Is HBV RNA a new endpoint of HBV cure?

The strong association between viral replication and development of hepatocellular carcinoma (HCC) among patients with chronic hepatitis B virus (HBV) infection has been established for almost 20 years since the publication of the REVEAL study.[1] Nucleos(t)ide analog (NA), which can offer potent blockade of HBV reverse transcriptase and suppression of HBV DNA, have been proven a powerful means to reduce the risk of HCC. Patients who have their HBV DNA fully suppressed by NA, preferably to an undetectable level, enjoy a lower risk of HCC than those who have incomplete viral suppression.[2] With a deeper understanding of the viral life cycle, a wave searching for new HBV therapeutics aiming for a higher goal of functional cure was started in the last decade. Functional cure is defined as hepatitis B surface (HBsAg) seroclearance, which has been shown to be associated with excellent prognosis in the long-term. Despite arduous efforts of clinical research, the new HBV therapies can hardly overcome the high barriers to HBV cure, namely refractory reservoir of covalently closed circular DNA (cccDNA) and integrated HBV DNA, high viral burden of HBV DNA and HBsAg, as well as impaired host innate and adaptive immune responses against the virus.[3] A compromised intermediate target, called partial cure, was defined in an AASLD-EASL treatment endpoint conference as HBsAg <100 IU/ml with undetectable HBV DNA at 24 weeks off treatment.[4] Whether this endpoint is sustained and valid for all new HBV therapeutics remains to be tested. Pre-genomic HBV RNA is a transcriptional product from cccDNA that cannot be directly suppressed by NA. Serum HBV RNA reflects well the intrahepatic HBV RNA among both untreated and NA-treated patients.[5] Over 75% of NA-suppressed patients with undetectable HBV DNA may still have quantifiable serum HBV RNA after 2 years of treatment.[6] An obvious question is whether complete suppression of HBV RNA, even in the presence of HBsAg, is associated with a reduced HCC risk. A new class of antiviral agents, capsid assembly inhibitors, can suppress HBV RNA much more readily than NA.[3] However, it may be another decade before we could expect to see the benefit in HCC reduction, if any, from clinical data of capsid assembly inhibitors. In this issue of the Journal, Wang and colleagues from China compared the HBV DNA and HBV RNA levels among 36 pairs of propensity score-matched HCC patients and cirrhotic controls.[7] They found that patients with HCC, compared to cirrhotic controls, had significantly higher HBV DNA (median 2.4 logs vs. 0.0 log IU/ml) and HBV RNA (median 3.1 logs vs. 2.0 log copies/ml) levels. Even among the 9 HCC patients and 29 patients with HBV DNA negativity by ultrasensitive HBV DNA assay, the HBV RNA levels were significantly higher in the HCC group (median 3.0 log copies/ml) vs. the cirrhosis group (median 0.0 copies/ml). These findings highlight the potential importance of HBV RNA as a viral marker indicating persistent cccDNA transcriptional activity despite full HBV DNA suppression by NA. Before the role of HBV RNA can be recommended as an integral part of HBV treatment endpoint, a few questions need to be addressed. First, commercial standardized assays have to be available in clinics. For now, highly sensitive HBV RNA assays with detection limits down to 10 copies/ml are under investigation and are about to launch.[8,9] Second, the confounding effect of detectable HBV DNA has to be eliminated. In the study by Wang and colleagues, only 16 HCC patients had received NA, and 14 of them had been treated for over 12 months. However, the majority of HCC patients were untreated with both elevated HBV DNA and HBV RNA levels.[7] In another study from China, among 2,974 chronic hepatitis B patients on NA followed up for up to 5 years, detectable baseline HBV RNA was found to be associate with increased risk of HCC among the patient subgroups with cirrhosis and detectable HBV DNA.[10] To confirm the importance of HBV RNA on HCC development, a large cohort of NA-treated chronic HBV-infected patients with undetectable HBV DNA should be studied. Third, one needs to address whether undetectable HBV RNA will improve the sustainability of disease remission after treatment is stopped. Based on published data so far, undetectable HBV RNA may predict post-treatment hepatitis flare but not viral relapse among patients who have stopped NA therapy.[11,12] Nonetheless, if complete suppression of HBV RNA in addition to HBV DNA on continuous antiviral therapy can further reduce the risk of HCC, it may become a new therapeutic goal before a finite HBV cure remedy is available. In parallel with advances in HBV diagnostic tests, we are pushing our limit on the therapeutic target of antiviral therapy. Residual viremia is becoming less acceptable with the availability of very sensitive HBV DNA and HBV RNA assays. In mice models, ongoing viral replication and production of infectious viruses have been demonstrated in serum samples of chronic hepatitis B patients on NA with HBV DNA suppressed to below the limit of quantification (but target detected).[13] Now, we start to see reports suggesting HBV RNA must also to be suppressed to reduce the risk of HCC to a minimum. All this evidence calls for more effective anti-HBV therapy that can completely abolish the activity of cccDNA on top of NA. Development of new HBV therapeutics aiming for a functional cure is therefore not only aiming for a finite duration of therapy but also to reduce the risk of HBV-related complications.