Discovery of oseltamivir-based novel PROTACs as degraders targeting neuraminidase to combat H1N1 influenza virus

Development of novel antivrial agents that induce the degradation of the main protease of human-infecting coronaviruses

The family of human-infecting coronaviruses (HCoVs) poses a serious threat to global health and includes several highly pathogenic strains that cause severe respiratory illnesses. It is essential that we develop effective broad-spectrum anti-HCoV agents to prepare for future outbreaks. In this study, we used PROteolysis TArgeting Chimera (PROTAC) technology focused on degradation of the HCoV main protease (Mpro), a conserved enzyme essential for viral replication and pathogenicity. By adapting the Mpro inhibitor GC376, we produced two novel PROTACs, P2 and P3, which showed relatively broad-spectrum activity against the human-infecting CoVs HCoV-229E, HCoV-OC43, and SARS-CoV-2. The concentrations of these PROTACs that reduced virus replication by 50 % ranged from 0.71 to 4.6 μM, and neither showed cytotoxicity at 100 μM. Furthermore, mechanistic binding studies demonstrated that P2 and P3 effectively targeted HCoV-229E, HCoV-OC43, and SARS-CoV-2 by degrading Mpro within cells in vitro. This study highlights the potential of PROTAC technology in the development of broad-spectrum anti-HCoVs agents, presenting a novel approach for dealing with future viral outbreaks, particularly those stemming from CoVs.

Discovery of L15 as a novel Vif PROTAC degrader with antiviral activity against HIV-1

Viral infectivity factor (Vif) has been recognized as a new therapeutic target for human immunodeficiency virus-1 (HIV-1) infected patients. In our previous work, we have synthesized a novel class of Vif inhibitors with 2-amino-N-(5-hydroxy-2-methoxyphenyl)-6-((4-nitrophenyl)thio)benzamide scaffold, which show obvious activity in HIV-1 infected cells and are also effective against drug-resistant strains. Proteolytic targeting chimera (PROTAC) utilizes the ubiquitin-proteasome system to degrade target proteins, which is well established in the field of cancer, but the antiviral PROTAC molecules are rarely reported. In order to explore the effectiveness of PROTAC in the antiviral area, we designed and synthesized a series of degrader of HIV-1 Vif based on 2-amino-N-(5-hydroxy-2-methoxyphenyl)-6-((4-nitrophenyl)thio)benzamide scaffold. Among them, L15 can degrade Vif protein obviously in a dose-dependent manner and shows certain antivirus activity. Meanwhile, molecular dynamics simulation indicated that the ternary complex formed by L15, Vif, and E3 ligase adopted a reasonable binding mode and maintained a stable interaction. This provided a molecular basis and prerequisite for the selective degradation of the Vif protein by L15. This study reports the HIV-1 Vif PROTAC for the first time and represents the proof-of-concept of PROTACs-based antiviral drug discovery in the field of HIV/ acquired immune deficiency syndrome (AIDS).

Broad-spectrum activity against mosquito-borne flaviviruses achieved by a targeted protein degradation mechanism

Viral genetic diversity presents significant challenges in developing antivirals with broad-spectrum activity and high barriers to resistance. Here we report development of proteolysis targeting chimeras (PROTACs) targeting the dengue virus envelope (E) protein through coupling of known E fusion inhibitors to ligands of the CRL4CRBN E3 ubiquitin ligase. The resulting small molecules block viral entry through inhibition of E-mediated membrane fusion and interfere with viral particle production by depleting intracellular E in infected Huh 7.5 cells. This activity is retained in the presence of point mutations previously shown to confer partial resistance to the parental inhibitors due to decreased inhibitor-binding. The E PROTACs also exhibit broadened spectrum of activity compared to the parental E inhibitors against a panel of mosquito-borne flaviviruses. These findings encourage further exploration of targeted protein degradation as a differentiated and potentially advantageous modality for development of broad-spectrum direct-acting antivirals.

