Magnetic beads-based neuraminidase enzyme microreactor as a drug discovery tool for screening inhibitors from compound libraries and fishing ligands from natural products

Screening anti-anaphylactoid components in Polygonum cuspidatum via cell membrane chromatography with LC-MS targeting Mas-related G protein-coupled receptor X2

Mas-related G protein-coupled receptor X2 (MrgprX2) is acknowledged as a mast cell-specific receptor, playing a crucial role in orchestrating anaphylactoid responses through mast cell degranulation. It holds promise as a target for regulating allergic and inflammatory diseases mediated by mast cells. Polygonum cuspidatum (PC) has shown notable anti-anaphylactoid effects, while its pharmacologically active components remain unclear. In this study, we successfully utilized MrgprX2 high-expressing cell membrane chromatography (CMC), in conjunction with liquid chromatography-mass spectrometry (LC-MS), to identify active anti-anaphylactoid components in PC. Our study pinpointed polydatin, resveratrol, and emodin-8-O-β-d-glucoside as potential anti-anaphylactoid compounds in PC. Their anti-anaphylactoid activities were evaluated through β-aminohexosidase and histamine release assays, demonstrating a concentration-dependent inhibition for both β-aminohexosidase and histamine release. This approach, integrating MrgprX2 high-expression CMC with LC-MS, proves effective in screening potential anti-anaphylactoid ingredients in natural herbal medicines. The findings from this study illuminated the anti-anaphylactoid properties of specific components in PC and provided an efficient method for the drug development of natural products.

A magnetic beads-based ligand fishing method for rapid discovery of monoterpene indoles as monoamine oxidase A inhibitors from Hunteria zeylanica

In this study, a novel magnetic bead-based ligand fishing method was developed for rapid discovery of monoterpene indoles as monoamine oxidase A inhibitors from natural products. In order to improve the screening efficiency, two different magnetic beads, i.e. amine and carboxyl terminated magnetic beads, were comprehensively compared in terms of their ability to immobilize monoamine oxidase A (MAOA), biocatalytic activity and specific adsorption rates for affinity ligands. Carboxyl terminated magnetic beads performed better for MAOA immobilization and demonstrated superior performance in ligand fishing. The MAOA immobilized magnetic beads were applied to screen novel monoamine oxidase inhibitors in an alkaloid-rich plant, Hunteria zeylanica. Twelve MAOA affinity ligands were screened out, and ten of them were identified as monoterpene indole alkaloids by HPLC-Obitrap-MS/MS. Among them, six ligands, namely geissoschizol, vobasinol, yohimbol, dihydrocorynanthenol, eburnamine and (+)-isoeburnamine which exhibited inhibitory activity against MAOA with low IC50 values. To further explore their inhibitory mechanism, enzyme kinetic analysis and molecular docking studies were conducted.

Recent advances in bioaffinity strategies for preclinical and clinical drug discovery: Screening natural products, small molecules and antibodies

Bioaffinity drug screening strategies have gained popularity in preclinical and clinical drug discovery for natural products, small molecules and antibodies owing to their superior selectivity, the large number of compounds to be screened and their ability to minimize the time and expenses of the drug discovery process. This paper provides a systematic summary of the principles of commonly used bioaffinity-based screening methods, elaborates on the success of bioaffinity in clinical drug development and summarizes the active compounds, preclinical drugs and marketed drugs obtained through affinity screening methods. Owing to the high demand for new drugs, bioaffinity-guided screening techniques will play a greater part in clinical drug development.

