Marie-Louise Vachon, M.D., M.Sc.; Douglas T. Dieterich, M.D.
Posted: 02/15/2012; Semin Liver Dis. 2011;31(4):399-409. © 2011 Thieme Medical Publishers
Abstract and Introduction
Abstract
The year 2011 marks the dawn of the new era of direct-acting antivirals for hepatitis C. For the first time since 1998, the U.S. Food and Drug Administration approved two new antiviral drugs for the treatment of chronic hepatitis C virus genotype 1. Dual therapy with pegylated interferon and ribavirin is no longer the standard of care for genotype 1. The new treatment paradigm includes one direct-acting antiviral, a protease inhibitor, in combination with pegylated interferon and ribavirin. This combination nearly doubles the chances of response to treatment, but at the cost of increased toxicity. Many agents with different mechanisms of action and improved safety profiles are in clinical development. The holy grail of HCV treatment is an all oral, interferon-free treatment. The ideal regimen will be potent, well tolerated, with minimal drug-drug interactions and once daily. This article covers new concepts of treatment of hepatitis C with DAAs and gives an overview of the recent highlights in direct-acting antiviral development.
Introduction
In May 2011, telaprevir and boceprevir were approved by the U.S. Food and Drug Administration (FDA) for treatment of chronic hepatitis C virus (HCV) genotype 1. Dual therapy with pegylated interferon (pegIFN) and ribavirin (RBV) is no longer the standard of care for genotype 1 HCV. The new treatment paradigm includes one direct-acting antiviral (DAA), a protease inhibitor (PI), in combination with pegIFN and RBV. The addition of a DAA to pegIFN/RBV nearly doubles the chances of response to treatment at the cost of increased toxicity. This is only the first wave of DAA use since many agents with different mechanisms of action and improved safety profiles are in phase I, II, and III of clinical development (Table 1 and Table 2). The use of three agents to treat HCV mirrors human immunodeficiency virus (HIV) triple combination treatment in many ways. Combination treatment seems inevitable to prevent emergence of resistance in HCV. Clinical trials are ongoing to identify the ideal regimen which would be potent, well tolerated, with minimal drug-drug interactions, once daily, all oral, and for as short a duration as possible (Table 3). Two major differences between HIV and the treatment of HCV are that treatment of HCV is for a definite duration and that HCV is curable. In this article, we examine new concepts of treatment of HCV with DAAs in general and review investigational compounds that have entered phase II of clinical development.
New Concepts of DAA Use
Several new concepts arose with the development of DAAs to treat hepatitis C. Because DAAs are true antivirals that target critical steps of HCV replication, similar to antiretrovirals inhibiting HIV, selection of resistant mutants is inevitable with monotherapy.[1] The HCV replication cycle and the different sites at which DAAs can interfere with HCV replication are thoroughly reviewed elsewhere.[2,3] RNA viruses like HCV inherently possess an error-prone RNA-dependent RNA polymerase that lacks the proofreading function. As a result, and with the rapid HCV viral turnover (up to 1012 virions produced each day), it is estimated that one mutation is contained in every single genome copied.[4] There are thus many variant populations coexisting in a given individual, the patient's quasispecies. Most of these variant populations are susceptible to DAAs because wild-type viruses usually have the advantage of fitness, but some of these preexisting variants are drug-resistant at baseline.[5,6] When selection pressure is applied with the use of a DAA in monotherapy, these preexistent variants are rapidly selected and can become the predominant circulating population, potentially leading to treatment failure.[7–9] Resistance profiles differ between drug classes. Some drugs have different resistance mutations within the same class. A new concept that emerged as part of PI development is the difference in resistance profile between genotype 1a and genotype 1b. For example, the R155K substitution typically emerges when HCV genotype 1a is exposed to telaprevir.[8] Only one nucleotide change is required for the 1a subtype to develop resistance whereas two nucleotide changes must occur in genotype 1b.[10] This is not specific to PI use and has been described with other drug classes, for example, nonnucleoside polymerase inhibitors.[10] As a result, antiviral responses can vary between HCV genotype 1 subtypes during treatment with DAAs, while response was similar when treated with pegIFN and RBV dual therapy.
