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

An overview on the antimalarial activity of 1,2,4-trioxanes, 1,2,4-trioxolanes and 1,2,4,5-tetraoxanes

The demand for novel, fast-acting, and effective antimalarial medications is increasing exponentially. Multidrug resistant forms of malarial parasites, which are rapidly spreading, pose a serious threat to global health. Drug resistance has been addressed using a variety of strategies, such as targeted therapies, the hybrid drug idea, the development of advanced analogues of pre-existing drugs, and the hybrid model of resistant strains control mechanisms. Additionally, the demand for discovering new potent drugs grows due to the prolonged life cycle of conventional therapy brought on by the emergence of resistant strains and ongoing changes in existing therapies. The 1,2,4-trioxane ring system in artemisinin (ART) is the most significant endoperoxide structural scaffold and is thought to be the key pharmacophoric moiety required for the pharmacodynamic potential of endoperoxide-based antimalarials. Several derivatives of artemisinin have also been found as potential treatments for multidrug-resistant strain in this area. Many 1,2,4-trioxanes, 1,2,4-trioxolanes, and 1,2,4,5-tetraoxanes derivatives have been synthesised as a result, and many of these have shown promise antimalarial activity both in vivo and in vitro against Plasmodium parasites. As a consequence, efforts to develop a functionally straight-forward, less expensive, and vastly more effective synthetic pathway to trioxanes continue. This study aims to give a thorough examination of the biological properties and mode of action of endoperoxide compounds derived from 1,2,4-trioxane-based functional scaffolds. The present system of 1,2,4-trioxane, 1,2,4-trioxolane, and 1,2,4,5-tetraoxane compounds and dimers with potentially antimalarial activity will be highlighted in this systematic review (January 1963-December 2022).

Chemo-proteomics in antimalarial target identification and engagement

Humans have lived in tenuous battle with malaria over millennia. Today, while much of the world is free of the disease, areas of South America, Asia, and Africa still wage this war with substantial impacts on their social and economic development. The threat of widespread resistance to all currently available antimalarial therapies continues to raise concern. Therefore, it is imperative that novel antimalarial chemotypes be developed to populate the pipeline going forward. Phenotypic screening has been responsible for the majority of the new chemotypes emerging in the past few decades. However, this can result in limited information on the molecular target of these compounds which may serve as an unknown variable complicating their progression into clinical development. Target identification and validation is a process that incorporates techniques from a range of different disciplines. Chemical biology and more specifically chemo-proteomics have been heavily utilized for this purpose. This review provides an in-depth summary of the application of chemo-proteomics in antimalarial development. Here we focus particularly on the methodology, practicalities, merits, and limitations of designing these experiments. Together this provides learnings on the future use of chemo-proteomics in antimalarial development.

Peroxide Antimalarial Drugs Target Redox Homeostasis in Plasmodium falciparum Infected Red Blood Cells

Plasmodium falciparum causes the most lethal form of malaria. Peroxide antimalarials based on artemisinin underpin the frontline treatments for malaria, but artemisinin resistance is rapidly spreading. Synthetic peroxide antimalarials, known as ozonides, are in clinical development and offer a potential alternative. Here, we used chemoproteomics to investigate the protein alkylation targets of artemisinin and ozonide probes, including an analogue of the ozonide clinical candidate, artefenomel. We greatly expanded the list of proteins alkylated by peroxide antimalarials and identified significant enrichment of redox-related proteins for both artemisinins and ozonides. Disrupted redox homeostasis was confirmed by dynamic live imaging of the glutathione redox potential using a genetically encoded redox-sensitive fluorescence-based biosensor. Targeted liquid chromatography-mass spectrometry (LC-MS)-based thiol metabolomics also confirmed changes in cellular thiol levels. This work shows that peroxide antimalarials disproportionately alkylate proteins involved in redox homeostasis and that disrupted redox processes are involved in the mechanism of action of these important antimalarials.