Deep eutectic solvent extraction combined with magnetic bead ligand fishing for identification of α-glucosidase inhibitors from Pueraria lobata

In this study, a deep eutectic solvent (DES) extraction combined with a magnetic bead ligand affinity analytical method was developed and used for α-glucosidase inhibitor identification from Pueraria lobata. Several critical parameters affecting the analysis performance, including the type of DES, molar ratio, water amount, pH, salt concentration, and volume of DES, were investigated. The selected analytical sample preparation conditions were as follows. The composition of DES is choline chloride-1,4-butanediol (1:3), the water content is 40%, pH is 7.0 and the volume of extraction solution is 2 mL. The obtained sample extraction solution was analyzed directly using α-glucosidase immobilized magnetic beads (GMBs). Three α-glucosidase inhibitors in Pueraria lobata, including puerarin, daidzin, and daidzein, were identified. Luteolin was used as a positive control to evaluate the method's selectivity. Results showed it could selectively bond to the GMBs in the DES. As the affinity analysis was performed directly in a DES, the solution-removing process could be avoided. The intra-day and inter-day precisions of the method are 5.21% and 6.38%, respectively. The solvent amount was 1/50-1/2000 of that used in traditional methods.

Bioassay guided isolation of caffeoylquinic acids from the leaves of Ilex pubescens Hook. et Arn. and investigation of their anti-influenza mechanism

ETHNOPHARMACOLOGICAL RELEVANCE

Ilex pubescens Hook. et Arn. (Maodongqing, MDQ) is a common herbal tea ingredient in Southern China for heat clearance and anti-inflammation. Our preliminary screening showed that 50% ethanol extract of its leaves has anti-influenza virus activity. In this report, we proceed to identify the active components and clarify the related anti-influenza mechanisms.

AIM

We aim to isolate and identify the anti-influenza virus phytochemicals from the extract of the MDQ leaves, and study their anti-influenza virus mechanism.

MATERIAL AND METHODS

Plaque reduction assay was used to test the anti-influenza virus activity of fractions and compounds. Neuraminidase inhibitory assay was used to confirm the target protein. Molecular docking and reverse genetics were used to confirm the acting site of caffeoylquinic acids (CQAs) on viral neuraminidase.

RESULTS

Eight CQAs, 3,5-di-O-caffeoylquinic acid methyl ester (Me 3,5-DCQA), 3,4-di-O-caffeoylquinic acid methyl ester (Me 3,4-DCQA), 3,4,5-tri-O-caffeoylquinic acid methyl ester (Me 3,4,5-TCQA), 3,4,5-tri-O-caffeoylquinic acid (3,4,5-TCQA), 4,5-di-O-caffeoylquinic acid (4,5-DCQA), 3,5-di-O-caffeoylquinic acid (3,5-DCQA), 3,4-di-O-caffeoylquinic acid (3,4-DCQA), and 3,5-di-O-caffeoyl-epi-quinic acid (3,5-epi-DCQA) were identified from the MDQ leaves, in which Me 3,5-DCQA, 3,4,5-TCQA and 3,5-epi-DCQA were isolated for the first time. All these eight compounds were found to inhibit neuraminidase (NA) of influenza A virus. The results of molecular docking and reverse genetics indicated that 3,4,5-TCQA interacted with Tyr100, Gln412 and Arg419 of influenza NA, and a novel NA binding groove was found.

CONCLUSION

Eight CQAs isolated from the leaves of MDQ were found to inhibit influenza A virus. 3,4,5-TCQA was found to interact with Tyr100, Gln412 and Arg419 of influenza NA. This study provided scientific evidence on the use of MDQ for treating influenza virus infection, and laid the foundation for the development of CQA derivatives as potential antiviral agents.

Fighting the flu: a brief review on anti-influenza agents

The influenza virus causes one of the most prevalent and lethal infectious viral diseases of the respiratory system; the disease progression varies from acute self-limiting mild fever to disease chronicity and death. Although both the preventive and treatment measures have been vital in protecting humans against seasonal epidemics or sporadic pandemics, there are several challenges to curb the influenza virus such as limited or poor cross-protection against circulating virus strains, moderate protection in immune-compromised patients, and rapid emergence of resistance. Currently, there are four US-FDA-approved anti-influenza drugs to treat flu infection, viz. Rapivab, Relenza, Tamiflu, and Xofluza. These drugs are classified based on their mode of action against the viral replication cycle with the first three being Neuraminidase inhibitors, and the fourth one targeting the viral polymerase. The emergence of the drug-resistant strains of influenza, however, underscores the need for continuous innovation towards development and discovery of new anti-influenza agents with enhanced antiviral effects, greater safety, and improved tolerability. Here in this review, we highlighted commercially available antiviral agents besides those that are at different stages of development including under clinical trials, with a brief account of their antiviral mechanisms.

