Design of a SARS-CoV-2 papain-like protease inhibitor with antiviral efficacy in a mouse model.

The emergence of SARS-CoV-2 variants and drug-resistant mutants calls for additional oral antivirals. The SARS-CoV-2 papain-like protease (PLpro) is a promising but challenging drug target. We designed and synthesized 85 noncovalent PLpro inhibitors that bind to a recently discovered ubiquitin binding site and the known BL2 groove pocket near the S4 subsite. Leads inhibited PLpro with the inhibitory constant Ki values from 13.2 to 88.2 nanomolar. The co-crystal structures of PLpro with eight leads revealed their interaction modes. The in vivo lead Jun12682 inhibited SARS-CoV-2 and its variants, including nirmatrelvir-resistant strains with EC50 from 0.44 to 2.02 micromolar. Oral treatment with Jun12682 improved survival and reduced lung viral loads and lesions in a SARS-CoV-2 infection mouse model, suggesting that PLpro inhibitors are promising oral SARS-CoV-2 antiviral candidates.

Structure-based design of SARS-CoV-2 papain-like protease inhibitors.

The COVID-19 pandemic is caused by SARS-CoV-2, an RNA virus with high transmissibility and mutation rate. Given the paucity of orally bioavailable antiviral drugs to combat SARS-CoV-2 infection, there is a critical need for additional antivirals with alternative mechanisms of action. Papain-like protease (PLpro) is one of the two SARS-CoV-2 encoded viral cysteine proteases essential for viral replication. PLpro cleaves at three sites of the viral polyproteins. In addition, PLpro antagonizes the host immune response upon viral infection by cleaving ISG15 and ubiquitin from host proteins. Therefore, PLpro is a validated antiviral drug target. In this study, we report the X-ray crystal structures of papain-like protease (PLpro) with two potent inhibitors, Jun9722 and Jun9843. Subsequently, we designed and synthesized several series of analogs to explore the structure-activity relationship, which led to the discovery of PLpro inhibitors with potent enzymatic inhibitory activity and antiviral activity against SARS-CoV-2. Together, the lead compounds are promising drug candidates for further development.

Design of SARS-CoV-2 papain-like protease inhibitor with antiviral efficacy in a mouse model.

The emergence of SARS-CoV-2 variants and drug-resistant mutants calls for additional oral antivirals. The SARS-CoV-2 papain-like protease (PLpro) is a promising but challenging drug target. In this study, we designed and synthesized 85 noncovalent PLpro inhibitors that bind to the newly discovered Val70Ub site and the known BL2 groove pocket. Potent compounds inhibited PLpro with inhibitory constant Ki values from 13.2 to 88.2 nM. The co-crystal structures of PLpro with eight leads revealed their interaction modes. The in vivo lead Jun12682 inhibited SARS-CoV-2 and its variants, including nirmatrelvir-resistant strains with EC50 from 0.44 to 2.02 µM. Oral treatment with Jun12682 significantly improved survival and reduced lung viral loads and lesions in a SARS-CoV-2 infection mouse model, suggesting PLpro inhibitors are promising oral SARS-CoV-2 antiviral candidates.

Structure-Based Lead Optimization of Enterovirus D68 2A Protease Inhibitors.

Enterovirus D68 (EV-D68) virus is a nonpolio enterovirus that typically causes respiratory illness and, in severe cases, can lead to paralysis and death in children. There is currently no vaccine or antiviral for EV-D68. We previously discovered the viral 2A protease (2Apro) as a viable antiviral drug target and identified telaprevir as a 2Apro inhibitor. 2Apro is a viral cysteine protease that cleaves the viral VP1-2A polyprotein junction. In this study, we report the X-ray crystal structures of EV-D68 2Apro, wild-type, and the C107A mutant and the structure-based lead optimization of telaprevir. Guided by the X-ray crystal structure, we predicted the binding pose of telaprevir in 2Apro using molecular dynamics simulations. We then utilized this model to inform structure-based optimization of the telaprevir's reactive warhead and P1-P4 substitutions. These efforts led to the discovery of 2Apro inhibitors with improved antiviral activity than telaprevir. These compounds represent promising lead compounds for further development as EV-D68 antivirals.

Nitrosoarenes Implement Cascade Cyclization of 1-Allenyl-2-alkynylbenzenes through Diradical Mechanism.

This work reports cascade cyclization between 1-allenyl-2-alkynylbenzenes and nitrosoarenes. When these two components reacted alone under N2, N,O-functionalized indane-fused isoxazolidines 3 were obtained selectively. DFT calculations verify that this reaction sequence involves unprecedented nitrone/alkyne cycloadditions, followed by diradical rearrangement.

Progress and Challenges in Targeting the SARS-CoV-2 Papain-like Protease.

SARS-CoV-2 is the causative agent of the COVID-19 pandemic. The approval of vaccines and small-molecule antivirals is vital in combating the pandemic. The viral polymerase inhibitors remdesivir and molnupiravir and the viral main protease inhibitor nirmatrelvir/ritonavir have been approved by the U.S. FDA. However, the emergence of variants of concern/interest calls for additional antivirals with novel mechanisms of action. The SARS-CoV-2 papain-like protease (PLpro) mediates the cleavage of viral polyprotein and modulates the host's innate immune response upon viral infection, rendering it a promising antiviral drug target. This Perspective highlights major achievements in structure-based design and high-throughput screening of SARS-CoV-2 PLpro inhibitors since the beginning of the pandemic. Encouraging progress includes the design of non-covalent PLpro inhibitors with favorable pharmacokinetic properties and the first-in-class covalent PLpro inhibitors. In addition, we offer our opinion on the knowledge gaps that need to be filled to advance PLpro inhibitors to the clinic.

