Nitazoxanide: Beyond Parasites toward a Novel Agent for Hepatitis C

January 23, 2009

See “Improved virologic response in chronic hepatitis C genotype 4 patients given nitazoxanide, peginterferon and ribavirin” by Rossignol JF, Elfert A, El-Gohary Y, et al on page 856.

Nitazoxanide (NTZ), a thiazolide with activity against anaerobic bacteria, protozoa and viruses, was the first drug licensed in the United States for the treatment of Cryptosporidium parvum.1 Its antiviral properties against hepatitis B (HBV) and hepatitis C (HCV) were coincidentally discovered when the drug was being evaluated to treat cryptosporidiosis in HIV patients coinfected with HBV or HCV.2 Evaluation of NTZ for chronic HCV was facilitated by studies of the compound for the treatment of infectious diarrhea in Egypt, where there is also very high prevalence of HCV infection, predominantly genotype 4, which resulted from mass immunization for schistosomiasis in the 1950s.3

Although there is substantial literature on NTZ as a safe and effective antiparasitic agent, the initial report of its antiviral properties in HCV-infected patients came in 2008 from Rossignol et al.4 This study was a double-blind, placebo-controlled trial of 50 patients with HCV genotype 4 treated with NTZ monotherapy (500 mg twice daily) for 24 weeks. Remarkably, seven of 23 patients (30 percent) had an end-of-treatment response and four of 23 (17 percent) had a sustained virologic response (SVR) compared with none in the placebo arm. Of note, all patients attaining an SVR had a baseline HCV RNA less than 400,000 IU/mL.

In this issue of GASTROENTEROLOGY, Rossignol et al5 present the results of a randomized, controlled trial evaluating the efficacy of NTZ in combination with peginterferon and ribavirin. Patients with genotype 4 from two Egyptian centers were randomized to receive NTZ monotherapy for 12 weeks followed by either peginterferon plus NTZ (dual therapy) or peginterferon plus ribavirin plus NTZ (triple therapy) for an additional 36 weeks. A control arm received standard dosing of peginterferon and ribavirin for 48 weeks.

Rapid Virologic Response (RVR; undetectable HCV RNA at week four of combination therapy) was observed in 38 percent of the standard of care group, 54 percent of the dual therapy group, and 64 percent of the triple therapy group. Complete early virologic response (undetectable HCV RNA at 12 weeks of combination therapy) occurred in 70 percent, 68 percent and 86 percent, respectively .The SVR rates at 24 weeks posttreatment were 50 percent, 61 percent and 79 percent.

RVR and SVR rates were significantly higher in patients receiving triple therapy compared with the standard of care. Adverse events were equal among treatment groups except for more anemia noted with ribavirin use. The authors conclude NTZ with peginterferon and ribavirin increases RVR and SVR rates compared with standard therapy without increasing adverse events.

The rationale for the 12-week lead-in phase with NTZ monotherapy is interesting and relates to the proposed mechanism of action of NTZ. Cellular rather than direct antiviral pathways were hypothesized to be affected by NTZ. NTZ effectively inhibits HCV replication in the replicon systems for genotypes 1a and 1b.6 Pretreatment with three days of NTZ followed by a combination of NTZ plus interferon resulted in a three-fold decrease in viral RNA.7 Elazar et al8 have recently proposed NTZ increases phosphorylation of dsRNA-dependent protein kinase (PKR) and induces eukaryotic initiation factor 2 alpha (eIF2a), which is an important factor in the innate immune response (see figure).

Among the molecules with important biological functions induced by type I interferons, PKR plays a critical role in the antiviral defense mechanism of the host. PKR phosphorylates eIF2a in response to viral infection of a cell, which ultimately inhibits translation of viral RNA.9 Several studies have shown HCV proteins may directly bind to PKR, thereby inhibiting autophosphorylation and obscuring the interferon sensitivity-determining region.10,11 This proposed virus–host interaction may be important; NTZ seems to improve interferon activity against the virus.

We look forward to further elucidation of the role of NTZ in this critical antiviral immune pathway and whether this proposed mechanism is operative in vivo. The authors acknowledge several shortcomings in the present study, including the fact it was not blinded to physicians or participants, and it was conducted only in genotype 4 patients.

