Pharmasset Announces the Expansion of the ELECTRON Trial in Chronic Hepatitis C

Pharmasset adding arms to it's genotype 2/3 ELECTRON trial looking at shorter durations of therapy in addition to inteferon-free monotherapy for it's PSI-7977 nucleotide analog. Nucs traditionally have a very high barrier to resistance in comparison to non-nucs, but with a virus capable of making every possibly mutation of itself in the course of a day, is nuc monotherapy without an immune system modulator or another viral life cycle-specific inhibitor enough, even in the relatively easy-to-treat G2 and 3 virus? If it's not, what are the odds of a PSI-7977 resistant quasispecies developing and the corresponding fitness level compared to wild type of those variants? Guess we'll find out soon enough. If anyone can shed further enlightenment, please do.

PRINCETON, N.J., June 8, 2011 /PRNewswire/ -- Pharmasset, Inc. (Nasdaq: VRUS) announced today the addition of three treatment cohorts to the ELECTRON trial of PSI-7977, a nucleotide analog polymerase inhibitor, for the treatment of chronic hepatitis C (HCV). The rapid and consistent antiviral effects and high barrier to resistance demonstrated with PSI-7977 to date provide the rationale for additional exploratory regimens in this setting. This amendment will add one arm exploring 12 weeks of PSI-7977 monotherapy (without peginterferon and ribavirin) and two arms of interferon-sparing therapy: one for 8 weeks of PSI-7977 plus peginterferon and ribavirin (Peg-IFN/RBV) in patients with HCV genotype 2 (GT2) or 3 (GT3) and one for 12 weeks of PSI-7977 plus Peg-IFN/RBV in patients with HCV genotype 1 (GT1) prior null responses.

"The combination data reported at EASL demonstrated that SVRs were achievable with two oral DAAs in the absence of peginterferon and ribavirin," stated Bill Symonds, PharmD, Pharmasset's Senior Vice President of Clinical Pharmacology and Translational Medicine, "We continue to explore the potential for removing peginterferon and ribavirin from the HCV treatment regimen. Given the encouraging data we are seeing in ELECTRON, we have decided to expand the study to investigate PSI-7977 monotherapy, as well as shorter treatment regimens based on the promising data we reported at EASL from PROTON."

Pharmasset anticipates reporting results from the first four arms of the trial (n=40) during the second half of 2011. We have submitted a number of abstracts to the 2011 American Association for the Study of Liver Diseases (AASLD) meeting, including data from the ELECTRON and PROTON trials.

About the Trial

The ELECTRON trial is an exploratory study of PSI-7977 for the treatment of chronic HCV infection. Part 1 of the trial is evaluating 12-week regimens of PSI-7977 400mg QD in combination with ribavirin (RBV) only, and in separate arms with abbreviated durations of Peg-IFN for 4, 8, or 12 weeks in treatment-naive patients with HCV GT2 or GT3. The primary endpoint of the trial is the safety and tolerability of PSI-7977 400mg QD and RBV for 12 weeks, administered with or without Peg-IFN. On May 11, 2011, Pharmasset announced the completed enrollment of Part 1 of ELECTRON in patients with HCV GT 2 or GT 3:

* PSI-7977 400mg with RBV for 12 weeks (no peginterferon);
* PSI-7977 400mg with RBV for 12 weeks; Peg-IFN weeks 1-4 only;
* PSI-7977 400mg with RBV for 12 weeks, Peg-IFN weeks 1-8 only;
* PSI-7977 400mg with Peg-IFN and RBV for 12 weeks.


In Part 2 of ELECTRON, Pharmasset will enroll an additional 30 patients into exploratory regimens of monotherapy and abbreviated durations of total therapy. Following on the first four Cohorts of ELECTRON a 5th cohort will be added to explore 7977 400mg monotherapy in treatment-naive patients with HCV GT2 or GT3:

* PSI-7977 400mg monotherapy for 12 weeks.


With the previously reported 100% SVR12 in naive GT2/3 subjects in PROTON, a 6th and 7th cohort will be added to ELECTRON to explore shorter treatment durations in both GT2/3 naive subjects and HCV GT1 subjects who have documented null responses (less than 2 log(10) IU/mL reduction in HCV RNA after 12 weeks of Peg-IFN/RBV):

* PSI-7977 400mg with Peg-IFN/RBV for 8 weeks
* PSI-7977 400mg QD with Peg-IFN/RBV for 12 weeks.


