Jean-Michel Pawlotsky, MD, PhD
Professor, Department of Virology
Henri Mondor Hospital
Université Paris Est
Créteil, France
Introduction
Hepatitis C virus (HCV) resistance to a direct-acting antiviral (DAA) agent corresponds to the selection during treatment of viral variants that bear amino acid substitutions that alter the drug target; therefore, they are less susceptible to the inhibitory activity of the drug. These drug-resistant variants preexist as minor populations within the patient’s HCV quasispecies. Drug exposure profoundly inhibits replication of the dominant “wild-type” drug-sensitive viral population, and the resistant variants gradually occupy the vacant replication space. Moreover, viruses with low-level or partial resistance that can continue to replicate in the presence of drug, often favored by suboptimal drug exposure, may accumulate further mutations, leading to stepwise decreases in drug susceptibility, albeit often at a cost of reduced replicative capacity. If insufficient antiviral activity is provided because of suboptimal dosing or adherence, inadequate virologic suppression and the selection of resistance is inevitable. Therefore, to reduce the development of resistance, it is essential to achieve optimal drug concentrations through proper dosing and maximal adherence.
Factors That Influence Viral Resistance in vivo
In vivo, viral resistance is influenced by 3 major related factors: the genetic barrier to resistance, the in vivo fitness of the resistant viral population, and drug exposure.[1]
The genetic barrier to resistance is defined as the number of amino acid substitutions needed for a viral variant to acquire full resistance to the drug in question. If a single substitution is sufficient to confer high-level resistance to a specific drug, the drug is considered to have a low genetic barrier to resistance, whereas 3 or more substitutions are required to confer resistance to a drug with a high genetic barrier. There is a low likelihood that variants bearing a large number of resistance substitutions will preexist in a given patient and be fit enough to replicate at high levels when an antiviral drug is administered. Therefore, drugs with a high genetic barrier to resistance are less likely to be associated with clinically meaningful resistance.
The in vivo fitness of the viral variant is defined as its ability to survive and grow in the replicative environment. A selected resistant variant must have the capacity to propagate to fill in the replication space left vacant by the elimination of a susceptible wild-type virus during drug exposure. Thus, a highly resistant but poorly “fit” virus will be less clinically significant than a less resistant but “fitter” virus that can replicate efficiently in the presence of the drug. The acquisition of compensatory mutations may restore the fitness of a resistant variant and allow it to replicate efficiently in the presence of the drug, possibly allowing it to persist after drug withdrawal.
Finally, drug exposure affects the development of drug resistance. The degree of drug resistance of a variant can be measured in vitro as the fold increase in the 50% and 90% inhibitory concentrations (IC50 and IC90 in cell-free assays) or the 50% and 90% effective concentrations (EC50 and EC90 in cell-culture systems), that is, the drug concentrations that inhibit the tested enzyme function or viral replication by 50% and 90%, respectively. Drug exposure is defined as the drug concentration achieved in vivo relative to the IC50, IC90, EC50, or EC90 of resistant variants. This measurement is a key determinant of the development of resistance in vivo. Indeed, if drug levels achieved in vivo are far above these IC/EC values, then resistant variants will be effectively inhibited even if they are far less susceptible than the wild-type virus in vitro. Therefore, the pharmacokinetics of the antiviral drugs and adherence to therapy are key in preventing treatment failure due to viral resistance.
Importance of Pharmacodynamics/Adherence to Interferon-Containing DAA Regimens
The importance of achieving high blood DAA concentrations to prevent the emergence of resistance was demonstrated in the first phase Ib trial with telaprevir, an NS3/4A protease inhibitor (PI) with a low genetic barrier to resistance, administered as monotherapy for 14 days.[2] In this trial, patients were more likely to achieve either an HCV RNA plateau or virologic breakthrough during the dosing period due to selection of telaprevir-resistant variants if they received lower telaprevir doses (450 mg every 8 hours or 1250 mg every 12 hours) than if they received the higher dose (750 mg every 8 hours).[2] However, outgrowth of resistant viral populations was only delayed in the latter group. In vivo fitness assessments showed that less resistant, but “fitter” variants were more likely to become the dominant species than more resistant, less “fit” HCV variants.[2] These findings led the American and European regulatory agencies to limit monotherapy studies involving DAAs with low genetic barriers to resistance to only a few days. Therefore, no other studies provided sufficiently long administration to accurately assess the effect of drug exposure on the emergence of resistance.
