Potential conflict of interest: H. W. R. serves as advisor for Roche Molecular Diagnostics, Anadys, Merck, Arrows, and Gilead; consults for PRA International, Tibotec, GlaxoSmithKline, Chiron Novartis, and Roche Therapeutics; and receives grant and research support from Schering-Plough, PRA International, and Roche. H. L. A. J. serves as advisor for Roche, Novartis, Gilead, Schering-Plough, and Bristol-Myers Squibb and receives grant and research support from Roche, Novartis, Gilead, Schering-Plough, and Bristol-Myers Squibb. X. T., J. L., M. A. T., and E. A. H. are employees of Schering-Plough Research Institute. R. J. d. K. serves as an advisor for Schering-Plough; receives grant and research support from Schering-Plough, Roche, and Medtronic; and speaks and teaches for Schering-Plough. J. d. B., J. F. B., C. J. W., R. M., J. S., J. J. v. L., and A. A. v. V. report no conflicts of interest.
Narlaprevir (SCH 900518) is a potent inhibitor of the hepatitis C virus (HCV) nonstructural protein 3 serine protease that is primarily metabolized by the cytochrome P450-3A4 system. In order to explore the use of ritonavir-based pharmacokinetic enhancement of an HCV protease inhibitor, this study investigated the safety, tolerability, pharmacokinetics, and antiviral activity of narlaprevir (with or without ritonavir) administered as monotherapy and as combination therapy with pegylated interferon-α-2b (PEG-IFN-α-2b) to HCV genotype 1–infected patients. This was a randomized, placebo-controlled, two-period, blinded study in 40 HCV genotype 1–infected patients (naïve and treatment-experienced). In period 1, narlaprevir was administered for 7 days as 800 mg three times daily without ritonavir or 400 mg twice daily with 200 mg ritonavir twice daily. In period 2, after a 4-week washout, the same dose and regimen of narlaprevir was administered in combination with PEG-IFN-α-2b for 14 days. Upon completion of period 2, all patients initiated PEG-IFN-α-2b and ribavirin treatment. A rapid and persistent decline in plasma HCV-RNA was observed in both treatment-experienced and treatment-naïve patients during period 1, with a mean viral load decline of at least 4 log10 in all treatment groups. A high percentage of both treatment-experienced (50%) and treatment-naïve (≥60%) patients had undetectable HCV-RNA (<25 IU/mL) after period 2. Standard of care resulted in sustained virological response (SVR) rates of 38% and 81% in treatment-experienced and treatment-naïve patients, respectively. Narlaprevir (with or without ritonavir) alone or in combination with PEG-IFN-α-2b was safe and well tolerated. Conclusion: Narlaprevir administration resulted in a robust HCV-RNA decline and high SVR rates when followed by standard of care in both treatment-experienced and treatment-naïve HCV genotype 1–infected patients. (HEPATOLOGY 2010
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Chronic hepatitis C virus (HCV) infection is a major cause of liver cirrhosis and hepatocellular carcinoma. HCV-related end-stage liver disease is now the main indication for liver transplantation in North America and western Europe.1 Estimates suggest that there are 170 million HCV-infected patients worldwide and that 3 to 4 million people are newly infected each year.2 Approximately 80% of patients who become infected with HCV develop chronic hepatitis C.3 The current standard of care (SOC), combination therapy of pegylated interferon-α- (PEG-IFN-α-2b) and ribavirin (RBV), achieves a sustained virological response (SVR) in only approximately 40% of patients infected with HCV genotype 1.4, 5
The HCV nonstructural protein 3 (NS3) gene encodes a serine protease critical for viral replication and is thought to have a dual role in establishing chronic HCV infection. The protease mediates the cleavage of the HCV polyprotein into functional viral proteins required for replication and may also play a role in viral evasion of the immune system by preventing expression of IFN response genes.6, 7 Direct-acting antiviral agents such as NS3 protease inhibitors are currently being evaluated in phase 3 clinical trials in combination with PEG-IFN-α and RBV. The addition of these first-generation protease inhibitors (VX-950, telaprevir; SCH 503034, boceprevir)8, 9 to the backbone therapy of PEG-IFN-α and RBV has improved the treatment outcomes significantly for HCV genotype 1–infected patients.10, 11