Discovery of Novel ERα and Aromatase Dual-Targeting PROTAC Degraders to Overcome Endocrine-Resistant Breast Cancer

Estrogen receptor α (ERα) plays a pivotal role in the proliferation, differentiation, and migration of breast cancer (BC) cells, and aromatase (ARO) is a crucial enzyme in estrogen synthesis. Hence, it is necessary to inhibit estrogen production or the activity of ERα for the treatment of estrogen receptor-positive (ER+) BC. Herein, we present a new category of dual-targeting PROTAC degraders designed to specifically target ERα and ARO. Among them, compound 18c bifunctionally degrades and inhibits ERα/ARO, thus effectively suppressing the proliferation of MCF-7 cells while showing negligible cytotoxicity to normal cells. In vivo, 18c promotes the degradation of ERα and ARO and inhibits the growth of MCF-7 xenograft tumors. Finally, compound 18c demonstrates promising antiproliferative and ERα degradation activity against the ERαMUT cells. These findings suggest that 18c, being the inaugural dual-targeting degrader for ERα and ARO, warrants further advancement for the management of BC and the surmounting of endocrine resistance.

Novel Acyl Thiourea-Based Hydrophobic Tagging Degraders Exert Potent Anti-Influenza Activity through Two Distinct Endonuclease Polymerase Acidic-Targeted Degradation Pathways

The spread of the influenza virus has caused devastating pandemics and huge economic losses worldwide. Antiviral drugs with diverse action modes are urgently required to overcome the challenges of viral mutation and drug resistance, and targeted protein degradation strategies constitute excellent candidates for this purpose. Herein, the first degradation of the influenza virus polymerase acidic (PA) protein using small-molecule degraders developed by hydrophobic tagging (HyT) technology to effectively combat the influenza virus was reported. The SAR results revealed that compound 19b with Boc2-(L)-Lys demonstrated excellent inhibitory activity against A/WSN/33/H1N1 (EC50 = 0.015 μM) and amantadine-resistant strain (A/PR/8/H1N1), low cytotoxicity, high selectivity, substantial degradation ability, and good drug-like properties. Mechanistic studies demonstrated that the proteasome system and autophagic lysosome pathway were the potential drivers of these HyT degraders. Thus, this study provides a powerful tool for investigating the targeted degradation of influenza virus proteins and for antiviral drug development.

Discovery of Potent Degraders of the Dengue Virus Envelope Protein

Targeted protein degradation has been widely adopted as a new approach to eliminate both established and previously recalcitrant therapeutic targets. Here we report the development of small molecule degraders of the envelope (E) protein of dengue virus. We developed two classes of bivalent E-degraders, linking two previously reported E-binding small molecules, GNF-2 and CVM-2-12-2, to a glutarimide-based recruiter of the CRL4CRBN ligase to effect proteosome-mediated degradation of the E protein. ZXH-2-107 (based on GNF-2) is an E degrader with ABL inhibition while ZXH-8-004 (based on CVM-2-12-2) is a selective and potent E-degrader. These two compounds provide proof-of-concept that difficult-to-drug targets such as a viral envelope protein can be effectively eliminated using a bivalent degrader and provide starting points for the future development of a new class antiviral drugs.

Modular Biomimetic Strategy Enables Discovery and SAR Exploration of Oxime Macrocycles as Influenza A Virus (H1N1) Inhibitors

Although vaccination remains the prevalent prophylactic means for controlling Influenza A virus (IAV) infections, novel structural antivirus small-molecule drugs with new mechanisms of action for treating IAV are highly desirable. Herein, we describe a modular biomimetic strategy to expeditiously achieve a new class of macrocycles featuring oxime, which might target the hemagglutinin (HA)-mediated IAV entry into the host cells. SAR analysis revealed that the size and linker of the macrocycles play an important role in improving potency. Particularly, as a 14-membered macrocyclic oxime, 37 exhibited potent inhibitory activity against IAV H1N1 with an EC50 value of 23 nM and low cytotoxicity, which alleviated cytopathic effects and protected cell survival obviously after H1N1 infection. Furthermore, 37 showed significant synergistic activity with neuraminidase inhibitor oseltamivir in vitro.

