Antimalarial cyclic peroxy ketals

Exploration of artemisinin derivatives and synthetic peroxides in antimalarial drug discovery research

Malaria is a life-threatening infectious disease caused by protozoal parasites belonging to the genus Plasmodium. It caused an estimated 405,000 deaths and 228 million malaria cases globally in 2018 as per the World Malaria Report released by World Health Organization (WHO) in 2019. Artemisinin (ART), a "Nobel medicine" and its derivatives have proven potential application in antimalarial drug discovery programs. In this review, antimalarial activity of the most active artemisinin derivatives modified at C-10/C-11/C-16/C-6 positions and synthetic peroxides (endoperoxides, 1,2,4-trioxolanes, 1,2,4-trioxanes, and 1,2,4,5-tetraoxanes) are systematically summarized. The developmental trend of ART derivatives, and cyclic peroxides along with their antimalarial activity and how the activity is affected by structural variations on different sites of the compounds are discussed. This compilation would be very useful towards scaffold hopping aimed at avoiding the unnecessary complexity in cyclic peroxides, and ultimately act as a handy resource for the development of potential chemotherapeutics against Plasmodium species.

Glycosyl hydroperoxides: a new class of potential antimalarial agents

Motivated by the antimalarial properties observed in organic peroxides, an extensive series of glycosyl hydroperoxides was prepared with the aim of identifying new bioactive molecules. Selected compounds were tested against a Plasmodium falciparum culture (chloroquine-susceptible strain D10 and chloroquine-resistant strain W2). Screening results indicated that the factors critical for antimalarial activity were the presence of a hydroperoxide moiety and solubility in water at pH 5.0. Moreover, the ability to inhibit β-hematin formation in vitro has been evaluated (BHIA Assay).

A new class of antimalarial dioxanes obtained through a simple two-step synthetic approach: rational design and structure-activity relationship studies

A new series of simple endoperoxides, characterized by a 3-methoxy-1,2-dioxane scaffold, was designed on the basis of a previously developed pharmacophore. Through a simplified and versatile scheme of synthesis, which utilizes cheap and commercially available starting materials, it was possible to obtain several structurally and stereochemically different compounds that were tested against P. falciparum. Most of compounds showed antimalarial activity in the low micromolar range and no cellular toxicity, all being significantly more active on chloroquine resistant (CQ-R) than on chloroquine sensitive (CQ-S) strains. Resulting structure-activity relationships were analyzed by means of experimental and computational techniques, validating our design rationale and tailoring it for the new scaffold. Our study demonstrated that according to the hypothesized mechanism of action, the antimalarial activity can be improved through rational structural modifications, paving the way for the development of new simplified antimalarial endoperoxides.

Malaria-infected mice are cured by a single oral dose of new dimeric trioxane sulfones which are also selectively and powerfully cytotoxic to cancer cells

A new series of 6 dimeric trioxane sulfones has been prepared from the natural trioxane artemisinin in five or six chemical steps. One of these thermally and hydrolytically stable new chemical entities (4c) completely cured malaria-infected mice via a single oral dose of 144 mg/kg. At a much lower single oral dose of only 54 mg/kg combined with 13 mg/kg of mefloquine hydrochloride, this trioxane dimer 4c as well as its parent trioxane dimer 4b also completely cured malaria-infected mice. Both dimers 4c and 4b were potently and selectively cytotoxic toward five cancer cell lines.

Artemisinin and a series of novel endoperoxide antimalarials exert early effects on digestive vacuole morphology