Resistance to antiviral drugs is classically prevented by combining several drugs with potent antiviral activity and no cross-resistance. Indeed, HCV resistance to DAAs is observed significantly less frequently when one of these drugs is administered in combination with peginterferon and ribavirin.[3,4] Therefore, the triple combination of peginterferon, ribavirin, and a PI—telaprevir or boceprevir—has become the new standard-of-care therapy for both treatment-naive and treatment-experienced patients with genotype 1 HCV infection.[5-8] For the reasons defined above, it is crucial that optimal exposure to all 3 drugs in the regimen be achieved for these patients. Telaprevir must be taken at a dose of 750 mg every 8 hours with fatty food, whereas boceprevir must be taken at the dose of 800 mg every 8 hours with food. Patients must fully adhere to the regimen for the entire treatment period because any prolonged interruption in PI administration would inevitably result in a resurgence of wild-type viruses and the opportunity for resistant variants to acquire fitness, especially if the antiviral pressure exerted by peginterferon and ribavirin is only modest. However, if the patient does miss a dose of telaprevir or boceprevir, the package inserts provide guidance on how to manage these short interruptions. For telaprevir, the prescribing information recommends that a missed dose should be skipped if more than 4 hours have passed since the time it is usually taken; however, if it is within 4 hours of the time that it is usually taken, the missed dose should be taken immediately with high-fat food.[5,6] The recommendation for boceprevir is similar, but the timing is different. A missed boceprevir dose should be skipped if it is fewer than 2 hours before the next dose is scheduled; if it is more than 2 hours before the next scheduled dose, the missed dose should be taken immediately with food.[7,8]
The importance of adherence to peginterferon and ribavirin has been demonstrated in the absence of DAAs; these data showed that optimal response rates were observed in patients who achieved more than 80% of their prescribed peginterferon and ribavirin doses for more than 80% of the time.[9] The impact of poor adherence to, or dose reductions of, peginterferon, ribavirin, or both has not been extensively studied in clinical trials with the triple combination of peginterferon, ribavirin, and telaprevir or boceprevir. Nevertheless, maintaining the dose of peginterferon is likely to be key in interferon-responsive patients treated with triple therapy because treatment failure primarily results from an inadequate response to peginterferon, leading to the uncontrolled outgrowth of resistant variants selected by the PI.[1,10-12] By contrast, a retrospective analysis of patients completing 48 weeks of peginterferon/ribavirin therapy suggested that reducing the dose of ribavirin has a negative impact on the outcome of therapy only before HCV RNA becomes undetectable, whereas the impact is modest after HCV replication is controlled.[13] More recent studies in patients receiving PI-based therapy have shown that modest ribavirin dose reductions do not impair the likelihood of treatment success.[14,15] Finally, a recent study showed that the cost effectiveness of triple therapy is dependent on optimal adherence.[16]
There are several strategies that can be used to optimize adherence rates in patients receiving DAA-based therapy, some of which are under investigation. Current strategies include patient education on the importance of adherence, interventions to reduce the adverse effects of therapy, addressing comorbidities that may affect adherence to therapy, and using a multidisciplinary team to help physicians implement all of the aforementioned strategies. Regarding patient education, a prospective, multicenter study was conducted to determine the influence of patient education on adherence to peginterferon plus ribavirin therapy in patients infected with HCV.[17] Investigators showed that patient education can significantly influence adherence to treatment as evidenced by adherence rates to ribavirin of 56% at 6 months in 175 patients not receiving education vs 70% in 208 patients receiving education (P = .006).[17] Therapeutic education included intervention by healthcare professionals other than the prescribing physician as well as the distribution of support documents and educational materials. Another study found that physician’s treatment experience and patient motivation were associated with improved adherence to HCV therapy, indicating that empowering patients to take charge of their own treatment can impact adherence.[18] It should be noted, however, that no data are yet available on how these strategies may affect adherence to triple combinations with the HCV PIs.
Importance of Pharmacodynamics/Adherence to Interferon-Free DAA Regimens
Maintaining optimal adherence has been shown to be an extremely effective way of minimizing the development of resistance in the HIV field.[19] Strategies under investigation that may help optimize adherence rates to interferon-free DAA therapy include less frequent dosing and ritonavir boosting. Many DAA agents in development have half-lives that may allow for twice- or even once-daily dosing, which is encouraging.