Screening of trypsin inhibitors in Cotinus coggygria Scop. extract using at-line nanofractionation coupled with semi-preparative reverse-phase liquid chromatography

In this study, an at-line nanofractionation (ANF) platform was successfully fabricated in parallel with mass spectrometry and trypsin inhibitory bioactivity assessment for rapid screening of trypsin inhibitors (TIs) from natural products for the first time. After systematic optimization, the ANF platform was applied to screen and identify TIs in the extract of a traditional Chinese herb, i.e., Cotinus coggygria Scop. The semi-preparative reverse-phase liquid chromatography was used subsequently to further simplify and enrich the insufficiently separated components. After comprehensive evaluation and validation, the ANF platform successfully identified 12 compounds as potential TIs, including 8 flavonoids and 2 organic acids. Additionally, a comparison study was conducted using two other ligand fishing approaches, i.e., capillary monolithic and magnetic beads-based trypsin-immobilized enzyme microreactors, which successfully identified 8 identical flavonoids as TIs. Importantly, the molecular docking study showed the molecular interactions between enzymes and inhibitors, thus strongly supporting the experimental results. Overall, this work has fully demonstrated the feasibility of the established ANF platform for screening TIs from Cotinus coggygria Scop., and proved its great prospects for screening bioactive components from natural products.

Rapid screening of neuraminidase inhibitors using an at-line nanofractionation platform involving parallel oseltamivir-sensitive/resistant neuraminidase bioassays

In this study, an advanced at-line nanofractionation based screening platform was developed to screen potential neuraminidase inhibitors (NAIs) from Lonicera japonica Thunb by involving two parallel bioassays, for determining both oseltamivir-sensitive neuraminidase (NAS) and oseltamivir-resistant neuraminidase (NAR) inhibitory activities. 20 potential NAIs with both NAS and NAR inhibitory effects were screened from Lonicera japonica Thunb and identified by mass spectrometer, including 11 phenolic acids, 8 flavonoids and one iridoid glycoside. The proposed at-line nanofractionation based screening platform for NAIs was also used to rapidly screen nine batches of water extracts of Lonicera japonica Thunb or its similar species. Clear differences in the number and content of active components were easily observed, demonstrating that the proposed method possesses great potential for the quality control of herb medicines.

High-Resolution Bioassay Profiling with Complemented Sensitivity and Resolution for Pancreatic Lipase Inhibitor Screening

How to rapidly and accurately screen bioactive components from complex natural products remains a major challenge. In this study, a screening platform for pancreatic lipase (PL) inhibitors was established by combining magnetic beads-based ligand fishing and high-resolution bioassay profiling. This platform was well validated using a mixture of standard compounds, i.e., (-)- epigallocatechin gallate (EGCG), luteolin and schisandrin. The dose-effect relationship of high-resolution bioassay profiling was demonstrated by the standard mixture with different concentrations for each compound. The screening of PL inhibitors from green tea extract at the concentrations of 0.2, 0.5 and 1.0 mg/mL by independent high-resolution bioassay profiling was performed. After sample pre-treatment by ligand fishing, green tea extract at the concentration of 0.2 mg/mL was specifically enriched and simplified, and consequently screened through the high-resolution bioassay profiling. As a result, three PL inhibitors, i.e., EGCG, (-)-Gallocatechin gallate (GCG) and (-)-Epicatechin gallate (ECG), were rapidly identified from the complex matrix. The established platform proved to be capable of enriching affinity binders and eliminating nonbinders in sample pre-treatment by ligand fishing, which overcame the technical challenges of high-resolution bioassay profiling in the aspects of sensitivity and resolution. Meanwhile, the high-resolution bioassay profiling possesses the ability of direct bioactive assessment, parallel structural analysis and identification after separation. The established platform allowed more accurate and rapid screening of PL inhibitors, which greatly facilitated natural product-based drug screening.