Response-guided therapy (RGT) refers to the use of on-treatment virologic response to tailor the duration of therapy for an individual patient.[11] This concept was used with pegIFN/RBV treatment but really emerged in the era of DAAs. Both telaprevir and boceprevir can be used for a shorter treatment duration in patients who achieve HCV RNA undetectability early on in treatment.[12,13] This is RGT. In the telaprevir studies, extended rapid virologic response (eRVR) was used to determine if RGT could be used.[12,14] The definition of eRVR was HCV RNA <10 IU/mL at week 4 and 12. In the boceprevir studies, RGT was used when HCV RNA was less than 9.3 IU/mL at week 8 through week 24.[13] Most phase II and III clinical trials of DAAs are exploring this possibility of shortening treatment duration in patients who achieve eRVR.
The concept of lead-in is also widely discussed in the DAA literature. Lead-in is the use of pegIFN and RBV for a short duration preceding the initiation of DAA. Lead-in was initially introduced in an attempt to lower HCV RNA levels before PI exposure to minimize the emergence of resistance. Also, by achieving proper pegIFN and RBV drug levels before the initiation of the DAA, it would avoid functional monotherapy. Studies of boceprevir used a 4-week lead-in of pegIFN and RBV. It turns out the lead-in was most useful in the assessment of interferon responsiveness. In the SPRINT-2 study evaluating boceprevir in combination with pegIFN/RBV in previously untreated patients with chronic HCV genotype 1, a decrease in the HCV RNA level by ≥1 log10 IU/mL after the 4-week lead-in increased the chances of sustained virologic response (SVR; undetectable HCV RNA 24 weeks after the end of treatment) ninefold (adjusted odds ratio [AOR], 9.0; 95% confidence interval [CI], 6.3–12.8; P <.001).[13] There was no advantage to lead-in when used with other DAAs, for example, with telaprevir and BI 201335.[14,15]
Lower Limit of Detection (LLOD) Versus Lower Limit of Quantification (LLOQ)
The LLOD is the HCV RNA concentration at which less than 5% of the samples that contain a known amount of an RNA standard yield a signal that can be detected. The LLOQ refers to the lowest HCV RNA concentration that is within the validated quantitative range of an assay. An undetectable result indicates that HCV RNA was not detected in the sample. The limit of detection and the limit of quantification vary according to the assay used. Both the boceprevir and telaprevir phase III studies used the COBAS TaqMan HCV RNA assay, version 2.0 (Roche), with a LLOD of 10 IU/mL and a LLOQ of 25 IU/mL. Eligibility for RGT was based on the lower limit of detection (<10 IU/mL) of the assay. There is thus a range in which HCV RNA can be detectable, but falls below the level of quantification. This situation is not uncommon during HCV treatment and is associated with lower SVR rates. In the SPRINT-2 trial, 53% of the 1071 patients enrolled had at least one on-treatment result showing detectable HCV RNA, but below the level of quantification.[13] For example, in the patients who received boceprevir and who had detectable HCV RNA at week 6 (any level), detectable HCV RNA at week 8 but below level of quantification, and undetectable HCV RNA at week 10, the SVR rate was 74%. In the patients who received boceprevir and who had detectable HCV RNA at week 6 (any level) but undetectable HCV RNA at weeks 8 and 10, the SVR rate was 86%. When assessing eligibility to response-guided therapy with DAAs, an undetectable HCV RNA (<LLOD) is not equivalent to a detectable HCV RNA that is below the limit of quantification of the assay (detectable but <LLOQ). The package inserts of telaprevir and boceprevir reflect this fact.
NS3/4A PIs
HCV NS3/4A PIs are often divided in two classes. The first generation PIs include the linear α-ketoamide derivatives, boceprevir and telaprevir. They bind the catalytic site of the enzyme covalently in a reversible reaction. Boceprevir and telaprevir are considered the first wave of the first-generation PIs. These two PIs are the first DAAs to have completed phase III clinical trials for treatment of HCV genotype 1 and to have received FDA approval. Second-wave PIs are mostly linear and macrocyclic noncovalent inhibitors of the NS3/4A enzyme. Both waves are highly potent inhibitors of the NS3/4A enzyme. The advantages of second wave PIs over first wave PIs are their convenience and improved side effect profile. Unfortunately, they share the same basic resistance mutations that are generated by the first wave of PIs. The truly second-generation PIs are the two drugs MK-5172 and the ACH-2684. They do not share the same resistance mutations and are pangenotypic. There are currently two first-generation, second-wave PIs that recently initiated phase III of clinical development: TMC435 and BI-201335.