Targeted protein degradation: from mechanisms to clinic

Targeted protein degradation refers to the use of small molecules to induce the selective degradation of proteins. In its most common form, this degradation is achieved through ligand-mediated neo-interactions between ubiquitin E3 ligases - the principal waste disposal machines of a cell - and the protein targets of interest, resulting in ubiquitylation and subsequent proteasomal degradation. Notable advances have been made in biological and mechanistic understanding of serendipitously discovered degraders. This improved understanding and novel chemistry has not only provided clinical proof of concept for targeted protein degradation but has also led to rapid growth of the field, with dozens of investigational drugs in active clinical trials. Two distinct classes of protein degradation therapeutics are being widely explored: bifunctional PROTACs and molecular glue degraders, both of which have their unique advantages and challenges. Here, we review the current landscape of targeted protein degradation approaches and how they have parallels in biological processes. We also outline the ongoing clinical exploration of novel degraders and provide some perspectives on the directions the field might take.

Design, synthesis, and biological evaluation of first-in-class indomethacin-based PROTACs degrading SARS-CoV-2 main protease and with broad-spectrum antiviral activity

To date, Proteolysis Targeting Chimera (PROTAC) technology has been successfully applied to mediate proteasomal-induced degradation of several pharmaceutical targets mainly related to oncology, immune disorders, and neurodegenerative diseases. On the other hand, its exploitation in the field of antiviral drug discovery is still in its infancy. Recently, we described two indomethacin (INM)-based PROTACs displaying broad-spectrum antiviral activity against coronaviruses. Here, we report the design, synthesis, and characterization of a novel series of INM-based PROTACs that recruit either Von-Hippel Lindau (VHL) or cereblon (CRBN) E3 ligases. The panel of INM-based PROTACs was also enlarged by varying the linker moiety. The antiviral activity resulted very susceptible to this modification, particularly for PROTACs hijacking VHL as E3 ligase, with one piperazine-based compound (PROTAC 6) showing potent anti-SARS-CoV-2 activity in infected human lung cells. Interestingly, degradation assays in both uninfected and virus-infected cells with the most promising PROTACs emerged so far (PROTACs 5 and 6) demonstrated that INM-PROTACs do not degrade human PGES-2 protein, as initially hypothesized, but induce the concentration-dependent degradation of SARS-CoV-2 main protease (Mpro) both in Mpro-transfected and in SARS-CoV-2-infected cells. Importantly, thanks to the target degradation, INM-PROTACs exhibited a considerable enhancement in antiviral activity with respect to indomethacin, with EC50 values in the low-micromolar/nanomolar range. Finally, kinetic solubility as well as metabolic and chemical stability were measured for PROTACs 5 and 6. Altogether, the identification of INM-based PROTACs as the first class of SARS-CoV-2 Mpro degraders demonstrating activity also in SARS-CoV-2-infected cells represents a significant advance in the development of effective, broad-spectrum anti-coronavirus strategies.

Fluorescence theranostic PROTACs for real-time visualization of ERα degradation

Proteolysis targeting chimera (PROTAC) technology, a groundbreaking strategy for degradation of pathogenic proteins by hijacking of the ubiquitin-proteasome-system has become a promising strategy in drug design. However, the real-time monitoring and visualization of protein degradation processes have been long-standing challenges in the realm of drug development. In this research, we sought to amalgamate the highly efficient protein-degrading capabilities of PROTAC technology with the visualization attributes of fluorescent probes, with the potential to pave the path for the design and development of a novel class of visual PROTACs. These novel PROTACs uniquely possess both fluorescence imaging and therapeutic characteristics, all with the goal of enabling real-time observations of protein degradation processes. Our approach involved the utilization of a high ER-targeting fluorescent probe, previously reported in our laboratory, which served as a warhead that specifically binds to the protein of interest (POI). Additionally, a VHL ligand for recruiting E3 ligase and linkers of various lengths were incorporated to synthesize a series of novel ER-inherent fluorescence PROTACs. Among them, compound A3 demonstrated remarkable efficiency in degrading ERα proteins (DC50 = 0.12 μM) and displaying exceptional anti-proliferative activity against MCF-7 cells (IC50 = 0.051 μM). Furthermore, it exhibited impressive fluorescence imaging performance, boasting an emission wavelength of up to 582 nm, a Stokes shift of 116 nm, and consistent optical properties. These attributes make it especially suitable for the real-time, in situ tracking of ERα protein degradation processes, thus may serve as a privileged visual theranostic PROTAC for ERα+ breast cancer. This study not only broadens the application spectrum of PROTAC technology but also introduces a novel approach for real-time visualization of protein degradation processes, ultimately enhancing the diagnostic and treatment efficacy of PROTACs.

