Polymorphic Structure Determination of the Macrocyclic Drug Paritaprevir by MicroED.

Paritaprevir is an orally bioavailable, macrocyclic drug used for treating chronic Hepatitis C virus (HCV) infection. Its structures have been elusive to the public until recently when one of the crystal forms is solved by microcrystal electron diffraction (MicroED). In this work, the MicroED structures of two distinct polymorphic crystal forms of paritaprevir are reported from the same experiment. The different polymorphs show conformational changes in the macrocyclic core, as well as the cyclopropyl sulfonamide and methyl pyrazinamide substituents. Molecular docking shows that one of the conformations fits well into the active site pocket of the HCV non-structural 3/4A (NS3/4A) serine protease target, and can interact with the pocket and catalytic triad via hydrophobic interactions and hydrogen bonds. These results can provide further insight for optimization of the binding of acyl sulfonamide inhibitors to the HCV NS3/4A serine protease. In addition, this also demonstrates the opportunity to derive different polymorphs and distinct macrocycle conformations from the same experiments using MicroED.

Polymorphic Structure Determination of the Macrocyclic Drug Paritaprevir by MicroED.

Paritaprevir is an orally bioavailable, macrocyclic drug used for treating chronic Hepatitis C virus infection. Its structures had been elusive to the public until recently when one of the crystal forms was solved by MicroED. In this work, we report the MicroED structures of two distinct polymorphic crystal forms of paritaprevir from the same experiment. The different polymorphs show conformational changes in the macrocyclic core, as well as the cyclopropylsulfonamide and methylpyrazinamide substituents. Molecular docking shows that one of the conformations fits well into the active site pocket of the NS3/4A serine protease target, and can interact with the pocket and catalytic triad via hydrophobic interactions and hydrogen bonds. These results can provide further insight for optimization of the binding of acylsulfonamide inhibitors to the NS3/4A serine protease. In addition, this also demonstrate the opportunity of deriving different polymorphs and distinct macrocycle conformations from the same experiments using MicroED.

Modulation of Kinase Activities In Vitro by Hepatitis C Virus Protease NS3/NS4A Mediated-Cleavage of Key Immune Modulator Kinases.

Hepatitis C Virus NS3/NS4A, a serine protease complex, has been found to interact with many host proteins and cause various adverse effects on cellular function and immune response. For example, the cleavage of important immune factors by NS3/NS4A has been suggested as a mechanism for the hepatitis C virus to evade innate immunity. The spectrum of susceptible substrates for NS3/NS4A cleavage certainly includes important immune modulator kinases such as IKKα, IKKß, IKKε, and TBK1, as demonstrated in this paper. We show that the kinase activities of these four host kinases were transformed in unexpected ways by NS3/NS4A. Treatment with NS3/NS4A caused a significant reduction in the kinase activities of both IKKα and IKKß, suggesting that HCV might use its NS3/NS4A protease activity to deactivate the NF-κB-associated innate immune responses. In contrast, the kinase activities of both IKKε and TBK1 were enhanced after NS3/NS4A treatment, and more strikingly, the enhancement was more than 10-fold within 20 min of treatment. Our mass spectroscopic results suggested that the cleavage after Cys89 in the kinase domain of IKKε by NS3/NS4A led to their higher kinase activities, and three potential mechanisms were discussed. The observed kinase activity enhancement might facilitate the activation of both IKKε- and TBK1-dependent cellular antiviral pathways, likely contributing to spontaneous clearance of the virus and observed acute HCV infection. After longer than 20 min cleavage, both IKKε- and TBK1 gradually lost their kinase activities and the relevant antiviral pathways were expected to be inactivated, facilitating the establishment of chronic HCV infection.

Viral Interference of Hepatitis C and E Virus Replication in Novel Experimental Co-Infection Systems.

