RESUMO
Monoclonal antibodies are an efficacious therapy against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). However, rapid viral mutagenesis led to escape from most of these therapies, outlining the need for an antibody cocktail with a broad neutralizing potency. Using an unbiased interrogation of the memory B cell repertoire of patients with convalescent COVID-19, we identified human antibodies with broad antiviral activity in vitro and efficacy in vivo against all tested SARS-CoV-2 variants of concern, including Delta and Omicron BA.1 and BA.2. Here, we describe an antibody cocktail, IMM-BCP-01, that consists of three patient-derived broadly neutralizing antibodies directed at nonoverlapping surfaces on the SARS-CoV-2 Spike protein. Two antibodies, IMM20184 and IMM20190, directly blocked Spike binding to the ACE2 receptor. Binding of the third antibody, IMM20253, to its cryptic epitope on the outer surface of RBD altered the conformation of the Spike Trimer, promoting the release of Spike monomers. These antibodies decreased Omicron SARS-CoV-2 infection in the lungs of Syrian golden hamsters in vivo and potently induced antiviral effector response in vitro, including phagocytosis, ADCC, and complement pathway activation. Our preclinical data demonstrated that the three-antibody cocktail IMM-BCP-01 could be a promising means for preventing or treating infection of SARS-CoV-2 variants of concern, including Omicron BA.1 and BA.2, in susceptible individuals.
Assuntos
COVID-19 , SARS-CoV-2 , Animais , Anticorpos Antivirais , Cricetinae , Humanos , Glicoproteína da Espícula de Coronavírus/genéticaRESUMO
Patients who recover from SARS-CoV-2 infections produce antibodies and antigen-specific T cells against multiple viral proteins. Here, an unbiased interrogation of the anti-viral memory B cell repertoire of convalescent patients has been performed by generating large, stable hybridoma libraries and screening thousands of monoclonal antibodies to identify specific, high-affinity immunoglobulins (Igs) directed at distinct viral components. As expected, a significant number of antibodies were directed at the Spike (S) protein, a majority of which recognized the full-length protein. These full-length Spike specific antibodies included a group of somatically hypermutated IgMs. Further, all but one of the six COVID-19 convalescent patients produced class-switched antibodies to a soluble form of the receptor-binding domain (RBD) of S protein. Functional properties of anti-Spike antibodies were confirmed in a pseudovirus neutralization assay. Importantly, more than half of all of the antibodies generated were directed at non-S viral proteins, including structural nucleocapsid (N) and membrane (M) proteins, as well as auxiliary open reading frame-encoded (ORF) proteins. The antibodies were generally characterized as having variable levels of somatic hypermutations (SHM) in all Ig classes and sub-types, and a diversity of VL and VH gene usage. These findings demonstrated that an unbiased, function-based approach towards interrogating the COVID-19 patient memory B cell response may have distinct advantages relative to genomics-based approaches when identifying highly effective anti-viral antibodies directed at SARS-CoV-2.
RESUMO
Single-stranded silencing RNAs (ss siRNA), while not as potent as duplex RNAs, have the potential to become a novel platform technology in RNA interference based gene silencing by virtue of their simplicity and plausibly favorable characteristics in pharmacokinetics and biodistribution. Like other therapeutic pharmaceutical agents, ss siRNA can be optimized to achieve higher potency through a structure-activity based approach. Systematic chemical modification at each position of a 21-mer oligonucleotide identified 2',5'-linked 3'-deoxythymidine (3dT) at position 1 and locked nucleic acids (LNAs) at the seed region as key components to afford significant enhancement in knockdown activity both in vitro and in vivo. Further optimization by additional chemical modifications should enable ss siRNA as an alternative gene silencing modality.
Assuntos
Inativação Gênica , RNA Mensageiro/genética , RNA Interferente Pequeno/genética , beta Catenina/genética , Células HEK293 , HumanosRESUMO
Short interfering RNAs (siRNAs) are a valuable tool for gene silencing with applications in both target validation and therapeutics. Many advances have recently been made to improve potency and specificity, and reduce toxicity and immunostimulation. However, siRNA delivery to a variety of tissues remains an obstacle for this technology. To date, siRNA delivery to muscle has only been achieved by local administration or by methods with limited potential use in the clinic. We report systemic delivery of a highly chemically modified cholesterol-conjugated siRNA targeting muscle-specific gene myostatin (Mstn) to a full range of muscles in mice. Following a single intravenous injection, we observe 85-95% knockdown of Mstn mRNA in skeletal muscle and >65% reduction in circulating Mstn protein sustained for >21 days. This level of Mstn knockdown is also accompanied by a functional effect on skeletal muscle, with animals showing an increase in muscle mass, size, and strength. The cholesterol-conjugated siRNA platform described here could have major implications for treatment of a variety of muscle disorders, including muscular atrophic diseases, muscular dystrophy, and type II diabetes.