Target-based drug design strategies to overcome resistance to antiviral agents: opportunities and challenges

Viral infections have a major impact in human health. Ongoing viral transmission and escalating selective pressure have the potential to favor the emergence of vaccine- and antiviral drug-resistant viruses. Target-based approaches for the design of antiviral drugs can play a pivotal role in combating drug-resistant challenges. Drug design computational tools facilitate the discovery of novel drugs. This review provides a comprehensive overview of current drug design strategies employed in the field of antiviral drug resistance, illustrated through the description of a series of successful applications. These strategies include technologies that enhance compound-target affinity while minimizing interactions with mutated binding pockets. Furthermore, emerging approaches such as virtual screening, targeted protein/RNA degradation, and resistance analysis during drug design have been harnessed to curtail the emergence of drug resistance. Additionally, host targeting antiviral drugs offer a promising avenue for circumventing viral mutation. The widespread adoption of these refined drug design strategies will effectively address the prevailing challenge posed by antiviral drug resistance.

Targeting SARS-CoV-2 nonstructural protein 3: Function, structure, inhibition, and perspective in drug discovery

As a highly contagious human pathogen, severe acute respiratory syndrome-associated coronavirus-2 (SARS-CoV-2) has infected billions of people worldwide with more than 6 million deaths. With several effective vaccines and antiviral drugs now available, the SARS-CoV-2 pandemic been brought under control. However, a new pathogenic coronavirus could emerge in the future, given the zoonotic nature of this virus. Natural evolution and drug-induced mutations of SARS-CoV-2 also require continued efforts for new anti-coronavirus drugs. Nonstructural protein (nsp) 3 of CoVs is a large, multifunctional protein, containing a papain-like protease (PLpro) and a macrodomain (Mac1), which are essential for viral replication. Here, we provide a comprehensive review of the function, structure, and inhibition of SARS-CoV/-CoV-2 PLpro and Mac1. We also discuss advances in, and challenges to, the discovery of drugs against these targets.

Intracellular Degradation of SARS-CoV-2 N-Protein Caused by Modular Nanotransporters Containing Anti-N-Protein Monobody and a Sequence That Recruits the Keap1 E3 Ligase

The proper viral assembly relies on both nucleic acids and structural viral proteins. Thus a biologically active agent that provides the degradation of one of these key proteins and/or destroys the viral factory could suppress viral replication efficiently. The nucleocapsid protein (N-protein) is a key protein for the SARS-CoV-2 virus. As a bioactive agent, we offer a modular nanotransporter (MNT) developed by us, which, in addition to an antibody mimetic to the N-protein, contains an amino acid sequence for the attraction of the Keap1 E3 ubiquitin ligase. This should lead to the subsequent degradation of the N-protein. We have shown that the functional properties of modules within the MNT permit its internalization into target cells, endosome escape into the cytosol, and binding to the N-protein. Using flow cytometry and western blotting, we demonstrated significant degradation of N-protein when A549 and A431 cells transfected with a plasmid coding for N-protein were incubated with the developed MNTs. The proposed MNTs open up a new approach for the treatment of viral diseases.

