Salix Pharmaceuticals rebounded from a string of regulatory setbacks this week with the Food and Drug Administration’s approval of Fulyzaq, its treatment for chronic diarrhea in HIV/AIDS patients.

Fulyzaq was approved Monday by the FDA, nearly four months after the agency delayed making a decision on Salix’s new drug application.

That temporary setback followed the agency’s request in September for additional test data on Relistor, a treatment for constipation in patients taking pain medicines. In addition, in November 2011 the FDA raised unexpected concerns about the Raleigh company’s efforts to get Xifaxan approved for the treatment of irritable bowel syndrome. Salix still hopes ultimately to win approval for these drugs as well.

“One down, two more to go,” spokesman Mike Freeman said.

Salix shares closed at $42.48 on Wednesday, up $2. The company’s shares are down from a 52 week high, recorded in July, of $55.60.

Fulyzaq, which the company expects to launch in March, will be Salix’s 15th prescription drug to hit the market. The company, which typically licenses the rights to drugs invented by others and then ushers them through development, won approval of its first drug in July 2000.

“To have come this far in such a short period of time, it’s a testament to this current management team,” Jefferies & Co. analyst Corey Davis said. “Just getting anything through the FDA these days is a huge accomplishment.”

He added that the management team’s credibility took a hit with its recent setbacks, and Fulyzaq’s approval demonstrates that they can still get a drug approved by the agency.

“Very few drugs go through clinical trials (and the approval process) without any kind of hitches,” Davis said.

Although Davis is optimistic that Salix also will eventually win approval of Xifaxan, he doesn’t expect Relistor to make the grade. The additional clinical trial that the FDA would like Salix to perform would be so large that it’s prohibitively expensive and probably isn’t even feasible, he said, although Salix is trying to persuade the agency to change its position.

Salix, as well as some analysts, project that Fulyzaq could achieve peak annual sales of $150 million to $200 million. But Davis isn’t nearly so upbeat, anticipating that peak annual sales will be “something closer to $50 million.”

“Everyone has a different opinion on these things,” Davis said.

Even analysts such as Tim Lugo of William Blair & Co., who endorses the $150 million-to-$200 million projection, anticipates that Fulyzaq sales won’t be “meaningful” to the company in the near-term, given the ramp-up time needed for the sale of new drugs.

Salix, which has 220 employees in Raleigh and 530 in all, has forecast that its revenue for 2012 will total $735 million, a 36 percent increase over 2011.

Salix’s ambitions for Fulyzaq, however, aren’t limited to the HIV/AIDS market. The company also is exploring the possibility of conducting tests that would enable it to seek approval of the drug as a treatment for “garden-variety diarrhea,” Freeman said.

That’s similar to the game plan that Salix has been following for Xifaxan, its top-selling drug. The company initially won approval for Xifaxan as a treatment for travelers’ diarrhea, and then obtained the OK to tout it as a treatment for a rare liver condition – in addition to its pending application to treat irritable bowel syndrome.

In 2008, Salix acquired the rights to sell Fulyzaq in North America, Europe and Japan from Napo Pharmaceuticals for a $5 million licensing fee and future milestone payments. Napo has been seeking to terminate that licensing agreement in a still-pending lawsuit that complains that Salix intentionally delayed the development of the drug. Salix contends the lawsuit has no merit.

Napo executives weren’t available to comment on the latest development.

Fulyzaq has a minimum of five years of patent protection under U.S. law, and Salix will seek additional protection based on the amount of time it took to develop the drug, Freeman said.

Patent protection aside, however, Freeman said, it’s unlikely that a cheaper generic version of Fulyzaq will ever be produced.

Fulyzaq is derived from red goo found in Croton lechleri trees in South American, and is just the second “botanical” prescription drug the FDA ever approved.

“This being a botanical product, the process to get it from the sap of the tree and (create) the final product is so complicated that we are almost 100 percent confident that no one will ever be able to” replicate the process, Freeman said.

Some analysts agree.

“We believe that its manufacturing process should represent a strong barrier to entry” for generic companies, Cantor Fitzgerald analyst Irina Rivkind wrote in a research note.

Ranii: 919-829-4877