Anticancer activity of artemisinin-derived trioxanes

The recent scientific and patent literature on therapeutically promising anticancer trioxanes derived from the antimalarial artemisinin are reviewed through mid-2006. Several of these new chemical entities, especially some trioxane dimers, have selective and very potent anticancer activity even at low nanomolar concentrations. A major opportunity exists for the development of these and related trioxanes into anticancer drug candidates.     

Extraordinarily potent antimalarial compounds: new, structurally simple, easily synthesized, tricyclic 1,2,4-trioxanes.

New, racemic, tricyclic trioxane alcohol 3 was designed and synthesized as a structurally simple analog of clinically useful, tetracyclic, antimalarial artemisinin. A series of 20 ester and ether derivatives of alcohol 3 were prepared easily, without destruction of the essential trioxane system. Chemical structure-antimalarial activity for each derivative was evaluated in vitro against chloroquine-resistant and chloroquine-sensitive Plasmodium falciparum parasites. Many of these derivatives were highly efficacious; carboxylate ester 9f, carbamate ester 10a, and sulfonate ester 12a had antimalarial potency similar to that of artemisinin, and carboxylate esters 9b and 9d, carbamate esters 10b and 10c, and phosphate esters 11a-c had antimalarial potency up to 7 times higher than that of artemisinin. Several of these most active analogs (e.g., carboxylate 9b and carbamates 10a and 10c) are stable crystalline solids, a feature of considerable practical value for any new drug candidate.     

The therapeutic potential of semi-synthetic artemisinin and synthetic endoperoxide antimalarial agents

Artemisinin derivatives such as artesunate, dihydroartemisinin and artemether are playing an increasing role in the treatment of drug-resistant malaria. They are the most potent antimalarials available, rapidly killing all asexual stages of the parasite Plasmodium falciparum. This review highlights the recent developments in the area of improved second-generation semi-synthetic artemisinin derivatives and fully synthetic antimalarial endoperoxide drugs. In pursuit of synthetic analogues of the artemisinins, one of the major challenges for chemists in this area has been the non-trivial development of techniques for the introduction of the peroxide bridge into candidate drugs. Although chemical research has enabled chemists to incorporate the endoperoxide ‘warhead’ into synthetic analogues of artemisinin, significant drawbacks with many candidates have included comparatively poor antimalarial activity, non-stereoselective syntheses and chemical approaches that are not readily amenable to scale up. However, very recent progress with synthetic 1,2,4-trioxolanes provides a new benchmark for future medicinal chemistry efforts in this area.     

Artemisia annua L.: a source of novel antimalarial drugs

Artemisia annua L. contains artemisinin, an endoperoxide sesquiterpene lactone, mainly in its leaves and inflorescences. This compound and a series of derivatives have attracted attention because of their potential value as antimalarial drugs. In this review a survey of the currently available literature data is given. It includes phytochemical aspects, such as constituents ofA. annua, the artemisinin content during the development of the plant and its biosynthesis, isolation, analysis and stability. Total chemical synthesis of artemisinin is referred to, as well as structure—activity relationships of derivatives and simplified analogues. Pharmacological studies are summarized, including the mechanism of action, interaction of the antimalarial activity with other drugs, possible occurrence of resistance to artemisinin, clinical results, toxicological aspects, metabolism and pharmacokinetics. Finally, plant cell biotechnologyy is mentioned as a possible means to obtain plants and cell cultures with higher artemisinin contents, allowing an industrial production of pharmaceuticals containing this novel drug.     

