mutant to the inhibitors, but also exhibited higher kcat/Km value than the single mutant. The antifolate WR99210 was equally effective against both the wild-type and SP21 (S58R+S117N) mutant DHFRs, but was much less effective against some of the single mutants. Data on kinetic parameters and inhibitory constant suggest that the wild-type P. vivax is susceptible to antimalarial antifolates and that point mutations in the DHFR domain of P. vivax are responsible for antifolate resistance.

Plasmodium falciparum: differential display analysis of gene expression during gametocytogenesis

With the Plasmodium falciparum genome sequencing near completion, functional analysis of individual parasite genes has become the major task of the postgenomic era. Understanding the expression patterns of individual genes is the initial step toward this goal. In this report, we have examined gene expression during gametocytogenesis of the malaria parasite, P. falciparum, using a modified differential display (DD) method. The modifications of this method include adjusting the dNTP mix, using upstream primers with higher AT contents, and reducing the extension temperature of the polymerase chain reaction (PCR). With a combination of 16 arbitrary upstream primers and 3 one-base-anchored oligo(dT) primers, we have successfully cloned 80 unique cDNA tags from stage IV-V gametocytes. Further analysis by dot blots and semiquantitative reverse transcriptase-PCR showed that at least 49 cDNAs had induced or elevated levels of expression in gametocytes. These results indicate that this modified DD procedure is suitable for large-scale identification of developmentally regulated genes in the AT-rich Plasmodium genome.

Profiling the malaria genome: a gene survey of three species of malaria parasite with comparison to other apicomplexan species

We have undertaken the first comparative pilot gene discovery analysis of approximately 25,000 random genomic and expressed sequence tags (ESTs) from three species of Plasmodium, the infectious agent that causes malaria. A total of 5482 genome survey sequences (GSSs) and 5582 ESTs were generated from mung bean nuclease (MBN) and cDNA libraries, respectively, of the ANKA line of the rodent malaria parasite Plasmodium berghei, and 10,874 GSSs generated from MBN libraries of the Salvador I and Belem lines of Plasmodium vivax, the most geographically wide-spread human malaria pathogen. These tags, together with 2438 Plasmodium falciparum sequences present in GenBank, were used to perform first-pass assembly and transcript reconstruction, and non-redundant consensus sequence datasets created. The datasets were compared against public protein databases and more than 1000 putative new Plasmodium proteins identified based on sequence similarity. Homologs of previously characterized Plasmodium genes were also identified, increasing the number of P. vivax and P. berghei sequences in public databases at least 10-fold. Comparative studies with other species of Apicomplexa identified interesting homologs of possible therapeutic or diagnostic value. A gene prediction program, Phat, was used to predict probable open reading frames for proteins in all three datasets. Predicted and non-redundant BLAST-matched proteins were submitted to InterPro, an integrated database of protein domains, signatures and families, for functional classification. Thus a partial predicted proteome was created for each species. This first comparative analysis of Plasmodium protein coding sequences represents a valuable resource for further studies on the biology of this important pathogen.

Modelling parasite drug resistance: lessons for management and control strategies

Mathematical models of the evolution of drug resistance in infectious diseases are predominantly concentrated in three main areas: antimalarial, antibiotic and anthelmintic resistance. There appears to be little or no cross-reference between them. This literature was examined to identify factors that influence the evolution of drug resistance irrespective of the species and drug under study. The aim is to provide non-technical readers with a basic qualitative understanding of the issues and pitfalls involved in designing drug treatment regimens to minimize the evolution of resistance. The principal factors determining the rate at which resistance evolves appear to be (i) the starting frequency of resistance, (ii) the level and pattern of drug use, (iii) the drug's pharmacokinetic properties, (iv) the number of genes required to encode resistance, (v) the level of sexual recombination in the parasite population, (vi) intrahost dynamics and, in particular, whether 'crowding' effects are present, (vii) the genetic basis of resistance and (viii) the number of individual parasites in an infection. The relative importance of these factors depends on the biology of the organisms under consideration and external factors such as the extent to which the infrastructure of health care delivery constrains the practicalities of drug regimens.

Plasmodium falciparum: linkage disequilibrium between loci in chromosomes 7 and 5 and chloroquine selective pressure in Northern Nigeria

In view of the recent discovery (Molecular Cell 6, 861-871) of a (Lys76Thr) codon change in gene pfcrt on chromosome 7 which determines in vitro chloroquine resistance in Plasmodium falciparum, we have re-examined samples taken before treatment in our study in Zaria, Northern Nigeria (Parasitology, 119, 343-348). Drug resistance was present in 5/5 cases where the pfcrt 76Thr codon change was seen (100% positive predictive value). Drug sensitivity was found in 26/28 cases where the change was absent (93% negative predictive value). Allele pfcrt 76Thr showed strong linkage disequilibrium with pfmdr1 Tyr86 on chromosome 5, more complete than that between pfcrt and cg2 alleles situated between recombination cross-over points on chromosome 7. Physical linkage of cg2 with pfcrt may account for linkage disequilibrium between their alleles but in the case of genes pfmdr1 and pfcrt, on different chromosomes, it is likely that this is maintained epistatically through the selective pressure of chloroquine.

