Nearly one third of patients with ulcerative colitis who received an experimental Pfizer Inc drug in a clinical trial were in remission after a year of treatment, the drugmaker said on Tuesday.

The drug, etrasimod, was the key asset Pfizer picked up in its $6.7 billion deal for Arena Pharmaceuticals last year. Analysts have estimated that peak sales could top $3 billion a year.

Ulcerative colitis (UC) is an inflammatory bowel disease. It affects around 3.8 million people in North America and Europe, according to the company, which plans to seek U.S. approval for etrasimod to treat UC in the second half of this year.

In its 52-week, 409-patient study, 32.1% of those who received etrasimod were in clinical remission after one year, compared with 6.7% for those who got a placebo.

Twelve weeks into that trial, clinical remission was 27% among etrasimod recipients versus 7.4% in the placebo arm.

In a separate 12-week study, Pfizer said clinical remission was achieved among 24.8% of patients receiving etrasimod compared to 15.2% of patients receiving a placebo.

Etrasimod belongs to a class of drugs called sphingosine 1-phosphate (S1P) receptor modulators. If approved, etrasimod would compete with Bristol Myers Squibb's S1P UC drug Zeposia.

"We've always been interested in S1P," Michael Corbo, Pfizer’s chief development officer for inflammation and immunology, said in an interview. "We think that we've gotten one that is really one of the best that we're going to find."

Pfizer had previously said both etrasimod trials succeeded, but did not disclose the actual data at that time.