An Endoperoxide‐Based Hybrid Approach to Deliver Falcipain Inhibitors Inside Malaria Parasites

The vinyl sulfone motif as a structural unit for novel drug design and discovery

INTRODUCTION

Vinyl sulfones are a special sulfur-containing structural unit that have attracted considerable attention, owing to their important role in serving as key structural motifs of various biologically active compounds as well as serving as versatile building blocks for organic transformations. The synthetic strategy of vinyl sulfone derivatives has been substantially upgraded over the past 30 years, and the wide application of this functional group in drug design and discovery has been promoted.

AREA COVERED

In this review, the authors review the application of vinyl sulfones in drug discovery and select optimized compounds which might have significant impact or potential inspiration for drug design.

EXPERT OPINION

Vinyl sulfones have been reported to target various macromolecular targets via non-covalent or covalent interactions, including multiple kinases, tubulin, cysteine protease, transcription factor, and so on. Thus, it has been significantly applied as a privileged scaffold in the design of anticancer, anti-infective, anti-inflammatory, and neuroprotective agents. However, much work remains to be done to improve the drug-like properties, such as chemical and metabolic stability, ADME, and toxicity. Besides, the chemical space of vinyl sulfones needs to be expanded, including but not limited to the design of constrained endocyclic and exocyclic vinyl sulfones.

Falcipains: Biochemistry, target validation and structure-activity relationship studies of inhibitors as antimalarials

Malaria is a tropical disease with significant morbidity and mortality burden caused by Plasmodium species in Africa, the Middle East, Asia, and South America. Pathogenic Plasmodium species have lately become increasingly resistant to approved chemotherapeutics and combination therapies. Therefore, there is an emergent need for identifying new druggable targets and novel chemical classes against the parasite. Falcipains, cysteine proteases required for heme metabolism in the erythrocytic stage, have emerged as promising drug targets against Plasmodium species that infect humans. This perspective discusses the biology, biochemistry, structural features, and genetics of falcipains. The efforts to identify selective or dual inhibitors and their structure-activity relationships are reviewed to give a perspective on the design of novel compounds targeting falcipains for antimalarial activity evaluating reasons for hits and misses for this important target.

The antimalarial activity of 1,2,4-trioxolane/trioxane hybrids and dimers: A review

Malaria, a mosquito-borne parasitic infection caused by protozoan parasites belonging to the genus Plasmodium, is a dangerous disease that contributes to millions of hospital visits and hundreds and thousands of deaths across the world, especially in Sub-Saharan Africa. Antimalarial agents are vital for treating malaria and controlling transmission, and 1,2,4-trioxolane/trioxane-containing agents, especially artemisinin and its derivatives, own antimalarial efficacy and low toxicity with unique mechanisms of action. Moreover, artemisinin-based combination therapies were recommended by the World Health Organization as the first-line treatment for uncomplicated malaria infection and have remained as the mainstay of the treatment of malaria, demonstrating that 1,2,4-trioxolane/trioxane derivatives are useful prototypes for the control and eradication of malaria. However, malaria parasites have already developed resistance to almost all of the currently available antimalarial agents, creating an urgent need for the search of novel pharmaceutical interventions for malaria. The purpose of this review article is to provide an emphasis on the current scenario (January 2012 to January 2022) of 1,2,4-trioxolane/trioxane hybrids and dimers with potential antimalarial activity and the structure-activity relationships are also discussed to facilitate further rational design of more effective candidates.

Synthesis of Non-symmetrical Dispiro-1,2,4,5-Tetraoxanes and Dispiro-1,2,4-Trioxanes Catalyzed by Silica Sulfuric Acid

A novel protocol for the preparation of non-symmetrical 1,2,4,5-tetraoxanes and 1,2,4-trioxanes, promoted by the heterogeneous silica sulfuric acid (SSA) catalyst, is reported. Different ketones react under mild conditions with gem-dihydroperoxides or peroxysilyl alcohols/β-hydroperoxy alcohols to generate the corresponding endoperoxides in good yields. Our mechanistic proposal, assisted by molecular orbital calculations, at the ωB97XD/def2-TZVPP/PCM(DCM)//B3LYP/6-31G(d) level of theory, enhances the role of SSA in the cyclocondensation step. This novel procedure differs from previously reported methods by using readily available and inexpensive reagents, with recyclable properties, thereby establishing a valid alternative approach for the synthesis of new biologically active endoperoxides.

