When the Food and Drug Administration delivered a favorable ruling last week on Human Genome Sciences' drug Benlysta, the outcome wasn't exactly unexpected. Executives had expressed hushed confidence in its approval. Analysts were bullish on its prospects as the first new treatment for systemic lupus in half a century.

But many watching the drug's development from inside and outside the company's Rockville headquarters say its true potential may only be revealed over time as researchers discover ways to refine the benefits for lupus patients and explore its use in treating other ailments.

“That's really the underappreciated opportunity for this company,” said Avik Roy, a health care analyst at equity research firm Monness Crespi Hardt. “The market is under-appreciating the peak sales potential for this drug in lupus but there's also enormous opportunity for this drug potentially in other diseases.”

Stock in Human Genome Sciences closed at $27.94 on Friday afternoon, an 8.8 percent increase over its closing price on Wednesday just before the FDA issued its decision.

The drug's approval provides a validation of sorts for Human Genome Sciences and chief executive H. Thomas Watkins, who when hired in 2004 transformed the company's research-heavy heritage into a business fixated on commercial products.

Of the three drugs in development that executives pushed toward market, even at the expense of other projects, Benlysta was its last hope for blockbuster success. An anthrax treatment called Raxibacumab only had the government as a customer and a hepatitis C drug called Zalbin was abandoned last fall.

Researchers began work on Benlysta as early as 1996. “You can appreciate both the human and financial resources that goes along with that. That's what it takes to develop a drug these days,” Watkins said.

The payoff stands to be huge. The company priced the drug at $35,000 per person per year and anticipates a U.S. market of about 325,000 eligible patients. Once the drug is made available around the world, analysts predict multibillion-dollar revenues.

Human Genome Sciences is prepared to unleash a sales force that will total 150 people when partnered with the team at its development partner GlaxoSmithKline. Rumors that GSK might move to buy the company have swirled periodically throughout the development and regulatory process.

Health care analyst Roy said the benefits of such an acquisition would be mixed. Human Genome Sciences' value stands to rise as Benlysta is unfurled, so a sale now might be premature, he said. GSK, on the other hand, has two drugs in development that were licensed from Human Genome Sciences and if approved could mean large royalty payments each year.

Nevertheless, Watkins said, the firm is moving forward as an independent entity. He said the company has hired a staff of 50 at new offices across Europe, where regulators are expected to rule on Benlysta later this year.