Investigating the antimalarial action of 1,2,4-trioxolanes with fluorescent chemical probes

The 1,2,4-trioxolanes are a new class of synthetic peroxidic antimalarials currently in human clinical trials. The well-known reactivity of the 1,2,4-trioxolane ring toward inorganic ferrous iron and ferrous iron heme is proposed to play a role in the antimalarial action of this class of compounds. We have designed structurally relevant fluorescent chemical probes to study the subcellular localization of 1,2,4-trioxolanes in cultured Plasmodium falciparum parasites. Microscopy experiments revealed that a probe fluorescently labeled on the adamantane ring accumulated specifically in digestive vacuole-associated neutral lipid bodies within the parasite while an isosteric, but nonperoxidic, congener did not. Probes fluorescently labeled on the cyclohexane ring showed no distinct localization pattern. In their subcellular localization and peroxidative effects, 1,2,4-trioxolane probes behave much like artemisinin-based probes studied previously. Our results are consistent with a role for adamantane-derived carbon-centered radicals in the antimalarial action of 1,2,4-trioxolanes, as hypothesized previously on the basis of chemical reactivity studies.     

The therapeutic potential of semi-synthetic artemisinin and synthetic endoperoxide antimalarial agents

Artemisinin derivatives such as artesunate, dihydroartemisinin and artemether are playing an increasing role in the treatment of drug-resistant malaria. They are the most potent antimalarials available, rapidly killing all asexual stages of the parasite Plasmodium falciparum. This review highlights the recent developments in the area of improved second-generation semi-synthetic artemisinin derivatives and fully synthetic antimalarial endoperoxide drugs. In pursuit of synthetic analogues of the artemisinins, one of the major challenges for chemists in this area has been the non-trivial development of techniques for the introduction of the peroxide bridge into candidate drugs. Although chemical research has enabled chemists to incorporate the endoperoxide 'warhead' into synthetic analogues of artemisinin, significant drawbacks with many candidates have included comparatively poor antimalarial activity, non-stereoselective syntheses and chemical approaches that are not readily amenable to scale up. However, very recent progress with synthetic 1,2,4-trioxolanes provides a new benchmark for future medicinal chemistry efforts in this area.     

Fourier transform infrared investigation of non-heme Fe(III) and Fe(II) decomposition of artemisinin and of a simplified trioxane alcohol.

Fourier transform infrared spectra are reported for the Fe(III)- and Fe(II)-mediated activation of the antimalarial agents artemisinin 1 and its simplified synthetic analogue, trioxane alcohol 2. By monitoring the frequencies of the newly established marker lines in the FTIR spectra, the products of the Fe(II) and Fe(III) reactions have been characterized. In both reactions, artemisinin is activated giving a product mixture of a ring-contracted tetrahydrofuran acetatal 3, C(4)-hydroxy deoxyartemisinin 4, and deoxyartemisinin 5. These data illustrate that the oxidation state of the iron places no restrictions on the endoperoxide reduction mechanism. The FTIR difference (light - dark) spectra indicate that the endoperoxide moiety of artemisinin is photolabile and that the resulted products have the same vibrational characteristics as those observed in the reactions with Fe(II) and Fe(III). The use of 18O-18O enriched endoperoxide in 2 has allowed us to identify two oxygen sensitive modes in the reactions with Fe(II). The reduction of the peroxide bond by Fe(II) in trioxane alcohol 2 follows both the C-C cleavage and 1,5-H shift pathways and produces a ring-contracted tetrahydrofuran acetal 6 which is converted to tetrahydrofuran aldehyde 7 and C(4)-hydroxy deoxytrioxane alcohol 8, respectively. The cleavage of the O-O bond in 1 and 2 by iron and the ability to correlate vibrational properties of the reaction products with structural properties of the isolated products suggest that infrared spectroscopy is an appropriate tool to study the mode of action of antimalarial endoperoxides.     

Artemisinin derivatives: a patent review (2006 – present)

Introduction: The isolation of artemisinin from an ancient Chinese remedy in the early 1970s heralded the beginning of a new era in antimalarial drug therapy culminating in artemisinin-based combination therapies currently being the mainstay of malaria treatment worldwide. Ongoing research on this compound and its derivatives has revealed its potential use in treating other infectious and noninfectious diseases.

Areas covered: This review provides a summary of patents published globally from January 2006 to June 2012 covering promising artemisinin derivatives and artemisinin-based drug combinations developed for use in various therapeutic areas.