TMC435
In phase I studies, TMC435 was generally safe and well tolerated. It showed potent anti-HCV activity. After 5 days of TMC-435 at a dose of 200 mg once daily, HCV RNA decreased by a median of 3.9 log10 IU/mL in HCV genotype 1-infected patients who failed prior interferon-based therapy.[16] The phase IIb PILLAR study is ongoing and week 24 results have recently been presented.[17] PILLAR is investigating the use of once-daily TMC435 at two different dosages (75 mg vs 150 mg) for 12 versus 24 weeks in combination with pegIFN/RBV for 24 vs 48 weeks (vs placebo/pegIFN/RBV for 48 weeks), in 386 patients with HCV genotype 1 naïve to HCV treatment. Patients with HCV RNA less than 25 IU/mL from week 4 to week 20 were eligible for RGT and stopped treatment at week 24. Of the patients receiving TMC435, 83% achieved eRVR and 94 to 97% achieved undetectable HCV RNA at week 24 of treatment compared with a high 82% in the pegIFN/RBV control group. Of the 83% of patients who achieved eRVR and stopped therapy at week 24, 88 to 97% achieved SVR12 (undetectable HCV RNA 12 weeks after the end of treatment). Discontinuation of TMC435 or placebo occurred in 7.1% of patients receiving TMC435/pegIFN/RBV versus 7.8% of patients receiving placebo/pegIFN/RBV. The most common adverse events occurred in a similar proportion of patients receiving or not receiving TMC435. TMC435 at the 150 mg once-daily dose was associated with mild and reversible increases in direct and indirect bilirubin. Elevation in bilirubin is thought to occur through inhibition of the two transporters: organic anion transporting polypeptide 1B1 (OATP1B1) and multidrug resistance-associated protein 2 (MRP2). OATP1B1 is responsible for bilirubin uptake into hepatocytes (influx) and MRP2 is responsible for efflux into bile. No inhibition of bilirubin conjugation has been observed.[18]
The ASPIRE study is the ongoing phase IIb study in prior partial responders (≥2 log10 drop in HCV RNA at week 12 but detectable HCV RNA at week 24), prior null responders (<2 log10 drop in HCV RNA at week 12 of treatment) and prior relapsers (undetectable HCV RNA at the end of treatment, but detectable HCV RNA within 24 weeks of end of treatment) to interferon-based therapy. Patients received TMC435 at doses of 100 mg versus 150 mg once daily in combination with pegIFN/RBV for 12, 24, or 48 weeks. In all seven arms of the trial, patients received pegIFN/RBV to complete 48 weeks of treatment. There was no RGT. The week 24 interim analysis showed that 92 to 96% of prior relapsers who received TMC435 had undetectable HCV RNA at week 24 compared with 83% of patients in the control group. Of the prior partial responders receiving TMC435, 83 to 89% had undetectable HCV RNA at week 24 versus 20% of the control group. Of the prior null responders, 70 to 87% had undetectable HCV RNA at week 24 versus 45% of the control group (higher than expected). The prior null responders typically have the lowest response rates to retreatment. The end of treatment and SVR data are eagerly awaited.
In summary, TMC435 is a highly potent once-daily dosing PI. Duration of treatment can be shortened to 24 weeks for the majority of patients; viral breakthroughs are low when used in combination with pegIFN and RBV. The 150 mg once-daily dose is being used in phase III trials in both patients who are naïve to HCV treatment and patients who previously relapsed to an interferon-based therapy. Patients will receive once-daily 150 mg of TMC435 during 12 weeks in combination with pegIFN/RBV for a 24 versus 48 weeks. A trial for HIV-infected patients has started enrollment in the second half of 2011.