Genotype 4 virus, although it is the most genetically diverse of the HCV viruses, is more similar to genotype 1 than to the other genotypes. The SVR rate to peginterferon and ribavirin for patients with genotype 4, ranging between 65 and 79 percent,3,12 is intermediate to those reported for genotypes 2 and 3. Middle Eastern patients seem to have a higher response rate of their genotype 4 infections, perhaps related to ethnicity or to the 4a subtype.

In the featured study by Rossignol et al,5 the standard of care arm had an SVR of only 50 percent, which is slightly lower than several series of Middle Eastern patients with genotype 4a infection. Their standard of care group also had a higher body mass index than either the dual or triple therapy groups with NTZ, which can adversely affect response to peginterferon and ribavirin therapy.13

Genotype 1 infection, with an SVR rate of approximately 50 percent, has remained the major focus of clinical trials utilizing intensified regimens of peginterferon and ribavirin or new classes of drugs. The U.S. population has proven repeatedly to be a more difficult group to treat than many others with lower SVR rates, perhaps due in part to higher body mass indexes and a greater racial variation.

African Americans harbor predominantly genotype 1 virus and have remarkably lower overall response rates to peginterferon and ribavirin (approximately 26–28 percent) compared with Caucasians. 14,15 Based on data from the Virahep-C study group, African Americans may have “blunted” immune responses to interferon-based therapy when compared with Caucasians. 16 As investigations of the role of NTZ in anti-HCV treatment continue, we hope adequate numbers of African Americans will be recruited to determine whether this drug can improve outcomes in African-American patients with chronic HCV.

Overall, only about 5 percent of patients with advanced fibrosis or cirrhosis were included in the current study. These represent inadequate numbers to comment extensively on the safety and efficacy of NTZ in the setting of cirrhosis. Both of the Phase II studies in genotype 1, treatment-naïve, and nonresponder patients being conducted in the United States will include subjects with well-compensated cirrhosis. This will hopefully provide further safety and efficacy data on this important group of patients who are known to have lower response rates to peginterferon and ribavirin.17,18 Since NTZ’s antiviral properties were first observed in patients coinfected with HCV and HIV and there are long-term safety data in this population, it would also be a logical progression to include this group in a future study. The coinfected population has lower response rates to peginter feron and ribavirin, and often more pronounced side effects, 19–21 so studying NTZ in this population is warranted.

A key secondary endpoint in this study was RVR, which revealed triple therapy had a significantly higher RVR than standard of care (64 vs 38 percent respectively; P = .048). It is worth noting during the 12-week NTZ monotherapy lead-in phase, both the double and triple therapy arms had a very modest but significant decrease in viral load from baseline, yet only the triple therapy group, which contained ribavirin, had a significant difference in RVR. Although RVR is a more important predictor of treatment response to peginterferon and ribavirin than is genotype,12 the validity of using it as a predictor after a 12-week lead-in phase (week 16 on treatment) will need to be explored.

From this study, it is difficult to ascertain the role of ribavirin as the relapse rates were 15 percent and 4 percent in the dual therapy and triple therapy arms, respectively, but significantly higher in the standard of care group at 33 percent, thereby suggesting NTZ was associated with reduced relapse rates. Rossignol et al22 recently presented preliminary data from 40 treatment-naïve patients who were all treated with four weeks of NTZ followed by 36 weeks of dual therapy with peginterferon alpha 2a and NTZ.22 Utilizing historical controls, SVR rates with the abbreviated lead-in phase and no ribavirin were 80 percent, comparable with the triple therapy arm from the present study. Based on these data, the lead-in phase in Phase II studies currently underway will be only four weeks.