For many years, human immunodeficiency virus (HIV)-specific protease inhibitors have been widely used as part of highly active antiretroviral therapy.12 Ritonavir is frequently prescribed with highly active antiretroviral therapy, not necessarily for its antiviral activity but for its ability to inhibit cytochrome P450-3A4 (CYP3A4). Inhibition of CYP3A4 by ritonavir leads to higher plasma concentrations of the coadministered HIV protease inhibitors, allowing a lower dose and a less frequent dosing schedule of HIV protease inhibitors.13 This discovery has significantly improved dosing convenience for patients and has resulted in increased efficacy of protease inhibitors for HIV treatment.14, 15
Narlaprevir (SCH 900518) is a novel potent oral direct-acting antiviral agent that prevents viral replication in infected host cells by inhibiting the HCV NS3 protease. The mechanism of inhibition involves the covalent, yet reversible, binding of narlaprevir to the NS3 protease active site serine through a ketoamide functional group. In the replicon system, the 50% and 90% maximal effective concentration for suppression of the HCV genotype 1b is approximately 20 ± 6 nM and 40 ± 10 nM (∼28 ng/mL), respectively.16 These data indicate that narlaprevir is approximately 10-fold more potent in vitro than other protease inhibitors currently in phase 3 trials (telaprevir and boceprevir).17, 18 The replicon data also suggest that combination therapy with IFN-α may enhance HCV-RNA reduction and may suppress the selection of resistant HCV mutations in a clinical setting.16
Additional in vitro studies have identified that narlaprevir is primarily metabolized by CYP3A4, and phase 1 clinical studies have confirmed that significantly increased plasma exposures of narlaprevir in healthy volunteers can be achieved when coadministered with ritonavir (data on file at Schering-Plough Research Institute). Effective telaprevir minimum were plasma concentrations (Cmin) approximately five-fold higher than the 90% maximal effective concentration (EC90) as determined by the replicon system.8 Therefore, in order to achieve narlaprevir exposures that would demonstrate potent antiviral activity, the doses administered in this study (800 mg narlaprevir three times daily and 400 mg narlaprevir with 200 mg ritonavir twice daily) were targeted to attain a therapeutic exposure and a mean Cmin at least five- to 10-fold above the replicon assay EC90 value of 40 nM (≈28 ng/mL).
We report the safety and tolerability of narlaprevir administered at two dose levels as monotherapy and in combination with PEG-IFN-α-2b in 40 treatment-naïve and treatment-experienced patients infected with HCV genotype 1. We also present the antiviral activity and pharmacokinetic profile of narlaprevir and the response to SOC (PEG-IFN-α-2b/RBV) following the completion of narlaprevir administration.
AE, adverse event; Cmin, minimum plasma concentration; CYP3A4, cytochrome P450-3A4; EC90, 90% maximal effective concentration; HCV, hepatitis C virus; HIV, human immunodeficiency virus; NS3, nonstructural protein 3; PEG-IFN-α-2b, pegylated interferon α-2b; RBV, ribavirin; RVR, rapid viral response; SOC, standard of care; SVR, sustained virological response.
Patients and Methods
Study Design and Organization.
This randomized, placebo-controlled, double-blind, two-period phase 1b study was conducted at three sites in The Netherlands. Narlaprevir dosing was conducted at a single site as an inpatient study; SOC was administered on an outpatient basis. Study medication (PegIntron, Rebetol, and narlaprevir) was supplied by Schering-Plough Research Institute. Ritonavir (Norvir; Abbott Laboratories) was also used in this study. Narlaprevir and matched-placebo were administered as an amorphous suspension.
