Peginterferon and ribavirin are likely to remain the backbone of antiviral treatment for chronic HCV for many years, even once new direct antivirals become available8, 9
. As a result, improving the response to interferon-based therapy remains a major priority. Results from this pilot study show that addition of SAMe to standard therapy improves early viral kinetics, likely by improving interferon signaling.
data suggested that SAMe may improve interferon signaling and antiviral effects through increased methylation of STAT1 leading to enhanced STAT1-DNA binding and thus greater ISG expression. This effect is particularly relevant in HCV infection because of the ability of HCV to induce PP2A, which may interfere with STAT1 methylation16, 17
. This study was undertaken to determine if the effects of SAMe observed in model systems would actually lead to improved treatment responses in patients with HCV infection.
The study design allowed patients to serve as their own controls to tease apart the effects of SAMe on early viral kinetics in a cohort of genotype 1 non-responders. The addition of SAMe to peginterferon and ribavirin had no effect on the first phase but improved the second phase slope of viral decline, which is thought to represent clearance of infected cells26
. SAMe treatment was also associated with greater induction of ISGs in PBMCs. The augmentation of ISG induction might be expected to affect the first phase decline, which is thought to be a measure of the effectiveness of interferon at blocking the production of new virions26
. However, no early effect was seen with the addition of SAMe. The consistency of the findings in different ISGs with different kinetics suggests that the effect of SAMe on ISG induction is likely broad, which also fits with the proposed mechanism of improved STAT1-DNA binding, a common event for known ISGs16
. Which ISGs are actually responsible for HCV clearance is currently unknown. It is possible that genes with later peak induction or delayed antiviral effects are responsible for the improvement in second phase kinetics. Ribavirin has recently been postulated to work through induction of a subset of ISGs and, like SAMe, ribavirin treatment leads to an improvement in second phase slope with no effect on first phase decline27–29
. Notably, in the in vitro
experiments, the effect of the addition of SAMe on viral clearance was not apparent until 72 hours after interferon treatment despite an effect on ISG mRNA expression as early as 4 hours. Clearly an improved understanding of how ISGs lead to viral clearance and specifically which ISGs affect which phases of viral decline, is needed.
In numerous retreatment trials, response rates among previous non-responders have been very poor. In the REPEAT trial, the largest retreatment study to date, Jensen and colleagues reported that 41% of previous non-responders receiving standard therapy achieved a pEVR and 13% had undetectable HCV RNA at week 12 (cEVR)5
. In the lead-in phase to the HALT-C trial, patients were retreated with peginterferon and ribavirin and 33% had undetectable HCV RNA by week 2030
. In this study with the addition of SAMe, 10 of 21 (48%) patients had a pEVR and 19% achieved a cEVR, including 1 patient with an RVR. By week 20, 10 of 21 patients (48%) had undetectable HCV RNA. These results suggest that SAMe can improve viral response rates in patients who are relatively resistant to interferon therapy. The important question, however, is whether improvement in these early responses will translate into higher rates of SVR. However it is important to acknowledge that this study was not designed nor powered to detect a difference in SVR but rather to explore whether SAMe improved interferon signaling and early viral kinetics.
Clearly with SVR rates of 19–33%, the addition of SAMe will not be sufficient on its own for the majority of non-responders. However ISG data from patients as well as the in-vitro mechanistic data suggest that SAMe works by improving interferon signaling and ISG induction. Improved interferon signaling may be particularly relevant for the treatment of non-responders as we enter the era of targeted antivirals. There has been great concern about the use of direct antivirals in prior non-responders because of the risk of developing resistance with ‘effective monotherapy’ due to the lack of effect of peginterferon and ribavirin. Even a marginal improvement in interferon efficacy may be enough to prevent the development of resistance, potentially leading to improved rates of viral clearance and ultimately of SVR. This strategy will have to be evaluated directly in clinical trials. SAMe may also improve outcomes in treatment naïve patients, although this was not addressed in this study.
The cell culture data provide support for the proposed mechanism of action of SAMe. Improved ISG induction was seen at physiologic concentrations of SAMe added to interferon. The co-immunoprecipitation data support the notion that SAMe increases STAT1 methylation in-vitro and in-vivo. Treatment with SAMe with or without interferon reduced the interaction of PIAS1 and STAT1. Most importantly, SAMe pretreatment led to an enhancement of the antiviral effect of interferon, particularly at later time-points, very similar to what was observed in study patients.
This study has some notable limitations. Second phase slope was assessed at 2 weeks rather than the standard 4 weeks26
. Asking patients to undergo therapy for a full 4 weeks before stopping treatment during the control course was felt to be problematic. Importantly, the 2-week viral kinetic data correlated very well with 4-week data in Course B suggesting that this is a reasonable surrogate (supplementary figure 1
). In addition, ISG induction was measured in PBMCs rather than liver, due to the impracticality of obtaining repeated liver specimens. The mechanism by which SAMe improves interferon signaling is not likely organ specific, however given that HCV affects interferon signaling differently between compartments31
, it is possible that the effect of SAMe on STAT1 methylation would be more pronounced in HCV-infected hepatocytes. It is notable that ISG induction in PBMCs correlates with treatment outcome32
In summary, the addition of SAMe to peginterferon and ribavirin led to improved early viral kinetics, enhanced ISG induction in PBMCs and higher rates of early and sustained viral clearance in a cohort of previous genotype 1 non-responders. SAMe was well tolerated and in-vitro data suggest that it works by increasing STAT1 methylation, leading to improved STAT1-DNA binding, and ultimately to enhanced ISG expression. Thus SAMe represents the first interferon sensitizing agent with in-vivo efficacy. SAMe appears to be a useful adjunct to interferon-based therapy, a factor that may be particularly important in the era of direct antivirals for HCV infection.