The goal of antiviral therapy for chronic hepatitis B (HBV) with oral nucleoside or nucleotide analogues is to ensure sustained suppression of HBV virologic activity as measured by quantitation of HBV DNA and ultimately, to prevent cirrhosis, hepatic failure, and hepatocellular cancer. Unfortunately, prolonged therapy with these agents can result in the emergence of antiviral resistance. The development of resistance depends on a number of variables: pre-treatment HBV DNA levels, rapidity of viral suppression (often correlated with potency of agent), degree of genetic barrier to resistance of agent (threshold probability the virus will mutate under selective pressure from the drug), duration of treatment, and prior exposure to oral antiviral therapy. Among the nucleoside analogue agents used to treat chronic HBV, different barriers to genetic resistance exist (Figure 1) and different rates of resistance have been reported to occur during treatment[1,3]. In this discussion, we will address the approach to patients with HBV who develop resistance to nucleoside analogues; HBV resistance to interferon and peginterferon preparations is not known to occur and will not be addressed.
Virologic breakthrough: a greater than 1 log10 (10-fold) rise in serum HBV DNA above nadir viral load in a patient who achieved initial viral suppression (during continued treatment).
Viral rebound: an increase in HBV DNA to greater than 20,000 IU/ml (or an increase above the pretreatment level) in a patient who attained an initial virologic response (during continued treatment).
Biochemical breakthrough: is characterized by a rise in serum ALT above the upper limit of normal after the patient achieves ALT normalization (during continued treatment).
Genotypic resistance: refers to specific DNA mutations known (based on in vitro studies) to confer resistance to a nucleoside antiviral agent.
Phenotypic resistance: refers to decreased in vitro susceptibility (as shown by an increase in inhibitory concentrations) to a nucleoside antiviral agent.
Consequences of Antiviral Resistance
The development of drug resistance has important clinical implications. In a study of cirrhotic patients with chronic hepatitis B who were treated with lamivudine versus placebo, disease progression (as measured by a composite endpoint of death and end-stage liver disease complications) occurred in 13% of patients who subsequently developed resistance on lamivudine compared with only 5% patients who did not develop resistance. This observation is consistent with the finding that HBV replication is a major risk factor for cirrhosis and hepatocellular carcinoma in HBV-infected patients older than 40 years of age. Thus, development of resistance leads to poorly controlled HBV, which then (if not dealt with) leads to increased risk of cirrhosis and hepatocellular carcinoma. In addition, the development of resistance can impact future treatment with other nucleoside analogues.
Evaluation of Patients with Virologic Breakthrough
In general, a rise in the HBV DNA level (virologic breakthrough) is the first sign of viral resistance and it precedes any biochemical breakthrough or symptomatic flare in hepatitis by weeks to months (Figure 2). Initial virologic breakthrough usually consists of low-level viremia (because the resistant HBV mutants have decreased replication fitness), but with continued treatment, compensatory mutations develop that can restore replication fitness and result in progressive increases in HBV DNA levels. Patients with virologic breakthrough should first be questioned about their medication adherence. If a major problem with adherence is identified, we recommend making a strong effort to help the patient remedy this situation and then reassess HBV DNA levels 12 weeks later. If virologic breakthrough has occurred under optimal adherence, or if elevated HBV DNA levels persist after increased adherence efforts, then testing for HBV resistance should be performed to confirm antiviral resistance. Genotypic resistance assays are the most commonly used resistance test in this setting. Several commercial genotypic assays are available that use hybridization technology (such as the line probe assay) or direct PCR sequencing. Reliable performance of these assays requires a HBV DNA level greater than 1000 copies/ml.
Resistance Profiles with Nucleoside Antiviral Agents
Genotypic testing can identify mutations in specific regions of the viral reverse transcriptase gene (Figure 3) that correlate with varying levels of resistance to specific agents (Figure 4). Treatment with lamivudine commonly leads to a mutation within the tyrosine-methionine-aspartate-aspartate (YMDD) motif in the reverse transcriptase region of the HBV polymerase gene. The primary mutation in this motif consists of the replacement of methionine by valine or isoleucine at the 204 codon, designated as M204V/I. The nomenclature M204V/I is preferred to YMDD because it conveys more accurate information regarding where the specific mutation has occurred. If the M204V/I mutation is present, lamivudine will not be effective. In addition, the M204V/I mutation results in cross-resistance to other nucleoside analogues (telbivudine, entecavir, and emtricitabine), which reduces their efficacy. The development of the additional mutations S202I and M250V further decrease the efficacy of entecavir. The A181T mutation confers resistance to lamivudine, adefovir, and tenofovir. The N236T mutation confers no resistance to lamivudine and entecavir but a 3-5-fold increased resistance to adefovir and tenofovir.
General Principles in the Management of Resistance
Therapy should be altered before a biochemical breakthrough occurs since biochemical breakthrough can result in a symptomatic hepatitis flare with marked ALT elevation (greater than 5 times upper limit of normal) and rarely hepatic decompensation[6,7]. Management of drug-resistant HBV most often consists of adding a second agent, but, in some instances, involves switching to an alternate agent (Figure 5). Adding an antiviral agent usually provides a more effective long-term response than switching, presumably because some HBV quasispecies continue to circulate that are susceptible to the original drug used; in addition, the original agent used may have some protective effect against resistance developing to the agent added. It is important to consider the resistance profiles of the various agents (and potential cross-resistance) when making a choice in a patient who has developed resistance. Ideally, the next agent will have the greatest potency, least cross-resistance, and a high genetic barrier to resistance. Entecavir and tenofovir have the highest barrier to resistance among oral antiviral therapies to date, a result of the multiple mutations that need to occur before these antivirals lose their efficacy[3,8].