BACKGROUND: Hepatitis C virus (HCV) constitutes a global health problem, while hepatitis E virus (HEV) is the major cause of acute viral hepatitis globally. HCV/HEV co-infections have been poorly characterized, as they are hampered by the lack of robust HEV cell culture systems. This study developed experimental models to study HCV/HEV co-infections and investigate viral interference in cells and humanized mice. METHODS: We used state-of-the art human hepatocytes tissue culture models to assess HEV and HCV replication in co- or super-transfection settings. Findings were confirmed by co- and super-infection experiments in human hepatocytes and in vivo in human liver chimeric mice. RESULTS: HEV was inhibited by concurrent HCV replication in human hepatocytes. This exclusion phenotype was linked to the protease activity of HCV. These findings were corroborated by the fact that in HEV on HCV super-infected mice, HEV viral loads were reduced in individual mice. Similarly, HCV on HEV super-infected mice showed reduced HCV viral loads. CONCLUSION: Direct interference of both viruses with HCV NS3/4A as the determinant was observed. In vivo, we detected reduced replication of both viruses after super-infection in individual mice. These findings provide new insights into the pathogenesis of HCV-HEV co-infections and should contribute to its clinical management in the future.

Hepatitis C virus drugs that inhibit SARS-CoV-2 papain-like protease synergize with remdesivir to suppress viral replication in cell culture.

Effective control of COVID-19 requires antivirals directed against SARS-CoV-2. We assessed 10 hepatitis C virus (HCV) protease-inhibitor drugs as potential SARS-CoV-2 antivirals. There is a striking structural similarity of the substrate binding clefts of SARS-CoV-2 main protease (Mpro) and HCV NS3/4A protease. Virtual docking experiments show that these HCV drugs can potentially bind into the Mpro substrate-binding cleft. We show that seven HCV drugs inhibit both SARS-CoV-2 Mpro protease activity and SARS-CoV-2 virus replication in Vero and/or human cells. However, their Mpro inhibiting activities did not correlate with their antiviral activities. This conundrum is resolved by demonstrating that four HCV protease inhibitor drugs, simeprevir, vaniprevir, paritaprevir, and grazoprevir inhibit the SARS CoV-2 papain-like protease (PLpro). HCV drugs that inhibit PLpro synergize with the viral polymerase inhibitor remdesivir to inhibit virus replication, increasing remdesivir's antiviral activity as much as 10-fold, while those that only inhibit Mpro do not synergize with remdesivir.

Treatment of hepatitis C virus infection.

Hepatitis C virus infection affects 71 million people worldwide. It is at the origin of a multi-organ disease associating hepatic manifestations, cryoglobulinemic vasculitis and general manifestations linked to chronic inflammation (diabetes, cardio-, reno- or cerebrovascular manifestations, extra-hepatic cancers including non-Hodgkin's lymphoma). The significant morbidity and mortality linked to the hepatitis C virus therefore justify its screening and access to treatments which have increased considerably over the past two decades. Understanding the replicative cycle of the hepatitis C virus has enabled the development of direct HCV-specific antivirals targeting viral proteins (NS3/4A protease, NS5B polymerase and the multifunctional NS5A replication complex). The combination of two to three specific inhibitors often co-formulated in a capsule, without pegylated interferon and most often without ribavirin, allows high antiviral efficacy (more than 97% cure) for a treatment duration of 8-12 weeks with satisfactory tolerance. HCV infection is the only chronic viral infection that can be cured and the hepatic or extrahepatic manifestations are mainly reversible. This underlines the importance of strengthening screening and access to care policies in order to achieve the elimination of viral infection C in the short term, in 2030, as expected from the program of the World Health Organization. If this elimination is possible in some countries (Iceland, France, Germany …), it seems compromised in others where prevention (USA), screening and/or access to care are still insufficient (Sub-Saharan Africa, Russia…).

In Silico Evaluation of the Effectivity of Approved Protease Inhibitors against the Main Protease of the Novel SARS-CoV-2 Virus.

The coronavirus disease, COVID-19, caused by the novel coronavirus SARS-CoV-2, which first emerged in Wuhan, China and was made known to the World in December 2019 turned into a pandemic causing more than 126,124 deaths worldwide up to April 16th, 2020. It has 79.5% sequence identity with SARS-CoV-1 and the same strategy for host cell invasion through the ACE-2 surface protein. Since the development of novel drugs is a long-lasting process, researchers look for effective substances among drugs already approved or developed for other purposes. The 3D structure of the SARS-CoV-2 main protease was compared with the 3D structures of seven proteases, which are drug targets, and docking analysis to the SARS-CoV-2 protease structure of thirty four approved and on-trial protease inhibitors was performed. Increased 3D structural similarity between the SARS-CoV-2 main protease, the HCV protease and α-thrombin was found. According to docking analysis the most promising results were found for HCV protease, DPP-4, α-thrombin and coagulation Factor Xa known inhibitors, with several of them exhibiting estimated free binding energy lower than -8.00 kcal/mol and better prediction results than reference compounds. Since some of the compounds are well-tolerated drugs, the promising in silico results may warrant further evaluation for viral anticipation. DPP-4 inhibitors with anti-viral action may be more useful for infected patients with diabetes, while anti-coagulant treatment is proposed in severe SARS-CoV-2 induced pneumonia.

