New praziquantel derivatives containing NO-donor furoxans and related furazans as active agents against Schistosoma mansoni

Fragment library screening by X-ray crystallography and binding site analysis on thioredoxin glutathione reductase of Schistosoma mansoni

Schistosomiasis is caused by parasites of the genus Schistosoma, which infect more than 200 million people. Praziquantel (PZQ) has been the main drug for controlling schistosomiasis for over four decades, but despite that it is ineffective against juvenile worms and size and taste issues with its pharmaceutical forms impose challenges for treating school-aged children. It is also important to note that PZQ resistant strains can be generated in laboratory conditions and observed in the field, hence its extensive use in mass drug administration programs raises concerns about resistance, highlighting the need to search for new schistosomicidal drugs. Schistosomes survival relies on the redox enzyme thioredoxin glutathione reductase (TGR), a validated target for the development of new anti-schistosomal drugs. Here we report a high-throughput fragment screening campaign of 768 compounds against S. mansoni TGR (SmTGR) using X-ray crystallography. We observed 49 binding events involving 35 distinct molecular fragments which were found to be distributed across 16 binding sites. Most sites are described for the first time within SmTGR, a noteworthy exception being the "doorstop pocket" near the NADPH binding site. We have compared results from hotspots and pocket druggability analysis of SmTGR with the experimental binding sites found in this work, with our results indicating only limited coincidence between experimental and computational results. Finally, we discuss that binding sites at the doorstop/NADPH binding site and in the SmTGR dimer interface, should be prioritized for developing SmTGR inhibitors as new antischistosomal drugs.

Small change, big difference: A promising praziquantel derivative designated P96 with broad-spectrum antischistosomal activity for chemotherapy of schistosomiasis japonica

BACKGROUND

Praziquantel (PZQ) has been the first line antischistosomal drug for all species of Schistosoma, and the only available drug for schistosomiasis japonica, without any alternative drugs since the 1980s. However, PZQ cannot prevent reinfection, and cannot cure schistosomiasis thoroughly because of its poor activity against juvenile schistosomes. In addition, reliance on a single drug is extremely dangerous, the development and spread of resistance to PZQ is becoming a great concern. Therefore, development of novel drug candidates for treatment and control of schistosomiasis is urgently needed.

METHODOLOGYS/PRINCIPAL FINDINGS

One of the PZQ derivative christened P96 with the substitution of cyclohexyl by cyclopentyl was synthesized by School of Pharmaceutical Sciences of Shandong University. We investigated the in vitro and in vivo activities of P96 against different developmental stages of S. japonicum. Parasitological studies and scanning electron microscopy were used to study the primary action characteristics of P96 in vitro. Both mouse and rabbit models were employed to evaluate schistosomicidal efficacy of P96 in vivo. Besides calculation of worm reduction rate and egg reduction rate, quantitative real-time PCR was used to evaluate the in vivo antischistosomal activity of P96 at molecular level. In vitro, after 24h exposure, P96 demonstrated the highest activities against both juvenile and adult worm of S. japonicum in comparison to PZQ. The antischistosomal efficacy was concentration-dependent, with P96 at 50μM demonstrating the most evident schistosomicidal effect. Scanning electron microscopy demonstrated that P96 caused more severe damages to schistosomula and adult worm tegument compared to PZQ. In vivo, our results showed that P96 was effective against S. japonicum at all developmental stages. Notably, its efficacy against young stage worms was significantly improved compared to PZQ. Moreover, P96 retained the high activity comparable to PZQ against the adult worm of S. japonicum.

CONCLUSIONS

P96 is a promising drug candidate for chemotherapy of schistosomiasis japonica, which has broad spectrum of action against various developmental stage, potentially addressing the deficiency of PZQ. It might be promoted as a drug candidate for use either alone or in combination with PZQ for the treatment of schistosomiasis.