The study was conducted as a two-period, fixed-sequence study in 40 HCV genotype 1–infected patients enrolled in four cohorts (Fig. 1). Cohorts 1 and 3 each included 10 patients naïve to HCV treatment; cohorts 2 and 4 each included 10 HCV treatment-experienced patients. In each cohort, patients were randomized in a 4:1 ratio to either narlaprevir (n = 8) or placebo (n = 2) according to a computer-generated random code. Treatment was prepared and dispensed in a blinded fashion by a third party for administration to the patients. The third party was not involved with the study procedures, assessments, or data recording and did not reveal the randomization during the study according to the Consolidated Standards of Reporting Trials guidelines.19
During period 1, patients received either 800 mg narlaprevir (or placebo) three times daily (cohorts 1 and 2) as monotherapy or 400 mg narlaprevir (or placebo) in combination with 200 mg ritonavir twice daily (cohorts 3 and 4) for 7 consecutive days. There was a washout period of approximately 4 weeks between the final treatment administration in period 1 and the first treatment in period 2. During the washout period, patients returned to the study site for assessments on days 14 and 21. Period 2 consisted of 14 consecutive days of dosing with the same dosing regimen as in period 1 in combination with 1.5 μg/kg/week PEG-IFN-α-2b (days 1 and 8). Upon completion of the second treatment period, patients were offered SOC with 1.5 μg/kg/week PEG-IFN-α-2b and daily weight-based RBV (800-1,400 mg) for 24 or 48 weeks. Initiation of SOC began immediately after confinement at the clinical site. Patients were treated for 24 (only if rapid viral response [RVR] was achieved) or 48 weeks at the discretion of the patients, provided standard stopping rules did not require premature discontinuation. Rapid viral response (RVR) was defined as HCV-RNA undetectable after 4 weeks of SOC. This study was conducted in accordance with Good Clinical Practice and with the Declaration of Helsinki after approval by each center's institutional review board. All patients provided written informed consent before participating in the study.
Key inclusion criteria included men and women between 18 and 65 years with body mass indexes of 18-40 kg/m2, HCV genotype 1 (any subtype), and HCV-RNA level >1 × 105 copies/mL (or equivalent international units). Chronic hepatitis C patients were naïve, nonresponders or relapsers to previous IFN-based treatment. Relapse was defined as undetectable HCV-RNA upon completion of a previous IFN-based treatment, but positive HCV-RNA during follow-up. Nonresponse was defined as positive HCV-RNA at the end of a previous IFN-based treatment or <2-log decline in HCV-RNA levels at 12 weeks and discontinued treatment. Key exclusion criteria included decompensated liver disease, findings consistent with Child-Pugh class B or C liver cirrhosis, and coinfection with HIV or hepatitis B virus. Patients with chronic stable hemophilia or on stable methadone substitution treatment were eligible for the study.
The Truegene assay was used to determine the genotype and subtype of all patients. Multiple samples for determination of plasma HCV-RNA levels and viral sequencing were obtained in both periods on day 1, followed by daily sample collection. HCV-RNA was measured during the SOC treatment at the start or treatment; at treatment weeks 4, 12, and 24; at end of treatment; and 24 weeks after treatment cessation. HCV-RNA levels during the narlaprevir treatment phase of the study were measured using the Roche Cobas TaqMan HCV/HPS assay version 2.0 (Covance, Switzerland) with a lower limit of quantification of 25 IU/mL and a lower limit of detection of 9.3 IU/mL. Plasma HCV-RNA levels during SOC were assessed at the Academic Medical Center (Amsterdam, The Netherlands) using the Roche Cobas Ampliprep/Cobas TaqMan assay version 1.0 with a lower limit of detection of 15 IU/mL. Viral population sequencing of the NS3 protease domain (amino acids 1-181) was performed for all patients at all time points collected if sufficient RNA was available. Viral RNA was extracted from human plasma samples using a commercially available silica-gel membrane based kit (Qiagen, Valencia, CA) and processed on an automated BioRobot 9604 system (Qiagen). Reverse-transcription of RNA was performed using a SuperScript III First Strand Synthesis Supermix kit (Invitrogen, Carlsbad, CA) with random hexamers according to the manufacturer's instructions. Polymerase chain reaction was conducted with a Platinum PCR SuperMix kit (Invitrogen). Reaction products were purified on a Biomek FX system (Beckman Coulter, Fullerton, CA) using a magnetic bead kit (Agencourt Bioscience Corporation, Beverly, MA). DNA sequencing of purified material was conducted on a 3730xl DNA Analyzer (Applied Biosystems).
