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

Rockville, MD -- June 19, 2009 -- Human Genome Sciences, Inc. (Nasdaq: HGSI) today announced that Novartis has completed enrollment and initial dosing in a Phase 2b clinical trial to 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.

"It is estimated that approximately 170 million people worldwide are infected with hepatitis C virus, including nearly four million in the United States," said Stephen Pianko, MD, FRACP, PhD, Monash University, Melbourne, Australia. "Even In developed countries, fewer than half of those who have been diagnosed with chronic hepatitis C have undertaken treatment - 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."

Mani Subramanian, MD, PhD, Executive Director, Clinical Research -- Infectious Diseases, HGS, said, "Novartis has made excellent progress in advancing the study of albinterferon alfa-2b dosed every four weeks. This dosing regimen of albinterferon alfa-2b, with a total of six injections, could offer an important treatment option if it demonstrates safety and efficacy comparable to peginterferon alfa-2a dosed once every week with a total of 24 injections."

About the Design of the Phase 2b Monthly Dosing Trial

This Phase 2b trial is a randomized, open-label, multi-center, active-controlled, dose-ranging study to evaluate the safety and efficacy of albinterferon alfa-2b administered every four weeks plus daily ribavirin in treatment-naïve patients with genotypes 2 and 3 chronic hepatitis C. 391 patients were 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 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.

The Company's primary focus is rapid progress toward the commercialization of its two lead drugs, Albuferon (albinterferon alfa-2b) for hepatitis C and BENLYSTA (belimumab, formerly LymphoStat-B) for lupus. Albuferon has now completed Phase 3 development, and the filing of global marketing applications is expected in fall 2009. Two Phase 3 trials of BENLYSTA are ongoing, with results expected in July and November 2009.

In April 2009, HGS completed delivery of 20,000 doses of raxibacumab (ABthrax) to the U.S. Strategic National Stockpile for use in the event of an emergency for the treatment of inhalation anthrax. The Company also has several drugs in earlier stages of clinical development for the treatment of cancer, led by the TRAIL receptor antibody HGS-ETR1 and a small-molecule antagonist of IAP (inhibitor of apoptosis) proteins. In addition, HGS has substantial financial rights to certain products in the GSK clinical pipeline including darapladib, currently in Phase 3 development as a potential treatment for coronary heart disease, and Syncria (albiglutide), currently in Phase 3 development as a potential treatment for type 2 diabetes.

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

Pharmasset Initiates Phase 1b Multiple
Ascending Dose Clinical Trial of PSI-7851
in Chronic Hepatitis C Patients

PSI-7851 was generally safe and well tolerated in Phase 1a single ascending dose trial

Further results from single ascending and multiple ascending dose trials are expected in second half 2009

Princeton, NJ -- June 9, 2009 -- Pharmasset, Inc. (Nasdaq: VRUS) announced today that it had completed the single ascending dose study and begun dosing in a multiple ascending dose trial with PSI-7851, a 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-7851 over 3 days in HCV-infected individuals.

"We are encouraged by the safety and pharmacokinetics of PSI-7851 thus far," stated Michelle Berrey, MD, MPH, Pharmasset's Chief Medical Officer. "We believe PSI-7851, Pharmasset's lead second generation nucleotide, has the potential to be administered once a day at low milligram doses, while also continuing to demonstrate the many benefits nucleos(t)ides have over other classes of HCV direct acting antivirals, including a high barrier to resistance, pan-genotype potency, and ability to combine with other classes of compounds."

PSI-7851 Phase 1 Program Overview

The Phase 1 program is investigating the safety, tolerability and pharmacokinetics of PSI-7851 in healthy subjects following single doses (Phase 1a) and in patients chronically infected with HCV genotype 1 following repeat dosing for 3 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-7851 ranging from 25mg to 800mg or a matching placebo. Preliminary data from the phase 1a single ascending dose study demonstrated:

- No serious adverse events or discontinuations;
- No dose-related adverse events;
- No grade III / IV lab abnormalities;
- No clinically significant changes in vital signs or ECGs.

A Phase 1b multiple ascending dose trial has now been initiated in patients with chronic HCV genotype 1 infection. Subjects will be enrolled at multiple centers and randomized to PSI-7851 (8 per cohort) or placebo (2 per cohort). Based upon the results from the SAD study, the first dose of PSI-7851 to be tested will be 50mg once daily. The primary objective is to assess the safety, tolerability and pharmacokinetics of PSI-7851 after repeat dosing over 3 days. The secondary objective is to evaluate the decrease in HCV RNA.

Results from both studies are expected in the second half of 2009.

About PSI-7851

PSI-7851 is a uridine nucleotide analog currently in development for the treatment of chronic HCV infection. PSI-7851 has demonstrated potent in vitro anti-HCV activity with EC50 values of 90 +/- 60 nM, which is approximately 15- to 20-fold more potent than Pharmasset's first generation nucleoside polymerase inhibitor, R7128. In vitro studies of PSI-7851 have not shown evidence of any mitochondrial or other cellular toxicities that may be associated with some nucleoside analogs. The half-life of the triphosphate in primary human hepatocytes is approximately 38 hours, which suggests the possibility for once-daily dosing. Like R7128, PSI-7851 has demonstrated in vitro activity against all of the most common HCV genotypes.

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) and, secondarily, on the development of Racivir(R) for the treatment of human immunodeficiency virus (HIV). Our research and development efforts focus on nucleos(t)ide analogs, a class of compounds which act to inhibit the enzymes required for viral replication. We currently have three clinical-stage product candidates: R7128, a nucleoside analog for chronic HCV infections, has initiated a Phase 2b clinical trial in combination with Pegasys plus Copegus through a strategic collaboration with Roche; PSI-7851, an unpartnered, next generation HCV nucleotide analog, which recently began Phase 1 clinical studies and Racivir, for the treatment of HIV that has completed a Phase 2 clinical trial.

For more information, see the web site at www.pharmasset.com.