As a cautionary note, ribavirin has continued to be necessary to decrease the relapse rate, even in combination with newer direct antivirals like the NS3/4 protease inhibitor telaprevir.23 Clearly, the role of ribavirin in NTZ-containing regimens need careful evaluation. Overall, the current study revealed provocative data in genotype 4 HCV infection that will need additional confirmation in larger studies. Thus, NTZ may add to our armamentarium of agents that have activity against HCV, although numerous questions arise about how best to utilize this oral agent with many similar antiviral effects of interferon. Is there any role for NTZ monotherapy? The modest antiviral response to NTZ monotherapy in the present study contrasts sharply with the antiviral response from an earlier study where 30 percent of patients had undetectable viremia after 24 weeks of NTZ monotherapy.4 It is unlikely the longer duration of NTZ monotherapy in the earlier study would fully explain these differences and it is unlikely NTZ monotherapy will be of great benefit for the majority of patients.

The more realistic role for NTZ in the near future would be in combination with peginterferon and ribavirin, as is currently being evaluated. Shortened therapy may be possible if the RVR rates continue to be high in subsequent studies. The biggest unanswered question remains as to whether NTZ is sufficiently potent to enhance therapeutic response in patients with difficult-to-treat characteristics, such as genotype 1, high viral levels, African Americans and previous nonresponders to peginterferon and ribavirin, namely, those frequently found in the U.S. population. Combining NTZ with specific targeted antiviral therapy in HCV (STAT-C agents) is also of interest.

Recent in vitro data of NTZ with NS5b polymerase inhibitors and the NS3/4 protease inhibitor telaprevir have yielded promising results.24 NTZ seems to have a high barrier to resistance, synergistic effects on HCV replication when combined with direct antivirals, and some activity against common, drug-resistant variants that form under selective pressure of the polymerase or protease inhibitors.7 Peginterferon and ribavirin remain the cornerstones of therapy, although it is exciting (but a bit premature) to speculate what niche NTZ could potentially fill in future combination therapy. As we continue our quest for an oral “HCV cocktail,” NTZ may find its way into the mix, but first will need to further prove itself in additional, well-designed studies.

JAMA M. DARLING
MICHAEL W. FRIED
Division of Gastroenterology and Hepatology
University of North Carolina at Chapel Hill
Chapel Hill, North Carolina

References

1. Rossignol JF. Nitazoxanide in the treatment of acquired immune deficiency syndrome-related cryptosporidiosis: results of the United States compassionate use program in 365 patients. Aliment Pharmacol Ther 2006;24:887–894.


2. Rossignol JF, Keeffe EB. Thiazolides: a new class of drugs for the treatment of chronic hepatitis B and C. Future Microbiol 2008;3: 539–545.


3. Kamal SM, Nasser IA. Hepatitis C genotype 4: what we know and what we don’t yet know. Hepatology 2008;47:1371–1383.


4. Rossignol JF, Kabil SM, El-Gohary Y, et al. Clinical trial: randomized, double-blind, placebo-controlled study of nitazoxanide monotherapy for the treatment of patients with chronic hepatitis C genotype 4. Aliment Pharmacol Ther 2008;28:574–580.


5. Rossignol JF, Elfert A, El-Gohary Y, et al Improved virologic response in chronic hepatitis C genotype 4 patients given nitazoxanide, peginterferon, and ribavirin. Gastroenterology 2009;136:856–862.


6. Korba BE, Elazar M, Lui P, et al. Potential for hepatitis C virus resistance to nitazoxanide or tizoxanide. Antimicrob Agents Chemother 2008;52:4069–4071.


7. Korba BE, Montero AB, Farrar K, et al. Nitazoxanide, tizoxanide and other thiazolides are potent inhibitors of hepatitis B virus and hepatitis C virus replication. Antiviral Res 2008;77:56–63.


8. Elazar MLM, McKenna S, et al. Nitazoxanide (NTZ) is an inducer of eIF2a and PKR phosphorylation. Hepatology 2008;48:1151A.


9. Garcia MA, Gil J, Ventoso I, et al. Impact of protein kinase PKR in cell biology: from antiviral to antiproliferative action. Microbiol Mol Biol Rev 2006;70:1032–1060.


10. Gale MJ Jr, Korth MJ, Tang NM, et al. Evidence that hepatitis C virus resistance to interferon is mediated through repression of the PKR protein kinase by the nonstructural 5A protein. Virology 1997;230:217–227.

Figure. NTZ increases phosphorylation of PKR and induces eIF2a, which ultimately inhibits translation of viral RNA.