Boceprevir: A Protease Inhibitor for the Treatment of Hepatitis C

Mei H. Chang, BA, PharmD, BCPS; Lori A. Gordon, PharmD; and Horatio B. Fung, PharmD, BCPS
Pharmacy Service, James J. Peters Veterans Affairs Medical Center, Bronx, New York

ABSTRACT
Background: Boceprevir is a protease inhibitor indi- cated for the treatment of chronic hepatitis C virus (HCV) genotype 1 infection in combination with peginterferon and ribavirin for treatment-naive pa- tients and those who previously failed to improve with interferon and ribavirin treatment.
Objective: This article provides an overview of the mechanism of action, pharmacologic and pharmacoki- netic properties, clinical efficacy, and tolerability of boceprevir.
Methods: Relevant information was identified through a search of PubMed (1990–July 2012), EMBASE (1990–July 2012), International Pharma- ceutical Abstracts (1970–July 2012), and Google Scholar using the key words boceprevir, SCH 503034, non-structural protein 3 (NS3) serine protease inhibi- tor, and direct-acting antiviral agent (DAA). Addi- tional information was obtained from the US Food and Drug Administration’s Web site, review of the refer- ence lists of identified articles, and posters and ab- stracts from scientific meetings.
Results: Clinical efficacy of boceprevir was assessed in 2 Phase III trials, Serine Protease Inhibitor Ther- apy–2 (SPRINT-2) for treatment-naive patients and Retreatment with HCV Serine Protease Inhibitor Bo- ceprevir and PegIntron/Rebetol 2 (RESPOND-2) for treatment-experienced patients. In SPRINT-2, patients were randomized to receive peginterferon ti ribavirin (PR) or peginterferon ti ribavirin ti boceprevir (PRB); duration of boceprevir therapy varied from 24, 32, to 44 weeks on the basis of HCV RNA results. The pri- mary endpoint was achievement of sustained virologic response (SVR; lower limit of detection, 9.3 IU/mL). The addition of boceprevir was shown to be superior, with overall SVR rates ranging from 63% to 66% com- pared with 38% with PR (P ti 0.001). Results of SVR
in SPRINT-2 were also reorganized to monitor SVRs in black and non-black patients. Treatment-experienced patients were assessed in RESPOND-2; however, null responders were excluded. Patients were again ran- domized to PR or PRB; duration of boceprevir therapy varied from 32 to 44 weeks on the basis of HCV RNA results. SVR was significantly higher in patients receiv- ing boceprevir (59%–66% vs 21% with PR; P ti 0.001). This benefit was seen in both previous nonre- sponders (SVR, 40%–52% vs 7% with PR), as well as previous relapsers (SVR, 69%–75% vs 29% with PR). Importantly, SVR could be attained with a shortened course of therapy in almost one half of all treated pa- tients in SPRINT-2 (44%) and RESPOND-2 (46%).
Conclusions: Boceprevir was well tolerated in clini- cal trials and a welcomed addition to our HCV armamentarium. (Clin Ther. 2012;34:2021–2038) Published by Elsevier HS Journals, Inc.
Key words: boceprevir, direct-acting antiviral agent, nonstructural protein 3 serine protease inhib- itor, SCH 503034.

INTRODUCTION
After peaking in the late 1980s, the incidence of hepa- titis C virus (HCV) infection steadily declined through- out the 1990s; however, since 2003, these rates have remained constant.1 With an estimated 3.2 million people living with HCV, it is now recognized as the most common chronic bloodborne infection in the United States, accounting for 1.3% of the population.2 The continued prevalence is largely driven by the aging population infected during the peak of new cases.3

Accepted for publication August 17, 2012. http://dx.doi.org/10.1016/j.clinthera.2012.08.009 0149-2918/$ – see front matter
Published by Elsevier HS Journals, Inc.

However, this number is most likely an underestimate of the true burden of the disease because many infected individuals are often asymptomatic and hence un- aware of their status. Moreover, individuals who are at the highest risk for infection, intravenous drug abusers, may not access the health care system.3,4 Approxi- mately 70% to 85% of those acutely infected will de- velop a chronic infection.2 Of these patients, 60% to 70% will develop active liver disease and 25% will develop cirrhosis; therefore, HCV-related cirrhosis ac- counts for ti40% of all cases of cirrhosis.2–6 Further- more, HCV has an estimated 5-year cumulative risk for hepatocellular carcinoma of 17%.7 Finally, HCV re- mains the leading cause for liver transplantation.2–4
First identified in 1989 as non-A, non-B hepatitis, HCV is a positive, single-stranded, RNA virus of the family Flaviviridae.3,5,8–11 The virus exists as ti6 ma- jor genotypes, which can be further classified into ti70 subtypes.9,12,13 These genotypes have varying geo- graphical distributions, with genotype 1 being most commonly found in the United States, followed by ge- notypes 2 and 3.1 Encoded in a 3000 amino acid se- quence is the polyprotein that contains key structural and nonstructural proteins.5,8,11,14 The nonstructural proteins responsible for HCV replication are activated when cleavage occurs at 4 specific sites, a process cat- alyzed by the nonstructural protein 3 (NS3) protease;

antiviral therapy for hepatitis C compounds, a focused approach in which therapy is targeted for various stages of the HCV life cycle, possibly resulting in the ultimate removal of peginterferon and ribavirin as the backbone of therapy.6,20–22 Currently, there are ti50 direct-acting antiviral agents (DAAs), exhibiting vary- ing potency against HCV genotypes, at assorted stages of preclinical and clinical development.20
Nonetheless, this management approach brings its own set of challenges. The practitioner involved in the management of the patient with chronic HCV must become familiar with specific treatment response ter- minology (Table I) as he or she embarks on treatment or retreatment.3 A clear understanding of such terms facilitates intraprovider communication. More impor- tantly, response-guided therapy (RGT), in which treat- ment duration is decided on the basis of HCV RNA response at prespecified intervals, is the current corner- stone of HCV treatment.20 In addition, the HCV pro- vider must learn the complexities of baseline and on-

Table I. Virologic response during treatment of hepatitis C virus (HCV).3

Virologic
Response Definition

hence, the NS3 protease is a key target in the arrest of HCV replication.8,11,14
Since 2001, with the pegylation of interferon, there have been no significant changes in the management of chronic HCV infection, a disheartening fact given the low cure rates in the predominant genotype seen in the United States.4–7,13,15 The standard of care (SOC) with peginterferon and ribavirin for 48 weeks of therapy resulted in sustained virologic response (SVR) of 30% to
Rapid virologic response
Early virologic response
End-of-treatment response Sustained virologic
response
HCV RNA negative by treatment week 4
HCV RNA negative by treatment week 12
HCV RNA negative at end of treatment period
HCV RNA negative 24 weeks’ posttreatment

50%.4–7,12,13,15 A major limitation to advances in re- search was the lack of a durable replication model.10,16 Fortunately, in 1999, the chimpanzee model was ex- changed for a cell culture replication model system.8,10,11 The proof of concept paper for the NS3 protease inhibi- tor, BI 2061, in which HCV RNA declined by ti4-log10, provided new hope, although it was short-lived because of cardiotoxicity associated with this compound.10,16,17
Null response Decrease in HCV RNA by ti2- log10 from baseline by week 24
Partial response Decrease in HCV RNA by ti2- log10 from baseline without HCV RNA negative at week 24
Breakthrough HCV RNA negative with reappearance of HCV RNA during treatment

