Mutagenic effects of ribavirin in vivo

Phylogeny and molecular evolution of the hepatitis C virus

The hepatitis C virus (HCV) is a globally prevalent human pathogen that causes persistent liver infections in most infected individuals. HCV is classified into seven phylogenetically distinct genotypes, which have different geographical distributions and levels of genetic diversity. Some of these genotypes are endemic and highly divergent, whereas others disseminate rapidly on an epidemic scale but display lower variability. HCV phylogeny has an important impact on disease epidemiology and clinical practice because the viral genotype may determine the pathogenesis and severity of the resultant chronic liver disease. In addition, there is a clear association between the HCV genotype and its susceptibility to antiviral treatment. Similarly to other RNA viruses, in a single host, HCV exists as a combination of related but genetically different variants. The whole formation is the actual target of selection exerted by a host organism and antiviral therapeutics. The genetic structure of the viral population is largely shaped by mutations that are constantly introduced during an error-prone replication. However, it appears that genetic recombination may also contribute to this process. This heterogeneous collection of variants has a significant ability to evolve towards the fitness optimum. Interestingly, negative selection, which restricts diversity, emerges as an essential force that drives HCV evolution. It is becoming clear that HCV evolves to become stably adapted to the host environment. In this article we review the HCV phylogeny and molecular evolution in the context of host-virus interactions.

Management and therapy of chronic hepatitis C in HIV

PURPOSE OF REVIEW

Chronic hepatitis C is currently one of the leading causes of hospitalization and death in HIV+ persons. Treatment is particularly challenging in coinfected patients due to lower efficacy and more side effects.

RECENT FINDINGS

The combination of pegylated interferon plus ribavirin is the current treatment of choice. In the absence of contraindications, treatment should be provided with no restrictions up front (e.g., asking unnecessarily for a liver biopsy) and revisited at weeks 4 and 12. Treatment should only be continued in early virological responders. The use of standard ribavirin doses (1000-1200 mg/day) and for at least 12 months is crucial to maximize the effect of therapy. In patients with rapid virological response (undetectable viraemia at week 4), shorter periods of therapy (24 weeks) may be advisable for hepatitis C virus genotypes 2 and 3. Patients with low CD4 percentages should defer treatment and prioritize highly active antiretroviral therapy. Didanosine should never be co-administered with ribavirin due to potential life-threatening complications. When possible, zidovudine, stavudine and abacavir should be replaced by other agents having no deleterious interactions with ribavirin.

SUMMARY

The treatment of chronic hepatitis C has become a priority in hepatitis C virus/HIV-coinfected patients, and the best results are obtained by tailoring therapy to the individual patient's characteristics.

Role of weight-based ribavirin dosing and extended duration of therapy in chronic hepatitis C in HIV-infected patients: the PRESCO trial

The response to pegylated interferon (pegIFN) plus ribavirin (RBV) as treatment of chronic hepatitis C virus (HCV) infection is lower in HIV-coinfected than in HCV-monoinfected patients and could be due to suboptimal RBV dosing and/or insufficient duration of therapy in prior trials. In a prospective, multicenter, open, comparative trial, HCV/HIV-coinfected patients received pegIFN plus weight-based RBV for 48 or 72 weeks (HCV genotypes 1 and 4) and 24 or 48 weeks (HCV genotypes 2 and 3). Use of didanosine was not allowed. Out of 389 patients included in the trial, 61% were infected by HCV-1/4 and 67% had serum HCV-RNA >500,000 IU/ml. Sustained virological response (SVR) was achieved by 49.6%, significantly higher in HCV-2/3 than HCV-1/4 (72.4% vs. 35%; p < 0.0001). A high drop-out rate in the longer treatment arms precluded obtaining definitive conclusions about the efficacy of prolonging therapy. Premature treatment discontinuations due to serious adverse events occurred in 8.2%. Infection with HCV-2/3, lower baseline HCV-RNA, and negative HCV-RNA at week 12 were all independent predictors of SVR in the multivariate analysis. The use of RBV 1000-1200 mg/day plus pegIFN is relatively safe and provides SVR in nearly half of coinfected patients, twice as high in HCV-2/3 than HCV-1/4.

Critical role of ribavirin for the achievement of early virological response to HCV therapy in HCV/HIV-coinfected patients

The response to hepatitis C virus (HCV) therapy seems to be lower in HCV/HIV-coinfected patients than in HCV-monoinfected individuals. Given that most pivotal trials conducted in coinfected patients have used the combination of pegylated interferon (pegIFN) along with fixed low doses (800 mg/day) of ribavirin (RBV), it is unclear whether HIV itself and/or suboptimal RBV exposure could explain this poorer outcome. Two well-defined end points of early virological response were evaluated in Peginterferon Ribavirina España Coinfección (PRESCO), a multicentre trial in which the combination of pegIFN plus RBV (1000 mg if body weight <75 kg and 1200 mg if >75 kg) was prescribed to coinfected patients. For comparisons, we used unpublished data from early kinetics in two other large trials, one performed in HIV-negative patients [Pegasys International Study Group (PISG)] in which RBV 1000-1200 mg/day was used and another [AIDS Pegasys Ribavirin Coinfection Trial (APRICOT)] in which HIV-positive patients received fixed low RBV doses (800 mg/day). A total of 348 HCV/HIV-coinfected patients from the PRESCO trial were analysed as well as all patients treated with pegIFN plus RBV, who completed 12 weeks of therapy in the comparative studies (435 in PISG and 268 in APRICOT). Negative serum HCV-RNA at week 4 (which has the highest positive predictive value of sustained virological response, SVR) was attained in 33.3%, 31.2% and 13% of treated patients with HCV genotype 1, respectively, in PRESCO, PISG and APRICOT. For HCV genotypes 2/3, responses were 83.7%, 84.2% and 37%, respectively. A decline lower than 2 log(10) at week 12 (which has the highest negative predictive value of SVR) was seen in 25.5%, 19.5% and 37% of HCV genotype-1-infected patients, and in 2.1%, 2.9% and 12% of genotypes-2/3-infected patients, respectively. Prescription of high RBV doses enhances the early virological response to HCV therapy in HCV/HIV-coinfected patients, with results approaching those seen in HCV-monoinfected patients.