4-Amino-3-(1-{[amino(3-methyl-2-oxido-1,2,5-oxadiazol-4-yl)methylene]hydrazinylidene}ethyl)-1,2,5-oxadiazole 2-Oxide

Functionally substituted 1,2,5-oxadiazole 2-oxides (furoxans) are important pharmaceutical scaffolds used for the preparation of various pharmacologically active substances. Furoxans bearing hydrazone functionality are considered as promising drug candidates for the treatment of neglected diseases. However, pharmacologically oriented hydrazones derived from (furoxanyl)amidrazones and acetylfuroxans have remained unknown so far. In this communication, a simple method for the synthesis of 4-amino-3-(1-{[amino(3-methyl-2-oxido-1,2,5-oxadiazol-4-yl)methylene]hydrazinylidene}ethyl)-1,2,5-oxadiazole 2-oxide is described. The structure of the synthesized compound was established by elemental analysis, high-resolution mass spectrometry, 1H, 13C NMR and IR spectroscopy.

Recent Advances in the Synthesis and Biomedical Applications of Heterocyclic NO-Donors

Nitric oxide (NO) is a key signaling molecule that acts in various physiological processes such as cellular metabolism, vasodilation and transmission of nerve impulses. A wide number of vascular diseases as well as various immune and neurodegenerative disorders were found to be directly associated with a disruption of NO production in living organisms. These issues justify a constant search of novel NO-donors with improved pharmacokinetic profiles and prolonged action. In a series of known structural classes capable of NO release, heterocyclic NO-donors are of special importance due to their increased hydrolytic stability and low toxicity. It is no wonder that synthetic and biochemical investigations of heterocyclic NO-donors have emerged significantly in recent years. In this review, we summarized recent advances in the synthesis, reactivity and biomedical applications of promising heterocyclic NO-donors (furoxans, sydnone imines, pyridazine dioxides, azasydnones). The synthetic potential of each heterocyclic system along with biochemical mechanisms of action are emphasized.

Synthesis of oxadiazole-2-oxide derivatives as potential drug candidates for schistosomiasis targeting SjTGR

BACKGROUND

Schistosomiasis is a chronic parasitic disease that affects millions of people's health worldwide. Because of the increasing drug resistance to praziquantel (PZQ), which is the primary drug for schistosomiasis, developing new drugs to treat schistosomiasis is crucial. Oxadiazole-2-oxides have been identified as potential anti-schistosomiasis reagents targeting thioredoxin glutathione reductase (TGR).

METHODS

In this work, one of the oxadiazole-2-oxides derivatives furoxan was used as the lead compound to exploit a series of novel furoxan derivatives for studying inhibitory activity against both recombinant Schistosoma japonicum TGR containing selenium (rSjTGR-Sec) and soluble worm antigen protein (SWAP) containing wild-type Schistosoma japonicum TGR (wtSjTGR), in order to develop a new leading compound for schistosomiasis. Thirty-nine novel derivatives were prepared to test their activity toward both enzymes. The docking method was used to detect the binding site between the active molecule and SjTGR. The structure-activity relationship (SAR) of these novel furoxan derivatives was preliminarily analyzed.

RESULTS

It was found that several new derivatives, including compounds 6a-6d, 9ab, 9bd and 9be, demonstrated greater activity toward rSjTGR-Sec or SWAP containing wtSjTGR than did furoxan. Interestingly, all intermediates bearing hydroxy (6a-6d) showed excellent inhibitory activity against both enzymes. In particular, compound 6d with trifluoromethyl on a pyridine ring was found to have much higher inhibition toward both rSjTGR-Sec (half-maximal inhibitory concentration, IC50,7.5nM) and SWAP containing wtSjTGR (IC50 55.8nM) than furoxan. Additionally, the docking method identified the possible matching sites between 6d and Schistosoma japonicum TGR (SjTGR), which theoretically lends support to the inhibitory activity of 6d.

CONCLUSION

The data obtained herein showed that 6d with trifluoromethyl on a pyridine ring could be a valuable leading compound for further study.