Ten years later, with the approval of boceprevir18 and telaprevir19 in 2011, the treatment of HCV is now experiencing a similar revelation in treatment options that a different viral infection, HIV, saw ti15 years ago. The paradigm has shifted to specifically targeted
Relapse
HCV RNA negative at end of treatment period with reappearance of HCV RNA 24 weeks’ posttreatment

treatment resistance patterns. Unlike nonviral chronic comorbidities, HCV is a dynamic entity that quickly adapts in response to drug pressure, largely due to its prolific, yet error-prone, nature.23 The close manage- ment of adherence, adverse drug events, and drug in- teractions will become increasingly important, clearing an excellent niche for specialists with expertise in this area.
In the future, questions will need to be addressed regarding the approach of patients with varying failed responses to a regimen containing peginterferon, riba- virin, and DAA or even regimens containing multiple DAAs.24 Further research efforts must still be pursued in difficult-to-treat patients, such as those of African- American ethnicity or with advanced fibrosis, decom- pensated cirrhosis, reinfection of liver transplant, and coinfection with HIV.7,25,26 Aside from the clinical concerns mentioned here, another looming issue re- mains how the health care system will absolve the in- crease in health care costs.23 In a phenomenon called “warehousing,” many patients have been waiting, and will continue to wait, to start therapy with regimens containing newly approved agents promising im- proved cure rates.22 As new treatments become available, difficult decisions must be made regarding who to treat first: patients who may benefit the most due to advanced liver disease but may not tolerate therapy to completion or patients who have multiple positive prognostic factors, and hence an increased likelihood of achieving an SVR in a shortened treat- ment cycle?3
Boceprevir is a novel DAA, specifically a NS3 pro- tease inhibitor, that was approved by the US Food and Drug Administration (FDA) on May 13, 2011, for the treatment of chronic HCV genotype 1 infection in combination with peginterferon and ribavirin.18 Cur- rently, it is 1 of only 2 FDA-approved DAAs available to treat adult (age ti18 years) patients with compen- sated liver diseases who are either treatment naive or previously failed to improve with SOC therapy.18,19 It is not indicated for monotherapy due to resistance is-

stracts (1970–July 2012), and Google Scholar using the key words boceprevir, SCH 503034, non-struc- tural protein 3 (NS3) serine protease inhibitor, and direct-acting antiviral agent (DAA) identified relevant information pertaining to boceprevir. To retrieve all available information on boceprevir, neither limits nor inclusion/exclusion criteria were applied. Additional information was obtained from the FDA Web site and by a thorough review of the reference lists of identified articles. Posters and abstracts from meetings of the American Association for the Study of Liver Diseases, European Association for the Study of the Liver, Infec- tious Diseases Society of America, American Society for Microbiology, Conference on Retroviruses and Opportunistic Infections, and the European Society of Clinical Microbiology and Infectious Diseases from 2000 to 2011 were also consulted.

RESULTS Chemical Structure
Boceprevir (Figure) is a NS3 serine protease inhibitor of HCV.8,18 Chemically, it is (1R,5S)-N-[3-Amino-1-(cy- clobutylmethyl)-2,3-dioxopropyl]-3-[2(S)-[[[(1,1-dim- ethylethyl)amino]carbonyl]amino]-3,3-dimethyl-1- oxobutyl]-6,6-dimethyl-3-azabicyclo[3.1.0]hexan-2 (S)-carboxamide.18 The empirical formula for boceprevir is C27H45N5O5, with a molecular weight of 519.7 dal- tons.18 It is manufactured as a white to off-white amor- phous powder containing a mixture of 2 diastereomers, in which the S-configuration is the active isomer.18,27

Mechanism of Action
The NS3 serine protease is vital to the life cycle of HCV, without which the virus will not be able to rep- licate inside the host cell.8,27 This virally encoded pro- tease is responsible for cleaving the polyprotein gener-

O

sues.18 The current article provides an overview of the mechanism of action, pharmacologic and pharmacoki-

N
H
N
NH2

netic properties, clinical efficacy, and tolerability of boceprevir.
HN
H
N

O
O
O

O

METHODS

A search of PubMed (1990–July 2012), EMBASE (1990–July 2012), International Pharmaceutical Ab-
Figure. Structural formula of boceprevir.8,18

ated from the HCV genome template. Cleavage allows for the release of functional proteins and generation of mature proteins. Boceprevir is a covalent reversible inhibitor of the HCV NS3 serine protease, binding at the active site through an ti-ketoamide functional group.8,18,27 Inhibition of the NS3 protease will sup- press RNA replication and ultimately the production of virions.8,27 As such, boceprevir is described as a DAA.

In Vitro Activity
The in vitro activity of boceprevir was evaluated by using 2 biological assays, the enzyme assay and the cell-based replicon assay.8,27 The assays are used to assess the potency of inhibition and subsequent effi- cacy of HCV protease inhibitors. In the HCV NS3 bio- chemical assay against genotype 1a and 1b, boceprevir was shown to be a potent inhibitor with an average binding constant Ki of 14 nM.18,27,28 The values for the boceprevir concentration at which the reaction of an organism was 50% or 90% less than the control value were ti200 and 400 nM, respectively, in a 72- hour cell-based replicon assay consisting of only geno- type 1b.8,18,27,28 In addition, the cross-reactivity of bo- ceprevir was tested in vitro against a broad number of other serine proteases and general enzymes, and showed no cross-reactivity or identification of no ma- jor issues.8,27 Final results all indicated that boceprevir was highly selective toward the HCV NS3 protease inhibitor.8,18,27,28

Pharmacokinetics and Pharmacodynamics
In preclinical studies in rats and dogs, boceprevir demonstrated good oral bioavailabilities of 26% and 30%, respectively.8,27,28 Based on target organ analy- sis in rats, boceprevir was also found to concentrate well in the liver, with a 30-fold liver/plasma concentra- tion ratio. 8,27,28

steady state did not increase proportionally to doses, whereas exposures for individual subjects overlapped at 800 and 1200 mg, indicating diminished absorption at higher doses. Compared with the fasting state, food increased the AUC of boceprevir by 65%. The type of meal (high-fat vs low-fat) and timing of meal did not make a difference in drug exposure. Currently, all doses are advised to be taken with a meal or a light snack to enhance the exposure of boceprevir.18 At 800 mg TID, steady state was achieved after 1 day with minimal accumulation, 0.8-fold to 1.5-fold.18 The vol- ume of distribution was ti772 L at steady state with 75% bound to plasma protein.18,29 Boceprevir primar- ily undergoes metabolism via the aldo-keto reductase pathway to inactive ketone-reduced metabolites.18,34 To a lesser extent, it also undergoes oxidative metab- olism via cytochrome P450 (CYP) 3A4 and 3A5.18,34 After a single 800-mg radiocarbon dose of boceprevir, ti79% of the dose was excreted in the feces and 9% in the urine.34 Of these amounts, only 8% (feces) and 3% (urine) were eliminated as boceprevir, suggesting that the liver is the primary elimination route. Elimination t½ values ranged from 7 to 15 hours across a total daily dose range of 50 to 1200 mg.29 The mean plasma t½ is ti3.4 hours.18 These values found in healthy volun- teers were similar to those obtained from patients and are summarized in Table II.18,29–33 No significant dif- ferences were found in any pharmacokinetic parame- ters in studies conducted in patients with genotype 1 infection who were nonresponders to interferon with or without ribavirin and patients with varying degrees of hepatic and renal impairment.30–33

Table II. Pharmacokinetic parameters* of boce- previr.18,29

Variable Value

In humans, single-dose and multiple ascending-dose pharmacokinetic studies have been conducted in healthy volunteers, patients, and those with varying degrees of hepatic and renal impairment.29–33 In trials with healthy volunteers, pharmacokinetic results were obtained from subjects who received 800 mg of boce- previr monotherapy 3 TID.18,29 On oral administra-

Cmax Cmin Tmax Vdss
t
1⁄2
AUC0–8

1723 ng/mL
88 ng/mL
2h 772 L 3.4 h
5408 ng · h/mL

tion, boceprevir was rapidly absorbed, with a median Tmax of 2 hours, Cmax of 1723 ng/mL, Cmin of 88 ng/
mL, and AUC0–8 of 5408 ng · h/mL. Additional dose studies indicated that AUC, Cmax, and Cmin values at
Vdss ti apparent volume of distribution at steady state. *Values based on healthy volunteers who received 800 mg
of boceprevir TID.