Sequential structures provide insights into the fidelity of RNA replication

RNA virus replication is an error-prone event caused by the low fidelity of viral RNA-dependent RNA polymerases. Replication fidelity can be decreased further by the use of mutagenic ribonucleoside analogs to a point where viral genetic information can no longer be maintained. For foot-and-mouth disease virus, the antiviral analogs ribavirin and 5-fluorouracil have been shown to be mutagenic, contributing to virus extinction through lethal mutagenesis. Here, we report the x-ray structure of four elongation complexes of foot-and-mouth disease virus polymerase 3D obtained in presence of natural substrates, ATP and UTP, or mutagenic nucleotides, ribavirin triphosphate and 5-fluorouridine triphosphate with different RNAs as template-primer molecules. The ability of these complexes to synthesize RNA in crystals allowed us to capture different successive replication events and to define the critical amino acids involved in (i) the recognition and positioning of the incoming nucleotide or analog; (ii) the positioning of the acceptor base of the template strand; and (iii) the positioning of the 3'-OH group of the primer nucleotide during RNA replication. The structures identify key interactions involved in viral RNA replication and provide insights into the molecular basis of the low fidelity of viral RNA polymerases.

Mutagenic effect of ribavirin on hepatitis C nonstructural 5B quasispecies in vitro and during antiviral therapy

BACKGROUND AND AIMS

Addition of ribavirin to interferon alfa treatment has substantially increased sustained virologic response rates in patients with chronic hepatitis C (CHC). Ribavirin acts as an RNA virus mutagen in vitro, thereby leading to error catastrophe. However, data in CHC are controversial.

METHODS

The nonstructural (NS) 5B quasi-species heterogeneity was analyzed in Huh7 cells harboring a subgenomic hepatitis C virus (HCV) replicon system treated with ribavirin or levovirin. Accordingly, NS5B quasi-species were studied in 14 patients with CHC who received ribavirin alone or combined with pegylated interferon alfa both at baseline and during the first weeks of therapy. Analysis of NS3 quasi-species served as control.

RESULTS

Cultivation of HCV replicon cells with ribavirin led to higher NS5B mutational frequencies compared with levovirin-treated or untreated cells (P < .05). Patients receiving ribavirin monotherapy showed higher overall mutational frequencies within NS3 and NS5B during therapy as compared with baseline (P < .01). Proportions of ribavirin-specific G-to-A and C-to-T transitions increased (P < .01). Paired analysis confirmed significant mean increases of mutational frequencies of approximately 5%. Ribavirin serum concentrations were positively correlated with mutational frequency changes (P < .05). In patients receiving combination therapy, a decrease of NS5B mutational frequencies ( approximately 10%) and lower proportions of G-to-A and T-to-C mutations (P < .01) were detectable.

CONCLUSIONS

Ribavirin, but not its L-enantiomer levovirin, is a mutagen in HCV replicon cells. In patients with CHC, ribavirin monotherapy exhibits a moderate mutagenic effect early during therapy that is not detectable in combination with pegylated interferon alfa.

Present and future therapy for hepatitis C virus

The treatment of hepatitis C virus (HCV) infection has developed enormously over recent years. Early treatment of acute HCV infection with interferon-alpha can prevent chronicity and a significant proportion of patients with chronic HCV can be cured with the current standard therapy consisting of pegylated interferon-alpha and ribavirin. However, the improvement of current treatment regimens and the development of new antiviral drugs will be essential within the next few years. This review focuses on the present and future concepts for treating HCV infection, including prevention of infection, antiviral therapy of acute and chronic HCV and the management of patients after liver transplantation.

The anti-yellow fever virus activity of ribavirin is independent of error-prone replication

The precise mechanism by which the broad-spectrum anti-RNA virus agent ribavirin elicits its in vitro antiviral effect has remained a matter of debate. We have demonstrated that inhibition of cellular inosine monophosphate dehydrogenase (IMPDH) activity, and thus depletion of intracellular GTP pools, is the predominant mechanism by which ribavirin inhibits the replication of four flavi- and two paramyxoviruses (J Virol 79:1943-1947, 2005). As a consequence, induction of error catastrophe, which has been proposed as a mechanism by which ribavirin may elicit its anti-RNA virus activity, may be expected to have little, if any, impact on its antiviral effect. The flavivirus yellow fever virus (17D vaccine strain) was cultured for five consecutive passages in the presence of 1-beta-D-ribofuranosyl-1,2,4-triazole-3-carboxamide (ribavirin), 5-ethynyl-1-beta-D-ribo-furanosylimidazole-4-carboxamide (EICAR) (the 5-ethynyl analog of ribavirin), or mycophenolic acid (MPA; a compound that exclusively inhibits IMPDH). The reduction in infectious virus yield brought about by ribavirin (as well as MPA and EICAR) was paralleled by a similar reduction in viral RNA yield; in case of error-prone replication, the infectious virus yield is expected to decrease significantly faster than the viral RNA yield. In addition, pre-extinction populations of the virus that has suffered a maximum impact of treatment with ribavirin did not accumulate an increased number of mutations. Very similar observations were obtained with EICAR and with MPA, a molecule that cannot be incorporated into viral RNA. These data thus allow us to conclude that the in vitro anti-yellow fever virus activity of ribavirin is independent of error-prone replication.