In vitro antiplasmodial activity and cytotoxicity of ethnobotanically selected South African plants

The resistance of Plasmodium spp. to currently used drugs has become a serious problem and efforts are being directed in obtaining new drugs with different structural features. One option favoured is the search for new plant derived antimalarial drugs. Bark and leaves of 20 extracts from 14 South African plant species were tested for in vitro antiplasmodial activity by means of the flow cytometric test. The most active extract of each species giving more than 70% inhibition at 50 microg/ml was selected for determination of IC(50) values. Two extracts had IC(50) values below 2 microg/ml, another seven had IC(50) values between 2 and 5 microg/ml while one had an IC(50) of 10.1 microg/ml. Chloroquine had an IC(50) of 0.043 microg/ml. Cytotoxicities of the five most active extracts at 50 microg/ml were determined with the monkey kidney cell toxicity test and the ID(50) values ranged between 35 and 100 microg/ml.

Homology modeling of wild type and pyrimethamine/cycloguanil-cross resistant mutant type Plasmodium falciparum dihydrofolate reductase. A model for antimalarial chemotherapy resistance

We propose a low-resolution model for both the wild type and the pyrimethamine (Pyr)/cycloguanil (Cyc) cross-resistant mutant type Plasmodium falciparum DHFR (PfDHFR), based on homology modeling using chicken liver DHFR as a template. The built models contain five alpha-helices, eight beta-sheets, eight tight turns and several loops. The Ramachandran plot for the models shows 95.3 and 100% of the amino acid residues in the favorable regions for the whole enzymes and for the active sites, respectively. Furthermore, we made a preliminary analysis of the complexes Pyr/Cyc-wild DHFR and Pyr/Cyc-mutant DHFR in order to explain the probable mechanism of resistance. Our results show that the steric factor may be the main structural cause of P. falciparum resistance toward antifolate drugs.

Mutation rates in the dihydrofolate reductase gene of Plasmodium falciparum

A new method has been established to define the limits on a spontaneous mutation rate for a gene in Plasmodium falciparum. The method combines mathematical modelling and large-scale in vitro culturing and calculates the difference in mutant frequencies at 2 separate time-points. We measured the mutation rate at 2 positions in the dihydrofolate reductase (DHFR) gene of 3D7, a pyrimethamine-sensitive line of P. falciparum. This line was re-cloned and an effectively large population was treated with a selective pyrimethamine concentration of 40 nM. We detected point mutations at codon-46 (TTA to TCA) and codon-108 (AGC to AAC), resulting in serine replacing leucine and asparagine replacing serine respectively in the corresponding gene product. The substitutions caused a decrease in pyrimethamine sensitivity. By mathematical modelling we determined that the mutation rate at a given position in DHFR was low and occurred at less than 2.5 x 10(-9) mutations/DHFR gene/replication. This result has important implications for Plasmodium genetic diversity and antimalarial drug therapy by demonstrating that even with low mutation rates anti-malarial resistance will inevitably arise when mutant alleles are selected under drug pressure.

Genome comparison of progressively drug resistant Plasmodium falciparum lines derived from drug sensitive clone

Chloroquine has been the mainstay of malaria chemotherapy for the past five decades, but resistance is now widespread. Pyrimethamine or proguanil form an important component of some alternate drug combinations being used for treatment of uncomplicated Plasmodium falciparum infections in areas of chloroquine resistance. Both pyrimethamine and proguanil are dihydrofolate reductase (DHFR) inhibitors, the proguanil acting primarily through its major metabolite cycloguanil. Resistance to these drugs arises due to specific point mutations in the dhfr gene. Cross resistance between cycloguanil and pyrimethamine is not absolute. It is, therefore, important to investigate mutation rates in P. falciparum for pyrimethamine and proguanil so that DHFR inhibitor with less mutation rate is favored in drug combinations. Hence, we have compared mutation rates in P. falciparum genome for pyrimethamine and cycloguanil. Using erythrocytic stages of P. falciparum cultures, progressively drug resistant lines were selected in vitro and comparing their RFLP profile with a repeat sequence. Our finding suggests that pyrimethamine has higher mutation rate compared to cycloguanil. It enhances the degree of genomic polymorphism leading to diversity of natural parasite population which in turn is predisposes the parasites for faster selection of resistance to some other antimalarial drugs.