Expert opinion: The diversity of semi-synthetic artemisinin derivatives has been limited to the same design strategy of modifying the artemisinin molecule at the same positions due to inherent synthetic challenges. To address this, future endeavors should include: the use of biotransformation strategies to modify other positions in the sesquiterpene ring while retaining the endoperoxide bridge; the design and synthesis of synthetic ozonides based on the pharmacophoric endoperoxide motif and drug repositioning approaches to artemisinin-based combination therapy. A better understanding of the mechanism of action of artemisinin derivatives and their biomolecular targets may provide an invaluable tool for the development of derivatives with a wider array of activity and greater clinical utility than currently appreciated.     

Synthesis and antimalarial assessment of a new series of orally active amino-functionalized spiro 1,2,4-trioxanes

Keto-trioxanes 7a-d, easily accessible in two steps from allylic alcohols 5a-d, underwent reductive amination with substituted anilines to furnish amino-functionalized trioxanes 8a-i, 9a-i, 10a-i, and 11a-i. All these new trioxanes were assessed for their oral antimalarial activity against multidrug-resistant Plasmodium yoelii nigeriensis in Swiss mice. 2-Naphthalene-based trioxanes 9c and 9i, the most active compounds of the series, provided 100% protection to the malaria-infected mice at 24 mg/kg × 4 days, while the related trioxane 9b and phenanthrene-based trioxane 11e provided a similar level of protection at 48 mg/kg × 4 days. All other trioxanes, except 10c, 10d, and 10g, provided 100% protection at 96 mg/kg × 4 days. In this model, β-arteether provided 100% protection at 48 mg/kg × 4 days and 20% protection at 24 mg/kg × 4 days.     

Trioxaquines: hybrid molecules for the treatment of malaria

Artemisinin, with its 1,2,4-trioxane as active motif, is now the first-line treatment for multidrug-resistant malaria. The endoperoxide ring is essential for the antimalarial activity of artemisinin. Based on its mechanism of action, new hybrid molecules named trioxaquines with a dual mode of action have been designed. Trioxaquines are made by the covalent attachment of a trioxane, having alkylating ability, to a quinoline, known to easily penetrate within infected erythrocytes. This review discusses the importance of various hybrid molecules of artemisinin and 4-aminoquinoline in the treatment of malaria and the evolution of a trioxaquine hybrid as a promising antimalarial drug candidate.     

Chloroquine and artemisinin: six decades of research--what next?

Over the next decade drugs will remain the focus of continuous efforts to control malaria, with a contribution from pharmacogenomic development. Quinine, extracted from Cinchona bark, has been the source for aminoquinoline drugs such as chloroquine; more recently, artemisinin extracted from Artemisia allowed the design of artemisinin mimics containing a trioxane structure. Here, we examine parallels between chloroquine and artemisinin in terms of pharmacological target discovery, mechanism of action and parasite resistance. The widespread use of chloroquine has dramatically reduced its therapeutic response, thus recent strategies are based on artemisinin combinations.     

Synthetic peroxides as potent antimalarials. News and views

The present review describes the development of synthetic cyclic peroxides, which are designed to surpass the antimalarial activity of the lead molecule, the natural product (+)-artemisinin and some of its C10 derivatives. To begin with, tricyclic and bicyclic 1,2,4-trioxanes are taken to show how the pharmacophore was identified and chirality proved to be irrelevant. The action of ferrous salts on trioxanes illustrates the structural elements that are needed so that reductive breaking of the peroxide bond leads to C-centered radicals, the alleged parasiticidal agents. Views are expressed on how heme, Plasmodium SERCA, and plain ferrous ions, either as targets or activators, could be implicated in the mode of action. Thereafter, news about 1,2,4-trioxolanes, 1,2,4-trioxanes, 1,2,4,5-tetraoxanes, 1,2-dioxolanes, and 1,2-dioxanes is recounted, emphasizing aspects of design, mechanism, and the importance of the adamantane entity for buttressing activity. News about compounds made up of a trioxane covalently bound to aminoquinoline, so-called hybrid molecules, is reported together with a view that they might be better than mechanical mixtures. No new antimalarial can be considered without a word about the risk posed by the parasite developing resistance. The review is not intended to be exhaustive. Some gaps prior to 2009 are filled in, while the later literature up to the end of July 2011 has been covered. Artemisinin and its derivatives fall outside the scope of the review. Nevertheless, some mechanistic insights garnered from artemisinin, which are relevant to synthetic peroxides, are included.     

