Induction of Multidrug Tolerance in Plasmodium falciparum by Extended Artemisinin Pressure

Plasmodium falciparum resistance to artemisinin derivatives in Southeast Asia threatens global malaria control strategies. Whether delayed parasite clearance, which exposes larger parasite numbers to artemisinins for longer times, selects higher-grade resistance remains unexplored. We investigated whether long-lasting artemisinin pressure selects a novel multidrug-tolerance profile. Although 50% inhibitory concentrations for 10 antimalarial drugs tested were unchanged, drug-tolerant parasites showed higher recrudescence rates for endoperoxides, quinolones, and an antifolate, including partner drugs of recommended combination therapies, but remained susceptible to atovaquone. Moreover, the age range of intraerythrocytic stages able to resist artemisinin was extended to older ring forms and trophozoites. Multidrug tolerance results from drug-induced quiescence, which enables parasites to survive exposure to unrelated antimalarial drugs that inhibit a variety of metabolic pathways. This novel resistance pattern should be urgently monitored in the field because this pattern is not detected by current assays and represents a major threat to antimalarial drug policy.     

Antimalarial drugs: current status and new developments

Malaria continues to be a major threat in the developing world, with > 1 million clinical episodes and 3000 deaths every day. In the last century, malaria claimed between 150 and 300 million lives, accounting for 2 – 5% of all deaths. Currently ~ 40% of the world population resides in areas of active malaria transmission. The disease symptoms are most severe in young children and pregnant women. A total of 90% of the disease-associated mortality occurs in Subsaharan Africa, despite the fact that malaria is indigenous to most tropical regions. A licensed vaccine for malaria has not become a reality and antimalarial drugs are the only available method of treatment. Although chloroquine, the first synthetically developed antimalarial, proved to be an almost magical cure for > 30 years, the emergence and spread of chloroquine-resistant parasites has made it virtually ineffective in most parts of the world. Currently, artemisinin, a plant-derived antimalarial, is the only available drug that is globally effective against the parasite. Although several new drugs have been introduced in the past 30 years, widespread or isolated cases of resistance indicate that their window of effectiveness will be limited. Thus, there is an urgent need to develop new therapeutics and regimens for malaria control. This article presents an overview of the currently available antimalarial chemotherapy options and the efforts being undertaken to develop new drugs based on both the recent technological advances and modifications to the old remedies, and on combination therapies.     

An alkaline comet assay study on the antimalarial drug atovaquone in human peripheral blood lymphocytes: a study based on clinically relevant concentrations

Atovaquone, a hydroxynaphthoquinone, is an anti‐parasite drug, selectively targeting the mitochondrial respiratory chain of malaria parasite. It is used for both the treatment and prevention of malaria, usually in a fixed combination with proguanil. Although atovaquone has not often been associated with severe adverse reactions in the recommended dosages and has a relatively favorable side effect profile, the present study was undertaken to evaluate its cytogenotoxic potential towards human peripheral blood lymphocytes. Two different concentrations of atovaquone found in plasma when used in fixed‐dose combination with proguanile hydrochloride were used with and without S9 metabolic activation: 2950 ng ml^?1 used for prophylactic treatment and 11 800 ng ml^?1 used in treatment of malaria. The results showed that lymphocyte viability was not affected after the treatment, suggesting that atovaquone was not cytotoxic in the given concentrations. With the alkaline comet assay we demonstrated that in human peripheral blood lymphocytes no significant changes in comet parameters occurred after the treatment. There were no differences in tested parameters with the addition of S9 metabolic activation, indicating that atovaquone either has no metabolite or it is not toxic in the given concentrations. Since no effects were observed after the treatment, it is to be concluded that atovaquone is safe from the aspect of genototoxicity in the recommended dosages. Copyright © 2011 John Wiley & Sons, Ltd.     

克服肿瘤乏氧的超分子纳米粒的制备及其增强光动力疗效

肿瘤乏氧的微环境使光动力治疗的疗效降低,抑制肿瘤细胞自身呼吸耗氧比增加氧供给更能有效地克服肿瘤乏氧,提高光动力治疗疗效。为了实现这一策略,本研究采用纳米沉淀法制备了装载光敏剂维替泊芬(verteporfin,VER)、耗氧抑制剂阿托伐醌(atovaquone,ATO)及稳定剂聚乙烯吡咯烷酮(PVP)-K30的超分子纳米粒(VER-ATOsupramolecular nanoparticles,VER-ATO-SMN)。以粒径作为评价指标,采用单因素实验确定VER-ATO-SMN的最佳处方为:VER∶ATO(w/w)=1∶1,PVP-K30用量100 mg,N,N-二甲基甲酰胺∶水(v/v)=1∶10;对超分子纳米粒的形态、大小、粒径分布及包封率等进行表征。结果显示,所制得超分子纳米粒为规则球形,分散性良好,水合动力学直径为101.21±4.30 nm,VER和ATO的包封率分别为70.86%和77.52%;纳米粒光稳定性良好,且在模拟生理条件溶液中稳定。与游离VER和VER脂质体相比,VER-ATO-SMN在细胞水平上表现出更强的治疗效果,其机制是能够更有效地进入肿瘤细胞,并通过降低细胞的呼吸耗氧,提高细胞内的氧浓度,从而增加了VER介导的光动力治疗产生的活性氧的数量。本文还建立了4T1荷瘤小鼠模型(动物实验符合中国实验动物护理和使用准则,并经联勤保障部队第九〇〇医院实验动物伦理委员会批准)进行体内药效实验。结果表明,VER-ATO-SMN可有效抑制肿瘤生长甚至完全消融肿瘤。     