Clinical Efficacy
The safety and efficacy of boceprevir was evaluated in previously untreated patients as well as in previously treated genotype 1 patients with chronic HCV infection.

Treatment-Naive Patients
Kwo et al evaluated the safety and efficacy of boce- previr in a Phase II, multicenter, randomized, open- label trial, Serine Protease Inhibitor Therapy–1 (SPRINT-1).4,26 A total of 520 patients were random- ized in a 1:1:1:1:1 ratio to the following treatment groups: PR48 (SOC, peginterferon alfa-2b 1.5 tig/kg weekly ti ribavirin 800–1400 mg/d, dosed according to weight, for 48 weeks), PRB28 (SOC ti boceprevir 800 mg TID, for 28 weeks), PR4B24 (SOC lead-in; 4 weeks and SOC ti boceprevir 800 mg TID, 24 addi- tional weeks), PRB48 (SOC ti boceprevir 800 mg TID, for 48 weeks), and PR4B44 (SOC lead-in; 4 weeks and SOC ti boceprevir 800 mg TID, 44 additional weeks).4,10,16,26,35–38 Patients were stratified according to race (black vs non-black) and cirrhosis status (pres- ent vs not present) as determined by results of biopsy within the previous 5 years.4,10,26 A separate second part of the study evaluated the influence of ribavirin dose on efficacy, comparing a control group who re- ceived SOC ti boceprevir 800 mg TID for 48 weeks with a treatment group who received peginterferon alfa-2b 1.5 tig/kg weekly ti low-dose ribavirin 400 to 1000 mg/d, dosed according to weight ti boceprevir 800 mg TID for 48 weeks.4,10,26
The primary efficacy endpoint in SPRINT-1, and subsequent trials, was SVR, defined as patients with undetectable HCV RNA (lower limit of detection, 15 IU/mL) at 24 weeks after completion of therapy.26 The addition of boceprevir demonstrated significant im- provement in rates of SVR over SOC, ranging from 54% to 75% compared with 38% with SOC (P values, 0.013 to ti 0.0001).4,5,10,16,21,26,35–37,39 Moreover, the rates of SVR were better in boceprevir treatment groups that included the SOC 4-week lead-in (56% and 75% vs 54% and 67%, respectively). Attainment of SVR was highly associated with a rapid virologic response (RVR), defined as HCV RNA undetectable by treatment week 4.10,26,37 Attainment of SVR was also positively associated with a ti1.5-log10 reduction after the SOC lead-in.7,26 Of patients who attained an RVR, 74% to 100% in each treatment group achieved even- tual SVR, with a large number of those patients in the boceprevir treatment arms (180 of 416 vs 8 of 104 in

the SOC arm) and especially the 2 boceprevir arms that included SOC lead-in (116/221).10,26,37 Overall, of the 373 who attained undetectable HCV RNA levels by treatment week 12, almost 80% (78.6%) achieved SVR.10,26 Conversely, if HCV RNA was detectable af- ter week 12, the addition of boceprevir did not provide a clinical advantage, with only 1 additional patient in the PRB48 group achieving SVR. The probability of attaining SVR was highest if there was a ti4-log10 de- crease in HCV RNA at week 4 and lowest if there was a ti1.5-log10 decrease in HCV RNA at week 4 regard- less of treatment duration.40 These benefits were seen in difficult-to-treat patients, such as black patients (SVR rates ranging from 29% to 53% with boceprevir compared with 13% with SOC) and cirrhotic patients (SVR rates ranging from 57% to 78% with boceprevir compared with 25% with SOC), albeit in a small sam- ple size.10,26 Results from previous clinical trials had shown that blacks and cirrhotics achieved lower SVR rates as compared to non-blacks and non-cirrhotics.25,26
In part 2 of SPRINT-1, the researchers demon- strated that standard ribavirin dosing was necessary for optimal treatment efficacy. SVR was 50% with standard ribavirin dosing compared with 36% with low-dose ribavirin dosing.4,5 There was also a higher relapse rate associated with the decreased ribavirin dose (22% with low-dosing vs 11% with standard dosing).
Poordad et al25 evaluated the safety and efficacy of boceprevir in a Phase III, multicenter, randomized, blinded, placebo-controlled trial, Serine Protease In- hibitor Therapy–2 (SPRINT-2). A total of 1097 pa- tients were separated into 2 cohorts (157 black patients vs 940 non-black patients) and then randomized in a 1:1:1 ratio to the following treatment groups: PR48 (SOC, peginterferon alfa-2b 1.5 tig/kg weekly ti riba- virin 800–1400 mg/d, dosed according to weight; for 48 weeks and matching placebo), PR4B24 (SOC lead-in; 4 weeks and SOC ti boceprevir 800 mg TID; 24 addi- tional weeks ti SOC/placebo; 20 additional weeks if HCV RNA detectable after treatment week 8), and PR4B44 (SOC lead-in; 4 weeks and SOC ti boceprevir 800 mg TID; 44 additional weeks).4,25 In all 3 groups, treatment was stopped according to a standard futility rule, defined as detectable HCV RNA at week 24. Pa- tients were stratified according to baseline HCV RNA (ti400,000 vs ti400,000 IU/mL) and genotype 1 sub- type (1a vs 1b).25

The primary endpoint of SVR (lower limit of detec- tion, 9.3 IU/mL) allowed HCV RNA measurements from posttreatment week 12 to be carried forward if posttreatment week 24 measurements were unavail- able.25 The addition of boceprevir was shown to be superior over SOC, resulting in overall SVR rates rang- ing from 63% to 66% compared with 38% with SOC (P ti 0.001).4,10,25,36,37,41,42 Rates of SVR were signif- icantly higher in both cohorts (67%–68% compared with 40% with SOC in non-black patients, P ti 0.001; 42%–53% compared with 23% with SOC in black patients, P ti 0.04 for PR4B24 and P ti 0.004 for PR4B44).4,5,7,10,16,24,25,37,38,43–45 Overall rates of SVR were also significantly higher in patients with mild to moderate fibrosis (F0–F2), with an exception in black patients with mild to moderate fibrosis not quite reaching statistical significance (P ti 0.03).25 Importantly, SVR could be attained with a shortened course of therapy in almost one half of all treated patients (44%).7,16,46 An important exception was seen in treatment-experienced patients with cirrhosis who had a higher rate of SVR with an entire year of treatment (42% vs 31%).25
Attainment of SVR was highly associated with an RVR (ti89%) regardless of treatment group or race.25 However, SVR was also positively associated with a ti1-log10 reduction after the SOC lead-in (SVR 82% compared with 39% in those with a ti1-log10 reduc- tion).20 This finding held especially true for patients with advanced fibrosis (F3–F4). Attainment of SVR was also highly associated with undetectable HCV RNA at treatment week 8 (week 4 of boceprevir or matching placebo) (ti75%) regardless of treatment group or race.25 However, a larger number of patients met these criteria if treated with boceprevir (368 of 734 vs 51 of 363 in the SOC arm). Moreover, for patients who did not achieve RVR, the addition of boceprevir significantly increased overall rates of SVR (65%– 66% with boceprevir vs 34%). Even in those with ti1- log10 reduction (poor interferon responsiveness) after SOC lead-in, the addition of boceprevir resulted in sig- nificantly increased rates of SVR (28%–38% vs 4% with SOC). This was similarly significant in patients with mild to moderate, but not advanced, fibrosis (P ti 0.001 versus P values: 0.31 – 1.00, respectively). Re- sults from SPRINT-2 are summarized in Table III.25