Progress in the research of artemisinin-related antimalarials: An update

Artemisinin, a sesquiterpene lactone endoperoxide isolated fromArtemisia annua L., and a number of its semisynthetic derivatives have shown to possess antimalarial properties. They are all eflective againstPlasmodium parasites that are resistant to the newest and commonly used antimalarial drugs. This article gives a survey of the literature dealing with artemisinin-relaled antimalarial issues that have appeared from the end of 1989 up to the beginning of 1994. A broad range of medical and pharmaceutical disciplines is covered, including phytochemical aspects like the selection of high-producing plants, analytical procedures, and plant biotechnology. Furthermore, the organic synthesis of artemisinin derivatives is discussed, as well as their mechanism of action and antimalarial activity, metabolism and pharmacokinetics, clinical studies, sideeffects and toxicology, and biological activities other than antimalarial activity.     

Anticancer properties of artemisinin derivatives and their targeted delivery by transferrin conjugation

Artemisinin and its derivatives are well known antimalaria drugs and particularly useful for the treatment of infection of Plasmodium falciparum malaria parasites resistant to traditional antimalarials. Artemisinin has an endoperoxide bridge that is activated by intraparasitic heme-iron to form free radicals, which kill malaria parasites by alkylating biomolecules. In recent years, there are many reports of anticancer activities of artemisinins both in vitro and in vivo. Artemisinins have inhibitory effects on cancer cell growth, including many drug- and radiation-resistant cancer cell lines. The cytotoxic effect of artemisinin is specific to cancer cells because most cancer cells express a high concentration of transferrin receptors on cell surface and have higher iron ion influx than normal cells via transferrin mechanism. In addition, some artemisinin analogs have been shown to have antiangiogenesis activity. Artemisinin tagged to transferrin via carbohydrate chain has also been shown to have high potency and specificity against cancer cells. The conjugation enables targeted delivery of artemisinin into cancer cells. In this review, we discuss the anticancer activities and mechanisms of action of artemisinins and the transferrin-conjugate.     

Artemisia annua L.: a source of novel antimalarial drugs

Artemisia annua L. contains artemisinin, an endoperoxide sesquiterpene lactone, mainly in its leaves and inflorescences. This compound and a series of derivatives have attracted attention because of their potential value as antimalarial drugs. In this review a survey of the currently available literature data is given. It includes phytochemical aspects, such as constituents ofA. annua, the artemisinin content during the development of the plant and its biosynthesis, isolation, analysis and stability. Total chemical synthesis of artemisinin is referred to, as well as structure—activity relationships of derivatives and simplified analogues. Pharmacological studies are summarized, including the mechanism of action, interaction of the antimalarial activity with other drugs, possible occurrence of resistance to artemisinin, clinical results, toxicological aspects, metabolism and pharmacokinetics. Finally, plant cell biotechnologyy is mentioned as a possible means to obtain plants and cell cultures with higher artemisinin contents, allowing an industrial production of pharmaceuticals containing this novel drug.     