Structure-based Approaches Targeting Parasite Cysteine Proteases

Cysteine proteases are essential hydrolytic enzymes present in the majority of organisms, including viruses and unicellular parasites. Despite the high sequence identity displayed among these proteins, specific structural features across different species grant distinct functions to these biomolecules, frequently related to pathological conditions. Consequently, their relevance as promising targets for potential specific inhibitors has been highlighted and occasionally validated in recent decades. In this review, we discuss the recent outcomes of structure-based campaigns aiming the discovery of new inhibitor prototypes against cruzain and falcipain, as alternative therapeutic tools for Chagas disease and malaria treatments, respectively. Computational and synthetic approaches have been combined on hit optimization strategies and are also discussed herein. These rationales are extended to additional tropical infectious and neglected pathologies, such as schistosomiasis, leishmaniasis and babesiosis, and also to Alzheimer's Disease, a widespread neurodegenerative disease poorly managed by currently available drugs and recently linked to particular physiopathological roles of human cysteine proteases.

Malaria Hybrids: A Chronological Evolution

Malaria, an upsetting malaise caused by a diverse class of Plasmodium species affects about 40% of the world's population. The distress associated with it has reached colossal scales owing to the development of resistance to most of the clinically available agents. Hence, the search for newer molecules for malaria treatment and cure is an incessant process. After the era of a single molecule for malaria treatment ended, there was an advent of combination therapy. However, lately there had been reports of the development of resistance to many of these agents as well. Subsequently, at present most of the peer groups working on malaria treatment aim to develop novel molecules, which may act on more than one biological processes of the parasite life cycle, and these scaffolds have been aptly termed as Hybrid Molecules or Double Drugs. These molecules may hold the key to hitherto unknown ways of showing a detrimental effect on the parasite. This review enlists a few of the recent advances made in malaria treatment by these hybrid molecules in a sequential manner.

Endoperoxide-8-aminoquinoline hybrids as dual-stage antimalarial agents with enhanced metabolic stability

Hybrid compounds may play a critical role in the context of the malaria eradication agenda, which will benefit from therapeutic tools active against the symptomatic erythrocytic stage of Plasmodium infection, and also capable of eliminating liver stage parasites. To address the need for efficient multistage antiplasmodial compounds, a small library of 1,2,4,5-tetraoxane-8- aminoquinoline hybrids, with the metabolically labile C-5 position of the 8-aminoquinoline moiety blocked with aryl groups, was synthesized and screened for antiplasmodial activity and metabolic stability. The hybrid compounds inhibited development of intra-erythrocytic forms of the multidrug-resistant Plasmodium falciparum W2 strain, with EC₅₀ values in the nM range, and with low cytotoxicity against mammalian cells. The compounds also inhibited the development of P. berghei liver stage parasites, with the most potent compounds displaying EC₅₀ values in the low μM range. SAR analysis revealed that unbranched linkers between the endoperoxide and 8-aminoquinoline pharmacophores are most beneficial for dual antiplasmodial activity. Importantly, hybrids were significantly more potent than a 1:1 mixture of 8-aminoquinoline-tetraoxane, highlighting the superiority of the hybrid approach over the combination therapy. Furthermore, aryl substituents at C-5 of the 8-aminoquinoline moiety improve the compounds' metabolic stability when compared with their primaquine (i.e. C-5 unsubstituted) counterparts. Overall, this study reveals that blocking the quinoline C-5 position does not result in loss of dual-stage antimalarial activity, and that tetraoxane-8- aminoquinoline hybrids are an attractive approach to achieve elimination of exo- and intraerythrocytic parasites, thus with the potential to be used in malaria eradication campaigns.