BI 201335
BI201335 is a HCV NS3 PI given once daily currently in phase III. The phase IIb SILEN-C1 trial was conducted in patients with HCV genotype 1 naïve to HCV treatment to evaluate safety and efficacy of BI 201335 given once daily at a dose of 120 mg or 240 mg for 24 weeks in combination with pegIFN and RBV for 24 vs 48 weeks.[14] A lead-in of pegIFN and RBV for 3 days was also evaluated in 2 of the 4 arms (with 120 mg and 240 mg once daily). In the two arms using BI 201335 at a dose of 240 mg (with and without a lead-in), patients achieving eRVR were rerandomized to either stop treatment at week 24 or continue with pegIFN/RBV for a total of 48 weeks. Patients receiving 240 mg once daily without a lead-in achieved the highest eRVR rate of 87% and were thus eligible for shortened treatment duration. As expected with this high eRVR rate, this arm also had the highest SVR rate of 83% versus 73% of patients receiving 240 mg with a lead-in and 56% of patients in the pegIFN/RBV control group. Prolonging treatment to 48 weeks in those patients achieving eRVR did not result in higher SVR rates. Of those who completed 24 weeks, 93% achieved SVR versus 90% of those who completed 48 weeks. Viral breakthroughs occurred in 2.8 to 5.8% of patients receiving BI 201335 with the highest rate in those of the 120 mg daily with lead-in arm.
The phase IIb SILEN-C2 trial evaluated BI 201335 for 24 weeks in combination with pegIFN/RBV for 24 versus 48 weeks, with or without a 3-day lead-in of pegIFN/RBV in previous partial and null responders infected with HCV genotype 1.[19] The 240 mg once-daily dose (with and without a lead-in) was compared with 240 mg twice daily with a lead-in. Patients of the 240 mg once-daily group with lead-in achieving eRVR were rerandomized to stopping therapy or continuing 48 weeks with pegIFN/RBV. Similar to the SILEN-C1 trial, the lead-in did not appear to be useful. The 240 mg once-daily dosing without a lead-in led to the highest SVR rates. Overall, eRVR was achieved by 45% of patients and SVR was achieved by 27 to 41% of patients. The lowest SVR rate was observed in the 240 mg once daily with the lead-in arm, the one group that used RGT for those achieving eRVR. In comparison to the good results observed with 24 weeks of treatment in the naïve patients achieving eRVR in the SILEN-C1 trial,[14] prior partial and null responders achieving eRVR in SILEN-C2 achieved lower SVR rates when stopped at week 24. Only 40% of patients achieved SVR when stopped at week 24 compared with 72% of those who completed 48 weeks of treatment. The additional 24 weeks of peg/RBV greatly impacted the relapse rate. Sixty percent of those who stopped at week 24 relapsed compared with 21% of those who completed 48 weeks of treatment. Viral breakthroughs occurred predominantly on BI 201335 compared with peg/RBV (17–28% vs 5–7%). Several adverse events were reported in a higher proportion of patients receiving BI 201335 compared with those on placebo and were dose-dependent. Jaundice, skin manifestations including rash, photosensitivity reactions, pruritus and dry skin, and gastrointestinal side effects, mostly nausea, vomiting, and diarrhea were reported in the BI 201335 arms in a proportion exceeding 10% of the placebo/peg/RBV group. Jaundice was secondary to predominantly indirect or unconjugated hyperbilirubinemia. This was dose-dependent, rapidly reversible in all cases at cessation of BI 201335, and not associated with liver injury. The mechanism of action is inhibition of hepatic uptake of uridine diphosphate glucuronosyltransferase 1 family polypeptide A1 (UGT1A1).[20]
ACH-0141625 (ACH-1625)
ACH-1625 is an inhibitor of the HCV NS3 protease. ACH-1625 exhibits rapid and selective distribution to the liver and has high liver/plasma ratios. In phase Ib, 5 days of ACH-1625 monotherapy at doses ranging from 200 to 600 mg twice daily or 400 to 600 mg once daily led to mean maximal reductions in HCV RNA ranging from 3.1 log10 to 4.25 log10.[21]
This PI is currently in phase IIa of clinical development to evaluate its safety, tolerability, and antiviral activity in combination with pegIFN and RBV in patients with HCV genotype 1 naïve to treatment. This study has two segments. Segment 1 compares three different dosages of ACH-1625, 200 mg, 400 mg, and 800 mg once daily with placebo in combination with pegIFN/RBV for 28 days followed by pegIFN/RBV for a total duration of 48 weeks in 64 patients with HCV genotype 1. Segment 2 compares the same three dosing regimens to placebo for 12 weeks followed by 36 weeks of pegIFN/RBV. Week 4 results of Segment 1 showed achievement of rapid virologic response (RVR) in 74 to 81% of patients compared with 20% of patients receiving placebo/pegIFN/RBV. Reductions in HCV RNA on ACH-1625 ranged from 4.63 log10 to 4.96 log10 IU/mL after 4 weeks of triple therapy compared with 2.25 log10 IU/mL with placebo/pegIFN/RBV. It is worth noting that the majority of the patients enrolled were carriers of the unfavorable IL28B CT or TT genotypes and infected with HCV genotype 1a. The safety profile was comparable between all groups. No viral breakthroughs were observed during the first 4 weeks.