RESUMO
Herein, we describe the development of a functionally selective liver X receptor ß (LXRß) agonist series optimized for Emax selectivity, solubility, and physical properties to allow efficacy and safety studies in vivo. Compound 9 showed central pharmacodynamic effects in rodent models, evidenced by statistically significant increases in apolipoprotein E (apoE) and ATP-binding cassette transporter levels in the brain, along with a greatly improved peripheral lipid safety profile when compared to those of full dual agonists. These findings were replicated by subchronic dosing studies in non-human primates, where cerebrospinal fluid levels of apoE and amyloid-ß peptides were increased concomitantly with an improved peripheral lipid profile relative to that of nonselective compounds. These results suggest that optimization of LXR agonists for Emax selectivity may have the potential to circumvent the adverse lipid-related effects of hepatic LXR activity.
Assuntos
Transportadores de Cassetes de Ligação de ATP/metabolismo , Doença de Alzheimer/tratamento farmacológico , Peptídeos beta-Amiloides/líquido cefalorraquidiano , Apolipoproteínas E/líquido cefalorraquidiano , Benzamidas/química , Benzamidas/farmacologia , Receptores Nucleares Órfãos/agonistas , Piperidinas/química , Piperidinas/farmacologia , Animais , Encéfalo/efeitos dos fármacos , Encéfalo/metabolismo , Cães , Células Hep G2 , Humanos , Lipídeos/análise , Fígado/efeitos dos fármacos , Fígado/metabolismo , Receptores X do Fígado , Locomoção/efeitos dos fármacos , Macaca mulatta , Células Madin Darby de Rim Canino , Camundongos , Camundongos TransgênicosRESUMO
With the goal of identifying inhibitors of hepatitis C virus (HCV) NS3/4a protease that are potent against a wide range of genotypes and clinically relevant mutant viruses, several subseries of macrocycles were investigated based on observations made during the discovery of MK-5172. Quinazolinone-containing macrocycles were identified as promising leads, and optimization for superior cross-genotype and mutant enzyme potency as well as rat liver and plasma concentrations following oral dosing, led to the development of MK-2748. Additional investigation of a series of bis-macrocycles containing a fused 18- and 15-membered ring system were also optimized for the same properties, leading to the discovery of MK-6325. Both compounds display the broad genotype and mutant potency necessary for clinical development as next-generation HCV NS3/4a protease inhibitors.
Assuntos
Antivirais/farmacologia , Hepacivirus/enzimologia , Compostos Macrocíclicos/farmacologia , Quinazolinonas/farmacologia , Sulfonas/farmacologia , Proteínas não Estruturais Virais/antagonistas & inibidores , Animais , Antivirais/química , Antivirais/farmacocinética , Cristalografia por Raios X , Descoberta de Drogas , Hepacivirus/efeitos dos fármacos , Hepacivirus/genética , Hepatite C/tratamento farmacológico , Hepatite C/virologia , Humanos , Compostos Macrocíclicos/química , Compostos Macrocíclicos/farmacocinética , Modelos Moleculares , Mutação , Quinazolinonas/química , Quinazolinonas/farmacocinética , Ratos , Sulfonas/farmacocinética , Proteínas não Estruturais Virais/genéticaRESUMO
The present study aimed at establishing feasibility of delivering short interfering RNA (siRNA) to target the coagulation cascade in rat and rabbit, two commonly used species for studying thrombosis and hemostasis. siRNAs that produced over 90% mRNA knockdown of rat plasma prekallikrein and rabbit Factor X (FX) were identified from in vitro screens. An ionizable amino lipid based lipid nanoparticle (LNP) formulation for siRNA in vivo delivery was characterized as tolerable and exerting no appreciable effect on coagulability at day 7 postdosing in both species. Both prekallikrein siRNA-LNP and FX siRNA-LNP resulted in dose-dependent and selective knockdown of target gene mRNA in the liver with maximum reduction of over 90% on day 7 following a single dose of siRNA-LNP. Knockdown of plasma prekallikrein was associated with modest clot weight reduction in the rat arteriovenous shunt thrombosis model and no increase in the cuticle bleeding time. Knockdown of FX in the rabbit was accompanied with prolongation in ex vivo clotting times. Results fit the expectations with both targets and demonstrate for the first time, the feasibility of targeting coagulation factors in rat, and, more broadly, targeting a gene of interest in rabbit, via systemic delivery of ionizable LNP formulated siRNA.