Novel series of 1,2,4-trioxane derivatives as antimalarial agents

Among three series of 1,2,4-trioxane derivatives, five compounds showed good in vitro antimalarial activity, three compounds of which exhibited better activity against P. falciparum resistant (RKL9) strain than the sensitive (3D7) one. Two best compounds were one from aryl series and the other from heteroaryl series with IC₅₀ values of 1.24 µM and 1.24 µM and 1.06 µM and 1.17 µM, against sensitive and resistant strains, respectively. Further, trioxane derivatives exhibited good binding affinity for the P. falciparum cysteine protease falcipain 2 receptor (PDB id: 3BPF) with well defined drug-like and pharmacokinetic properties based on Lipinski's rule of five with additional physicochemical and ADMET parameters. In view of having antimalarial potential, 1,2,4-trioxane derivative(s) reported herein may be useful as novel antimalarial lead(s) in the discovery and development of future antimalarial drug candidates as P. falciparum falcipain 2 inhibitors against resistant malaria.

Are Antimalarial Hybrid Molecules a Close Reality or a Distant Dream?

Emergence of drug-resistant Plasmodium falciparum strains has led to a situation of haste in the scientific and pharmaceutical communities. Hence, all their efforts are redirected toward finding alternative chemotherapeutic agents that are capable of combating multidrug-resistant parasite strains. In light of this situation, scientists have come up with the concept of hybridization of two or more active pharmacophores into a single chemical entity, resulting in "antimalarial hybrids." The approach has been applied widely for generation of lead compounds against deadly diseases such as cancer and AIDS, with a proven potential for use as novel drugs, but is comparatively new in the sphere of antimalarial drug discovery. A sudden surge has been evidenced in the number of studies on the design and synthesis of hybrids for treating malaria and may be regarded as proof of their potential advantages over artemisinin-based combination therapy (ACT). However, it is evident from recent studies that most of the potential advantages of antimalarial hybrids, such as lower toxicity, better pharmacokinetics, and easier formulation, have yet to be realized. A number of questions left unaddressed at present need to be answered before this approach can progress to the late stages of clinical development and prove their worth in the clinic. To the best of our knowledge, this compilation is the first attempt to shed light on the shortcomings that are surfacing as more and more studies on molecular hybridization of the active pharmacophores of known antimalarials are being published.

Dipeptidyl Vinyl Sulfone as a Novel Chemical Tool to Inhibit HMGB1/NLRP3-Inflammasome and Inflamma-miRs in Aβ-Mediated Microglial Inflammation

Rapid microglial activation and associated inflammatory pathways contribute to immune-defense and tissue repair in the central nervous system (CNS). However, persistent activation of these cells will ultimately result in vast production of pro-inflammatory mediators and other neurotoxic factors, which may induce neuronal damage and contribute to chronic neurodegenerative diseases, as Alzheimer's disease (AD). Therefore, small molecules with immunomodulatory effects on microglia may be considered as potential tools to counteract their proinflammatory phenotype and neuroimmune dysregulation in such disorders. Indeed, reducing amyloid-β (Aβ)-induced microglia activation is believed to be effective in treating AD. In this study, we investigated whether dipeptidyl vinyl sulfone (VS) was able to attenuate Aβ-mediated inflammatory response using a mouse microglial (N9) cell line and a solution containing a mixture of Aβ aggregates. We show that low levels of VS are able to prevent cell death while reducing microglia phagocytosis upon Aβ treatment. VS also suppressed Aβ-induced expression of inflammatory mediators in microglia, such as matrix metalloproteinase (MMP)-2 and MMP-9, as well as high-mobility group box protein-1 (HMGB1), nod-like receptor protein 3 (NLRP3)-inflammasome, and interleukin (IL)-1β. Interestingly, increased expression of the two critical inflammation-related microRNAs (miR)-155 and miR-146a in microglia upon Aβ treatment was also prevented by VS coincubation. Taken together, VS emerges as a potential new therapeutic strategy worthy of further investigation in improved cellular and animal models of AD.