Danoprevir (RG7227/ITMN-191)
Danoprevir is a potent macrocyclic inhibitor of the HCV NS3/4A serine protease. In phase 1b studies in treatment naïve patients with HCV genotype 1, administration of danoprevir for 14 days was associated with a maximal median reduction of HCV RNA of 3.8 log10 in monotherapy and 5.7 log10 IU/mL in combination therapy with pegIFN and RBV.[22] In subsequent studies, danoprevir was boosted by ritonavir, a strong inhibitor of the CYP3A4 enzyme. A phase 1b study evaluated multiple ascending doses of ritonavir-boosted danoprevir in combination with pegIFN/RBV in 30 patients with HCV genotype 1 naïve to treatment.[23] More patients using boosted danoprevir (72%, 18/25 and 100%, 8/8 in the group receiving danoprevir 200 mg/ritonavir 100 mg twice daily) achieved undetectable HCV RNA (<15 IU/mL) at day 15 compared with patients who had previously received high-dose unboosted danoprevir (14%, 1/7) or placebo (20%, 1/5). This boosting allowed the use of a significantly lower dose of danoprevir, resulting in lower area under the curve (AUC) and maximum concentration (Cmax) of danoprevir. This reduced systemic exposure can improve the safety profile and reduce the probability of grade 4 ALT elevations, which had been seen with unboosted danoprevir at a dose of 900 mg twice daily.[24] Danoprevir/ritonavir is now being evaluated in several phase II clinical trials. A randomized, open-label study is evaluating SVR of danoprevir/ritonavir in combination with pegIFN/RBV in treatment of naïve patients with HCV genotype 1. Week 12 results of a substudy of 24 prior null responders treated with open-label danoprevir/ritonavir 100 mg/100 mg twice daily with pegIFN and RBV were recently presented.[25] Patients received 12 weeks of triple therapy after which they continued on pegIFN/RBV for a total of 48 weeks. At week 12, results showed a significant disparity between patients with HCV genotype 1a versus 1b. Fifty percent (50%) of patients with HCV genotype 1a achieved EVR versus 88% of those with HCV genotype 1b. Four of the eight patients with HCV genotype 1a experienced viral breakthrough with selection of the R155K mutation compared with 6% of patients with HCV genotype 1b.
A randomized open-label phase II study is evaluating SVR with danoprevir/ritonavir and RBV in combination with mericitabine (RG7128), a polymerase inhibitor, and/or pegIFN in patients with HCV genotype 1 who failed previous standard therapies. In this study, the six study arms contain RBV, but two of the six study arms are pegIFN-free.
A second phase II study, INFORM-SVR, is recruiting patients to evaluate the combination of danoprevir/ritonavir plus mericitabine with and without RBV in patients with HCV genotype 1. This is a pegIFN-free trial. In two arms, interferon-naïve patients will receive danoprevir 100 mg/ritonavir 100 mg twice daily with mericitabine 1000 mg twice daily with or without RBV for 12 weeks or 24 weeks. The third arm will enroll interferon-unable/intolerant patients who will receive an open-label combination of danoprevir/ritonavir/mericitabine plus RBV for 24 weeks. The combination of danoprevir/ritonavir with mericitabine looks promising and results of these phase II trials are awaited.
