Pharmasset and Roche Obtain FDA Consent to Start a Phase 2b Study with R7128 in Treatment Naive HCV Patients

Princeton, NJ -- January 12, 2009 -- Pharmasset, Inc. (Nasdaq: VRUS) announced today that they and their development partner, Roche, have agreed with the FDA on the final design for a phase 2b trial with R7128, a nucleoside inhibitor of hepatitis C (HCV), slated to initiate in the first quarter of this year.

"We are pleased that R7128 is advancing into a large phase 2b trial," stated Michelle Berrey, MD, MPH, Pharmasset's Chief Medical Officer. "R7128 is the most advanced nucleoside polymerase inhibitor in development and we believe this class of drug brings a number of advantages to HCV-infected patients. R7128's higher barrier to resistance and activity across multiple viral genotypes, as well as the promising short-term safety and tolerability, may bring patients a new option for therapy. We look for this trial to better define the optimal treatment duration with R7128 in combination with the standard of care."

The phase 2b trial is anticipated to enroll about 400 treatment-naive, genotype-1 or genotype 4 HCV-infected patients. The trial will evaluate the dose and duration of treatment of R7128 in combination with Pegasys [pegylated interferon alfa-2a] plus Copegus [ribavirin]. The primary efficacy endpoint of the trial will be the proportion of patients that achieve a sustained virologic response (SVR), defined as undetectable (measured by Roche TaqMan assay) HCV RNA 24 weeks after completion of treatment.

Patients will be enrolled into one of 5 arms:

• 24 weeks of total treatment, with R7128 500 mg bid [twice-daily] in combination with pegylated interferon and ribavirin for 12 weeks, followed by 12 weeks of pegylated interferon and ribavirin

• 24 weeks of total treatment, with R7128 1000 mg bid in combination with pegylated interferon and ribavirin for 12 weeks, followed by 12 weeks of pegylated interferon and ribavirin

• 24 weeks of total treatment, with R7128 1000 mg bid in combination with pegylated interferon and ribavirin for 8 weeks, followed by a further 16 weeks of pegylated interferon and ribavirin

• 48 weeks of total treatment, with R7128 1000 mg bid in combination with pegylated interferon and ribavirin for 12 weeks, followed by a further 36 weeks of pegylated interferon and ribavirin.

• A control arm with pegylated interferon and ribavirin for 48 weeks.

Patients in the 24 week arms will discontinue treatment at week 24 if they achieved a rapid virological response (RVR), defined as undetectable level of HCV RNA at week 4 ("RVR-guided"). Patients that do not achieve an RVR will continue on the standard of care until week 48.

According to the current study design, patients will be enrolled as two cohorts, with randomization of the second larger cohort being initiated based on 12 week safety data of the first cohort.

During 2009, we expect to provide updates on the progress of the trial.

About R7128

R7128 is being developed for the treatment of chronic HCV infection. R7128 is a prodrug of PSI-6130, a cytidine nucleoside analog inhibitor of HCV RNA polymerase. A prodrug is a chemically modified form of a molecule designed to enhance the absorption, distribution and metabolic properties of that molecule. R7128 has shown in vitro activity against all of the most common HCV genotypes (1, 2, 3 and 4).

In a 4-week phase 1 combination study that was conducted in 81 treatment-naive patients chronically infected with HCV genotype 1, R7128 demonstrated significant short-term antiviral activity with safety and tolerability comparable to placebo with SOC [standard of care]. Results from the 500 mg, 1500 mg and 1000 mg dose cohorts (cohorts 1, 2 and 3) in 81 treatment-naive patients infected with HCV genotype 1 indicated:

• Results with R7128 500 mg bid, 1000 mg bid and 1500 mg bid with SOC achieved a mean 3.8 log10 IU/mL (30% RVR, 6 of 20), 5.0 log10 IU/mL (88% RVR, 22 of 25) and 5.1 log10 IU/mL (85% RVR, 17 of 20) decrease in HCV RNA, respectively.

• Results with placebo with SOC indicated patients achieved a mean 2.9 log10 IU/mL decrease in HCV RNA and 18.75% (3 of 16) patients achieved RVR (< 15 IU/ml).

