Biogen and Belgian partner UCB said their experimental lupus treatment met the main goal of a late-stage trial in patients with the chronic disease, a surprise result after the drug failed an earlier mid-stage study.

The unexpected success, if replicated in a second late-stage study, could offer lupus patients a new alternative in a field littered with trial failures and limited approved therapies. The companies plan to start the second study later this year.

The drug, called dapirolizumab pegol, showed a more significant reduction in disease activity when given along with treatments such as corticosteroids and drugs that suppress the immune system, compared to a placebo in the 321-patient trial, the companies said.

The disease, which mostly affects women, is unpredictable as it causes a range of symptoms. Approved treatments include AstraZeneca's Saphnelo and GSK's Benlysta.

"There has been substantial progress for our patients, especially with the approval of some of these new therapies. But the reality is, the unmet need remains," Adam Meyers, the head of Biogen's immunology and new disease areas division, said in an interview prior to the data.

"We commonly hear things about modest efficacy or slower onset of action, and our goal is to try to improve upon treatment options for our patients," Meyers added.

The drug is being tested in patients with the most common form of lupus, systemic lupus erythematosus, where the patient's immune system attacks the body's own tissues and potentially leads to organ failure.