BMS-650032
One of the most important results, if not the most important, in the study of DAAs in the year 2011 was the result of the phase IIa study evaluating quadruple therapy with the PI BMS-650032 (600 mg twice daily) and BMS-790052 (an HCV NS5A replication complex inhibitor; 60 mg once daily) with and without pegIFN/RBV for 24 weeks in patients with HCV genotype 1 who were prior null responders to IFN-based therapy.[26] Prior null responders are typically the most difficult-to-retreat patient population. In this study four (36%) of the 11 null responders who received BMS-650032 and BMS-790052 alone for 24 weeks achieved SVR. This result is proof of the concept that HCV can be cured without pegIFN/RBV. Of the 11 patients in this arm, six had a viral breakthrough and resistant variants to both drugs were detected.[27] In four of these six, HCV RNA became undetectable when pegIFN/RBV was added at the time of viral breakthrough. All 10 patients who received the quadruple therapy were cured. There was 100% SVR12 and 90% SVR24 (one patient had SVR12, detectable HCV RNA 6 months posttreatment and undetectable HCV RNA when retested later on). These two DAAs are currently being evaluated in a phase IIb study in combination with pegIFN lambda (BMS-914143) with and without ribavirin for 24 weeks in patients with chronic HCV genotype 1 naïve to treatment. This study also compares the use of a single DAA (either BMS-650032 or BMS-790052) with RBV and either pegIFN-lambda or pegIFN-alfa. It will be very interesting to learn the role of RBV when these two DAAs are combined with pegIFN.
GS-9451 and GS-9256
GS-9451 is a potent macrocyclic HCV NS3 PI that achieved a median maximal change in HCV RNA of 3.6 log10 IU/mL (range, -4.7 log10 to -3.1 log10 IU/mL) following 3-day monotherapy in treatment-naïve patients with HCV genotype 1 infection during phase I.[28] It is currently being evaluated in phase II studies in combination with other DAAs. A phase IIb study with RGT will evaluate the efficacy and safety of 16 and 24 weeks of a four-drug regimen with GS-9451 and tegobuvir (a nonnucleoside HCV polymerase inhibitor) and 24 weeks of a three-drug regimen of GS-9451 without tegobuvir, all with pegIFN and RBV. Other phase II studies evaluating different DAA combinations that include the NS5A inhibitor GS-5885 are ongoing.
GS-9451 has additive to synergistic antiviral activity when combined with pegIFN, RBV, NS5A inhibitors, or polymerase inhibitors. Although a PI, GS-9451 retains activity against V36M and T54S, two NS3 mutations. However, R155K, A156T, and D168V are cross-resistant to GS-9451. The NS3 resistance mutations selected during treatment with GS-9451 are fully susceptible to other HCV inhibitor classes.[29] This supports its use in combination with other DAAs of the company's pipeline. A new study evaluating a four all-oral drug regimen is currently recruiting patients. In this study, GS-9451 is administered with GS-5885 (a NS5A inhibitor given at two different dosages), tegobuvir, and RBV for 12 or 24 weeks in patients with chronic HCV genotype 1 infection. This type of combination could completely revolutionize HCV treatment if found potent and well tolerated.
GS-9256 is also a potent PI that was being evaluated until recently. Preliminary results of the phase II study evaluating GS-9256 in combination with tegobuvir ± RBV and ± pegIFN for 28 days have been presented.[28] With the three oral drugs, 38% achieved RVR, 100% then achieved complete early virologic response (cEVR; undetectable HCV RNA at week 12) and maintained undetectability at week 24. With four drugs, 100% achieved RVR and were still undetectable at week 24. Without RBV and pegIFN, GS-9256 and tegobuvir led to RVR in only 7% of patients (1/15). Of these 15 patients, 12 achieved cEVR following the addition of pegIFN/RBV. All combinations were well tolerated. It was decided that GS-9256 would not be further developed, in part due to its higher potential to inhibit the transport and metabolism of bilirubin compared with GS-9451.[30]
Nucleoside and Nucleotide NS5B Polymerase Inhibitors
The nucleoside and nucleotide analog inhibitors of the HCV polymerase target the catalytic site of the enzyme. When they incorporate in the RNA chain in lieu of the natural substrate, they cause RNA chain termination. Nucleoside analogs must be phosphorylated three times by cellular kinases to become active as the triphosphate form. Nucleotide analogs are already in the active form. Because the NS5B target is highly conserved between HCV genotypes, polymerase inhibitors usually have pangenotypic activity (Fig. 1).
Figure 1. Characteristics of the five classes of direct-acting antivirals (DAAs). *Varies with the generation. **Palm II are effective against multiple genotypes. (DAA, direct acting antiviral; NS3/4A, nonstructural 3/4A protein; PI, protease inhibitor; NS5A, nonstructural 5A protein; NS5B, nonstructural 5B protein; Nuc, nucleoside.)