Endoperoxide antimalarials: development, structural diversity and pharmacodynamic aspects with reference to 1,2,4-trioxane-based structural scaffold

Malaria disease continues to be a major health problem worldwide due to the emergence of multidrug-resistant strains of Plasmodium falciparum. In recent days, artemisinin (ART)-based drugs and combination therapies remain the drugs of choice for resistant P. falciparum malaria. However, resistance to ART-based drugs has begun to appear in some parts of the world. Endoperoxide compounds (natural/semisynthetic/synthetic) representing a huge number of antimalarial agents possess a wide structural diversity with a desired antimalarial effectiveness against resistant P. falciparum malaria. The 1,2,4-trioxane ring system lacking the lactone ring that constitutes the most important endoperoxide structural scaffold is believed to be the key pharmacophoric moiety and is primarily responsible for the pharmacodynamic potential of endoperoxide-based antimalarials. Due to this reason, research into endoperoxide, particularly 1,2,4-trioxane-, 1,2,4-trioxolane- and 1,2,4,5-teraoxane-based scaffolds, has gained significant interest in recent years for developing antimalarial drugs against resistant malaria. In this paper, a comprehensive effort has been made to review the development of endoperoxide antimalarials from traditional antimalarial leads (natural/semisynthetic) and structural diversity of endoperoxide molecules derived from 1,2,4-trioxane-, 1,2,4-trioxolane- and 1,2,4,5-teraoxane-based structural scaffolds, including their chimeric (hybrid) molecules, which are newer and potent antimalarial agents.

Artemisinin-based antimalarial research: application of biotechnology to the production of artemisinin, its mode of action, and the mechanism of resistance of Plasmodium parasites

Malaria is a worldwide disease caused by Plasmodium parasites. A sesquiterpene endoperoxide artemisinin isolated from Artemisia annua was discovered and has been accepted for its use in artemisinin-based combinatorial therapies, as the most effective current antimalarial treatment. However, the quantity of this compound produced from the A. annua plant is very low, and the availability of artemisinin is insufficient to treat all infected patients. In addition, the emergence of artemisinin-resistant Plasmodium has been reported recently. Several techniques have been applied to enhance artemisinin availability, and studies related to its mode of action and the mechanism of resistance of malaria-causing parasites are ongoing. In this review, we summarize the application of modern technologies to improve the production of artemisinin, including our ongoing research on artemisinin biosynthetic genes in other Artemisia species. The current understanding of the mode of action of artemisinin as well as the mechanism of resistance against this compound in Plasmodium parasites is also presented. Finally, the current situation of malaria infection and the future direction of antimalarial drug development are discussed.

Pyrimidine-based antimalarials: design strategies and antiplasmodial effects

The versatility in the design strategies of pyrimidine scaffold offer considerable opportunity for developing antimalarials capable of hitting different biological targets.

A structure guided drug-discovery approach towards identification of Plasmodium inhibitors

This article provides a comprehensive review of inhibitors from natural, semisynthetic or synthetic sources against key targets ofPlasmodium falciparum.

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.

Synthesis, antiplasmodial activity and mechanistic studies of pyrimidine-5-carbonitrile and quinoline hybrids

A series of hybrids comprising of 5-cyanopyrimidine and quinoline moiety were synthesized and tested for in vitro antiplasmodial activity against NF54 and Dd2 strains of Plasmodium falciparum. Hybrid bearing m-nitrophenyl substituent at C-4 of pyrimidine displayed the highest antiplasmodial activity [IC50 = 56 nM] against the CQ(R) (Dd2) strain, which is four-fold greater than CQ.

From hybrid compounds to targeted drug delivery in antimalarial therapy

The discovery of new drugs to treat malaria is a continuous effort for medicinal chemists due to the emergence and spread of resistant strains of Plasmodium falciparum to nearly all used antimalarials. The rapid adaptation of the malaria parasite remains a major limitation to disease control. Development of hybrid antimalarial agents has been actively pursued as a promising strategy to overcome the emergence of resistant parasite strains. This review presents the journey that started with simple combinations of two active moieties into one chemical entity and progressed into a delivery/targeted system based on major antimalarial classes of drugs. The rationale for providing different mechanisms of action against a single or additional targets involved in the multiple stages of the parasite's life-cycle is highlighted. Finally, a perspective for this polypharmacologic approach is presented.

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.