青蒿素生物合成的研究进展

青蒿素是上世纪70年代初中国科学家从药用植物黄花蒿(Artemisia annua L.)中分离得到的、含有过氧基团的新型倍半萜内酯抗疟药物。笔者综述了近年来青蒿素生物合成机理的研究进展,从青蒿素的前体如青蒿乙素和青蒿酸到青蒿素的合成,讨论了青蒿素生物合成的基本途径;从参与青蒿素合成的关键酶组成和功能,探讨了合成路线的主要构成,并介绍了青蒿素合成代谢调控的基因工程手段,为青蒿素的生物技术开发提供参考。     

In-vitro Interaction of Artemisinin with Intact Human Erythrocytes,Erythrocyte Ghosts,Haemoglobin and Carbonic Anhydrase

The kinetics of the interaction of the antimalarial compound artemisinin with human erythrocytes, erythrocyte ghosts, haemoglobin and carbonic anhydrase were evaluated in-vitro. Artemisinin plasma concentrations, measured by HPLC (high pressure liquid chromatography), decreased with time during incubations with whole blood and erythrocyte suspensions of varying haematocrit. Artemisinin concentrations declined more rapidly during incubations under oxygen-poor as compared to oxygen-rich conditions. Artemisinin concentrations did not decrease during incubation with erythrocyte ghosts suspended in plasma suggesting that the drug does not bind avidly to red blood cell membranes. There was no decline in concentrations of artemisinin in the presence of carbonic anhydrase. The disappearance of the drug in solutions containing haemoglobin was very rapid and was even more so when the incubation was performed under an argon- instead of oxygen-rich atmosphere. The results suggest that drug blood clearance may be considered for inclusion in a pharmacokinetic model, but does not invalidate in-vivo plasma concentration-time data and their relevance for clinical effects. Furthermore, caution is advised when relating measurements of in-vitro potency to drug levels in patients. Finally, the enhanced artemisinin disappearance when oxygen tension is low may contribute towards the explanation of the selective toxicity of the endoperoxide drugs to Plasmodium falciparum parasite.     

New developments in synthetic peroxidic drugs as artemisinin mimics

The present review describes the current status of synthetic cyclic peroxides, trioxanes and trioxolanes that show significant promise as antimalarial drugs because of their artemisinin-like activity. The literature from 1996 onwards is critically surveyed to provide an update on how an age-old, persistent, debilitating and frequently deadly disease could be treated by new, affordable and effective medicines possessing the peroxide pharmacophore. The review is not exhaustive and does not cover recent progress on the lead structure artemisinin and its derivatives. Nevertheless, some mechanistic aspects gleaned from artemisinin that have relevance to synthetic peroxides are discussed.     

Evidence of a complex between adriamycin derivatives and cardiolipin: Possible role in cardiotoxicity

Most of the mitochondrial damage induced by antimitotic drugs of the adriamycin family could be due to the high affinity of these drugs for the membrane. The prime interaction between the anthracycline drug and this membrane would explain specific alterations observed on mitochondria. Cardiolipin has been proposed as a privileged target. We have tested this hypothesis here. Model membranes (lipid monolayers, liposomes) were used to demonstrate the interaction between these anthracycline drugs and different phospholipids. A new surface potential technique showed the specificity of adriamycin derivatives for cardiolipin whereas no complexation was observed with neutral phospholipids (dipalmitoyl lecithin and egg lecithin). Association constants were evaluated and a good correlation was obtained between the mitochondrial toxicity of each drug and its affinity for cardiolipin. Fluorescence measurements were carried out in order to locate precisely the position of the drug in the lipid bilayer. Perturbations of the lipid organization after complex formation were analysed using phospholipase A₂ as an enzymic probe.     

Lipid bodies in innate immune response to bacterial and parasite infections

Lipid bodies (also known as lipid droplets, adiposomes) are dynamic organelles with key roles in regulating storage and turnover of lipids in different cells and organisms. The emerging role of lipid bodies as inflammatory organelles raises lipid body status to critical regulators of different inflammatory and infectious diseases and key markers of cell activation. Notably, lipid body biogenesis is highly regulated and is cell and stimuli specific. Lipid body structural features, including lipid and protein composition may vary according to the cell type, activation state and inflammatory environment and thus may determine different cellular functions for lipid bodies. Here we will review the morphological and structural aspects of lipid bodies, the regulated mechanisms of formation, as well as lipid body functions in cells involved in the innate immune response during bacterial and parasite infections.