About Pharmasset

Pharmasset is a clinical-stage pharmaceutical company committed to discovering, developing, and commercializing novel drugs to treat viral infections. Pharmasset's primary focus is the development of oral therapeutics for the treatment of hepatitis C virus (HCV) infection. Our research and development efforts are focused on nucleoside/tide analogs, a class of compounds which act as alternative substrates for the viral polymerase, thus inhibiting viral replication. We currently have three clinical-stage product candidates advancing in trials in various populations. Our pyrimidine, PSI-7977, an unpartnered uracil nucleotide analog, is currently under study in three Phase 2b trials in patients with HCV genotypes 1 through 6, including abbreviated duration interferon and interferon-free regimens. Our purine, PSI-938, an unpartnered guanosine nucleotide analog, recently reported safety and efficacy data from 14 days of monotherapy as well as 14 days in combination with the pyrimidine, PSI-7977. An SVR-endpoint study of the purine-pyrimidine combination is anticipated to begin in the third quarter of 2011. Mericitabine (RG7128) continues in two Phase 2b trials and one interferon-free trial being conducted through a strategic collaboration with Roche.

Contact
Richard E. T. Smith, Ph.D.
VP, Investor Relations and Corporate Communications
Office: +1 (609) 865-0693

Forward-Looking Statements

Pharmasset "Safe Harbor" Statement under the Private Securities Litigation Reform Act of 1995: Statements in this press release that are not historical facts are "forward-looking statements," that involve risks, uncertainties, and other important factors, including, without limitation, the risk of cessation or delay of any of the ongoing or planned clinical trials and/or our development of our product candidates, the risk that the results of previously conducted studies involving our product candidates will not be repeated or observed in ongoing or future studies involving our product candidates, the risk that our collaboration with Roche will not continue or will not be successful, and the risk that any one or more of our product candidates will not be successfully developed and commercialized. For a discussion of risks, uncertainties, and other important factors, any of which could cause our actual results to differ from those contained in the forward-looking statements, see the section entitled "Risk Factors" in our Annual Report on Form 10-K for the fiscal year ended September 30, 2010 and our Quarterly Reports on Form 10-Q filed with the Securities and Exchange Commission and discussions of potential risks, uncertainties, and other important factors in our subsequent filings with the Securities and Exchange Commission.

SOURCE Pharmasset, Inc.

Pharmasset exploring interferon-sparing Phase II trial for it's nucelotide analog PSI-7977 for Genotype 2/3 HCV...

Possible great niche for PSI-7977 in Genotype 2/3 subjects, possibly cutting the duration of therapy in half for these patients and possibly taking the interferon component out of the equation. -Chris

Pharmasset Initiates Exploratory Interferon Sparing Clinical Trial of PSI-7977 for Chronic Hepatitis C


Exploratory phase 2 trial in patients with HCV genotype 2 or 3 to receive PSI-7977 400mg QD and ribavirin, with 0-12 weeks of pegylated interferon

Interim data expected in first half of 2011

PRINCETON, N.J., Dec. 14, 2010 /PRNewswire/ -- Pharmasset, Inc. (Nasdaq: VRUS) announced today that dosing has begun in an exploratory study of PSI-7977, a nucleotide analog polymerase inhibitor, for the treatment of chronic hepatitis C (HCV). The trial will evaluate PSI-7977 400mg QD in combination with ribavirin (RBV), with 0, 4, 8, or 12 weeks of pegylated interferon alfa 2a (Peg-IFN) in treatment-naive patients infected with HCV genotype 2 or 3.
"Based on the encouraging efficacy, safety and resistance data from the Phase 2a trial with PSI-7977 in combination with pegylated interferon and ribavirin, we initiated a study to explore shorter durations of interferon, including an interferon-free regimen in treatment naïve patients with HCV genotype 2 or 3," stated Michelle Berrey, MD, MPH, Pharmasset's Chief Medical Officer. "An important limitation to the current treatment of HCV patients is the use of interferon with its associated side effect profile. We believe that PSI-7977's high barrier to resistance and the lack of a significant effect of IL28B genotype in the Phase 2a trial provide us an opportunity with PSI-7977 to explore shorter durations of interferon, and potentially even removing it from the treatment regimen altogether."

About the Trial
The trial is anticipated to enroll approximately 40 patients infected with HCV genotype 2 or 3 who have not previously been treated. The primary endpoint of the trial will be the assessment of safety and tolerability of PSI-7977 400mg QD and RBV for 12 weeks, administered with or without pegylated interferon (Peg-IFN) in treatment naïve patients with HCV genotypes 2 or 3. The trial will be conducted in New Zealand. Patients will be randomized into one of 4 arms:

-- PSI-7977 400mg QD in combination with RBV for 12 weeks (no interferon);
-- PSI-7977 400mg QD in combination with RBV for 12 weeks, with only 4 weeks of Peg-IFN;
-- PSI-7977 400mg QD in combination with RBV for 12 weeks, with only 8 weeks of Peg-IFN;
-- PSI-7977 400mg QD in combination with Peg-IFN and RBV for 12 weeks.

Patients will be stratified by HCV genotype and IL28B status to ensure balance across cohorts.
Pharmasset anticipates reporting interim data from this trial in the first half of 2011.