Mannich Bases: Centrality in Cytotoxic Drug Design.

Mannich bases identified by Professor Carl Mannich have been the most extensively explored scaffolds for more than 100 years now. The versatile biological roles that they play have promoted their applications in many clinical conditions. The present review highlights the application of Mannich bases as cytotoxic agents, categorizing them into synthetic, semisynthetic, and prodrugs classes, and gives an exhaustive account of the work reported in the last two decades. The methods of synthesis of these cytotoxic agents, their anti-cancer potential in various cell lines, and promising leads for future drug development have also been discussed. Structure-activity relationships, along with the targets on which these cytotoxic Mannich bases act, have been included as well.

Development of a [2 + 2]-Nitroso/Alkene Cycloaddition Using Sodium Tetrakis[3,5-bis(trifluoromethyl)phenyl]borate Catalyst: Controlled Chemoselectivity of Two Equilibrating Isomeric Intermediates.

Sodium tetrakis[3,5-bis(trifluoromethyl)-phenyl]borate (NaBArF) catalyzes the [2 + 2] cycloaddition of 1,4-disubstituted cyclopenta-1,3-dien-2-yl esters with nitrsobenzene in toluene, affording two isolable regioisomers of 6-oxa-7-azabicyclo[3.2.0] heptanes, which thermally rearrange into the same 4-aminocyclopent-1-en-3-ones. In the case of 4-substituted cyclopenta-1,3-dien-2-yl esters, their initial [2 + 2] cycloaddition intermediates undergo a rapid ring expansion to afford six-membered piperidone derivatives efficiently.

Gold-catalyzed (4 + 2)-annulations between α-alkyl alkenylgold carbenes and benzisoxazoles with reactive alkyl groups.

This work reports new (4 + 2)-annulations of α-alkyl vinylgold carbenes with benzisoxazoles to afford 3,4-dihydroquinoline derivatives with high anti-stereoselectivity. The annulations are operable with carbenes in both acyclic and cyclic forms. This reaction sequence involves an initial formation of imines from α-alkylgold carbenes and benzisoxazoles, followed by a novel carbonyl-enamine reaction to yield 3,4-dihydroquinoline derivatives. This system presents the first alkyl C-H reactivity of α-alkyl gold carbenes with an external substrate.

Stereoselective annulation between an allene, an alkene, and two nitrosoarenes to access bis(isoxazoliodine) derivatives.

This work reports metal-free annulations between one allene, two nitrosoarenes and one electron-deficient alkene to afford bis(isoxazolidine) derivatives stereoselectively. This process involves an initial formation of isoxazolidin-4-imine oxides, followed by their dipolar [3 + 2]-cycloaddition with electron-deficient alkenes. To highlight the utility, the annulations of 5-alleneyl-1-enes with nitrosoarenes were also feasible to afford the desired bis(isoxazolidine) products with excellent stereocontrol. The resulting bis(isoxazolidine) products produced from two systems were reduced with Zn/MeOH to induce reductive N-O cleavages, yielding branched polyaminols stereoselectively.

Copper-Mediated [3+2] Annulation of 3-N-Hydroxyallylamines with Nitrosoarenes.

Cu-mediated annulations of N-hydroxyallylamines with nitrosoarenes proceed through unprecedented formal [3+2] cycloadditions of N-hydroxyaminoallyl radicals with nitrosoarenes. Our mechanistic analysis opposes a 5-endo-trig cyclization involved in the final ring-closure step. To manifest the reaction utility, chemical elaborations of resulting isoxazolidinyl products into 2- or 3-substituted quinoline N-oxides and acyclic 1,3-diamino-2-ols are also described.

Total Syntheses and Biological Evaluation of (±)-Botryosphaeridione, (±)-Pleodendione, 4-epi-Periconianone B, and Analogues.

The total syntheses of (±)-botryosphaeridione, (±)-pleodendione, (±)-hoaensieremodione, 4-epi-periconianone B, and their analogues have been accomplished for the first time. All the synthesized target compounds were screened in neural anti-inflammatory assays using LPS induced microglia cells (N9). Among them, compounds 1 and 21 were identified as potential lead compounds for further profiling.

Total synthesis of an anticancer norsesquiterpene alkaloid isolated from the fungus Flammulina velutipes.

The first total synthesis of a norsesquiterpene alkaloid (R)-8-hydroxy-4,7,7-trimethyl-7,8-dihydrocyclopenta[e]isoindole-1,3(2H,6H)-dione, isolated from the mushroom-forming fungus Flammulina velutipes, in both racemic and enantiomeric pure forms, is reported. The (-)-enantiomer of the natural product has been synthesized from the D-(-)-pantolactone chiral pool. The synthesis features a one-pot, three-step reaction sequence comprising an enyne RCM/Diels-Alder/aromatization to construct the desired indane skeleton present in the natural product. Our synthesis further confirms the assigned structure and absolute configuration of the natural product.