Letter to the Editor: Examining HBV‐RNA Kinetics During NA Treatment—Are NAs Multifunctional Antiviral Agents?
Authors: Harel Dahari, Andrew Vaillant, Scott J. Cotler
Journal: Hepatology (2020)

Potential conflict of interest: Dr. Vaillant is employed and owns stock in Replicor. Supported by the U.S. National Institutes of Health Grant Nos. R01‐AI144112 and R01‐AI146917. Carey et al.(1) recently proposed that HBV pregenomic RNA (pgRNA) is a sensitive indicator of covalently closed circular DNA (cccDNA) transcription status in patients receiving nucleos(t)ide analogue therapy (NA) for hepatitis B. The pgRNA indicator is transcribed from the cccDNA minichromosome and therefore reflects a product of cccDNA transcriptional activity and its prevalence within HBV‐infected cells. They observed that some patients reached pgRNA negativity within 12 months of NA, suggesting a surprisingly rapid decline in transcriptional activity/prevalence of cccDNA. Moreover, studies by Bazinet et al.(2) and Liu et al.(3) found that pgRNA and total HBV RNA (totRNA) declined from pretreatment levels as early as 6 months and 3 months of NA, respectively. In both of these studies, pgRNA/totRNA decline lagged behind the HBV DNA (virus) decline, in agreement with the findings in Carey et al.(1) Interestingly, Liu et al.(3) observed a biphasic decline in both totRNA and virus in some rapid responder patients, which is surprising because NA could only be expected to cause similar rates of totRNA and virus decline if an antiviral activity in addition to reverse‐transcriptase inhibition was present (Fig. 1).FIG. 1: Established and potential antiviral effects of nucleos(t)ide analogue (NA) in HBV infection. Previously described inhibition of reverse transcription by the HBV reverse transcriptase (RT) provides a mechanism for inhibiting secretion of HBV DNA (red) and recycling of HBV DNA (purple) consistent with reduction of HBV DNA but not HBV pregenomic RNA (pgRNA) in the blood. Potential mechanisms consistent with reduction of HBV pgRNA in the blood include transcriptional inactivation/degradation of covalently closed circular DNA (cccDNA) (1), inhibition of RT‐binding to pgRNA (2) potentially required for the encapsidation of pgRNA (3). MVB, multivesicular body; rcDNA, relaxed circular DNA.Biphasic virus decline during NA typically consists of a rapid first phase followed by a slower second phase. Mathematical modeling suggests that this is a result of NA blocking virus production, with the first phase driven by its t1/2 in blood. The second phase is driven by the rate of HBV‐infected cell loss/death.(4) Assuming similar serum t1/2 for pgRNA/totRNA and virus, the first phase observed by Liu et al.(3) might suggest that NAs immediately inhibit pgRNA/totRNA production by novel mechanisms, whereas the second phase could reflect inhibition of pgRNA/totRNA and possibly cccDNA loss and/or HBV‐infected cell loss/death (Fig. 1). While persistence of pgRNA is an indicator of ongoing cccDNA transcription in patients with undetectable serum viral levels, and quantifiable pgRNA is associated with virus rebound and alanine aminotransferase flare when NA is discontinued,(1) undetectable pgRNA does not necessarily indicate cccDNA clearance. Future studies with very early and frequent pgRNA/totRNA and viral sampling will be required to characterize pgRNA/totRNA kinetics and perform theoretical modeling, to elucidate pgRNA/totRNA dynamics during infection and NA and whether NA promotes cccDNA clearance.