In a harder to treat, non-responder, genotype 2 and 3 HCV patient population treated for 4 weeks:

• Results with R7128 1500 mg bid with SOC led to 90% of patients achieving an RVR (<15 IU/mL) compared to 60% in the standard of care arm.

In November 2008, Pharmasset, Roche and InterMune initiated the INFORM-1 trial to investigate the combination of R7128 with InterMune's R7227 (ITMN-191), a protease inhibitor in HCV patients in the absence of pegylated interferon and ribavirin. Patients will receive a maximum of 14 days of combination treatment, followed by Pegasys plus Copegus for a further 46 weeks. The aim of the study is to investigate the safety and antiviral activity of the combination.

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 B virus (HBV), hepatitis C virus (HCV) and human immunodeficiency virus (HIV).

For further information, visit www.pharmasset.com.

Idenix Pharmaceuticals Initiates Proof-of-Concept Study of IDX184 for the Treatment of Hepatitis C Virus (HCV)

Cambridge, MA -- January 12, 2009 -- Idenix Pharmaceuticals, Inc. (Nasdaq: IDIX), a biopharmaceutical company engaged in the discovery and development of drugs for the treatment of human viral and other infectious diseases, today announced that it has initiated a proof-of-concept study of IDX184, a liver-targeted nucleotide prodrug candidate for the treatment of HCV under an Investigational New Drug application (IND) with the U.S. Food and Drug Administration (FDA). The study is evaluating IDX184 in treatment-naive hepatitis C genotype-1 infected patients.

"We are very pleased with the progress that we have made in our hepatitis C discovery program in 2008, having successfully advanced IDX184 from IND to a proof-of-concept study, while simultaneously advancing two additional HCV discovery programs into IND-enabling preclinical studies," said Jean-Pierre Sommadossi, PhD, chief executive officer of Idenix. "As we work to finalize a partnership for our lead HIV drug candidate, our goal in 2009 is to become the first biopharmaceutical company with innovative drugs from three major classes of direct-acting hepatitis C antivirals in clinical development."

The proof-of-concept trial in HCV-infected patients is being conducted at multiple centers around the world. The trial design is a phase 2/3, double-blind, placebo-controlled, dose-escalation study to evaluate the safety and antiviral activity of IDX184 in treatment-naive adult patients infected with chronic hepatitis C. The study will evaluate four doses of IDX184, ranging from 25 to 100 mg once-per-day, administered for three days. Each cohort of the study will evaluate ten patients randomized eight to IDX184 and two to placebo.

About IDX184

IDX184 is a once-daily, oral nucleotide prodrug candidate based on Idenix's proprietary liver-targeting technology. This technology enables the delivery of high levels of nucleoside triphosphate in the liver, potentially maximizing drug efficacy and limiting systemic side effects. In HCV genotype-1 infected chimpanzees, once-daily oral administration of 10 mg/kg/day of IDX184 produced a mean viral load reduction of 2.3 log10 after four days of dosing. In a phase I study in healthy volunteers evaluating doses ranging from 5 to 100 mg/day, IDX184 was safe and well-tolerated; the most common adverse event reported in this study was dizziness and it was more frequently reported in subjects receiving placebo.

About Idenix

Idenix Pharmaceuticals, Inc., headquartered in Cambridge, Massachusetts, is a biopharmaceutical company engaged in the discovery and development of drugs for the treatment of human viral and other infectious diseases. Idenix's current focus is on the treatment of infections caused by hepatitis C virus and HIV.

For further information about Idenix, please refer to www.idenix.com.

Human Genome Sciences Announces Initiation of Phase 2b Trial 0f Albuferon Dosed Monthly in Chronic Hepatitis C

Trial conducted by Novartis to evaluate safety and efficacy of Albuferon administered every four weeks in combination with ribavirin in patients with genotypes 2 and 3 hepatitis C

Rockville, MD -- January 12, 2009 -- Human Genome Sciences, Inc. (Nasdaq: HGSI) today announced that Novartis has initiated dosing in a Phase 2b trial that will evaluate the safety and efficacy of Albuferon (albinterferon alfa-2b) administered monthly in combination with ribavirin in treatment-naive patients with genotypes 2 and 3 chronic hepatitis C. Albuferon is being developed by HGS and Novartis under an exclusive worldwide co-development and commercialization agreement entered into in June 2006.