To investigate a potential correlation between exposure to narlaprevir and antiviral activity, plasma samples were collected over the course of the study for pharmacokinetic profiling of narlaprevir with or without ritonavir. In period 1, serial blood narlaprevir pharmacokinetic samples were taken on days 1 and 7. Additional narlaprevir pharmacokinetic sampling was performed on days 2, 5, 6, and 7 for trough level (Cmin) determination. In period 2, serial blood narlaprevir pharmacokinetic samples were collected on days 1, 7, and 14, and additional pharmacokinetic samples for Cmin determination were obtained on days 2, 8, 10, 12, and 14. Plasma concentrations of narlaprevir were determined using a validated liquid chromatographic–tandem mass spectrometric method with a limit of quantification of 6.08 ng/mL.
Patients were monitored for safety and tolerability at regular intervals from the start of dosing through a follow-up visit 24 weeks after completion of SOC. Safety assessments included physical examination, vital signs, clinical laboratory tests, electrocardiograms, and the recording of all adverse events.
Demographic and Baseline Characteristics.
Forty-one patients (10 patients each in cohorts 1-3 and 11 patients in cohort 4) were enrolled in the study. One patient in cohort 4 discontinued immediately after the first dose on day 1 because of intolerance to the drug suspension; study medication was stopped at the discretion of the investigator, and this patient was replaced. A total of 40 patients completed the narlaprevir treatment phase of the study and initiated SOC immediately after period 2. Treatment-experienced patients consisted of 12 relapse patients and eight nonresponders. Demographic and other baseline characteristics of the randomized patients are shown in Table 1.
Table 1. Baseline Characteristics
800 mg Narlaprevir TID
400 mg Narlaprevir BID+ Ritonavir
Cohort 1: Tx-Naïve (n =10)
Cohort 2: Tx-Experienced (n = 10)
Cohort 3: Tx-Naïve (n =10)
Cohort 4: Tx-Experienced (n = 10)
Data are presented as n (%) or mean (SD).
Abbreviations: BID, two times daily; BMI, body mass index; TID, three times daily; Tx, treatment.
Patients receiving methadone
Patients with hemophilia
4.8 × 106 (3.2 × 106)
6.3 × 106 (4.4 × 106)
3.8 × 106 (2.9 × 106)
4.3 × 106 (4.6 × 106)
The pharmacokinetic profile of narlaprevir 800 mg three times daily or 400 mg with ritonavir two times daily was characterized (Table 2). Exposure to narlaprevir with and without coadministration of PEG-IFN-α-2b for both treatment-naïve and treatment-experienced patients was comparable. Narlaprevir was eliminated more slowly when coadministered with ritonavir than when administered alone. Dose-normalized daily exposures (area under the curve) on day 14 in the presence of PEG-IFN-α-2b and ritonavir increased 7.6- and 7.1-fold in treatment-naïve and treatment-experienced patients, respectively, compared with narlaprevir monotherapy. Based on the pharmacokinetic methodology employed in this trial, the narlaprevir terminal T1/2 could not be determined for all treatment groups.
Table 2. Pharmacokinetic Parameters of Narlaprevir After 14 Days of Narlaprevir With PEG-IFN-α-2b With or Without Ritonavir (Period 2) in Chronis Hepatitis C Patients
Abbreviations: AUC 0-τ, area under the plasma concentration time curve over the dosing interval; BID, two times daily; Cmax, maximum plasma concentration; CV, coefficient of variance; TID, three times daily; Tmax, time of maximum plasma concentration; Tx, treatment.
One patient was excluded because no pharmacokinetic samples were available.