Drug discovery from natural products - Old problems and novel solutions for the treatment of neurodegenerative diseases

The use of natural products has been shown to be a fruitful approach in the discovery of novel pharmaceuticals. In fact, many currently approved drugs originated from compounds that were first identified in nature. Chemical diversity of natural compounds cannot be matched by man-made libraries of chemically synthesized molecules. Many natural compounds interact with and modulate regulatory protein targets and can be considered evolutionarily-optimized drug-like molecules. Despite this, many pharmaceutical companies have reduced or eliminated their natural product discovery programs in the last two decades. Screening natural products for pharmacologically active compounds is a challenging task that requires high resource commitment. Novel approaches at the early stage of the drug discovery pipeline are needed to allow for rapid screening and identification of the most promising molecules. Here, we review the possible evolutionary roots for drug-like characteristics of numerous natural compounds. Since many of these compounds target evolutionarily conserved cellular signaling pathways, we propose novel, early-stage drug discovery approaches to identify drug candidates that can be used for the potential prevention and treatment of neurodegenerative diseases. Invertebrate in vivo animal models of neurodegenerative diseases and innovative tools used within these models are proposed here as a screening funnel to identify new drug candidates and to shuttle these hits into further stages of the drug discovery pipeline.

Efficient discovery of potential inhibitors for SARS-CoV-2 3C-like protease from herbal extracts using a native MS-based affinity-selection method

The 3C-like protease (3CLpro) of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is essential to the virus life cycle and is supposed to be a potential target for the treatment of coronaviral infection. Traditional Chinese medicines (TCMs) have played an impressive role in the treatment of COVID-19 in China. The effectiveness of TCM formulations prompts scientists to take continuous effort on searching for bioactive small molecules from the ancient resources. Herein, we developed a native mass spectrometry-based affinity-selection method for rapid screening of active small molecules from crude herbal extracts applied for COVID-19 therapy. Six common herbs named Lonicera japonica, Scutellaria baicalensis, Forsythia suspensa, Glycyrrhiza uralensis, Cirsium japonicum, and Andrographis paniculata were investigated. After preliminary separation of the crude extracts, the fractions were incubated with 3CLpro. A native MS-based affinity screening assay was then conducted to search for the protein-ligand complexes. A UHPLC-Q/TOF-MS with UNIFI data acquisition and data processing software was applied to identify the hit compounds. Standard compounds were used to verify the outcomes. Among the 16 hits, three flavonoids, baicalein, scutellarein and ganhuangenin, were identified as potential noncovalent inhibitors against 3CLpro with IC₅₀ values of 0.94, 3.02, and 0.84 μM, respectively. Their binding affinities were further characterized by native MS, with K_d values being 1.43, 3.85, and 1.09 μM, respectively. Overall, we established an efficient native MS-based strategy for discovering 3CLpro ligands from crude mixtures, which supplies a potential strategy of small molecule lead discovery from TCMs.

Magnetic particles for enzyme immobilization: A versatile support for ligand screening

Enzyme inhibitors represent a substantial fraction of all small molecules currently in clinical use. Therefore, the early stage of drug-discovery process and development efforts are focused on the identification of new enzyme inhibitors through screening assays. The use of immobilized enzymes on solid supports to probe ligand-enzyme interactions have been employed with success not only to identify and characterize but also to isolate new ligands from complex mixtures. Between the available solid supports, magnetic particles have emerged as a promising support for enzyme immobilization due to the high superficial area, easy separation from the reaction medium and versatility. Particularly, the ligand fishing assay has been employed as a very useful tool to rapidly isolate bioactive compounds from complex mixtures, and hence the use of magnetic particles for enzyme immobilization has been widespread. Thus, this review provides a critical overview of the screening assays using immobilized enzymes on magnetic particles between 2006 and 2021.