Intrinsic fluorescence properties of antimalarial pyrido[1,2-a]benzimidazoles facilitate subcellular accumulation and mechanistic studies in the human malaria parasite Plasmodium falciparum

The intrinsic fluorescence properties of two related pyrido[1,2-a]benzimidazole antimalarial compounds suitable for the cellular imaging of the human malaria parasite Plasmodium falciparum without the need to attach extrinsic fluorophores are described. Although these compounds are structurally related, they have been shown by confocal microscopy to not only accumulate selectively within P. falciparum but to also accumulate differently in the organelles investigated. Localization to the digestive vacuole and nearby neutral lipids was observed for compound 2 which was shown to inhibit hemozoin formation using a cellular fractionation assay indicating that this is a contributing mechanism of action. By contrast, compound 1, which differs from compound 2 by the replacement of the imidazole[1,2-a:4,5-b']dipyridine core with the benzimidazole core as well as the presence of Cl substituents, shows very different localisation patterns and shows no evidence of hemozoin inhibition, suggesting a different mechanism of antimalarial action. Docking profiles of both compounds on the hemozoin surface further provided insight into their mechanisms of action.

System-wide biochemical analysis reveals ozonide antimalarials initially act by disrupting Plasmodium falciparum haemoglobin digestion

Ozonide antimalarials, OZ277 (arterolane) and OZ439 (artefenomel), are synthetic peroxide-based antimalarials with potent activity against the deadliest malaria parasite, Plasmodium falciparum. Here we used a "multi-omics" workflow, in combination with activity-based protein profiling (ABPP), to demonstrate that peroxide antimalarials initially target the haemoglobin (Hb) digestion pathway to kill malaria parasites. Time-dependent metabolomic profiling of ozonide-treated P. falciparum infected red blood cells revealed a rapid depletion of short Hb-derived peptides followed by subsequent alterations in lipid and nucleotide metabolism, while untargeted peptidomics showed accumulation of longer Hb-derived peptides. Quantitative proteomics and ABPP assays demonstrated that Hb-digesting proteases were increased in abundance and activity following treatment, respectively. Ozonide-induced depletion of short Hb-derived peptides was less extensive in a drug-treated K13-mutant artemisinin resistant parasite line (Cam3.IIR539T) than in the drug-treated isogenic sensitive strain (Cam3.IIrev), further confirming the association between ozonide activity and Hb catabolism. To demonstrate that compromised Hb catabolism may be a primary mechanism involved in ozonide antimalarial activity, we showed that parasites forced to rely solely on Hb digestion for amino acids became hypersensitive to short ozonide exposures. Quantitative proteomics analysis also revealed parasite proteins involved in translation and the ubiquitin-proteasome system were enriched following drug treatment, suggestive of the parasite engaging a stress response to mitigate ozonide-induced damage. Taken together, these data point to a mechanism of action involving initial impairment of Hb catabolism, and indicate that the parasite regulates protein turnover to manage ozonide-induced damage.

Stochastic Protein Alkylation by Antimalarial Peroxides

Antimalarial peroxides such as the phytochemical artemisinin or the synthetic ozonides arterolane and artefenomel undergo reductive cleavage of the pharmacophoric peroxide bond by ferrous heme, released by parasite hemoglobin digestion. The generated carbon-centered radicals alkylate heme in an intramolecular reaction and proteins in an intermolecular reaction. Here, we determine the proteinaceous alkylation signatures of artemisinin and synthetic ozonides in Plasmodium falciparum using alkyne click chemistry probes to identify target proteins by affinity purification and mass spectrometry-based proteomics. Using stringent controls and purification procedures, we identified 25 P. falciparum proteins that were alkylated by the antimalarial peroxides in a peroxide-dependent manner, but the alkylation patterns were more random than we had anticipated. Moreover, there was little overlap in the alkylation signatures identified in this work and those disclosed in previous studies. Our findings suggest that alkylation of parasite proteins by antimalarial peroxides is likely to be a nonspecific, stochastic process.

Ozonide Antimalarial Activity in the Context of Artemisinin-Resistant Malaria

The ozonides are one of the most advanced drug classes in the antimalarial development pipeline and were designed to improve on limitations associated with current front-line artemisinin-based therapies. Like the artemisinins, the pharmacophoric peroxide bond of ozonides is essential for activity, and it appears that these antimalarials share a similar mode of action, raising the possibility of cross-resistance. Resistance to artemisinins is associated with Plasmodium falciparum mutations that allow resistant parasites to escape short-term artemisinin-mediated damage (elimination half-life ~1 h). Importantly, some ozonides (e.g., OZ439) have a sustained in vivo drug exposure profile, providing a major pharmacokinetic advantage over the artemisinin derivatives. Here, we describe recent progress made towards understanding ozonide antimalarial activity and discuss ozonide utility within the context of artemisinin resistance.