Tmax, hours (range)
Cmax, ng/mL (CV %)
AUC 0-τ, ng/hour/mL (CV %)
A rapid and persistent decline in plasma HCV-RNA levels was observed that was strikingly similar in all cohorts. Therapy with narlaprevir with or without ritonavir (period 1) resulted in a mean >4 log10 IU/mL decline in plasma HCV-RNA levels in all treatment groups (Fig. 2A). The mean HCV-RNA changes from baseline in all narlaprevir-treated patients are listed in Table 3. All groups demonstrated a similar return of viral load to baseline during the 4-week washout period after period 1. No significant changes in HCV-RNA levels were observed in patients who received placebo.
Table 3. Mean Changes of HCV-RNA Levels (log10 IU/mL) from Baseline After Each Treatment Period in Narlaprevir-Treated Patients
800 mg Narlaprevir TID
400 mg Narlaprevir BID + Ritonavir
Cohort 1: Tx-Naïve (n = 8)
Cohort 2: Tx-Experienced (n = 8)
Cohort 3: Tx-Naïve (n = 8)
Cohort 4: Tx-Experienced (n = 8)
Abbreviations: BID, two times daily; TID, three times daily; Tx, treatment.
End of period 1
End of period 2
Patients with HCV-RNA <25 IU/mL at end of period 2, n (%)
5 (63 )
When narlaprevir ± ritonavir was coadministered with PEG-IFN-α-2b, similar declines in HCV viral load were achieved across all treatment groups (Fig. 2B). All patients achieved a > 3 log10 IU/mL decline in HCV-RNA levels, and the majority of patients had a maximal HCV-RNA decline of 4-5 log10 IU/mL. The mean HCV-RNA changes from baseline for each treatment cohort are listed in Table 3. Patients randomized to the placebo group demonstrated a mean HCV decline of 0.45 log10 IU/mL in response to PEG-IFN-α-2b treatment (Fig. 2B).
All 40 patients completed period 2 and initiated SOC within 1 day after the last narlaprevir dose. Patients were treated with SOC for 24 weeks at the discretion of the patient if HCV-RNA was undetectable after 4 weeks of SOC. The treatment outcomes (SVR, relapse, nonresponse, or breakthrough) according to prior treatment history of all patients are listed in Table 4. Treatment-naïve patients treated with narlaprevir had an overall SVR rate of 81% (13/16) compared with 38% (6/16) in the treatment-experienced group. In the placebo group, 38% (3/8) of patients achieved SVR after at least 48 weeks of treatment; all responders were treatment naïve. Of the six treatment-experienced patients who achieved SVR, five patients were previous relapsers, and one was a previous nonresponder. Nine treatment-naïve patients treated with narlaprevir had an RVR and subsequently achieved SVR (100%) with 24 weeks (six patients) or 48 weeks (three patients) of SOC. Seven treatment-experienced patients treated with narlaprevir had an RVR, of whom six achieved SVR (86%) after 48 weeks of SOC.
Table 4. Therapy Outcome After Treatment with PEG-IFN-α-2b and RBV in Patients Treated with Narlaprevir or Placebo
Treatment-Naïve Patients (n = 20)
Treatment-Experienced Patients (n = 20)
Narlaprevir (n = 16)
Placebo (n = 4)
Narlaprevir (n = 16)
Placebo (n = 4)
Data are presented as n (%).
Undetectable HCV-RNA 24 weeks after completion of treatment.
Positive HCV-RNA at end of treatment.
Undetectable HCV-RNA at completion of treatment, but relapsed during follow-up.
1 log10 increase of HCV-RNA from nadir during SOC.
In this study, viral variants were detected in all cohorts and in both treatment- naïve and treatment-experienced patients. Treatment-emergent variants known to be associated with resistance to narlaprevir, characterized in biochemical and cell-based assays, were observed in five patients (Table 5). These were observed at loci V36, R155, and A156. Susceptibility to narlaprevir has not been characterized for the treatment-emergent mutation R155T.16 Of the 40 patients enrolled, 24 had an isoleucine-170 (I170) polymorphism detected in pretreatment samples. This variant is not known to confer reduced susceptibility to narlaprevir. All patients with treatment-emergent resistance variants failed to achieve undetectable viral HCV-RNA levels. Virological breakthrough was observed in four patients; one previous nonresponder appeared to be a nonresponder again during SOC. One treatment-experienced patient with a serine-54 polymorphism at baseline associated with reduced susceptibility to narlaprevir achieved undetectable viral load levels in period 2 (cohort 2). This patient remained HCV-RNA undetectable during SOC but relapsed after 24 weeks of treatment.