Treatment-Experienced Patients
An initial Phase I trial evaluated varying doses of boceprevir (100–400 mg daily) for 2 weeks in treat-

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Volume 34 Number 10

ment-experienced patients, which resulted in a mean 2.06-log10 reduction in HCV RNA for the highest dose of boceprevir.17,35 Investigators evaluated the efficacy of boceprevir in a Phase Ib, randomized-to-sequence, open- label, 2-dose level, crossover study.4,10,17,35,36,47–49 Twenty-six previous nonresponse patients, defined as ti2-log10 reduction in HCV RNA after 12 weeks of peginterferon alfa-2b ti ribavirin, received boceprevir TID as monotherapy for 7 days, peginterferon alfa-2b 1.5 tig/kg once weekly as monotherapy for 14 days, and combination therapy for 14 days.4,10,17,35,47 A 14- day washout period interrupted each treatment inter- vention to allow for HCV RNA to return to baseline values. Fourteen patients received boceprevir at 200 mg per dose and 12 patients received boceprevir at 400 mg per dose.4,10,17,35,36,47–49
Boceprevir was shown to have clinical activity as illus- trated by a maximum mean 1.08-log10 decrease in HCV RNA after low-dose boceprevir monotherapy and a max- imum mean 1.95-log10 and 2.28-log10 decrease in HCV RNA after combination therapy for 1 and 2 weeks, re- spectively (compared with a 1.08-log10 decrease with peginterferon alfa-2b monotherapy).4,10,17,35,36,47–49 Boceprevir 400 mg TID resulted in a maximum mean 1.60-log10 decrease in HCV RNA after low-dose boce- previr monotherapy and a maximum mean 2.48-log10 and 2.68-log10 decrease in HCV RNA after combina- tion therapy for 1 and 2 weeks, respectively (compared with a 1.26-log10 decrease with peginterferon alfa-2b monotherapy). HCV RNA became undetectable in 8 patients within the 2-week combination therapy period.47
The safety and efficacy of boceprevir was evaluated in a Phase III, multicenter, randomized, double-blinded, placebo-controlled trial: Retreatment with HCV Serine Protease Inhibitor Boceprevir and PegIntron/Rebetol 2 (RESPOND-2).5,13,16,50 Patients had to have demon- strated responsiveness to interferon (minimum dura- tion of therapy, 12 weeks); thus, null-response patients were not included.5,7,10,13,24,41,43,45,46 Previous nonre- sponse was defined as ti2-log10 decrease by week 12 but detectable HCV RNA throughout therapy.4,7,13,42 Relapse was defined as undetectable HCV RNA at the end of treatment without attainment of SVR.4,13 A to- tal of 403 patients were randomized in a 1:2:2 ratio to the following treatment groups: PR48 (SOC: peginter- feron alfa-2b 1.5 tig/kg weekly ti ribavirin 800–1400 mg/d, dosed according to weight; for 48 weeks and matching placebo), PR4B32 (SOC lead-in; 4 weeks and

SOC ti boceprevir 800 mg TID; 32 additional weeks ti SOC/placebo; 12 additional weeks if HCV RNA detectable after treatment week 8), and PR4B44 (SOC lead-in; 4 weeks and SOC ti boceprevir 800mg TID; 44 addi- tional weeks).4,5,7,10,13,16,38,41,43,45,50,51 In all 3 groups, treatment was stopped according to a standard futility rule, defined as detectable HCV RNA at week 12.4,13,46 Patients were stratified according to previous response (nonresponse vs relapse) and genotype 1 subtype (1a vs 1b).13 The majority of patients were previous relapse pa- tients (64%).13
SVR was significantly higher with the addition of boceprevir (59%–66% vs 21% with SOC; P ti 0.001).4,5,7,10,13,16,24,37,42–44,46,50 This increase in SVR was largely fueled by the higher end of treatment responses rates (70%–77% with boceprevir vs 31% with SOC) coupled with lower rates of relapse (12%– 25% vs 32%).4,5,7,10,13,16,24,37,42–44,46,50 This benefit was seen in both previous nonresponse patients (SVR 40%–52% vs 7% with SOC), as well as previous relapse patients (69%–75% vs 29% with SOC).4,5,7,13,41,45,46,51 Importantly, SVR could be attained with a shortened course of therapy in almost one half of all treated pa- tients (46%).46 An important exception was seen in treatment-experienced patients with cirrhosis who had a higher rate of SVR with an entire year of treatment (77% vs 35%).13 In patients who were responsive to SOC lead-in, boceprevir conferred higher response rates (SVR, 73%–79%; SOC, 25%).10,13,43,46 Even in the 25% of patients who were poorly responsive to SOC lead-in, boceprevir resulted in attainment of SVR in 33% to 34% of patients compared with 0% of pa- tients treated with SOC.10,13,20,45,46 Attainment of SVR was highly associated with undetectable HCV RNA at treatment week 8 (week 4 of boceprevir or matching placebo) (ti86%) regardless of treatment group.4,13,45,50 However, a larger proportion of pa- tients met these criteria if treated with boceprevir (46%–52% vs 9% in the SOC arm) .4,13,45,50 Table IV summarizes the results of SVR from RESPOND-2.13
SPRINT-1, SPRINT-2, and RESPOND-2 excluded patients who had liver diseases from other causes, de- compensated cirrhosis, HIV coinfection, hepatitis B vi- rus coinfection, previous organ transplantation, re- nal insufficiency, pre-existing psychiatric disease, seizure disorder, cardiovascular disease, poorly con- trolled diabetes, pregnancy or breastfeeding, hemo- globinopathies, autoimmune disease, active sub- stance abuse, and active cancer.4,13,16,25,26 Future

studies in some of these populations will be critical to decision making in clinical practice. A Phase IIb study involving HIV/HCV coinfected, treatment-na- ive patients is currently evaluating the utility of bo- ceprevir in this population.4,20

Adverse Events
A total of 2098 patients were exposed to at least 1 dose of boceprevir, ranging from a total daily dose of 300 to 2400 mg.52 Most of these patients (91%) re- ceived the currently approved dose of boceprevir (2400 mg/d). The tolerability of boceprevir, in combination with peginterferon and ribavirin, was assessed in 2095 subjects between 1 Phase II trial and 2 Phase III trials, representing a median exposure of 201 days of boce- previr among subjects from these pooled data.18,52 Be- cause boceprevir was studied, and subsequently ap- proved, in combination with peginterferon and ribavirin, it may be difficult to establish a definite cau- sality relationship between this agent and a particular adverse event.
Adverse events occurred in almost 100% of all study patients.25,51 Common adverse effects shared by all treatment groups, regardless of addition of bocepre- vir, include: fatigue (60% vs 53%–57% with bocepre- vir), headache (42% vs 46%), nausea (42% vs 43%– 48%), pyrexia (33% vs 32%–33%), chills (28% vs 33%–36%), insomnia (32% vs 33%), anorexia (25%– 26% vs 16%–24%), diarrhea (24%–25% vs 16%– 22%), and alopecia (22%–27% vs 16%– 27%).4,5,13,18,25,26,37 Rash or other skin disorders were not associated with boceprevir.4,26,43,50
Overall, rates of discontinuation due to adverse drug reactions were higher among patients who re- ceived boceprevir (8%–19% vs 2%–16%; P values, 0.02–0.15).4,25,26,36,45 Dose modifications of pegin- terferon or ribavirin due to adverse drug reactions also were more common among patients who re- ceived boceprevir (39% vs 24%).18 Rates of serious adverse events were similar between SOC and boce- previr regimens (5%–12% vs 9%–14%, respectively; P values, 0.03–0.23).13,24,25,37,43,45 Eight deaths were reported in the completed clinical trials; 2 of these deaths, both suicides, were possibly related to a study drug (most likely peginterferon), with 1 oc- curring in a patient who received boceprevir and 1 occurring in a patient who had not received boceprevir.52

