Study on the biochemical basis of mefloquine resistant Plasmodium falciparum

Functional characterization of a unique cytochrome P450 in Toxoplasma gondii

The basic metabolic cytochrome P450 (CYP) proteins are essential for the biotransformation of sterols and xenobiotics. By contrast, the Toxoplasma gondii genome contains only one CYP gene, and the role of this enzyme in the physiology and biochemistry of apicomplexan parasites is unknown. Because it is a potential resistance gene, identifying the functionality of P450 in T. gondii is particularly important. Knocking out Tg-P450 had no significant effect on T. gondii survival, but mice infected with parasites overexpressing Tg-P450 exhibited significantly enhanced pathogenicity. Enzyme activity analyses demonstrated that this protein has mammalian CYP2B and CYP3A enzymatic activity. In addition, T. gondii lacking the P450 gene exhibited reduced resistance to quinine, mefloquine and clarithromycin compared with parasites overexpressing Tg-P450. These results suggest that P450 functions in T. gondii metabolism and detoxification is involved in vitally important processes in parasitic organisms, making this enzyme a potential drug target.

Patient age does not affect mefloquine concentrations in erythrocytes and plasma during the acute phase of falciparum malaria

Objective

To evaluate whether patient age has a significant impact on mefloquine concentrations in the plasma and erythrocytes over the course of treatment for uncomplicated falciparum malaria.

Methods

A total of 20 children aged between 8 and 11 years and 20 adult males aged between 22 and 41 years with uncomplicated falciparum malaria were enrolled in the study. Mefloquine was administered to patients in both age groups at a dose of 20 mg kg⁻¹. The steady-state drug concentrations were measured by reversed-phase high performance liquid chromatography.

Results

All patients had an undetectable mefloquine concentration on day 0. In adults, the plasma mefloquine concentrations ranged from 770 to 2930 ng mL⁻¹ and the erythrocyte concentrations ranged from 2000 to 6030 ng mL⁻¹. In children, plasma mefloquine concentrations ranged from 881 to 3300 ng mL⁻¹ and erythrocyte concentrations ranged from 3000 to 4920 ng mL⁻¹. There was no significant correlation between mefloquine concentrations in the plasma and erythrocytes in either adults or children.

Conclusion

In the present study, we observed no effect of patient age on the steady-state concentrations of mefloquine in the plasma and erythrocytes. We found that the mefloquine concentration in the erythrocytes was approximately 2.8-times higher than in the plasma. There were no significant correlations between mefloquine concentrations in the erythrocytes and plasma for either age group.

Protein profiling of mefloquine resistant Plasmodium falciparum using mass spectrometry-based proteomics

Malaria is a mosquito borne infectious disease caused by protozoa of genus Plasmodium. There are five species of Plasmodium that are found to infect humans. Plasmodium falciparum can cause severe malaria leading to higher morbidity and mortality of malaria than the other four species. Antimalarial resistance is the major obstacle to control malaria. Mefloquine was used in combination with Artesunate for uncomplicated P. falciparum in South East Asia and it has developed and established mefloquine resistance in this region. Here, gel-enhanced liquid chromatography/tandem mass spectrometry (GeLC-MS/MS)-based proteomics and label-free quantification were used to explore the protein profiles of mefloquine-sensitive and -induced resistant P. falciparum. A Thai P. falciparum isolate (S066) was used as a model in this research. Our data revealed for the first time that 69 proteins exhibited at least 2-fold differences in their expression levels between the two parasite lines. Of these, 36 were up-regulated and 33 were down-regulated in the mefloquine-resistant line compared with the mefloquine-sensitive line. These findings are consistent with those of past studies, where the multidrug resistance protein Pgh1 showed an up-regulation pattern consistent with that expected from its average 3-copy pfmdr1 gene number. Pgh1 and eight other up-regulated proteins (i.e., histo-aspartyl protease protein, exportin 1, eukaryotic translation initiation factor 3 subunit 8, peptidyl-prolyl cis-trans isomerase, serine rich protein homologue, exported protein 1, ATP synthase beta chain and phospholipid scramblase 1) were further validated for their expression levels using reverse transcriptase quantitative real-time PCR. The data support the up-regulation status in the mefloquine-resistant parasite line of all the candidate genes referred to above. Therefore, GeLC-MS/MS-based proteomics combined with label-free quantification is a reliable approach for exploring mefloquine resistance biomarkers in P. falciparum. Identification of these proteins leads to better understanding of mefloquine resistant mechanisms in malaria parasites.