Emerging drug design strategies in anti-influenza drug discovery

Influenza is an acute respiratory infection caused by influenza viruses (IFV), According to the World Health Organization (WHO), seasonal IFV epidemics result in approximately 3-5 million cases of severe illness, leading to about half a million deaths worldwide, along with severe economic losses and social burdens. Unfortunately, frequent mutations in IFV lead to a certain lag in vaccine development as well as resistance to existing antiviral drugs. Therefore, it is of great importance to develop anti-IFV drugs with high efficiency against wild-type and resistant strains, needed in the fight against current and future outbreaks caused by different IFV strains. In this review, we summarize general strategies used for the discovery and development of antiviral agents targeting multiple IFV strains (including those resistant to available drugs). Structure-based drug design, mechanism-based drug design, multivalent interaction-based drug design and drug repurposing are amongst the most relevant strategies that provide a framework for the development of antiviral drugs targeting IFV.

Targeted protein modification as a paradigm shift in drug discovery

Targeted Protein Modification (TPM) is an umbrella term encompassing numerous tools and approaches that use bifunctional agents to induce a desired modification over the POI. The most well-known TPM mechanism is PROTAC-directed protein ubiquitination. PROTAC-based targeted degradation offers several advantages over conventional small-molecule inhibitors, has shifted the drug discovery paradigm, and is acquiring increasing interest as over ten PROTACs have entered clinical trials in the past few years. Targeting the protein of interest for proteasomal degradation by PROTACS was the pioneer of various toolboxes for selective protein degradation. Nowadays, the ever-increasing number of tools and strategies for modulating and modifying the POI has expanded far beyond protein degradation, which phosphorylation and de-phosphorylation of the protein of interest, targeted acetylation, and selective modification of protein O-GlcNAcylation are among them. These novel strategies have opened new avenues for achieving more precise outcomes while remaining feasible and minimizing side effects. This field, however, is still in its infancy and has a long way to precede widespread use and translation into clinical practice. Herein, we investigate the pros and cons of these novel strategies by exploring the latest advancements in this field. Ultimately, we briefly discuss the emerging potential applications of these innovations in cancer therapy, neurodegeneration, viral infections, and autoimmune and inflammatory diseases.

Rational design and optimization of acylthioureas as novel potent influenza virus non-nucleoside polymerase inhibitors

Evidence suggests that rapidly evolving virus subvariants risk rendering current vaccines and anti-influenza drugs ineffective. Hence, exploring novel scaffolds or new targets of anti-influenza drugs is of great urgency. Herein, we report the discovery of a series of acylthiourea derivatives produced via a scaffold-hopping strategy as potent antiviral agents against influenza A and B subtypes. The most effective compound 10m displayed subnanomolar activity against H1N1 proliferation (EC50 = 0.8 nM) and exhibited inhibitory activity toward other influenza strains, including influenza B virus and H1N1 variant (H1N1, H274Y). Additionally, druggability evaluation revealed that 10m exhibited favorable pharmacokinetic properties and was metabolically stable in liver microsome preparations from three different species as well as in human plasma. In vitro and in vivo toxicity studies confirmed that 10m demonstrated a high safety profile. Furthermore, 10m exhibited satisfactory antiviral activity in a lethal influenza virus mouse model. Moreover, mechanistic studies indicated that these acylthiourea derivatives inhibited influenza virus proliferation by targeting influenza virus RNA-dependent RNA polymerase. Thus, 10m is a potential lead compound for the further exploration of treatment options for influenza.

Ubiquitination and cell-autonomous immunity

Cell-autonomous immunity is the first line of defense by which cells recognize and contribute to eliminating invasive pathogens. It is composed of immune signaling networks that sense microbial pathogens, promote pathogen restriction, and stimulate their elimination, including host cell death. Ubiquitination is a pivotal orchestrator of these pathways, by changing the activity of signal transducers and effector proteins in an efficient way. In this review, we will focus on how ubiquitin connects the pathways that sense pathogens to the cellular responses to invaders and shed light on how ubiquitination impacts the microenvironment around the infected cell, stimulating the appropriate immune response. Finally, we discuss therapeutic options directed at favoring cell-autonomous immune responses to infection.