RESUMO
PURPOSE: To develop a tool based on siRNA-mediated knockdown of hepatic P450 oxidoreductase (POR) to decrease the CYP-mediated metabolism of small molecule drugs that suffer from rapid metabolism in vivo, with the aim of improving plasma exposure of these drugs. METHODS: siRNA against the POR gene was delivered using lipid nanoparticles (LNPs) into rats. The time course of POR mRNA knockdown, POR protein knockdown, and loss of POR enzyme activity was monitored. The rat livers were harvested to produce microsomes to determine the impact of POR knockdown on the metabolism of several probe substrates. Midazolam (a CYP3A substrate with high intrinsic clearance) was administered into LNP-treated rats to determine the impact of POR knockdown on midazolam pharmacokinetics. RESULTS: Hepatic POR mRNA and protein levels were significantly reduced by administering siRNA and the maximum POR enzyme activity reduction (~85%) occurred 2 weeks post-dose. In vitro analysis showed significant reductions in metabolism of probe substrates due to POR knockdown in liver, and in vivo POR knockdown resulted in greater than 10-fold increases in midazolam plasma concentrations following oral dosing. CONCLUSIONS: Anti-POR siRNA can be used to significantly reduce hepatic metabolism by various CYPs as well as greatly increase the bioavailability of high clearance compounds following an oral dose, thus enabling it to be used as a tool to increase drug exposure in vivo.
Assuntos
Sistema Enzimático do Citocromo P-450/genética , Técnicas de Silenciamento de Genes/métodos , RNA Interferente Pequeno/farmacologia , Animais , Química Farmacêutica , Diclofenaco/metabolismo , Técnicas In Vitro , Masculino , Microssomos/efeitos dos fármacos , Microssomos/enzimologia , Microssomos Hepáticos/efeitos dos fármacos , Microssomos Hepáticos/enzimologia , Midazolam/metabolismo , Nanopartículas , Ligação Proteica , RatosRESUMO
A series of macrocyclic compounds containing a cyclic constraint in the P2-P4 linker region have been discovered and shown to exhibit excellent HCV NS3/4a genotype 3a and genotype 1b R155K, A156T, A156V, and D168V mutant activity while maintaining high rat liver exposure. The effect of the constraint is most dramatic against gt 1b A156 mutants where ~20-fold improvements in potency are achieved by introduction of a variety of ring systems into the P2-P4 linker.
Assuntos
Proteínas de Transporte/antagonistas & inibidores , Hepacivirus/enzimologia , Compostos Macrocíclicos/química , Inibidores de Proteases/química , Proteínas não Estruturais Virais/antagonistas & inibidores , Animais , Sítios de Ligação , Proteínas de Transporte/metabolismo , Domínio Catalítico , Ciclização , Genótipo , Meia-Vida , Hepacivirus/genética , Peptídeos e Proteínas de Sinalização Intracelular , Cinética , Fígado/metabolismo , Compostos Macrocíclicos/síntese química , Compostos Macrocíclicos/farmacocinética , Simulação de Acoplamento Molecular , Mutação , Inibidores de Proteases/síntese química , Inibidores de Proteases/farmacocinética , Ratos , Relação Estrutura-Atividade , Proteínas não Estruturais Virais/metabolismoRESUMO
A series of macrocyclic compounds containing 2-substituted-quinoline moieties have been discovered and shown to exhibit excellent HCV NS3/4a genotype 3a and genotype 1b R155K mutant activity while maintaining the high rat liver exposure. Cyclization of the 2-substituted quinoline substituent led to a series of tricyclic P2 compounds which also display superb gt3a potency.