Clarithromycin enhances the antimalarial efficacy of mefloquine via its increased bioavailability and disrupting P. falciparum apicoplast

AIM

Many important drugs like mefloquine are not being used because of the development of resistance and other related issues. In the present study, we aimed to control drug resistance by using combination therapy and tried to understand the mechanism involved.

MATERIAL AND METHODS

We have explored in vitro interaction of clarithromycin (CLTR), and mefloquine (MQ) against Pf3D7 and PfK1 strains. Bioavailability of MQ in parasitized RBC lysate was checked in the presence/absence of CLTR using HPLC method. Further tufA mRNA/protein expression was investigated to know the effect of both drugs on apicoplast by using qPCR and Western blotting.

KEY FINDINGS

MQ and CLTR inhibited growth of Pf3D7 and PfK1. CLTR showed its delayed antimalarial effect by its low IC50 values in the second cycle which indicates its effect on apicoplast. Downregulation of tufA expression on both mRNA and protein level supports this hypothesis. MQ and CLTR showed synergism/additiveness (mean ∑FICs = 0.89 and 1.26) against Pf3D7 and PfK1 respectively. It is evidenced from HPLC data that CLTR might have reduced metabolism of MQ in Plasmodium falciparum, leading to increased levels of MQ to produce enhanced antimalarial activity. The metabolism of CLTR is also reduced may be due to competitive metabolism of MQ via CYP3A4.

SIGNIFICANCE

The present study reveals that broad spectrum biological activities (i.e. antimalarial and antiviral) of MQ can be saved by using suitable partner drug like CLTR. This study also shows that CLTR increases the concentration of MQ and disrupts the apicoplast.

Confutation of the existence of sequence-conserved cytochrome P450 enzymes in Plasmodium falciparum

The aim of the present study was to find evidence for a homologous protein of the mammalian cytochrome P450 family member CYP2B1/B2 in Plasmodium falciparum at the nucleic acid level. Prior research had demonstrated enzyme activity in the parasite comparable to mammalian CYP1A, 2A, 2B and 2E enzymes and presence of CYP enzymes by spectrophotometric and electrophoretic analyses. In recent years, the transcriptome/proteome data of P. falciparum and other Plasmodium spp. have been published and we performed an in silico analysis to identify putative cytochrome P450 family members in the parasite. This analysis failed to identify homologs to CYP1A, 2A, 2B and 2E enzymes in Plasmodium. A prior study had also claimed the presence of a conserved CYP2B1/B2 gene in the parasite by using Northern analysis with a rat CYP2B1/B2 probe. We have repeated this analysis by cloning a rat CYP2B1/B2 cDNA and using it as a hybridization probe against total RNA extracted from P. falciparum K1 and 3D7 clones but did not obtain positive results. This is consistent with the transcriptome/proteome sequence data and suggests that the genus Plasmodium contains either only highly diverged CYP proteins which are not easily identified by their primary sequence or that they have been functionally replaced by other enzymes. It is suggested that further studies are performed that allow isolation and identification of such proteins through their functional activities.

In vitro antimalarial interactions between mefloquine and cytochrome P450 inhibitors

The treatment and control of malaria is becoming increasingly difficult due to resistance of Plasmodium falciparum strains resistance to commonly used antimalarials. Combination therapy is currently the strategy for combating multi-drug resistant falciparum malaria, through exploiting phamacodynamic synergistic effect and delaying the emergence of drug resistance. The objective of the present study was to investigate antimalarial activity of inhibitors of cytochrome P450 (CYP) enzyme including their interactions with the antimalarial mefloquine against chloroquine-resistant (K1) and chloroquine-sensitive (3D7) P. falciparum clones in vitro. Results showed IC(50) (drug concentration which produces 50% schizont maturation inhibition) values [mean (range)] of mefloquine against K1 and 3D7 clones to be 8.6 (8.0-9.3) and 12.1 (10.5-13.8) nM, respectively. The corresponding values for the IC(50) of quinidine were 32.2 (31.9-32.5) and 28.7 (28.4-29.0) nM, and for ketoconazole were 3.9 (3.7-4.1) and 4.8 (4.6-5.1) microM, respectively. Analysis of isobologram revealed a trend of decreasing of fraction IC(50) (FIC), which indicates synergistics of the either quinidine or ketoconazole with mefloquine for both chloroquine-resistant and chloroquine-sensitive clones.