Fluorescent labeling of ursolic acid with FITC for investigation of its cytotoxic activity using confocal microscopy

Fluorescent labeling is a widely-used approach in the study of intracellular processes. This method is becoming increasingly popular for studying small bioactive molecules of natural origin; it allows us to estimate the vital intracellular changes which occur under their influence. We propose a new approach for visualization of the intracellular distribution of triterpene acids, based on fluorescent labeling by fluoresceine isothiocyanate. As a model compound we took the most widely-used and best-studied acid in the ursane series - ursolic acid, as this enabled us to compare the results obtained during our research with the available data, in order to evaluate the validity of the proposed method. Experimental tracing of the dynamics of penetration and distribution of the labeled ursolic acid has shown that when the acid enters the cell, it initially localizes on the inner membranes where the predicted target Akt1/protein kinase B - a protein that inhibits apoptosis - is located.

Virtual design of novel Plasmodium falciparum cysteine protease falcipain-2 hybrid lactone-chalcone and isatin-chalcone inhibitors probing the S2 active site pocket

We report computer-aided design of new lactone–chalcone and isatin–chalcone (HLCIC) inhibitors of the falcipain-2 (PfFP-2). 3D models of 15 FP-2:HLCIC1-15 complexes with known observed activity (IC₅₀^exp) were prepared to establish a quantitative structure–activity (QSAR) model and linear correlation between relative Gibbs free energy of enzyme:inhibitor complex formation (ΔΔG_com) and IC₅₀^exp: pIC₅₀^exp = −0.0236 × ΔΔG_com+5.082(#); R² = 0.93. A 3D pharmacophore model (PH4) derived from the QSAR directed our effort to design novel HLCIC analogues. During the design, an initial virtual library of 2621440 HLCIC was focused down to 18288 drug-like compounds and finally, PH4 screened to identify 81 promising compounds. Thirty-three others were added from an intuitive substitution approach intended to fill better the enzyme S2 pocket. One hundred and fourteen theoretical IC₅₀ (IC₅₀^pre) values were predicted by means of (#) and their pharmacokinetics (ADME) profiles. More than 30 putative HLCICs display IC₅₀^pre 100 times superior to that of the published most active training set inhibitor HLCIC1.

In vitro activity of anti-malarial ozonides against an artemisinin-resistant isolate

BACKGROUND

Recently published data suggest that artemisinin derivatives and synthetic peroxides, such as the ozonides OZ277 and OZ439, have a similar mode of action. Here the cross-resistance of OZ277 and OZ439 and four additional next-generation ozonides was probed against the artemisinin-resistant clinical isolate Plasmodium falciparum Cam3.I, which carries the K13-propeller mutation R539T (Cam3.IR539T).

METHODS

The previously described in vitro ring-stage survival assay (RSA0-3h) was employed and a simplified variation of the original protocol was developed.

RESULTS

At the pharmacologically relevant concentration of 700 nM, all six ozonides were highly effective against the dihydroartemisinin-resistant P. falciparum Cam3.IR539T parasites, showing a per cent survival ranging from <0.01 to 1.83%. A simplified version of the original RSA0-3h method was developed and gave similar results, thus providing a practical drug discovery tool for further optimization of next-generation anti-malarial peroxides.

CONCLUSION

The absence of in vitro cross-resistance against the artemisinin-resistant clinical isolate Cam3.IR539T suggests that ozonides could be effective against artemisinin-resistant P. falciparum. How this will translate to the human situation in clinical settings remains to be investigated.