Ligand screening assay for the enzyme kallikrein immobilized on NHS-activated Sepharose

Human tissue kallikreins (KLKs) are serine proteases involved in various physiological and pathological conditions, including cancer and neurological disorders. These enzymes constitute attractive drug targets, which has stimulated the search for new KLK inhibitors. In this study, we have covalently immobilized porcine pancreas KLK on an NHS-activated Sepharose matrix, to obtain KLK-Sepharose-NHS. The immobilized enzyme showed high recovered activity and maintained the ability of free KLK to recognize the synthetic substrate Z-Phe-Arg-AMC (K_Mapp = 10.3 ± 0.9 μM). As proof of concept, we used leupeptin as a reference inhibitor to perform inhibition studies for KLK-Sepharose-NHS and to determine the half-maximal inhibitory concentration (IC₅₀ = 0.13 ± 0.01 μM), the inhibition constant (K_i = 0.06 μM), and the leupeptin inhibition mechanism. We evaluated several complex matrixes (plant crude extract) by the same bioassay, to demonstrate their applicability. The species Solanum lycocarpum, Stryphnodendron adstringens, and Psychotria carthagenensis gave the best results. KLK-Sepharose-NHS was fully active after six consecutive reaction cycles and retained about 60 % of its initial activity after being used for at least five months, so the bioassay developed herein is a promising strategy to screen and to identify KLK ligands.

LABEL-FREE BIO-AFFINITY MASS SPECTROMETRY FOR SCREENING AND LOCATING BIOACTIVE MOLECULES

Despite the recent increase in the development of bioactive molecules in the drug industry, the enormous chemical space and lack of productivity are still important issues. Additional alternative approaches to screen and locate bioactive molecules are urgently needed. Label-free bio-affinity mass spectrometry (BA-MS) provides opportunities for the discovery and development of innovative drugs. This review provides a comprehensive portrayal of BA-MS techniques and of their applications in screening and locating bioactive molecules. After introducing the basic principles, alongside some application notes, the current state-of-the-art of BA-MS-assisted drug discovery is discussed, including native MS, size-exclusion chromatography-MS, ultrafiltration-MS, solid-phase micro-extraction-MS, and cell membrane chromatography-MS. Finally, several challenges and limitations of the current methods are summarized, with a view to potential future directions for BA-MS-assisted drug discovery. © 2019 John Wiley & Sons Ltd. Mass Spec Rev.

Screening of β-secretase inhibitors from Dendrobii Caulis by covalently enzyme-immobilized magnetic beads coupled with ultra-high-performance liquid chromatography

In the present study, to effectively discover potential β-secretase inhibitors from Dendrobii Caulis, β-secretase was immobilized on magnetic beads via direct covalent connection and coupled with ultra-high-performance liquid chromatography. Mechanochemical-assisted extraction was used to extract active ingredients from Dendrobii Caulis. The main reaction conditions, including screening enzyme inhibitors, were evaluated, and the most important enzyme kinetic parameters were also investigated across the UV-vis spectrophotometer. Five compounds (rutin, scoparone, naringenin, dendrophenol, and erianin) with high binding affinity to magnetic beads were removed from the extract. The results indicated that β-secretase was successfully immobilized and screened out five potential inhibitor compounds by ligand fishing. The lowest IC₅₀ values were noted for rutin (5.437 μM) and erianin (2.039 μM). This is the first report on immobilized β-secretase on magnetic beads for the identification of potential active compounds against Alzheimer's disease from complex biological mixtures.