Medicinal chemistry perspectives of 1,2,3,4-tetrahydroisoquinoline analogs - biological activities and SAR studies

Isoquinoline alkaloids are a large group of natural products in which 1,2,3,4-tetrahydroisoquinolines (THIQ) form an important class. THIQ based natural and synthetic compounds exert diverse biological activities against various infective pathogens and neurodegenerative disorders. Due to these reasons, the THIQ heterocyclic scaffold has garnered a lot of attention in the scientific community which has resulted in the development of novel THIQ analogs with potent biological activity. The present review provides a much-needed update on the biological potential of THIQ analogs, their structural-activity relationship (SAR), and their mechanism of action. In addition, a note on commonly used synthetic strategies for constructing the core scaffold has also been discussed.

Radicicol, a Novel Lead Compound against the Migratory-Stage Schistosomula of Schistosoma japonicum

Schistosomiasis poses a serious threat to human health and remains a major tropical and parasitic disease in more than 70 countries. Praziquantel (PZQ) has been the primary treatment for schistosomiasis for nearly 4 decades. However, its efficacy against migratory-stage schistosomula is limited. Radicicol (RAD), a β-resorcylic acid lactone derived from Paecilomyces sp. strain SC0924, was investigated as an alternative treatment for Schistosoma japonicum In vitro tests showed that within 72 h, RAD (10 μmol/liter) completely killed schistosomula of both skin and liver stages with an efficacy significantly higher than that of PZQ, although it was less potent against adult worms than PZQ. In vivo, RAD reduced worm burdens and liver eggs by 91.18% and 86.01%, respectively, by killing migratory-stage schistosomula. Optical microscopy and scanning electron microscopy revealed that RAD damaged the epiderm and tegument morphology of S. japonicum worms at various stages and altered their motility to different degrees. RAD exhibited schistosomicidal effects at different stages in vitro and in vivo, especially at the migratory stage, implying that its mechanism could be different from that of PZQ. Collectively, these results showed that RAD is promising as a lead for the development of drugs to control the migratory-stage schistosomula of S. japonicum.

A Machine Learning Strategy for Drug Discovery Identifies Anti-Schistosomal Small Molecules

Schistosomiasis is a chronic and painful disease of poverty caused by the flatworm parasite Schistosoma. Drug discovery for antischistosomal compounds predominantly employs in vitro whole organism (phenotypic) screens against two developmental stages of Schistosoma mansoni, post-infective larvae (somules) and adults. We generated two rule books and associated scoring systems to normalize 3898 phenotypic data points to enable machine learning. The data were used to generate eight Bayesian machine learning models with the Assay Central software according to parasite’s developmental stage and experimental time point (≤24, 48, 72, and >72 h). The models helped predict 56 active and nonactive compounds from commercial compound libraries for testing. When these were screened against S. mansoni in vitro, the prediction accuracy for active and inactives was 61% and 56% for somules and adults, respectively; also, hit rates were 48% and 34%, respectively, far exceeding the typical 1–2% hit rate for traditional high throughput screens.

Direct Synthesis of N-(1,2,5-Oxadiazolyl)hydrazones through a Diazotization/Reduction/Condensation Cascade

A straightforward synthesis of a series of previously unknown N-(1,2,5-oxadiazolyl)hydrazones through the diazotization/reduction/condensation cascade of amino-1,2,5-oxadiazoles was accomplished. The described protocol was suitable for a wide array of target hydrazones, which were prepared in good to high yields under smooth reaction conditions with very good functional group tolerance. Importantly, the presented approach unveils a direct route to in situ generation of previously inaccessible (1,2,5-oxadiazolyl)hydrazines. In addition, a first example of the ionic structure incorporating a protonated hydrazone motif linked to the 1,2,5-oxadiazole 2-oxide subunit was synthesized, indicating the stability of prepared compounds toward acid-promoted hydrolysis. Overall, this method provides a direct access to the isosteric analogues of drug candidates for treatment of various neglected diseases, thus enabling their potential application in medicinal chemistry and drug design.