Artermisinin and its derivatives are now the mainstays of antimalarial treatment; however, their mechanism of action is only poorly understood. We report on the synthesis of a novel series of epoxy-endoperoxides that can be prepared in high yields from simple starting materials. Endoperoxides that are disubstituted with alkyl or benzyl side chains show efficient inhibition of the growth of both chloroquine-sensitive and -resistant strains of Plasmodium falciparum. A trans-epoxide with respect to the peroxide linkage increases the activity compared to that of its cis-epoxy counterpart or the parent endoperoxide. The novel endoperoxides do not show a strong interaction with artemisinin. We have compared the mechanism of action of the novel endoperoxides with that of artemisinin. Electron microscopy reveals that the novel endoperoxides cause the early accumulation of endocytic vesicles, while artemisinin causes the disruption of the digestive vacuole membrane. At longer incubation times artemisinin causes extensive loss of organellar structures, while the novel endoperoxides cause myelin body formation as well as the accumulation of endocytic vesicles. An early event following endoperoxide treatment is the redistribution of the pH-sensitive probe LysoSensor Blue from the digestive vacuole to punctate structures. By contrast, neither artemisinin nor the novel endoperoxides caused alterations in the morphology of the endoplasmic reticulum nor showed antagonistic antimalarial activity when they were used with thapsigargin. Analysis of rhodamine 123 uptake by P. falciparum suggests that disruption of the mitochondrial membrane potential occurs as a downstream effect rather than as an initiator of parasite killing. The data suggest that the digestive vacuole is an important initial site of endoperoxide antimalarial activity.

Drug discovery and development for neglected parasitic diseases

Diseases caused by tropical parasites affect hundreds of millions of people worldwide but have been largely neglected for drug development because they affect poor people in poor regions of the world. Most of the current drugs used to treat these diseases are decades old and have many limitations, including the emergence of drug resistance. This review will summarize efforts to reinvigorate the drug development pipeline for these diseases, which is driven in large part by support from major philanthropies. The organisms responsible for these diseases have a fascinating biology, and many potential biochemical targets are now apparent. These neglected diseases present unique challenges to drug development that are being addressed by new consortia of scientists from academia and industry.

Fe(II)-induced reduction of labelled endoperoxides. NMR degradation studies on G3 factor and its methyl ether

The behavior of G3 factor and of its methylated or fluorinated analogues G3Me and G3F, was studied under Fe(II) conditions. Degradation products were isolated and characterized in each case. The use of labelled compounds allowed us to propose mechanisms in which a tertiary radical is involved. This radical rearranges by 5-exo-trig cyclization, or disproportionates in the case of G3Me. A correlation between antiplasmodial activity and stability of this radical is proposed.

Non-stochastic and stochastic linear indices of the ‘molecular pseudograph’s atom adjacency matrix’: application to ‘in silico’ studies for the rational discovery of new antimalarial compounds

Malaria is one of the most deadly diseases, affecting million of people especially in developing countries. Because of the rapidly increasing threat worldwide of malaria epidemics multidrugs resistant to therapies, there is an urgent global need to discover new classes of antimalarial compounds. In an effort to overcome this problem, we have investigated the use of structure-based classification models for the 'rational' selection/identification or design/optimization of new lead antimalarials from virtual combinatorial data sets. In this sense, TOpological MOlecular COMputer Design strategy (TOMOCOMD approach) has been introduced in order to obtain two quantitative models for the discrimination of antimalarials. A collected data set containing 597 antimalarial compounds is presented as a helpful tool not only for theoretical chemist but for other researchers in this area. The validated models (including non-stochastic and stochastic indices) classify correctly more than 90% of compounds in both training and external prediction data sets. They showed high Matthews' correlation coefficients; 0.87 and 0.82 for training and 0.86 and 0.79 for test set. The TOMOCOMD-CARDD approach implemented in this work was successfully compared with two of the most useful models for antimalarials selection reported so far. Thus we expect that these two QSAR models can be used in the identification of previously un-known antimalarials compounds.

Development of artemisinin and its structurally simplified trioxane derivatives as antimalarial drugs

Artemisinin and simplified trioxane analogs constitute a promising class of antimalarial chemotherapeutic agents. Their development since the early 1970s into clinical trials and clinical use has drawn much attention from medical scientists worldwide although the crude extract containing artemisinin has been used in China for treatment of fever for many centuries. Many research groups have independently and collaboratively conducted various studies on the artemisinin system both in search for the new compounds more antimalarially active than the parent artemisinin and in an attempt to understand its molecular mechanism(s) of action. Ongoing studies have provided a better understanding of the putative intermediates essential for the antimalarial activity and have led to designer trioxanes whose chemical structures have been simplified and modified to increase efficacy while lowering toxicity. Other desirable features beneficial to clinical uses such as bioavailability, drug stability and water solubility have been considered, and portions of the trioxane skeleton have been added or modified to accommodate these parameters accordingly.