Adsorbed hollow fiber immobilized tyrosinase for the screening of enzyme inhibitors from Pueraria lobata extract

In this study, a method based on adsorbed hollow fiber immobilized tyrosinase (TYR) was developed to screening potential TYR inhibitors from Pueraria lobate extract. Kojic acid and ranitidine were used as positive and negative control to verify the reliability of the proposed method, respectively. Several significant parameters of the screening process, including the amount of P. lobata extract, adsorption time and incubation time, were optimized. After investigating the repeatability of the developed method, seven potential active compounds in P. lobata extract were successfully detected and their chemical structures were tentatively identified by liquid chromatography - mass spectrometry analysis. Furthermore, the inhibitory activity of four identified compounds on TYR was tested in vitro, and three of them, namely, puerarin, puerarin-6″-O-xyloside and puerarin apioside were verified to have good TYR inhibitory activity with IC₅₀ value of 478.5, 513.8, and 877.3 μM, respectively. In addition, the molecular docking results indicated that these compounds could bind to the amino acid residues in TYR catalytic pocket. These results proved that the proposed method is a feasible approach for screening of TYR inhibitors from plant extract.

Rapid identification and isolation of neuraminidase inhibitors from mockstrawberry (Duchesnea indica Andr.) based on ligand fishing combined with HR-ESI-Q-TOF-MS

Neuraminidase inhibitors (NAIs) are the mainstay antiviral drugs against influenza infection. In this study, a ligand fishing protocol was developed to screen NAIs using neuraminidase immobilized magnetic beads (NA-MB). After verifying the feasibility of NA-MB with an artificial mixture including NA inhibitors and non-inhibitors, the developed ligand fishing protocol was applied to screen NAIs from the crude extracts of Duchesnea indica Andr. Twenty-four NA binding compounds were identified from the normal butanol (n-BuOH) extract of D. indica as potential NAIs by high-performance liquid chromatography coupled with quadrupole time-of-flight mass spectrometry (HPLC–Q-TOF-MS) assisted with Compound Structure Identification (CSI):FingerID, including 12 ellagitannins, 4 brevifolin derivatives, 3 ellagic acid derivatives, and 4 flavonoids. Among them, 9 compounds were isolated and tested for in vitro NA inhibitory activities against NA from Clostridium perfringens, and from oseltamivir sensitive and resistant influenza A virus strains. The results indicate that compound B23 has the NA inhibitory activities in both the oseltamivir sensitive and resistant viral NA, with half maximal inhibitory concentration (IC₅₀) values of 197.9 and 125.4 μmol/L, respectively. Moreover, B23 can obviously reduce the replication of oseltamivir sensitive and resistant viruses in Madin–Darby canine kidney (MDCK) cells at the concentrations of 40 and 200 μmol/L.

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An efficient ligand fishing protocol was developed to rapidly screen the neuraminidase inhibitors from natural sources.

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24 potential neuraminidase inhibitors were identified from Duchesnea indica as potential NAIs by HPLC-Q-TOF-MS.

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One compound can inhibit neuraminidase activities in both the oseltamivir sensitive and resistant virus strains.

Recent advances in screening active components from natural products based on bioaffinity techniques

Natural products have provided numerous lead compounds for drug discovery. However, the traditional analytical methods cannot detect most of these active components, especially at their usual low concentrations, from complex natural products. Herein, we reviewed the recent technological advances (2015–2019) related to the separation and screening bioactive components from natural resources, especially the emerging screening methods based on the bioaffinity techniques, including biological chromatography, affinity electrophoresis, affinity mass spectroscopy, and the latest magnetic and optical methods. These screening methods are uniquely advanced compared to other traditional methods, and they can fish out the active components from complex natural products because of the affinity between target and components, without tedious separation works. Therefore, these new tools can reduce the time and cost of the drug discovery process and accelerate the development of more effective and better-targeted therapeutic agents.

Natural products against acute respiratory infections: Strategies and lessons learned

ETHNOPHARMACOLOGICAL RELEVANCE

A wide variety of traditional herbal remedies have been used throughout history for the treatment of symptoms related to acute respiratory infections (ARIs).

AIM OF THE REVIEW

The present work provides a timely overview of natural products affecting the most common pathogens involved in ARIs, in particular influenza viruses and rhinoviruses as well as bacteria involved in co-infections, their molecular targets, their role in drug discovery, and the current portfolio of available naturally derived anti-ARI drugs.