2028 Volume 34 Number 10

Anemia
Anemia was the principal adverse drug effect associ- ated with boceprevir.24 In clinical trials, rates of anemia, defined as a hemoglobin (Hgb) level ti10 g/dL, were higher in patients who received boceprevir (49%–55% vs 20%–34%; P ti 0.001).4,5,7,16,24–26,35,37,38,41,43–45,51,53 The majority of cases were classified as grade 1 or 2 ane- mia (97%).10,25 Grade 3 or 4 anemia (ie, Hgb ti8 g/dL), although rare, was higher in groups treated with bocepre- vir (22 of 734 vs 6 of 363).13,25,26,45 The addition of boceprevir usually results in an additional 1-g/dL de- crease in Hgb levels.25,44
Investigators were allowed to reduce the dose of ribavirin and/or use erythropoietin-stimulating agents (ESAs) at their discretion.10,25,26 The dose of ribavirin was reduced in 13% to 21% of patients taking boce- previr.5,25,45,50 These rates were similar to those of patients who were not taking boceprevir.26 In patients who received boceprevir, there was also an increased utilization of these ESAs (41%–46% vs 21%–24%; P ti 0.001).4,5,7,24,25,43–45,50,53 In SPRINT-2, the mean duration of ESAs was 94 to 156 weeks in patients re- ceiving boceprevir compared with 121 weeks in those treated with SOC.53 Red blood cell transfusions, while rare, were higher among patients treated with bocepre- vir (20 of 734 vs 2 of 363).25,41 These interventions ultimately resulted in low rates of discontinuation due to anemia (0%–3%).7,10,13,24,37,38,45
Interestingly, in some trials, patients who developed anemia also exhibited increased rates of SVR.25,26,44–46 Patients with ti3-g/dL decreases in Hgb were more likely to attain SVR (43% vs 30%; P ti 0.001).16 Anemia may potentially serve as a surrogate marker of increased riba- virin concentrations; hence, this clinical correla- tion.10,16,26 If true, then administration of ESAs may be a preferred intervention in patients who have ane- mia.10,26 However, this association elucidated by SPRINT-1 was not seen in SPRINT-2; SVR rates were similar, regardless of the anemia management strategy used.26,44,54 Furthermore, ESAs are not without risk and are associated with an increased chance of throm- boembolic events.44,54 An ongoing study is evaluating ribavirin dose reduction versus ESA administration in boceprevir regimens.37,44,46

Other Boceprevir-Associated Adverse Drug Reactions
Dysgeusia was the second more frequent adverse drug effect associated with boceprevir.24 Among healthy volunteers, the incidence of dysgeusia in-

creased from ti10% in those receiving boceprevir ti1200 mg/d to ti25% in those who received bocepre- vir 2400 to 3600 mg/d.52 In clinical trials, rates of dysgeusia were higher in patients who received boce- previr (27%–49% vs 9%–29%; P ti 0.001).13,24–26 Dry skin was also reported more frequently in patients treated with boceprevir (21%–22% vs 8%; P values, 0.004–0.009).4,13,25,45
Incidences of grade 1 neutropenia and thrombocy- topenia were also higher with boceprevir (85%–86% and 28%–33%, respectively) compared with placebo (77% and 13%, respectively).25 Rates of grade 3 neu- tropenia (absolute neutrophil count, 500–750 cells/
tiL) was significantly higher in the boceprevir treat- ment groups compared with SOC (24%–25% vs 14%; P ti 0.001).25 Five cases of severe or life-threatening neutropenia were reported collectively from all clinical data, all occurring among patients who received boce- previr; in each case, neutropenia resolved with dis- continuation of all study drugs.52 Three cases of grade 4 thrombocytopenia (platelets ti 25,000/mL) occurred among patients receiving boceprevir com- pared with none among SOC. Nonetheless, these cases did not require additional intervention for management.

Resistance
The development of viral resistance is influenced by
3major factors: the genetic barrier, or number of amino acid substitutions required, to resistance; the in vivo fitness of the viral variant, or how capable it is in filling the replication space left by the wild-type virus; and drug exposure relative to the inhibitory concentra- tion for the viral variant.40,55 Successful treatment is compromised by either incomplete viral suppression during therapy or pre-existing viral variants present before therapy.56
HCV has a high rate of replication (1012 virions daily).5,9,11,15,23,55,57–59 This high rate, coupled with a lack of proofreading by the RNA-dependent-RNA- polymerase (NS5B), results in a high error rate during replication and, consequently, a variety of genetically distinct but closely related strains, known as quasi- species.5,7,57–59,60,61 In genotype 1, the subtype 1a virus seems to have a lower genetic barrier to resistance for protease inhibitors compared with genotype 1b, resulting in greater rates of treatment failure.7,20,40,45,55 This find- ing may be a significant area of consternation because replicon studies have been based on an HCV 1b sub-

type consensus strain.39 The difference between the 2 subtypes can result in the development of different re- sistance profiles. 39,42,50,56 Two nucleotide mutations must occur at position R155K (CGG ¡ AAG) for re- sistance to develop in genotype 1b, whereas only 1 mutation is required in genotype 1a (R155K [AGG ¡ AAG]).20,39,55,62 Hence, in vivo, genotype 1a resis- tance tends to be predominated with R155K/T, V36M, and T54A/S mutations; genotype 1b, on the other hand, most often reveals D168A/V, A156S, T54A/S, and I170A as common mutations.42
Some of these mutations affect the active site of bo- ceprevir, rendering the drug ineffective, without neces- sarily affecting viral substrate binding.5,58,60,63,64 Bo- ceprevir protrudes extensively from substrate envelope regions that correlate with known sites of resistance mutations.61 Many of the primary drug resistance mu- tations can be explained by disruption of atomic inter- actions at the catalytic triad site.40,61 In addition, mu- tations outside of the substrate envelope may also affect the inhibitor’s affinity without altering the bio- logical function of the enzyme.61 Among the host of DAAs being developed, the NS3 protease inhibitors and the NS5A inhibitors seem to have low genetic bar- riers to resistance; single amino acid substitutions re- sult in compromised binding ability of these com- pounds.20,40,62 At present, it is unclear the extent to which mutations within the NS4A cofactor sites may also contribute to resistance.39 Combination therapy with peginterferon greatly reduces the potential for the emergence of resistance, as many resistant variants re- main sensitive to peginterferon.4,7,41,57,65 Mathemati- cal modeling indicates that at least triple DAA therapy, with nonoverlapping resistance profiles, will be re- quired to prevent resistance emergence.20,42,55,62–64,66
Resistance to boceprevir is rare in treatment-naive patients.4,39,56 The effect of IL28 genotype on resis- tance development has yet to be analyzed.35 Naturally occurring resistance is reported to range anywhere from ti1% to 10%.37,55,56,67 For example, there is re- port of a treatment-naive, genotype 1a patient with A156T present with a 0.78% frequency.59,60 Nonethe- less, this small percentage actually equates to a large number of RNA molecules, ti106 to 107 RNA mole- cules per gram of liver carrying this mutation.59 The low incidence of resistance may be related to the repli- cative fitness of these variants.9,40,56,58,61,64 Nonethe- less, it has been seen that some treatment-naive pa- tients are host to a variety of mutations at baseline; for