Generation of host-directed and virus-specific antivirals using targeted protein degradation promoted by small molecules and viral RNA mimics

Targeted protein degradation (TPD), as exemplified by proteolysis-targeting chimera (PROTAC), is an emerging drug discovery platform. PROTAC molecules, which typically contain a target protein ligand linked to an E3 ligase ligand, recruit a target protein to the E3 ligase to induce its ubiquitination and degradation. Here, we applied PROTAC approaches to develop broad-spectrum antivirals targeting key host factors for many viruses and virus-specific antivirals targeting unique viral proteins. For host-directed antivirals, we identified a small-molecule degrader, FM-74-103, that elicits selective degradation of human GSPT1, a translation termination factor. FM-74-103-mediated GSPT1 degradation inhibits both RNA and DNA viruses. Among virus-specific antivirals, we developed viral RNA oligonucleotide-based bifunctional molecules (Destroyers). As a proof of principle, RNA mimics of viral promoter sequences were used as heterobifunctional molecules to recruit and target influenza viral polymerase for degradation. This work highlights the broad utility of TPD to rationally design and develop next-generation antivirals.

T-705-Derived Prodrugs Show High Antiviral Efficacies against a Broad Range of Influenza A Viruses with Synergistic Effects When Combined with Oseltamivir

Emerging influenza A viruses (IAV) bear the potential to cause pandemics with unpredictable consequences for global human health. In particular, the WHO has declared avian H5 and H7 subtypes as high-risk candidates, and continuous surveillance of these viruses as well as the development of novel, broadly acting antivirals, are key for pandemic preparedness. In this study, we sought to design T-705 (Favipiravir) related inhibitors that target the RNA-dependent RNA polymerase and evaluate their antiviral efficacies against a broad range of IAVs. Therefore, we synthesized a library of derivatives of T-705 ribonucleoside analogues (called T-1106 pronucleotides) and tested their ability to inhibit both seasonal and highly pathogenic avian influenza viruses in vitro. We further showed that diphosphate (DP) prodrugs of T-1106 are potent inhibitors of H1N1, H3N2, H5N1, and H7N9 IAV replication. Importantly, in comparison to T-705, these DP derivatives achieved 5- to 10-fold higher antiviral activity and were non-cytotoxic at the therapeutically active concentrations. Moreover, our lead DP prodrug candidate showed drug synergy with the neuraminidase inhibitor oseltamivir, thus opening up another avenue for combinational antiviral therapy against IAV infections. Our findings may serve as a basis for further pre-clinical development of T-1106 prodrugs as an effective countermeasure against emerging IAVs with pandemic potential.

Antiviral PROTACs: Opportunity borne with challenge

Proteolysis targeting chimera (PROTAC) degradation of pathogenic proteins by hijacking of the ubiquitin-proteasome-system has become a promising strategy in drug design. The overwhelming advantages of PROTAC technology have ensured a rapid and wide usage, and multiple PROTACs have entered clinical trials. Several antiviral PROTACs have been developed with promising bioactivities against various pathogenic viruses. However, the number of reported antiviral PROTACs is far less than that of other diseases, e.g., cancers, immune disorders, and neurodegenerative diseases, possibly because of the common deficiencies of PROTAC technology (e.g., limited available ligands and poor membrane permeability) plus the complex mechanism involved and the high tendency of viral mutation during transmission and replication, which may challenge the successful development of effective antiviral PROTACs. This review highlights the important advances in this rapidly growing field and critical limitations encountered in developing antiviral PROTACs by analyzing the current status and representative examples of antiviral PROTACs and other PROTAC-like antiviral agents. We also summarize and analyze the general principles and strategies for antiviral PROTAC design and optimization with the intent of indicating the potential strategic directions for future progress.

Conformationally locked sugar derivatives and analogues as potential neuraminidase inhibitors