Monoclonal Antibodies That Recognize the Alkylation Signature of Antimalarial Ozonides OZ277 (Arterolane) and OZ439 (Artefenomel)

The singular structure of artemisinin, with its embedded 1,2,4-trioxane heterocycle, has inspired the discovery of numerous semisynthetic artemisinin and structurally diverse synthetic peroxide antimalarials, including ozonides OZ277 (arterolane) and OZ439 (artefenomel). Despite the critical importance of artemisinin combination therapies (ACTs), the precise mode of action of peroxidic antimalarials is not fully understood. However, it has long been proposed that the peroxide bond in artemisinin and other antimalarial peroxides undergoes reductive activation by ferrous heme released during hemoglobin digestion to produce carbon-centered radicals that alkylate heme and parasite proteins. To probe the mode of action of OZ277 and OZ439, this paper now describes initial studies with monoclonal antibodies that recognize the alkylation signature (sum of heme and protein alkylation) of these synthetic peroxides. Immunofluorescence experiments conducted with ozonide-treated parasite cultures showed that ozonide alkylation is restricted to the parasite, as no signal was found in the erythrocyte or its membrane. In Western blot experiments with ozonide-treated Plasmodium falciparum malaria parasites, distinct protein bands were observed. Significantly, no protein bands were detected in parallel Western blot experiments performed with lysates from ozonide-treated Babesia divergens, parasites that also proliferate inside erythrocytes but, in contrast to P. falciparum, do not catabolize hemoglobin. However, subsequent immunoprecipitation experiments with these antibodies failed to identify the P. falciparum proteins alkylated by OZ277 and OZ439. To the best of the authors’ knowledge, this shows for the first time that antimalarial ozonides, such as the artemisinins, alkylate proteins in P. falciparum.

Oxidative stress-mediated antimalarial activity of plakortin, a natural endoperoxide from the tropical sponge Plakortis simplex

Plakortin, a polyketide endoperoxide from the sponge Plakortis simplex has antiparasitic activity against P. falciparum. Similar to artemisinin, its activity depends on the peroxide functionality. Plakortin induced stage-, dose- and time-dependent morphologic anomalies, early maturation delay, ROS generation and lipid peroxidation in the parasite. Ring damage by 1 and 10 µM plakortin led to parasite death before schizogony at 20 and 95%, respectively. Treatment of late schizonts with 1, 2, 5 and 10 µM plakortin resulted in decreased reinfection rates by 30, 50, 61 and 65%, respectively. In both rings and trophozoites, plakortin induced a dose- and time-dependent ROS production as well as a significant lipid peroxidation and up to 4-fold increase of the lipoperoxide breakdown product 4-hydroxynonenal (4-HNE). Antioxidants and the free radical scavengers trolox and N-acetylcysteine significantly attenuated the parasite damage. Plakortin generated 4-HNE conjugates with the P. falciparum proteins: heat shock protein Hsp70-1, endoplasmatic reticulum-standing Hsp70-2 (BiP analogue), V-type proton ATPase catalytic subunit A, enolase, the putative vacuolar protein sorting-associated protein 11, and the dynein heavy chain-like protein, whose specific binding sites were identified by mass spectrometry. These proteins are crucially involved in protein trafficking, transmembrane and vesicular transport and parasite survival. We hypothesize that binding of 4-HNE to functionally relevant parasite proteins may explain the observed plakortin-induced morphologic aberrations and parasite death. The identification of 4-HNE-protein conjugates may generate a novel paradigm to explain the mechanism of action of pro-oxidant, peroxide-based antimalarials such as plakortin, artemisinins and synthetic endoperoxides.

Trioxolane-Mediated Delivery of Mefloquine Limits Brain Exposure in a Mouse Model of Malaria

Peroxidic antimalarial agents including the sequiterpene artemisinins and the synthetic 1,2,4-trioxolanes function via initial intraparasitic reduction of an endoperoxide bond. By chemically coupling this reduction to release of a tethered drug species it is possible to confer two distinct pharmacological effects in a parasite-selective fashion, both in vitro and in vivo. Here we demonstrate the trioxolane-mediated delivery of the antimalarial agent mefloquine in a mouse malaria model. Selective partitioning of the trioxolane-mefloquine conjugate in parasitized erythrocytes, combined with effective exclusion of the conjugate from brain significantly reduced brain exposure as compared to mice directly administered mefloquine. These studies suggest the potential of trioxolane-mediated drug delivery to mitigate off-target effects of existing drugs, including the adverse neuropsychiatric effects of mefloquine use in therapeutic and chemoprophylactic settings.