"Patients undergoing treatment for chronic hepatitis C often find it challenging to participate in normal daily activities, especially in the days following dose administration," said Stephen Pianko, MD, FRACP, PhD, Monash University, Melbourne, Australia. "Pegylated interferons, the current standard of care, require administration once every week. Albinterferon alfa-2b dosed every four weeks with a total of six injections could offer an important treatment option, if it demonstrates comparable safety and efficacy vs. peginterferon alfa-2a dosed weekly with a total of 24 injections."

In December 2008, HGS announced that Albuferon met its primary endpoint of non-inferiority to peginterferon alfa-2a (Pegasys) in ACHIEVE 2/3, a Phase 3 clinical trial of Albuferon in combination with ribavirin in treatment-naive patients with genotypes 2 and 3 chronic hepatitis C. In these patients, the Phase 3 study showed that 900-mcg Albuferon administered every two weeks had efficacy comparable to peginterferon alfa-2a [Pegasys], with comparable rates of severe and/or serious adverse events and discontinuations due to adverse events.

"Hepatitis C is the most common chronic blood-borne infection in the developed world, and there continues to be a significant need for more effective and better tolerated treatments," said Mani Subramanian, MD, PhD, Executive Director, Clinical Research - Infectious Diseases, HGS. "Only an estimated 40% of U.S. patients diagnosed with chronic hepatitis C have undertaken treatment to date - in part due to the side effects associated with interferon injections, which are currently required on a weekly basis. A monthly dosing schedule with Albuferon may well result in more patients choosing to be treated."

About the Design of the Phase 2b Monthly Dosing Trial

This Phase 2b trial is a randomized, open-label, multi-center, active-controlled, adaptive-design dose-ranging study to evaluate the safety and efficacy of albinterferon alfa-2b administered every four weeks plus daily ribavirin in treatment-naive patients with genotypes 2 and 3 chronic hepatitis C. Approximately 375 patients will be randomized in a 4:4:4:3 ratio into four treatment groups, including three that will receive albinterferon alfa-2b administered once every four weeks (900 mcg, 1200 mcg or 1500 mcg), in addition to the active-control group, which will receive peginterferon alfa-2a at the standard 180-mcg dose once every week. All patients in the study will receive 800-mg daily oral ribavirin. The total duration of treatment will be 24 weeks. The primary efficacy endpoint is sustained virologic response (SVR) at Week 48 (24 weeks following the end of treatment).

About Albinterferon Alfa-2b (Albuferon)

Albinterferon alfa-2b is a novel, longer-acting form of interferon alfa that was created using the proprietary HGS albumin-fusion technology. Human albumin is the most prevalent naturally occurring blood protein in the human circulatory system, persisting in circulation in the body for approximately 19 days. Research has shown that genetic fusion of therapeutic proteins to human albumin decreases clearance and prolongs the half-life of the therapeutic proteins. Albuferon results from the genetic fusion of human albumin and interferon alfa.

Albuferon is being developed by HGS and Novartis for the treatment of chronic hepatitis C under an exclusive worldwide co-development and commercialization agreement entered into in June 2006. HGS and Novartis will co-commercialize Albuferon in the United States and will share clinical development costs, U.S. commercialization costs and U.S. profits equally. Novartis will be responsible for commercialization in the rest of the world and will pay HGS a royalty on those sales. Clinical development, commercial milestone and other payments to HGS could total as much as $507.5 million, including $132.5 million received to date.

About Human Genome Sciences

The mission of HGS is to apply great science and great medicine to bring innovative drugs to patients with unmet medical needs. The HGS clinical development pipeline includes novel drugs to treat hepatitis C, lupus, inhalation anthrax and cancer.

For more information about HGS, please visit the Company's web site at www.hgsi.com.