Molecular Mechanisms of Resistance and Structure-Based Drug Design in Homodimeric Viral Proteases

Drug resistance is a global health threat costing society billions of dollars and impacting millions of lives each year. Current drug design strategies are inadequate because they focus on disrupting target activity and not restricting the evolutionary pathways to resistance. Improved strategies would exploit the structural and dynamic changes in the enzyme–inhibitor system integrating data from many inhibitors and variants. Using HIV-1 protease as a model system, I aimed to elucidate the underlying resistance mechanisms, characterize conserved protease-inhibitor interactions, and generate more robust inhibitors by applying these insights. For primary mechanisms of resistance, comparing interactions at the protease–inhibitor interface showed how specific modifications affected potency. For mutations distal to the active site, molecular dynamics simulations were necessary to elucidate how changes propagated to reduce inhibitor binding. These insights informed inhibitor design to improve potency against highly resistant variants by optimizing hydrogen bonding. A series of hybrid inhibitors was also designed that showed excellent potency by combining key moieties of multiple FDA-approved inhibitors. I characterized the structural basis for alterations in binding affinity in HIV-1 protease both from mutations and inhibitors. I applied these strategies to HTLV-1 protease, a potential drug target. I identified the HIV-1 inhibitor darunavir as a viable scaffold and evaluated analogues, leading to a low-nanomolar compound with potential for optimization. Hopefully, insights from this thesis will lead to the development of potent HTLV-1 protease inhibitors. More broadly, these inhibitor design strategies are applicable to other rapidly evolving targets, thereby reducing drug resistance rates in the future

Crystal Structure of SARS-CoV-2 Main Protease in Complex with the Non-Covalent Inhibitor ML188

Viral proteases are critical enzymes for the maturation of many human pathogenic viruses and thus are key targets for direct acting antivirals (DAAs). The current viral pandemic caused by SARS-CoV-2 is in dire need of DAAs. The Main protease (M(pro)) is the focus of extensive structure-based drug design efforts which are mostly covalent inhibitors targeting the catalytic cysteine. ML188 is a non-covalent inhibitor designed to target SARS-CoV-1 M(pro), and provides an initial scaffold for the creation of effective pan-coronavirus inhibitors. In the current study, we found that ML188 inhibits SARS-CoV-2 M(pro) at 2.5 microM, which is more potent than against SAR-CoV-1 M(pro). We determined the crystal structure of ML188 in complex with SARS-CoV-2 M(pro) to 2.39 A resolution. Sharing 96% sequence identity, structural comparison of the two complexes only shows subtle differences. Non-covalent protease inhibitors complement the design of covalent inhibitors against SARS-CoV-2 main protease and are critical initial steps in the design of DAAs to treat CoVID 19

Avoiding Drug Resistance by Substrate Envelope-Guided Design: Toward Potent and Robust HCV NS3/4A Protease Inhibitors

Hepatitis C virus (HCV) infects millions of people worldwide, causing chronic liver disease that can lead to cirrhosis, hepatocellular carcinoma, and liver transplant. In the last several years, the advent of direct-acting antivirals, including NS3/4A protease inhibitors (PIs), has remarkably improved treatment outcomes of HCV-infected patients. However, selection of resistance-associated substitutions and polymorphisms among genotypes can lead to drug resistance and in some cases treatment failure. A proactive strategy to combat resistance is to constrain PIs within evolutionarily conserved regions in the protease active site. Designing PIs using the substrate envelope is a rational strategy to decrease the susceptibility to resistance by using the constraints of substrate recognition. We successfully designed two series of HCV NS3/4A PIs to leverage unexploited areas in the substrate envelope to improve potency, specifically against resistance-associated substitutions at D168. Our design strategy achieved better resistance profiles over both the FDA-approved NS3/4A PI grazoprevir and the parent compound against the clinically relevant D168A substitution. Crystallographic structural analysis and inhibition assays confirmed that optimally filling the substrate envelope is critical to improve inhibitor potency while avoiding resistance. Specifically, inhibitors that enhanced hydrophobic packing in the S4 pocket and avoided an energetically frustrated pocket performed the best. Thus, the HCV substrate envelope proved to be a powerful tool to design robust PIs, offering a strategy that can be translated to other targets for rational design of inhibitors with improved potency and resistance profiles.IMPORTANCE Despite significant progress, hepatitis C virus (HCV) continues to be a major health problem with millions of people infected worldwide and thousands dying annually due to resulting complications. Recent antiviral combinations can achieve \u3e 95% cure, but late diagnosis, low access to treatment, and treatment failure due to drug resistance continue to be roadblocks against eradication of the virus. We report the rational design of two series of HCV NS3/4A protease inhibitors with improved resistance profiles by exploiting evolutionarily constrained regions of the active site using the substrate envelope model. Optimally filling the S4 pocket is critical to avoid resistance and improve potency. Our results provide drug design strategies to avoid resistance that are applicable to other quickly evolving viral drug targets