Preparation and physicochemical characterization of atovaquone‐containing liposomes

This work describes the preparation and the physicochemical properties of atovaquone‐loaded liposomes. It also describes drug release from the liposomes. As many factors can influence liposome stability, we studied several formulations, including different concentrations of atovaquone, phospholipids, and cholesterol. The effect of atovaquone (ATV) concentration was also evaluated. The highest binding percentage (100±2.5%) was obtained under alkaline conditions for a 2 mg/ml concentration of ATV. The percentage of encapsulation decreased significantly when drug concentrations increased. Drug uptake (expressed per unit mass of phospholipids) was nonlinearly related to equilibrium ATV concentration. A Langmuir‐type sorption was suggested (r = 0.978). In acidic or neutral buffer, the binding percentage reached 42.1 ± 0.02%. The increase of phospholipids and cholesterol concentrations did not significantly improve the ATV binding yield for the lowest ATV concentration. Conversely, ATV binding was significantly increased for the highest ATV concentration. All the formulations tested gave monodispersed liposomes with a mean diameter around 260 nm. The dilution (1/5–1/20) of liposomes in alkaline conditions induced a significant release of ATV (74% release). In acidic or neutral buffer no release was observed, suggesting an encapsulation process. Drug Dev. Res. 47:155–161, 1999. © 1999 Wiley‐Liss, Inc.     

Biorelevant Dissolution Testing to Predict the Plasma Profile of Lipophilic Drugs After Oral Administration

Purpose. To quantitatively compare in vitro dissolution data in biorelevant and compendial media, to investigate whether in vitro differences are reflected in the simulated plasma profile and to specify under which circumstances prediction of the plasma profile of orally administered lipophilic drugs can be achieved. Methods. Previously published dissolution data from seven products of four lipophilic drugs were compared using the first order model, the RRSBW distribution, and a model based on the Noyes-Whitney theory. Simulated plasma profiles were then obtained using a model-dependent approach. Simulated and observed plasma profiles were compared with the difference factor, f ₁. Results. No model consistently provided the best fit to the in vitrodata, which varied significantly with medium composition. Prediction of the plasma profile was possible (9.6 f ₁ 34.2) in seven out of eleven cases. Conclusions. Although prediction of the plasma profile of lipophilic drugs solely on the basis of in vitro data remains an ambitious target, this study shows that the plasma profile of a lipophilic drug can be predicted with appropriate in vitro dissolution data, provided that the absolute bioavailability of the drug is known and the drug has dissolution limited absorption.     

Short communication: Prevalence of mutations associated with resistance to atovaquone and to the antifolate effect of proguanil in Plasmodium falciparum isolates from northern Ghana

Atovaquone-proguanil has recently been introduced for the treatment and prophylaxis of malaria. However, resistance of Plasmodium falciparum is increasingly reported. We assessed P. falciparum polymorphisms associated with resistance to atovaquone (cytochrome b, cytb) and to cycloguanil, the active compound of proguanil (dihydrofolate reductase, dhfr) in 100 isolates from northern Ghana. None of these exhibited cytb codon 268 mutations. Moreover, no dhfr V16A, S108T or I164L mutations linked with cycloguanil resistance were detected. However, dhfr triple mutants (S108N-I51L-C59R) conferring resistance to proguanil and sulphadoxine-pyrimethamine were seen in 51% of the isolates. In northern Ghana, P. falciparum cytb codon 268 mutations associated with atovaquone resistance are absent. Although proguanil appears to act synergistically with atovaquone in a way different from its antifolate property, the abundance of dhfr polymorphisms will likely compromise the prevention of dissemination of atovaquone-resistant parasites once emerged.     

Failure of primary atovaquone prophylaxis for prevention of toxoplasmosis in hematopoietic cell transplant recipients

The efficacy of primary prophylaxis with atovaquone in preventing Toxoplasma reactivation and disease in hematopoietic cell transplant (HCT) recipients is unknown. We describe 2 cases of atovaquone prophylaxis failure in pre‐HCT Toxoplasma‐seropositive (pre‐HCTSP) recipients who underwent allogeneic HCT (allo‐HCT) and review the literature on atovaquone prophylaxis in HCT recipients.