Management of Lamivudine Resistance
As discussed in other cases, lamivudine has a high rate of resistance, approaching 70% after 4 years of therapy[3,9]. The M204I/V mutation is the primary resistance mutation that develops in patients treated with lamivudine. Other mutations can develop with significant frequency, including the double mutations L180M + M204I and L180M + M204V; it appears the L180M mutation develops as a compensatory change after the M204I/V mutation. The preferred management strategy with lamivudine resistance consists of adding an antiviral agent with a different resistance profile, typically adefovir or tenofovir. Several studies involving lamivudine-resistant patients have shown adding adefovir to lamivudine is superior to switching lamivudine to adefovir in terms of lower rates of adefovir resistance, higher rates of viral suppression (greater than 80% undetectable after 4 years), and more frequent ALT normalization[10,11,12]. Similarly, adding tenofovir to lamivudine can suppress lamivudine-resistant HBV and available data suggest tenofovir is more effective than adefovir in this setting[13,14,15]. From a practical standpoint, if tenofovir is going to be added, the lamivudine can be switched to emtricitabine (a drug equivalent to lamivudine) so the patient can receive the more convenient tenofovir-emtricitabine (Truvada) fixed drug combination. Entecavir at a higher dosage (1.0 mg) can provide clinical benefit to patients with lamivudine resistance. Entecavir, however, is structurally similar to lamivudine and thus shares cross-resistance. As a result, lamivudine-resistant patients who receive entecavir 1.0 mg daily develop entecavir resistance at a rate of 11-15% and 27-36% by 2 and 3 years, respectively[17,18]. When entecavir is used in lamivudine-resistant patients, the lamivudine should be discontinued. Telbivudine generally does not have adequate activity against lamivudine-resistant HBV and thus should not be used to treat patients with lamivudine resistance.
Management of Telbivudine Resistance
Telbivudine is associated with a moderate rate of resistance when used as monotherapy, but at a lower rate than with lamivudine treatment. Similar to lamivudine, the rates increase substantially after the first year of therapy. The M204I mutation is the primary mutation identified with telbivudine resistance. Thus, telbivudine and lamivudine share a similar resistance profile and have high-level cross-resistance. For patients on telbivudine who develop a M204V/I mutation, the approach should, in general, be the same as outlined above for lamivudine resistance.
Management of Entecavir Resistance
In patients who have not previously received lamivudine therapy, development of resistance with entecavir therapy only rarely occurs. In contrast, patients with prior lamivudine therapy and lamivudine resistance development resistance when treated with entecavir at a significant rate, even when the higher 1.0 mg/day dose of entecavir is used. It appears that entecavir resistance results from a "2-hit" mechanism, with the first hit involving lamivudine-associated mutations (that result in the selection of these strains during treatment with entecavir because they are less susceptible to entecavir) and the second hit consisting of additional entecavir mutations that develop in these prior selected strains. Obvious virologic failure does not generally occur until the second set of mutations develop. The most common resistance patterns observed with entecavir are the quadruple mutation patterns: I169T + L180M + M204V + M250V or L180M + T184G + S202I + M204V. Other resistance patterns have been identified, but occur less frequently. When full entecavir resistance occurs, adding tenofovir is recommended, but limited clinical data are available. Adding adefovir is also an option, but usually not recommended because it is less potent than tenofovir.
Management of Adefovir Resistance
The development of adefovir resistance is most often associated with the N236T and/or A181T/V mutations. These mutations generate partial cross-resistance to tenofovir, but apparently do not cause resistance to entecavir. Isolates with the N236T show in vitro susceptibility to lamivudine whereas those with A181T/V show resistance. In patients who develop adefovir resistance, there are several options. One option is to add entecavir if the patient has not received prior lamivudine therapy. For patients with prior lamivudine exposure, switching to tenofovir plus entecavir, or the more convenient tenofovir-emtricitabine (Truvada) fixed-drug combination is probably the best option. Tenofovir has a higher barrier to resistance and greater potency compared to adefovir.
Management of Tenofovir Resistance
Very little is known regarding HBV resistance to tenofovir and management of tenofovir-resistant isolates. In a large trial of tenofovir versus adefovir in patients with chronic HBV, after 48 weeks of treatment, none of the 426 tenofovir-treated patients developed mutations in the DNA polymerase region associated with decreased sensitivity to tenofovir. Several reports have shown development of HBV tenofovir resistance in patients co-infected with HIV associated with the mutations A194T combined with lamivudine mutations (L180M and M204V)[21,22]. The adefovir resistance is most often associated with the N236T and/or A181T/V mutations. These mutations generate partial cross-resistance to tenofovir, but apparently do not cause resistance to entecavir.
Management of Multidrug Resistance
In patients who have developed resistance to both lamivudine and adefovir, therapy should be switched to tenofovir-emtricitabine (Truvada) or tenofovir plus entecavir. If there is resistance to both lamivudine and entecavir, the therapy should be changed to tenofovir alone or tenofovir-emtricitabine (Truvada). In principle, it is important to switch to a nucleoside (or nucleotide) analogue from a different structural class whenever possible. Combination therapy with agents that have complementary cross-resistance profiles is preferred in this setting. If options are limited for oral antiviral therapy, peginterferon should be considered, keeping in mind the contraindications for peginterferon therapy.