Lignans, flavonoids and coumarins from Viola philippica and their α-glucosidase and HCV protease inhibitory activities.

Two lignans including a new one, five flavonoids and five coumarins were isolated from the whole plant of Viola philippica (synonymised as Viola yedoensis Makino). The new compound was structurally determined as (7R,8S,8'S) -3,3'-dimethoxy- 4,4',9-trihydroxy- 7,9'-epoxy-8,8'-lignan 9-O-rutinoside by analysis of its NMR, MS and CD spectroscopic data. The known compounds were characterised by comparing their NMR and MS data with those reported. Among the known compounds, 5-hydroxy-4'-methoxyflavone-7-O- rutinoside, 6,7-di-O-ß-D- glucopyranosylesculetin, and 7R,8S-dihydrodehydrodiconiferyl alcohol 4-O-ß-D- glucopyranoside were isolated and identified from this genus for the first time. Of these compounds, 5-hydroxy-4'-methoxyflavone-7-O-rutinoside and (7R,8S,8'S) -3,3'-dimethoxy- 4,4',9-trihydroxy- 7,9'-epoxy-8,8'-lignan 9-O-rutinoside were potently active against α-glucosidase, while the two dimeric coumarins, 5, 5'-bi (6, 7-dihydroxycoumarin) and 6,6',7,7'-tetrahydroxy-5,8'-bicoumarin potently inhibited HCV protease.

In vitro inhibition of Hepatitis C virus protease and antioxidant by flavonoid glycosides from the Saudi costal plant Sarcocornia fruticosa.

A new flavonol triglycoside, rhamnazin 3-O-2G-rhamnorutinoside or rhamnazin 3-O-(2″,6″-O-α-di-rhamnosyl)-ß-glucoside (1) was isolated along with known flavonols, rhamnazin 3-O-rutinoside (2), rhamnazin 3-O-(6″-O-α-rhamnosyl)-ß-galactoside (3), isorhamnetin 3-O-(6″-O-α-rhamnosyl)-ß-galactoside (4), isorhamnetin 3-O-(2″,6″-O-α-di-rhamnosyl)-ß-galactoside (5), and isorhamnetin (6), and allantoin (7) from the aqueous methanol extract of Sarcocornia fruticosa leaves. Spectral analyses (UV, MS, and NMR) and acid hydrolysis were used to determine the structures. These compounds in this study except 6 were reported for the first time from the genus Sarcocornia. The extract and flavonol glycosides (1-5) were evaluated for antioxidant and inhibition of HCV protease enzyme. Rhamnazin triglycoside (1) was shown to have a potent HCV protease inhibitor with IC50 value 8.9 µM, while isorhamnetin di- and triglycosides (4 and 5) were effectively scavenged DPPH radicals with IC50 values 3.8 and 4.3 µM, respectively.

Grazoprevir/elbasvir combination therapy for HCV infection.

Interferon-free regimens combine different second-wave direct-acting antiviral agents (DAAs), which target the main viral proteins involved in the replication cycle of hepatitis C virus (HCV): NS3/4A protease inhibitors (simeprevir or paritaprevir boosted by ritonavir), NS5B nucleos(t)idic (sofosbuvir) and nonnucleos(t)idic (dasabuvir) polymerase inhibitors, NS5A replication complex inhibitors (daclatasvir, ledipasvir, elbasvir, velpatasvir). Combinations of two or three DAAs, given for 8-24 weeks reach sustained virology response (SVR) rates greater than 90% with good tolerance. SVR rates and safety are similar in clinical trials and in real life, usually higher than 95% in the per-protocol analysis. Next-generation DAAs are now expected. To be competitive, these new combinations need to prove their added value regarding the pill burden, the reduced duration of treatment, the drug-drug interaction profile and safety. Zepatier is a fixed-dose combination product coformulating MK-5172 [grazoprevir (GZR), 100 mg QD] and MK-8742 [elbasvir or (EBR) 50 mg QD]: it combines highly potent inhibitors of the HCV NS3/4A protease and NS5A replication complex, respectively. This review provides a summary of the main evidence available for the use of GZR/EBR and highlights the strength of this combination.