Nitric oxide debilitates the neuropathogenic schistosome Trichobilharzia regenti in mice, partly by inhibiting its vital peptidases

Background

Avian schistosomes, the causative agents of human cercarial dermatitis (or swimmer’s itch), die in mammals but the mechanisms responsible for parasite elimination are unknown. Here we examined the role of reactive nitrogen species, nitric oxide (NO) and peroxynitrite, in the immune response of mice experimentally infected with Trichobilharzia regenti, a model species of avian schistosomes remarkable for its neuropathogenicity.

Methods

Inducible NO synthase (iNOS) was localized by immunohistochemistry in the skin and the spinal cord of mice infected by T. regenti. The impact of iNOS inhibition by aminoguanidine on parasite burden and growth was then evaluated in vivo. The vulnerability of T. regenti schistosomula to NO and peroxynitrite was assessed in vitro by viability assays and electron microscopy. Additionally, the effect of NO on the activity of T. regenti peptidases was tested using a fluorogenic substrate.

Results

iNOS was detected around the parasites in the epidermis 8 h post-infection and also in the spinal cord 3 days post-infection (dpi). Inhibition of iNOS resulted in slower parasite growth 3 dpi, but the opposite effect was observed 7 dpi. At the latter time point, moderately increased parasite burden was also noticed in the spinal cord. In vitro, NO did not impair the parasites, but inhibited the activity of T. regenti cathepsins B1.1 and B2, the peptidases essential for parasite migration and digestion. Peroxynitrite severely damaged the surface tegument of the parasites and decreased their viability in vitro, but rather did not participate in parasite clearance in vivo.

Conclusions

Reactive nitrogen species, specifically NO, do not directly kill T. regenti in mice. NO promotes the parasite growth soon after penetration (3 dpi), but prevents it later (7 dpi) when also suspends the parasite migration in the CNS. NO-related disruption of the parasite proteolytic machinery is partly responsible for this effect.

The Schistosoma mansoni tegumental allergen protein, SmTAL1: Binding to an IQ-motif from a voltage-gated ion channel and effects of praziquantel

SmTAL1 is a calcium binding protein from the parasitic worm, Schistosoma mansoni. Structurally it is comprised of two domains - an N-terminal EF-hand domain and a C-terminal dynein light chain (DLC)-like domain. The protein has previously been shown to interact with the anti-schistosomal drug, praziquantel (PZQ). Here, we demonstrated that both EF-hands in the N-terminal domain are functional calcium ion binding sites. The second EF-hand appears to be more important in dictating affinity and mediating the conformational changes which occur on calcium ion binding. There is positive cooperativity between the four calcium ion binding sites in the dimeric form of SmTAL1. Both the EF-hand domain and the DLC-domain dimerise independently suggesting that both play a role in forming the SmTAL1 dimer. SmTAL1 binds non-cooperatively to PZQ and cooperatively to an IQ-motif from SmCa_v1B, a voltage-gated calcium channel. PZQ tends to strengthen this interaction, although the relationship is complex. These data suggest the hypothesis that SmTAL1 regulates at least one voltage-gated calcium channel and PZQ interferes with this process. This may be important in the molecular mechanism of this drug. It also suggests that compounds which bind SmTAL1, such as six from the Medicines for Malaria Box identified in this work, may represent possible leads for the discovery of novel antagonists.

Searching for drugs for Chagas disease, leishmaniasis and schistosomiasis: a review

Chagas disease, leishmaniasis and schistosomiasis are neglected diseases (NDs) and are a considerable global challenge. Despite the huge number of people infected, NDs do not create interest from pharmaceutical companies because the associated revenue is generally low. Most of the research on these diseases has been conducted in academic institutions. The chemotherapeutic armamentarium for NDs is scarce and inefficient and better drugs are needed. Researchers have found some promising potential drug candidates using medicinal chemistry and computational approaches. Most of these compounds are synthetic but some are from natural sources or are semi-synthetic. Drug repurposing or repositioning has also been greatly stimulated for NDs. This review considers some potential drug candidates and provides details of their design, discovery and activity.

