Analysts see promise in Pharmasset's lead candidate, a pill labeled PSI-7977, which has just started late-stage testing. They say it could become a preferred option for care if it works without the injectable drug interferon, which can leave patients with flulike symptoms that last for months.
Before the drug reaches the market, though, it must pass through the largest and most expensive stage of clinical testing and receive Food and Drug Administration approval. Analyst Erik Gordon of University of Michigan's Ross School of Business said the price Gilead agreed to pay multiplies the risk in that process.
"At that price, everything had better work perfectly," he said.
Hepatitis C treatments represent a ripe opportunity for drugmakers. The virus can lead to life-threatening liver damage and is the main cause of liver transplants in the United States.
Analysts expect the disease to become a bigger health problem because of the large, aging population of U.S. baby boomers, including some people who used intravenous drugs when they were younger.
Pharmasset has three potential hepatitis C treatments in clinical testing, including PSI-7977. That one has already demonstrated its effectiveness in hepatitis C patients with genotypes 2 and 3, but analysts want to see how it works in genotype 1, the most prevalent subset in the Western world and the hardest to treat.
Earlier this year, the FDA approved two pills that offered the first new breakthrough treatments for the liver disease in 20 years. The Vertex Pharmaceuticals Inc. hepatitis C pill Incivek rang up about $420 million in this year's third quarter. Another pill, Merck & Co. Inc.'s Victrelis had $31 million in sales in the quarter.
Shares of Pharmasset soared 85 percent, or $61.47, to close Monday at $134.14, while Gilead's stock dropped more than 9 percent, or $3.62, to close at $36.26.