MATERIALS AND METHODS

Literature of the last ten years was evaluated for natural products active against influenza viruses and rhinoviruses. The collected bioactive agents were further investigated for reported activities against ARI-relevant bacteria, and analysed for the chemical space they cover in relation to currently known natural products and approved drugs.

RESULTS

An overview of (i) natural compounds active in target-based and/or phenotypic assays relevant to ARIs, (ii) extracts, and (iii) in vivo data are provided, offering not only a starting point for further in-depth phytochemical and antimicrobial studies, but also revealing insights into the most relevant anti-ARI scaffolds and compound classes. Investigations of the chemical space of bioactive natural products based on principal component analysis show that many of these compounds are drug-like. However, some bioactive natural products are substantially larger and have more polar groups than most approved drugs. A workflow with various strategies for the discovery of novel antiviral agents is suggested, thereby evaluating the merit of in silico techniques, the use of complementary assays, and the relevance of ethnopharmacological knowledge on the exploration of the therapeutic potential of natural products.

CONCLUSIONS

The longstanding ethnopharmacological tradition of natural remedies against ARIs highlights their therapeutic impact and remains a highly valuable selection criterion for natural materials to be investigated in the search for novel anti-ARI acting concepts. We observe a tendency towards assaying for broad-spectrum antivirals and antibacterials mainly discovered in interdisciplinary academic settings, and ascertain a clear demand for more translational studies to strengthen efforts for the development of effective and safe therapeutic agents for patients suffering from ARIs.

Solid-Supported Proteins in the Liquid Chromatography Domain to Probe Ligand-Target Interactions

Ligand-target interactions play a central role in drug discovery processes because these interactions are crucial in biological systems. Small molecules-proteins interactions can regulate and modulate protein function and activity through conformational changes. Therefore, bioanalytical tools to screen new ligands have focused mainly on probing ligand-target interactions. These interactions have been evaluated by using solid-supported proteins, which provide advantages like increased protein stability and easier protein extraction from the reaction medium, which enables protein reuse. In some specific approaches, precisely in the ligand fishing assay, the bioanalytical method allows the ligands to be directly isolated from complex mixtures, including combinatorial libraries and natural products extracts without prior purification or fractionation steps. Most of these screening assays are based on liquid chromatography separation, and the binding events can be monitored through on-line or off-line methods. In the on-line approaches, solid supports containing the immobilized biological target are used as chromatographic columns most of the time. Several terms have been used to refer to such approaches, such as weak affinity chromatography, high-performance affinity chromatography, on-flow activity assays, and high-performance liquid affinity chromatography. On the other hand, in the off-line approaches, the binding event occurs outside the liquid chromatography system and may encompass affinity and activity-based assays in which the biological target is immobilized on magnetic particles or monolithic silica, among others. After the incubation step, the supernatant or the eluate from the binding assay is analyzed by liquid chromatography coupled to various detectors. Regardless of the selected bioanalytical approach, the use of solid supported proteins has significantly contributed to the development of automated and reliable screening methods that enable ligands to be isolated and characterized in complex matrixes without purification, thereby reducing costs and avoiding time-laborious steps. This review provides a critical overview of recently developed assays.

Inhibition of sialidase activity as a therapeutic approach

The demand for novel anti-influenza drugs persists, which is highlighted by the recent pandemics of influenza affecting thousands of people across the globe. One of the approaches to block the virus spreading is inhibiting viral sialidase (neuraminidase). This enzyme cleaves the sialic acid link between the newly formed virions and the host cell surface liberating the virions from the cell and maintaining the cycle of infection. Viral neuraminidases appear therefore as attractive therapeutic targets for preventing further spread of influenza infection. Compared to ion channel blockers that were the first approved anti-influenza drugs, neuraminidase inhibitors are well tolerated and target both influenza A and B viruses. Moreover, neuraminidase/sialidase inhibitors may be useful for managing some other human pathologies, such as cancer. In this review, we discuss the available knowledge on neuraminidase or sialidase inhibitors, their design, clinical application, and the current challenges.