Assuntos
Proteínas de Transporte/antagonistas & inibidores , Hepacivirus/enzimologia , Compostos Macrocíclicos/química , Inibidores de Proteases/química , Proteínas não Estruturais Virais/antagonistas & inibidores , Animais , Proteínas de Transporte/metabolismo , Ciclização , Genótipo , Meia-Vida , Hepacivirus/genética , Peptídeos e Proteínas de Sinalização Intracelular , Cinética , Fígado/metabolismo , Compostos Macrocíclicos/síntese química , Compostos Macrocíclicos/farmacocinética , Inibidores de Proteases/síntese química , Inibidores de Proteases/farmacocinética , Quinolinas/química , Ratos , Relação Estrutura-Atividade , Proteínas não Estruturais Virais/metabolismoRESUMO
HCV NS3/4a protease inhibitors are proven therapeutic agents against chronic hepatitis C virus infection, with boceprevir and telaprevir having recently received regulatory approval as add-on therapy to pegylated interferon/ribavirin for patients harboring genotype 1 infections. Overcoming antiviral resistance, broad genotype coverage, and a convenient dosing regimen are important attributes for future agents to be used in combinations without interferon. In this communication, we report the preclinical profile of MK-5172, a novel P2-P4 quinoxaline macrocyclic NS3/4a protease inhibitor currently in clinical development. The compound demonstrates subnanomolar activity against a broad enzyme panel encompassing major hepatitis C virus (HCV) genotypes as well as variants resistant to earlier protease inhibitors. In replicon selections, MK-5172 exerted high selective pressure, which yielded few resistant colonies. In both rat and dog, MK-5172 demonstrates good plasma and liver exposures, with 24-h liver levels suggestive of once-daily dosing. When administered to HCV-infected chimpanzees harboring chronic gt1a or gt1b infections, MK-5172 suppressed viral load between 4 to 5 logs at a dose of 1 mg/kg of body weight twice daily (b.i.d.) for 7 days. Based on its preclinical profile, MK-5172 is anticipated to be broadly active against multiple HCV genotypes and clinically important resistance variants and highly suited for incorporation into newer all-oral regimens.
Assuntos
Hepacivirus/efeitos dos fármacos , Inibidores de Proteases/farmacologia , Quinoxalinas/farmacologia , Quinoxalinas/farmacocinética , Proteínas não Estruturais Virais/antagonistas & inibidores , Amidas , Animais , Antivirais/farmacologia , Carbamatos , Ciclopropanos , Cães , Farmacorresistência Viral , Genótipo , Hepacivirus/enzimologia , Hepacivirus/genética , Hepatite C Crônica/tratamento farmacológico , Hepatite C Crônica/virologia , Fígado/efeitos dos fármacos , Pan troglodytes , Quinoxalinas/metabolismo , Ratos , Sulfonamidas , Carga Viral/efeitos dos fármacosRESUMO
A new class of HCV NS3/4a protease inhibitors containing a P2 to P4 macrocyclic constraint was designed using a molecular modeling-derived strategy. Building on the profile of previous clinical compounds and exploring the P2 and linker regions of the series allowed for optimization of broad genotype and mutant enzyme potency, cellular activity, and rat liver exposure following oral dosing. These studies led to the identification of clinical candidate 15 (MK-5172), which is active against genotype 1-3 NS3/4a and clinically relevant mutant enzymes and has good plasma exposure and excellent liver exposure in multiple species.
RESUMO
The discovery of MK-1220 is reported along with the development of a series of HCV NS3/4A protease inhibitors containing a P2 to P4 macrocyclic constraint with improved preclinical pharmacokinetics. Optimization of the P2 heterocycle substitution pattern as well as the P3 amino acid led to compounds with greatly improved plasma exposure following oral dosing in both rats and dogs while maintaining excellent enzyme potency and cellular activity. These studies led to the identification of MK-1220.
RESUMO
A new class of HCV NS3/4a protease inhibitors which contain a P2 to P4 macrocyclic constraint was designed using a molecular-modeling derived strategy. Exploration of the P2 heterocyclic region, the P2 to P4 linker, and the P1 side chain of this class of compounds via a modular synthetic strategy allowed for the optimization of enzyme potency, cellular activity, and rat liver exposure following oral dosing. These studies led to the identification of clinical candidate 35b (vaniprevir, MK-7009), which is active against both the genotype 1 and genotype 2 NS3/4a protease enzymes and has good plasma exposure and excellent liver exposure in multiple species.