example, 1 patient was found to have V36A, A156V, and R155S at baseline, whereas another patient in the same study had V36A, A156T, and T54A at base- line.68 Of greater concern, mutants resistant to NS3 protease and NS5B nonnucleoside inhibitors have been detected as the pre-existing dominant strain in a small percentage of patients (0.2%–2.8%), which may have significant implications in the future of HCV treatment.55–58
Viral resistance develops rapidly with NS3 protease inhibitor monotherapy, even within the first days of treatment.5,42,55,57 The initial drop in HCV RNA is attributable to inhibition of wild-type virus, which leads to the uncovering of pre-existing resistant vari- ants that cause treatment failure with subsequent cross-resistance.37,39,40,56,60,67 Single mutations may be selected initially and then replaced by more fit com- bination mutants.60 Mutations conferring resistance to NS3 protease inhibitors have been selected in vitro, with these same mutations observed in clinical trial patients as well.58,65 A156T confers the highest level of resistance to boceprevir.35,39,58,59,65,69 However, rep- licons carrying the A156T mutation were significantly less fit than replicons with other mutations.5,65,70 Overall, there is an inverse correlation between resis- tance potential and variant fitness (ie, the 156 mutation had the lowest relative fitness). 39,55,64 In vitro replicon fitness of various mutants follows this order: A156T ti R155K ti V36M ti T54A.58 However, some muta- tions, such as Q41R, significantly increase replicon fit- ness. All of the following mutations confer low to me- dium levels of resistance to boceprevir: V36M/A, T54A/S, V55A, R155G/K/L/T, A156S, and V170A/
T.35,36,39,45,55,58,59,63,64,69 However, some variants that show low resistance in vitro may be more damag- ing in vivo.40 Novel mutations with unknown clinical significance at present have also been isolated at V48I, V55A, T72I, and I153V.39
Overall, with boceprevir monotherapy, mutations have been detected at the following positions: 7, 8, 14, 18, 33, 36, 46, 48, 49, 54, 55, 61, 71, 72, 80, 86, 87, 91, 117, 119, 122, 127, 131, 146, 147, 153, 155, 156, 170, 174, and 179.39,45,63–65 In a previous nonre- sponse patient treated in a Phase Ib trial, the following mutations were detected after collecting ti90 clones: V36I, Q41R, F43S/C, T54A, V170A (before treatment with boceprevir), and A156S (and the end of follow- up).56 Major mutations observed in Phase II trials in- cluded R155K (64%), T54S (54%), and V36M

(54%).4,10,26,50 Mutations observed in ti25% of the obtained samples included T54A, V55A, R155T, A156S, V158I, and V170A.10,26 In clinical trials, resis- tant variants were found in 28% to 47% in patients who had a ti1-log10 decrease in HCV RNA after SOC lead-in, compared with only 4% to 8% in patients who had a ti1-log10 decrease in HCV RNA.7,10,13,24,45 Nearly 100% (107 of 109) of patients who developed variants with reduced susceptibility to boceprevir ex- perienced nonresponse, relapse, or breakthrough.27
The fold increase in resistance with double muta- tions was approximately equal to the multiplicative product of individual mutations.58,65 However, the replicon fitness of these mutants was typically lower than those of single mutants (ie, V36M ti A156T).58 Nonetheless, double mutations observed after Phase Ib trials with boceprevir included T54S ti R155K and V36M ti R155K.57 These double mutations, and oth- ers seen in vitro (A156T ti P89Q, A156T ti Q86P), may be the evolution of resistant variants coupled with compensatory mutations to confer higher replicative fitness and thus result in long-term persistence at higher frequencies.40,55–57,59 It is possible that the R155K substitution present in conjunction with other substitutions may allow improved fitness. 39,56 A sec- ond mutation would improve relative fitness (V36A ti R155K).39
In patients failing to improve with triple therapy, the dominant viral population at time of breakthrough or relapse was resistant to the protease inhibitor, with subsequent disappearance of this variant as it was re- placed by the wild-type virus over time without drug selection pressure.4,5,24,40,50,57,60,63 This finding rein- forces the importance of adhering to treatment futility recommendations for DAAs in patients who have in- adequate viral responses as the development of treat- ment-emergent resistance is expected if therapy were to continue. Triple HCV therapy with boceprevir should be discontinued in all patients with either HCV RNA levels ti100 IU/mL at treatment week 12 or detectable HCV RNA levels at treatment week 24.18 Similarly for telaprevir, HCV therapy should be discontinued in all patients with either HCV RNA levels ti1000 IU/mL at treatment week 4 or week 12 or detectable HCV RNA levels at treatment week 24.19 Without the drug selec- tion pressure, the reversion to wild-type virus took place within a few weeks to months, depending on the type of mutation; however, some variants may persist for even longer.4,5,24,37,50 For example, the median

time for reversion for V36M was 0.5 year, the median time for R155K was 1.19 years, and the median time for T54S was 1.43 years.4,5,24,50 It is likely that these variants remain at low levels, beyond the limit of de- tection, potentially limiting future treatment op- tions.5,24 However, to date, none of these patients has been rechallenged with an alternative NS3 protease inhibitor.50 Interestingly, the mutational pattern at long-term follow-up was not easily predicted from end-of-treatment analyses, as mentioned earlier.56,57 Because HCV is a RNA virus reproducing entirely in the cytoplasm, no true archiving takes place, as with retrovirus HIV, so it may be possible that resistant variants are completely cleared due to competition with more fit, drug-sensitive viruses.40
Key mutations in boceprevir exhibit cross-resistance with other agents.5,39,40,49 The most frequent telapre- vir substitution is R155K.35,62,70 Other mutations that have been seen in clinical trials include V36A/M, R155K, T54A, A156S/T/V, and V36M ti R155K.50,56 Similar to boceprevir, A156T/V or double mutations at position 36 ti 155 or 36ti 156 confer high-level resis- tance to telaprevir, and V36A/M, T54A, R155K, and A156S confer low-level resistance to telapre- vir.7,39,56,58,69 However, Q41R, F43C/S, and V170A affect boceprevir more than telaprevir.39,58 These dif- ferences are probably due to the structural differences between the 2 agents, affecting van der Waals interac- tions differently with the enzyme.58 This is important as the development of other DAAs continue. A156T also confers a high level of resistance to SCH 900518 (narlaprevir), with additional resistance noted with the rather benign (to boceprevir) mutations R155K and A156S/T.35,65 Although boceprevir remained fully ac- tive against mutations at D168, the mutations D168V and D168I led to significant decreases in the activity (ti2000-fold) of TMC 435 (medivir), BI 201335, and ITMN 191 (danoprevir).40,56,63,69,70 Reduced suscep- tibility was seen in MK 7009 (vaniprevir) with the fol- lowing mutations: F43S, Q41R, R155K, A156T, and D168Y.56,63,69 HCV nucleoside inhibitors, which seem to possess a higher genetic barrier to resistance, show less cross-reactivity with NS3 protease inhibi- tors.66 NM 107 had unaltered activity against HCV replicons carrying boceprevir resistance (A156T/S, V170A, and T54A), and boceprevir had unaltered ac- tivity against HCV replicons carrying nucleoside resis- tance (S282T).66 However, resistance to MK 5172 has been noted at position 156.40 Currently, resistance

testing at baseline is not recommended in clinical prac- tice because there are insufficient data to determine the cost-effectiveness of such measures.55,63 With future combination regimens with DAAs, this standard of practice may change.

