UCB SA (UCB.BT) is shifting its business focus to immunology and the central nervous system as the Belgian pharmaceutical company seeks to offset patent expirations on blockbuster drugs like allergy drug Zyrtec and epilepsy drug Keppra in the U.S.

Cimzia is one of three core products on which the company is betting, together with Parkinson's patch Neupro and epilepsy Vimpat. UCB's share price slumped Jan. 6 on the news the U.S. Food and Drug Administration had asked for more information before deciding whether to approve the drug as a treatment for rheumatoid arthritis.

THE DRUG: UCB wants to sell Cimzia to treat Crohn's disease, a painful inflammation of the bowel, and rheumatoid arthritis. The drug has been approved by the U.S. Food and Drug Administration to cure Crohn's disease in April 2008. It had been approved in Switzerland in Dec. 2007, while it is still awaiting a green light in Europe.

Rheumatoid arthritis, which causes chronic inflammation of the joints, offers a much bigger market potential than Crohn's disease.

Any slowdown on the drug's approval will have an impact on the company's restructuring because it will delay income from a new, key drug which should replace revenue from older products that lose their patent exclusivity. "Any delay is going to widen this earnings gap," said Degroof analyst Thibaud Rutsaert.

THE OPPORTUNITY: The Crohn's disease treatment is a $1 billion market, while rheumatoid arthritis is estimated to be worth around $4 billion in sales for drugs comparable with Cimzia. The US market accounts for as much as 75% of that potential market.

UCB estimates that about five million people suffer from rheumatoid arthritis worldwide, with up to 1% of the population in industrialized countries being affected.

PERFORMANCE: Cimzia has been on sale for Crohn's disease in Switzerland since January 2008, and in the U.S. since April. Sales last year totaled EUR10 million, mainly driven by new patients in the U.S.

Abbott Laboratories' (ABT) Humira is the closest competitor, on which Cimzia has a price advantage, as UCB priced it 5% lower for Crohn's disease in the U.S. The drug retails at about $1,300 for one month's treatment.

WHAT UCB SAYS: Cimzia has the potential to become a blockbuster drug, which means reaching sales around $1 billion per year. Given that it is a drug for treating chronic diseases, Cimzia is expected to have a "slow uptake" in sales.

The FDA's request for a new safety update isn't unusual for the approval of such a complex drug. The process is on schedule, as UCB submitted a request in Feb. 2008 and approval usually takes between 18 and 24 months.

WHAT ANALYSTS SAY: The FDA request will delay Cimzia's approval for rheumatoid arthritis to late 2009 at least. Many analysts expect eventual approval to come between 12 and 18 months after UCB's filing.

WHAT'S NEXT: UCB will submit the requested additional information to the FDA in the second quarter. Meanwhile sales for Crohn's disease are expected to slowly increase in the U.S. In Europe, UCB will await a response on rheumatoid arthritis, for which indication it filed for approval in July, before resuming discussions with authorities on Crohn's disease indication.

-By Alessandro Torello, Dow Jones Newswires; +32 2 741 14 88; alessandro.torello@dowjones.com