The influenza virus remains a major health concern for mankind because it tends to mutate frequently and cause high morbidity. Influenza prevention and treatment are greatly aided by the use of antivirals. One such class of antivirals is neuraminidase inhibitors (NAIs), effective against influenza viruses. A neuraminidase on the virus's surface serves a vital function in viral propogation by assisting in the release of viruses from infected host cells. Neuraminidase inhibitors are the backbone in stoping such virus propagation thus helps in the treatment of influenza viruses infections. Two NAI medicines are licensed globally: Oseltamivir (Tamiflu™) and Zanamivir (Relanza™). There are two molecules that have acquired Japanese approval recently: Peramivir and Laninamivir, whereas Laninamivir octanoate is in Phase III clinical trials. The need for novel NAIs is due to frequent mutations in viruses and the rise in resistance against existing medication. The NA inhibitors (NAIs) are designed to have (oxa)cyclohexene scaffolds (a sugar scaffold) to mimic the oxonium transition state in the enzymatic cleavage of sialic acid. This review discusses in details and comprises all such conformationally locked (oxa)cyclohexene scaffolds and their analogues which have been recently designed and synthesized as potential neuraminidase inhibitors, thus as antiviral molecules. The structure-activity relationship of such diverese molecules has also been discussed in this review.

Targeted Protein Degradation for Infectious Diseases: from Basic Biology to Drug Discovery

Targeted protein degradation (TPD) is emerging as one of the most innovative strategies to tackle infectious diseases. Particularly, proteolysis-targeting chimera (PROTAC)-mediated protein degradation may offer several benefits over classical anti-infective small-molecule drugs. Because of their peculiar and catalytic mechanism of action, anti-infective PROTACs might be advantageous in terms of efficacy, toxicity, and selectivity. Importantly, PROTACs may also overcome the emergence of antimicrobial resistance. Furthermore, anti-infective PROTACs might have the potential to (i) modulate "undruggable" targets, (ii) "recycle" inhibitors from classical drug discovery approaches, and (iii) open new scenarios for combination therapies. Here, we try to address these points by discussing selected case studies of antiviral PROTACs and the first-in-class antibacterial PROTACs. Finally, we discuss how the field of PROTAC-mediated TPD might be exploited in parasitic diseases. Since no antiparasitic PROTAC has been reported yet, we also describe the parasite proteasome system. While in its infancy and with many challenges ahead, we hope that PROTAC-mediated protein degradation for infectious diseases may lead to the development of next-generation anti-infective drugs.

Recent Advances in PROTAC-Based Antiviral Strategies

Numerous mysteries of cell and molecular biology have been resolved through extensive research into intracellular processes, which has also resulted in the development of innovative technologies for the treatment of infectious and non-infectious diseases. Some of the deadliest diseases, accounting for a staggering number of deaths, have been caused by viruses. Conventional antiviral therapies have been unable to achieve a feat in combating viral infections. As a result, the healthcare system has come under tremendous pressure globally. Therefore, there is an urgent need to discover and develop newer therapeutic approaches against viruses. One such innovative approach that has recently garnered attention in the research world and can be exploited for developing antiviral therapeutic strategies is the PROteolysis TArgeting Chimeras (PROTAC) technology, in which heterobifunctional compounds are employed for the selective degradation of target proteins by the intracellular protein degradation machinery. This review covers the most recent advancements in PROTAC technology, its diversity and mode of action, and how it can be applied to open up new possibilities for creating cutting-edge antiviral treatments and vaccines.

A Perspective on Newly Emerging Proteolysis-Targeting Strategies in Antimicrobial Drug Discovery

Targeted protein degradation is a new aspect in the field of drug discovery. Traditionally, developing an antibiotic includes tedious and expensive processes, such as drug screening, lead optimization, and formulation. Proteolysis-targeting chimeras (PROTACs) are new-generation drugs that use the proteolytic mechanism to selectively degrade and eliminate proteins involved in human diseases. The application of PROTACs is explored immensely in the field of cancer, and various PROTACs are in clinical trials. Thus, researchers have a profound interest in pursuing PROTAC technology as a new weapon to fight pathogenic viruses and bacteria. This review highlights the importance of antimicrobial PROTACs and other similar "PROTAC-like" techniques to degrade pathogenic target proteins (i.e., viral/bacterial proteins). These techniques can perform specific protein degradation of the pathogenic protein to avoid resistance caused by mutations or abnormal expression of the pathogenic protein. PROTAC-based antimicrobial therapeutics have the advantage of high specificity and the ability to degrade "undruggable" proteins, such as nonenzymatic and structural proteins.