PSI-352938 (PSI-938)
PSI-938 is a purine (guanosine) nucleotide analog polymerase inhibitor of HCV. In earlier phases of development, PSI-938 was shown to have pangenotypic coverage, high liver to plasma ratios, residual activity against S282T variants (substitution selected by and associated with resistance to the 2'-methyl nucleosides), and low risk of drug-drug interactions. The 14-day results of a phase II study, the NUCLEAR study, were recently presented.[31] The NUCLEAR study compared different combinations of once daily PSI-938 plus PSI-7977 (a second polymerase inhibitor) to PSI-938 monotherapy for 14 days in treatment naïve patients with HCV genotype 1. It is the first to evaluate the combination of two nucleotide analogs for the treatment of HCV infection. Of the 24 patients who received combination treatment, 22 (92%) achieved HCV RNA <15 IU after 14 days of treatment. The two drugs are known to have complementary resistant profiles and not surprisingly, no viral breakthroughs were observed. Treatment was well tolerated. An interferon-free combination trial of PSI-938 and PSI-7977, the QUANTUM trial, has been initiated. PSI-938 was granted the fast track designation by the FDA for treatment of chronic HCV infection in August 2011.
PSI-7977
PSI-7977 is a pyrimidine (uridine) nucleotide analog active against all HCV genotypes. The dramatic results of the phase 2b study PROTON assessing safety and efficacy of PSI-7977 in combination with pegIFN/RBV against HCV genotypes 2 and 3 were presented earlier in 2011.[32] PROTON enrolled 25 treatment-naïve patients. One patient was lost to follow-up early in the study. Among the 24 patients who completed 12 weeks of triple therapy, 24 (100%) achieved SVR. These results suggest that therapy can be significantly shortened in patients infected with HCV genotypes 2 and 3 without compromising the chances of response. A phase II study in patients with HCV genotype 2 and 3 is ongoing to explore the use of PSI-7977 in monotherapy for 12 weeks versus PSI-7977 in combination with peg/RBV for 8 weeks.
Nonnucleoside NS5B Polymerase Inhibitors
Whereas the nucleoside inhibitors bind to the polymerase's active site, the nonnucleoside inhibitors bind to allosteric sites of the enzyme. This induces conformational changes that downregulate the polymerase's activity. Different binding sites disposed in a right hand motif with the thumb (thumb 1 and thumb 2), finger and palm (palm 1 and palm 2) domains are potential targets of nonnucleoside inhibitors. As a result of different target sites, mechanism of inhibition, and potency differences, nonnucleoside inhibitors have a low genetic barrier to resistance compared with nucleoside/nucleotide analogs (Fig. 1).[33]
SETROBUVIR (ANA598)
Setrobuvir (ANA598) is a potent nonnucleoside inhibitor and the most advanced in development. In a phase II combination study with pegIFN and RBV, 72% of patients achieved undetectable HCV RNA at week 8.[34] It is currently in a phase IIb study for the treatment of chronic HCV infection. In this ongoing study, 133 patients naïve to HCV treatment and 141 previously treated patients (n = 133) have been enrolled to receive setrobuvir 200 mg twice daily in combination with pegIFN/RBV. Patients naïve to HCV treatment with undetectable HCV RNA at week 8 and at subsequent visits will complete treatment at week 28 (RGT). Previously treated patients will receive 48 weeks of treatment. Future trials combining ANA598 with DAAs of different classes will likely offer the best SVR results.
Nonstructural Protein 5A (NS5A) Replication Complex Inhibitors
Inhibitors of NS5A block viral production at an early stage of assembly. The exact mechanism of action of the NS5A protein is unknown.[35] Without having an enzymatic function, this multifunctional protein is essential for replication and assembly of HCV and has no human homologs.[36,37]
DACLATASVIR (BMS-790052)
BMS-790052 is the first NS5A inhibitor with proof-of-concept in the clinic. The results of the first placebo-controlled, multiple ascending-dose clinical study evaluate its antiviral activity, resistance profile, pharmacokinetics, safety, and tolerability in 30 patients with chronic HCV infection infected with HCV genotype 1 were recently published.[38] Its pharmacokinetic profile supports once-daily dosing and the drug was well tolerated. Patients received BMS-790052 for 14 days. The mean maximum decline from baseline in HCV RNA ranged from 2.8 to 4.1 log10 IU/mL. Most patients experienced viral rebound during the first 7 days of BMS-790052 monotherapy. Viral breakthroughs were associated with mutations that had been previously found in the NS5A at baseline and at the time of resistance development.[39,40]
Cyclophilin Inhibitors
Cyclophilin inhibitors are derived from cyclosporine A, but lack calcineurin-binding properties and thus do not exhibit immunosuppressive effects.[41,42] Alisporivir (Debio 025) is the first-in-class cyclophilin inhibitor that recently initiated a phase III trial. It binds to cyclophilin A, an essential cofactor for HCV replication and shows additive antiviral effect with pegIFN in patients with genotype 1 and 4 HCV.[41] Cyclophilin inhibitors are sometimes referred to as host-targeted agents, but can also be part of the DAAs because they are known to interact with the NS5A protein.