Assuntos
Hepacivirus/enzimologia , Indóis/farmacologia , Inibidores de Serina Proteinase/farmacologia , Proteínas não Estruturais Virais/antagonistas & inibidores , Animais , Área Sob a Curva , Proteínas de Transporte/antagonistas & inibidores , Proteínas de Transporte/metabolismo , Ciclopropanos , Cães , Descoberta de Drogas , Avaliação Pré-Clínica de Medicamentos , Indóis/química , Indóis/farmacocinética , Concentração Inibidora 50 , Peptídeos e Proteínas de Sinalização Intracelular , Isoindóis , Lactamas Macrocíclicas , Leucina/análogos & derivados , Fígado/metabolismo , Macaca mulatta , Compostos Macrocíclicos/química , Compostos Macrocíclicos/farmacocinética , Compostos Macrocíclicos/farmacologia , Taxa de Depuração Metabólica , Modelos Químicos , Estrutura Molecular , Pan troglodytes , Prolina/análogos & derivados , Ratos , Inibidores de Serina Proteinase/química , Inibidores de Serina Proteinase/farmacocinética , Relação Estrutura-Atividade , Sulfonamidas , Proteínas não Estruturais Virais/metabolismo , Proteínas Virais/antagonistas & inibidores , Proteínas Virais/metabolismoRESUMO
The administration of hepatitis C virus (HCV) NS3/4A protease inhibitors to patients with chronic HCV infections has demonstrated that they have dramatic antiviral effects and that compounds acting via this mechanism are likely to form a key component of future anti-HCV therapy. We report here on the preclinical profile of MK-7009, an inhibitor of genotype 1a and 1b proteases at subnanomolar concentrations with modestly shifted potency against genotype 2a and 2b proteases at low nanomolar concentrations. Potent activity was also observed in a cell-based HCV replicon assay in the presence of added human serum (50%). In multiple species evaluated in preclinical studies, the MK-7009 concentrations in the liver were maintained at a significant multiple of the cell-based replicon 50% effective concentration over 12 to 24 h following the administration of moderate oral doses (5 to 10 mg per kg of body weight). MK-7009 also had excellent selectivity against both a range of human proteases and a broad panel of pharmacologically relevant ion channels, receptors, and enzymes. On the basis of this favorable profile, MK-7009 was selected for clinical development and is currently being evaluated in controlled clinical trials with both healthy volunteers and HCV-infected patients.
Assuntos
Antivirais/farmacologia , Hepacivirus/efeitos dos fármacos , Indóis/farmacologia , Inibidores de Proteases/farmacologia , Proteínas não Estruturais Virais/antagonistas & inibidores , Animais , Antivirais/farmacocinética , Área Sob a Curva , Linhagem Celular , Ciclopropanos , Cães , Genótipo , Meia-Vida , Hepacivirus/enzimologia , Hepacivirus/genética , Humanos , Indóis/farmacocinética , Interferon alfa-2 , Interferon-alfa/farmacologia , Isoindóis , Lactamas Macrocíclicas , Leucina/análogos & derivados , Macaca mulatta , Pan troglodytes , Prolina/análogos & derivados , Inibidores de Proteases/farmacocinética , Ratos , Proteínas Recombinantes , Replicon , Especificidade por Substrato , Sulfonamidas , Proteínas não Estruturais Virais/genéticaRESUMO
In a follow-up to our recent disclosure of P2-P4 macrocyclic inhibitors of the hepatitis C virus (HCV) NS3 protease (e.g., 1, Chart 1), we report a new but related compound series featuring a basic amine at the N-terminus of the P3-amino acid residue. Replacement of the electroneutral P3-amino acid capping group (which is a feature of almost all tripeptide-like inhibitors of NS3 reported to date) with a basic group is not only tolerated but can result in advantageous cell based potency. Optimization of this new class of P3-amine based inhibitors gave compounds such as 25 and 26 that combine excellent cell based activity with pharmacokinetic properties that are attractive for an antiviral targeting HCV.
Assuntos
Aminas/síntese química , Antivirais/síntese química , Proteínas não Estruturais Virais/antagonistas & inibidores , Aminas/farmacocinética , Aminas/farmacologia , Animais , Antivirais/farmacocinética , Antivirais/farmacologia , Cães , Descoberta de Drogas , Masculino , Ratos , Ratos Sprague-Dawley , Relação Estrutura-AtividadeRESUMO
A novel series of hepatitis C virus (HCV) NS3/4A protease inhibitors bearing a P2-P4 macrocycle and a P1-P1' alpha-ketoamide serine trap is reported. The NS3 protease, which is essential for viral replication, is considered one of the most attractive targets for developing novel anti-HCV therapies. The optimization of both the macrocycle and the warhead portions led to the discovery of compounds 8b and 8 g with a good activity both in the enzyme as well as in the cell based (replicon) assays with favorable PK profile in a preclinical species.