About Pharmasset
Pharmasset is a clinical-stage pharmaceutical company committed to discovering, developing, and commercializing novel drugs to treat viral infections. Pharmasset's primary focus is on the development of oral therapeutics for the treatment of hepatitis C virus (HCV). Our research and development efforts focus on nucleoside/tide analogs, a class of compounds which act as alternative substrates for the viral polymerase, thus inhibiting viral replication. We currently have four clinical-stage product candidates. RG7128, a cytosine nucleoside analog for chronic HCV infection, is in two Phase 2b clinical studies in combination with Pegasys(R) plus Copegus(R) and is also in the INFORM studies, the first series of studies designed to assess the potential of combinations of small molecules without Pegasys(R) and Copegus(R) to treat chronic HCV. These clinical studies are being conducted through a strategic collaboration with Roche. Our other clinical stage HCV candidates include PSI-7977, an unpartnered uracil nucleotide analog that has recently initiated dosing in a Phase 2b study in patients with HCV genotypes 1, 2, or 3, and PSI-938, an unpartnered guanosine nucleotide analog which recently completed a 7-day monotherapy study. We also have in our pipeline an additional purine nucleotide analog, PSI-661, in advanced preclinical development.

Pharmasset Initiates Phase 1b Multiple Ascending Dose Clinical Trial of PSI-938 in Patients with Chronic Hepatitis C

-- PSI-938 single ascending dose study in healthy volunteers supports progression to 7 day monotherapy study in HCV infected patients -- PSI-938 was generally safe and well tolerated with no dose-limiting toxicity -- Further results from single ascending and multiple ascending dose trials are expected in the third quarter 2010

PRINCETON, N.J., July 28 /PRNewswire-FirstCall/ -- Pharmasset, Inc. (NASDAQ:VRUS) announced today that safety and pharmacokinetic data from the PSI-352938 ("PSI-938") single ascending dose study support progression to a multiple ascending dose trial with PSI-938, which has initiated dosing. PSI-938 is a guanine nucleotide analog polymerase inhibitor for the treatment of chronic hepatitis C virus (HCV) infection. This study is designed to assess the safety, tolerability and antiviral activity of PSI-938 monotherapy administered over 7 days in HCV-infected individuals.
 

"PSI-938 is the third differentiated nucleoside analog that Pharmasset has advanced into clinical development for HCV," stated Michelle Berrey, MD, MPH, Pharmasset's Chief Medical Officer. "As with RG7128 and PSI-7977, PSI-938 demonstrates a high barrier to resistance in vitro; but unlike the cytidine and uridine analogs, PSI-938 retains equivalent potency against the S282T mutant. As the HCV field moves toward IFN-sparing, all-oral DAA combinations, we continue to believe nucleosides could become the backbone of care given these differentiating attributes and have the potential to be combined with all DAA classes."
 

PSI-938 Phase 1 Program Overview
 

The Phase 1 program is investigating the safety, tolerability and pharmacokinetics of PSI-938 in healthy subjects following escalating single doses (Phase 1a), and in patients chronically infected with HCV genotype 1 following repeat dosing for 7 days (Phase 1b). The Phase 1b study will additionally investigate hepatitis C viral dynamics and monitor for the development of drug resistance.
 

Subjects in the Phase 1a single ascending dose study received single doses of PSI-938 ranging from 100 mg to 800 mg or a matching placebo. Preliminary data from the phase 1a single ascending dose study includes:
 

--  No serious adverse events or discontinuations;
  --  No dose-related adverse events or dose-limiting toxicity;
  --  No grade III / IV lab abnormalities;
  --  No clinically significant changes in vital signs or ECGs; and
  --  PK which supports QD dosing.

A Phase 1b multiple ascending dose trial has now been initiated in treatment-naïve patients with chronic HCV genotype 1 infection. Subjects will be enrolled at multiple centers in the US and randomized to PSI-938 or placebo. Based upon the results from the first time in human study, the first dose of PSI-938 to be tested will be 100 mg administered once daily. The primary objectives of this study are to assess the safety, tolerability, pharmacokinetics and viral dynamics of PSI-938 after repeat dosing over 7 days.
 

  Results from both studies are expected in the third quarter of 2010.

  Purine and Pyrimidine Analogs

Pharmasset's purine nucleoside/tide analogs share many of the benefits of pyrimidine nucleoside/tide analogs, such as RG7128 and PSI-7977, in that they have demonstrated in vitro activity across multiple HCV genotypes, have a higher barrier to resistance than other classes of HCV small molecules in development, and, in spite of the prodrug technology, have no CYP 3A4 liability and thus a lower risk of drug interactions when combined with other direct acting antivirals targeting HCV. In addition, Pharmasset's purine analogs retain equivalent potency against wild type HCV and virus with the S282T mutation associated with in vitro resistance in other nucleoside/tide analogs in development. Furthermore, the purines are metabolized to the active triphosphate form through a different phosphorylation pathway than the pyrimidine analogs, thus decreasing the risk of competition between the two analogs for conversion to the active triphosphate. Given these characteristics, Pharmasset's purine and pyrimidine analogs have the potential to be combined as part of a future treatment regimen, which will be the focus of upcoming trials.