In a phase II study of patients with HCV genotype 1, 2, 3, and 4 naïve to HCV treatment, alisporivir doses of 200, 600, and 1,000 mg/day in combination with pegIFN for 4 weeks were compared with monotherapy with alisporivir 1,000 mg/day or pegIFN.[42] In patients with genotypes 1 and 4, the 600- and 1,000-mg combination treatments reduced HCV RNA by up to 4.61 ( ± 1.88) log10 IU/ml and 4.75 (±2.19) log10 IU/mL at week 4, respectively. In patients with genotypes 2 and 3, HCV RNA levels were reduced by -5.91 (±1.11) log10 IU/mL and -5.89 (±0.43) log10 IU/mL at week 4, respectively, with the same treatment regimens. Alisporivir 1000 mg/day was associated with a higher incidence of isolated hyperbilirubinemia. SVR results of the phase II ESSENTIAL study were recently presented.[43] Alisporivir (600 mg twice daily during one week followed by once-daily dosing) with pegIFN/RBV led to SVR in 76% of patients taking the triple therapy for 48 weeks compared with 55% of the control group (P = .008). Triple therapy with alisporivir for 24 weeks was as effective as pegIFN/RBV dual therapy for 48 weeks with 53% SVR compared with 55%, respectively. In the RGT arm in which patients could stop triple therapy at week 24 if they achieved RVR, the SVR rate was 69%. Alisporivir demonstrates a high barrier to resistance and interestingly, the resistance mutation identified with its use (D320E) is mainly located in the NS5A domain II. Recent findings indicate that alisporivir inhibits hepatitis C virus replication by preventing a cyclophilin A induced cis-trans isomerization in domain II of NS5A.[44] Metabolism is through cytochrome P450 3A4, which can compromise its ability to be given concomitantly with substrates, inhibitors, or inducers of this cytochrome. Cyclophilin inhibitors could be part of a potent DAA combination in patients not taking concomitant P450 3A4 medications.
Conclusion
After many years of little or no progress in the development of HCV DAAs, 2011 was a watershed year for several reasons. The most clinically significant development was the approval of boceprevir and telaprevir for the treatment of HCV genotype 1. That will revolutionize the treatment of HCV in the clinic and change the landscape of HCV treatment forever because of the dramatically increased SVR demonstrated by both drugs. The Berlin EASL meeting in March of 2011 showcased some equally dramatic and positive results of drugs in development. Among many huge advances in the field, we saw the first 100% SVR for pegIFN/RBV plus PSI-7977 for HCV genotypes 2 and 3, with a shortened course of treatment. The most revolutionary results were revealed by the combination of the PI BMS-650032 and the NS5A inhibitor BMS-790052. In previous null responders to interferon treated with quadruple therapy consisting of a PI, a NS5A inhibitor, pegIFN and RBV, 100% SVR was achieved. This alone would merit a special mention, but the truly groundbreaking results were in the other arm of the study, which combined only the PI and the NS5A compound without pegIFN/RBV. In that arm, an SVR of 36% was achieved demonstrating for first time ever, an interferon-free, RBV-free cure of HCV.
Not only did we have the first FDA, European Medicines Agency (EMEA), and Canadian approvals for the PIs telaprevir and boceprevir in 2011, but we saw the proof of principle that the Holy Grail of HCV therapy is achievable; SVR without interferon. There are dozens of new drugs in clinical development now and many will fall by the wayside, but there are clearly enough that will be approved to reassure us that the future is very bright indeed for DAA treatment of HCV.
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