Table 5. Treatment-Emergent Mutations at the NS3 Domain (Defined as Changes from Reference Sequence) Observed Throughout the Narlaprevir Treatment Phase and SOC Based on Population Sequencing
V36M, R155T, A156T
V36M, R155K, A156S
V36M, R155K, A156T
V36M, R155K, A156S
No severe or serious adverse events (AEs) and no dosing interruptions or discontinuations were reported during narlaprevir dosing. A complete listing of the most frequently reported AEs recorded for both period 1 and period 2 is provided in Table 6. During period 1, the most commonly reported AEs were gastrointestinal symptoms (diarrhea, anorectal discomfort, abdominal discomfort, abdominal distension). Gastrointestinal symptoms were reported in 25 (76%) patients who received narlaprevir and 4 (50%) patients who received placebo. During period 2, when PEG-IFN-α-2b was added to the treatment regimen, the most commonly reported AE was influenza-like illness, which was observed in 30 (94%) patients who received narlaprevir and 6 (75%) patients who received placebo. Also during period 2, there was an elevated rate of gastrointestinal symptoms. Gastrointestinal-related AEs were reported by 24 (75%) patients who received narlaprevir, compared with no patients in the placebo group. No significant difference in AEs was noted between patients that were treatment-naïve versus treatment-experienced. Ritonavir coadministration did not significantly affect the AE profile. Three serious AEs (one instance of elevated CRP and two instances of pyrexia) occurred during SOC administration. All three events occurred in the same patient and required hospital admission, but they were not considered related to narlaprevir treatment. No clinically significant changes in blood chemistry or hematological parameters, vital signs, or electrocardiograms occurred in any treatment group.
Table 6. Summary of Most Frequently Reported AEs (≥10% of Patients) During 7 Days of Narlaprevir Monotherapy with or Without Ritonavir (Period 1) and After 14 Days of Narlaprevir with PEG-IFN-α-2b with or Without Ritonavir (Period 2)
800 mg Narlaprevir TID (n = 16)
400 mg Narlaprevir BID + Ritonavir (n = 16)
Placebo (n = 4)
Placebo + Ritonavir (n = 4)
AEs are summarized by body system and preferred term regardless of severity and drug relatedness. No AEs were considered probably or very likely related to study medication. All AEs reported were grade 1.
Abbreviations: BID, two times daily; TID, three times daily.
The present study was the first clinical trial to evaluate narlaprevir in chronic hepatitis C patients and to evaluate a treatment regimen that used a pharmacokinetic enhancer (ritonavir) in combination with an HCV NS3 protease inhibitor for the treatment of hepatitis C. In addition, this was one of the first phase 1b studies to offer treatment with PEG-IFN-α-2b and RBV to all patients following treatment with narlaprevir in order to explore the potential of increasing the RVR and, consequently, the SVR rates. Finally, the first clinical mutational analysis of narlaprevir was performed to investigate the development of NS3/4 genome sequence changes during and after narlaprevir treatment.
The primary objective of this study was to assess the safety and tolerability of narlaprevir with or without ritonavir and PEG-IFN-α-2b in chronic hepatitis C patients. During narlaprevir dosing, there were no treatment discontinuations and no serious AEs. The most frequently reported AEs were gastrointestinal symptoms and influenza-like illness. Addition of PEG-IFN-α-2b to the treatment regimen increased the frequency of AEs, however, these AEs (flu-like symptoms) were consistent with those expected for pegylated IFN. Combination with ritonavir did not significantly affect the AE profile. Most AEs reported in patients receiving narlaprevir were mild or moderate in severity. None of these moderate events was considered to be related to the study drug. Consistent with the results in healthy volunteers, narlaprevir appeared to be safe and well tolerated in all patients.