D-Glucose-Derived 1,2,4-Trioxepanes: Synthesis, Conformational Study, and Antimalarial Activity

New enantiomerically pure 1,2,4-trioxepanes 10a,b/11a,b were synthesized from D-glucose. Their conformational behavior was studied by low-temperature NMR and substantiated by DFT calculations. On evaluation of in vitro antimalarial activity, the adamantyl derivative 11b showed IC50 values in the low micromolar range, particularly against the W2 chloroquine-resistant Plasmodium falciparum strain (IC50 = 0.15 ± 0.12 μM).

Drug delivery to the malaria parasite using an arterolane-like scaffold

Antimalarial agents artemisinin and arterolane act via initial reduction of a peroxide bond in a process likely mediated by ferrous iron sources in the parasite. Here, we report the synthesis and antiplasmodial activity of arterolane-like 1,2,4-trioxolanes specifically designed to release a tethered drug species within the malaria parasite. Compared with our earlier drug delivery scaffolds, these new arterolane-inspired systems are of significantly decreased molecular weight and possess superior metabolic stability. We describe an efficient, concise and scalable synthesis of the new systems, and demonstrate the use of the aminonucleoside antibiotic puromycin as a chemo/biomarker to validate successful drug release in live Plasmodium falciparum parasites. Together, the improved drug-like properties, more efficient synthesis, and proof of concept using puromycin, suggests these new molecules as improved vehicles for targeted drug delivery to the malaria parasite.

Hemozoin and antimalarial drug discovery

Recent initiatives to develop more effective and affordable drugs, controlling mosquitoes and development of a preventative vaccine have been launched with the goal of completely eradicating malaria. To this end, Novartis (Surrey, UK) and GlaxoSmithKline (Middlesex, UK) screened their chemical libraries of approximately two million small molecules for antimalarial properties, which resulted in a set of over 20,000 'highly druggable' initial hits. Efforts in academia are centered on specific pathway targets. One such high-throughput screening effort has been focused on hemozoin formation, a unique heme detoxification pathway found in the malaria parasite. This review discusses the current approaches and limitations of high-throughput screening discovery of hemozoin inhibitors. In the future, new methods must be developed to validate the mechanism of action of these hit compounds within the parasite.

Synthesis of five- and six-membered cyclic organic peroxides: Key transformations into peroxide ring-retaining products

The present review describes the current status of synthetic five and six-membered cyclic peroxides such as 1,2-dioxolanes, 1,2,4-trioxolanes (ozonides), 1,2-dioxanes, 1,2-dioxenes, 1,2,4-trioxanes, and 1,2,4,5-tetraoxanes. The literature from 2000 onwards is surveyed to provide an update on synthesis of cyclic peroxides. The indicated period of time is, on the whole, characterized by the development of new efficient and scale-up methods for the preparation of these cyclic compounds. It was shown that cyclic peroxides remain unchanged throughout the course of a wide range of fundamental organic reactions. Due to these properties, the molecular structures can be greatly modified to give peroxide ring-retaining products. The chemistry of cyclic peroxides has attracted considerable attention, because these compounds are used in medicine for the design of antimalarial, antihelminthic, and antitumor agents.

Antimalarial activity of 4-metoxychalcones: docking studies as falcipain/plasmepsin inhibitors, ADMET and lipophilic efficiency analysis to identify a putative oral lead candidate

Herein, we report the antimalarial activity of nine 4-methoxychalcone derivatives 1a-i and an initial analysis of their ADMET properties. All compounds showed potent activity against the P. falciparum chloroquine-resistant clone W2, with IC50 values ranging from 1.96 µM to 10.99 µM, with moderate or low cytotoxicity against the HeLa cell line. The compound 1a (IC50 = 2.06 µM) had the best selectivity index (9.0). All the sulfonamide 4-metychalcone derivatives synthesized had cLogP values between 2 and 5 (mean value 3.79) and molecular weights (MWs) below 500. The substitution of the pyrrolidine group in 1i by a morpholine group in 1a reduced the cLogP value from 3.05 in compound 1i to 2.34 in compound 1a. Indeed, compound 1a had the highest LipE value. The binding free energy of compound 1a showed it to be the most optimal chalcone derivative for plasmepsin-2 (-7.3 Kcal/mol). The physicochemical properties and LipE analysis of the dataset allowed us to establish that compound 1a is the highest quality compound of the series and a potential oral lead candidate.

