Reply
Authors: Jeremie Guedj, Laetitia Canini, Scott Cotler, Harel Dahari
Journal: Hepatology (2014)

Potential conflict of interest: Nothing to report. This work was supported by the National Institutes of Health (grant no.: P20‐GM103452) and the U.S. Department of Energy (contract no.: DE‐AC52‐06NA25396). Author names in bold designated shared co‐first authorship. We thank Ashish Goyal and John Murray for their interest in our study.1 However, we disagree with their interpretation of the mechanism of action of pegylated interferon (IFN)‐alpha against hepatitis D virus (HDV), the nature of the long delay, t0, before IFN had an effect in reducing HDV‐RNA levels, and the estimation of HDV half‐life, t1/2, in blood. First, Goyal and Murray incorrectly stated that we "concluded that IFN impacted only on viral production." Rather, we explained that, in our model,1 an additional effect of IFN in blocking viral infection, θ, has only a minimal impact on viral decline (and can hardly be estimated) as long as blocking of viral production, ε, is high (Fig. 1, dotted line), and this is why, for simplification, we set θ = 0. On the other hand, if the main effect of IFN was blocking viral infection with no or minimal effect on blocking viral production, then one should observe a continuous monophasic viral decline (Fig. 1, solid line) and not a biphasic viral decline as reported in our study (Fig. 1, dashed line). Interestingly, we recently showed2 that a biphasic viral decline was also observed in patients treated with lonafarnib, a prenylation inhibitor, which was previously shown to block HDV assembly/secretion.3 The similarity in the pattern of viral decline reinforces our conclusion that an additional effect in blocking viral infection cannot be ruled out, but that the main mechanism of action of IFN is blocking viral production.Figure 1: Viral load (black circles) and HBsAg data (gray circles) for patient 1 and best fit of our HDV model (1) assuming that IFN blocks only viral infection (i.e., ε = 0 and θ = 95% solid lines) or only viral production (dashed lines: ε = 93% and θ = 0; see other model parameters in Table 1 in our study1). No major effect is predicted for the latter when assuming an additional effect in blocking infection (i.e., ε = 93% and θ = 95%, dotted lines). Abbreviation: HBsAg, hepatitis B surface antigen.An interesting argument of Goyal and Murray is that the further a drug acts from viral production, the longer the time before its effect will be observed.4 Accordingly, they suggest that the long delay observed here (median t0 = 8.5 days) indicates that IFN acts mainly on blocking new infection, rather than blocking viral production, and that the intracellular infection life cycle (termed here eclipse phase) of HDV or of hepatitis B virus (HBV) should be close to 8.5 days. However, a study of severe combined immunodeficiency mice with humanized liver indicates that the HDV and HBV eclipse phases are shorter than 8.5 days. High‐titer serum HDV‐RNA levels were already established at the first time point assayed—7 days postinoculation—making it highly likely that viremia was present at even earlier time points (Jeffrey Glenn, unpublished data). In the case of HBV infection, we recently showed that the eclipse phase length was approximately 3 days.5 Although we agree that the long delay observed in the antiviral response is surprising, it does not necessarily reflect the mechanism of action of IFN and the length of the eclipse phase. The delay might be partly explained by in vitro findings that HDV inhibits IFN signaling.6 Regarding the estimation of the HDV t1/2 in blood, our previous viral modeling study suggested that a more precise estimate of virus could be obtained by analyzing the early viral decay with drugs inhibiting viral assembly/secretion.7 Interestingly, initiation of lonafarnib treatment led to a short delay of approximately 1 day and a rapid viral decline, reflecting a shorter t1/2 with lonafarnib (t1/2 = 1.4 day), compared to IFN (t1/2 = 2.9 day). However, the new estimate remained within a similar order of magnitude.2 In summary, we welcome the interest in, and scrutiny of, our modeling approach. This healthy debate indicates a new era for HDV kinetic modeling and treatment.