Assuntos
Antivirais/síntese química , Proteínas de Transporte/antagonistas & inibidores , Hepacivirus/enzimologia , Compostos Macrocíclicos/síntese química , Inibidores de Proteases/química , Proteínas não Estruturais Virais/antagonistas & inibidores , Proteínas Virais/antagonistas & inibidores , Amidas/síntese química , Animais , Antivirais/metabolismo , Proteínas de Transporte/metabolismo , Hepacivirus/efeitos dos fármacos , Peptídeos e Proteínas de Sinalização Intracelular , Cetonas/síntese química , Compostos Macrocíclicos/metabolismo , Masculino , Inibidores de Proteases/metabolismo , Inibidores de Proteases/farmacologia , Ratos , Ratos Wistar , Inibidores de Serina Proteinase/síntese química , Inibidores de Serina Proteinase/metabolismo , Proteínas não Estruturais Virais/metabolismo , Proteínas Virais/metabolismoAssuntos
Antivirais/química , Proteínas de Transporte/antagonistas & inibidores , Hepacivirus/enzimologia , Compostos Macrocíclicos/química , Inibidores de Proteases/química , Quinolinas/química , Proteínas não Estruturais Virais/antagonistas & inibidores , Proteínas Virais/antagonistas & inibidores , Antivirais/síntese química , Antivirais/farmacologia , Carbamatos/química , Carbamatos/farmacologia , Hepacivirus/efeitos dos fármacos , Peptídeos e Proteínas de Sinalização Intracelular , Compostos Macrocíclicos/síntese química , Compostos Macrocíclicos/farmacologia , Inibidores de Proteases/síntese química , Inibidores de Proteases/farmacologia , Quinolinas/síntese química , Quinolinas/farmacologia , Relação Estrutura-Atividade , Tiazóis/química , Tiazóis/farmacologiaRESUMO
Molecular modeling of inhibitor bound full length HCV NS3/4A protease structures proved to be a valuable tool in the design of a new series of potent NS3 protease inhibitors. Optimization of initial compounds provided 25a. The in vitro activity and selectivity as well as the rat pharmacokinetic profile of 25a compare favorably with the data for other NS3/4A protease inhibitors currently in clinical development for the treatment of HCV.
Assuntos
Hepacivirus/enzimologia , Compostos Macrocíclicos/química , Inibidores de Serina Proteinase/química , Proteínas não Estruturais Virais/antagonistas & inibidores , Animais , Compostos Macrocíclicos/síntese química , Compostos Macrocíclicos/farmacocinética , Compostos Macrocíclicos/farmacologia , Modelos Moleculares , Ratos , Inibidores de Serina Proteinase/síntese química , Inibidores de Serina Proteinase/farmacocinética , Inibidores de Serina Proteinase/farmacologia , Proteínas não Estruturais Virais/químicaRESUMO
The hepatitis C virus (HCV) nonstructural protein 3 (NS3) with its cofactor NS4A is a pivotal enzyme for the replication of HCV. Inhibition of NS3-4A protease activity has been validated as an antiviral target in clinical studies of inhibitors of the enzyme. We have developed a sensitive time-resolved fluorescence (TRF) assay capable of detecting very low NS3-4A concentrations. A depsipeptide substrate is used that contains a europium-cryptate moiety and an efficient quenching group, QSY-7. The TRF assay is at least 30-fold more sensitive than a fluorescence energy transfer (FRET) assay and allows evaluation of NS3 protease inhibitors in reactions catalyzed by low enzyme concentrations (30 pM). Use of low enzyme concentrations allows for accurate measurement of inhibition by compounds with subnanomolar inhibition constants. The inhibitory potency of the potent protease inhibitor, BILN-2061, is significantly greater than previously reported. The ability to accurately determine inhibitory potency in reactions with low picomolar concentrations of NS3-4A is crucially important to allow valid comparisons between potent inhibitors. Studies of the interaction of NS3 with its NS4A cofactor at low enzyme concentration also reveal that the protease activity is salt dependent. This salt dependence of the enzyme activity is not present when high enzyme concentrations are used in the FRET assay.