Structural Analysis of Potent Hybrid HIV-1 Protease Inhibitors Containing Bis-Tetrahydrofuran in a Pseudo-Symmetric Dipeptide Isostere

The design, synthesis, and X-ray structural analysis of hybrid HIV-1 protease inhibitors (PIs) containing bis-tetrahydrofuran (bis-THF) in a pseudo-C2-symmetric dipeptide isostere are described. A series of PIs were synthesized by incorporating bis-THF of darunavir on either side of the Phe-Phe isostere of lopinavir in combination with hydrophobic amino acids on the opposite P2/P2\u27 position. Structure-activity relationship studies indicated that the bis-THF moiety can be attached at either the P2 or P2\u27 position without significantly affecting potency. However, the group on the opposite P2/P2\u27 position had a dramatic effect on potency depending on the size and shape of the side chain. Cocrystal structures of inhibitors with wild-type HIV-1 protease revealed that the bis-THF moiety retained similar interactions as observed in the darunavir-protease complex regardless of position on the Phe-Phe isostere. Analyses of cocrystal structures and molecular dynamics simulations provide insights for optimizing HIV-1 PIs containing bis-THF in non-sulfonamide dipeptide isosteres

Unique structural solution from a VH3-30 antibody targeting the hemagglutinin stem of influenza A viruses

Broadly neutralizing antibodies (bnAbs) targeting conserved influenza A virus (IAV) hemagglutinin (HA) epitopes can provide valuable information for accelerating universal vaccine designs. Here, we report structural details for heterosubtypic recognition of HA from circulating and emerging IAVs by the human antibody 3I14. Somatic hypermutations play a critical role in shaping the HCDR3, which alone and uniquely among VH3-30 derived antibodies, forms contacts with five sub-pockets within the HA-stem hydrophobic groove. 3I14 light-chain interactions are also key for binding HA and contribute a large buried surface area spanning two HA protomers. Comparison of 3I14 to bnAbs from several defined classes provide insights to the bias selection of VH3-30 antibodies and reveals that 3I14 represents a novel structural solution within the VH3-30 repertoire. The structures reported here improve our understanding of cross-group heterosubtypic binding activity, providing the basis for advancing immunogen designs aimed at eliciting a broadly protective response to IAV

Optimizing the refinement of merohedrally twinned P61 HIV-1 protease-inhibitor cocrystal structures

Twinning is a crystal-growth anomaly in which protein monomers exist in different orientations but are related in a specific way, causing diffraction reflections to overlap. Twinning imposes additional symmetry on the data, often leading to the assignment of a higher symmetry space group. Specifically, in merohedral twinning, reflections from each monomer overlap and require a twin law to model unique structural data from overlapping reflections. Neglecting twinning in the crystallographic analysis of quasi-rotationally symmetric homo-oligomeric protein structures can mask the degree of structural non-identity between monomers. In particular, any deviations from perfect symmetry will be lost if higher than appropriate symmetry is applied during crystallographic analysis. Such cases warrant choosing between the highest symmetry space group possible or determining whether the monomers have distinguishable structural asymmetries and thus require a lower symmetry space group and a twin law. Using hexagonal cocrystals of HIV-1 protease, a C2-symmetric homodimer whose symmetry is broken by bound ligand, it is shown that both assigning a lower symmetry space group and applying a twin law during refinement are critical to achieving a structural model that more accurately fits the electron density. By re-analyzing three recently published HIV-1 protease structures, improvements in nearly every crystallographic metric are demonstrated. Most importantly, a procedure is demonstrated where the inhibitor can be reliably modeled in a single orientation. This protocol may be applicable to many other homo-oligomers in the PDB