Drug Interactions
Boceprevir is metabolized via CYP3A4/5 and the aldo-keto reductase pathway, specifically AKR1C2 and AKR1C3.6,20,34,44 Boceprevir is also a substrate of p-glycoprotein.6,53 A brief summary of potential drug interactions with boceprevir is provided in Table V.6
In vitro studies with known CYP3A4/5 inhibitors, such as ketoconazole, described the decreased forma- tion of boceprevir metabolites by 36% to 68%.4,34 In

vitro studies exhibited 100% inhibition of boceprevir with ritonavir, with a small increase in boceprevir plasma trough concentrations (4%); however, overall boceprevir drug exposure was decreased by 19%. 6,34,71 Likewise, potent inducers of CYP3A4/5, such as St. John’s wort or rifampin, should also be avoided to reduce the potential of decreased boceprevir levels and subsequent treatment failure.6,50 The coadministration of boceprevir and efavirenz resulted in decreased boce- previr plasma trough concentrations by 44% and de- creased drug exposure by 19%, hence precluding this combination in HCV/HIV coinfected patients at present.6,50,65,71
Boceprevir additionally acts as a strong, reversible inhibitor of CYP3A4 and p-glycoprotein.4,6,44,50 Hence, current recommendations contraindicate the coadministration of boceprevir with drugs, such as simvastatin or certain immunosuppressive agents, that

Table V. Potential drug interactions with boce- previr.6

Contraindicated
Anticonvulsants (carbamazepine, phenobarbital, phenytoin) Antimycobacterials (rifampin, rifabutin) ti-receptor agonists (salmeterol)
HMG-CoA reductase inhibitors (simvastatin, lovastatin)
PDE-5 inhibitors, in pulmonary hypertension (sildenafil, tadalafil)
Caution
Antiarrhythmics (amiodarone, flecainide) Antidepressants (trazodone)
Antifungals (ketoconazole, itraconazole, posaconazole, voriconazole)
Anxiolytics (alprazolam) Corticosteroids, inhaled (budesonide, fluticasone)
Dihydropyridine calcium channel blockers (nifedipine, felodipine, nicardipine)
HMG-CoA reductase inhibitors (atorvastatin) Immunosuppressants (cyclosporine, tacrolimus) Opioids (methadone, buprenorphine)
PDE-5 inhibitors, in erectile dysfunction (sildenafil, tadalafil, vardenafil) Warfarin

HMG-CoA ti 3-hydroxy-3-methylglutaryl coenzyme A; PDE-5 ti phosphodiesterase type 5.
are highly dependent on CYP3A4/5 for clearance and in which high levels are associated with serious or life- threatening effects.6,50,72
Because biotransformation is conducted via 2 differ- ent major enzyme pathways, boceprevir may be less affected by CYP3A4 interactions.34 In studies con- ducted with aldo-keto reductase inhibitors, such as ibuprofen, boceprevir exposure was not significant de- spite the inhibition of metabolites by 55% to 91%.6,34 This finding may possibly be related to the fact that various isoforms of aldo-keto reductase are present in multiple tissues, resulting in decreased saturation.6
Aside from pharmacokinetic interactions, certain pharmacodynamic interactions must be considered as well. Zidovudine should be avoided due to an in- creased potential for anemia.6,73 Certainly, because boceprevir is currently indicated to be given with peginterferon and ribavirin, this remains a strong consideration. Medications such as efavirenz, which might have cumulative psychiatric effects with peginterferon, may be an additional area of pharma- codynamic concern.73

Dosage and Administration
Boceprevir is available as 200-mg capsules.18 The recommended dosage of boceprevir is 4 capsules (800 mg) taken TID (every 7–9 hours). The dose of bocepre- vir should not be adjusted to manage adverse drug reactions.18,44 In clinical trials, depending on the sever- ity of adverse event, investigators were able to adjust the dose of peginterferon alfa-2b (hold for up to

2 weeks or 2-step reduction to 1 and 0.5 tig/kg) or ribavirin (step-wise 200 mg reductions).13,26 Patients with contraindications to or toxicity associated with peginterferon or ribavirin should not receive bocepre- vir as monotherapy.44,74 Each dose of boceprevir should be taken with food because of an increased AUC by up to 65%.18 Adherence to time interval is especially important in treatment-experienced pa- tients, in whom ti60% adherence to time intervals re- sulted in decreased rates of SVR.75
The clinical trials with boceprevir included a 4-week lead-in period with peginterferon alfa-2b and ribavi- rin.26 There is a recently completed trial using boceprevir with peginterferon alfa-2a, the results of which are ex- pected to be similar.4,24 The theory behind this choice was to allow both backbone components to reach opti- mum steady-state concentrations, resulting in a signifi- cant HCV viral load reduction, with the intention of therefore reducing the likelihood of the emergence of drug-resistant mutations.7,10,26,35,36,38,45,67 However, the true benefit of this lead-in strategy may be its utility as an early on-treatment predictor of response to guide consideration for the addition of DAAs.7,10,20,24,25,44 Moreover, this lead-in period can provide some indi- cation of patient tolerability and adherence.16,25
The results of SPRINT-2 suggest that non-black pa- tients with RVR can be treated with peginterferon alfa-2b and ribavirin without compromising chances of SVR (albeit, at the cost of a longer course of treat-

ment but with the potential benefit of decreased ad- verse effects and health care costs).16,46 Data from pre- vious work also suggest that treatment-naive patients with IL28B C/C genotype, indicating favorable re- sponse to SOC, could potentially be treated similarly by using dual therapy.16
However, the new treatment paradigm for chronic HCV genotype 1 infection is RGT. RGT is based on the lower limit of detection, not the lower limit of quanti- tation; the 2 terms are not synonymous, and further analysis suggests that differences between the measures are clinically relevant.76 In treatment-naive patients without cirrhosis, HCV RNA undetectable at weeks 8 and 24 results in 28 weeks of therapy (24 weeks of triple therapy).10,16,24,25,44,50,53 For treatment-naive patients whose levels remain detectable at week 8 but are undetectable at week 24, triple therapy continues until week 36 (32 weeks of triple therapy), followed by SOC through week 48 (12 additional weeks of pegin- terferon and ribavirin).10,25,44,50,53 Table VI provides an algorithm summarizing RGT.18
In treatment-experienced patients (previous partial re- sponse or previous relapse), patients receive 36 weeks of treatment (32 weeks of triple therapy) if HCV RNA are undetectable at weeks 8 and 24.10,13,16,24,50,53 Similarly, for treatment-experienced patients whose levels re- main detectable at week 8 but are undetectable at week 24, triple therapy continues until week 36 (32 weeks of triple therapy), followed by SOC through week 48

Table VI. Response-guided therapy treatment algorithm.18

Treatment Response (HCV RNA)
Treatment
Patient Group Week 8 Week 12 Week 24 Duration*

Treatment-naive ND ND ND 28 weeks†
Detectable ti100 IU/mL ND 48 weeks†
Prior partial response/prior relapse ND ND ND 36 weeks*
Detectable ti100 IU/mL ND 48 weeks†

Compensated cirrhosis

HCV ti hepatitis C virus; ND ti nondetectable.
48 weeks†‡

*All treatment regimens begin with a 4-week lead-in period with peginterferon and ribavirin, without boceprevir. Peginterferon and ribavirin are continued, with boceprevir, for the remaining duration of treatment. Boceprevir is stopped at week 36 if treatment duration is 48 weeks.
†If HCV RNA ti100 IU/mL at week 12, triple therapy should be discontinued. If HCV RNA detectable at week 24, triple therapy should be discontinued.
‡Boceprevir is continued for 44 weeks after the 4-week lead-in period with peginterferon and ribavirin.