The secondary objectives were to investigate the antiviral activity and pharmacokinetic profile of narlaprevir. Narlaprevir demonstrated a profound antiviral activity in both treatment-naïve and treatment-experienced patients. A rapid and persistent mean HCV-RNA decline of at least 4 log10 IU/mL was achieved in all patients whether narlaprevir was administered for 7 days alone or with ritonavir. HCV-RNA levels returned to baseline at the end of a 4-week washout period. During 14 days of treatment with narlaprevir with or without ritonavir in combination with PEG-IFN-α-2b, plasma HCV-RNA levels declined in two phases: a rapid decline within the first day followed by a more gradual viral decline thereafter. Four patients who received narlaprevir achieved undetectable HCV-RNA (<15 IU/mL) after 14 days. Follow-up treatment with PEG-IFN-α-2b and RBV resulted in high SVR rates of 81% (13/16) in treatment-naïve patients and 38% (6/16) in treatment-experienced patients treated with narlaprevir (with or without ritonavir). A rapid viral response was a strong positive predictor for SVR in treatment-naïve (9/9) and treatment-experienced patients (6/7). These results demonstrate that the rapid and profound decline in HCV-RNA that was observed after a short initial period (14 days) of narlaprevir dosing could result in an increased RVR rate and subsequently an increased SVR rate in both treatment-naïve and treatment-experienced patients compared with regular SOC.4, 5, 20 This finding suggests that SVR rates may be further enhanced when the dosing period of narlaprevir is extended to a 12-week regimen, which is currently being assessed in a phase 2a trial.21
The pharmacokinetic objective of this study was to generate a mean Cmin at least five- to 10-fold above the replicon assay EC90 value of 40 nM (≈28 ng/mL). Analysis of the pharmacokinetic profile of narlaprevir monotherapy revealed plasma concentrations at least six times the EC90 at trough in all treatment groups after a 7-day dosing period. A quartile distribution of median Cmin of 170, 296, 1,150, and 1,725 ng/mL represented a median Cmin six- to 62-fold higher than the EC90 for narlaprevir. Although these doses generated an extraordinary range of narlaprevir plasma trough exposures, the antiviral activity observed was similar for all of the quartiles. The enhanced trough levels observed when narlaprevir was administered with ritonavir and the associated robust antiviral activity observed in this study provided a proof of principle for the use of pharmacokinetic enhancement in HCV therapy. This study justified and guided the further clinical investigation of a once daily dosing regimen of narlaprevir (200 mg and 400 mg) in combination with low-dose ritonavir (100 mg) in a phase 2a study.21
Although the results of this phase 1b study demonstrate the great potential of narlaprevir to improve therapy for HCV-infected patients, several limitations should be considered. Clearly, the short duration of narlaprevir dosing influenced its potential impact on SVR rates following SOC. However, despite this limitation, administration of narlaprevir before initiation of SOC still appeared to benefit the patients significantly. In addition to the short duration of narlaprevir dosing, the study was limited by a heterogenous and small patient population. A further complication was secondary to the sequential dosing periods interrupted by a 1-month washout period. To address these study limitations, several modifications to future study designs could be employed. First, the small size (10 patients per cohort) and heterogeneity (differences in treatment history, baseline HCV-RNA, wide range of body mass index, different ethnic groups, and patients with hemophilia) of the study population could have biased the treatment effect estimate. A larger and more restricted study population could remove this potential bias. Such changes were implemented in a subsequent phase 2a study of narlaprevir.21 Second, the approach of two sequential dosing periods separated by a washout period was chosen to investigate narlaprevir monotherapy and viral rebound after removal of drug pressure, as well as to attempt to demonstrate the additional antiviral effect of narlaprevir when used in combination with PEG-IFN-α-2b. However, as shown in other studies with protease inhibitor monotherapy,22, 23 7 days of narlaprevir monotherapy most likely induced resistant variants with reduced susceptibility and complicated the interpretation of combination therapy during period 2 of the study. Detection of single variants (A156T), double variants (V36M together with R155K), and in one case a triple variant (V36M and R155K together with A156S) showed that the treatment regimens in this study selected for virus variants with a high level of resistance to narlaprevir. Based on population sequencing during the washout period, one patient had a viral population consisting of V36M, R155T, and A156T associated with high levels of resistance to narlaprevir (Table 5). This patient had a less profound HCV-RNA decline during period 2, and HCV-RNA even increased after day 8. In total, sequence analysis of the NS3/4A protease domain showed that viral variants (R155K, V36L/M, A156T/S) associated with reduced susceptibility to narlaprevir were present in five patients. Treatment outcome of these five genotype 1a patients included a viral breakthrough in four patients, and one patient appeared to be a nonresponder. Longer duration of narlaprevir treatment in combination with PEG-IFN-α-2b and RBV may increase the durability of antiviral response to this treatment regimen and add protection against potential viral breakthrough and emergence of viral variants.10 Longer follow-up and clonal analysis is needed to fully understand the kinetics of these resistance variants.