Antimalarial chemotherapeutic peroxides: artemisinin, yingzhaosu A and related compounds

Mechanism-based rational design and gram-scale chemical synthesis have produced some new trioxane and endoperoxide antimalarial drug candidates that are efficacious and safe. This review summarises recent achievements in this area of peroxide drug development for malaria chemotherapy.

Synthetic studies toward bioactive cyclic peroxides from the marine sponge Plakortis angulospiculatus

[reaction: see text] The total synthesis of two stereoisomers of a bioactive cyclic peroxide isolated from the marine sponge Plakortis angulospiculatus has been achieved in 18 steps with an overall yield of 2.8%. Diels-Alder addition of singlet oxygen to an acyclic triene carboxylic acid precursor was used to construct the 3,6-dihydro-1,2-dioxin ring. By comparing spectral data of the synthesized compounds and the natural material, we tentatively assign the absolute stereochemistry for the natural product as 3S,6R,8S,10R.

Antimalarial activity of synthetic 1,2,4-trioxanes and cyclic peroxy ketals, a quantum similarity study

In this work, the antimalarial activity of two series of 20 and 7 synthetic 1,2,4-trioxanes and a set of 20 cyclic peroxy ketals are tested for correlation search by means of Molecular Quantum Similarity Measures (MQSM). QSAR models, dealing with different biological responses (IC90, IC50 and ED90) of the parasite Plasmodium Falciparum, are constructed using MQSM as molecular descriptors and are satisfactorily correlated. The statistical results of the 20 1,2,4-trioxanes are deeply analyzed to elucidate the relevant structural features in the biological activity, revealing the importance of phenyl substitutions.

Antimalarial sulfide, sulfone, and sulfonamide trioxanes

A series of trioxanes featuring sulfide, sulfone, and sulfonamide substituents in diverse positions has been prepared. Structure-activity relationship (SAR) generalizations highlight two major factors controlling the antimalarial potency of these new chemical entities: (1) the proximity of the sulfur-containing substituent to the crucial peroxide bond and (2) the oxidation state of the sulfur-containing substituent. Generally, sulfones are more antimalarially potent than the corresponding sulfides.

Glutathione and trypanothione in parasitic hydroperoxide metabolism

Thiol-dependent hydroperoxide metabolism in parasites is reviewed in respect to potential therapeutic strategies. The hydroperoxide metabolism of Crithidia fasciculata has been characterized to comprise a cascade of three enzymes, trypanothione reductase, tryparedoxin, and tryparedoxin peroxidase, plus two supportive enzymes to synthesize the redox mediator trypanothione from glutathione and spermidine. The essentiality of the system in respect to parasite vitality and virulence has been verified by genetic approaches. The system appears to be common to all genera of the Kinetoplastida. The terminal peroxidase of the system belongs to the protein family of peroxiredoxins which is also represented in Entamoeba and a variety of metazoan parasites. Plasmodial hydroperoxide metabolism displays similarities to the mammalian system in comprising glutathione biosynthesis, glutathione reductase, and at least one glutathione peroxidase homolog having the active site selenocysteine replaced by cysteine. Nothing precise is known about the antioxidant defence systems of Giardia, Toxoplasma, and Trichomonas species. Also, the role of ovothiols and mycothiols reportedly present in several parasites remains to be established. Scrutinizing known enzymes of parasitic antioxidant defence for suitability as drug targets leaves only those of the trypanosomatid system as directly or indirectly validated. By generally accepted criteria of target selection and feasibility considerations tryparedoxin and tryparedoxin peroxidase can at present be rated as the most appealing target structures for the development of antiparasitic drugs.