In HIV therapy, the use of low-dose ritonavir to improve the pharmacokinetics of HIV PIs (so-called ritonavir boosting) has become standard practice. NS3/4A PIs are cytochrome P450 3A substrates; therefore, their plasma concentrations can also be substantially improved when coadministered with low-dose ritonavir, which may allow for prolonged dosing intervals and subsequent increased adherence rates. Boosting has been studied with 3 PIs—danoprevir, narlaprevir, and ABT-450—with encouraging results that suggest such strategies may support once-daily dosing, reduce adverse events (by lowering the required dose of the HCV PI), and reduce the risk of resistance.[20-23] Results with ABT-450 showed that higher plasma trough levels were associated with a lower likelihood of selecting resistant HCV variants over a short course of administration of 3 days.[23] However, a number of first-generation PIs can achieve high and well-tolerated plasma concentrations with once- or twice-daily dosing without ritonavir boosting.
Interferon-free regimens will reduce toxicity and adverse effects associated with peginterferon and ribavirin therapy, possibly improving adherence rates. The results of a short-term study combining the NS3/4A PI GS-9256 and the nonnucleoside RNA-dependent RNA polymerase inhibitor tegobuvir has been disappointing because this combination of agents has a low genetic barrier to resistance and frequent early virologic breakthroughs were observed.[24] Additional studies which have combined agents with low genetic barriers to resistance include the SOUND-C1 and SOUND-C2 studies. These trials combined the NS3/4A protease inhibitor BI 201335 and nonnucleoside polymerase inhibitor BI 207127, with or without ribavirin.[25,26] The ZENITH study combined the NS5B polymerase inhibitor VX-222 plus telaprevir.[27] In the SOUND-C2 study, treatment response rates, and likely the selection of resistant HCV variants, appeared to be influenced by the genetic background of the host (IL28B genotype).[26] Regimens comprising agents with higher barriers to resistance, such as the cyclophilin inhibitor alisporivir,[28] the combination of the nucleoside analogue inhibitor mericitabine and the PI danoprevir,[29] and the combination of the nucleotide inhibitor PSI-7977 with ribavirin have shown more promising results.[30] Indeed, the latter combination resulted in undetectable HCV RNA at 12 weeks post-therapy (SVR12) for 10 out of 10 genotype 2/3 HCV–infected treatment-naive patients receiving this regimen for 12 weeks.
Another interferon-free regimen comprising agents with low barriers to resistance—the first-generation NS3/4A PI asunaprevir and the NS5A inhibitor daclatasvir—has provided valuable information on the importance of drug exposure and how this differs depending on HCV subtype.[31,32] Protease inhibitors have a low genetic barrier to resistance; they can select fit variants that are poorly controlled at the drug concentrations achieved by doses used in clinical trials and practice. This is also true for NS5A inhibitors in genotype 1a HCV, as suggested by in vitro by studies showing a major shift in the IC50s induced by single amino acid substitutions in the NS5A sequence.[33] As a result, virologic breakthrough due to the selection of HCV variants resistant to both drugs was observed within a few days to weeks in 6 of the 9 patients infected with genotype 1a HCV.[31] By contrast, in genotype 1b HCV models in vitro, daclatasvir retains subnanomolar potency against all variants with single amino acid substitutions; the fold-change in IC50s conferred by these substitutions in the presence of the inhibitor was substantially lower than in genotype 1a.[33] Consequently, the drug concentrations achieved in vivo in genotype 1b HCV–infected patients were able to control NS5A variants carrying these substitutions, both in the absence or presence of associated substitutions conferring resistance to the PI. As a result, the combination of asunaprevir and daclatasvir was shown to lead to high sustained viral eradication rates (~ 90%) in patients infected with genotype 1b HCV.[31,32] To date, the available data on interferon-free regimens are more informative regarding pharmacodynamics and exposure than adherence. Additional data are awaited.
Conclusions
Although data are scarce regarding drug exposure and adherence in the context of HCV treatment with new DAA-based therapies, preliminary data and lessons learned from the HIV field suggest that optimal drug exposure and maximal adherence will be crucial to success with DAA-based therapies. In addition, the data from interferon-free regimens emphasize the importance of drug exposure, through optimal dosing and strict adherence to the prescribed regimen, when using combinations of drugs with a low to moderate barrier to resistance. Drugs with a higher barrier to resistance, such as nucleos(t)ide analogues, cyclophilin inhibitors, or second-generation PIs may theoretically be more tolerant to weaker adherence, but data are lacking thus far. When these therapies are available in clinical practice, virologic failures may be observed more often than in strictly controlled clinical trials. Vigilance and thorough patient education will be required to ensure high cure rates.
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