Combination of protease inhibitor–based regimens with SOC (PEG-IFN-α-2b and RBV) has dramatically improved chronic hepatitis C treatment outcomes.10, 11 Telaprevir and boceprevir, both of which are HCV-specific NS3 protease inhibitors, are currently being evaluated in phase 3 clinical trials with a three times daily dosing regimen. The requirement of these compounds for high frequency dosing may lead to a lack of adherence and consequently lowered protease inhibitor exposure that could potentially lead to the development of resistant virus and a failure to achieve SVR.24 Since the mid-1990s, combining a pharmacokinetic enhancer with protease inhibitors in antiretroviral drug regimens has provided HIV patients with potent therapies that durably suppress HIV replication to undetectable levels and reduce the likelihood of generating drug resistance.25 Inhibition of the CYP-450 (3A4) metabolic pathway by ritonavir provides the basis for pharmacokinetic enhancement of concomitantly administered HIV protease inhibitors. CYP3A4 is present in the intestinal tract and liver, where it plays a key role in protease inhibitor first-pass metabolism.26 A once daily dosing regimen of narlaprevir and ritonavir could be a major advantage, because the pill burden will likely increase with the addition of future direct-acting antiviral agents to the current SOC.
The potential of undesired effects of ritonavir during HCV treatment is low due to a possibility for a shorter treatment duration (compared with HIV treatment), administration of a low dose, and reduced dosing frequency (once daily). However, coadministration of a metabolic enhancer will require attention to possible interactions with other medications metabolized by CYP3A4 (such as statins and benzodiazepines).26 Other protease inhibitors such as TMC435 have demonstrated potent antiviral activity with once daily dosing without ritonavir boosting.27 It is therefore uncertain if ritonavir boosting will be useful in future treatment regimens that potentially include three or four drug combinations. Nevertheless, knowledge about the coadministration of HCV protease inhibitors with ritonavir will be important in the large HIV-coinfected subpopulation of patients.
In conclusion, narlaprevir demonstrated potent antiviral activity when administered as monotherapy (with and without ritonavir) and in combination with PEG-IFN-α-2b. For the first time during the treatment of HCV infection, pharmacokinetic and pharmacodynamic modeling supports once daily dosing of an HCV NS3 protease inhibitor (narlaprevir) with low-dose ritonavir in chronic hepatitis C patients.21 An ongoing study is currently assessing the efficacy of once daily dosing (narlaprevir and ritonavir), the impact on viral resistance, and the possibility of a shorter SOC treatment duration due to the potency of the compound.
We thank the patients who agreed to participate in this clinical study. We also acknowledge the contributions of the following individuals: M. W. Peters (study nurse, Academic Medical Center); X. Thomas, S. Menting, and S. P. H. Rebers (laboratory staff, Section of Virology, Academic Medical Center); C. van der Ent and I. Brings (Clinical Research Bureau, Erasmus MC University Hospital); and Marleen Ypey and Arjen Akkermann (PRA International).