Tracking HCV protease population diversity during transmission and susceptibility of founder populations to antiviral therapy.

Due to the highly restricted species-tropism of Hepatitis C virus (HCV) a limited number of animal models exist for pre-clinical evaluation of vaccines and antiviral compounds. The human-liver chimeric mouse model allows heterologous challenge with clinically relevant strains derived from patients. However, to date, the transmission and longitudinal evolution of founder viral populations in this model have not been characterized in-depth using state-of-the-art sequencing technologies. Focusing on NS3 protease encoding region of the viral genome, mutant spectra in a donor inoculum and individual recipient mice were determined via Illumina sequencing and compared, to determine the effects of transmission on founder viral population complexity. In all transmissions, a genetic bottleneck was observed, although diverse viral populations were transmitted in each case. A low frequency cloud of mutations (<1%) was detectable in the donor inoculum and recipient mice, with single nucleotide variants (SNVs) > 1% restricted to a subset of nucleotides. The population of SNVs >1% was reduced upon transmission while the low frequency SNV cloud remained stable. Fixation of multiple identical synonymous substitutions was apparent in independent transmissions, and no evidence for reversion of T-cell epitopes was observed. In addition, susceptibility of founder populations to antiviral therapy was assessed. Animals were treated with protease inhibitor (PI) monotherapy to track resistance associated substitution (RAS) emergence. Longitudinal analyses revealed a decline in population diversity under therapy, with no detectable RAS >1% prior to therapy commencement. Despite inoculation from a common source and identical therapeutic regimens, unique RAS emergence profiles were identified in different hosts prior to and during therapeutic failure, with complex mutational signatures at protease residues 155, 156 and 168 detected. Together these analyses track viral population complexity at high-resolution in the human-liver chimeric mouse model post-transmission and under therapeutic intervention, revealing novel insights into the evolutionary processes which shape viral protease population composition at various critical stages of the viral life-cycle.

Different Culture Metabolites of the Red Sea Fungus Fusarium equiseti Optimize the Inhibition of Hepatitis C Virus NS3/4A Protease (HCV PR).

The endophytic fungus Fusarium equiseti was isolated from the brown alga Padina pavonica, collected from the Red Sea. The fungus was identified by its morphology and 18S rDNA. Cultivation of this fungal strain in biomalt-peptone medium led to isolation of 12 known metabolites of diketopeprazines and anthraquinones. The organic extract and isolated compounds were screened for their inhibition of hepatitis C virus NS3/4A protease (HCV PR). As a result, the fungal metabolites showed inhibition of HCV protease (IC50 from 19 to 77 µM), and the fungus was subjected to culture on Czapek's (Cz) media, with a yield of nine metabolites with potent HCV protease inhibition ranging from IC50 10 to 37 µM. The Cz culture extract exhibited high-level inhibition of HCV protease (IC50 27.6 µg/mL) compared to the biomalt culture extract (IC50 56 µg/mL), and the most potent HCV PR isolated compound (Griseoxanthone C, IC50 19.8 µM) from the bio-malt culture extract showed less of an inhibitory effect compared to isolated ω-hydroxyemodin (IC50 10.7 µM) from the optimized Cz culture extract. Both HCV PR active inhibitors ω-hydroxyemodin and griseoxanthone C were considered as the lowest selective safe constituents against Trypsin inhibitory effect with IC50 48.5 and 51.3 µM, respectively.

Development of a simple HPLC-UV method for the determination of the hepatitis C virus inhibitor simeprevir in human plasma.

