One measure of VX 950's promise: wide licensing early in its lifecycle. Eli Lilly agreed in 1997 to provide up to $50 million in research support for Vertex's development of orally active protease inhibitors. In 2004, Mitsubishi purchased rights to develop and commercialize VX 950 in Japan and other Far East countries.

Also poised to enter the hepatitis C market is valpocitabine (NM 283), an RNA polymerase inhibitor currently in Phase II studies. Designed as a once-a-day oral therapy, also probably in combination with interferon, the compound has proven active against genotype 1 of HCV, the most treatment-resistant strain of the virus.

"It's early days for these drugs," Molowa says. "So we have to wait and see. But interferon is the only game in town now. It's a multibillion-dollar market, so there's a nice opportunity for additional add-on therapies."