Novel alleles of Plasmodium falciparum dhfr that confer resistance to chlorcycloguanil

Fitness trade-offs in the evolution of dihydrofolate reductase and drug resistance in Plasmodium falciparum

BACKGROUND

Patterns of emerging drug resistance reflect the underlying adaptive landscapes for specific drugs. In Plasmodium falciparum, the parasite that causes the most serious form of malaria, antifolate drugs inhibit the function of essential enzymes in the folate pathway. However, a handful of mutations in the gene coding for one such enzyme, dihydrofolate reductase, confer drug resistance. Understanding how evolution proceeds from drug susceptibility to drug resistance is critical if new antifolate treatments are to have sustained usefulness.

METHODOLOGY/PRINCIPAL FINDINGS

We use a transgenic yeast expression system to build on previous studies that described the adaptive landscape for the antifolate drug pyrimethamine, and we describe the most likely evolutionary trajectories for the evolution of drug resistance to the antifolate chlorcycloguanil. We find that the adaptive landscape for chlorcycloguanil is multi-peaked, not all highly resistant alleles are equally accessible by evolution, and there are both commonalities and differences in adaptive landscapes for chlorcycloguanil and pyrimethamine.

CONCLUSIONS/SIGNIFICANCE

Our findings suggest that cross-resistance between drugs targeting the same enzyme reflect the fitness landscapes associated with each particular drug and the position of the genotype on both landscapes. The possible public health implications of these findings are discussed.

Quantitative structure activity relationship study of 2,4,6-trisubstituted-s-triazine derivatives as antimalarial inhibitors of Plasmodium falciparum dihydrofolate reductase

This study presents a quantitative structure activity relationships (QSAR) study on a pool of 19 bio-active s-triazine compounds. Molecular descriptors, kappa {¹κ}, chi {³χ}, x component of the dipole moment (μ(x) ), Coulson charge (q(N) ) on the nitrogen atom sandwiched between the two substituted carbons of the triazine ring, and total energy (E(T) ) obtained from AM1 calculations provide valuable information and have a significant role in the assessment of dihydrofolate reductase (DHFR) inhibitory activity of the compounds. By using the Genetic Function Approach (GFA) technique, five QSAR models have been drawn up with the help of these calculated descriptors and DHFR inhibitory activity data of the molecules. Among the obtained QSAR models presented in the study, statistically the most significant one is a four-parameter linear equation with the Lack-of-Fit value 0.5624, squared correlation coefficient R² value of 0.7697, and the squared cross-validated correlation coefficient R²(CV) value of 0.6469. The results are discussed in light of the main factors that influence the DHFR inhibitory activity.

Chemoprevention by Pyrimethamine

The goal of the current research was to investigate the chemopreventive potency of an antimalaria drug, pyrimethamine, in in vitro conditions. The fibrosarcoma (WEHI-164) cell line was used for evaluating cytotoxicity, matrix metalloproteinase 2 (MMP-2) activity, and apoptosis. Pyrimethamine and methotrexate were used at concentrations of 0-8 microg/ml in triplicate and 2-fold dilutions. MMP-2 activity was assessed using zymoanalysis method. For assessment of apoptosis, the terminal deoxyribonucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) method was used. Cytotoxicity analysis of pyrimethamine showed a greater tolerability than methotrexate at concentrations of 1-8 microg/ml. The dose-dependent inhibitory effect of pyrimethamine on MMP-2 activity was significantly less than that of methotrexate at concentrations of 1-8 microg/ml. Moreover, the rate of apoptosis for pyrimethamine-treated cells at different doses (0.1, 1, and 10 microg/ml) was 3.30%, 9.42%, and 11.32%, respectively. Our data suggest that pyrimethamine enables suppression of MMP-2 activity and induces apoptosis that could be assumed for chemoprevention therapy.

Malaria Chemotherapeutics Part I: History of Antimalarial Drug Development, Currently Used Therapeutics, and Drugs in Clinical Development

Since ancient times, humankind has had to struggle against the persistent onslaught of pathogenic microorganisms. Nowadays, malaria is still the most important infectious disease worldwide. Considerable success in gaining control over malaria was achieved in the 1950s and 60s through landscaping measures, vector control with the insecticide DDT, and the widespread administration of chloroquine, the most important antimalarial agent ever. In the late 1960s, the final victory over malaria was believed to be within reach. However, the parasites could not be eradicated because they developed resistance against the most widely used and affordable drugs of that time. Today, cases of malaria infections are on the rise and have reached record numbers. This review gives a short description of the malaria disease, briefly addresses the history of antimalarial drug development, and focuses on drugs currently available for malaria therapy. The present knowledge regarding their mode of action and the mechanisms of resistance are explained, as are the attempts made by numerous research groups to overcome the resistance problem within classes of existing drugs and in some novel classes. Finally, this review covers all classes of antimalarials for which at least one drug candidate is in clinical development. Antimalarial agents that are solely in early development stages will be addressed in a separate review.

Folate metabolism as a source of molecular targets for antimalarials

Folate metabolism of the malaria parasites provides two targets for current antimalarials: dihydrofolate reductase and dihydropteroate synthase. Dihydrofolate reductase inhibitors have been used as antimalarials over the past few decades, often in combination with dihydropteroate synthase inhibitors. Resistance to these antifolate drugs developed through mutations in both target enzymes. However, limited mutation possibilities gave opportunities for the development of new drugs. Furthermore, other enzymes in the folate and related pathways are potential new targets that remain to be exploited. These include thymidylate synthase, an enzyme fused with dihydrofolate reductase in the same protein chain, serine hydroxymethyltransferase, methylene tetrahydrofolate dehydrogenase, methionine synthase and enzymes in the glycine cleavage pathway.