A simple high-performance liquid chromatography method for the determination of the hepatitis C virus protease inhibitor simeprevir in human plasma was developed and validated. The method involved a rapid and simple solid-phase extraction of simeprevir using Oasis HLB 1cc cartridges, isocratic reversed-phase liquid chromatography on an XTerra RP18 (150 mm×4.6 mm, 3.5 µm) column, and ultraviolet detection at 225 nm. The mobile phase consisted of phosphate buffer (pH 6, 52.5 mM) and acetonitrile (30:70, v/v). This assay proved to be sensitive (lower limit of quantification of 0.05 µg/mL), linear (correlation coefficients ≥0.99), specific (no interference with various potentially co-administrated drugs), reproducible (both intra-day and inter-day coefficients of variation ≤8.3%), and accurate (deviations ranged from -8.0 to 1.2% and from -3.3 to 6.0% for intra-day and inter-day analysis, respectively). The method was applied to therapeutic monitoring of patients undergoing simeprevir treatment for hepatitis C and proved to be robust and reliable. Thus, this method provides a simple, sensitive, precise and reproducible assay for dosing simeprevir that can be readily adaptable to routine use by clinical laboratories with standard equipment.

HCV viral load at baseline and at week 4 of telaprevir/boceprevir based triple therapies are associated with virological outcome in HIV/hepatitis C co-infected patients.

BACKGROUND: As first generation HCV-specific protease inhibitors, boceprevir (BOC) or telaprevir (TVR) can achieve 60% to 70% sustained virological response (SVR) for HCV infected patients with genotype 1 infections, they could remain temporary a therapeutic option in patients living in resources limited countries with limited access to the new anti-HCV direct acting antiviral (DAA) drugs, such as sofosbuvir. OBJECTIVES AND STUDY DESIGN: Here we evaluated in a routine practice setting, the treatment responses, tolerance and factors associated with SVR of a triple therapy with BOC or TVR, combined with pegylated interferon and ribavirin (PegIFN/RBV) in HIV/HCV co-infected patients, included in a large cohort of HIV/HCV coinfected patients (ANRS CO13-HEPAVIH). RESULTS: Among the 89 HIV/HCV coinfected patients treated, 65% of whom were previous non-responders to PegIFN/RBV therapy, 65%, 55% and 41% had at baseline genotype 1a, a high baseline HCV-RNA (≥800,000 IU/ml) and a cirrhosis, respectively. The SVR12 rate was 63% overall, 53% for BOC-based regimen and 66% for TVR-based regimen. In multivariate analysis, two factors were significantly associated with HCV SVR: HCV viral load <800,000 IU/mL at treatment initiation versus ≥800,000 IU/mL (OR 4.403, 95% CI 1.29-15.04; p=0.018) and virological response at W4 (HCV-RNA undetectable after 4 weeks of triple therapy) (OR 3.35, 95% CI 1.07-10.48; p=0.038). CONCLUSIONS: Overall SVR12 was 63% and our results suggest that HIV/HCV coinfected patients with low HCV viral load (<800,000 IU/mL) and undetectable HCV-RNA after 4 weeks of triple therapy with TVR or BOC-based regimen have a higher probability of treatment success.

Plasma apolipoprotein H limits HCV replication and associates with response to NS3 protease inhibitors-based therapy.

BACKGROUND & AIMS: Chronic infection with HCV remains a public health problem with approximately 150 million people infected worldwide. HCV intersects with lipid metabolism for replication and entry; and plasma concentrations of apolipoproteins have been identified as predictors for response to therapy. Herein, we conducted a screen of plasma proteins, including all apolipoproteins, to identify correlates of response to pegylated-interferon/ribavirin (PR) and HCV non-structural protein 3 (NS3) inhibitors (i.e., telaprevir/boceprevir) therapy in treatment-experienced cirrhotic patients from the ANRS CUPIC cohort. METHODS: We analysed 220 baseline plasma protein concentrations in 189 patients using Luminex technology and analyzed results. RESULTS: We identified baseline levels of apolipoprotein H (apoH) as a surrogate marker for sustained virological response (SVR). Notably, increased plasma concentration of apoH, used in combination with known clinical parameters, established a robust model with improved classification of patients as likely to achieve SVR (AUC = 0.77, Se = 66%, Sp = 72%, NRI = 39%). Moreover, we provide mechanistic information that indicates a previously unidentified role for apoH during viral entry. Using a human liver slices HCV infection model, we demonstrate that apoH limits replication. CONCLUSION: These data support testing of new biomarker strategies for the management of cirrhotic HCV patients and expand our understanding of how apoH may intersect with HCV infection.

Patients treated with first-generation HCV protease inhibitors exhibit high ribavirin concentrations.