(12 additional weeks of peginterferon and ribavi- rin).10,13,24,50,53 All patients with compensated cir- rhosis are treated for 48 weeks (44 weeks of triple therapy).13 Discontinuation of boceprevir is recom- mended in all patients with HCV RNA ti 100 IU/mL at treatment week 12 or detectable HCV RNA at week 24.13,18,25
Further information regarding dosing in the following populations is necessary: HIV coinfection, pre- and post- transplant patients, low platelets, decompensated liver disease/cirrhosis, renal failure, and pediatrics.16,18,43,74

Pharmacoeconomic Considerations
The pharmacoeconomics of boceprevir therapy have been examined in the published literature.53,77,78 Cammà et al77 created a Markov decision model to assess the cost-effectiveness of triple therapy for HCV (peginterferon-alfa, ribavirin, and boceprevir or tel- aprevir) versus SOC (peginterferon, ribavirin) dual therapy in treatment-naive patients with genotype 1 viruses. Their model consisted of treatment-naive white patients aged 50 years, weighing 70 kg, with genotype 1 HCV, and a Metavir score of F2 for liver fibrosis over a time horizon of 20 years. The Markov decision model used 5 competing strategies: (1) boceprevir RGT; (2) bo- ceprevir IL28B genotype–guided therapy; (3) boceprevir RVR-guided therapy; (4) telaprevir RGT; and (5) telapre- vir IL28B genotype–guided therapy, whereby the IL28B genotype was a predictor of virologic response to ther- apy. Outcomes analyzed included life-years gained (LYG), costs (in 2011 euros), and incremental cost-effec- tiveness ratio. According to Cammà et al, the most effec- tive and cost-effective strategies were boceprevir RVR- guided therapy and telaprevir IL28B genotype–guided therapy, which resulted in 4.04 and 4.42 LYG and incre- mental cost-effectiveness ratios of €8.304 per LYG and

€11.455 per LYG, respectively. This model was highly sensitive to IL28B genotype, likelihood of RVR and SVR, and prices of boceprevir/telaprevir. Cammà et al con- cluded that triple therapy was cost-effective when com- pared with dual therapy.
The discount rate is the pharmacoeconomic method used to convert future clinical benefits into present value. Because the clinical benefit of HCV treatment may take place years after therapy, another study78 examined the long-term benefits of dual therapy (SVR set at 40%) compared with triple therapy (SVR set at 70%) in simulation models with a choice of different values for yearly discount rates. Messori el al78 accom- plished this by measuring the long-term benefits under different conditions based on their standard model and used quality-adjusted life-years (QALYs) to quantify the benefits. The gain they saw with triple versus dual therapy decreased from 0.45 QALYs with a 0% dis- count rate to 0.22 QALYs with a 6% discount rate. As expected, Messori et al found that varying the discount rate affected the magnitude of estimated benefit and therefore should be analyzed closely when determining the value-based price for the newer HCV treatments.
Tungol et al53 identified factors that decision makers and payers should consider before making a formulary choice of which protease inhibitor to include. In addition to SVR rates compared with SOC, decision makers should consider increased treatment costs, patient man- agement and adherence, comparative safety and efficacy, and appropriate utilization of management controls.53
Currently, there are 2 protease inhibitors available as part of a triple-therapy regimen for patients with geno- type 1 chronic HCV infections: boceprevir and telapre- vir.18,19 The 2011 average wholesale prices (in US $) for a course of each agent are listed in Table VII.79 The cost for a course of boceprevir therapy varies based on length

Table VII. Average wholesale price (AWP) of boceprevir and telaprevir treatment courses.79

Generic (Brand) Name Regimen Acquisition Cost (AWP, 2011 US $)

Boceprevir* 800 mg PO q8h ti 24 weeks 31,671.36
Boceprevir 800 mg PO q8h ti 32 weeks 42,228.48
Boceprevir 800 mg PO q8h ti 44 weeks 58,064.16
Telaprevir† 750 mg PO q8h ti 12 weeks 59,038.56

*Trademark: Victrelis® (Merck & Co, Inc, Whitehouse Station, New Jersey). †Trademark: Incivek® (Vertex Pharmaceuticals Inc, Cambridge, Massachusetts).

of therapy according to RGT (range, $31,671.36–
$58,064.16). A full course of therapy for telaprevir is always 12 weeks ($59,038.56). To date, there are no head-to-head comparisons of the 2 available agents. As described in the previous pharmacoeconomic stud- ies,53,77,78 in addition to drug costs, multiple other factors must be considered to justify the cost-effectiveness of therapy. For example, patient adherence (a difficult mea- sure even in clinical trials) is a crucial factor to consider because nonadherence to these agents will lead to treat- ment failure and resistance development. It will be pru- dent to believe that successfully treating eligible patients with these new DAAs will not only result in clinical ben- efits but cost savings from reduced morbidity as well. At this time, additional pharmacoeconomic studies are nec- essary to examine this further.

DISCUSSION
A paradigm shift in the management of chronic HCV infection occurred with the introduction of com- pounds that were specifically targeted as antiviral ther- apy for hepatitis C.6,20–22 Since 2001, the SOC with peginterferon and ribavirin only offered low cure rates (SVR range, 30%–50%) for patients infected with ge- notype 1 HCV, the predominant genotype seen in the United States.4–7,13,15 With the addition of bocepre- vir to the HCV armamentarium, the SVR rates have been increased to ti 70% for treatment-naive and treat- ment-experienced patients.13,25 With ti50 DAAs at varying stages of development, boceprevir offers a glimpse of hope for what the next few years may offer in HCV management.20 Boceprevir is a potent inhibitor (Ki of 14 nM18,27,28) of NS3 protease indi- cated for the treatment of chronic HCV genotype 1 infection in combination with peginterferon and ribavirin for treatment-naive patients and patients who failed to improve with previous interferon and ribavirin treatment.18
Results of Phase III clinical trials allowed for impor- tant insights in the utilization of boceprevir. In addi- tion to higher SVR rates with boceprevir triple therapy, SPRINT-21 and RESPOND-22 used a 4-week lead-in phase with peginterferon and ribavirin. The lead-in phase is crucial in decreasing viral relapse, decreasing emergence of resistance, predicting patients’ likelihood of SVR, weighing the risks and benefits of continuing therapy, and addressing adherence.7,10,20,26,35,67 With the exception of compensated cirrhotic patients (44 weeks of triple therapy after a 4-week lead-in), RGT is

indicated for most patients eligible for boceprevir ther- apy, allowing for a chance of shorter therapy dura- tion.13,18 Treatment futility rules at weeks 12 and 24 are important to follow to avoid unnecessary drug ex- posure and adverse events.
Boceprevir was well tolerated in clinical trials; how- ever, rates of anemia (defined as Hgb ti10 g/dL) and dysgeusia were higher in patients who received boce- previr (49%–55% vs 20%–34% [P ti 0.001]; 27%– 49% vs 9%–29% [P ti 0.001]).13,24–26
Boceprevir should not be used in nongenotype 1 HCV infections or in patients with contraindications to peginterferon or ribavirin therapy. Further informa- tion regarding dosing in the following populations is necessary: HIV coinfection, pre- and posttransplant patients, low platelets, decompensated liver disease/
cirrhosis, renal failure, and pediatrics.
Precautions for drug interactions will continue to grow as new studies are available because boceprevir is metabolized via CYP3A4/5 and the aldo-keto reduc- tase pathway and is also a substrate of p-glycopro- tein.6,20,34,44,53 Drug–drug interactions with bocepre- vir and many other drugs have not yet been fully elucidated. Practitioners should continue to seek out additional information as more data become available. The close management of adherence, adverse drug events, and drug interactions will become increasingly important as complexities arise, clearing an excellent niche for specialists with expertise in the management of HCV.

CONCLUSIONS
Boceprevir is a covalent reversible inhibitor of the HCV NS3 serine protease indicated for the treatment of chronic HCV genotype 1 infection in combination with peginterferon and ribavirin. After almost a decade of no new HCV drugs since the introduction of pegy- lated interferon in 2001, boceprevir is a welcome ad- dition in our battle against HCV eradication. It is cur- rently 1 of only 2 FDA-approved DAAs available to treat adult (age ti18 years) patients with compensated liver diseases who are either treatment naive or previ- ously failed to improve with interferon and ribavirin therapy.

ACKNOWLEDGEMENTS
All authors contributed equally to the literature search, data interpretation, figure creation, and writing of the manuscript.

CONFLICTS OF INTEREST
The authors have indicated that they have no conflicts of interest regarding the content of this article.

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Address correspondence to: Mei H. Chang, BA, PharmD, BCPS, Pharmacy Service, James J. Peters Veterans Affairs Medical Center, 130 West Kings- bridge Road (119), Bronx, NY 10468. E-mail: [email protected]