Ferrous iron-dependent drug delivery enables controlled and selective release of therapeutic agents in vivo

The precise targeting of cytotoxic agents to specific cell types or cellular compartments is of significant interest in medicine, with particular relevance for infectious diseases and cancer. Here, we describe a method to exploit aberrant levels of mobile ferrous iron (Fe(II)) for selective drug delivery in vivo. This approach makes use of a 1,2,4-trioxolane moiety, which serves as an Fe(II)-sensitive "trigger," making drug release contingent on Fe(II)-promoted trioxolane fragmentation. We demonstrate in vivo validation of this approach with the Plasmodium berghei model of murine malaria. Malaria parasites produce high concentrations of mobile ferrous iron as a consequence of their catabolism of host hemoglobin in the infected erythrocyte. Using activity-based probes, we successfully demonstrate the Fe(II)-dependent and parasite-selective delivery of a potent dipeptidyl aminopeptidase inhibitor. We find that delivery of the compound in its Fe(II)-targeted form leads to more sustained target inhibition with greatly reduced off-target inhibition of mammalian cathepsins. This selective drug delivery translates into improved efficacy and tolerability. These findings demonstrate the utility of a purely chemical means to achieve selective drug targeting in vivo. This approach may find useful application in parasitic infections and more broadly in any disease state characterized by aberrant production of reactive ferrous iron.

Ferrous iron-dependent delivery of therapeutic agents to the malaria parasite

BACKGROUND

The malaria parasites Plasmodium falciparum and Plasmodium vivax generate significant concentrations of free unbound ferrous iron heme as a side product of hemoglobin degradation. The presence of these chemically reactive forms of iron, rare in healthy cells, presents an opportunity for parasite-selective drug delivery. Accordingly, our group is developing technologies for the targeted delivery of therapeutics to the intra-erythrocytic malaria parasite. These so-called 'fragmenting hybrids' employ a 1,2,4-trioxolane ring system as an iron(II)-sensing 'trigger' moiety and a 'traceless' retro-Michael linker to which a variety of partner drug species may be attached. After ferrous iron-promoted activation in the parasite, the partner drug is released via a β-elimination reaction.

METHODS

In this report, we describe three orthogonal experimental approaches that were explored in order to generate in vitro proof-of-concept for ferrous iron-dependent drug delivery from a prototypical fragmenting hybrid.

CONCLUSION

Studies of two fragmenting hybrids by orthogonal approaches confirm that a partner drug species can be delivered to live P. falciparum parasites. A key advantage of this approach is the potential to mask a partner drug's intrinsic bioactivity prior to release in the parasite.

Boronated carbohydrate derivatives as potential boron neutron capture therapy reagents

The treatment of cancer remains one of the most challenging problems for humanity. Boron neutron capture therapy is a binary approach for cancer treatment that is particularly attractive in treating high-grade gliomas and metastatic brain tumors. Among the types of boron-containing molecules used as boron neutron capture therapy agents, boronated carbohydrate derivatives have received significant attention because of their preferential uptake by growing tumor cells. This review provides a summary of the recent developments in the chemistry of carborane-containing carbohydrates.

Comparative antimalarial activities and ADME profiles of ozonides (1,2,4-trioxolanes) OZ277, OZ439, and their 1,2-dioxolane, 1,2,4-trioxane, and 1,2,4,5-tetraoxane isosteres

To ascertain the structure-activity relationship of the core 1,2,4-trioxolane substructure of dispiro ozonides OZ277 and OZ439, we compared the antimalarial activities and ADME profiles of the 1,2-dioxolane, 1,2,4-trioxane, and 1,2,4,5-tetraoxane isosteres. Consistent with previous data, both dioxolanes had very weak antimalarial properties. For the OZ277 series, the trioxane isostere had the best ADME profile, but its overall antimalarial efficacy was not superior to that of the trioxolane or tetraoxane isosteres. For the OZ439 series, there was a good correlation between the antimalarial efficacy and ADME profiles in the rank order trioxolane > trioxane > tetraoxane. As we have previously observed for OZ439 versus OZ277, the OZ439 series peroxides had superior exposure and efficacy in mice compared to the corresponding OZ277 series peroxides.