The recent development of direct-acting antiviral (DAA) molecules that are active on hepatitis C virus (HCV) has raised the concern that resistance may undermine therapy based on DAAs. A new standard-of-care treatment is now available for treatment-naive patients and treatment-experienced patients infected with HCV genotype 1. This treatment is based on a triple combination of pegylated interferon α (IFNα), ribavirin and a protease inhibitor (either telaprevir or boceprevir).
Principles of HCV Resistance to DAAs
Viral resistance to a DAA drug corresponds to the selection (during treatment) of viral variants that bear amino acid substitutions altering the drug target and that are, therefore, less susceptible to the drug's inhibitory activity. These DAA-resistant variants preexist as minor populations within the patient's quasispecies (i.e., the ensemble of all viral variant populations present in a given individual). However, preexisting drug-resistant variants are rarely detected with current techniques before therapy because the amino acid substitutions that confer resistance also generally reduce replicative capacity in the absence of the drug.
Drug exposure profoundly inhibits the replication of the dominant, wild-type, drug-sensitive viral population, and the resistant variants gradually occupy the vacant replication space. Resistance is usually associated with a typical escape pattern (with a rapid recovery of pretreatment levels of viral replication) when amino acid substitutions confer a high level of resistance without impairing fitness in the presence of the drug (Fig. 1). Viral replication may resume more gradually if the resistant virus is not very fit. Cross-resistance (i.e., overlapping resistance) between two antiviral drugs that target the same site or function is due to amino acid substitutions that confer reduced susceptibility to both drugs.
Resistance to Telaprevir or Boceprevir Monotherapy
Telaprevir and boceprevir bind tightly to the catalytic site of the HCV protease, compete with its natural substrates, the polyprotein cleavage sites, and thereby inhibit polyprotein processing (i.e., the generation of mature viral proteins). HCV protease inhibitors have a low genetic barrier to resistance, and they have been shown to select resistant HCV variants in vitro and in vivo.1, 2 These variants bear amino acid substitutions that are located in close vicinity to the protease catalytic triad, alter the affinity of the drugs for the enzyme's catalytic site, and thereby attenuate their inhibitory activity. Boceprevir and telaprevir share extensive cross-resistance. The most frequent resistance substitutions that they select are V36A/M, T54A/S, R155K/T/Q, V36A/M+R155K/T, A156V/T, and V36A/M+A156V/T; however, numerous other substitutions have been reported during the administration of these drugs. HCV subtypes 1a and 1b are associated with slightly different resistance patterns.
Mechanisms of Treatment Failure During Triple-Combination Therapy With Pegylated IFNα, Ribavirin, and Telaprevir or Boceprevir
In treatment-adherent patients, the failure of the triple combination of pegylated IFNα, ribavirin, and a protease inhibitor to eradicate an HCV infection results primarily from an inadequate response to pegylated IFNα and ribavirin, which leads to uncontrolled outgrowth of resistant variants selected by the protease inhibitor. Indeed, phase 2 and 3 clinical trials have shown that the outcome of triple-combination therapy strongly depends on the ability of pegylated IFNα and ribavirin to induce a sufficiently strong antiviral response in host cells,3-6 as detailed in Table 1. Figure 2 shows schematic representations of the dynamics of wild-type (sensitive) and resistant HCV variants during treatment with a DAA with a low barrier to resistance (e.g., telaprevir or boceprevir) that is administered in combination with pegylated IFNα and ribavirin according to the antiviral effects of IFNα and ribavirin.
|HCV RNA Decline During Lead-In (log10 IU/mL)||Treatment Failure Rate (%): SPRINT-1 (Phase 2, Treatment-Naive, BOC)*|
|28-Week Arm (n = 103)||48-Week Arm (n = 103)|
|0.5 to <1.0||76||38|
|1.0 to <1.5||70||35|
|1.5 to <2.0||27||20|
|2.0 to <3.0||33||21|
|3.0 to <4.0||17||18|
|Undetectable (<15 IU/mL)||0||0|
|Treatment Failure Rate (%): SPRINT-2 (Phase 3, Treatment-Naive, BOC)†|
|BOC/RGT Arm (n = 368)||BOC/PR48 Arm (n = 366)|
|Treatment Failure Rate (%): RESPOND-2 (Phase 3, Treatment-Experienced, BOC)‡|
|BOC/RGT Arm (n = 162)||BOC/PR48 Arm (n = 161)|
|Treatment Failure Rate (%): REALIZE (Phase 3, Treatment-Experienced, Telaprevir)§|
|T12PR48 Arm (n = 266)|
|Prior partial responders|
|Prior null responders|
In approximately 50% to 70% of patients whose treatment failed to eradicate the infection and who experienced a breakthrough during treatment or a relapse after the end of therapy in phase 2 and 3 clinical trials of telaprevir or boceprevir, protease inhibitor–resistant viral populations were dominant at the time of relapse.4, 8-10 In the remaining cases, the wild-type virus was still dominant when the protease inhibitor administration was stopped, but the protease inhibitor–resistant viral population had also been enriched.
Posttreatment Behavior of Protease Inhibitor–Resistant HCV Variants
Follow-up studies of patients who were included in phase 2 and 3 clinical trials of pegylated IFNα, ribavirin, and telaprevir or boceprevir and whose treatment failed to eradicate HCV showed that after several months to 2 years, the vast majority of the patients had lost the resistant virus, whereas the wild-type, telaprevir-sensitive virus had returned as the dominant population.11, 12 HCV variants resistant to the protease inhibitor still replicated but did so as minor viral populations (a situation close to the pretherapeutic one).
Consequences of Treatment Failure for the Outcome of Liver Disease
After triple-combination therapy fails to eradicate HCV, the rate of disease progression appears similar to the rate observed before treatment, at least in the short to medium term. This is explained by the fact that the circulating HCV RNA level does not correlate with the severity of liver disease, which progresses slowly. Because HCV is not cytopathic, there is no reason to believe that resistant HCV variants would be more aggressive than the wild-type virus for the liver. It is also unlikely that the selection of protease inhibitor–resistant variants present for years as minor viral populations could trigger a strong intrahepatic cellular immune response and the associated production of proinflammatory cytokines that would accelerate liver disease progression. Thus, although long-term follow-up studies of these patients are needed to draw firm conclusions, there is currently no reason to think that a failure to clear HCV with the triple combination of pegylated IFNα, ribavirin, and a protease inhibitor is harmful. Patients for whom this regimen has failed should be reassured because many other therapeutic options will be available in the near future.
Implications of Treatment Failure for Future DAA-Based Therapy
It is possible that protease inhibitor–resistant HCV variants acquire additional substitutions that further improve their fitness in the presence of a protease inhibitor during its administration. Nevertheless, 2 years after a triple-combination treatment failure, a situation close to the pretherapeutic one is restored, with a dominant wild-type viral population and the presence of minor variants bearing amino acid substitutions conferring resistance to protease inhibitors. Thus, a new treatment with telaprevir and boceprevir or a first-generation protease inhibitor sharing cross-resistance with these drugs is not contraindicated as long as the drug is combined with another drug or several drugs that potently inhibit HCV replication while sharing no cross-resistance with protease inhibitors (this excludes retreatment in combination with pegylated IFNα and ribavirin only). Two options can be envisaged: quadruple combinations with two non–cross-resistant DAAs, pegylated IFNα, and ribavirin and all-oral, IFN-free drug regimens. Ongoing studies will define the optimal retreatment strategies for patients for whom the triple combination of pegylated IFNα, ribavirin, and telaprevir or boceprevir fails to eradicate HCV.