Pan-3C Protease Inhibitor Rupintrivir Binds SARS-CoV-2 Main Protease in a Unique Binding Mode

Rupintrivir targets the 3C cysteine proteases of the picornaviridae family, which includes rhinoviruses and enteroviruses that cause a range of human diseases. Despite being a pan-3C protease inhibitor, rupintrivir activity is extremely weak against the homologous 3C-like protease of SARS-CoV-2. In this study, the crystal structures of rupintrivir were determined bound to enterovirus 68 (EV68) 3C protease and the 3C-like main protease (M(pro)) from SARS-CoV-2. While the EV68 3C protease-rupintrivir structure was similar to previously determined complexes with other picornavirus 3C proteases, rupintrivir bound in a unique conformation to the active site of SARS-CoV-2 M(pro) splitting the catalytic cysteine and histidine residues. This bifurcation of the catalytic dyad may provide a novel approach for inhibiting cysteine proteases

Crystal Structure of SARS-CoV-2 Main Protease in Complex with the Non-Covalent Inhibitor ML188

Viral proteases are critical enzymes for the maturation of many human pathogenic viruses and thus are key targets for direct acting antivirals (DAAs). The current viral pandemic caused by SARS-CoV-2 is in dire need of DAAs. The Main protease (Mpro) is the focus of extensive structure-based drug design efforts which are mostly covalent inhibitors targeting the catalytic cysteine. ML188 is a non-covalent inhibitor designed to target SARS-CoV-1 Mpro, and provides an initial scaffold for the creation of effective pan-coronavirus inhibitors. In the current study, we found that ML188 inhibits SARS-CoV-2 Mpro at 2.5 µM, which is more potent than against SAR-CoV-1 Mpro. We determined the crystal structure of ML188 in complex with SARS-CoV-2 Mpro to 2.39 Å resolution. Sharing 96% sequence identity, structural comparison of the two complexes only shows subtle differences. Non-covalent protease inhibitors complement the design of covalent inhibitors against SARS-CoV-2 main protease and are critical initial steps in the design of DAAs to treat CoVID 19

Defining the substrate envelope of SARS-CoV-2 main protease to predict and avoid drug resistance.

Coronaviruses can evolve and spread rapidly to cause severe disease morbidity and mortality, as exemplified by SARS-CoV-2 variants of the COVID-19 pandemic. Although currently available vaccines remain mostly effective against SARS-CoV-2 variants, additional treatment strategies are needed. Inhibitors that target essential viral enzymes, such as proteases and polymerases, represent key classes of antivirals. However, clinical use of antiviral therapies inevitably leads to emergence of drug resistance. In this study we implemented a strategy to pre-emptively address drug resistance to protease inhibitors targeting the main protease (Mpro) of SARS-CoV-2, an essential enzyme that promotes viral maturation. We solved nine high-resolution cocrystal structures of SARS-CoV-2 Mpro bound to substrate peptides and six structures with cleavage products. These structures enabled us to define the substrate envelope of Mpro, map the critical recognition elements, and identify evolutionarily vulnerable sites that may be susceptible to resistance mutations that would compromise binding of the newly developed Mpro inhibitors. Our results suggest strategies for developing robust inhibitors against SARS-CoV-2 that will retain longer-lasting efficacy against this evolving viral pathogen

Quinoxaline-Based Linear HCV NS3/4A Protease Inhibitors Exhibit Potent Activity against Drug Resistant Variants

A series of linear HCV NS3/4A protease inhibitors was designed by eliminating the P2-P4 macrocyclic linker in grazoprevir, which, in addition to conferring conformational flexibility, allowed structure-activity relationship (SAR) exploration of diverse quinoxalines at the P2 position. Biochemical and replicon data indicated preference for small hydrophobic groups at the 3-position of P2 quinoxaline for maintaining potency against resistant variants R155K, A156T, and D168A/V. The linear inhibitors, though generally less potent than the corresponding macrocyclic analogues, were relatively easier to synthesize and less susceptible to drug resistance. Three inhibitor cocrystal structures bound to wild-type NS3/4A protease revealed a conformation with subtle changes in the binding of P2 quinoxaline, depending on the 3-position substituent, likely impacting both inhibitor potency and resistance profile. The SAR and structural analysis highlight inhibitor features that strengthen interactions of the P2 moiety with the catalytic triad residues, providing valuable insights to improve potency against resistant variants