Expanding the Antimalarial Drug Arsenal—Now, But How?

The number of available and effective antimalarial drugs is quickly dwindling. This is mainly because a number of drug resistance-associated mutations in malaria parasite genes, such as crt, mdr1, dhfr/dhps, and others, have led to widespread resistance to all known classes of antimalarial compounds. Unfortunately, malaria parasites have started to exhibit some level of resistance in Southeast Asia even to the most recently introduced class of drugs, artemisinins. While there is much need, the antimalarial drug development pipeline remains woefully thin, with little chemical diversity, and there is currently no alternative to the precious artemisinins. It is difficult to predict where the next generation of antimalarial drugs will come from; however, there are six major approaches: (i) re-optimizing the use of existing antimalarials by either replacement/rotation or combination approach; (ii) repurposing drugs that are currently used to treat other infections or diseases; (iii) chemically modifying existing antimalarial compounds; (iv) exploring natural sources; (v) large-scale screening of diverse chemical libraries; and (vi) through parasite genome-based (“targeted”) discoveries. When any newly discovered effective antimalarial treatment is used by the populus, we must maintain constant vigilance for both parasite-specific and human-related factors that are likely to hamper its success. This article is neither comprehensive nor conclusive. Our purpose is to provide an overview of antimalarial drug resistance, associated parasite genetic factors (1. Introduction; 2. Emergence of artemisinin resistance in P. falciparum), and the antimalarial drug development pipeline (3. Overview of the global pipeline of antimalarial drugs), and highlight some examples of the aforementioned approaches to future antimalarial treatment. These approaches can be categorized into “short term” (4. Feasible options for now) and “long term” (5. Next generation of antimalarial treatment—Approaches and candidates). However, these two categories are interrelated, and the approaches in both should be implemented in parallel with focus on developing a successful, long-lasting antimalarial chemotherapy

Glucuronidation of the Antiretroviral Drug Efavirenz by UGT2B7 and an in Vitro Investigation of Drug-Drug Interaction with ZidovudineS⃞

The non-nucleoside reverse transcriptase inhibitor efavirenz (EFV) is directly conjugated by the UDP-glucuronosyltransferase (UGT) pathway to form EFV-N-glucuronide (EFV-G), but the enzyme(s) involved has not yet been identified. The glucuronidation of EFV was screened with UGT1A and UGT2B enzymes expressed in a heterologous system, and UGT2B7 was shown to be the only reactive enzyme. The apparent Km value of UGT2B7 (21 μM) is similar to the value observed for human liver microsomes (24 μM), whereas the variant allozyme UGT2B7*2 (Tyr268) displayed similar kinetic parameters. Because 3′-azido-3′-deoxythymidine (AZT), one of the most current nucleotide reverse transcriptase inhibitors prescribed in combination with EFV, is also conjugated by UGT2B7, the potential metabolic interaction between EFV and AZT has been studied using human liver microsomes. Glucuronidation of both drugs was inhibited by one another, in a concentration-dependent manner. At Km values (25 and 1000 μM for EFV and AZT, respectively), EFV inhibited AZT glucuronidation by 47%, whereas AZT inhibited EFV glucuronidation by 23%. With a Ki value of 17 μM for AZT-glucuronide formation, EFV appears to be one of the most selective and potent competitive inhibitor of AZT glucuronidation in vitro. Moreover, assuming that concentrations of EFV achieved in plasma (Cmax = 12.9 μM) are in a range similar to its Ki value, it was estimated that EFV could produce a theoretical 43% inhibition of AZT glucuronidation in vivo. We conclude that UGT2B7 has a major role in EFV glucuronidation and that EFV could potentially interfere with the hepatic glucuronidation of AZT

Treatment with Coartem (Artemether-Lumefantrine) in Papua New Guinea

A recent drug efficacy trial reported Coartem (artemether-lumefantrine) to be highly effective against Plasmodium falciparum in children less than 5 years of age in Papua New Guinea (PNG). In contrast, we have observed high levels of treatment failures in non-trial conditions in a longitudinal cohort study in the same age group in PNG. Recrudescences were confirmed by genotyping of three different marker genes to provide optimal discrimination power between parasite clones. After excluding genetic host factors by genotyping potentially relevant cytochrome P450 loci, the high number of treatment failures in our study is best explained by poor adherence to complex dosing regimens in combination with insufficient fat supplementation, which are both crucial parameters for the outcome of Coartem treatment. In contrast to the situation in classic drug trials with ideal treatment conditions, our field survey highlights potential problems with unsupervised usage of Coartem in routine clinical practice and under program conditions

Microsatellite polymorphism within <it>pfcrt </it>provides evidence of continuing evolution of chloroquine-resistant alleles in Papua New Guinea

<p>Abstract</p> <p>Background</p> <p>Polymorphism in the <it>pfcrt </it>gene underlies <it>Plasmodium falciparum </it>chloroquine resistance (CQR), as sensitive strains consistently carry lysine (K), while CQR strains carry threonine (T) at the codon 76. Previous studies have shown that microsatellite (MS) haplotype variation can be used to study the evolution of CQR polymorphism and to characterize intra- and inter-population dispersal of CQR in Papua New Guinea (PNG).</p> <p>Methods</p> <p>Here, following identification of new polymorphic MS in introns 2 and 3 within the <it>pfcrt </it>gene (msint2 and msint3, respectively), locus-by-locus and haplotype heterozygosity (<it>H</it>) analyses were performed to determine the distribution of this intronic polymorphism among <it>pfcrt </it>chloroquine-sensitive and CQR alleles.</p> <p>Results</p> <p>For MS flanking the <it>pfcrt </it>CQR allele, <it>H </it>ranged from 0.07 (B5M77, -18 kb) to 0.094 (9B12, +2 kb) suggesting that CQ selection pressure was responsible for strong homogenisation of this gene locus. In a survey of 206 <it>pfcrt</it>-SVMNT allele-containing field samples from malaria-endemic regions of PNG, <it>H </it>for msint2 was 0.201. This observation suggests that <it>pfcrt </it>msint2 exhibits a higher level of diversity than what is expected from the analyses of <it>pfcrt </it>flanking MS. Further analyses showed that one of the three haplotypes present in the early 1980's samples has become the predominant haplotype (frequency = 0.901) in CQR parasite populations collected after 1995 from three PNG sites, when CQR had spread throughout malaria-endemic regions of PNG. Apparent localized diversification of <it>pfcrt </it>haplotypes at each site was also observed among samples collected after 1995, where minor CQR-associated haplotypes were found to be unique to each site.</p> <p>Conclusion</p> <p>In this study, a higher level of diversity at MS loci within the <it>pfcrt </it>gene was observed when compared with the level of diversity at <it>pfcrt </it>flanking MS. While <it>pfcrt </it>(K76T) and its immediate flanking region indicate homogenisation in PNG CQR parasite populations, <it>pfcrt </it>intronic MS variation provides evidence that the locus is still evolving. Further studies are needed to determine whether these intronic MS introduce the underlying genetic mechanisms that may generate <it>pfcrt </it>allelic diversity.</p