Could we expect new praziquantel derivatives? A meta pharmacometrics/pharmacoinformatics analysis of all antischistosomal praziquantel derivatives found in the literature

Praziquantel (PZQ) is the first line drug for the treatment of human Schistosoma spp. worm infections. However, it suffers from low activity towards immature stages of the worm, and its prolonged use induces resistance/tolerance. During the last 40 years, 263 PZQ analogues have been synthesized and tested against Schistosoma spp. worms, but less than 10% of them showed significant activity. Here, we propose a rationalization of the chemical space of the PZQ derivatives by a ligand-based approach. First, we constructed an in-house database with all PZQ derivatives available in the literature. This analysis shows a high heterogeneity in the data. Fortunately, all studies include PZQ as a reference, permitting the classification of compounds into three classes according to their activities. Models involving ligand-based pharmacophore and logistic regression were performed. Five physicochemical parameters were identified as the best to explain the biological activity. In the end, we proposed new PZQ derivatives with modifications at positions 1 and 7, we analysed them with our models, and we observed that they can be more active than the previously synthesized derivatives. The main goal of this work was to conduct the most valuable meta-pharmacometrics/pharmacoinformatics analysis with all Praziquantel medicinal chemistry data available in the literature.

Praziquantel for Schistosomiasis: Single-Drug Metabolism Revisited, Mode of Action, and Resistance

Schistosomiasis, a major neglected tropical disease, affects more than 250 million people worldwide. Treatment of schistosomiasis has relied on the anthelmintic drug praziquantel (PZQ) for more than a generation. PZQ is the drug of choice for the treatment of schistosomiasis; it is effective against all major forms of schistosomiasis, although it is less active against juvenile than mature parasites. A pyrazino-isoquinoline derivative, PZQ is not considered to be toxic and generally causes few or transient, mild side effects. Increasingly, mass drug administration targeting populations in sub-Saharan Africa where schistosomiasis is endemic has led to the appearance of reduced efficacy of PZQ, which portends the selection of drug-resistant forms of these pathogens. The synthesis of improved derivatives of PZQ is attracting attention, e.g., in the (i) synthesis of drug analogues, (ii) rational design of pharmacophores, and (iii) discovery of new compounds from large-scale screening programs. This article reviews reports from the 1970s to the present on the metabolism and mechanism of action of PZQ and its derivatives against schistosomes.

Brønsted acid-mediated cyclization-dehydrosulfonylation/reduction sequences: An easy access to pyrazinoisoquinolines and pyridopyrazines

An efficient and alternative synthetic approach has been developed to prepare various N-(arylethyl)piperazine-2,6-diones from 4-benzenesulfonyliminodiacetic acid and primary amines using carbonyldiimidazole in the presence of a catalytic amount of DMAP at ambient temperature. Piperazine-2,6-diones are successfully transformed to pharmaceutically useful pyridopyrazines or pyrazinoisoquinolines and ene-diamides via an imide carbonyl group activation strategy using a Brønsted acid. Subsequent dehydrosulfonylation reactions of the ene-diamides, in a one pot manner, smoothly transformed them to substituted pyrazinones. A concise synthesis of praziquantel (1) has also been achieved through this method.

Design of hybrid heterocyclic systems with a furoxanylpyridine core via tandem hetero-Diels–Alder/retro-Diels–Alder reactions of (1,2,4-triazin-3-yl)furoxans

Synthetic strategy for the synthesis of hybrid heterocyclic systems with the furoxanylpyridine core based on the tandem hetero-Diels–Alder/retro-Diels–Alder reactions of (1,2,4-triazin-3-yl)furoxans with enamine and norbornadiene has been developed.

Efficient assembly of mono- and bis(1,2,4-oxadiazol-3-yl)furoxan scaffolds via tandem reactions of furoxanylamidoximes

Tandem protocols for the synthesis of new types of hybrid molecules – (1,2,4-oxadiazol-3-yl)furoxans based on the furoxanylamidoximes heterocyclization have been developed.