Anemia is a well-known RBV-related event in HCV therapy which is exacerbated by the addition of telaprevir and boceprevir. This retrospective study evaluated and compared ribavirin exposure and parameters able to influence hemoglobin decrease in a large population of patients treated with dual or triple therapy. Patients on triple therapy had higher ribavirin concentrations at week 12 of treatment (3460 ng/mL vs. 1843 ng/mL; P < .0001). An association was also observed between week 12 eGFR and ribavirin concentration only for patients on triple therapy (P = .002). The proportion of patients with a >20 mL/min/1.73 m(2) decrease in eGFR at week 12 was higher among patients on triple therapy: 32%, 14%, and 5% for boceprevir, telaprevir, and dual therapy, respectively (P = .025 and .026). No correlation was observed between boceprevir and telaprevir concentrations and hemoglobin or eGFR decrease. Exacerbation of anemia in patients on triple therapy is related to higher ribavirin concentrations. We provide an explanation for this increase in plasma RBV concentration. Triple therapy with PEG-IFN, RBV, and telaprevir or boceprevir will remain the only HCV treatment option for many patients. Our data show that the RBV dose can be decreased while maintaining adequate plasma concentrations and reducing anemia.

Provision of clinical pharmacist services for individuals with chronic hepatitis C viral infection: Joint Opinion of the GI/Liver/Nutrition and Infectious Diseases Practice and Research Networks of the American College of Clinical Pharmacy.

The objective of this opinion paper was to identify and describe potential clinical pharmacists' services for the prevention and management of patients infected with the hepatitis C virus (HCV). The goals of this paper are to guide the establishment and development of pharmacy services for patients infected with HCV and to highlight HCV research and educational opportunities. Recommendations were based on the following: a review of published data on clinical pharmacist involvement in the treatment and management of HCV-infected patients; a consensus of clinical pharmacists who provide direct patient care to HCV-infected patients and practice in different pharmacy models, including community-based and academic settings; and a review of published guidelines and literature focusing on the treatment and management of HCV infections. The recommendations provided in this opinion paper define the areas of clinical pharmacist involvement and clinical pharmacy practice in the treatment and management of patients with HCV. Clinical pharmacists can promote preventive measures and education about reducing HCV transmission, improve medication adherence, assist in monitoring clinical and adverse effects, recommend treatment strategies to minimize adverse effects and drug interactions, and facilitate medication acquisition and logistics that positively improve patient outcomes and reduce the health care system costs.

Hepatitis C virus inhibitory hydrolysable tannins from the fruits of Terminalia chebula.

Two new hydrolysable tannins, chebumeinin A (1) and chebumeinin B (2), together with eight known related compounds (3-10), were isolated from the fruits of Terminalia chebula. The new compounds were structurally determined by analysis of their spectroscopic data and the known compounds characterized by comparing their spectroscopic data with literature values. All isolates were evaluated by an HCV protease inhibition assay, and some compounds were found to be potently active.

Inhibition of HCV replication by cyclophilin antagonists is linked to replication fitness and occurs by inhibition of membranous web formation.

BACKGROUND & AIMS: Replication of hepatitis C virus (HCV) requires host cell factors, such as cyclophilin A (CypA). CypA binds to HCV's nonstructural protein (NS)5A to promote replication of viral RNA. CypA antagonists, such as cyclosporines, are potent inhibitors of HCV replication. NS2 modulates sensitivity of HCV to cyclosporines. We investigated why cyclosporines require NS2 to increase their inhibitory effect and how they block HCV replication. METHODS: We determined replication fitness and sensitivity of various HCV replicons, containing or lacking NS2, to cyclosporine and other direct-acting antiviral agents. We also analyzed the effects of cyclosporine on membranous web formation by electron microscopy. RESULTS: NS2-5B replicons of genotype 2a (JFH1), but not genotype 1b, had increased sensitivity to cyclosporine. This difference was lost with replication-attenuated NS3-5B JFH1 RNAs, showing that cyclosporine sensitivity is linked to reduced replication fitness of NS2-containing HCV RNAs. Fitness also determined sensitivity to a nucleoside analogue and an NS5A inhibitor, but not to telaprevir. Cyclosporine blocked de novo formation of the membranous web, but had little effect on established membranous replication factories. This block was prevented by cyclosporine resistance mutations in NS5A. CONCLUSIONS: Cleavage at the NS2/3 junction is a rate-limiting step in replication of particular HCV isolates and determines their sensitivity to CypA inhibitors. These drugs target de novo formation of the membranous web and RNA replication.